Installation of Pipelines by Horizontal Directional Drilling An Engineering Design Guide [PDF]

Zitiervorschau

Catalog No. L51730

Installation of Pipelines by Horizontal Directional Drilling An Engineering Design Guide Contract PR-227-9424

Prepared for the Design Applications Supervisory Committee (Off/On Shore Supervisory Committee) Pipeline Research Council International, Inc.

Prepared by the following Research Agencies: J.D. Hair and Associates, Inc. Louis J. Capozzi & Associates, Inc. Stress Engineering Services, Inc.

Author: Paul D. Watson

Publication Date: April 15, 1995

“This report is furnished to Pipeline Research Council International, Inc. (PRCI) under the terms of PRCI PR-227-9424, between PRCI and J.D. Hair and Associates, Inc., Louis J. Capozzi & Associates, Inc., Stress Engineering Services, Inc.. The contents of this report are published as received from J.D. Hair and Associates, Inc., Louis J. Capozzi & Associates, Inc., Stress Engineering Services, Inc.. The opinions, findings, and conclusions expressed in the report are those of the authors and not necessarily those of PRCI, its member companies, or their representatives. Publication and dissemination of this report by PRCI should not be considered an endorsement by PRCI or J.D. Hair and Associates, Inc., Louis J. Capozzi & Associates, Inc., Stress Engineering Services, Inc., or the accuracy or validity of any opinions, findings, or conclusions expressed herein. In publishing this report, PRCI makes no warranty or representation, expressed or implied, with respect to the accuracy, completeness, usefulness, or fitness for purpose of the information contained herein, or that the use of any information, method, process, or apparatus disclosed in this report may not infringe on privately owned rights. PRCI assumes no liability with respect to the use of , or for damages resulting from the use of, any information, method, process, or apparatus disclosed in this report. The text of this publication, or any part thereof, may not be reproduced or transmitted in any form by any means, electronic or mechanical, including photocopying, recording, storage in an information retrieval system, or otherwise, without the prior, written approval of PRCI.”

Pipeline Research Council International Catalog No. L51730 Price: $995 Copyright, 1995 All Rights Reserved by Pipeline Research Council International, Inc. PRCI Reports are Published by Technical Toolboxes, Inc. 3801 Kirby Drive, Suite 340 Houston, Texas 77098 Tel: 713-630-0505 Fax: 713-630-0560 Email: [email protected]

PIPELINE RESEARCH COUNCIL INTERNATIONAL

G. L. Walker, Pacific Gas Transmission Company (Chairman) E. E. Thomas, Southern Natural Gas Company (Vice Chairman) P. S. Anderson, Foothills Pipe Lines Ltd. R. L. Brown, Natural Gas Pipeline Company of America E. Herløe, Statoil R. C. Hesje, Transportadora de Gas de1 Norte M. C. Hocking, Transcontinental Gas Pipe Line Corp. D. L. Johnson, Enron Operations Corp W. A. Johnson II, El Paso Natural Gas Company D. F. Keprta, ARCO Oil and Gas Company R. E. Keyser, Panhandle Eastern Corporation R. W. Little, Union Gas Limited J. P. Lucido, ANR Pipeline Company H. A. Madariaga, Southern California Gas Company J. K. McDonald, East Australian Pipeline Ltd. D. J. McNiel, Tenneco Gas M. Merrill, BP Pipelines (Alaska) Inc. K. J. Naarding, N. V. Nederlandse Gasunie C. W. Petersen, Exxon Production Research Company D. E. Reid, TransCanada PipeLines, Ltd. P. R. Smullen, Shell Development Company B. J. Sokoloski, CNG Transmission Corporation P. M. Sørensen, Dansk Olie og Naturgas A/S B. C. Sosinski, Consumers Power Company R. J. Turner, NOVA Gas Transmission Ltd. D. C. Walker, Oklahoma Natural Gas Company T. L. Willke, Gas Research Institute K. F. Wrenn, Jr., Columbia Gas Transmission Corp. T. F. Murphy, American Gas Association (PRC Staff) A. G. Cotterman, American Gas Association (PRC Staff)

OFFSHORE AND ONSHORE DESIGN APPLICATIONS SUPERVISORY COMMITTEE R. E. Keyser, Panhandle Eastern Corporation (Chairman) *D. W. Allen, Shell Development Company *J. A. Barbalich, Tenneco Gas *S. T. Barbas, Exxon Production Research Company *R. L. Barron, Texas Gas Transmission Corp. *L. M. Bums, Colorado Interstate Gas Company T. D. Caldwell, BP Exploration, Inc. J. C. Chao, Exxon Production Research Company G. W. Connors, Union Gas Limited *M. J. Coyne, Shell Oil Company D. A. Degenhardt, Natural Gas Pipeline Co. of America J. P. Dunne, ANR Pipeline Company J. R. Ellwood, Foothills Pipe Lines Ltd. R. W. Gailing, Southern California Gas Company R. E. Hoepner, Transcontinental Gas Pipe Line Corp. W. C. Kazokas, Jr., ARCO Exploration and Production Technology *J. Kleinhans, BP Exploration, Inc. *F. Kopp, Shell Oil Company S. W. Lambright, Consumers Power Company *C. G. Langner, Shell Development Company W. R. Ledbetter, Tenneco Gas C. Lee, Pacific Gas and Electric Company S. Lund, Statoil S. N. Marr, TransCanada PipeLines, Ltd. O. Medina, El Paso Natural Gas Company *J. E. Meyer, Panhandle Eastern Corporation *K. C. Peters, Southern Natural Gas Company M. Rizkalla, NOVA Gas Transmission Ltd. *L. A. Salinas, Tenneco Gas *O. R. Samdal, Statoil J. Spiekhout, N. V. Nederlandse Gasunie J. E. Thygesen, Dansk Olie og Naturgas A/S *R. Verley, Statoil L. D. Walker, Southern Natural Gas Company A. G. Cotterman, American Gas Association (PRC Staff) *Alternate or Ad Hoc Group Member only Special thanks to the following PR-227-9321 ad hoc group members: R. E. Hoepner, Transcontinental Gas Pipe Line Corp. (Chairman) J. A. Barbalich, Tenneco Gas D. A. Degenhardt, Natural Gas Pipeline Co. of America J. P. Dunne, ANR Pipeline Company R. W. Gailing, Southern California Gas Company J. E. Meyer, Panhandle Eastern Corporation M. Rizkalla, NOVA Gas Transmission Ltd. J. Spiekhout, N. V. Nederlandse Gasunie

Table of Contents

Executive Summary 1. The Horizontal Directional Drilling Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Pilot Hole Directional Drilling, 1; Jetting, 3; Downhole Motors, 3; Wash Pipe, 3; Downhole Surveying, 3; Surface Monitoring, 4; Reaming & Pulling Back, 5; Prereaming, 5; Pulling Back, 7; Buoyancy Control, 7. 2. Feasibility Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Technical Feasibility, 8; Subsurface Soil Material, 9; Contractual Feasibility, 10; Economic Feasibility, 12; Cost Estimating, 12; Estimating Parameters, 12; Shift Cost Summary, 16; Estimate Recap, 18; Owner’s Cost, 18. 3. Site Characterization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Geological Factors, 21; Topographic and Hydrographic Details, 22; Geotechnical Aspects, 22; Unified Classification System for Soil Type, 22; Soil Condition Parameters, 25; Rock Condition Parameters, 26; Material Strengths, 27; Deformation Potential, 27; Groundwater, 27; Subsurface Stratification, 28; Site Characterization Study Contents, 28; Responsibility for Site Characterization, 28; Definition of the Obstacle, 29; Site Exploration, 29; Surface Survey, 30; Subsurface Survey, 30. 4. General Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Definition of the Obstacle, 32; Drilled Path Design, 32; Definition of Curves, 34; Entry and Exit Points, 34; Entry and Exit Angles, 34; Depth of Cover; 34; Design Radius of Curvature, 35; Directional Accuracy and Tolerances, 35; Pipe Specification, 35; External Pipe Coating, 36; Multiple Line Installation, 36.

5. Pipe Stress Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Installation Loads and Stresses, 37; Pulling Load Calculation Method, 38; Drilled Path Analysis, 38; Pulling Loads, 38; Installation Stress Analysis, 45; Individual Loads, 45; Combined Loads, 47; Example Pulling Load Calculation, 48; Example Installation Stress Analysis, 54; Operating Loads and Stresses, 56; Combined Stresses and Limitations, 57; Example Operating Stress Analysis, 58; Spreadsheet - Load and Stress Analysis, 59. 6. Construction Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Workspace, 61; Horizontal Drilling Rig, 61; Pull Section Fabrication, 63; Drilling Fluids, 65; Functions, 66; Composition, 66; Quantity Estimating Calculations, 67; Recommended Disposal Methods, 70; Environmental Impact, 73. 7. Contractual Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Lump Sum Contracts, 77; Pricing, 77; Unknown Subsurface Condition Risk, 78; Technical Specification, 78; Plan & Profile Drawing, 78; Daywork Contract, 83; Uniform Daywork Bid Sheet, 83; Equipment Failure Risk, 83. 8. Construction Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Drilled Path, 87; Construction Staking, 87; Pilot Hole, 87; Directional Drilling Performance, 92; Downhole Survey Calculations, 92; Radius of Curvature Calculations, 94; TruTracker Surface Monitoring System, 95; Asbuilt Error Distribution, 96; Pipe Installation, 96; Pull Section Handling, 96; Buoyancy Control, 96; Coating Integrity, 96; Drilling Fluid Flow, 96. Bibliography Metric “SI” Unit Conversion Table Glossary

EXECUTIVE SUMMARY This engineering design guide is the principal product of PRC project PR-227-9424. Its purpose is to serve as a step by step guide for engineers engaged in the evaluation, design, and management of natural gas pipeline construction by Horizontal Directional Drilling (HDD). It is not intended to replace sound engineering judgment in the design process nor can it possibly address every question which might arise in the design of any specific crossing. HDD pipeline design involves sophisticated engineering principles and should be performed under the supervision of a qualified professional engineer. The guide contains eight sections which address the following general topics. 1. A description of the HDD installation process; 2. Feasibility considerations including the state of the art in HDD, factors which limit its use, and a method for estimating the detailed cost of HDD installations under various conditions; 3. Components of a site characterization required for HDD design and bidding including geological factors, geotechnical aspects, and field survey requirements; 4. General considerations relative to drilled path design, pipe specification, external pipe coating, and multiple line installation; 5. Methods for analyzing pipe stresses both during installation and under operating conditions including a method for calculating pulling loads involved with pull back; 6. The impact of HDD operations on the environment including a discussion of drilling fluid functions, composition, quantities, and disposal methods; 7. General considerations relative to contract form, unknown subsurface condition risk, technical specifications, design drawings; and 8. Inspection requirements during construction including a detailed discussion of downhole survey calculation methods. Cost estimating, pipe stress, and drilling fluid quantity calculation methods are presented in a Lotus l-2-3 spreadsheet format and demonstrated with sample problems. A diskette containing spreadsheet tiles is fixed to the inside back cover of the guide. Photographs and sketches have been included where appropriate to illustrate construction operations.

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SECTION 1 - THE HORIZONTAL DIRECTIONAL, DRILLING PROCESS

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SECTION 1 THE HORIZONTAL DIRECTIONAL DRILLING PROCESS The horizontal directional drilling process represents a significant improvement over traditional cut and cover methods for installing pipelines beneath obstructions, such as rivers or shorelines, which warrant specialized construction attention. In order to take full advantage of the benefits offered by horizontal directional drilling (HDD) and produce designs which can be efficiently executed in the field, design engineers should have a working knowledge of the process. This section presents a general description of the HDD process. The tools and techniques used in the HDD process are an outgrowth of the oil well drilling industry. The components of a horizontal drilling rig used for pipeline construction are similar to those of an oil well drilling rig with the major exception being that a horizontal drilling rig is equipped with an inclined ramp as opposed to a vertical mast. HDD pilot hole operations are not unlike those involved in drilling a directional oil well. Drill pipe and downhole tools are generally interchangeable and drilling fluid is used throughout the operation to transport drilled spoil, reduce friction, stabilize the hole, etc. Because of these similarities, the process is generally referred to as drilling as opposed to boring. Installation of a pipeline by HDD is generally accomplished in two stages as illustrated in Figure l-l. The first stage consists of directionally drilling a small diameter pilot hole along a designed directional path. The second stage involves enlarging this pilot hole to a diameter which will accommodate the pipeline and pulling the pipeline back into the enlarged hole. Pilot Hole Directional Drilling Pilot hole directional control is achieved by using a non-rotating drill string with an asymmetrical leading edge. The asymmetry of the leading edge creates a steering bias while the non-rotating aspect of the drill string allows the steering bias to be held in a specific position while drilling. If a change in direction is required, the drill string is rolled so that the direction of bias is the same as the desired change in direction. The direction of bias is referred to as the tool face. Straight progress may be achieved by drilling with a series of offsetting tool face positions. The drill string may also be continually rotated where directional control is not required. Leading edge asymmetry can be accomplished by several methods. Typically, the leading edge will have an angular offset created by a bent sub or bent motor housing. This is illustrated schematically in Figure l-2.

SECTION 1 - THE HORIZONTAL DIRECTIONAL DRILLING PROCESS

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STAGE 1, PILOT HOLE DIRECTIONAL DRILLING HORIZONTAL DRILLING RIG DRILLING FLUID RETURNS

EXIT POINT

DESIGNED DRILLED PATH THEORETICAL ANNULUS GENERAL DIRECTION OF PROGRESS PILOT HOLE DRILLING

STAGE 2, REAMING & PULLING BACK

THEORETICAL ANNULUS GENERAL DIRECTION OF PROGRESS PREREAMING

GENERAL DIRECTION OF PROGRESS PULLING BACK

Figure l-l The HDD Process

SECTION 1 - THE HORIZONTAL DIRECTIONAL DRILLING PROCESS

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DOWNHOLE MOTOR BIT

BENT SUB NON-MAGNETIC COLLAR

Figure 1-2 Bottom Hole Assembly Jetting It is common in soft soils to achieve drilling progress by hydraulic cutting with a jet nozzle. In this case, the direction of flow from the nozzle can be offset from the central axis of the drill string thereby creating a steering bias. This may be accomplished by blocking selected nozzles on a standard roller cone bit or by custom fabricating a jet deflection bit. If hard spots are encountered, the drill string may be rotated to drill without directional control until the hard spot has been penetrated. Downhole Motors Downhole mechanical cutting action required for harder soils is provided by downhole hydraulic motors. Downhole hydraulic motors, commonly referred to as mud motors, convert hydraulic energy from drilling mud pumped from the surface to mechanical energy at the bit. This allows for bit rotation without drill string rotation. There are two basic types of mud motors; positive displacement and turbine. Positive displacement motors are typically used in HDD applications. Basically, a positive displacement mud motor consists of a spiralshaped stator containing a sinusoidal shaped rotor. Mud flow through the stator imparts rotation to the rotor which is in turn connected through a linkage to the bit. Wash Pipe In some cases, a larger diameter wash pipe may be rotated concentrically over the nonrotating steerable drill string. This serves to prevent sticking of the steerable string and allows its tool face to be freely oriented. It also maintains the pilot hole if it becomes necessary to withdraw the steerable string. Downhole Surveying The actual path of the pilot hole is monitored during drilling by taking periodic readings of the inclination and azimuth of the leading edge. Readings are taken with an instrument,

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SECTION 1 - THE HORIZONTAL DIRECTIONAL DRILLING PROCESS

commonly referred to as a probe, inserted in a drill collar as close as possible to the drill bit. Transmission of downhole probe survey readings to the surface is generally accomplished through a wire running inside the drill string. These readings, in conjunction with measurements of the distance drilled since the last survey, are used to calculate the horizontal and vertical coordinates along the pilot hole relative to the initial entry point on the surface. Survey calculation methods are discussed in detail in Section 8. Azimuth readings are taken from the earth’s magnetic field and are subject to interference from downhole tools, drill pipe, and magnetic fields created by adjacent structures. Therefore, the probe must be inserted in a non magnetic collar and positioned in the string so that it is adequately isolated from downhole tools and drill pipe. The combination of bit, mud motor (if used), subs, survey probe, and non magnetic collars is referred to as the Bottom Hole Assembly or BHA. A typical bottom hole assembly is shown as Figure 1-2. Surface Monitoring

The pilot hole path may also be tracked using a surface monitoring system. Surface monitoring systems determine the location of the probe downhole by taking measurements from a grid or point on the surface. An example of this is the TruTracker® System. This system uses a surface coil of known location to induce a magnetic field. The probe senses its location relative to this induced magnetic field and communicates this information to the surface. This is shown schematically in Figure l-3.

KNOWN CORNER LOCATIONS

SURFACE COIL

Figure l-3

TruTracker Surface Monitoring System (TruTracker is a Trademark of Sharewell, Inc.)

SECTION 1 - THE HORIZONTAL DIRECTIONAL DRILLING PROCESS

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Reaming & PulIing Back Enlarging the pilot hole is accomplished using either prereaming passes prior to pipe installation or simultaneously during pipe installation. Reaming tools typically consist of a circular array of cutters and drilling fluid jets and are often custom made by contractors for a particular hole size or type of soil. Examples of different types of reaming tools are shown in Figures l-4, l-5, 1-6. Prereaming Most contractors will opt to preream a pilot hole before attempting to install pipe. For a prereaming pass, reamers attached to the drill string at the exit point are rotated and drawn to the drilling rig thus enlarging the pilot hole. Drill pipe is added behind the reamers as they progress toward the drill rig. This insures that a string of pipe is always maintained in the drilled hole. It is also possible to ream away from the drill rig. In this case, reamers fitted into the drill string at the rig are rotated and thrust away from it.

Figure l-4 44 inch hole opener typically used for rock crossings. (photo courtesy of Specialty Drilling Services)

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SECTION 1 - THE HORIZONTAL DIRECTIONAL DRILLING PROCESS

Figure l-5 Soft soil flycutter emerging from exit point with pull section attached. (photo courtesy of Michels Pipeline Construction Co.)

Figure 1-6 42 inch barrel reamer typically used in prereamed holes. (photo courtesy of Specialty Drilling Services)

SECTION 1 - THE HORIZONTAL DIRECTIONAL DRILLING PROCESS

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Pulling Back Pipe installation is accomplished by attaching the prefabricated pipeline pull section behind a reaming assembly at the exit point and pulling the reaming assembly and pull section back to the drilling rig. This is undertaken after completion of prereaming or, for smaller diameter lines in soft soils, directly after completion of the pilot hole. A swivel is utilized to connect the pull section to the leading reaming assembly to minimize torsion transmitted to the pipe (refer to Figure 1-5). The pull section is supported using some combination of roller stands, pipe handling equipment, or a flotation ditch to minimize tension and prevent damage to the pipe. Buoyancy Control Uplift forces resulting from the buoyancy of larger diameter lines can be very substantial. High pulling forces may be required to overcome drag resulting from buoyancy uplift. Therefore, contractors will often implement measures to control the buoyancy of pipe 30 inches or over in diameter. The most common method of controlling buoyancy is to fill the pipe with water as it enters the hole. This requires an internal fill line to discharge water at the leading edge of the pull section (after the breakover point). An air line may also be required to break the vacuum which may form at the leading edge as the pull section is pulled up to the rig. The amount of water placed in the pipe is controlled to provide the most advantageous distribution of buoyant forces. Some contractors may choose to establish a constant buoyancy. This can be accomplished by inserting a smaller diameter line into the pull section and filling the smaller line with water. The smaller line is sized to hold the volume of water required per lineal foot to offset the uplift forces. References Microtunneling & Horizontal Directional Drilling, Proceedings of the First Trenchless Excavation Center (TEC) Symposium, November 13-15, 1990, Houston, Texas. Rotary Drilling, Controlled Directional Drilling, Unit III, Lesson 1, Courtesy, Petroleum Extension Service (PETEX), The University of Texas at Austin.

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SECTION 2 - FEASIBLITY CONSIDERATIONS

SECTION 2 FEASIBILITY CONSIDERATIONS Three standards may be used to assess the feasibility of HDD for a given crossing. These are technical, contractual, and economic. First, a crossing is technically feasible if it can be installed using existing tools and techniques regardless of uncertainties surrounding the cost of installation. Second, a crossing is contractually feasible if the cost of installation can be accurately estimated in advance allowing contractors to submit lump sum bids. Third, a crossing is economically feasible if its installation cost is less than the cost of an equivalent construction method. Technical Feasibility For a pipeline to be installed by HDD, one of two conditions must be achieved downhole. Either an open hole must be cut into the subsurface material to such an extent that installation of a pipeline by the pull back method is possible, or the soil properties must be modified so that it behaves in fluid manner allowing a pipeline to be pulled through it. The possibility of achieving either of these conditions downhole is dependent primarily on subsurface soil conditions. The open hole condition is similar to that achieved in a typical oil well. A cylindrical hole is cut in the subsurface. Drilling fluid flows to the surface in the annulus between the pipe and the hole wall. Drilled spoil is transported in the drilling fluid to the surface. This is generally applicable to rock and cohesive soils. It may also apply to some sandy or silty soils depending on the density of the material, the specific makeup of the coarse fraction, and the binding or structural capacity of the fine fraction. It is probable that loose cohesionless soils will not support an open hole over a long horizontally drilled length. This does not, however, prevent the installation of a pipeline. The mechanical agitation of the reaming tool coupled with the injection of bentonitic drilling fluid will cause the soil to experience a decrease in shear strength. If the resulting shear strength is low enough, the soil will behave in a fluid manner allowing a pipe to be pulled through it. The fluid behavior of loose sands, commonly referred to as quicksand, is defined by geotechnical engineers as liquefaction. If either an open hole or fluid condition can be achieved downhole and the stresses imposed on the pipe and tooling are not excessive, installation by HDD is technically feasible. The technical feasibility of a proposed HDD installation can be predicted by comparing it to past installations in three basic parameters: drilled length, pipe diameter, and subsurface soil material. These three parameters work in combination to limit what can be achieved at a

SECTION 2 - FEASIBILITY CONSIDERATIONS

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given location. Installations which define the state of the art in length and diameter as of 1994 are presented in Table 2 - 1. Table 2 - l State of the Art Installations as of 1994 Location

Length

Diameter

Soil Material

Date

Wormley Creek Yorktown, VA

5,850 ft. (1,783 m)

10 in. (DN 250)

Alluvial

1994

SB Elizabeth River Norfolk, VA

2,160 ft (658 m)

48 in. (DN 1200)

Alluvial

1993

Limitations with respect to length and diameter are primarily due to limits on the capacity of existing tools and drill pipe. Present technology involves thrusting pipe from the surface to advance a pilot hole. The flexibility of relatively slender drill pipe does not allow an unlimited amount of thrust to be applied. Control of the leading edge diminishes over long lengths. Present technology also involves rotating pipe at the surface to rotate reamers downhole. The capacity of drill pipe for the transmission of torsion is limited. Installation of a 48 inch pipe will typically require completion of a 60 inch reaming pass. While development of new tools and techniques which increase load bearing and energy transmission capacities of drill pipe is possible, economic factors come into play. The market for HDD installation of pipe over longer lengths or larger diameters than those presented in Table 2-l has not been defined. Subsurface Soil Material While length, diameter, and subsurface soil material work in combination to limit the technical feasibility of an HDD installation, technical feasibility is primarily limited by subsurface soil material. Two material characteristics prevent successful establishment of either an open hole or fluid condition. These are large grain content (i.e. gravel, cobbles) and excessive rock strength and hardness. Soils consisting principally of coarse grained material present a serious restriction on the feasibility of HDD. Coarse material cannot be readily fluidized by the drilling fluid. Neither is it stable enough to be cut and removed in a drilling fluid stream through an open hole as is the case in a crossing drilled in competent rock. A boulder or cluster of cobbles will remain in the drilled path and present an obstruction to a bit, reamer, or pipeline. They must be mechanically displaced during hole enlargement. Displacement may be radially outward into voids formed by the entrainment of finer grained (sand and smaller size) material. However, naturally dense, high gravel percentage soils contain little entrainable material and insufficient voids may be developed to permit passage by larger diameter reamers or pipe. Coarse material may also migrate to low spots on the drilled path forming impenetrable blocks.

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SECTION 2 - FEASIBILITY CONSIDERATIONS

Exceptionally strong and hard rock will hamper all phases of an HDD project. Experience has shown competent rock with unconfined compressive strengths exceeding 12,000 psi and Mohs Scale of Hardness factors ranging somewhat above 7 can be negotiated with today’s technology. However, entry of such materials at depth is usually difficult. The directional drilling string tends to deflect rather than penetrate. Conversely, poor quality (extensively fractured or jointed) rock can present the same problems as coarse granular deposits. Two of the most significant crossings installed to date in rock were completed in the Fall of 1991. The longest, at approximately 3,000 feet (914 m) was installed beneath the Niagara River near Niagara Falls, New York. This 30 inch (DN 750) crossing was placed through a soft shale. An additional installation in harder rock was completed in 1991 beneath the Housatonic River near Shelton, Connecticut. This 24 inch (DN 600) line penetrated approximately 1,200 feet (366 m) of hard, fine-grained schist in a total horizontal drilled length of approximately 1,732 feet (528 m). General guidelines for assessing the feasibility of prospective HDD installations based on earth material type and gravel percent by weight are presented in Table 2-2. Earth material type and gravel percent by weight are determined in the site characterization phase of HDD installation design discussed in Section 3.0. Engineering judgment based on a foundation of practical experience must be applied when using the guidelines presented in Table 2.2. Knowledge of subsurface conditions will be based on extrapolation of measured properties from discreet soil borings generally taken by individuals not involved in HDD construction. A crossing may be placed in competent rock beneath a river. Nevertheless, overburden soils will probably have to be penetrated before the rock stratum is entered. A crossing installed in the lower Mississippi River flood plain may encounter clays, silts, sands, and gravels of varying relative densities in a relatively short distance. Only the general character of the subsurface material will be known in advance of construction. Contractual Feasibility

Once the technical feasibility of a prospective HDD installation has been established, its contractual feasibility can be assessed. This assessment is accomplished in the same way as technical feasibility, by comparing it to past installations. If the crossing falls near the limits of the state of the art in any of the basic parameters; length, diameter, or soil conditions, it is possible that it may be viewed by contractors as too risky to undertake for a fixed lump sum price. It should be understood, however, that determination of contractual feasibility is very subjective and will vary for individual contractors based on their experience and commercial situation. In today’s market for HDD services, most crossings that are technically feasible will be bid on a lump sum basis by at least one contractor. Nonetheless, it is not unusual to receive only one lump sum bid for state of the art crossings or for lump sum bids received to be very high. If contractual feasibility is questionable, the benefits of a day work contract or an alternate construction method should be considered. Contractual considerations, including day work contracts, are discussed in Section 7.

SECTION 2 - FEASIBILITY CONSIDERATIONS

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Table 2-2 HDD Feasibility Assessment Guidelines Earth Material Type

Gravel % by Weight

HDD Feasibility

Very soft to hard strength, possibly slickensided, clay.

N/A

Good To Excellent. Plugging of the annulus surrounding the drill stem during pilot hole drilling may produce inadvertent drilling fluid returns through slickensides. Penetration of strong clay surrounded by considerably weaker or looser soils may result in the pilot bit “skipping” along the interface. Pilot hole steering difficulties are likely to result during passage through very soft layers.

Very loose to very dense sand with or without gravel traces.

0 to 30

Good to Excellent. Gravel may cause slight steering problems. Some steering imprecision may also result during passage through very loose material.

Very loose to very dense gravelly sand.

30 to 50 Marginally Acceptable. Drilling fluid characteristics and handling are critical to success. Pilot hole steering may be imprecise.

Very loose to very dense sandy gravel.

50 to 85

Very loose to very dense gravel.

85 to 100 Unacceptable. With present technology and experience, horizontal penetration, especially in the denser strata, is almost impossible. Such materials must be avoided or penetrated at a steep angle.

Rock.

N/A

Questionable. Horizontal penetration for any appreciable distance will be extremely difficult regardless of drilling fluid quality. Pilot hole steering may be imprecise.

Excellent to Unacceptable. Softer or partially weathered materials offer HDD performance akin to that of hard strength clay. Technology is available to drill through more competent rock, especially in the weaker horizontal plane. Penetrating solid rock after passing through soil may be difficult due to the bit’s tendency to “skip” along the lower hard surface. If in “rounded” cobble form, competent rock is virtually impossible to drill.

12

SECTION 2 - FEASIBILITY CONSIDERATIONS

Economic Feasibility Determining the economic feasibility of a prospective HDD installation is a fairly straightforward exercise involving comparison of the estimated cost of HDD with the estimated cost of an alternate installation method. If the HDD estimate is less, it is economically feasible. When making this comparison, it is important to estimate the cost of equivalent designs and to include all costs associated with each method. For example, it would not be valid to compare the cost of a pipeline river crossing installed by open excavation with 3 feet of cover against the cost of a drilled installation providing 25 feet of cover without including some adjustment in the excavated estimate to account for possible future remedial work brought on by the relatively shallow 3 foot cover. Restoration costs and the costs associated with environmental impact for each method considered must also be included. The environmental impact associated with HDD construction operations is discussed in Section 6. A procedure estimating the cost of specialized HDD services is presented in the following paragraphs. Cost Estimating The first step in accurately estimating the cost of specialized HDD services is to estimate the contractor’s direct job costs. Direct job costs are composed of two components, daily shift costs and non-daily costs. Daily shift costs are those costs which are dependent on the number of days operations are conducted. They are determined by calculating the cost of a given operation per shift and multiplying that figure by the number of shifts required to complete the operation. Non-daily costs are those costs which are not dependent the duration of operations such as equipment hauling involved with mobilization and demobilization. The cost to the owner is determined by adding a mark-up to the contractor’s direct costs. This mark-up covers the contractor‘s overhead, contingencies, and profit. A Lotus spreadsheet routine which performs these calculations is included in the file labeled ESTIMATE.WK4/WK3 on the diskette attached to the back cover of this manual. The routine is designed to estimate costs involved with the operation of a typical horizontal drilling rig. Included at the end of this section is the printout from an example estimate which has been performed using this routine. The example calculates an owner’s cost estimate for a 24 inch pipeline river crossing with a drilled length of 2,500 feet in soft alluvial deposits (silts, sands, clays). An explanation of how the routine operates is presented on the following pages. Estimating Parameters The routine performs calculations using estimating parameters input by the operator. Based upon the length, diameter, and probable subsurface conditions, rates for pilot hole production, reaming and pull back penetration, and mud flow can be selected from the tables included in this section. The drilled segment length should be based on a preliminary design which takes into account standard horizontal drilling practices with respect to deflection angles and radius of curvature. Four general classifications of subsurface conditions are

SECTION 2 - FEASIBILITY CONSIDERATIONS

13

listed in the tables. The general classification which is most descriptive of the anticipated conditions at the subject crossing should be used to select input parameters. Operational durations and drilling mud quantities are calculated by the routine using the input parameters. Calculations are organized by operational phase: pilot hole, prereaming, and pull back. Pilot Hole. The pilot hole production rate is truly a production rate as opposed to a penetration rate. It takes into account time spent redrilling, surveying, adding pipe, etc., and is dependent upon the subsurface conditions and required pilot hole tolerance. Typical values are given in the following tables. Table 2-3 Pilot Hole Production Rate in feet per hour for pipe diameters less than 30 inches Drilled Length. ft. < 2,000 2,000 - 3,000 > 3,000

Silt. Sand. Clay 60 55 50

Soft Rock 30 25 20

Gravel 45 40 35

Hard Rock 15 10 questionable

Table 2-4 Pilot Hole Production Rate in feet per hour for pipe diameters 30 inches and greater Drilled Length. ft. < 2,000 2,000 - 3,000 > 3,000

Silt. Sand, Clay 50 45 40

Gravel 40 35 30

Soft Rock 25 20 15

Hard Rock 10 questionable questionable

Pilot hole duration is determined by dividing the production rate into the drilled length to determine total hours and converting total hours to shifts using the number of hours per shift. Pilot hole mud flow rate is dependent upon whether a jetting assembly or downhole motor is used. A jetting assembly flow rate of 5 barrels/minute is used for silts, sands, and clays. A downhole motor flow rate of 10 barrels/minute is used for gravels, soft rock, and hard rock. The circulation loss factor adjusts drilling mud quantity calculations to account for mud which is unable to be recovered for recirculation. For example, a circulation loss factor of 0.2 indicates that 20% of the fluid pumped downhole will be lost and only 80% will be available for recirculation. The circulation loss factor is primarily dependent on subsurface conditions. It is difficult to predict and can range from near 0 to 1. Circulation loss factors used for cost estimating purposes are listed in the following table. Table 2-5 Circulation Loss Factors Silt, Sand, Clay Gravel Soft Rock Hard Rock

0.5 0.8 0.2 0.2

SECTION 2 - FEASIBILITY CONSIDERATIONS

14

Pilot hole mud consumed is determined by multiplying the circulation loss factor by the total quantity of mud pumped downhole during pilot hole drilling. Mud pumped downhole during pilot hole drilling is the product of the pilot hole mud flow rate, the pilot hole duration, and a pumping factor. A pumping factor of 35 minutes per hour is used to account for time during pilot hole drilling when drilling fluid is not being pumped. Pilot hole mud consumed in barrels is converted to 100 pound sacks of high yield bentonite by dividing by a typical yield of 200 barrels of drilling mud per ton of dry bentonite. Prereaming. The prereaming penetration rate is the speed at which the reamer is being pulled along the pilot hole. It is dependent upon soil conditions and the diameter of the reamer. Typical values are given in the following table. Table 2-6 Prereaming Penetration Rate in feet per minute Pipe Diameter, in. < 24 24 - 32 > 32

Silt. Sand. Clay 3.0 2.5 2.0

Gravel 2.0 1.5 questionable

Soft Rock 1.0 0.5 0.3

Hard Rock 0.5 questionable questionable

The number of prereaming passes to be used is dependent upon the subsurface conditions and the pipe diameter. For estimating purposes, it can be assumed that all crossings will be prereamed at least once. If the pipe diameter is between 30 inches and 42 inches, the use of a second prereaming pass is probable. If the pipe diameter is greater than 42 inches, the use of a third prereaming pass is probable. If the crossing is being installed in soft rock or hard rock, an additional two passes should be used in the estimate. Prereaming duration is determined by dividing the length by the penetration rate to establish the actual reaming time in minutes, adding two minutes per joint (30 foot drill pipe) to break and make up drill pipe, converting total minutes to shifts using the number of hours per shift, and adding estimated rig-up time of one half shift. This gives the duration for a single prereaming pass which is multiplied by the number of passes to give a total duration for the prereaming operation. Prereaming mud flow rate is primarily a function of diameter and can be estimated from the following table. Table 2-7 Ream & Pull Back Mud Flow Rate in barrels per minute Pipe Diameter. in. < 24 24 - 32 > 32

Silt. Sand. Clay 7 10 15

Gravel 10 13 questionable

Soft Rock 7 10 15

Hard Rock 7 questionable questionable

SECTION 2 - FEASIBILITY CONSIDERATIONS

15

Circulation loss factors used for prereaming are the same as those used for pilot hole drilling and can be found in Table 2-5. Prereaming mud consumed is determined by multiplying the circulation loss factor by the total quantity of mud pumped downhole during prereaming. Mud pumped downhole during prereaming is the product of the drilled length, the prereaming mud flow rate, and the number of prereaming passes, divided by the prereaming penetration rate. Prereaming mud consumed in barrels is converted to 100 pound sacks of high yield bentonite by dividing by a typical yield of 200 barrels per ton of dry bentonite. Pull Back. The pull back penetration rate is the speed at which the pipe is being pulled into the reamed hole. It is dependent primarily on pipe diameter but can also be affected by the quality of the reamed hole. Typical values are given in the following table. Table 2-8 Pull Back Penetration Rate in feet per minute Pipe Diameter. in < 24 24-32 > 32

10 8 6

Pull back duration is determined by dividing the length by the penetration rate to establish the actual pull back time in minutes, adding two minutes per joint (30 foot drill pipe) to break and make up drill pipe, converting total minutes to shifts using the number of hours per shift, and adding estimated rig-up time of one shift. Drilling mud flow rates used during the pull back operation are essentially the same as those used in prereaming and are given in Table 2-7. Circulation loss factors used for pull back are the same as those used for pilot hole drilling and can be found in Table 2-5. Pull Back mud consumed is determined by multiplying the circulation loss factor by the total quantity of mud pumped downhole during pullback. Mud pumped downhole during pull back is the product of the drilled length and the pull back mud flow rate, divided by the pull back penetration rate. Pull back mud consumed in barrels is converted to 100 pound sacks of high yield bentonite by dividing by a typical yield of 200 barrels of drilling mud per ton of dry bentonite. Total Mud Consumed. Drilling program calculations conclude with a determination of the total amount of drilling mud consumed. The total is the sum of the consumed amounts calculated for each operational phase plus 1,000 barrels. The addition of 1,000 barrels accounts for the drilling mud system “line fill”. For convenience in pricing, mud consumed in barrels is converted to 100 pound sacks of high yield bentonite by dividing by a typical yield of 200 barrels of drilling mud per ton of dry bentonite.

SECTION 2 - FEASIBILITY CONSIDERATIONS

16

Shift Cost Summary For clarity, the routine calculates total direct job cost by breaking the job into a series of functional tasks. These tasks are defined below. l

Mobilization. Transportation of men and equipment to the jobsite.

l

Rig-Up. Erection of the drilling rig at the jobsite ready for pilot hole drilling.

l

Pilot Hole. Directional drilling of the small diameter pilot hole complete for reaming and pulling back. Ream & Pull Back. Reaming the pilot hole and pulling the prefabricated pull section back through it to the drill rig.

l

Rig-Down. Disassembly of the drilling rig at the jobsite ready for demobilization.

l

Demobilization. Transportation of men and equipment from the jobsite.

l

Drilling Mud. The cost of drilling mud used in crossing installation.

Additional tasks which are not addressed in the routine but which may need to be estimated are defined below. These tasks do not involve specialized drilling activities. They are accomplished using standard pipeline construction methods. l

l

Site Preparation. Clearing and grading of the jobsite on both river banks ready for construction operations. Pull Section Fabrication. Stringing, welding, coating and pretesting the pull section and preparing the section for installation.

l

Final Hydrostatic Test. Final hydrostatic test of the inplace pull section.

l

Site Restoration. Clean-up, etc. of the work location.

Shift Cost. Labor and equipment costs per shift are determined by identifying the individual laborers and equipment necessary to complete a specific task and assigning hourly or per shift rates to each laborer and equipment item. The labor and equipment costs per shift for a specific task can then be calculated. The routine uses two standard crews, a horizontal drilling crew and a pull back support crew. The horizontal drilling crew is structured to perform horizontal drilling activities while the pull back support crew is designed to handle the pull section during installation. Shift costs for these crews are detailed in Tables 2-9 and 2-10. Allocation of the crew costs to the defined tasks is presented in the routine under the heading “Shift Cost Summary”.

SECTION 2 - FEASIBILITY CONSIDERATIONS

17

Table 2-9 Horizontal Drilling Crew cost per 10 hour shift Description Superintendent Driller Surveyor Mud Man Crane Operator Ramp Laborer Mud Laborer

1 1 1 1 1 2 2

Labor Total

9

Horizontal Drilling Spread (fuel & maintenance) Downhole Survey System Crane (fuel & maintenance) Backhoe Loader (fuel & maintenance) Pick-up Trucks (fuel & maintenance)

1 1 1 1 2

Unit/shift 600 500 500 500 500 300 300

Total/shift 600 500 500 500 500 600 600 $3,800

2,500 500 1,500 250 25 250 25 50 10

Equipment Total

2,500 500 1,500 250 25 250 25 100 20 $5,170

Crew Total

$8,970

Table 2-10 Pull Back Support Crew cost per 10 hour shift Description Foreman Sideboom Operator Backhoe Operator Common Laborer

1 2 1 6

Labor Total

10

Sideboom Tractor (fuel & maintenance) Track Mounted Backhoe (fuel & maintenance) Roller Stands Pick-up Trucks (fuel & maintenance)

2 1 1 set 2

Unit/shift 550 500 500 300

Total/shift 550 1,000 500 1,800 $3,850

500 200 300 150 200 50 10

Equipment Total

1,000 400 300 150 200 100 20 $2,170

Crew Total

$6,020

18

SECTION 2 - FEASIBILITY CONSIDERATIONS

Estimate Recap The routine presents calculated direct costs, broken down by task, under the heading “Estimate Recap”. Calculated direct costs are a combination of shift costs, determined by multiplying the number of shifts by the single shift cost, and non-shift costs, which are not tied to duration. Non-shift costs included by the routine are drilling mud and transportation. Drilling mud cost is calculated in the drilling program using a per sack price of $12.00. A lump sum for transportation of $20,000 is included in both the mobilization and demobilization tasks. Durations for pilot hole and ream & pull back are calculated in the drilling program. Durations for rig-up/rig-down and mobilization/demobilization are set at a constant 2 days each. Owner’s Cost The estimated owner’s cost is calculated by adding a mark-up to total direct costs. This mark-up covers the contractor’s overhead, profit, and risk contingencies. The components for overhead and profit are held constant at 10% and 15%, respectively. The component for risk must be evaluated for each crossing taking into account the possibilities for operational problems posed by the length, diameter, and subsurface conditions. Risk may be logically evaluated by estimating the cost and frequency of possible operational problems. For example, encountering a single random boulder during pilot hole drilling may force the contractor to redrill a portion of the pilot hole to avoid the boulder. This redrill may add two days to the duration of pilot hole drilling resulting in an increase in direct cost of $17,940.00 (i.e., twice the shift cost of the horizontal drilling crew). A contractor’s experience in a given region or subsurface material may indicate that a boulder, or some type of obstruction requiring a two day redrill, will be encountered once in every 2,000 feet drilled. A logical contingency cost for encountering an obstruction during pilot hole drilling may then be calculated for a specific job by dividing the designed drilled length by 2,000 feet and multiplying the result times $17,940.00. These calculations illustrate a logical method for evaluating one risk scenario. However, operational problem scenarios and costs vary and are difficult to predict. This is illustrated by extending the previous example. The contractor encounters a boulder and redrills around it. The redrilled path just misses another boulder. During prereaming the boulder is encountered but it is displaced slightly and the reaming tool “walks” around it. The boulder is encountered again during pull back. This time the rigid pipeline will not “walk” around the boulder and the pipe becomes stuck. The contractor works for five days to free the pipe before twisting off the drill pipe in front of the reamer. He cannot free the pipeline and must abandon it beneath the waterway. He has now spent close to his entire operational budget, has lost his reaming tools, some drill pipe, and owes the owner for the pipeline abandoned beneath the waterway. He must drill a new pilot hole along a different path, purchase new pipe and fabricate a pull section, and begin the ream and pull back process again. His risks have not been diminished. The geology has not changed. He may fail again.

SECTION 2 - FEASIBILITY CONSIDERATIONS

19

Typical values for the risk component of mark-up due to length and diameter in varying soil conditions are presented in the following tables. These values are added to the previously mentioned 25% for overhead (10%) and profit (15%) to determine the mark-up. Table 2-11 Mark-up for Risk associated with drilled length Drilled Length. ft. < 2,000 2,000 - 3,000 > 3,000

Silt. Sand. Clay 0% 10% 20%

Gravel 20% 40% 60%

Soft Rock 10% 20% 30%

Hard Rock 30% 50% questionable

Table 2-12 Mark-up for Risk associated with pipe diameter Pine Diameter. in. < 24 24-32 > 32

Silt, Sand. Clay 0% 10% 20%

Gravel 30% 50% questionable

Soft Rock 20% 30% 40%

Hard Rock 50% questionable questionable

References Drilling Fluids in Pipeline Installation by Horizontal Directional Drilling, Prepared for the Offshore and Onshore Design Applications Supervisory Committee of the Pipeline Research Committee at the American Gas Association, October 31, 1994.

EXAMPLE ANALYSIS - OWNER’S COST ESTIMATE, DRILLING SERVICES ONLY ESTIMATING PARAMETERS WORK SCHEDULE LENGTH PILOT HOLE PROD RATE DRILLING MUD FLOW RATE PILOT HOLE DURATION CIRCULATION LOSS PILOT HOLE MUD QTY PREREAM PASSES PREREAM TRAVEL SPEED PREREAM MUD FLOW RATE PREREAMING DURATION CIRCULATION LOSS PREREAMING MUD QTY PULL BACK TRAVEL SPEED PULL BACK MUD FLOW RATE PULLBACK DURATION CIRCULATION LOSS PULLBACK MUD QTY MUD COST TOTAL MUD QTY

10.0 7.0 2,500 55.0 5 4.5 50% 398 1 2.50 10 24.4 2.4 50% 500 8.00 10 18.0 1.8 50% 156 12.00 1,154

Hours/Shift Shifts/Week Feet Feet/Hour bpm Shifts Sacks Quantity Feet/Min bpm Hours Shifts Sacks Feet/Min bpm Hours Shifts Sacks $/Sack (100 lb) Sacks (100 lb)

SHIFT COST SUMMARY

FUNCTIONAL TASK - (Crews Required) MOBILIZATION - (Drilling Crew) RIG-UP - (Drilling Crew) PILOT HOLE - (Drilling Crew) REAM & PULL BACK - (Drilling & P.B. Support Crews) RIG-DOWN - (Drilling Crew) DEMOBILIZATION - (Drilling Crew)

NUMBER OF PERSONNEL

LABOR COST

EQUIPMENT COST

CREW TOTAL

9 9 9 19 9 9

3,800.00 3,800.00 3,800.00 7,850.00 3,800.00 3,800.00

5.170.00 5.170.00 5,170.00 7,340.00 5,170.00 5,170.00

8,970.00 8,970.00 8,970.00 14,990.00 8,970.00 8,970.00

LABOR EQUIPMENT COST COST

NON-SHIFT COST

TASK TOTAL

ESTIMATE RECAP

FUNCTIONAL TASK

SHIFTS

MOBILIZATION RIG-UP PILOT HOLE REAM (1 PULL BACK RIG-DOWN DEMOBILIZATION DRILLING MUD

TOTALS

2.0 2.0 4.5 4.2 2.0 2.0 N/A

7,600.00 7,800.00 17,272.73 32,459.38 7,600.00 7,600.00 N/A

10,340.00 10,340.00 23,500.00 31,144.03 10,340.00 10,340.00 N/A

20,000.00 0.00 0.00 0.00 0.00 20,000.00 13,847.73

37,940.00 17,940.00 40,772.73 63,603.40 17,940.00 37,940.00 13.847.73

16.8

$80.132.10

$96,004.03

$53,847.73

229,983.86

ESTIMATED COST CONTRACTOR’S DIRECT JOB COST = ESTIMATED MARK-UP @ 45% ESTIMATED OWNER’S COST =

$229,984 U.S. DOLLARS $103,493 U.S. DOLLARS $333,477 U.S. DOLLARS

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21

SECTION 3 - SITE CHARACTERIZATION

SECTION 3 SITE CHARACTERIZATION Selection of HDD for use on a particular crossing should be predicated on a thorough understanding of the site’s characteristics. If HDD is the selected construction method, the design, permitting, and execution of the crossing should be governed by the conditions at the crossing site. Existing features, both natural and man-made, should dictate the manner in which an HDD crossing is configured. Application of the HDD construction process will elicit responses from the site’s features during both the short and long terms. Site conditions can be divided into two major groups, passive and active. Passive conditions are the site’s characteristics prior to construction. Examples of passive conditions are the earth material types, stratification, and groundwater conditions. Also included in this category are the various aspects of the site’s surface such as the topographic/hydrographic relief and the presence of humankind’s activities. In essence, passive conditions are the site’s inplace characteristics regardless of the selected method of crossing installation. Active conditions are broadly defined as the products of the HDD construction process. This category includes the condition of the drilled hole, the various procedures necessary to complete the HDD installation, the response of the passive conditions to the HDD process, and the short and long term effects on the installed pipe. Simply stated, active conditions are the construction dependent phenomena at a given location. Active conditions are discussed in other sections of this design guide. Passive conditions are described in this section along with procedures for characterizing the passive conditions at a specific crossing site. Primary passive condition considerations are geological factors, topographic and hydrographic details, and geotechnical aspects. Geological Factors In assessing the suitability of HDD for a specific location, an understanding of the site’s origin is fundamental This is important not only to project the site’s effects on HDD, but also to plan an effective site characterization study. Understanding the mechanism by which the site was developed, whether by aeolian (airborne), colluvial (gravity), alluvial (river), lacustrine (lake), glacial, or marine (saltwater sea) depositional processes, will forecast the types of materials to be expected as well as the potential for anomalous impediments (boulders, cobble fields, buried logs, stumps, etc.) which influence the HDD construction process. Geological evaluation thus provides the background for assessing the obstacle to be crossed.

SECTION 3 - SITE CHARACTERIZATION

22

Topographic and Hydrographic Details

Essential information stemming from topographic and hydrographic considerations is the site’s surface configuration. Not only does this information allow definition of the obstacle to be crossed, it also provides a basis for decisions concerning the arrangement of construction operations. Informational products include the dry land and underwater configuration of the site or obstacle as well as any existing man-made features. Geotechnical Aspects

Geotechnical aspects can be divided into two classifications: earth material parameters and subsurface stratification. Earth material parameters define the type and condition of the material at the site. Subsurface stratification defines the manner in which the earth material is distributed throughout the site. For construction purposes, earth materials fall into two broad categories, soil and rock. Soil can be defined as material made up of distinct particles which interact to form a mass. The particles may vary in size and may contain water or air in the interstitial spaces. Soil can generally be excavated without drilling or blasting. In contrast to soil, rock is a consolidated and hardened material which generally requires drilling or blasting for excavation. Rock cannot be easily separated into distinct particles. The dividing line between soil and rock, however, is not definite. Unified Classification System for Soil Type

The type of soil at a crossing site should be classified using a standard classification system. For a soil classification system to be effective in the HDD industry, it must be widely used,’ simple, inexpensive, and based on parameters which impact the HDD process. The Unified Soil Classification System satisfies these standards. This system was developed by A. Casagrande and jointly adopted by the United States Corps of Engineers and Bureau of Reclamation in 1952. It is described in detail in ASTM Standard D 2487. The ASTM Standard bases classification on laboratory tests performed on the portion of a soil sample passing the 3 inch (75 mm) sieve. Pertinent definitions from ASTM D 2487 are presented below. Cobbles. particles of rock that will pass a 12 inch (300 mm) square opening and be retained on a 3 inch (75 mm) U.S. standard sieve. Boulders. particles of rock that will not pass a 12 inch (300 mm) square opening. Gravel. particles of rock that will pass a 3 inch (75 mm) sieve and be retained on a No. 4 (4.75 mm) U.S. standard sieve with the following subdivisions: Coarse. passes 3 inch (75 mm) sieve and retained on 3/4 inch (19 mm) sieve, and Fine. passes 3/4 inch (19 mm) sieve and retained on No. 4 (4.75 mm) sieve.

SECTION 3 - SITE CHARACTERIZATION

23

Sand. particles of rock that will pass a No. 4 (4.75 mm) sieve and be retained on a No. 200 (75 µm) U.S. standard sieve with the following subdivisions: Coarse. passes No. 4 (4.75 mm) sieve and retained on No. 10 (2.00 mm) sieve, Medium. passes No. 10 (2.00 mm) sieve and retained on No. 40 (425 µm) sieve, and Fine. passes No. 40 (425 µm) sieve and retained on No. 200 (75 µm) sieve. Clay. soil passing a No. 200 (75 µm) U.S. standard sieve that can be made to exhibit plasticity (putty-like properties) within a range of water contents and that exhibits considerable strength when air dry. Silt. soil passing a No. 200 (75 µm) U.S. standard sieve that is nonplastic or very slightly plastic and that exhibits little or no strength when air dry. Organic clay. a clay with sufficient organic content to influence the soil properties. Organic silt. a silt with sufficient organic content to influence the soil properties, Peat, a soil composed of vegetable tissue in various stages of decomposition usually with an organic odor, a dark-brown to black color, a spongy consistency, and a texture ranging from fibrous to amorphous.

General criteria for assigning group symbols from ASTM D 2487 are summarized in Figure 3-l. Details with respect to laboratory classification may be found in ASTM D 2487. A general discussion of the Unified Soil Classification System is presented in Soil Engineering (Third Edition, pp. 306-310) by Spangler and Handy. Excerpts from this discussion are included below. All soils are classified into fifteen groups, each group being designated by two letters. These letters are abbreviations of certain soil characteristics, as follows... G S M C Pt

- Gravel - Sand - Nonplastic or low plasticity fines - Plastic fines - Peat, humus, swamp soils

O W P L H

-

Organic Well graded Poorly graded Low liquid limit High liquid limit

GW and SW Groups. These groups comprise well-graded gravelly and sandy soils which contain less than 5% of nonplastic fines passing the No. 200 sieve. Fines which are present must not noticeably change the strength characteristics of the coarse-grained fraction and must not interfere with its free-draining characteristic... GP and SP Groups. These groups are poorly graded gravels and sands containing less than 5% of nonplastic fines. They may consist of uniform gravels, uniform sands, or nonuniform mixtures of very coarse material and very fine sand with intermediate sizes lacking. Materials of this latter type are sometimes referred to as skip-graded, gap-graded, or step-graded.

SUMMARIZED CRITERIA FOR ASSIGNING GROUP SYMBOLS AND GROUP NAMES COARSE-GRAINED SOILS More than 50% retained on No. 200 sieve

GRAVELS More than 50% of coarse fraction retained on No. 4 sieve

CLEAN GRAVELS Less than 5% fines GRAVELS WITH FINES More than 12% fines CLEAN SANDS Less than 5% fines

SANDS 50% or more of coarse fraction passes No. 4 sieve FINE-GRAINED SOILS 50% or more passes the No. 200 sieve

SANDS WITH FINES More than 12% fines INORGANIC

SILTS AND CLAYS Liquid limit less than 50

ORGANIC SILTS AND CLAYS Liquid limit 50 or more

OL CH MH

ORGANIC HIGHLY ORGANIC SOILS

SOIL GROUP SYMBOL GW GP GM GC SW SP SM SC CL ML

Primarily organic matter, dark in color, and organic odor Figure 3-1 Soil Classification Chart Extracted with permission from the Annual Book of ASTM Standards (ASTM Designation: D 2487-93, p. 208), copyright American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103.

OH PT

CLASSIFICATION GROUP NAME Well-graded gravel Poorly graded gravel Silty gravel Clayey gravel Well-graded sand Poorly graded sand Silty sand Clayey sand Lean clay Silt Organic clay Organic silt Fat clay Elastic silt Organic clay Organic silt Peat

SECTION 3 - SITE CHARACTERIZATION

25

GM and SM Groups. In general, these groups include gravels or sands which contain more than 12% of fines having little or no plasticity. Gradation is not important, and both well-graded and poorly graded materials are included. Some sands and gravels in these groups may have a binder composed of natural cementing agents, so proportioned that the mixture shows negligible swelling or shrinkage. Thus the dry strength is provided by a small amount of soil binder or by cementation of calcareous materials or iron oxide. The fine fraction of noncemented materials may be composed of silts or rock-flour types having little or no plasticity, and the mixture will exhibit no dry strength... GC and SC Groups. These groups comprise gravelly or sandy soils with more than 12% of fines which exhibit either low or high plasticity. Gradation of these materials is not important. The plasticity of the binder fraction has more influence on the behavior of the soils than does variation in gradation. The fine fraction is generally composed of clays... ML and MH Groups. These groups include the predominantly silty materials and micaceous or diatomaceous soils. Soils in these groups are sandy silts, clayey silts, or inorganic silts with relatively low plasticity. Also included are loessial soils and rock flours. Micaceous and diatomaceous soils generally fall within the MH group but may extend into the ML group. The same is true for certain types of kaolin clays and some illite clays having relatively low plasticity... CL and CH Groups. The CL and CH groups embrace clays with low and high liquid limits, respectively. They are primarily inorganic clays. Low-plasticity clays are classified as CL and are usually lean clays, sandy clays, or silty clays. The medium-plasticity and high-plasticity clays are classified as CH. These include the fat clays, gumbo clays, certain volcanic clays, and bentonite. The glacial clays of the northern United States cover a wide band in the CL and CH groups. OL and OH Groups. The soils in these groups are characterized by the presence of organic matter, including organic silts and clays. They have a plasticity range which corresponds with the ML and MH groups. Pt Group. Highly organic soils which are very compressible and have undesirable construction characteristics are classified in one group with the symbol Pt. Peat, humus, and swamp soils with a highly organic texture are typical of the group. Particles of leaves, grass, branches of bushes, or other fibrous vegetable matter are common components of these soils. Borderline Classifications. Soils in the GW, SW, GP and SP groups are nonplastic materials having less than 5% passing the No. 200 sieve, while GM, SM, GC and SC soils have more than 12% passing the No. 200 sieve. When these coarse-grained materials contain between 5% and 12% of fines, they are classified as borderline and are designated by a dual symbol, such as GW-GM. Similarly, coarse-grained soils which have less than 5% passing the No. 200 sieve, but which are not free draining or in which the fine fraction exhibits plasticity, are also classed as borderline and given a dual symbol...

Soil Condition Parameters Parameters which determine a soil’s condition and aid in its classification vary depending on the soil type. For clay soils, the unit weight, moisture content, and Atterberg limits should be determined. For granular soils, the in situ density (Standard Penetration Test “blow counts”)

SECTION 3 - SITE CHARACTERIZATION

26

and grain size distribution should be determined. Standard procedures for measuring these parameters are listed below. EM1110-2-1906 ASTM D-2216 ASTM D-4318 ASTM D-1586 ASTM D-422

Unit Weight Moisture Content Atterberg Limits Standard Penetration Test Sieve Analysis

Note: ASTM refers to The American Society for Testing and Materials EM denotes Engineer Manual, Laboratory Soils Testing, U.S. Army Corps of Engineers Rock Condition Parameters

In the case of rock, measurements of unit weight, hardness, and in situ condition are necessary. Rock hardness is measured by Mohs Scale of Hardness. Details of Mohs Scale are listed in Table 3-l below. Table 3-l

Mohs Scale Of Hardness Original Version Talc Gypsum Calcite Fluorite Apatite Orthoclase Quartz Topaz Corundum Diamond

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Revised And Expanded Version Talc Gypsum Calcite Fluorite Apatite Orthoclase Vitreous Pure Silica Quartz Topaz Garnet Fused Zirconium Oxide Fused Alumina Silicon Carbide Boron Carbide Diamond

In situ rock quality is indicated by a modified core recovery ratio known as the Rock Quality Designation (RQD). This ratio is determined by considering only pieces of core that are at least 4 inches long and are hard and sound. The percentage ratio between the total length of such core recovered and the length of core drilled on a given run is the RQD. Breaks obviously caused by drilling are ignored. The diameter of the core should preferably not be less than 2 l/8 inch. Rock quality descriptions related to RQD are listed in Table 3-2.

SECTION 3 - SITE CHARACTERIZATION

27

Table 3-2

Rock Quality Descriptions ROD 90 75 50 25 0 -

(%) 100 90 75 50 25

Rock Quality Excellent Good Fair Poor Very Poor

Material Strengths

Shear strength determination through laboratory unconfined compression testing of undisturbed clay and rock specimens usually provides adequate definition for HDD construction. Especially important in the analysis of clay strength is determination of sensitivity: the material’s initial strength compared to its remolded strength. High strength clay generally features healed prefractures. Termed slickensides, these passive condition anomalies may result in loss of drilling fluid circulation and inadvertent surface returns. Deformation Potential

Construction related short term, or “immediate”, earth material deformations, both elastic (recoverable) and plastic (permanent), can be assessed through various numerical techniques such as finite element analysis. Moduli determined from unconfined compression and triaxial shear testing should be used. Assessment of longer term, time dependent deformational behavior (i.e., settlement) should be determined by conducting incremental or constant rate of strain consolidation tests. Performance of incremental rate of strain consolidation testing, in which a load increment is held through several cycles of “secondary” consolidation, will also allow evaluation of ultra long term deflection (i.e., “creep”) characteristics. Groundwater

Trenchless construction operations, as well as in-service performance of the completed project, will largely depend upon proximity to (whether above or below) the free water surface. Consequently, the potential for fluctuation of the groundwater table, due to natural as well as man-made causes such as rainfall, river stage variation, and human induced area dewatering, must be determined. The potential for a perched water table must also be assessed since unchecked borehole flow during HDD operations could jeopardize successful construction completion. Facility design and execution must also consider both total as well as buoyant soil unit weights. Finally, because regulatory bodies are beginning to question the effects of directional drilling on groundwater quality, study efforts varying from cursory to extensive have been evoked. In light of these considerations, earth material permeability is a parameter which should be assessed.

SECTION 3 - SITE CHARACTERIZATION

28

Normally, the phreatic surface is measured in place. However, the potential for variation must be derived from review of long term site specific records. Permeability can be determined through laboratory testing; either through direct measurement (falling head, constant head, or triaxial permeability testing) or extracted from consolidation test time rate analysis. Subsurface Stratification

Once geotechnical material parameters have been defined, the manner in which they are dispersed throughout the site (i.e., the subsurface profile) can be determined. In essence, earth materials will form two types of interfaces: material and conditional. A material interface is the demarcation between two different classifications (i.e., clay and sand, rock and gravel), while a conditional interface is the differentiation within a particular earth material type (i.e., loose and dense sand, soft and hard clay). Another part of stratification determination is assessment of the possibilities for natural as well as manmade anomalous obstacles to HDD operations. Buried logs, stumps, gravel pockets, cobble fields, and boulders exemplify natural anomalies. Manmade impediments consist of existing pipelines, sunken barges, bulkhead/bridge pier piling, etc. In essence, determination of the subsurface profile, incorporating the site’s geological, potamological, and geotechnical aspects, completes definition of the site’s passive conditions relative to HDD. Site Characterization Study Contents

The objective of the site characterization study inherent to HDD construction, or for that matter, the engineering investigation involved in any project, is to determine and portray the site conditions relevant to selecting, designing, and executing the installation. In accomplishing this objective, the typical site characterization process produces three classes of data: l

Raw Data. Direct measurements.

l

Processed Data. Information stemming from test results or computations performed on

raw data. l

Information stated in the form of construction plans, drawings, specifications, bid documents, permit applications, etc.

Evaluated Data.

The site characterization process develops information, through a sequentially staged generation of raw, processed, and evaluated data, which is used in the production of detailed construction plans and specifications necessary to execute an HDD installation. Responsibility for Site Characterization

Because full utilization of the site characterization study spans a particular project from conception to completion, the owner is generally the party best able to bear responsibility for study execution. However, due to the “emerging technology” nature of HDD, construction

29

SECTION 3 - SITE CHARACTERIZATION

contractors have often assumed, or been charged with, provision of site characterization details. While this may be viewed as placing responsibility for the construction and liability for failure directly on the appropriate party’s shoulders, the benefits of the detailed site characterization study occurring prior to the construction method selection, design, and permitting processes are lost. Likewise, a complete definition of the obstacle itself is denied the owner’s engineers. The permitting process, an increasingly involved and intricate phase of any constructed facility, is thus placed on the critical path in terms of project accomplishment, rather than being set in a parallel mode with contractor selection. Consequently, aside from the provision of site specific supplementary data, responsibility for execution of the site characterization study can be most efficiently discharged by, and should be placed with, the owner. Definition of the Obstacle

Basically, two classes of obstacles are negotiated via HDD. l

Time Dependent. Obstacles such as rivers (alluvial) and zones of migrating subsurface

contamination possessing the capability of expanding and/or relocating with the passage of time. l

Obstacles such as highway or railroad embankments, flood protection levees, and environmentally sensitive surface areas having temporally fixed boundaries.

Feature Dependent.

The primary concern in evaluating either type of obstacle is determination of its spatial extent. In the former case, such determination must include assessment of the obstacle’s boundaries throughout the design life of the HDD installation. Potamology (the study of rivers) yields an alluvial obstacle’s potential for horizontal displacement and vertical penetration (i.e., the stream’s meandering and scouring characteristics) during a selected time span. By the same token, non-alluvial obstacle effects with the passage of time, such as uncompleted consolidation settlement of a massive highway embankment or integrity maintenance of a flood protection levee, must also be evaluated. In concert with a site’s geotechnically related passive conditions, a thorough definition of the obstacle to be crossed will dictate geometry of the directionally drilled path plus the steps necessary to restore site integrity following HDD completion. Site Exploration

The primary component of a site characterization study is a site exploration. An appropriate site exploration will consist of both surface and subsurface surveys. Although each survey may be performed by different specialized engineering consultants, it is important that the results be integrated onto a single plan and profile drawing which will form the basis of any contract and be used to price, plan, and execute the crossing. Since this drawing will also be

SECTION 3 - SITE CHARACTERIZATION

30

used to make the working profile which will be the basis for downhole navigation, accurate measurements are essential. Surface Survey

A topographic survey should be conducted to accurately describe the working areas where construction activities will take place. Both horizontal and vertical control must be established for use in referencing hydrographic and geotechnical data. A typical survey should include overbank profiles on the design path centerline extending from approximately 300 feet (91 m) landward of the entry point to the length of the prefabricated pull section landward of the exit point. Survey ties should also be made to topographic features in the vicinity of the crossing. For significant waterways, a hydrographic survey will be required to accurately describe the bottom contours. Typically, it should consist of fathometer readings along the design path centerline, and along parallel paths approximately 200 feet (60m) upstream and downstream of the centerline. This scope can be expanded to include more upstream and/or downstream ranges if this data is required to analyze future river activity. Subsurface Survey

The subsurface survey is directed at determining: l

l

Material Interfaces. Differentiation between different types of earth materials. Conditional Interfaces.

Differentiation between different states of a single earth

material. l

Discrete inclusions of dissimilar and/or conditionally different materials from within the enveloping earth material mass.

Inplace Anomalies.

Definitions of such items, when brought together in the context of one another, produce a subsurface profile. In turn, such a profile enables assessment of the obstacle’s bounds, both time dependent as well as feature dependent, plus the site specific efficacy of the HDD process. Concerted presentation of such information will allow efficient execution and will expedite pre-construction permitting and post-construction certification processes. At the present time, subsurface surveying for HDD installations mainly involves taking vertical borings to produce specimens for physical testing. Borehole conduct procedures, spacing, depth, and sampling frequency generally depend on a project’s extent and the subsurface profile’s potential for variation (as defined by the previously mentioned geological and potamological evaluations). Material properties of clay and rock are determined through securing undisturbed test specimens. Granular materials, such as silt, sand, and gravel, are subjected to in situ density determinations (primarily Standard Penetration Testing) which also produce samples for laboratory classification (mainly grain size analyses). Of particular concern in the exploration of granular soils is that a hydraulic gradient outward from the

SECTION 3 - SITE CHARACTERIZATION

31

borehole is maintained at all times. An inward gradient risks “quickening” the in situ soils to produce a false sense of what is actually there. At any rate, material and conditional interfaces are then established through interpolation between boreholes. Field sampling specifications are listed below. Standard Penetration Test Thin walled Tube Sampling Rock Coring

ASTM D-1586 ASTM D-1587 ASTM D-2113

As an alternative to the Standard Penetration Test when soft soils are encountered, the Dutch Cone Penetration Test can be used. Although not yet extensively employed in the United States, this test is used in Europe and is a recognized “ISO-test”. Other non-sampled intrusive procedures (cross-borehole electrical resistivity/conductivity and shear wave analyses, etc.) as well as non-intrusive, near surface geophysical techniques (reflective/refractive surveying, sub-bottom acoustic profiling, ground penetrating radar, etc.) are possible candidates to expand field exploration utilities. Generally speaking, these exploration methods can enhance data from boreholes by providing a more precise definition of material and conditional interfaces. Expansion of a sampled borehole program through the utilization of non-sampled soundings and/or non-intrusive examinations will improve site characterization efforts. Sampled boreholes, however, are likely to remain the cornerstone of any field investigation because of drawbacks to using these non-traditional exploration procedures, such as regulatory considerations requiring site surface integrity restoration and the lack of physical specimens. Laboratory Testing. As previously detailed, earth material parameter determination relies

heavily on laboratory testing. In contrast to field procedures, laboratory evaluation offers better control of the test conditions plus the ability to impose a variety of stress systems. In this manner, the site’s overall “performance” (its active conditions during HDD as well as its post-construction responses) can be better simulated. By contrast, many passive conditions are more precisely defined through field procedures (the disturbance associated with sampling any non-lithified earth material is not a factor). Therefore, a complete investigative program should be based on laboratory testing results in concert with data from field procedures. References

ASTM Standard D 2487 - 93, Classification of Soils for Engineering Purposes (Unified Soil Classification System). (Extracted, with permission, from the Annual Book of ASTM Standards, copyright American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103 .) Merlin G. Spangler and Richard L. Handy, Soil Engineering, Third Edition, (New York, New York; Intext Press, Inc., 1973)

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32

SECTION 4 - GENERAL DESIGN CONSIDERATIONS

SECTION 4 GENERAL DESIGN CONSIDERATIONS Horizontal directional drilling is a specialized pipeline construction method that is used to cross obstacles which cannot be readily crossed by open cut excavation. Considerations relative to the technical, contractual, and economic feasibility which govern the use of HDD have been discussed in Section 2. This section presents general considerations relative to detailed drilled crossing design. In planning a drilled crossing, the designer should seek to minimize its installation cost while satisfying the objectives of the owner. The primary way to minimize the installation cost is to minimize the drilled length. Typically, the objectives of the owner are satisfied if the following conditions are met: construction must not adversely impact the environment, construction must not damage the pipeline, and the crossing must remain intact and functional over its design life. Definition of the Obstacle To maximize the advantages offered by HDD, primary design consideration should be given to defining the obstacle to be crossed. For example, a river is a dynamic entity. Not only should the water’s width and depth be considered, the potential for bank migration and scour during the design life of the crossing should also be taken into account. It should always be remembered that flexibility in locating a pipeline to be installed by HDD exists not only in the horizontal plane but in the vertical plane as well. An engineering definition of the obstacle results from site characterization efforts which have been discussed in Section 3. Drilled Path Design A designed drilled path consists of a series of straight lines and curves. The straight lines are referred to as tangents and the curves are typically sag bend, over bends, or side bends depending on their axial plane. Compound bends may also be used but are generally avoided to simplify drilling. The location and configuration of a drilled profile are defined by its entry and exit points, entry and exit angles, radius of curvature, and points of curvature and tangency. The relationship of these parameters to each other is shown in Figure 4-l. These parameters, or their limiting values, should be specified on the contract plan & profile drawing.

EXISTING GRADE

/ ENTRY POINT

PT

PC

PT

EXIT POINT

RADIUS OF RADIUS OF CURVATURE, R

CURVATURE, R EXIT ANGLE

DESIGNED DRILLED PROFILE DIRECTION OF HORLZONTAL COORDINATES / POINT OF CURVATURE, PC POINT OF TANGENCY, PT

Figure 4-l Definition of Drilled Path Curves

34

SECTION 4 - GENERAL DESIGN CONSIDERATIONS

Definition of Curves Designed drilled segment sag bends, over bends, and side bends are generally defined as simple circular curves using standard surveying relationships. Nomenclature is shown in Figure 4-1. Entry and Exit Points The entry and exit points are the end points of the drilled profile. The drilling rig is positioned at the entry point. The pipeline is pulled into the exit point and back to the entry point. The relative location of the entry and exit points, and consequently the direction of pilot hole drilling, reaming, and pulling back, should be established by the site’s geotechnical and topographical conditions. When choosing the relative locations of the entry and exit points, it is important to note that steering precision and drilling effectiveness are greater close to the drilling rig. Where possible, the entry point should be located close to anticipated adverse subsurface conditions. An additional consideration is the availability of workspace for pull section fabrication. It is preferable to have workspace in line with the drilled segment and extending back from the exit point the length of the pull section plus 200 feet (61 m). This will allow the pull section to be prefabricated in one continuous length prior to installation. If space is not available, the pull section may be fabricated in two or more sections which are welded together during installation. However, welding during installation slows the process and will increase costs. A slow installation also increases the chances of getting the pipe stuck. Entry and Exit Angles Entry angles should be held between 8º and 20º with horizontal. These boundaries are due chiefly to equipment limitations. Horizontal drilling rigs are typically manufactured to operate at 10º to 12º. Exit angles should be designed to allow easy breakover support. That is, the exit angle should not be so steep that the pull section must be severely elevated in order to guide it into the drilled hole. This will generally be less than 10º for larger diameter lines. Depth of Cover The depth of cover should be governed by the definition of the obstacle. Adequate cover should be provided to maintain crossing integrity over its design life. Geotechnical drillability factors may also be considered when selecting the vertical position of the pipeline. A minimum depth of cover of 15 feet (4.6m) should be maintained in designing drilled profiles. This aids in reducing inadvertent returns, provides a margin for error in existing grade elevation, and allows for future changes in grade elevation.

SECTION 4 - GENERAL DESIGN CONSIDERATIONS

35

Design Radius of Curvature Industry Standard. The design radius of curvature in feet for circular bends used in HDD installations is determined by the following formula. R = lOO*Dnom

(5.1)

where, R

=

The radius of curvature of circular bends in feet.

Dnom

=

The nominal diameter of the pipe in inches.

This relationship has been developed over a period of years in the horizontal drilling industry and is based on experience with constructability as opposed to any theoretical analysis. Pipe Stress Criteria. The design radius of curvature may be reduced from the industry standard. However, a reduction in radius will increase bending stresses and pulling tension. Methods for analyzing the effect of radius of curvature variations on the integrity of the pipeline and the pulling tension are discussed in detail in Section 5. Directional Accuracy and Tolerances Downhole survey instruments are typically magnetic and are therefore subject to some inaccuracy. Additionally, some pilot hole deviation from the designed drilled path may be experienced due to soil reaction. Therefore, a tolerance should be allowed in actual versus designed pilot hole course. For magnetic instruments, an error in alignment of 1% of the drilled length is not unusual. Error can be reduced by using a surface monitoring system or redrilling the pilot hole, but this will increase the cost of the installation. Magnetic error may also be eliminated through the use of a gyroscopic survey system. However, gyroscopic systems are susceptible to mechanical failure in HDD applications and have been used only on a limited basis in North America. Therefore, tolerances should be considered when locating a drilled segment near existing facilities and purchasing easements. Construction specifications with respect to pilot hole accuracy are discussed in Section 7. Pipe Specification The primary criteria governing the specification of pipe to be installed by HDD is its service. In most cases, the wall thickness and specified minimum yield strength will be determined by applicable codes and regulations. However, stresses and loads imposed by the installation method should be reviewed and, where prudent, analyzed in combination with operating stresses to insure that acceptable limits are not exceeded. Methods for analyzing the loads and stresses imposed on a steel pipeline installed by HDD are discussed in detail in Section 5.

36

SECTION 4 - GENERAL DESIGN CONSIDERATIONS

External Pipe Coating External coatings used in HDD installations should be smooth and resistant to abrasion. Historically, pipelines installed by HDD in alluvial soils have been coated with corrosion coating only. Weight coating is generally not required. The deep, undisturbed cover provided by HDD installation has proven adequate to restrain buoyant pipelines. The corrosion coating most often used on HDD crossings is thin film fusion bonded epoxy ranging in thickness from 14 to 22 mils. This coating is popular not only because it is a highly durable system, but also because the field joints can be coated using a compatible fusion bonded epoxy system. If problems are experienced with a fusion bonded epoxy field joint coating system on larger diameter and wall thickness pipe, glass fiber reinforced epoxy joint coating may be used. This joint coating system has been used with success in Europe. However, the acceptable radius of curvature for bends should be increased to avoid micro cracks in the glass fiber reinforced coating. Pipe coated with extruded coatings may also be used with shrink sleeves on field joints. This system is acceptable from an installation standpoint as long as care is taken to insure that the shrink sleeves are properly applied and suitably bonded. Tape coating for pipe or field joints should not be used because of its tendency to be rolled off during pull back. For crossings installed in rock, highly abrasive soils, or soil conditions which might involve point loads, a reinforcing coating should be used in addition to the corrosion coating. The reinforcing coating need not have corrosion prevention properties, but should provide mechanical protection to the underlying corrosion coating. Multiple Line Installation Multiple pipelines may be installed in a single drilled hole by joining them to a common pulling head for pull back. The lines do not need to be banded together but should follow the head freely as they are pulled into the drilled hole. Installation of multiple pipelines in a single hole in this manner is fairly common. If separation of the pipelines is required for cathodic protection purposes, this may be accomplished using rubber spacers or by providing the pipelines with a thick resilient coating. The pipelines will roll during installation. Therefore, provisions should be made outside of the drilled segment to allow the pipelines to be properly positioned for tie-in.

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SECTION 5 - PIPE STRESS ANALYSIS

37

SECTION 5 PIPE STRESS ANALYSIS Load and stress analysis for an HDD pipeline is different from similar analyses of conventionally buried or laid pipelines because of the relatively high tension loads, sometimes severe bending, and external fluid pressures felt by the pipeline during the installation process. In some cases these loads may be higher than the design service loads. Pipeline properties (wall thickness, material grade) and pilot hole profiles must be selected such that the pipeline can be installed and operated without risk of damage. A pipeline installed by HDD can be examined for load and stress states by first breaking the problem into two distinct events: installation and operation. Installation Loads and Stresses The focus here is to examine the loads experienced during installation of the pipeline as it is pulled into a prereamed pilot hole, or one that is being back reamed immediately ahead of the incoming pipeline. During installation the pipeline is subjected to: tension required to pull the pipe into the pilot hole and around curved sections in the hole, made up of, frictional drag due to wetted friction between pipe and wall of hole, fluidic drag of pipe pulled through the viscous drilling mud trapped in the hole annulus, unbalanced gravity (weight) effects of pulling the pipe into and out of a hole at different elevations, bending as the pipe is forced to negotiate the curves in the hole, external hoop from the pressure exerted by the presence of the drilling mud in the annulus around the pipe (unless the pipe is flooded with a fluid at a similar pressure). The stresses and failure potential of the pipe are a result of the interaction of these loads; therefore, calculation of the individual effects does not give an accurate picture of the stress limitations (Fowler and Langer) (Loh). The purpose of this section is to describe a reasonably simple means to estimate installation loads, calculate the resulting stresses, and determine if the overall HDD design is adequate. Loads and stresses experienced during installation of the pipeline are distinct from loads and stresses experienced during the service life of the pipeline and call for specific calculations

38

SECTION 5 - PIPE STRESS ANALYSIS

and design checks. It is assumed in the method that follows that the pilot hole has been reamed approximately 12 inches larger than the pipeline diameter and that the annulus between the diameter and the reamed hole is filled with drilling mud of a known (or assumable) density. Reconsolidation of the formation surrounding the pilot hole will undoubtedly occur over time but if any significant formation pressure loads were exerted on the pipe during the pull back process, it is not expected that the pipe could be pulled in at all. Therefore, formation pressures are not considered in the analysis of the installation loads, Pulling Load Calculation Method Drilled Path Analysis The first step in calculating pulling load is to analyze the drilled path. This analysis can be based on the designed drilled path or the “asbuilt” pilot hole. The drilled path centerline should be plotted in the two-dimensional vertical and horizontal planes with pipe length along the x-axis and distance from the reference line the y-axis. The entire drilled path should be broken into discrete sections: straight or curved. As many sections as necessary can be defined and there is no upper or lower bound on the length or arc length of any chosen section. Straight sections are those in which hole curvature is ideally zero but may actually have very slight curvature. Any pipe section with a net curvature less than that necessary to make the pipe deviate beyond the walls of the hole, which is roughly 12 inches larger in diameter than the pipe itself, can be considered a straight section. Curved sections should be short enough to assume one constant radius for the entire sweep of that section. If the radius of curvature in a particular curved section is variable, break that curved length into small enough sections to justify the assumption of constant radius of curvature in each smaller section. A plot of a simple designed drilled profile is shown in Figure 5-1. As few sections as possible should be designated for the entire drilled path but as many as necessary to completely define its shape. The junctions between sections are assumed to be continuous (no sudden non-linearities in the shape of the pipe) and free of externally applied moment. The junction between a straight and curved section will constitute the beginning of the curvature for the curved section. It will later be modeled in this analysis as the end point of a simply supported beam. Curved sections may join straight sections or other curved sections but straight sections will always join curved sections on both ends since there is no reason to sub-divide long straight sections. Pulling Loads The load and stress calculation method described here begins with an elementary finite difference calculation of the pull force required to completely install the pipeline section from exit to entry point of the reamed pilot hole. The calculation is done in a way to define the maximum pulling loads which are assumed to occur at the moment the pipeline emerges from the entry point. Axial load in the pipe during the last instant of the pull back process will be distributed along its length from entry to exit. Cumulative axial load is composed of discrete axial loads occurring in each section of the hole due to friction between the pipe and

Figure 5-l HDD Designed Drilled Profile

40

SECTION 5 - PIPE STRESS ANALYSIS

the hole wall plus dynamic fluid friction required to make the pipe move through the viscous drilling mud. Because the hole wall/pipe frictional components are caused by the shape of the hole, the axial tension in the pipe at any given point can conveniently be considered to be confined to that particular location in the hole, regardless of which portion of the pipe is passing that point in the hole during any instant in the pull back process. This fact allows for worst case loads anywhere in the hole to be calculated only for the case where the pipe has just emerged from the hole. Calculation - Straight Sections. The pipe is assumed to be pulled from the right to the left (as viewed in Figure 5-1) in all of the following models and calculations. The total pull force required to install the pipe is determined by summing the individual forces required to pull the pipe through each of the straight and curved sections defined in the hole profile. The modeling and calculation process must be done sequentially from right to left (i.e. from pipe side to rig side). Each straight section is modeled with variables as shown in Figure 5-2. Figure 5-2 Straight Section Model

For any straight section, the left end tension, T2, is found from the static force balance, T2 = T1 + IfrictI + DRAG + Ws x L x sin

(5.l)

The ± term is resolved as follows: (-) if T2 tends downhole, (+) if T2 tends upslope, (0) if the hole section is horizontal, 8 = 0. where, T2

=

tension at the left end of the section, in lbs, required to over come drag and friction.

SECTION 5 - PIPE STRESS ANALYSIS

41

T1

=

tension at the right end of the section in lbs. This may be zero for the first section of the hole (Point A in Figure 5-1), or it may be determined by the drag of the pipe remaining on the rollers.

frict

=

friction between pipe and soil in lbs.

DRAG =

fluidic drag between pipe and viscous drilling fluid in lbs.

Ws

=

effective (submerged) weight per foot of the pipeline plus internal contents (if filled with water) in lbs/ft.

L

=

length of section in feet.

=

angle of the axis of the straight hole section relative to horizontal (zero equals horizontal, 90° is vertical)

Relationships defining the friction and fluidic drag terms are presented below. The absolute value for frict is used to insure that the it always acts in the proper direction relative to T2. (5.2) (5.3) where, µsoil

=

average coefficient of friction between pipe and soil; recommended values between 0.21-0.30 (Maidla)

D

=

outside diameter of pipe in inches.

µmud =

fluid drag coefficient for steel tube pulled through bentonite mud; recommended value 0.05 psi (NEN 3650).

Calculation - Curved Sections. Each curved hole section selected in the hole profile is modelled as shown in Figure 5-3 with the variables the same as the straight section, plus additional variables as defined below, R

=

(constant) radius of curvature of the section in feet,

=

included angle of the curved section in degrees.

1

=

angle in degrees from horizontal of T1, at right end of section.

2

=

angle in degrees from horizontal of T2, at left end of section,

=

(5.4)

SECTION 5 - PIPE STRESS ANALYSIS

42

L is replaced by Larc = R x 8 x (n/180). Figure 5-3 Curved Section Model

N1, N, & N2 = normal contact forces at right, center, & left points, respectively. frict1, frict, & frict2 = frictional forces associated with normal forces at right, center, & left points, respectively. The distributed submerged weight of the pipe and contents are approximated to operate vertically at the center of the section despite the curvature of the pipe section. To determine the normal forces of contact at the center and ends, each curved section is modelled as a beam in 3-point bending as shown in Figure 5-4. Loads on the beam are axial tension, T, plus distributed, submerged weight, Ws. A beam in 3-point, simply supported bending will not assume a constant radial, circular shape. For the bent pipe to fit in the circular hole section it must deflect enough to place its center at a point matching the displacement, h, of a circular arc with radius, R, where, h = R x [ 1 - cos

]

(5.5)

Note that this approximation of beam behavior in each curved section cannot be expected to be strictly accurate since more than three points of contact are likely for virtually any

SECTION 5 - PIPE STRESS ANALYSIS

43

pipe/hole shape combination. However, since the objective is to determine normal contact forces and then calculate frictional forces exerted on the pipe as it is pulled through a curved section, this method is an adequate approximation and accounts for pipe stiffness relative to curve radius. The frictional forces become components in a later calculation of total pulling tension, Ttot. Sensitivity checks on the effect of Ttot in the overall stress analysis of the pipeline show that small variations in the value for Ttot do not grossly affect calculated stresses. Also, frictional loads are significantly affected by the assumed coefficients of friction, µsoil, and drag, µmud. Figure 5-4 3 -Point Bending

Nl N2

The solution to the beam model to find N uses the vertical component of distributed weight, Ws x cos 8, as the main load and the arc length of the pipe section, L,,, in place of L. From Roark’s solution for elastic beam deflection, N = [12 x T x h - (ws/12) x cos 8 x Y]/X

(5.6)

SECTION 5 - PIPE STRESS ANALYSIS

44

where, X = 3 x Larc - j/2 x tanh(U/2) 2

(5.7)

2

Y = 18 x (Larc) - j x [ 1 - l/cosh(U/2)] j = (E x I/T)

(5.8)

1/2

(5.9)

where, 7

E

=

Young’s Modulus (2.9 x 10 psi for steel)

I

=

Bending Moment of Inertia in inch

U

=

12 x Larc/j

tanh

=

hyperbolic tangent

cosh =

4

(5.10)

hyperbolic cosine

A value for T must be used to calculate both N and j. The proper value for T is the average of T1 and T2; therefore, an iterative solution is required to solve for T2 with accuracy. For hand calculations a few sensitivity checks on the value of T2 as the average, or Tave, varies are quick to show that the assumed Tave, need not be exactly the average of T1 and T2. frict = N x µsoil

(5.11)

Therefore, end reactions are assumed to be N/2 and end friction forces are assumed to be f/2. Where N is a positive value (defined as downward acting as in Figure 5-4) it shows that the bending resistance and/or buoyancy of the pipe is sufficient to require a normal force acting against the top of the hole in order to get the pipe to displace downward by an amount equal to h. Where N is negative, the submerged pipe weight is sufficient to carry the pipe to the bottom of the curved section where an upward acting normal force is felt at the point of contact. Whether N is positive or negative in value, all friction values are taken as positive, acting in opposition to T2. Assume forces acting along the curved path of the pipeline can be added as if acting in a straight line (as along a highly flexible, rope-like member). Then, T2 = T1 + 2 x [frict] + DRAG ± Ws x Larc x sin The ± term is resolved as follows: (-) if T2 tends downhole,

(5.12)

SECTION 5 - PIPE STRESS ANALYSIS

45

(+) if T2 tends upslope, (0) if the hole section is horizontal, 0 = 0. The absolute value of frict is used because friction always retards pipe movement caused by T2. Calculation - Total Pulling Load. The total force, Ttot, required to pull the entire pipeline into the reamed pilot hole is the sum of all straight and curved section values for AT, (T2 T1). T tot

=

Ci (T2 - T1), for i sections

(5.13)

Installation Stress Analysis The worst case stress condition for the pipe will be located where the most serious combination of tensile, bending and/or hoop stresses occur simultaneously. This is not always obvious in looking at the hole profile because the interactions of the three loading conditions is not necessarily intuitive. To be sure that the point with the worst case condition is isolated it may be necessary to do a critical stress analysis for several suspect locations. In general, highest stresses will be felt at locations of tight radius bending, high tension (closer to the rig side), and high hydrostatic head (deepest point). The approach that follows is taken from the API Recommended Practice 2A-WSD. Lower case f’s represent actual stresses, upper case F’s represent allowable stresses. Individual Loads For any selected location in the drilled path profile that is suspected of being a critical stress location, first calculate the individual stresses for the specific loading conditions (tensile, bending, hoop stresses) and compare against allowable levels for these stress states. If no individual stress condition appears to cause overstress failure, the combined stress state is compared in two interaction equations presented as unity checks. That is, the combined stresses in the interaction equation must be less than 1.0 for the pipe to be safe from collapse by bending or hoop collapse in all regimes (plastic, elastic and transition). Tensile stress. f t =T/A

(5.14)

where, T A

= =

tension at the point of interest in lbs. cross-sectional area of pipe wall in inches.

SECTION 5 - PIPE STRESS ANALYSIS

46

Bending stress. fb = (E x D)/(24 x R)

(5.15)

Hoop Stress. fh = (

x D)/(2 x t)

(5.16)

where, t

=

pipe wall thickness in inches

and where Ap (psi) is equal to the difference between hydrostatic pressure exerted by the drilling mud in the hole acting on the outside of the pipe and the pressure from water, mud or air acting on the inside of the pipe, at the depth of the point of interest (Ap producing a compressive external hoop stress is taken as positive), external mud pressure = mud wt (ppg) x depth (ft)/19.25

(5.17)

Compare each actual stress, f, to its allowable stress, F, as follows: Tension. Ft = 0.9 x SMYS

(5.18)

where, SMYS =

Specified Minimum Yield Strength in psi

Bending. Fb = 0.75 x SMYS for D/t I 1,5OO,OOO/SMYS

(5.19)

Fb = [0.84 - {1.74 x SMYS x D/(E x t)}] x SMYS for 1,5OO,OOO/SMYS < D/t I 3,OOO,OOO/SMYS

(5.20)

Fb = [0.72 - (0.58 x SMYS x D/(E x t)}] x SMYS for 3,OOO,OOO/SMYS < D/t I 300,000

(5.21)

Hoop Buckling Stress. fh < Fhc/l.5

(5.22)

where Fhc, critical hoop buckling stress, is a function of Fhe, elastic hoop buckling stress as follows:

SECTION 5 - PIPE STRESS ANALYSIS

2

47

Fhe = 0.88 x E x (t/D) (for long, unstiffened cylinders)

(5.23)

Fhc = Fhe for Fhe Ij 0.55 x SMYS

(5.24)

and,

For inelastic hoop buckling, Fhc = 0.45 x SMYS + 0.18 x Fhe for 0.55 x SMYS < Fhe I 1.6 x SMYS

(5.25)

Fhc = 1.31 x SMYS/[l.15 + (SMYS/Fhe)] for 1.6 x SMYS < Fhe < 6.2 x SMYS

(5.26)

Fhc = SMYS for Fhe > 6.2 X SMYS

(5.27)

Combined Loads If all preliminary checks indicate that the loading on the pipe will not cause failure (overstress or buckling) due to a single load condition, the suspect stress locations must be checked for safety under interactive combined loading by conducting two unity checks; first a dual load condition (tension plus bending) and finally a full interactive load unity check which must be satisfied for combined tensile, bending and hoop stresses. The unity check for combined stresses, tensile and bending, is: ft/(0.9 x SMYS) + fb/Fb I 1.0

(5.28)

The unity check for full interaction of tensile, bending and external hoop stresses is: A2 + B2 + 2v x [A] x B 1

(5.29)

A = (ft + fb - 0.5 x fh) x 1.25/SMYS

(5.30)

B = 1.5 x fh/Fhc

(5.31)

where,

v = Poisson’s ratio (0.3 for steel) Satisfying the unity check equation for combined loading at all particular locations of suspect serious stresses, after first satisfying all single-load condition stress cases at those

48

SECTION 5- PIPE STRESS ANALYSIS

locations, is sufficient to qualify the design for an HDD pipeline installation. If the unity check results in a value greater than one it does not mean that the pipeline will necessarily fail (by overstress or buckling) but it does indicate that the combined stress state places the design in a range where some test specimens under similar stress states have been found to be subject to failure. The combined stress interaction analysis described above is useful for finding solutions to field problems where conditions differ from original design expectations. One typical case might be if a pilot hole has a few spots where the radius is tighter than designed and, thus, the unity check exceeds one. In this case it is possible to find a modified solution to the installation problem by varying one load parameter and checking its effect on the stress interaction. For example, if a certain HDD design profile fails the unity check with the pipe installed empty it may pass the unity check when filled with water, thus reducing hoop stress and decreasing buoyancy. Example Pulling Load Calculation Use the example HDD pilot hole profile as shown in Figure 5-5. For this example the hole is assumed to occupy only a single plane, i.e. there is no significant curvature into or out of the plane of the paper. The pipe side (right) and rig side (left) are at the same elevation and 1,500 feet apart. The total depth for the horizontal straight section is 100 feet below the entry/exit datum elevation. The total arc length of the centerline of the hole profile is 1,525 feet. The right curved section is a 20° arc at a 1,000 foot radius. The left curved section is a 14° arc at a 1,200 foot radius. The following particulars are given for this example installation: D = 12.75 in Pipe: Grade B Steel µsoil = 0.3 Mud wt. = 12 ppg

t = 0.25 in Ws = -46.21 lb/ft SMYS = 35,000 psi E = 2.9 x l07 psi µmud = 0.05 psi This is a conservative (high drilled solids content assumed) mud weight value. 12 ppg = 89.76 lb/ft 3 Formation is predominately soft clay Right side tension (pull-back) on the pipe as it enters the hole = 0 Pipe is installed empty

Examination of the geometry of the pilot hole allows for it to be broken down into five convenient sections for calculating pull forces. From right to left they are: Section

Type

A to B B to C C to D D to E E to F

straight curved, R = 1000 ft straight curved, R = 1200 ft straight

Angle = 20° = 10° = 20° =0 = 7° = 14° = 14°

Length L L arc L L arc L

= 116.1 ft = 349.1 ft = 500.3 ft = 293.2 ft = 266.2 ft

T2

1500

RIG SIDE

PIPE SIDE

R =1200'

20”

/---J

Figure 5-5

Example, HDD Pilot Hole Profile

SECTION 5 - PIPE STRESS ANALYSIS

50

Pulling Loads Straight Section at Point B. = TB - TA = [frict] + DRAG - Ws x L x sin 8 [frict] = Ws x L x cos x µs o i l = (-46.21 lb/ft) ( 116.1 ft) (cos 20º) (0.3) = 1,512 lb DRAG = 12 x x D x L x µmud = (12) (12.75 in) (ll6.1 ft) (0.05 lb/in2) = 2,790 lb Ws x L x sin 8 = (-46.21 lb/ft) (ll6.1 ft) (sin 20°) = -1,835 lb ATBA = 1,512 lb+ 2,790 lb - (-1,835 lb) = 6,137 lb TB = + TA = 6,137 lb

Pull Load at Point B

Curved Section at Point C. h = R x [l - cos = (1000 ft)(l - cos 10o) = 15.19ft I = x (D - t)3 x t/8 = (12.75 in - 0.25 in)3(0.25 in)/8 = 191.75 in4 Assume Tave for section = 10,000 lb to start iterative solution. j = (E x I/Tave)1/2 = [2.9 x 107psi (191.75 in4)/10,000 lb]1/2 = 745.7 in U = 12 x Larc/j = (12)(349.1 ft)/(745.7 in) = 5.62

SECTION 5 - PIPE STRESS ANALYSIS

51

X = 3 x Larc - (j/2) x tanh(U/2) = (3)(349.1 ft) - (0.5)(745.7 in)tanh(5.62/2) = 677.15 in 2

Y = 18 x (Larc) - j2 x [l - l/cosh(U/2)] 2 2 = (18)(349.1 ft) - (745.7 in) x [l - l/cosh(5.62/2)] 2 = 1,704,320 in N = [12 x Tave x h - (Ws/12) x cos x Y]/X = [(12)(10,000 lb)(15.19 ft) -(l/12)(-46.21 lb/ft)(cos 10°)(1,704,320 in2)]/677.15 in = 12,237 lb The positive value indicates that N acting down is the reaction normal force required at the top of the hole to bend the buoyant pipe (in this example) into the curve required to match the pilot hole. = T c -T b = 2 x [frict] + DRAG - Ws x Larc x sin [frict] = N x µsoil = (12,237 lb)(0.3) = 3,671 lb DRAG = 12 x x D x Larc x µmud = (12) (12.75 in)(349.1 ft)(0.05 lb/in2) = 8,389 lb WS x Larc x sin 8 = (-46.21 lb/ft)(349.1 ft)(sin 10º) = -2,801 lb AT,, = 2(3,671 lb) + 8,389 lb - (-2,801 lb) = 18,533 lb Tc = ATo, + TB = 24,670 lb

Pull Load at Point C before Tave assumption check

Check accuracy of assuming Tave = 10,000 at the beginning of this iterative solution. T ave = (Tc + TB)/2 = (24,670 lb + 6,137 lb)/2 = 15,404 lb Percent difference is (15,404 - 10,000)/10,000 x l00%, which is equal to 54%. This does not fall within an acceptable level of l0%, so the iteration process begins. Select 15,404 as

52

SECTION 5 - PIPE STRESS ANALYSIS

the value for the assumed Tave. Run through the same calculations and once again compare the calculated Tave with the assumed Tave. Continue the iteration until the percent difference is equal to or below 10%. In this example, a little iteration reveals that a more exact value for Tave is 15,698 lb, and thus, = 2(3,966 lb) + 8,389 lb - (-2,801 lb) = 19,122 lb T c = 25,259 lb

Pull Load at Point C

Straight Section at Point D. ATDc = T D - T c = [frict] + DRAG - Ws x L x sin [frict] = Ws x L x cos 6 x µsoil = (-46.21 lb/ft)(500.3 ft)(cos 0°)(0.3) = 6,936 lb DRAG = 12 x x D x L x µmud = (12) (12.75 in)(500.3 ft)(0.05 lb/in2) = 12,024 lb Ws x L x sin

= (-46.21 lb/ft)(500.3 ft)(sin 0°) = 0 lb = 6,936 lb + 12,024 lb - 0 lb = 18,960 lb

T D = AT, + TC = 44,219 lb

Pull Load at Point D

Curved section at Point E. The iterative solution will yield Tave = 51,545 lb. The calculation is as follows: h = R x [l - cos(a/2)] = (1200 ft)(l - cos 7°) = 8.94 ft I = 191.75 in4 j = (E x I/Tave)1/2 = [(2.9 x l07 psi)(191.75 in4)/(51,545 lb)]1/2

53

SECTION 5 - PIPE STRESS ANALYSIS

= 328.45 in U = 12 x Larc /j = (12)(293.2 ft)/(328.45 in) = 10.71 X = 3 x Larc - (j/2) x tanh(U/2) = (3)(293.2 ft) - (0.5)(328.45 in)tanh(10.71/2) = 715.38 in 2

2

Y = 18 x (Larc) - j x [ 1 - l/cosh(U/2)] = [(18)(293.2 ft)2 - (328.45 in)2 x [l - l/cosh(10.71/2)] = 1,440,532 in2 N = [12 x Tave x h - (Ws/12) x cos x Y]/X = [(12)(51,545 lb)(8.94 ft) - (1/12)(-46.21 lb/ft)(cos 7°)(1,440,532 in2)]/715.38 in = 15,427 lb The positive value indicates that N acting down is the reaction normal force required at the top of the hole to bend the buoyant pipe (in this example) into the curve required to match the pilot hole. ATED = TE - TD = 2 x [frict] + DRAG + Ws x L x sin 8 [frict] = N x µsoil = (15,427 lb)(0.3) = 4,628 lb DRAG = 12 x x D x L x µmud ( 12.75 in)(293.2 ft)(0.05 lb/in2) = (12) = 7,047 lb Ws x Larc x sin 9 = (-46.21 lb/ft)(293.2 ft)(sin 7°) = -1,651 lbf ATED = 2(4,628 lb) + 7,047 lb + (-1,651 lb) = 14,652 lb TD TE = = 58,871 lb

Pull Load at Point E

SECTION 5 - PIPE STRESS ANALYSIS

54

Straight Section at Point F. ATFE = TF - TE = [frict] + DRAG + Ws x L x sin [frict] = Ws x L x cos 8 x µsoil = (-46.21 lb/ft)(266.2 ft)(cos 14°)(0.3) = 3,581 lb DRAG = 12 x x D x L x µmud 2 = (12) (12.75 in)(266.2 ft)(0.05 lb/in ) = 6,398 lb Ws x L x sin

= (-46.21 lb/ft)(266.2 ft)(sin 14°) = -2,976 lb

ATFE = 3,581 lb + 6,398 lb + (-2,976 lb) = 7,003 lb T F = ATE + TE = 65,874 lb

Pull Load at Point F

Total Pull Load, Ttot. The total pulling load is simply the sum of all the individual loads which is equal to the pulling load at point F. T tot = AT,, + AT,, + AT,, + ATED + AT, = TF = 65,874 lb Example Installation Stress Analysis A complete analysis of the installation stresses experienced by the pipe requires stress calculations for any point where the combined stresses may be near a maximum. For this example case, examination of the pilot hole plot shows that the most likely location for high stress due to combined loading is at point E. This point is closer to the rig side and, therefore, will have relatively high local tension. It also is part of a tight radius curve (R = 1200 A) and is near the deepest point with the highest hydrostatic mud pressure. For completeness and confidence, stresses at points C and D should also be checked. Individual Stresses at Point E From Figure 5-5, the depth at Point E is 64.4 ft below datum elevation.

SECTION 5 - PIPE STRESS ANALYSIS

55

Tensile stress. ft = TE/A 2 2 = (58,871 lb) / [ (n/4) (12.75 - 12.25 )] = 5,997 psi Bending stress. fb = (E x D)/(24)(R) 7 = (2.9 x 10 )(12.75)/[(24)(1200 ft)] = 12,839 psi External Hoop Stress. fh = x D)/(2 x t) = (40.15 psi)(12.75 in)/[(2)(0.25 in)] = 1,024 psi = (12 ppg)(64.4 ft)/(l9.25) = 40.15 psi Allowable Tension. Ft = 0.9 x SMYS = (0.9)(35,000 psi) = 31,500 psi Note that ft is less than 31,500 psi, so tension is within allowable limits. Allowable Bending. F b = [0.84 - { 1.74 x SMYS x D/(E x t)}] x SMYS for 1,500,000/SMYS < D/t fe 3,000,000/SMYS Fb = [0.84 - {(1.74)(35,000 psi)(12.75 in)/(2.9 x 107 psi x 0.25 in)}](35,000 psi) = 25,652 psi Note that fb is less than 25,652 psi, so bending is within allowable limits. Allowable Elastic Hoop Buckling. 2 F he = 0.88 x E x (t/D) = 0.88(2.9 x 107)(0.25 in/12.75 in)2 = 9,812 psi

and,

SECTION 5 - PIPE STRESS ANALYSIS

56

Fhc = Fhe for Fhe 5 0.55 X 35,000 psi F hc = 9,812 psi Note that fh is less than Fhc/l.5 = 6,541 psi, so external hoop stress is within allowable limits for buckling. Combined Load Interactions at Point E Since all individual stress checks are acceptable, the combined load interaction checks will now be examined. Tensile and Bending. ft/(0.9 x SMYS) + (fb/Fb) 5 1.0

[Unity check]

5,997 psi/(0.9 x 35,000 psi) + (12,839 psi/25,652 psi) = 0.69 0.69 < 1.0 so combined tensile and bending at Point E is acceptable. Tensile, Bending, and External Hoop.

A = [ft + fb - 0.5 x fh] x 1.25/SMYS = [5,997 psi + 12,839 psi - (0.5)(1,024 psi)](1.25)/(35,000 psi) = 0.654 B = 1.5 fh/Fhc = (1.5)(1,024 psi)/(9,812 psi) = 0.157 (0.654)2 + (0. 157)2 + (2)(0.3)(0.654)(0.157) = 0.51 0.51 < 1.0 so combined stresses at Point E are acceptable. Operating Loads and Stresses With one exception, the operating loads and stresses in a pipeline installed by HDD are not materially different from those experienced by pipelines installed by cut and cover techniques; therefore, past procedures for calculating and limiting stresses can be applied. The exception involves elastic bending. A pipeline installed by HDD will contain elastic bends. The pipe will not be bent to conform to the drilled hole as a pipeline installed by cut and cover is bent to conform to the ditch. Bending stresses imposed by the HDD installation

SECTION 5 - PIPE STRESS ANALYSIS

57

method will generally not be severe. However, they should be checked in combination with other longitudinal and hoop stresses to insure that acceptable limits are not exceeded. The operating loads imposed on a pipeline installed by HDD are listed below: internal pressure from the fluid flowing in it, elastic bending as the pipe conforms to the shape of the drilled hole, thermal resulting from the difference between the constructed (locked in) temperature and the operating temperature. Formulas for calculating stresses produced by internal pressure and elastic bending have been previously listed (5.15 and 5.16) and are repeated below. Bending stress. fb = (E x D)/(24 x R)

(5.15)

Hoop Stress. fh =

x D)/(2 x t)

(5.16)

In this case Ap is equal to the difference between hydrostatic pressure exerted by groundwater acting on the outside of the pipe and the pressure from the fluid (gas) flowing inside of the pipe. Note that for this analysis Ap producing a tensile external hoop stress is taken as positive. The formula for calculating thermal stresses is taken from ASME/ANSI B31.4 Thermal Stress. ft = (E x k) x (T1-T2)

(5.3 1)

where, k

=

the coefficient of thermal expansion for steel (0.0000065 inches per inch per ºF)(ANSI B31.4)

TI

=

Constructed temperature in ºF

T2

=

Operating temperature in ºF

Combined Stresses and Limitations Combined stresses can be analyzed by calculating the maximum shear stress on a small element in the pipeline. This maximum shear stress should be limited to 45% of the SMYS

SECTION 5 - PIPE STRESS ANALYSIS

58

of the pipe (ASME/ANSI B31.4). The maximum shear stress at any element is calculated using the following formula (Timoshenko and Gere). (5.32)

fv = (fhoop - flong)/2 where, fhoop =

the total hoop stress acting on the element

flong =

the total longitudinal stress acting on the element

Note that in this analysis all tensile stresses are positive and all compressive stresses are negative. The total hoop stress is determined using equation 5.16. The total longitudinal stress is determined by taking the sum of the longitudinal stresses resulting from bending (equation 5.15), thermal (equation 5.31), and internal pressure (equation 5.33). Longitudinal stress from internal pressure is calculated as follows: fp = fh x v

(5.33)

where, v = Poisson’s ratio (0.3 for steel) The maximum shear stress will occur in an element on the compressive side of an elastic bend and at the maximum distance from the neutral axis of the bend. Example Operating Stress Analysis Using the same example previously analyzed for installation stresses, check the combined operating stresses. Relevant data are listed below: D = 12.75 in Pipe: Grade B Steel

t = 0.25 in SMYS = 35,000 psi

E = 2.9 x 107 psi

The total depth for the horizontal straight section is 100 feet below the entry/exit datum elevation and the groundwater table is assumed to be 10 feet below the entry/exit datum. The shortest radius of curvature is 1,000 feet on the right curve. The maximum allowable operating pressure is 720 psi. The construction temperature is 60 ºF and the operating temperature is 80 ºF. Bending Stress. fb = (E x D)/(24 x R) = (2.9 x 107 psi)(12.75 in)/(24)(1000 ft)

SECTION 5 - PIPE STRESS ANALYSIS

59

= ±15,406 psi Hoop Stress. = 720 psi - (90)(0.4333) 681 psi fh = x D)/(2 x t) = (681 psi)(12.75 in)/(2)(0.25 in) = +17,366 psi Thermal Stress. f t = (E x k) x (Tl - T2) = (2.9 x 107 psi)(0.0000065)(60 ºF- 80 ºF) = -3,770 psi Total Longitudinal Compressive Stress. f long = -15,406 psi + -3,770 psi + (17,366 psi)(.3) = -13,966 psi Maximum Shear Stress fv = (fhoop - flong)/2 = [17,366 psi - (-13,966 psi)]/2 = 15,666 psi The allowable shear stress is 15,750 psi (45% of 35,000 psi) which is greater than 15,666 psi. Therefore, the pipe specification is acceptable. It should be noted that violation of the 45% limit on shear stress does not necessarily mean that the pipeline will fail. Bending loads resulting from HDD installation are not sustained. That is, they may be relieved by plastic deformation. However, operating stresses should be maintained in the elastic range to provide a conservative design. Spreadsheet - Load and Stress Analysis Hand calculations for a given pipeline installation case can be done; however, software to assist and do alternate scenario calculations would obviously be helpful for this analysis method. A Lotus spreadsheet routine for performing the installation load and stress calculations is included in the file PULL.WK4AVK3 on the diskette attached to the back cover of this manual. An example of this spreadsheet is attached at the end of this section for the pilot hole and stress point calculations explained in the previous subsections. By using this spreadsheet, a few alternate scenario cases can be calculated to observe effects of changing key parameters. For example, pull forces and combined stresses for this case can be significantly altered by assuming the pipe is pulled in flooded with water instead of empty.

SECTION 5 - PIPE STRESS ANALYSIS

60

The spreadsheet results of this alternate scenario are attached at the end of this section. Other interesting cases to examine are the effects of a tighter than designed radius in the curved sections, varying the mud weight in the hole, and using thicker walled pipe. A Lotus spreadsheet routine for performing the operating load and stress calculations is included in the file STRESS.WK4WK3 on the diskette attached to the back cover of this manual. An example of this spreadsheet is attached at the end of this section for the calculations explained in the previous subsections. References API RP 2A-WSD, Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms -- Working Stress Design, Twentieth Edition, (Dallas, Texas; American Petroleum Institute, 1993) Fowler, J.R. and Langner, C.G., “Performance Limits for Deepwater Pipelines”, OTC 6757, 23rd Annual Offshore Technology Conference, Houston, TX, May 6-9, 1991. Loh, J.T., “A Unified Design Procedure for Tubular Members”, OTC 6310, 22nd Annual Offshore Technology Conference, Houston, TX, May 7-10, 1990. Maidla, E.E., “Borehole Friction Assessment and Application to Oilfield Casing Design in Directional Wells”, doctoral dissertation Louisiana State University, Department of Petroleum Engineering, Baton Rouge, LA, December 1987. Meijers, P., “Review of a Calculation Method for Earth Pressure on Pipelines Installed by Directional Drilling”, Delft Geotechnics, Report CO-341850/4 commissioned by N.V. Netherlands Gasunie, March 1993. NEN 3650, “Requirements for Steel Pipeline Transportation Systems”, unofficial translation, Government/Industry Standards Committee 343 20, The Netherlands, 1992. NEN 3651, “Supplementary Requirements for Steel Pipelines Crossing Major Public Works (Dykes, High Level Canals, Waterways, Roads)“, unofficial translation, Government/Industry Standards Committee 343 20, The Netherlands, February 1994. Roark, R. J., Formulas for Stress and Strain, Second Edition & Fifth Edition. (New York, New York; McGraw-Hill, 1943, 1965) ASME/ANSI B31.4-1986 Edition with 1987 Addenda, Liquid Transportation Systems for Hydrocarbons, Liquid Petroleum Gas, Anhydrous Ammonia, and Alcohols. (New York, New York; The American Society of Mechanical Engineers, 1987) Timoshenko, S. P. and Gere, James M., Mechanics of Materials. (New York, New York; Van Nostrand Reinhold Company, 1972)

Date: 04/26/95

Page 1 of 2

ANALYSIS OF INSTALLATION LOADS AND STRESSES PROJECT:

HDD Design Guide, Example Calculation

GENERAL DATA

Pipe Diameter: Wall Thickness: SMYS: Young’s Modulus: Total Pipe Length: Moment of Inertia: Pipe Face Surface Area: Diameter/wall thickness ratio: Poisson’s ratio: Mud Weight: Coeff. of Soil Fric.: Fluid Drag Coeff.:

PIPE WEIGHT DATA

12.75 0.250 35,000 2.9E+07 1,525 191.75 9.82 51.00 0.30 89.76 0.30 0.05

Inches Inches Psi Psi ft Inches^4 Inches^2

Lb/cu.ft Psi

Pipe Weight in Air: 33.38 Lb/ft Pipe Interior Vol.: 0.82 cu.ft/ft Pipe Exterior Vol.: 0.89 cu.ft/ft Air Line Weight: 0.00 Lb/ft Air Line Diameter: 0.00 Inches Air Line Ext. Vol.: 0.000 cu.ft/ft Weight of Water: 0.00 Lb/ft Displaced Mud Weight: 79.58 Lb/ft Water density (enter 0 for no buoyancy control): 0.00 Lb/cu.ft Effective Wt. of pipe: -46.21 Lb/ft Note: positive value indicates downward force

ANALYSIS OF LOADS FOR STRAIGHT SECTION PULLED DOWNSLOPE

Measured Length: Angle of Inclination: =

116.10 ft 20.00 degrees 0.35 radians

Axial Tension limited by RP2A-WSD Comparison: Longitudinal Bending limited by RP2A-WSD Comparison:

Drag Forces from Mud: Friction from Soil: Effective Weight of Pipe:

2,790 Lb 1,512 Lb (1,835) Lb

Tension on section: Cumulative Force exerted:

6,138 Lb 6,138 Lb

625 Psi

< 31,500

0 Psi

< 25,652

External Hoop Stress limited by RP2A-WSD Comparison: 631 Psi

< 6,541

Combined Stresses, Tensile & Bending, limited by RP2A-WSD Comparison: 1.00 0.02 < Combined Stresses, Tensile, Bending & Hoop limited by RP2A-WSD < Comparison: 0.01 1.00

ANALYSIS OF LOADS FOR CURVILINEAR SECTION PULLED DOWNSLOPE

Measured Length: Change in Inclination Angle: = Radius of Curvature: Center Displacement:

349.07 20.00 0.35 1000.00 15.19

ft degrees radians ft ft

Assumed Average Tension:

15,698 Lb

Normal Force: Drag Forces from Mud: Friction from Soil: Effective Weight of Pipe:

13,218 Lb 8,389 Lb 7,931 Lb (2,801) Lb

Tension on section: Average Tension: Cumulative Force exerted:

19,121 Lb 15,698 Lb 25,259 Lb

Axial Tension limited by RP2A-WSD Comparison:

2,573 Psi

< 31,500

Longitudinal Bending limited by RP2A-WSD Comparison: 15,406 Psi

< 25,652

External Hoop Stress limited by RP2A-WSD Comparison: 1,595 Psi < 6,541 Combined Stresses, Tensile & Bending, limited by RP2A-WSD Comparison: 0.68 < 1.00 Combined Stresses, Tensile, Bending & Hoop limited by RP2A-WSD Comparison: 1.00 0.53