Micro-Tunnelling Method Statement [PDF]

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Appendix S Information on Micro-tunnelling S(1): Preface to Appendix S S(2): Typical Method statement - Direct Pipe Technique S(3): Review of Risks associated with Micro-Tunneling

Appendix S(1)

Preface to Appendix S

Appendix S: Further Information on Micro-tunnelling Preface The two main Sruwaddacon Bay crossings will be executed using micro-tunnelling technology. Section 5.5.2 of the Onshore Pipeline EIS discusses the main principles of this method, and the potential environmental impacts associated with this work are discussed in the various specialist chapters including Chapters, 12, 13 and 14. The possible application of contingencies in the event of unforeseen circumstances during construction are also outlined in the Geotechnical Risk Register which is described in Chapter 15 and presented in Appendix M. In addition to the information provided in the Corrib Onshore Pipeline EIS on the proposed construction method for both crossings of Sruwaddacon Bay, the following technical documents are included here as follows: 1. Typical Method Statement – Direct Pipe Technique. 2. Review of Risks Associated with Micro-tunnelling. De la Motte & Partners GmbH, specialist consultants in trenchless construction, prepared the Method Statement for the Direct Pipe technique on the basis of their extensive experience with trenchless construction methods, including the Direct Pipe method. The review of risks associated with micro-tunnelling discusses the probabilities of encountering operational difficulties, such as physical obstructions or difficult ground conditions during the tunnelling process. It also discusses possible technical difficulties which could cause contingency measures to be applied. The document concludes that the probability of requiring the installation of an intermediate pit for either of the two crossings is very low. It also concludes that in the unlikely event that such an intervention is required, it is not predicted to have significant impact on the habitat or the fauna in the area. Therefore seasonal constraints in association with the proposed tunnelling operations should not be necessary. SEPIL is currently awaiting the return of tenders for the provision of specialist construction services to undertake these crossings. It is expected that proposals will be made by contractors providing various types of micro-tunnelling techniques including the Direct Pipe and/or similar methods.

Appendix S(2)

Typical Method statement - Direct Pipe Technique

Document Title:

TYPICAL METHOD STATEMENT DIRECT PIPE TECHNIQUE

Project:

Crossing Sruwaddacon Bay

Customer:

Shell E&P Ireland Ltd.

Corrib House 52 Lower Leeson Street Dublin 2, Ireland Contractor:

Birkenweg 11, 21465 Reinbek Tel. 040 / 711 10 03 E-Mail:

[email protected]

Internet:

www.delaMotte-Partner.de

Document Title:

TYPICAL METHOD STATEMENT DIRECT PIPE TECHNIQUE Project:

Crossing Sruwaddacon Bay

Customer:

Shell E&P Ireland Ltd.

Birkenweg 11, 21465 Reinbek Telefon: 040/711 10 03 – Fax: 040/710 57 03 Revised version

2008-08-28

2

E. Lord

de la Motte

Revised version

2008-02-08

1

Meins

de la Motte

Initial release

2007-12-21

0

de la Motte

de la Motte

Date

Rev.

Created

Checked

Version

Project No.:

20803500

Approved

File Path:

Pages:

Text:

Drawings:

Drawings:

-

Enclosures:

27 -

Typical Method Statement Direct Pipe Technique Shell E&P Ireland Ltd.

Crossing Sruwaddacon Bay

Birkenweg 11 / 21465 Reinbek Tel.+49 40 711 10 03

CONTENTS

1

General description

4

2

Main components

15

2.1

The tunnel boring machine (TBM)

15

2.2

Sleeving Pipe

16

2.3

Adapter- (joint-) pipes

17

2.4

Inside the pipe installed equipment, feeding pipes/hoses.

17

2.5

The steering unit

17

2.6

Pipe Thruster

18

2.7

Annulus

19

2.8

Drilling Fluid Handling Unit

19

3

PRE-construction ACTIVITIES

20

3.1

Soil conditions

20

3.2

Implementation drawings

20

3.3

Construction description / construction schedule

20

3.4

Engineering

21

3.5

Volume / mass balance

22

3.6

Obstacles

22

3.7

Recovery of TBM

24

3.8

Machinery breakdown

24

3.9

Availabilty of personnel and equipment

24

Appendix A: Principles of Micro-tunnelling by means of Pipe Jacking

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1

GENERAL DESCRIPTION The DIRECT PIPE® -method is a trenchless pipe tunnelling method which combines the drilling of a subsurface hole along a selected route and the installation of a pipe within it, in one operation. The pipeline is installed in one single working step without requiring vertical access from the surface along the route of the pipeline. The DIRECT PIPE® method can be applied for river and estuary crossings where the pipeline can be assembled in its full length or in parts on at least one side of the crossing. The DIRECT PIPE® method combines the advantages of the well-established construction methods of Micro-tunnelling (pipe jacking) and Horizontal Directional Drilling (HDD). (See figure 1.01.) This attachment provides the key difference between the DIRECT PIPE®-method and the Pipe Jacking method.

Figure 1.01 General illustration of a river crossing using the DIRECT PIPE® - method [Herrenknecht AG]

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In the Direct Pipe Method, the jacking force is applied via a so-called Pipe Thruster (Hydraulic pushing/pulling equipment) directly on to the pipe, which is to be installed. In the case of the Sruwaddacon Bay crossings, this is a sleeving pipe in which the gas pipeline and services will be installed. This sleeving pipe will be assembled and placed in a launch area in its full length or in sections. The thruster’s force is developed by a set of hydraulic cylinders and is transmitted onto the pipe in such a way that any external coating is not damaged. The Pipe Thruster is safely anchored in the ground. A Tunnel Boring Machine (TBM) is mounted at the front end of the pipe string. Power lines and hoses, essential for driving and controlling theTBM will be guided through the sleeving pipe and connected via flexible cables to the appropriate supply and steering equipment i.e. steering cabin, energy supply etc (see Figure 1.02) before commencing the tunneling process. In the starting pit, the Pipe Thruster will push the TBM through the start sealing arrangement and into the ground. The sleeving pipe transmits the essential jacking force for the drilling process to the TBM and absorbs pipe torsion. The TBM represents well proven tunnel boring technology which has been used over the last 20 years in micro-tunnelling.

Figure 1.02: General illustration of the equipment at start-up including drill head, sleeving pipe string, pipe thruster, steering cabin, energy supply, drilling fluid handling unit [Herrenknecht AG]

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The pipe thruster pushes the TBM in a sloped direction via the sleeving pipe into the ground along a pre-determined access gradient. The machine is steered by steering cylinders which are integral to the TBM. Surveying/locating with an accuracy of up to one centimetre in the vertical direction and a few centimetres in the horizontal direction is provided by a gyro system in the TBM.

Figure 1.03 Pipe Thruster (Herrenknecht AG)

Excavation is carried out by a cutting head situated in the front end of the TBM. The detailed configuration of the cutting head will vary depending on the geology. Drilling fluid consisting of water or an aqeuous bentonite solution will be provided at the cutting head through a supply hose to aid the removal of excavated material. The material excavated is pumped through a conveyor pipe to a separator station near the starting pit. Here the soil will be separated from the drilling fluid by screens and hydrocyclons. The soil can be removed and transported to a suitably licensed disposal facility. The drilling fluid will be reused.

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n

o p r

q

n Cutting head o Steering cylinder p Drive unit qRemoval pipe r Feeding pipe

Figure 1.04: General illustration of the drilling fluid circulation in the TBM [Herrenknecht AG]

The TBM has a slightly larger diameter than the sleeving pipe. The small annulus created by this difference (varying from approximately 20 mm at the front end to 50 mm further back), will be filled with drilling fluid to prevent collapse of the excavated hole and to minimise friction between the sleeving pipe and the surrounding soils. Very small quantities of drilling fluid are used at static head pressure only. This minimises the amount of fluid which potentially could infiltrate the soil (see Figures 1.04 and 1.05)

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Figure 1.05: Hydraulic System within TBMs

Shell E&P Ireland Ltd.

Typical Method Statement Direct Pipe Technique Tel.+49 40 711 10 03

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Once the TBM and the sleeving pipe reach the reception pit, the TBM is disconnected from all essential utilities and recovered (Figure 1.06. The supply hoses, pipe thrusters and auxiliary equipment will be removed. The gas pipeline and services can then be installed into the sleeving pipe. This is done by pulling the pipeline bundle through the sleeving pipe by a winch located in the reception pit.

Figure 1.06: Removing the cutter head after having laid the pipe [Herrenknecht AG]

All individual system components used in the Direct Pipe Method were have been developed and successfully used in many micro-tunnelling and HDD projects by Herrenknecht AG, an expert company in the fabrication of tunnelling and micro-tunnelling machines, located in Schwanau, Germany.

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Figure 1.07: Pipe thrusting of steel sleeving pipe by Direct Pipe Method [de la Motte & Partner GmbH]

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The principle and sequence of the Direct Pipe method is shown clearly in the following sketches.

Figure 1.08: Procedure of DIRECT PIPE® Method[de la Motte & Partner GmbH]

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The main benefits of applying the DIRECT PIPE® - method for the Sruwaddacon Bay crossings are as follows: ˆ The risk of failure caused by unexpected soil conditions is minimised. ˆ A pipe is permanently in the bore hole and thus holding the borehole open so that a collapse of the hole is impossible. ˆ The cutting head of the excavation machine can be designed and equipped with the appropriate tools for nearly all geological conditions. ˆ Rocks can be crushed and removed safely through supply lines within the sleeving pipe and out of the borehole; therefore deposits in the borehole do not occur. ˆ Large bending radii can be achieved with high accuracy due to the improved steering mechanism; thereby avoiding excessive bending stress (inadmissible low bending radii of the pipeline) and excessive laying forces. ˆ A large working area is only required on the start side of the crossing. ˆ Only a small working area for a short time is necessary on the target site (to allow for the dismantling of the cutting head and pulling of the pipeline bundle). ˆ A minimum volume of drilling fluid is required and the amount of excavated material is minimised. ˆ Due to the continuous pipe jacking operation the effective tunnelling time is low and high installation performances can be achieved. ˆ The sleeving pipe can be prepared and assembled in parts.

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Figure 1.09: Starting pit with Pipe Thruster and a 48” pipeline river Rhein crossing in Germany

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Figure1.10: Reception pit with the cutting head of TBM

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2

MAIN COMPONENTS The main components for the application of the DIRECT PIPE® - method in Sruwaddacon Bay are as follows: ˆ Tunnel boring machine (TBM) ˆ Sleeving pipe ˆ Swivel joint ˆ Adapter- (joint-) pipes ˆ Inside the pipe installed equipment, feeding pipes/hoses ˆ Steering unit (control cabinet) ˆ Pipe Thruster ˆ Pipeline bundle (gas pipeline and services) to be installated ˆ Drilling fluid handling unit.

Each single component represents proven technology. By joining these single components to make up the DIRECT PIPE® - method, the tunneling of pipelines without surface interruption is possible.

2.1

The tunnel boring machine (TBM) At the cutting face the soil/rock is loosened by the cutting head which is fitted with excavation tools, the design and configuration of which depending on the local geology. The excavated soil is then transferred into the TBM. The cutting head is supported by a drilling fluid, containing either water or an aqueous bentonite suspension. Hard soil layers, conglomerates and rock as well as unstable soils (sand and gravel) can be excavated.

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Figure 2.01 Excavation and transport in TBM Figure 2.01 illustrates how the excavated soil mixed with the drilling fluid is pumped via a slurry pump through the TBM and the slurry pipes/hoses within the sleeving pipe to the separator/recycling unit located at the starting pit. Torsion is transmitted from the cutting head, into the ground. To avoid transmitting torsion along the longitudinal axis from the TBM into the sleeving pipe and vice versa, the TBM is connected to the pipeline via an active swivel joint.

2.2

Sleeving Pipe A sleeving pipe of at least 42 inch internal diameter is required for the DIRECT PIPE® tunnelling proposed for the Sruwaddacon Bay crossings. This will ensure that all of the necessary pipes, hoses, cables and equipment can be accommodated. The sleeving pipe must be designed to handle the forces created by the Pipe Thruster.

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2.3

Adapter- (joint) pipes Up to three adapter pipes will be installed between the TBM and the sleeving pipe in order to optimise steerability. The adapter pipes are connected by a special construction. These connections allow horizontal and vertical bending. The arrangement of steering cylinders in combination with the adapter joint connections provides a further increase of steerability of the TBM.

2.4

Inside the pipe installed equipment, feeding pipes/hoses. All of the pipes, hoses and cables installed within the sleeving pipe are supported by special clamps and rollers. No direct contact to the inner pipe wall is required and thus damage is avoided. The advantage of this design is that equipment can easily be removed by a cable winch and disassembled. The design also incorporates rails for a battery driven crawler which provides a secure means of transportation of personnel (if required) and material through the sleeve pipe to the TBM and back. The crawler will be designed in accordance with relevant safety- and testing requirements.

2.5

The steering unit The steering unit is situated in an air-conditioned part of a container located next to the starting pit. All essential steering and control tasks can be operated and remotecontrolled via the steering unit. The hydraulic cylinders, pumps, motors and cameras are remotely controlled from the steering unit. In addition measurements and monitoring of turning moment, speed, mass, distance, pressure at important locations, slopes and positions are registered and recorded via the steering unit. A trained operator performs all control and steering tasks. The steering of the TBM is executed by 3 hydraulic cylinders, equally circumferentially spaced in the TBM, and is based on a continuous accurate survey of its position. The survey is a combined system of an electronic barometric level and a gyro compass. The data from these two survey devices determines the position of the TBM and compared with the target position at any particular point. This

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enables the operator to drive the TBM along the required curvature and to aim for and reach the target area with great accuracy.

2.6

Pipe Thruster The Pipe Thruster clamps the sleeving pipe by its clamping units which enable the thruster to transfer thrust and traction forces into the sleeving pipe and subsequently to the TBM. The clamping device is suitable for pipelines from 20” up to 60”, and the clamping units can be applied to all kind of pipes and coatings. Once the Pipe Thruster has clamped the pipe, two large hydraulic cylinders move the pipeline forwards or backwards as required. These cylinders are able to create, with a stroke of 5m and a moving speed of 5m/min, a maximum pull/push force of 5,000 kN - 7,500 kN. The clamping unit and the hydraulic cylinders can pivot on their supports, so that flatter or steeper moving angles are possible. The Pipe Thruster has a minimum weight of approximately 45 tonnes (5,000kN unit) and can be disassembled into modules for transportation. The Pipe Thruster requires secure anchoring to be able to transfer the forces into the ground. See Figure 1.09 which shows the set up of the pipe thruster for a 48” sleeving pipe in the starting pit.

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2.7

Annulus In order to reduce the thrust forces (friction between pipe and soil) an annulus between the jacking pipe and the soil is created. This is filled with a lubricant. This is an aqeuous bentonite solution. The annulus is created by selecting a TBM diameter slightly larger than the diameter of the sleeving pipe. The size of the annular gap between the sleeving pipe and the surrounding soil will depend on the geology present and the relationship between pipe stiffness and required driving curve. The creation of a clean annular gap is very important. Therefore the TBM, the sleeving pipe and the adapter pipes are designed conically to slowly establish the required annular gap. Directly behind the cutting head a lubricant is introduced into this annular gap . Over the length of the transition area, from adapter pipe to the sleeving pipe, the annular gap is completely filled with this fluid. To avoid an uncontrolled release of this fluid into the soil and a collapse of the annular gap during the tunnelling process, the annular gap is connected to a tank located near the starting pit. This atmospheric tank contains drilling fluid and ensures that the fluid pressure in the annulus is kept at hydrostatic head along the drilling curve at all times.

2.8

Drilling Fluid Handling Unit A containerised drilling fluid handling unit is located in the area of the starting pit. This recycling unit (see figure 1.02) is used to produce bentonite (mud) water suspension as and when required, depending on the characteristics of the crossing and the local geology. The unit includes pumping facilities to provide drilling fluid at the cutting head and to inject the fluid which serves as a lubricant in the annular gap. The unit is also used to separate the soil cuttings/solids from the slurry pumped back from the cutting head. These solids are removed and the remaining drilling fluid is treated, tested and re-used.

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3

PRE-CONSTRUCTION ACTIVITIES Prior to commencing activities, a number of design and engineering activities will be carried out. An essential part of this is the examination of soil conditions.

3.1

Soil conditions ˆ Review of available regional soils data. ˆ Site specific borehole survey ˆ Identify likely construction hazards and conduct risk assessment in relation to the general and specific hazards of the construction process. Develop and establish contingency measures as required.

3.2

Implementation drawings As a mimimum, the following site specific drawings will be prepared: ˆ Longitudinal section of the designed crossing. ˆ Layout with illustration of intended crossing route. ˆ Site equipment with terrain requirement

The actual profile of the terrain is integrated in the longitudinal section of the crossing. For the crossing of rivers the actual profile of the river bed in the crossing area will be included. If a recent profile of the terrain / river bed is not available a new survey of the riverbed should be carried out.

3.3

Construction description / construction schedule In sufficient time a construction description and a construction schedule including critical steps and activities have to be developed.

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The schedule for the application of the DIRECT PIPE method incorporates the following steps: ˆ Site preparation. ˆ Surveys. ˆ Site and equipment mobilisation ˆ Installation of starting and reception pits with entrance and exit walls. ˆ Installation of sleeving pipe by the DIRECT PIPE method ˆ Pull-in of the bundle of the 20” gas pipeline and services. ˆ Quality control and quality assurance activities ˆ Demobilisation of equipment ˆ Site reinstatement ˆ Issue as-built documentation

3.4

Engineering The following calculations, in accordance with current standards are performed: ˆ Pipe static according to ATV A 161 – for the sleeving pipe. ˆ Calculation / proof of allowed bending radius of sleevinging pipe. ˆ Calculation / proof of the Thruster force required for the thrusting of the pipe and the TBM. ˆ Calculation / proof of the Pipe Thruster anchoring requirements. ˆ Necessary supporting pressure in the annular gap. ˆ Calculation / proof of force transmission from the Pipe Thruster to the sleeving pipe without damaging the coating

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ˆ Design calculation required to demonstrate safe installation of the gas pipeline and services in the sleeving pipe. This will include calculating the permitted pulling forces and the safety margin against buckling for each pipe for the installation within the sleeving pipe ˆ Dimensioning of rollers and determination of distances during installation and driving-in of sleeving pipe ˆ Drilling mud system design details including fluid characteristics, system capacity and dimensions and operating pressures.

3.5

Volume / mass balance The quantity of soil recovered should correspond to the volume of the borehole excavated. The volume balance of the drilling fluid pumped to the TBM, introduced into annulus as a lubricant, the anticipated volume of drilling fluid that may escape to the surrounding soils, and the volume returning to the surface treatment facilities shall be recorded on a continuous basis.

3.6

Obstacles

3.6.1

GENERAL In the unlikely event of encountering a previously undetected obstacle whilst crossing Sruwaddacon Bay, the following considerations will be made via micro-tunnelling For the evaluation of risks in case an obstacle is found it is essential to apply appropriate engineering approaches and develop the appropriate measures. Accurate site preparation and investigation of the ground has been undertaken. The possibility to handle an obstacle and finish crossing the Bay is almost always given by the machine technology intended to be used. Possible recovery methodologies are discussed below.

3.6.2

RECOVERING OBSTACLES WITH ACCESSIBLE TBM AND •56” PIPE If a sleeving pipe of 56’’ diameter or larger is used the TBM at the tunnel face is accessible by manned intervention. A pressurised work environment

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can be created in the TBM. The excavated hole in front of the TBM can be kept stable by compressed air. The obstacle can be handled manually; manual breaking of the obstruction using hydraulic tools and transported to the starting pit through the sleeving pipe. In predominant dry earth conditions the use of a pressurised work environment is not necessary. Detailed safety precautions for the deployed personnel will be developed and strictly adhered to in all situations Having this option, it is considered that the majority of foreseen obstacles can be dealt with without vertical surface intervention.

3.6.3

OBSTACLES WHICH DO NOT MOVE WHEN APPROCHED BY THE TBM In exceptional circumstances, one could encounter obstacles which are embedded rigidly and which cannot be moved from their position when approached via the TBM. In this case the fixed obstacle can be drilled through or the part of the obstacle which is in the way can be drilled through by the TBM.

3.6.4

OBSTACLES WHICH MOVE WHEN APPROCHED BY THE TBM These are obstacles which start to move in the ground when approached by the TBM. These obstacles are not fixed in place and can therefore not be driven at or through or hacked by the TBM, as the TBM excavation tools cannot provide enough pushing force and moment on the obstacle. In this situation trained personnel will fix the obstacle tight at the cutting head, before the drilling process is re-commenced. Should the obstacle start to move again, then trained personnel will enter the sleeving pipe and via the hatch in the TBM to break the obstacle by a mechanical splitter tools. If this is not successful the next option is to pull the sleeving pipe with TBM back over a certain distance and commence tunnelling along a new trajectory. In the unlikely situation that this is not successful the obstacle may need to be removed from the surface. In this worst case scenario the TBM is pulled backwards a little by the Pipe Thruster in order not to damage the TBM when recovering the obstacle. In this unlikely event, removal of the obstacle through surface intervention can be undertaken. An intervention pit may be required.

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3.6.5

OBSTACLES WITH A SIZE OF 0.42 M Obstacles, e.g. stones in size less than 30% of the sleeving pipe diameter can be broken by an integrated stone breaker and discharged hydraulically via the returning drilling fluid.

3.6.6

CONCLUSION Detailed site specific design and appropriate soils data will provide sufficient basis to handle every natural obstacle and complete the DIRECT PIPE® work successfully.

3.7

Recovery of TBM If a recovery of the obstacle is not possible, the sleeving pipe with the TBM is pulled back. The abandoned tunnel-shaped borehole is plugged filled with a self-hardening special mixture, which is environmentally inert. The tunnelling operation can then be executed along a new re-designed trajectory.

3.8

Machinery breakdown Equipment in the TBM that fails, can be manually substituted by trained mechanics onsite and the machine can be started up again. If repair based on manned entry is not possible, the pipe and TBM must be pulled back.. The plugging of the borehole is executed as described in 3.7.

3.9

Availabilty of personnel and equipment Tunnelling using the DIRECT PIPE® method requires 24 hr operation. Therefore sufficient operators and service personnel and spare equipment must be mobilised.

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Appendix A : Principles of Micro-tunnelling by means of Pipe Jacking Pipe Jacking is a trenchless pipe tunnelling method whereby pipes are installed pipe by pipe from the launch pit (shaft) to the target shaft forming one sub-surface conduit crossing, without requiring vertical access from the surface, into which pipelines and/or services can be installed. As for the DIRECT PIPE®-method, the Pipe Jacking method can be applied for river and estuary crossings where the pipeline can be assembled on at least one side of the crossing. The DIRECT PIPE® and Pipe Jacking methods make use of the same equipment as follows: x

Tunnel Boring Machine (TBM)

x

Drilling liquid and liquid handling units

x

TBM positioning System

x

Control cabinets

x

Equipment to push the drilling string with the TBM in the front of it.

The key difference compared to the DIRECT PIPE®-method is that the TBM is pushed by the use of hydraulic jacks. The principle is shown on Figure A1 below:

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Figure A1: Pipe Jacking Process

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Typical Method Statement Direct Pipe Technique Shell E&P Ireland Ltd.

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Hydraulic jacks push pipe segments, each with a length between 2.0 - 3.0 m, with the TBM in front, to the end of their stroke. Then the jacks stroke is reversed, a new pipe segment is inserted and tunnelling of the crossing continues again until the end of the hydraulic jack’s stroke. This process is repeated until the end of the crossing is reached. This method is normally executed with concrete pipes or cast steel pipes but in special cases pre designed steel pipes can be used.

Figure A2: Pipe Jacking in progress showing inside of launch pit. (Note hydraulic jacking frame and concrete pipe sections. One pipe section is being jacked forward in front of the red assembly. Another pipe section is ready for placing into the blue jacking frame when the red assembly has been retracted. This process is repeated until the crossing has been completed).

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Appendix S(3)

Review of Risks associated with Micro-Tunneling

Corrib Onshore Pipeline Sruwaddacon Bay Crossings Review of Risks Associated with Micro-tunnelling

January, 2009

Corrib Onshore Pipeline

Review of Risks Associated with Micro-tunnelling

TABLE OF CONTENTS 1

INTRODUCTION ........................................................................................................................ 1

2

GROUND CONDITIONS ............................................................................................................ 2 2.1

2.2 3

GEOTECHNICAL HAZARDS AND ASSOCIATED CONTINGENCY MEASURES ............................. 2 2.1.1

Obstructions ..................................................................................................... 3

2.1.2

Contingency Measures..................................................................................... 3

EQUIPMENT RISK ............................................................................................................. 5

EXPERIENCE WITH DIRECT PIPE MICRO-TUNNELLING..................................................... 6 3.1

CROSSING LENGTH .......................................................................................................... 6 3.1.1

Design of cutting head to tunnel the required distance without failure due to wear and tear.................................................................................................... 7

3.1.2

Availability of sufficient thrust to progress the sleeve pipe and TBM to the reception pit. ..................................................................................................... 7

3.2 4

DURATION OF MICRO-TUNNELLING OPERATIONS ................................................................ 7

ENVIRONMENTAL RISKS ........................................................................................................ 8 4.1

DRILLING FLUID – BENTONITE........................................................................................... 8

4.2

RISK REVIEW ................................................................................................................... 8 4.2.1

Likelihood of a release of drilling fluid from the tunnel head ............................ 8

4.2.2

The likelihood of a release of bentonite from the annulus ............................... 9

4.2.3

The likelihood of a release of bentonite from the handling unit in the starting pit...................................................................................................................... 9

4.2.4 5

6

Potential consequences of a release ............................................................... 9

INTERVENTION PIT ................................................................................................................ 11 5.1

INTRODUCTION .............................................................................................................. 11

5.2

MARINE INTERVENTION .................................................................................................. 11 5.2.1

Size of the Emergency Intervention Pit .......................................................... 11

5.2.2

Installation of the Emergency Intervention Pit................................................ 11

5.2.3

Dewatering of Emergency Intervention Pit..................................................... 11

5.2.4

Duration of Intervention Works....................................................................... 12

5.3

SCOUR .......................................................................................................................... 12

5.4

TIMING .......................................................................................................................... 12

CONCLUSIONS ....................................................................................................................... 14

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1 INTRODUCTION A review of the risks associated with micro-tunnelling using the Direct Pipe or similar method for the Sruwaddacon Bay crossings has been made and is summarised below. This review is based on information available on micro-tunnelling technology and methodology, and on information gained from extensive site investigation works in the bay over the last two summer seasons. This document should be read in conjunction with Chapter 5 and the Geotechnical Risk Register (Appendix M4) of the Onshore Pipeline EIS. The Geotechnical Risk Register sets out the various aspects of the micro-tunnelling operation in terms of: x

Hazards

x

Causes

x

Risk (before and after Risk Control Measures)

x

Risk Control Measures (Design Controls and Construction Controls)

x

Contingency measures

The hazards identified for the tunnelling operations (Items 24 – 29) range from unexpected ground conditions, to loss of drilling fluids and machinery breakdown. This document provides further information on the ground conditions encountered during site investigation works, as well as more detailed information on the technical aspects of the tunnelling equipment. It discusses the probability of encountering such hazards, and also expands on the contingency measures available. Finally it provides a summary and a qualitative assessment of the risks associated with the works, including the potential need to construct an emergency intermediate intervention pit during tunnelling operations.

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2 GROUND CONDITIONS A significant amount of site investigation data from the underlying ground conditions in and around Sruwaddacon Bay was acquired by SEPIL / RPS during 2007 and 2008. This includes: Geophysical survey of Sruwaddacon Bay (Osiris, 2007): x

Multi-beam bathymetry

x

Side Scan Sonar

x

Sub-bottom Profiling

x

Land Based Seismic Refraction

x

Magnetometer

Geotechnical surveys at land based locations around Sruwaddacon Bay (GES, 2007): x

15 Boreholes (Rotary core)

Geotechnical survey of proposed crossings of Sruwaddacon Bay (IDL, 2008): x

14 boreholes (Cone penetration test, Shell & auger and Rotary core).

The results of the 2007 geophysical survey provided information on the likely composition and depth of overburden for the proposed tunnelled crossings. This indicated that micro-tunnelling would be a feasible technology for crossing the bay. The results of the 2007 and 2008 borehole surveys have confirmed depth and composition of overburden and provided further information in relation to underlying rock. The dominant ground conditions encountered are sands and gravels over psammite bedrock with rock at both sides of the estuary. This information has now been examined by technical experts in trenchless construction de la Motte & Partner GmbH. Their conclusion is that the ground conditions encountered in the two proposed crossings of Sruwaddacon Bay (predominantly sand and gravels with some silt / clay deposits) are suited to micro-tunnelling.

2.1

GEOTECHNICAL MEASURES

HAZARDS

AND

ASSOCIATED

CONTINGENCY

Local variations in ground conditions are managed firstly by the selection and configuration of an appropriate cutting head on the Tunnel Boring Machine (TBM). The borehole data obtained to date now provides sufficient information for the contractor to design the TBM and the associated cutting heads.

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Review of Risks Associated with Micro-tunnelling

Obstructions

Site investigation surveys cannot completely rule out the potential presence of obstructions in the proposed path of the tunnelling operation. The probability of encountering such objects, and possible contingency measures are discussed in the following sections.

2.1.1.1

Boulders

Isolated boulders generally do not pose a problem for normal tunnelling. The cutting equipment on the TBM will be designed to cater for rock as it will be encountered at both sides of Sruwaddacon Bay. The site investigation surveys have not identified features which could represent large boulders, however, the presence of such obstacles cannot be completely ruled out. If a boulder should be of a particular size relative to the diameter of the TBM (between 30% and 100% of the TBM diameter), it has the potential to become loosened in the surrounding ground and can begin rotating with the cutting head of the TBM. This would stop the progress of the TBM, and the TBM operator will be aware that there is a problem.

2.1.1.2

Tree trunks

If a tree trunk was to be encountered, the cutting head of the TBM could become blocked with wooden fragments. Once blocked, it may not be possible for the TBM to progress particularly if the trunk is large and only partially removed from in front of the TBM. Should this occur, it will immediately become apparent to the operator of the TBM. There are no indications from the site surveys that tree trunks will be encountered, however, it cannot be ruled out completely.

2.1.1.3

Peat

Areas of deep peat strata, isolated lenses of peat or other soft soil types do not pose a significant problem for the TBM, if the TBM is welded to the ridged steel sleeving pipes. Should peat or other soft soil types be encountered, progress will still be made by the TBM. The site investigation data has not identified any extensive area of peat in the proposed crossings.

2.1.1.4

Metallic objects

As a result of the site investigation activities, which included magnetometer surveys, it is considered that metallic objects, which could potentially create problems for micro-tunnelling, are highly unlikely to be encountered. There is little reason to believe that metallic objects would be found at depth in an area that has not been subject to significant industrial or transport activity, and the proposed depths of the two crossings should mean that metallic obstacles should not be encountered.

2.1.2

Contingency Measures

In the event that difficulties outlined above should occur, solutions that do not require surface intervention are available. These are set out in Table 1 below.

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Obstruction

Comments

Risk

Boulders

Cutting head of TBM is rotated in alternate directions which may gradually break up the obstruction.

Low probability of encountering boulders and mitigation measures in place to tunnel through boulders. Risk of failure due to boulders is considered very low.

The diameter of the TBM has been chosen so that manned intervention (see Note 1) at the cutting head of the machine is possible. If a boulder were to be encountered, visual inspection through the hatch door in the TBM is possible. Subsequent to inspection and subject to all the relevant safety precautions, the splitting of boulders by mechanical means can be executed whereafter boulder fragments will be crushed with the cone crusher built-in into the cutting head of the TBM. If this is unsuccessful there remains the option to retract the TBM and sleeve pipe for the distance necessary to adjust its forward trajectory to steer past the obstruction (see Note 2 below). Tree trunks or other wooden objects

Cutting head of TBM is rotated in alternate directions which may gradually break up the obstruction. The diameter of the TBM has been chosen so that manned intervention at the cutting face of the machine is possible. This will allow inspection of the cutting head and tunnel face from within the TBM. Removal of wooden material from in front of the cutting head will also be possible from within the machine. Depending on the size of the tree trunk (and its orientation) this operation may need to be repeated until the obstruction is passed.

Low probability of encountering wooden objects and mitigation measures in place to tunnel through such objects. Risk of failure due to wooden objects is considered very low.

If this is unsuccessful there remains the option to retract the TBM and sleeve pipe for the distance necessary to adjust its forward trajectory to steer past the obstruction. Peat

Peat was only encountered in one borehole out of fourteen that were drilled, so there is no evidence of pervasive peat areas that could cause problems. The sleeving pipe to which the TBM is welded has a high degree of stiffness which will assist in maintaining the alignment of the tunnel through the area of peat.

Probability of encountering extensive areas of peat is low and the direct pipe method allows the tunnel direction to be maintained through such areas. Risk of failure due to peat is considered negligible.

Table 1: Risk of Failure due to Unexpected Ground Conditions Note 1 The TBM can be accessed via the sleeving pipe by specially trained operatives if necessary. The diameter of the sleeving pipe proposed for both crossings of Sruwaddacon Bay is in the range 1.4m to 1.8m. At this size, there is sufficient space for an operative to access the TBM and for inspections to be made of the cutting head and tunnel face. This inspection will provide the operator with the additional information necessary to decide the next course of action.

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Once it has been confirmed that there is no technical malfunction of a type which cannot be determined from the control cabin, it will be apparent that the problem is due to an obstruction of some kind. In this case, the operator will be able to open a specially designed hatch via an airlock within the TBM or the sleeving pipe that allows safe inspection of the cutting head and the area immediately in front of it. In this way, it will be possible to identify the nature of the obstruction. Trenchless solutions to possible obstructions are relatively simple and can be repeated. Manned operations within the sleeve pipe and TBM will be carried out according to strict safety procedures. Specialist staff with specific training will be used and in this way any potential risks to personnel can be kept to a minimum. Note 2 A unique feature of the proposed Direct Pipe micro-tunnelling method is that the TBM and the entire pipe string can be retracted together using the same equipment and process as is used to drive it forward. This provides additional flexibility, not available in conventional microtunnelling or ‘pipe-jacking’, which can only work in the forward direction. The rigid steel sleeving pipe used for the Direct Pipe method also allows much greater accuracy and control than the concrete casing pipe sections used in pipe jacking. Should it be necessary to avoid an obstacle in this way, the operation will be carried out according to a set procedure and the abandoned void space plugged with a suitable approved material.

2.2

EQUIPMENT RISK

Micro-tunnelling is a successful technology with a long track record of more than 20 years. The basic mechanical elements of the process are robustly designed for working in tough environments. With the appropriate choice of cutting head, micro-tunnelling can be used in a wide range of ground conditions. This flexibility means that the process is also widely used. Micro-tunnelling systems are also designed such that they can be maintained and repaired. In the case of the proposed crossings of Sruwaddacon Bay where the sleeve pipe diameter will be in the range 1.4m to 1.8m, it will also be possible to carry out maintenance of the TBM (e.g. replacement of wearing parts) from within the machine (using specialist staff and working procedures). Other elements of the trenchless systems (bentonite handling, pipe thruster installation etc.) are based around standard industrial machinery and equipment and can be maintained relatively easily. The equipment to be used on the proposed trenchless crossings of Sruwaddacon Bay will be fully inspected and tested prior to use to ensure that the chance of mechanical or other technical failure is minimised or eliminated. The risk to the process from technological / equipment factors is therefore very low.

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3 EXPERIENCE WITH DIRECT PIPE MICRO-TUNNELLING The proposed crossings of Sruwaddacon Bay are approximately 600m long (Lower Crossing) and approximately 1,000m long (Upper Crossing). Micro-tunnelling has been used successfully (with concrete sleeve pipes) over similar distances and ground conditions to those in Sruwaddacon Bay. Examples of micro-tunnelling projects are shown on Table 2 below. No.

Country

Geology

Dimensions (diameter x length)

1

Apeldoorn, Netherlands

Fine sand

1,500mm x 930m

2

Biarritz, France

Sand, gravel, marl, soft rock, boulders

1,200mm x 780m

3

Israel

Silt, sand, fine gravel

1,200mm x 760m

4

Amsterdam, The Netherlands

Fine sand

1,000mm x 416m

5

Ramsgate, United Kingdom

Sand, limestone

1,200mm x 530m

6

Heidelberg, Germany

Sand, sandstone, clay, gravel

1,200mm x 430m

7

Los Angeles, United States

Sand

1,500mm x 500m

8

Port of Manilla, Phillippines

Rock

1,500mm x 550m

9

Wurzburg, Germany

Sand, marl, mudstone

1,200mm x 500m

10

Zurich, Switzerland

Marl, mudstone, sand

1,200mm x 500m

Table 2: Examples of Micro-Tunnelling Projects (completed in the period 2005 – 2008). The first application of Direct Pipe micro-tunnelling took place beneath the River Rhine in Worms, Germany during 2007. It was completed on schedule and without encountering technical difficulties in the tunnelling operation. The sleeving pipe diameter was 48” (1,200mm) and the crossing length was 464m.

3.1

CROSSING LENGTH

The crossing length for the Worms project is shorter than the crossings proposed for Sruwaddacon Bay. The diameter of the Worms project is also smaller than that proposed for Sruwaddacon Bay crossings. These points have been considered further in the assesment of risk associated with using Direct Pipe method in Sruwaddacon Bay. Assuming that an appropriate specification for the TBM cutting head is used (e.g. correctly designed and configured on the basis of site investigation data), the most important factor in determining the achievable crossing length is the available thrust forces. The Worms crossing was achieved without approaching the maximum thrust limit of the pipe thruster. The maximum available thrust force was 500 tonnes, but only approximately 100 tonnes were required. This indicates that, for the conditions prevailing at Worms, the maximum possible crossing length could have been more than twice the 464 metres covered. Two factors govern the ability of a micro-tunnelling system to achieve a given length of crossing: 1. Design of TBM cutting head

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2. Available thrust forces These are discussed below:

3.1.1

Design of cutting head to tunnel the required distance without failure due to wear and tear.

The TBM cutting head for each crossing will be specifically designed by the TBM manufacturer to match the soil conditions determined by the geotechnical site investigations. This should ensure that the cutting head is capable of completing the crossing without requiring replacement. The diameter of the sleeving pipe will be in the range 1.4m to 1.8m to ensure that potential replacement of the cutting teeth, should it be necessary, is possible from within the tunnel.

3.1.2

Availability of sufficient thrust to progress the sleeve pipe and TBM to the reception pit.

The cutting head needs to be forced against the tunnel face in order to function. Force is also required to overcome the friction of the sleeving pipe against the side of the borehole. This friction increases as the length of the tunnel increases. The pipe thruster specified for the Sruwaddacon Bay tunnelling operations has a capacity of 750 tonnes. The estimated maximum thrust requirements for the two proposed crossings are as follows: x

Lower crossing (approximately 600m): 360 tonnes

x

Upper crossing (approximately 1,000m): 480 tonnes

The 750 tonnes available is considered to provide an adequate safety margin. Inter-jacking stations can be used on long crossings where there is a risk that friction forces will reduce the thrust available at the cutting head to a point where the tunnelling ceases to progress. Inter-jacking stations are installed at intervals in the sleeving pipe to maintain the thrust available at the cutting head. This is a well-proven technique also used in conventional micro-tunnelling. Inter-jacks do not provide additional thrust from the launch pit, but do so from a point within the sleeving pipe string. The use of inter-jacks requires a phased thrust sequence whereby thrust is provided alternately by the inter-jacks and the main pipe thruster. In this way, the TBM and first length of sleeve pipe is advanced by the inter-jack section, while the inter-jack section and following length of sleeve pipe is then advanced by the pipe thruster. The tunnelling process continues in this way, advancing each time by a length equal to the distance of travel of the hydraulic rams on the inter-jack assembly. This process is slower than if inter-jacks were not required.

3.2

DURATION OF MICRO-TUNNELLING OPERATIONS

Once the launch pits have been constructed and all the necessary equipment and materials are ready for the installation of the sleeving pipe, it is estimated that each of the crossings will be completed within approximately 2 – 5 weeks when taking good ground conditions into consideration and no obstructions are encountered. As an example, the crossing of the River Rhine at Worms, Germany described above was completed in 11 days.

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4 ENVIRONMENTAL RISKS 4.1

DRILLING FLUID – BENTONITE

The quantities of bentonite used in micro-tunnelling operations are very much lower than would be used with a similarly sized Horizontal Directional Drilling (HDD) operation. It is also used at much lower pressures. Bentonite is not used to keep the tunnel open, only to minimise the friction in the annulus between the sleeve pipe and the tunnel created by the TBM. The annulus is approximately 50mm wide. Aqueous bentonite is a natural non-toxic clay material that is used in the micro-tunnelling process for: x

Lubrication of the TBM cutting head (for Corrib this will most likely be done with water instead of bentonite mixture).

x

Lubrication of the annulus between the sleeving pipe and the tunnel.

x

Transport of excavated cuttings by a hydraulic system from the TBM. (for Corrib this will most likely be done with water instead of bentonite mixture).

The configuration of drilling fluid facilities within the tunnel boring set-up is illustrated in Figure 1.05 in the Typical Direct Pipe Method Statement (see EIS Appendix S). The drilling fluid used to cool and lubricate the TBM is injected at the cutting head via hoses within the sleeving pipe. This fluid is likely to be mainly water, but may also be an aqueous bentonite solution if required. The maximum pressure required at the cutting head is that required to balance the surrounding ground pressure i.e. the ambient pressure at tunnel face, and is monitored and controlled at all stages. The drilling fluid becomes mixed with drill cuttings and is pumped via a return pipe to a drilling fluid handling unit at the launch pit for recovery and recycling. The annulus between the sleeve pipe and surrounding ground is lubricated with bentonite to reduce friction. This bentonite mix is made to the required consistency. It is introduced into the annulus at intervals. The pressure of the bentonite in the annulus is maintained by static head from an atmospheric head tank located near the launch pit.

4.2

RISK REVIEW

The risk of an uncontrolled release of bentonite into watercourses is discussed below.

4.2.1

Likelihood of a release of drilling fluid from the tunnel head

The drilling fluid (most likely water) is contained in hoses until it arrives at the cutting head. The quantities are controlled by the pumping rate and monitored through the returns processed in the drilling fluid handling unit. In the event that drilling fluid starts to migrate into the surrounding soil, the constant monitoring of drilling fluid would show an imbalance. Drilling fluid pumped to the cutting head would then be diverted via a bypass inside the TBM into the return line, thereby establishing a closed loop and eliminating the risk of further escape. The static pressure of water will prevent any significant release of drilling fluid into the water column.

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The likelihood of a release of bentonite from the annulus

The small volume of bentonite provided in the annulus is at static pressure controlled by the head tank. The bentonite flows into the annulus via lubricating nipples located at the TBM. Although there will be a transitional zone where the surrounding soil will contain small quantities of bentonite, there will not be any significant pressure available for the bentonite to escape upwards and reach the watercourse. Potential loss of bentonite is monitored by mass balance.

4.2.3

The likelihood of a release of bentonite from the handling unit in the starting pit

Significant quantities of bentonite will be present in the area of the drilling fluid handling unit which is located in a bunded area on land. Spill prevention procedures will be put into place and will form part of the environmental management system on site, and spill response procedures will be in place to contain any potential release from the process.

4.2.4

Potential consequences of a release

The potential consequences of a release of bentonite into the water column is discussed in the Onshore Pipeline EIS (Sections 13.4, 14.4). Table 3 below summarises the risks associated with the use of bentonite in the micro-tunnelling process: Description

Comments

Risk

Depth of overburden

Adequate overburden to minimise probability of loss of bentonite – i.e. not sufficient overpressure in the bentonite system to allow fluid to reach seabed.

Low

Bentonite volumes

Smaller volumes of bentonite than with HDD.

Low

The bentonite (mud) water suspension is used a) to lubricate the annulus between the borehole and the sleeving pipe and, b) to carry cuttings into the suction of the pump at the TBM, which circulates the spoil back to the shore based handling facility via hoses located in the sleeve pipe. In either case, large volumes of bentonite at high pressure are therefore not present and therefore a significant escape of bentonite is unlikely. Bentonite pressure at cutting head

Currently it is planned to use water as drilling fluid at the cutting head

Low

Bentonite pressure in annulus

Bentonite runs in the annulus to reduce friction. This pressure is maintained at static head connection to tank with bentonite at ground level or by means of a pump with a pressure limiter. There will not be any significant pressure available for the bentonite to escape. Potential fluid losses are

Low

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Description

Comments monitored, and significant uncontrolled releases of bentonite into the marine environment are not possible.

Risk

Environmental characteristics of Bentonite

Bentonite in small quantities is unlikely to have a significant adverse effect on the marine environment.

Low

Bentonite storage and handling

Bentonite is stored and handled on land in a controlled manner at a bunded location to prevent spills to the watercourse.

Low

Table 3: Summary of Risks Associated with Bentonite

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5 INTERVENTION PIT 5.1

INTRODUCTION

Depending on the location of the obstruction or difficulty encountered in the tunnel, it may be possible to use land based equipment to excavate from the surface to remove an obstruction at periods of low tide. This approach has the advantage of of reducing disturbance and impact to the environment; it is possible for the intervention works to be completed within days or even hours. However, this approach would only be feasible where the depth of cover to the obstruction was less than approximately 5m. Some sheet piling may be also necessary to ensure that the excavation work can be carried out safely.

5.2

MARINE INTERVENTION

In the event that recovery cannot be achieved as described above, it may be necessary to establish a temporary emergency intervention pit in the intertidal or subtidal area to access the tunnel vertically from the surface. This is considered to represent the worst-case scenario in terms of disturbance in Sruwaddacon Bay. Aspects of this are described below.

5.2.1

Size of the Emergency Intervention Pit

A typical intervention pit will have a footprint of approximately 10m x 12m (120m2). The emergency intervention pit is likely to be constructed using an inner and outer perimeter of sheet piles. Material excavated from within the inner perimeter is stored between the inner and outer sheet piles. The area of disturbance associated with this type of intervention pit is small compared to the overall length of either of the crossings of Sruwaddacon Bay.

5.2.2

Installation of the Emergency Intervention Pit

An emergency intervention pit in the intertidal or subtidal area requires installation via a floating / jackup platform. This would be of a size similar to that used during the borehole survey carried out in Sruwaddacon Bay in 2008. The necessary plant and support vessels would be operating in accordance with detailed environmental and safety procedures. A temporary jetty may also have to be constructed. The location of such a jetty has been outlined on drawing no Dg0306.

5.2.3

Dewatering of Emergency Intervention Pit

Once constructed and excavated, it will be necessary to keep the emergency intervention pit dewatered and to ensure that there is no escape of bentonite into the surrounding areas of the bay. Dewatering will be managed by pumping from the nearby jack-up platform/barge. Sedimentation tanks will be used if necessary. Careful environmental management of the works during the intervention process will ensure that disturbance to the bay is minimised.

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Duration of Intervention Works

The duration of marine intervention works (if required) will depend on the location and nature of the obstruction or difficulty encountered. It is expected that once plant and equipment has been mobilised to Sruwaddacon Bay, the construction of a temporary intervention pit can be completed within 1 week. Excavation works and removal of the problem would be programmed to take place over a further week. Backfilling and removal of sheet piles can be completed within a week. Therefore, it is expected that works can be completed within a period of approximately 3 weeks. If it is necessary to mobilise equipment to construct a temporary intervention pit, it is likely that this equipment would remain on standby within Sruwaddacon Bay until both crossings have been completed.

5.3

SCOUR

There will be a potential for seabed scour within Sruwaddacon Bay if a temporary emergency intervention pit is installed in either of the two crossings. The likelihood, characteristics and impacts of scour depend on a number of factors including: x

Nature and characteristics of material in the area of the pit

x

Current velocities

x

Size of pit

x

Duration of intervention works

Observations made during the site investigation works during 2008 using a jack-up platform in Sruwaddacon Bay at a number of locations on both proposed crossings of Sruwaddacon Bay demonstrated that scour effects were limited.. Any temporary structure placed within Sruwaddacon Bay will be monitored regularly to assess if it is causing scour. If scouring should occur, mitigation e.g. placing of sand bags or concrete mats at points around the structure where scouring is observed, will be employed. This measure is often sufficient to prevent scouring and is easily removed once works are completed. Monitoring will continue during the works to ensure that any scour effects are mitigated. The size of the potential temporary emergency intervention pit is relatively small compared to the overall area of water flow at either of the proposed crossings. It represents approximately 2.7% of the width of Sruwaddacon Bay at the Lower Crossing and 1.3% of the width of Sruwaddacon Bay at the Upper and Lower crossings respectively.

5.4

TIMING

In the unlikely event of the worst-case scenario where an emergency intervention pit is required for the tunnelling operation in Sruwaddacon Bay, there will be a temporary impact on the surrounding environment. The aspects of this in the context of seasonal sensitivities are discussed below.

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It is estimated that the total duration of a marine surface intervention as described above can be completed within a period of approximately 3 weeks. It is proposed that the activity is subject to detailed consultation with the relevant agencies e.g. NPWS and NWRFB in order to eliminate or minimise as required any potential impact on the habitat, the birds using the bay or the fish populations. Periods when particular caution needs to be employed include the period of smolt migration in the spring, the winter feeding period for birds, and to some extent the salmon run in the late summer (although a limited size emergency intervention pit is unlikely to have a material impact on migrating adult salmon). Open cut methods have traditionally been used for similar estuary crossings for pipelines in other parts of Ireland. The impacts of an open cut crossing are such that seasonal constraints may be required to protect sensitive habitat and fauna. However, the proposed Direct Pipe micro-tunnelling method has been selected specifically to minimise the potential risk to the habitat and fauna of Sruwaddacon Bay. Therefore, seasonal constraints may not be required for the main activities. However, it is recognised that in the event of needing an emergency intervention pit, specific mitigation measures, including certain seasonal constraints may be appropriate. Table 4 below lists proposed seasonal mitigation options in the event that marine surface intervention should be required: Description

Comments

Tunnelling – micro-tunnelling – Normal operation

No seasonal constraints required. No likely significant impact on SPA or SAC as a result of normal tunnelling operation.

Installation of emergency intervention pit in Sruwaddacon Bay, dewatering, removal of tunnelling obstruction, removal of pit.

Presence of marine plant. Construction of temporary jetty. Installation of sheet piles. Excavation and deposition of bay material within sheet-piled area. Discharge of water from dewatering. Removal of tunnelling obstruction and recovery. Removal of pit and demobilisation of marine plant. Estimated duration: 3 weeks. Any intervention activity should be subject to detailed consultation with NPWS and NWRFB and approval of necessary mitigation measures to be employed in periods of high sensitivity.

Scour

No seasonal constraints identified.

Table 4: Seasonal Constraints

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6 CONCLUSIONS The main risks associated with the use of Direct Pipe micro-tunnelling technology for two crossings of Sruwaddacon Bay have been evaluated and discussed. Through site investigation in combination with thorough design and selection of technology, the main risks have been substantially mitigated. The conclusions of this assessment of risks are as follows: 1. The technical risks associated with the use of Direct Pipe micro-tunnelling in Sruwaddacon Bay are low. It is expected that both proposed crossings of Sruwaddacon Bay will be completed without surface intervention. 2. The risks to the environment of the proposed trenchless construction method are low. 3. The risk of encountering obstructions on either of the two crossings is low. 4. In the event that tunnelling through an obstruction cannot be achieved after manned intervention, or after realignment of the trajectory of the crossing, it may be necessary to establish a temporary emergency intervention pit in the intertidal or subtidal area to access the obstruction from the surface. The probability of this being necessary is very low. 5. The environmental risks associated with properly managed surface intervention are low.

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