Special Seminar LNG Tanks [PDF]

Zitiervorschau

Dept. of Ship Technology, CUSAT, Batch-XL

CHAPTER 11 SPECIAL SEMINAR REPORT

“Evolution of cargo handling in LNG Carrier”

Source: www.aukevisser.nl

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CONTENT 

List of Abbreviations

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Introduction

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Spherical Moss tank (Kvaerner-Moss Spherical tank)

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Membrane tank

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New CS-1 Cargo containment system

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World LNG demand

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Onboard procedure for loading LNG Carrier

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Next Generation LNG carrier

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Apple shaped LNG tanks

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Sayaendo LNG carrier: Main characteristics

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Conclusion

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References

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LIST OF ABBREVIATIONS LNG: Liquified Natural Gas BOG: Boil Off Gases BOR: Boil Off Rate MHI: Mitsubishi Heavy Industries DHIM: Direct Loading Analysis AIP: Approval in Principle LR: Lloyds’ Register DNV: Det Norske Veritas Class NK: NIPPON KAIJI KYOKAI ABS: American Bureau of Shipping RAO: Response Amplitude Operator UST: Ultra Steam Plant IGC code: International Gas Carrier Code IMO: International Maritime Organization

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INTRODUCTION The LNG carriers are designed, constructed and equipped to carry cryogenic liquefied natural gas (LNG) stored at a minimum temperature of -163oC and atmospheric pressure with density of 470 kg/m3. The spherical and membrane types are accepted worldwide as cryogenic cargo containment systems. A number of technologies to design the cargo hold and handling systems have entered into practice for LNG carriers. Evaluation for ensuring the structural integrity and the precise design of the cargo system is the most important concern. Numerous economical reasons lead to a significant increase in tank capacity and innovations in cargo handling systems such as BOG reliquefication systems and propulsion systems such as dual fuel electric driven or diesel driven engines, etc. The most critical concern of the LNG transportation societies is how to meet and manage the new environmental and economical challenges.

Figure1: LNG carrier with membrane tank (Source: www.aukevisser.nl)

METHANE PIONEER was the first ship to carry LNG internationally, on a voyage from the Trunkline Terminal at Lake Charles, Louisiana to the British Gas facility on Canvey Island in the UK in 1959. METHANE PIONEER was a converted freighter, fitted with 5 tanks with balsa wood and glass fibre insulation. Her successful crossing of the Atlantic with 5000 m3 of LNG conclusively demonstrated the feasibility of internationally traded LNG and marked the start of the LNG era. The first two commercially viable methane carriers; METHANE PROGRESS and METHANE PRINCESS, entered their service in 1964. Each of them carried 27400m3 of gas.

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SPHERICAL MOSS TANK (KVAERNER-MOSS SPHERICAL TANK) The spherical independent tank (by Kvaerner-Moss Technology) consists of insulated single wall spherical tank, supported by a vertically built skirt. The skirt is connected with the tank around the periphery of the equator. The cargo tank material is aluminium alloy. Each cargo tank is seated in a separate cargo hold with the tank skirt mounted directly on the foundation deck. The leak protection system prevents hull structural members from direct contact with cryogenic liquid cargo.

Figure 2: Moss tank cut section (Source: www.liquefiedgascarrier.com) 

The tanks are encased within void spaces and situated in-line from forward to aft within the hull. The spaces between the inner hull and outer hull are used for ballast and also provide protection to the cargo tanks in the event of an emergency situation, such as a collision or grounding.

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There is no secondary barrier as the tanks, primarily due to their spherical construction, have a high degree of safety against fracture or failure. The tanks are heavily insulated with approximately 220 mm of polystyrene foam to reduce boil-off to a minimum.

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Each tank is covered by a spherical steel tank cover, the main purpose being for tank and insulation weather protection. The cover also permits control of the hold space atmosphere. The lower edge of each cover is welded to the weather deck, forming a

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watertight seal. A flexible rubber seal is used at the point where the tank dome protrudes out from the cover.

Figure 3: Kvaerner Moss Spherical Tank (Source: www.marineinsight.com)

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The tanks are each supported by a metal skirt from the equatorial ring, which transmits the weight of the tank and the cargo to the lower hull. The skirt is stiffened in the upper part by horizontal rings and the lower part by vertical corrugated stiffeners.

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In the case of a crack occurring in the tank material, a small leakage of LNG within the insulation will be detected at an early stage by the gas detection system fitted at the equatorial ring area and at the drip pan. The drip pan, installed directly below each cargo tank, is fitted with temperature sensors to detect the presence of LNG.

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Thus, following are the advantages of Kvaerner-Moss Spherical tanks:   

It enables space between the inner and outer hull and this can be used for ballast and provided protection to cargo in case of side-ward collision damages. The spherical shape allows even distribution of stress, therefore reducing the risk of fracture or failure. Since ‘Leak before Failure’ concept is used in the design, it presumes and ensures that the primary barrier (tank shell) will fail progressively and not catastrophically. This allows crack generation to occur before it propagates and causes ultimate failure.

MEMBRANE TANK The cargo containment system consists of insulated cargo tanks encased within the inner hull and situated in line from forward to aft. The spaces between the inner hull and outer hull are used for ballast and will also protect the cargo tanks in the event of an emergency situation, such as collision or grounding. The cargo tanks are separated from other compartments, and from each other, by transverse cofferdams which are dry compartments. The tank is made of:    

A thin flexible membrane, called the primary membrane, which is in contact with the cargo. This is fabricated from Invar and has a typical thickness of 0.7mm. A layer of plywood boxes filled with Perlite, called the primary insulation, typically of approximately 230 mm thickness. A second flexible membrane similar to the first one, called the secondary membrane. Also, of Invar and having a typical thickness of 0.7mm. A second layer of boxes, also filled with Perlite, and in contact with the inner hull, called the secondary insulation. This layer is typically of approximately 300 mm thickness.

The function of the membranes is to prevent leakage, while the insulation supports and transmits the loads and, in addition, minimises heat exchange between the cargo and the inner hull. The secondary membrane, sandwiched between the two layers of insulation, not only provides a safety barrier between the two layers of insulation, but also reduces convection currents within the insulation.

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Figure 4: Membrane design (Source: www.marineinsight.com)

Figure 5: Gas Transport Technigaz (GTT) –GT96 (Source: GTT training manual)

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Some of the advantages of membrane tanks are as follows:   

They are generally of smaller gross tonnage, that is the space occupied within the hull is lower for a given cargo volume. Due to the above reason, maximum space in the hold can be used for cargo containment. Since the height of tanks above the main deck is significantly lesser compared to the cases of Moss tanks, membrane tanks provide allow visibility from the navigational bridge. This also allows a lower wheelhouse.

Figure 6: Parts of a membrane tank (Source: www.marineinsight.com)

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Dept. of Ship Technology, CUSAT, Batch-XL

New CS-1 CARGO CONTAINMENT SYSTEM In recent years, a new CS-1 cargo containment system has been developed by GTT in which the advantages of both Gaz Transport and Technigaz systems have been combined together into one system. The basic structure of the system uses  Invar (Gaz Transport system) for a membrane,  Reinforced plastic foam (Technigaz system) for insulation, and  Triplex (material formed by aluminum sheet reinforced with glass cloth: Technigaz system) for secondary barrieer.

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WORLD LNG DEMAND After steady growth in recent years, global LNG trade increased sharply in 2017, rising by 35.2 MT to reach 293.1 MT. This marks the fourth consecutive year of incremental growth, and the secondlargest annual increase ever (behind only 2010). The increase was driven by higher production at liquefaction plants in Australia, as well as fullyear production and new trains at Sabine Pass LNG in the United States. The continued addition of supply in the Pacific Basin, primarily in Australia, as well as the start of exports from the United States Gulf of Mexico enabled this increase. Demand growth was most pronounced in Asia; China, India, and Pakistan added a combined 13.0 MT in incremental LNG demand. Inter basin LNG trade flows have declined, particularly as Pacific Basin supplies continued to catch up with high demand in that region.

Figure 9 : Major LNG exporting countries (Source: World LNG report 2018)

China was a clear driver of LNG import growth during 2017, accounting for over one-third of net growth, rising by 12.7 MT. Also in the Pacific region, South Korea recorded the second-largest increase, with LNG demand supported by the power sector throughout the year to rise by 4.9 MT to 38.6 MT (2nd highest annual total for the country).

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Fig 10 : Global LNG trade (Source: World LNG report 2018)

The most common size of LNG carriers delivered or on order is between 120,000-180,000 m3, and often referred to as Conventional. The demand for lower LNG transportation costs is most effectively met by increasing the LNG capacity of the LNG carriers. Thus, the LNG carriers to and from Qatar ordered over the last few years are of the large sizes of approx. 210,000 m3 and 265,000 m3 and referred to as Q-flex and Q-max, respectively.

Figure 11 : Global LNG Fleet by Year of Delivery (Source: World LNG report 2018)

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ONBOARD PROCEDURES FOR LOADING LNG CARGO LNG is liquefied natural gas, which is the very cold liquid form of natural gas.LNG carriers are generally specialised ships transporting LNG at its atmospheric pressure boiling point of approximately -162 degree C, depending on the cargo grade.LNG carriers were typically in the range 80-135,000 m3 up until 2006. Before loading operations begin, the pre-operational ship/shore procedures must be thoroughly discussed and followed. Appropriate information exchange is required and the relevant parts of the ship/shore safety check list should be completed.

Line Cool Down The terminal should be instructed to begin pumping at a slow rate for approximately 15 minutes, in order to gradually cool down the terminal piping and the ship’s headers. Slowly increase the terminal pumping rate until theliquid main and spray headers have cooled down (approximately 15/20 minutes). Cargo tank pressures should be monitored closely and if required the HD compressor should be adjusted in order to maintain a constant vapour pressure. Note ! In order to avoid the possibility of pipe sections hogging, (contracting at the bottom more than at the top and thus causing flanges and long pipe sections to be stressed) the liquid header and crossovers must be cooled down and filled as quickly as possible. Prior to commencing the loading operation the cargo pipelines have to be cooled. The primary reasons for cooling the cargo lines are: i) To minimize the possibility of leaks being created at joints with valves or other sections of pipeline as they contract when cargo is passed. ii) To reduce the possibility of sudden shock loadings on bellows as pipes contract rapidly. iii) To avoid the formation of vapor locks in the pipelines when cargo is introduced. If LNG is introduced into a warm pipeline the initial cargo will vaporize, create a large pressure that can ‘block’ the loading of the liquid. It is then possible that this vapour will then condense very rapidly as the temperature reduces below the condensation point, allowing the liquid to surge along the pipeline possibly resulting in damage to the pipelines, valves or connections.

Air purge of loading arms After the connection of loading arms, air should be purged from the loading arms and the tips of manifold pipes. N2 gas is lead into the loading arms from injection lines connected to the arms, and then pressurize up to about 4 to 6 kg/cm2G. After pressurization, the ship’s liquid manifold vent valve and vapor manifold vent/drain valve are opened to release air and N2 gas into the atmosphere. While this operation is repeated two or three times, a leak test (with soap solution) is conducted at the same time. Air purge comes to an end when the oxygen content of the purged gas has dropped below 2%.

Loading Arms Cool Down The cool down of the loading arms is performed from shore side by use of a small capacity pump. At a discharge port, the arms are cooled down by sending in LNG by ship’s spray pump.

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Loading Operation LNG is loaded via the loading manifolds to the liquid header and then to each tank filling line. The boil-off and displaced vapour leave each tank via the vapour suction to the vapour header. The vapour is initially free-flowed to shore via vapour crossover manifold and, as tank pressure rises, one compressor is brought into operation to increase the gas flow to shore and limit the vapour main and cargo tank pressure. As the loading rate increases, it is important to monitor the tank pressures and to start one HD compressor. If the compressors are unable to cope with the volume of boil-off and displaced gas, it will be necessary to reduce the loading rate.

Bulk Loading When all lines and valves are fully cooled the vessel can commence ramping up the loading rate in the sequence agreed with the terminal. Deballasting should be commenced in accordance with the cargo plan. The cargo should be evenly distributed during the loading. Ensure the HD compressors are adjusted in line with loading rate to ensure that the tank vapour pressure remains at a level safely below the lifting pressure of the relief valves. Ensure Nitrogen system is performing correctly. Moss vessels will require the temperature gradient (with particular reference to the equator) to remain within certain limits, the tank temperatures are therefore to be closely monitored. Hourly temperatures are to be recorded in order that if required the vessel can verify that temperature has stayed within the manufacturers tolerances. If not already started membrane ships should start appropriate cofferdam heating. Communications with the terminal should be tested on a frequent basis. Remote gauging devices and v alve position indicators should be verified against local readouts at regular intervals during the operation. Moorings should be diligently attended and vessel movement with respect to loading arms closely monitored, if required additional persons are to be called to assist with the moorings. If at any time the OOW is in doubt a senior officer or the Master should be called.

Topping off As the vessel approaches completion of cargo operations the tanks should be staggered in line with the cargo plan, typically this would leave a gap of 10 to 15 minutes between completion of each tank. The terminal is to be notified well in advance and in line with the agreed procedure that the vessel is topping of and will need to reduce loading rate. Notification should be made at least 30 minutes before reducing rate. Note: Membrane tanks normally fill to 98% where as Moss vessels normally fill to 99.5%. On all vessels the independent alarms activate at preset filling levels, the upper alarm activates the ESD if previous alarms are ignored.

Deballasting The deballasting operation is carried out simultaneously with the cargo loading operation. Before any de-ballasting commences, all ballast surfaces should be visually checked and confirmed as free from oil or other pollutants. This check must be carried out through inspection hatches / tank lids. This is particularly important for ballast tanks which are situated adjacent to fuel oil tanks. If fitted, gas detection / sampling systems may not indicate the presence of hydrocarbons particularly in small quantities. Deballasting is initially carried out by gravity discharge until the level in the ballast tanks approach the vessels water line when the ballast pumps are used.

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The ballast should be adjusted to keep a small stern trim to aid with the stripping of the ballast tanks. The flow rate of the ballast should be adjusted to keep the ship within 1 meter of the arrival draft or as specified by the terminal. Deballasting should normally be completed before the start of the topping off of the cargo tanks.

Behaviour of LNG in the cargo tanks When loaded in the cargo tanks, the pressure of the vapour phase is maintained substantially constant, slightly above atmospheric pressure. The external heat passing through the tank insulation generates convection currents within the bulk cargo, causing heated LNG to rise to the surface where it vaporizes. The heat necessary for vaporization comes from the LNG, and as long as the vapour is continuously removed by maintaining the pressure as substantially constant, the LNG remains at its boiling temperature. If the vapour pressure is reduced by removing more vapour that is generated, the LNG temperature will decrease. In order to make up the equilibrium pressure corresponding to its temperature, the vaporization of LNG is accelerated, resulting in an increase heat transfer from LNG to vapour. If the vapour pressure is increased by removing less vapour than is generated, the LNG temperature will increase. In order to reduce the pressure to a level corresponding to the equilibrium with its temperature, the vaporization of LNG is slowed down and the heat transfer from LNG to vapour is reduced. LNG is a mixture of several components with different physical properties, particularly the vaporization rates; the more volatile fraction of the cargo vaporizes at a greater rate that the less volatile fraction. The vapour generated by the boiling of the cargo contains a higher concentration of the more volatile fraction than the LNG.

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NEXT GENERATION LNG CARRIER MHI (Mitsubishi Heavy Industries) has developed a next-generation spherical tank LNG carrier with a continuous tank cover, nicknamed SAYAENDO, meaning “peas in a pod”. Whereas a conventional spherical tank carrier is equipped with hemispherical shaped domes above the weather deck protecting each tank as separate structures, SAYAENDO has a continuous structure protecting all tanks, and this structure is fully integrated with the main hull.

Figure 12 : SAYAENDO (Peas) (Source: en.wikipedia.org)

The product name SAYAENDO, or peapod in Japanese, aptly describes the continuous weather cover for the cargo tanks that is integrated with the ship’s hull, constituting a visual and conceptual distinction from the ubiquitous hemispherical covers found on conventional Moss LNG carriers. The continuous tank cover significantly contributes to the overall hull strength. It also allows more optimal layout of steel needed to maintain strength thus enabling more compact ship dimensions. Such structural characteristics made possible a reduction in the hull steel weight over a conventional design of the same cargo capacity thus achieving improved fuel consumption due to reduced displacement, better terminal compatibility, and improvements in safety, reliability and maintainability. These advantages also make the SAYAENDO design ideally suited for a large spherical tank LNG carrier. The continuous tank cover also removes the need for complex arrangements that support tank-top piping, cables and passages, thus contributing to improved maintenance. Other merits inherent to the SAYANEDO, such as reduced exposure of supporting structures and outfitting, and higher protection to maintain global strength and cargo tank integrity against ice impact loads, also make the new design uniquely suited for operating in cold regions and in ice-bound seas.

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This report presents how state-of-the-art engineering verification methodologies were applied to validate the new design to meet the stringent technical, regulatory and safety requirements of the LNG shipping industry, as well as the design of SAYAENDO itself. This next generation design incorporates the advantages of structurally proven Moss tank design and the aerodynamic and continuous tank cover of the membrane tank design.

Figure 13: MHI next generation LNG carrier under sea trials (Source: MHI Ltd Japan)

APPLE SHAPED LNG TANKS The apple-shaped LNG tanks are based on a highly reliable MOSS-type tank. MHI improved the MOSS-type tank into an apple like shape to increase the volumetric efficiency and maximize the cargo capacity.

Figure 14: Comparison between conventional and Sayaendo tank (Source: Paper “DESIGN OF THE EVOLUTIONARY LNG CARRIER”)

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The ordinary tank consists of semi-spheres and a cylinder, and the apple-shaped tank consists of a donut-shaped torus as well as semi-spheres and a cylinder. Because of the lower height, the centre of gravity of the apple-shaped tank is at a lower position than the ordinary one despite the same volumetric capacity. The outer surface of the aluminium tanks is covered with heat insulator. The heat insulator thickness is different in each place depending on the surrounding structures and conditions to keep the predetermined heat insulation for the whole tanks.

SAYAENDO LNG CARRIER: MAIN CHARACTERISTICS  Increased cargo capacity via stretched tanks The capacity to transport 8000m3 more LNG than a typical 147000 m3 carrier is achieved without increasing the beam by using apple shaped tanks that maintain the same tank diameter. Thus, the new design provides a higher cargo capacity while having the same air draft.

 Reduced hull weight and compact design In conventional LNG Carriers, hemispherical covers provide little contribution to the overall strength, which is constituted by other hull structures. By employing a continuous cover to house the four spherical tanks, the cover is able to perform as a primary hull strength element, and the 155300m3 SAYAENDO was able to achieve greater overall strength while achieving a reduction in steel weight.

 Lower fuel consumption A significant reduction in fuel consumption is achieved through reduction in steel weight and improvements in propulsion performance, as is a reduction of head wind force with the use of the continuous tank cover

 Low boil-off rate (BOR) The rates of naturally generated boil off gas (BOG), which is caused by heat ingress into LNG storage tanks, can be readily optimized according to operational requirements by altering the thickness of the thermal insulation. Instead of a typical BOR of 0.15%/d achieved in conventional LNG carriers, the SAYAENDO is capable of 0.08%/d.

 Lower maintenance costs In conventional hemispherical tank cover ships, piping, electric cables, and passages on top the covers are supported by complex structures. The continuous tank cover makes such supporting structures unnecessary, thus significantly improving maintainability.

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 Highly versatile cargo capacity A larger cargo capacity increases economic competitiveness by lowering the unit cost of operation; however, due to limitations on LNG storage capacity at receiving terminals, LNG carriers that are too large may pose operational difficulties. A total tank capacity of 155300 m3 offers highly versatile terminal compatibility worldwide.

Figure 15: Complex Piping and cabling over Moss tank type LNG carrier (Source: www.aukevisser.nl)

 Environmental performance With improvements in propulsion performance, a reduction of head wind force, and lowered fuel consumption via adoption of the duel fuel engines and the UST plant, the SAYAENDO is expected to achieve a CO2 reduction of approximately 25% per cargo unit during actual operations compared with a conventional 147000 m3 carrier

Figure 16: CO2 Reductions per Cargo Unit Source: Paper “DESIGN OF THE EVOLUTIONARY LNG CARRIER

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Figure 17: PACIFIC MIMOSA (Source: www.aukevisser.nl)

CONCLUSION Safety and reliability of LNG carriers have always been important demands to the design of this type of ship, as can also be seen from the stringent standards given by IMO and IGC codes. Thus, the most widely used LNG containment systems, the Moss and the membrane have been applied for many years because of their reliability. Today, the membrane type LNG carrier seems to take over the major market share because of its better utilisation of the ship’s hull volume. In today’s world efforts to minimize carbon footprint and fuel costs, improvements in energy efficiency are being vigorously sought in the marine transportation industry, thus Sayaendo design (Next generation LNG carrier) that features a continuous cover integrated with the hull and a reliable spherical tank system, which can be suitable for enlargement and is appropriate for use in cold regions. The advantages of Sayaendo were confirmed through various technological evaluations.

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REFERENCES 1. DESIGN OF THE EVOLUTIONARY LNG CARRIER “SAYAENDO” by Koichi Sato, Mitsubishi Heavy Industries (MHI), Ltd. Japan 2. Watanabe, M., Structural Design and Construction Method for “Apple Shaped LNG Cargo Tank” 3. Sato, K. et al., MHI-DILAM (Direct Loading Analysis Method) - An Advanced Structural Analysis Method for Ships and Offshore Structures

4. ShipRight Construction Monitoring Procedure- www.lr.org

5. www.marineinsight.com 6. “Development of High Efficiency Marine Propulsion Plant (Ultra Steam Turbine)” by Makoto Ito, MHI Ltd. Japan 7. “LNGC with Continuous Integrated Tank Cover” by Koichi Sato and Sai Hiramatsu, Mitsubishi Heavy Industries (MHI), Ltd. Japan 8. www.aukevisser.nl 9.

World LNG report 2018

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