Microsoft Word - Session 14 - Moss Maritime - Iversen, Oivin.docØivin Iversen &
Roy-Inge Sørensen
Moss Maritime
© Gastech 2005 Iversen & Sørensen 2
Abstract: The single barrier Moss Spherical Tank Design has through a service period of more than 30 years built up an unparalleled record regarding safety, reliability and availability. Nevertheless, as new design tools become available the concept is continuously being re-examined in search of enhanced safety as well as improved technical and economical performance. In this paper we will outline the results of development work carried out during the last few years. We believe this work has resulted in design improvements, which makes the Moss design even better able to meet the future challenges and demands for safe and economical LNG transport in a world-wide market.
The following key areas will be highlighted:
• Larger Carriers: Large sized LNG projects where larger volumes of LNG are to be delivered to the same receiving
terminal allows the use of larger LNG carriers. Are there technical limitations to the size of the carrier? Moss LNG carrier designs will be presented.
• Alternative propulsion systems: The focus on transportation cost has made other propulsion systems than the steam plant interesting.
• Cargo handling: Introduction of the slow-speed diesel engine as propulsion plant has also pressed forward alternative handling of the boil off gas. The Moss® Reliquefaction System will be presented.
• Ship vibrations: Slow-speed diesel engines expose the carrier to other vibration frequencies. A vibration analysis has therefore been performed for Moss type LNG carriers.
• Improved operational procedures: New cool-down estimates based on stress and cargo-handling calculations will be presented.
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1 INTRODUCTION
The original Moss® LNG carrier design incorporated some unique features: • Single barrier containment • Leak before failure principle • Full access for external and internal inspection The result was a containment design which for more than 3 decades in world-wide operation has shown unparalleled dependability and availability. The first Moss ship, the Normal Lady was commissioned in 1973 and is still in service, fulfilling a charter which will end in 2021. The Lady will then be 48 years old. The steady improvement in computer software for analysis and simulation since the 1970’s together with lessons learned from general ship operation is the basis for design scrutiny and search for improvements. During the last few years, Moss Maritime has invested in development work in order to enhance our concept and meet future requirments from owners and operators. Some results of this work are described in the paper. Some of these developments and their advantages are described in this paper.
2 THE BASIS OF THE MOSS TYPE LNG CARRIERS
Advances in design must build on established configurations and operating experience. Developments should be concentrated in areas where enhancements seem possible in order to ensure technical and economical progress. A review of the background and performance of the Moss type carrier is therefore appropriate before describing the results of the development work. The international regulatory situation regarding liquefied gas ships during the period 1960 to 1970 was fragmented and in a state of formation and emergence, to say the least. Each of the several ship classification societies had its own rules, with each differing from the others in numerous important respects. Until 1965 there were no uniform international rules to be observed for the transport of liquefied gases. In 1965 The US Coast Guard issued a regulation entitled “Navigation and Vessel Inspection Circular No. 13-65: Foreign Vessels Carrying Bulk Liquid Cargoes Which Involve Potential Unusual Operating Risks; Requirements for Plan Review and Inspection”. The logic behind this regulation which was immediately understood and accepted worldwide, was that if a hazardous cargo ship had problems in a U.S. port, the danger was to that U.S. port specifically, regardless of the vessel’s flag of registry. From then on universal / international rules for LNG ships were in effect via the Coast Guard’s regulations and these rules were strictly enforced. Thus, the international hazardous cargo ships ‘playing field’ was levelled, and all ship builders and ship owners, irrespective of flag or class, had to meet the same rules if they intended ever to trade to the U.S. The Moss design was originated in the late 1960’s and early 1970’s when overseas LNG transport was in its infancy. Even though the owners of the first Moss carriers were indeed pioneers and should be credited for vision and courage, the designers felt that the design incorporated adequate safety margins. The reason being that accurate strength analysis methods were available and extensive testing was carried out. Time has indeed proven the integrity of the Moss design as a review of available operating records easily verifies. As of July 2004 the statistics shows the following: MOSS GT TGZ Other No. of ships in operation 79 56 25 6 Accumulated ship years 1033 716 373 239 Accumulated tank years 4946 3630 1929 ---- Average days unscheduled offhire pr. shipyear 0.4 4.1 1.8 1.4
Table 2-1 Fleet statistics as of July 2004
All ships and tanks of Moss type ever built are still in service. There is not detected leak from any of the 365 Moss spherical tanks. Unscheduled offhire for Moss type of ships are significant lower than for any other LNG ship design.
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Figure 2-1 Annual unscheduled offhire
The statistics show that the existing Moss design is reliable and endurable and all research and development performed have had the same focus; safety, reliability and availability.
3 LNG CARRIER SIZING
Traditionally the restrictions to the carrier size are due to other factors than the technological limitations of the ships. It has been the terminals and the access to these terminals that have dictated the ship sizes. The Bay of Tokyo has a maximum displacement restriction of 105 000 tonnes. However, new projects are being developed, where Energy Majors have interests in all parts of the LNG chain. Larger carriers giving reduced transportation costs, which will most probably be dedicated to one trade through out their lifetime, are of outmost interest. Both the liquefaction plant and the receiving terminal can be designed for larger vessels to take advantage of the economy of scale factor. So, what are the technical limitations to the size of a Moss type LNG carrier? Theoretically, the largest possible Moss type LNG carrier will be the defined by the largest possible diameter of the sphere multiplied by number of tanks. Today, the largest existing sphere is almost 42 meters in diameter. Carriers with spheres of up to 46 meters have been approved by class. So, is this the maximum diameter? We do not believe so. It will be difficult to answer this question without performing comprehensive analyses of a specific design. It should be mentioned that for offshore applications spheres of up to 56 meter in diameter has been evaluated.
3.1 RANGE OF MOSS TYPE LNG CARRIERS
The last years developments have resulted in a range of Moss type LNG carriers which can meet future demands. All carriers are designed for 40 years world-wide/20 years North Atlantic operation and all filling levels.
3.1.1 Moss 147k m3 LNG carrier
A standard 147k m3 Moss type carrier has 4 tanks each with a diameter of approximately 41.5 meters and a volume of 37 500 m3.
Figure 3-1 147k m3 Moss type LNG carrier
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The main parameter of the 147k m3 design are: • Length, o.a. 289.00 m • Length, p.p. 274.00 m • Breadth, moulded 49.00 m • Depth, moulded 27.00 m • Draught, design 11.40 m
3.1.2 Moss 200k m3 LNG carrier
Using the sphere diameter as for a 147k m3 LNG carrier with a vertical cylindrical section above equator additional cargo volume can be obtained. A design consisting of 5 tanks with a diameter of 41.6 meters, 4 of them with a cylindrical insert has been developed giving a total carrying capacity of 200 000 m3.
Figure 3-2 200k m3 Moss type LNG carrier
The main parameter of the 200k m3 design are: • Length, o.a. 315.00 m • Length, p.p. 300.00 m • Breadth, moulded 50.00 m • Depth, moulded 29.00 m • Draught, design 12.00 m
3.1.3 Moss 235k m3 LNG carrier
Extensive analyses have also been performed for larger diameter spheres. A tank with a diameter of 46 meters has been examined and approved by Class. Such tank will have a cargo volume of more than 50 000 m3. A concept consisting of 4 such tanks together with one “standard” 40.44 meters tank has been developed and approved. This design has a carrying capacity of 235 000 m3.
Figure 3-3 235k m3 Moss type LNG Carrier
The main parameters of the 235k m3 design are: • Length, o.a. 344.90 m • Length, p.p. 328.50 m • Breadth, moulded 55.00 m • Depth, moulded 32.50 m • Draught, design 12.00 m
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3.1.4 Moss 250k m3 LNG carrier
A further development of the 235k m3 design has also been performed arriving at the carrying capacity of 250 000 m3. The diameter of the 4 aft tanks is increased to 47 meters.
The main parameters of the 250k m3 design are: • Length, o.a. 349.40 m • Length, p.p. 333.00 m • Breadth, moulded 57.00 m • Depth, moulded 32.50 m • Draught, design 13.00 m
3.1.5 Hull performance
When developing new carriers, Moss puts a lot of effort into the hull design. Shaping of the hull is important for the overall performance of the carrier. For the range of carriers described above new twin skeg twin propeller hullforms where developed. No carrier of today has a twin skeg hull and it was then even more important for Moss to present hullforms with performance characterises which could match that of existing carriers. Therefore, Moss has done extensive test of both the new 147k m3 and the new 235k m3 hullforms in a model basin. The results of the tests are presented in figure 3-5.
Theoretically, a hull with twin skeg will have a higher resistance that a similar hull with single skeg due to a larger wetted surface. However, a better propulsion efficiency can be obtained with a twin skeg arrangement. When comparing the new twin skeg 147k LNG carrier hullform ( white curve) with existing single skeg 137k LNG carriers (yellow curve) it can be seen that a 10% reduction in required power is achieved even with a 7% higher cargo intake.
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Figure 3-4 250k m3 Moss LNG carrier
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4 ALTERNATIVE PROPULSION SYSTEMS
4.1 PROPULSION STATUS
Propulsion systems of today’s merchant ships are dominated by diesel engines. A configuration of a single slow-speed diesel is the overall preferred choice for high sea shipping of any cargo, and especially for larger vessels. The configuration of multiple medium-speed diesels is the preferred choice for passenger vessels, special trades and short sea shippingi). The steam turbine solution is the choice only for LNG carriers and naval vessels. Gas turbine solutions are found mainly in naval vessels and fast ferries. Also Cruise ships of the latest generation use gas turbines to some extent. For LNG carriers the propulsion status is that all existing carriers are equipped with steam propulsion except two small 30-year-old LNG carriers, the Havfru and Century and the new Gaz de France Energy. Havfru and Century are equipped with diesel engines while Gaz de France Energy is equipped with a diesel electric propulsion system. For the LNG carriers in order, the status is as follows: • 2 carriers with Diesel electric propulsion at Chantier de l’Atlanique, • carriers with Diesel electric propulsion at Hyundai Heavy Industries • 8 carriers with slow speed diesel propulsion at Hyundai Heavy Industries, Samsung and Daewoo.
4.2 ALTERNATIVE PROPULSION SYSTEMS FOR LNG CARRIERS
Alternatives to the traditional steam turbine plant has been discussed and evaluated for use onboard LNG carriers for decades. The main reason is the significant difference in thermal efficiency as shown in figure 4-1 below and as relative fuel consumption in table 4-1.
PROPULSION TYPE FUEL
CONSUMPTION RELATIVE
CONSUMPTION [g/kWh] [%]
CONV. STEAM PLANT 290 100 COMBINED CYCLE GT 200 69 DIESEL ELECTRIC 200 69 MEDIUM SPEED DIESEL 180 62 SLOW SPEED DIESEL 170 59
Table 4-1 Relative fuel consumption
Source: Man B&W
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All relevant propulsion system alternatives give higher fuel efficiency than the steam plant. The most interesting alternatives are: • Slow speed diesels with reliquefaction system • Dual-fuel diesel electric propulsion • Gas turbines The required power for propulsion can be generated by single, twin or multiple, fuel or gas driven, prime movers. The engines can either be directly coupled to the propeller(s), geared or coupled to an electrical generator. Single or twin propellers can be chosen. The choice will depend on economical and operational aspects. Historically, the assessment of these factors for the option of propulsion technology have for ordinary larger cargo vessels of the following categories; container vessels, bulk carriers and tankers, ended at the selection of a single, heavy fuel-burning, slow speed diesel engine in more than 90% of contemporary vessels. The LNG carrier is in fact a large bulk carrier for liquid cargo, operating at somewhat higher speed. The Moss type LNG carriers described in section 3 have all been design for using slow speed diesels for propulsion. The arrangements consist of two diesels directly coupled to separate propeller shafts. However, other types of propulsion systems can also be installed.
4.3 SLOW SPEED DIESEL PROPULSION
In a diesel engine driven LNG carrier, the energy requirement is less due to the higher efficiency. So, the supplementary energy by boil-off or fuel oil can be reduced significantly as shown in figure 4-2. For a LNG carrier with slow speed diesel propulsion the natural boil off can be handled by a reliquefaction system as described in section 6. Moss Maritime has for our new LNG carrier designs proposed to install two slow speed diesel engines and twin propeller. Each engine directly coupled to a separate propeller shaft. Thus, the gear is redundant. The reliability of the slow speed diesel is also as good as steam propulsion. Statistics from one particular ship owner, operating both steam propelled LNG carriers and other single diesel driven merchant ships shows that the annual unscheduled stoppage at sea is insignificantly different. The statistics is presented in table 4-2. Applying this for a twin diesel arrangement will give an availability of the propulsion plant of close to 100%.
Figure 4-2 Propulsion alternative - energy need for propulsion
Source: Man B&W
5 VIBRATION ANALYSIS
Installation of slow speed diesel engines as prime mover for propulsion in LNG carriers means introduction of additional excitation sources acting on a wide range of frequencies. In order to disclose unforeseen problems, a detailed investigation of vibration response of cargo tank structure as well as the global hull structure has been conducted. The base case to be studied has been a large carrier with cargo capacity of 235k m3. The design comprises four spherical cargo tanks with diameter 46 meters and one smaller tank forward with diameter 41.6 meters. The vessel is provided with twin skeg afterbody with two, seven cylinder, slow speed diesel engines installed. Two four bladed, fixed propellers are directly connected to the propeller shafts. The main parameters of the vessel is as follows: Hull Length, o.a 344.90 m Length, p.p. 328.50 m Breadth, moulded 55.00 m Depth, moulded 32.50 m Draught, design 12.00 m Ship speed 20 knots Main Engines Number of sets 2 Type Slow Speed Diesel Engine MAN B&W 7S70MC-C MCR 2 x 19.5 MW Propellers Number of 2 Number of blades 4 Diameter 7800 mm Revolutions, max 80 Both full load condition as well as ballast condition with empty cargo tanks have been assumed. The seven cylinder engines are considered to represent an optimum installation for the actual vessel. However, transfer functions covering all actual categories of excitation forces and moments have been performed. This makes it possible to estimate vibration response for alternative number of cylinders as well as different engine designs. The finite element model of the hull girder with deck houses and tank covers consist of 24 super elements. The model is shown on fig. 5-1. The containment system comprising tank shell, tank skirt and pipe tower consists of abt. 2700 basis elements sufficient for defining the stiffness of the subject structure. This element model is illustrated in fig. 5-2.
Table 4-2 Availability of propulsion plant
Steam 99.9% Single Diesel 99.8% Twin diesel: one of two engines running 100 % Twin diesel: both engines running 99.7% Source: LNG ship owner/operator
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For a slow speed, 2-stroke diesel engine, the forces and moments may be divided in the following categories: • Free moments and forces due to rotating and oscillating parts • Guide force moments due to reaction forces in the engines crosshead and bearings. The engine forces were applied as couples to the engine structure as shown on fig. 5-3. The pressure field from the propellers was applied as shown on fig. 5-4.
Figure 5-1 Finite element model
Figure 5-2 Model of sphere, skirt and tank
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The different excitation sources, which have been analyzed, are the following: • 2ndv = 2nd order free moment, vertical • 4thv = 4th order free moment, vertical • 7thH = 7th order H-moment • 4thX = 4th order X-moment • 4thPr = 4th order propeller excitation In addition the response was calculated for the 1st order free moments in vertical and horizontal direction. The vibration levels were however found to be below 0.1 mm/s, and are not presented. The inphase and antiphase modes of the two diesel engines have been investigated. For the present study, phasing of the engines and/or propellers has not been found necessary for obtaining acceptable response. However devices are available for keeping the phase constant between the two propulsion units if found favorable for other projects. Further, influence by adopting active as well as inactive top stays have also been evaluated. The response has been investigated for the following location and type of structure: • Natural frequency of the hull girder and girder system inside • Lower tank skirt connection to the foundation deck • Cargo tank shell structure • Pipe tower tank structure • Tank cover structure • Top of deckhouse
Figure 5-4 Propeller forces acting on the hull
Figure 5-3 Forces applied to engine structure
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The vibration levels for the various excitation sources have been compared with the response caused by a normal “good” propeller design. It is recognized that the response representing the diesel engines are of the same magnitude or less then the response created by propellers. It is also acknowledged through long service experience that Moss LNG carriers equipped with proven and well adapted propeller designs do not suffer any harm from vibrations. The overall maximum vibration levels as calculated are presented in figure 5-5 and 5-6 for loaded and ballasted condition respectively. The highest vibration levels are caused by local resonance of the structure and may easy been avoided by minor counter measures if found necessary.
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6 MOSS RELIQUEFACTION SYSTEM
Reliquefaction systems have been proposed and considered at regular intervals ever since the building of LNG carriers started in earnest around 1970. The systems proposed in the past were rejected due to high cost, complexity and large space requirement. However, during the last 30 years, the efficiency of diesel-based propulsion plants has increased dramatically compared with the steam turbine plants used on LNG carriers. It is now more than 10 years since Moss Maritime decided to develop a reliquefaction system of their own that they could offer to their licensing yards. The Moss RS concept was first presented in 1995. Since then, further development work has been carried out and the system has been refined and simplified. Present status Recently, the Qatargas placed an order for eight large LNG carriers at Daewoo (4 carriers), Hyundai (2 carriers) and Samsung (2 carriers). The order is part of the Qatargas II project. This is the first order ever of a LNG carrier equipped with a reliquefaction system and slow speed diesel engines. It is fair to say that this order inaugurates a new era for the LNG carrier design. All eight carriers will be equipped with the Moss Reliquefaction System (Moss RS). Although the reliquefaction system has already been described in several media, we feel that the new order mentioned above justifies a brief presentation of the system here. Reliquefaction concept The Moss Reliquefaction System is based on a closed nitrogen expansion cycle extracting heat from the boil- off gas (BOG). Several novel features such as partial liquefaction, separation of non-condensables and N2- compressor/expander unit mounted on a common gear have resulted in a compact system with reasonable power consumption. The diagram in Figure 6-1 shows the equipment located in the cargo machinery deckhouse. The reliquefaction plant is configured with 2x100% Moss RS one cryogenic heat exchanger (cold box). The 2x100% high duty (HD) compressors are connected to a combined HD heater and vaporizer. A BOG heater, required in case of discharge directly from the low duty compressor to the thermal oxidizer, is not shown in the diagram. The high duty (HD) compressors and combined HD heater/vaporizer will be used for the same operations as in conventional LNG carriers. The combined HD heater and vaporizer are described in more detail in a separate section.
Figure 6-1 Diagram of the cargo system inside the machinery deckhouse
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Figure 6-2 shows a model of a cargo machinery deckhouse with equipment as outlined in Figure 6.1. The nitrogen compressors/expanders and cold box are located on the first floor while the BOG compressors, high duty compressors and the combined high duty heater and vaporizer are located on the second floor. The nitrogen reservoir is located on the roof of the deckhouse. The equipment is fitted in a deckhouse with the same “footprint” as a standard 135.000 m3 Moss LNG carrier.
Figure 6-2 Arrangement of the reliquefaction plant inside the machinery deckhouse Boil-off gas cycle BOG is removed from the cargo tanks by means of conventional LD compressors. The LD compressor discharge pressure is 4.5 bar and the BOG is cooled and condensed to LNG at this elevated pressure in a 3- stream plate-fin cryogenic heat exchanger (cold box). Non-condensibles, mainly nitrogen, are removed from the separator vessel maintained at 4.5 bar and transferred to a thermal oxidizer or other acceptable means of disposal. From the separator, the LNG is forced or pumped back to the cargo tanks. Nitrogen cycle The cryogenic temperature inside the cold box is produced by means of a nitrogen compression-expansion cycle. Nitrogen at 14 bar with ambient temperature is compressed in a 3-stage centrifugal compressor. Exit pressure from the third stage is 57 bar. Three seawater coolers handle inter-cooling and after-cooling. From here, the gas is pre-cooled by passing through the warm part of the cold box. It is then cooled by use of the expander reducing the pressure to 14.5 bar to cryogenic temperature -163ºC. The cooled nitrogen is then led to the cold part of the cold box where it cools and condenses the BOG. After having processed the BOG, the nitrogen returns to the warm part for pre-cooling before it is returned to the nitrogen compressor. This completes the cycle.
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Technical merits: • The system only uses proven components with extensive references from air-separation, peak-shaving
plants world-wide. • The onshore LNG production plant at Snurrevarden, Norway, started production at the beginning of 2003.
This plant operates with a nitrogen/expander unit identical to the one that will be installed on LNG carriers.
• The total quantity of Btu’s loaded can now be delivered to the customer. • The LNG nitrogen content is reduced during the voyage, thus facilitating tank pressure control and
reducing power consumption for the reliquefaction plant. • The system is prefabricated in skid modules for installation and hook-up on board. • No increase in cargo machinery space is necessary. • The system has automatic capacity control. • The system can be stopped when the cargo pumps are in operation. This will reduce the need for extra
generator capacity. • Several alternatives of the system can be offered, all of which fulfil the rule requirement for redundancy. • No extra personnel are required for operation and maintenance. Economical merits: • Increased cargo quantity delivered. • Reduced heel required on ballast voyage • Large savings in total fuel consumption • Improved propulsion redundancy • More qualified crew available for handling of the propulsion plant • More flexibility in contract negotiations
Redundancy Several alternatives of the reliquefaction system can be offered, all in compliance with the IGC requirements for redundancy. The alternatives are: • 2x100% Moss RS with one cold box • 3x50% RS plant with two cold boxes Another alternative to reliquefaction is using a thermal oxidizer to burn the boil-off gas. Combining reliquefaction and gas burning gives other alternatives like: • 1x100% Moss RS and one 100% thermal oxidizer • 2x50% Moss RS plant with one 100% thermal oxidizer • 2x100% Moss RS with one or two cold boxes, in combination with a thermal oxidizer The eight LNG carriers mentioned above for the Qatargas II project are all specified with 2x100% Moss RS with one cold box and one 100% thermal oxidizer. The equipment specified gives a redundancy above the rule requirement. Operation and control The reliquefaction unit is equipped for automatic capacity control with a theoretic turn down to zero capacity in idling mode. The LD compressor capacity is adjusted automatically in accordance with the BOR, i.e. pressure control, or it can be operated at a fixed capacity. Increasing or decreasing the pressure in the nitrogen cycle controls the plant. Sensors and transmitters provide the required input signals to a programmable logic controller (part of the main vessel control system). Conditioned output signals are led to control valves and inlet guide vane actuators. Start/stop is manual, but with automatic turndown. Start-up time from warm condition is 6 hours after pre-cooling with BOG circulation. Start-up from cold condition will vary from one to six hours. Dynamic simulation At the beginning of 2004, a comprehensive new dynamic model was developed in co-operation with Kongsberg Maritime and Hamworthy Gas Systems for further verification and simulation of the reliquefaction plan behavior. The main purpose is to confirm the stability and convergence of the proposed control system and to check system response to transients such as load changes, warm start-up, cold start and emergency shut down. The model will also function as a training platform for operations of the system. Figure 6-3 shows the operator interface of the model.
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Size range The guaranteed boil off rate (BOR) is traditionally calculated based on pure methane and maximum filling limit of the tank. This is also taken as design capacity for the reliquefaction plant. Before the design capacity is fixed, an evaluation of the heat ingress from the piping between tanks and reliquefaction plant and BOR when sailing in rough seas, should be conducted. The presented reliquefaction system is easy to adapt to new capacities and will therefore fit LNG carriers of any size. As per today, the capacity of the Snurrevarden project in Norway is 2500 kg/h and will for the Qatargas II project be about 5600 kg/h.
7 IMPROVED OPERATIONAL PROCEDURES
7.1 COOL DOWN OF CARGO TANK
Background All LNG carriers with Moss containment systems must follow a procedure for tank cool down and subsequent loading. The present restrictions, forming the procedure for tank cool down (from warm condition), are as follows: • Progressive cool down of tank by means of LNG spraying before loading starts • When the average sensor temperature has reached -110°C, loading is permitted • The time to reach 50% tank filling shall not be less than 6 hours • Loading can then proceed at a constant rate which corresponds to full tank after 12 hours It has been recognised that if the requirement of –110°C at the sensors can be increased, operational flexibility will improve. The advantages of such a modification are: • Reduced requirement for the heel • Reduced spraying during the ballast leg and in the best case no spraying, subject to the length of ballast
leg as well as ambient temperatures • Reduced cool down time when commissioning the carrier • Earlier loading start
Figure 6-3 Interface panel of the dynamic model
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A study has therefore been conducted to investigate if it would be feasible to start loading cargo tanks earlier than accepted in previous procedures. Basics of the study The operational restrictions are related mainly to stress levels appearing in the equatorial groove through transient thermal loads in addition to gravity loads during the cool down and loading phase. The thermal analysis has been based on the assumption that the loading of LNG can start when the average temperature for the sensors in the equator area is –80 °C, which is 30°C higher than currently accepted. There are four sensors located on the outside of the tank skirt. They are equally distributed over the circumference and located 200 mm below the equatorial plane. The analysis was also based on the following operational cool down procedure: Spraying 0-6 hours 40 m3/hour Spraying 6-12 hours 20 m3/hours Loading 6-12 hours up to 50% filling Loading 12-18 hours up to 100% filling The study was performed on the basis of a four-tank LNG carrier with a cargo capacity of 137 000 m3 and a tank diameter of 40.44 m. The tank material is aluminium and the supporting skirt is constructed partly of aluminium, partly of stainless steel and high tensile mild steel. The cryogenic insulation material is mostly polystyrene. The transient temperature and stress distributions in the tank structure, including the equatorial profile, have been calculated by means of ANSYS Professional Software package. The ANSYS program is a 3-dimensional element program suitable for solving non-linear multi-physic problems like complex transient thermal and mechanical behaviour of all types of structures. The tank geometry is axis symmetrical about the vertical tank centre. All other properties like material properties, constraints as well as the major part of the actual loads are symmetrical around the same axis. Therefore, it has derived advantages from applying a fully axis- symmetrical 2-dimensional model. Such analyses will be as accurate as those created from an equivalent 3- dimensional model. Material properties as function of temperature are used. The instant values during the transient temperature progress are accounted for. The structural modelling is defined and a calculating mesh is generated in sufficient detail to allow the program to model the physics of the system. Figure 7-1 shows a slice model picture of one-half of the tank, including the skirt and the mesh density used in the groove area of the equator profile.
Figure 7-1 Geometry of the tank / skirt model in ANSYS
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Stress basics During cool down, the skirt side of the equatorial profile groove experiences tensile stresses, whereas during service with full load, compressive stresses arise. The full-load service condition includes static as well as dynamic loads on probability level Q=10-8 (service life 20 years). Consequently, the groove area is considered to be the most important field to be investigated in order to verify the extreme stress range. The criterion for permissible stresses during cool down is a “stress range” criterion. It means that the total stress variation experienced at any point in the equator groove, during the lifetime, has to be within a specified range: σ = /σe /service + /σe /cool down ≤ cF where: σ is variation in equivalent stress according to von Mises yield criterion. However, since this stress is always
a positive scalar, it has to be assigned a positive or negative sign corresponding to the dominant component of the bi-axial surface stress in the groove.
F is design equivalent stress as defined in the DNV rules and in the IGC Code. c is a value which is a subject for discussion. However, in the present study c=2.8 is used. The stresses are calculated in terms of von Mises equivalent stress criteria versus time after start of cool-down and up to 28 hours where the tank is fully loaded and the temperature in the construction is considered to be close to “steady state” condition. In order to define the correct sign of the von Mises values, the meridional stresses, considered to be the dominant stress component, are also calculated. The value of this component will disclose whether tensile or compressive stresses is dominant. The maximum service stresses appearing during the whole service life of the vessel are also calculated for the same locations as in the cooling down analysis. The same ANSYS element model is used. The stresses recorded after 27 hours are representative. At this time, the temperature in the structure will be close to balance with only minor adjustments. Further, the loads at the same moment of time include appropriate accelerations as well as internal overpressure. The model was not able to absorb interaction forces from the hull. For this reason, an allowance of 15% was added to the calculated stress values to verify the correct stress range. This allowance is based on experience from previous studies. Temperature gradients The temperature gradients are illustrated in Figures 7-2 to 7-4 for several locations. The cool down process starts at +25°C and has implemented the restrictions for cool down as outlined above. The loading however, starts when the temperature in the equator profile has reached –80°C, about 5.5 hours after start. The sudden change in cool down rate occurring after 11 hours is due to the liquid reaching the level of the equator profile.
Figure 7-2 Transient temperature profile at location of the sensors
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Figure 7-3 Transient temperature at tank side of groove
Figure 7-4 Transient temperature at skirt side of groove Illustrations defining typical temperature distribution in the skirt and shell are shown Figures 7-5 and 7-6.
Figure 7-5 Temperature distribution at part of tank shell/skirt 100% loaded
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Figure 7-6 Temperature distribution at skirt 100% loaded Stress results Figure 7-7 illustrates the maximum von Mises stresses appearing in the whole groove area during the complete process, regardless of the specific location. The stresses are recorded in MPa units. Part of the curve, representing the time after 24 hours, records the maximum service condition where the temperature gradient is considered to be more or less levelled out, and where the maximum dynamic accelerations are included.
Figure 7-7 Maximum appearing von Mise stress in groove area
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Figures 7-8 and 7-9 illustrate stress plots of von Mises stresses at various progress phases. The stresses are recorded in Pa units.
Fig. 7-8 von Mise stresses after 10 hours
Fig. 7-9 von Mise stresses after 20 hours
Ta n
k si
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Conclusion The conclusion is that the stress range margin is significant, indicating that the present minimum temperature requirement of –110°C in the equator profile can be increased. The study paves the way for further work in order to predict temperature limits and quantify the impact on operations and costs for a full round trip. Application of the results on existing LNG carriers The results from this study will in principle be applicable for existing Moss LNG carriers. However, a separate study is required for each carrier in order to make sure that the carrier can increase the loading temperature as outlined in the present study. In addition, adequate documentation must be prepared for verification and approval by the classification society.
7.2 COMBINED HIGH DUTY HEAT EXCHANGER
Conventional LNG carriers are equipped with separate heat exchangers for high duty (HD) LNG vaporising and gas heating. A simplified flow diagram for the heat exchanger system is shown in figure 7-10. The high duty heater is operated as part of the decommissioning and provides the gas heating for the following operations: 1. Boil down of LNG residual 2. Warm up of the cargo tanks The LNG vaporizer is operated as part of the commissioning and general cargo handling. The vaporizer provides the heating requirements for the following cargo operations. 1. Gassing up the cargo tanks following inert gas purging 2. Replacement gas while discharging cargo at full rate in the absence of vapor return from shore 3. Pressurizing the cargo tanks for emergency discharge, if appropriate. The above listing shows that the heat exchangers are dedicated to different operation procedures. Onboard an LNG carrier, the nature of these procedures excludes simultaneous operation of the heat exchangers. This allows the installation of one combined heat exchanger that handles both vaporizing and superheating of LNG.
Figure 7-10
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A simplified flow diagram for the combined heat exchanger is shown in Figure 7-11. Moss Maritime has developed and patented such a combined heat exchanger of shell and tube type, with separate u-bundles for vaporizing and superheating. The head of the heat exchanger is fitted with a combined outlet chamber and two separate chambers for LNG and vapor at the inlet side. The gas temperature at the exit of the heat exchanger is kept constant at all time by means of bypass flow control. Steam is used as a heating medium and is applied on the shell side. The steam supply valve is left fully open and is not controlled during operation. The combined heat exchanger can be adapted for any LNG carrier size or design data specification. A sketch of the heat exchanger is shown in Figure 7-12.
Application of the combined heat exchanger results in a simpler overall installation. One of the challenges in the present Moss carrier design has been the cargo machinery room piping arrangement. This is because the large diameter piping must be arranged to connect several equipment units, such as compressors and heat exchangers, in a very concentrated area. During ship operation, there are many operational modes. This results in different patterns of thermal loading. Thermal gradients up to 180°C must be accommodated without excessive pipe stress and loads on equipment nozzles. In addition, the piping must be arranged so that severe vibrations are avoided at all times. Consequently, many of the existing carriers have cargo machinery rooms with an abundance of bellows compensators, spring supports and vibration dampers. This limits access to valves and instrumentation and installation and maintenance work is difficult and costly. As a result of the combined heat exchanger installation, there are fewer components, and the challenges outlined above are reduced. The total piping length will be reduced as well as the number of bellows compensators and spring supports. Vibration dampers can be omitted. Insulation requirements will also be reduced, and for the steam side, the number of valves and steam tramps will be halved. The cargo machinery
Figure 7-12 Combined High Duty Heat Exchanger
Figure 7-11
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room will appear more “organized” with improved access to valves and instrumentation. The installation and maintenance costs will be lowered due to the combined heat exchanger. A budgetary cost comparison between the combined high duty heat exchanger arrangement and the conventional heat exchanger arrangement included costs for relevant hardware and installation. The comparison is based on a 145k m3 LNG carrier. The net savings in connection with applying the combined heat exchanger solution are USD 155 000 per vessel.
7.3 SIMPLIFIED BOG SYSTEM
The boil-off gas (BOG) system on conventional LNG carriers consists of BOG compressor, forcing vaporizer, gas heater and mist separator. If the steam boilers are operating in the 100% gas mode, the forcing vaporizer is used to supplement the natural boil off. Since the BOG compressor is located downstream of the forcing vaporizer, it must be sized to handle a gas quantity equal to the full boiler load. This means about 7800 kg/h for a 135,000 m3 LNG Carrier. However, with zero forcing, which is the most frequently used operating mode, the compressor will handle only about 3200 kg/h. In other words, the BOG compressor runs more or less permanently with a turn down of more than 60%. This is not an ideal arrangement and has led to control problems and operation with permanently open anti-surge valve. Figure 7-13 shows a simplified flow diagram for the conventional BOG system.
As for the high duty heater case, Moss Maritime has patented a combined forcing vaporizer and gas heater. In this unit, LNG for additional BOG production (forcing) is vaporized and mixed with the BOG from the cargo tanks to produce a common superheated vapor. The unit will condition LNG or BOG alone or in any combination up to the design capacity.
Figure 7-13
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The combined heat exchanger is of shell and tube type, with separate u-bundles for vaporizing and BOG heating. The head of the heat exchanger is fitted with a combined outlet chamber and two separate chambers for LNG and vapor at the inlet side. The gas temperature at the exit of the heat exchanger is kept constant at all times by means of bypass flow control. Steam is used as a heating medium and is applied on the shell side. The steam supply valve is left fully open and is not controlled during operation. The combined heat exchanger can be adapted for any LNG carrier size or design data specification. Figure 7-14 shows a sketch of the heat exchanger.
Figure 7-15 shows a new arrangement of the BOG system.
Figure 7-14 Combined Low Duty Heat Exchanger
Figure 7-15
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The arrangement is based on use of the patented combined vaporizer and heater in combination with the BOG compressor located upstream of the heat exchanger. The BOG compressor handles the boil off only, unlike the conventional system where the BOG compressor handles both BOG and forcing vapor. This arrangement means that the BOG compressor can be sized for the maximum boil-off rate instead of for combined duty. For a 135 000 m3 LNG carrier, this may for example be 4 000 kg/h. The normal BOG quantity of 3 200 kg/h will then be well within the operating envelope with a discharge pressure of up to 1.9 bara. The advantages with this arrangement are considerable and for some cases identical to those described above for the combined high duty heat exchanger. • Smaller BOG compressor and electric motor • Split range control by speed and inlet guide vanes are not required • Discharge pressure will be high enough for easy downstream throttling control • The mist separator used today can be omitted • Simpler piping arrangements with fewer supports, insulation and bellows compensators • Reduced investment cost and improved operational reliability
A budgetary cost comparison between the combined BOG/forcing heat exchanger arrangement and the conventional heat exchanger arrangement including cost for relevant hardware and installation has been conducted. The comparison is based on a 145k m3 LNG carrier. The net savings by applying the combined heat exchanger solution are USD 450 000 per vessel.
8 TRANSPORTATION COST ASSESSMENT
A simplified cost assessment of some of the new developments has been done. The comparison has been made with the basis of the same cargo tank volume regardless of the propulsion system. There has been argued that some of the alternative propulsion systems required reduced engine room length and thus allows for larger carrying capacity within the same main parameters. However, Moss together with others believes that this is a very theoretical approach as a hull can be easily modified to a marginally cost to obtain the same carrying capacity. Fuel cost: The unit cost of fuel, whether it is gas or oil, is not constant. The heavy fuel oil cost fluctuates and is as of 5th January 2005 in the range of 140-230 USD/ton. The cost of Marine Diesel oil is in the range of 330- 470 USD/ton. The cost of fuel differs from port to port. In this cost assessment the heavy fuel cost is set to 150 USD/ton. The value to be set for BOG used in the propulsion plant is even more debated. At the loading port LNG value is defined by the FOB price. However, the LNG value at the receiving terminal is set by the CIF cost. Which value shall be used for the BOG? It is our belief that all LNG loaded on a carrier and not used for propulsion can be sold to CIF cost, and that is what should be used to define the BOG value. The results from the cost comparison are given in figure 8-1.
Mo s s 137k S team
BOG+HFO Mo s s 147 Steam
100% BOG
BOG+HFO Mo s s 147 Twin Dies e l
Mo s s 235K Twin Dies e l
-40.0 %
-35.0 %
-30.0 %
-25.0 %
-20.0 %
-15.0 %
-10.0 %
-5.0 %
0.0 %
% c
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Comparing a 147k m LNG carrier with steam propulsion with an equally sized carrier with twin screw twin slow speed diesel propulsion and a reliquefaction plant installed, a significant saving can be achieved. The savings are in the range of 7%. For an annual transportation of 1 million tons of LNG, cost reduction is in the range of 3 million USD. If a higher service speed of 21 knots can be accommodated in your trade, an additional 3% cost saving is possible. At the last LNG14 conference in 2004 Mr. Ohira of Mitsubishi Heavy Industries presented a graph saying that for LNG/HFO price ratios of 0.5 and above a gas fired propulsion plant is less economical than a for a slow speed diesel plant with reliquefaction. Mr. Ohira's graph is presented below.
9 CONCLUSIONS
Research and development are mainly done to improve safety and availability or to reduce the costs. In the LNG industry safety comes first. Any new development must not compromise safety or reliability. In this paper, it is described some of the research and development work done for the Moss type LNG carrier in the recent years. Development of larger diameter of the spherical tanks has shown that the Moss type LNG carrier can be designed for capacities of 200k-250k m3. Even larger carriers can be designed, as the maximum diameter of the sphere has not been reached. By carefully developing the hull lines, significant reduction in required propulsion power can be obtained, as the model tests of the Moss 147k m3 LNG carrier with twin skeg - twin propeller has shown. Alternative propulsion systems such as diesel electrical plants or slow speed diesels combined with a reliquefaction system can significantly reduce the transportation cost of a LNG carrier. Such systems have already been ordered and we will most probably see more carriers being ordered with other propulsion systems than the traditional steam plant. Introduction of slow speed diesel engines for propulsion means introduction of additional excitation sources acting on a wide range of frequencies. In order to disclose unforeseen problems, a detailed investigation of vibration response of cargo tank structure as well as the global hull structure has been conducted. The analysis shows that the response representing the diesel engines are of the same magnitude or less than the response created by the propellers. It is also acknowledged through long service experience that Moss type LNG carriers equipped with proven and well adapted propeller designs do not suffer any harm from vibrations. Recently an order for eight large LNG carriers was placed at three yards in South Korea as a part of the Qatargas II project. This is the first order ever of a LNG carrier equipped with reliquefaction system. In view of that, a brief presentation of the Moss Reliquefaction system is given. It has been recognised that increased operational flexibility will be achieved if the restriction concerning the temperature in the equator profile can be relaxed. The study performed has shown positive results. The
Figure 8-2 Fuel cost graph from Mitsubishi H. I.
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present minimum temperature requirement of –110°C in the equator profile can be increased, improving the operational flexibility of the containment system further. Presentation of new arrangements for BOG handling and high duty heater and vaporiser are given. By applying combined type heat exchanger, improvements are achieved in way of layout, operation and cost. Overall transportation cost assessments indicates that a LNG carrier equipped with slow speed diesel engines and reliquefaction system has a potential for significantly reducing the transportation costs over a wide range of LNG/HFO price ratios.
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