Challenges of Mitsubishi Membrane LNG Carrier

Kazuaki YuasaMitsubishi Heavy Industries, Ltd.

Japan

Co-author:KATSUYA UWATOKO, Naval Architect

KIYOKAZU KAWABATA, Engineering ManagerHIROSHI SHIRAKIHARA, Engineering Manager

MASARU KODAMA, Senior ResearcherSEIICHI HAMA, System Engineer

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1. Introduction Mitsubishi Heavy Industries, Ltd., (MHI) is a pioneer in liquefied gas carriers, having built the world sfirst large sized refrigerated LPG carrier, the Bridgestone- maru , in 1962. Nowadays, MHI has the topmarket share in the construction of both large sized LPG carriers and LNG carriers. Since 1983, he has construct ed twenty-one (21) Moss spherical tank LNG carriers and is recognized ashaving the most advanced Moss shipyard. He introduced the so-called second generation LNG carrierconcept characterized by a lower boil-off rate (BOR) with a forcing vaporizer system, applying it in theAustralian North West Shelf project in 1989. This concept has since become the world standard becauseof its economical merits and operational flexibility. MHI has also been studying Gaz Transport membrane tank LNG Carriers in details since a licenseagreement in 1973 in order to realize more advanced membrane ships. A chronological list of researchand development by MHI is shown in Fig. 1-1. In 1999 MHI was awarded the contract for the membraneLNG carriers of a Malaysian project as the lead yard in cooperation with Mitsui Engineering andShipbuilding Co., Ltd., (MES). These membrane ships are now under constructions at the shipyards of thetwo companies. Although new generation GTT membrane ships have been constructed since 1994 by various shipyards,we have introduced cutting-edge technology to achieve the further advances GTT membrane ships.Therefore, this paper focuses on the challenges faced in building Mitsubishi GTT membrane LNGcarriers, issues such as the incorporation of the latest hydrodynamic technology in the design of anoptimal propeller and hull form with simple shaped cargo tanks, a system of detailed structuralassessment for safe and reliable construction, production technology advances involving computerizedlogistics and quality control, an interference checking system and a newly developed automatic TIGwelder. The latest delivery and order list of Mitsubishi LNG carriers is shown in Table 1-1.

2. Hull Form Development by CFD Computational Fluid Dynamics (CFD) has developed dramatically during the past two decades in allengineering fields where fluid flow phenomena takes place. CFD is used not only for research purposesbut also for the design of hull forms and propellers in the field of ship hydrodynamics. The reliability ofour CFD methodology has been verified through comparison with the accumulated model ship data.

2.1 Hull Form Design A finite volume method as the Navier-Stokes solver has been developed to evaluate free surface viscousflow around the hull. This CFD code can predict not only the flow field but also wave patterns (Fig. 2-1),resistance, pressure distribution on hull surface (Fig. 2-2) and self-propulsion factors. These calculationsresult in important information for improvement of ship s hull form. The propulsive performance can bejudged by the correlation between CFD results and model ship data. The use of this CFD methodologyhas enabled the development of superior hull form and better prediction of propulsion performance. In thepast, a huge number of ship model tests had to be carried out in order to develop optimum hull form, butsuch labor intensive work can be eliminated through the introduction of advanced CFD technique. For Mitsubishi GTT membrane ships, an optimal hull form has been developed to accommodate asimpler cargo tank configuration.

2.2 Propeller Design The surface panel method, based on potential theory, has been developed to analyze the hydrodynamiccharacteristics of propellers operating in uniform and non-uniform flows. This CFD method is applicableto the simulation of the flow field around a rotating propeller, the pressure distribution on the blade andhub surface, the phenomenon of unsteady cavitation, and the vibration force induced by the propeller.

3. Structural Verification We have completed the hull structural design of Mitsubishi GTT membrane ships, and also finalized

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detail design of insulation system based on GTT s key plan. Throughout these design processes, extensivestructural analyses using the latest technologies have been performed for each structural member of hulland insulation system in order to ensure the highest structural reliability.

3.1. Stress and Fatigue Sufficient strength of the inner hull and insulation structure is particularly important for membrane LNGcarriers, and must be maintained throughout a vessel s service life. The basic strength of the inner hull has been verified in the global stress analysis by a Finite Element(F.E.) method for entire hull model that complies with LR SDA (SDA : Structural Design Assessment)procedure. Fig. 3-1 shows an example of stress distribution with exaggeratedly deformed shape under atypical loading condition, in which stresses and buckling strength are confirmed to be within the criteria.Such special areas as the connection between the trunk top and the accommodation have also beenanalyzed. The fatigue strength of the inner hull and contiguous structure, which is one of the most importantfeatures of the membrane LNG carrier, has been verified by hot spot stress assessment using a fine-meshF.E. analysis, as shown in Fig. 3-2, that conforms to LR FDA (FDA : Fatigue Design Assessment)notation. The fatigue strength of square corners as stress concentration parts has been verified, as well asthat of the hopper. In addition, Discrete Analysis Method (DISAM), which has been developed by MHI,was carried out using the same F.E. model. DISAM performs simulation of direct wave pressure, andlong-term prediction of stresses on the specific trading route of a subject vessel, taking the complexeffects of several dynamic load components into account. The fatigue strength of the structure has beenconfirmed to be sufficient by DISAM, too. Each structure of the insulation system has also been verified from the strength point of view. Since it isthe first membrane ship by MHI, careful strength assessments have been conducted for each insulationelement, such as the coupler system to secure insulation boxes and the welding joint of the INVARmembrane. These assessments have verified the sufficiency of strength against maximum load during thevessel s life, as well as fatigue. As for the junction of INVAR membrane and the hull structure, detailedF.E. analysis has been carried out to ensure sufficient fatigue life of the INVAR strake joint at thejunction.

3.2. Vibration Special care must be taken to prevent excessive vibration of the inner hull structure and to ensure thesafety of the insulation system. It is, therefore, important to predict at the early design stage the actualvibration behavior for this purpose and achieving a low vibration vessel. Fig. 3-3 shows an example offree vibration mode in the whole ship F.E. vibration analysis, which was used to estimate the naturalfrequency and vibratory response at sea. Vibration of the hull girder, superstructure and double hullstructure including inner hull have been confirmed the relevant vibration characteristics through thisanalysis.

3.3. Sloshing GTT provides guidelines o n the shape and dimension of cargo tank s and the filling limit of cargo againstsloshing. Sloshing analysis to simulate the actual liquid motion at sea, as shown in Fig. 3-4 has confirmedthe reliability of the insulation system. The strength of the inner hull steel structure against liquid motionhas also been evaluated and confirmed.

4. New Production TechnologyNew production technologies have been introduced to achieve better quality and productivity of cargocontainment system. The following are some examples.

4.1. Computerized network control system of logistics and quality control (LOGIQ) for the cargocontainment system

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The GTT membrane cargo containment system consists of insulation boxes, membrane INVAR sheets,INVAR components, resin ropes, coupler base sockets, fixing bolts, etc. This means that GTT systemrequires many kinds of components and a huge number of elements, namely more than 700,000. Therefore, the precise and exact control for construction of the cargo containment system is a keytechnology to monitor and control the enormous number of work processes. Each component must bechecked for position and accuracy of installation. A system called LOGIQ was developed for logistics and quality control in order to meet this need. 4.1.1 Logistics

The key function of the control system for construction process is to deliver the components to theappropriate place and at the appropriate time, in accordance with the planned construction schedule.

An outline of the system and the process flow of product logistics is illustrated in Fig. 4-1. Rawmaterials and components are shipped to the shipyard warehouse, and then moved to storehouse at quayside and handled in the cargo tank for installation at the appropriate time. During these processes, allmaterial and components are constantly monitored, with status acknowledgement and orders given forthe next step of the work process and control checks made against the planned construction scheduledisplayed on VDUs. This system also has an automatic construction plan rescheduling function that takesinto account all changes of surrounding conditions.

4.1.2 Quality control The key function of the quality control system is to maintain and evaluate recorded quality data,including technical inspection results, from the production of a component to the completion of itsinstallation. LOGIQ provides a system that is capable of storing and monitoring data for materials,products and installation inspection results. A bar code system is used to identify components. Example ofquality control and assurance using the LOGIQ system is shown in Fig. 4-2 for secondary insulationboxes. This system also provides the repair history of each component in order to support future maintenancework.

4.2 PRO-Engineering for advanced systematic product design of insulation components GTT membrane tanks require many kinds and quantities of components. Based on GTT s basic technicaldocuments, the shipyard must produce the detailed drawings of all component to facilitate purchasing. Acomputer-based design system, namely PRO-Engineering, has been introduced to improve and ensureprecise component design. It is also capable of reducing workloads and shortening the design time. This system offers a user-friendly work procedure. Designers can start dimensional alteration work oneach component as well as assemble components based on a 3-dimensional view on displayed on VDU.When all the design information of a component has been defined, its 3D design is complete, as shown inFig. 4-6. The shearing of design work is possible, i.e., the work done on one VDU can be extracted andincorporated into other work done on another VDU. This system makes possible concurrent designactivities to reduce the design workload, and provides the following production drawings.a) Layout of all components for cargo tank insulation work, as shown in Fig. 4-3.b) Detailed component drawings including tag numbering and material, weight, etc. as shown in Fig. 4-4. The latter drawings are used for material purchasing.

4.3 Interference checking system for production facility and scaffolding In operat ing production facilities that include manipulator s and welding machine s, on scaf folding in acargo tank, it is essential to avoid the interference between the production facilities and scaffolding inproduction process. Therefore we have checked the interference by application of dynamic digital mock-up software. This systems allow predicted interference in a construction process to be displayed on aVDU before the actual work is started. It analyzes the interference or clearance between the facilities andthe scaffolding using 3D digital models. Once 3D model data has been inputted from the 3D CAD

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system, it is possible to simulate various situations in detail. Fig. 4-5 shows an example of interference checking. The system operator places each welding machineon INVAR sheets along each tongue and checks for scaffolding interference on the VDU. If interferenceis found, the system warns the operator. At the same time, the clearance and working space can beconfirmed visually. Fig. 4-6 shows an example of interference checking during operation of the manipulator near thetransverse bulkhead of the cargo tank. The system operator positions a manipulator near the inner hulland, visually checking on the VDU, makes sure it fits insulation boxes in the virtual.

5. Automatic TIG welding machine for end strake raised edge GTT membrane LNG carriers are characterized by their INV AR membranes, whose thicknes s is no morethan 3.0 mm. Highly accurate TIG welding has to be used to weld these thin INVAR sheets. AlthoughTIG welding has been mechanized for simple areas, manual welding by skilled welding operators is stillrequired for the more complicated area and thus still relies on highly skilled welders. Under suchcircumstances, MHI developed an automatic TIG welding machine for end strake raised edges, whosejoint configuration is shown in Fig. 5-1. This welded joint is not of a simple configuration. There are three different types of welded joints withina narrow welding length of about 600 mm : two overlaps (1.5/1.5mm), five overlaps(1.5/0.7/0.6/0.7/1.5mm) and three overlaps (0.7/0.5/0.7mm). Furthermore, the welding line is verycomplicated, as shown in Fig. 5-1. End strake raised edges occur all over the tank, so there is a wide rangeof welding positions -- flat, inclined, horizontal and overhead. The following key factors have to be taken into account for successful automated welding of such endstrake raised edges.a) Tracking of the welding lineb) Setting of the automatic welding conditions for each welded configurationsc) Lighter weight/compact design Fig. 5-2 shows the configuration of the automatic TIG welding machine developed by MHI. This welding machine is basically composed of four units, as follows.a) A guide rail equipped with a clamp mechanism capable of clamping firmly on the end strake edge.

The entire guide rail is consists of a honeycomb structure made of aluminum alloy to achieve acompact design and a weight of less than 7 kgf.

b) The carriageThere are three fixtures : a fixture that moves the welding torch so that it oscillates back and forthacross the welding line ; a fixture that guides the welding torch along the desired height and direction ;and a fixture that facilitates carriage travel. The weight of the carriage is about 3 kgf. In order to easehandling even in an overhead position, its size is small enough that it can be placed on the palm of ahand.

c) The controller mounted on the welding power supply unitIt controls the welding sequence, the automatic sensing conditions, the welding conditions, theautomatic setting parameters, etc.

d) The remote control keypadThe operator can manipulate the position of the welding torch or change the welding conditions byusing this remote control keypad, while at the same time observing the welding performance. Fig. 5-3shows the view of welding by this automatic TIG welding machine.

Fig. 5-4 shows the cross-sectional macrostructures of the welded joints, as viewed from this machine.This welding machine is applicable to the continuous weldings of three different welded joints throughthe selection of appropriate welding conditions. Furthermore, automatic learning of welding is possible. Good welding quality is maintained for all kinds of welding positions and joint configurations, as shownin Fig. 5-4.

6. Concluding Remarks

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Now MHI is building Gaz Transpor t membrane tank LNG carriers having a cargo capacity of 137,100m3 and incorporating the various challenges described in this paper. The hull construction assembly will be finished by the time this paper is presented and the insulationwork for cargo tanks will have started. In overcoming the challenges of this project, close cooperation among our staf f, the Owners, the licenserand the classification society has been essential. We would like to take this opportunity to thank MISC,Petronas Tankers, GTT and LRS. MHI is building its latest LNGCs with both major containment systems, namely, the Moss spherical tanktype and the GTT membrane tank type. Finally we would like to say that we look forward to continuing tosupply LNGCs that meet the needs of worldwide owners.

1970 1980 1990 2000

1. General Hull Form Wind Force / Maneuverability Machinery & Fittings2. Moss Spherical Tank Strength Welding Insulation Construction Life Time Maintenance3. GT Membrane Membrane Sloshing Insulation Construction Life Time Maintenance4. TGZ Membrane Membrane Sloshing Insulation Construction Life Time Maintenance

DRL,DFD

CRP S/T,RL

Investigation of Design(License)

Thermal brake New STJ

Mock-up model

Long panel

Indonesia I (4) Indonesia II (3) Australia (4) Indonesia IV (1)

(License)Investigation of Design (No.86) (No.96)

Kalingas project Malaysia III project

Mock-up modelMock-up model for

Qatar project Mock-up model

Brunei (2) Malaysia I (5) Malaysia II (5)

(License)Investigation of Design (MK-I) (MK-III)

1,100m3 ship Mock-up model for NWS project

Brunei (5)

Fig.1-1 Research and Development for LNG Carrier

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Name of Ship Project Owner or Operator Tank TypeCargo Capa.

(m3)Del.

Banshu MaruEchigo MaruDewa Maru

Indonesia IINYK LineNYK LineKawasaki Line

125,542125,568125,631

198319831984

Northwest SanderingNorthwest SwiftNorthwest SeaeagleNorthwest Stormpetrel

AustraliaNorth West Shelf

ALSOCNYK LineShellALSOC

127,362127,427127,452127,443

1989199019921994

EkaputraDwiputraLNG Vesta

Indonesia IIIIndonesiaIndonesia IV

Cometco ShippingPacific LNG TransportMitsui OSK Lines

137,012127,386127,386

199019941994

Ish Abu Dhabi Abu Dhabi National Oil Company 137,304 1995Al KhorAl WajbahDohaAl Jasra

Qatar

NYK LineMitsui OSK LinesNYK LineNYK Line

137,354137,309137,262137,227

1996199719992000

Golar Mazo Indonesia/Taiwan Faraway Maritime Shipping Company 136,867 2000(undecided) Oman Osaka Gas I. T. /NYK Line 137,000 2000(undecided) Enron/India Enron/Mitsui OSK Lines

MossSpherical

137,100 2001(undecided)(undecided)

Malaysia M.I.S.C.GT

Membrane137,100137,100

20022003

(undecided) Brunei Brunei Gas Carriers Sdn. Bhd 137,000 2002(undecided)(undecided)

ShellMoss

Spherical137,000137,000

20022003

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Table.1-1 Delivery and Order List of LNG Carriers

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Fig.2-1 Wave Pattern

Fig.2-2 Pressure Distribution on Hull Surface

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Fig.3-1 Global Stress Analysis

Fig.3-2 Detailed F.E. Models for Hull Fatigue Analysis

Fig.3-3 Whole Ship Vibration Analysis

Fig.3-4 Liquid Motion and Pressure in Sloshing Analysis

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Fig.4-1 Outline of LOGIQ System

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Fig.4-2 QC & QA Control of Insulation Boxes

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Fig.4-3 Layout of Components of Cargo Tank Insulation

Fig.4-4 Detailed Component Drawing

Scaffolding

Insulation BoxesInterference Check

Manipulator

Scaffolding

Interference Check

Insulation Boxes

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Fig.4-5 Interference Check between Scaffolding and Welding Machine

Fig.4-6 Interference Check between Scaffolding and Manipulator

Fig.5-1 Joint Configuration of End Strake Raised Edges

Fig.5-2 Configuration of Automatic TIG Welding Machine

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Fig.5-3 View of Welding by Automatic TIG Welding Machine

Fig.5-4 Cross-sectional Macrostructure

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