After nearly 20 years of accumulation, China Classification Society (CCS) has established the service capacity of liquefied gas carriers, and comprehensively formed the standard system of liquefied gas carriers, including the supporting software and service products. Up to now, there are 80 CCS-classed large liquefied gas carriers (orders inclusive), and the classed ship types cover large membrane-type LNG carriers (NO96 and MARK containment systems), large A-type independent tank LPG carriers (VLGC), B-type independent tank LNG carriers, large ethane carriers (VLEC), C-type independent tank LNG carriers, etc .

This paper will focus on the main technical points of LNG carriers, and the achievements of CCS in the key technologies of various types of LNG carriers (membrane, A-type, B-type and C-type) in recent years, such as hull structure, liquid cargo containment system, sloshing analysis, fatigue strength assessment, application of new materials, risk assessment and industry organization requirements .

Figure 1 Type of CCS-classed Liquefied Gas Carriers

Technical Key Points of Various LNG Carriers

1. Membrane-type LNG carrier

The membrane-type liquid cargo tank used in membrane-type LNG carriers is non-self-supported, which needs to be in closing contact with the hull and supported by the hull structure. Due to the unique flatness laying requirements of the containment system of the membrane tank, full protection to the cryogenic liquid cargo and deformation control of the containment system, the hull in the cargo tank area is constructed as a complete double-hull structure with double-layer bottom, double side, double-layer deck and double-layer transverse bulkhead. All adjacent supporting members (including longitudinal skeleton, stiffeners and main support structures) of the liquid cargo tank are all arranged at the double-layer bottom, double hull, convex deck and double-layer transverse bulkhead facing outside the liquid cargo tank.

Figure 2: Hull Structure of Cargo Tank Area of Membrane-type  Liquefied Gas Carrier (Left) and Membrane Containment System (Right)

Figure 3 Direct Calculation of Structure in Membrane-type Liquefied Gas Tank Area

Figure 4 Joint between Double-layer Transverse Bulkhead and Double-layer Bottom Longitudinal Truss

Figure 5 Joint between Vertical Truss and Deck Longitudinal Truss of Double-layer Transverse Bulkhead

Figure 6 Joint between Horizontal Truss and Transverse Bulkhead between Double Hulls

At present, the rules and technical standards of CCS’s membrane-type LNG carriers have fully covered: the general longitudinal  strength requirements,  standard  dimension requirements, direct calculation of cargo tank area structure, direct calculation of whole ship structure, fatigue strength evaluation, spectral analysis based fatigue strength evaluation, strength evaluation under sloshing load, pump tower vibration and temperature field analysis requirements. CCS has the service capability for single classing the membrane-type LNG carriers.

Deformation Control of Containment System

In addition to the typical key nodes of traditional liquid cargo carriers, such as the upper/lower folding angle of bottom side tank, the lower folding angle of top side tank, and the intersection of large elbow plate of double-hull horizontal longitudinal truss and bulkhead, membrane-type LNG carrier also includes the high stress parts such as the joint between the double-layer transverse bulkhead vertical truss and the double-layer bottom longitudinal truss, the joint between the double-layer transverse bulkhead vertical truss and the deck longitudinal truss, and the joint between the horizontal truss and transverse bulkhead between double hulls. These parts are similar to the T-shaped intersection joints. In order to keep the area facing the inner side of liquid cargo tank smooth, these T-shaped intersection joints must be strengthened by inserting local thickening plates to reduce the stress level and prolong the fatigue life. At the initial stage of design, it is necessary to design these T-shaped intersections reasonably to control the relative deformation.

Figure 7 Load Distribution of Sloshing Non-impact Motion and Sloshing Impact Motion

Sloshing Analysis of Containment System

For a long time, sloshing load prediction and structural response is one of the technical challenges in the design of LNG cargo tank containment system. CCS’s Guidelines for Evaluation of Sloshing Load and Component Dimension of Liquid Tank (referred to as the Sloshing Guidelines) classifies the sloshing motion into horizontal Class I, horizontal Class II and horizontal Class III according to the intensity from low to high based on the sloshing resonance of the ship and liquid cargo tank, among which horizontal Class III includes the impact motion, and corresponding loads are given respectively.

Figure 8 Structural Strength Evaluation of Pump Tower and Accessories

For small liquid tanks with high resonance and large liquid tanks with low resonance, the Sloshing Guidelines gives the hull structure check requirements under non-impact sloshing load by standard formulas. For large liquid tanks with high resonance, the Sloshing Guidelines stipulates that the load shall be calculated directly for sloshing impact load, and the results of sloshing model test can be used as sloshing design load after approval . The Sloshing Guidelines adopts the method of "equivalent static load", introduces dynamic amplification factor, transforms the sloshing impact load of liquid cargo tank into the equivalent static load directly acting on the hull structure, and properly considers the "buffer effect" of the containment system. For the loading and unloading equipment of the membrane LNG tank containment system - pump tower and its accessory structure, the Sloshing Guidelines clarifies the content and process of structural strength evaluation of pump tower, providing guidance for the structural design and plan approval of pump tower in the industry.

Fatigue Strength Assessment (Hull Structure + Cargo Containment System)

The LNG carriers generally have a service life of 40~45 years, and the fatigue strength assessment of key parts in cargo tank area is also the focus of attention in the design stage of membrane LNG carrier. CCS’s Guidelines for Fatigue Strength of Hull Structure (referred to as Fatigue Guidelines) specifies the fatigue strength assessment requirements for membrane LNG carrier and other parts based on equivalent design wave load system. Among them, the folding angle area under the bottom side tank and the joint between the double-layer bottom longitudinal truss and the double- layer transverse bulkhead are particularly important.

Figure 9-1 Upper and Lower Folding Angles of Inclined Plate in Bottom Side Tank Figure

9-2 Joint between Double-layer Bottom Longitudinal Truss and Double-layer Transverse Bulkhead

Figure 9-3 Joint between Double- hull Horizontal Truss and Transverse Bulkhead

Figure 9-4 Joint between Liquid/Air Chamber Opening and Convex Deck

Figure 10 Convex Deck Rear End Elbow Plate and Deckhouse Opening

Figure 11 Fatigue Strength Check Position of NO96 Membrane Containment System

Based on the principle of direct load prediction and fatigue spectrum analysis, CCS issued the Guidelines for Fatigue Strength Assessment of Hull Structures based on Spectral Analysis , and gave a "personalized" fatigue strength analysis solution suitable for membrane LNG carriers on specific routes. Besides the key nodes in the cargo tank area, the evaluation sites further include the toe end of the elbow plate at the rear end of the convex deck in the bow and tail areas of the ship, and the entrance and exit corners above the deckhouse side walls connected with the longitudinal walls participating in the total longitudinal strength.

In addition to the hull structure, the fatigue strength evaluation of cargo containment system shall be paid attention to in the design process of membrane LNG carrier. According to the Rules for Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (referred to as Bulk Liquid Rules), CCS has formulated the "Guidance Document for Plan Approval of NO96 Membrane- type Cargo Containment System". According to the Miner linear cumulative damage criterion and North Atlantic environmental conditions, this document stipulates that the stress distribution obeys the rules of Weibull distribution, with the excess probability of 10~8, and the fatigue life criterion of joints is 40 years. The loading conditions considered include full load, ballast, complete cooling (considering 20 years, once a year) and partial cooling. The loads include hull girder load and temperature load. The evaluation positions include: the joint between the invar anchoring flat steel and the top of transverse bulkhead; the joint between the invar pipe and the lower end of the transverse bulkhead; the joint between the second layer lap lath of invar steel and the lower end of the transverse bulkhead; the reinforced joint of liquid cargo tank at trihedron of inner bottom plate, transverse bulkhead and the inclined plate of bottom side tank. Different joint forms shall be evaluated by different SN curves.

2. A/B-type LNG carrier

The A-type independent liquid cargo tank used by A-type liquefied gas carrier is self-supported, with complete secondary panel. A-type liquefied gas carriers are divided into super-large fully cooled VLGC carriers suitable for loading liquefied petroleum gas (LPG) with a minimum temperature not lower than -55℃, and LNT A-BOX LNG carriers suitable for loading liquefied natural gas (LNG) with a design temperature of -163℃. B-type liquefied gas carrier is similar to A-type liquefied gas carrier. But B-type independent liquid cargo tank can adopt some secondary panels, which shall be checked by model test and fine calculation, such as crack propagation and leakage analysis of liquid cargo tank based on fracture mechanics. Some of its secondary panel shall meet the protection requirements of liquid cargo leakage at sea for at least 15 days, and shall still make the secondary panel fulfill its functional requirements at the static heeling angle of 30° .

In recent years, CCS has continuously carried out full- scale ship verification of A/B-type carriers, and formed a complete specification and technical standard system of A/ B-type independent tank liquefied gas carriers, including the key technologies such as specification check, direct calculation and fracture mechanics crack propagation of B-type tank.

Independent Liquid Cargo Tank Support Structure

Independent liquid cargo tanks do not constitute the hull structure, but they shall be supported by the hull. Various loads are transmitted between prism A/B-type liquid cargo tank and hull structure through several support structures. When the liquid cargo tank is subjected to static and dynamic loads, the supporting devices and structures shall be able to prevent the liquid cargo tank from moving, and shall not cause excessive stress on the hull and liquid cargo tank.

Figure 12 Cross Section of A/B-type Liquefied Gas Carrier and Finite Element Model of Tank Section

Figure 13 Design of Tank Saddle-hull Planar Laminated Wood Design of Tank Saddle-hull Curved Laminated Wood

The support structures transfer load in contact mode. When A/B-type liquefied gas carrier navigates in the complex and changeable wave environment, the load transfer between the support structures will become very complicated. The concrete manifestation is that one part of the support devices is contact, while the other is non-contact. In order to consider the influence of hull deformation on support in motion, it is necessary to obtain the support force distribution of hull, liquid cargo tank and support structure by continuous contact iteration calculation in direct calculation of tank section. Generally, the contact between support devices can be modeled by rod element simulation. How to simulate the contact accurately in the tank section and refinement model and how to analyze the structural strength of each support structure under different working conditions are the difficult points in the calculation of A/B-type liquefied gas carrier.

Analysis of Fatigue Crack Growth and Leakage

The B-type independent liquid cargo tank used by B-type liquefied gas carrier is self-supported with partial secondary panel. In order to reasonably reduce the size of the secondary panel, fracture mechanics analysis of fatigue crack growth shall be carried out to determine: the crack propagation path of liquid cargo tank structure, the crack growth rate, the time required for crack propagation to cause liquid cargo tank leakage, the crack size and shape

through thickness, and the time required for a detectable crack to reach a critical state. Therefore, crack propagation assessment and leakage analysis of fracture mechanics have been the core technology of B-type hold LNG carrier design.

In 2020, relying on the project of "Deepening Research on Liquefied Gas Carrier", CCS achieved this key technical breakthrough with unremitting efforts, filled the major technical gap, and developed the supporting fracture mechanics analysis software. Based on LBB (leakage before breaking) criterion, CCS’s Bulk Liquid Rules specify the technical requirements for crack propagation assessment analysis and leakage rate calculation of B-type hold (see "Fatigue Crack Growth and Leakage Analysis of B-type Hold" in the technical application part of this journal for details) by adopting BS7910 failure assessment diagram method and modifying the Paris crack propagation formula. Based on the above research results, CCS realized the single class of B-type cargo hold LNG carrier in 2022.

3. C-type LNG carrier

The C-type independent liquid cargo tanks used by C-type LNG carriers are usually cylindrical, which are designed based on the revised pressure vessel criteria, including crack propagation criteria, to ensure that the initial surface cracks will not propagate more than half of the hull thickness during the service life of the liquid cargo tanks.

There are generally two design methods for the contact between the supporting seats of C-type independent liquid cargo tank, that is, the contact between saddle and the hull. One is to weld the liquid cargo tank and the saddle into a whole, and set planar laminated wood between the saddle and the inner bottom of the hull to transmit the contact load. The other is to set curved laminated wood between the liquid cargo tank and the saddle to transmit contact load, and connect the lower end of the saddle with the hull. At present, the latter is adopted in most saddle design of C-type independent liquid cargo tank in China.

Figure 14 Strength Evaluation Model of Laminated Wood Structure Supporting the Tank

For C-type liquefied gas carrier, the strength evaluation of laminated wood structure supporting the tank is one of the key points in design. According to the layout and material characteristics of laminated wood, CCS developed a verification method of laminated wood based on nonlinear finite element method, which is more accurate than the previous empirical formulas, mainly including model range, boundary conditions, load conditions and allowable criteria.

The finite element analysis model takes a complete tank and its support structure as the goal, and the support of the hull structure to the tank as the boundary condition. But no consideration is given to the influence of the total longitudinal deformation of the hull on the tank. Besides, the plate and beam elements are adopted for the tank and its support structure, and the solid elements for the laminated wood.

Strength Evaluation of Y-joint in C-type Tank

Through long-term research, CCS has formulated the dynamic pressure calculation formula and structural strength evaluation method conforming to IGC rules and the fatigue strength evaluation method based on linear cumulative damage model for the key point "Y-joint" in bi-lobe tank design. The fatigue strength evaluation method of Y-joint of independent C-type tri-lobe tank can quickly screen out the positions less than the allowable damage value by screening the fatigue evaluation positions according to ASME VIII-2, and simplify the criterion requirements of allowable stress of liquid cargo tank in IGC. Thus, CCS developed a leading advanced technical standard system and implementation means in the field of C-type tri-lobe tank liquefied gas carrier type.

Common Technology of Various LNG Carriers

1. Application of new materials

Low temperature containment system of liquefied gas carrier is mainly made of the metal materials, including the low temperature carbon manganese steel, invar steel, stainless steel, 9% nickel steel, aluminum alloy, and high manganese steel. CCS gives full play to its technical advantages to promote the application of fracture mechanics in LNG marine materials. CCS also devotes itself to the application research of new materials for marine LNG low-temperature storage tanks, and the research on the evaluation mechanism, evaluation method and criterion of fracture toughness of materials. Based on the research results of new materials, the technical standards of new materials such as low-temperature carbon manganese steel, high manganese steel and 7Ni steel were supplemented, and the actual ship verification was completed through VLGC, membrane-type LNG carrier and C-type LNG carrier. The new application of domestic new materials on LNG carriers has broken through the technical barriers, which provides assistance for promoting the development of LNG industry and enhancing China's core competitiveness in the field of LNG high-end shipbuilding.

Figure 15 Layout of LNG Carrier Cargo Operating System

2. Cargo transfer and handling system

The loading and unloading system of LNG carrier is arranged on the top deck, cargo compressor room and cargo tank. It mainly consists of the following components: submerged liquid cargo pump (two per tank), tank sweeping/spray pump (one per tank), auxiliary (emergency) liquid cargo pump, as well as compressor, vacuum pump, gasifier and heater, safety valve, pipeline, valve parts, etc. of cargo compressor room. The cargo operating system functions for: cargo loading/unloading and transfer, drying, inerting and sweeping of cargo tanks, and the above two operations for any cargo tank in the absence of a base station.

Because the liquid temperature of LNG reaches -163℃, the cargo pipeline in contact with LNG is generally made of 316L high-quality stainless steel. Liquid cargo is loaded/unloaded through two cross pipes in the ship, and is transfered forward and backward through liquid manifold (arranged in the length direction of the ship) located on the top deck and connecting the liquid domes of each cargo tank. The cross pipe in the ship is divided into two loading/unloading joints on each side, and thus four loading/ unloading joints are arranged on each side of the ship. The cargo tank air domes are connected with each other by gas manifolds arranged along the top deck. The gas manifold is connected by a return air compressor with a cross pipe (one joint on each side) located in the ship to control the cargo tank pressure during loading and unloading.

During loading, the gas displaced from the cargo tank is recovered to the base station through the gas manifold, cross pipe and the return cargo compressor. During unloading, the gas manifold is used together with the cross pipe or evaporator to supply gas to the cargo tank, to make up for the pressure reduction in the cargo tank caused by the outflow of liquid cargo. In this way, the cargo tank pressure can be controlled within a reasonable range during loading and unloading.

The sweep/spray pipe is connected with the liquid cross pipe, which is used for sweeping or cooling the cargo tanks, and for spraying (replenishment gas) when the return steam is insufficient during unloading. The spray device of each liquid cargo tank is installed in the air dome of the cargo tank, which is used to disperse the liquid to each nozzle to help evaporate and accelerate the cooling speed .

The gas manifold connects the air domes to discharge the boil-off gas, and the excess vapor is generally discharged to the atmosphere through No.1 ventilation mast (because it is farthest from the superstructure). The gas manifold also sends the Boil-Off Gas (BOG) into the engine room for combustion through the gas compressor and gas heater (or heater).

3. Risk assessment

In recent years, CCS has carried out the application research of safety reliability and risk assessment methods for LNG carriers, and grasped the current situation of risk sources for LNG carriers. CCS also carries out research on typical risk scenarios. Based on the construction of LNG carrier risk scenarios, CCS carries out risk analysis on the main risk factors with reference to the results of risk quantification, and puts forward appropriate risk management and control measures to comprehensively reduce safety risks from the macro level. Further, CCS assisted the shipowners in carrying out complete risk analysis on 14,000m3  LNG bunkering ships in Hudong-Zhonghua Shipyard, providing risk control measures for design stage and full-scale ship operation stage.

4. Requirements of industry organizations

In 2021, CCS carried out research on the requirements of OCIMF, USCG, SIGTTO and other industry organizations, as well as the competent authorities, for LNG carriers through the "China-US Energy Transportation Applicable Ship Type Research Project". At present, the project has been successfully completed and related service products have been formed, helping the shipowners to better cope with the inspections by the oil and gas companies and the flag states, and providing better value-added services for shipowners.