LNGC Project Factors and Cargo Containment System

The development of LNGC (Liquefied Natural Gas Carrier) project involves a myriad of factors that collectively shape their design, construction, and operation. Among these factors, market demand serves as a fundamental driver, with projections for LNG consumption and trade routes influencing the decision to invest in new LNG carriers. Economic considerations, including fuel prices, financing options, and operational costs, also weigh heavily in project planning. Furthermore, technological advancements in cargo containment systems, propulsion technologies, and safety features play a crucial role in enhancing the efficiency, reliability, and environmental performance of LNGCs. Regulatory compliance with international maritime standards and safety regulations is another critical factor, ensuring that LNGC projects meet stringent requirements for vessel design, construction, and operation. Additionally, logistical factors such as port infrastructure, LNG terminal availability, and shipping routes are essential considerations that influence project feasibility and operational efficiency.

The cargo containment system is a key component of LNGC projects, essential for the safe and efficient transportation of liquefied natural gas. Various factors influence the selection of cargo containment systems, including vessel size, intended trade routes, LNG cargo volume, and operational requirements. Common types of cargo containment systems include membrane – type, spherical, and independent tank systems, each offering unique advantages in terms of structural integrity, thermal insulation, and operational flexibility. Engineers and designers carefully evaluate these options, considering factors such as cryogenic temperature maintenance, pressure resistance, and compatibility with other vessel systems. Additionally, cost – effectiveness, maintenance requirements, and regulatory compliance play significant roles in determining the most suitable cargo containment system for LNGC projects. Ultimately, the selection of an appropriate cargo containment system is critical to ensuring the safety, reliability, and environmental sustainability of LNG transportation operations.

LNGC Project

Factors Involved In Selections of LNGC Project

Up to this date more than 160 LNG carriers have been built, since the first tentative in the late fifties. Their cargo carrying capacity varies from less than 20 000 m³ to about 150 000 m³. Also different cargo containment system concepts have been adopted.

In fact many interrelated factors are involved in the selection of a liquefied gas carrier for a specific project.

Cargo Containment System Selection

The factors to be considered in the selection of the cargo containment system include:

a) COST

Operating and capital cost. Long-term operating and repair costs and containment system reliability have considerable economic impact in the overall gas ship operation.

b) SHIPYARD AVAILABILITY

Shipyards have licenses to build certain types of containment systems. Therefore shipyard availability could become a determining factor for the selection of the containment system.

c) OPERATING EXPERIENCE

Long-term operating experience with the conventional hull and containment system is to be considered.

d) MATERIALS

Besides the adoption of mild steel or high tensile steel for the hull, the selected tank and insulating material should be of a proven reliability.

e) BOIL OFF

Boil-off has a significant effect on the project’s economic. Investigation into the use of a reliquification plant should also be considered.

f) SHIP OPERATIONAL FACTORS

Visibility from the bridge, voyage maintenance, easy access to operating equipment, easy access to important areas during operations, surveys and maintenance is considered.

Cargo Containment Systems

Developing a containment system adequate for the The business of LNG and historical involvement in maritime transportation of gastransportation of liquefied gas by sea imposed unique engineering demands on the marine industry.

The IGC Code takes into account several types of cargo containment systems for gas carriers. However not all the cargo containment systems described by the Code are suitable for LNG vessels.

Intregral Tanks

Integral tanks form a structural part of the ship’s hull and are influenced in the same manner and by the same loads, which stress the adjacent hull structure. Integral tanks are in general allowed for cargoes having vapor pressure not exceeding 0,25 bar (0,7 bar in certain particular cases) and boiling point not less than minus 10 °C. Of course, this type of cargo containment system is not suitable for carriage of natural gases, which are to be transported at a temperature lower than minus 160 °C.

Membrane Tanks

Membrane tanks are non-self-supporting tanks, which consist of a thin layer (membrane) supported by the adjacent inner hull structure through the insulation. The membrane is designed in such a way as to limit the thermal and other expansion/contraction stresses. The membrane is, in general, allowed for cargoes having a vapor pressure not exceeding 0,25 bar (0,7 bar in certain particular cases) and for any boiling points. This type of cargo containment system is allowed for LNG vessels.

Semi-Membrane Tanks

Semi-membrane tanks are tanks, which are non-self-supporting in loaded condition, similarly to the membrane tanks; however, the rounded parts at the corners of the tanks are designed in such a way as to accommodate the thermal and other expansion/contraction stresses. The semi-membrane tanks can carry the same cargoes as the membrane tanks.

Independent Tanks

Independent tanks are self-supporting tanks that do not form part of the ship’s hull structure and are not essential for the hull strength. Origin, Applicability, Requirement of IMO Gas Code The IGC Code contemplates three categories of independent tanks:

a) TYPE “A” INDEPENDENT TANKS

These are tanks, which are designed primarily using recognized standards of classical ship-structural analysis procedures. Where these tanks are essentially made of flat surfaces, they can carry loads having a vapor pressure not exceeding 0,7 bar and any boiling temperature. These tanks may be used for LNG vessels; however, there are no recent LNG vessels with this kind of cargo containment system due to the necessity of a complete structural secondary barrier.

b) TYPE “B” INDEPENDENT TANKS

These are tanks, which are designed using model tests, refined analytical tools and analysis method to determine stress levels, fatigue life and crack propagation characteristics. These tanks can carry loads having a vapor pressure not exceeding 0,7 bar and any boiling temperature.

c) TYPE “C” INDEPENDENT TANKS

Internal Insulation Tanks

Internal insulation tanks are non self-supporting and consists of thermal insulation materials, which contribute to the cargo containment and are supported by the structure of the adjacent inner hull or of an independent tank. The inner surface of the insulation is exposed to the cargo. The internal insulation tank is, in general, allowed for cargoes having a vapor pressure not exceeding 0,25 bar (0,7 bar in certain particular cases). The IGC Code contemplates two types of internal insulation tanks:

a) TYPE 1 TANKS

These are tanks in, which the insulation or the combination of the insulation with one or more liners A liner is a non-self supporting layer of any material, which is part of the insulation to enhance some of its characteristics, for instance the mechanical characteristics. A liner differs from a membrane has it does not have the function to be the only element containing the cargo.x functions only as primary barrier.

b) TYPE 2 TANKS

These are tanks in, which the insulation or the combination of the insulation with one or more liners functions as primary barrier and secondary barrier and the two barriers are clearly distinguishable.

Secondary Barriers

The IGC Code prescribes in certain cases a secondary barrier to contain any leakage from the layer of Cargo containment system of gas vesselthe cargo containment system, which is in contact with the cargo. This layer is commonly known as primary barrier. In principle the secondary barrier is another containment layer arranged at a certain distance from the primary barrier.

The secondary barrier is to be designed in such a way as to contain any envisaged leak for 15 days, unless different requirements apply to certain specific routes, to prevent the lowering of the temperature of the supporting steel structure below the design temperature allowed for the materials. It is to be independent from the primary barrier so that any mechanism of failure in the primary barrier does not cause failure of the secondary barrier.

The secondary barrier may be complete (i. e., completely surrounding the primary barrier) or can be partial. The partial secondary barrier is a kind of one or more drip trays arranged in such a way as that all possible leakages can be collected therein and of a size sufficient to keep all the envisaged leakage for the period of 15 days. When it is intended to use a secondary barrier, it is necessary to know the maximum quantity of leakage that can be expected in the 15 days period. Sophisticated calculations of fatigue and crack propagation analysis are necessary to realistically envision the worst possible case of failure of the primary barrier and the maximum possible leakage.

The IGC Code requirements relative to the secondary barrier are summarized in Table. The requirements applicable to LNGC have been highlighted yellow in the Table. The requirements highlighted in red are applicable to other types of gas carriers (LPGC). The color orange has been used for systems that even though might be theoretically used, are never used for LNGC.