scope:

Introduction

General

As energy demand increases there is a commensurate increase in the need for liquefied gas carriers/bunkers/barges. These include liquefied natural gas (LNG), liquefied petroleum gas (LPG) and liquefied ethane gas (LEG) vessels. Liquefied gas fuel tanks will be needed for gas transportation and gas fueled ships, respectively. As described in the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) and the International Code of Safety for Ships using Gases or other Low-flashpoint Fuels (IGF Code), there are several categories of cargo/fuel containment systems including membrane type and independent type (Type A, Type B, and Type C) systems.

Among the very large LNG cargo vessel fleet, the most common tank type is the membrane-type LNG vessel, in which the tank is directly formed by the inner hull structure. For this type of vessel, sloshing is a key issue since it could potentially cause damage to the cargo containment systems due to the smooth surfaces inside the tank.

Other types of large liquefied gas carriers are independent Type B, including spherical and prismatic vessels. They are designed to minimize sloshing loads using a spherical shape or the internal member structures inside the Type B containment system. For large LPG vessels, the independent Type A tank is usually applied utilizing the hull structure as the full secondary barrier.

In recent years, the small-scale liquefied gas cargo market has expanded for short/medium distance transportation (e.g., domestic trade). One of the promising solutions to meet this growing need is to use an independent Type C containment system, which is commonly used for small scale liquefied gas cargo transportation. For example, for a bi-lobe Type C LNG/LPG/LEG vessel, there may be four bi-lobe tanks installed inside a carrier, with each tank possessing end saddle supports. One end is fixed (all degrees of freedom are restrained) and the other end is designed to be able to slide in a longitudinal direction to compensate for the effect of thermal contraction/expansion caused by the temperature change of the tanks.

The global shipping industry also faces a challenge as legislation for Emission Control Areas (ECA) has limited the allowable sulfur emissions of ships significantly, firstly in North America and northern Europe. Natural gas is a potential solution for meeting these emission requirements since it is a cleaner burning fuel which can reduce sulfur oxide (SO₂) emissions by 90% to 95% and carbon dioxide (CO₂) emissions by 20% to 25%. Emission regulations have promoted the use of alternative marine fuels, such as natural gas. Most early adopters of natural gas as fuel have utilized Type C fuel containment systems, considering the advantages of boil-off gas management and associated operational flexibility.

Type C cargo/fuel tanks are also known as "pressure vessels" and are designed and built to meet the requirements of recognized pressure vessel standards or codes such as the ASME BPVC, which are supplemented by additional Class Society requirements and statutory regulations. Since the Type C tank is designed to be independent of the vessel's hull, it is not essential for maintaining the hull strength or the integrity of the vessel. However, the liquefied gas cargo/fuel tank itself must be designed to sustain all static and dynamic loads (e.g., weight, wave-induced loads, sloshing loads, etc.) during its service life. In general, there are two categories for Type C tanks applied in liquefied gas cargo vessels or gas fueled ships: foam-insulated single-shell tanks (e.g., cylindrical, bi-lobe, tri-lobe, etc.), as shown in Section 1, Figures 1 and 2, and vacuum-insulated double-shell tanks, as shown in Section 1, Figure 3, respectively. These Guidance Notes address the strength evaluation of these two kinds of Type C tanks and their supporting structures.

In the ABS Guide for Building and Classing Liquefied Gas Carriers with Independent Tanks (LGC Guide), the procedure for the strength evaluation of hull, tank, and support structures has been developed for gas carriers with independent type gas tanks. However, the LGC Guide puts increased emphasis to the hull structure and the independent Type B prismatic tanks in gas carriers. These Guidance Notes provide a procedure for the structural assessment of Type C independent cargo/fuel tank and supporting structures under static and dynamic loads to supplement the LGC Guide and Part 5C, Chapter 8 and Part 5C, Chapter 13 of the Marine Vessel Rules.

These Guidance Notes define design load cases, which include standard load conditions, accidental load conditions, and test load conditions for yielding and buckling strength assessment, and also design load cases including wave-induced high cycle fatigue load and cargo/fuel loading/unloading induced low cycle fatigue load conditions for fatigue assessment. The finite element (FE) analysis-based direct calculation is required for structural analysis of the tank and support structures in various design load cases. Finally, an analysis procedure is provided for the strength evaluation of the Type C tank and support structures for the yielding, buckling, and fatigue failure modes.