The potential of Independent Type C tanks for new applications requires designers to fully understand the relevant rules and guidance, writes Dr Bo Wang, ABS
Industry announcement: New fuels are set to play an important role in reducing shipping’s contribution to greenhouse gas production, but as these new energy sources are considered, it is important to maintain a focus on safety. Designers, owners and manufacturers must ensure that the chosen containment systems specified meet all applicable standards for new fuels.
The potential to use independent Type C liquefied gas cargo tanks to carry ammonia and hydrogen in a liquid state is seen as a promising solution for transporting these fuels. For example, vacuum-insulated Type C tanks with double shells can contribute flexibility in fuel choices with the most popular design for gas fuel containment.
With cleaner energy demand increasing, there has already been a commensurate increase in the demand for gas carriers capable of carrying LNG, LPG and Liquefied Ethane Gas. Market demand has been strong for small and medium sized gas carriers and bunker barges in both short/medium distance trades.
Growing fuel demand has been met with independent Type C tanks; typically in a cylindrical or bi-lobe configuration, each with two end saddle supports for gas carriers. On gas fuelled ships, vacuum-insulated Type C gas fuel tanks with double shells are the most popular design for the gas fuel containment system.
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 design of independent Type C liquefied gas cargo tanks must meet the requirements of the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code). Type C fuel tanks must satisfy the International Code of Safety for Ships Using Gases or Other Low Flashpoint Fuels (IGF Code) requirements.
In general, independent Type C tanks – also known as pressure vessels – are designed and built to meet the requirements of recognized pressure vessel standards such as the ASME Boiler and Pressure Vessel Code (BPVC), as well as additional classification society requirements and statutory regulations.
To support the safety of the energy transition, ABS has developed Guidance Notes to provide procedures for determining design loads on Type C tanks and performing the strength evaluation of the tank and supporting structures.
Liquefied gas cargo/fuel tanks must be designed to sustain all static and dynamic loads such as weight, wave-induced loads and sloshing loads during their service life. This includes liquefied gas cargo tanks on gas carriers, barges or offshore terminals and liquefied gas fuel tanks on gas fuelled ships. The technical approach adopted in these procedures is based on the direct calculation method using the Finite Element (FE) analysis to assess tank and supporting structures subject to static and dynamic loads.
Design load cases including standard, accidental and test load conditions are defined for yielding, buckling and fatigue evaluation. Finally, a strength assessment procedure for different failure modes is provided for tank and 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.
The new 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 the ABS Marine Vessel Rules.
The notes consider design load cases including wave-induced high cycle fatigue load and cargo/fuel loading/unloading induced low cycle fatigue load conditions for fatigue assessment.
