ABS revises criteria for liquefied gas carriers with independent tanks

Class society ABS has revised and reissued its Guide for Building and Classing Liquefied Gas Carriers With Independent Tanks. Type-A independent tanks were used on the first generation of LNG carriers and are now used for fully refrigerated LPG containment systems for vessels with capacities of up to 90,000 m3, of which several are currently on order.  The Self-supporting Prismatic Type-B (SPB) independent tanks for LNG vessels are receiving new attention as the interest in floating LNG terminals and regasification units increases.  Type-B spherical tanks continue to be selected for LNG carriers and considered for new floating LNG units.  Type-C independent tanks used for fully pressurized LPG carriers and the new generation of small coastwise LNG carriers are also covered in the Guide.  

The newly revised ABS Guide provides a multi-level approach to the evaluation of the structure of a proposed design. This roadmap creates a structural evaluation hierarchy so that a practical, deterministic assessment of initial designs for these carriers can be made more quickly and easily.  Where required, increased design confidence also can be gained by applying more advanced structural analyses, including the dynamic loading approach (DLA) and spectral fatigue analysis (SFA).  
Criteria for the strength assessment of the hull and cargo tank structures should also take account of their interactions, explains Wei Biao (Bill) Shi, ABS Director, Engineering Support.  The current criteria, he says, encompass initial scantling evaluation, design load criteria and load combinations, structural response prediction and strength acceptance criteria.

“Industry has been calling for the use of advanced predictive models while at the same time asking for a format that will lend itself to quick determinations of initial designs and construction cycles,” say Shi.  “A necessary balance must be struck between the incorporation of highly advanced technology methods and the need for practical, deterministic applications.”  

Liquefied gas carriers with independent tanks present their own set of technical challenges, for example the system that secures the independent cargo tanks within the hull structure. “The interaction between the hull and independent cargo tank structures needs to be explicitly considered during the strength assessment,” says Shi.  “For prismatic tanks, the preliminary strength assessment of vertical supports, anti-roll chocks, anti-pitch chocks, collision chocks and anti-flotation chocks can be achieved at the initial design stage with the use of critical dynamic and accidental load cases when detailed engineering models are not yet developed.” 

In its newly revised Guide, ABS has developed fifteen (15) load cases for the strength assessment of the hull and cargo tank structures.
ABS provides a systematic way to evaluate the hull, cargo tank structures and support and securing systems with the use of specially developed engineering software systems. “Through the complete cycle of the structural evaluation, critical areas can be identified in both hull and independent cargo tank structures,” says Shi. These identified critical areas can also be used to guide enhanced monitoring during construction and scheduled surveys.  

Shi says large liquefied gas carriers with independent tanks are high-value assets requiring a high level of structural safety, robustness and longevity. The strength criteria contained in the Guide is used to verify compliance with the structural analysis requirements in the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) and as a condition of classification.

Source: Transportweekly