Feature 1 | METHODS & MATERIALS ABS revises LNG criteria
Revised liquefied natural gas carrier construction guidance sees ABS develop new criteria stretching from initial scantling evaluation, design load criteria and load combinations, through structural response prediction and strength acceptance criteria.
Independent Tanks. , in the process developing 15 load cases for the strength assessment of the hull and cargo tank structures. Type-A independent tanks were used on the
C
first generation of LNG carriers and are now used for fully refrigerated LPG containment systems for vessels with capacities of up to 90,000m3
, of which several are currently on
order. Latterly, Self-supporting Prismatic Type-B (SPB) independent tanks for LNG vessels have received special attention as the interest in floating LNG terminals and regasification unit 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 pressurised LPG carriers and the new generation small coastwise LNG carriers are also covered in the Guide. Te newly revised ABS Guide provides a
multi-level approach to the evaluation of the structure of a proposed design. Tis 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 will also be
lass society ABS has revised and reissued its Guide for Building and Classing Liquefied Gas Carriers With
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. Te 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,” says Mr 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. Exemplary is the system that secures the independent cargo tanks within the hull structure. “Te interaction between the hull and independent cargo tank structures needs to be explicitly considered during the strength assessment,” says Mr
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.”, he adds. 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. “Trough the complete cycle of the structural evaluation, critical areas can be identified in both hull and independent cargo tank structures,” says Mr Shi. These identified critical areas can also be used to guide enhanced monitoring during construction and scheduled surveys. Mr 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 revised 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. NA
Dust extractor for welding safety
Potentially dangerous radio-toxic dust generated during grinding of some types of welding electrode can now be collected and disposed of in complete safety, according to UK-based Huntingdon Fusion Techniques Limited (HFT). The company has released a new vacuum dust extraction system for its TEG -3 tungsten electrode grinding
machine. This more comprehensive Tungsten Grinder, has all the features of the non-extraction version with the added extraction system. In re-designing the transport packaging for the Tungsten Grinding Machine, to accommodate the extraction
system, HFT has taken the opportunity to introduce a new robust container that meets the requirements for international transportation.
32 The Naval Architect July/August 2009
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