Reducing Vessel Weight & Operating Costs

Ultra-Large liquefied natural gas (LNG) Carriers introduce a number of unique problems due to their size. Liquid sloshing limits the carriage of LNG in large side to side membrane tanks to be either over 80% or less than 10% full to avoid damage to the tank lining and insulation. The Cubic Donut Tank System (CDT) was first patented by Prof. Thomas Lamb (Prof. Emeritus, Univ. of Mich.) in 1976 and is less expensive to construct than spherical and membrane LNG tanks. Altair ProductDesign recognized the merits of the invention but noted that the patent in is original form faced several structural issues in regard to hydrostatic and sloshing loads.

Altair ProductDesign worked with Prof. Lamb to improve the original design. Using Altair’s own optimization technology, Optistruct, Altair ProductDesign developed an innovative central interconnecting system that simultaneously mitigated hydrostatic loads while keeping the sloshing to a minimum. This design improvement retains all the advantages of the originally conceived idea of confining the liquid in a series curved shells so as to ensure that the structure is primarily loaded in a membrane mode rather than in a bending mode.

Using Altair’s Radioss solver, a series of sloshing simulations were performed to evaluate the sloshing loads assuming the vessels roll period. This analysis was repeated for head seas, bow seas, and beam seas. In all cases the sloshing loads were lower with the new CDT. This combined with the lower hydrostatic loading, allowed for a lighter free standing tank that can be integrated more easily into a vessel’s hull.

When integrated into a hull, the lighter weight and free standing CDT results in a smaller vessel for the same volume of LNG. This results in lower operational cost in addition to the lower acquisition cost.