Simple dynamic loading analysis available for LNG designs

A simpler, faster alternative to a full DLA (Dynamic Loading Approach) analysis of LNG carrier designs is being made available by ABS through its unique ABS SafeHull program.

Drawing on its experience in classing more than 40 large LNG carriers, the classification society has developed new criteria that will permit a SafeHull evaluation of Gaz Transport and Technigaz membrane designs. A vessel that meets the criteria will be issued a SafeHull (SH) notation.

SafeHull compliant ships are also automatically eligible for inclusion in the ABS SafeShip, through-life integrity maintenance program.

"Traditionally LNG carriers have been designed to meet the prescriptive requirements of a classification society," said Dr Donald Liu, senior vice president technology at ABS. "Because of the specialist nature of these vessels, either the shipyard or the owner would then request a full DLA analysis. This is a time consuming and expensive process."

By using ABS SafeHull the dynamic loads that will be placed on the vessel's hull and containment system are automatically considered during the development of the design.

"This has the added advantage," Liu explained, "of using the net scantling criteria which is then automatically checked against the most severe dynamic loads that the vessel is likely to encounter in service, permitting an accurate assessment of the fatigue life of the structure. By using a SafeHull evaluation, shipyards will be able to optimize their designs and improve production efficiencies."

The ABS SafeHull criteria also automatically consider the requirements of the IMO's IGC code, and the maximum allowable bending stress of the inner hull as determined by the containment system designer. "SafeHull provides for a much quicker and more reliable evaluation of all the structural details," Liu said.

Other issues that have been considered in the development of SafeHull for LNGCs are sloshing, particularly from partial loads, and the dynamic loads placed on the hull by these vessels normal 20 knot service speed.

"Although LNG's have some similarities to tankers, their service speed and sections of their hull form are more similar to container carriers," said Hoseong Lee, ABS project manager for LNG SafeHull development. "Since we have already developed SafeHull applications for both these ship types we have been able to rapidly correlate this experience with our understanding of LNGCs from our almost 40 year involvement with these specialized ships. The result, we believe, is the most advanced gas carrier design evaluation system available."

Most recently ABS classed the series of three 138,200 cu m LNG carriers delivered by Samsung HI to SK Shipping. These vessels incorporated the Technigaz containment system.

"The membrane systems are proving the most popular with owners at present," Lee noted. "Yet it is the membrane system that places the greatest stresses on the hull structure, particularly on the bulkheads and inner bottom. That is the advantage of SafeHull. It is able to consider the full range of dynamic loads that the structure will encounter and establish net scantlings that are specifically tailored to meet those loads."

With shipowners looking for 30-40 year effective lifespans for these vessels, they need to know that the hull structure will have a fatigue life that corresponds to the expected commercial life. "Only ABS SafeHull can provide that analysis in a quick, simple and cost saving manner," Dr Liu stressed.