Sloshing Fluid Dynamics Advanced by ABS & Daewoo Study

By George Backwell at March 27, 2011 03:56
Filed Under: Research & Development

One of the greatest challenges that naval architects currently face – the prediction of sloshing loads and resulting structural response in LNG (liquid natural gas) carriers – is a complicated puzzle in fluid dynamics whose eventual solution will add significantly to the safety of crews aboard the new generation of giant LNG carriers.

On 23, March 2011, classification society ABS and Daewoo Shipbuilding & Marine Engineering (DSME) announced they had established a new methodology which will contribute to the study of the phenomenon; but before going into that, what exactly is 'sloshing'? And why is an understanding of it of such significance?

Sloshing in LNG Tanks

Sloshing (familiar to all who have walked with a half-full jerrycan) is the motion of a fluid in a partially filled tank which is also undergoing motion. In the tanks of large LNG ships it may become a potent force capable of causing a violent wave in certain conditions, depending on the interplay of such variables as the tank shape, the fill height and the actual motion of the ship. When sloshing becomes violent, the insulating membrane that lines the interior of LNG cargo tanks is liable to become damaged, with potentially disastrous consequences.

The purpose of this membrane is specifically to prevent liquified natural gas, at a temperature of -163C, from coming into contact with the ship’s steel structure, with catastrophic effects on its structural integrity; not only that, but high impact sloshing loads are passed through the containment system into the hull structure itself with unpredictable consequences. Hence the strong interest of classification societies and LNG ship designers in the fluid dynamics of sloshing and the amelioration of its harmful effects.

LNG Ship Cargo Tank: Photo credit: Thinforth

Shipping LNG in Huge Ships a Challenge

Gulf state Qatar is the world’s biggest producer of LNG, and to achieve economies of scale its shipping arm ‘Qatargas’ already has in operation (and on order) the largest LNG ships yet built. Their ‘Q-Max’ type (maximum possible size to fit the ‘Q’ for Qatar terminal) has a length overall of 245 meters (1,132 ft) and an LNG capacity of 266,000 cubic meters (9,400.000 cu.ft).

The introduction of these new giant LNG ships poses a technical challenge for the shipbuilding industry, as the move to larger ships carries with it the potential of higher cargo sloshing loads in larger ship tanks, with a wider-scale calamity the unfortunate outcome should the LNG cargo containment system fail.

ABS & Daewoo Research Findings

It is beyond the scope of this article to say more than that the new ABS-DSME methodology proposes a pre-screening procedure that would enable naval architects to select the most severe critical sea states to use before the start of a sloshing model test, or in a software sloshing flow simulation.

The definitive last word is with Yung Sup Shin, Head of the Containment System Group, ABS Corporate Technology: “Model tests cannot cover all possible wave height, period, heading and filling levels since there are just too many combinations to determine the life-time maximum sloshing loads. However, we have developed a robust way to determine a finite number of critical sloshing conditions consistently and efficiently compared to other existing methods.”

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