ABS issues new criteria for FLG terminals

03 May 2010
A graphic of the ABS FLGT software

A graphic of the ABS FLGT software

ABS has announced the release of its latest Guide for Floating Offshore Liquefied Gas Terminals, reflecting the latest structural design and analysis developments in gas handling, storage and transportation.

ABS has been involved either directly as the class society or in the design review for several groundbreaking floating LNG (FLNG) and floating storage and re-gasification unit (FSRU) concepts. The new Guide provides criteria that can be applied to the classification of the hull structure of floating offshore liquefied gas terminals (FLGTs) with membrane tanks or independent prismatic tanks.

The newly released criteria from ABS addresses liquefied gas terminals with ship-shaped or barge-shaped hull forms, having single center cargo tanks or two cargo tanks abreast arranged along the center-line of the terminal’s hull. This new release is based on the design and analyses experience gained by the society from classing membrane tank LNG carriers, liquefied LNG and LPG gas carriers with independent tanks and FPSOs.

Structural design challenges are being driven by the increase in the size of terminal hulls, shallow water load effects, frequent partial filling, offloading operations and critical interfaces between the hull and topside structure and between the hull and position mooring system.

FLGT concepts have broached the possibility of hull structures up to 450m in length and 70m in breadth, which would make them the largest ship-shaped units to be built. With the hull structure so large, designs with two cargo tanks abreast are being proposed to minimize the internal load effects, particularly from sloshing within the partially-filled tanks during loading and discharge operations, said Xiaozhi (Christina) Wang, ABS structures manager, offshore technology.

The onsite environment is typically close to shore so shallow water effects, which can place more severe environmental loads on the hull structure than when it is in deeper water depth, need to be considered. Frequent partial tank filling is also an important factor in establishing adequate strength to resist the dynamic loads from sloshing. “Unlike trading LNG carriers, which can avoid critical filling levels, a floating terminal operates at partial tank filling levels so the structure must be able to sustain various sloshing scenarios,” explained Wang.

Two other important considerations in the structural analysis include offloading operations and hull and topside interface. Offloading operations, either side-by-side or in tandem, have an impact on a floating terminal’s response motions as the coupling effects and relative motions between the terminal’s hull and offloading vessel must be taken into consideration. Wang also said that analyses of the hull and topside interface are critical as the size and weight of the topsides modules is significant. “The massive size of newbuild terminals calls for careful integration of the topside structure with the hull structure. For ABS, this means a detailed review of the loads as well as calculations of the hull’s yield, buckling and fatigue strength,” said Wang.

To support the technical guidance it is now providing for FLGTs, ABS has developed proprietary software. The software provides calculations for evaluating structures considering buckling, yielding, ultimate strength and fatigue strength. Importantly, the floating terminal structural criteria takes into account low cycle fatigue which factors in the cyclic and more frequent loading and discharge nature of a floating terminal as compared to a trading LNG carrier.

ABS’ evaluation of a floating gas project is based upon the application of prescriptive requirements, sea-keeping studies, structural strength and fatigue analysis, assessment of the containment system, including sloshing analyses and an evaluation of the station keeping systems. As applicable, ABS will review the topsides, the gas processing and liquefaction plants or the re-gasification modules and use advanced risk analysis to verify that accepted safety standards are attained.

A number of ABS Guides and Guidance Notes are referred to when establishing compliance for a floating gas facility including the ABS Guide for Building and Classing Offshore LNG Terminals as well as international standards such as the International Maritime Organization’s (IMO) Gas Code as applicable.

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