Leading offshore classification society ABS has revised its Guide for Floating Offshore Liquefied Gas Terminals, reflecting the growth in size of gas handling, storage and transportation systems and the resulting impact on structural design and analysis requirements.
Technical challenges in this sector are being driven by a combination of design and operational issues. These include 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.
Offshore Floating Liquefied Gas Terminals (FLGT) concepts have introduced the possibility of hull structures up to 450 meters in length and 90 meters in breadth, which would make them the largest ship-shaped units yet 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 in the partially-filled tanks during loading and discharge operations
“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,” explains Xiaozhi (Christina) Wang, Director, ABS Corporate Marine Technology. “Frequent partial tank filling is also an important factor in establishing adequate strength to withstand the dynamic loads from sloshing.”
In addition, says Wang, operations are typically carried out 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, also need to be considered.
Two other important considerations within 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 notes 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,” she adds.
ABS has been involved either directly as the class society or in the design review for several ground-breaking Floating LNG (FLNG) and Floating Storage and Re-gasification Unit (FSRU) projects. The Guide provides criteria that can be applied to the classification of the hull structure of FLGTs with membrane tanks or independent prismatic tanks.
The 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 centerline of the terminal’s hull. This new release of criteria 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.
To support the technical guidance ABS provides for FLGTs, the class society has developed proprietary software which 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, station keeping system evaluation and containment system assessment, including sloshing analyses. 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.
Founded in 1862, ABS is a leading international classification society devoted to promoting the security of life, property and the marine environment through the development and verification of standards for the design, construction and operational maintenance of marine-related facilities.
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Source: ABS, March 22, 2011;