ABS adopts new FPSO structural requirements

28 Jan 2009

The American Bureau of Shipping (ABS) has adopted new structural requirements for the evaluation of FPSO units. The criteria are contained in a revised version of the ABS Guide for Building and Classing Floating Production Installations. The new standards will be mandatory effective July 1, 2009. Companion requirements for newbuild FPSOs are scheduled for release later in 2009.

ABS has been working with industry for the past year to develop the new criteria. Leading FPSO designers and operators who have participated in the development and reviewed the new ABS criteria say it better reflects the true operational nature of a FPSO. "The methodology and practices for the new FPSO conversion criteria are offshore-centred," says Xiaozhi (Christina) Wang, senior managing principal engineer, ABS Research & Product Development. "The new requirements apply FPSO-specific loading conditions and prescribed strength assessment procedures that are to be followed."

ABS says the criteria allows for better prediction of environmental loads using more realistic load cases. Realistic tank load patterns as well as appropriate load combination factors were used in the development of the criteria. Low cycle fatigue was also factored into the criteria acknowledging the cyclic, more frequent loading and discharge nature of FPSOs as compared to trading tankers.

According to Wang, the revised fatigue assessment approach takes account of actual FPSO operations, including the variations in tank loadings due to the many loading and offloading cycles, as well as sea waves and swell. The wave and swell loadings subject the hull structure to high cycle fatigue loads, whereas the loading and unloading of the cargo tanks subject the structure to low cycle fatigue loads.

In addition to varying loading patterns, FPSOs are intended to operate at a specific site for a numbers of years without dry docking. Normal maintenance, inspections and repair are carried out on-site. To reflect this, the new standards do not require port down time for repairs as is the case for trading tankers.

Much heavier and larger topside production facilities are also being developed for FPSOs and the new requirements outline a topside and hull interaction analysis procedure requiring finite element analysis. "The weight of the topside structure and its interaction with the hull is an important consideration," says Wang. "Close review of the loads is needed as well as calculations of the interaction forces and their effects on the hull's strength and fatigue life."

The new standards also provide guidance for establishing the renewal scantlings for the FPSO as it starts its new life.

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