Leading offshore classification society ABS has adopted new structural requirements for the evaluation of converted floating production, storage and offloading (FPSO) units. The criteria are contained in a revised version of the society's ABS Guide for Building and Classing Floating Production Installations. The society also provided a preview of their soon-to-be released standards for newbuild FPSOs during a press conference at the industry's Offshore Technology Conference (OTC) in Houston, Texas.
ABS has been working with industry for the past year in a two-phase approach toward revising the FPSO structural criteria: phase one relates to conversions and phase two relates to newbuilds. Leading FPSO designers and operators who participated in the phase one development and reviewed the conversion criteria say it better reflects the true operational nature of an FPSO.
"The methodology and practices for the new FPSO conversion criteria are offshore-centered," said Xiaozhi (Christina) Wang, Senior Managing Principal Engineer, ABS Corporate Technology. "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 calculation 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.
For phase two -- the upcoming newbuild criteria -- Wang says it includes all the features as developed for conversions such as more realistic loading cases, high cycle and low cycle fatigue strength assessment, hull girder ultimate strength assessment and hull interaction analysis with topside structure. Wang says the newbuild criteria is currently being finalized and will be released to the industry shortly.
ABS has extensive experience classing FPSOs with more than 50 FPSO units, both newbuilds and conversions, currently in class. The society drew upon a substantial database of information regarding converted, in-service FPSOs in selecting loading patterns and developing fatigue calculations for the new criteria. ABS has also classed several newbuild FPSOs including Chevron's Agbami FPSO operating offshore Nigeria.
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