New ABS structural requirements for FPSO's

Source: Marinelog

ABS is adopting new structural requirements for the analysis of floating production, storage and offloading units (FPSOs).

Contained in a soon-to-be-released revision and update of the society's Guide for Floating Production Installations, the new requirements have been developed from experience ABS has gained from classing the 50 FPSO units-- both newbuilds and conversions -- it has in class.

Speaking on the opening day of the Offshore Technology Conference (OTC) in Houston, Texas, Xiaozhi (Christina) Wang, Managing Principal Engineer, ABS Corporate Research & Product Development, said that the new requirements apply FPSO-specific loading conditions and prescribe strength assessment procedures to be followed.

"The FPSOs of today and the future require us to think differently," Wang noted. "Previous class requirements applicable to these vessels have been based on the Rules for trading tankers. The approach has served the industry well for many years but our knowledge base and new technology now allow us to more explicitly consider these specialized vessels."

Continuing demand for single and double hull tanker conversions, specialized environmental loads and operational loading conditions, and the structural and loads interface between the much larger and heavier topsides production facilities and the hull were among the many issues that spurred the decision by ABS to re-evaluate the Rules that specifically apply to FPSOs.

The new guide will require hull girder ultimate strength evaluation and finite element analysis as part of the tanker conversion to FPSO requirements to receive Class.

Wang says her team of hydrodynamic and structural engineers has been working on criteria development and validation, addressing inspection and repair load cases, refining operational loading conditions, revising fatigue assessment to include low cycle fatigue, developing a new topside and hull interaction analysis procedure, and a new position mooring and hull interface analysis procedure.

The ABS Sea Environment Assessment System (SEAS) software has also been improved to better address the effects of environmental loads for the strength and fatigue assessment for FPSO conversions, taking into account the prior trading routes as a tanker and the site and transit conditions as an FPSO.

For conversions, the strength evaluation consists of evaluating the hull structure as a trading tanker prior to conversion and then as an FPSO after conversion with topside loads and mooring system, explains Wang.

This involves consideration of both the sea environment encountered as a trading tanker, the transit condition to the FPSO site location, and the sea environment at the FPSO site.

The loads encountered in FPSO operations include the variations in tank loadings due to the many loading and offloading cycles, as well as the 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. FPSO operations are also different than those of tankers, as they are intended to operate at a specific site for a numbers of years without dry docking. Normal inspections and repairs are carried out at the operational site.

ABS has been working with industry--both designers and operators-- in the development of the new criteria.

A draft of the new guide that covers conversions of existing tankers is currently being prepared for final industry review and comment before being released. Requirements for newbuild FPSOs are being developed and will also be released to industry for review and comment in the second half of the year.

"All the projections are for a sustained level of contracting for FPSO units," says Wang. The fleet is expected to grow from 136 to 161 units in 2008 with forward high-low projections varying between an additional 106 and 126 units in the next five years. "We believe that the new approach that we have developed will better position ABS to assist the designers and operators of these new units Ð whether conversions or newbuilds," Wang adds.

The ABS share of the world production fleet, which includes spars, TLPs, FPSOs and semisubmersibles built or certified to ABS standards, currently stands at a market leading 47 percent with a similar proportion of all new construction contracts for these units also specifying ABS class.