Corrosion and the structural safety of FPSO mooring systems in Tropical waters

As oil and gas production increasingly moves to deeper waters the operating platforms of choice are FPSOs (Floating production storage and off-loading) ('floaters'). Station keeping is a crucial operational requirement since failure can have disastrous implications. Typically a highly redundant mooring system, consisting of chain and wire rope or synthetic rope is used. The chains have links made from 70 mm diam steel (420 mm link length) to 185 mm (1.1 m link length). Their design employs high 'factors of safety', based largely on North Sea operational experience. This reflects the high variability in loading conditions and in structural capacity as a result of fatigue and corrosion. Concern has been expressed that present design requirements may not be sufficient for operations in the Tropics. As a result, a major international Joint Industry Research project was established to study corrosion loss and pitting and also wear of mooring chain and of wire rope. The experimental work is based largely in Australia. It will provide data for revision of design and operational guidelines using modern structural reliability principles and state-of-the-art understanding and models for corrosion metal loss and pitting. The paper outlines the issues and provides the framework for the on-going project work.