Life Extension of Ageing FPSO and SPM Calm Buoy Hulls and Deck Plates Under Corrosion Effects - Part 1B

Corrosion wastages occurs in the form of pitting corrosion or uniform/general corrosion. However, it was observed that previous academic studies relied on data obtained from shuttle tankers and imposed such to FPSO for the purpose of studies. While other scholars took data obtained from laboratory in corrosion coupon for the purpose of research. The state-of-the art is the use of data obtained from FPSO and SPM calm buoy in real marine environment operational location. It was revealed that most of the offshore structural design codes for floating assets in tropical waters or temperate region were still based on the codes for North Sea installations, even when both were known to have obvious different environmental conditions. These data are a true representation of the state of marine corrosion wastages associated with the floating assets. The data obtained from laboratory is usually simulated and controlled while that of marine environment occurs naturally. It was also observed that the validity of data suitable for life extension studies is placed at 18 months by some class societies, while some oil majors allows data obtained within 60 months and provided that such data will be scrutinized by relevant subject matter expert. Usually a minimum of five points thickness gauging and maximum of eight-point thickness measurements where there is marginal wastage is required per strake of a square metre. It was also observed that the collection, collation, analysis and interpretation of these data drives the technical aspect of the intelligent business decision aimed at making life extension repairs decisions of FPSO and SPM calm buoy. Although, it was remarked that corrosion wastages do not alter the chemical and mechanical properties of steel but only changes their geometry. Thus, the remaining thickness is a representation of the remaining strength of the steel plate. It was also remarked that these repairs are done according to class society requirements and supervisions from material selection, verifications and repairs. Steel plate grade, heat number, attending surveyors work order number, mill test certificate and reports signed at steel yard during manufacturing testing by the surveyor are some of the requirements. Others are the steel plates welding processes, sighting of procured steel plates and welders’ certificates at repair sites. Thus, steel plates repairs on classed vessels must be done with classed steel plates of the same grade and thickness to the badly corroded one being renewed. Although, in the absence of the same thickness, a higher thickness can be used but must be approved by the class society covering such asset. Regrettably, steel plates of different grades mean that they have constituents’ elements percentage composition and hence microstructure which can pose a threat of galvanic corrosion if applied in such marine environment.