For years operators have reported accelerated rates of corrosion in wire rope and chain in warmer waters. Mooring design codes specify corrosion endurance of these components predominantly based on experience from the colder waters of the North Sea. The scope of the SCORCH JIP was to investigate and characterize corrosion of steel chain and wire rope moorings for a wide range of operating Floating Production Units (FPUs) and Floating Production Storage and Offtake vessels (FPSOs) in warm waters. A database was compiled of detailed corrosion measurements of in-service and retired mooring chains and wire ropes from about 30 FPUs operating in warm waters off Asia, Africa and the Americas. A set of standardized procedures were developed for forensic examinations of retrieved chain and wire rope, including photogrammetry and laboratory measurements that allowed 3D reconstruction and statistical analysis of corroded surfaces. The database was complemented by destructive tensile tests of a number of samples in order to correlate the observed degradation with residual breaking load. The SCORCH JIP also investigated the impact of sea temperature, water velocity, depth and oxygenation, steel grade and chain and wire rope construction through over 750 sample and full-scale tests spanning 3.5 years at sites around Australia. Additional laboratory tests were conducted into Microbiologically Influenced Corrosion (MIC) and the combined effect of corrosion and wear of mooring chains. The SCORCH JIP has produced a series of findings with long-lasting implications for the industry, including detailed investigations of factors in corrosion such MIC, chain pitting, chain wear, wire rope blocking compound efficacy, and the effect of environmental and operating conditions. The SCORCH JIP has produced tabulated predictions for corrosion in varying temperatures, at positions in the mooring line and for nutrient levels that could encourage MIC. The predictions are underpinned by a large number of field experiments and results from operating FPUs, and are supplemented by detailed operational guidance and recommendations for maximizing mooring corrosion endurance.

Forensic investigations on severely corroded (pitted) chains recovered from the mooring systems of two FPUs in West Africa and Indonesia are described. During the course of the SCORCH JIP, it became apparent that a similar phenomenon had been experienced by other JIP participants indicating that it may be a common problem deserving attention. The conclusion of the present investigation is that the large pits most likely can be attributed to Microbiologically Influenced Corrosion (MIC). Where pitting occurred, the pit depth growth rate was found in instances to be up to 10 times the uniform corrosion rate, with pits distributed approximately uniformly but randomly around the link and the bar circumference. Destructive tensile tests of the samples indicated residual strength that was at worst 25% below as-manufactured estimated As-new Breaking Load (ABL). The correlation of loss of strength with loss of metallic area due to uniform and pitting corrosion indicated that breaking load cannot be treated simplistically, and requires more sophisticated consideration of the pit geometry and its location with respect to areas of high stress within the link.

A recently identified problem facing operators and designers in the integrity of mooring chain is the effect of highly localized Microbiologically Influenced Corrosion (MIC) in tropical waters manifesting in large pits, as identified in the SCORCH JIP. These pits, if assessed against API-RP-2I, would necessitate immediate removal and replacement of the chain links. The conventional discard criteria in mooring codes were formulated based on uniform corrosion, and not highly localized material loss. The application of rigorous Finite Element techniques to determine the residual strength of these degraded links has not been recognised in existing codes of practice as an acceptable technique for assessing whether or not the chains can meet operational requirements. The Chain FEARS (Finite Element Analysis of Residual Strength) JIP aims to address this by developing guidance for the determination of discard criteria for mooring chains. This will assist in improving the reliability of prediction of the remaining life of degraded chain and better inform mooring replacement programs. This paper outlines the scope, objectives and an overview of the results of the first phase of the Chain FEARS JIP; investigating Finite Element methods for determining the strength loss of degraded mooring chains. This investigation has resolved issues including the effects of degradation on strength reduction and the challenges of establishing a robust methodology for assessing the remaining strength of degraded chains. The scope of the second phase of the Chain FEARS JIP, to develop tools for the prediction of the residual fatigue life of degraded chain, is also outlined. The findings of the Chain FEARS JIP are applicable to a wide variety of FPUs reliant on chain moorings and will enable the application of a more rational guidance for remaining chain life prediction and mooring line replacement.

Welded Joints long-term integrity is of particular importance to large vessels, such as frigates or bulk carriers. Downtime due to corrosion concerns can be onerous. Ship classification organisations, such as The American Bureau of Shipping (ABS), standardises processes and product approval programmes for materials manufacturers. High-strength steels are of particular interest due to their high strength to cost ratio for large marine vessel construction. The corrosion performance of such steels has been studied, but data on the specific degradation of such metals when welded and for the welds themselves are scarce. The present paper reports marine immersion corrosion pit depth data for several ABS steel grade weld permutations and exposed to different sites for up to 2.5 years. The implications of the presented results are discussed as well as the possible influence of alloying elements and microstructural differences on the underlying corrosion kinetics.

The most cost-effective solution to ongoing production in a low oil price environment is often to extend the life of ageing producing assets past the original design intent. The offshore oil and gas industry therefore faces the challenge of extending the useful production life of ageing assets whilst ensuring their integrity. The challenges to increased life include ensuring integrity given potential modifications to the floating or fixed assets, modifications to the loading design conditions over their service life, as well as the cumulative effect of fatigue loading on key structural components. Operators and designers increasingly have access to relatively low-cost, high-accuracy instrumentation systems that can be used to monitor the loading and dynamic response of FPSOs and offshore platforms. This paper details a suite of cost-effective ongoing health monitoring measures that can be used to extend the life of producing assets. A design verification process is envisaged to validate original design assumptions and update them against more detailed local site knowledge. Instrumentation can supply health-monitoring data that can be analysed in close to real-time, and which can signal to operators the advent of elevated risk scenarios requiring a higher level of intervention and inspection. This facilitates a methodology for applying risk-based inspections to key components, under which production can be extended while relatively riskier locations are targeted for priority inspections and, if necessary, repair or modification. The technologies investigated here provide cost-effective methods for implementing the concepts of Mooring Integrity Management (MIM) recently expounded by DeepStar®, as well Risk-Based Inspections (RBI). Use of these procedures can allow safe, extended operation of productive assets beyond their original design life at minimum ongoing cost. The techniques will be of significant benefit to offshore operators with ageing assets, as well as to FPSO and fixed platform designers and contractors.