Rapid Residual Strength Assessment of Degraded Mooring Chains Using an FEA Based Response Surface Modelling Tool

Abstract

Abstract Design of moorings with chain sections typically assume a level of uniform through-life corrosion and wear. The associated reduction in strength is, in simplistic terms, estimated by assuming a smaller as-new chain diameter. Where inspections show chain links to have localised material loss the resultant reduction in strength is harder to account for as it is dependent on the volume, shape and location of the loss. Discard criteria found in design standards are necessarily conservative as they must encompass the most damaging combinations of size, shape, and location of material loss. As a result, operators have turned to Finite Element Analysis (FEA) to directly assess the effect of localised chain material loss. Whilst this approach has been shown to be effective it incurs significant computational cost and is time consuming and expensive when compared to the application of these simple discard criteria. The Corroded Chain Analysis and Structural Evaluation (CCASE) tool has been developed to leverage the benefits of FEA while reducing significantly the time and cost associated with a chain anomaly assessment, providing a rapid prediction of the degraded chain’s residual strength within minutes. CCASE utilises the results of FEA conducted previously for the Chain FEARS JIP as well as newly performed FEA for pitting, seabed, hawsepipe, and interlink wear anomalies to generate a Response Surface Model (RSM) of Break Load Prediction. The geometry of the degraded chain links is input into CCASE via a point cloud file typically obtainable by surface or subsea photogrammetry scanning methods. These chains are then characterised by the tool, relative to an idealised chain link, at a number of cross sections around the geometry. These characteristic parameters, when used with the Response Surface Model (RSM), predict the break load of the degraded chain link. The CCASE tool was able to predict the break load of test degradation geometries, independent of those used to develop the RSM, to within approximately 5% of the break load predicted by the FEA approach for the cases tested and within a similar margin for two cases involving real-world degraded chain pulled to destruction on a test rig. CCASE can thus be used to perform rapid, cost effective engineering assessments of chain anomalies in place of traditional conservative discard criteria. This gives an operator a clearer decision path to continue to monitor chain anomalies or commit to expensive replacement works.