Investigations Into Break Strength of Offshore Mooring Chains

Abstract

Abstract The first phase of the Chain FEARS (Finite Element Analysis of Residual Strength) Joint Industry Project (JIP) aimed to develop guidance for the determination of a rational discard criteria for mooring chains subject to severe pitting corrosion which, based on current code requirements, would otherwise require immediate removal and replacement. Critical to the ability to evaluate the residual strength of a degraded chain, is to have an accurate estimate of the chain in its as-new condition, providing a benchmark for any loss in strength associated with severe corrosion or wear. A Finite Element Analyses (FEA) residual capacity assessment method was developed and correlated against available break strength test data of degraded links as part of the JIP. FEA were conducted of as-new chain links in accordance with this validated methods, which when compared with current Class Rules specified Minimum Break Load (MBL) demonstrated a significant disparity between Predicted Break Load (PBL) and the codified MBL, particularly for larger chain sizes. It was identified, consequent on this inconsistency in MBL with respect to chain size that inconsistency in proof loading would also result for chains of larger size. Consequently further investigations were carried out with the objective of:Determining an alternative formulation for the break strength of studlink and studless chain, thereby allowing comparison with the current Class Rules break strength formulation.Assessing the extent and effect of inconsistencies in the Conventional Proof Load FormulationEstablishing the origin and the technical basis for the Conventional MBL formulae.Assessing the validity of the Code MBL with respect to Actual Break Load (ABL) of common studlink mooring chain and the implications of using an alternative Amended Formulation for as-new chain strength. The basis for the determination of the current Class Rules on chain strength is established and in doing so serves to explain the a significant disparity between PBL and the codified MBL that is evident particularly for larger chain sizes. An alternative formulation for the break strength of as-new studlink and studless chain is presented. It is recommended that further investigation be conducted to establish an alternative proof load formulation optimized for fatigue endurance.