Correlation of Finite Element Analysis FEA Predicted Residual Strength of Degraded Offshore Mooring Chains with Test Data

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 establish rational discard criteria, is the ability to accurately predict the residual strength of degraded chain, and to have as a benchmark for loss in strength, an accurate estimate of the chain in its as-new condition. With a correlated FEA method for residual strength prediction and a benchmark for as-new condition capacity, it would then be possible to establish a theoretical relationship between different types of degradation and mooring chain capacity loss, from which rational discard criteria would be derived. To this end the Chain FEARS JIP first developed a Finite Element Analyses (FEA) residual capacity assessment method to accurately predict the residual strength of degraded chains. A number of assessments were carried out to establish the sensitivity of the Predicted Break Load (PBL) to both engineering parameters such as friction coefficient, and numerical modelling techniques. The developed method was validated by the correlation of the PBL against a number of physical break tests. This paper presents a review of the break strength test data of pitting corrosion degraded chain links. The FEA modelling methodology based Predicted Break Load (PBL) are compared with the test data Actual Break Load (ABL) along with the sensitivity of engineering parameters and numerical model modelling techniques on predictions. The developed FEA method accurately predicts the location of the ‘failure’ within the chain string and the ductile necking failure mode, determined to be the prevalent mode of failure for the chain links samples considered in this study. The degree of correlation between PBL and ABL confirms that accurate prediction of the effects of corrosion degradation consequent on uniform and large pitting corrosion can be accurately predicted by use of the Finite Element Method. The developed FEA method was also employed to establish a benchmark for the strength capacity of as-new condition links as presented in [1], the basis for assessing the relationship between corrosion degradation and residual chain link capacity [2] and a basis for a multi-axial fatigue assessment method to establish the fatigue capacity of as-new and degraded chain links [3,4,5].