Stress Intensity Factors in Fitness-for-Service Assessment of Cracks in Mooring Chains

Abstract

Chain corrosion-fatigue often governs the design of the mooring systems of floating production units, and it is assessed based on a stress-life or S-N approach. For integrity management along the lifetime of the assets, a Fracture Mechanics (FM) approach can be more appropriate, especially if cracks were detected during in-service inspections. An essential parameter of FM is the Stress Intensity Factor (SIF), which becomes fundamental to evaluate the response of a cracked link. The aim of this paper is to present SIF results for chains under different combinations of cracks and loads. Analytic or handbook solutions exist and provide accurate SIFs for simple geometries and test specimens. However, the particular geometry of chain links makes the Finite Element Method (FEM) more appropriate. The authors utilized the contour integral method together with the general purpose nonlinear FEM code Abaqus, to carry out a large amount of analyses to obtain SIFs for different chain sizes, material grades, crack shapes, crack depths and crack locations along the links. In addition to this, SIFs were derived in combination with other degradation phenomena such as large pitting or interlink wear. As an outcome, empirical equations were developed to predict the SIFs of propagating cracks in mooring chains under a large variety of scenarios. This allows engineers to assess the criticality of fatigue cracks using suitable crack growth models, and hence evaluate their fitness-for-service or need to implement remedial actions.