Cyclic Cohesive Zone Model (CCZM)-Based Fatigue Crack Initiation Prediction for a Mooring Chain

Offshore mooring chains are generally subjected to cyclic loads induced by the ocean environment and fatigue failure of mooring chains occurs with high frequency. For safety reason, it is necessary to perform fatigue assessment for offshore mooring chains during the design phase. In this study, a cyclic cohesive zone model (CCZM) is constructed for fatigue crack initiation prediction for a mooring chain. Different from the traditional approaches such as S-N curves and T-N curves, the material fatigue is regarded as an irreversible and continuous process and it can continuously take into account stiffness degradation of the material and non-linear accumulation of fatigue damage during the fatigue process. In the cyclic cohesive zone model, the constitutive equation called as traction-separation law is adjusted to linear unloading according to the stress states of a mooring chain, and the accumulated fatigue damage is then measured by a damage evolution equation where the fatigue damage is quantified by material deformation weighted by loading. A case study on fatigue crack initiation prediction for a mooring chain is performed to demonstrate the capability of the cyclic cohesive zone model. The life of fatigue crack initiation predicted by the cyclic cohesive zone model is then compared with experimental data and the life calculated by method from industrial standard. Furthermore, an analysis is carried out to predict the fatigue crack initiation of a mooring chain with a corrosion pit. Results of this study demonstrate that the cyclic cohesive zone model has the capability to reasonably predict the fatigue crack initiation of an offshore mooring chain.