Fatigue is one of the main causes of failures in mechanical and structural systems. Offshore installations, in particular, are susceptible to fatigue failure due to their exposure to the combination of wind loads, wave loads and currents. In order to assess the safety of the components of these installations, the expected lifetime of the component needs to be estimated. The fatigue life is the sum of the number of loading cycles required for a fatigue crack to initiate, and the number of cycles required for the crack to propagate before sudden fracture occurs. Since analytical determination of the fatigue crack propagation life in real geometries is rarely viable, crack propagation problems are normally solved using some computational method. In this review the use of the finite element method (FEM) and the extended finite element method (XFEM) to model fatigue crack propagation is discussed. The basic techniques are presented, together with some of the recent developments.

As offshore structures are reaching their original design life, effective repair and life-extension techniques are required, in order to ensure continued safe operation. A stop hole may be drilled at the end of a crack, in order to delay further fatigue crack propagation. To compare the stop hole technique with other relevant repair techniques, its effect needs to be modelled. Here, a procedure for modelling the stop hole-induced fatigue crack growth delay for a crack propagating under mixed-mode I + II conditions is presented. The procedure combines the S–N curve for fatigue crack initiation and the Paris law for fatigue crack propagation, with models for mixed-mode crack propagation and finite-element analysis of the cracked geometry. The failure criterion inherent in the S–N curves is used to define the transition between initiation and propagation. The procedure is implemented on an example, and the importance of considering the mixed-mode conditions is indicated.