Prediction of fatigue crack growth in offshore structures using a sea state equivalent stress concept

Fatigue testing under simulated service conditions has advantages over constant amplitude tests. For example the variability in both amplitude and frequency content of service loading can he reproduced hy carrying out tests under simulated service conditions. This means that the complex interactions between environment and loading which govern fatigue crack growth mechanisms in offshore structures are taken into account. Existing fracture mechanics models and fatigue crack growth prediction methods generally rely on using the overall equivalent stress range with it suitable crack growth law for fatigue crack growth prediction under variable amplitude lending. For S-N type analysis this method is by far the best when dealing with variable amplitude sequences. However, for fracture mechanics (FM) crack growth prediction employed after an in-service inspection schedule, the use of the overall sequence equivalent stress: range will not allow for sequence effects to he accounted for. These effects can he significant under service loading conditions as crack growth is largely dependent on the stress intensity factor range-which is a function of stress range and crack size. It is possible that the use of the overall sequence equivalent stress concept in a fracture mechanics analysis procedure may not he robust enough to handle the high degree of variability observed in service as crack growth acceleration and retardation can not account for. A different and mole realistic fracture mechanics based approach is required. This paper presents a new fracture mechanics based model for predicting fatigue crack growth in offshore structures. The model relies on the use of measurable sea state properties to determine crack growth associated with each sea state over its duration. It uses the principle of equivalent stress range hut limited to a sea state. This approach is more representative of fatigue damage under service conditions anti has the added capability of allowing the inclusion of any sea state interaction effects which may he anticipated.