The effect of structural compliance on fatigue crack growth in jacket structures

Quantifying the fatigue crack growth and remaining life in joints making up jacket structures forms one of the basic requirements of a sub sea structural integrity assurance scheme. The accurate prediction of the likely failure time of welds allows a realistic estimate of the risk of structural collapse at any stage in a structure's life. It is current practice to consider the welds making up the member as individual components rather than looking at the whole compliant system of welds, joints, members, and structural framing arrangement. In this approach, the nominal loads in any one member are determined from an analysis of the undamaged structure and are then applied to a crack growth solution using handbook stress intensity factor solutions to determine the fatigue life of that component. This method assumes that the applied load is purely load-controlled whereas in reality it is a combination of both load and displacement controlled. A study was performed to investigate the influence of the surrounding structure on crack growth in tubular members located in jacket structures. The aim of the study was to verify whether the traditional approach, which uses stresses from undamaged structures to evaluate crack growth in individual components, is appropriate. The findings of the study showed that structural compliance has only a beneficial effect on fatigue growth in the latter stage of the crack growth process with crack lengths greater than 40% circumference. It was shown that the beneficial effect of structural compliance on fatigue crack growth in the later stage of the growth process does not significantly increase the overall fatigue life. It was concluded, that the current approach, which uses stresses from undamaged structures to evaluate crack growth in individual components, is valid and safe to use.