The problem of adequate yield load solutions in the context of proof tests on a damaged subsea umbilical

Abstract

The residual lifetime and fitness for service of a pre-damaged tube of a subsea umbilical was determined. The tube had experienced large ovalisation and re-rounding by applying a boost pressure prior to service. The aim of this paper was to describe and to critically discuss the methodology of the fracture mechanics analysis required in addition to leak tightness testing. Special emphasis was put on the reversal of conservatism between the two steps: (a) Determination of the maximum crack size in the component which just would not have caused component failure under the boost loading applied for re-rounding the tubes, and (b) Prediction of the residual lifetime of the component under in-service loading assuming the existence of the maximum possible crack size of step (a). A number of yield load solutions for axial cracks in pressurised hollow cylinders was applied which yielded quite different results. The fatigue crack extension analyses were performed for various crack depth-crack length combinations which just would have survived the boost pressure. For both the simulation of crack propagation under service load and the prediction of the end-of-life state residual stresses due to the pre-history of the tube were considered. The information on these stresses was provided by finite element simulation.