A Method for Estimating the Fatigue life of a Conductor Pipe
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The purpose of this new method is to calculate the probability of failure by fatigue of a free standing conductor pipe subjected to waves. This combines two approaches: a stochastic approach using statistical distribution of sea wave conditions and of fatigue crack initiation time, and a deterministic approach using standard nonlinear finite element analyses to compute the nonlinear transient response of a conductor pipe to a given set of irregular waves. Two types of nonlinearity are of importance: Soil rheology leading to a nonlinear stiffness matrix, and wave loading due to the nonlinear term in the Morison equation which is proportional to the square of the relative velocity between the flexible structure and the water particles. The method described takes into account the sources of randomness, the uncertainties and the nonlinearities. A fatigue analysis using this method has been carried out for a typical conductor pipe in the Gulf of Guinea. A dozen dynamic analyses using generated waves described by different wave heights and associated period are needed because of nonlinearity and since the stochastic approach required computing the response of the structure to any irregular wave. For this study, results showed that the conductor pipe is still safe while the usual deterministic approach leads to a damage structure.