Fatigue and fracture reliability of TLP tethers under random loading

Abstract

Tension leg platform (TLP), a deep water oil exploration offshore compliant system, is vulnerable to considerable fatigue damage over its design life period due to the dynamic excitations caused by the oscillating waves and wind. In the presence of random loading like, that produced by the wind and waves, reliability against fatigue and fracture failure becomes important. The reliability assessment against fatigue failure considers the uncertainties associated with the parameters and procedures employed for the fatigue damage estimation. This fatigue estimate requires a dynamic response analysis under various environmental loadings. In the present study a non-linear dynamic analysis of the platform has been carried out for response calculations. The response histories so obtained have been employed for the study of fatigue reliability analysis of TLP tethers under long crested random sea and associated wind. Fatigue damage estimation of tether joints is made using Palmgren–Miner's rule ( S– N curve approach) and fracture mechanics approaches. The stress ranges employed are described by Rayleigh distribution. Non-linear limit state functions based on above two approaches are derived in terms of random variables. First order reliability method (FORM) and Monte Carlo simulation technique have been employed for reliability estimation. The influence of various random variables on overall probability of failure has been studied through sensitivity analysis.