Fatigue reliability analysis of tension leg platform

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 waves and oscillating wind, reliability against fatigue failure becomes important. There are many uncertainties involved in the probabilistic nature of this random process. 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 loading. 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 tether under long crested random sea. Fatigue damage estimation of tether joints is made using Palmgren-Miner's rule (S-N curve approach). The stress ranges employed are described by Rayleigh distribution. Nonlinear limit state functions based on above approach is derived in terms of random variables. First Order Reliability Method (FORM) and Monte Carlo simulation methods have been employed for reliability estimation. The sensitivity of various random variables in overall probability of failure are studied through sensitivity analysis.