Stochastic Fatique Analysis of Jacket Type Offshore Structures

摘要

In this paper a stochastic reliability assessment for jacket type offshore structures subjected to wave loads in deep water environments is outlined. In the realiability assessment, structural and loading uncertainties are taken into account by means of some stochastic variables. To estimate statistical measures of structural stress variations the model spectral analysis method is applied. The analysis is divided into four steps: a description of the sea state, (including a short-term and a long-term model), a description of the wave loading, a structural analysis and a fatigue analysis. A method to estimate the reliability of offshore structures subjected to wave loads in deep water environments is presented. Failure modes corresponding to fatigue failure are used. The reliability is estimated using a first-order reliability method. The statistically measured of structural stress variations is estimated by using model spectral analysis method. The damage in the failure elements is estimated by using Miner's rule and S-N approach. The distribution function of the stress amplitudes, for a given sea state, is estimated by Rayleigh distribution, Rice distribution, RFC-method and RC-method. For most of the sea states under consideration the stress process became broad-banded and therefore, the results by using Rayleigh distribution cannot be expected to give satisfactory results. The RFC and RC methods give a different estimation of the distribution function of the stress amplitudes, resulting in a major difference in the estimate of the safety indices. This means that the order and definition of the stress amplitudes must be taken into account in the damage accumulation model. For this purpose more experimental and theoretical work are needed. (AB) 16 refs.