The Ergodicity Assumption for Sea States in the Reliability Estimation of Offshore Structures

Abstract

Various schemes for the computation of structural reliability in the presence of ergodic wave load processes, ergodic sea-state sequences and simple (nonergodic) random variables such as strength parameters or statistical uncertainties are investigated for both nondeteriorating and deteriorating structural properties. It is assumed that at least conditional outcrossing rates can be computed. It is found that rigorous formulations can require substantial numerical effort even in the context of FORM / SORM computation schemes as they require nested integration. However, for high reliability and not too large variabilities of the nonergodic variables, simpler computation schemes can be used. In the nondeteriorating case, the ergodic theorem can be used with advantage, while in the deteriorating case, Jensen’s inequality can be applied to arrive at simple and appropriate computation schemes.