ABSTRACT:

This paper deals with the probabilistic analysis on the quasi-static strength and the natural vibration of composite and slender offshore structures for deep water, which are treated as a simple tower model by replacing its tower frameworks and platform with the beam-columns with equivalent stiffness and strength, respectively. First, reliability assessment of the structure under the quasi-static extreme wave loads is performed by using the reliability analysis program package developed for spatial frameworks based on ultimate strength analysis. Next, the effect of uncertainty on the natural frequencies is discussed by using a probabilistic sensitivity analysis method. Finally, the applicability of the proposed methods is demonstrated through the numerical example for a 450m guyed tower.

INTRODUCTION

In this paper, a method is presented for the probabilistic analysis on the quasi-static strength and the natural vibration of slender and composite offshore structures for deep water. The composite structure is dealt as a simple tower model made by replacing its tower frameworks and platform with the beam-columns with equivalent stiffness and strength[I-9]. First, the reliability assessment of the structure under quasi-static extreme wave loads is performed by using the reliability analysis program package developed for spatial frameworks based on ultimate strength analysis. Probabilistic properties of quasi-static extreme wave loads are estimated by using Morison" s formula and the First-Order SecondMoment method. Collapse strength of the structure under a notional extreme sea-state is evaluated by using a matrix method combined with a plastic node method. Probabilistically dominant failure modes are selected by applying the so-called branch-and-bound method [10-12]. Next, the effect of uncertainty on the natural frequencies is discussed by using a probabilistic sensitivity analysis method. Finally, a numerical example of a guyed tower is provided to demonstrate the applicability of the proposed methods.

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