ABSTRACT

The paper investigates the performance of a simplified procedure called Turkstra's rule for estimating the extreme values of a linear combination of load effects By applying this procedure to the case of the horizontal surge oscillations of ega TLP structure it is shown that it may lead to significant under prediction of the total extreme response

INTRODUCTION

In design considerations of eg offshore structures, a typical difficulty is the load effect combination problem, which may be of the form How to estimate m a correct manner an extreme combined load effect due to the environmental forces on the structure during a designs situation, for example a 100- year storm The complexity of this problem is demonstrated by the fact that no generally valid accurate procedures that are practically useful, have so far been developed Instead, several rather coarse approximate procedures have been proposed, see e g Pearce and Wen (1981) for a discussion of some of the most Important combination rules for application In design

In the development of procedures for combination of extreme load effects to be used In codified design, one is often faced with two conflicting demands - on the one hand that of simplicity and robustness of application, and on the other hand that of accuracy The picture may be further complicated by the fact that one is not able to fully verify the Inherent accuracy of a proposed procedure for estimating design values of a load effect combination formula There may of course be several factors contributing to this, of which the following two are often important 1) Lack of precise knowledge of the properties of the load effect components and their combination 2) An accurate analysis of the extreme values of the combined load effect is beyond practical reach

In this paper the aim is to investigate the performance of a class of simplified procedures for estimating the extreme values of the combined first-order and second-order, slowly-varying wave Induced response of compliant offshore structures Specifically, the man object of study is a class of procedures whose members are usually referred to as a Turkstra's rule

THE LOAD EFFECT COMBINATION PROBLEM

The load effect combination problem to be investigated In the following can be expressed as Equation (1) (Available in full paper)

where Z1(t) and Z2 (t) are assumed to be stationary stochastic processes In particular we shall be concerned with the case where Z(t) is the total wave induced surge motion of e g a TLP structure Z1(t) then denotes the first-order response and &(t) the slow-drift response

The goal of the subsequent analyses is to discuss some specific methods for predicting extreme values of the total response process Z(t) Specifically, the performance of the so-called Turkstra's rule (Turkstra, 1970, Wen and Pearce, 1981) will be assessed

Let the largest value of Z(t) during a time Interval of length T be denoted by Z(t), that is Equation (2) (Available in full paper)

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