ABSTRACT:

The evaluation of risk associated with offshore jackets can be quantified only by a satisfactory assessment of the reliability of the structure. In this paper a methodology for the computation of the reliability of the jacket against collapse using point estimation technique is presented. A plane truss under wave loads is used as a vehicle to illustrate the calculation of the reliability. Methods for the computation of the reliabilities of damaged and repaired tubular members are then presented. The evaluation of the reliabilities of the damaged and repaired jacket can be used to justify the need for repairs and to estimate the efficacy of repair.

INTRODUCTION

For offshore jackets subjected to forces of nature, there exists an uncertainty in applied wave loads and strength of the structure. The evaluation of the risk can only be done by computing the reliability (probability of non-failure) of the jacket. An over-designed jacket (reliability too high) will have few structural failures during its lifetime but the huge capital that has been invested can be considered an economic failure. On the other hand, an under-designed jacket (reliability too low) may experience too many structural failures resulting in frequent repairs the cost of which is unacceptable. The optimum balance between the costs and risks cannot therefore be achieved without a consideration of the reliability. Reliabilities of the jacket have been evaluated by Bouma (1979), Rajagopalan (1986), Olufsen (1992) and many others in various limit states, such as elastic, fatigue, buckling and joint failures, collapse etc. This paper presents a method of computing the reliability of the jacket platform in one of its failure modes assumed dictated by the collapse of one of its members. Reliabilities of the jacket due to the damage of the member and subsequent repairs are also evaluated.

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