ABSTRACT:

In this paper a method is presented for reliability assessment of aged jacket structures considering increase of extreme wave loads due to marine growth and corrosion and deterioration of rigidity and resistance due to corrosion. Structures are modelled as spatial frames subjected to quasi-static extreme loads. and reliability assessment is performed based on ultimate strength analysis. The applicability of the proposed method is demonstrated through a numerical example for a typical platform under a notional extreme seastate.

INTRODUCTION

This paper deals with the system reliability of aged jacket structures considering increase of extreme wave loads due to marine growth and corrosion and deterioration of rigidity and resistance due to corrosion. The process of reliability assessment of aged jacket structures is demonstrated in Fig.1 and the conceptual variation of system reliability with age of structures is also illustrated in Fig. 2[1-6]. Extreme wave loads are estimated by using Stokes wave theory and Morison" s formula considering aged effects. Probabilistic properties are evaluated by means of the First-Order Second-moment method [6]. Structures are modeled as spatial frameworks and reliability assessment is carried out based on ultimate strength analysis. Ultimate collapse is evaluated by using a linearized failure condition of the section under the combined effect of biaxial bending moment and axial force to generate safety margins. 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[1.6]. Those methods are applied to a typical jacket structure under a notional extreme seastate. Through a numerical example, the applicability of the proposed methods is demonstrated.

ESTIMATION OF EXTREME WAVE LOADS

The coefficients are dependent on many factors. such as Reynolds number, Keulegan-Carpenter number the relative roughness of member surfaces. etc. which are difficult to exactly estimate in practice.

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