ABSTRACT

A reliability approach for the selection of design criteria of rigid fixed offshore platforms under wave actions is presented. Severe storm occurrence probabilities using the uniform Poisson process are calculated. The load intensity is described by the distribution of wave heights generated by severe storms. The estimated design life and the reliability required can be used as input parameter of the model.

INTRODUCTION

It is generally recognized that design of offshore structures requires a resolution of uncertainties. Developing design criteria in view of these uncertainties a certain risk has to be accepted /1,2/. The formulation of such criteria involves the prediction of expected load intensities of acting forces based on extrapolation from past experience. The load intensities may be caused by a combinatiol of forces resulting from waves, wind, currents, earthquakes etc .. Since it may be assumed the critical forces on offshore structures like fixed platforms are the lateral forces developed by waves it is first attempted to treat the aspect of risk assessment due to wave action. Wind-induced waves of engineering 'significance may occur during severe storms, i.e. hurricanes, cyclones etc. from which load intensity distributions may be derived. Since. the integrity of a structure is require for a period of time (i.e. design lif.e of the structure) rather for a specific storm the frequency of arrival of significant storms must be estimated and considered in the formulation.

In this analysis structures are assumed to be rigid with a deterministic resistance. Due to its simplicity in application the concept of the return period is still widely used as a risk criterion among engineer although it is prone to misinterpretation /3/. A more refined model, the reliability function /4/, will be introduced and applied here for that purpose.

RELIABILITY CONCEPT
General

The reliability concept /4/ is based on the fact that the resistance R of a structure associated with a specified type of applied load may be considered a random variable, and the applied load S, such as wave loading, is also a random variable. The probability of failure Pf under a single load application is then defined as the probability that the resistance R, chosen at random from the population described by the cumulative distribution function FR (x) is smaller than the load S selected at random from the population FS (x). If stochastic independence between Sand R is assumed, then

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where pea) is the probability of the event A and fS (x) is the probability density function (PDF) of the load S. In many cases it may be assumed that the standard deviation of the resistance R is small compared with that of the load S. In other words, the resistance R is not a random variable but a constant Ro.

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