ABSTRACT

The use of ‘Joint probability’ in the derivation of environmental design criteria for offshore structures is a subject of increasing interest to offshore engineers, and to those responsible for certification. However, it must not be regarded simply as a means of refining environmental criteria. The use of joint probability concepts rises questions which affect the whole of the conventional design process, from the manipulation and interpretation of measured data through to the design levels of risk which are acceptable for offshore structures. Following a review of selected literature, the paper considers wave/current joint probability in the southern North Sea, and its effect on the choice of environmental design criteria. Example current speed criteria, to be applied simultaneously with the design wave criteria, have been estimated using a simple method based on the bi-variate Weibull distribution, making the assumption that the waves and currents occur independently of one another. The results are used to demonstrate that ‘Joint probability’ cannot be applied without taking account of the design return period, and hence of the issues of risk and reliability.

INTRODUCTION

The Department of Energy (DEn), in ‘Offshore Installations Guidance on Design and Construction, Meteorological and Oceanographic Design Parameters, Revision of Part II, Section 2, ‘Environmental Considerations’, 2nd distributed version, May 1989’, widely known as ‘the Draft Guidance Notes’, states the following:

"Statutory Instrument 1974 No 289, The Offshore Installations (Construction and Survey) Regulations 1974, Schedule 2, Part II, requires that a ‘competent person’ shall assess the combination of metocean parameters to which an (offshore) installation may foreseeably be subjected on the basis of individual parameters which ‘shall not be less than those likely to be exceeded on average once only in any period of 50 years’

In Section 4, ‘Combinations of Extreme Parameters’, DEn offers the following advice

"It is unlikely that extreme values of all individual metocean parameters (wave, current, wind, etc) will occur simultaneously and produce the worst structural effect Allowance may therefore be made for the reduced probability of two or more uncorrelated or partially correlated parameters acting together. However, in making such allowance, due account should be taken of any reduction m those safety factors which are otherwise implicit in designs in which it is assumed that extreme values with a 50 year return period occur simultaneously, which tend to compensate for the fact that individual values with a 50 year return period have a significant probability of occurrence m any year (0 02) Thus, if account is taken of joint probabilities, then unless safety factors elsewhere in the design process are adequate, consideration should be given to the use of individual values with a longer return period than 50 years, together with the appropriate joint probability factors Such factors should be based on adequate information about the joint probabilities of extreme events. Account should also be taken of any uncertainties in measuring or estimating the individual parameters"

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