Accurate and reliable estimates of probabilities of rare, extreme metocean events are critical for optimal design of offshore facilities. They ensure facilities are neither over- nor under-designed, allowing target reliability levels to be achieved without undue conservatism. Engineering design requires metocean parameters to be specified with a return-period of 100 years, but often specification to a return period of 10,000 years is required. Modern hindcast data bases, typically used to derive extremal criteria, can be of limited extent, consisting of data for a few decades; hindcasts with periods of more than 50 years remain unusual. In addition, metocean criteria are often stratified by covariate - seasonality or directionality are common examples. Further, specification of joint occurrence of parameters at long return periods is necessary to avoid excessive conservatism, and such criteria may also need to be specified as functions of one or more covariates. Methods used by practitioners to meet these requirements are often somewhat ad-hoc, based on experience and intuition. In this paper we review recent applications which add statistical rigour and consistency to the estimation of design values. In particular, we present methods for maximising the benefit of limited data sets and deriving consistent extremal criteria with covariate, resulting in criteria that are consistent with respect to multiple covariates, including space, time and directionality.

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