  Environmental contours are used in structural reliability analysis of marine
and coastal structures as an approximate means to locate the boundary of the
distribution of environmental variables, and hence sets of environmental
conditions giving rise to extreme structural loads and responses. Outline
guidance concerning the application of environmental contour methods is given
in recent design guidelines from many organisations. However there is lack of
clarity concerning the differences between approaches to environmental contour
estimation reported in the literature, and regarding the relationship between
the environmental contour, corresponding to some return period, and the extreme
structural response for the same period. Hence there is uncertainty about
precisely when environmental contours should be used, and how they should be
used well. This article seeks to provide some assistance in understanding the
fundamental issues regarding environmental contours and their use in structural
reliability analysis. Approaches to estimating the joint distribution of
environmental variables, and to estimating environmental contours based on that
distribution, are described. Simple software for estimation of the joint
distribution, and hence environmental contours, is illustrated (and is freely
available from the authors). Extra assumptions required to relate the
characteristics of environmental contour to structural failure are outlined.
Alternative response-based methods not requiring environmental contours are
summarised. The results of an informal survey of the metocean user community
regarding environmental contours are presented. Finally, recommendations about
when and how environmental contour methods should be used are made.
