  Extreme value analysis (EVA) uses data to estimate long-term extreme
environmental conditions for variables such as significant wave height and
period, for the design of marine structures. Together with models for the
short-term evolution of the ocean environment and for wave-structure
interaction, EVA provides a basis for full probabilistic design analysis.
Alternatively, environmental contours provide an approximate approach to
estimating structural integrity, without requiring structural knowledge. These
contour methods also exploit statistical models, including EVA, but avoid the
need for structural modelling by making what are believed to be conservative
assumptions about the shape of the structural failure boundary in the
environment space. These assumptions, however, may not always be appropriate,
or may lead to unnecessary wasted resources from over design. We demonstrate a
methodology for efficient fully probabilistic analysis of structural failure.
From this, we estimate the joint conditional probability density of the
environment (CDE), given the occurrence of an extreme structural response. We
use CDE as a diagnostic to highlight the deficiencies of environmental contour
methods for design; none of the IFORM environmental contours considered
characterise CDE well for three example structures.
