Estimating Metocean Environments Associated with Extreme Structural Response to Demonstrate the Dangers of Environmental Contour Methods

TL;DR

This paper critically evaluates environmental contour methods for marine structure design, demonstrating that they often fail to accurately represent the joint environmental conditions associated with extreme structural responses, and proposes a probabilistic diagnostic approach.

Contribution

It introduces a methodology for estimating the joint conditional probability density of environmental variables given extreme structural responses, revealing limitations of existing contour methods.

Findings

Environmental contours often do not accurately characterize the joint environment for extreme responses.

The proposed CDE diagnostic exposes deficiencies in contour-based design approaches.

Full probabilistic analysis provides better insights into environmental conditions linked to structural failure.

Abstract

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.