Inverse Directional Simulation: an environmental contour method providing an exact return period

This study presents an improved approach for drawing environmental contours, derived from the inverse First Order Reliability Method (iFORM). It is shown that the approach used in the iFORM method is not accurate as it only takes into account a fraction of the probability space behind the contour line-which means that the iFORM method provides inherently non-conservative environmental contours. iDS, an alternative solution based on the chi-square distribution is suggested, and its adequacy is demonstrated with a Monte Carlo simulation.

An expression is derived for computing sector-constrained environmental contours where the exceedance probability is contained within specific range of interest for one or more variables. The newly suggested environmental contour methods are applied on measured wind and turbulence time series and compared with the iFORM method.