Global distribution and risk to shipping of very extreme sea states (VESS)

The impact of extreme sea states on offshore infrastructure is of intense interest in the ocean engineering community at the present time. In this study, a new quality-controlled database of global satellite-derived estimates of significant wave height (HS) and surface marine wind speed from seven missions spanning the period August 1991-March 2010, known as GlobWave, is scanned to yield over 5000 ocean basin specific orbit segments with peak HS > 12 m. This population was subsequently distilled to a population of 120 individual storms [so-called very extreme sea states (VESS) storms], in which there was at least one altimeter estimate of HS > 16 m. The highest HSs were observed in the Northern Hemisphere with ten orbit segments in the North Atlantic Ocean (NAO) with a peak HS of >18 m followed by four segments in the North Pacific Ocean (NPO). Only three HS peaks >18 m were seen in the entire Southern Oceans. Three of the >5000 orbit segments had a peak HS >20 m with the highest at 20.6 m. The number of VESS storms detected is greatest in the NAO (the smallest basin), a result that appears to be consistent with general circulation studies of extratropical cyclogenesis frequency and intensity in general atmospheric circulation models. A new continuous 33-year global wave hindcast (GROW2012) based on a new atmospheric reanalysis wind field product appears to provide unbiased estimates of the probability of exceedance of VESS in extratropical storms and small-basin dependent biases (0.5-1.5 m) of peak HS greater than approximate to 16 m. GROW2012 was, therefore, applied with a voyage simulator for nine trade routes to assess the risk of a merchant vessel that does not avail itself of weather routing of encountering VESS. The highest monthly exceedance probabilities (MEPs) at the VESS threshold of 14 m are found in the NAO and NPO at approximate to 0.1% during winter months. The alternative statistical distribution of the MEP of the single maximum peak sea state to be expected for a voyage for any month (MEPm) shows that the highest MEPm is for the month of December in the NAO along the great circle route between the middle US East Coast and entrance to English Channel and in the NPO along the Yokohama-Seattle route, at about 3%. Still, the overall probability of a vessel encountering sea states that may contain waves capable of catastrophic damage over a 33-year lifetime is quite small with mean number of hours of exposure to a vessel typically less than approximate to 3 h for six of the nine routes and a maximum of 10 h for two of the routes.