SS:Metocean "Extreme Hurricane Design Criteria for LNG Developments: Experience Using a Long Synthetic Database"
Authors: Mcconochie Jason Dane , Stroud Stan , Mason Luciano 
Published: July 2010

Tropical cyclones (hurricanes) dominate the extreme wave climate off North West (NW) Australia between latitudes 5 to 25 degrees south, with a storm occurrence rate of 4.5 storms per season. On average, two of these storms attain Category 5 intensity. The NW Australian region of interest has an area equal to the northern half of the Gulf of Mexico, and it appears that hurricane return period wave height criteria in the southern part of the NW Australian region (approximately 20 degrees south), produce return period wave heights of the same magnitude as for the central region of the Gulf of Mexico. A reliable historical hurricane database for that region exists only for the post-satellite era (since 1970), consequently the historical storm database is relatively short.

Use of short historical storm databases for forecasting storm wind and wave fields, and application of extrapolation methods (e.g. peak-over-threshold) for determining return period estimates out to say the 100 year return period are well founded. However extension of this method to obtain estimates at very low probabilities (e.g. to the 1 in 10,000 year return period) involves extrapolating the tail of the distribution. It is recognized this provides highly uncertain estimates at low probability levels.

To overcome the problem of extrapolation, a synthetic database has been developed consisting of approximately 450,000 individual synthetic storms (100,000 years) to represent the hurricane population of the study region. This paper describes development of the synthetic tropical cyclone (hurricane) storm database, and the distinct advantages that it offers as the associated time histories of the wind and wave fields are available at all return periods. This allows examination of storm and wave characteristics at say the 100, 1000 and 10,000 year levels, and application of the storm time histories to obtain response-based criteria.

Introduction

Generally metocean design criteria are established using a database of historical hurricane tracks with central pressures and use of wind and wave field model simulations. Wind and wave maxima from each storm are then subject to extrapolation in logarithmic space. This method has a series of deficiencies. Use of different extreme value methods (e.g. Weibull, Pareto), different fitting techniques (e.g. method of moments, maximum likelihood), and choice of data truncation level, result in different return period estimates, for the same data set at long return periods. Here, the primary deficiency of this method is that it involves extrapolation of the tail of the distribution, where there are few data points. This results in large confidence intervals on the extrapolated return period curves.

In addition, implicit in the extrapolation method is that the behavior of the wind and wave fields do not change from the population of historical storms to that of rare storms. For example, in continental shelf water depths, the effects of wave refraction and water depth wave height limitations present in the hindcasts of historical storms, are not taken into proper account in the extrapolation process. This paper reports on the results and application of a synthetic tropical cyclone study which largely removes difficulties associated with extreme value analysis methods.