Implications of Hurricane Ivan on Deepwater Gulf of Mexico Metocean Design Criteria

摘要

Hurricane Ivan (September 2004) generated the highest wavesrnever measured or hindcast in the Gulf of Mexico. Thernimplications of Ivan on deepwater Gulf of Mexico metoceanrndesign criteria are examined from three aspects: (1) whetherrnthere is an obvious deficiency in the ODGP wind and wavernhindcast model, (2) whether Ivan generated “freak” conditionsrnthat break the NTS/API gust factor relationships and windrnspectra characterizations or the Forristall wave height andrncrest height distributions, and (3) whether Ivan is, from arnstatistical standpoint, truly an “unexpected” event. To addressrnthe first two issues, wind and wave measurements collected onrntwo industry platforms near Ivan’s track are examined.rnExcellent comparisons are found between the industryrnmeasurements and the ODGP hindcast model, which furtherrncorroborate the findings of [2]. Analysis of the industry windrnmeasurements suggests NTS/API gust factors are valid forrnIvan, while the NTS/API spectra are found to agree reasonablyrnwell with the measured spectra except perhaps for a tendencyrnof API to overestimate low-frequency energy. Measured wavernheights and wave crests are found to fit the Forristallrndistributions quite well, providing no support for “freak”rnwaves during Ivan. To address the last issue, the Gulf ofrnMexico hindcast record is examined. The maximum wavernheights generated by Ivan correspond to roughly a 2,500-yearrnlocal return period, assuming the extremal distribution beforernIvan was correct. Adding Ivan to the existing hindcastrndatabase is found to raise 100-year Hs values close to thernstorm track by 1.0 m or more. Through consideration ofrnhurricane encounters in a finer spatial sense (100 km zones)rnrather than simply considering the entire Gulf as a singlernencounter zone, a strong argument can be made that thernconditions generated by Ivan could reasonably be expected atrna local level within the context of 105 years of hurricanernexperience throughout the Gulf. However, while Ivan may berndescribed as an expected event, prudence dictates that windsrnand waves generated by Ivan be included in the hindcastrndatabase used for future Gulf of Mexico designs, as opposedrnto treating them as local outliers to existing extremalrndistributions and neglecting them.