Development of Revised Gulf of Mexico Metocean Hurricane Conditions for Reference by API Recommended Practices

Abstract

The hurricane metocean conditions presently contained in the 21st edition of API RP2A are in the process of being revised, to account for both the effects of recent major hurricanes as well as changes in understanding regarding the occurrence of severe hurricanes that has taken place since the RP2A conditions were last updated in 1993. This paper contains information on the process used to develop new hurricane metocean conditions, including the hurricane hindcast information used as the basis for the conditions and the statistical methodology used to analyze the data. A summary of the present draft version of the new conditions is also included. The new conditions, when accepted by API, will eventually be published as part of a separate stand-alone API recommended practice on metocean conditions which can then in turn be referenced by other API recommended practices covering offshore structures and operations, such as those addressing shallow-water fixed platforms, jackups, deepwater platforms, and floating MODUs. New draft hurricane metocean conditions are provided for four regions spanning the northern Gulf of Mexico from southern Florida to southern Texas, for all water depths 7.5 m and deeper. The new conditions between 86.5o and 90o W are significantly higher than the conditions currently contained in RP2A, due to the recent severe storms, most notably Opal (1995), Ivan (2004), and Katrina (2005); however, the new conditions in the other three regions are similar to those currently in RP2A. The new conditions are based on the hurricane record from 1950 to the 2005. Unlike common practice in previous hurricane metocean studies, storms before 1950 were excluded from the statistical analysis because the quantity and quality of data available for those storms is considered too uncertain. The division into four regions recognizes the geographical variability of hurricane conditions across the Gulf of Mexico and is an improvement over the uniform conditions described in RP2A. However, a site-specific metocean study is the only way to more thoroughly account for regional variability in developing design and operational metocean criteria for major development projects. Introduction The arrival of major hurricanes Ivan, Katrina, and Rita, three of the most severe storms on record in the Gulf of Mexico, within two hurricane seasons (2004 and 2005), caused massive interruptions in the production of oil and gas in the Gulf of Mexico due to damage to facilities. Given the impact of these storms, and changes that have occurred in the understanding of hurricane conditions in the Gulf of Mexico since 1993 (the year the conditions in API RP2A were last updated), it was decided by the API Hurricane Evaluation and Assessment Team (HEAT) that the hurricane metocean conditions presently contained in the 21st edition of API RP2A (1) should be re-evaluated and, if necessary, revised. An API Task Group, composed of the authors of this paper, was formed to develop new hurricane conditions. It was also decided that the new hurricane conditions would not be included in API RP2A but would be included in a stand-alone API recommended practice document on metocean conditions, which could then in turn be referenced by other API recommended practices.

Recent structural design considerations related to floating production systems

“…The impact of these hurricanes was considered by Versowski et al (2006), Ward and Gebara (2007), Wisch (2006), Berek et al (2007), Forristal (2007) and Wisch and Ward (2007a,b). Prior to the 2004 hurricane season, offshore practices and standards continued to evolve by incorporating new technology and experience.…”

Parametric Adjustments to the Rankine Vortex Wind Model for Gulf of Mexico Hurricanes

“…At other locations, smaller SWHs were recorded, but they still exceeded the 100-year estimates by several meters. As a result, it has become necessary to reassess the extreme wave climatology in the GOM region (e.g., Berek et al [7]). Similar efforts are underway to reassess extreme storm surge and coastal inundation effects in this region.…”

Detection of changes in variance of oceanographic time-series using changepoint analysis

“…They also note the presence of a near-decadal cycle with peaks at around 1970, 1980, 1995 and 2004-5. As a result, the veracity of the pre-1950 portion of the data set is in doubt, and a number of practitioners currently only use post-1950 data in design (see for example Berek et al (2007); American Petroleum Institute ( 2007)). Similar discussions also surround other databases, see for example Landsea et al (2004).…”