Use of RP 2MET Annex Gulf Metocean Conditions with 2A and 2SIM
Authors: Stear James 
Published: April 2012

The new API RP 2MET contains indicative metocean conditions in a manner similarto those previously provided in the 21st and earlier editions of RP 2A. Whileuse of site-specific metocean conditions is preferred for all design andassessment purposes, as they reflect the best estimate of the environmentalhazard, the indicative 2MET conditions may be used with the new editions of 2Aand 2SIM for certain structure classes and water depths as an alternative tousing those from a site-specific study. This paper briefly reviews the basis ofthe RP 2MET Gulf of Mexico hurricane and winter storm annex values, comparesthem with previous RP 2A Gulf metocean conditions, including those previouslycontained in Section 17, and provides examples of their application.

Introduction

RP 2MET, a new API recommended practice applicable to the development ofmetocean design and operating conditions, is planned for publication in 2012. Layout of the new document has been previously described in OTC 20693 (Stear,2010). Aligned with ISO 19901–1 " Petroleum and Natural Gas Industries -Specific Requirements for Offshore Structures - Part 1: Metocean Design andOperating Considerations," the core of the document describes various metoceanphenomena that must be considered when designing offshore structures, andprovides guidance for using measurement and hindcast data sets to developdesign criteria. In addition to the core guidance, a series of regional annexescontaining indicative metocean conditions for many areas of the world are alsoprovided. The quality of the numbers on the regional annexes variessubstantially, with most being indicative only, i.e. suitable for conceptstudies but not to be used for actual design.

An exception to this are the hurricane and winter storm conditions provided inthe Gulf of Mexico regional annex. Following on the directions of API SC 2, these have been developed to replace the indicative metocean conditionspreviously included with 21st and earlier editions of RP 2A, and as with thoseearlier values may be used with the new editions of 2A and 2SIM for certainstructure classes and water depths as opposed to using metocean criteria from asite-specific study. Use of these conditions for final design of deep water ordynamic systems is not acceptable, as they do not represent the full range ofconditions and variability that could be encountered.

This paper briefly reviews the basis of the RP 2MET Gulf of Mexico hurricaneand winter storm annex values, and compares them with RP 2A 21st edition Gulfmetocean conditions including those previously contained in Section 17. Severalexamples of their extraction for use with new and existing fixed platforms inwater depths less than 120 m are provided.

Basis of 2MET Gulf Annex Values

RP 2MET Annex C.4 contains indicative Gulf of Mexico extreme metoceanconditions resulting from hurricanes, winter storms, the Loop Current, eddiesand Topographic Rossby waves, along with example air and sea temperatures. Anexample Hs-Tp frequency table of the long-term wave environment and two monthlywind roses are also provided. These values are intended to provide an overviewof conditions which may be encountered in the Gulf, and are available forreference by other API recommended practices. The hurricane and winter stormconditions, which govern fixed platform design and assessment, are summarizedbelow. These conditions replace the indicative values previously published inRP 2A and Bulletin 2INT-MET.