API BULL 2INT-DG
Interim Guidance for Design of Offshore Structures for Hurricane Conditions
|Publication Date:||1 May 2007|
Hurricane Ivan in 2004 and hurricanes Katrina and Rita in 2005 resulted in considerable damage and destruction to fixed and floating facilities in the Gulf of Mexico (GOM). Several API Committees are in the process of revising and updating standards to incorporate learnings from these and other recent, large, intense storms like Opal (1995).
Also included are other improvements occurring over the past 15 years to the industry's understanding of hurricanes. One major change is a complete revision to the hurricane conditions presently contained in API RP 2A-WSD, 21st Edition, recognizing the higher level of hazard in certain parts of the GOM. Another is the revised understanding of the potential for local wave-in-deck damage. While work on standards development continues, in the interim the following documents are being issued to provide immediate guidance for the design and assessment of offshore Gulf of Mexico fixed and floating facilities in hurricane conditions:
• API Bulletin 2INT-MET Interim Guidance on Hurricane Conditions in the Gulf of Mexico, May 2007.
• API Bulletin 2INT-DG Interim Guidance for Design of Offshore Structures for Hurricane Conditions, May 2007.
• API Bulletin 2INT-EX Interim Guidance for Assessment of Existing Offshore Structures for Hurricane Conditions, May 2007.
API Bull 2INT-DG is being issued concurrently with API Bull 2INT-MET to give guidance, at a high level, on how to utilize the updated hurricane winds, waves, surge and current conditions in API Bull 2INT-MET for the design of offshore structures.
The hurricane metocean conditions presently contained in the 21st Edition of API RP 2A-WSD have not been updated since 1993. Since that time, several major severe storms, most notably Opal (1995), Ivan (2004) and Katrina (2005), have affected the Gulf. Most importantly, however, industry's understanding of hurricane risk has continued to evolve. Strong evidence now exists for a regional dependence for the large, intense wave-making storms. Also, investigations into the underlying hurricane record, HURDAT, used as the foundation for the industry's storm hindcast database, have revealed that storms from the early period of the database are probably biased low in terms of intensity.
API Bull 2INT-MET presents new hurricane conditions for four GOM regions: West, West Central, Central and East, all based on the 1950 through 2005 period of the industry's hindcast database. Differences from hurricane conditions in API RP 2A-WSD, 21st Edition are most pronounced in the Central region; the updated deepwater 100-year return period significant wave height in the Central region is 15.8 m (52 ft), in contrast with the 12 m (40 ft) value implied by API RP 2A-WSD. The differences are primarily driven by the high frequency of intense storms experienced by this region, and to a lesser degree the elimination of the less trusted (pre-1950) portion of the historical hindcast record. Conditions in the other three regions vary slightly from each other, but are close to the values in API RP 2A-WSD.
The main objective of this Bulletin is to provide updated guidance for the use of hurricane metocean conditions in the GOM, particularly in the Central Region and its adjoining transitions.
The content of API Bull 2INT-MET is undergoing extensive review and evaluation. The final results are planned to be included in a new, stand-alone document (API RP 2MET) which will contain the metocean conditions for use with other API design standards. API RP 2MET will also serve as the basis for a revised U.S. Regional Annex in ISO 19901-1.
The focus of this Bulletin is the design of the structures of new, permanent offshore drilling/production platforms in the Gulf of Mexico. The reassessment of the structures of existing permanent production platforms is in the companion API Bull 2INT-EX. MODUs and Jack-ups are discussed in separate documents (API RP 95F and API RP 95J, 2006) and are not included here.
This document is intended to cover the design of the structural systems of the following types of offshore platforms:
1. Steel jacket or template platforms, towers and compliant towers.
2. Minimum non-jacket and special structures (including caissons) defined in API RP 2A-WSD.
3. Tension Leg Platforms.
4. Moored, Floating platforms (semi-submersible shaped, spar shaped, ship shaped).
This Bulletin is organized with a separate section for each of the applicable reference standards. While some issues are common, this provides a path for independently revising or withdrawing parts of this Bulletin.
A part of this Bulletin shall be considered withdrawn only if:
1. The Bulletin is withdrawn in its entirety, or
2. A standard listed in 1.5 is revised, and the new edition contains a specific statement declaring the relevant part of this Bulletin superseded.
A commentary is included to explain the reasons for selecting the values for this Bulletin.
As for the future, the API Hurricane Evaluation & Assessment
Team (HEAT) is continuing its orderly work on metocean conditions,