API BULL 2INT-EX: Interim Guidance for Assessment of Existing Offshore Structures for Hurricane Conditions
SDO: API: American Petroleum Institute
DOD Adopted ANSI Approved Approved
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) as well as other improvements to the industry's understanding of hurricane risk which have occurred over the past 15 years. 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.
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) that 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.
API Bull 2INT-EX 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 assessment of existing offshore structures. The design of new permanent structures is contained in the companion API Bull 2INT-DG.
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, resulting in increases to local extremes in the areas affected by these storms. Most importantly, however, industry's understanding of hurricane risk has continued to evolve. Strong evidence now exists for there being a regional dependence for 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 for existing structures, particularly in the Central Region and its adjoining transition regions.
This document is intended to cover the design of the structural systems of the following types of offshore structures:
1. Steel template platforms and compliant towers.
2. Minimum non-jacketed and special structures (including caissons) defined in API RP 2A-WSD.
3. Tension Leg Platforms (TLPs).
4. Moored, floating platforms (semi-submersible shaped, spar shaped, ship shaped).
USE OF EXISTING ASSESSMENT RESULTS
Specific assessment approaches using structural analyses are described in this document. In many cases, results from an existing assessment or structural analysis that was previously performed on the structure or a similar structure can be used in lieu of the assessments described herein. In such cases, the previous studies should be representative of the structure's current configuration and condition.
This Bulletin is applicable for existing offshore structures located in the Gulf of Mexico at the time of the publication of this Bulletin. The guidance in this document is not intended for use in designing new platforms. For the design of new platforms, see API Bull 2INT-DG.
Platforms designed according to API Bull 2INT-DG should not use this guide for assessment once the structure is installed, unless some assessment initiator other than functional expansion occurs.
This Bulletin is organized depending upon whether the structure is fixed or floating and upon its location. Section 2 describes the initiators that are used to determine if a structure should be assessed. Once this is determined, Section 3 describes the assessment approach for fixed structures and Section 4 describes the assessment approach for floating structures. Section 5 provides general recommendations that should be applied where appropriate to all offshore structures in the GOM in order to reduce the risks and consequences of damage from hurricanes.
A commentary is included to provide additional guidance and explain the reasons for selecting the values for 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.
As for the future, the API Hurricane Evaluation & Assessment Team (HEAT) is continuing its orderly work on metocean conditions, platform robustness/fragility assessment & calibration, learnings on the direct and indirect economic impact of platform failures, and safety issue mitigations beyond current personnel evacuation, SCSSV, and P&A practices. Modifications to these Interim Guidelines may be expected in terms of practical tradeoffs, evolving practices, and revisions of the referenced standards.
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