Api Bull 2Int-Dg-2007 (American Petroleum Institute).Pdf

2INT DG fm Interim Guidance for Design of Offshore Structures for Hurricane Conditions API BULLETIN 2INT DG MAY 2007 Interim Guidance for Design of Offshore Structures for Hurricane Conditions Upstrea[.]

Interim Guidance for Design

of Offshore Structures for Hurricane Conditions

Interim Guidance for Design

of Offshore Structures for Hurricane Conditions

User’s of this Bulletin should not rely exclusively on the information contained in this ment Sound business, scientific, engineering, and safety judgement should be used inemploying the information contained herein.

docu-API publications may be used by anyone desiring to do so Every effort has been made bythe Institute to assure the accuracy and reliability of the data contained in them; however, theInstitute makes no representation, warranty, or guarantee in connection with this publicationand hereby expressly disclaims any liability or responsibility for loss or damage resultingfrom its use or for the violation of any authorities having jurisdiction with which this publi-cation may conflict

API publications are published to facilitate the broad availability of proven, sound ing and operating practices These publications are not intended to obviate the need forapplying sound engineering judgment regarding when and where these publications should

engineer-be utilized The formulation and publication of API publications is not intended in any way

to inhibit anyone from using any other practices

Any manufacturer marking equipment or materials in conformance with the markingrequirements of an API standard is solely responsible for complying with all the applicablerequirements of that standard API does not represent, warrant, or guarantee that such prod-ucts do in fact conform to the applicable API standard

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Services, 1220 L Street, N.W., Washington, D.C 20005.

Copyright © 2007 American Petroleum Institute

This Bulletin is under the jurisdiction of the API Subcommittee on Offshore Structures.Nothing contained in any API publication is to be construed as granting any right, by impli-cation or otherwise, for the manufacture, sale, or use of any method, apparatus, or productcovered by letters patent Neither should anything contained in the publication be construed

as insuring anyone against liability for infringement of letters patent

This document was produced under API standardization procedures that ensure appropriatenotification and participation in the developmental process and is designated as an API Stan-dard Questions concerning the interpretation of the content of this publication or commentsand questions concerning the procedures under which this publication was developed should

be directed in writing to the Director of Standards, American Petroleum Institute, 1220 LStreet, N.W., Washington, D.C 20005 Requests for permission to reproduce or translate all

or any part of the material published herein should also be addressed to the director.Generally, API Standards are reviewed and revised, reaffirmed, or withdrawn at least everyfive years A one-time extension of up to two years may be added to this review cycle Status

of the publication can be ascertained from the API Standards Department, telephone (202)682-8000 A catalog of API publications and materials is published annually and updatedquarterly by API, 1220 L Street, N.W., Washington, D.C 20005

Suggested revisions are invited and should be submitted to the Standards and PublicationsDepartment, API, 1220 L Street, NW, Washington, D.C 20005, standards@api.org

Page

1 SCOPE 1

1.1 Preface 1

1.2 Purpose .1

1.3 Background 1

1.4 Applicability .1

1.5 Reference Standards .2

1.6 Organization .2

2 GENERAL GUIDANCE FOR NEW STRUCTURES 2

3 API RP 2A-WSD—INTERIM GUIDANCE FOR FIXED PLATFORMS 3

3.1 Return Periods 3

3.2 Deck Elevation and Forces on Decks 3

4 API BULL 2TD—INTERIM GUIDANCE FOR TIE-DOWNS .3

4.1 Design Wind Speeds and Loads 3

4.2 Motions 4

4.3 Drilling Structure Tie-down Systems 4

5 API RP 2T—INTERIM GUIDANCE FOR TENSION LEG PLATFORMS .5

5.1 Return Periods for Metocean Parameters 5

5.2 Deck Elevation and Forces on Decks 5

5.3 Minimum Tension 5

5.4 TLP System Robustness 6

5.5 Other Recommendations for TLP Design .6

6 API RP 2FPS—INTERIM GUIDANCE FLOATING PRODUCTION SYSTEMS 6

6.1 General 6

6.2 Air Gap Design 6

7 API RP 2SK— INTERIM GUIDANCE FOR STATION KEEPING 7

8 API RP 2RD—INTERIM GUIDANCE FOR DYNAMIC RISERS .7

9 COMMENTARY 7

9.1 Commentary General .7

9.2 Deck Elevation and Local Random Wave Crests 7

9.3 (C3.1) Commentary for API RP 2A-WSD 8

9.4 (C4.1) Commentary on Wind for Drilling Structures 8

9.5 (C5) Commentary on TLPs (API RP 2T) 9

9.6 (C6) Commentary Floating Production Systems (API RP 2FPS) 9

Tables 4.1.1 Offshore Design Reference Wind Speed for Drilling Structures .4

C4.1.2 Design Wind Speeds Used for Existing Drilling Structures 8

Interim Guidance for Design of Offshore Structures for Hurricane Conditions

1 Scope

1.1 PREFACE

Hurricane Ivan in 2004 and hurricanes Katrina and Rita in 2005 resulted in considerable damage and destruction to fixed andfloating facilities in the Gulf of Mexico (GOM) Several API Committees are in the process of revising and updating standards toincorporate 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 majorchange is a complete revision to the hurricane conditions presently contained in API RP 2A-WSD, 21st Edition, recognizing thehigher level of hazard in certain parts of the GOM Another is the revised understanding of the potential for local wave-in-deck dam-age While work on standards development continues, in the interim the following documents are being issued to provideimmediate 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

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 theGulf Most importantly, however, industry’s understanding of hurricane risk has continued to evolve Strong evidence now existsfor 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 ofthe 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 onthe 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 inthe 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 pri-marily driven by the high frequency of intense storms experienced by this region, and to a lesser degree the elimination of the lesstrusted (pre-1950) portion of the historical hindcast record Conditions in the other three regions vary slightly from each other, butare 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, ticularly in the Central Region and its adjoining transitions

par-The content of API Bull 2INT-MET is undergoing extensive review and evaluation par-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 dards API RP 2MET will also serve as the basis for a revised U.S Regional Annex in ISO 19901-1

stan-1.4 APPLICABILITY

2 API B ULLETIN 2INT-DG

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)

1.5 REFERENCE STANDARDS

This document is intended to explain how to use the content of API Bull 2INT-MET in conjunction with the following designdocuments:

API

RP 2A-WSD Planning, Designing and Constructing Fixed Offshore Platforms—Working Stress Design, 21st Edition,

December 2000 through Supplement 3, June 2007

RP 2FPS Recommended Practice for Planning, Designing and Constructing Floating Production Systems, 1st Edition,

March 2001

RP 2RD Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms (TLPs), 1st Edition, June

1998

RP 2SK Design and Analysis of Stationkeeping Systems for Floating Structures, 3rd Edition, October 2005

RP 2T Planning Designing and Constructing Tension Leg Platforms, 2nd Edition, August 1997

Bull 2TD Guidelines for Tie-downs on Offshore Production Facilities for Hurricane Season, 1st Edition, June 2006

These standards are applicable to the types of structures in 1.4, have been actively applied in designs in U.S waters, and includeguidance, methods and criteria to apply metocean conditions

Nothing in this Bulletin is intended to suggest, recommend or endorse a relaxation of provisions in existing API Standards, which remain in effect The more severe of the metocean conditions in API Bull 2INT-MET or the metocean conditions in existing API Standards should be applied, unless metocean conditions derived from a valid site specific investigation are used

1.6 ORGANIZATION

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 letin superseded

Bul-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 tions, platform robustness/fragility assessment & calibration, learnings on the direct and indirect economic impact of platformfailures, and safety issue mitigations beyond current personnel evacuation, SCSSV, and P&A practices Modifications to theseInterim Guidelines may be expected in terms of practical tradeoffs, evolving practices, and revisions of the referenced standards

condi-2 General Guidance for New Structures

Several principles have been applied during the development of this document:

1 The underlying philosophy is to continue to use the same formulae and design rules in the relevant API Standards, but stitute, for hurricane conditions, the parameters for waves, wind, surge and current indicated in API Bull 2INT-MET

sub-2 API Bull 2INT-MET is expressed in return period, and some current API provisions are in terms of specific wave heights

or crests; thus some translation is necessary The intention is to achieve the same return period (stated or implied) as the rent API provisions (The updated Hurricane Conditions in API Bull 2INT-MET are taken as reflecting a change in theunderstanding of the hurricane environment.)

cur-3 The design of floating structures in particular utilizes more metocean parameters than described in API Bull 2INT-MET.Detailed, site-specific studies should be conducted to develop the simultaneous wind, wave spectra (periods and heights), cur-rents and directions needed for design (see “Guidelines for Site-Specific Metocean Studies” in API Bull 2INT-MET)

I NTERIM G UIDANCE FOR D ESIGN OF O FFSHORE S TRUCTURES FOR H URRICANE C ONDITIONS 3

4 Other significant learnings from the recent hurricane experiences related to the performance of structures in hurricanes areincluded in this Bulletin as interim updates to the relevant standards

5 Metocean conditions not associated with tropical storm events, e.g., winter storm, or metocean parameters for operatingand service conditions are not updated in API Bull 2INT-MET (except as noted), and design practice should remain as perexisting standards

The sections that follow offer specific items for each of the relevant design standards

3 API RP 2A-WSD—Interim Guidance for Fixed Platforms

Referenced Edition: API RP 2A-WSD Planning, Designing and Constructing Fixed Offshore Platforms—Working Stress Design,

21st Edition, December 2000 through Supplement 3, June 2007

3.2 DECK ELEVATION AND FORCES ON DECKS

The recommendations for deck elevation are the same for L-1 and L-2 platforms

As already stated in API RP 2A-WSD, deck elevations should be selected taking account of expected platform subsidence over itsservice life, both regional and that due to hydrocarbon extraction

The second sentence in API RP 2A-WSD, Section 2.3.4.d.3 is deleted and the referenced Figure 2.3.4-8 is not applicable Theprovisions of API RP 2A-WSD, Sections 2.3.4.d.3 and 2.3.4.g on deck elevations for L-1 and L-2 platforms should be interpreted

as follows: the minimum elevation of the underside of the deck should equal or exceed the “hurricane water elevation” which isdefined as the 100-year maximum crest elevation, as defined in API Bull 2INT-MET, plus 15% for the local random wave crest,plus a minimum of 1.5 m (5 ft) of safety margin or air gap

Alternatively, the added 15% for the local random wave crest may be omitted in establishing the minimum deck elevation, but, inthis case, deck structure and any deck components and equipment that lie below the ‘hurricane water elevation’ should bedesigned to withstand the local wave force associated with the local random wave crest

The wave kinematics and subsequent force to be applied to the structure for global design purposes should not include waveheight or period contributions due to the local random wave crest

For further information on the local random wave phenomenon, deck elevation, and wave force computation, see Commentary 9.2

4 API Bull 2TD—Interim Guidance for Tie-downs

Referenced Edition: API Bull 2TD Guidelines for Tie-downs on Offshore Production Facilities for Hurricane Season, 1st Edition,

June 2006

4.1 DESIGN WIND SPEEDS AND LOADS

For the Gulf of Mexico, wind speeds for designing tie-downs should be taken from API Bull 2INT-MET, with recurrence vals from full population or sudden storm data as noted in Table 4.1.1