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 API BULLETIN 2INT-DG MAY 2007 Interim Guidance for Design of Offshore Structures for Hurricane Conditions Upstream Segment API BULLETIN 2INT-DG MAY 2007 SPECIAL NOTES API publications necessarily address problems of a general nature With respect to particular circumstances, local, state, and federal laws and regulations should be reviewed Neither API nor any of API’s employees, subcontractors, consultants, committees, or other assignees make any warranty or representation, either express or implied, with respect to the accuracy, completeness, or usefulness of the information contained herein, or assume any liability or responsibility for any use, or the results of such use, of any information or process disclosed in this publication Neither API nor any of API’s employees, subcontractors, consultants, or other assignees represent that use of this publication would not infringe upon privately owned rights User’s of this Bulletin should not rely exclusively on the information contained in this document Sound business, scientific, engineering, and safety judgement should be used in employing the information contained herein API publications may be used by anyone desiring to so Every effort has been made by the Institute to assure the accuracy and reliability of the data contained in them; however, the Institute makes no representation, warranty, or guarantee in connection with this publication and hereby expressly disclaims any liability or responsibility for loss or damage resulting from its use or for the violation of any authorities having jurisdiction with which this publication may conflict API publications are published to facilitate the broad availability of proven, sound engineering and operating practices These publications are not intended to obviate the need for applying sound engineering judgment regarding when and where these publications should 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 marking requirements of an API standard is solely responsible for complying with all the applicable requirements of that standard API does not represent, warrant, or guarantee that such products in fact conform to the applicable API standard All rights reserved No part of this work may be reproduced, stored in a retrieval system, or transmitted by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher Contact the Publisher, API Publishing Services, 1220 L Street, N.W., Washington, D.C 20005 Copyright © 2007 American Petroleum Institute FOREWORD 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 implication or otherwise, for the manufacture, sale, or use of any method, apparatus, or product covered 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 appropriate notification and participation in the developmental process and is designated as an API Standard Questions concerning the interpretation of the content of this publication or comments and questions concerning the procedures under which this publication was developed should be directed in writing to the Director of Standards, American Petroleum Institute, 1220 L Street, 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 every five 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 updated quarterly by API, 1220 L Street, N.W., Washington, D.C 20005 Suggested revisions are invited and should be submitted to the Standards and Publications Department, API, 1220 L Street, NW, Washington, D.C 20005, standards@api.org iii CONTENTS Page SCOPE 1.1 Preface 1.2 Purpose .1 1.3 Background 1.4 Applicability .1 1.5 Reference Standards .2 1.6 Organization .2 GENERAL GUIDANCE FOR NEW STRUCTURES API RP 2A-WSD—INTERIM GUIDANCE FOR FIXED PLATFORMS 3.1 Return Periods 3.2 Deck Elevation and Forces on Decks API BULL 2TD—INTERIM GUIDANCE FOR TIE-DOWNS .3 4.1 Design Wind Speeds and Loads 4.2 Motions 4.3 Drilling Structure Tie-down Systems API RP 2T—INTERIM GUIDANCE FOR TENSION LEG PLATFORMS .5 5.1 Return Periods for Metocean Parameters 5.2 Deck Elevation and Forces on Decks 5.3 Minimum Tension 5.4 TLP System Robustness 5.5 Other Recommendations for TLP Design .6 API RP 2FPS—INTERIM GUIDANCE FLOATING PRODUCTION SYSTEMS 6.1 General 6.2 Air Gap Design API RP 2SK— INTERIM GUIDANCE FOR STATION KEEPING API RP 2RD—INTERIM GUIDANCE FOR DYNAMIC RISERS .7 COMMENTARY 9.1 Commentary General .7 9.2 Deck Elevation and Local Random Wave Crests 9.3 (C3.1) Commentary for API RP 2A-WSD 9.4 (C4.1) Commentary on Wind for Drilling Structures 9.5 (C5) Commentary on TLPs (API RP 2T) 9.6 (C6) Commentary Floating Production Systems (API RP 2FPS) Tables 4.1.1 Offshore Design Reference Wind Speed for Drilling Structures .4 C4.1.2 Design Wind Speeds Used for Existing Drilling Structures v Interim Guidance for Design of Offshore Structures for Hurricane Conditions Scope 1.1 PREFACE 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 1.2 PURPOSE 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 1.3 BACKGROUND 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 1.4 APPLICABILITY 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 API BULLETIN 2INT-DG This document is intended to cover the design of the structural systems of the following types of offshore platforms: Steel jacket or template platforms, towers and compliant towers Minimum non-jacket and special structures (including caissons) defined in API RP 2A-WSD Tension Leg Platforms 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 design documents: API RP 2A-WSD RP 2FPS RP 2RD RP 2SK RP 2T Bull 2TD Planning, Designing and Constructing Fixed Offshore Platforms—Working Stress Design, 21st Edition, December 2000 through Supplement 3, June 2007 Recommended Practice for Planning, Designing and Constructing Floating Production Systems, 1st Edition, March 2001 Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms (TLPs), 1st Edition, June 1998 Design and Analysis of Stationkeeping Systems for Floating Structures, 3rd Edition, October 2005 Planning Designing and Constructing Tension Leg Platforms, 2nd Edition, August 1997 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 include guidance, 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: The Bulletin is withdrawn in its entirety, or 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, 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 General Guidance for New Structures Several principles have been applied during the development of this document: The underlying philosophy is to continue to use the same formulae and design rules in the relevant API Standards, but substitute, for hurricane conditions, the parameters for waves, wind, surge and current indicated in API Bull 2INT-MET 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 current API provisions (The updated Hurricane Conditions in API Bull 2INT-MET are taken as reflecting a change in the understanding of the hurricane environment.) 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), currents and directions needed for design (see “Guidelines for Site-Specific Metocean Studies” in API Bull 2INT-MET) INTERIM GUIDANCE FOR DESIGN OF OFFSHORE STRUCTURES FOR HURRICANE CONDITIONS Other significant learnings from the recent hurricane experiences related to the performance of structures in hurricanes are included in this Bulletin as interim updates to the relevant standards Metocean conditions not associated with tropical storm events, e.g., winter storm, or metocean parameters for operating and service conditions are not updated in API Bull 2INT-MET (except as noted), and design practice should remain as per existing standards The sections that follow offer specific items for each of the relevant design standards 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 3.1 RETURN PERIODS In applying Section 2.3.4, “Hydrodynamic Force Guidelines for U.S Waters,” the values of specific wave, wind and current parameters for Gulf of Mexico should be replaced with values from API Bull 2INT-MET For L-1 (High Consequence) structures, a 100-year return period should be used For L-2 (Medium Consequence) structures, a return period equal to or greater than 50 years should be used For L-3 (Low Consequence) structures, a return period equal to or greater than 25 years should be used For L-1 and L-2 structures it is presumed that platforms in the U.S Gulf of Mexico are manned-evacuated 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 its service 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 The provisions 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 is defined 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, in this case, deck structure and any deck components and equipment that lie below the ‘hurricane water elevation’ should be designed 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 wave height 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 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 intervals from full population or sudden storm data as noted in Table 4.1.1 If the drilling structure is a new rig, purpose-built for a specific location, metocean conditions for the specific Gulf of Mexico Region may be used If the new rig is intended to work anywhere in the Gulf, then Central Region metocean conditions should be used The same conditions would also apply for the tie-down of an existing drilling structure Design wind speeds for drilling derricks are defined as the 3-second gust at the standard reference height (10 m) Guidance on the wind profile variation with elevation can be found in API Bull 2INT-MET 4 API BULLETIN 2INT-DG Table 4.1.1—Offshore Design Reference Wind Speed for Drilling Structures Environment Return Period Speed Operating 44 Knots* Wind—Expected 100-year Full Population Hurricanes See API Bull 2INT-MET Wind—Unexpected 100-year Sudden Hurricanes Note: *This operating wind speed in a hour average at 10 m elevation It should be converted to a 3-second gust before application API Spec 4F provides guidance and a methodology for computing wind loads on derricks and masts The projected area method is recommended and all structural members and appurtenances should be considered when computing wind loads The projected area method is expected to be conservative in that the effects of global shielding and gust effect factors are not considered 4.2 MOTIONS The design of tie-downs for drilling structures should include the effects of platform or vessel motion in combination with wind, dead and live loads, as applicable The use of accelerations from platform specific analyses is recommended Responses including mean, slow drift, wave frequency, and high-frequency contributions should be combined with the effect of inclination with respect to the direction of gravity Peak response (action) is 3.5 to 4.0 times the spectral RMS If application specific values are not available, the following representative values of deck acceleration during design hurricanes (including rotation and tilt effects) are suggested Fixed platforms Waves clear deck, wd300m 04g – 06g Waves impact deck 18g Compliant towers 05g – 07g Tension leg platforms 20g – 33g Semisubmersibles 20g – 36g Spars 27g – 38g It should be noted that the tie-down forces for a drilling rig (drill floor/substructure skid beam) could be significantly different from those acting on its supporting substructure (substructure/deck skid beam) Combination rules are currently not defined Peak wind plus peak dynamics may be taken as moderately conservative 4.3 DRILLING STRUCTURE TIE-DOWN SYSTEMS Drilling structures should be secured to the capping beams or deck of the platform using suitable tie-down systems or means to prevent overturning and sliding These tie-down systems should be rated to resist overturning and sliding loads calculated using design lateral wind and dynamic forces factored by a value of 1.25, at AISC allowable stress levels without the 1/3 increase for wind or dynamic loading For the calculations, dead weights of the drilling structures should be based on 90% of the rig minimum weight The calculation of minimum weight may assume the removal of all optional structures and equipment, and fluid tanks may be considered empty unless otherwise specified in the rig operations manual for storm preparations The distribution of foundation support reactions should be limited to comply with design allowable bearing loadings for the supporting structure For tie-down systems that utilize clamps to prevent rig movement, the maximum allowable static coefficient of friction to be used in overturning or inadvertent rig sliding calculations of drilling structures supported by steel foundations should generally be less than or equal to 0.12 Alternative values for the above coefficient of friction may be used, provided such values have been validated through testing and are consistent with rig skidding procedures For example, if the design of an offshore sliding rig incorporates a coefficient of friction consistent with ungreased surfaces, the owner/operator should maintain and inspect the beams to ensure that they are not inadvertently greased INTERIM GUIDANCE FOR DESIGN OF OFFSHORE STRUCTURES FOR HURRICANE CONDITIONS Welded tie-downs may be sized according to the normal rules for topside structure design Welded stops and mechanical dogs with appreciable clearances should be designed for the corresponding impact forces Tie-downs for equipment should also satisfy wave into deck requirements as discussed in 3.2 above API RP 2T—Interim Guidance for Tension Leg Platforms Referenced Edition: API RP 2T Planning Designing and Constructing Tension Leg Platforms, 2nd Edition, August 1997 5.1 RETURN PERIODS FOR METOCEAN PARAMETERS Sections in API RP 2T that reference “extreme” conditions should generally be taken as 100-year return period conditions based on API Bull 2INT-MET values Reduced extreme conditions (e.g., damaged conditions) should not be less than equivalent 10-year return period conditions For TLP design, it is recommended that a full set of metocean parameters be developed, using the site specific methods described in API Bull 2INT-MET, rather than conducting the design with just the metocean parameters printed in API Bull 2INT-MET Specifically, Section 5.4.3.2 of API RP 2T shall be taken to read as follows: “The development of metocean parameters should generally be done in accordance with API Bull 2INT-MET or successor documents.” 5.2 DECK ELEVATION AND FORCES ON DECKS Replace Section 4.6.1.2f with the following: f Air gap The minimum clearance between the lowest deck and a wave crest is an important parameter in the design of the TLP The air gap has an effect on the center of gravity and in turn the maximum and minimum tendon tensions The recommended minimum deck elevation should result in a 1.5 m (5 ft) air gap to main steel in extreme conditions (100-year return period), combined with zero or greater air gap in survival (1000-year minimum return period) conditions with no margins Local wave effects in these conditions can be dealt with by local strength design If the designer chooses to design for wave impact on deck rather than ensure sufficient clearance to avoid impact, the TLP system, including deck, hull, tendons, and foundations should be designed for the anticipated local and global wave forces (including slamming) and resulting responses Given recent Gulf of Mexico large storm observations, it may be good practice to provide tripping brackets on major deck girders even with designs meeting the above criteria API Bull 2INT-MET states that the crest elevation can be as much as 15% higher somewhere in the boundaries of a platform deck For TLP design, this should be interpreted as follows: The calculation of global wave forces may omit consideration of the 15% increase in crest elevation However, structures and other platform components and equipment that lie at elevations between the crest and the crest plus 15% plus air gap should be designed for local wave forces 5.3 MINIMUM TENSION The Minimum Tension calculation should be performed for 100-year extreme conditions and 1000-year survival conditions, with the associated parameters determined in accordance with API Bull 2INT-MET procedures For extreme condition safety category B response, the minimum tendon tension in at least one (1) tendon per corner should remain non-negative For Category S, minimum tension in at least three (3) corner groups of tendons should maintain non-negative tension in the 1000-year return period response environment For Category S, if tension is not maintained in the 1000-year return period response environment, then a comprehensive coupled analysis of the tendon system performance under loss of tension should be performed to demonstrate adequate robustness against subsequent snap loading The analysis should address detailed load sequences induced in all components (top and bottom) on all tendons to ensure load capacities are not exceeded and components function as intended in order to prevent tendon disconnect 6 API BULLETIN 2INT-DG 5.4 TLP SYSTEM ROBUSTNESS A survival level check of the TLP system should be performed for purposes of ensuring the robustness of the system The objective is to evaluate the TLP in conditions beyond design “extreme” and design “survival” conditions in order to identify any failure thresholds that are close to the design points This may be performed through an analysis of 1000-year return period or greater conditions Overall survival of the tendon system and support points should be ensured without exceeding yield or causing mechanical disconnect of the system Both vertical and lateral wave loads on the deck should be considered in the modeling as warranted Meeting these criteria will in principle ensure a robust design for components that have non-brittle failure modes Good material selection and fabrication procedures are likely to ensure ductile behavior of structural components For example, tendon pipe should have a very ductile failure mode Overloading the tendon with forces associated with a 1000-year return event should not lead to catastrophic failure However, some mechanical components may have “brittle” failure modes in that going beyond a certain load may cause movement of mechanical pieces, or complete or partial disengagement of the component An important example for a TLP is a typical tendon bottom connector, which is designed to engage or disengage by stroking the tendon downwards beyond the in-service load shoulder This functionality is necessary for installation and removal A margin of safety is designed into the system in the distance of stroke required to disengage the connector Through careful global performance analysis, one can accurately analyze the motion and stress behavior of the tendon during a down-stroke event However, the behavior of the mechanical component may not be as predictable Care should be taken in the design of such components to insure that the device will indeed re-engage properly under actual service conditions Consideration should be given to corrosion and sedimentation and their potential effect on the connector’s mechanical performance Consideration should also be given to the speed at which the mechanical parts are able to move with respect to the movement of the tendon Specific measures should be taken to demonstrate the robustness of the design to overload, including prototype testing and analytical modeling as appropriate Mechanically securing the connector in place may also be considered as a means of ensuring robustness, although one should also check the effect of securing the lower connector on the upper connector assembly Other tendon components should also be examined in the same light to demonstrate that there are no failure modes that can lead to a sudden failure or catastrophic damage at or beyond survival load cases 5.5 OTHER RECOMMENDATIONS FOR TLP DESIGN Down flooding points, access hatches, and other points of potential water ingress within the splash zone should be weather tight and structurally designed to withstand wave impact loads and full immersion in the 100-year design wave crest API RP 2FPS—Interim Guidance Floating Production Systems Referenced Edition: API RP 2FPS Recommended Practice for Planning, Designing and Constructing Floating Production Systems, 1st Edition, March 2001 6.1 GENERAL Design of new Category FPS units and their mooring systems should continue to be based on more metocean parameters than are defined in API Bull 2INT-MET (see Section 2) For hurricane conditions, such metocean parameters should be developed in accordance with the requirements of API Bull 2INT-MET or successor document for conducting site-specific studies Environmental criteria for Category and applications that operate under MODU practice defined in API RP 2FPS should follow the requirements of API RP 95F 6.2 AIR GAP DESIGN Current guidance in API RP 2FPS for air gap design for semi-submersibles and spars refers to API RP 2T The contents of API RP 2FPS, Sections 3.2.6 and 5.2.5 should be interpreted as follows: When assessing air gap, the following effects should be considered: • wave crest elevation, including wave asymmetry; INTERIM GUIDANCE FOR DESIGN OF OFFSHORE STRUCTURES FOR HURRICANE CONDITIONS • • • • wave/structure interaction effects (e.g., wave enhancement, run-up, etc.); global rigid body motions (including dynamic effects); effects of interacting systems (e.g., mooring and riser systems); maximum/minimum operating drafts Structures, parts of structures, equipment and supports that are not designed for the effects of direct wave action (wave impact, slamming, etc.) shall be located at elevations that provide an air gap equal to or greater than 1.5 m (5 ft) in the 100-year design event For robustness, consideration should be given to choosing a more rare, but still possible, event, e.g., 1000-year wave crest, and setting the elevations such that the air gap ≥ 0.0m (0.0 ft) in that event API RP 2SK— Interim Guidance for Station Keeping Referenced Edition: API RP 2SK Design and Analysis of Stationkeeping Systems for Floating Structures, 3rd Edition, October 2005 Design of new permanent mooring systems should continue to be based on more metocean parameters than are defined in API Bull 2INT-MET (see Section 2) For hurricane conditions, such metocean parameters should be developed in accordance with the requirements of API Bull 2INT-MET or successor document for conducting site-specific studies In addition to the API RP 2SK safety factor check, a mooring sensitivity or weak point analysis should be considered The objective of this analysis is to determine the probable failure mode of the mooring system Mitigation measures for the probable modes should be investigated to achieve a balanced design, if feasible A mooring sensitivity analysis is a useful tool for comparing different mooring systems for a given design criteria Such an analysis can provide useful information for risk assessment and mitigation strategies particularly in the light of other potential failure modes, such as bending over the fairlead, wire fretting, elastoplastic fatigue damage, etc As such, there are no defined acceptance criteria for the results of this sensitivity check As a minimum the mooring sensitivity or weak point analysis should address riser offset limits, line tensions, and anchor loads, and should be conducted for both the intact and damaged conditions API RP 2RD—Interim Guidance for Dynamic Risers Referenced Edition: API RP 2RD Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms (TLPs), 1st Edition, June 1998 See Section 2, “General Guidance for New Structures.” The way Design Cases and Loading Conditions are described in API RP 2RD, the information in API Bull 2INT-MET may be introduced for extreme conditions with no additional requirements Return periods for selecting metocean conditions remain as in API RP 2RD Commentary 9.1 COMMENTARY GENERAL The target is to maintain the same reliability inherent with the reliability in the referenced API Recommended Practices listed in 1.5, but to adjust for the updated understanding of metocean conditions during hurricanes This is a prudent approach while the final evaluation and updating of other design standards is ongoing New structure designs have some flexibility to accommodate this interim guidance The API Bull 2INT-MET includes wave heights, surge and other metocean parameters for return periods for 200, 1000, 2000 and 10,000 years This information is generally not explicitly called for in the design practices used in U.S waters, although it is in some European and Canadian codes If, as we are recommending here, the new hurricane conditions at the 100-year level are applied, there is no general reason at this time to invoke additional criteria that focus on overload, beyond what is already in the design RPs There is some new material in this interim guidance on design for robustness 9.2 DECK ELEVATION AND LOCAL RANDOM WAVE CRESTS API Bull 2INT-MET states that the crest elevation can be as much as 15% higher somewhere in the boundaries of a platform deck This is predominantly the random field effect—a wave staff at any corner of a hypothetical platform will record the same general statistics, yet the expected maximum of all points within the rectangle will be higher than the single point estimate Over a typical platform size area and for directional wave spectra in large hurricanes, the crest can be 10% – 14% higher at the worst API BULLETIN 2INT-DG point (Ref: Forristal, G Z., “Wave Crest Heights and Deck Damage in Hurricanes Ivan, Katrina, and Rita,” Proceedings, Offshore Technology Conference, OTC 18620, Houston, TX, May 2007.) Additionally, with an actual platform in place, up-wave measurements are about 10% higher than down-wave measurements This is to be interpreted for design guidance slightly differently for fixed and floating structures, but the principle is the same: 1) include the 15% for setting deck elevation, 2) not include the 15% in the calculation of global loads, and 3) design structure and other platform components and equipment that lie at elevations between the design wave crest and the crest plus 15% plus air gap for a local random wave crest force As an approximation, the local forces imposed by the local random wave crest may be modeled by moving the wave crest pressure up in elevation as required For any equipment and other platform components such as utilities including instruments, communication, control and power, drains, etc that would be below the recommended elevation, consideration should be given to a design that would permit uninterrupted functionality even with water in the region below the recommended elevation Risers, appurtenances, and localized components of offshore structures (fixed or floating platforms) are exposed to local increases in wave pressure due to irregularity of waves and proximity to columns, and should be designed accordingly In no case should the local design wave pressures for risers and appurtenances be less than those used for global structure design anywhere at the same elevation 9.3 (C3.1) COMMENTARY FOR API RP 2A-WSD The hydrodynamic and aerodynamic force “recipes” in API RP 2A-WSD, Section 2.3 “Design Loads” should remain as stated, except the wind speeds, wave heights and periods, and current velocities are from API Bull 2INT-MET In particular, Figure 2.3.1-2 “Doppler Shift Due to Steady Current” in API RP 2A-WSD should be applied with the metocean conditions specified in API Bull 2INT-MET 9.4 (C4.1) COMMENTARY ON WIND FOR DRILLING STRUCTURES The information in Table C4.1.2 is offered as typical for drilling structures designed and manufactured to API Spec 4F API Spec 4F specifies design wind speeds that are independent of a return period or gust averaging period; for the purposes of this document, they are presumed to be equivalent to a 3-second averaging period Table C4.1.2—Design Wind Speeds Used for Existing Drilling Structures Bare Survival Rig with Setback Existing API Spec 4F Equivalent 1-hour Existing API Spec 4F Equivalent 1-hour “3-sec gust” in knots, (m/s) speed in knots, (m/s) “3-sec gust” knots, (m/s) speed in knots, (m/s) Workover Mast with Guylines 60 (31) 45 (23) 60 (31) 45 (23) Drilling Mast 93 (48) 65 (33) 70 (36) 51 (42) Large Derrick (>18A) 107 (55) 73 (38) 93 (48) 65 (33) Note: An “18A” derrick is from Appendix B of API Spec 4F; this derrick has a 30 ft × 30 ft base with a 136 ft height (to bottom of crown beams) Rig Type Table C4.1.2 is shown in mixed units—knots to relate to API Spec 4F, and meters per second (m/s) to relate to API Bull 2INTMET charts From API Bull 2INT-MET, 100-year return period values of the 1- hour wind speed are 48 m/s (Central), 38 m/s (W Central), 40 m/s (West), and 38 m/s (East) The 1-hour speed for the large derrick, bare survival specified in API Spec 4F is 38 m/s, which according to API Bull 2INT-MET has a return period of about 100-years in the West, W Central, and East regions, and less than 25 years in the Central region Approximating the force as proportional to velocity squared, the ratio of 100-year loads in the Central Region to the API Spec 4F load is about 482/382 or ~ 1.6 Noting that the API Spec 4F recipe has some intentional conservatism, the Central Region overload might be more like 1.3 to 1.4 The API Spec 4F Work Group is completing a revision to the methodology for computing wind loads on drilling structures This methodology, based on other codes and wind tunnel test data, includes consideration of global shielding and gust effect factors that the existing specification does not include These factors may mitigate some of the concern about potential overloading of derrick structures in the Central region under the updated metocean criteria of API Bull 2INT-MET Particular care should be taken in evaluating existing structures that are designed to the lower wind speeds allowed for unguyed masts (93 knot [33 m/s] survival wind versus 107 knot [38 m/s] for large derricks) for use in any region of the GOM INTERIM GUIDANCE FOR DESIGN OF OFFSHORE STRUCTURES FOR HURRICANE CONDITIONS Reference for tie-down design: Research Council on Structural Connections—Specification for Structural Joints using ASTM A325 or A490 Bolts—June, 2004 (c/o American Institute of Steel Construction, Inc.) 9.5 (C5) COMMENTARY ON TLPS (API RP 2T) This Bulletin includes more material on tension leg platforms than on most other topics This is the consequence of ongoing development of the next revision of API RP 2T—which is nearly complete The next revision will also have provisions that address findings from TLP responses in Hurricanes Ivan, Katrina, and Rita In the interest of a timely dissemination of improved practices, selected topics related to design of TLPs for hurricane conditions and being considered for the next revision of API RP 2T are included in this Bulletin as an interim update Section 5.2 updates the deck elevation determination It is worth noting that both the 100-year check and the 1000-year check include safety margins—the air gap in the first case and the longer return period in the second 9.6 (C6) COMMENTARY FLOATING PRODUCTION SYSTEMS (API RP 2FPS) 9.6.1 (C6.1) General In line with the risk assessment guidance for the entire FPS described in API RP 2FPS, a structural system robustness check is recommended for any floater design This check might be performed using hurricane conditions with a longer return period The objective of this system robustness check is to ensure that the designer and the operator have a clear understanding of the redundancy or robustness of the design The system robustness check should include as a minimum the mooring system as well as the production and export risers The focus of this robustness check is maintaining station keeping, structural integrity and floating stability Key design considerations when performing a system robustness check are: • • • • Degradation of floating stability through compartment flooding or otherwise Positioning of down-stop and up-stop of riser support systems and vessel offset effects on risers Capacity and ductility of key riser components Mooring line and anchor Safety Factors, and the capacity of key mooring components as well as the capacity and ductility of their support structures • Key Structural components, such as: deck to hull connection, truss to hard tank connections of a spar, or pontoon to column connections on a semi-submersible 9.6.2 (C6.2) Air Gap Design Recognizing the interactions of larger floating bodies and wave fields, a smaller value of air gap may be justified if all the linear and non-linear effects described in 6.2 are accounted for and the action of the water that may contact the structure is localized and accounted for (See Commentary 9.2, “Deck Elevation and Local Wave Crests.”) Effective January 1, 2007 API Members receive a 30% discount where applicable The member discount does not apply to purchases made for the purpose of resale or for incorporation into commercial products, training courses, workshops, or other commercial enterprises 2007 Publications Order Form Available through IHS: Date: ❏ API Member (Check if Yes) Invoice To (❏ Check here if same as “Ship To”) Ship To (UPS will not deliver to a P.O Box) Name: Name: Title: Title: Company: Company: Department: Department: Address: Address: Phone Orders: Fax Orders: Online Orders: 1-800-854-7179 (Toll-free in the U.S and Canada) 303-397-7956 (Local and International) 303-397-2740 global.ihs.com City: State/Province: City: State/Province: Zip/Postal Code: Country: Zip/Postal Code: Country: Telephone: Telephone: Fax: Fax: E-Mail: E-Mail: Quantity Title G2AWSD RP 2A-WSD, Planning, Designing and Constructing Fixed Offshore PlatformsWorking Stress Design $315.00 G2FPS1 RP 2FPS, Recommended Practice for Planning, Designing, and Constructing Floating Production Systems $152.00 G02RD1 RP 2RD, Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms (TLPs) $187.00 G2SK03 RP 2SK, Design and Analysis of Stationkeeping Systems for Floating Structures $113.00 G02T02 RP 2T, Planning, Designing and Constructing Tension Leg Platforms $167.00 G2TD01 Bull 2TD, Guidelines for Tie-Downs on Offshore Production Facilities for Hurricane Season $44.00 ❏ Payment Enclosed ❏ MasterCard SO★ ❏ P.O No (Enclose Copy) Subtotal ❏ American Express ❏ Diners Club ❏ Discover Rush Shipping Fee (see below) Shipping and Handling (see below) Credit Card No.: Print Name (As It Appears on Card): Expiration Date: Signature: Total Applicable Sales Tax (see below) ❏ Charge My IHS Account No ❏ VISA Unit Price Product Number Total (in U.S Dollars) ★ To be placed on Standing Order for future editions of this publication, place a check mark in the SO column and sign here: Pricing and availability subject to change without notice Mail Orders – Payment by check or money order in U.S dollars is required except for established accounts State and local taxes, $10 processing fee, and 5% shipping must be added Send mail orders to: API Publications, IHS, 15 Inverness Way East, c/o Retail Sales, Englewood, CO 80112-5776, USA Purchase Orders – Purchase orders are accepted from established accounts Invoice will include actual freight cost, a $10 processing fee, plus state and local taxes Telephone Orders – If ordering by telephone, a $10 processing fee and actual freight costs will be added to the order Sales Tax – All U.S purchases must include applicable state and local sales tax Customers claiming tax-exempt status must provide IHS with a copy of their exemption certificate Shipping (U.S Orders) – Orders shipped within the U.S are sent via traceable means Most orders are shipped the same day Subscription updates are sent by First-Class Mail Other options, including next-day service, air service, and fax transmission are available at additional cost Call 1-800-854-7179 for more information Shipping (International Orders) – Standard international shipping is by air express courier service Subscription updates are sent by World Mail Normal delivery is 3-4 days from shipping date Rush Shipping Fee – Next Day Delivery orders charge is $20 in addition to the carrier charges Next Day Delivery orders must be placed by 2:00 p.m MST to ensure overnight delivery Returns – All returns must be pre-approved by calling the IHS Customer Service Department at 1-800-624-3974 for information and assistance There may be a 15% restocking fee Special order items, electronic documents, and age-dated materials are non-returnable There’s more where this came from The American Petroleum Institute provides additional resources and programs to the oil and natural gas industry which are based on API® Standards For more information, contact: • API Monogram® Licensing Program Phone: Fax: 202-962-4791 202-682-8070 • American Petroleum Institute Quality Registrar (APIQR®) Phone: Fax: 202-962-4791 202-682-8070 • API Spec Q1® Registration Phone: Fax: 202-962-4791 202-682-8070 • API Perforator Design Registration Phone: Fax: 202-962-4791 202-682-8070 • API ISO/TS 29001 Registration Phone: Fax: 202-962-4791 202-682-8070 • API Training Provider Certification Program Phone: Fax: 202-682-8490 202-682-8070 • Individual Certification Programs Phone: Fax: 202-682-8064 202-682-8348 • Engine Oil Licensing and Certification System (EOLCS) Phone: Fax: 202-682-8516 202-962-4739 • API PetroTEAM™ (Training, Education and Meetings) 202-682-8195 202-682-8222 Phone: Fax: Check out the API Publications, Programs, and Services Catalog online at www.api.org Helping You Get The Job Done Right.®