This draft document is not an api standard; it is under consideration within an api technical committee but has not received all approvals required to become an




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This draft document is not an API Standard; it is under consideration within an API technical committee but has not received all approvals required to become an Addendum to an API Standard. It shall not be reproduced or circulated or quoted, in whole or in part, outside of API committee activities except with the approval of the Chairman of the committee having jurisdiction and staff of the API Standards Dept. Copyright API. All rights reserved.


Ballot 2
January 2010












Derivation of Metocean Design and Operating Conditions

API RECOMMENDED PRACTICE 2MET
FIRST EDITION, XXX 200X



ISO 19901-1:20054 (Modified) Petroleum and natural gas industries — Specific requirements for offshore structures — Part 1: Metocean design and operating considerations


This draft is for committee balloting purposes only.


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.

API publications may be used by anyone desiring to do 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 do in fact conform to the applicable API standard.

Foreword

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.

Standards referenced herein may be replaced by other international or national standards that can be shown to meet or exceed the requirements of the referenced standard.

Suggested revisions are invited and should be submitted to the API, Standards Department, 1220 L Street, NW, Washington, DC 20005, or by email to standards@api.org.

This American National Standard is under the jurisdiction of the API Subcommittee on Offshore Structures. This standard modified from the English version of ISO 19901-2:200419901-1:2005. ISO 19901-2 1 was prepared by ISO Technical Committee ISO/TC I67, Materials, equipment and offshore structures for petroleum, petrochemical and natural gas industries, Subcommittee SC 7, Offshore Structures.

Contents Page

Special Notes 2

Foreword 3

Foreword 5

Introduction 7

1)Scope 9

2)those that relate to the determination of environmental conditions in general, together with the metocean parameters that are required to adequately describe them; 9

3)those that relate to the characterization and use of metocean parameters for the design, the construction activities or the operation of offshore structures. 9

4)Normative references 10

5)Terms and definitions 10

6)Symbols and abbreviated terms 17

7)Determining the relevant metocean parameters 20

8)Long-term distributions of metocean parameters, in the form of cumulative conditional or marginal statistics. These parameters are used 20

9)Normal environmental conditions, which are required 20

10)Short-term descriptions of one or a number of different design sea states, in conjunction with one or more design currents. A design sea state shall be described by a wave spectrum together with the significant wave height, a representative frequency or period, and a mean wave direction. Where appropriate, the wave spectrum may be supplemented with a directional spreading function, see 8.7. A design current is specified by a surface velocity and its velocity profile over the water column, including its direction, see Clause 9. 21

11)One or a number of individual design waves, in conjunction with one or more design currents. A design wave shall be specified by its height and period, together with an appropriate wave theory from which the wave kinematics can be derived, as well as (an) associated direction(s), see Clause 8. A design current is specified by a surface velocity and its velocity profile over the water column including its direction, see Clause 9. 21

1)placement on the seabed, and 24

12)Water depth, tides and storm surges 25

13)Wind 26

14)Waves 28

15)Currents 31

16)Other environmental factors 33


(informative)

Additional information and guidance 36


Scope 36

Normative references 36

Terms and definitions 36

Symbols and abbreviations 36

Determining the relevant metocean parameters 36

17)Specified return period wave height combined with the wind speed and the current velocity with the same specified return period, all determined by extrapolation of the individual parameters considered independently. 38

18)Any “reasonable” combination of wave height and period, wind speed and current velocity that results in 38

19)The deductive method breaks a storm into a series of simplified sub-models with specified probabilities. The sub-models are eventually combined using probabilistic laws to develop events with much lower overall probabilities. Deductive models have been used to estimate extremes of earthquakes, storm surges and, to a lesser degree, winds and waves[14]. The method has recently received renewed interest because it can potentially provide more accurate extreme estimates than the historical method for very rare events having return periods from about 1 000 to 10 000 years. 41

Water depth, tides and storm surges 45

Wind 46

20)For normal and short-term conditions, data should be given concerning the following. 47

Waves 51

21)For extreme and abnormal conditions 54

Estimated extreme and abnormal wave heights from specified directions should be developed and presented as a function of their return periods. Other data that should be developed include 54

Currents 68

Other environmental factors 72


(informative)

Discussion of wave frequency spectra 75


The Pierson-Moskowitz spectrum 75

The JONSWAP spectrum 78

Comparison of Pierson-Moskowitz and JONSWAP spectra 80

Ochi-Hubble spectra 81


(informative)

Regional information 85


1.General 85

2.North-west Europe 85

3.West coast of Africa 96

4.US Gulf of Mexico 109

22)Areas in water depths less than 15 m between 87.75° W and 90° W, those close to or inside barrier islands and those around the Mississippi Delta. These areas will be subject to sheltering, limited fetch, and possible attenuation of waves by interaction with a soft seafloor, and may have complicated surge and current patterns, while areas east of the barrier islands will be subject to complicated currents. 121

1)Surface speed: current speed at the surface (depth = 0) of the ocean, including any surge and tide. 139

23)Speed at mid-profile: current speed at a depth halfway between the surface (depth = 0) and the depth of the bottom of the profile. 139

24)Bottom of profile: the depth, measured from the surface, at which the current speed decays to background values (assumed 0.1 m/s). 139

25)1 000- and 2 000-year sudden hurricane load cases should be developed using the combination factors in the 100-year column. The 1 000- and 2 000-year sudden hurricane directional wave conditions may be approximated using Figure C.25. 145

26)The seasonal conditions are for the full population of early and late season tropical cyclones. They do not include winter storms, which should be treated as a separate storm population with its own set of derived extremes. Some of the extremes presented in this section, particularly in the post-peak period, may not represent the highest storm-driven n-year wind or wave conditions which could be encountered in the periods described. Application of these conditions should include a comprehensive risk assessment accounting for all storm conditions which could be encountered during the period of operation considered, in order to evaluate total risk. 150

27)The conditions in this section should be treated as complete load cases, and the wind, waves, and current should be treated as omni-directional. Do not use the factors in Figure C.25 to adjust the wave heights shown in the seasonal tables; however, the seasonal wave heights should not be higher in any given direction than the annual extreme waves adjusted for direction using Figure C.25. 150

28)Planning for operations in the pre-peak hurricane season should consider the possibility of delayed completion due to late arrival of equipment at the beginning of the operation, contingencies during the operation itself, delays due to Loop Current intrusions, and delays due to tropical storm occurrences. Wind, waves, and current corresponding to the latest likely completion date should be used in planning. 150

29)Planning for operations in the post-peak hurricane season should consider the possibility of an early start due to early availability of equipment. Wind, waves, and current corresponding to the earliest likely start date should be used in planning, or the operator should be in a position to terminate any operation and take precautionary measures in response to realistic tropical cyclone forecast conditions. 150

30)When adjusting the wind speeds to different averaging intervals and/or elevations, or when developing wind spectra, the formulas in A.7.3 and A.7.4 should be used. 153

31)Wave conditions are provided in the form of Hs, Hmax, and ηmax as well as associated Tp and THmax. When assessing systems with dynamic sensitivity, a ±10 % variation in the wave period should be considered. 153

32)The crest elevations ηmax provided include associated surge and tide. The crest elevations provided do not include any artificial air gap allowance like the 1.5 m previously recommended in API 2A-WSD or any allowance for local crest variation. When selecting a crest for an n-year condition, the higher of either the n-year crest provided or the crest of the n-year wave as determined by an appropriate high-order wave theory should be used. 153

33)Winter storm-driven seas can be reasonably represented by the JONSWAP spectrum with a γ of 1.0-3.0. Wave spreading can be represented using the form cosn(θ), with n equal to 4.0. A wave kinematics factor of 0.91 is considered representative. 153

34)The current profile shape is a uniform speed over the depth indicated. 153

35)When determining the alignment of currents relative to the waves, the component of current in-line with the waves should never be less than 0.1 m/s over all depths regardless of the relative offset angle. 153

36)The current profile should be constant from the bottom to the top of the Escarpment, and then decay linearly to background surface values (may assume 0.25 m/s). 157

37)The current heading should be toward a westerly direction but aligned with the local isobaths averaged over roughly a 30 km along-slope section. 157

38)Currents generated by TRWs often appear as a series of two pulses each 7 to 14 days in duration, with the duration dependent upon the location along the Escarpment. The increase and decay of the pulses can be characterized by sin(t/D) for 0 ≤ t ≤ D, with t in days, and D is the duration (7 to 14 days). 157

39)The potential for simultaneous occurrence of TRW and LCE currents at a site should be considered. 157

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