Derivation of Metocean Design and Operating Conditions

ANSI/API RECOMMENDED PRACTICE 2MET FIRST EDITION, NOVEMBER 2014

ISO 19901-1:2005 (Modified), Petroleum and natural gas industries—Specific requirements for offshore structures— Part 1: Metocean design and operating considerations

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API Foreword

The verbal forms used to express the provisions in this specification are as follows:

• the term “shall” denotes a minimum requirement in order to conform to the specification;

  
  

• the term “should” denotes a recommendation or that which is advised but not required in order to conform to the specification;

  
  

• the term “may” is used to express permission or a provision that is optional;

  
  

• the term “can” is used to express possibility or capability.

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Contents Page

API Foreword iii

Foreword vii

Introduction viii

1. Scope. 1

2. Normative references 2

3. Terms and definitions 2

4. Symbols and abbreviated terms 9

   1. Main symbols 9

   2. Abbreviated terms 11

5. Determining the relevant metocean parameters 12

   1. General 12

   2. Expert interpretation of the metocean database 12

   3. Selecting appropriate parameters for determining design actions or action effects 13

   4. The metocean database 13

   5. Storm types in a region 14

   6. Directionality 14

   7. Extrapolation to rare conditions 14

   8. Metocean parameters for fatigue assessments 15

   9. Metocean parameters for short-term activities 15

6. Water depth, tides and storm surges 16

   1. General 16

   2. Tides 17

   3. Storm surge 18

7. Wind 18

   1. General 18

   2. Wind actions and action effects 19

   3. Wind profile and time-averaged wind speed 19

   4. Wind spectra 19

8. Waves 20

   1. General 20

   2. Wave actions and action effects 20

   3. Intrinsic, apparent and encounter wave periods 21

   4. Two-dimensional wave kinematics 21

   5. Maximum height of an individual wave for long return periods 22

   6. Wave spectra 22

   7. Wave directional spreading function and spreading factor 22

   8. Wave crest elevation 22

9. Currents 23

   1. General 23

   2. Current velocities 23

   3. Current profile 24

   4. Current profile stretching 24

   5. Current blockage 24

10. Other environmental factors 24

    1. Marine growth 24

    2. Tsunamis 25

    3. Seiches 25

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    4. Sea ice and icebergs 25

    5. Snow and ice accretion 26

    6. Miscellaneous 26

Annex A (informative) Additional information and guidance 27

Annex B (informative) Discussion of wave frequency spectra 66

Annex C (informative) Regional information 76

Bibliography 160

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Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights.

ISO 19901-1 was prepared by Technical Committee ISO/TC 67, Materials, equipment and offshore structures for petroleum, petrochemical and natural gas industries, Subcommittee SC 7, Offshore structures.

ISO 19901 consists of the following parts, under the general title Petroleum and natural gas industries — Specific requirements for offshore structures:

• Part 1: Metocean design and operating considerations

• Part 2: Seismic design procedures and criteria

• Part 3: Topsides structure

• Part 4: Geotechnical and foundation design considerations

• Part 5: Weight control during engineering and construction

• Part 6: Marine operations

• Part 7: Stationkeeping systems for floating offshore structures and mobile offshore units

The following parts are under preparation:

• Part 8: Marine soil investigations

• Part 9: Structural integrity management

  
  

  ISO 19901 is one of a series of standards for offshore structures. The full series consists of the following International Standards.

• ISO 19900, Petroleum and natural gas industries — General requirements for offshore structures

• ISO 19901 (all parts), Petroleum and natural gas industries — Specific requirements for offshore structures

• ISO 19902, Petroleum and natural gas industries — Fixed steel offshore structures

• ISO 19903, Petroleum and natural gas industries — Fixed concrete offshore structures

• ISO 19904, Petroleum and natural gas industries — Floating offshore structures

• ISO 19905, Petroleum and natural gas industries — Site-specific assessment of mobile offshore units

• ISO 19906, Petroleum and natural gas industries — Arctic offshore structures

  
  

  
  

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  Introduction

  
  

  The series of International Standards applicable to types of offshore structure, ISO 19900 to ISO 19906, constitutes a common basis covering those aspects that address design requirements and assessments of all offshore structures used by the petroleum and natural gas industries worldwide. Through their application the intention is to achieve reliability levels appropriate for manned and unmanned offshore structures, whatever the type of structure and the nature or combination of the materials used.

  
  

  It is important to recognize that structural integrity is an overall concept comprising models for describing actions, structural analyses, design rules, safety elements, workmanship, quality control procedures and national requirements, all of which are mutually dependent. The modification of one aspect of design in isolation can disturb the balance of reliability inherent in the overall concept or structural system. The implications involved in modifications, therefore, need to be considered in relation to the overall reliability of all offshore structural systems.

  
  

  The series of International Standards applicable to types of offshore structure is intended to provide a wide latitude in the choice of structural configurations, materials and techniques without hindering innovation. Sound engineering judgment is therefore necessary in the use of these International Standards.

  
  

  The overall concept of structural integrity is described above. Some additional considerations apply for metocean design and operating conditions. The term “metocean” is short for “meteorological and oceanographic” and refers to the discipline concerned with the establishment of relevant environmental conditions for the design and operation of offshore structures. A major consideration in the design and operation of such a structure is the determination of actions on, and the behavior of, the structure as a result of winds, waves and currents.

  
  

  Environmental conditions vary widely around the world. For the majority of offshore locations there are little numerical data from historic conditions; comprehensive data often only start being collected when there is a specific need, for example, when exploration for hydrocarbons is being considered. Despite the usually short duration for which data are available, designers of offshore structures need estimates of extreme and abnormal environmental conditions (with an individual or joint probability of the order of

  1  102 / year and 1  103 to 1  104 / year, respectively).

  
  

  Even for areas like the Gulf of Mexico, offshore Indonesia and the North Sea, where there are up to 30 years of fairly reliable measurements available, the data are insufficient for rigorous statistical determination of appropriate extreme and abnormal environmental conditions. The determination of relevant design parameters has therefore to rely on the interpretation of the available data by specialists, together with an assessment of any other information, such as prevailing weather systems, ocean wave creation and regional and local bathymetry, coupled with consideration of data from comparable locations. It is hence important to employ specialists from both the metocean and structural communities in the determination of design parameters for offshore structures, particularly since setting of appropriate environmental conditions depends on the chosen option for the offshore structure.

  
  

  This part of ISO 19901 provides procedures and guidance for the determination of environmental conditions and their relevant parameters. Requirements for the determination of the actions on, and the behavior of, a structure in these environmental conditions are given in ISO 19901-3, ISO 19901-6, ISO 19901-7, ISO 19902, ISO 19903, ISO 19904, ISO 19905 and ISO 19906.

  
  

  Some background to, and guidance on, the use of this part of ISO 19901 is provided in informative Annex A. The clause numbering in Annex A is the same as in the normative text to facilitate cross- referencing.

  
  

  A discussion on wave spectra is provided in informative Annex B. Regional information, where available, is provided in informative Annex C.

  
  

  
  

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  ANSI/API Recommended Practice 2MET/ISO 19901-1:2005

  
  

  
  

  Petroleum and natural gas industries — Specific requirements for offshore structures —

  Part 1:

  Metocean design and operating considerations

  
  

  1 Scope

  
  

  This part of ISO 19901 gives general requirements for the determination and use of meteorological and oceanographic (metocean) conditions for the design, construction and operation of offshore structures of all types used in the petroleum and natural gas industries.

  
  

  The requirements are divided into two broad types:

  
  

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

     
     

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

  
  

  The environmental conditions and metocean parameters discussed comprise

  
  

• extreme and abnormal values of metocean parameters that recur with given return periods that are considerably longer than the design service life of the structure,

  
  

• long-term distributions of metocean parameters, in the form of cumulative, conditional, marginal or joint statistics of metocean parameters, and

  
  

• normal environmental conditions that are expected to occur frequently during the design service life of the structure.

  
  

  Metocean parameters are applicable to

  
  

• the determination of actions and action effects for the design of new structures,

  
  

• the determination of actions and action effects for the assessment of existing structures,

  
  

• the site-specific assessment of mobile offshore units,

  
  

• the determination of limiting environmental conditions, weather windows, actions and action effects for pre-service and post-service situations (i.e. fabrication, transportation and installation or decommissioning and removal of a structure), and

  
  

• the operation of the platform, where appropriate.

NOTE Specific metocean requirements for tension leg platforms are to be contained in API 2T[1], for site-specific assessment of jack-ups in ISO 19905-1[2], for arctic structures in ISO 19906[3] and for topsides structures in ISO 19901-3[4].

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