ISO/FDIS 19901-1

Oil and gas industries including lower carbon energy — Specific requirements for offshore structures — Part 1: Metocean design and operating considerations

ISO 19901-1:2015 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: - those that relate to the determination of environmental conditions in general, together with the metocean parameters that are required to adequately describe them; - 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 are: - 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 for the design of new structures, - the determination of actions 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 site-specific assessment of jack-ups are contained in ISO 19905‑1, for arctic offshore structures in ISO 19906 and for topside structures in ISO 19901‑3.

Industries du pétrole et du gaz, y compris les énergies à faible teneur en carbone — Exigences spécifiques relatives aux structures en mer — Partie 1: Dispositions océano-météorologiques pour la conception et l'exploitation

La présente partie de l'ISO 19901 fournit des exigences générales relatives à la détermination et à l'utilisation des conditions océanographiques et météorologiques (« océano-météorologiques ») pour la conception, la construction et l'exploitation des structures en mer de tous les types utilisés dans les industries du pétrole et du gaz naturel. Les exigences sont scindées en deux grandes catégories : — celles liées à la détermination des conditions environnementales en général, associées aux paramètres océano-météorologiques requis pour les décrire de manière appropriée ; — celles liées à la caractérisation et à l'utilisation des paramètres océano-météorologiques pour la conception, les activités de construction ou l'exploitation des structures en mer. Les conditions environnementales et les paramètres océano-météorologiques examinés sont : — les valeurs extrêmes et anormales des paramètres océano-météorologiques qui se répètent à des intervalles de récurrence donnés nettement plus longs que la durée de vie en service de la structure ; — les distributions à long terme des paramètres océano-météorologiques, sous forme de statistiques cumulées, conditionnelles, marginales ou conjointes ; et — les conditions environnementales normales dont l'occurrence fréquente est attendue tout au long de la durée de vie en service de la structure. Les paramètres océano-météorologiques sont applicables à : — la détermination des actions pour la conception des nouvelles structures ; — la détermination des actions pour l'évaluation des structures existantes ; — l'évaluation spécifique au site des unités mobiles en mer ; — la détermination des conditions environnementales restrictives, des fenêtres météorologiques, des actions et de leurs effets pour les situations de pré- et post-service (c'est-à-dire la fabrication, le transport et l'installation ou la mise hors service et l'enlèvement d'une structure) ; et — l'exploitation de la plate-forme, le cas échéant.

General Information

• Status: Not Published
• ICS: 75.180.10 - Exploratory, drilling and extraction equipment
• Technical Committee: ISO/TC 67/SC 7 - Offshore structures
• Drafting Committee: ISO/TC 67/SC 7 - Offshore structures
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 14-Apr-2026
• Completion Date: 14-Apr-2026

Relations

• Consolidates: FprEN ISO 19901-1 - Oil and gas industries including lower carbon energy - Specific requirements for offshore structures - Part 1: Metocean design and operating considerations (ISO/FDIS 19901-1:2026)
• Effective Date: 12-Feb-2026
• Revises: ISO 19901-1:2015 - Petroleum and natural gas industries — Specific requirements for offshore structures — Part 1: Metocean design and operating considerations
• Effective Date: 25-Jun-2022

Overview

ISO/FDIS 19901-1: Oil and gas industries including lower carbon energy - Specific requirements for offshore structures - Part 1: Metocean design and operating considerations is a key international standard developed by ISO for the oil, gas, and lower carbon energy sectors. This standard provides comprehensive guidance on determining and applying meteorological and oceanographic (metocean) conditions that are essential for the safe design, construction, and operation of all types of offshore structures. It covers the requirements for identifying, characterizing, and using metocean parameters, ensuring offshore structures are engineered to withstand both normal and extreme environmental conditions throughout their lifecycle.

By addressing the full spectrum of metocean issues, from data collection to the statistical treatment of extreme events, ISO/FDIS 19901-1 helps operators and designers reduce risk, enhance structural reliability, and meet regulatory requirements in the global offshore energy industry.

Key Topics

• Environmental Condition Determination: Requirements for the systematic identification of meteorological and oceanographic parameters, including wind, waves, tides, currents, storm surges, sea ice, and other relevant factors.
• Extreme and Abnormal Values: Guidance on estimating rare but critical events, such as severe storms or abnormal environmental loads, that may exceed the structure's typical design service life.
• Long-Term and Normal Data Distributions: Approaches to analyzing statistical distributions of metocean parameters, supporting fatigue assessments and planning for ongoing operations.
• Metocean Data Collection and Verification: Best practices for collecting, validating, and quality controlling meteorological and oceanographic data, including the use of expert assessments and regional datasets.
• Design and Operational Applications: How to apply metocean parameters during structural design, construction activities, operational planning, redevelopment, decommissioning, and for site-specific assessment of mobile offshore units.
• Regional Annexes: Informative guidance tailored for different global offshore regions, catering to the unique environmental characteristics and data availability in those areas.

Applications

ISO/FDIS 19901-1 is applied throughout the lifecycle of offshore structures in the oil and gas as well as renewables and lower carbon energy sectors. Its practical value is seen in the following areas:

• Design of New Offshore Structures: Ensures structures are engineered with reliable data on extreme and normal metocean conditions, optimizing safety and performance.
• Assessment of Existing Structures: Supports requalification and life extension by providing criteria for evaluating how older assets can withstand current or updated metocean loading.
• Site-Specific Assessment for Mobile Offshore Units: Enables the evaluation of floating or relocatable rigs (such as jack-ups) for particular locations, ensuring operational safety in variable environments.
• Planning for Construction, Installation, and Decommissioning: Defines environmental limits and weather windows for critical offshore operations, reducing downtime and enhancing safety.
• Operational Management: Assists in setting operational thresholds and guidelines, helping operators respond effectively to changing meteorological and oceanographic conditions.
• Compliance and Risk Management: Helps companies meet both regulatory and internal requirements for environmental risk, contributing to the sustainability and reliability of energy operations.

Related Standards

ISO/FDIS 19901-1 forms part of a comprehensive suite of offshore structural standards. Key related ISO standards include:

• ISO 19900: General requirements for offshore structures.
• ISO 19905-1: Site-specific assessment of mobile offshore units, including jack-ups.
• ISO 19906: Arctic offshore structures.
• ISO 19901-3: Specific requirements for topside structures.
• ISO 19902, ISO 19903, ISO 19904-1: Covering fixed steel, concrete, and floating offshore structures, respectively.
• ISO 19901-6: Marine operations.
• ISO 35106: Arctic operations - Metocean, ice, and seabed data.

By aligning with ISO/FDIS 19901-1 and the wider ISO 19901 series, organizations ensure that their offshore assets meet best-in-class international benchmarks for metocean consideration and operational resilience.

Standards Content (Sample)

FINAL DRAFT
International
Standard
ISO/TC 67/SC 7
Oil and gas industries including
Secretariat: BSI
lower carbon energy — Specific
Voting begins on:
requirements for offshore
2026-04-14
structures —
Voting terminates on:
2026-06-09
Part 1:
Metocean design and operating
considerations
Industries du pétrole et du gaz, y compris les énergies à faible
teneur en carbone — Exigences spécifiques relatives aux
structures en mer —
Partie 1: Dispositions océano-météorologiques pour la conception
et l'exploitation
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/CEN PARALLEL PROCESSING LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
FINAL DRAFT
International
Standard
ISO/TC 67/SC 7
Oil and gas industries including
Secretariat: BSI
lower carbon energy — Specific
Voting begins on:
requirements for offshore
structures —
Voting terminates on:
Part 1:
Metocean design and operating
considerations
Industries du pétrole et du gaz, y compris les énergies à faible
teneur en carbone — Exigences spécifiques relatives aux
structures en mer —
Partie 1: Dispositions océano-météorologiques pour la conception
et l'exploitation
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2026
IN ADDITION TO THEIR EVALUATION AS
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BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/CEN PARALLEL PROCESSING
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ii
Contents Page
Foreword .v
Introduction .vii
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols and abbreviated terms. 9
4.1 Symbols .9
4.2 Abbreviated terms . 12
5 Determining the relevant metocean parameters .12
5.1 General . 12
5.2 Expert development of metocean criteria. 13
5.3 Selecting appropriate parameters for determining design actions and action effects . 13
5.4 The metocean database .14
5.5 Storm types in a region . 15
5.6 Directionality . 15
5.7 Extrapolation to extreme and abnormal conditions . 15
5.8 Metocean parameters for fatigue assessments .16
5.9 Metocean parameters for short-term activities.16
5.10 Metocean parameters for medium-term activities .18
6 Water depth, tides and storm surges .18
6.1 General .18
6.2 Tides .18
6.3 Storm surges .19
6.4 Extreme water level .19
7 Wind .20
7.1 General . 20
7.2 Wind actions and action effects .21
7.3 Wind profile and time-averaged wind speed . 22
7.4 Wind spectra . 22
8 Waves .22
8.1 General . 22
8.2 Wave actions and action effects . 23
8.3 Sea-states — spectral waves .24
8.3.1 Wave spectrum .24
8.3.2 Directional spreading .24
8.3.3 Wave periods .24
8.3.4 Wave kinematics — velocities and accelerations . .24
8.4 Regular (periodic) waves . 25
8.4.1 General . 25
8.4.2 Wave period . 25
8.4.3 Wave kinematics — velocities and accelerations . . 25
8.4.4 Intrinsic, apparent and encounter wave periods . 26
8.5 Maximum height of an individual wave for long return periods .27
8.6 Linear and non-linear wave models .27
8.7 Wave crest elevation .27
9 Currents.28
9.1 General . 28
9.2 Current velocities . 28
9.3 Current profile . 29
9.4 Current profile stretching . 29
9.5 Current blockage . . 29

iii
9.6 Tidal currents . 29
10 Other environmental factors .30
10.1 Marine growth . 30
10.2 Tsunamis . 30
10.3 Seiches . 30
10.4 Sea ice and icebergs . .31
10.5 Snow and ice accretion .31
10.6 Thunderstorms and lightning .31
10.7 Rainfall .31
10.8 Squalls and downbursts .31
10.9 Internal waves and solitons .32
10.10 Shelf waves and eddies .32
10.11 Infra-gravity waves .32
10.12 Seawater temperature . 33
10.13 Miscellaneous . 33
11 Collection of metocean data .33
11.1 General . 33
11.2 Common requirements . 34
11.2.1 General . 34
11.2.2 Instrumentation. 34
11.3 Meteorology . 34
11.3.1 General . 34
11.3.2 Weather observation and reporting for helicopter operations . 34
11.3.3 Weather observation and reporting for weather forecasting services . 35
11.3.4 Weather observations for design purposes . 35
11.3.5 Weather observation and reporting for climatological purposes . 36
11.4 Oceanography . 36
11.4.1 General . 36
11.4.2 Measurements and observations. 36
11.5 Data quality control . 36
12 Verification of weather forecast information .37
12.1 General .37
13 Information concerning the annexes .37
13.1 Information concerning Annex A .37
13.2 Information concerning the regional annexes .37
Annex A (informative) Additional information and guidance .38
Annex B (informative) Northwest Europe .107
Annex C (informative) West Coast of Africa .117
Annex D (informative) Offshore Canada . 128
Annex E (informative) Sakhalin/Sea of Okhotsk . 154
Annex F (informative) Caspian Sea .178
Annex G (informative) South East Asian Sea .196
Annex H (informative) Mediterranean Sea .217
Annex I (informative) Brazil . 243
Annex J (informative) US Gulf of Mexico .256
Annex K (informative) US Coast of California .307
Annex L (informative) Other US Waters .312
Bibliography .316

iv
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.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 67, Oil and gas industries including lower
carbon energy, Subcommittee SC 7, Offshore structures, in collaboration with the European Committee for
Standardization (CEN) Technical Committee CEN/TC 12, Oil and gas industries including lower carbon energy,
in accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This third edition cancels and replaces the second edition (ISO 19901-1:2015), which has been technically
revised.
The main changes are as follows:
— clarification on the role of the metocean expert (5.2 and A.5.2);
— additional information related to the determination of associated criteria (5.3 and A.5.3);
— additional information related to the estimation of extreme or abnormal conditions (5.7 and A.5.7);
— alignment of the content related to wind with API RP 2MET (Clause 7 and Clause A.7);
— additional information related to breaking or non-breaking wave kinematic estimation (8.4.3 and
A.8.4.3);
— expansion of the content related to additional environment factors to be considered (Clause 10 and
Clause A.10);
— introduction of text related to the verification of weather forecast information (Clause 12 and Clause A.12);
— update to offshore Canada regional annex (Annex D);
— update to Sakhalin/Sea of Okhotsk regional annex (Annex E);
— update to Caspian Sea regional annex (Annex F);
— introduction of Mediterranean Sea regional annex (Annex H);

v
— introduction of Brazil regional annex (Annex I);
— re-introduction of US Gulf of Mexico regional annex (Annex J);
— re-introduction of coast of California regional annex (Annex K);
— re-introduction of the overview annex of regions excluding Gulf of Mexico and California (Annex L).
A list of all parts in the ISO 19901 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

vi
Introduction
The International Standards on offshore structures prepared by TC 67 (i.e. ISO 19900, the ISO 19901 series,
ISO 19902, ISO 19903, ISO 19904-1, the ISO 19905 series, ISO 19906) constitute a common basis covering
those aspects that address design requirements and assessments of all offshore structures used by the oil
and gas industries including lower carbon energy 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, should be considered in relation to the overall reliability of
all offshore structural systems.
The International Standards on offshore structures prepared by TC 67 are intended to provide a wide
latitude in the choice of structural configurations, materials and techniques without hindering innovation.
Sound engineering judgement is therefore necessary in the use of these International Standards.
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
behaviour 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
−2 −3 −4
1 × 10 /year and 1 × 10 to 1 × 10 /year, respectively).
Even for areas like the Gulf of Mexico, offshore Indonesia and the North Sea, where there are over 40 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 experts, 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. In particular, due
account should be taken of the uncertainties that arise from the analyses of limited datasets. It is hence
important to employ experts 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.
Requirements for the determination of the actions on, and the behaviour of, a structure in these
environmental conditions are given in ISO 19901-3, ISO 19901-6, ISO 19901-7, ISO 19902, ISO 19903,
ISO 19904-1, ISO 19905-1 and ISO 19906.
NOTE Specific metocean requirements for site-specific assessment of jack-ups are contained in ISO 19905-1,
for arctic offshore structures in ISO 19906, for arctic offshore operations in ISO 35106 and for topside structures in
ISO 19901-3.
Some background to, and guidance on, the use of this document is provided in Annex A. The clause numbering
in Annex A is the same as in the main text to facilitate cross-referencing.
Regional information, where available, is provided in the Annexes B to L. This information has been
developed by experts from the region or country concerned to supplement the guidance provided in this
document. Each regional annex provides regional or national data on environmental conditions for the area
concerned.
vii
FINAL DRAFT International Standard ISO/FDIS 19901-1:2026(en)
Oil and gas industries including lower carbon energy —
Specific requirements for offshore structures —
Part 1:
Metocean design and operating considerations
IMPORTANT — The electronic file of this document contains colours which are considered to be
useful for the correct understanding of the document. Users should therefore consider printing this
document using a colour printer.
1 Scope
This document provides two broad types of requirements for the determination and use of meteorological
and oceanographic (metocean) conditions for the design, construction and operation of offshore structures:
— those related to the determination of environmental conditions in general, together with the metocean
parameters that are required to adequately describe them;
— those related to the characterization and use of metocean parameters for the design, the construction
activities or the operation of offshore structures.
This document covers the following environmental conditions and metocean parameters:
— 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;
— normal environmental conditions that are expected to occur frequently during the design service life of
the structure.
Metocean parameters specified in this document are applicable to:
— the determination of actions for the design of new structures;
— the determination of actions 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);
— the operation of the platform, where appropriate.
It is beyond the scope of this document to provide detailed instructions that can be followed to produce
reliable estimates of extreme or abnormal conditions in all areas and in all cases.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.

ISO 19900, Petroleum and natural gas industries — General requirements for offshore structures
ISO 19906, Petroleum and natural gas industries — Arctic offshore structures
3 Terms and definitions
For the purpose of this document, the terms and definitions given in ISO 19900 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
abnormal value
design value of a parameter of abnormal severity used in accidental limit state checks in which a structure is
intended not to suffer complete loss of integrity
Note 1 to entry: Abnormal events are typically accidental and environmental (including seismic) events having
−3 −4
probabilities of exceedance of the order of 10 to 10 per annum.
3.2
chart datum
local datum used to fix water depths (3.51) on a chart or tidal heights over an area
Note 1 to entry: Chart datum is usually an approximation to the level of the lowest astronomical tide (3.17).
Note 2 to entry: Chart datum can differ from one chart to another and care is required if cross referencing sites that
are not on the same chart.
3.3
conditional probability
probability of the occurrence of a variable A, given that other variables B, C, … have certain assigned values
Note 1 to entry: The conditional probability of A given that B, C, … occur is written as P(A|B,C,…). The concept is
applicable to metocean parameters, as well as to actions and action effects.
EXAMPLE When considering wave parameters, A can be the individual crest elevation, B the water depth (3.51)
and C the significant wave height (3.36), and so on.
3.4
design crest elevation
extreme crest elevation measured relative to still water level (3.44)
Note 1 to entry: The design crest elevation is used in combination with information on astronomical tide, storm surge
(3.45), platform settlement, reservoir subsidence and water depth (3.51) uncertainty and is derived using extreme
value (3.7) analysis. Where simplified models are used to estimate the kinematics of the design wave (3.5), the design
crest elevation can be different from (usually somewhat greater than) the crest elevation of the design wave used to
calculate actions on the structure. In reality, the wave with the greatest trough-to-crest height and the wave with the
highest crest are different waves.
3.5
design wave
deterministic wave used for the design of an offshore structure
Note 1 to entry: The design wave is an engineering abstraction. Most often it is a periodic wave with suitable
characteristics (e.g. height H, period T, steepness, crest elevation). The choice of a design wave depends on:
— the design purpose(s) considered;
— the wave environment;
— the geometry of the structure;

— the type of action(s) or action effect(s) pursued.
Note 2 to entry: Normally, a design wave is only compatible with design situations in which the action effect(s) are
quasi-statically related to the associated wave actions on the structure.
3.6
extreme water level
combination of design crest elevation (3.4), astronomical tide and storm surge (3.45) referenced to either LAT
(3.17) or MSL (3.20)
3.7
extreme value
representative value of a parameter used in ultimate limit state checks
−2
Note 1 to entry: Extreme events have probabilities of the order of 10 per annum.
3.8
gravity wave
wave in a fluid or in the interface between two fluids for which the predominant restoring forces are gravity
and buoyancy
Note 1 to entry: Wind-generated surface waves.
3.9
gust
brief rise and fall in wind speed lasting less than 1 min
Note 1 to entry: In some countries, gusts are reported in meteorological observations if the maximum wind speed
exceeds approximately 8 m/s.
3.10
gust wind speed
maximum value of the wind speed of a gust (3.9) averaged over a short (3 s to 60 s) specified duration within
a longer (1 min to 1 h) specified duration, measured at a given elevation
Note 1 to entry: For design purposes, the specified duration depends on the dimensions and natural period of (part of)
the structure being designed such that the structure is designed for the most onerous conditions; thus, a small part
of a structure is designed for a shorter gust wind speed duration (and hence a higher gust wind speed) than a larger
(part of a) structure.
3.11
highest astronomical tide
HAT
level of high tide when all harmonic components causing the tides are in phase
Note 1 to entry: The harmonic components are in phase approximately once every 19 years, but these conditions are
approached several times each year.
3.12
hindcasting
method of simulating historical (metocean) data for a region through numerical modelling
3.13
infra-gravity wave
surface gravity wave (3.8) with a period in the range of approximately 25 s to 500 s
Note 1 to entry: In principle an infra-gravity wave is generated by different physical processes but is most commonly
associated with waves generated by non-linear second-order difference frequency interactions between different
swell (3.47) wave components.
3.14
internal wave
gravity wave (3.8) which propagates within a stratified water column

3.15
JONSWAP
Joint North Sea Project Spectrum
version of the Pierson-Moskowitz spectrum (3.42) which accounts for the continued development of the
spectrum through non-linear wave-wave interaction over time and space
3.16
long-term distribution
probability distribution of a variable over a long time scale
Note 1 to entry: The time scale exceeds the duration of a sea-state (3.32), in which the statistics are assumed constant
(see short-term distribution (3.35)). The time scale is hence comparable to a season or to the design service life of a
structure.
EXAMPLE Long-term distributions of:
— significant wave height (3.36) (based on, for example, storm peaks or all sea-states);
— significant wave height in the months May to September;
— individual wave heights;
— current speeds (such as for use in assessing vortex-induced vibrations of drilling risers);
— scatter diagrams (3.30) with the joint distribution of significant wave height and wave period (such as for
use in a fatigue analysis);
— a particular action effect;
— sea ice types and thickness;
— iceberg mass and velocity;
— storm maximum significant wave height.
3.17
lowest astronomical tide
LAT
level of low tide when all harmonic components causing the tides are in phase
Note 1 to entry: The harmonic components are in phase approximately once every 19 years, but these conditions are
approached several times each year.
3.18
marginal distribution
marginal probability
statistical distribution (probability) of the occurrence of a variable A independent of any other variable
Note 1 to entry: The marginal distribution is obtained by integrating the full distribution over all values of the other
variables B, C, … and is written as P(A). The concept is applicable to metocean parameters, as well as to actions and
action effects.
EXAMPLE When considering wave conditions, A can be the individual crest elevation for all mean zero-crossing
periods (3.22) B and all significant wave heights (3.36) C, occurring at a particular site.
3.19
marine growth
living organisms attached to an offshore structure

3.20
mean sea level
MSL
arithmetic mean of all sea levels measured over a long period, a minimum of 1 year
Note 1 to entry: Seasonal changes in mean level can be expected in some regions and over many years the mean sea
level can change.
3.21
mean wind speed
time-averaged wind speed, averaged over a specified time interval and at a specified elevation
Note 1 to entry: The mean wind speed varies with elevation above mean sea level (3.20) and the averaging time
interval; a standard reference elevation is 10 m and with an averaging time of 10 min. See also gust wind speed (3.10)
and sustained wind speed (3.46).
3.22
mean zero-crossing period
average period between (up or down) zero-crossing waves in a sea-state (3.32)
Note 1 to entry: In practice the mean zero-crossing period is often estimated from the zeroth and second moments of
the wave spectrum (3.52) as TT mf //mf 2mm  .
   
z 20 20 2
3.23
monsoon
seasonally reversing wind pattern, with associated pattern of rainfall
Note 1 to entry: The term was first applied to the winds over the Arabian Sea which blow for six months from northeast
and for six months from southwest, but it has been extended to similar winds in other parts of the world.
3.24
most probable maximum
value of the maximum of a variable with the highest probability of occurring
Note 1 to entry: The most probable maximum is the value for which the probability density function of the maxima of
the variable has its peak. It is also called the mode or modus of the statistical distribution.
3.25
operating conditions
most severe combination of environmental conditions under which a given operation is permitted to proceed
Note 1 to entry: Operating conditions are determined for operations that exert a significant action on the structure.
Operating conditions are usually a compromise: they are sufficiently severe that the operation can generally be
performed without excessive downtime, but they are not so severe that they have an undue impact on design or safety.
3.26
polar low
depression that forms in polar air, often near a boundary between ice and sea
3.27
residual current
part of the total current that is not constituted from harmonic tidal components (i.e. the tidal stream)
Note 1 to entry: Residual currents are caused by a variety of physical mechanisms and comprise a large range of
natural frequencies and magnitudes in different parts o
...

ISO/DISFDIS 19901-1:2025(en)
ISO/TC 67/SC 7
Secretariat: BSI
Date: 2025-11-052026-03-30
Oil and gas industries including lower carbon energy — Specific
requirements for offshore structures — —
Part 1:
Metocean design and operating considerations
Industries du pétrole et du gaz, y compris les énergies à faible teneur en carbone — Exigences spécifiques
relatives aux structures en mer — —
Partie 1: Dispositions océano-météorologiques pour la conception et l'exploitation
FDIS stage
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All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication
may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying,
or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO
at the address below or ISO’s member body in the country of the requester.
ISO copyright office
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CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
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Website: www.iso.org
Published in Switzerland
ii
Table of Contents
Foreword . v
Introduction . vii
1 Scope . 1
2 Normative references . 2
3 Terms and definitions . 3
4 Symbols and abbreviated terms . 11
4.1 Symbols . 11
4.2 Abbreviated terms . 13
5 Determining the relevant metocean parameters . 13
5.1 General. 13
5.2 Expert development of metocean criteria . 14
5.3 Selecting appropriate parameters for determining design actions and action effects . 14
5.4 The metocean database . 16
5.5 Storm types in a region . 16
5.6 Directionality . 16
5.7 Extrapolation to extreme and abnormal conditions . 17
5.8 Metocean parameters for fatigue assessments . 18
5.9 Metocean parameters for short-term activities . 18
5.10 Metocean parameters for medium-term activities . 20
6 Water depth, tides and storm surges . 20
6.1 General. 20
6.2 Tides . 20
6.3 Storm surges . 21
6.4 Extreme water level . 21
7 Wind. 22
7.1 General. 22
7.2 Wind actions and action effects . 23
7.3 Wind profile and time-averaged wind speed . 24
7.4 Wind spectra . 24
8 Waves . 24
8.1 General. 24
8.2 Wave actions and action effects . 25
8.3 Sea-states — spectral waves . 26
8.4 Regular (periodic) waves . 27
8.5 Maximum height of an individual wave for long return periods . 29
8.6 Linear and non-linear wave models . 30
8.7 Wave crest elevation . 30
9 Currents . 31
9.1 General. 31
9.2 Current velocities. 31
9.3 Current profile . 32
9.4 Current profile stretching . 32
9.5 Current blockage . 32
9.6 Tidal currents . 32
10 Other environmental factors . 32
10.1 Marine growth . 32
10.2 Tsunamis . 33
iii
10.3 Seiches . 33
10.4 Sea ice and icebergs . 33
10.5 Snow and ice accretion . 34
10.6 Thunderstorms and lightning . 34
10.7 Rainfall . 34
10.8 Squalls and downbursts . 34
10.9 Internal waves and solitons . 35
10.10 Shelf waves and eddies . 35
10.11 Infra-gravity waves . 35
10.12 Seawater temperature . 36
10.13 Miscellaneous . 36
11 Collection of metocean data . 36
11.1 General. 36
11.2 Common requirements . 37
11.3 Meteorology . 37
11.4 Oceanography . 39
11.5 Data quality control . 40
12 Verification of weather forecast information . 40
12.1 General. 40
13 Information concerning the annexes . 41
13.1 Information concerning Annex A . 41
13.2 Information concerning the regional annexes . 41
Annex A (informative) Additional information and guidance . 42
Annex B (informative) Northwest Europe . 127
Annex C (informative) West Coast of Africa . 141
Annex D (informative) Offshore Canada . 154
Annex E (informative) Sakhalin/Sea of Okhotsk . 196
Annex F (informative) Caspian Sea . 236
Annex G (informative) South East Asian Sea . 259
Annex H (informative) Mediterranean Sea. 292
Annex I (informative) Brazil . 329
Annex J (informative) US Gulf of Mexico . 353
Annex K (informative) US Coast of California . 424
Annex L (informative) Other US Waters . 430
Bibliography . 435

iv
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.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of
ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights
in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s)
which may be required to implement this document. However, implementers are cautioned that this may not
represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 67, Oil and gas industries including lower carbon
energy, Subcommittee SC 7, Offshore structures, in collaboration with the European Committee for
Standardization (CEN) Technical Committee CEN/TC 12, Oil and gas industries including lower carbon energy,
in accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This third edition cancels and replaces the second edition (ISO 19901-1:2015), which has been technically
revised.
The main changes are as follows:
— — clarification on the role of the metocean expert (5.2(5.2 and A.5.2A.5.2););
— — additional information related to the determination of associated criteria (5.3(5.3 and A.5.3A.5.3););
— — additional information related to the estimation of extreme or abnormal conditions (5.7(5.7 and
A.5.7A.5.7););
— — alignment of the content related to wind with API RP 2MET (Clause 7(Clause 7 and
Clause A.7Clause A.7););
— — additional information related to breaking or non-breaking wave kinematic estimation (8.4.3(8.4.3
and A.8.4.3A.8.4.3););
— — expansion of the content related to additional environment factors to be considered
(Clause 10(Clause 10 and Clause A.10Clause A.10););
v
— — introduction of text related to the verification of weather forecast information (Clause 12(Clause 12
and Clause A.12Clause A.12););
— — update to offshore Canada regional annex (Annex D(Annex D););
— — update to Sakhalin/Sea of Okhotsk regional annex (Annex E(Annex E););
— — update to Caspian Sea regional annex (Annex F(Annex F););
— — introduction of Mediterranean Sea regional annex (Annex H(Annex H););
— — introduction of Brazil regional annex (Annex I(Annex I););
— — re-introduction of US Gulf of Mexico regional annex (Annex J(Annex J););
— — re-introduction of coast of California regional annex (Annex K(Annex K););
— — re-introduction of the overview annex of regions excluding Gulf of Mexico and
California (Annex L(Annex L).).
A list of all parts in the ISO 19901 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
vi
Introduction
The International Standards on offshore structures prepared by TC 67 (i.e. ISO 19900, the ISO 19901 series,
ISO 19902, ISO 19903, ISO 19904-1, the ISO 19905 series, ISO 19906) constitute a common basis covering
those aspects that address design requirements and assessments of all offshore structures used by the oil and
gas industries including lower carbon energy 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, should be considered in relation to the overall reliability of
all offshore structural systems.
The International Standards on offshore structures prepared by TC 67 isare intended to provide a wide
latitude in the choice of structural configurations, materials and techniques without hindering innovation.
Sound engineering judgement is therefore necessary in the use of these International Standards.
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
behaviour 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
−2
abnormal environmental conditions (with an individual or joint probability of the order of 1 × 10 /year and
−3 −4
1 × 10 to 1 × 10 /year, respectively).
Even for areas like the Gulf of Mexico, offshore Indonesia and the North Sea, where there are over 40 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 experts, 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. In particular, due
account should be taken of the uncertainties that arise from the analyses of limited datasets. It is hence
important to employ experts 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.
Requirements for the determination of the actions on, and the behaviour 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--1,
ISO 19905--1 and ISO 19906.
NoteNOTE Specific metocean requirements for site-specific assessment of jack-ups are contained in ISO 19905--1, for
arctic offshore structures in ISO 19906, for arctic offshore operations in ISO 35106 and for topside structures in
ISO 19901--3.
Some background to, and guidance on, the use of this document is provided in Annex AAnnex A. The clause
numbering in Annex AAnnex A is the same as in the main text to facilitate cross-referencing.
vii
Regional information, where available, is provided in the Annexes BAnnexes B to LL. This information has
been developed by experts from the region or country concerned to supplement the guidance provided in this
document. Each regional annex provides regional or national data on environmental conditions for the area
concerned.
viii
Oil and gas industries including lower carbon energy — Specific
requirements for offshore structures — —
Part 1:
Metocean design and operating considerations
IMPORTANT — The electronic file of this document contains colours which are considered to be useful
for the correct understanding of the document. Users should therefore consider printing this
document using a colour printer.
1 Scope
This document provides two broad types of requirements for the determination and use of meteorological
and oceanographic (metocean) conditions for the design, construction and operation of offshore structures:
— — those related to the determination of environmental conditions in general, together with the metocean
parameters that are required to adequately describe them;
— — those related to the characterization and use of metocean parameters for the design, the construction
activities or the operation of offshore structures.
This document covers the following environmental conditions and metocean parameters:
— — 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;
normal environmental conditions that are expected to occur frequently during the design service life
— —
of the structure.
Metocean parameters specified in this document are applicable to:
— — the determination of actions for the design of new structures;
— — the determination of actions 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);
— — the operation of the platform, where appropriate.
It is beyond the scope of this part of ISO 19901documentdocument to provide detailed instructions that can
be followed to produce reliable estimates of extreme or abnormal conditions in all areas and in all cases.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 19900, Petroleum and natural gas industries — General requirements for offshore structures
ISO 19901 (all parts), Oil and gas industries including lower carbon energy — Specific requirements for offshore
structures:
— Part 1 Petroleum and natural gas industries. Specific requirements for offshore structures. Metocean
design and operating considerations
— Part 2 Petroleum and natural gas industries. Specific requirements for offshore structures. Seismic design
procedures and criteria
— Part 3 Petroleum and natural gas industries. Specific requirements for offshore structures. Topsides
structure
— Part 4 Petroleum and natural gas industries. Specific requirements for offshore structures. Geotechnical
and foundation design considerations
— Part 5 Petroleum and natural gas industries. Specific requirements for offshore structures. Weight control
during engineering and construction
— Part 6 Petroleum and natural gas industries. Specific requirements for offshore structures. Marine
operations
— Part 7 Petroleum and natural gas industries. Specific requirements for offshore structures. Stationkeeping
systems for floating offshore structures and mobile offshore units
— Part 8 Petroleum and natural gas industries. Specific requirements for offshore structures. Marine soil
investigations
— Part 9 Petroleum and natural gas industries. Specific requirements for offshore structures. Structural
integrity management
— Part 10 Petroleum and natural gas industries. Specific requirements for offshore structures. Marine
geophysical investigations:
ISO 19902, Petroleum and natural gas industries — Fixed steel offshore structures
ISO 19903, Petroleum and natural gas industries — Concrete offshore structures
ISO 19904-1, Petroleum and natural gas industries — Floating offshore structures — Part 1: Ship-shaped, semi-
submersible, spar and shallow-draught cylindrical structures
ISO 19905-1, Oil and gas industries including lower carbon energy — Site-specific assessment of mobile offshore
units — Part 1: Jack-ups: elevated at a site
ISO 19906, Petroleum and natural gas industries — Arctic offshore structures
ISO 35106, Petroleum and natural gas industries — Arctic operations — Metocean, ice, and seabed data
3 Terms and definitions
For the purpose of this document, the terms and definitions given in ISO 19900 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— — ISO Online browsing platform: available at https://www.iso.org/obp
— — IEC Electropedia: available at https://www.electropedia.org/
3.1 3.1
abnormal value
design value of a parameter of abnormal severity used in accidental limit state checks in which a structure is
intended not to suffer complete loss of integrity
Note 1 to entry: Abnormal events are typically accidental and environmental (including seismic) events having
−3 −4
probabilities of exceedance of the order of 10 to 10 per annum.
3.2 3.2
chart datum
local datum used to fix water depths (3.51(3.51)) on a chart or tidal heights over an area
Note 1 to entry: Chart datum is usually an approximation to the level of the lowest astronomical tide (3.17(3.17).).
Note 2 to entry: Chart datum can differ from one chart to another and care is required if cross referencing sites that are
not on the same chart.
3.3 3.3
conditional probability distribution
statistical distribution of the probability of the occurrence of a variable A, given that other variables B, C, …
have certain assigned values
Note 1 to entry: The conditional probability of A given that B, C, … occur is written as P(A|B,C,…). The concept is applicable
to metocean parameters, as well as to actions and action effects.
EXAMPLE When considering wave parameters, A can be the individual crest elevation, B the water depth
(3.51(3.51)) and C the significant wave height (3.36(3.36),), and so on.
3.4 3.4
design crest elevation
extreme crest elevation measured relative to still water level (3.44(3.44))
Note 1 to entry: The design crest elevation is used in combination with information on astronomical tide, storm surge
(3.45(3.45),), platform settlement, reservoir subsidence and water depth (3.51(3.51)) uncertainty and is derived using
extreme value (3.7(3.7)) analysis. Where simplified models are used to estimate the kinematics of the design wave
(3.5(3.5),), the design crest elevation can be different from (usually somewhat greater than) the crest elevation of the
design wave used to calculate actions on the structure. In reality, the wave with the greatest trough-to-crest height and
the wave with the highest crest will beare different waves.
3.5 3.5
design wave
deterministic wave used for the design of an offshore structure
Note 1 to entry: The design wave is an engineering abstraction. Most often it is a periodic wave with suitable
characteristics (e.g. height H, period T, steepness, crest elevation). The choice of a design wave depends on:
— — the design purpose(s) considered;
— — the wave environment;
— — the geometry of the structure;
— — the type of action(s) or action effect(s) pursued.
Note 2 to entry: Normally, a design wave is only compatible with design situations in which the action effect(s) are quasi-
statically related to the associated wave actions on the structure.
3.6 3.6
extreme water level
combination of design crest elevation (3.4(3.4),), astronomical tide and storm surge (3.45(3.45)) referenced to
either LAT (3.17(3.17)) or MSL (3.20(3.20))
3.7 3.7
extreme value
representative value of a parameter used in ultimate limit state checks
−2
Note 1 to entry: Extreme events have probabilities of the order of 10 per annum.
3.8 3.8
gravity wave
wave in a fluid or in the interface between two fluids for which the predominant restoring forces are gravity
and buoyancy
EXAMPLE Note 1 to entry: Wind-generated surface waves.
3.9 3.9
gust
brief rise and fall in wind speed lasting less than 1 min
Note 1 to entry: In some countries, gusts are reported in meteorological observations if the maximum wind speed
exceeds approximately 8 m/s.
3.10 3.10
gust wind speed
maximum value of the wind speed of a gust (3.9(3.9)) averaged over a short (3 s to 60 s) specified duration
within a longer (1 min to 1 h) specified duration, measured at a given elevation.
Note 1 to entry: For design purposes, the specified duration depends on the dimensions and natural period of (part of)
the structure being designed such that the structure is designed for the most onerous conditions; thus, a small part of a
structure is designed for a shorter gust wind speed duration (and hence a higher gust wind speed) than a larger (part of
a) structure.
3.11 3.11
highest astronomical tide
HAT
level of high tide when all harmonic components causing the tides are in phase
Note 1 to entry: The harmonic components are in phase approximately once every 19 years, but these conditions are
approached several times each year.
3.12 3.12
hindcasting
method of simulating historical (metocean) data for a region through numerical modelling
3.13 3.13
infra-gravity wave
surface gravity wave (3.8(3.8)) with a period in the range of approximately 25 s to 500 s
Note 1 to entry: In principle an infra-gravity wave is generated by different physical processes but is most commonly
associated with waves generated by non-linear second-order difference frequency interactions between different swell
(3.47(3.47)) wave components.
3.14 3.14
internal wave
gravity wave (3.8(3.8)) which propagates within a stratified water column
3.15 3.15
JONSWAP
Joint North Sea Project Spectrum
version of the Pierson-Moskowitz spectrum (3.42(3.42)) which accounts for the continued development of the
spectrum through non-linear wave-wave interaction over time and space
3.16 3.16
long-term distribution
probability distribution of a variable over a long time scale
Note 1 to entry: The time scale exceeds the duration of a sea-state (3.32(3.32),), in which the statistics are assumed
constant (see short-term distribution (3.35(3.35)).)). The time scale is hence comparable to a season or to the design
service life of a structure.
EXAMPLE Long-term distributions of:
— — significant wave height (3.36(3.36)) (based on, for example, storm peaks or all sea-states);
— — significant wave height in the months May to September;
— — individual wave heights;
— — current speeds (such as for use in assessing vortex-induced vibrations of drilling risers);
— — scatter diagrams (3.30(3.30)) with the joint distribution of significant wave height and wave period (such as for
use in a fatigue analysis);
— — a particular action effect;
— — sea ice types and thickness;
— — iceberg mass and velocity;
— — storm maximum significant wave height.
3.17 3.17
lowest astronomical tide
LAT
level of low tide when all harmonic components causing the tides are in phase
Note 1 to entry: The harmonic components are in phase approximately once every 19 years, but these conditions are
approached several times each year.
3.18 3.18
marginal distribution
marginal probability
statistical distribution (probability) of the occurrence of a variable A independent of any other variable
Note 1 to entry: The marginal distribution is obtained by integrating the full distribution over all values of the other
variables B, C, … and is written as P(A). The concept is applicable to metocean parameters, as well as to actions and action
effects.
EXAMPLE When considering wave conditions, A can be the individual crest elevation for all mean zero-crossing
periods (3.22(3.22)) B and all significant wave heights (3.36(3.36)) C, occurring at a particular site.
3.19 3.19
marine growth
living organisms attached to an offshore structure
3.20 3.20
mean sea level
MSL
arithmetic mean of all sea levels measured over a long period, a minimum of 1 year
Note 1 to entry: Seasonal changes in mean level can be expected in some regions and over many years the mean sea level
can change.
3.21 3.21
mean wind speed
time-averaged wind speed, averaged over a specified time interval and at a specified elevation
Note 1 to entry: The mean wind speed varies with elevation above mean sea level (3.20(3.20)) and the averaging time
interval; a standard reference elevation is 10 m and with an averaging time of 10 min. See also gust wind speed
(3.10(3.10)) and sustained wind speed (3.46(3.46).).
3.22 3.22
mean zero-crossing period
average period between (up or down) zero-crossing waves in a sea-state (3.32(3.32))
Note 1 to entry: In practice the mean zero-crossing period is often estimated from the zeroth and second moments of the
wave spectrum (3.52(3.52) as .) as 𝑇𝑇 =𝑇𝑇 =�𝑚𝑚 (𝑓𝑓)/𝑚𝑚 (𝑓𝑓) = 2𝜋𝜋�𝑚𝑚 (𝜔𝜔)/𝑚𝑚 (𝜔𝜔).
𝑧𝑧 2 0 2 0 2
3.23 3.23
monsoon
seasonally reversing wind pattern, with associated pattern of rainfall
Note 1 to entry: The term was first applied to the winds over the Arabian Sea which blow for six months from northeast
and for six months from southwest, but it has been extended to similar winds in other parts of the world.
3.24 3.24
most probable maximum
value of the maximum of a variable with the highest probability of occurring
Note 1 to entry: The most probable maximum is the value for which the probability density function of the maxima of the
variable has its peak. It is also called the mode or modus of the statistical distribution.
3.25 3.25
operating conditions
most severe combination of environmental conditions under which a given operation is permitted to proceed
Note 1 to entry: Operating conditions are determined for operations that exert a significant action on the structure.
Operating conditions are usually a compromise: they are sufficiently severe that the operation can generally be
performed without excessive downtime, but they are not so severe that they have an undue impact on design or safety.
3.26 3.26
polar low
depression that forms in polar air, often near a boundary between ice and sea
3.27 3.27
residual current
part of the total current that is not constituted from harmonic tidal components (i.e. the tidal stream)
Note 1 to entry: Residual currents are caused by a variety of physical mechanisms and comprise a large range of natural
frequencies and magnitudes in different parts of the world.
3.28 3.28
residual water level
part of the total water level that is not constituted from harmonic tidal components (i.e. the tidal height)
3.29
3.29
return period
average period between occurrences of an event or of a particular value being exceeded
Note 1 to entry: The offshore industry commonly uses a return period measured in years for environmental events. For
a rare event, the return period in years is equal to the reciprocal of the annual probability of exceedance of the event.
3.30 3.30
scatter diagram
joint probability of two or more (metocean) parameters
Note 1 to entry: A scatter diagram is especially used with wave parameters in the metocean context (for example in
fatigue assessments). The wave scatter diagram is commonly understood to be the probability of the joint occurrence of
the significant wave height (3.36(3.36)) (Hs) and a representative period (Tz or Tp).
3.31 3.31
sea floor
interface between the sea and the seabed (3.33(3.33)) and referring to the upper surface of all unconsolidated
material
3.32 3.32
sea-state
condition of the sea during a period in which its statistics remain approximately stationary and egodic
Note 1 to entry: In a statistical sense the sea-state does not change markedly within the period. The period during which
this condition exists depends on the particular weather condition at any given time, however it is often assumed to be
3 h in extra-tropical conditions, however it can be as short as 1 h in tropical cyclonic conditions.
3.33 3.33
seabed
materials below the sea in which a structure is founded, whether of soils such as sand, silt or clay, cemented
material or of rock
3.34 3.34
seiche
oscillation of a body of water at its natural period
3.35 3.35
short-term distribution
probability distribution of a variable within a short interval of time during which conditions are assumed to
be statistically stationary
Note 1 to entry: The interval chosen is most often the duration of a sea-state (3.32(3.32).).
3.36 3.36
significant wave height
statistical measure of the height of waves in a sea-state (3.32(3.32))
Note 1 to entry: The significant wave height was originally defined as the mean height of the highest one-third of the zero
up-crossing waves in a sea-state. In most offshore data acquisition systems the significant wave height is currently taken
as ,4 𝑚𝑚 , (where m0 is the zeroth spectral moment (3.40(3.40)))) or 4σ, where σ is the standard deviation of the time
�
series of water surface elevation over the duration of the measurement, typically a period of approximately 30 min.
3.37 3.37
site-averaging
averaging extreme values (3.7(3.7)) from a number of individual sites
Note 1 to entry: It is used to take account of the localised extent of phenomena such as tropical cyclones (3.49(3.49)) or
meso-scale eddies e.g. warm core rings in the Gulf of Mexico.
3.38 3.38
site-pooling
concatenating datasets from several sites into a single dataset for extremal analysis, with the length of the
dataset equating to the sum of the individual datasets.
Note 1 to entry: It is used to take account of the localised extent of phenomena such as tropical cyclones (3.49(3.49)) or
meso-scale eddies e.g. warm core rings in the Gulf of Mexico.
3.39 3.39
soliton
solitary wave or wave packet travelling on an internal density discontinuity which, as a result of the
cancellation of non-linear and dispersive effects, maintains its shape and speed over extended distances
EXAMPLE Internal tides which form on the density gradient within the water column can interact with the
continental slope and form internal solitary wave packets.
3.40 3.40
spectral moment
integral over frequency of the spectral density function (3.42(3.42)) multiplied by the nth power of the
∞
𝑛𝑛
frequency, either expressed in hertz (cycles per second) as 𝑚𝑚 (𝑓𝑓) =∫ 𝑓𝑓 𝑆𝑆(𝑓𝑓)𝑑𝑑𝑓𝑓 or expressed in circular
𝑛𝑛
∞
𝑛𝑛
frequency (radians/second) as 𝑚𝑚 (𝜔𝜔) =∫ 𝜔𝜔 𝑆𝑆(𝜔𝜔)d𝜔𝜔
𝑛𝑛
n
Note 1 to entry: As ω = 2 π f, the relationship between the two moment expressions is: mn(ω ) = (2π) mn(f).
Note 2 to entry: The integration extends over the entire frequency range from zero to infinity. In practice the integration
is often truncated at a frequency beyond which the contribution to the integral is negligible and/or the sensor no longer
responds accurately. Care should be taken when utilizing moments of order higher than 2, as for standard spectral
models, the 4th moment will not converge; the value is in effect determined by the choice of truncation.
3.41 3.41
spectral peak period
period of the maximum (peak) energy density in the spectrum (3.42(3.42))
...