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01-januar-2016
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Petroleum and natural gas industries - Specific requirements for offshore structures -
Part 1: Metocean design and operating considerations (ISO 19901-1:2015)
Erdöl- und Erdgasindustrie - Spezielle Anforderungen für Offshore-Anlagen - Teil 1:
Grundsätze für die Auslegung und den Betrieb auf dem offenen Meer (ISO 19901-
1:2015)
Industries du pétrole et du gaz naturel - Exigences spécifiques relatives aux structures
en mer - Partie 1: Dispositions océano-météorologiques pour la conception et
l'exploitation (ISO 19901-1:2015)
Ta slovenski standard je istoveten z: EN ISO 19901-1:2015
ICS:
75.180.10 Oprema za raziskovanje in Exploratory and extraction
odkopavanje equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 19901-1
EUROPEAN STANDARD
NORME EUROPÉENNE
November 2015
EUROPÄISCHE NORM
ICS 75.180.10 Supersedes EN ISO 19901-1:2005
English Version
Petroleum and natural gas industries - Specific
requirements for offshore structures - Part 1: Metocean
design and operating considerations (ISO 19901-1:2015)
Industries du pétrole et du gaz naturel - Exigences Erdöl- und Erdgasindustrie - Erdöl- und
spécifiques relatives aux structures en mer - Partie 1: Erdgasindustrie - Spezielle Anforderungen für
Dispositions océano-météorologiques pour la Offshore-Anlagen - Teil 1: Grundsätze für die
conception et l'exploitation (ISO 19901-1:2015) Auslegung und den Betrieb auf dem offenen Meer (ISO
19901-1:2015)
This European Standard was approved by CEN on 17 July 2015.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2015 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 19901-1:2015 E
worldwide for CEN national Members.

Contents Page
European Foreword . 3

European Foreword
This document (EN ISO 19901-1:2015) has been prepared by Technical Committee ISO/TC 67
"Materials, equipment and offshore structures for petroleum, petrochemical and natural gas industries"
in collaboration with Technical Committee CEN/TC 12 “Materials, equipment and offshore structures
for petroleum, petrochemical and natural gas industries” the secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by May 2016, and conflicting national standards shall be
withdrawn at the latest by May 2016.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent
rights.
This document supersedes EN ISO 19901-1:2005.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 19901-1:2015 has been approved by CEN as EN ISO 19901-1:2015 without any
modification.
INTERNATIONAL ISO
STANDARD 19901-1
Second edition
2015-10-15
Petroleum and natural gas
industries — Specific requirements
for offshore structures —
Part 1:
Metocean design and operating
considerations
Industries du pétrole et du gaz naturel — Exigences spécifiques
relatives aux structures en mer —
Partie 1: Dispositions océano-météorologiques pour la conception et
l’exploitation
Reference number
ISO 19901-1:2015(E)
©
ISO 2015
ISO 19901-1:2015(E)
© ISO 2015, Published in Switzerland
All rights reserved. Unless otherwise specified, 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, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2015 – All rights reserved

ISO 19901-1:2015(E)
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 .14
5.6 Directionality .14
5.7 Extrapolation to extreme and abnormal conditions .15
5.8 Metocean parameters for fatigue assessments .15
5.9 Metocean parameters for short-term activities.16
5.10 Metocean parameters for medium-term activities .17
6 Water depth, tides and storm surges .17
6.1 General .17
6.2 Tides .17
6.3 Storm surges .18
6.4 Extreme water level .18
7 Wind .19
7.1 General .19
7.2 Wind actions and action effects .20
7.3 Wind profile and time-averaged wind speed.21
7.4 Wind spectra .21
8 Waves .21
8.1 General .21
8.2 Wave actions and action effects .22
8.3 Sea states — Spectral waves .23
8.3.1 Wave spectrum .23
8.3.2 Directional spreading .23
8.3.3 Wave periods .23
8.3.4 Wave kinematics — Velocities and accelerations .23
8.4 Regular (periodic) waves .24
8.4.1 General.24
8.4.2 Wave period .24
8.4.3 Wave kinematics — Velocities and accelerations .24
8.4.4 Intrinsic, apparent and encounter wave periods .24
8.5 Maximum height of an individual wave for long return periods .25
8.6 Linear wave models .25
8.7 Wave crest elevation .25
9 Currents .26
9.1 General .26
9.2 Current velocities .26
9.3 Current profile .27
9.4 Current profile stretching .27
9.5 Current blockage .27
ISO 19901-1:2015(E)
10 Other environmental factors .27
10.1 Marine growth .27
10.2 Tsunamis .28
10.3 Seiches .28
10.4 Sea ice and icebergs .28
10.5 Snow and ice accretion .28
10.6 Miscellaneous .29
11 Collection of metocean data .29
11.1 General .29
11.2 Common requirements .30
11.2.1 General.30
11.2.2 Instrumentation .30
11.3 Meteorology .30
11.3.1 General.30
11.3.2 Weather observation and reporting for helicopter operations .30
11.3.3 Weather observation and reporting for weather forecasting services .31
11.3.4 Weather observation and reporting for climatological purposes .31
11.4 Oceanography .31
11.4.1 General.31
11.4.2 Measurements and observations .32
11.5 Data quality control .32
12 Information concerning the annexes .32
12.1 Information concerning Annex A .32
12.2 Information concerning the regional annexes .32
Annex A (informative) Additional information and guidance .33
Annex B (informative) Northwest Europe .82
Annex C (informative) West coast of Africa .92
Annex D (informative) Offshore Canada .103
Annex E (informative) Sakhalin/Sea of Okhotsk .131
Annex F (informative) Caspian Sea .155
Annex G (informative) Southern East Asian Sea .173
Bibliography .195
iv © ISO 2015 – All rights reserved

ISO 19901-1:2015(E)
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 documents 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).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 67, Materials, equipment and offshore structures
for petroleum, petrochemical and natural gas industries, Subcommittee SC 7, Offshore structures.
This second edition cancels and replaces the first edition (ISO 19901-1:2005), which has been
technically revised.
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 7: Stationkeeping systems for floating offshore structures and mobile offshore units
— Part 8: Marine soil investigations
The following parts are under preparation:
— Part 6: Marine operations
— 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 19901-1:2015(E)
— 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 19904-1, Petroleum and natural gas industries — Floating offshore structures — Part 1: Monohulls,
semi-submersibles and spars
— ISO 19905-1, Petroleum and natural gas industries — Site-specific assessment of mobile offshore units
— Part 1: Jack-ups
— ISO/TR 19905-2, Petroleum and natural gas industries — Site-specific assessment of mobile offshore
units — Part 2: Jack-ups commentary
1)
— ISO 19905-3 , Petroleum and natural gas industries — Site-specific assessment of mobile offshore
units — Part 3: Floating unit
— ISO 19906, Petroleum and natural gas industries — Arctic offshore structures
1) In preparation.
vi © ISO 2015 – All rights reserved

ISO 19901-1:2015(E)
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 judgement 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 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
−2 −3 −4
the order of 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 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 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 needs to be taken of the uncertainties that arise from the analyses of limited data
sets. 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.
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 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.
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 main text to facilitate cross-referencing.
Regional information, where available, is provided in the Regional Annexes B to G. This information has
been developed by experts from the region or country concerned to supplement the guidance provided
in this part of ISO 19901. Each Regional Annex provides regional or national data on environmental
conditions for the area concerned.
INTERNATIONAL STANDARD ISO 19901-1:2015(E)
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:
— 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.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. 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 19901-1:2015(E)
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 19904-1, Petroleum and natural gas industries — Floating offshore structures — Part 1: Monohulls,
semi-submersibles and spars
ISO 19905-1, Petroleum and natural gas industries — Site-specific assessment of mobile offshore units —
Part 1: Jack-ups
ISO 19906, Petroleum and natural gas industries — Arctic offshore structures
WMO-No. 306, Manual on Codes
3  Terms and definitions
For the purpose of this document, the terms and definitions given in ISO 19900 and the following apply.
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 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.
3.3
conditional probability
conditional distribution
statistical distribution (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
and C the significant wave height, and so on.
3.4
design crest elevation
extreme crest elevation measured relative to still water level
Note 1 to entry: The design crest elevation is used in combination with information on astronomical tide, storm
surge, platform settlement, reservoir subsidence and water depth uncertainty and is derived using extreme
value analysis. Where simplified models are used to estimate the kinematics of the design wave, 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 be different waves.
2 © ISO 2015 – All rights reserved

ISO 19901-1:2015(E)
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
expert
individual who through training and experience is competent to provide metocean advice
specific to the area or topic in question
3.7
extreme water level
EWL
combination of design crest elevation, astronomical tide and storm surge referenced to either LAT or MSL
3.8
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.
3.9
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 are an example of gravity waves.
3.10
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.
3.11
gust wind speed
maximum value of the wind speed of a gust averaged over a short (3 s to 60 s) specified duration within
a longer (1 min to 1 h) specified duration
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.
Note 2 to entry: The elevation of the measured gust should also be specified.
ISO 19901-1:2015(E)
3.12
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.
3.13
hindcasting
method of simulating historical (metocean) data for a region through numerical modelling
3.14
infra-gravity wave
surface gravity wave with a period in the range of approximately 25 s to 300 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 nonlinear second-order difference frequency interactions
between different swell wave components.
3.15
internal wave
gravity wave which propagates within a stratified water column
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, in which the statistics are assumed constant
(see 3.34 short-term distribution). 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 (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 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.
4 © ISO 2015 – All rights reserved

ISO 19901-1:2015(E)
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 B and all significant wave heights 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
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 and the averaging time interval;
a standard reference elevation is 10 m and a standard time interval is 1 h. See also 3.11 gust wind speed and
3.43 sustained wind speed.
3.22
mean zero-crossing period
average period between (up or down) zero-crossing waves in a sea state
Note 1 to entry: In practice the mean zero-crossing period is often estimated from the zeroth and second
moments of the wave spectrum asTT== mf()/(mf)(=2π mmωω)/ () .
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.
ISO 19901-1:2015(E)
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 of the world.
3.28
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 an
...