EN ISO 19905-3:2019

SLOVENSKI STANDARD
01-januar-2020
Industrija za predelavo nafte in zemeljskega plina - Ocenjevanje premičnih naftnih
ploščadi na področju postavitve - 3. del: Plavajoča enota (ISO 19905-3:2017)
Petroleum and natural gas industries - Site-specific assessment of mobile offshore units
- Part 3: Floating unit (ISO 19905-3:2017)
Erdöl- und Erdgasindustrie - Beurteilung von mobilen Offshore Einheiten bezüglich ihres
Einsatzgebietes - Teil 3: Schwimmende Einheiten (ISO 19905-3:2017)
Industries du pétrole et du gaz naturel - Évaluation spécifique au site d'unités mobiles en
mer - Partie 3: Unité flottante (ISO 19905-3:2017)
Ta slovenski standard je istoveten z: EN ISO 19905-3:2019
ICS:
75.180.10 Oprema za raziskovanje, Exploratory, drilling and
vrtanje in odkopavanje extraction equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 19905-3
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2019
EUROPÄISCHE NORM
ICS 75.180.10
English Version
Petroleum and natural gas industries - Site-specific
assessment of mobile offshore units - Part 3: Floating unit
(ISO 19905-3:2017)
Industries du pétrole et du gaz naturel - Évaluation Erdöl- und Erdgasindustrie - Beurteilung von mobilen
spécifique au site d'unités mobiles en mer - Partie 3: Offshore Einheiten bezüglich ihres Einsatzgebietes -
Unité flottante (ISO 19905-3:2017) Teil 3: Schwimmende Einheiten (ISO 19905-3:2017)
This European Standard was approved by CEN on 9 September 2019.

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, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, 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: Rue de la Science 23, B-1040 Brussels
© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 19905-3:2019 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
The text of ISO 19905-3:2017 has been prepared by Technical Committee ISO/TC 67 "Materials,
equipment and offshore structures for petroleum, petrochemical and natural gas industries” of the
International Organization for Standardization (ISO) and has been taken over as EN ISO 19905-3:2019
by Technical Committee CEN/TC 12 “Materials, equipment and offshore structures for petroleum,
petrochemical and natural gas industries” the secretariat of which is held by NEN.
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 April 2020, and conflicting national standards shall be
withdrawn at the latest by April 2020.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
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, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 19905-3:2017 has been approved by CEN as EN ISO 19905-3:2019 without any
modification.
INTERNATIONAL ISO
STANDARD 19905-3
First edition
2017-10
Petroleum and natural gas
industries — Site-specific assessment
of mobile offshore units —
Part 3:
Floating unit
Industries du pétrole et du gaz naturel — Évaluation spécifique au
site d'unités mobiles en mer —
Partie 3: Unité flottante
Reference number
ISO 19905-3:2017(E)
©
ISO 2017
ISO 19905-3:2017(E)
© ISO 2017, 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
Ch. de Blandonnet 8 • CP 401
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 2017 – All rights reserved

ISO 19905-3:2017(E)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 2
3  Terms and definitions . 2
4 Abbreviated terms . 4
5 Overall considerations . 5
5.1 General . 5
5.1.1 Competency . 5
5.1.2 Planning . 5
5.1.3 Reporting . 5
5.1.4 Regulations . 5
5.1.5 Classification of unit . 5
5.2 Assessment . 5
5.3 Exposure levels . 6
5.4 Selection of limit states . 6
5.5 Determination of assessment situations . 7
5.5.1 General. 7
5.5.2 Arctic operations and ice . 7
5.5.3 Earthquake . 8
5.6 Models and analytical tools . 8
6 Data to be assembled for each site . 8
6.1 Applicability . 8
6.2 Mobile floating unit data . 8
6.3 Stationkeeping data . 9
6.3.1 General. 9
6.3.2 Moored units . 9
6.3.3 Moored units with thruster assist. 9
6.3.4 Dynamically positioned units . 9
6.4 Site data . 9
6.5 Data on activity use limitations .10
6.5.1 General.10
6.5.2 Reassessment or modification of activity use limitations .10
6.5.3 Sources of data and types of activity use limitations .10
6.6 Post installation data .11
7 Actions .11
8 Hull of unit .11
8.1 Strength .11
8.1.1 General.11
8.1.2 Monohull .11
8.1.3 Semi-submersible .12
8.1.4 Other hull forms .12
8.2 Airgap and freeboard .12
8.2.1 General.12
8.2.2 Monohull .12
8.2.3 Semi-submersible .12
8.2.4 Other hull forms .13
8.3 Temperature .13
8.4 Stability .13
9 Stationkeeping system .13
9.1 General .13
ISO 19905-3:2017(E)
9.2 Moored .13
9.3 Thruster assisted mooring .13
9.4 Dynamic positioning systems .13
10  Activity specific assessments .14
10.1 General .14
10.2 Assessment of site-specific activities and equipment .14
10.2.1 General.14
10.2.2 Marine drilling riser assessment .14
10.3 Risk assessment .14
10.4 Activity specific operating guidelines .14
11  Confirmation of compatibility between analysis and as-installed condition .15
Annex A (informative) Outline of an activity specific operating guideline document for a
dynamically positioned unit and a moored unit .16
Annex B (informative) Suggested process for completing a site-specific assessment of a
mobile floating unit .19
Bibliography .21
iv © ISO 2017 – All rights reserved

ISO 19905-3:2017(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 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 the following
URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 67, Materials, equipment and offshore
structures for petroleum, petrochemical and natural gas industries, Subcommittee SC 7, Offshore
structures.
A list of all parts in the ISO 19905 series can be found on the ISO website.
ISO 19905-3:2017(E)
Introduction
The series of International Standards applicable to types of offshore structure, ISO 19900 to ISO 19906,
addresses design requirements and assessments for 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 or assessment rules, safety elements, workmanship,
quality control procedures and national requirements, all of which are mutually dependent. The
modification of one aspect of the design or assessment 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 offshore structural systems.
The series of International Standards applicable to the various 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.
This document states the general principles and basic requirements for the site-specific assessment of
mobile floating units. The technical information used in the assessment primarily resides in documents
referenced herein. This document is intended to be used for assessment and not for design.
Site-specific assessment is normally carried out when an existing mobile floating unit is to be installed
at a specific site. The assessment is not intended to provide a full evaluation of the unit; it is assumed
that aspects not addressed herein have been addressed at the design stage using other practices and
standards.
The purpose of the site-specific assessment is to demonstrate the adequacy of the mobile floating
unit, its stationkeeping system and any connected systems for the applicable assessment situations
and defined limit states, taking into account the consequences of failure. The results of a site-specific
assessment should be appropriately recorded and communicated to those persons required to know or
act on the conclusions and recommendations. Alternative approaches to the site-specific assessment
can be used provided that they have been shown to give a level of reliability equivalent, or superior, to
that implicit in this document.
In International Standards, the following verbal forms are used:
— “shall” and “shall not” are used to indicate requirements strictly to be followed in order to conform
to the document and from which no deviation is permitted;
— “should” and “should not” are used to indicate that, among several possibilities, one is recommended
as particularly suitable, without mentioning or excluding others, or that a certain course of action is
preferred but not necessarily required, or that (in the negative form) a certain possibility or course
of action is deprecated but not prohibited;
— “may” is used to indicate a course of action permissible within the limits of the document;
— “can” and “cannot” are used for statements of possibility and capability, whether material, physical
or causal.
vi © ISO 2017 – All rights reserved

INTERNATIONAL STANDARD ISO 19905-3:2017(E)
Petroleum and natural gas industries — Site-specific
assessment of mobile offshore units —
Part 3:
Floating unit
1 Scope
This document specifies requirements and gives guidance for the site-specific assessment of mobile
floating units for use in the petroleum and natural gas industries. It addresses the installed phase, at a
specific site, of manned non-evacuated, manned evacuated and unmanned mobile floating units.
This document addresses mobile floating units that are monohull (e.g. ship-shaped vessels or
barges); column-stabilized, commonly referred to as semi-submersibles; or other hull forms (e.g.
cylindrical/conical shaped). It is not applicable to tension leg platforms. Stationkeeping can be provided
by a mooring system, a thruster assisted mooring system, or dynamic positioning. The function of the
unit can be broad, including drilling, floatel, tender assist, etc. In situations where hydrocarbons are
being produced, there can be additional requirements.
The requirements of this document apply to the hull and stationkeeping system for all types of
mobile units. The activity specific operating guideline document requirements can be modified to be
appropriate to the situation being assessed.
This document does not address all site considerations, and certain specific locations can require
additional assessment.
This document is applicable only to mobile floating units that are structurally sound and adequately
maintained, which is normally demonstrated through holding a valid RCS classification certificate.
This document does not address design, transportation to and from site, or installation and removal
from site.
This document sets out the requirements for site-specific assessments, but generally relies on other
documents to supply the details of how the assessments are to be undertaken. In general:
— ISO 19901-7 is referenced for the assessment of the stationkeeping system;
— ISO 19904-1 is referenced to determine the metocean actions on the unit;
— ISO 19906 is referenced for arctic and cold regions;
— the hull structure and airgap are assessed by use of a comparison between the site-specific metocean
conditions and its design conditions, as set out in the RCS approved operations manual;
[1]
— ISO 13624-1 and ISO/TR 13624-2 are referenced for the assessment of the marine drilling riser of
mobile floating drilling units. Equivalent alternative methodologies can be used;
[5]
— IMCA M 220 is referenced for developing an activity specific operating guidelines. Agreed
alternative methodologies can be used.
NOTE 1 The scope of ISO 19904-1 specifically states that its requirements do not apply to mobile units, but the
methodologies given for assessing metocean actions can be used.
[4]
NOTE 2 RCS rules and the IMO MODU code provide guidance for design and general operation of mobile
floating units.
ISO 19905-3:2017(E)
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 13624-1:2009, Petroleum and natural gas industries — Drilling and production equipment — Part 1:
Design and operation of marine drilling riser equipment
ISO 19900:2013, Petroleum and natural gas industries — General requirements for offshore structures
ISO 19901-1, Petroleum and natural gas industries — Specific requirements for offshore structures —
Part 1: Metocean design and operating considerations
ISO 19901-7:2013, Petroleum and natural gas industries — Specific requirements for offshore structures —
Part 7: Stationkeeping systems for floating offshore structures and mobile offshore units
ISO 19904-1, Petroleum and natural gas industries — Floating offshore structures — Part 1: Monohulls,
semi-submersibles and spars
1)
ISO 19906:— , Petroleum and natural gas industries — Arctic offshore structures
3  Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 19900, ISO 19901-1,
ISO 19901-7, ISO 19904-1 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at http://www.iso.org/obp
— IEC Electropedia: available at http://www.electropedia.org/
3.1
activity specific operating guidelines
ASOG
document that sets out the activity specific actions to be taken at specific alert level thresholds (3.3)
Note 1 to entry: The ASOG for drilling operations is often called the well specific operating guideline document.
Note 2 to entry: An example ASOG for DP and moored units is given in Annex A.
3.2
alert level
condition when certain parameters are below the lower limit, or between limits, or above the upper limit
Note 1 to entry: Alert levels are often colour-coded. The colour-coding will often be green for normal, blue for
advisory, yellow alert, or red alert. Parameters affecting the change of colour-coded alert levels can be, for
example, limiting metocean conditions are reached, loss of equipment function, reduced power levels, offset
limits are reached [watch circles (3.17)], vessel motions, etc. Actions that need to be taken can include, for
example, discontinue drilling, disconnect riser, suspend lifts, etc.
3.3
alert level threshold
boundary between alert levels (3.2) which is crossed when certain parameters reach or exceed specific
limits and that triggers actions
1) To be published. Stage at time of publication ISO/DIS 19906:2017.
2 © ISO 2017 – All rights reserved

ISO 19905-3:2017(E)
3.4
assessment
site-specific assessment
evaluation of a mobile floating unit and activity specific equipment to determine compliance with the
requirements of this document
[SOURCE: ISO 19905-1:2016, 3.4, modified — the definition has been revised.]
3.5
assessment situation
mobile floating unit configuration together with the metocean and ice actions to be assessed
[SOURCE: ISO 19905-1:2016, 3.5, modified — the definition has been revised.]
3.6
assessor
entity performing the site-specific assessment
[SOURCE: ISO 19905-1:2016, 3.6]
3.7
drift-off
unintended move of a dynamically positioned or thruster-assisted vessel off its intended location [watch
circle (3.17)] relative to its set point, generally caused by loss of stationkeeping control or propulsion
[SOURCE: ISO 13624-1:2009, 3.1.25, modified — the definition has been revised.]
3.8
drive-off
unintended move of a dynamically positioned or thruster-assisted vessel off its intended location [watch
circle (3.17)] relative to its set point, driven by the vessel's main propulsion or stationkeeping thrusters
[SOURCE: ISO 13624-1:2009, 3.1.27, modified — the definition has been revised.]
3.9
extreme storm event
extreme combination of wind, wave and current conditions to which the structure can be subjected
during its deployment
Note 1 to entry: This is the metocean event used for ULS storm assessment and varies depending on what is being
assessed. For example, the metocean event for the ULS assessment of the mooring system can be different from
that used in the ULS assessment of the hull strength or airgap.
[SOURCE: ISO 19905-1:2016, 3.17, modified — Note 1 to entry has been revised.]
3.10
operating manual
marine operations manual
manual that defines the operational characteristics and capabilities of the mobile floating unit
Note 1 to entry: The assessor (3.6) is advised to ensure that the operations manual referenced is the latest
revision and that any updated weight data are provided.
[SOURCE: ISO 19905-1:2016, 3.45, modified — “jack-up” has been replaced by “mobile floating unit”.]
3.11
operator
representative of the companies leasing the site
Note 1 to entry: The operator is normally the oil company acting on behalf of co-licensees.
[SOURCE: ISO 19905-1:2016, 3.46]
ISO 19905-3:2017(E)
3.12
recognized classification society
RCS
member of the International Association of Classification Societies (IACS), with recognized and relevant
competence and experience in mobile floating units, and with established rules and procedures for
classification/certification of such units used in petroleum-related activities
[SOURCE: ISO 19901-7:2013, 3.23, modified — the definition has been revised.]
3.13
set point position
intended location of the unit (3.15)
3.14
sudden hurricane
hurricane that forms locally and, due to speed of formation and proximity to infrastructure at time of
formation, might not allow sufficient time to evacuate manned facilities
Note 1 to entry: The population of storms used to derive the sudden hurricane at a given site can be defined in
terms of the time horizon required to evacuate the site.
3.15
unit
platform
complete assembly, including hull structure, topsides, foundations and stationkeeping systems
[SOURCE: ISO 19900:2013, 3.35, modified — the definition has been revised.]
3.16
unit owner
representative of the companies owning or chartering the unit (3.15)
[SOURCE: ISO 19905-1:2016, 3.29, modified — “jack-up” has been replaced by “unit”.]
3.17
watch circles
concentric group of imaginary closed curves (e.g. circles) developed from the alert levels (3.2) that
indicate when specific actions need to be taken
Note 1 to entry: Watch circles, which are often colour coded to indicate the actions that need to be taken, are
normally described in the activity specific operating guidelines (3.1).
4 Abbreviated terms
ALS accidental limit state
ASOG activity specific operating guidelines
DP dynamic positioning
FLS fatigue limit state
FMEA failure mode and effects analysis
IACS International Association of Classification Societies
IMCA International Marine Contractors Association
IMO International Maritime Organization
4 © ISO 2017 – All rights reserved

ISO 19905-3:2017(E)
MODU mobile offshore drilling unit
MOU mobile offshore unit
RAO response amplitude operators
RCS recognized classification society
SLS serviceability limit state
ULS ultimate limit state
5 Overall considerations
5.1 General
5.1.1 Competency
Assessments undertaken in accordance with this document shall only be performed by persons
competent through education, training and experience in the relevant disciplines.
5.1.2 Planning
Adequate planning shall be undertaken before a site-specific assessment is started. The planning shall
include the determination of all assessment situations relevant for the site under consideration. The
assessment criteria shall be in accordance with the general requirements for assessment of existing
structures specified in ISO 19900, as far as relevant for mobile floating units.
5.1.3 Reporting
The assessor should prepare a report summarizing the inputs, assumptions and conclusions of the
assessment. Previous site-specific assessments may be taken into consideration when preparing
the report.
5.1.4 Regulations
Each country can have its own set of regulations concerning offshore operations. It is the responsibility
of the operator and the owner of the mobile floating unit to comply with relevant rules and regulations,
which can depend upon the site and type of activities to be conducted.
5.1.5  Classification of unit
This document is applicable only to mobile floating units that are structurally sound and adequately
maintained. To achieve this, the unit shall either
— hold valid classification society certification from an RCS, as defined in 3.12, throughout the duration
of the operation at the specific site subject to assessment, or
— have been verified by an independent competent body to be structurally fit for purpose for afloat
operations, and are subject to periodic inspection, both to the standards of an RCS.
Mobile floating units that do not comply with this requirement shall be assessed in accordance with the
provisions of ISO 19904-1, supplemented by methodologies from this document, where applicable.
5.2 Assessment
The objective of the assessment is to show that the acceptance criteria are met. Annex B provides a
diagrammatic example of the process to be used in the site-specific assessment of a mobile floating
ISO 19905-3:2017(E)
unit. Other approaches may be applied; they shall be shown to give a level of reliability equivalent, or
superior, to that implicit in this document.
In situations where hydrocarbons are being produced, the requirements of this document should be
supplemented with consideration of the additional risk and consequences that can result in additional
requirements.
5.3 Exposure levels
The exposure level for each site-specific assessment of a mobile floating unit shall be determined by the
owner and the operator and, where applicable, shall be agreed by the regulator and the operator and by
the regulator and operator(s) of adjacent facilities.
NOTE Adjacent facilities (workover platform, local platforms, transport lines, subsea facilities, etc.) are
those that are sufficiently close to the unit for there to be a potential for impact if the unit drifts from location.
Exposure levels for extreme storm assessments are given in Table 1.
Consequence categories and life safety categories are given in ISO 19900.
Table 1 — Determination of exposure level (from ISO 19900:2013)
Consequence category
Life-safety category
C1 High consequence C2 Medium consequence C3 Low consequence
S1 Manned non-evacuated L1 L1 L1
S2 Manned evacuated L1 L2 L2
S3 Unmanned L1 L2 L3
The stationkeeping system shall be assessed as per Clause 9 for levels L1 and L2.
The following apply to categories L1, L2 and L3:
— L1: The extreme storm referenced in Clause 8 shall be the 50 year independent extremes.
— L2: The extreme storm referenced in Clause 8 for exposure level L2 shall be the 50 year independent
extremes that could be reached at the site prior to evacuation being effected (e.g. 50 year 48 hour
notice sudden hurricane in tropical revolving storm areas).
— L3: The assessment criteria for the hull and stationkeeping system shall be agreed between the
owner and the operator.
5.4 Selection of limit states
ISO 19900 defines four limit state categories:
a) ultimate limit states (ULS);
b) serviceability limit states (SLS);
c) fatigue limit states (FLS);
d) accidental limit states (ALS).
The limit states enumerated above apply to the hull and stationkeeping system of the mobile floating
unit. They can also apply to other activity critical components for which assessments are required by
this document (see 10.2), e.g. marine drilling riser and wellhead on drilling units.
NOTE 1 The ULS is normally based on the unit being in a survival condition with, for example, the marine
drilling riser disconnected on drilling units or equivalent survival preparations being undertaken on other types
of unit.
6 © ISO 2017 – All rights reserved

ISO 19905-3:2017(E)
NOTE 2 The limits of the SLS are often based on, for example, the capability of the connected marine drilling
riser on drilling units, or other limiting equipment on other types of unit. The ASOG specifies the circumstances
under which the unit transitions from the SLS to the ULS conditions.
For moored units and thruster-assist moored units, the stationkeeping damaged case (e.g. a single line
damage or loss of thruster assistance component(s) as determined through FMEA) shall be assessed
(see ISO 19901-7) as part of the ULS condition. It can be acceptable to continue site-specific activities
with the unit in an impaired state (e.g. with a single line damaged); in such cases, it shall be shown that
— the unit in the impaired/damaged condition meets all the requirements of this document, including
the requirement for an additional stationkeeping redundancy case (e.g. another mooring line
damaged), and
— the failure was not the result of a systemic defect or error that can lead to other stationkeeping
components failing under non-extreme conditions.
The FLS is generally addressed at the design stage and fatigue need not be considered, unless the unit is
to be operated in conditions outside the design envelope.
The ALS are generally addressed at the design stage and need not be evaluated in the site-specific
assessment, unless there are unusual risks at the site under consideration that were not considered at
the design stage.
NOTE 3 Changes to the grade or strength of the mooring system can affect the fatigue capacity of certain hull
and equipment components, e.g. fairleads, mooring winches, etc.
5.5 Determination of assessment situations
5.5.1 General
Provisions related to metocean conditions and their application are given in ISO 19900, and these
shall be complied with in conjunction with the further requirements of ISO 19901-1 and those of this
document.
Assessment situations include all the service and operational requirements resulting from the intended
use of the floating structure and the metocean conditions that could affect the stationkeeping system,
and any activity specific requirement limitations, according to ISO 19900.
In particular, an environmental assessment situation consists of a set of actions induced by waves,
wind, current and ice (if any) on the floating structure, related systems (e.g. the mooring system, if
applicable), and activity specific equipment (e.g. risers), and is characterized by a given return period
for one or more environmental variables or for a contour of environmental variables.
Criteria to be met in the assessment can be directly related to the specific formulation of the assessment
situations. In this case, assessment situations, calculation process and assessment criteria are
interrelated and should not be separated from one another.
A mobile floating unit can be used in various modes at a single site (e.g. operating or survival mode,
etc.) and at a number of different alert levels. Any required restrictions on the mode of operations
shall be included in the ASOG. See also ISO 19901-7:2013, Clause 6 for additional discussion that can be
applicable to assessment situations.
If the deployment is to be of limited duration, applicable (seasonal) data may be used for the months
under consideration, including suitable contingency.
5.5.2 Arctic operations and ice
For mobile offshore floating units stationed or operating in arctic or cold regions, the mobile unit-
specific requirements of ISO 19906:—, Clauses 13 and 17 shall apply, subject to the following.
ISO 19905-3:2017(E)
The stationkeeping system of units stationed or operating in arctic or cold regions shall comply with
the requirements of ISO 19901-7 supplemented with additional req
...