ISO 11231:2019
(Main)Space systems — Probabilistic risk assessment (PRA)
Space systems — Probabilistic risk assessment (PRA)
This document supports and complements the implementation of the risk management process defined in ISO 17666 in situations when the application of a quantitative risk assessment is deemed necessary. This document defines the principles, process, implementation and requirements for conducting a quantitative risk assessment and explains the details of probabilistic risk assessment (PRA) as applied to safety. While PRA can be applied to project risk management involving cost and schedule, this application is outside the scope of this document. This document provides the basic requirements and procedures for the use of PRA techniques to assess safety or mission risk and success in space programmes and projects. This document is applicable to all international space projects involving: — the design of space vehicles for the transportation of personnel in space; — the design of space and non-terrestrial planetary stations inhabited by human beings; — the design of space and launch vehicles powered by, or carrying, nuclear materials; — other projects as directed by the authorities or clients. These types of projects generally involve scenarios, chains of events or activities that could result in the death of, or serious injury to, members of the public, astronauts or pilots, or the workforce, or the loss of critical or high-value equipment and property. For other types of projects, it is intended that PRA be performed at the discretion of the project management.
Systèmes spatiaux — Évaluation du risque probabiliste (PRA)
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INTERNATIONAL ISO
STANDARD 11231
Second edition
2019-05
Space systems — Probabilistic risk
assessment (PRA)
Systèmes spatiaux — Évaluation du risque probabiliste (PRA)
Reference number
©
ISO 2019
© ISO 2019
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
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Abbreviated terms . 4
4 Principles of probabilistic risk assessment . 4
4.1 General . 4
4.2 Mission success and system safety risk assessment concept . 4
4.3 PRA general process . 7
5 Objectives, uses and benefits of probabilistic risk assessment . 8
5.1 Objectives of a probabilistic risk assessment . 8
5.2 Probabilistic risk assessment results usage . 8
5.3 Benefits of a probabilistic risk assessment . 9
6 PRA requirements and detailed process . 9
6.1 Probabilistic risk assessment requirements . 9
6.2 Overview of the probabilistic risk assessment process . 9
6.3 Probabilistic risk assessment basic tasks .10
6.3.1 General.10
6.3.2 Task 1: Objectives and approach definition .10
6.3.3 Task 2: System familiarization .11
6.3.4 Task 3: Initiating event identification .11
6.3.5 Task 4: Scenario modelling .12
6.3.6 Task 5: Failure modelling .12
6.3.7 Task 6: Quantification .13
6.3.8 Task 7: Uncertainty analysis .13
6.3.9 Task 8: Sensitivity analysis .14
6.3.10 Task 9: Ranking .14
6.3.11 Data analysis .15
7 Peer review .15
7.1 General .15
7.2 Internal peer reviews .15
7.3 External peer reviews .15
8 Probabilistic risk assessment report — Data content requirements .16
Annex A (informative) Example of space systems unit-value/mission-criticality category
definitions .17
Annex B (informative) Capability-based PRA process tailoring guidance .18
Bibliography .22
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 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 20, Aircraft and space vehicles,
Subcommittee SC 14, Space systems and operations.
This second edition cancels and replaces the first edition (ISO 11231:2010), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— updated definitions of terms;
— simplification of Clause 4;
— updated figures and tables;
— addition of capability-based safety, reliability and quality assurance.
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.
iv © ISO 2019 – All rights reserved
Introduction
Structured risk management processes use qualitative and quantitative risk assessment techniques
to support optimal decisions regarding safety and the probability of mission success, as provided in
ISO 17666. The most systematic and comprehensive methodology for conducting these evaluations is
probabilistic risk assessment (PRA).
PRA has, over the past three decades, become the principal analytic method for identifying and
analysing risk from projects and complex systems. Its utility for risk management (RM) has been
proven in many industries, including aerospace, electricity generation, petrochemical and defence. PRA
is a methodology used to identify and evaluate risk, in order to facilitate RM activities by identifying
dominant contributors to risk, so that resources can be effectively allocated to address significant
risk drivers and are not wasted on items that contribute insignificantly to the risk. In addition to
analysing risk, PRA provides a framework to quantify uncertainties in events and event sequences that
are important to system safety. By enabling the quantification of uncertainty, PRA informs decision
makers on the sources of uncertainty and provides information on the worth of investment resources
in reducing uncertainty. In this way, PRA supplements traditional safety analyses that support
safety-related decisions. Through the use of PRA, safety analyses are capable of focusing on both the
probability and severity of events and consequences that adversely impact safety.
PRA differs from reliability analysis in two important respects:
a) PRA allows a more precise quantification of uncertainty both for individual events and for the
overall system;
b) PRA applies more informative evaluations that quantify metrics related to the occurrence of highly
adverse consequences (e.g. fatalities, loss of mission), as opposed to narrowly defined system
performance metrics (e.g. mean-time-to-failure).
PRA also differs from hazard analyses, which identifies and evaluates metrics related to the effects of
high-consequence and low-probability events, treating them as if they had happened, i.e. without regard
to their probability of occurrence. In addition, the completeness of the set of accident scenarios cannot
be assured in the conduct of a hazard analysis. PRA results are more diverse and directly applicable
to resource allocation and other RM decision-making based on a broader spectrum of consequence
metrics.
Through the PRA process, weaknesses and vulnerabilities of the system that can adversely impact
safety, performance and mission success are identified. These results in turn provide insights into viable
RM strategies to reduce risk and direct the decision maker to areas where expenditure of resources to
improve design and operation might be more effective.
The most useful applications of PRA have been in the risk evaluation of complex systems that can result
in low-probability and high-consequence scenarios, or the evaluation of complex scenarios consisting of
chains of events that collectively may adversely impact system safety more than individually.
INTERNATIONAL STANDARD ISO 11231:2019(E)
Space systems — Probabilistic risk assessment (PRA)
1 Scope
This document supports and complements the implementation of the risk management process defined
in ISO 17666 in situations when the application of a quantitative risk assessment is deemed necessary.
This document defines the principles, process, implementation and requirements for conducting a
quantitative risk assessment and explains the details of probabilistic risk assessment (PRA) as applied
to safety. While PRA can be applied to project risk management involving cost and schedule, this
application is outside the scope of this document.
This document provides the basic requirements and procedures for the use of PRA techniques to assess
safety or mission risk and success in space programmes and projects. This document is appli
...
INTERNATIONAL ISO
STANDARD 11231
Second edition
2019-05
Space systems — Probabilistic risk
assessment (PRA)
Systèmes spatiaux — Évaluation du risque probabiliste (PRA)
Reference number
©
ISO 2019
© ISO 2019
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
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Abbreviated terms . 4
4 Principles of probabilistic risk assessment . 4
4.1 General . 4
4.2 Mission success and system safety risk assessment concept . 4
4.3 PRA general process . 7
5 Objectives, uses and benefits of probabilistic risk assessment . 8
5.1 Objectives of a probabilistic risk assessment . 8
5.2 Probabilistic risk assessment results usage . 8
5.3 Benefits of a probabilistic risk assessment . 9
6 PRA requirements and detailed process . 9
6.1 Probabilistic risk assessment requirements . 9
6.2 Overview of the probabilistic risk assessment process . 9
6.3 Probabilistic risk assessment basic tasks .10
6.3.1 General.10
6.3.2 Task 1: Objectives and approach definition .10
6.3.3 Task 2: System familiarization .11
6.3.4 Task 3: Initiating event identification .11
6.3.5 Task 4: Scenario modelling .12
6.3.6 Task 5: Failure modelling .12
6.3.7 Task 6: Quantification .13
6.3.8 Task 7: Uncertainty analysis .13
6.3.9 Task 8: Sensitivity analysis .14
6.3.10 Task 9: Ranking .14
6.3.11 Data analysis .15
7 Peer review .15
7.1 General .15
7.2 Internal peer reviews .15
7.3 External peer reviews .15
8 Probabilistic risk assessment report — Data content requirements .16
Annex A (informative) Example of space systems unit-value/mission-criticality category
definitions .17
Annex B (informative) Capability-based PRA process tailoring guidance .18
Bibliography .22
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 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 20, Aircraft and space vehicles,
Subcommittee SC 14, Space systems and operations.
This second edition cancels and replaces the first edition (ISO 11231:2010), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— updated definitions of terms;
— simplification of Clause 4;
— updated figures and tables;
— addition of capability-based safety, reliability and quality assurance.
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.
iv © ISO 2019 – All rights reserved
Introduction
Structured risk management processes use qualitative and quantitative risk assessment techniques
to support optimal decisions regarding safety and the probability of mission success, as provided in
ISO 17666. The most systematic and comprehensive methodology for conducting these evaluations is
probabilistic risk assessment (PRA).
PRA has, over the past three decades, become the principal analytic method for identifying and
analysing risk from projects and complex systems. Its utility for risk management (RM) has been
proven in many industries, including aerospace, electricity generation, petrochemical and defence. PRA
is a methodology used to identify and evaluate risk, in order to facilitate RM activities by identifying
dominant contributors to risk, so that resources can be effectively allocated to address significant
risk drivers and are not wasted on items that contribute insignificantly to the risk. In addition to
analysing risk, PRA provides a framework to quantify uncertainties in events and event sequences that
are important to system safety. By enabling the quantification of uncertainty, PRA informs decision
makers on the sources of uncertainty and provides information on the worth of investment resources
in reducing uncertainty. In this way, PRA supplements traditional safety analyses that support
safety-related decisions. Through the use of PRA, safety analyses are capable of focusing on both the
probability and severity of events and consequences that adversely impact safety.
PRA differs from reliability analysis in two important respects:
a) PRA allows a more precise quantification of uncertainty both for individual events and for the
overall system;
b) PRA applies more informative evaluations that quantify metrics related to the occurrence of highly
adverse consequences (e.g. fatalities, loss of mission), as opposed to narrowly defined system
performance metrics (e.g. mean-time-to-failure).
PRA also differs from hazard analyses, which identifies and evaluates metrics related to the effects of
high-consequence and low-probability events, treating them as if they had happened, i.e. without regard
to their probability of occurrence. In addition, the completeness of the set of accident scenarios cannot
be assured in the conduct of a hazard analysis. PRA results are more diverse and directly applicable
to resource allocation and other RM decision-making based on a broader spectrum of consequence
metrics.
Through the PRA process, weaknesses and vulnerabilities of the system that can adversely impact
safety, performance and mission success are identified. These results in turn provide insights into viable
RM strategies to reduce risk and direct the decision maker to areas where expenditure of resources to
improve design and operation might be more effective.
The most useful applications of PRA have been in the risk evaluation of complex systems that can result
in low-probability and high-consequence scenarios, or the evaluation of complex scenarios consisting of
chains of events that collectively may adversely impact system safety more than individually.
INTERNATIONAL STANDARD ISO 11231:2019(E)
Space systems — Probabilistic risk assessment (PRA)
1 Scope
This document supports and complements the implementation of the risk management process defined
in ISO 17666 in situations when the application of a quantitative risk assessment is deemed necessary.
This document defines the principles, process, implementation and requirements for conducting a
quantitative risk assessment and explains the details of probabilistic risk assessment (PRA) as applied
to safety. While PRA can be applied to project risk management involving cost and schedule, this
application is outside the scope of this document.
This document provides the basic requirements and procedures for the use of PRA techniques to assess
safety or mission risk and success in space programmes and projects. This document is appli
...
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