EN ISO 19901-2:2017

SLOVENSKI STANDARD
01-marec-2018
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SIST EN ISO 19901-2:2005
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Petroleum and natural gas industries - Specific requirements for offshore structures -
Part 2: Seismic design procedures and criteria (ISO 19901-2:2017)
Erdöl- und Erdgasindustrie - Spezielle Anforderungen für Offshore-Anlagen - Teil 2:
Seismische Auslegungsverfahren und –kriterien (ISO 19901-2:2017)
Industries du pétrole et du gaz naturel - Exigences spécifiques relatives aux structures
en mer - Partie 2: Procédures de conception et critères sismiques (ISO 19901-2:2017)
Ta slovenski standard je istoveten z: EN ISO 19901-2:2017
ICS:
75.180.10 Oprema za raziskovanje, Exploratory, drilling and
vrtanje in odkopavanje extraction equipment
91.120.25 =DãþLWDSUHGSRWUHVLLQ Seismic and vibration
YLEUDFLMDPL protection
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 19901-2
EUROPEAN STANDARD
NORME EUROPÉENNE
December 2017
EUROPÄISCHE NORM
ICS 75.180.10 Supersedes EN ISO 19901-2:2004
English Version
Petroleum and natural gas industries - Specific
requirements for offshore structures - Part 2: Seismic
design procedures and criteria (ISO 19901-2:2017)
Industries du pétrole et du gaz naturel - Exigences Erdöl- und Erdgasindustrie - Spezielle Anforderungen
spécifiques relatives aux structures en mer - Partie 2: für Offshore-Anlagen - Teil 2: Seismische
Procédures de conception et critères sismiques (ISO Auslegungsverfahren und -kriterien (ISO 19901-
19901-2:2017) 2:2017)
This European Standard was approved by CEN on 18 November 2017.

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, 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
© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 19901-2:2017 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 19901-2:2017) 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 CYS.
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 June 2018, and conflicting national standards shall be
withdrawn at the latest by June 2018.
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.
This document supersedes EN ISO 19901-2:2004.
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, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 19901-2:2017 has been approved by CEN as EN ISO 19901-2:2017 without any
modification.
INTERNATIONAL ISO
STANDARD 19901-2
Second edition
2017-11
Petroleum and natural gas
industries — Specific requirements
for offshore structures —
Part 2:
Seismic design procedures and
criteria
Industries du pétrole et du gaz naturel — Exigences spécifiques
relatives aux structures en mer —
Partie 2: Procédures de conception et critères sismiques
Reference number
ISO 19901-2:2017(E)
©
ISO 2017
ISO 19901-2:2017(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 19901-2:2017(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3  Terms and definitions . 2
4 Symbols and abbreviated terms . 5
4.1 Symbols . 5
4.2 Abbreviated terms . 6
5  Earthquake hazards . 7
6 Seismic design principles and methodology . 7
6.1 Design principles . 7
6.2 Seismic design procedures . 8
6.2.1 General. 8
6.2.2 Extreme level earthquake design . 9
6.2.3 Abnormal level earthquake design .10
6.3 Spectral acceleration data .11
6.4 Seismic risk category .11
6.5 Seismic design requirements.12
6.6 Site investigation .13
7  Simplified seismic action procedure .13
7.1 Soil classification and spectral shape .13
7.2 Seismic action procedure .17
8 Detailed seismic action procedure .18
8.1 Site-specific seismic hazard assessment .18
8.2 Probabilistic seismic hazard analysis .18
8.3 Deterministic seismic hazard analysis .19
8.4 Seismic action procedure .21
8.5 Local site response analyses .24
9  Performance requirements .24
9.1 ELE performance.24
9.2 ALE performance .25
Annex A (informative) Additional information and guidance .26
Annex B (informative) Simplified action procedure spectral accelerations .35
Annex C (informative) Regional information .46
Bibliography .50
ISO 19901-2:2017(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.
This second edition cancels and replaces the first edition (ISO 19901-2:2004), which has been technically
revised.
A list of all parts in the ISO 19901 series can be found on the ISO website.
iv © ISO 2017 – All rights reserved

ISO 19901-2:2017(E)
Introduction
The series of International Standards applicable to types of offshore structure, ISO 19900 to ISO 19906,
addresses 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 or assessment rules, safety elements, workmanship,
quality control procedures and national requirements, all of which are mutually dependent. The
modification of one aspect of 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 all 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.
The overall concept of structural integrity is described above. Some additional considerations apply for
seismic design. These include the magnitude and probability of seismic events, the use and importance
of the offshore structure, the robustness of the structure under consideration and the allowable damage
due to seismic actions with different probabilities. All of these, and any other relevant information,
need to be considered in relation to the overall reliability of the structure.
Seismic conditions vary widely around the world, and the design criteria depend primarily on
observations of historical seismic events together with consideration of seismotectonics. In many
cases, site-specific seismic hazard assessments will be required to complete the design or assessment
of a structure.
This document is intended to provide general seismic design procedures for different types of offshore
structures, and a framework for the derivation of seismic design criteria. Further requirements are
contained within the general requirements standard, ISO 19900, and within the structure-specific
standards, ISO 19902, ISO 19903, ISO 19904 and ISO 19906. The consideration of seismic events in
connection with mobile offshore units is addressed in ISO 19905.
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 normative text to facilitate cross-referencing.
Regional information on expected seismic accelerations for offshore areas is provided in Annex B.
INTERNATIONAL STANDARD ISO 19901-2:2017(E)
Petroleum and natural gas industries — Specific
requirements for offshore structures —
Part 2:
Seismic design procedures and criteria
1 Scope
This document contains requirements for defining the seismic design procedures and criteria for
offshore structures; guidance on the requirements is included in Annex A. The requirements focus on
fixed steel offshore structures and fixed concrete offshore structures. The effects of seismic events on
floating structures and partially buoyant structures are briefly discussed. The site-specific assessment
of jack-ups in elevated condition is only covered in this document to the extent that the requirements
are applicable.
Only earthquake-induced ground motions are addressed in detail. Other geologically induced hazards
such as liquefaction, slope instability, faults, tsunamis, mud volcanoes and shock waves are mentioned
and briefly discussed.
The requirements are intended to reduce risks to persons, the environment, and assets to the lowest
levels that are reasonably practicable. This intent is achieved by using:
a) seismic design procedures which are dependent on the exposure level of the offshore structure and
the expected intensity of seismic events;
b) a two-level seismic design check in which the structure is designed to the ultimate limit state (ULS)
for strength and stiffness and then checked to abnormal environmental events or the abnormal
limit state (ALS) to ensure that it meets reserve strength and energy dissipation requirements.
Procedures and requirements for a site-specific probabilistic seismic hazard analysis (PSHA) are
addressed for offshore structures in high seismic areas and/or with high exposure levels. However, a
thorough explanation of PSHA procedures is not included.
Where a simplified design approach is allowed, worldwide offshore maps, which are included in
Annex B, show the intensity of ground shaking corresponding to a return period of 1 000 years. In
such cases, these maps may be used with corresponding scale factors to determine appropriate seismic
actions for the design of a structure.
For design of fixed steel offshore structures, further specific requirements and recommended values
of design parameters (e.g. partial action and resistance factors) are included in ISO 19902, while those
for fixed concrete offshore structures are contained in ISO 19903. Seismic requirements for floating
structures are contained in ISO 19904, for site-specific assessment of jack-ups and other MOUs in
ISO 19905 (all parts), for arctic structures in ISO 19906 and for topsides structures in ISO 19901-3.
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 19901-8, Petroleum and natural gas industries — Specific requirements for offshore structures –
Part 8: Marine soils Investigation
ISO 19901-2:2017(E)
ISO 19902, Petroleum and natural gas industries — Fixed steel offshore structures
ISO 19903, Petroleum and natural gas industries — Fixed concrete offshore structures
3  Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 19900 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
abnormal level earthquake
ALE
intense earthquake of abnormal severity with a very low probability of occurring during the life of the
structure
Note 1 to entry: The ALE event is comparable to the abnormal event in the design of fixed structures which are
described in ISO 19902 and ISO 19903.
3.2
attenuation
decay of seismic waves as they travel from the earthquake source to the site under consideration
3.3
directional combination
combination of response values due to each of the three orthogonal components of earthquake-induced
ground motions
3.4
escape and evacuation system
system provided on the offshore structure to facilitate escape and evacuation in an emergency
EXAMPLE Passageways, chutes, ladders, life rafts and helidecks.
3.5
extreme level earthquake
ELE
strong earthquake with a reasonable probability of occurring during the life of the structure
Note 1 to entry: The ELE event is comparable to the extreme environmental event in the design of fixed structures
which are described in ISO 19902 and ISO 19903.
3.6
fault movement
movement occurring on a fault during an earthquake
3.7
ground motion
accelerations, velocities or displacements of the ground produced by seismic waves radiating away
from earthquake sources
Note 1 to entry: A fixed offshore structure is founded in or on the seabed (3.17) and consequently only seabed
motions are of significance. The term ground motions is used rather than seabed motions for consistency of
terminology with seismic design for onshore structures.
Note 2 to entry: Ground motions can be at a specific depth or over a specific region within the seabed.
2 © ISO 2017 – All rights reserved

ISO 19901-2:2017(E)
3.8
liquefaction
fluidity of soil due to the increase in pore pressures caused by earthquake action under undrained
conditions
3.9
modal combination
combination of response values associated with each dynamic mode of a structure
3.10
mud volcano
diapiric intrusion of plastic clay causing high pressure gas-water seepages which carry mud, fragments
of rock (and occasionally oil) to the surface
Note 1 to entry: The surface expression of a mud volcano is a cone of mud with continuous or intermittent gas
escaping through the mud.
3.11
probabilistic seismic hazard analysis
PSHA
framework permitting the identification, quantification and rational combination of uncertainties in
earthquakes' intensity, location, rate of recurrence and variations in ground motion (3.7) characteristics
3.12
probability of exceedance
probability that a variable (or that an event) exceeds a specified reference level given exposure time
EXAMPLE Example of probability of exceedance during a given exposure time is the annual probability of
exceedance of a specified magnitude of ground acceleration, ground velocity or ground displacement.
3.13
response spectrum
function representing the peak elastic response for single degree of freedom oscillators with a specific
damping ratios in terms of absolute acceleration, pseudo velocity, or relative displacement values
against natural frequency or period of the oscillators
3.14
safety system
systems provided on the offshore structure to detect, control and mitigate hazardous situations
EXAMPLE Gas detection, emergency shutdown, fire protection, and their control systems.
3.15
sea floor
interface between the sea and the seabed (3.17)
3.16
seabed slide
failure of seabed (3.17) slopes
3.17
seabed
material below the sea floor (3.15) in which a structure is founded
3.18
seismic risk category
SRC
category defined from the exposure level and the expected intensity of seismic motions
ISO 19901-2:2017(E)
3.19
seismic hazard curve
curve showing the annual probability of exceedance (3.12) against a measure of seismic intensity
Note 1 to entry: The seismic intensity measures can include parameters such as peak ground acceleration,
spectral acceleration (3.22), or spectral velocity (3.23).
3.20
seismic reserve capacity factor
factor indicating the structure’s ability to sustain ground motions due to earthquakes beyond the level
of the ELE (3.5)
Note 1 to entry: The seismic reserve capacity factor is a structure specific property that is used to determine the
ELE acceleration from the ALE (3.1) acceleration.
3.21
site response analysis
wave propagation analysis permitting the evaluation of the effect of local geological and soil conditions
on the ground motions (3.7) as they propagate up from depth to the surface at the site
3.22
spectral acceleration
maximum absolute acceleration response of a single degree of freedom oscillator subjected to ground
motions (3.7) due to an earthquake
3.23
spectral velocity
maximum pseudo velocity response of a single degree of freedom oscillator subjected to ground motions
(3.7) due to an earthquake
Note 1 to entry: The pseudo velocity spectrum is computed by factoring the displacement or acceleration spectra
by the oscillator’s circular frequency or the inverse of its frequency, respectively. The pseudo spectrum is either
relative or absolute, depending on the type of response spectra that is factored.
3.24
spectral displacement
maximum relative displacement response of a single degree of freedom oscillator subjected to ground
motions (3.7) due to an earthquake
3.25
static pushover method
static pushover analysis
application and incremental increase of a global static pattern of actions on a structure, including
equivalent dynamic inertial actions, until a global failure mechanism occurs
3.26
tsunami
long period sea waves caused by rapid vertical movements of the sea floor (3.15)
Note 1 to entry: The vertical movement of the sea floor is often associated with fault rupture during earthquakes
or with seabed slides (3.16).
4 © ISO 2017 – All rights reserved

ISO 19901-2:2017(E)
4 Symbols and abbreviated terms
4.1 Symbols
a slope of the seismic hazard curve
R
C site coefficient, a correction factor applied to the acceleration part (shorter periods) of a
a
response spectrum
C correction factor applied to the spectral acceleration to account for uncertainties not
c
captured in a seismic hazard curve
C seismic reserve capacity factor; see Formulae (7) and (10)
r
C site coefficient, a correction factor applied to the velocity part (longer periods) of a
v
response spectrum
s undrained shear strength of the soil
u
s̅ average undrained shear strength of the soil in the effective seabed
u
D scaling factor for damping
G low amplitude shear modulus of the soil
max
g acceleration due to gravity
M magnitude of an earthquake measured by the energy released at its source
N scale factor for conversion of the site 1 000-year acceleration spectrum to the site ALE
ALE
acceleration spectrum
p atmospheric pressure
a
P annual probability of exceedance for the ALE event
ALE
P probability of exceedance
e
P annual probability of exceedance for the ELE event
ELE
P target annual probability of failure
f
q cone penetration resistance of sand
c
q normalized cone penetration resistance of sand
cl
average normalized cone penetration resistance of sand in the effective seabed
q
cl
S (T) spectral acceleration associated with a single degree of freedom oscillator period, T
a
mean spectral acceleration associated with a single degree of freedom oscillator period,
ST()
a
T; obtained from a PSHA
S (T) ALE spectral acceleration associated with a single degree of freedom oscillator period, T
a,ALE
mean spectral acceleration associated with a single degree of freedom oscillator period,
ST()
a,ALE
T; obtained from a PSHA
S (T) ELE spectral acceleration associated with a single degree of freedom oscillator period, T
a,ELE
ISO 19901-2:2017(E)
mean ELE spectral acceleration associated with a single degree of freedom oscillator
ST()
a,ELE
period, T; obtained from a PSHA
S (T) 1 000-year rock outcrop spectral acceleration obtained from maps associated with a
a,map
single degree of freedom oscillator period, T
NOTE  The maps included in Annex B are for oscillator periods of 0,2 s and 1,0 s.
mean spectral acceleration associated with a probability of exceedance, P , and a single
e
ST()
a,Pe
degree of freedom oscillator period, T, obtained from a PSHA
mean spectral acceleration associated with a target annual probability of failure, P , and
f
ST()
a,Pe
a single degree of freedom oscillator period, T, obtained from a PSHA
S (T) site spectral acceleration corresponding to a return period of 1 000 years and a single
a,site
degree of freedom oscillator period, T
T natural period of a simple, single degree of freedom oscillator
T dominant modal period of the structure
dom
T return period
return
u code utilization in time history analysis, i
i
median code utilization

u
V representative shear wave velocity
s
V̅ average of representative shear wave velocity in the effective seabed
s
ρ mass density of soil
η per cent of critical damping
σ logarithmic standard deviation of uncertainties not captured in a seismic hazard curve
LR
σ′ in situ vertical effective stress of soil
v0
4.2 Abbreviated terms
ALE abnormal level earthquake
ALS abnormal limit state
ELE extreme level earthquake
L1, L2, L3 exposure level derived in accordance with the International Standard applicable to the
1)
type of offshore structure
MOU mobile offshore unit
PGA peak ground acceleration
PSHA probabilistic seismic hazard analysis
1) International Standards applicable to types of offshore structure include ISO 19902, ISO 19903, ISO 19904 (all
parts), ISO 19905 (all parts) and ISO 19906.
6 © ISO 2017 – All rights reserved

ISO 19901-2:2017(E)
SRC seismic risk category
TLP tension leg platform
ULS ultimate limit state
5  Earthquake hazards
Actions and action effects due to seismic events shall be evaluated in the structural design of
offshore structures in seismically active areas. Areas are considered seismically active on the basis of
previous records of earthquake activity, both in frequency of occurrence and in magnitude. Annex B
provides maps of indicative seismic accelerations; however, for many areas, depending on indicative
accelerations and exposure levels, seismicity shall be determined on the basis of detailed seismic
hazard investigations (see Clause 8).
Evaluation of seismic events for seismically active regions shall include investigation of the
characteristics of ground motions and of the acceptable seismic risk for structures. Structures in
seismically active regions shall be designed for ground motions due to earthquakes. However, other
seismic hazards shall also be considered in the design and, when warranted, should be addressed by
special studies (e.g. mudflow loading, seabed deformation). The following hazards can be caused by a
seismic event:
— soil liquefaction;
— seabed slide;
— fault movement;
— tsunamis;
— mud volcanoes;
— shock waves.
Effects of seismic events on subsea equipment, pipelines and in-field flowlines shall be addressed by
special studies (e.g. simultaneous seabed and structure excitation, spatially varying motions).
6 Seismic design principles and methodology
6.1 Design principles
This clause addresses the design of structures against base excitations, i.e. accelerations, velocities and
displacements caused by ground motions.
Structures located in seismically active areas shall be designed to the ultimate limit state (ULS) for
low-intensity frequent earthquakes and to the abnormal limit state (ALS) for high-intensity rare
earthquakes.
The ULS requirements are intended to provide a structure which is adequately sized for strength and
stiffness to ensure that no significant structural damage occurs for a level of earthquake ground motion
with an adequately low likelihood of being exceeded during the design service life of the structure. The
seismic ULS design event is the extreme level earthquake (ELE). The structure shall be designed such
that an ELE event will cause little or no damage. It is recommended that the structure be inspected
subsequent to an ELE occurrence.
The ALS requirements are intended to ensure that the structure and foundation have sufficient reserve
strength, displacement and/or energy dissipation capacity to sustain large inelastic displacement
reversals without complete loss of integrity, although structural damage can occur. The seismic ALS
design event is the abnormal level earthquake (ALE). The ALE is an intense earthquake of abnormal
ISO 19901-2:2017(E)
severity with a very low probability of occurring during the structure's design service life. The ALE
can cause considerable damage to the structure; however, the structure shall be designed such that
overall structural integrity is maintained to avoid structural collapse causing loss of life and/or major
environmental damage.
Both ELE and ALE return periods depend on the exposure level and the expected intensity of seismic
events. The target annual failure probabilities given in 6.4 may be modified to meet targets set by
owners in consultation with regulators, or to meet regional requirements where they exist.
6.2 Seismic design procedures
6.2.1 General
Two alternative procedures for seismic design are provided. The simplified method may be used
where seismic considerations are unlikely to govern the design of a structure. The detailed method
shall be used where seismic considerations have a significant impact on the design. The selection of
the appropriate procedure depends on the exposure level of the structure and the expected intensity
and characteristics of seismic events. The simplified procedure (see Clause 7) allows the use of generic
seismic maps provided in Annex B; while the detailed procedure (see Clause 8) requires a site-specific
seismic hazard study. In all cases, the simplified procedure may be used to perform appraisal and
concept screening for a new offshore development.
When a structural design is asymmetric in geometric configuration or directional capacity, additional
analyses shall be included to demonstrate suitable performance in weaker directions. For time history
analyses, this may require different orientations of the earthquake horizontal records to demonstrate
performance requirements (see Clause 9).
Figure 1 presents a flowchart of the selection process and the steps associated with both procedures.
8 © ISO 2017 – All rights reserved

ISO 19901-2:2017(E)
a
SRC 3 structures may be designed using either the simplified or the detailed seismic action procedure (see
Table 4).
Figure 1 — Seismic design procedures
6.2.2  Extreme level earthquake design
During the ELE event, structural members and foundation components are permitted to sustain
localized and limited non-linear behaviour (e.g. yielding in steel, tensile cracking in concrete). As such,
ELE design procedures are primarily based on linear elastic methods of structural analysis with, for
example, non-linear soil-structure interaction effects being linearized. However, if seismic isolation or
passive energy dissipation devices are employed, non-linear time history procedures shall be used.
ISO 19901-2:2017(E)
For structures subjected to base excitations from seismic events, either of the following two methods of
analysis are allowed for the ELE design check:
a) the response spectrum analysis method;
b) the time history analysis method.
In both methods, the base excitations shall be composed of three motions, i.e. two orthogonal horizontal
motions and the vertical motion. Damping compatible with the ELE deformation levels should be used
in the ELE design, as guided by the recommendations in the International Standard applicable to the
2)
type of offshore structure . Higher values of damping due to hydrodynamics or soil deformation
(hysteretic and radiation) may be used; however, the damping used shall be substantiated with special
studies. The foundation may be modelled with equivalent elastic springs and, if necessary, mass and
damping elements; off-diagonal and frequency dependence can be significant. The foundation stiffness
and damping values shall be compatible with the ELE level of soil deformations.
In a response spectrum analysis, the methods for combining the responses in the three orthogonal
directions shall consider correlation between the modes of vibration. The complete quadratic
combination (CQC) method can be used to capture the correlation between closely spaced modes.
Sufficient modes should be included in the modal combination to obtain a least 90 % structural mass
participation in each horizontal direction. When responses due to each directional component of an
earthquake are calculated separately, the responses due to the three earthquake directions may be
combined using the square root of the sum of the squares method. Alternatively, the three directional
responses may be combined linearly assuming that one component is at its maximum while the other
two components are at 40 % of their respective maximum values. In this method, the sign of each
response parameter shall be selected such that the response combination is maximized.
If the time history analysis method is used, a minimum of 4 sets of time history records shall be used
to capture the randomness in seismic motions. The earthquake time history records shall be selected
such that they represent the dominating ELE events. Component code checks are calculated at each
time step and the maximum code utilization during each time history record shall be used to assess
the component performance. Satisfactory performance shall be achieved for either the greater of four
or half the total sets of time history records. Satisfactory performance of a given time history record,
constitutes all code utilizations being less than or equal to 1,0.
Equipment on the deck shall be designed to withstand motions that account for the transmission of
ground motions through the structure. The structure can amplify the ground motion such that the
deck accelerations are much higher than the earthquake excitation. The time history analysis method
is recommended for obtaining deck motions (especially relative motions) and deck motion response
spectra (typically absolute acceleration spectra).
The effects of ELE-induced motions on pipelines, conductors, risers and other safety-critical components
shall be considered.
6.2.3  Abnormal level earthquake design
In high seismic areas, it is uneconomic to design a structure such that the ALE event would be resisted
without non-linear behaviour. Therefore, the ALE design check allows non-linear methods of analysis,
e.g. structural elements are allowed to behave plastically, foundation piles are allowed to reach axial
capacity or develop plastic behaviour, and skirt foundations are allowed to slide. In effect, the design
depends on a combination of static reserve strength, ductility, and energy dissipation to resist the ALE
actions.
Structural and foundation models used in an ALE analysis shall include possible stiffness and
strength degradation of components under cyclic action reversals. The ALE analysis shall be based
on representative values of modelling parameters such as material strength, soil strength and soil
2) International Standards applicable to types of offshore structure include ISO 19902, ISO 19903, ISO 19904 (all
parts), ISO 19905 (all parts) and ISO 19906.
10 © ISO 2017 – All rights reserved

ISO 19901-2:2017(E)
stiffness. This can require reconsideration of the conservatism that is typically present in the ELE
design procedure.
For structures subjected to base excitations from seismic events, either of the following two methods of
analysis are allowed for the ALE design check:
a) the static pushover or extreme displacement method;
b) the non-linear time history analysis method.
The two methods can complement each other in most cases. The requirements in 6.2.2 for the
composition of base excitations from three orthogonal components of motion and for damping also
apply to the ALE design procedure.
The static pushover analysis method may be used to determine possible and controlling global
mechanisms of failure, or the global displacement of the structure (i.e. beyond the ELE). The latter may
be achieved by performing a displacement-controlled structural analysis.
The non-linear time history analysis method is the most accurate and recommended method of
ALE analysis. A minimum of four time history analyses shall be used to capture the randomness in
a seismic event. The earthquake time history records shall be selected such that they represent the
dominating ALE events. If seven or more time history records are used, global structure survival shall
be demonstrated in half or more of the time history analyses (see 9.2). If fewer than seven time history
records are used, global survival shall be demonstrated
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