ISO 19901-2:2004

Petroleum and natural gas industries — Specific requirements for offshore structures — Part 2: Seismic design procedures and criteria

ISO 19901-2:2004 contains requirements for defining the seismic design procedures and criteria for offshore structures; guidance on the requirements is included. The requirements are applicable to fixed steel structures and fixed concrete structures. The effects of seismic events on floating structures and partially buoyant structures are also briefly discussed. The site-specific assessment of jack-ups in elevated condition is only covered in ISO 19901-2:2004 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 seismic design procedures which are dependent on the platform's exposure level and the expected intensity of seismic events and 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 accidental limit state (ALS) to ensure that it meets reserve strength and energy dissipation requirements. For high seismic areas and/or high exposure level fixed structures, a site-specific seismic hazard assessment is required; for such cases, the procedures and requirements for a site-specific probabilistic seismic hazard analysis (PSHA) are addressed. However, a thorough explanation of PSHA procedures is not included. Where a simplified design approach is allowed, worldwide offshore maps are included that 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.

Industries du pétrole et du gaz naturel — Exigences spécifiques relatives aux structures en mer — Partie 2: Procédures de conception et critères sismiques

General Information

• Status: Withdrawn
• Publication Date: 17-Nov-2004
• Withdrawal Date: 17-Nov-2004
• ICS: 75.180.10 - Exploratory, drilling and extraction equipment
• Technical Committee: ISO/TC 67/SC 7 - Offshore structures
• Drafting Committee: ISO/TC 67/SC 7/WG 3 - Fixed steel structures
• Current Stage: 9599 - Withdrawal of International Standard
• Completion Date: 15-Nov-2017

Relations

• Revised: ISO 19901-2:2017 - Petroleum and natural gas industries — Specific requirements for offshore structures — Part 2: Seismic design procedures and criteria
• Effective Date: 10-Dec-2011
• Consolidates: ISO 12175:1994 - Space data and information transfer systems — Standard formatted data units — Structure and construction rules
• Effective Date: 06-Jun-2022

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 19901-2
First edition
2004-11-15


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 19901-2:2004(E)
©
ISO 2004

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ISO 19901-2:2004(E)
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ii © ISO 2004 – All rights reserved

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ISO 19901-2:2004(E)
Contents Page
Foreword. iv
Introduction . vi
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 2
4 Symbols and abbreviated terms. 4
4.1 Symbols . 4
4.2 Abbreviated terms. 6
5 Earthquake hazards . 6
6 Seismic design principles and methodology. 7
6.1 Design principles . 7
6.2 Seismic design procedures . 7
6.3 Spectral acceleration data. 10
6.4 Seismic risk category . 11
6.5 Seismic design requirements . 12
7 Simplified seismic action procedure . 12
7.1 Soil classification and spectral shape . 12
7.2 Seismic action procedure . 16
8 Detailed seismic action procedure. 16
8.1 Site-specific seismic hazard assessment . 16
8.2 Probabilistic seismic hazard analysis . 17
8.3 Deterministic seismic hazard analysis . 17
8.4 Seismic action procedure . 19
8.5 Local site response analyses . 21
9 Performance requirements . 22
9.1 ELE performance . 22
9.2 ALE performance . 22
Annex A (informative) Additional information and guidance. 23
Annex B (informative) Regional information . 32
Bibliography . 45

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ISO 19901-2:2004(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 19901-2 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.
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 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
The following parts of ISO 19901 are under preparation:
 Part 3: Topsides structure
 Part 6: Marine operations
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 (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
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ISO 19901-2:2004(E)
 ISO 19904-2, Petroleum and natural gas industries — Floating offshore structures — Part 2: Tension leg
platforms
 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
 ISO 19906, Petroleum and natural gas industries — Arctic offshore structures
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ISO 19901-2:2004(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 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
seismic design. These include the magnitude and probability of seismic events, the use and importance of the
platform, 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 part of ISO 19901 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 part of ISO 19901 is provided in informative 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 informative Annex B.

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INTERNATIONAL STANDARD ISO 19901-2:2004(E)

Petroleum and natural gas industries — Specific requirements
for offshore structures —
Part 2:
Seismic design procedures and criteria
1 Scope
This part of ISO 19901 contains requirements for defining the seismic design procedures and criteria for
offshore structures; guidance on the requirements is included in Annex A. The requirements are applicable to
fixed steel structures and fixed concrete structures. The effects of seismic events on floating structures and
partially buoyant structures are also briefly discussed. The site-specific assessment of jack-ups in elevated
condition is only covered in this part of ISO 19901 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 platform's exposure level 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 accidental limit state
(ALS) to ensure that it meets reserve strength and energy dissipation requirements.
For high seismic areas and/or high exposure level fixed structures, a site-specific seismic hazard assessment
is required; for such cases, the procedures and requirements for a site-specific probabilistic seismic hazard
analysis (PSHA) are addressed. However, a thorough explanation of PSHA procedures is not included.
Where a simplified design approach is allowed, worldwide offshore maps are included in Annex B that 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.
NOTE 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. Specific seismic requirements for floating structures are to be contained in
[2] [3]
ISO 19904 , for site-specific assessment of jack-ups and other MOUs in ISO 19905 , for arctic structures in
[4] [1]
ISO 19906 and for topsides structures in ISO 19901-3 .
2 Normative references
The following referenced documents are indispensable for the application 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
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ISO 19901-2:2004(E)
1)
ISO 19902 , Petroleum and natural gas industries — Fixed steel offshore structures
1)
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.
3.1
abnormal level earthquake
ALE
intense earthquake of abnormal severity under the action of which the structure should not suffer complete
loss of integrity
NOTE The ALE event is comparable to the abnormal event in the design of fixed structures which are described in
ISO 19902 and ISO 19903. When exposed to the ALE, a manned structure is supposed to maintain structural and/or
floatation integrity for a sufficient period of time to enable evacuation to take place.
3.2
attenuation
decay of seismic waves as they travel from a source to the site under consideration
3.3
directional combination
combination of response values due to each of the three orthogonal components of an earthquake motion
3.4
escape and evacuation systems
systems provided on a platform to facilitate escape and evacuation in an emergency
NOTE Escape and evacuation systems include passageways, chutes, ladders, life rafts and helidecks.
3.5
extreme level earthquake
ELE
earthquake with a severity which the structure should sustain without major damage
NOTE 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. When exposed to an ELE, a structure is supposed to retain its full capacity for all
subsequent conditions.
3.6
fault movement
movement occurring on a fault during an earthquake
3.7
ground motions
accelerations, velocities or displacements of the ground produced by seismic waves radiating away from
earthquake sources
NOTE A fixed offshore structure is founded in or on the seabed 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.

1) To be published.
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ISO 19901-2:2004(E)
3.8
liquefaction
fluidity of cohesionless 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 volcanoes
diapiric intrusion of plastic clay causing high pressure gas-water seepages which carry mud, fragments of rock
(and occasionally oil) to the surface
NOTE 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 characteristics
3.12
probability of exceedance
probability that a variable (or that an event) exceeds a specified reference level given exposure time
EXAMPLES Examples of probabilities of exceedance during a given exposure time are the annual probability of
exceedance of a specified magnitude of ground acceleration, ground velocity or ground displacement.
3.13
response spectrum
plot representing structural response in terms of absolute acceleration, pseudo velocity, or relative
displacement values against natural frequency or period
3.14
safety systems
systems provided on a platform to detect, control and mitigate hazardous situations
NOTE Safety systems include gas detection, emergency shutdown, fire protection, and their control systems.
3.15
sea floor
interface between the sea and the seabed
3.16
sea floor slide
failure of sea floor slopes
3.17
seabed
materials below the sea in which a structure is founded
NOTE The seabed can be considered as the half-space below the sea floor.
3.18
seismic risk category
SRC
category defined from the exposure level and the expected intensity of seismic motions
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ISO 19901-2:2004(E)
3.19
seismic hazard curve
curve showing the probability of exceedance against a measure of seismic intensity
NOTE The seismic intensity measures can include parameters such as peak ground acceleration, spectral
acceleration, or spectral velocity.
3.20
seismic reserve capacity factor
ratio of spectral acceleration which causes structural collapse or catastrophic system failure to the ELE
spectral acceleration
3.21
site response analysis
wave propagation analysis permitting the evaluation of the effect of local geological and soil conditions on the
design ground motions at a given site
NOTE The site response analysis results can include amplitude, frequency content and duration.
3.22
spectral acceleration
maximum absolute acceleration response of a single degree of freedom oscillator subjected to ground
motions due to an earthquake
3.23
spectral velocity
maximum pseudo velocity response of a single degree of freedom oscillator subjected to ground motions due
to an earthquake
3.24
spectral displacement
maximum relative displacement response of a single degree of freedom oscillator subjected to ground motions
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
NOTE The vertical movement of the sea floor is often associated with fault rupture during earthquakes or with
seabed mud slides.
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 of a response spectrum

a
C correction factor applied to the spectral acceleration to account for uncertainties not captured in a
c
seismic hazard curve
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ISO 19901-2:2004(E)
C seismic reserve capacity factor, see Equation (7)
r
C site coefficient, a correction factor applied to the velocity part of a response spectrum
v
c undrained shear strength of the soil
u
c average undrained shear strength of the soil of the top 30 m of the seabed
u
D scaling factor for damping
G low amplitude shear modulus of the soil
max
2
g acceleration due to gravity (9,81 m/s )
M magnitude of a given seismic 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
q average normalized cone penetration resistance of sand of the top 30 m of the seabed
cl
S (T) spectral acceleration associated with a single degree of freedom oscillator period T
a
ST() mean spectral acceleration associated with a single degree of freedom oscillator period T;
a
obtained from a PSHA
S (T) ALE spectral acceleration associated with a single degree of freedom oscillator period T
a,ALE
ST mean ALE spectral acceleration associated with a single degree of freedom oscillator period T;
a,ALE()
obtained from a PSHA
S (T) ELE spectral acceleration associated with a single degree of freedom oscillator period T
a,ELE
ST() mean ELE spectral acceleration associated with a single degree of freedom oscillator period T;
a,ELE
obtained from a PSHA
S (T) 1 000 year rock outcrop spectral acceleration obtained from maps associated with a single
a,map
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.
ST mean spectral acceleration associated with a probability of exceedance P and a single degree of
a,Pe()
e
freedom oscillator period T; obtained from a PSHA
ST mean spectral acceleration associated with a target annual probability of failure P and a single
a,Pf()
f
degree of freedom oscillator period T; obtained from a PSHA
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ISO 19901-2:2004(E)
S (T) site spectral acceleration corresponding to a return period of 1 000 years and a single degree of
a,site
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
uˆ median code utilization
v shear wave velocity
s
v average shear wave velocity of the top 30 m of the seabed
s
ρ mass density of soil
η percent of critical damping
σ logarithmic standard deviation of uncertainties not captured in a seismic hazard curve
LR
σ′ vertical effective stress of soil
v0
4.2 Abbreviated terms
ALE abnormal level earthquake
ALS accidental limit state
ELE extreme level earthquake
L1, L2, L3 exposure level derived in accordance with the International Standard applicable to the type of
2)
offshore structure
MOU mobile offshore unit
PGA peak ground acceleration
PSHA probabilistic seismic hazard analysis
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 considered 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 indicative of
seismic accelerations, however for many areas, depending on indicative accelerations and exposure levels,
seismicity shall be determined on the basis of detailed investigations, see 6.5.

2) International Standards applicable to types of offshore structure, include ISO 19902 and ISO 19903, and when
available, ISO 19904 (all parts), ISO 19905 (all parts) and ISO 19906. See the Bibliography.
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ISO 19901-2:2004(E)
Consideration of seismic events for seismically active regions shall include investigation of the characteristics
of ground motions and 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 should be addressed by special studies. The following hazards can be caused by a seismic
event:
 soil liquefaction;
 sea floor 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.
6 Seismic design principles and methodology
6.1 Design principles
Clause 6 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 for the ultimate limit state (ULS), abnormal
environmental events and the accidental limit state (ALS) using different levels of earthquake.
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. Shutdown of production operations is tolerable and the structure should 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 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. A simplified method may be used where seismic
considerations are unlikely to govern the design of a structure, while the detailed method shall
...