EN ISO 19901-4:2003

SLOVENSKI STANDARD
01-maj-2004
Petroleum and natural gas industries - Specific requirements for offshore
structures - Part 4: Geotechnical and foundation design considerations (ISO 19901
-4:2003)
Petroleum and natural gas industries - Specific requirements for offshore structures -
Part 4: Geotechnical and foundation design considerations (ISO 19901-4:2003)
Erdöl- und Erdgasindustrie - Offshore-Plattformen - Teil 4: Geotechnische und
Fundament-Auslegungsmerkmale (ISO 19901-4:2003)
Industries du pétrole et du gaz naturel - Exigences spécifiques relatives aux structures
en mer - Partie 4: Bases conceptuelles des fondations (ISO 19901-4:2003)
Ta slovenski standard je istoveten z: EN ISO 19901-4:2003
ICS:
75.180.10 Oprema za raziskovanje in Exploratory and extraction
odkopavanje equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 19901-4
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2003
ICS 75.180.10
English version
Petroleum and natural gas industries - Specific requirements for
offshore structures - Part 4: Geotechnical and foundation design
considerations (ISO 19901-4:2003)
Industries du pétrole et du gaz naturel - Exigences Erdöl- und Erdgasindustrie - Offshore-Plattformen - Teil 4:
spécifiques relatives aux structures en mer - Partie 4: Geotechnische und Fundament-Auslegungsmerkmale (ISO
Bases conceptuelles des fondations (ISO 19901-4:2003) 19901-4:2003)
This European Standard was approved by CEN on 10 July 2003.
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 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 Management Centre has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and United
Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2003 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 19901-4:2003 E
worldwide for CEN national Members.

Foreword
This document (EN ISO 19901-4:2003) has been prepared by Technical Committee ISO/TC 67
"Materials, equipment and offshore structures for petroleum and natural gas industries" in
collaboration with Technical Committee CEN/TC 12 "Materials, equipment and offshore
structures for petroleum and natural gas industries", the secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of
an identical text or by endorsement, at the latest by February 2004, and conflicting national
standards shall be withdrawn at the latest by February 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, Czech
Republic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and
the United Kingdom.
NOTE FROM CMC  The foreword is susceptible to be amended on reception of the German
language version. The confirmed or amended foreword, and when appropriate, the normative
annex ZA for the references to international publications with their relevant European
publications will be circulated with the German version.
Endorsement notice
The text of ISO 19901-4:2003 has been approved by CEN as EN ISO 19901-4:2003 without any
modifications.
INTERNATIONAL ISO
STANDARD 19901-4
First edition
2003-08-01
Petroleum and natural gas industries —
Specific requirements for offshore
structures —
Part 4:
Geotechnical and foundation design
considerations
Industries du pétrole et du gaz naturel — Exigences spécifiques
relatives aux structures en mer —
Partie 4: Bases conceptuelles des fondations

Reference number
ISO 19901-4:2003(E)
©
ISO 2003
ISO 19901-4:2003(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.

©  ISO 2003
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2003 — All rights reserved

ISO 19901-4:2003(E)
Contents Page
Foreword. v
Introduction . vii
1 Scope. 1
2 Normative references. 1
3 Terms and definitions. 2
4 Symbols. 3
5 General requirements. 4
5.1 General. 4
5.2 Testing and instrumentation. 4
5.3 Conductor installation and shallow well drilling . 5
6 Geotechnical data acquisition and integrated geoscience studies. 5
6.1 Geotechnical assessment. 5
6.2 Shallow geophysical investigation . 5
6.3 Geological modelling and identification of hazards.6
6.4 Geotechnical investigation. 8
7 Stability of shallow foundations. 9
7.1 General. 9
7.2 Principles. 10
7.3 Acceptance criteria. 10
7.4 Undrained bearing capacity — constant shear strength. 12
7.5 Undrained bearing capacity — linearly increasing shear strength . 13
7.6 Drained bearing capacity . 13
7.7 Shear strength used in bearing capacity calculations. 14
7.8 Settlements and displacements . 14
7.9 Dynamic behaviour. 14
7.10 Hydraulic stability. 15
7.11 Installation and removal. 15
7.12 Shallow foundations equipped with skirts. 15
7.13 Shallow foundations without skirts . 15
7.14 Installation effects. 16
Annex A (informative) Additional information and guidance. 17
A.1 Scope. 17
A.2 Normative references. 17
A.3 Terms and definitions. 17
A.4 Symboles. 17
A.5 General requirements. 17
A.6 Geotechnical data acquisition and integrated geoscience studies. 18
A.6.1 Geotechnical assessment. 18
A.6.2 Shallow geophysical investigation . 18
A.6.3 Geological modelling and identification of hazards.18
A.6.4 Geotechnical investigation. 18
A.7 Stability of shallow foundations. 20
A.7.1 General. 20
A.7.2 Principles. 20
A.7.3 Acceptance criteria. 22
A.7.4 Undrained bearing capacity — constant shear strength. 22
A.7.5 Undrained bearing capacity — linearly increasing shear strength . 23
A.7.6 Drained bearing capacity . 25
ISO 19901-4:2003(E)
A.7.7 Shear strength used in bearing capacity calculations.26
A.7.8 Settlements and displacements.26
A.7.9 Dynamic behaviour.26
A.7.10 Hydraulic stability.26
A.7.11 Installation and removal.26
A.7.12 Shallow foundations equipped with skirts .26
A.7.13 Shallow foundations without skirts.27
A.7.14 Installation effects.27
Annex B (informative) Carbonate soils.28
B.1 General.28
B.2 Characteristic features.28
B.3 Properties.29
B.4 Foundations.29
B.4.1 Driven piles.29
B.4.2 Other deep foundation alternatives.29
B.4.3 Shallow foundations.29
B.5 Assessment.30
Bibliography.31

iv © ISO 2003 — All rights reserved

ISO 19901-4:2003(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 19901-4 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 4: Geotechnical and foundation design considerations
 Part 5: Weight control during engineering and construction
The following parts of ISO 19901 are under preparation:
 Part 1: Metocean design and operating considerations
 Part 2: Seismic design procedures and criteria
 Part 3: Topsides structure
 Part 6: Marine operations
 Part 7: Stationkeeping systems for floating offshore structures and mobile offshore units
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, Petroleum and natural gas industries — Floating offshore structures
 ISO 19905-1, Petroleum and natural gas industries — Site-specific assessment of mobile offshore
units — Part 1: Jack-ups
ISO 19901-4:2003(E)
 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
vi © ISO 2003 — All rights reserved

ISO 19901-4:2003(E)
Introduction
The offshore structures International Standards ISO 19900 to ISO 19906 constitute a common basis covering
those aspects that address design requirements and assessments of all offshore structures used by the
petroleum and natural gas industries worldwide. Through their application the intention is to achieve reliability
levels appropriate for manned and unmanned offshore structures, whatever the type of structure and the
nature 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 offshore structures International Standards are 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. For foundations, some additional considerations
apply. These include the time, frequency and rate at which actions are applied, the method of foundation
installation, the properties of the surrounding soil, the overall behaviour of the seabed, effects from adjacent
structures and the results of drilling into the seabed. All of these, and any other relevant information, need to
be considered in relation to the overall reliability of the foundation.
The design practice for the foundations of offshore structures has proved to be an innovative and evolving
process over the years since the 1950s. This evolution is expected to continue and is encouraged. Therefore,
circumstances can arise when the procedures described herein or in the other International Standards
ISO 19902 to ISO 19906 (or elsewhere) are insufficient on their own to ensure that a safe and economical
foundation design is achieved.
Seabed soils vary. Experience gained at one location is not necessarily applicable at another. The scope of
the site investigation for one structure is not necessarily adequate for another. Extra caution is necessary
when dealing with unfamiliar soils or foundation concepts. This part of ISO 19901 is intended to provide wide
latitude in the choice of site investigation techniques and foundation solutions, without hindering innovation.
Sound engineering judgement is therefore necessary in the use of this part of ISO 19901.
For an offshore structure and its foundations, the action effects at the interface between the structure's
subsystem and the foundation's subsystem(s) are internal forces, moments and deformations. When
addressing the foundation's subsystem(s) in isolation, these internal forces, moments and deformations may
be considered as actions on the foundation's subsystem(s) and this approach is followed in this part of
ISO 19901.
To meet certain needs of industry for linking software to specific elements in this part of ISO 19901, a special
numbering system has been permitted for figures, tables and equations.
Some background to and guidance on the use of this part of ISO 19901 is provided for information in Annex A.
Guidance on foundations in carbonate soils is provided for information in Annex B. There is, as yet, insufficient
knowledge and understanding of such soils to produce normative requirements.

INTERNATIONAL STANDARD ISO 19901-4:2003(E)

Petroleum and natural gas industries — Specific requirements
for offshore structures —
Part 4:
Geotechnical and foundation design considerations
1 Scope
This part of ISO 19901 contains requirements and recommendations for those aspects of geoscience and
foundation engineering that are applicable to a broad range of offshore structures, rather than to a particular
structure type. Such aspects are
 site characterization,
 soil and rock characterization,
 design and installation of foundations supported by the seabed (shallow foundations), and
 identification of hazards.
Aspects of soil mechanics and foundation engineering that apply equally to offshore and onshore structures
are not addressed. The user of this part of ISO 19901 is expected to be familiar with such aspects.
NOTE 1 Particular requirements for the design of piled foundations, which have a traditional association with fixed steel
structures, are given in ISO 19902.
NOTE 2 Particular requirements for the design of shallow gravity foundations, which have a traditional association with
fixed concrete structures, are detailed in ISO 19903.
NOTE 3 Particular requirements for the anchor points of mooring systems of floating structures are detailed in
[65]
ISO 19901-7 .
NOTE 4 Particular requirements for the design of spud can foundations, which have a traditional association with jack-
up mobile offshore units (MOUs), are detailed in ISO 19905 (all parts).
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
ISO 19902, Petroleum and natural gas industries — Fixed steel offshore structures
ISO 19903, Petroleum and natural gas industries — Fixed concrete offshore structures
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 19901-4:2003(E)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 19900 and the following apply.
3.1
design actions
combination of representative actions and partial safety factors representing a design situation for use in
checking the acceptability of a design
3.2
drained condition
condition whereby the applied stresses and stress changes are supported by the soil skeleton and do not
cause a change in pore pressure
3.3
effective foundation area
reduced foundation area having its geometric centre at the point where the resultant action vector intersects
the foundation base level
3.4
material factor
partial safety factor applied to the strength of the soil
3.5
sea floor
interface between the sea and the seabed
3.6
seabed
materials below the sea in which a structure is founded, whether of soils such as sand, silt or clay, cemented
materials or of rock
NOTE 1 The seabed can be considered as the half-space below the sea floor.
NOTE 2 Offshore foundations are most commonly installed in soils, and the terminology in this part of ISO 19901
reflects this. However, the requirements equally apply to cemented seabed materials and rocks. Thus, the term “soil” does
not exclude any other material at or below the sea floor.
NOTE 3 As yet there are no universally accepted definitions of the various types of soil and rock, see A.6.4.3.
3.7
settlement
permanent downward movement of a structure as a result of its own weight and other actions
3.8
undrained condition
condition whereby the applied stresses and stress changes are supported by both the soil skeleton and the
pore fluid and do not cause a change in volume
3.9
undrained shear strength
maximum shear stress at yielding or at a specified maximum strain in an undrained condition
NOTE Yielding is the condition of a material in which a large plastic strain occurs at little or no stress increase.
2 © ISO 2003 — All rights reserved

ISO 19901-4:2003(E)
4 Symbols
Commonly used symbols are listed below, other symbols are defined in the text following the applicable
formula. It should be noted that symbols can have different meanings between formulae.
A total foundation area
A′ effective foundation area
A embedded vertical cross-sectional area of foundation
h
a soil attraction
B′ effective width of foundation
c undrained shear strength of clay
u
c average undrained shear strength between sea floor and base level for linearly increasing isotropic
u,ave
undrained shear strength with depth
c undrained shear strength at base level
u,0
D depth to base level
b
H factored horizontal total action on base area
b
K correction factor, which accounts for inclined actions, foundation shape, and depth of embedment
c
K drained horizontal soil reaction coefficient
rd
K undrained horizontal soil reaction coefficient
ru
L effective length of foundation area
′
p effective overburden stress at base level (skirt tip level when skirts are used)
Q design sliding resistance
d,h
Q design bearing capacity in the absence of horizontal actions
d,v
q design unit bearing capacity in the absence of horizontal actions
d,v
V factored vertical total action on base area
b
γ material factor
m
γ ′ submerged unit weight of soil
κ rate of increase of undrained shear strength with depth
φ ′ effective angle of internal friction
ISO 19901-4:2003(E)
5 General requirements
5.1 General
The foundation shall be designed to carry static and dynamic (repetitive as well as transient) actions without
causing excessive deformation of or vibrations in the structure. Special attention shall be given to the effects
of repetitive and transient actions on the structural response, as well as on the strength of the supporting soils.
The possibility of movement of the seabed shall be considered. Any actions resulting from such movements
on foundation members shall be considered in the design. The potential for disturbance to foundation soils by
conductor installation or shallow well drilling shall be assessed (see 5.3).
5.2 Testing and instrumentation
Where there is uncertainty regarding the behaviour of foundations, testing or instrumentation should be
undertaken. Possible methods include the following.
a) Load testing.
Load testing or large-scale field testing should be performed where there is particular uncertainty in the
foundation capacity and where safety and/or economy are of particular importance.
b) Model tests.
Model tests should be performed where
1) the foundation configuration differs significantly from earlier configurations where operational
experience exists,
2) the soil conditions differ significantly from those where operational experience exists,
3) new methods of installation or removal are envisaged, or
4) a high degree of uncertainty exists as to how the structure or its foundation will behave.
c) Temporary instrumentation.
Structures should be fitted with temporary instrumentation where
1) the installation method presupposes the existence of measured data for control of the operation, or
2) an installation method is to be applied with which little or no experience has been gained.
d) Permanent instrumentation.
Structures should be fitted with permanent instrumentation where
1) the safety or behaviour of the foundation is dependent on active operation,
EXAMPLE Where drainage systems are used, data shall be immediately accessible to the user.
2) the foundation configuration, the soil conditions, or the actions differ substantially from those with
which experience has been gained,
3) there is a need for monitoring of the whole foundation with regard to penetration, settlement, tilt, or
other behaviour, or
4) the method of removal presupposes the existence of measured data for control of the operation.
4 © ISO 2003 — All rights reserved

ISO 19901-4:2003(E)
5.3 Conductor installation and shallow well drilling
The planning for conductor installation and shallow well drilling shall take into account the potential for
disturbance to foundation soils and the consequent risk of a reduction in stability of the structure or of adjacent
conductors.
Soil disturbances during drilling operations can result from hydraulic fracture, washout (uncontrolled
enlargement of the drilled hole), or shallow gas pockets. Hydraulic fracture occurs where drilling fluid pressure
is too high and fluid is lost into the formation, possibly softening the surrounding soil. Washout generally
occurs in granular soils and can, in part, be induced by high drilling fluid circulation rates or drilling without
mud. Washout can produce large voids in the soil structure and lead to stress relief in the surrounding soils.
These incidents can be accompanied by loss of circulation of drilling fluids, return of these fluids to the sea
floor other than through the conductor, or the creation of sea floor craters. Thereby the stability of foundations
can be reduced and displacements increased. These detrimental effects can occur whether the drilling takes
place after installation of the structure or before, e.g. through a pre-installed template or for an exploration
well.
Records of conductor installation and shallow well drilling shall be available to the designer of the structure.
The implications for foundation soils of any incidents of inadequate grouting, excessive loss of circulation,
return of drilling fluids to the sea floor other than through the conductor, or creation of sea floor craters should
be assessed. The cuttings from the well drilling operation, if allowed to accumulate on the sea floor, should be
taken into account in the foundation design, installation procedure and structure removal.
6 Geotechnical data acquisition and integrated geoscience studies
6.1 Geotechnical assessment
The determination of geotechnical parameters and the assessment of geological hazards and constraints
result from an integrated study of the area using geophysics, geology and geotechnical engineering.
Geophysical data are acquired to develop a geological model so as to better understand depositional and
other processes and features of an area. The geophysical data are also used to help interpret the stratigraphy
from geotechnical boreholes, to define lateral variability across a site, and to provide guidance on optimizing
the location of the proposed facilities. Incorporation of geotechnical data into the geological model gives
insight into the potential impact of geological conditions on man-made facilities, such as structures, pipelines,
anchors and wellheads.
6.2 Shallow geophysical investigation
Shallow geophysical investigation can provide information about soil stratigraphy and evidence of geological
features, such as slumps, scarps, irregular or rough topography, mud volcanoes, mud lumps, collapse
features, sand waves, slides, faults, diapirs, erosional surfaces, gas bubbles in the sediments, gas seeps,
buried channels, and lateral variations in stratum thicknesses. The areal extent of shallow soil layers can
sometimes be mapped if good correspondence is established between the soil boring and in situ test
information and the results from the seabed surveys.
The types of equipment for performing shallow geophysical investigation that should be considered are
discussed below.
a) Echo sounders or swathe bathymetric systems (in which a series of sweeps of the bathymetric equipment
are used) define water depths and sea floor morphology. On complex sea floors, swathe systems have
the advantage of providing higher data density and better definition of variable topography. Seismic three-
dimensional data acquired for exploration purposes also provide useful data for developing water-bottom
(bathymetry) maps.
These data should only be used for preliminary evaluations because the resolution could be of the order
of a few metres depending on the variability of the topography.
b) Sub-bottom profilers (tuned transducers) define structural features within the near-surface sediments.
NOTE These systems can also provide data to develop water-bottom maps.
ISO 19901-4:2003(E)
c) Side-scan sonar defines sea floor features and sea floor reflectivity.
NOTE Backscatter measurements from some swathe systems can also provide morphological information.
d) Seismic sources, such as boomers or minisparkers, can define the structure to deeper depths up to
approximately 100 m below the sea floor and either single or tuned arrays of sparkers, air guns, water
guns or sleeve-exploders can define structure to deeper depths and can tie in with deep seismic data
from reservoir studies. Seismic source signals are received either with single channel analogue or multi-
channel hydrophones. Digital processing of the recorded signals enhances the quality of the images
recorded and removes extraneous noise and multiples from the recorded signals.
e) Seabed refraction equipment provides information on the stratification of the top few metres of the
seabed.
Shallow sampling of near surface sediments using drop, piston or grab sampling, or vibrocoring together with
cone penetrometer tests along geophysical tracklines can be useful for calibration of results and improved
definition of the shallow geology.
Direct observation of the sea floor using a remotely operated vehicle (ROV) or manned submersible can also
provide important confirmation or characterization of geological conditions.
6.3 Geological modelling and identification of hazards
6.3.1 General
The nature, magnitude, and return intervals of potential active geological processes should be evaluated by
site investigation techniques. Judicious use of analytical modelling can provide input for determination of the
effects of active geological processes on structures and foundations. Due to uncertainties associated with
definition of these processes, a parametric approach to studies is also helpful in the development of design
criteria.
A geological model is constructed by hypothesizing a depositional process. The geophysical data should be
mapped within the context of the hypotheses made. Features within the same geological period should be
mapped together. Features not associated with a particular process should be mapped separately. If
necessary, the mapping strategy should be adjusted to fit the model until agreement exists between the data
and the model. The results of the geological modelling phase should ideally allow the interpreter to discuss in
a report how features have developed over time, in order to allow assessment of how the features can affect
future man-made developments.
Some of the more familiar geological processes, events and conditions are discussed in 6.3.2 to 6.3.7.
6.3.2 Earthquakes
Seismic actions shall be considered in design of structures for areas that are determined to be seismically
active. Areas are considered seismically active on the basis of the historical record of earthquake activity, both
in frequency of occurrence and in magnitude, or on the basis of a tectonic review of the region (for more
[66]
details, see ISO 19901-2 ).
Seismic considerations for such areas shall include investigation of the subsurface soils for instability due to
liquefaction, submarine slides triggered by earthquake activity, proximity of the site to seismogenic faults, the
characteristics of the ground motions expected during the life of the structure, and the acceptable seismic risk
for the type of operation intended. Structures in shallow water that can be subjected to tsunamis shall be
investigated for the effects of the resulting actions.
6.3.3 Fault planes
In some offshore areas, fault planes can extend to the sea floor with the potential for vertical and horizontal
movement. Fault movement can occur as a result of tectonic activity, removal of fluids from deep reservoirs or
long-term creep related to large-scale sedimentation or erosion. Siting of facilities in close proximity to fault
6 © ISO 2003 — All rights reserved

ISO 19901-4:2003(E)
planes intersecting the sea floor should be avoided, if possible. If circumstances dictate siting structures near
potentially active faults, the magnitude and time scale of expected movement shall be estimated on the basis
of a geological study for use in the design of structures.
6.3.4 Sea floor instability
Movements of the sea floor can be caused by ocean wave pressures, earthquakes, soil self-weight, hydrates,
shallow gas, faults, or other geological processes. Weak, underconsolidated sediments occurring in areas
where wave pressures are significant at the sea floor are susceptible to wave-induced movement and can be
unstable under very small slope angles. Earthquakes can induce failure of sea floor slopes that are otherwise
stable under the existing soil self-weight and wave actions.
Rapid sedimentation (such as actively growing deltas), low soil strength, soil self-weight, and wave-induced
pressures are generally controlling factors for the geological processes that continually move sediment
downslope. Important design considerations under these conditions include the effects of large-scale
movement of sediment (i.e. mud slides and slumps) in areas subjected to strong wave pressures, downslope
creep movements in areas not directly affected by wave/sea floor interaction and the effects of sediment
erosion and/or deposition on structure performance.
The scope of site investigations in areas of potential instability shall focus on identification of metastable
geological features surrounding the site and definition of the soil engineering properties required for modelling
and estimating sea floor movements.
Estimates of soil movement as a function of depth below the sea floor based on geotechnical analyses can be
used to predict actions on structural members. Geological studies employing historical bathymetric data can
be useful for quantifying deposition rates during the design life of the facility.
6.3.5 Scour and sediment mobility
Scour is the removal of seabed soils by currents and waves. Such erosion can be due to a natural geological
process or can be caused by structural components interrupting the natural flow regime above the sea floor.
From observations, sea floor variations can usually be characterized as some combination of the following.
a) Local scour.
Steep-sided scour pits around foundation components such as piles and pile groups, as seen in flume
models.
b) Global scour.
Shallow scoured basins of large extent around a structure, possibly due to overall structure effects,
multiple structure interaction, or wave-soil-structure interaction.
c) Overall seabed movement of sand waves, ridges, and shoals that would also occur in the absence of a
structure.
Such movements can result in sea floor lowering or rising, or repeated cycles of these. The addition of
man-made structures often changes the local sediment transport regime that can aggravate erosion,
cause accumulation, or have no net effect.
Scour can result in removal of vertical and lateral support for foundations, causing undesirable settlements of
shallow foundations and overstressing of foundation components. Where scour is a possibility, it shall be
taken into account in design and/or its mitigation shall be considered.
ISO 19901-4:2003(E)
6.3.6 Shallow gas
The presence of either biogenic or petrogenic gas in the pore water of shallow soils is an important
consideration to the engineering of the foundation. In situ natural gas can be both gaseous or bound with
water to form a solid (known as hydrate). In addition to being a potential drilling hazard during both site
investigation soil borings and oil well drilling, the effects of shallow gas can be important to foundation
engineering. The e
...