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General Report of TC209
Offshore Geotechnics
Rapport Général du TC209
Géotechnique Offshore
Jewell R.A.
Fugro GeoConsulting
ABSTRACT: This general report introduces the discussion session organized by ISSMGE Technical Committee 209 (TC209)
“Offshore Geotechnics”. The main topics include offshore wind projects, pipelines and seabed structures, seabed soils, coastal and
nearshore work, and pile foundations.
RÉSUMÉ : Ce rapport général introduit la séance de discussion organisée par le Comité Technique 209 (TC209) “Géotechnique
Offshore” de la SIMSG. Les principaux thèmes abordés sont les projets d’éoliennes offshore, les pipelines et structures sous-marines,
les sols sous-marins, les travaux côtiers et nearshore et les fondations sur pieux
KEYWORDS: offshore, caisson, piles, pipes, cyclic load, stability diagram, lateral load, tests, numerical analysis, bearing capacity
1 INTRODUCTION.
The organizers of the 18th International Conference
“Challenges and Innovations in Geotechnics” have implemented
important changes to the conference format. One is the
inclusion of Offshore Geotechnics at this main ISSMGE forum.
Second is the focus given to the technical committees.
This general report covers the session organized by the
ISSMGE Technical Committee 209 (TC209) “Offshore
Geotechnics” chaired by Philippe Jeanjean. Participation by
TC209 at this 18
th
conference includes the 2
nd
ISSMGE
McClelland Lecture by Mark Randolph, this discussion session
and a workshop on recent research and development on piles
under cyclic loading.
The main difference in offshore geotechnics arises from the
conditions and environment offshore. There is a stark contrast
in access for site investigation, soil sampling, field testing,
installation and observation. Activities offshore often require
new tools. Soft soil conditions at seabed level are encountered
in deepwater, frequently with high carbonate content, unusual
mineralogy or biogenic activity. Combined and cyclic loading
usually dominate design, whether caused by waves and currents
acting on structures or by repeated expansion and contraction of
pipelines on the seabed. Large displacement is a feature of the
installation and operation of seabed pipelines. Many of the
above features of offshore geotechnics are discussed in papers
to this session.
This general report has been organized into five main subject
areas: Offshore Wind; Pipelines and Seabed Structures; Seabed
Soils; Coastal and Near Shore work; Pile Foundations.
Since papers on the cyclic loading of piles will be presented
and discussed at the TC209 workshop, these are highlighted in
this general report but will not be presented during the
discussion session. The main focus for presentations will be
Offshore Wind and large displacement as encountered with
Offshore Pipelines.
Only a limited selection of papers will be presented at the
discussion session. All the papers are in the proceedings and
many will be presented at the poster session. Participants are
strongly encouraged to attend the TC209 workshop where the
cyclic behavior and design of piles will be presented and
debated based around papers to this session.
2 OFFSHORE WIND.
2.1
Site investigation
Project development, engineering design and project
construction are three main phases for offshore wind farms. A
major challenge is to minimize geotechnical risk for foundation
engineering. In current practice, geotechnical risk is addressed
mainly during the engineering design phase P2 in Table 1. Ben-
Hazzine and Griffiths
(2013)
suggest that risk management may
be improved through more extensive geophysical survey and
preliminary site investigation during the project development
phase P1. The authors highlight various sources of geotechnical
risk such as inherent soil variability, measurement errors and
“transformation errors” caused by simple empirical
interpretation of data.
Table 1. Timing of Geotechnical Work for Offshore Windfarms
(Ben-Hazzine and Griffiths, 2013)
Muir Wood and Knight (2013) use experience from 15
offshore wind farm projects in the UK to illustrate the manner
in which geotechnical risk was managed and to define
categories of poor, mediocre and good practice (or
vice versa
).
Poor practice includes appointment of the foundation design
team after site investigation is completed, insufficient planning
and poor interpretation of geophysical and geotechnical surveys.
Mediocre practice often involves incorrect scope for
geophysical and geotechnical surveys causing extra cost and
increased risk. On projects with good practice the foundation
design team was appointed at the start of the project and
specified the site investigation work. The ground models were
t g r l
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