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Site investigation and geotechnical design strategy for offshore wind development
Investigation géotechniques et stratégie de conception pour le développement d’éoliennes
maritimes
Muir Wood A.
DONG Energy
Knight P.
Parsons Brinckerhoff
ABSTRACT: The development of multi billion euro Offshore Wind Farms presents geotechnical engineers with the opportunity to
create comprehensive detailed ground models incorporating a large variety of geotechnical hazards. However the political structure of
renewable energy projects often leads to a fragmented development team, with no one party appointed for the whole design process.
Inexperienced clients are often commissioning surveys because they think that they want to do a survey rather than for an engineering
reason. This leads to unclear specification, and a resulting survey that does not add the expected value to the project. The authors
demonstrate in this paper how site investigation and ground modelling practices that are followed as routine in the design cycle of
onshore projects can be adapted and applied to add significant value to offshore renewable projects. This paper seeks to set out a
structure for development of the ground model for offshore wind projects, and demonstrates how clients can ensure their surveys are
adding value to the design strategy for their projects.
RÉSUMÉ : Grâce au développement très coûteux de fermes éoliennes en mer, les ingénieurs géotechniciens, ont l’opportunité de
concevoir des modèles de sol détaillés et exhaustifs qui peuvent rendre compte d’un large éventail de risques géotechniques.
Cependant la structure politique conduit au fait que les projets concernant les énergies renouvelables se trouvent bien souvent menés
par diverses équipes de conception sans véritable coordination. Des clients inexpérimentés ont souvent recours à des sondages dans le
seul but de faire des sondages et non pour des raisons techniques. Cela conduit à avoir des cahiers des charges souvent imprécis qui
n’apportent rien à la valeur attendue du projet. Les auteurs démontrent dans cet article comment les études de terrain et les pratiques
de modélisation du terrain utilisées systématiquement dans les projets sur terre peuvent être adaptées et appliquées aux projets en mer
et peuvent augmenter leur valeur considérablement. Cet article propose une structure de développement des modèles terrestres pour
les champs d’éolienne en mer et démontre comment les clients peuvent s’assurer que leurs sondages valorisent les stratégies de
conception de leurs projets.
KEYWORDS: geotechnical hazard management, site investigation strategy, ground model development, offshore wind farm
1 INTRODUCTION
The offshore wind industry in Northern Europe started with the
development of small demonstration projects. In recent years
these have significantly increased in size, and many of the
current projects are now multi billion euro investments, with
development times in excess of five years. The industry can
demonstrate many examples of how lessons learnt and
knowledge gained from the earlier projects have been
incorporated into the recent larger projects, ultimately leading to
lower capital and operational expenditure per MW of power
generated.
Through this process the design methodology and codes (e.g.
DNV 2011) which were originally predominately based on the
experience of offshore oil and gas infrastructure have also
continued to develop and there are many examples of good
practice in current projects. However, for geotechnical site
investigations this learning process has not been completely
positive. It leads to a tendency to base the scope of the
investigation on a specification for a previous projects, rather
than on what is most appropriate for the site and specific
development.
The authors have been involved in the design of over 15
projects, which when built will total more than 8GW of power.
A review of these projects shows that they have all spent
comparable money on site investigation - typically
approximately 1% of the project capital costs (which on the
current large projects leads to investigations costing in excess of
€30M). However the success of the investigations in managing
the geotechnical risks and bringing value to the projects is
extremely variable.
2 EXAMPLE PROJECTS
Typical problems that the authors have experienced are:
investigations not planned to mitigate project specific
geotechnical hazards; poor recording and interpretation of
geological information; planning of surveys not based on the
results of proceeding investigations/studies; and surveys not
specified by the foundation designers.
Some specific examples of good and poor practice on
projects are detailed in the following sections. The project
names have not been stated however the approximate
construction cost of each project is given.
2.1
Poor practice
2.1.1
Project 1 – value ~€2bn
The site investigation comprised site wide sub-bottom
geophysical survey with three rounds of geotechnical works: (1)
met mast borehole; (2) site wide boreholes; (3) seabed CPTs on
every foundation location. Extensive advanced lab testing and
down hole geophysical methods and pressuremeter tests were
undertaken.
Issues included: the detailed designer was appointed after the
site investigation was completed; insufficient time was allowed
for the interpretation of the geophysical and geotechnical
