2303
Shallow foundations for offshore wind towers
Fondations superficielles pour des installations éoliennes maritimes
Arroyo M., Abadías D., Alcoverrro J., Gens A.
Dep. of Geotechnical Engineering and Geosciences, Technical University of Catalonia, Barcelona, Spain
ABSTRACT: Direct foundations are present in about 25% of the installed offshore wind power towers. The peculiarities of this type
of structure are well known: high dynamic sensitivity, complex couplings between environmental actions, machine operation and
structural response, complex installation and maintenance, difficult site investigation. There is a clear need for optimized foundation
design tools that would enable cost reduction and a more detailed assessment of the risk of every installation. One such tool is likely
to be the systematic use of failure envelopes for capacity checks. The paper explores the benefits of such an approach with various
realistic design examples.
RÉSUMÉ : Les fondations superficielles interviennent dans la réalisation de 25% des structures éoliennes maritimes. Les
particularités de ce type de structures sont bien connues: haute sensibilité dynamique, couplages complexes entre les actions
environnementales, le fonctionnement de la machine et la réponse structurelle, installation et maintenance difficiles, investigation
géotechniques onéreuses. Un besoin évident d'optimisation des outils de conception est nécessaire pour permettre la réduction des
coûts et une évaluation plus détaillée du risque de chaque installation. Le recours systématique à des enveloppes de rupture pour les
justifications de la capacité portante des fondations peut bien être un tel outil. Ce papier explore les avantages d'une telle approche
avec divers exemples de conception réalistes.
KEYWORDS: direct foundation, capacity, offshore, energy, wind farms
1 INTRODUCTION
Offshore wind is an increasingly large contributor to the energy
production mix of several European countries, particularly those
bordering the North and Baltic seas. An exponential increase in
installations is currently anticipated in this region. It is
reasonable to expect that other regions of the world will follow
suit.
Offshore wind turbines (OWT) are generally larger than
those installed on land, with 3 to 5 MW of nominal capacity
being now the norm, but with turbines of up to 10 Mw coming
soon to the market. Rotor diameters of more than 100 m and
nacelle locations 80 m above mean sea level are common. The
result is a relatively lightweight and slender structure,
supporting a rotating machine finely tuned to maximize power
production while minimizing structural loading.
While initial OWT installations took place near shore (< 10
km) at locations with relatively shallow water depths (< 20 m),
current developments are clearly located offshore (10 -100 km
from the nearer coast) with water depths of 20-50 m being
typical. Several floating support concepts are now being
developed; however, commercial installations are still always
supported by some kind of fixed structure. For these, the
foundation of choice would depend in any case on the particular
site conditions, construction equipment availability and, to a
certain extent, local traditions.
To this date pile foundations have been largely dominant,
mostly as single large (4-6 m diameter) monopile installations,
and lately also as smaller (1-2 m) piles for jackets and tripods.
However, examining the industry databases (e.g. Burton et al
2011) it appears that at the end of 2011 about 25% of the
installed power was supported by direct foundations or gravity
base substructures (GBS). Most of these GBS installations took
place in relatively shallow waters, but there are some examples
already at larger distances from the coastline and in deeper
waters. Perhaps the most significant is the Thornton Bank I
project, 27 km offshore Zeebrugge in Belgium, where 6 OWT
of 5 Mw were installed in water depths of 20-30 m. The
foundation design for this installation was described by Peire et
al (2009) and its outline is reproduced here in Figure 1. These
are large (44 m height; 23.5 m base diameter) concrete shells,
floated into place and later ballasted with a mixture of sand and
olivine with the base at 4 m below the original seafloor level.
The geotechnical profile at the site comprises medium and high
density sands and stiff tertiary clays.
2 DESIGN ISSUES FOR DIRECT OWT FOUNDATIONS
There are several specific standards dealing with OWT. Perhaps
the highest ranked is IEC 61400-3 (2009) which, from the point
of view of structural design, establishes design cases and site
ambient load specification procedures, introduces a safety
format and gives broad indications about structural design
procedures. However, detailed specification of structural and
foundation design procedures is deliberately referred to other
documents, like the ISO 1990X offshore standard series or
DNV-OS-J101 (2010).
As might be expected, the indications given by such
standards are, on the one hand, firmly based in conventional
design practice when being specific, and somewhat elusive with
problems that lack a clear conventional solution. An example of
the later is the consideration of fatigue or foundation failure
under cyclic loading. An example of the former is the
consideration of foundation bearing capacity which, for shallow
foundations, follows a conventional superposition and
correction procedure not very different from those outlined by
Brinch-Hansen (1970) or Vesic (1975).
When designing foundations for OWT, there will be of
course issues of geotechnical capacity under extreme loads.
However the design drivers might be sometimes related to other
considerations, such as dynamic characteristics of the whole
