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In recent years, the offshore wind industry has seen an important boost that is expected to continue in the coming years. In order for the offshore wind industry to achieve adequate development, it is essential to solve some existing uncertainties, some of which relate to foundations. These foundations are important for this type of project. As foundations represent approximately 35% of the total cost of an offshore wind project, it is essential that they receive special attention. There are different types of foundations that are used in the offshore wind industry. The most common types are steel monopiles, gravity-based structures (GBS), tripods, and jackets. However, there are some other types, such as suction caissons, tripiles, etc. For high water depths, the alternative to the previously mentioned foundations is the use of floating supports. Some offshore wind installations currently in operation have GBS-type foundations (also known as GBF: Gravity-based foundation). Although this typology has not been widely used until now, there is research that has highlighted its advantages over other types of foundation for both small and large water depth sites. There are no doubts over the importance of GBS. In fact, the offshore wind industry is trying to introduce improvements so as to turn GBF into a competitive foundation alternative, suitable for the widest ranges of water depth. The present article deals with GBS foundations. The article begins with the current state of the field, including not only the concepts of GBS constructed so far, but also other concepts that are in a less mature state of development. Furthermore, we also present a classification of this type of structure based on the GBS of offshore wind facilities that are currently in operation, as well as some reflections on future GBS alternatives.
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Journal of
Marine Science
and Engineering
Review
Gravity-Based Foundations in the Offshore
Wind Sector
M. Dolores Esteban *, José-Santos López-Gutiérrez and Vicente Negro
Research Group on Marine, Coastal and Port Environment and other Sensitive Areas, Universidad Politécnica
de Madrid, E28040 Madrid, Spain; josesantos.lopez@upm.es (J.-S.L.-G.); vicente.negro@upm.es (V.N.)
*Correspondence: mariadolores.esteban@upm.es
Received: 27 December 2018; Accepted: 24 January 2019; Published: 12 March 2019


Abstract:
In recent years, the offshore wind industry has seen an important boost that is expected
to continue in the coming years. In order for the offshore wind industry to achieve adequate
development, it is essential to solve some existing uncertainties, some of which relate to foundations.
These foundations are important for this type of project. As foundations represent approximately 35%
of the total cost of an offshore wind project, it is essential that they receive special attention. There
are different types of foundations that are used in the offshore wind industry. The most common
types are steel monopiles, gravity-based structures (GBS), tripods, and jackets. However, there are
some other types, such as suction caissons, tripiles, etc. For high water depths, the alternative to the
previously mentioned foundations is the use of floating supports. Some offshore wind installations
currently in operation have GBS-type foundations (also known as GBF: Gravity-based foundation).
Although this typology has not been widely used until now, there is research that has highlighted
its advantages over other types of foundation for both small and large water depth sites. There are
no doubts over the importance of GBS. In fact, the offshore wind industry is trying to introduce
improvements so as to turn GBF into a competitive foundation alternative, suitable for the widest
ranges of water depth. The present article deals with GBS foundations. The article begins with the
current state of the field, including not only the concepts of GBS constructed so far, but also other
concepts that are in a less mature state of development. Furthermore, we also present a classification
of this type of structure based on the GBS of offshore wind facilities that are currently in operation, as
well as some reflections on future GBS alternatives.
Keywords: support structures; gravity-based structures; GBS; GBF
1. Introduction
The offshore wind sector can now be considered to be in a commercial stage of development [
1
].
According to a WindEurope report [
2
], in Europe during 2017, the total output of offshore wind
turbines constructed during the year totaled 3148 megawatts (MW), which is the annual installed
power record since the creation of this technology (in 2015, the annual installed capacity was very close
to that of 2017, but the rest of the years do not exceed half of the installed capacity in 2017). At the
end of 2017, there was a total of 15,780 MW of generation in Europe (double that at the end of 2014),
installed in 92 offshore wind farms, located in 11 countries across Europe [2].
The total amount of installed power generation has been growing over time due to several factors,
for instance: The installation of higher power wind turbines, larger numbers of wind turbines in each
facility, the state of development of the offshore wind industry in different countries, the successes
achieved in the sector, etc. This was accomplished by, among other reasons, moving to locations
with greater depth, investigating new concepts for foundations and substructures, working to solve
J. Mar. Sci. Eng. 2019,7, 64; doi:10.3390/jmse7030064 www.mdpi.com/journal/jmse
J. Mar. Sci. Eng. 2019,7, 64 2 of 14
uncertainties in foundation design, implementing scour protection systems, and increasing the amount
of investment to connect wind farms to energy grids [3,4].
The foundation is the key component in an offshore wind facility, representing the way through
which the loads of the superstructure, in this case the wind turbine, are transmitted to the soil.
Therefore, soil properties are essential in the selection of the type of foundation used in these
facilities [5].
Different types of foundation have been developed for use in the offshore wind industry. The most
common types are steel monopiles, gravity-based structures (GBS), tripods, and jackets [
6
]. Other
types of foundations, such as suction caisson, concrete monopile, floating support, etc., are currently in
a less advanced phase of development. This article focuses on GBS, also known as GBF (gravity-based
foundations), being the type of foundation that is suitable in cases of soils with high bearing capacity.
This is because of the way gravity structures support loads and transmit them to the soil [7,8].
Soils with a high load-bearing capacity allow the installation of shallow foundations [
9
], such as
the GBS. On the other hand, in the case of low load-bearing capacity soils, the best solution is to use
deep foundations or propose soil improvement techniques, such as piles, either as a monopile or as
a foundation for jackets or tripods. Other key aspects when deciding on the type of foundation to
use are the depth of the water, the loads associated with the characteristics of the wind turbine, and
the climatic loads (wind, waves, marine currents, tidal range, means of manufacturing, installation,
operation, dismantling, etc.). In fact, there are many considerations to be measured when selecting the
most appropriate foundation for a wind turbine facility. The cost of the foundation is around 35% of
the overall cost of the project. This significant cost is an important factor when both designing and
choosing the foundation to be used.
Most types of foundation used in offshore wind, both direct to the ground foundation and floating
supports, are concepts inherited from the offshore oil and gas industries [
10
,
11
]. In particular, it is clear
that GBS foundation originated from the oil and gas industries, with a structure known as a Condeep
(concrete deep-water structure) [
12
]. These are used for different water depths, with the maximum
depth being 330 m (Troll Condeep, in the Norwegian sector of the North Sea, 1995). The first Condeep
structure ever built was Ekofist I, in 1973, in the North Sea [
13
,
14
]. A Condeep is usually constructed
in a fjord, given its good characteristics in terms of construction associated with sheltered waters and
high-water depths [15].
On the other hand, concrete structures are also used at sea in breakwaters and quay walls [
16
],
where many structures use the floating caisson technique [
17
]. In fact, some companies have tried to
use a similar concept for offshore wind farms. It is important to understand the differences that exist
between offshore wind structures and breakwaters. The function of offshore wind foundations is to
support the wind turbine, and the function of the breakwater is to shelter the interior area, so that ships
can securely carry out loading and unloading operations. It is for this reason that the breakwaters have
to stop a large amount of wave energy. Furthermore, in the case of the supporting structures of the
wind turbines, it is important that the wave loads be as low as possible and as transparent as possible
to waves, with the objective of reducing the cost of the structure.
According to WindEurope
'
s report, at the end of 2017, monopile foundation in Europe was at the
top of the classification, with 3720 units (81.7%), followed by the jacket, with 315 units, and the gravity
based foundation, with 283 units. There were only seven floating platforms constructed during this
period, six of which were SPARs and one which was semi-submersible.
According to those statistics, there are some offshore wind facilities operating in Europe that have
GBS-type foundations; however, the small percentage of such cases indicates that GBFs have not been
widely used in the industry up until now. This is due to the ease of use of monopiles, which represent
>80% of offshore wind foundations. This characteristic of monopiles, together with their reduced cost,
has displaced other types of foundations from a strategic position in the sector. However, as water
depth increases, some limitations appear around the use of monopiles, potentially causing other types
of foundations to increase in use.
J. Mar. Sci. Eng. 2019,7, 64 3 of 14
Alternatives to the monopile have to be considered in locations with a terrain where the driving-in
of monopiles is difficult, for instance, rocky soils. In such cases, the GBS is expected to work well.
The main advantages of the GBS are: The good behavior of similar structures in the oil and gas and
port engineering industries; its suitability as a foundation in rocky or sandy soils, with its high bearing
capacity, where pile driving can be complicated; and it being an alternative that can enrich market
competitiveness and therefore reduce of costs in any industry. The main disadvantages of the GBS are:
It has not had great acceptance in the wind industry up to now; it needs soil with specific geotechnical
properties, such as high bearing capacity; in general, previous soil preparation is needed for correct
support of the structure; the large occupation area in the seabed, with its associated environmental
impact; and the necessary means of manufacture, transport, and installation.
Although this typology has not previously been widely used, there are certain opinions that have
highlighted its advantages over other types of foundations for both shallow and deep water sites.
In fact, the offshore wind industry is trying to introduce improvements to turn it into a competitive
foundation alternative in the widest range of water depths. There has been recent open discussion in
some of the most important conferences on offshore wind energy about possible competition between
jacket, XXL monopile, and GBS foundation types in water depths of around 40 m.
The importance of the GBS foundation for the future of the offshore wind industry is not currently
discussed. As a consequence, this article is about GBS foundations, since it is fundamental to achieve
greater knowledge about this concept. The paper provides a review of the different existing GBS
foundation concepts. For this, on one hand, the foundations of wind farms in Europe that are already
in operation are analyzed, and on the other, the main existing concepts that are in a less mature
development phase, either at the research or prototype level, are identified. In addition, this article
includes a classification of GBS foundations, elaborated on by the authors, based on offshore wind
farms examples in operation. The paper also includes some reflections on the future of GBS alternatives.
2. Objectives and Research Methodology
The main aim of this paper is to show the different existing alternatives of GBS foundations
for offshore wind facilities, including the already constructed ones and others in an early stage
of development.
For that, it was necessary to find all the offshore wind farms in operation that have GBS
foundations, and to study each specific design. After that, the different GBS foundation concepts were
identified and classified. Then, GBS alternatives in an early phase of study were analyzed. For all of
this, an in-depth literature review was carried out.
Based on available information, a classification proposal for the already constructed GBS
foundations is elaborated on by the authors, in order to clarify the different existing general concepts
in operating wind farms. Furthermore, some reflections are given on other GBS concepts that have not
yet been proven, as well as an analysis on the future on this type of concept.
3. State of the Art and Discussion
This section includes two parts. The first (3.1) identifies the different offshore wind farms in
operation in Europe that have GBS foundations. The second (3.2) concerns the different concepts of
GBS foundations. This second part includes not only the collected state of the art, but also a discussion
on this information.
3.1. European Offshore Wind Farms with GBS Foundations
European offshore wind farms with GBS foundations were identified, even those already
dismantled (Table 1); these were mainly culled from different reliable Internet sources [18,19].
J. Mar. Sci. Eng. 2019,7, 64 4 of 14
Table 1.
List of European offshore wind farms in operation with gravity-based structure
(GBS) foundations.
Name of the Farm Country Year
(COMISSIO-NING)
Total Power
(MW) Turbine Model Depth (M)
Kårehamn Sweden 2013 48 Vestas
3 MW 6–20
Vindpark Vänern Sweden 2012 30 WWD
3 MW -
Avedφre Holme Denmark 2011 10.8 Siemens
3.6 MW 2
Rφdsan II (Nysted II) Denmark 2010 207 Siemens
2.3 MW 6–12
SprogφDenmark 2009 21 Vestas
3 MW 10–16
Thorntonbank Phase 1 Belgium 2009 30 Repower (Senvion)
5 MW 13–20
Lillgrund Sweden 2007 110 Siemens
2.3 MW 4–13
Breitling Germany 2006 2.5 Nordex
2.5 MW 0.5
Nysted I (Rφdsan I) Denmark 2003 166 Siemens
2.3 MW 6–10
Middelgrunden Denmark 2001 40 Bonus, Siemens
2 MW 3–6
TunφKnob Denmark 1995 5 Vestas
500 kW 4–7
Vindeby (dismantled) Denmark 1991 4.95 Bonus
450 kW 2–4
In analyzing Table 1, several conclusions can be drawn:
Total wind farms: 12, one dismantled. Seven are in Denmark, which is the current leader in the
use of GBS in offshore wind, three in Sweden, and one each in Belgium and Germany.
Regarding year of commissioning: Two farms in 2009, and one each in 1991 (dismantled), 1995,
2001, 2003, 2006, 2007, 2010, 2011, 2012, and 2013. After 2013, there have been no facilities
commissioned with GBS.
Regarding the total power, the minimum is Breitling, with 2.5 MW and only one Nordex turbine,
and the maximum is Nysted I (Rφdsan I), with 166 MW and more than 70 turbine units.
The nominal power of the wind turbines is between ~0.5 MW (Vindeby, with 0.45, dismantled,
and TunφKnob, with 0.5) and 5 MW (Thorntonbank Phase 1).
The water depths of the sites are between 0.5 m (Breitling) and 20 m (Thorntonbank Phase 1).
3.2. GBS Foundation Types
A review of the different types of GBS foundations installed in offshore wind facilities was carried
out. Section 3.2.1 includes the main information from this review. After that, Section 3.2.2, concerning
new concepts for GBS foundations, includes some ideas at an early stage of development.
3.2.1. Proven Concepts of GBS Foundations
The first offshore wind farm with a GBS foundation was Vindeby, commissioned in 1991,
dismantled in 2017, and located in between 2 and 4 m of water. Since then, different offshore wind
facilities have been constructed with GBS foundations.
As a result of the analysis carried out here, a basic classification of the different GBS types is
included. This classification includes first-, second-, and third-generation types of GBS foundations.
J. Mar. Sci. Eng. 2019,7, 64 5 of 14
The first-generation GBS foundations correspond to the first offshore wind facilities with GBS
foundations: Tun
φ
Knob, commissioned in 1995 and located in between 4 and 7 m of water; and
Middelgrunden (Figure 1), commissioned in 2001 and located in between 3 and 6 m of water. This
first generation also corresponds to the first designs that were made of this type of foundation for the
offshore wind industry.
J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 5 of 14
3.2.1. Proven Concepts of GBS Foundations
The first offshore wind farm with a GBS foundation was Vindeby, commissioned in 1991,
dismantled in 2017, and located in between 2 and 4 m of water. Since then, different offshore wind
facilities have been constructed with GBS foundations.
As a result of the analysis carried out here, a basic classification of the different GBS types is
included. This classification includes first-, second-, and third-generation types of GBS foundations.
The first-generation GBS foundations correspond to the first offshore wind facilities with GBS
foundations: Tunϕ Knob, commissioned in 1995 and located in between 4 and 7 m of water; and
Middelgrunden (Figure 1), commissioned in 2001 and located in between 3 and 6 m of water. This
first generation also corresponds to the first designs that were made of this type of foundation for the
offshore wind industry.
Figure 1. Middelgrunden offshore wind farm foundations, reproduced from [20], with permission
from Elsevier, 2019.
This type of GBS is a completely solid, reinforced concrete structure, without holes or cells. It is
composed of a large-diameter slab; in the case of Middelgrunden, this is between 16.7 and 17.6 m
and very thin. The said slab is attached to a small-diameter shaft that, in some cases, ends in the form
of a cone as an icebreaker (Figure 1).
Since it is a solid structure, the weight to be taken into account for the transport and installation
is very high compared to the weight of a structure with the same geometry, but with incorporated
holes or cells. This typology was possible designed because Tunϕ Knob and Middelgrunden offshore
wind farms were built in locations with very shallow waterbetween 3 and 7 m of water depth.
While this type of concept can be considered to be a suitable solution for shallow depths, it is not
viable for sites in deeper water. The main problem is that, in deeper water, the design of these
structures, which work based on their own weight once they are in operation, leads to greater
weights than if they are designed to incorporate holes or cells. These heavy structures are not easy to
transport to the site or to install, as they need barges and cranes with special requirements. This
makes this first generation of GBS unprofitable for greater depths.
Following this, designs corresponding to the second generation of GBS foundations were
developed. Examples of this second generation are: Nysted I (or Rϕdsan I) (Figure 2), commissioned
in 2003 and located in between 6 and 10 meters of water; Lillgrund (Figure 3), commissioned in 2007
and located in between 4 and 13 meters of water; Sprogϕ (Figure 4), commissioned in 2009 and
located in between 10 and 16 meters of water; Rϕdsan II (or Nysted II) (Figure 5), commissioned in
2010 and located in between 6 and 12 meters of water; and Kårehamn (Figure 6), commissioned in
2013 and located in between 6 and 20 meters of water.
Figure 1.
Middelgrunden offshore wind farm foundations, reproduced from [
20
], with permission
from Elsevier, 2019.
This type of GBS is a completely solid, reinforced concrete structure, without holes or cells. It is
composed of a large-diameter slab; in the case of Middelgrunden, this is between 16.7 and 17.6 m and
very thin. The said slab is attached to a small-diameter shaft that, in some cases, ends in the form of a
cone as an icebreaker (Figure 1).
Since it is a solid structure, the weight to be taken into account for the transport and installation
is very high compared to the weight of a structure with the same geometry, but with incorporated
holes or cells. This typology was possible designed because Tun
φ
Knob and Middelgrunden offshore
wind farms were built in locations with very shallow water—between 3 and 7 m of water depth. While
this type of concept can be considered to be a suitable solution for shallow depths, it is not viable for
sites in deeper water. The main problem is that, in deeper water, the design of these structures, which
work based on their own weight once they are in operation, leads to greater weights than if they are
designed to incorporate holes or cells. These heavy structures are not easy to transport to the site or to
install, as they need barges and cranes with special requirements. This makes this first generation of
GBS unprofitable for greater depths.
Following this, designs corresponding to the second generation of GBS foundations were
developed. Examples of this second generation are: Nysted I (or R
φ
dsan I) (Figure 2), commissioned
in 2003 and located in between 6 and 10 meters of water; Lillgrund (Figure 3), commissioned in 2007
and located in between 4 and 13 meters of water; Sprog
φ
(Figure 4), commissioned in 2009 and located
in between 10 and 16 meters of water; Rφdsan II (or Nysted II) (Figure 5), commissioned in 2010 and
located in between 6 and 12 meters of water; and Kårehamn (Figure 6), commissioned in 2013 and
located in between 6 and 20 meters of water.
J. Mar. Sci. Eng. 2019,7, 64 6 of 14
J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 6 of 14
Figure 2. Nysted I (or Rϕdsan I) offshore wind farm foundations, with permission from Tech-Marine,
2019.
Figure 3. Lillgrund offshore wind farm foundations reproduced from [21], with permission from
Tech-Marine, 2019]
Figure 2.
Nysted I (or R
φ
dsan I) offshore wind farm foundations, with permission from
Tech-Marine, 2019.
J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 6 of 14
Figure 2. Nysted I (or Rϕdsan I) offshore wind farm foundations, with permission from Tech-Marine,
2019.
Figure 3. Lillgrund offshore wind farm foundations reproduced from [21], with permission from
Tech-Marine, 2019]
Figure 3.
Lillgrund offshore wind farm foundations reproduced from [
21
], with permission from
Tech-Marine, 2019.
J. Mar. Sci. Eng. 2019,7, 64 7 of 14
Figure 4. Sprogφoffshore wind farm foundations [22], with permission from Aarsleff, 2019.
J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 7 of 14
Figure 4. Sprogϕ offshore wind farm foundations [22], with permission from Aarsleff, 2019.
Figure 5. Rϕdsan II (or Nysted II) offshore wind farm foundations, reproduced from [22], with
permission from Aarsleff, 2019.
Figure 5.
R
φ
dsan II (or Nysted II) offshore wind farm foundations, reproduced from [
22
], with
permission from Aarsleff, 2019.
J. Mar. Sci. Eng. 2019,7, 64 8 of 14
J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 8 of 14
Figure 6. Kårehamn offshore wind farm foundations, reproduced from [20], with permission from
Elsevier, 2019.
This type of GBS foundation is composed of a flat slab and a shaft, similar to that of the first
generation but with the main difference being that the slab contains holes or cells. This means that if
the weights of a first- and second-generation GBS structure were to be compared, both with the same
geometry, the weight of the latter would be much lower. This lower weight allows more units to be
transported in the same barge, from the port to the final location, in one trip. In addition, the cranes
used in installation have less demanding requirements. Once the GBS foundation is installed, the
holes or cells are filled with ballast, thus achieving the final design weight of the structure that
allows it to be stable and resistant to loads.
The third generation of GBS foundations are the latest concepts that were built. An example of
this generation is the first phase of the Thornton Bank offshore wind farm (Figure 7), commissioned
in 2009 and located in between 13 and 20 meters of water. As can be seen in that figure, the structure
has a conical shape in the lower part and a vertical shaft in the upper part. The structure is mostly
hollow inside, not only the slab or lower part. This structure was designed to be transported using a
semi-floating method, thus reducing the weight of the structure for that phase, which lowers the
requirements of the transport vessels and cranes used in their installation. Once the structures are in
place, the hollow area is filled with ballast, to provide the necessary weight to support the loads.
Figure 6.
Kårehamn offshore wind farm foundations, reproduced from [
20
], with permission from
Elsevier, 2019.
This type of GBS foundation is composed of a flat slab and a shaft, similar to that of the first
generation but with the main difference being that the slab contains holes or cells. This means that if
the weights of a first- and second-generation GBS structure were to be compared, both with the same
geometry, the weight of the latter would be much lower. This lower weight allows more units to be
transported in the same barge, from the port to the final location, in one trip. In addition, the cranes
used in installation have less demanding requirements. Once the GBS foundation is installed, the holes
or cells are filled with ballast, thus achieving the final design weight of the structure that allows it to
be stable and resistant to loads.
The third generation of GBS foundations are the latest concepts that were built. An example of
this generation is the first phase of the Thornton Bank offshore wind farm (Figure 7), commissioned in
2009 and located in between 13 and 20 meters of water. As can be seen in that figure, the structure
has a conical shape in the lower part and a vertical shaft in the upper part. The structure is mostly
hollow inside, not only the slab or lower part. This structure was designed to be transported using
a semi-floating method, thus reducing the weight of the structure for that phase, which lowers the
requirements of the transport vessels and cranes used in their installation. Once the structures are in
place, the hollow area is filled with ballast, to provide the necessary weight to support the loads.
J. Mar. Sci. Eng. 2019,7, 64 9 of 14
J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 9 of 14
Figure 7. Thornton Bank offshore wind farm foundations, reproduced from [20], with permission
from Elsevier, 2019.
The classification proposed for the GBS concepts used up to now in offshore wind farms
already in operation is shown in Figure 8, which includes an example, a conceptual draft, and a
photo of the different types of GBS foundations that correspond to the first, second, and third
generations.
First Generation
Example:
Middelgrunden
Second Generation
Example: Nysted I
(Rϕdsand 1)
Third Generation
Example: Thornton
Bank Phase 1
Figure 8. Classification proposal for the existing GBS concepts, reproduced from [20], with
permission from Elsevier, 2019.
Figure 7.
Thornton Bank offshore wind farm foundations, reproduced from [
20
], with permission from
Elsevier, 2019.
The classification proposed for the GBS concepts used up to now in offshore wind farms already
in operation is shown in Figure 8, which includes an example, a conceptual draft, and a photo of the
different types of GBS foundations that correspond to the first, second, and third generations.
J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 9 of 14
Figure 7. Thornton Bank offshore wind farm foundations, reproduced from [20], with permission
from Elsevier, 2019.
The classification proposed for the GBS concepts used up to now in offshore wind farms
already in operation is shown in Figure 8, which includes an example, a conceptual draft, and a
photo of the different types of GBS foundations that correspond to the first, second, and third
generations.
First Generation
Example:
Middelgrunden
Second Generation
Example: Nysted I
(Rϕdsand 1)
Third Generation
Example: Thornton
Bank Phase 1
Figure 8. Classification proposal for the existing GBS concepts, reproduced from [20], with
permission from Elsevier, 2019.
Figure 8.
Classification proposal for the existing GBS concepts, reproduced from [
20
], with permission
from Elsevier, 2019.
J. Mar. Sci. Eng. 2019,7, 64 10 of 14
3.2.2. New Concepts of GBS Foundations
Now that the different concepts of operational offshore wind farms have been outlined, the
following sections concern new types of GBS foundations that are in the research phase. Most of these
new concepts are based on the F2F (floated to fixed) concept, which refers to a structure that behaves
as a float during the transport phase from the port to its final location, and during the installation
phase. During transport, it is necessary to have the support of small tugboats. By floating the structure,
the need for vessels for transport is eliminated, except for the tugboats, thus reducing the cost of that
phase. Some of these types of GBS foundations need special-purpose vessels, with the objective of
transporting and installing the GBS structure and the wind turbine generator (WTG) together, with the
WTG being pre-assembled in the port.
Crane-Free Gravity Base (Seatower)
The crane-free gravity base concept is a concrete structure with a relatively thin slab, an
intermediate-length conical part, and a cylindrical shaft in the upper part. This concept was designed
to be transported by its own flotation ability, so it is hollow inside, for which it needs the support
of tugboats (Figure 9). It avoids the use of an expensive and weather-sensitive crane. According to
Reference [
23
], this concept has been optimized for the logistics, from the manufacturing through to
the decommissioning process.
J. Mar. Sci. Eng. 2018, 6, x FOR PEER REVIEW 10 of 14
3.2.2. New Concepts of GBS Foundations
Now that the different concepts of operational offshore wind farms have been outlined, the
following sections concern new types of GBS foundations that are in the research phase. Most of
these new concepts are based on the F2F (floated to fixed) concept, which refers to a structure that
behaves as a float during the transport phase from the port to its final location, and during the
installation phase. During transport, it is necessary to have the support of small tugboats. By floating
the structure, the need for vessels for transport is eliminated, except for the tugboats, thus reducing
the cost of that phase. Some of these types of GBS foundations need special-purpose vessels, with the
objective of transporting and installing the GBS structure and the wind turbine generator (WTG)
together, with the WTG being pre-assembled in the port.
Crane-Free Gravity Base (Seatower)
The crane-free gravity base concept is a concrete structure with a relatively thin slab, an
intermediate-length conical part, and a cylindrical shaft in the upper part. This concept was
designed to be transported by its own flotation ability, so it is hollow inside, for which it needs the
support of tugboats (Figure 9). It avoids the use of an expensive and weather-sensitive crane.
According to Reference [23], this concept has been optimized for the logistics, from the
manufacturing through to the decommissioning process.
Figure 9. Crane-free gravity base concept, reproduced from [23], with permission from Elsevier,
2019.
Gravitas Gravity Base (Arup/Costain/Hochtief)
The gravitas gravity base concept is shaped similarly to the crane-free gravity base type, also
having a relatively thin slab, intermediate-length cone, and cylindrical shaft in the upper part. This
structure is also self-floating and can be transported to its final location only with the assistance of
small tugboats. According to www.arup.com, some characteristics of this concepts are: It is a
reinforced concrete and ballasted gravity structure; it can be deployed in water depths up to 60 m; it
can hold turbines generating up to 8 MW; it requires minimal seabed preparation because it can
accommodate existing seabed slopes and surface sediments; its skirt has variants to suit specific
seabed sediment conditions; the collar design for the turbine mast connection can accommodate an
~2º vertical alignment tolerance; there is the potential to repower it without replacing the
foundation; the concrete base is configured for rapid construction using available construction skills;
its construction is an onshore activity, which is tailored for ease of subsequent installation; it does
not required deep water (10 m draft) for construction; the foundations are self-buoyant for ease of
deployment to the wind farm location; it uses available and abundant standard tugs to install the
foundations; installation is done by sinking, through a controlled influx of water, followed by
sand/aggregate ballasting; and it includes scour protection, designed for minimum maintenance
over the design life of the wind farm [24].
The key figures for a 35-m water depth, central North Sea environment conditions, and 6 MW
output are:
Air-gap concrete structure: 20 m.
Figure 9.
Crane-free gravity base concept, reproduced from [
23
], with permission from Elsevier, 2019.
Gravitas Gravity Base (Arup/Costain/Hochtief)
The gravitas gravity base concept is shaped similarly to the crane-free gravity base type, also
having a relatively thin slab, intermediate-length cone, and cylindrical shaft in the upper part. This
structure is also self-floating and can be transported to its final location only with the assistance
of small tugboats. According to www.arup.com, some characteristics of this concepts are: It is a
reinforced concrete and ballasted gravity structure; it can be deployed in water depths up to 60 m;
it can hold turbines generating up to 8 MW; it requires minimal seabed preparation because it can
accommodate existing seabed slopes and surface sediments; its skirt has variants to suit specific seabed
sediment conditions; the collar design for the turbine mast connection can accommodate an ~2
º
vertical
alignment tolerance; there is the potential to repower it without replacing the foundation; the concrete
base is configured for rapid construction using available construction skills; its construction is an
onshore activity, which is tailored for ease of subsequent installation; it does not required deep water
(10 m draft) for construction; the foundations are self-buoyant for ease of deployment to the wind farm
location; it uses available and abundant standard tugs to install the foundations; installation is done by
sinking, through a controlled influx of water, followed by sand/aggregate ballasting; and it includes
scour protection, designed for minimum maintenance over the design life of the wind farm [24].
The key figures for a 35-m water depth, central North Sea environment conditions, and 6 MW
output are:
J. Mar. Sci. Eng. 2019,7, 64 11 of 14
Air-gap concrete structure: 20 m.
Hub height above LAT (Lowest Astronomical Tide): 90 m.
Base outer diameter: 34 m.
Outer diameter, caisson: 31 m.
Outer diameter, top of shaft: 6 m.
Concrete volume: 1919 m3.
Steel reinforcements: 720 tons.
Strabag Gravity Base (STRABAG)
STRABAG has two different gravity-base concepts. Both of them have a geometrical slab and a
cylinder in the upper part. The concepts have in common joint transportation and installation of the
foundation and the wind turbine generator, with preassembly being performed in port, thus reducing
the number of operations carried out at sea during the installation phase. To be able to carry the
floating foundation and wind turbine together, a specifically-purposed vessel is used, called STRABAG
Carrier [25].
According to www.strabag-offshore.com, both these concepts use the pre-stressed concrete
technique, they are suitable for water depths up to ~45 m, and they can be completely disassembled.
GBF Gravity Base (Ramboll/BMT Nigel Gee and Freyssinet)
The GBF gravity base concept is a concrete structure with a circular slab, a conical intermediate
part, and a relatively small-diameter cylinder shaft in the upper part. This concept is not self-floating
and requires a specific barge for the transport and installation of the GBS structure and the wind
turbine generator together. This specific barge is called a transport and installation barge (TIB) [26].
The tower, nacelle, and rotor are assembled in the port quay before being lowered into the
water. The TIB is ballasted down to the level of the base, then, upon connection, will refloat to the
transportation depth. This concept was developed with the support of the Carbon Trust.
According to Reference [
27
], this type of foundation is suitable for water depths between 20 and
55 m, many seabed conditions, a distance offshore between 2 and 200 miles, and turbines with a unit
power between 3 and 10 MW.
Other Gravity Base Structure Concepts
Other gravity base structures are described in Reference [
28
], both of them having similar shapes
to the abovementioned concepts. These are, respectively, concepts from a collaboration between
BAM Wind Energie and Van Oord, and the consortia of Skanska, Smit Marine Projects, and Grontmij.
Both have two different parts—a slab and a shaft—and both are cylindrical, with a smooth transition
between both with a conical shape.
Other Related Concepts
Other concepts than can be considered to be related to the GBS foundation concept are the
Rockmat and the ocean brick system (OBS), both described below.
Rockmat (OFS: CETEAL/Cathie Associates/DVO) [
29
] is an innovative concept as a foundation
for a wind turbine generator that can be used in rocky soils. It is a technology for the interface between
the soil and different types of support structures, such as jackets, GBS, etc. It comprises a precast base
to make the entire foundation self-floating and supportable by tugboats and is installed by ballasting
with water and concrete. It is fixed in its final position through a combination of a grout injection
system associated with a jack levelling system. Irregularities in the contact between the foundation
and the seabed are filled with grout injections. After installation of the foundation, the wind turbine
generator is installed.
J. Mar. Sci. Eng. 2019,7, 64 12 of 14
According to www.rockmat.com, this concept has the following advantages: No need for previous
soil preparation; no costly barge crane with weather restrictions, with only tugboats necessary for its
installation; and reversible water ballasting. Installation is estimated to be 30 hours, with the support
of one 100-ton bollard-pull tugboat, three 10-ton bollard-pull tugboats, and a barge to supply the
concrete mixing unit and compressor.
The ocean brick system (OBS) (Technical University Braunschweig) is a modular system consisting
of hollow precast blocs (10 m
×
10 m
×
10 m), piled like interconnected cubes, to create a stiff, light,
strong structure. The structure can be constructed in a dry dock and floated to the site with the support
of tugboats [30].
4. Conclusions
A review of the different operational European offshore wind farms with GBS foundations was
carried out in this study. In total, there are only 13 of these, and they are located in Denmark, Germany,
Sweden, Finland, and Belgium. The deepest water in all of those wind farms is 20 m, in the case
of Thorntonbank Phase 1. Furthermore, it is important to note that since 2013, there have been no
commissioned GBS facilities.
The current strength of the monopile in the offshore wind-power sector is evident, compared to
other typologies. This is mainly due to the simplicity of the structure, which results in benefits during
manufacturing, installation, and maintenance, as well as cost. With its 80% representativeness, the
monopile has shown its clear dominance in the current market, where most sites have a water depth
not exceeding 20–30 m. At greater depths, there is a battle to be more competitive between the XXL
monopile, GBS, and jacket foundation types.
The different types of GBS foundations used in all the wind farms in operation in Europe
were analyzed. Based on this analysis, a classification was proposed for the different types, which
distinguishes between the first, second, and third generations.
Examples of first-generation GBS foundations are Tun
φ
Knob and Middelgrunden. These are
solid concrete structures, without cells or holes, corresponding to the first designs.
Examples of the second generation are Nysted I or R
φ
dsan I, Lillgrund, Sprog
φ
, R
φ
dsan II or
Nysted II, and Kårehamn. These foundations include holes or cells in the slab or lower part of the
structure, which reduces their weight for transport and installation. Once the GBS structure is installed,
the holes or cells are filled with ballast, achieving the final design weight that supports the design loads.
The only example of a third-generation structure is Thornton Bank. This concept has a conical
shape with a hole or cell inside and not only in the slab or lower part, as in the second-generation
models. This type of structure was planned to be semi-floating during the transport and installation
phases, decreasing the weight of the foundation and reducing the lifting requirements. Once this
foundation is placed on the seabed, the interior hole is filled with ballast to achieve the final design
weight to support the design loads.
Other, nonproven concepts were analyzed in this study, some of them based on the F2F (floated
to fixed) concept, which is a floating structure during the transport and installation phases, supported
by small tugboats, which decreases the costs because of the self-buoyancy of the foundation and
there being no need to use larger, more specific transport vessels. Another new concept needs
special-purpose vessels to transport and install the GBS and WTG structures together, with the WTG
being pre-assembled onshore. These transport vessels are designed specifically for each concept.
Some of these new concepts include the crane-free gravity base (Seatower), gravitas gravity base
(Arup/Costain/Hochtief), Strabag gravity base (STRABAG), GBF gravity base (Ramboll/BMT Nigel
Gee and Freyssinet), GBF gravity base (Ramboll/BMT Nigel Gee and Freyssinet), Rockmat (OFS:
CETEAL/Cathie Associates/DVO), ocean brick system (Technical University Braunschweig), etc.
As mentioned above, new locations with greater water depths will begin a battle between the
XXL monopile, GBS, and jacket types. The more real and well-analyzed options the market has, the
more competitive this scenario could become. This is why it is expected that solutions similar to the
J. Mar. Sci. Eng. 2019,7, 64 13 of 14
third-generation concepts, characteristic of the first phase of the Thornton Bank offshore wind farm,
will come online in the future, including more options to use F2F concepts where it is necessary to
build special-purpose vessels.
The trend will likely be to have the entire wind turbine pre-assembled on the structure at the port,
in order to reduce the number of operations carried out at sea, which involve greater costs and risks.
The GBS concept is very interesting in that respect, and one example is the ELISA project in the Canary
Islands, designed by Esteyco, which has a slab and a shaft, without a conical transition. The prototype
for a 5 MW-output concept will use special-purpose vessels to help in the transportation of the entire
GBS structure and wind turbine. As a nod to previous concepts, it has a telescopic tower and, using
hydraulic jacks, manages to raise the different sections into their final positions, with the nacelle and
rotor already installed on the last section of the tower.
Author Contributions:
Conceptualization, M.D.E. and J.-S.L.-G.; Methodology, M.D.E. and V.N.; Investigation,
M.D.E. and J.-S.L.-G.; Resources, V.N.; Writing—Original Draft Preparation, M.D.E.; Writing—Review and Editing,
J.-S.L.-G. and V.N.
Funding:
Authors give thanks the Agustín de Betancourt Foundation (FAB) for the support received over the
past few years.
Conflicts of Interest: The authors declare no conflict of interest.
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©
2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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