U.S. patent application number 13/000286 was filed with the patent office on 2011-12-15 for support structure for use in the offshore wind farm industry.
This patent application is currently assigned to SEATOWER AS. Invention is credited to Karel Karal, Sigurd Ramslie.
Application Number | 20110305523 13/000286 |
Document ID | / |
Family ID | 41434578 |
Filed Date | 2011-12-15 |
United States Patent
Application |
20110305523 |
Kind Code |
A1 |
Karal; Karel ; et
al. |
December 15, 2011 |
SUPPORT STRUCTURE FOR USE IN THE OFFSHORE WIND FARM INDUSTRY
Abstract
A support structure for use in the offshore wind farm industry,
and a method of manufacturing and installing same, including a
foundation for installation on a seabed below a body of water and a
tower connected to and extending upwards from the foundation and
being capable of supporting at least an equipment unit. The
foundation includes a bottom slab and a wall extending upwards from
the bottom slab, thereby defining a first cavity for holding
ballast and for providing buoyancy during tow-out and installation.
The foundation further includes a circumferential skirt extending
downwards from the bottom slab, thereby defining at least one
compartment underneath the foundation.
Inventors: |
Karal; Karel; (Oslo, NO)
; Ramslie; Sigurd; (Quinns Rocks, AU) |
Assignee: |
SEATOWER AS
Oslo
NO
|
Family ID: |
41434578 |
Appl. No.: |
13/000286 |
Filed: |
June 17, 2009 |
PCT Filed: |
June 17, 2009 |
PCT NO: |
PCT/NO2009/000226 |
371 Date: |
March 24, 2011 |
Current U.S.
Class: |
405/207 ;
405/195.1; 405/203 |
Current CPC
Class: |
E02B 2017/0091 20130101;
Y02P 70/50 20151101; E02B 17/025 20130101; F03D 13/22 20160501;
E02B 17/027 20130101; E02B 2017/0039 20130101; F05B 2240/95
20130101; Y02E 10/72 20130101; E02B 2017/0065 20130101; Y02E 10/727
20130101; F03D 13/10 20160501 |
Class at
Publication: |
405/207 ;
405/195.1; 405/203 |
International
Class: |
E02D 29/09 20060101
E02D029/09; E02D 5/20 20060101 E02D005/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 20, 2008 |
NO |
2008 2860 |
Claims
1-19. (canceled)
20. A support structure for use in the offshore wind farm industry,
comprising: a foundation for installation on a seabed below a body
of water; and a tower connected to and extending upwards from the
foundation and being capable of supporting at least an equipment
unit, wherein the foundation further comprises: a bottom slab
element and a wall extending upwards from the bottom slab element,
thereby defining a first cavity wherein the foundation comprises a
circumferential skirt extending downwards from the bottom slab,
thereby defining at least one compartment underneath the
foundation.
21. The support structure of claim 20, wherein the tower is
connected to the foundation via a lower part of the tower being
attached to the bottom slab element.
22. The support structure of claim 20, wherein the tower is
connected to the foundation via fixing elements connected to at
least an upper wall portion.
23. The support structure of claim 20, wherein the at least one
compartment is subdivided into compartments by means of skirts
extending downwards from the bottom slab and preferably extending
radially from a center portion of the bottom slab to respective
areas of the circumferential skirt.
24. The support structure of claim wherein the foundation comprises
a roof structure, extending between an upper wall portion and the
tower, thereby enclosing the first cavity.
25. The support structure of claim 24, wherein the roof structure
comprises an outer shell and an inner shell defining at least one
second cavity there between, said inner shell facing the first
cavity.
26. The support structure of claim 25, wherein the second cavity is
filled with a material such as concrete.
27. The support structure of claim 24, wherein the roof structure
is formed by concrete cast in conventional formwork, or by single
shell metal plates.
28. The support structure of claim 20, wherein the bottom slab and
the wall comprise an outer shell and an inner shell defining at
least one second cavity there between, said inner shell facing the
first cavity.
29. The support structure of claim 28, wherein the upper wall is
formed by slipform casting or by single shell metal plates.
30. The support structure of claim 24, further comprising a buoyant
stabilizing device releasably and slidably connected to the
foundation, whereby the stability of the structure is maintained
during installation when the roof structure is moved from a
position above the water to a fully submerged state.
31. The support structure of claim 30, wherein the buoyant
stabilizing device comprises a recessed portion having upper and
lower end stops for cooperation with a flange on the foundation,
whereby the buoyant stabilizing device slidable movement is
restricted by said upper and lower end stops.
32. The support structure of claim 30, wherein the buoyant
stabilizing device comprises at least one inner cavity for
selective addition and extraction of a ballasting fluid, such as
water.
33. A method of manufacturing the support structure of claim 20,
comprising the providing of a bottom slab having downwardly
extending skirts to an onshore fabrication site, comprising the
steps of: a) extending a circumferential lower wall from the bottom
slab to form a foundation lower part, said lower wall having a
vertical extension dimensioned according to the buoyancy
requirements for the completed support structure; b) placing the
foundation lower part in a floating position on the body of water;
c) extending an upper wall portion; and d) connecting the tower to
the foundation by attaching a lower part of the tower to the bottom
slab and by connecting a part of the tower via fixing elements to
at least the upper wall portion of the foundation.
34. The method of claim 33, wherein a roof structure is extended
between the upper wall and the tower, thereby enclosing the first
cavity.
35. A method of installing a support structure for use in the
offshore wind farm industry, comprising a foundation for
installation on a seabed below a body of water and a tower
connected to and extending upwards from the foundation and being
capable of supporting at least an equipment unit, comprising the
steps of: a) towing the structure in a floating state to the
installation location while controlling the buoyancy and center of
gravity of the support by means of a controlled addition of a
ballast material into a cavity defined by structural elements of
the support structure, whereby the need for separate buoyancy
elements and/or specialized vessels during transport is eliminated
or significantly reduced, and b) transferring the structure from a
floating state to an installed state, by filling a ballast material
into the cavity until the structure is installed on the seabed,
whereby the need for separate buoyancy elements and/or specialized
vessels and cranes during installation is eliminated or
significantly reduced.
36. The method of claim 35, wherein the support structure is moved
into a substantially level state on the seabed by injecting a
grouting material into selected compartments confined by skirts
below a bottom slab element of the support structure.
37. A method of installing a support structure for use in the
offshore wind farm industry, comprising a foundation for
installation on a seabed below a body of water and a tower
connected to and extending upwards from the foundation and being
capable of supporting at least an equipment unit, comprising the
steps of: a) towing the structure in a floating state to the
installation location while controlling the buoyancy and center of
gravity of the support by means of a cavity defined by structural
elements of the support structure, whereby the need for separate
buoyancy elements and/or specialized vessels during transport is
eliminated or significantly reduced, b) transferring the structure
from a floating state to an installed state, by filling a ballast
material into the cavity until the structure is installed on the
seabed, whereby the need for separate buoyancy elements and/or
specialized vessels and cranes during installation is eliminated or
significantly reduced; and c) moving the support structure into a
substantially level state on the seabed by injecting a grouting
material into selected compartments confined by skirts below a
bottom slab element of the support structure.
38. The method of claim 35, wherein the filling in step b comprises
the filling, at least partly, of water or a solid material or a
combination of both.
39. The method of claim 37, wherein the filling in step b comprises
the filling, at least partly, of water or a solid material or a
combination of both.
Description
[0001] The present invention relates to structures for supporting
offshore wind turbines and similar equipment. More specifically,
the invention relates to a support structure for use in the
offshore wind farm industry, comprising a foundation for
installation on a seabed below a body of water and a tower
connected to and extending upwards from the foundation and being
capable of supporting at least an equipment unit; as well as a
method of manufacturing the support structure and a method of
installing the support structure.
[0002] The increasing demand of exploitation of renewable energy
sources enhances the demand of offshore wind power generation where
the wind conditions are more favorable than onshore and the
environmental impact is much less. There is an increasing need for
structures that can support heavy wind turbines in a significant
height over the sea level. The support structure consists of
shaft/tower fixed to the seabed either directly by means of a
foundation or the structure is made floating and connected to the
seabed by means of a mooring. The present invention relates to the
former type, namely the fixed support structures.
[0003] Typical fixed support structures for wind turbines applied
in practice, planned for application, patented and described in
publicly accessible sources are, in general terms characterized by
the following: [0004] 1. Demanding installation where the tower is
deployed in-situ on a preinstalled foundation [0005] 2. The
foundation is fixed to the seabed by driven or drilled piles
[0006] Existing solutions using the gravity force to fix the
structure to the seabed instead of piles are known for their
considerable limits of application related to their weight, water
depth at the installation site as well as water depth at the
load-out locations and along the transport route.
[0007] EP 1 429 024 discloses a support structure for an offshore
wind turbine, comprising a caisson supported by several columns
embedded in the seabed and subjected to tension and pressure loads.
Selected columns are piled at an inclined angle with respect to the
vertical. The caisson is supported below the water surface but
above the seabed.
[0008] WO 03/080939 discloses a foundation structure for a wind
turbine tower or similar, for installation on the seabed. The
foundation structure can be manoeuvred to its offshore position
using a vessel and separate (and removable) buoyancy means. These
buoyancy elements must be rather large in order to maintain
stability. When in position, the structure is lowered to the seabed
and a pumping mechanism is used to sink a lower portion of the
structure (e.g. skirts) into the seabed. When the foundation
structure has been anchored (or piled) in position on the seabed,
it is capable of supporting the wind turbine tower.
[0009] By their nature, the above solutions tend to yield high
overall capital investment costs, i.e. the total costs for
fabrication, load-out, transport and installation.
[0010] It is therefore provided a support structure for use in the
offshore wind farm industry, comprising a foundation for
installation on a seabed below a body of water and a tower
connected to and extending upwards from the foundation and being
capable of supporting at least an equipment unit, characterized in
that the foundation comprises a bottom slab element and a wall
extending upwards from the bottom slab element, thereby defining a
first cavity for holding ballast and for providing buoyancy during
tow-out and installation.
[0011] The tower is preferably connected to the foundation via a
lower part of the tower being attached to the bottom slab element
and connected to the foundation via fixing elements connected to at
least an upper wall portion.
[0012] Preferably, the foundation comprises a circumferential skirt
extending downwards from the bottom slab, thereby defining at least
one compartment underneath the foundation. Preferably, the at least
one compartment is subdivided into compartments by means of skirts
extending downwards from the bottom slab and preferably extending
radially from a center portion of the bottom slab to respective
areas of the circumferential skirt.
[0013] In an embodiment, the foundation comprises a roof structure,
extending between the upper wall and the tower, thereby enclosing
the first cavity. In one embodiment, the roof structure comprises
an outer shell and an inner shell defining at least one second
cavity there between, said inner shell facing the first cavity. The
second cavity is preferably filled with a material such as
concrete. In another embodiment, the roof structure is formed by
concrete cast in conventional formwork, or by single shell metal
plates.
[0014] In one embodiment, the bottom slab element and the wall
comprise an outer shell and an inner shell defining at least one
second cavity there between, said inner shell facing the first
cavity. The second cavity is preferably filled with a material such
as concrete. In another embodiment, the upper wall is formed by
slipform casting or by single shell metal plates.
[0015] In one embodiment, the support structure comprises a buoyant
stabilizing device releasably and slidably connected to the
foundation, whereby the stability of the structure is maintained
during tow, and during installation when the roof structure is
moved from a position above the water to a partly or fully
submerged state. Preferably, the buoyant stabilizing device
comprises a recessed portion having upper and lower end stops for
cooperation with a flange on the foundation, whereby the buoyant
stabilizing device slidable movement is restricted by said upper
and lower end stops. Also, the buoyant stabilizing device
preferably comprises at least one inner cavity for selective
addition and extraction of a ballasting fluid, such as water.
[0016] It is also provided a method of manufacturing the invented
support structure, comprising the providing of a bottom slab having
downwardly extending skirts to an onshore fabrication site,
characterized by the steps of: [0017] a) extending a
circumferential lower wall from the bottom slab to form a
foundation lower part, said lower wall having a vertical extension
dimensioned according to the buoyancy requirements for the
completed support structure; [0018] b) placing the lower part in a
floating position on the body of water; [0019] c) extending the
upper wall; and [0020] d) connecting the tower to the foundation by
attaching a lower part of the tower to the bottom slab element and
by connecting a part of the tower via fixing elements to at least
an upper wall portion of the foundation.
[0021] In one embodiment of the invented method, a roof structure
is extended between the upper wall and the tower, thereby enclosing
the first cavity.
[0022] It is also provided a method of installing the invented
support structure, characterized by the steps of: towing the
structure in a floating state to the installation location, and
transferring the structure from a floating state to an installed
state, by filling ballast into the first cavity until the structure
is installed on the seabed.
[0023] If deemed necessary, the method installing comprises moving
the foundation into a substantially level state by injecting a
grouting material into selected ones of the compartments confined
by the skirts below the bottom slab element.
[0024] The present invention introduces a number of parameters and
structural compatibility by using different material types that can
be applied for optimizing the supply of ready-for-operation
structural supports for offshore wind farms. The following
advantageous aspects are achieved: [0025] 1. Large degree of
completion and commissioning work can be done at the fabrication
site instead at the offshore installation site, allowing
integration of the tower to the foundation, cabling work and
similar [0026] 2. Wider material selection and range of structural
dimensions [0027] 3. Transport to the site on deck of barges and
vessels is eliminated or significantly reduced [0028] 4. Separate
buoyancy elements during tow-out are not required [0029] 5.
Deployment into the position (transfer from the transport position
to the operation position) by adding ballast, not by lifting [0030]
6. No piling or other forms of "fixing" to the seabed is needed
[0031] 7. Design and outfitting for removal can be easily
implemented [0032] 8. Need for large offshore cranes is avoided
[0033] In addition to lower overall costs the present invention
resolves shortcomings associated with the known solutions by:
[0034] 1. Enabling delivery of the supports from fabrication sites
allowing operation of shallow draft vessels thus widening the
selection of fabrication sites [0035] 2. Reducing needs for
specialized vessels [0036] 3. Allowing superstructure (tower, wind
generator, etc) to be fitted to foundation structure at shore,
prior to tow to installation location [0037] 4. Allowing foundation
structure to be leveled following installation on seabed, to
prevent unpredictable inclination of the installed support [0038]
5. Reducing or eliminating hydrodynamic loads acting directly on
the tower [0039] 6. Resistance to heavy ice loads
[0040] These and other characteristics of the invention will be
clear from the following description of a preferential form of
embodiment, given as a non-restrictive example, with reference to
the attached drawings wherein:
[0041] FIG. 1 is a schematic side view of a first embodiment of the
invention, illustrating the principle of the invention where a
portion of the foundation is protruding above the water
surface;
[0042] FIG. 2 is a section through a lower part of the structure
shown in FIG. 1, along the section line A-A in FIG. 3;
[0043] FIG. 3 is a section along the section line B-B in FIG.
2;
[0044] FIG. 4 is a schematic side view of a second embodiment of
the invention, illustrating the principle of the invention where a
portion of the foundation is below the water surface
[0045] FIG. 5 is a section through a lower part of the structure
shown in FIG. 4, along a section line similar to section line A-A
in FIG. 3;
[0046] FIG. 6 is a section through a lower portion of the
foundation structure, along a section line similar to section line
A-A in FIG. 3, placed on shore;
[0047] FIG. 7 is a cut out of lower portion of the foundation
structure illustrating composition of the main load bearing plate
elements;
[0048] FIG. 8 is a side view of the lower portion of the foundation
structure shown in FIG. 6, while being lifted from shore;
[0049] FIG. 9 is a side view of the lower portion of the foundation
structure shown in FIG. 6, floating on water;
[0050] FIG. 10 shows the embodiment shown in FIG. 5, in a floating
state on water and partially filled with ballast;
[0051] FIGS. 11 to 13 show the main operations in the transport and
installation of the support structure;
[0052] FIG. 14 is a horizontal section at through line C-C in FIG.
15, and shows the second embodiment of the invention fitted with a
reusable floating stability device;
[0053] FIG. 15 is a section through a lower part of the structure
shown in FIG. 4, along the section line A-A in FIG. 14, floating in
the water and fitted with a reusable floating stability device;
[0054] FIG. 16 shows the same structure and floating stability
device as in FIG. 14, but where the floating stability device is
shown in a state detached from the floating supporting structure,
e.g. prior to attachment to the structure;
[0055] FIG. 17 shows a vertical section of the same structure and
floating stability device as in FIG. 15 during lowering to seabed;
and
[0056] FIG. 18 shows the same structure and floating stability
device as in FIGS. 15 and 17, where the structure has been deployed
onto seabed and the floating stability device has been flooded for
detaching from the structure and further retrieval.
[0057] FIG. 1 is a side view of a first embodiment of the support
structure, generally denoted by the reference numeral 1 and
hereinafter also referred to as a "structure". The support
structure 1, which comprises a tower 7 and a foundation 4 is
illustrated placed in a body of water 2 and resting on a seabed 3
via the foundation 4. The support structure supports a turbine 5
with rotor blades 6a-c. The turbine is mounted on top of the tower
7 that is supported by and fixed to the foundation 4 by means of a
fixing structure 8. In this embodiment the foundation 4 protrudes
above the water level 9, which is a typical arrangement for sites
in shallow water. The foundation 4 comprises a bottom slab 14 (see
FIG. 2) and a wall extending upwards from the bottom slab. For
reasons which will become apparent later, when the manufacturing
process is described, the foundation wall is conveniently denoted
"lower wall 23" and upper wall 54", as indicated in FIG. 1.
[0058] Although not mandatory, it is advantageous to give the
foundation 4 a circular shape that can efficiently resist
environmental loads in various phases during fabrication, transport
and operation; typically hydrostatic water pressure, wave loads
and--in some cases--, ice loads. The tower 7 is fixed to the
foundation by means of a multiple-legged structure 8.
[0059] FIG. 2 is a vertical section through the foundation 4 along
two vertical planes A-A as shown in FIG. 3. The multiple-legged
fixing structure 8, fixing the tower 7 to the foundation 4
comprises upper struts 10, vertical columns 11 and--if
necessary--lower struts 12. A lower part 13 of the tower 7 may be
embedded into the bottom slab 14 of the foundation in order to
facilitate transfer of shear loads from the tower 7 into the
foundation. The space 15 inside the foundation 4 is used to control
the buoyancy and center of gravity of the structure during
fabrication, transport to the field and installation by being
either air-filled or, to certain degree either filled by water or
solid ballast or combination water and solid ballast.
[0060] FIG. 3 is a horizontal section along section line B-B in
FIG. 2, through the part of the foundation that is embedded in soil
when the structure has been installed. Radial skirts 16 divide the
confined space within the outer (circular) skirt 18 into a number
of compartments 17. As an example, FIG. 3 shows three such
compartments 17a-c, divided by radial skirts 16a-c. The skirts
improve the load bearing capacity of the foundation by transferring
the outer loads into deeper soils strata, and the outer skirt
prevents deteriorating effects from possible scour of the seabed
along the periphery. Upon embedding the skirts into seabed, grout
or similar substance is filled into the compartments 17a-c in order
to avoid water filled pockets to be trapped between bottom of the
foundation and seabed. Grouting can be utilized to ensure that the
foundation 4 is leveled (horizontally) and thereby ensuring
verticality of the tower by controlling the grouting pressure thus
the inserted grout volume in the individual compartments 17a-c.
Dividing the base into three individual compartments as shown in
FIG. 3 or into multiple of three groups of pressure-connected
compartments, the leveling can be accomplished.
[0061] FIG. 4 is a side view of a second embodiment of the support
structure 1 placed in a body of water 2 and resting on a seabed 3
via a foundation 4'. In this embodiment the foundation 4' does not
protrude above the water level 9. This is a typical arrangement for
sites in deeper waters where this arrangement is less costly
compared to the structure shown in FIG. 1, and this embodiment of
the foundation 4' comprises a roof structure 52, connected to the
upper wall 54 and thus enclosing the internal space 15 (see FIG.
5). The skilled person will understand that the transition between
the tower 7 and the roof structure 52 may--if required--be sealed
by conventional means. The roof structure 52 is preferably slanted,
as shown in FIG. 4.
[0062] FIG. 5 is a vertical section through the foundation 4' along
two vertical planes A-A similar to that shown in FIG. 3 for the
first embodiment. It is seen the interior 15 of the foundation 4
can be used for ballast, i.e. water and/or solid ballast 19 (see
FIG. 10).
[0063] The inventive fabrication, transport and installation
procedure is illustrated in FIGS. 6 to 17 and explained in the
following.
[0064] FIG. 6 shows the first stage of fabrication that takes place
on a quayside. However, a similar fabrication procedure and method
would be possible by using more expensive facilities such as dry or
graving dock, or a submersible barge and by applying very similar
procedure and materials. Therefore the further explanation will
focus on the quayside fabrication and only comments will be
provided, where appropriate, for the other optional fabrication
methods. On quayside 20 temporary supports 21a-c are established as
necessary to support the lower part 22 of the foundation. This part
of the process is similar irrespective of whether the first
embodiment foundation 4 or the second embodiment foundation 4' is
used. The fabrication of the lower part 22 of the foundation 4; 4',
comprises fabrication of outer skirt 18, radial skirts 16a-c (cf.
FIG. 3; not shown in FIGS. 6 and 8 to 17), and pre-assembly of
bottom slab 14 and vertical walls 23 is carried on until a
necessary predetermined height of the walls 23 is achieved and at
the same time the capacity of load-out device is not exceeded (see
explanation related to FIG. 8). The required height of the lower
walls 23 is governed by the required minimum acceptable freeboard
in the next phase in which the structure under construction is
floating. In order to achieve as low weight as possible in order to
avoid expensive cranes, according to this invention, both the
bottom slab 14 and the vertical walls 23 or their lower part are
fabricated as double steel shell structure having an outer shell 44
and an inner shell 42, defining a cavity 46 there between, and
where the shells 42, 44 are held in desired distance from each
other by transversal spacing plates or rods 48, as shown in FIG. 7.
The cavity 46 is intended to, in a later phase of fabrication, be
filled with concrete in order to achieve desirable strength of the
completed shell structure. However, the hollow double shell steel
structure is designed with strength sufficient to carry all loads
occurring in the initial phases of fabrication. It is also possible
to use prefabricated double shell sections commercially available
on the market under brand name Bi-Steel. The use of this sandwich
type of structure may not be required if the fabrication is
performed in dock or on a submersible barge where the weight may
not be a limiting parameter.
[0065] Upon completion of the lower part 22 of the foundation 4;
4', it is transferred (e.g. lifted; cf. FIG. 8) from the quayside
20 into a floating state on a surface of water 2 as shown in FIG.
9. The lifting can be performed using slings 50 attached to a
floating or land based crane (not shown).
[0066] Upon lowering into water the lower part 22 of foundation 4;
4' floats with an appropriate freeboard that allows safe work with
continuation of the fabrication, which may continue either with
extending the vertical walls in order to increase the freeboard of
the floating body or filling the cavity between the shells by
concrete to increase strength of the lower part of the foundation
4; 4'. Completion of the foundation 4; 4' involves construction of
walls up to the their final height and for the foundation 4' shown
in FIG. 4, also constructing the tight roof 52, thus creating a
barrier between the internal space 15 and the outside water.
Construction of the walls up to their final height can either
continue with the double-shell configuration or continue using
standard concrete structure building methods such as slipforming or
the use of conventional formwork and concrete casting. In order to
achieve desired weigh and weight distribution, concrete of
different density can be used in different sections or concrete can
be entirely or in sections replaced by steel. This fabrication
phase is shown in FIG. 9 where the lower part of the foundation 22
floats on surface of water mass 2 and construction of walls and
other parts inside the foundation is in progress.
[0067] In FIG. 10 the foundation 4' has been completed; the tower 7
has been inserted into the foundation 4, aligned with the
foundation 4' and connected to it as necessary, and the complete
structure is readied for tow-out. FIG. 10 shows adjustments of the
draft and the center of buoyancy that both are important for
stability of the complete structure during tow to the site. It is
seen that in this selected case solid ballast 19 is being added
into the space 15 of the foundation 4' via a suitable opening 37 in
the roof structure 52. In the design, the outer dimensions of the
foundation 4; 4' are important parameters the design engineer can
adjust to control the floating stability. The ultimate goal would
be achieving sufficient stability with the tower 7 and equipment
units 5, 6a-c installed before tow-out. This may however lead to a
cost inefficient solution in which the additional cost for the
extended structure cannot counterweigh the technical and economical
gains from installing the tower and installing of all equipment and
outfitting at the shore fabrication site. Therefore, compromises
may be required to be introduced such as: [0068] 1. Postpone
installation of equipment units 5, 6a-c to the phase where the
support structure 4; 4' and tower 7 have been installed at the
offshore site. This reduces significantly the height of centre of
gravity above the center of buoyancy, reduces the wind loads on the
structure during this phase, and reduces the draft during tow.
[0069] 2. Use a telescopic tower (such as disclosed in Norwegian
patent application no. 20073363) where the upper part is inserted
into the lower part during tow and installation and thereafter
retracted (pushed out by hydrostatic pressure). The effects of this
are of the same character as in item 1 described above. [0070] 3.
Design and fabricate the tower 7 or its upper part from lighter
materials than steel, e.g. from high strength reinforced
plastics.
[0071] FIG. 11 shows the transport of the support structure 1 from
the assembly site to the installation site by towage of the
structure by means of a towing vessel 23 connected by tow line 24
to the foundation 4'. Depending on the maritime conditions along to
towing route, such as ship traffic, tow route curvature, additional
vessel(s) may be needed to perform this task.
[0072] FIG. 12 shows the structure 1 being transferred from the
towing position to seabed that is achieved by adding weight (solid
ballast 19, or water) into the space 15 of the foundation 4' from
the installation vessel 23 via a connection 25.
[0073] FIG. 13 shows the structure 1 deployed on the seabed 3 with
outer skirts 18 and radial skirts 17a,c penetrated into the seabed.
The grouting, i.e. filling of voids where is not contact between
the base of the foundation 4' and the seabed and, if needed,
filling of additional grout in selected compartment(s) 17a-c thus
to align the tower 7 with vertical by leveling the foundation 4',
is illustrated as being performed by adding grouting substance via
a suitable conduit 33. Depending on the initial inclination with
the respect to position of the skirt compartments the grout is
filled into one or two of the three compartments.
[0074] FIG. 14 is top view of the foundation 4' along section line
C-C in FIG. 15, in the region of the water line, and illustrates a
floating stability device 26 attached to the foundation 4'. The
purpose of the floating stability device 26 is to provide
additional water plane area to the floating foundation 4' and hence
make it stable during tow-out and installation. The floating
stability device 26 may be designed as a hollow body, preferably in
a shape embracing the structure along its periphery, e.g. circular
as seen in the figure. In the preferred embodiment the device 26
comprises two segments 27a,b connected to each other via a joint 28
and a locking mechanism 29.
[0075] FIG. 16 is a vertical section through a portion of the
structure 1, illustrating the foundation 4' and the floating
stability device 26 along two vertical planes A-A as defined in
FIG. 14. In the cross section it is seen that the foundation 4' is
provided with a flange 30 that fits into a recess 31 in the inner
wall 32 of the floating stability device 26. Interaction between
the floating stability device 26 and the foundation 4' takes place
when the foundation 4' is ballasted down so that the flange 30
rests on the recess 31, as shown in FIG. 17. The typical situations
when the floating stability element 26 is needed are (a) tow to
site and lowering/ballasting to seabed of top-heavy structure 1 and
(b) lowering/ballasting to seabed of structure 1 for deeper water
where the vertical wall of the foundation 4' or entire foundation
is submerged below water surface 9.
[0076] FIG. 16 is a top view of foundation 4' and the floating
stability device 26 with is segments 27a,b connected through the
hinge 28, locking mechanism 29 disconnected and the segments 27a,b
separated so that the device 26 can be maneuvered toward the
foundation 4' with aim of embracing the foundation by the segments
27a and 27b. When these are in position, the locking mechanism 29
can be engaged thus creating an assembly that after increasing of
draft of the structure 1 behaves as one body from the floating
stability point of view.
[0077] FIG. 17 shows the lowering of the structure 1 to the seabed
3 being in progress. Ballast is gradually added into the space 15
of the foundation 4' thus the assembly of the structure 1 and the
floating stability device 26 is submerging deeper into water. In
order to achieve structural strength the device 26 is strengthened
by internal reinforcement indicated by structural members 33a and
33b.
[0078] In FIG. 18 the assembly made of the structure 1 and the
floating device 26 has been by adding ballast deployed onto the
seabed 3 with the skirts 18 penetrating into it. For detachment
from the foundation the floating stability device 26 is outfitted
with equipment for ballasting and deballasting by means of sea
water. In order to reduce the free surface area of the ballast
water inside the floating stability device, the interior of it is
divided with vertical bulkheads (not shown). The figure shows that
ballast water 34 has been filled into the device so that the device
26 is floating without contact between the recess 30 and the flange
31, hence the floating stability device 26 can be easily disengaged
and removed.
[0079] The invention is particularly suitable for suitable for
shallow waters in particular in the interval between 8 m and 30 m.
The system can preferably be designed in the soft-stiff dynamic
response regime.
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