WO2011137903A2

WO2011137903A2 - Semi-submerged multiple wind-turbine system

Info

Publication number: WO2011137903A2
Application number: PCT/DK2011/000043
Authority: WO
Inventors: Stephan Moellgaard Henriksen
Original assignee: Stephan Moellgaard Henriksen
Priority date: 2010-05-05
Filing date: 2011-05-05
Publication date: 2011-11-10
Also published as: WO2011137903A3

Abstract

The invention relates to a semi-submerged multiple wind-turbine system for use in offshore wind power production implying that it is a floating system with means to maintain a permanent position at sea. The invention increases the technical and economical feasibility of multiple wind-turbine systems most significantly by the reduction and/or control of the loads or forces on to and within the system with derived advantages on structural induced loadings and cost as a consequence. This is achieved by featuring certain relations between the main elements of the system and featuring certain means of control of the system wherein the distribution of buoyancy and weights which is enabled by among other means the use of compartments in the pontoon structures are of a particular benefit to the system.

Description

SEMI-SUBMERGED MULTIPLE WIND-TURBINE SYSTEM

TECHNICAL FIELD

The present invention relates to a semi-submerged multiple wind-turbine system for use in offshore wind power production.

BACKGROUND OF THE INVENTION

Floating systems are being considered for offshore wind power production because they all alike become increasingly feasible compared to the alternatives when power plants are installed at increasing water depths. Other reasons to consider floating systems is their potential of mobility which can be utilized to lower construction cost and risk.

While some systems seems to comprise wind turbines which are particularly tolerant to motions and displacements as induced by for example wind and waves it is in general and for most types of wind turbines a significant advantage with a system which operate with limited motions and displacements in respect of a geometric fix point.

Examples of systems comprising one wind-turbine per vessel can be given with : a) "International Publication Number WO 2009/131826 A2" titled "Column- stabilized offshore platform with water-entrapment plates and asymmetric mooring system for support of offshore wind turbines". This is principally a "column stabilized unit". b) "International Publication Number WO 2006/132539 Al" titled "Floating wind turbine installation". This is in principally a "deep draught floater". c) "International Publication Number WO 2010/029766 Al" titled "Tension- moored floating body and method for towing and installation tension-moored floating body". This is principally a "tension leg platform". d) "International Publication Number WO 2004/097217 Al" titled "Wind Power Station". This is principally a "deep draught floater" which distinguish itself from other common applications by for example the positioning system which comprises one line only which is provided with a swivel joint.

Examples of systems comprising more than one wind-turbine per vessel herein called MWS (Multiple Wind-turbine System) can be found in : a) "International Publication Number WO 2009/067023 Al" titled "Floating Wind Power Apperatus". This is principally a "column stabilized unit" which distinguishes itself from the majority of other floating systems of wind turbines by for example having wind turbines which are based on towers which are vertically inclined and this is claimed as specific feature of the system. b) "International Publication Number WO 02/073032 Al" titled "Offshore

Floating Wind Power Generation Plant". This is principally a "column stabilized unit" which distinguishes itself from other common applications by not having for example horizontal oriented main structural elements above the waterline such that submerged main structural elements constitute the only connection between a number of columns having a water plane area and where said columns each form base for one wind turbine.

These examples cover most principal vessel types and positioning system types considered in conjunction with wind-turbines, however, the examples do not include for example the ship shaped types.

Although "International Publication Number WO 02/073032 A12 could be seen as an example of the opposite then a MWS would preferably be designed such that wind-turbine induced wake interference is avoided. This may lead to a relatively large marine structure in terms of geometric extent in the horizontal plane. It seems logical to design such a system with a ratio between water plane area moment of inertia [I,wp] and water plane area [A,wp] that is [I,wp]/[A,wp] which is relatively high in order to achieve advantages in relation to stability and motions and such a system could be based on for example the "column stabilized unit" just like prior examples herein and now referred to as a SMWS (Semi-submerged Multiple Wind-turbine System) .

The technical and/or economical feasibility of the SMWS is, however, compromised and the issues related hereto has been discussed and assessed by numerous and industrious publishers for more than a decade. The conclusions are, however, varying and made subject but in general that a main issue in relation to the feasibility of the SMWS is structural requirements although requirements related to the particulars of for example the positioning systems are also pointed out as an area of concern. If a cost break-down of a SMWS is made in to "wind-turbine" and "vessel" and "positioning system" then the fraction of the "wind turbine" has for example been estimated to be in the region of say 10 to 25% and this is a low fraction in particular when considering that there are other cost involved for the fully operational wind power plant such as for example power transmission system costs. Moreover, the known SMWS are designed in a way where the forces from wind and waves may induce failure in the construction materials due to fatigue or the impact of loading of the construction due hydrostatics and/or hydrodynamics.

One object of the present invention is to design a semi-submerged multiple wind- turbine system for use in offshore wind power production in which the loads on the structures are minimized.

A further object is to increase the feasibility of the SMWS and more particularly to investigate design basis and principles for a better understanding of the parameters which distinct a floating system comprising wind-turbines and which are significant to economical feasibility. Moreover, an object of this invention is to device a SMWS which consider both vertical axis and horizontal axis types of wind-turbines characteristics and which can be constructed and deployed also in areas of relatively low water depth.

These and further objects are achieved by the features that are disclosed and described for the SMWS below and in the claims.

DISCLOSURE OF THE INVENTION

The present invention provides a semi-submerged multiple wind-turbine system for use in offshore wind power production implying that it is a floating system with means to maintain a permanent position at sea. The present invention increases the technical and economical feasibility of multiple wind-turbine systems most significantly by the reduction and/or control of the loads or forces on to and within the system with derived advantages on structural induced loadings and cost as a consequence. This is achieved by featuring certain relations between the main elements of the system and featuring certain means of control of the system wherein the distribution of buoyancy and weights which is enabled by among other means the use of compartments in the pontoon structures are of a particular benefit to the system.

Consequently, the present invention relates to a semi-submerged multiple wind- turbine system comprising three semi-submersed essentially vertical oriented support columns, each being adapted to carry a wind-turbine assembly and at least two submerged essentially horizontal oriented pontoon structures where said support columns and pontoon structures are connected to form a structural entity where in a projection on the horizontal plane the geometric relations between the support columns are equal to the geometric relations of vertices of a triangle and the semi-submerged multiple wind-turbine system further comprises a positioning system for transferring forces between said structural entity and one or more geostationary positions, wherein each of the support columns comprises at least one compartment and said pontoon structures comprises a number of longitudinally distributed compartments and a number of said compartments are hydro or hydrostatic connected or connectable to a fluid to allow one or more of the compartments to be filled with the fluid.

The one or more compartments that are filled with fluid, such as water or gas are preferably totally filled with the water or optional gas to balance the pressure in the structure with the pressure from the water surrounding the submerged pontoon structure.

The term "compartment" is to be understood as a chamber or tank that may provide ballast or buoyancy in a marine structure and may contain liquid or gas or a combination thereof. A compartment normally is connected to the exterior environment or other compartments via pipes and valves or similar means. In this context a compartment may also feature permanent openings to the exterior, e.g. a hole or a flexible boundary between the interior and the exterior such as

membranes.

In this context the term fluid should be construed as comprising liquid and gas and the liquid may be sea water or water from a lake or a river in case the invention is used in such locations. However, the liquid may also be other liquids, such as fresh water or oil or mixtures of water and oil or other additives which may be used to inhibit corrosion. The gas may be air or other gases it may be desired to use, e.g. helium for some specific purposes.

According to the invention it is possible that one or more of the compartments can be filled with gas. The gas is preferably air and it may be pressurized for the purpose of balancing the pressure from the water surrounding the submerged pontoon structure.

A pontoon structure according to the invention may be relatively long, e.g. between 50 to 300 meters and have a diameter between 6 and 10 meters, and the pontoon structure comprises at least 2 compartments and preferably from about 4 to about 80 compartments, and more preferably from about 10 to about 50 compartments. The exact number of compartments in a pontoon structure will depend on the size and the specific use of the pontoon structure, however, the number of

compartment should be sufficient to allow the structure to compensate undesired loads be controlling the ratio between ballast and buoyancy effect of the

compartments. Some of the compartments may be filled with water and other may filled with air and by adjusting the ratio between water filled and air filled compartments in the pontoon structure it is possible to minimize undesired load on the pontoon structure and the structural entity forming the semi-submerged multiple wind-turbine system. Undesired loads are e.g. vibrations due to wind and waves that may lead to fatigue and failure of the materials in the system.

To further improve the strength of the semi-submerged multiple wind-turbine system it is preferred that the pontoon structure comprise one or more

reinforcement members. The reinforcement members may e.g. be ribs or crossbars. The pontoon structure may also be made from corrugated plate members, which may also increase the strength.

In an alternative embodiment the pontoon structure comprises at least one compartment extending in the longitudinal direction of the pontoon structure. In case the exterior pontoon wall has the shape of a tube, the compartment may be considered as another tube with smaller diameter running inside the pontoon. The space between the interior pontoon wall and the exterior compartment wall may be filled with water and the interior of the compartment may be filled with air to provide a balance between ballast and buoyancy and thereby serve to neutralize undesired loads on the system.

Moreover, in an embodiment of the semi-submerged multiple wind-turbine system according to the invention the pressure in the one ore more compartments is controlled. The pressure in a compartment may be controlled to correspond to the pressure in the water surrounding the pontoon or the pressure may be controlled to be higher or lower than the pressure in the water surrounding the pontoon. In an uncomplicated embodiment the pressure in a compartment can be controlled by the level of fluid as the level of fluid such as water may have a certain stand in the closed compartment and thereby determine the pressure in the compartment. The pressure may also be controlled by use of a fluid such as a gas, e.g. air. The pressure control system preferably includes pumps and valves to facilitate the control of the pressure. In a simple embodiment that ensures the same pressure inside and outside the compartment there is a hole in the wall of the compartment or a membrane allowing transport or water.

The semi-submerged multiple wind-turbine system may include a piping system comprising pipes and optional pumps and valves that allow fluid to be transferred between two or more compartments. The piping system may also allow direct communication between one or more compartments and the exterior environment such as the sea. Thereby easy control of ballast and buoyancy can be obtained.

By use of such a system the submerged depth of the semi-submerged multiple wind-turbine system according to the invention can easy be controlled and adjusted in relation to the water depth at the location, e.g. a harbour or open sea.

Preferably the structural entity comprising columns and pontoons in the semi- submerged multiple wind-turbine system is able to move in respect of the one or more geostationary positions. The geostationary positions are, e.g. anchoring positions on the sea bed whereto the structural entity is attached by cables or similar means. In the structural entity the turret morring system allows the structural entity floating at sea level to move in respect of wave and wind direction, whereby it is possible to optimize the power output from the system and also reduce undesired load on the system.

In the following disclosure of preferred embodiments according to the invention is used the terms and understood principles that are commonly used and well known within marine and naval construction design, see e.g. "Principles of Naval

Architecture Volumes 1-3"; published by The Society of Naval Architects and Marine Engineers (SNAME). It should, moreover, be understood that the properties or relations listed in the following refer to the operating conditions of the semi- submersible multiple wind-turbine system.

Consequently, in a preferred embodiment of the semi-submerged multiple wind- turbine system according to the invention the three semi-submersed support columns each have a water plane area [A,c-wp] and a water volume displacement weight [V,c];

the submerged pontoon structures each have a water volume displacement weight [V,p];

the support columns have a combined water plane area moment of inertia [I,c-wp] and said semi-submersible wind-turbine system is having a water plane area moment of inertia [I,s-wp] where [I,c-wp] and [I,s-wp] are both taken along any neutral axis of [I,s-wp] and [I,c-wp]/[I,s-wp] = 1.00;

and the pontoon structures have a water volume displacement weight [V,p] and a weight in air [W,p] and said compartments in the pontoon structures have a water weight capacity [W,p-c] so when [W,p] + [W,p-c] = [W,p-a] then 1.4 >

[V,p]/[W,p-a] > 0.6.

Moreover, it is preferred that the water plane area [A,c-wp] of a support column has a geometric centre and the horizontal distance between the geometric centre of the water plane areas of two support columns is [S] and wherein [I,c-wp] includes the water plane area moment of inertia in way of structures which may or may not be conceived as part of support columns and having a water plane area which lies within a horizontal distance of 0.1 x [S] from the geometric centre of the water plane area of a support column where in case there are different distances [S] between water plane areas then the largest distances apply.

Preferably one or more of the support columns comprises more than one water plane area having the combined properties of [I,c-wp] as per the definition of areas included in [I,c-wp].

The pontoon structures may have a cross sectional area of the longitudinal load bearing structure where off a fraction [A,p-sd] is above still water line and a fraction [A,p-sw] is below still water line at a position along said pontoon structures, preferably such as [A,p-sd]/[A,p-sw] <0.1, more preferred such as [A,p- sd]/[A,p-sw]<0.25, and even more preferred such as [A,p-sd]/[A,p-sw]<0.5. Furthermore, it is preferred that the [I,c-wp]/[I,s-wp]≠ 1, such as [I,c-wp]/[I,s- wp] > 0.95, preferably such as [I,c-wp]/[I,s-wp] > 0.90, more preferably such as [I,c-wp]/[I,s-wp] > 0.80, and even more preferably such as [I,c-wp]/[I,s-wp] > 0.7. In an embodiment of the semi-submerged multiple wind-turbine system the weight of the water weight capacity [W,p-c] in part or in full comprises solid weights.

For the purpose of optimizing the properties of the semi-submerged multiple wind- turbine system it is preferred that the positioning system transfer forces in to the structural entity in a position within a horizontal distance [D] from the geometric centre of the water plane area of a support column where [D] < 0.1 x [S], preferably such as [D] < 0.2 x [S], more preferably such as [D] < 0.5 x [S].

In a further embodiment the support columns and/or pontoon structures comprise submerged structural appendages with a horizontal plane projection area [A,h] where the areas are horizontally distributed in the vicinity of said main columns and/or in the vicinity of the longitudinal mid point of the pontoon structures and/or in the vicinity of the longitudinal quarter points of the pontoon structures in order to feature damping of motions and/or vibrations of or within the system. To improve the possibilities for service and maintenance of the semi-submerged multiple wind-turbine system the system may comprise embodiments where one or more of the wind-turbine assemblies are substituted and/or complemented by facilities such as for example boat facilities, helicopter landing platform,

accommodation facilities or electrical gear such a transformer stations.

To increase the efficiency of the semi-submerged multiple wind-turbine system one or more of the wind-turbines assemblies comprise more than one rotor. In this way it may be possible to optimize the power output of the system.

In a preferred embodiment of the semi-submerged multiple wind-turbine system according to the present invention the thrust and/or dynamic loadings frequency is controlled by change of rotor sway or change of rotor blade pitch or change of rotor speed of revolutions such that resulting forces of the semi-submerged multiple wind-turbine system can be managed to control heading and structural loading of the semi-submerged multiple wind-turbine system.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in details with reference to some figures in which :

Fig. 1 shows a semi-submerged multiple wind-turbine system according to the invention with turret for mooring located in the support column below a wind turbine;

Fig. 2 shows a semi-submerged multiple wind-turbine system according to the invention with turret in a column that does not support a wind turbine ;

Fig. 3 shows a preferred embodiment of a semi-submerged multiple wind-turbine system according to the invention;

Fig. 4 shows an alternative embodiment of a semi-submerged multiple wind-turbine system according to the invention;

Fig. 5 shows yet an alternative embodiment of a semi-submerged multiple wind- turbine system according to the invention;

Fig. 6 shows an embodiment of a semi-submerged multiple wind-turbine system according to the invention;

Fig. 7 shows a semi-submerged multiple wind-turbine system according to the invention with platform and walkways;

Fig. 8 shows a section of a pontoon structure according to the invention; and Fig. 9 shows a section of another pontoon structure according to the invention.

The possible designs of the semi-submerged multiple wind-turbine system may vary within a wide range and in the following figures a few possible designs are schematically shown. In the figures the same reference numbers are used for the same parts.

Figure 1 shows an embodiment of a semi-submerged multiple wind-turbine system 1 according to the invention. The semi-submerged multiple wind-turbine system 1 comprises three support columns 2 connected by three pontoon structures 3. Each support column 2 is equipped with a wind turbine assembly 4. A mooring system 5 is attached to one support column 2, and the turret is located inside the support column 2. Besides fixing the semi-submerged multiple wind-turbine system 1 the mooring system 5 may also comprise cables that can transport the electric power produced by the system. The attachment of the mooring system 5 to one of the support columns 2 provides the option that the entire system can turn in respect of the wind whereby it may be possible to exclude yaw rings from the wind turbines 4 and, thereby, reduce the costs.

Figure 2 shows a similar embodiment of a semi-submerged multiple wind-turbine system 1 according to the invention. The semi-submerged multiple wind-turbine system 1 also comprises three support columns 2 connected by three pontoon structures 3. Each support column 2 is equipped with a wind turbine assembly 4. The mooring system 5 is, however, attached to a turret enclosed in a column 6 in the central part of the system.

In figure 1 and 2 the pontoon structures 3 connect the support columns 2 in a star- shaped configuration.

Figure 3 shows a system 1 where the pontoon structures 3 connect the support columns 2 in a triangular configuration. This embodiment is often preferred as it is simple and provides high strength in the system.

Figure 4 also shows a system 1 where the pontoon structures 3 are in a triangular configuration, however, the connect the pontoon structures 3 with the support columns 2 further pontoon structures 7 are applied. This embodiment also provides good strength in the system, while less material may be required for the pontoon structures 3 and 7.

The mooring system 5 in the embodiments of figure 3 and 4 are attached to one support column 2.

In contrast to this mooring figure 5 shows an embodiment of a semi-submerged multiple wind-turbine system 1, which for the major part corresponds to the system depicted in figure 4. However, in the system 1 depicted in figure 5 the mooring system 5 is attached to a turret enclosed in a column 6 located at the point where a pontoon structure 7 is connected with the triangular structure formed be the pontoon structures 3. Figure 6 shows a semi-submerged multiple wind-turbine system 1 according to the invention where the three support columns 2 are connected by only two pontoon structures 3 in a V-shaped configurations. The mooring system 5 is attached to the support column in the top point of the V. Although this embodiment offers the option of material savings due to the fact that only two pontoon structures 3 are used the configuration has, however, less strength and should only be used in less harsh environments.

Figure 7 shows a semi-submerged multiple wind-turbine system 1 where the support columns 2 are connected by pontoon structures 3 in star-shaped

configuration. The mooring system 5 is attached to a turret in a central column 6 which carries a platform 8, which may serve as a helicopter landing platform. The support columns 2 and the platform 8 are connected by gangways 9 which are supported by the pontoon structures 3. This sophisticated embodiment is very useful for many locations, but also more costly to produce.

Figure 8 shows a section of a pontoon structure 10 according to the invention. The pontoon structure 10 comprises a tubular element 11 having internal compartments 12 formed by dividing walls 13. In use the compartments 12 may be filled with water or they may be filled with gas. Whether or not a specific compartment 12 is filled with water or gas depends on the specific use of the pontoon structure 12 and the actual forces involved. To provide increased strength the pontoon structure 10 is provided with reinforcement ribs 14.

Figure 9 shows a section of an alternative pontoon structure 20, which is

constructed with beams 21 with reinforcing crossbars 22. In this particular pontoon structure the compartments 23 are placed within the structure formed by the beams 21 and the crossbars 22. Although the beams 21 and the crossbars 22 may be hollow, i.e. constructed from pipes or similar tubular material and, consequently, are able to contribute to buoyancy they are not considered as compartments in this context as they are not connected with pipes, valves, pumps etc.

Which of the pontoon structures 10 or 20 that are preferred depend on the conditions the semi-submerged multiple wind-turbine system in which the pontoon structures are forming parts of is designed to be exposed too.

The following more detailed description of the invention is also related to some specific embodiments which more specifically relate to e.g. the conditions and the design basis for the implementation of the invention. As it is understood the present invention is implementable in a variety of embodiments and only some of these embodiments are described herein.

The pontoon structure may be of shell plate construction or of beam element construction where a beam element construction is also often referred to as e.g. a lattice or jacket or space frame construction and the invention may be implemented using a combination of these constructions. The wind-turbine assemblies of the SMWS may be distanced in both the horizontal plane and the vertical plane or in a combination hereof to reduce or avoid wind- turbine induced wake interference.

The semi-submerged multiple wind-turbine system may be featured with a positioning system comprising a geo stationary structure extending from sea bed which allow the floating structures of the semi-submerged multiple wind-turbine system to rotate around one geo stationary axis perpendicular to the horizontal plane and with essentially no degree of freedom to move in the horizontal plane. Such systems may be feasible at lower water depths.

The compartments in the pontoon structures may or may not be conceived as tanks as for example ballast tanks of a vessel and arranged for sea water containment and/or transfer but compartments may contain other fluids such as fresh water and may be connected or connectable to sea for transfer of fluids and/or hydrostatic forces via piping or via flexible membranes or by a simple hole in a plating forming a boundary between a compartment and sea. Such fluid weights as contained in the compartments may be substituted by other solid weights as also claimed with this invention. An important function of weight is to control loads and vibrations in operating condition although shift able weights such as sea water weights may be practical in the perspective of the fabrication and deployment of the SMWS.

To expand the use and value of the SMWS it may in substitution or addition to wind-turbine assemblies comprise other features of relevance to an offshore wind power plant or installation such as helicopter landing platforms and/or

accommodation modules and/or electrical equipment.

The SMWS may under special conditions be designed to a limit state where motions but in more particular vibrations or large global low frequency deflections of the structures all predominantly in the vertical direction needs consideration in relation to hydro dynamical damping and the features to provide for this is claimed with this invention as structural appendages with a horizontal plane projection area. Such features although not highly efficient at the amplitudes/frequencies which are expected for this particular SMWS are specified at certain characteristic locations being at columns due to motions and at mid point and quarter point of pontoons due to vibrations.

As wind-turbine assemblies are distinct in terms of both thrust, direction of thrust and dynamic loads and since this invention allows for a design of a very light structure but which may be sensitive to certain loading conditions then the control of the wind-turbine assemblies may be an use full operational mean to control loading conditions. Such loading conditions are influenced by coupled parameters and obviously the thrust of the wind-turbine assemblies but also the rate of revolutions of the wind-turbines and for example the heading of the SMWS in relation to waves. Features to control the wind-turbine assemblies are claimed as a part of this invention not because there is a mechanical coincidence of means, but because of the physical relations between forces of the wind-turbine assemblies and the SMWS which is of relevance to the loading of the SMWS. The pontoon structures and the support columns may be configured such that they in a projection on to the horizontal plane appear to constitute a triangle or a V or a star or a combination hereof.

The SMWS may be outfitted in such a way that for example walkways between the support columns above waterline are being support by the pontoon structures by secondary structures.

The SMWS may be configured such that wind-turbine assemblies have a structural connection to the pontoon structures in the form of for example bracings.

This disclosure is made using an example of a specific embodiment and using typical 3-bladed horizontal axis wind-turbines such that the more significant features and their advantages are described in basic terms and such that figures which are given for illustrative purposes only relate to same conceptual steel structure SMWS design with wind-turbines of a rating in the region of about 3.5 MW each.

In this examp-e a basic SMWS comprises three semi-submerged support columns and three identical wind-turbine assemblies and three submerged pontoon structures and a turret mooring positioning system wherein the wind-turbine assemblies are founded on a support column each and the support columns and the pontoon structures are connected to constitute a closed triangle and the turret interfaces with one of the support columns which then becomes the upwind support column where the turret mooring system enables weather vaning capability of the SMWS.

The horizontal distance between support columns corresponds to about 2 times the wind-turbine induced wake tunnel diameter, e.g. about 260 meter and the hub height of the wind-turbines above still water line is e.g. 85 meter.

The remaining features are determined on basis of complex coupled parameters here first described on basis of the some of the features of the parts or part- assemblies which in a full assembly constitutes the SMWS.

Support columns: The support column is an essentially closed shell plate construction which is cylindrical from above operational still waterline and down and here of a diameter [D,c] of say 8 meter and of a draft [T,c-a] say 15 meter. It features at least one chamber for tihe containment of ballast water weight. A free floating and detached support colurwn in upright position loaded by own weight including ballast weight and loaded with weight of a wind-turbine assembly and for one of the support columns loaded with weight of turret and subjected to mooring forces has got a certain draft [ ,c-f] at hydrostatic equilibrium. When coupled to a pontoon structure which is equaBy in hydrostatic equilibrium then the transfer of shear forces will be nil. This state is only achievable in a still water condition but optimum in terms of global loading applied on to the pontoon structure. It is a feature of the support column that a certain shear force at the connection between the support column and the pontoon structure can be applied for a given draft [T,c-a] by adding or removing weight such that the pontoon structure then not in an equilibrium can be structurally preloaded as a mean to reduce the stresses and deflections resulting of the combined global loads of the pontoon structure when in full SMWS assembly. This is a relation between water volume displacement weight of column in free floating condition [V,c-f] and in SMWS assembly condition [V,c-a].

Pontoon structure:

The pontoon structure is an essentially closed shell plate construction which is cylindrical and here of a diameter [D,p] of e.g. about 8 meter. It features a number of longitudinally distributed compartments for containment of ballast water weight. A pontoon structure has a weight [W,p-a] including ballast water where in weight can be added or removed such that it can couple according to the description for the columns above. This is a relation between water volume displacement weight of fully submerged pontoon structure [V,p] and weight of pontoon structure in SMWS assembly condition [W,p-a]. The longitudinally distributed compartments features control of the weight distribution of the pontoon structure and thereby the means to control the stresses and deflections resulting from the combined loads and in particular the longitudinal structural loads of the pontoon structure. The combined loads comprise static loads and cyclic loads as induced by for example waves and wind-turbine assemblies and these loads are also considered in the perspective of structural vibration. The first order natural frequency of a pontoon structure with the lowest practicable ratio of [W,p-a]/[V,p] could be say 0.7 Hz and this may coincide with the predominant induced or exiting forces of the wind-turbine assembly whereas when in-expensive weight such as water can be added to this system then frequency can be adjusted within a certain range here down till say 0.2 Hz. When considering vibration alone then said compartments enables the distribution of weight as feasible for actual exiting forces and modes or orders of vibration. The resulting hydrostatic loading of the shell plate area forming a boundary to a compartment which is directly connected to sea is nil and this is a mean to reduce local structural loads which is enabled by the use of the distributed compartments where they contain water at a hydrostatic pressure corresponding to or near that of the surrounding sea. In the perspective of local hydrostatic loads alone then a compartment containing water could facilitate reduction of scantling of the structure and this is part of the combined optimization of weights, loads, strength and stiffness. The compartments further contribute to stiffness of the SMWS in relation to motions and to hydrostatic stability of the SMWS as they restrict the effect of the free water plane areas of any water contained in a the pontoon structure. Turret mooring system:

The turret mooring system comprises a turret integrated in a column, which may be a support column, connected to number of spread mooring lines connected to a number of geostationary anchoring points wherein the significant feature in relation to the turret is in that it accommodate for example a part of the electrical power transmission system comprising power swivels and cables extending from SMWS to sea bed.

Wind-turbine assemblies:

The wind-turbine assemblies are here typical 3-bladed horizontal axis type wherein the most significant characteristic in relation to the SMWS is in the frequency of their exiting forces and in that they for example do not necessarily need a yawing mechanism and what is thereto related.

SMWS assembly:

The SMWS is devised with particular attention to loads of structures and positioning system where 3 support columns in a triangular configuration with a high ratio [I,wp]/[A,wp] is a feature to achieve high hydrostatic restoring forces while at the same time reducing the wave loads and where relevant the ice loads following a low water plane area. The configuration further allow for a high utilization of masses to obtain a high inertia of masses in the direction of rotational movements as the weights [V,c-a] has maximized distance to centre of flotation of the SMWS. If the structure was infinite stiff then the pontoon structure would be loaded and stressed to a maximum, however, due to the features of the pontoon structures which allow for a structure with controlled deflections whether that is static or dynamic it is possible to utilize the inertia of the weights [V,c-a] to minimize stresses in the pontoon structures.

When the features as described for the parts in the above are implemented in conjunction with in particular the conclusions of motion and vibration analysis of the full SMWS assembly then it is possible to construct the most significant structural elements in this example being the pontoon structures connecting towards the positioning system to near the strength requirements resulting from the bending moment induced by the wind-turbines alone. Since a wind-turbine also impose a force in the horizontal plane on to the SMWS of a magnitude of about the trust of the rotor of the wind-turbine then a SMWS would be increasingly loaded structurally and in particularly in the region of for example a turret of a positioning system with an increased number of turbines which is one of reasons why the present invention comprises only 3 turbines and not more.

The coupling of features and design principles has a continuously positive spin off as the then less massive structures are less be subjected to environmental loadings also from for example current such that materials consumption and positioning system loads are reduced and such that the SMWS can be deployed and will become more feasible also in areas at relatively low water depth where wave loads are even typically reduced with lower water depth. This is achievable because both loads within the range which distinct wind-turbine applications and the consequence of the desired reduced structural scantlings due to reduced loads which is in particular the consequences in relation to vibrations and structural stability has been considered and has found a technical solution which has not been addresses in prior art.

Claims

1. A semi-submerged multiple wind-turbine system comprising three semi- submersed essentially vertical oriented support columns, each being adapted to carry a wind-turbine assembly and at least two submerged essentially horizontal oriented pontoon structures where said support columns and pontoon structures are connected to form a structural entity where in a projection on the horizontal plane the geometric relations between the support columns are equal to the geometric relations of vertices of a triangle and the semi-submerged multiple wind- turbine system further comprises a positioning system for transferring forces between said structural entity and one or more geostationary positions, wherein each of the support columns comprises at least one compartment and said pontoon structures comprises a number of longitudinally distributed compartments and a number of said compartments are hydro or hydrostatic connected or connectable to a fluid to allow one or more of the compartments to be filled with the fluid.

2. A semi-submerged multiple wind-turbine system according to claim 1, wherein the one or more of the compartments can be filled with gas or liquid.

3. A semi-submerged multiple wind-turbine system according to claim 1 or 2, wherein a pontoon structure comprises at least 2 compartments, preferably from about 4 to about 80 compartments, suitable from about 10 to about 50

compartments.

4. A semi-submerged multiple wind-turbine system according to any of the claims 1 or 2, wherein the pontoon structure comprises at least one compartment extending in the longitudinal direction of the pontoon structure.

5. A semi-submerged multiple wind-turbine system according to any of the preceding claims, wherein the pressure in the one ore more compartments is controlled.

6. A semi-submerged multiple wind-turbine system according to any of the preceding claims, wherein the structural entity is able to move in respect of the one or more geostationary positions.

7. A semi-submerged multiple wind-turbine system according to any of the preceding claims, wherein, the three semi-submersed support columns each have a water plane area [A,c-wp] and a water volume displacement weight [V,c];

the submerged pontoon structures each have a water volume displacement weight [V,p]; the support columns have a combined water plane area moment of inertia [I,c-wp] and said semi-submersible wind-turbine system is having a water plane area moment of inertia [I,s-wp] where [I,c-wp] and [I,s-wp] are both taken along any neutral axis of [I,s-wp] and [I,c-wp]/[I,s-wp] = 1.00; and the pontoon structures have a weight [W,p-c] and a water volume displacement weight [V,p] and a weight in air [W,p] and said compartments in the pontoon structures have a water weight capacity [W,p-c] so when [W,p] + [W,p-c] = [W,p-a] then 1.4 >

[V,p]/[W,p-a] > 0.6

8. A semi-submerged multiple wind-turbine system according to any of the preceding claims, wherein the water plane area [A,c-wp] of a support column has a geometric centre and the horizontal distance between the geometric centre of the water plane areas of two support columns is [S] and wherein [I,c-wp] includes the water plane area moment of inertia in way of structures which may or may not be conceived as part of support columns and having a water plane area which lies within a horizontal distance of 0.1 x [S] from the geometric centre of the water plane area of a support column where in case there are different distances [S] between water plane areas then the largest distances apply.

9. A semi-submerged multiple wind-turbine system according to any of the preceding claims, wherein one or more of said support columns comprises more than one water plane area having the combined properties of [I,c-wp] as per the definition of areas included in [I,c-wp]

10. A semi-submerged multiple wind-turbine system according to any of the preceding claims, wherein said pontoon structures have a cross sectional area of the longitudinal load bearing structure where off a fraction [A,p-sd] is above still water line and a fraction [A,p-sw] is below still water line at a position along said pontoon structures, preferably such as [A,p-sd]/[A,p-sw]<0.1, more preferred such as [A,p-sd]/[A,p-sw]<0.25, and even more preferred such as [A,p-sd]/[A,p- sw]<0.5

11. A semi-submerged multiple wind-turbine system according to any of the preceding claims, wherein said [I,c-wp]/[I,s-wp]≠ 1, such as [I,c-wp]/[I,s-wp] > 0.95, preferably such as [I,c-wp]/[I,s-wp] > 0.90, more preferably such as [I,c- wp]/[I,s-wp] > 0.80, and even more preferably such as [I,c-wp]/[I,s-wp] > 0.7.

12. A semi-submerged multiple wind-turbine system according to any of the preceding claims, wherein the weight of said water weight capacity [W,p-c] in part or in full comprises solid weights.

13. A semi-submerged multiple wind-turbine system according to any of the preceding claims, wherein said positioning system transfer forces in to said structural entity in a position within a horizontal distance [D] from the geometric centre of the water plane area of a support column where [D] < 0.1 x [S], preferably such as [D] < 0.2 x [S], more preferably such as [D] < 0.5 x [S].

14. A semi-submerged multiple wind-turbine system according to any of the preceding claims, wherein said support columns and/or pontoon structures comprise submerged structural appendages with a horizontal plane projection area [A,h] where said areas are horizontally distributed in the vicinity of said main columns and/or in the vicinity of the longitudinal mid point of said pontoon structures and/or in the vicinity of the longitudinal quarter points of said pontoon structures.

15. A semi-submerged multiple wind-turbine system according to any of the preceding claims, wherein one or more of said wind-turbine assemblies are substituted and/or complemented by facilities such as for example boat facilities, helicopter landing platform, accommodation facilities or electrical gear such a transformer stations.

16. A semi-submerged multiple wind-turbine system according to any of the preceding claims, wherein thrust and/or dynamic loadings frequency is controlled by change of rotor sway or change of rotor blade pitch or change of rotor speed of revolutions such that resulting forces of the semi-submerged multiple wind-turbine system can be managed to control heading and structural loading of the semi- submerged multiple wind-turbine system.