IE20090271A1 - Floating platform - Google Patents

Abstract

A stable deep water floating platform for floating in a body of water, the floating platform comprising: a. a semi-submersible hull defining a substantially streamlined body with opposing narrowed end portions; and b. a passive stabilising arrangement comprising 1. an anti-pitching element coupled to the hull, the anti-pitching element comprising a substantially planar body defining a first plane; 2. an anti-rolling element comprising a substantially planar body defining a second plane, the anti-rolling element being coupled to the hull such that the first and second planes are substantially perpendicular to one another; and 3. an anti-heave element comprising a substantially planar body defining a third plane, the anti-heave element being coupled to the hull such that the third plane is substantially perpendicular to the first and second planes, wherein, in use, the floating platform sits beneath the water surface with the stabilising arrangement located beneath the hull such that the shape and orientation of the anti-pitching, anti-rolling and anti-heave elements dampen the loads acting on the platform whilst the streamlined shape of the hull minimises the destabilising impact of the wave action on the floating platform. <Figure 1>

Description

FLOATING PLATFORM Background to the invention The present invention relates to floating platforms for the support of equipment 5 such as but not limited to wind turbines and the like, for location offshore.

Increasing pressure to generate more electricity from renewable sources such as wind energy in turn increases the requirement for sites at which to locate wind turbines and the like. However, such developments are often considered unsightly and the noise that they generate considered intrusive. It is therefore common for planning for such sites to be vigorously opposed by local residents and lovers of the countryside. Locating such sites offshore not only avoid such opposition, but also provides more optimal wind conditions, including higher mean wind speed and tower turbulence.

However, as many conventional offshore wind farms are located in water shallow enough to allow the supporting platforms to stand on the seabed, the resulting installation often remains in sight of land and is therefore subject to objections from local residents, Furthermore, such installations are not possible in some countries, such as Norway, as the coastal waters are generally too deep to allow the supporting platforms to stand on the seabed.

One way to overcome the problem of siting such installations in deep coastal waters is to use floating platforms anchored to the sea bed, rather than supporting platforms that stand on the sea bed.

The use of floating platforms not only allows the installations to be sited in areas of deep water, thereby increasing the number of suitable sites, it also allows the installations to be sited further from shore and thus out of sight of the shore i.e. beyond the horizon.

Unfortunately however, not all equipment and devices can tolerate being installed 5 on a moving platform without expensive additional design. In order to avoid such expense and increase the potential of offshore sites it is desirable that the platform should remain stable in rough seas with waves up to sixteen meters high.

Stability can be achieved using active or passive stabilisers. Active stabilisers are 10 not preferred due to their requirement for large amounts of energy in rough seas and their generally higher maintenance costs. Traditionally passive stabilisers have comprised massive structures below the platform to lower the centre of gravity.

Such massive structures require specialist equipment for their construction, such as floating cranes and barges, the expense of which often makes projects involving floating platforms unfeasible.

There is therefore a need for a floating structure with increased stability and which avoids the need for using active stabilising arrangements with associated increased cost, complexity and energy requirements.

By designing stabilisers which rely on their passive hydrodynamic properties including their entrained mass to provide stability, the component parts of the floating structure can be made much lighter and the need for specialist construction equipment and associated expense avoided, Summary of the Invention The present invention seeks to address the problems of the prior art.

Accordingly, a first aspect of die present invention provides a stable deep water 30 floating platform for floating in a body of water, the floating platform comprising: IE 0 9 Ο 2 71 a. a semi-submersible bull defining a substantially streamlined body with opposing narrowed end portions; and b. a passive stabilising arrangement comprising: i. an anti-pitching element coupled to the hull, the anti-pitching element comprising a substantially planar body defining a first plane; ii. an anti-rolling element comprising a substantially planar body defining a second plane, the anti-rolling element being coupled to the hull such that the first and second planes are substantially perpendicular to one another; and iii. an anti-heave element comprising a substantially planar body defining a third plane, the anti-heave element being coupled to the hull such that the third plane is substantially perpendicular to the first and second planes, wherein, in use, the floating platform sits beneath the water surface with the stabilising arrangement located beneath the hull such that the shape and orientation of the anti-pitching, anti-rolling and anti-heave elements dampen the loads acting on the platform whilst the streamlined shape of the hull and its elevational form minimises the destabilising impact of the wave action on the floating platform.

The stable deep water floating platform is preferably hydrostatically and hydrodynamical ly stable.

Preferably, the loads acting on the platform include wind and wave actions. For example, the platform may be capable of resisting wind loads derived from the presence of a wind turbine mounted on the platform or some other structure mounted on the platform that may attract wind forces.

The semi-submersible hull is optimally shaped to be as streamlined as possible, thereby reducing the incident wave loads to a minimum. This is also assisted by IE 0 9 0 2 7 1 the elevational shape of the hull. In use, the hull is located below the water surface and wave line in order to reduce wave loads. Preferably, the hull is preferably located at least 50m below the water surface and wave line.

The components of the passive stabilising arrangement i.e. the anti-pitching, antirolling and anti-heave elements, are preferably supported on the hull by means of cables or supporting struts or the like, and are preferably sited lower in the water than the hull itself. Preferably, the elements of the passive stabilising arrangement are sited 50m lower in the water than the hull itself.

The substantially planar body of the elements of the passive stabilising arrangement allows the elements to entrain a large mass of sea water whilst having a very high form drag.

The strategic location of the elements of the passive stabilising arrangement allows minimisation of heaving, pitching, surging, rolling, swaying and/or yawing motions of the platform due to wind and wave action. In addition, the position of the elements of the passive stabilisation arrangement lowers the centre of gravity of the platform, thereby preventing capsizing of the structure in strong winds and strong wind and wave action.

The term ‘heaving* is intended to mean the vertical motion of the structure The term ‘pitching* is intended to mean rotation of the floating platform about its centre of buoyancy in a fore to aft direction.

The term ‘surging* is intended to mean a forwards and backwards movement of the floating platform in a fore to aft direction.

IE 0 9 0 2 71 The term ‘rolling* is intended to mean rotation of the floating platform about its centre of buoyancy in an orthogonal direction (when compared to ‘pitching’), i.e. in a port to starboard direction.

The term ‘swaying’ is intended to mean linear movement of the floating platform from side to side, i.e. in the port to starboard direction.

The term ‘yawing* is intended to mean the rotation in plan of the platform.

The platform may further comprise mooring engagement means for attachment to a cable secured to the sea bed. A single mooring is preferred as several moorings increase the exclusion zone around the floating platform. Thus, for example, where several floating platforms are sited together, such as a wind farm installation or the like, the large resultant exclusion zone would be unfeasible as it would interfere with normal fishing of the area.

In addition, the single mooring allows the floating platform to rotate downwind. The single mooring arrangement also allows trawlers and the like to fish the seas around the installation if desired.

Preferably, the mooring is provided with streamlined floatation devices. This allows the floating platform to sit closer to the moorings in light winds. The floating mooring prevents the platform from crossing the mooring point and thereby prevents the mooring rope from becoming snagged or trapped under the structure In one embodiment, the semi-submersible hull is substantially circular. However, it will be appreciated that any other suitable shape known to the skilled person and appropriately streamlined could be used. For example, the semi-submersible hull could be substantially elliptical in shape. 1ϊ 0 9 0 2 7 1 A platform in accordance with the present invention may comprise one or more ballast tanks coupled to the hull.

Having several ballast tanks permits re-ballasting during storms to reduce the angle at which the floating platform sits in the water i.e. the tilt of the platform. In addition the presence of several ballast tanks assists in keeping the structure upright during construction and decommissioning.

In addition, the platform may comprise one or more floatation tanks coupled to the hull.

The ballast tanks either contain air or sea water or stored fresh water or a combination of the two. A tank may be partially filled with air to provide buoyancy, but that same tank may have a partly filled bladder containing fresh water. So a combination of all three variables may be possible.

In a preferred embodiment, the platform is provided with several permanent floatation tanks and ballast tanks.

Water movement into and out of the ballast tanks may be controlled by any suitable means, such as an electro-pneumatic system. However, it will be appreciated that any other suitable control system known to the skilled person could be used instead of or in additional to such an electro-pneumatic system.

Preferably, the floating platform includes a deck level supported on the hull. The deck level may be supported on the hull by means of supporting columns extending from the hull, for example, extending from the upper surface of the hull. Alternatively, any other suitable support structure known to the skilled person may be employed.

IE Ο 9 Ο 2 7 1 Where supporting columns are used, preferably the column end portions adjacent the hull are of narrowed dimensions and streamlined in plan profile so as to attract the minimal wave load.

As an alternative or a supplement to the energy provided by, for example, a wind turbine mounted on the floating platform, preferably, at least a portion of the semisubmersible hull streamlined body defines a chamber, for example, for use as a plant room to house electrolysis equipment and/or compression equipment and the like. Having such a chamber located in the hull effectively acts as a ballast chamber, thereby keeping the hull volume to a minimum. Together with the ballast tanks, the chamber serves to reduce the amount of topside structure that would otherwise have to resist wind and wave loads.

Where the chamber is used in such a manner, the ballast tanks are preferably fresh water containers that supply the electrolysis equipment (housed in the hull). The gases produced as a result of the electrolysis process could then be supplied to shore or alternatively stored in storage tanks for subsequent transportation to shore.

The gas storage tanks may be located at the mooring.

In this way, the floating platform of the present invention may comprise several sections which can be floated (using floats or other alternative inherent floatation properties) to the required mooring location and assembled 'on site without the need for floating cranes and/or barges and the like.

Preferably, the mooring cable is provided with streamlined floatation devices. This allows the floating platform to sit closer to the moorings in light winds. By having a buoyant mooring cable the cable will float to the surface in light winds when it IE 0 9 0 2 7 1. has a lesser load on it and hence pull the platform towards the mooring point. The buoyant cable also prevents the cable from becoming snagged under the platform.

It will be appreciated that the energy provided from a wind turbine mounted on the floating platform could be provided to shore using a cable from the floating installation to the shore. However, energy storage is possible using the plant room described above coupled with the storage tanks or in the event a large installation of platforms be linked by pipeline to shore.

The floating platform may be provided with a wind turbine, a helipad, a small building or any other suitable structure desired which can be supported on the platform within the limits of stability and dimension of the platform.

Brief Description of the Drawings An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 is an isometric view of a first embodiment of a semi submersible support structure shown supporting a wind turbine; Figure 2 is a side view of the embodiment of figure 1; Figure 3 is a side elevation of the submergible support structure of figure 2 detailing sections AA, BB, CC and DD; Figure 4 is an end view of the embodiment of figure 1; Figure 5 is a plan of section AA as shown in figure 3; Figure 6 is a plan of section BB as shown in figure 3; Figure 7 is an end view of section CC as shown in figure 3; and Figure 8 is an end view of section DD as shown in figure 3.

IE Ο 9 Ο 2 7 1 Detailed Description of the Invention An embodiment of the present invention will now be described with reference to all of figures 1 to 8, in which common features are indicated by common figure references.

Figures 1, 2 and 4 show a submergible support structure comprising platform 9 attached to which is an example application comprising of a lattice tower 10 which in turn supports a wind turbine 11. Platform 9 is a substantially planar structure its surface being orientated substantially in the horizontal plane when the submergible support structure 12 is in equilibrium. Several columns 8, 8’ extend from the top of submerged hull 6 to the under side of platform 9 above the surface of the water. Columns 8, 8’ pass through the surface of the water 13 and are therefore subject to forces created by waves hitting them. In order to minimise the load placed on columns 8, 8’ by passing waves the horizontal cross-section (see figure 6) is a streamline shape. When the longitudinal axis of columns 8, 8’ is aligned with the direction of wave travel, the surface area on which waves impact is thus minimised, thereby minimising the force of wave action on columns 8, 8\ The hull 6, although submerged, will also be subject to wave action, albeit to a lesser extent than columns 8, 8’, Therefore hull 6 also has an elongate streamline horizontal cross20 section (see figure 5) the longitudinal axis of which is substantially parallel to the longitudinal axes of columns 8, 8’. The sides of hull 6 are substantially planar and substantially parallel to each other.

The plane of the sides of hull 6 is orthogonal to the normal direction of wave travel due to pitching and surging actions on the structure. By minimising the area of hull perpendicular to the oncoming waves minimises the wave load action on the device.

The large plane of the sides of hull 6 is also orthogonal to yaw actions on the structure thereby providing a large resistance to movement. The entire mass of the IE 0 9 0 2 71 hull including the entrained mass and the forces in the mooring rope provide the remaining resistance to yaw movement. Yaw forces created by wind action on equipment located on platform 9 will thus be resisted.

Forces created by wind and wave action on the hull are resisted by cable 14 which splits and attaches to opposite sides of hull 6 on the lateral axis of the semi submersible support structure. The attachment positions of cable 14 are displaced from the vertical axis of the semi submersible support structure, set up a torque which resists rotation about the vertical axis.

Passive stability components 1, 2, 2’, 3, 3’ are located at a water depth such that there is substantially no wave action acting directly upon them and are attached to hull 6 by superstructure comprising one or more of struts 4, 5 and cables 7. Passive stability components 1,2,2’ 3,3' lower the centre of gravity of the whole structure increasing stability and reducing the possibility of capsize.

Anti-heave device 1 is a substantially planar member located directly below the hull 6 such that when the semi submersible support structure is in equilibrium, the anti-heave member is horizontal. Its dimensions are such that it entrains adequate water to provide an inertial load sufficient to restrain hull 6 from vertical motion (heave) due to transitory water level changes created by passing waves. The anti heave device t also acts as a strut preventing devices 3 and 3’ and 2 and 2’ from moving together when forces act on them.

Anti-pitch devices 2, 2* are attached to each end of anti-heave device 1. Anti-pitch devices 2, 2’ are shaped to maximise the surface area in the plane orthogonal to their direction of travel due to pitching. The entrainment of water within and around the device and due to its form shape is thus maximised as is the resistance to the pitch and surge motions acting on the device. The combined resistive and inertial forces of anti-pitch devices 2, 2’ is sufficient to minimise oscillations of IE 0 9 0 2 71 hull 6 about its lateral, (port to starboard), axis due to transitory water level changes such as created by passing waves. The anti pitch devices are located far enough fore and aft of the structure such that the wave actions on it are in opposite directions to that acting on the hull.

Anti-roll devices 3, 3* are provided, located centrally under the hull 6. Anti-roll devices 3, 3’ are shaped to maximise the surface area in the plane orthogonal to their direction of travel due to rolling and swaying actions. The entrainment of water within the device and due to its form shape maximises the resistance to the roll and sway motions acting on the structure. The combined resistive and inertial forces of anti-roll devices 3, 3’ is sufficient to minimise oscillations of hull 6 about its longitudinal, (fore to aft), axis due to transitory water level changes such as created by passing waves. The anti roll devices are located far enough port and starboard of the structure such that the wave actions on it are in opposite directions to that acting on the hull.

The semi submersible support structure 12 is moored to a single anchorage 15 by cable 14. This arrangement allows the semi submersible support structure 12 to rotate about anchorage 15 and minimises the area of sea obstructed at any one time when compared to a multi-point mooring arrangement. Rotating also allows the structure to align itself with the waves and minimises lateral wave loads that cause roll. Buoyancy devices, not shown, are fitted to the cable 14. The buoyancy of the cable 14 is such that in light winds a vertical force will be applied to it sufficient to pull the semi submersible support structure 12 towards anchorage 15 resulting in the area of obstructed sea being further reduced. The buoyant mooring also removes the possibility of die structure 12 snagging itself on the cable 14. The position of anchorage 15 is indicated by a surface marker buoy 17. Apparatus 16 is provided in order to prevent structure 12 from crossing over die mooring point and is also present in order to store gases that may be generated from the equipment in the hull. Device 16 may be attached directly to the anchorage 15.

IE 0 9 0 2 7 t Figures 5, 7 and 8 show sections through hull 6. Hull 6 comprises a plant room 18 for housing plant and machinery required by the application of the submergible support structure. Taking the application of a wind turbine as an example, electrolysis equipment powered by the electrical output of the wind turbine, may be located in the plant room 18, the output gases from which would then be temporarily stored in apparatus 16 before being transported ashore. The plant room 18 being below the surface of the water 13 provides floatation and reduces the amount of structure above the surface of the water which would be subject to wind and wave action. When the structure is floated slightly and device 21 is above water level this will be opened to allow plant to be installed or removed.

Hull 6 further comprises ballast tanks 19. Apertures defined in the hull 6 allow sea water to enter ballast tanks 19. The amount of sea water in ballast tanks 19 is controlled by an electro pneumatic system which displaces the sea water with compressed air. Varying the water ballast of ballast tanks 19 allows adjustment of the position of the hull 6 about its longitudinal and lateral axes and also the position of hull 6 relative to the surface of the water 13. Adjusting the position of hull 6 in this way may facilitate construction, maintenance and decommissioning operations and provide improved stability in the event of storms. One or more ballast tanks may further comprise a flexible bladder for the containment of fresh water which may be required by machinery located in plant room 18. As fresh water is withdrawn the flexible bladder will collapse and sea water will enter through apertures defined in hull 6 to fill the vacated space, thereby maintaining the structures overall buoyancy. Several of the ballast tanks, 19 will become permanent flotation tanks after construction.

Figure 6 shows a horizontal cross-section of columns 8, 8*. At least two of the columns 8, 8’ comprises personnel access 20 which allows personnel to gain access to plant room 18 from platform 9. At least two of the columns 8, 8’ will provide IE Ο 9 Ο 2 7 1 ventilation to the plant room 18. At least two of the columns 8, 8’ will provide electrical and mechanical risers to the wind turbine 11 via the lattice tower 10.

It will be appreciated that the floating installation may be modular and therefore readily transported to the desired offshore location and assembled in situ.

Although aspects of the invention have been described with reference to the embodiment shown in the accompanying drawings, it is to be understood that the invention is not limited to the precise embodiment shown and that various changes and modifications may be effected without further inventive skill and effort, for example, the semi submersible support structure 12 may be used with alternative applications such as but not limited to a helipad, a floating accommodation block and/or a floating processing platform.

Claims (8)

CLAIMS 1. A stable deep water floating platform for floating in a body of water, the floating platform comprising: a) a semi-submersible hull defining a substantially streamlined body with opposing narrowed end portions; and b) a passive stabilising arrangement comprising: i) an anti-pitching element coupled to the hull, the anti-pitching element comprising a substantially planar body defining a first plane; ii) an anti-rolling element comprising a substantially planar body defining a second plane, the anti-rolling element being coupled to the hull such that the first and second planes are substantially perpendicular to one another; and iii) an anti-heave element comprising a substantially planar body defining a third plane, the anti-heave element being coupled to the hull such that the third plane is substantially perpendicular to the first and second planes, wherein, in use, the floating platform sits beneath the water surface with the stabilising arrangement located beneath the hull such that the shape and orientation of the anti-pitching, anti-rolling and anti-heave elements dampen the loads acting on the platform whilst the streamlined shape of the hull minimises the destabilising impact of the wave action on the floating platform. 2. A platform according to Claim I, further comprising mooring engagement means for attachment to a cable secured to the sea bed. 3. A platform according to Claim I or Claim 2, wherein the semi-submersible hull is substantially circular. 4. A platform according to any preceding Claim, wherein the semi-submersible hull is a streamlined elongate shape in cross-section. IE Ο 9 Ο 2 7 1 5. A platform according to any preceding Claim, further comprising one or more ballast tanks coupled to the hull 5 6. A platform according to any preceding Claim, further comprising one or more floatation tanks coupled to the hull. 7. A platform according to any preceding Claim, wherein at least a portion of the semi-submersible hull streamlined body defines a chamber. 8. A platform according to any preceding Claim, further comprising a wind turbine. 9. A platform substantially as hereinbefore described and with reference to the 15 accompanying drawings. The following amended Claims were filed on 8 ,h February 2010 claims /£ 0 9 0 2 7 j I. A stable deep water floating platform for floating in a body of water, the floating platform comprising: a) a semi-submersible hull defining a substantially streamlined body with opposing narrowed end portions; and b) a passive stabilising arrangement comprising: i) an anti-pitching element coupled to the hull, the anti-pitching element comprising a substantially planar body acting in a vertical plane aligned fore aft; ii) an anti-rolling element comprising a substantially planar body acting in a second vertical plane, the anti-rolling element being coupled to the hull such that its vertical plane is substantially perpendicular to that of the anti-pitching element* s; and iii) an anti-heave element comprising a substantially planar body defining another plane, the anti-heave element being coupled to the hull such that its horizontal plane is substantially perpendicular to that of the anitp itch ing and anti-rolling planes, wherein, in use, the floating platform sits beneath the water surface with the stabilising arrangement located beneath the hull such that the shape and orientation of the anti-pitching, anti-rolling and anti-heave elements dampen the loads acting on the platform whilst the streamlined shape of the hull minimises the destabilising impact of the wave action on the floating platform.

1. A platform according to Claim 1, further comprising mooring engagement means for attachment to a cable secured to the sea bed.

2. A platform according to Claim 1 or Claim 2, wherein the semi-submersible hull is substantially circular. IE 0 9

3. A platform according to any preceding Claim, wherein the semi-submersible hull is a streamlined shape in cross-section in both a vertical plane that lies perpendicular to the longitudinal axis of the semisubmersible hull, and the horizontal plane.

4. A platform according to any preceding Claim, further comprising one or more ballast tanks coupled to the hull.

5. A platform according to any preceding Claim, further comprising one or more 10 floatation tanks coupled to the hull.

6. A platform according to any preceding Claim, wherein at least a portion of the semi-submersible hull's streamlined body defines a chamber. 15

7. A platform according to any preceding Claim, further comprising a wind turbine.

8. A platform substantially as hereinbefore described and with reference to the accompanying drawings.