Docking method

Abstract

A docking system (29) for docking a first floating object (1) anchored at sea with a first anchor (9) with a second floating object (18) comprising a vertical movement system providing vertical forces and a horizontal movement system providing horizontal forces. The vertical movement system comprises lifting means (31 ) located on the second floating object (18) providing a force for lifting the first floating object (1), and the horizontal movement system comprises retracting means (35) containing a connecting cable (36) providing a force for shortening the distance between the two floating objects and resilient separating means (37) encircling the two floating objects providing a resilient counterforce to prevent the two floating objects from colliding.

Description

[0001] method

[0002] TECHNICAL FIELD

[0003] The present invention concerns docking of two floating objects at sea. One such object may comprise a floating platform for carrying a tower with a wind power generator. Another such object may be a floating rig for maintenance works at sea. In particular the invention concerns docking of a lightweight floating platform with a rig having lifting equipment installed. By the expression “docking” should be understood to join two objects mechanically while at sea. By the expression "rig” should be understood a semi-submersible platform with tackle, equipment, or machinery fitted for a specified purpose.

[0004] BACKGROUND OF THE INVENTION

[0005] There are known a plurality of concepts for offshore floating wind turbine platforms. Generally, these fall into three main categories: Spars; Tension Leg Platforms (TLP); and semi-submersible systems. When comparing different types of offshore wind turbine structures, wave and wind induced motions are not the only elements of performance to consider. Economics play a significant role. It is therefore important to carefully study the fabrication, installation, commissioning costs and ease of access for maintenance methodologies. Semi-submersible concepts with a shallow draught and good stability in operational and transit conditions are significantly cheaper to tow out, install and commission.

[0006] Semi-submersible platforms were first use in the oil drilling industry. The semisubmersible platform got its name from the fact that it is literally semi-submerged - half above the water and half below. The upper and lower halves are connected by vertical pillars. The objectives are low water drag and astonishing stability in rough sea. When in an operation position the platform is submersed. The hull is ballasted by filling water into ballast tanks of the hull. For transportation to and from a shipyard facility the draught must be shallow. While in transit the semi-submersible platform is deballasted and thus the hull ballast tanks are filled with air. Then the platform takes a transport position in order to reach shallow areas. Floating platforms carrying high towers tends to have a spread out platform construction. To stabilize a big tower at sea the foundation platform must provide sufficient stabilizing forces in all directions. This often results in platforms having equal dimensions in all direction. Thus the width is equal to the length. However shipyards are intended for building ships where the width is a fraction of the length. Consequently shipyards are seldom suited for building wide platforms. Instead the offshore industry looks for facilities outside shipyards to build platforms. This may sometimes be very costly since the use of heavy lifting mobile cranes are not only very costly but also rare since they are few.

[0007] Crane operations at sea are challenging. In a working site at sea there may be a platform to be built and a vessel for delivering the building modules. A crane is provided to lift modules from the vessel to the platform while building. All three objects are floating in the sea. All three objects have different modes of moving behavior in the sea due to the waves. Thus when one object is moving upwards another object is moving downwards. It is easily understood that a challenge is to make two floating objects to move synchronously in the sea. In a first mode of operation lifting must be performed in a weather window where the wind and sea heave is small. In a second mode of operation the crane is attached to the floating platform being built to form a unit. However there is still a challenge in picking and lifting modules from a vessel while moving in the sea.

[0008] Semi-submersible vessels having protruding lifting arrangement is previously known. In one embodiment the lifting arrangement comprises a fork-like shelf. The fork is arranged to protrude under the object to be lifted. By help of deballasting the vessel the floating object may be lifted. To make sure that the lifted object does not slide off the vessel the fork comprises a plurality of friction establishing pads. These pads may also comprise shock-absorbing facilities. However the relative movement caused by the waves between the vessel and the object to be lifted may be huge. Since the lifting operation is very slow the floating object may bounce or collide many times with the fork before the object is finally lifted. In a situation where both objects are heavy such collisions may deform the objects. Severe damage may then occur. A submersible vessel having at the bow part a slot for housing gripping means is previously known. The gripping means comprises an open ring arrangement to receive a spar type column. The ring arrangement comprises a horizontally oriented pronged unit which is arranged to grip a flange section arranged on the column. The vessel itself contains a crane and equipment for building a wind power unit. At first the ship is ballasted in order to lower the gripping means under the flange of the spar. Having gripped the flange of the spar buoy the vessel is deballasted to raise in the sea thereby lifting the spar buoy. While holding the spar buoy with the gripping means like a pendulum a tower and a wind turbine is mounted by help of the crane. However the lifting operation is very slow and both floating objects are affected by different horizontal wave forces. A plurality of bounces and collisions may be expected. As a result, large relative movement between the spar buoy and the vessel may exist during installation. For big structures which may be heavy and fragile the risk of collision and deformation is evident. The lifting operation may be very difficult and dangerous.

[0009] Periodically, the floating wind power platform must undergo maintenance. Currently part of the maintenance process requires that the floating wind turbine is transported to a harbor. The tethers must be removed and a service vessel or tug boat must be engaged. Towing the floating wind power platform is time consuming and requires a weather window with calm sea and small wind forces. Looking for such a window may last for several days and entails that the floating wind turbine is out of commission for the time that is required for towing and maintenance. Besides the cost for hiring a service vessel may be considerable. There is thus a need for maintenance services on site of the wind power platform.

[0010] From US 20130152840 (Awashima) a working system for floating structure is previously known the object of which is to provide installation or maintenance of the floating structure to be performed in a stable manner. The working system includes a spar type floating structure and a working ship configured to perform installation or maintenance of the floating structure. The floating structure has a column section whose peripheral surface is located at a waterline when the floating structure is floating, a ballast section arranged below the column section and a flange section arranged at an intermediate portion of the column section. The working ship has a gripping section capable of engaging with the flange section, and a raising or lowering device configured to raise and lower the floating structure.

[0011] The floating structure or the working ship includes a ballast adjustment device configured to raise and lower the floating structure or the working ship. The gripping section of the working ship engage with the flange section of the floating structure. The floating structure is lowered or the working ship is raised to secure the floating structure to the working ship. The gripping section have a pronged unit with a recess corresponding in shape to a circumferential half of the peripheral surface of the column section and is arranged at a bow of the working ship. The gripping section has an elastic member arranged at a portion thereof where the gripping section comes into contact with the floating structure configured to restrict horizontal movement of the flange section.

[0012] From DK 202100330 (Hasholt) a semi-submersible service vessel for a floating wind turbine is previously known the object of which is to stably fixing the wind turbine during heavy seas. The semi-submersible service vessel has a hull and a ballasting system. The ballasting system is arranged to selectively lower the hull to a first draft and raise the hull to a second draft. The second draft is smaller than the first draft. A submersed elongate lifting fork is fixed to the hull and is configured to extend across the underside of the floating wind turbine and engage the underside of the floating wind turbine when the hull is raised from the first draft to the second draft. The lifting fork is arranged to lift the entire floating wind turbine when the hull is raised from the first draft to the second draft.

[0013] From US 2019071830 (Lindblade) an offshore structure mating system and installation method is previously known the object of which it to install a wind turbine generator on a floating hull offshore without using a large offshore crane. The system comprises a floating vessel having a pair of forks defining a slot. A gimbal table, defining an opening, is positioned within the slot and connected to the vessel and a locking collar is mounted to the gimbal table. A mating member is attached to the spar buoy. The vessel is maneuvered to bring the spar buoy within the gimbal table opening. With the spar buoy positioned within the gimbal table opening, the locking collar is arranged and designed to releasably attach to the mating member of the spar buoy and restrict relative vertical movement between the spar buoy and the vessel while the gimbal table allows the floating vessel to roll and pitch without driving these motions into the spar buoy.

[0014] From SE2100014 (Moritz) a Wind Power Plant is previously known the object of which is to improv a floating wind power platform. The wind power plant comprises a lightweight construction containing a tower carrying a nacelle and three arms for stabilizing the tower. Each arm consists of two tensional resisting elements that form with part of the tower a triangle. The platform is anchored in deep sea and makes use of the anchoring forces to keep the tensional resisting elements stretched. Each distal end of the arm comprises a float for stabilizing the platform. The tensional resisting elements comprises catenary elements, such as a rope or a wire. In an embodiment the tensional resisting element comprises a beam.

[0015] SUMMARY OF THE INVENTION

[0016] A primary object of the present invention is to seek ways to improve the building and maintenance of a wind power platform at sea. Especially the invention seeks ways to dock two floating object at sea.

[0017] This object is achieved according to the invention by a docking system characterized by the features in the independent claim 1 , or by a method characterized by the steps in the independent claim 9. Preferred embodiments are described in the dependent claims.

[0018] Two floating objects are dockable if their movements are synchronized. This may be cumbersome since the two floating objects comprise heavy weight structures which are freely moveable in the sea. A collision between the two may cause severe damages. A damaged object may have to be taken out of production and may need to be repaired. To be able to dock two floating objects they ought to be stably aligned towards each other and their movement synchronized. However in the sea each floating object has its own pattern of movement caused by hydrodynamic forces. The sea heave causes vertical movements and the waves cause horizontal movements. According to the invention the two objects are aligned by lifting the first floating object by the second floating object. The more the first floating object is lifted the less influence has the sea heave. When the lifting force is greater than the additional buoyancy force caused by the sea heave the influence of the sea heave is eliminated and the first floating object is isolated from the hydrodynamic forces. The first floating object needs not be lifted completely out of the water. It needs only be lifted to a point where the first floating object is isolated from the movement of the sea heave.

[0019] According to the invention the first floating object and the second floating object is docked by help of a docking system comprising a vertical movement system providing vertical forces and a horizontal movement system providing horizontal forces. In an embodiment the vertical movement system comprises lifting means located on the second floating object for lifting the first floating object. In an embodiment of the invention the horizontal movement system comprises retracting means providing forces for shortening the distance between the two floating objects and resilient separating means providing a resilient counterforce to keep the two floating objects resiliently apart. In an embodiment the retracting means comprises a stiff winchable connection between the two floating objects. In an embodiment the resilient separating means comprises anchor cables attached on each distal side of the two floating objects to provide a resilient separating counterforce between the two floating objects. In an embodiment a prestressed anchoring line is formed by the retracting means and the resilient separating means.

[0020] In an embodiment the docking system comprises a locking system. The locking system comprises mechanical attachment means to stably lock the first floating object to the second floating object to form an integrated unit where the lifting means on the second floating object may perform precision activities on the top of the first floating object.

[0021] According to an embodiment of the invention a process of docking involves lifting the first floating object and shortening the distance between the first and second object. In a final stage the first floating object is mechanically locked to the second floating object. In an embodiment the first object is lifted by vertical forces in a vertical movement until it is isolated from the influence of the sea heave. In an embodiment the distance between the two objects is shortened by an active and a resilient reactive horizontal forces in a horizontal movement. In an embodiment the process comprises a vertical movement followed by a horizontal motion. In an embodiment the process comprises a plurality of infinitesimal vertical movements each followed by an infinitesimal horizontal movement. The size of the infinitesimal movements may differ. According to the invention this process prevail until the first floating object is stably positioned in front of the second floating object. In this position the locking of the first floating object to the second floating object will be commenced. A resilient separating counterforce between the floating objects may keep the two floating objects apart during the lifting process.

[0022] In an embodiment of the invention the vertical movement system comprises a crane with a lifting cable mounted on the second floating object. In an embodiment the lifting cable may be attached to the top of the first floating object. In an embodiment the lifting cable is attached to a forklike tool which by hooking the first floating object may lift the first floating object. Preferably the lifting cable comprises a cable such as a catenary element. In an embodiment the a cable may contain resilient means to avoid snatches in the floating objects. In an embodiment the resilient means comprises stroke limiting means. Thus at a predetermined stroke the resilient means is rigidly blocked from further expansion and thus act as a stiff cable.

[0023] A catenary element is flexible in bending but capable of transferring traction forces. Catenary is defined by the curve assumed by a cord of uniform density and cross section that is perfectly flexible but not capable of being stretched and that hangs freely from two fixed points. Thus a catenary element may comprise a cable which will be stiff when stressed.

[0024] In an embodiment the retracting means of the horizontal movement system comprises a stiff connecting cable attached between the two floating objects. The connection cable comprises a winchable catenary element. In an embodiment the connection cable is be shortened by winch activity. In an embodiment the winch means is a part of the second floating object. To keep the two floating objects resiliently apart each floating object is affected by a resilient tension force in an opposite direction. In an embodiment the resilient separating means of the horizontal movement system provides a resilient counterforce affecting the two floating objects in opposite directions. In an embodiment the resilient counterforce is provided by anchor forces attached to a distal side of each floating object. Since anchor lines are long and hanging down in the water they may be reckoned to provide a resilient counterforce. The resilient separating means is holding the two floating objects apart and thus preventing collision. In an embodiment the resilient counterforce may be provided by propeller means of the second floating object. Thus the horizontal movement system comprises on the one hand a retracting force to shorten the distance between the two floating object and on the other hand a force to resiliently keep the two floating objects apart.

[0025] In an embodiment the first floating object comprises a central tower and three arms each anchored by an anchor cable at sea. According to the invention the second floating object is navigating towards the tower of the first floating object such that an anchor cable of the first floating object, the first floating object, the second floating object and its propeller means or its anchor are positioned in a straight line. A retracting means with a predetermined but retractable length is arranged to connect the two floating objects. Having been connected the two floating object is held apart by a resilient counterforce provided by an anchor cable or propeller force of the second floating object. By controlling the propeller speed or adjusting the length of the anchor cable the line of objects will be subjected to a predetermined resilient tensile counterforce. Thus the straight line will become a prestressed straight anchoring line. In an embodiment the straight line is formed by the retracting means and the resilient separating means. In the docking action the retraction force between the two floating object matches the resilient counterforce holding them apart. Thus by hauling in the cable to shorten the distance between the two floating objects the anchor cable must be continuously slacken to allow the two floating objects to mate in a controlled manner. According to the invention the winch means on the second floating object may be used for lifting the first floating object. In this embodiment the horizontal movement system comprises an inclined connecting cable attached between the two floating objects which when winching provides a vertical force component as well as a horizontal force component. In an embodiment the connecting cable is inclined from the first floating object upwards to the second floating object. Thus by inclining the connection cable it may be used for providing both lifting and retracting forces. The connecting cable is attached to a lower part of the first floating object to a winch on the second floating object. In an embodiment the cable is wound over a pully. Both floating objects are held apart by forces from the resilient separating means. When winching the connecting cable a force is created in a somewhat inclined direction towards the second floating object. In this embodiment the force comprises a vertical component for lifting the first floating object and a horizontal component for shortening the distance between the two floating objects. The horizontal component matches the resilient anchor force or the propeller force of the resilient separating means. In case of propeller the resilient counterforce may be controlled by speed adjustment of the propeller. In case of anchor the resilient separating counterforce may be controlled by adjusting the length of the anchor cable in small steps to match the shortening movement caused by the horizontal component of the lifting force.

[0026] The first floating object is lifted by the second object. At first the lifting force is small but increases due to the height position of the first floating object. The more the first floating object is lifted out of the water the more increases the lifting force. Using the winch lifting method the horizontal force component is also increased. In a situation where the lifting cable is inclined 45 degrees the force of the horizontal component is equal to the lifting force. Thus when the retracting force between the two floating object is increasing the force of the resilient separating means must also increase to keep the two floating objects apart and prevent collision. In an embodiment of the winch lifting method according to the invention the resilient counterforce of the resilient separating means is first increased to secure that the two floating objects is held apart and then gradually decrease to make possible the two floating objects to come closer in order to dock. According to the invention the force of the horizontal component of the lifting cable and the resilient counterforce of the resilient separating means is kept in balance. At all timed the distance between the two floating objects is defined by a predetermined distance controlled by either propeller force or anchor force.

[0027] In an embodiment the locking system comprises protruding brackets on the second floating object and locking means to firmly attach the first floating object to the brackets. In an embodiment the locking means comprises articulated arms. In an embodiment the arms are hydraulically maneuverable. In an embodiment the locking system comprises two horizontally protruding bowl-shaped brackets for receiving a cylindric column of the first floating object and embracing means for tightening the column to the bracket. By using two brackets arranged in a vertical line the locked tower is stably attached to the second floating object whereby the crane on the second floating object may perform precision work at the top of the tower. In an embodiment an embracing means comprises a catenary element or loop and a winch for tightening the element. In an embodiment the embracing means comprises articulated arms to push the column against the bracket.

[0028] Isolated from the vertical movement of the sea heave the first floating object is still affected by wave forces in a horizontal direction. However the tower of the first floating object is kept in an upright position by its stabilizing arms. According to an embodiment of the invention the relative horizontal movement between the two floating objects is decreased by the resilient separating system. All items between the anchors of the resilient separating means are kept in a prestressed straight anchoring line. Since the two floating objects are affected by a separating resilient counterforce in a longitudinal direction the stiff connection between them makes the two objects linearly aligned. Thus the affection of longitudinal movement of the waves is decreased. By the prestressed anchoring line also the affection of traversal wave motion is decreased. According to the invention the resilient counterforce is stretching the two floating object and thereby provide a transversal force that keep the objects in the line. Any movement of an object out of the prestressed straight line will be affected by a transversal force due to the prestressing that will cause the object to move into the line. This transversal force will reduce the hydrodynamic horizontal forces affecting the hanging first floating object. Thus the two objects can be docked by shortening the stiff cable between the two objects. By winching a catenary element such as a stiff cable attached between the two floating objects the two floating objects are drawn to each other. When in contact the first floating object is resting against the protruding brackets of the second floating object. A final stage of the docking process is to lock the first object to the second object. Hence by locking the first floating object to the second floating object the two floating objects are unified to an autonomous unit and thus the crane on the second floating object may make precision tasks on the first floating object.

[0029] In an embodiment the lifting means comprises flexible speed capacity where the lifting force may be controlled by sensing the tension of the lifting cable. This will become handy since the tower will experience an oscillating vertical movement caused by the sea heave. Thus when the lifting means senses a slack in the lifting cable the lifting speed will increase to eliminate the slack. This will restrict the movement of the platform. Thus by having the lifting cable stretched at all times no slack appears. No slack will imply no snatching forces that can harm the tower or the rig.

[0030] Before a docking process the two floating objects are positioned stably in the vicinity to each other. When the docking is performed between a three armed floating platform and a square-formed rig a corner of the rig is being positioned between two of the arms of the floating platform. In this embodiment the docking process makes use of the resilience performance of the anchor cable on the floating platform and an opposite propeller force of the rig to keep the two floating objects at a predetermined distance from each other. By adjusting the propeller force the distance between the two objects may be shortened in an controlled manner. The reversable propulsion force may be replaced by a anchor cable.

[0031] In an embodiment of the invention the first floating object may comprise a floating wind power platform with a tower and a plurality of stabilizing arms. In an embodiment each arm consists of an upper catenary element and a lower strut element which form with part of the tower a stabilization triangular construction. Each arm is at its distal end flexibly connected to a balancing body penetrating the water surface. In an embodiment each balancing body comprises a counterweight to keep the catenary element stretched. In an embodiment the floating wind power platform is moored to a mooring system in deep water. The mooring system comprises a plurality of anchor lines which provide resilient counterforces to keep the platform in a geographic determined position. The second floating object may be a semi-submersible rig. In an embodiment of the invention the semi-submersible rig comprises lifting facilities such as a crane. The rig is self-propelled by a plurality of propellers and also contains hydraulic motors for driving tools such as winches. The rig may also comprise a dynamic positioning system. The propeller means or anchor lines may provide a resiling force to keep the rig in a desired position.

[0032] A strut element may comprise a beam, a framework, a lattice structure etc. In an embodiment the strut element is made of steel. In an embodiment the strut element of the first floating object is positioned under water. Thus a strut element, such as a beam, may make use of the Archimedes principle and may also comprise cavities which may be filled with a mixture of water and air to reduce gravity forces. Thereby such a beam may comprise a slender design.

[0033] A balancing body comprises an elongated cylinder flexibly attached to the outer end of the arm. The balancing body is vertically aligned and penetrates the surface of the sea. In an embodiment the balancing body comprises a container made of steel. In an embodiment the balancing body comprises a concrete construction. In an embodiment the container is watertight and filled with a mixture of ballast and gas. The ballast may be water and the gas may be air. In an embodiment the height level in the sea of the balancing body is defined by controlling the mixture of water and gas in the container. Increasing the ballast material may be understood as ballasting the balancing body.

[0034] In a first aspect the object is achieved by a docking system for docking a first floating object anchored at sea with a first anchor with a second floating object comprising a vertical movement system providing vertical forces and a horizontal movement system providing horizontal forces, wherein the vertical movement system comprises lifting means located on the second floating object providing a force for lifting the first floating object, and that the horizontal movement system comprises retracting means containing a connecting cable providing a force for shortening the distance between the two floating objects and resilient separating means encircling the two floating objects providing a resilient counterforce to prevent the two floating objects from colliding.

[0035] In further embodiments the retracting means comprises a retractable connecting cable attached between a tower of the first floating object and a winch on the second floating object. The resilient separating means comprises a first anchor cable connected to the first floating object and a second anchor cable connected to the second floating object. The two floating object, the retracting means and the resilient separating means form an anchoring line which when prestressed keep the first and the second floating object apart to prevent collision. The connecting cable is inclined from the first floating object to the second floating object providing an inclined force, whereby the retracting means comprises the horizontal force component of the inclined force and the lifting means comprises the vertical force component of the inclined force. The vertical movement system comprises a crane and a lifting cable attached to the top of the tower. The vertical movement system comprises a crane and a lifting cable attached to a fork-like tool for lifting the platform by the tower. The docking system comprises locking means containing protruding brackets on the second floating object and articulated arms for firmly embracing the tower.

[0036] In a second aspect the object is achieved by a method of docking a first floating object anchored at sea with a first anchor with a second floating object, the method comprises connecting the two floating objects with a retracting means providing a force for shortening the distance between the two floating objects, encircling the two floating objects with a resilient separating means providing a resilient counterforce, forming an anchoring line comprising the first anchor the first floating object and the second floating object, prestressing the anchoring line to prevent the two floating objects from colliding, lifting the first floating object by a lifting means located on the second floating object, shortening the distance between the two floating objects by the retracting means, and locking the first floating object to the second floating object.

[0037] BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the present invention will become more apparent to a person skilled in the art from the following detailed description in conjunction with the appended drawings in which: fig 1. is a plane view of a floating platform and a semi-submerged rig with a docking system according to the invention, fig 2. is a side view of the floating platform and a semi-submerged rig with a docking system according to the invention, fig 3. is a plane view of a floating platform and a semi-submerged rig with a docking system according to a second embodiment of the invention, fig 4. is a side view of the floating platform and a semi-submerged rig with a docking system according to the second embodiment of the invention, fig 5. is a plane view of a floating platform and a semi-submerged rig with a docking system according to a third embodiment of the invention, and fig 6. is a side view of the floating platform and a semi-submerged rig with a docking system according to the third embodiment of the invention.

[0038] DESCRIPTION OF PREFERRED EMBODIMENTS

[0039] A docking system 29 for docking a first floating object 1 with a second floating object 18 is depicted in fig 1 - 6. In the embodiments shown the first floating object comprises a floating wind power platform 1 and the second floating object comprises a semisubmersible rig 18. The floating wind power platform comprises a semi-submersible platform containing a tower 2 and three arms 3. Each arm consists of a beam 4 and a catenary element 5 between the tower and the distal end 6 of the beam. As depicted in fig 2 the beam and the catenary element form with part of the tower a triangle to stabilize the tower. The beam and the catenary element are flexibly connected to the tower such that the arms are foldably connected to the tower. The distal end of the beam is flexibly connected to a balancing body 7 which in turn is flexible connected by a mooring link 14 to a mooring buoy 8 of an anchor system represented by a first anchor cable 9. Due to symmetry only one arm is described. Thus the floating platform is stably moored by three mooring buoys and kept in a geographic positing by the anchor system. The three mooring buoys are connected with each other by a connecting line 15 to form a mooring unit 17. The connecting line 15 is positioned below the sea surface 16 at a sufficient depth in the water to provide a draught allowing floating vessels to pass.

[0040] At the bottom of the tower the floating platform comprises a buoyancy element 10 to provide floating capacity to the platform. The buoyancy element comprises a container fillable with a mixture of water and air to ballasting the platform. In the embodiment shown the buoyancy element comprises a tri star-formed design in order not to interfere with the flexibility of the arms. In the embodiment shown the balancing body 7 comprises an elongated cylindric container for balancing the platform according to Archimedes principle. At its lower end the balancing body comprises a counter weight 11 to keep the catenary elements 5 stretched. In an embodiment the counter weight comprises iron ore. In an embodiment the balancing body comprises a container fillable with a mixture of water and air for ballasting its height in the sea. The platform 1 comprises a star-formed top bracket 12 to flexibly connect the catenary elements to the tower. The platform comprises a star-formed bottom bracket 13 to flexibly connect the beams to the tower. Due to symmetry only one arm arrangement is described.

[0041] The semi-submersible rig 18 comprises a large deck 19 to provide working capacity and for keeping spare parts and maintenance equipment. The rig comprises a crane 20 to perform lifting actions. The rig also comprises a plurality of winches 21 to perform lifting actions or tightening actions. The rig comprises propeller means 22 to make the rig self-propelled. The propeller means may comprise a plurality of propellers and thruster means. Preferably the rig contains an automatic positioning system. In the embodiment shown the rig comprises an upper body 24 and a lower body 25 connected with each other by a plurality of columns 26 thereby providing semisubmersible capacity.

[0042] In the embodiment shown the rig is anchored with a second anchor cable 23 and connected with the platform by a connecting cable 36. In the embodiment the connecting cable comprises a stiff retractable cable. The rig is anchored such that the rig and the tower is oriented in line with the first anchor cable 9. An anchor cable is long and hanging down in the water and thus may represent a spring element 30 being part of the anchor cable 9. Since an anchor cable is a resilient element the two anchor cables comprise a resilient separating means 37 which keeps the platform and the rig apart. In the embodiment the first anchor cable 9, the floating platform 1 , the connecting cable 36, the rig 18 and the second anchor cable 23 form a straight anchoring line 28. By stretching the anchoring line the platform and the rig is held apart in order not to collide. The stretching may be performed by hauling in the second anchor cable or reversing the propeller force. The stretching of the anchor cables provide a traction force of the anchoring line 28 which will keep the two floating objects in a prestressed straight line and thereby reducing the influence of horizontal wave forces on the anchored floating objects.

[0043] An embodiment of the docking system is depicted in in fig 1 and 2. The docking system 29 comprises lifting means 31 for lifting the tower 1 , retracting means 35 for shortening the distance between the tower 1 and the rig 18 and may include locking means 38 to mechanically join the tower and the rig. The lifting means 31 contains a lifting cable 32 operated by the crane 20. In the embodiment shown in fig 1 and 2 the lifting cable 32 is attached to the top of the tower 2 of the floating platform 1. The retracting means 35 comprises a connecting cable 36 between the tower and a winch 21 on the deck 19 of the rig. In the embodiment shown the retractable stiff cable comprises a catenary element.

[0044] The locking means comprises a first 40 and second 41 protruding bracket on the rig. In the embodiment shown the rig comprises an upper body 24 and a lower body 25 connected with each other by a plurality of columns 26. In the embodiment the first bracket 40 is located on the upper body 24 of the rig and the second bracket 41 is located on the lower body 25. The first an the second bracket are oriented in a vertical line to make the tower aligned in an upright position. In the embodiment shown the locking system further comprises a locking means in the form of articulated arms 39 to firmly hold the tower of the floating platform rigidly to the rig.

[0045] Affected by the sea heave the platform 1 will be moving in a vertical direction.

[0046] According to the invention the platform will be isolated from the influences of the sea heave motion by lifting the platform sufficiently. Thus by using the lifting means 31 the tower is lifted high enough to be isolated from the sea motion. In an embodiment the lifting means comprises flexible speed capacity where the lifting force may be controlled by sensing the tension of the lifting cable. This will become handy since the tower will experience an oscillating vertical movement caused by the sea heave. Thus when the lifting means senses a slack in the lifting cable the lifting speed will increase to eliminate the slack. This will restrict the movement of the platform. Thus by having the lifting cable stretched at all times no slack appears. No slack means no snatching forces that can harm the tower or the rig. Further lifting will step by step increase the lifting force until the platform is isolated from the motion of the sea heave.

[0047] Lifted and isolated from the sea heave the tower of the platform will be hanging like a pendulum. Since the bottom part of the tower is still in the water it will be affected by horizontal wave forces. According to the invention the influence of these horizontal wave forces may be reduced by prestressing the anchoring line 28. The prestressed anchoring line 28 will provide a longitudinal force that will decrease the influence of the wave forces on the tower. In the embodiment the prestressed anchoring line 28 comprises the contracting means 35 and the resilient separating means 37. The contracting means 35 comprises a retractable connecting cable 36 to make possible the shortening of the distance between the tower and the rig. The resilient separating means 37 comprises the first anchor cable 9 and the second anchor cable 23. The prestressing may be accomplished by shortening the second anchor cable or reversing the propeller force. By prestressing the anchoring line 28 the tower and the rig will be vertically aligned in front of each other. Being stably positioned by the prestressed anchoring line the tower will be prevented from movement caused by the waves. Any wave forces back and forth in the longitudinal direction will be decreased by the prestressed force of the connecting cable 36 between the tower and the rig.

[0048] According to the invention the prestressed anchoring line 28 will orient all objects in a stretched straight line. The straight line is created by the prestressed force of the anchoring line. If an object moves outside this line it will experience a transversal force that will make the object move back into the line. In the embodiment shown the prestressing is preferable performed by shortening the second anchor cable 23. To perform a distance shortening action the connecting cable is winched to bring the two floating objects closer while simultaneously the prestressed anchor cable 23 is continuously slackened.

[0049] The connecting cable 36 is very stiff and fully determining the distance between platform 1 and the rig 18. In an embodiment the distance shortening process can be performed by attaching the connecting cable 36 or a loop to the tower and slowly winch the tower towards the rig. When close to the rig the tower will be steered into two bowl-formed protruding brackets 40 and 41 of the rig. Caught to the brackets the tower is locked to the rig by locking means such as a pair of articulated arms 39. The final result is a mating of the two floating objects 1 and 18, so they become one single unit without any relative motion. At this state the lifting crane can perform heavy lifts of components with high spatial accuracy.

[0050] In an embodiment of the docking system according to the invention the winch is used to lift the tower. As depicted in fig 3 and 4 the lifting means 31 comprises an inclined connecting cable 36 and a winch 21 on the rig 18. The connecting cable is attached to a lower part 43 of tower 2. The connecting cable is preferably attached to a point on the tower 2 at the same vertical coordinate as the mooring link 14 is attached to the balancing body 7. This eliminates a tilting moment on the tower. The other end of the lifting cable is attached to the winch 21 on the rig 18. In the embodiment shown the winch is positioned on the underside of the upper body 24 of the rig. In the embodiment the connecting cable is laid over a pully 27 and wound on the winch. As can be seen in fig 4 the connecting cable is somewhat inclined towards the rig.

[0051] The force provided by the inclined connecting cable 36 comprises a vertical component and a horizontal component. The vertical component is equal to the force of the lifting means 31. The horizonal component is equal to the force of the retracting means 35. Thus in this embodiment the lifting means 31 comprises the vertical force component of the inclined connecting cable 36. Thus in this embodiment the retracting means 35 comprises the horizontal force component of the inclined connecting cable 36. As previously described the platform and the rig is arranged in a prestressed anchoring line 28. The prestressed anchoring line comprises the retracting means 35 and the resilient separating means 37. Thus the distance shortening process as well as the lifting process is accomplished by pulling the inclined connection cable. The lifting movement is determined by the vertical force component of the connecting cable. The horizontal movement is determined by the horizontal force component of the connecting cable and the resilient counterforce of the second anchor cable 23 when continuously slackened. Also in this embodiment the winch means comprises flexible speed capacity where the lifting force is controlled by sensing the tension of the lifting cable. Thus no slacks appear int the lifting cable.

[0052] An embodiment of a docking system according to the invention is shown I fig 5 and 6. The docking system according to the invention is also useable to perform maintenance operation at site with a built up platform. In this embodiment the lifting means is attached at the side of the tower to allow a sufficient space for the lifting cable alongside the tower. A specially designed elongated fork-like tool 44 is used. The tool comprises a preferably horizontally aligned fork with two prongs 45. The opening of the fork is somewhat bigger than the diameter of the tower 2. In the embodiment the tool is hooked under the top bracket 12 of the tower. The lifting cable 32 of the lifting means 31 is attached to a lifting point 46 of the tool 44 in close vicinity of the tower. The distal end 47 of the tool 44 is attached by a counter cable 48 to a maneuvering position 49 on the deck 19 of the rig.

[0053] In the embodiment shown the lifting means may contain a resilient means in the form of a spring 33 to reduce snatches in the lifting cable 32. In the embodiment the spring comprise stroke limiting means 34. Thus at a predetermined stroke the resilient means is rigidly blocked from further expansion and thus act as a stiff cable.

[0054] The lifting operation is performed in the same way as shown in fig 1 and 2. Thus the lifting operation is performed by lifting the tower by help of the tool to a point where the tower is isolated from the sea heave. The distance reducing operation is performed by the help of the fork-like tool 44. As soon as the platform has been lifted and the tower is hung on the prongs 45 of the fork-like tool 44 the fork-like tool may be maneuvered by the counter cable 48 in all horizontal directions in order to move the tower in contact with the protruding brackets 40 and 41. As described above the tower is locked to the rig by embracing the tower with locking means 38 such as articulated arms 39.

[0055] The docking operation according to the invention may be performed on large structures. In an embodiment the rig comprises a used rig for oil drilling. Such a rig may be 40 m high and have a deck in order of 50 by 40 meters. The floating platform may be very big. According to the invention the total height of the tower including the buoyancy element may be 130-200 m. The length of an arm may be in the range of 80- 150 m. Hence the ratio between the arm and the tower would be almost one. The height of the balancing body may be in the range of 20-70 m and the diameter of the mid portion in the range of 5-12 m.

[0056] Although favorable the scope of the invention must not be limited by the embodiments presented but also contain embodiments obvious to a person skilled in the art. For instance the platform could comprise more than three stabilizing arms. The lifting operation may be performed by ballasting or deballasting the platform and the rig respectively. The resilient element may comprise rubber cables with sufficient stroke. The protruding brackets may comprise rubber lining as also the locking elements.

Claims

CLAIMS1 . A docking system (29) for docking a first floating object (1 ) anchored at sea with a first anchor (9) with a second floating object (18) comprising a vertical movement system providing vertical forces and a horizontal movement system providing horizontal forces, c h a ra cte ri zed i n that the vertical movement system comprises lifting means (31 ) located on the second floating object (18) providing a force for lifting the first floating object (1), and that the horizontal movement system comprises retracting means (35) containing a connecting cable (36) providing a force for shortening the distance between the two floating objects and resilient separating means (37) providing a resilient counterforce to prevent the two floating objects from colliding.

2. A docking system according to claim 1 , wherein the connecting cable (36) is attached between a tower (2) of the first floating object (1 ) and a winch (21 ) on the second floating object (18).

3. A docking system according to claim 1-2, wherein the resilient separating means (37) comprises the first anchor cable (9) connected to the first floating object (1 ) and a second anchor cable (23) connected to the second floating object (18).

4. A docking system according to any of the preceding claims, wherein the two floating object, the retracting means (35) and the resilient separating means (37) form an anchoring line (28) which when prestressed keep the first (1 ) and the second (18) floating object apart to prevent collision.

5. A docking system according to any of the preceding claims, wherein the connecting cable (36) is inclined upwards from the first floating object (1 ) to the second floating object (18) providing an inclined force, whereby the retracting means (35) comprises the horizontal force component of the inclined force and the lifting means (31) comprises the vertical force component.

6. A docking system according to claim 2, wherein the vertical movement system comprises a crane (20) and a lifting cable (32) attached to the top of the tower (2).

7. A docking system according to any of the claims 2-5, wherein the vertical movement system comprises a crane and a lifting cable attached to a fork-like tool (44) for lifting the platform (1) by the tower (2).

8. A docking system according to any of the claims 2-7, wherein the docking system comprises locking means (38) containing protruding brackets (40, 41 ) on the second floating object (18) and articulated arms (39) for firmly embracing the tower (2).

9. A method of docking a first floating object (1 ) anchored at sea with a first anchor (9) with a second floating object (18), c h a ra cte ri ze d by connecting the two floating objects with a retracting means (35), forming an anchoring line (28) comprising the first anchor (9) the first floating object (1 ), the retracting means (35), and the second floating object (18), prestressing the anchoring line (28) to prevent the two floating objects from colliding, lifting the first floating object (1 ) by a lifting means (31) located on the second floating object (18), shortening the distance between the two floating objects by the retracting means (35).

10. Using the docking method according to claims 1-9 or a method according to claim 10 for docking a floating wind power platform with a semi-submersible rig.

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