Patent Description:
Crane operations at sea are challenging, especially when the unit on which the operations are to be done is a floating unit. There are in principle two ways to do this. The first is to deploy a vessel having a suitable crane next to the floating unit and use this crane to move objects between the vessel (or a separate vessel) and the floating unit. The second way is to arrange a crane on the floating unit and use this crane to move objects between a support vessel and the floating unit as well as performing operations, such as installations on board the floating unit.

In the first alternative, all operations have to be done while both the vessel and the unit are moving due to wave motions. This requires sophisticated heave compensation and can only be done during relatively calm conditions.

The second alternative requires heave compensation only during loading or unloading of the vessel, but it requires the installation of a crane on board the floating unit having the wind turbine.

There are several examples of a floating wind turbine having a crane installed thereon:.

<CIT> shows a wind turbine where a crane is temporarily attached to the tower of the wind turbine. The crane is capable of moving up and down along the tower.

<CIT> shows a similar crane that is capable of installing further tower sections on top of the tower section to which the crane is attached.

<CIT> also shows a crane that is attached to the tower of the wind turbine and is capable of moving up and down the tower. The crane is adapted to replace the blades of the wind turbine.

<CIT> shows yet another crane that is attached to a tower structure.

<CIT> shows a wind turbine where a small service crane is arranged inside the nacelle. A hatch is opened through which the crane can emerge when there is a need for the service crane.

<CIT> shows a larger service crane arranged on top of the nacelle.

<CIT> shows examples where a crane is arranged on a support vessel, but the support vessel is attached to the foundation of the wind turbine.

<CIT> shows a support vessel where at least a portion of a sole crane is lifted onto a support structure above the vessel. The system compensates for the relative motion between the vessel and the support structure.

<CIT> relates to a method for installing a crane on a portion of an offshore wind turbine generator from a vessel, where a removable crane adapter having a first coupling to the portion of the offshore wind turbine generator.

In all of the known solutions, except for <CIT>, the crane is arranged on the tower of the wind turbine. This means that the tower must be designed to carry both the crane and the load of the crane. It also must have an outer surface to which the crane can be attached. Moreover, the reach of the crane is limited by this placement, and care must be taken so that the crane does not interfere with the rotor and the blades.

If the crane is not permanently attached to the tower, it will be cumbersome to install and remove the crane each time it is needed. If it is permanently attached, maintenance is a challenge.

It is therefore a need of a crane that is more versatile, and which can be used on a wide range of floating wind turbines without the tower being designed to carry the crane. It is also a desire that the crane is easy to install without requiring a particularly heavy-lift crane.

Further, <CIT> describes a process for installing an offshore tower, where a gravity-based structure is resting on the sea floor, and a crane is placed on this structure. This publication does not describe a semi-submersible sub-structure floating in the water. Further, since the crane is placed on the gravity-based structure, this solution does not have take into account any relative movements between the crane and the substructure, caused by wave motion.

These objectives are achieved by an operations and maintenance arrangement for floating wind turbines, comprising a floating sub-structure on which at least one wind turbine unit is situated, a service operation vessel and a portable crane, said floating sub-structure having an interface capable of receiving and fixedly locking said crane to said sub-structure, said service operation vessel having a ship crane capable of lifting said portable crane from said vessel and onto said sub-structure.

According to the invention the arrangement comprises a support to receive components, such as wind turbine blades or gear units, to be installed on the wind turbine, said support having an interface to mate and fixedly lock with an interface on said floating sub-structure. This ensures that the portable crane may lift the components without having to take relative movement due to wave action into account.

If the component is wind turbine blades, it is preferred that it is arranged in a cradle, and if there are several blades, that they are stacked in a multiple of cradles.

In the case of the component being several blades, it is preferred that they are stacked in height in the cradles. This takes up less space on the deck of the support vessel and the stack can be lifted as a unit.

Conveniently, the support is a truss beam, which results in low weight but high strength.

If the floating sub structure has at least two pontoons, one pontoon can receive the portable crane and the other the support with the components.

By remotely operating the portable crane from the support vessel, the risk of having personnel on board the floating wind turbine is eliminated.

According to a method of replacing components, such as wind turbine blades or gear units, of a floating wind turbine, it comprises the steps of deploying a service operation vessel next to a floating wind turbine, lifting, using a ship crane on said vessel, a portable crane from said vessel onto a sub-structure of said floating wind turbine, fixedly locking said portable crane to said sub-structure, lifting component support onto said sub-structure, fixedly locking said component support to said sub-structure, lifting a component to be replaced from said wind turbine onto said component support, using said portable crane, lifting said component from said component support onto said vessel, lifting a new component onto said component support, lifting said new component onto said wind turbine and attaching it to where said component to be replaced used to be, lifting said empty support onto said vessel, and lifting said portable crane onto said vessel, using said ship crane.

In a preferred embodiment, adapted for replacement of wind turbine blades, it comprises the steps of lifting worn blades one by one from a rotor of said wind turbine onto said support, using said portable crane, lifting a stack of said worn blades from said blade support onto said vessel, lifting a stack of new blades onto said blade support, and lifting said new blades one by one to said rotor, using said portable crane.

The invention will now be described in further detail by an exemplary embodiment shown in the enclosed drawings, in which:.

Although the following description of an embodiment describes in detail a replacement of turbine blades, it should be understood that the arrangement and method can be used for other types of maintenance, such as replacement of gear units or other components having a weight that does not exceed the lifting capacity of the portable crane. The portable crane may, e.g., have a lifting capacity of <NUM> metric tons.

In <FIG> is shown a floating wind turbine unit <NUM>. It comprises a floating sub-structure <NUM> and a wind turbine <NUM>. The floating substructure is triangular in shape and comprises three cylindrical pontoons <NUM>, <NUM>, <NUM>, which are connected by tubular beams <NUM>. The wind turbine <NUM> has a tower <NUM>, which is attached to one of the pontoons <NUM>, and a rotor <NUM> with blades <NUM>.

The shown wind turbine unit is only an example. It may have other designs, such as the one shown in co-pending <CIT>. The only requirement is that it can carry a crane, which will be described below.

Next to the floating wind turbine unit <NUM> (hereafter just referred to as "unit") is a support vessel <NUM>. The vessel <NUM> is equipped with a ship crane <NUM>, which is permanently attached to the vessel <NUM>. It also has a cargo area <NUM> on the deck. As can be seen, the cargo is initially a portable crane <NUM>, a truss beam <NUM>, a first set of empty blade cradles <NUM> and a second set of blade cradles <NUM> with blades <NUM> which will all be explained further below.

The vessel <NUM> can be anchored next to the unit or may be held in position by DP (dynamic positioning).

<FIG> show step by step an operation to replace the blades <NUM> of the wind turbine <NUM> with new blades <NUM>. This is an operation that has to be done at intervals as the blades get worn by erosion from wind, rain, ice, heating and cooling, insects, birds and lightning strikes. Depending on the local conditions, the blades have to be replaced every <NUM>-<NUM> year.

<FIG> shows a first step in the blade replacement operation. The ship crane <NUM> has started to lift the portable crane <NUM> from the cargo deck <NUM>.

In <FIG> the portable crane <NUM> has been placed on top of one of the pontoons <NUM>. The pontoon <NUM> has been equipped with an interface that readily can mate with the underside of the crane <NUM>. The crane <NUM> is securely locked onto the pontoon <NUM> via the interface. How the interface and locking means are designed will be a choice to the person of skill. Many options are readily available in the field.

The portable crane may have its own power supply, such as by an onboard battery or generator, or a cable may extend from the vessel <NUM> to the crane <NUM>. The crane <NUM> is conveniently remotely operated from the vessel <NUM>. The operator may then be at a safe distance and follow the operation of the crane via cameras installed on the crane <NUM>.

With the crane <NUM> in place on the unit <NUM> it can be used to lift items from the vessel <NUM> to the unit. However, the crane <NUM> on the vessel may also be used, if it is more convenient.

<FIG> shows a truss beam <NUM> having been lifted off from the vessel and installed on top of a pontoon <NUM>. The interface between the beam <NUM> and the pontoon <NUM> can be the same as the interface between the crane <NUM> and the pontoon <NUM>. On top of the beam <NUM> a first cradle comprising two supports 18a and 18b.

Now the removal of the work blades <NUM> can start. In <FIG> a first blade 10a has been lifted off from the rotor <NUM> by the crane <NUM> and placed in the cradle 18a, 18b. Next a new cradle comprising supports 18c, 18d is placed on top of the first cradle, and a second worn blade 10b is lifted off the rotor <NUM> by the crane <NUM> and placed in the cradle 18c, 18b, as shown in <FIG>.

The last blade 10c is lifted and placed in a cradle in the same way as the previous blade 10b, so that, as shown in <FIG>, all blades are orderly stacked on top of the beam <NUM>.

As shown in <FIG>, the stack of blades 10a, b, c is lifted off from the beam <NUM>. This can be done by the ship crane <NUM>, or by the portable crane <NUM>. It is, however, in general more convenient to use the crane that is on the unit where the load is to be placed. This is because it is easier to time the lifting off of the load than the putting down with the wave movements.

In <FIG> the stack of work blades 10a, b, c has been placed on the cargo deck <NUM> of the vessel <NUM>.

Now the stack of new blades <NUM> is lifted from the vessel <NUM> and placed on the truss beam <NUM>. Now the portable crane can lift the blades <NUM> to the nacelle for attachment. As each blade <NUM> is lifted, the cradles on which it was resting are lifted on board the vessel <NUM>.

In <FIG>, the new blades <NUM> have all been installed. Now the truss beam <NUM> is lifted on board the vessel. When this has been completed, the crane <NUM> itself is released and lifted on board the vessel by the ship crane <NUM>. The operation is now completed, as shown in <FIG>. The vessel <NUM> can now travel back to port to unload the worn blades <NUM>, and if needed, take a new stack of blades on board for replacement of blades of another wind turbine.

The truss beam <NUM> may of course be replaced by another support that reliably supports the blades, such as a platform. The blades may also be arranged side by side on the support. However, stacking is more convenient as it saves space on the deck.

Claim 1:
An operations and maintenance arrangement for floating wind turbines, comprising a floating sub-structure (<NUM>) on which at least one wind turbine unit (<NUM>) is situated, a service operation vessel (<NUM>) and a portable crane (<NUM>), said floating sub-structure (<NUM>) having an interface capable of receiving and fixedly locking said crane (<NUM>) to said sub-structure (<NUM>), said service operation vessel (<NUM>) having a ship crane (<NUM>) capable of lifting said portable crane (<NUM>) from said vessel (<NUM>) and onto said sub-structure (<NUM>), characterized in that said arrangement further comprising a support to receive components, such as wind turbine blades (<NUM>, <NUM>) or gear units, to be installed on the wind turbine (<NUM>), said support having an interface to mate and fixedly lock with an interface on said floating sub-structure (<NUM>).