The invention relates to a vessel provided with a retractable thruster. Vessels with retractable thrusters are known, wherein one or more thrusters can be extended up to about 4 metres below keel level of the vessel. These thrusters are often used for dynamic positioning purposes and for maintaining the vessel in a fixed orientation with respect to prevailing wave and current directions. However, while sailing to a working location or while entering a harbour, the positioning thrusters are retracted flush with keel level of the vessel in order to decrease the water resistance and to not interfere with the sea bed in shallow waters.

The known retractable thrusters are relatively difficult to inspect and to maintain as maintenance can only be carried out in a dry dock or by means of divers.

Furthermore, the known thrusters, which may comprise an integrated containerized unit having a drive control unit, a propeller, cooling etc., take a long time to install and can only be mounted with relative difficulty.

It is a purpose of the present invention to provide for a retractable thruster that can be easily installed and on which maintenance can be carried out in an easy manner. It is a further object of the present invention to provide a system that is particularly suited for conversion of existing vessels to vessels comprising one or more thrusters for dynamic positioning purposes.

Hereto the vessel according to the invention is characterised in that the thruster is connected to a lifting device for retracting the thruster in two stages, the lifting device in the first stage lifting the thruster to a level at or near the keel of the vessel, the second stage comprising lifting the thruster to an access level, located above the first level. By mounting the thruster for instance through a hole in the vessel (moonpool-like), the thruster can be retracted in the first stage to be flush with the hull of the vessel for purposes of entering shallow waters or for sailing the vessel at high speed. In case maintenance is necessary, the thruster is lifted to the second level for easy access via for instance a tween deck. By mounting the thruster, that is preferably connected to a frame or container, in a hole (moonpool-like) in the vessel, a relatively simple construction is obtained wherein the thruster can be put in position when the vessel is already afloat. Hereby relatively short construction times can be achieved.

In one embodiment, the vessel or the frame of the thruster is provided with a

rack. The other one of the vessel or the frame comprises a pinion movable along the rack, wherein in the second stage, the rack is extended by means of a separate rack extension means which is attachable to the rack. By means of for instance a double- toothed rack which is connected to the vessel and by pinions connected to the thruster frame, the thruster can be retracted to keel level. For maintenance purposes the rack extension means is temporarily connected to the main deck so that the track for the pinions is extended and the thruster can be displaced along the second stage up to the level of a tween deck for maintenance purposes. A temporary workfloor can be constructed across the shaft in the vessel wherein the thruster can be displaced for access of a maintenance crew to the thruster.

In an alternative embodiment, the lifting means are comprised of hydraulic cylinders. The frame of the thruster is detachably coupled to the cylinders. The thruster can be lifted to the first stage at for instance a full stroke of the cylinders, and thereafter be decoupled therefrom. Then the cylinders are extended and connected to the frame at a lower position. Upon retraction of the cylinders in the second stage, the thruster can be further retracted into the vessel, wherein the frame may come to extend above the main deck.

The invention will be explained in detail with reference to the accompanying drawings. In the drawings: Figure 1 shows a top view of a moonpool according to the invention wherein a container or frame comprising a thruster may be mounted, Figure 2 shows the first embodiment of a retractable thruster comprising a rack and pinion in the working position, Figure 3 shows the thruster according to figure 2 in the first stage of retraction, Figure 4 shows the thruster according to figure 2 in the second stage of retraction, Figure 5 shows the sealing arrangement of the frame on an enlarged scale, Figure 6 shows a retractable thruster comprising hydraulic lifting means in the working position, Figure 7 shows the thruster of figure 6 in the first stage of retraction, Figure 8 shows a displacement of the coupling ring along the frame of the thruster,

Figure 9 shows the thruster according to figure 6 in a further retracted position, Figure 10 shows the thruster of figure 6 in the second stage of retraction for inspection or maintenance purposes, and Figures 10 and 11 show the locking mechanism for the frame in the locked and unlocked position, respectively.

Figure 1 shows a top view of a deck 1 of a vessel comprising a shaft or moonpool 2 in which a thruster can be placed. Along two double-toothed racks 3,3', two sets 4,4' of each four electrically driven pinion drives can be vertically displaced.

As shown in figure 2, the thruster 5, as well as its drive motor, gear mechanism and rotational drive unit 5'are connected to a frame 6. In figure 2, the thruster 5 extends below keel level 7. The pinion drives 4,4' are connected to the top part of the frame 6.

A locking device, as is shown in figure 11, is incorporated at the top end of the frame 6, preferably integrated with the pinion drives 4,4', for fixing the frame 6 in a play- free position with respect to the vessel. Hereto the racks 3,3' are at regular positions provided with holes 3a, 3b, into which a locking pin of the locking device may be inserted.

The racks 3,3' extend up to deck level 8. In the working position, the frame 6 is in its lowest position with respect to the shaft 2 and is supported on support members 9,9' connected to the tween deck lO. The upper part of the frame 6 has a radially extending support ring 11 for co-operating with the support members 9,9'. An axial seal 23, as shown in more detail in figure 5, is comprised between the support members 9,9' and the support ring 11. The inner wall of the shaft 2 is near the upper part provided with radial seals 16,16'. The frame 6 comprises at the position corresponding to the position of the radial seals 16,16'in figure 2, two upper radial sealing projections 17,17', and two lower radial sealing projections 18,18'. These radial sealing projections are shown in more detail in figure 5. When the frame 6 is retracted upwardly, the lower radial sealing projections 18,18' are brought into engagement with the seals 16,16', while the upper radial sealing projections 17,17' are moved upward, as shown in figure 3.

Figure 3 shows the vertical displacement of the pinion drives 4,4' along the rack 3,3' to retract the thruster 5 to the first stage wherein the thruster is flush with keel level 7.

Figure 4 shows a temporary rack extension means 12 connected to the racks 3,3' for displacing the thruster 5 to a maintenance or inspection position. In this position a temporary workfloor 13 can be positioned on the support members 9 for allowing access to the thruster 5 by a maintenance crew.

Figure 5 shows in more detail the construction of the axial seal 23 which is connected to a support surface 22 of the support member 9. In the lowest position of the frame 6, wherein the thruster 5 is placed below keel level, the support ring 11 rests on the rim 25 of the support surface 22 and compresses the axial seal 23, such that water is prevented from entering via the space between the shaft wall 19 and the frame wall 20.

Preferably, the frame 6 is formed by a closed container having buoyancy. On the shaft wall 19, the radial sealing element 16 is comprised, which is compressed by the upper radial projection 17 on the frame wall 20. When the frame 6 is retracted upwardly, the slanting surfaces of the projection 17 slide past the sealing element 16, which expands radially outwardly when no longer radially restricted by the protrusion 17. In its expanded state, the sealing element 16 is provided with slanting surfaces, such that the lower projection 18 can be slid into engagement with the sealing element 16 upon moving the frame 6 further upward, until the sealing element 16 is again fully compressed by the lower projection 18.

Figure 6 shows an embodiment wherein the lifting means of the thruster 5 are formed by hydraulic cylinders 14,14'. The frame 6 of the thruster 5 is mounted in a ring 15 and is connected thereto by means of separate connection pins or bolts and holes in the frame 6. When the cylinders 14,14' are completely retracted as shown in figure 7, the thruster 5 is located at keel level 7. For further lifting the thruster, the ring 15 is disconnected from the frame 6 and is slid along said frame by extending the cylinders 14,14' until the ring 15 abuts against the support members 9,9'. At the position shown in figure 8 the ring 15 is reattached to the frame 6 which is thereto provided with a second set of holes for connecting to the coupling pins of the ring 15.

Figure 9 shows a further retracted position of the thruster 5. In this position again the frame 6 is fixed relative to the vessel whereafter the ring 15 is disconnected from the frame 6 and slid to the lowest position of the frame 6 to be reattached in that position. Thereafter the cylinders 14,14' are retracted for the last time so that the frame 6 comes to extend above deck level 8, as shown in figure 10. The lifting system is designed to be able to lift the thruster which above water level, including the weight of the container or frame 6, has a weight of about 100 tons.

Figures 11 and 12 show the locking device 30, which is integrated with the pinion drive 4, in the locked and unlocked positions, respectively. The locking pin 31 is via a lever 32 displaceable from and towards the rack 3, and can be entered into a hole or recess 3a in the rack 3. The electric drive motor of the pinion drive 4 is mounted in the same housing as the locking pin 31. On the housing, a first arm 33 and a second arm 34 are hingingly connected. The first arm 33 can be moved by means of a spindle 36, which engages a threaded bush 37 at the end of the arm 33. The spindle 36 may be manually operated, for instance by means of a hand wheel. By raising the arm 33, the arm 34 is pressed with its end into engagement with a curved pressing surface 35 of the rack 3, such that the locking device 30 is pressed downward with respect of the rack 3. As the frame 6 carrying the thruster 5 has buoyancy, and is preferably formed as a closed container, the downward force exerted by the locking device 30 eliminates any play in the suspension of the frame within the shaft 2.