WO2008040924A1

WO2008040924A1 - Floatable dry docks

Info

Publication number: WO2008040924A1
Application number: PCT/GB2006/003742
Authority: WO
Inventors: Donald Scott Thom
Original assignee: Welcome Inn Investments N.V.
Priority date: 2006-10-06
Filing date: 2006-10-06
Publication date: 2008-04-10

Abstract

The present invention provides a floatable dry dock (10) and a water-plane tower (22) for a floatable dry dock and a method of raising a vessel in a floatable dry dock. Previous floatable dry docks have been found to be particularly unstable when a pivotally mounted lifting cradle (11) used to lift a vessel becomes submerged. The present invention provides a floatable dry dock that has a cradle with at least one stabilising portion (21, 22) that remains above the water when the remainder of the cradle is submerged to receive a vessel. The at least one stabilising portions provide water-plane for stabilising the cradle. The at least one stabilising portion may comprise a water-plane tower.

Description

Floatable Dry Docks

This invention relates to floatable dry docks for use in lifting vessels out of the water for maintenance or repair purposes. Typically these types of docks can lift anything from one to several hundred tonnes.

An existing type of floatable dry dock comprises a lifting cradle attached to a catamaran, which lifting cradle can be pivotally lowered to receive a vessel and raised to lift the vessel out of the water. The cradle may be raised and lowered by respectively increasing and decreasing the buoyancy of the lifting cradle by flooding and emptying buoyancy tanks of the lifting cradle with water. The buoyancy tanks are emptied of water by filling the tanks with compressed air. During lifting and lowering of the vessel the combined "water plane" of the vessel, catamaran and cradle remains reasonably constant and is not dramatically affected by the reduction in "water plane" from the hull of the vessel being lifted out of the water.

Surprisingly, however, it has been found that such a system is actually unstable when the lifting cradle is submerged under water. A small decrease in the buoyancy of the cradle causes the cradle to lower in an unstable manner. Once the cradle has been lowered slightly, it experiences greater water pressure acting upon it. This causes the cradle to be compressed and its density increases. Hence, the cradle tends to lower further and an unstable lowering cycle is set up. Conversely, the lifting cradle is raised in an unstable manner by putting compressed air in the buoyancy tanks of the cradle. This causes the buoyancy of the cradle to increase and for the cradle to be raised slightly. This decreases the water pressure acting on the lifting cradle and causes the cradle to expand and reduce its density. This causes the cradle to rise further and an opposite unstable cycle is set up. Hence the system is difficult to control.

Furthermore, tidal forces, waves and movement of the catamaran cause the cradle to move around in the water. This movement makes the cradles difficult to control and increases the likelihood of it becoming unstable.

An object of the present invention is to alleviate some of the problems of the prior art.

According to the present invention, there is provided a floatable dry dock comprising a lifting cradle of variable buoyancy pivotally mounted about a pivot axis on a base, wherein the lifting cradle can be partially submerged to receive a vessel and can be raised to at least partially lift a vessel received in the cradle out of the water, wherein the cradle is shaped and configured so that when the cradle is partially submerged to receive a vessel, at least one stabilising portion of the cradle remains above the water, which at least one stabilising portion provides water plane for stabilising the cradle.

For the purposes of the present application, the "water plane" of an object is the area of water at the water/air interface which is displaced by the object. For an object at sea, the upwards acting buoyancy force from the partially submerged object is equal to the weight of the fluid that is displaced by the object. Hence, as more of an object is submerged, the buoyancy force from that object increases until the object is entirely submerged. At this point the volume of the fluid displaced is at its maximum.

Buoyancy force increases approximately linearly with the volume of cradle that is submerged. Hence, the increase or decrease in this force is proportional to the change in volume of the submerged parts of the cradle as the cradle is submerged or raised. When the cradle is partially submerged to receive a vessel, the change in volume comes from the submerging of the parts of the cradle that were above the water.

For a small increase in depth of the cradle, the increase in upwards buoyancy force is proportional to the area of the newly submerged parts of the cradle. Conversely, for a small decrease in depth of the cradle, the decrease in upwards buoyancy force is proportional to the area of the newly raised parts of the cradle and hence proportional to their water plane.

The cradle may be considered to be relatively stable if forces acting on the cradle are counteracted by changes in buoyancy within a fairly narrow range of movement of the cradle. In view of the above explanation it may be seen that this can be achieved by maintaining a sufficiently large water plane to keep the cradle stable. Preferably, the stabilising portion is provided radially separated from the pivot axis of the lifting cradle, The separation from the pivot axis provides greater torque about the pivot to keep the cradle stable. The pivot axis of the lifting cradle may be provided towards one end of the cradle. In some embodiments, the stabilising portion is provided towards an opposite end of the lifting cradle from the pivot axis.

Preferably, at least one of the stabilising portions comprise a water-plane tower. Preferably, the water-plane tower comprises a sealed tower of buoyant material. The use of a water-plane tower allows the bottom of the tower to be submerged up to a depth equal to the length of the tower while the top of the tower still remains above the water and allows water plane to be maintained as the cradle is raised and lowered. The buoyant material could be air.

Preferably, the water-plane tower is pivotally connected to the remainder of the cradle.

Most preferably, the floating dry dock is arranged such that the tower tends to remain in a generally vertical position when the tower is partially or fully submerged. This allows the water-plane tower to naturally remain in an upright position when partly submerged due to the upwardly acting buoyancy force.

The water-plane tower may be connected to orientation control means for controlling the orientation of the water-plane tower relative to the remainder of the lifting cradle. This allows the orientation of the tower to be controlled even when the tower is substantially or fully out of the water. In this situation, the tower cannot be kept vertical by the use of buoyancy force as there is no or very little buoyancy force acting on the tower. Without the use of orientation control means, the tower will tend to pivot to enable its centre of gravity to reach its lowest point. Whilst this is not important once the tower is out of the water, it is preferable to return the tower to a generally upright orientation before lowering the cradle.

The orientation control means may comprise a hydraulic ram connected between the water-plane tower and the lifting cradle such that the orientation of the water-plane tower can be controlled by the ram.

In alternative embodiments the water-plane tower may be freely pivotally connected to the remainder of the lifting cradle so that it may freely rotate about the remainder of the lifting cradle.

Most preferably, the water-plane tower is pivotally mounted such that the tower can pivot between two extremes with respect to the lifting cradle and wherein limiting means are provided to ensure the water-plane tower does not pivot further than the two extremes. This prevents the water-plane tower from pivoting more than a given amount away from a generally upright orientation. It cannot be flipped over by tidal currents. This reduces the work the orientation control means have to do in order to bring the tower back to an upright position. It may also mean that orientation control means are unnecessary as the limiting means provided may contain the tower such that when submerged the tower will gradually return to a vertical orientation as it is submerged. It also means the water-plane tower does not extend outwards from the rest of the cradle and therefore does not occupy unnecessary space on the water.

The water-plane tower is preferably provided with a ballast below its pivotal connection to the remainder of the cradle so that the tower tends to remain in an upright position when no part of the water-plane tower is submerged in the water. The ballast may cause the centre of gravity of the tower to be below the pivot and may act to prevent the tower from flipping over and inverting itself. When using limiting means, it also puts less demand on the limiting means as the tower itself will naturally want to stay within limits of the limiting means.

The tower may be arranged to allow the weight of the ballast to be varied. The variable weight ballast may allow the weight of the ballast to be reduced during lifting so that the floating dry dock may lift greater weight. The floating dry dock can lift a vessel of increased weight as the weight of the ballast is reduced.

The ballast may be formed of a generally hollow ballast section of the tower which may be at least partially filled with water. The ballast section may form a plurality of holes to allow water to enter the ballast section when it is at least partly submerged. This allows the ballast section to be filled while it is in the water and does not require it to be filled before use. Some of the holes may be provided with one-way valves to allow water to flow into the ballast section at a greater rate than the water can drain out of the ballast section. The valves may comprise rubber seals. Secondary holes may be provided to allow water to drain out of the ballast section. The secondary holes may be smaller than the holes with valves so that water may flow into the ballast section more rapidly than it may escape.

The water-plane tower may be raised out of the water by the lifting cradle quicker than the water can drain out of the ballast section, thus providing a ballast even when the water-plane tower is out of the water. This means the tower will tend to remain in a generally vertical position while the cradle is being raised out of the water.

Preferably, the water-plane tower forms a path for air to enter and exit the ballast section. This allows the water to flow into the ballast section effectively as air exits the section. It also allows the water to drain out effectively and be replaced by incoming air entering the section.

The air path may be provided by a breather pipe in the water-plane tower.

Preferably, the base the lifting cradle is mounted on is a vessel, and most preferably, a catamaran. According to another aspect of the invention, there is provided a water-plane tower for mounting on a lifting cradle of variable buoyancy. The water-plane tower may be provided with pivotal mounting means to enable it to be pivotally mounted to the lifting cradle at a pivot.

The water-plane tower is preferably provided with a ballast below the pivotal mounting means such that when pivotally mounted the tower tends to remain in an upright position when no part of the water-plane tower is submerged in the water.

The ballast may comprise a variable ballast such that the weight of the ballast can vary. The variable ballast may allow the weight of the ballast to be reduced during lifting so that the floating dry dock can lift greater weight. The ballast may be formed of a generally hollow ballast section of the tower at least partly filled with water. The ballast section may form a plurality of holes to allow water to enter the ballast section when it is at least partly submerged. Some of the holes may be provided with one-way valves to allow water to flow into the ballast section at a greater rate than the water can drain out of the ballast section. The valves may comprise rubber seals. Secondary holes may be provided to allow water to drain out of the ballast section. The secondary holes may be smaller than the holes with valves so that water may flow into the ballast section more rapidly than it may escape.

Preferably, the water-plane tower forms a path for air to enter and exit the ballast section. The air path may be provided by a breather pipe in the water-plane tower. According to a further aspect of the invention, there is provided a method of raising a vessel in a floating dry dock, said method including the steps of:

- providing a lifting cradle - providing a water-plane tower with a ballast, the ballast comprising a generally hollow ballast section, said section having at least one hole such that water can enter and exit the section, said tower being mounted on the lifting cradle

- partially submerging the lifting cradle for receiving a vessel, wherein the at least one hole in the ballast section allows water to flow into the ballast section of the tower

- raising the lifting cradle to lift the vessel partially or fully out of the water, wherein the lifting cradle is raised at a rate to allow water in the ballast section of the tower to drain out of the ballast section while the cradle is being lifted so that the weight of the ballast is reduced during raising of the lifting cradle.

Thus, the method provides the water-plane tower with a ballast even when the water- plane tower is out of the water. This means the tower will tend to remain in a generally vertical position while the cradle is being raised out of the water.

Preferably, the ballast section is provided with at least one entry hole to allow water to enter the section quickly and at least one smaller exit hole to allow the water to drain out slowly. Most preferably the at least one entry hole is provided with a one-way valve to allow water to enter but not exit the entry hole. A floating dry dock, which embodies the present invention, will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic plan view of a floatable dry dock with two water-plane towers,

Figure 2 is a schematic side view of a floatable dry dock of figure 1, and

Figure 3 is a schematic view of a water-plane tower attached to an arm of a lifting cradle of the floatable dry dock of figures 1 and 2.

Referring to Figure 1, there is shown a dry dock 10 that has one lifting cradle 11 pivotally mounted on a base in the form of a catamaran 12. The dry dock 10 could equally have two lifting cradles 11. As shown in figure 2, the catamaran 12 is in the form of an elongate twin-hull catamaran made of lightweight marine alloy or steel. In other embodiments the base could be a mono-hull or a cylindrical float or other floatable structure such as, for example, a trimaran.

The lifting cradle 11 comprises two arms 15, each arm pivotally mounted at a respective opposite end 14 of the catamaran 12 via pivotal mountings 13.

The pivotal mountings 13 are located on an axis between the two hulls of the catamaran base 12 that extends in a direction along the length of the hulls of the base. The arms 15 are made of a lightweight marine alloy or steel construction and are of an arcuate shape. Elongate buoyancy tanks 16 to 19 are provided extending parallel with the catamaran hulls between the two arms 15. When lowered, the lifting cradle 11 is shaped to permit a vessel (not shown) to be floated in from one end.

The tanks 16 to 19 have means for selectively flooding the tanks 16 to 19 with water in sequence to cause the cradle 11 to submerge and cause the arms 15 to pivot about pivotal mountings 13 and become submerged.

Platform support means (not shown) are provided on the arms 15 in the form of an arcuate track running along, and adjacent to, the concave edge of the arms 15 for supporting a lifting platform for receiving the vessel.

Referring to figure 3, the cradle 11 also comprises two water-plane towers 21, 22; one mounted on the outside of each arm 15 at a distal end thereof. In the following description, only one of the towers 22 will be described in detail but it is to be understood that the other water-plane tower 21 is of an identical or similar construction.

The water-plane tower 22 is of generally cylindrical shape and, except where otherwise indicated below, is made of steel.

The tower 22 is mounted towards a lower end thereof to the remainder of the cradle 11 on pivotal mountings 24 so the tower can pivot about an axis parallel to the axis extending between the hulls of the catamaran 12. The tower 22 is also connected to the remainder of the cradle 11 by a hydraulic ram (not shown) that can control the orientation of the tower 22 with respect to the remainder of the cradle.

A bobbin 25 is provided connected to the water-plane tower 22 and the arm 15 on which the tower is mounted. The bobbin 25 is generally circular in shape with a tab portion, as can be seen most clearly in figure 2. The bobbin 25 is attached to the arm 15 by bolts. The tab portion has a hole 30 through it. A pivot rod 31 is slotted through this hole and fixed at one end to the arm 15 so they cannot rotate with respect to one another. The rod 31 also extends through a corresponding hole through the diameter of the tower 22 so that the opposite end of the rod 31 is on the other side of the tower 22 than the bobbin 25. The tower 22 is free to pivot about the rod 31. A brace member 34 is connected to the tab portion of the bobbin 25 and faces inwardly towards the tower 22. The brace member 34 has arms that extend into recesses in the surface of the water-plane tower 22 so that the arms are constrained to move only in the recesses in the tower surface.

At the opposite end of the rod 31, there is a second smaller bobbin 32. The rod 31 is fixed to the second bobbin 32 by slotting it through a hole in the tab portion of the bobbin and fixing it to the bobbin 32 so the rod 32 and second bobbin 32 cannot rotate with respect to each other. The bobbin is orientated so that the circular portion thereof is above the tab portion and rod 31. The circular portion 32 of the bobbin 32 is connected to a second brace member 33 which faces inwardly towards the tower 22. The second brace member 33 has arms that extend into recesses in the surface of the water-plane tower 22 so that the anus are constrained to move only in the recesses in the tower surface. Hence the tower 22 can rotate with respect to the bobbin 32 (and consequently rod 31, bobbin 25 and arm 15) to an extent determined by the abutment of the arms of the brace member 33 on the edges of the recesses and the abutment of the arms of brace member 34 against the corresponding edges of the recesses on the other side of the tower 22.

The upper part of the cylindrical water-plane tower 22 forms a sealed chamber of air.

The lower part of the cylindrical water-plane tower 22 i.e. the part generally below the hole for rod 31 forms a generally hollow lower ballast section 26, sealed from the remainder of the tower. The ballast section 26 is formed with holes 40, 41 that allow water to enter the ballast section 26. The ballast section is also provided with a breather pipe 27 extending from the ballast section 26, up through the tower 22 to an upper region of the tower 22 where it is exposed to the outside. The pipe 27 allows air to enter and exit the ballast section 26.

The ballast section 26 is provided with two types of holes. Larger holes 41 allow water to enter the ballast section 26 rapidly to provide a weighted ballast. These larger holes 41 are provided with one-way valves made with rubber seals. Hence, these larger holes only allow water to enter the ballast section 26. Water cannot escape from these larger, valved holes 41. Small, secondary holes 40 are provided to allow water to enter and exit the ballast section 26. These holes are not provided with valves. Hence, water escapes from the ballast section 26 at a slower rate than it can enter as it can only escape through the smaller secondary holes 40.

In operation, the dry dock 10 is floated out to where the vessel to be lifted is located, or the vessel is floated to the vicinity of the dry dock 10. The dry dock is positioned either astern or ahead of vessel.

The hydraulic ram (not shown) is used to ensure the tower is orientated in a generally vertical position before the cradle 11 is lowered.

The tanks 16 to 19 of the cradle 11 are flooded with water to submerge the platform (not shown) to a position where the vessel can be floated into position from one end of the cradle 11.

As the cradle 11 is lowered into this position, water enters the ballast section 26 of the water-plane tower 22 through the larger 41 (and smaller secondary 40) holes. Air that was contained in the ballast section 26 can escape through breather pipe 27. Hence, the tower 22 is provided with a weighted ballast to increase its stability. When the lifting cradle 11 is in the position in which the vessel is received, the top of the tower 22 remains above the surface of the water 28. The water-plane provided by the parts of the cradle above the surface of the water 28 provide sufficient combined water-plane to stabilise the cradle. In this example, the combined water-plane is substantially provided by the two water-plane towers 21 , 22.

With the vessel in place, the tanks 16 to 19 are sequentially purged of water by pumping in compressed air to increase the buoyancy of the cradle 11 in a controlled manner. Firstly, tank 16 is supplied with compressed air then tank 17 followed in sequence by the tanks 18 and 19.

This causes the arms 15 to rise by pivoting about the pivotal mountings 13. The water-plane tower 22 also rises. Initially, a large proportion of the tower 22 is submerged and the tower 22 naturally stays in a vertical orientation due to buoyancy of the upper part of tower 22. As more of the tower 22 is lifted out of the water, the buoyancy force that maintains the vertical orientation decreases and the tower 22 tends to flip over. The tower 22 is prevented from tipping past the pre-determined limits determined by the abutment of the arms of brace members 33, 34 on the edges of the recesses in the water-plane tower surface. The hydraulic ram (not shown) is also used to control the orientation of the tower 22 to keep it in a generally vertical orientation. As the ballast section 26 is lifted out of the water, the water contained within the section 26 drains out from secondary holes 40. The water cannot drain out of the larger, valved holes 41. As water leaves the ballast section 26, air enters to take its place through the breather pipe 27.

The water in the ballast section 26 drains out slowly from the smaller secondary holes 40 so that a weighted ballast is maintained for a significant period as the cradle 11 is lifted. Hence, the tower 22 tends to remain in a vertical orientation as it is lifted out of the water, even without the use of the brace members 33, 34 or the hydraulic ram (not shown).

As the water drains out of the ballast section 26, the weight of the cradle 11 is reduced and hence, when the cradle 11 is lifted slowly enough to allow water to drain from the ballast section 26, a larger vessel can be lifted than if a permanent weighted ballast was used.

In order to lower the vessel after repair and maintenance, the cradle 11 is lowered. Before this is done, the hydraulic ram (not shown) is used to ensure the tower is orientated in a generally vertical position. Alternatively, the tower can be retained within limits provided by the brace members 33, 34 and be allowed to naturally return to a vertical orientation as it is lowered into the water. In order to lower the cradle 11, the tanks 16 to 19 are flooded with water in the reverse order, starting first with tank 19 and then progressing in sequence by flooding tanks 18, 17 and then finally tank 16.

Li an alternative embodiment, the hydraulic ram is not provided. In this case, the tower 22 is constrained to move within the limits provided by the brace members 33, 34 and will naturally return to a vertical orientation as it is lowered into the water.

In a further alternative embodiment, the brace members 33, 34 are not provided to constrain the movement of the tower 22. In this case, the tower 22 is constrained by the hydraulic ram (not shown) in a vertical orientation as it is lowered into the water.

In yet a further alternative, neither the hydraulic ram nor the brace members 33, 34 are provided. In this case, the tower 22 is free to pivot about rod 31 with respect to the rest of the cradle and will tend to lie flat against the water surface 28. As the cradle 11 is lowered, the tower 22 naturally returns to a generally vertical orientation as it is lowered into the water.

Claims

1. A floatable dry dock comprising a lifting cradle of variable buoyancy pivotally mounted on a base, wherein the lifting cradle can be partially submerged to receive a vessel and can be raised to at least partially lift a vessel received in the cradle out of the water, wherein the cradle is shaped and configured so that when the cradle is partially submerged to receive a vessel, at least one stabilising portion of the cradle remains above the water, which at least one stabilising portion provides water plane for stabilising the cradle.

2. A floatable dry dock as claimed in claim 1 wherein the lifting cradle is pivotally mounted about an axis and stabilising portion is provided radially separated from the pivot axis of the lifting cradle.

3. A floatable dry dock as claimed in claim 1 or claim 2 wherein the at least one stabilising portion comprises a water-plane tower.

4. A floatable dry dock as claimed in claim 2 or claim 3 wherein the water-plane tower comprises a sealed tower of buoyant material.

5. A floatable dry dock as claimed in claim 4 wherein the buoyant material is air.

6. A floatable dry dock as claimed in any of claims 2 to 5 wherein the water- plane tower is pivotally connected to the remainder of the cradle.

7. A floatable dry dock as claimed in claim 6 wherein the floatable dry dock is arranged such that the tower tends to remain in a generally vertical position when the tower is partially or fully submerged.

8. A floatable dry dock as claimed in claim 6 or claim 7 wherein the water-plane tower is connected to orientation control means for controlling the orientation of the water-plane tower relative to the remainder of the lifting cradle.

9. A floatable dry dock as claimed in claim 8 wherein the orientation control means comprises a hydraulic ram connected between the water-plane tower and the lifting cradle such that the orientation of the water-plane tower is controllable by the ram.

10. A floatable dry dock as claimed in claim 6 or claim 7 wherein the water-plane tower is freely pivotally connected to the remainder of the lifting cradle so that it is freely rotateable about the remainder of the lifting cradle.

11. A floatable dry dock as claimed in any of claims 7 to 10 wherein the water- plane tower is pivotally mounted such that the tower is pivotable between two extremes with respect to the remainder of the lifting cradle and wherein limiting means are provided to ensure the water-plane tower does not pivot further than the two extremes.

12. A floatable dry dock as claimed in any of claims 6 to 11 wherein the water- plane tower is provided with a ballast below its pivotal connection to the remainder of the cradle so that the tower tends to remain in an upright position when no part of the water-plane tower is submerged in the water.

13. A floatable dry dock as claimed in claim 12 wherein the ballast causes the centre of gravity of the tower to be below the pivot and acts to prevent the tower from flipping over and inverting itself.

14. A floatable dry dock as claimed in claim 12 or 13 wherein the tower is arranged to allow the weight of the ballast to be varied.

15. A floatable dry dock as claimed in claim 14 wherein the ballast is formed of a generally hollow ballast section of the tower which may be at least partially filled with water.

16. A floatable dry dock as claimed in claim 15 wherein the ballast section forms a plurality of holes to allow water to enter the ballast section when it is at least partly submerged.

17. A floatable dry dock as claimed in claim 16 wherein some of the holes are provided with one-way valves to allow water to flow into the ballast section at a greater rate than the water can drain out of the ballast section.

18. A floatable dry dock as claimed in claim 16 or 17 wherein secondary holes are provided to allow water to drain out of the ballast section.

19. A floatable dry dock as claimed in claim 18 when dependant on claim 17 wherein the secondary holes may be smaller than the holes with valves so that water flows into the ballast section more rapidly than it escapes.

20. A floatable dry dock as claimed in any preceding claim wherein the base the lifting cradle is mounted on is a vessel, and most preferably, a catamaran.

21. A water-plane tower for mounting on a lifting cradle of variable buoyancy.

22. A water-plane tower as claimed in claim 21 wherein the water-plane tower is provided with pivotal mounting means to enable it to be pivotally mounted to the lifting cradle at a pivot.

23. A water-plane tower as claimed in claim 22 wherein the water-plane tower is provided with a ballast below the pivotal mounting means such that when pivotally mounted the tower tends to remain in an upright position when no part of the water- plane tower is submerged in the water.

24. A water-plane tower as claimed in claim 23 wherein the ballast comprises a variable ballast such that the weight of the ballast is variable.

25. A method of raising a vessel in a floating dry dock, said method including the steps of:

26. A method of raising a vessel in a floating dry dock as claimed in claim 25 wherein the ballast section is provided with at least one entry hole to allow water to enter the section quickly and at least one smaller exit hole to allow the water to drain out slowly.

27. A method of raising a vessel in a floating dry dock as claimed in claim 26 wherein the at least one entry hole is provided with a one-way valve to allow water to enter but not exit the entry hole.

28. A floatable dry dock or water-plane tower substantially as hereinbefore

described with reference to and/or as shown in the accompanying drawings.