Abstract:
A floatable dry dock includes a lifting cradle having two spaced arms pivotally mounted on a buoyant base and, one or more flotation tanks interconnecting the arms. A platform is mounted on the arms and a platform support is operable to ensure that the platform remains horizontal when the arms pivot about their pivotal attachment to the base.

Description:
This application is the US national phase of international application PCT/GB2004/003455, filed 11 Aug. 2004, which designated the U.S. and claims priority of GB 0319019.6, filed 13 Aug. 2003, the entire contents of each of which are hereby incorporated by reference. 
   BACKGROUND 
   1. Technical Field 
   This invention relates to dry dock systems 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. 
   2. Related Art 
   There are basically two types of dry dock. There are those comprising a lock that has at least one closable door into which the vessel is floated, and the water is drained from the lock to leave the vessel high and dry. 
   A second type of dry dock system comprises a floating dock that consist of a raft that is floated to a region ahead or astern of the vessel and submerged so as to be positioned beneath the vessel. The raft has floatation chambers built into the walls of the raft so that they can be purged of water by displacing the water with compressed air. A major problem with this type of dock is that the amount of required “water plane” makes these types of docks highly unstable. “Water plane” is defined as the area of water at the water air interface which is displaced by a part of the dock. In general the greater the “water plane” the greater will be the stability of the dock. As these docks lift a boat out of the water, there is considerable “water plane” provided by the engagement of the boat hull with the water, but it becomes particularly dangerous as the “water plane” decreases when the hull is lifted out of the water and eventually loses contact with the water. As the boat leaves the water this adds considerable weight to the dock with a considerable and rapid decrease in the “water plane” making the whole system extremely unstable, in the final stages of the lifting operation. 
   To remain within the bounds of stability, it is traditional to design the dock system so that it lifts vessels of about one half of the weight of the dock itself. 
   There is a need to provide dry dock facilities for small boats at local harbours, moorings, club harbours or lagoons and the like. There is also a need for providing a much cheaper design of floating dock than has been possible before and one that is easily moveable from one location to another. There is also a need to be able to produce a dry dock system that can be used to lift vessels out of the water rapidly thus saving valuable time and cost. Conventional dry dock systems do not permit the rapid lifting of vessels because of the problems due to the unstable designs associated with the “water plane” problem mentioned above. 
   BRIEF SUMMARY 
   An object of the present invention is to provide a floating dry dock that is both stable and quick to operate and which can lift vessels of up to twice its own weight. 
   According to an exemplary embodiment of the present invention there is provided a floatable dry dock comprising a lifting cradle having two spaced arms pivotally mounted on a buoyant base, one or more floatation tanks interconnecting the arms, and a platform mounted on the arms, and platform support means operable to ensure that the platform remains horizontal when the arms pivot about their pivotal attachment to the base. 
   Preferably the platform has wheels at an extremity of the platform and the platform support means comprises an arcuate track on each arm along which the wheels of the platform run when the arms are pivoted whilst maintaining the platform in a horizontal altitude. 
   Preferably the arms are of an arcuate shape and there is a plurality of elongate floatation tanks extending between the arms to define a part cylindrical cradle. 
   The base may comprise one or more elongate hulls. For example the base comprises a catamaran vessel. The base may comprise a sidewall located at each end of the hulls of the base and the pivot about which the arms rotate may be located on an axis between the hulls that extends along the length of the hulls. 
   There may be a single floatable cradle mounted on the base or there could be two spaced floatable cradles are mounted on the base. 
   The arms may also comprise inflatable buoyancy tanks. 
   According to a further aspect of the present invention the platform may be pivotally mounted between the arms and the platform support means may comprise pairs of extendable and contractable links, one of each pair of links being operable to expand when the other link of the pair contracts and the links being operable to ensure that the platform remains horizontal relative to its axis of pivotal mounting on the arms. 
   In this latter mentioned embodiment the platform may be of generally rectangular shape and one link of each pair of links is provided at a corner of the platform and the other link of each pair of links is provided at a respective opposite corner of the platform. 
   Again in this latter mentioned embodiment the arms may be elongate arms mounted at one end on the base and having a buoyancy tank provide at a second end of the arms, and the platform is mounted on a pivot at a region intermediate the ends of the arm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example with reference to the accompanying drawings in which; 
       FIG. 1  is a schematic side view of a floatable dry dock constructed in accordance with the present invention having two lifting cradles, and 
       FIG. 2  shows a part-sectional view through a wheel and track of one of the arms of the dry dock shown in  FIG. 1 , and 
       FIG. 3  is a side view of a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Referring to  FIG. 1 , there is shown a dry dock  10  that has two lifting cradles  11  mounted on a common floatable base  12 . However, it is to be understood that the present invention is applicable to dry docks  10  where there is only one lifting cradle mounted on the base  12 . In the following description only one of the lifting cradles  11  will be described in detail but it is to be understood that the other lifting cradle  11  is of an identical or similar construction unless the context says otherwise. 
   Referring specifically to  FIG. 1 , the base  12  is in the form of an elongate twin-hull catamaran made of lightweight marine alloy or steel. The base  12  could be a mono-hull or a cylindrical float or other floatable structure such as for example a trimaran. 
   Mounted on the base  12  are the engines and , propulsion equipment (not shown), and all the controls and services  13  for piloting the base  12  to a location adjacent a vessel  14  to be lifted. The services  13  include pumps for flooding and emptying buoyancy tanks of the lifting cradles (to be described hereinafter) and other services. 
   Each lifting cradle  11  comprises to arms  15  pivotally mounted on pivotal mountings  12 ( a ) in sidewalls of the base  12 . The pivots ( 12 ( a )are located on an axis between the two hulls 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 and have elongate buoyancy tanks  16  to  20  (shown dotted) extending between the two arms  15  to define a part-cylindrical cradle  11 , which when lowered (as will be explained later), enables the vessel  14  to be floated in from one end of the cradle  11 . 
   The tanks  16  to  20  have means for selectively flooding the tanks  16  to  20  with water in sequence to cause the cradle  11  to submerge and cause the arms  15  to pivot about pivots  12 ( a )and become submerged. The tanks are connected to a source  24  of compressed air whereby they can be purged of water and filled with compressed air to vary the buoyancy of the cradle  11 . The arms  15  may also incorporate buoyancy tanks (not shown). 
   The arms  15  have a platform support means in the form of an arcuate track  26  running along, and adjacent to, the concave edge of the arms  15  for supporting a lifting platform  22 . The lifting platform  22  has wheels  25  at each lateral extremity (see  FIG. 2 ) that run in the tracks  26 . The shape of the arcuate tracks  26 , and the position of the wheels  25  on the platform  22 , is arranged so that the platform  22  remains stable and horizontal as the arms  15  rotate about the pivotal means  12 ( a ). 
   As the arms  15  pivot upwards and downwards, the platform  22 , whilst remaining horizontal moves in a horizontal direction towards or away from the base  12 . 
   In order to stabilise the vessel  14  during lifting or lowering of the arms  15 , the platform  22  is provided with supports  27  that are initially spaced apart and secured to the platform  22  at a width slightly wider than the width of the vessel  14 . The supports  27  can be of a height that enables them to project out of the water (as shown on the left hand side of  FIG. 1 ) so that the pilot can steer the vessel  14  into position between the supports  27  when the cradle  11  is submerged. The supports  27  are positioned at equal distance from a plane of symmetry of the platform  22  so that the vessel  14  is located above the centre of gravity of the platform  22  to avoid tilting of the platform  22  during lifting or lowering of the arms  15 . 
   In operation, the dry dock  10  is floated out to where the vessel  14  to be lifted is located, or the vessel  14  is floated to the vicinity of the dry dock  10 . The dry dock is positioned either astern or ahead of vessel  14 . The tanks  16  to  20  of the cradle  11  are flooded with water to submerge the platform  22  to a position where the vessel  14  can be floated into position between the supports  27  from one end of the cradle  11 . This position is shown in the left hand side of  FIG. 1 . 
   With the vessel  14  in place above the platform  22 , the tanks  16  to  20  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 ,  19 , and  20 . This causes the arms  15  to rise by pivoting about the pivotal connection  12 ( a ). The upward movement of the arms  15  from a submerged position as shown in the left hand side of  FIG. 1  towards the position shown in the right hand side of  FIG. 1  is continued until the vessel  14  is lifted clear of the water surface  28 . 
   In order to lower the vessel  14  after repair and maintenance from the position shown in the right hand side of  FIG. 1  to the position shown in the left hand side of  FIG. 1 , the above procedure is reversed. That is to say, the tanks  16  to  20  are flooded with water in the reverse order, starting first with tank  20  and then progressing in sequence by flooding tanks  19 ,  18 ,  17  and then finally tank  16 . 
   During lifting and lowering of the vessel  14 , the combined “water plane” (that is to say the area at the interface between the water surface and the air) of the vessel  14 , the catamaran  12 , the arms  15 , and the tanks  16  to  20  remains reasonably constant and hence the whole of the dry dock  10  together with the vessel is very stable. 
   The stability of the dry dock  10  is such that it is possible to reverse the traditional factor of safety of 2:1 (that is to say the conventional limit of lifting vessels  14  of one half of the displacement of the dry dock  10 ). Thus with each dry dock  10  constructed in accordance with the present invention, it is possible to lift vessels  14  of twice the weight of the dry dock. This offers a significant advantage over all prior known floating dry docks. 
   Furthermore, each of the two cradles  11  shown in  FIG. 1  can be operated independently of the other. In other words, it is unnecessary to counterbalance the lifting of one vessel  14  by lifting a second vessel  14  with the other cradle. In fact, the provision of two cradles  11  on one catamaran  12 , improves stability of each, because the total “water plane” is the sum total of the “water plane” of both cradles  11 , the base  12  and the vessel  14  and not just the “water plane” of one cradle  11 . In situations with floating dry docks  10  that have two lifting cradles  11 , where one cradle  11  is raised and the other lowered as shown in  FIG. 1 , the raised cradle  11  effectively converts the catamaran base  12  into a trimaran with an outer rigger formed by the raised cradle  11 . Therefore, since each cradle  11  is very stable to start with (compared with prior known dry docks) the stability of the whole is further enhanced with two lifting cradles  11 . 
   In  FIG. 1  there is shown two cradles  11 , but as explained above, it is not essential to build two cradles on each base  12 . 
   In the above example the platform  22  has wheels  25  that run in arcuate tracks  26  on the arms  15 . Whilst this is the preferred way of mounting the platform  22 , it is possible to mount the platform  22  on pivots  31  at each end of its axis of symmetry instead of mounting them in the arcuate tracks  26 . This is shown schematically in  FIG. 3 . 
   Referring to  FIG. 3  the platform  22  is of generally rectangular shape and the arms  15  need not be of an arcuate shape but could simply be elongate arms  15  as shown. In this case, the cradle  11  may simply comprise the two arms  15  interconnected by a single buoyancy tank  34  at a free end of the arms  15 . 
   In order to maintain the platform  22  in a horizontal and stable state, the corners of the platform  22  are interconnected to each of the arms  15  by way of a platform support means in the form of pairs of links  36 ,  37 . The links  36 ,  37  of each pair may be in the form of hydraulic pistons that are interlinked so that the links  36  expand whilst the links  37  contract when the arm  55  is raised by introducing compressed air into the tank  34 . During lowering of the cradle  11  the tank  34  is flooded in a controlled manner and the links  37  expand whilst the links  36  contract thereby ensuring that the platform  22  remains horizontal throughout all movements of the arms  15 . In this case, the centre of gravity of the platform  22  remains at a fixed radius relative to the pivot about which the arms  15  rotate.