Abstract:
A skidding pallet ( 200 ) for use on the skid rails ( 110, 115 ) of a vessel is disclosed, the pallet comprising a supporting means such as a frame ( 230 ), a mounting means for the mounting of an object thereon, and at least one actuation means, such as a cylinder ( 240 ), attached to one of the supporting means or mounting means. In use, the supporting means interfaces with the skid rails and the actuation means are actuatable to cause the mounting means to rotate in relation to said supporting means and the skid rails.

Description:
[0001]    This invention relates to deployment or recovery of objects at sea (such as modules) and in particular to skidding pallets used during said deployment or recovery of objects. 
         [0002]    Vessels, such as the Far Saga, which can be used for subsea installation and module handling operations, are commonly equipped with a moonpool and module handling system tower (MHS tower). These are used to recover/install subsea modules (chokes, control pods, subsea pumps, gas compressor or any other assembly which functions as a unit but can be joined with other units for increasing the function) from/to subsea templates. A subsea template is essentially a guide frame to aid alignment of the modules. The module or object to be deployed will commonly be positioned under the MHS tower (and above the moonpool) by a skidding pallet on skidding rails. 
         [0003]    It is normal on module deployment operations for the module and its running tool to be loaded onboard the vessel when at quay, and seafastened to the skidding pallet on the skidding rails prior to departure to the installation site. The running tool is for carrying the module to the template and, in operation, runs on guidewires connecting the vessel to the template. 
         [0004]    During deployment the vessel&#39;s bow should always be orientated against the weather. Furthermore, the orientation of the module is important as modules can only be installed in a set position and orientation on the template. Problems can arise when the orientation of the module (or the running tool), set during the mobilisation of the vessel, no longer coincides with the preferred vessel heading when on the installation site. This situation often leads to postponement of the operation until weather conditions are more suitable. The module and running tool should therefore be placed on the pallet with an orientation which coincides with the final installation position on the template, when the vessel lies with its bow against the weather. 
         [0005]    When there is too large an offset between the module installation heading and the weather direction, the guidewires will start to twist, making it harder/impossible to run the running tool/module on the guidewires. Usually a  45  degree offset from the module installation heading can be dealt with if the water depth is sufficiently deep (approx 200 m plus). However the current and the size of the module (drag forces) will also have an impact on the ability to install with offset. 
         [0006]    Therefore, the decision of what position/orientation the module shall have on the pallet needs to be based on the weather forecast. But direction of the weather often changes during the time between mobilisation and reaching the installation site. Also weather forecasting is never totally reliable. 
         [0007]    It would therefore be desirable to have a way of changing the position of the running tool/module (or any other object) on the skidding pallet in a safe and simple manner while at sea. It would also be desirable that any solution does not require any significant changes to the vessel design or apparatus. 
         [0008]    In a first aspect of the invention there is provided a pallet for use on the skid rails of a vessel, said pallet comprising a supporting means, a mounting means, for the mounting of an object thereon, and at least one actuation means attached to one of said frame members, wherein in use, said supporting means interfaces with the skid rails and said actuation means are actuatable to cause the mounting means to rotate in relation to said supporting means and said skid rails. 
         [0009]    This ability to rotate the pallet with an object such as a running tool and module mounted thereon means that the vessel can take on the heading that is best for the circumstances and weather conditions while the pallet is simply repositioned to coincide with the subsea template as required. 
         [0010]    At least part of the supporting means on either side may, in use, abut the inside edge of one of the skid rails such that its is moveable only in the direction of the skid rails. In one embodiment there is four abutments, one in the vicinity of each corner of the supporting means. 
         [0011]    Said mounting means may be rotatably mounted on said guiding frame such that said mounting means is rotatable around its centre. Said mounting means may be connected to said supporting means by a pivot mounting located centrally to both frames. 
         [0012]    Said pallet may have the same dimensions and interfaces as standard skidding pallets. 
         [0013]    Said actuation means may comprise a hydraulic cylinder. Said hydraulic cylinder may be mounted at a first end to one of said supporting means or via mounting means at appoint on the cylinder body, with a second end of said hydraulic cylinder able to act against the other frame member such that when said ram is extended, one of said frame members rotates in relation to the other. Said hydraulic cylinder may be attached to said supporting means at a first end while said mounting means is provided with abutments evenly spaced around it, for the second end of said hydraulic cylinder to act against. Said abutments may comprise blocks or holes. Said blocks may each be provided with a recess. Said hydraulic cylinder may have a head for location in said holes or recesses. Said head may have an inclined edge, to prevent it locating in and/or abutting the edge of any abutments when being retracted. 
         [0014]    The main pallet structure may be comprised in either the supporting means or the mounting means. If comprised in the mounting means, said supporting means may comprise a guiding frame having a width similar to the width between the inside edges of the two skid rails. In this case said mounting means may be arranged to sit on the skid rails, when in use. Extending from each corner of said guiding frame there may be wheel or roller means for running along a substantially circular track on an inside surface of said pallet structure, to allow the main pallet structure to rotate in relation to the guiding frame. The abutments for said hydraulic cylinder may also be located on an inside surface of said pallet structure. Said hydraulic cylinder may be pivotably mounted to said pallet structure and provided with a guiding means to guide its second end against said abutments. Said guiding means may be a secondary cylinder. In one embodiment, two main hydraulic cylinders are provided, each attached to opposite corners of said supporting means, each having a secondary cylinder. 
         [0015]    If the main pallet structure is comprised in the supporting means, then said supporting means may be arranged to sit on the skid rails, when in use. The mounting means may then comprise a substantially flat panel. Said abutments may be located on the underside of said panel, arranged at regular intervals in a substantially circular configuration. Said hydraulic cylinder may be attached to said supporting means such that it allows movement in two axes, thus allowing the second end of said hydraulic cylinder to be held substantially on the circular path made by said abutments, and to engage/disengage with said abutments. This may be achieved by attaching the cylinder via a mounting. Said second end of said hydraulic cylinder may be arranged to raise up against the underside of the mounting means due to the action of gravity on the first end of said cylinder. The head of said hydraulic cylinder may have an inclined edge such that it does not engage with the abutments when being retracted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]    Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which: 
           [0017]      FIG. 1   a  shows a portion of the main deck of a vessel suitable for handling modules; 
           [0018]      FIG. 1   b  shows the Module Handling System Tower of the vessel of  FIG. 1   a;    
           [0019]      FIGS. 2   a  to  2   d  show a pallet according to a first embodiment of the invention from above, in cross section, from one side, and in perspective view respectively; 
           [0020]      FIGS. 3   a  to  3   d  show, in cross section, the pallet of  FIGS. 2   a - 2   d  in various stages of rotation; 
           [0021]      FIGS. 4   a  and  4   b  show a pallet according to a second embodiment of the invention from below and from one side; 
           [0022]      FIGS. 5   a  - 5   c  show, in cross section, perspective view and a detail, the pallet of 
           [0023]      FIGS. 4   a  and  4   b  being rotated; and 
           [0024]      FIGS. 6   a  to  6   d  show different views of an alternative form of actuating means for turning the pallet. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0025]      FIGS. 1   a  and  1   b  in conjunction show a general arrangement of a vessel equipped with a moonpool  130 , module handling system tower (MHS tower)  140  and skid rails  110 , 115 .  FIG. 1   a  shows a portion of the main deck from which can be clearly seen skid rails main track  110 , leading to the moonpool  130  area under the tower. There are two other sets of skid rails, side tracks  115 , perpendicular with these.  FIG. 1   b  shows the MHS tower in detail. Its legs  125  are positioned at sites  120  ( FIG. 1   a ) on the vessel deck. The tower is equipped with guide wire winches  150 . 
         [0026]    In normal operations, when the vessel is above the template, the pallet, with module/running tool, is skidded along the skid rails  110  into the MHS tower, over the moonpool. The MHS tower is then used to lift the module and allow the empty pallet to be skidded out the tower. The moonpool hatch can then be opened. The running tool with the module is then lowered on guidewires down to the template. 
         [0027]      FIGS. 2   a  to  2   d  show the pallet  200  from above, in cross section, from one side, and in perspective view respectively. In each case the pallet  200  is shown on the skid rails  110 . The pallet  200  consists of a guiding frame  220 , a main pallet frame  230  and two main hydraulic cylinders  240 , each with associated secondary guiding secondary cylinder  260 . The guiding frame is provided with protrusions  250  (visible on  FIG. 2   c ) in each corner. Spaced around the inside main frame  230  are a number of gripper blocks  270 . Extending out from each corner of the guiding frame are wheels  280 , and around the inside edge  285  is complementary track  290 . 
         [0028]    The guiding frame  220  is approximately as wide as the gap between the skid rails  110 . The protrusions  250  are essentially a large bolt through each corner of the guiding frame  220  and a washer (and nut) underneath the rail flange. Due to the protrusions the guiding frame  220  can only move along the rails  110  and cannot change its orientation. The main pallet frame  230  sits on the skid rails  110  and can be skidded along them. It is attached to the guiding frame  220  by the location of wheels  280  into the tracks  290  such that they can be rotated relative to each other, the wheels  280  turning in the tracks  290 . In effect this means that the main pallet frame  230  can be rotated relative to the guiding frame  220  and the rails  110 , as the guiding frame cannot be turned due to the protrusions  250 . 
         [0029]    The overall pallet  200  has the same external dimensions and interfaces as standard skidding pallets. It is designed for a design load of 20Te and is moved on the skidding beams in the same way as the standard skidding pallets by the use of PPU&#39;s (Push-Pull Units). 
         [0030]    There are two main hydraulic cylinders  240  for rotating the main pallet frame, each with one end (in this case the cylinder body end) pivotably attached to opposing corners of the guiding frame. The other end of each main hydraulic cylinder  240  (the rod) is forced to engage with one of the gripper blocks  270  due to the action of the guiding cylinders  260 . The main hydraulic cylinders  240  can then be activated to push against the gripper blocks  270  causing the main pallet frame  230  to rotate. 
         [0031]      FIGS. 3   a  to  3   d  show cutaway drawings of the pallet  200  as it is being rotated. To operate the pallet  200 , a hydraulic supply with a valve pack is connected to the pallet  200  through quick connectors. In  FIG. 3   a  the main hydraulic cylinders  240  are each engaged against a gripper block  270   a,    270   a ′.  FIG. 3   b  shows the main hydraulic cylinders after they have been extended to their full stroke. The result of this is that the main pallet frame  230  is rotated relative to the fixed guiding frame  220 . For each full-stroke cylinder  260  push, the pallet  200  is rotated approximately 22 degrees. Each main hydraulic cylinder  240  can then be controlled to retract and engage again with the next gripper block  270   b,    270   b ′ as shown in  FIG. 3   c  and, should further rotation be required, be extended again, against these gripper blocks  270   b,    270   b ′.  FIG. 3   d  shows the pallet  200  after such a further extension. These steps can be repeated as many times as necessary. Eight pushes in total are needed for a full 180-degree rotation. 
         [0032]      FIGS. 4   a  and  4   b  show a pallet (from below and one side respectively)  400  according to a further embodiment. It comprises pallet frame  410  and, rotatably mounted to this via central pivot  415 , pallet top  420 . A hydraulic cylinder  430  is attached to said pallet frame via mounting  450 . At the end of the cylinder rod is a head  440 , which abuts against recessed gripper blocks  460 . In this embodiment the pallet frame makes up the support structure. It is this structure which sits on the rails, and has protrusions (not shown) similar to the protrusions  250  of  FIG. 2   c,  to hold it to the rail in the same way as before. The pallet top  420  is essentially a flat panel with gripper blocks  460 , arranged on its underside in a circular configuration as shown. 
         [0033]      FIGS. 5   a  and  5   b  show the pallet (from below) after an extension of the hydraulic cylinder  430 . The principle of how this pallet works is similar to the previous embodiment. The mounting  450  allows the cylinder to pivot in 2 directions, one allowing the head  440  of the cylinder rod to follow the circular pattern of gripping blocks. The other direction is for tilting the cylinder to allow it to engage the gripper blocks. The head  440  of the hydraulic cylinder acts against the gripper block  460 , the gripper block having a recess to match the head  440  shape, under the influence of gravity as the cylinder housing is heavier than the piston/rod with head. This is because the mounting  450  is close to the rod end of the cylinder and when the cylinder  430  is retracted, there is little weight on the cylinder head  440  side of the mounting  450 , and more weight on the other end. The cylinder will therefore tilt until the head is in contact with pallet top  420 . Also, when the cylinder is extended, while more weight is transferred to the head side of the attachment and the cylinder head  440  also gets a longer moment around the attachment, the volume in the cylinder is filled by hydraulic oil, ensuring that the centre of gravity remains on the same side of the mounting  450 . 
         [0034]    This causes the pallet top to rotate around its axis approximately 30 degrees as shown. The cylinder  430  rod can then be retracted. The head has an inclined back edge such that it does not engage with the next gripper rod as it is retracted, but is deflected under it. When fully retracted, it can then be extended against this next gripper block as before. This process can be repeated as often as necessary. 
         [0035]      FIG. 5   c  shows a close up of the cylinder mounting  450  for this embodiment. It shows the cylinder  430  being retained in a bracket  470 , which is pivotably mounted inside said mounting  430 . This allows the cylinder to be moveable around two axes as previously explained. 
         [0036]      FIG. 6   a  to  6   d  show a number of views of an alternative actuation system for turning the pallet top  420 . It comprises a hydraulic cylinder  600  with cylinder rod  610  and a head  620 . This cylinder is largely similar to the hydraulic cylinder  430  of  FIG. 4 , and can be mounted to the pallet in the same way. Also provided a number of holes  640  in the underside of the pallet top for the head  620  to locate in. When located in a hole  640  the rod  610  can be extended as before, the head  620  abutting the edge of the hole, causing the pallet top  420  to rotate. These holes  640  may be bored straight into the pallet top  420  or into a strip  630 , as shown here, the strip then attached to the pallet top  420  underside. Said holes should form a circular configuration as with the gripper blocks. The head  620  has an inclined edge  650 , to prevent it locating in and/or abutting the edge of any holes  640  when being retracted, and is instead deflected out of the hole. A similar actuation system can also be envisaged for the pallet of  FIG. 2 , having holes along the inside edge of the main pallet frame  230 . 
         [0037]    The above example is for illustration only and further embodiments and variations can be envisaged without departing from the spirit and scope of the invention. For example there may be different means utilised to turn the main pallet frame, other than hydraulic cylinders. Any number of hydraulic cylinders may be used, as may any number of gripper blocks/holes.