Abstract:
A method for offshore overboarding a structure on a vessel for subsequent lowering the structure to the sea floor, is using a crane ( 13 ) to lift the structure at its storage place on a deck of the vessel and move it by means of the crane to a position free of the vessel before lowering it into the sea. During at least a part of the overboarding movement of the structure ( 1 ), the structure ( 1 ) is forced to follow a track ( 2 ) along the deck of the vessel. Two apparatus for performing the method are also presented.

Description:
[0001]    The present invention relates to equipment and a method for offshore overboarding of a structure for safe and efficient lifting and manoeuvring of the structure over the deck of a surface vessel. 
       BACKGROUND 
       [0002]    Conventional offshore installation methods are normally based on the use of tugger lines directly connecting the structure to winches or handled manually by on-deck personnel. A common challenge for such operations is to obtain adequate control of the movement of the structure and the rotation in particular when the vessel is influenced by wave loads. Especially in situations where the structure is large and there is limited deck space, the overboarding operation is critical due to the risk of clashing of structure into deck equipment. Consequently, the limiting sea state for such operations is often quite low. 
         [0003]    A number of constant tension winches, typically five to eight winches, have previously been used to guide and overboard heavy structures. The winches have been arranged such that the structure is guided step by step by a system of winch wires connected between the structure and deck. As the winches at times will pull against each other, wires have ruptured as a result of lack of control due to the complexity of the system. 
         [0004]    GB 2502379 A, which was not publically available at the priority date of the present application, shows overboarding a structure by means of a crane while keeping the object stationary with respect to the crane axis by means of telescopic arms extending between the structure and a rotatable ring on the pedestal of the crane. The arms are maintained in compression in order to reduce pendulum movements of the structure caused by ship movements due to the sea state. This system is complicated and cannot easily be decommissioned for use on other vessels. Besides, it cannot be used for unwieldy structures and structures stored outside the relatively short reach of the arms. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention comprises a method and an apparatus as defined in the appended claims. 
         [0006]    With the present invention, the challenges mentioned above are reduced to a large extent. The main reason for this is that the structure is connected to one or more sliders or “sleds” moving along a track or rail which is fixed to the vessel deck. The sled(s) slide along the rail. For large loads, two sleds will often be used, whereby one of the sleds will be connected to one line from each corner of the structure and will be close to the horizontal position of the structure&#39;s center of gravity. A second sled will be situated at the tail end and will guide the rotation of the structure. For smaller loads, one sled might be sufficient. The lines attached to the sleds can consist of wire and polyamide slings for shock absorption. The operation starts by lifting the structure off deck with the crane. Then, the crane can start to function and thereby pull the structure along the rail. The sled(s) function as a moving guidance system for the structure. This increases the control of the structure and thereby reduces the risk of undesired structure movements, rotations and excessive forces in any tugger lines. Consequently, the sea-state operability can be increased in some cases. 
         [0007]    The guide rail, which is a key part of the invention, is designed in a specific manner dependent on the structure dimensions, crane properties and the deck layout. The main idea is to introduce a rail pattern which shape is fitted for the specific structure geometry. Although the rail design is not limited to a circular pattern, it may be beneficial since it eliminates the need for crane boom in/out when the crane centre corresponds with the centre of curvature of the rail. Straight rails and a combination of straight and curved rails are also included in the invention. 
         [0008]    The present invention can be applied on a large variety of offshore construction lifts, such as, but not limited to templates, manifolds, spools, suction anchors regardless of weight and shape. Typically, large and irregularly shaped structures can be handled. 
         [0009]    The guide rail is a more safe mechanically passive system, using a track rather than a number of winches to control horizontal movement of the lifted object during over-boarding. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    For better understanding of the invention, it will be described in more detail with reference to the exemplifying embodiments shown in the appended drawings, wherein 
           [0011]      FIG. 1  shows is a perspective view showing a first structure suspended in a crane, 
           [0012]      FIG. 2  is a perspective view of a section of a track for use in the present invention, 
           [0013]      FIG. 3  is a perspective view of a slider for use with the track of  FIG. 2 , 
           [0014]      FIG. 4  is a vertical cross section of an assembly of the track in  FIG. 2  and the slider in  FIG. 3 , 
           [0015]      FIG. 5  is a plan view, partly in phantom lines, of the arrangement of  FIG. 1  in a first position, 
           [0016]      FIG. 6  is a plan view like  FIG. 5  in solid lines, 
           [0017]      FIG. 7  is a plan view like  FIG. 6  with the first structure in a second position, 
           [0018]      FIG. 8  is a perspective view of a second structure in a first position suspended in a crane, 
           [0019]      FIG. 9  is a perspective view at a larger scale of parts of the second structure in  FIG. 8  in a second position, 
           [0020]      FIG. 10  is a plan view of the second structure in the second position, 
           [0021]      FIG. 11  is a plan view like  FIG. 10  with the second structure in a third position. 
           [0022]      FIG. 12  is a plan view like  FIG. 10  with the second structure in a fourth position. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  shows a first structure  1 , such as a subsea template, on the deck  16  of a surface vessel suspended in a crane  13 . The structure is straddling a track or rail  2  welded to the deck  16  of the vessel. The structure  1  is seen connected by means of lines  15  to a slider or sled  3  which can slide in the rail  2 . Hidden from view is a second sled  4  connected at a peripheral position of the structure  1 . 
         [0024]      FIG. 2  shows a segment of the rail  2 . As also show in the cross section of  FIG. 4 , the rail comprises two curved, vertically oriented parallel plates  12  with a slot between them. These plates are held in position above a base plate  10  by brackets  11  which are welded to the base plate  10  and the respective plate  12 , leaving a gap between the lower edges of the parallel plates  12  and the base plate  10 .  FIG. 2  also shows further brackets  17  which keep the rail fixed to the deck  16 . 
         [0025]      FIG. 3  shows details of the slide or sled  3 ,  4 . It comprises a vertical plate  6  that will fit glidingly in the slot between the parallel plates  12  of the rail  2 . It further has a top plate  7  with holes for line connections, and a bottom plate  8  to fit in the gap between the lower edges of the parallel plates  12  and the base plate  10  and hold the sled captive in the rail  2 . The vertical plate  6  may be slightly curved to fit the curvature of the rail  2 . 
         [0026]    In addition,  FIG. 4  shows shackles  18  for connecting tension lines  15  to the sled. 
         [0027]      FIG. 5  shows the layout of the rail  2  on the deck of the vessel and the position of the sleds  3 ,  4  and lines  15 , with the structure  1  and crane  13  shown only in phantom lines for clarity. The rail  2  comprises three of the segments shown in  FIG. 2 . 
         [0028]    In  FIG. 6 , the structure  1  is in the starting position where it is lifted off deck with wires connecting the crane  13  to top of structure. The bottom of the structure  1  is connected to the rail sleds  3 ,  4  with lines  15 . There are two sleds, one  4  is situated aft of the structure, whereas the other  3  is below the center (not visible on this figure). The sleds are designed such that they slide smoothly within the rail  2 . The curvature of the rail  2  is designed to have its center of curvature on or close to the slewing axis  14  of the crane, such that the crane only needs to slew around its center and not change the boom angle in order to maneuver the structure off deck. 
         [0029]    For this particular structure  1 , the large size relative to available deck space creates a high demand on accuracy in the positioning of the structure during the over boarding operation. The guide rail system provides a passive positioning and rotation control during the overboard phase. 
         [0030]      FIG. 7  shows the structure  1  half way from starting position to the deployment position outside the vessel hull  5 . The crane  13  is slewing slowly while the lines  15  connecting the structure  1  to the rail sleds  3 ,  4  insure passive position and rotation control of the structure. 
         [0031]    As the slewing continues, the sleds  3 ,  4  will eventually leave the rail  2  as the structure  1  comes clear of the deck  16 . In this situation, the orientation of the structure is controlled by lines from tugger winches on deck. Subsequently, the structure is lowered into the water, with the sleds  3 ,  4  hanging in their lines  15 . When the structure  1  has reached a suitable depth, the crane  13  is set to heave compensation, and tugger lines and sleds are disconnected from the structure  1  by an ROV and retrieved to the vessel. 
         [0032]      FIG. 8  shows a second structure  20 , here a spool connected to a squared termination device  21 , on the deck  16  of a surface vessel, suspended in a crane  13  in a first position. The structure is located substantially to one side of a track or rail  19  welded or in other ways attached to the deck  16  of the vessel. 
         [0033]      FIG. 9  shows the structure  20  in a second position, lifted off deck  16  with the crane  13 . The structure is connected by lines  15  to a sled  4 , which is gliding in the rail track  19  attached firmly to the vessel deck  16 . 
         [0034]    In  FIG. 8 , the structure  20  is in the starting position, where it is lifted off deck with wires connecting the crane  13  to top of structure. The bottom of the structure  20 , at the end termination device  21 , is connected to the rail sled  4  with lines  15 , as shown in  FIG. 9 . There is only one sled, which is situated aft at the trailing end of the structure. The sled is designed such that it slides smoothly within the rail  19 . The path of the rail  19  is designed for optimal guidance of structure  20  over the deck  16  with respect to clearance to other deck structures. 
         [0035]    For this particular structure  20 , the large size relative to available deck space creates a high demand on accuracy in the positioning of the structure  20  during the overboarding operation. The guide rail system provides a passive positioning control during the overboard phase. 
         [0036]      FIG. 10  shows the structure  20  in the second position in  FIG. 9 , half way from the starting position to the deployment position outside the vessel hull  5 . The crane  13  is moving slowly in the direction of the track  19  while the lines  15  connecting the structure  20  to the rail sled  4  insure passive position control of the structure  20 . 
         [0037]      FIG. 11  shows the structure  20  in a third and final position before leaving the vessel deck  16 . As the moving of the crane continues, the sled  4  will leave the rail  19  as the structure  20  comes clear of the deck  16 . In this situation, which is shown in  FIG. 12 , the orientation of the structure  20  is controlled by lines from tugger winches (not shown) on deck. Subsequently, the structure is lowered into the water, with the sled  4  hanging in its lines  15 . When the structure  20  has reached a suitable depth, the crane  13  is set to heave compensation, and tugger lines and the sled are disconnected from the structure  20  by an ROV and retrieved to the vessel. 
         [0038]    It will be understood that the invention is not limited to the exemplifying embodiment described above, but can be varied and modified by the skilled person within the scope of the following claims. For example, the track can take various forms, such as that of a railroad rail. In this case, the slider could take the form of a trolley straddling the head of the rail and having wheels or other low friction elements engaging under either side of the head. Furthermore, the track can easily be removed from the deck of the vessel after completion of the overboarding operation, e.g. for use on another or the same vessel on a later occasion. For ease of handling and storage, the track may be built in manageable sections, which are joined together in a suitable manner, e.g. with a pin and socket connection, during installation on the deck.