Abstract:
Equipment and method for lowering equipment ( 10 ) to the seabed from a vessel ( 12 ) using man made fibre rope ( 16 ) and a winch ( 20 ) employing wire rope ( 24 ), the fibre rope being paid out (or when lifting, drawn in) in sections by repeated operation of the winch ( 20 ) and wire rope ( 24 ). This is preferably done by holding the fibre rope ( 16 ) with holding means ( 32 ), while the wire rope ( 24 ) is detached for connection to a next section. The holding means ( 32 ) may be arranged to engage an attachment point ( 18 ) between adjacent rope sections but distinct from that engaged by the hoist platform ( 22 ).

Description:
[0001]     The invention relates to methods and apparatuses for deploying articles to great depth beneath the sea surface, for example to the seabed in deep waters.  
         [0002]     Cranes and winches employing wire rope have been used to deploy loads to the seabed in modest water depth for many years. Some of these cranes and winch systems are fitted with, or used in conjunction with, heave compensators, which take-up and pay out the rope dynamically, to compensate vertical motion (heave) of the ship, barge or other platform from which the rope is supported.  
         [0003]     As water depth increases, the weight of wire needed to lower equipment to the seabed increases until it becomes such a significant part of the total load that the method becomes impractical. Man made fibre rope can be almost neutrally buoyant and have strength and elastic characteristics similar to wire rope and is therefore potentially a suitable replacement for wire. Man made fibre rope, however, has a poor tolerance to the fatigue induced by bend cycling under load, and is thus unsuitable for use with current designs of heave compensator or with heave compensated drum winches. The same problem exists for winch systems without heave compensation, although the bend cycling will typically be less severe than in heave-compensated systems. Alternative systems can also be envisaged which do not increase bend cycling for the purpose of heave compensation, but bend cycling for the basic lifting/lowering operation is harder to avoid.  
         [0004]     The invention aims to obtain the weight benefits of using fibre rope, while avoiding the need for bend cycling fibre rope under load, when deploying loads from a vessel at sea.  
         [0005]     Broadly stated the invention provides equipment and method for lowering equipment to the seabed from a vessel using man made fibre rope and a winch employing wire rope, the fibre rope being paid out (or when lifting, drawn in) in sections by repeated operation of the winch and wire rope.  
         [0006]     The fibre rope may be continuous and provided with eyes, stoppers or other attachment points at regular intervals.  
         [0007]     The fibre rope may alternatively comprise discrete sections terminated with eyes or other attachment points, connected together to form the required length. In this case, the connections between sections may be made (or un-made) in the course of paying out (drawing in), or the entire length may be connected in advance and reeled during operation.  
         [0008]     Each fibre rope section may be terminated by looping around a thimble comprising a body with a peripheral channel for the rope, and two distinct openings, one opening receiving a connection (directly or indirectly) to the next rope sections, the other receiving a connection (directly or indirectly) to the hoist mechanism. In a particular embodiment disclosed in detail below, the second opening receives a bolt which fixes a stopper to the thimble, the stopper being temporarily engaged by the hoist mechanism.  
         [0009]     The stopper comprises a part-conical member formed in two halves held together by the bolt. In other arrangements, it can be envisaged that the hoist mechanism engages the thimble directly.  
         [0010]     The thimble having two distinct apertures and optionally a stopper permanently attachable thereto is also an independent aspect of the invention, for which novelty is claimed.  
         [0011]     The fibre rope may be provided between adjacent rope sections with stoppers engaged and released by a hoist mechanism openable by remote control to engage and release the fibre rope.  
         [0012]     In a preferred embodiment, the wire rope is arranged to raise and lower a hoist mechanism, the hoist mechanism being operable by remote control to allow the mechanism to pass freely up and down the rope, and then to engage the rope by means of said stopper.  
         [0013]     The fibre rope may be held at a holding means, while the wire rope is detached for connection to a next section. The holding means may be arranged to engage an attachment point between adjacent rope sections but distinct from that engaged by the hoist platform. A pair of collars or other stoppers is sufficient, spaced longitudinally to allow sufficient clearance between hoist mechanism and the holding means.  
         [0014]     Preferably the wire rope winch is heave compensated, permitting operation in a wider range of sea states and reducing strain on the components of the apparatus.  
         [0015]     The wire rope winch and holding means may both be heave-compensated, such that heave compensation can be maintained throughout the process of transferring the load from one to the other. The holding means and wire rope winch may be heave compensated in parallel by a common heave compensator, for example comprising a hydraulic ram.  
         [0016]     The winch and wire rope may be arranged in a double fall arrangement, with the end of the wire rope fixed on board the vessel while a running block is lowered and raised, alternately connected to and disconnected from the fibre rope.  
         [0017]     The invention further provides an apparatus comprising a rope store, hoist means and holding means adapted for deployment from a sea-going vessel for implementing a method of raising or lowering according to the invention as set forth above.  
         [0018]     The invention further provides a fibre rope assembly comprising plural rope sections and load-bearing stoppers connected between the sections, the rope assembly being adapted for use with a method according to the invention as set forth above.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
         [0020]      FIG. 1  shows the general arrangement of a lifting apparatus according to one embodiment of the present invention;  
         [0021]      FIG. 2  shows in more detail a hoist system within the apparatus of  FIG. 1 ;  
         [0022]      FIG. 3  shows in more detail the holding means including a hang-off platform in the apparatus of  FIG. 1 ;  
         [0023]      FIG. 4  shows a fibre rope system within the apparatus of  FIG. 1 , including enlarged detail of a joint between two rope sections;  
         [0024]      FIGS. 5, 6 ,  7 ,  8  and  9  illustrates successive stages in the operation of the apparatus, lowering a load to the seabed by a method according to the present invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0000]     General Arrangement  
         [0025]      FIG. 1  shows the general arrangement of a heave compensated lifting apparatus lowering a load  10  over the side of a vessel (not shown in full) in deep water. The load in this case is shown as a simple clump weight, but of course could be any module or tool which requires to be deployed to a great depth beneath the sea surface. Depths well in excess of 1000 m are routinely encountered in offshore developments, although the invention is not limited to any particular range of depth. A portion of the deck of the vessel is indicated schematically at  12 , while the side wall of the vessel is indicated at  14 . This could equally be the side wall of a moonpool, depending for example on the size of the load to be deployed.  
         [0026]     Load  10  is suspended chiefly on a man-made fibre rope  16 , made in segments with joints  18  along its length. A drum winch  20  mounted on deck  12  stores sufficient rope  16  for the depth of operation. The length of each segment may be 50 m, 100 m, or  300  m for example.  
         [0027]     A hoist platform  22  is suspended over the side of the vessel by a double-drop steel wire  24  a hoist winch  26  provided on deck  12  stores the wire  24 . Rope  16  passes through hoist jaws  28  mounted on hoist platform  22 , and engages part of a joint  18 , such that the weight of the load  10  is carried by hoist platform  22  and will stop the tension caused by the weight of the load is thus not experienced by portions of rope  16  above the level of platform  22 .  
         [0028]     A hang-off platform  28  is also suspended by wires  30   a  and  30   b  hang-off platform  28  carries a latch  32  which can be opened and closed to engage another portion of the joint  18  in rope  16 , which passes through an aperture in hang-off platform  28 . Hang-off platform  28  acts as a holding means and is arranged to stay at a relatively fixed height in relation to this sea surface, while hoist platform  22 , by operation of hoist system winch  26 , travels from a level just below hang-off platform  28  to a depth at least corresponding to the length of one segment in fibre rope  16 .  
         [0029]     Finally in this embodiment,  FIG. 1  shows a heave compensator  34 , comprising a number of sheaves (pulley wheels) mounted on a cross head  36  which is connected to the deck by a hydraulic ram  38 . Rope  16  and wires  24  and  30  all pass over respective sheaves mounted on cross head  36 . In operation, ram  38  is controlled automatically in response to heave (vertical motion) of the vessel, so as to isolate high-off platform  28  and hoist platform  22 , and hence load  10 , from this vertical motion. The principles of heave compensation are well-known, and appropriate systems are available from a number of manufacturers. The detail of the heave compensation system will not be described further. Moreover, it will be appreciated that heave compensator  34  can be omitted if it is not necessary in a given application.  
         [0030]     The arrangement shown in  FIG. 1  can be adapted to function without heave compensation, simply by eliminating ram  38  and envisaging the sheaves carried by cross head  36  being mounted at a fixed height above deck  12 .  
         [0031]     The various components of the lifting apparatus shown in  FIG. 1  will be described in more detail below. It should be appreciated that the detailed arrangement is only one possible example.  
         [0000]     Hoist System  
         [0032]      FIG. 12  shows in more detail the hoist system platform and the manner in which it is suspended by wire  24 . Heave compensator  34  is shown in broken outline, to avoid obscuring relevant detail. In this drawing, platform  22  is shown as a simple steel plate, which may of course be reinforced according to its load-bearing function. Aperture  40  can been seen, through which rope  16  and joint  18  can pass freely (the aperture is shown smaller than life in the figure to improve its visibility). Jaws  27   a  and  b  are provided each with half-conical bearing surfaces  42   a  and  42   b  for engaging part of a joint  18  in rope  16 . The detailed form of the joints in the rope will be described below in relation to  FIG. 4 . Jaws  27   a  and  27   b  are open and closed by means of hydraulic rams  44   a  and  44   b . An inset detail  46  shows the jaws in the closed position and engaging a joint  18  of rope  16 .  
         [0033]     As mentioned already, hoist platform  22  is supported in operation by wire rope  24 . Three sheaves  48 ,  50  and  52  are provided on hoist platform  22 , for guiding the wire  24  from two points of suspension at either end of the platform around the area of the aperture  40 . By this arrangement, vertical load on the jaws  27   a ,  27   b  acts substantially on the line between the two portions of wire  24 .  
         [0034]     Also associated with the hoist system platform and wire rope  24  are hoist winch  20  already mentioned, a strong point  53  on deck  12 , first and second sheaves  54  and  56  mounted on the heave compensator cross head  36  and a fixed sheave  58  mounted on the deck. The mounting and bearings of these sheaves are omitted for clarity. The functioning of the hoist will be described later.  
         [0000]     Holding Means/Hang-off Platform  
         [0035]      FIG. 3  shows in more detail the hang-off platform  28  and associated components. Again this comprises essentially a steel plate with an aperture  60  for passage of fibre rope  16  and joints  18 . Latch  32  comprises first and second latch pieces  62   a  and  62   b  which can be moved by small rams  64   a  and  64   b  to close off the aperture as shown inset at  66 . Wires  30   a  and  30   b  of fixed length support platform  28 , being terminated at two further strong points  68  and  70  on deck  12 . Wires  30   a  and  30   b  pass over to additional sheaves  72  and  74  respectively which are mounted on cross head  36  of heave compensator  34 .  
         [0000]     Fibre Rope System  
         [0036]      FIG. 4  shows the fibre rope system in more detail, with enlarged detail of a typical joint  18  shown inset at  80 . In this figure successive sections  16   a ,  16   b  and  16   c  of man-made fibre rope  16  are labelled separately. While these sections may in practice be 100 m or 300 m in length, they are shown shorter in the drawings for the purpose of illustration. The fibre rope used is far lighter, under water, than wire rope  24 . If desired, and particularly for extreme depths, neutrally-buoyant rope may be specified, such that the load on the hoist system is effectively independent of depth.  
         [0037]     In the condition shown, the bulk of rope  16  including joints  18  is wound on the drum of winch  20 . Winch  20 , serving as a rope store rather than a hoist, is driven to take up and pay out rope  16 , though not to lift the weight of load  10 . Further sheaves  82  and  84  are mounted respectively on heave compensator cross head  36  and deck  12  to pass the rope  16  from drum winch  20  over the side of the vessel, with heave compensation as required. The flanges of sheaves  82  and  84  are set wide enough to allow free passage of joints  18 , it being understood that the portions  16   b  and  16   c  of fibre rope  16  are, in operation, not subject to the weight of load  10  to reduce fatigue in the fibre rope as it bends.  
         [0038]     Referring to the inset detail  80 , each joint  18  in this embodiment comprises a symmetrical arrangement of components, permitting segmented rope  16  to be used without regard to the direction in which it has been wound on drum  20 . At the centre of each joint  18  is a circular plate or collar  86 , of a size suitable for engagement by the latched  32  of hang-off platform  28  (see detail  46  in  FIG. 3 ). Rope segments  16   a  and  16   b  either side of the joint are terminated by respective thimbles  88   a  and  88   b , which are connected to either side of collar  86  by shackles  90   a  and  90   b . Attached rigidly to each thimble  88   a ,  88   b  is a respective part-conical stopper  92   a ,  92   b . Each cone widens in the direction from the rope segment  16   a  to the terminating eye  88   a . The lower stopper  92   a  is thereby adapted to be engaged by the part-conical surfaces  42   a  and  42   b  of the hoist platform jaws  27   a  and  27   b , as seen in the detailed inset at  46  in  FIG. 2 .  
         [0039]     Each stopper  92   a ,  92   b  is formed by two halves of steel, clamped to the respective thimble  88   a ,  88   b  by a bolt  93 . The lower stopper  92   a  is shown with one half removed, to reveal the form of the thimble  88   a . This comprises a solid metal piece, with a channel guiding the rope  16  along path  16 ′ shown in broken lines. The rope doubles back and is spliced to itself in conventional fashion, the splice extending perhaps 3 or 4 m for security from the thimble. Unlike conventional thimbles, the thimbles  88   a  and  88   b  provide two distinct apertures in a solid body. The larger aperture allows passage of the shackle  90   a ,  90   b  which connects, via collar  86  and other parts, to the next rope segment. A smaller aperture  91  allows passage of the bolt  93  which, indirectly in this embodiment, allows connection of the rope segment  16   a  to the hoist platform  22 . The body can be formed entirely by casting, or assembled from a split tube and other pieces.  
         [0040]     Finally in relation to  FIG. 4 , it can be seen that a further cone stopper  94  is provided at the connection between the lowermost rope segment  16   a  and the load  10 . Stopper  94  again is dimensioned and oriented to be engaged by the jaws  27   a  and  27   b  of hoist platform  22 , as part of the lowering or lifting sequence, which will now be described in more detail.  
         [0000]     Set-up for Lifting and Lowering  
         [0041]     Summarising the configuration of the apparatus just described with reference to FIGS.  1  to  4 , an ordinary, cylinder-based heave compensator  34  is provided, comprised of a hydraulic ram  38  with cross head  36  upon which sheaves are mounted, together with a winch system ( 26 ,  22 ) wherein the hoist medium is wire rope ( 24 ).  
         [0042]     The wire  24  is taken from the winch  26  and passed through sheave  58  on the deck. It is then reeved through first sheave  54  on the heave compensator cross head and through a lowerable block in the form of platform  22  and sheaves  48 ,  50 ,  52 . The standing part is then returned to the ship and reeved through the second sheave  56  on the heave compensator and subsequently made fast to strongpoint  53  on the deck. This provides a heave compensated double-fall lowering (and lifting) system.  
         [0043]     As described above with reference to  FIG. 3 , wires  30   a  and  30   b  are led from two further strong points  68 ,  70  on the deck to hang-off platform  28  via the two additional sheaves  72 ,  74 . The platform  28  is thus heave compensated at the same rate and in phase with the lowering system.  
         [0044]     Man made fibre rope  16  (details in  FIG. 4 ) is provided in lengths that are the same length (or shorter) as the travel of the lowering system hoist platform  22 . These are terminated in eyes and joined together to form a continuous length suitable for the depth of the work and wound on to reel  20 . The rope  16  is then reeved through the deck and cross head sheaves  84 ,  82 .  
         [0000]     Lowering Operation  
         [0045]     With the complete apparatus constructed and prepared as just described, operation of the apparatus for lowering of a heavy load to the seabed proceeds as will now be described with reference to FIGS.  5  to  9 . It will be appreciated that lifting operations can be performed by a simple reversal of the lowering process.  
         [0046]      FIG. 5  shows the apparatus in an initial condition. Load  10  has previously been passed overboard by a suitable crane (not shown), and suspended by stopper  94  in the jaws of hoist platform  22 . Rope  16  has been passed through the aperture  60  in hang-off platform  28 , and connected to stopper  94  by a shackle. The exact sequence of these operations can be varied to suit. Either platform  22  or  28  may be made openable to facilitate entry of the rope into the respective aperture  40 ,  60 .  
         [0047]     By operation of wire winch  26  paying out wire  24 , the platform  22  and load  10  are lowered to the maximum depth of the lowering system with the rope  16  attached, thus drawing the rope  16  off the reel  20 .  
         [0048]     Reaching the position shown in  FIG. 6 , the first joint  18   a  in the rope  16  will now be at the hang-off platform  28  and a latch  32  is engaged which grips the collar of the joint.  
         [0049]     Referring now to  FIG. 7 , the load is now transferred to the rope  16  and hang-off platform  28 , while the jaws  27   a ,  27   b  on the travelling block (platform  22 ) are released and recovered to the surface ( FIG. 8 ). Once the hoist platform  22  is appropriately close beneath the hang-off platform  28 , it connects with the lower stopper  92   a  of joint  18   a  in the rope  16  and takes the load again. The joint&#39;s collar  86  is released from the platform  22  and lowering may once again be undertaken.  
         [0050]     In this manner the rope is never subjected to more than nominal load while being bend cycled through the winch, sheaves and especially the heave compensator. At the same time, all transfer operations are heave compensated.  
         [0051]     Support for the process may be provided by a remotely operated vehicle (ROV) and/or divers, not shown. These may be stationed by the lowest position of the hoist platform  22 , for example, where video observation and occasional intervention may be required to ensure reliable engagement and disengagement of the hoist from the rope stoppers  92   a  etc. Adequate observation may also or alternatively be provided by cameras mounted on the platform  22  and/or  28 .  
         [0052]     The method can be applied beneficially in oil &amp; gas field development (sub-sea construction) in depths beyond 300 m. General lifting and lowering operations can also be envisaged in depths down to full oceanic depth, for example for Salvage, Oceanography, and Military purposes.