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[0001]    The invention relates to well operations, especially in connection with production of hydrocarbons from underground formations. More specifically, the invention relates to a device for heave compensation of equipment on a moving vessel, as disclosed in the preamble of the independent claims. 
       BACKGROUND OF THE INVENTION 
       [0002]    During the recovery of hydrocarbons from wells in underground formations, the operator must sometimes carry out work in the wells. Such work may be maintenance or other technical operations, such as perforating, replacing or reperforating pipes, changing flow regulators, isolating production zones, monitoring production and logging pressure, flow and temperature. A motivating factor for the work is to increase the recovery rate of the well. This work, which is referred to by the collective term ‘well intervention’, is difficult to perform from conventional surface platforms, especially when subsea wells are involved. It is well known that subsea wells are less maintained than platform wells and hence have lower recovery rates. It is therefore desirable to have systems that are suitable for maintenance of subsea wells. 
         [0003]    A distinction is made between light well intervention and heavy well intervention. Today, light well intervention is carried out with the aid of wireline operations from a ship. Heavy well intervention (which includes the whole spectrum of intervention work) is usually carried out from a special moored, semi-submersible rig that is connected to the wells via a riser (pipe for gas and/or liquid). The special rig fills the gap between the ordinary drilling rigs and the vessels that carry out light well intervention using wireline operations, and has, inter alia, substantially lower costs than an ordinary drilling rig. 
         [0004]    To further optimise costs, it is therefore desirable that such special rigs are of maximum flexibility, such that, for example, in addition to carrying out heavy well intervention they can also carry out light well intervention using wireline-operated tools. 
         [0005]    Known floating intervention rigs have a drilling machine suspended from a tower via a heave compensation system for compensating for wave motions, which may comprise a combined hoisting and compensation cylinder. The tower and the cylinder are mounted on deck. In an ordinary device, the drilling machine is suspended from wires (so-called drill line), which run over guide pulleys at the top of the mast, via reversing pulleys at the top of the cylinder, to a fixed point at the top of the mast. The drilling machine can therefore be hoisted up and down by moving the cylinder, and the cylinder also compensates—within certain tolerances—for the motions of the rig such that the drilling machine is kept as steady as possible when carrying out intervention operations. A combined hoisting and compensation cylinder of this kind is capable of handling large loads, typically in the order of 250 tonnes. 
         [0006]    This device is used, for example, when landing large weights on the seabed or inside the drilling riser, as well as during drilling, and can be operated in passive or active compensation mode when there is a need for greater accuracy and control of the compensation. 
         [0007]    The prior art comprises WO 2007/145503 A1, which describes a device and a method for heave compensation. A mast is mounted on a floating vessel, a first compensation means that is attached via wire means on one side of the mast, and the other end of the wire means is further connected to a second heave compensation means attached to a tool unit for carrying out drilling operations. The second heave compensation means and the tool unit are moved along guide rails in the mast with the aid of dollies/lever arms. 
         [0008]    There are stringent requirements as regards the ability of the well intervention rig to—very accurately—compensate for the movements of the rig. Heave compensation requirements are often concretised as an ability to compensate weight change, positioning accuracy and speed limitation. In known well intervention rigs, however, the size of the combined hoisting and compensation cylinder, and the friction in the guide pulleys and reversing pulleys make it difficult to satisfy one or more of these requirements. There is therefore a need for a device that is capable of obtaining greater accuracy in heave compensation than is the case with known well intervention rigs. The invention meets this need and has other advantages in addition. 
       SUMMARY OF THE INVENTION 
       [0009]    Therefore, a device is provided for heave compensation of a tool unit which is suspended via one or more wire means in a mast mounted on a platform, each wire means at a first end being attached to the mast via an attachment and running via a first heave compensation unit, and each wire means at its other end being attached to a second heave compensation unit that is connected to the tool unit, characterised in that the second heave compensation unit comprises a movable compensation means which at its first end is attached to the wire means and which at its second end is attached to the tool unit, and that the second heave compensation unit comprises a releasable locking means with which the motions of the compensation means can selectively be prevented and allowed. 
         [0010]    In an embodiment, the compensation means is attached to the wire means via a connecting unit and is attached to the tool unit via a link element, the connecting unit and the link element being movable in relation to one another, and where the locking means is movable for selectively and releasably limiting the motions of the connecting unit and the link element in relation to one another. 
         [0011]    In an embodiment, the locking means comprises a locking bolt with a central, narrowed portion and broad shoulder portions at each end of the bolt. 
         [0012]    In an embodiment, the link element comprises a through hole having a first portion and a second portion, the first portion having a larger opening than the second portion. The first portion has a dimension that allows passage of the shoulder portions whilst the second portion has a dimension that does not allow passage of the shoulder portions, but does allow passage of the central portion. In an embodiment, the first portion has a circular cross-section and the second portion is elongate. 
         [0013]    In an embodiment, the wire means runs between the second heave compensation unit and the first heave compensation unit via guide pulleys that are mounted at an upper part of the mast, and run via the first heave compensation unit between the guide pulleys and the attachment in the mast. 
         [0014]    In an embodiment, the first heave compensation unit is connected at a first end to the platform and at a second end is movably attached to said wire means via reversing pulleys and a cylinder. 
         [0015]    In an embodiment, the device comprises locking means with which the movements of the compensation units can selectively be prevented, and the compensation units transfer loads between the tool unit and the wire unit as substantially rigid bodies. 
         [0016]    In an embodiment, the first heave compensation unit functions as an active heave compensator whilst the second heave compensation unit functions as a passive heave compensator. 
         [0017]    In an embodiment, the second heave compensation unit is an integral part of the tool unit. The first and second heave compensation units are preferably hydraulically operated. 
         [0018]    The second heave compensation unit can quickly and simply be installed on any drilling machine, without structural modifications of the drilling machine. 
     
    
     
       OVERVIEW OF THE FIGURES 
         [0019]    These and other characteristic features of the invention will be elucidated in the following description of preferred, non-limiting embodiments, with reference to the accompanying schematic drawings, wherein: 
           [0020]      FIG. 1  shows a mast with a compensation system, seen from a side; 
           [0021]      FIG. 2  shows a mast with the compensation means according to the invention, seen from the front; 
           [0022]      FIG. 3  shows the mast and the compensation means shown in  FIG. 2 , but from another side; 
           [0023]      FIG. 4   a  shows a section of a first embodiment of the invention, seen from a side, where the cylinders in the second heave compensation unit are in a retracted position; 
           [0024]      FIG. 4   b  shows the section shown in  FIG. 4   a , seen from the front; 
           [0025]      FIG. 4   c  corresponds to  FIG. 4   b , but shows the cylinders in the second heave compensation unit in a fully extended position; 
           [0026]      FIG. 4   d  corresponds to  FIG. 4   b , but shows the cylinders in the second heave compensation unit in a mid position; 
           [0027]      FIG. 5   a  shows a section of a second embodiment of the invention, seen from a side, where the cylinder in the second heave compensation unit is in a retracted position; 
           [0028]      FIG. 5   b  shows the section shown in  FIG. 5   a , seen from the front; 
           [0029]      FIG. 5   c  corresponds to  FIG. 5   b , but shows the cylinders in the second heave compensation unit in a fully extended position; 
           [0030]      FIG. 5   d  corresponds to  FIG. 5   b , but shows the cylinders in the second heave compensation unit in a mid position; 
           [0031]      FIG. 6   a  shows a section of a third embodiment of the invention, seen from a side, where the cylinder in the second heave compensation unit is in a retracted position; 
           [0032]      FIG. 6   b  shows the section shown in  FIG. 6   a , seen from the front; 
           [0033]      FIGS. 7   a  and  7   b  show the third embodiment of the heave compensation unit in an unlocked state, seen respectively from the front and from a side; 
           [0034]      FIGS. 7   c  and  7   d  are an enlarged section of  FIGS. 7   a  and  7   b  respectively, and  FIG. 7   e  is a sectional view of that shown in  FIG. 7   d ; 
           [0035]      FIGS. 8   a  and  8   b  shows a locked state, where the motion damper has not been activated; 
           [0036]      FIGS. 8   c  and  8   d  are enlarged sections of  FIGS. 8   a  and  8   b  respectively, and  FIG. 8   e  is a sectional view of that shown in  FIG. 8   d ; 
           [0037]      FIGS. 9   a  and  9   b  show a state in which it is possible to pull the locking bolt out of the keyhole, and  FIG. 9   c  is an enlarged sectional view of that shown in  FIG. 9   b ; and 
           [0038]      FIGS. 10   a  and  10   b  shows a position for weight reduction by screwing together drilling machine and drill string, a so-called “thread-saver” function, and 
           [0039]      FIG. 10   c  is an enlarged sectional view of that shown in  FIG. 10   b.    
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0040]      FIG. 1  shows a mast  1  located on a deck  2 , for example, on an intervention rig or drilling rig (not shown). An active heave compensation cylinder  7  and accumulator  8  and a combined hoisting and heave compensation cylinder  4  are mounted on the deck  2 . The heave compensation cylinder  4  is equipped at its upper end with reversing pulleys  5  for hoisting wires  6 . It is usual to have four or six parallel running hoisting wires. 
         [0041]    With reference to  FIG. 2 , the combined hoisting and heave compensation cylinder  4  has a length of stroke d, where the cylinder rod  12  is in a fully extended position. The fully retracted and extended positions, respectively, of the reversing pulleys are indicated by the reference numerals  5  and  5 ′. 
         [0042]    In the illustrated embodiments, the mast is constructed of a lattice structure in a known way, and will therefore not be described in more detail here. Similarly, the heave compensation units are driven by fluid reservoirs, regulating valves and systems, gas tanks and hydraulic power units. These components are well known to the skilled person and are therefore not referred to in more detail here. 
         [0043]    The mast is equipped with guide rails  9  for a drilling machine  11  in a known way. Furthermore, with reference to  FIGS. 2 and 3 , the wires  6  are connected to the upper part of the mast via an anchorage  10 . The wires  6  run from the anchorage  10  via the reversing pulleys  5  on the cylinder rod  12  of the combined hoisting and heave compensation cylinder  4 , onward via guide pulleys  3  at the top of the mast and then down to a compensator  20 , which is connected to the drilling machine  11 .  FIG. 2  shows the combined drilling machine  11  and compensator  20  in a lower position against the deck  2 , whilst these units in  FIG. 3  are raised slightly from the deck. 
         [0044]    Three embodiments of the compensator are described below. 
       First Embodiment 
       [0045]    Referring to  FIGS. 4   a - d , the compensator  20  comprises two cylinders  22   a,b  that are directly connected between the drilling machine  11  and the connecting link  24  of the hoisting wires. The hoisting wires (not shown in  FIGS. 4   a - c ) are connected to the connecting link  24  via suitable attachments  25 .  FIG. 4   a  further shows a dolly  21  attached to the drilling machine  11  for engagement with the tower guide rail as described above. 
         [0046]    The cylinders  22   a,b  are equipped with a lock  23  with which the cylinders can be locked in the retracted position when the system is not in use. The load from the drilling machine  11  is transferred to the lifting wires via the cylinders  22   a,b  and the connecting link  24 . 
         [0047]      FIGS. 4   a,b  show the cylinders  22   a,b  in a retracted position,  FIG. 4   c  shows the cylinders  22   a,b  in a fully extended position, whilst  FIG. 4   d  shows the cylinders  22   a,b  in a mid position. The reference numeral  26  indicates the respective cylinder rods of the cylinders. 
         [0048]    Fluid reservoirs and control units for the heave compensation cylinders  22   a,b  are in accordance with the prior art and are therefore not discussed in more detail here. 
       Second Embodiment 
       [0049]    Referring to  FIGS. 5   a - d , the compensator  20 ′ comprises one cylinder  27  which is connected between the drilling machine  11  (via a link  29 ) and a connecting link  28  for the hoisting wires (only attachments  25  for the wires are shown in  FIGS. 5   b - d ).  FIG. 5   a  further shows a dolly  21  attached to the drilling machine  11 , for engagement with the tower guide rail as described above. 
         [0050]    The cylinder  27  is equipped with lock  23  with which the cylinders can be locked in collapsed position when the compensator  20 ′ is not in use. The load from the drilling machine  11  is transferred to the hoisting wires via the cylinder  27  and the connecting link  28 . 
         [0051]    The  FIGS. 5   a,b  show the cylinder  27  in a retracted position,  FIG. 5   c  shows the cylinder  27  in a fully extended position, whilst  FIG. 5   d  shows the cylinder  27  in a mid position. The reference numeral  26  indicates the cylinder rod of the cylinder. Fluid reservoirs and control units for the heave compensation cylinder  27  are in accordance with the prior art and are therefore not discussed in more detail here. 
       Third Embodiment 
       [0052]    With reference to  FIGS. 6   a  to  10   c , the compensator  20 ″ comprises one compensator cylinder  31  (also referred to as a motion damper) that is mounted to a connecting piece  32  for the hoisting wires.  FIGS. 6   a  and  6   b  further show a dolly  21  attached to the drilling machine  11 , for engagement with the tower guide rail as described above. On the connecting piece  32  there is mounted a lock  30  (which, for example, is hydraulically operated via an actuator  86 ) that is capable of connecting together the connecting piece  32  and the below-lying link element  33  from which the drilling machine  11  is suspended. The lock  30  bears all load when it is in the locked position and the compensator  20 ″ is not in use, such that the load from the drilling machine  11  is transferred to the hoisting wires via the link element  33 , the hydraulically operated lock  30  and the connecting piece  32 .  FIGS. 6   a,b  show such a locked position, in which the cylinder  31  is in a retracted position and is inactive. Fluid reservoirs and control units for the compensation cylinder  31  are in accordance with the prior art and are therefore not discussed in more detail here. 
         [0053]      FIGS. 7   a  to  10   c  are further illustrations of the compensator cylinder  31 , the connecting piece  32 , the link element  33  and the connection between these components in different configurations. 
         [0054]    The connecting piece  32 , to which the compensator cylinder  31  and the wires  6  are fastened, comprise two plate elements  32   a,b  arranged spaced apart and fastened together by means of upper bolts  81  and lower bolts  83 . The lock  30 , with its hydraulic actuating mechanism, is also attached to the connecting piece (hydraulic connecting lines are not shown, as they are prior art). The lock  30  comprises a housing  30 ′ with a locking bolt  87 , a locking bolt cylinder  88  and a position sensor  89  for the locking bolt. The locking bolt  87  has a central narrowed portion  87   b,  and broad portions (flanges)  87   a  at each end. 
         [0055]    In this illustrated embodiment, the link element  33  also has a plate form, and is disposed between the plate elements  32   a,b  of the connecting piece in such a way as to be movable. The cylinder rod  26  of the compensator cylinder  31  (whose housing is fastened to the connecting piece) is secured to the link element  33  via a fastening bolt  84 . The link element  33  is provided with a through “keyhole”  82 , which is adapted for receiving the locking bolt  87 . The keyhole  82  is elongate and has a lower portion  82   a  that has a larger opening than the above-lying, slightly narrower portion  82   b  of the keyhole. 
         [0056]    The broad end portions  87   a  of the locking bolt  87  have a cross-sectional dimension that allows passage through the lower, broad portion  82   a  of the keyhole and into the respective support holes  37  in the side plates  32   a,b , but does not allow passage through the above-lying portion  82   b.  The central, narrowed portion  87   b  of the locking bolt has a cross-sectional dimension that allows movement of the locking bolt up and down in the keyhole, also in the slightly narrower portion  82   b.    
         [0057]      FIGS. 7   a - e  show an unlocked state. The locking bolt  87  has been fully withdrawn from the keyhole  82  in the link element  33 , thereby allowing the link element  33  to move between the two side plates  32   a,b  in the connecting piece  32 . The link element  33 , which is connected to the compensator cylinder  31  via the cylinder rod  26 , can move between a lower position (as shown) and an upper position, limited by, respectively, the upper shoulders  85  and lower shoulders  90  and the lower (stop) bolts  83 . The figures show that when the link element  33  is in the full lower position, it will rest on the two lower bolts  83  via the shoulders  85 , which will prevent the drilling machine from falling if the compensator cylinder  31  should fail. When the cylinder stroke is reduced to about half stroke (compared with that shown in  FIGS. 7   a - d ), the system will be in the position for motion damping. 
         [0058]      FIGS. 8   a - e  show a locked stated wherein the motion damper  31  is not in use. The system is locked, such that all load passes through the locking bolt without affecting the motion damper. The load is suspended from the drilling machine (not shown) and is transferred to the link element  33 . The link element  33  is suspended from the locking bolt  87  that is seated in holes in the side plates  32   a,b  of the connecting piece  32 . From the locking bolt  87 , the load passes through these two side plates up to the upper bolts  81 , which connect the hoisting wires to the connecting piece  32 . This is a normal configuration of the suspension system for drilling and lifting/lowering the drill string. 
         [0059]      FIGS. 9   a - c  show a state in which the lower link element  33  has been lifted up to a maximum height in the keyhole  82 , such that the locking bolt shoulders  87   a  and the broad portion  82   a  of the keyhole are aligned with one another. This is the only position in which it is possible to pull the locking bolt  87  out of the keyhole  82 . The locking bolt is moved (pulled) horizontally by means of a cylinder  88  equipped with a stroke sensor  89  such that there is control of whether the bolt is in lockable engagement with the keyhole or not. This lifting of the link element  33  is done with the aid of the compensator cylinder  31 . As described above,  FIGS. 9   a - c  also show that the lower dimension (diameter) of the keyhole is so great that the locking bolt can be moved horizontally through the keyhole. In the upper, narrower part of the keyhole, it is not possible to move the locking bolt in a horizontal direction owing to the shoulders having increased diameter at the ends. However, the locking bolt is free to be moved vertically in the keyhole, throughout the length of the keyhole. The sectional view in  FIG. 9   c  shows the locking bolt half out of engagement. It can be seen that the locking bolt has a shoulder  87   a  with a larger diameter at both ends than the diameter of the central portion  87   b.  The diameter of the shoulders fits in the supporting holes  37  of the connecting piece  32  and the lower part  82   a  of the keyhole in the link element. 
         [0060]      FIGS. 10   a - c  shows a state in which the locking bolt  87  is locked, but the damping cylinder  31  (not shown in  FIGS. 10   a - c ) is actuated with a small stroke such that the locking bolt is roughly in the middle of the keyhole  82 . This is a position for weight reduction on screwing together the drilling machine and the drill string, a so-called “thread-saver” function. 
         [0061]    The task of the compensator  21 ″ is to hold tool that has been lowered into the well in an accurate position without subjecting equipment installed in the well to weights greater than typically ±500 kg whilst the main heave compensation is in progress with the aid of the combined hoisting and heave compensation cylinder  4 , with associated accumulator tanks and other necessary, known equipment (not shown). The compensator  21 ″ with compensation cylinder  31  takes the “peaks” of the damping from the main compensator  4 . 
         [0062]    The compensator  21 ″ may thus have at least the following two functions:
   a) Damping/minimising vertical motion and controlling/minimising load against components inside the well from tools that may be lowered down into it; and   b) Reducing the load between the shaft of the drilling machine and the top of the drill pipe when they are to be screwed together (thread-saver).   
 
         [0065]    Although the compensator  21 ″ is described here as being made up of a lower link element  33  that is movably arranged between the two side plates  32   a,b  of the connecting piece  32 , the invention should not be limited to such designs, as a variant may be an inverted configuration where the lower link element has two side plates and the connecting piece comprises one element that is movably arranged therebetween. The invention should also not be limited to plate-shaped elements. 
       Features Common to the Embodiments 
       [0066]    The combined hoisting and heave compensation cylinder  4  and associated components (in the following also referred to as Stage 1) are used when landing large loads on the seabed or inside the drilling riser, and during drilling. In such situations, the compensator  20 ;  20 ′;  20 ″ is not necessarily in use, i.e., the cylinders are locked via their respective locking mechanisms  23 ;  30 . 
         [0067]    For heave compensation, Stage 1 can be operated in passive compensation mode or in active compensation mode. 
         [0068]    In situations where greater accuracy and control of the compensation are required, as for example, during well intervention, Stage 1 will be operated in active compensation mode. Stage 1 will therefore be able to achieve heave compensation down to a certain minimum level. 
         [0069]    The compensator  20 ;  20 ′;  20 ″ (in the following also referred to as Stage 2) is used together with Stage 1 in order to further increase sensitivity and accuracy, and to ensure that the power of the drilling machine does not exceed a defined minimum value. Stage 2 then functions as a passive heave compensator. The compensator in Stage 2, which may have a relatively short stroke length, is constructed such that the cylinder piston is held stationary until loaded with a predefined weight. When such a predefined weight has been reached, the compensator in Stage 2 will compensate by either retracting or extending the cylinder rod  26 . 
         [0070]    Examples of situations in which the need for a Stage 2 is present include landing of lighter equipment within the casing and subsea safety valves. 
         [0071]    A two-stage heave compensator of this kind can thus—very accurately—compensate for the motions of the rig. Within given operational parameters (e.g., max heave motion of rig), Stage 1 and Stage 2 in combination can compensate for a relatively small weight change and obtain major positioning accuracy at a limited speed. As mentioned above, a combined hoisting and compensation cylinder can typically handle loads of the order of 250 tonnes. In such a connection, a relatively small weight change may be of the order of ±500 kg, and the positioning accuracy can be of the order of ±10 cm. 
         [0072]    Stage 1 can handle large loads and most of the heave. The compensator  20 ;  20 ′;  20 ″ (Stage 2) is however substantially smaller than Stage 1 and thus generates less packing friction. In addition, the compensator in Stage 2 is located on top of the drilling machine  11  such that it does not take with it other friction than that in the compensator  20 ;  20 ′;  20 ″, and possibly some from the well. 
         [0073]    The compensator  20 ;  20 ′;  20 ″ is thus able to reduce the load amplitude from Stage 1 to a load oscillation that is within the requirement for weight change compensation. 
         [0074]    The device according to the invention functions in this way as a two-stage heave compensator, where the combined hoisting and heave compensation cylinder  4  (Stage 1) handles the large loads, whilst the compensator  20 ;  20 ′;  20 ″ (Stage 2, which has better sensitivity and greater accuracy) is able to compensate for loads that are smaller than Stage 1 is adapted to compensate for. 
         [0075]    In an embodiment, the load for which the compensator  20 ;  20 ′;  20 ″ is designed to compensate may be of the order of 8-10% of the load capacity of the hoisting system. 
         [0076]    It will be appreciated that the device for heave compensation can be used for purposes other than well intervention. 
         [0077]    The numerical values in the description above have been included to illustrate the application of the invention, and should not be regarded as a limitation of the invention.

Summary:
A device for heave compensation of a tool unit that is suspended via one or more wires from a mast mounted on a platform, each wire at a first end being attached to the mast via an attachment and running via a first heave compensation unit, and each wire at a second end being attached to a second heave compensation unit that is connected to the tool unit. The second heave compensation unit includes a movable compensation mechanism, which at its first end is attached to the wire and which at its second end is attached to the tool unit. The second heave compensation unit includes a releasable locking device with which the motions of the compensation mechanism can be selectively prevented and allowed.