Patent Publication Number: US-9422791-B2

Title: Heave compensation and tensioning apparatus, and method of use thereof

Description:
TECHNICAL FIELD 
     A heave compensation and tensioning apparatus is disclosed for use on floating drilling vessels such as drilling ships and semi-submersible drilling vessels. Particularly, though not exclusively, a heave compensation and tensioning apparatus is disclosed for use as a primary heave compensator or back up to the rig&#39;s drill string compensator. 
     BACKGROUND 
     As oil and gas offshore exploration and production operations are increasingly established in deeper waters, it has become more common for drilling activities to be performed from rigs that float on the surface of the water, such as drilling vessels or semi-submersible drilling rigs. Unlike fixed rigs or jack-up rigs, floating rigs are subject to wave motion, causing up- and down motion, which must be compensated for during drill, well completions, well interventions and other operations. Wave motion is of particular concern during “locked-to-bottom” operations (i.e. well completion, well testing and well intervention) where a landing string is physically connected to the wellhead at the seabed. Loss of heave compensation can lead to severe consequences. 
     Apart from the operational difficulties arising from the up-and-down motion of the floating rig, significant safety issues also arise, in particular the potential for the landing string to fracture or buckle and cause a blowout. Indeed, safety standards in offshore operations demand that a heave compensation system be regarded as an essential component of a floating rig during locked-to-bottom operations. 
     Known heave compensation systems may be described as employing passive heave compensation or active heave compensation. 
     A simple passive heave compensator is a soft spring which effectively strokes in and out in response to string loads as the vessel heaves up and down while effectively holding constant tension on the string. Exemplary types of simple passive heave compensators are crown-mounted compensators or drill string compensators. Passive heave compensators employ hydraulic cylinders and associated gas accumulators to store and dissipate the wave energy. 
     Active heave compensation differs from passive heave compensation by having an externally powered control system that actively tries to compensate for any movement at a specific point. Exemplary types of active heave compensation include active heave draw works which employ electric or hydraulic winch systems to raise and lower the top drive in response to the vessel motion. 
     Active and passive heave compensation systems may be combined to provide semi-active heave compensation systems. 
     The essential nature of the heave compensation function to a floating rig is such that safety standards also demand that they be designed such that no single component failure shall lead to overall failure of the system. They should also be “fail to safety” meaning that in the event of any failure the system defaults to a compensating state, which is the safest state during locked-to-bottom operations. While active heave draw works have numerous benefits, they fail to a “locked condition”. Safe operations and industry standards require additional means of safety to be implemented in the system/equipment configuration. Additional means of safety may include an in-line tensioner, design of a weak link in the riser/landing string, limiting operation parameters to be within the stretch limit of the riser, and so forth. 
     Generally, these operating parameters place constraints on operators which have direct impact on productivity and efficiency. All these existing options have limitations. In the case of an inline tensioner there are concerns about the how the inline tensioner behaves when run in series with the active heave draw works. In the case of the weak link in the riser, they typically only provide protection in an over-tensioned case and once broken, they provide no support to the landing string thereafter. In the case of limiting operating parameters to within the stretch of the riser, this can impose considerable downtime during offshore operations. 
     There is therefore a need for an alternative or improved heave compensation apparatus which may operate as a primary heave compensator or as a back-up to the rig&#39;s drill string compensator in the event of failure or disablement of the rig&#39;s drill string compensator. 
     There is also a need for an improved heave compensation apparatus which can be used as a lift frame for the installation of pressure control equipment during well testing/well intervention work, as those components are installed in the congested space of the drilling derrick. 
     The above references to background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the heave compensation and tensioning apparatus as disclosed herein. 
     SUMMARY 
     Generally, a heave compensation and tensioning apparatus, and a method of use thereof, is disclosed. The heave compensation and tensioning apparatus may be employed as a primary heave compensation and tensioning apparatus or as back-up to the rig&#39;s primary compensator in the event of failure or disablement of the rig&#39;s primary compensator. The primary compensator may be in the form of a drill string compensator. According to one aspect, there is disclosed a heave compensation and tensioning apparatus for a floating vessel, said apparatus comprising:
         one or more cylinders, the one or more cylinders having a respective piston head and a piston rod therein, a free end of the one or more piston rods being operatively associated with a top drive system of the floating vessel and a fixed end of the one or more cylinders being fixed relative to a flowhead assembly;   one or more piston accumulators;   a primary hydraulic fluid line interconnecting the one or more piston accumulators and the one or more cylinders;   a control valve assembly arranged to control fluid flow through the primary hydraulic fluid line and thereby retract or extend the one or more piston rods in response to, and to compensate for, heave of the floating vessel, the control valve assembly further comprising an isolation valve capable of locking the one or more piston rods at any point along their respective stroke path, thereby forming a rigid link between the top drive system and the flowhead assembly when said apparatus is operated as a back up to a primary compensator;   the primary hydraulic fluid line being further provided with a bypass fluid flowpath comprising a first bypass line and a second bypass line, each bypass line being configured to bypass the isolation valve, whereby the control valve assembly is configured to redirect fluid flow through the first bypass line when pressure in the cylinder exceeds an upper pressure threshold and through the second bypass line when pressure in the cylinders falls below a lower pressure threshold, thereby allowing the apparatus to continue to compensate for heave of the floating vessel.       

     In one embodiment said heave compensation and tensioning apparatus for a floating vessel comprises:
         a frame having an upper section adapted for attachment to a top drive system and a lower section adapted to interface with a flowhead assembly;   a pair of cylinders fixed to the lower section of the frame, each cylinder having a piston head and a piston rod therein, a free end of the piston rod being operatively associated with the upper section of the frame;   a pair of piston accumulators;   a primary hydraulic fluid line interconnecting the piston accumulators and the cylinders;   a control valve assembly arranged to control fluid flow through the primary hydraulic fluid line and thereby retract or extend the piston rods in response to, and to compensate for, heave of the floating vessel, the control valve assembly further comprising an isolation valve capable of locking the piston rods at any point along their respective stroke path, thereby forming a rigid link between the flowhead assembly and the top drive system when said apparatus is operated as a back up to a primary compensator;   the primary hydraulic fluid line being further provided with a bypass fluid flowpath comprising a first bypass line and a second bypass line, each bypass line being configured to bypass the isolation valve, whereby the control valve assembly is configured to redirect fluid flow through the first bypass line when pressure in the cylinders exceeds a upper pressure threshold and through the second bypass line when pressure in the cylinders falls below a lower pressure threshold, thereby allowing the apparatus to continue to compensate for heave of the floating vessel.       

     In another embodiment said heave compensation and tensioning apparatus for a floating vessel comprises:
         a frame having an upper section and a lower section adapted to interface with a flowhead assembly;   a cylinder fixed to the upper section of the frame, the cylinder having a piston head and a piston rod therein, a free end of the piston rod being adapted for attachment to a top drive system;   a piston accumulator;   a primary hydraulic fluid line interconnecting the piston accumulator and the cylinder;   a control valve assembly arranged to control fluid flow through the primary hydraulic fluid line and thereby retract or extend the piston rod in response to, and to compensate for, heave of the floating vessel, the control valve assembly further comprising an isolation valve capable of locking the piston rod at any point along its respective stroke path, thereby forming a rigid link between the top drive system and the flow head assembly when said apparatus is operated as a back up to a primary compensator;   the primary hydraulic fluid line being further provided with a bypass fluid flowpath comprising a first bypass line and a second bypass line, each bypass line being configured to bypass the isolation valve, whereby the control valve assembly is configured to redirect fluid flow through the first bypass line when pressure in the cylinder exceeds a upper pressure threshold and through the second bypass line when pressure in the cylinder falls below a lower pressure threshold, thereby allowing the apparatus to continue to compensate for heave of the floating vessel.       

     Where the isolation valve is closed unintentionally, consequently hydraulically locking the system, the apparatus will automatically revert to a compensating state. Further, in embodiments where the apparatus comprises a back up to the primary compensator, the apparatus is adapted to rapidly activate and provide compensation if the primary compensator fails. 
     In one embodiment the piston accumulator comprises a cylinder barrel having a hydraulic chamber and a pneumatic chamber defined therein, wherein the pneumatic chamber is in communication with a pneumatic pressure vessel via a pneumatic fluid line. The primary hydraulic fluid line may interconnect the hydraulic chambers of the piston accumulators and the cylinders. 
     In another embodiment the primary hydraulic fluid line may be further provided with an anti-recoil valve having a restricted flow path, whereby under sudden loss of load the anti-recoil valve is configured to close to a restricted opening, limiting the maximum flow of hydraulic fluid from the piston accumulator(s) to the cylinder(s). In further embodiments, the restricted flow path may be provided in a further bypass line. Sudden loss of load may be indicated by a vertical acceleration of the frame. The valve may be triggered to close when the frame velocity exceeds the maximum normal operating heave velocity. 
     The disclosure also describes a method of activating a back-up compensator to provide heave compensation for a floating vessel equipped with a primary compensator when said primary compensator fails, the method comprising:
         providing the back-up compensator according to the first aspect as defined above;   locating the one or more cylinders of the back-up compensator between the top drive system and the flow head assembly in a manner whereby the free ends of the piston rods are operatively associated with the top drive system and the fixed ends of the cylinders are fixed relative to the flow head assembly;   locking the isolation valve of the back-up compensator to form a rigid link between the top drive system and the flow head assembly;   when said primary compensator fails thereby causing the pressure in the one or more cylinders to exceed an upper pressure threshold or fall below a lower pressure threshold, redirecting fluid flow to bypass the locked isolation valve via the first bypass line or the second bypass line, respectively; and,   allowing the piston rods of the one or more cylinders to retract or extend in response to, and to compensate for, heave of the floating vessel.       

     It will be appreciated that there are load and space constraints in installing compensators into the drill derrick. In respect of heave compensators employing hydraulic cylinders, respective low pressure air accumulators in fluid communication with the blind side of the cylinders are generally provided to accommodate fluctuation in the gas pressure of the blind side of the cylinder when the piston rod of the cylinder retracts or extends in response to, and to compensate for, heave of the floating vessel. The heave compensation and tensioning apparatus described herein provides an apparatus where such additional low pressure air accumulators are redundant, thereby resulting in a reduced weight and footprint in comparison with prior art heave compensators. 
     Accordingly, there is disclosed a heave compensation and tensioning apparatus for a floating vessel, said apparatus comprising
         one or more cylinders having a respective piston head and piston rod therein, wherein each piston rod is hollow and in fluid communication with a respective blind side of each respective cylinder via an aperture in the piston head, the hollow piston rod thereby defining a low pressure air accumulator therein to accommodate fluctuation in gas pressure in the blind side of the cylinder when the piston rod retracts or extends in response to, and to compensate for, heave of the floating vessel.       

     A hydraulic cylinder for use in a heave compensating and tensioning apparatus for a floating vessel, the cylinder having a piston rod and a piston head therein, a free end of the piston rod capable of being operatively associated with a fixed point on the floating vessel, wherein each piston rod is hollow and in fluid communication with a respective blind side of each respective cylinder via an aperture in the piston head, the hollow piston rod thereby defining a low pressure air accumulator therein to accommodate fluctuation in gas pressure in the blind side of the cylinder when the piston rod retracts or extends in response to, and to compensate for, heave of the floating vessel. 
     The disclosure also provides a method of compensating for heave of a floating vessel, the method comprising:
         providing a heave compensation apparatus comprising:   one or more cylinders, each cylinder having a respective piston rod and piston head, a free end of the one or more piston rods being operatively associated with a top drive system of the floating vessel and a fixed end of the one or more cylinders being fixed relative to a flowhead assembly;   wherein each piston rod is hollow and in fluid communication with a respective blind side of each respective cylinder via an aperture in the piston head, the hollow piston rod thereby defining a gas accumulator therein to accommodate fluctuation in gas pressure in the blind side of the cylinder when the piston rod retracts or extends in response to, and to compensate for, heave of the floating vessel;   locating the one or more cylinders between the top drive system and the flowhead assembly in a manner whereby the free ends of the piston rods are operatively associated with the top drive system and the fixed ends of the cylinders are fixed relative to the flow head assembly; and,   allowing the piston rods of the cylinders to retract or extend in response to, and to compensate for, heave of the floating vessel.       

    
    
     
       DESCRIPTION OF THE FIGURES 
       Notwithstanding any other forms which may fall within the scope of the apparatus as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a partial schematic representation of a derrick and drill floor of a floating vessel showing a heave compensation and tensioning apparatus in accordance with one embodiment configured in-line with various components used in locked-to-bottom operations for oil and gas reserves offshore; 
         FIG. 2  is a schematic representation of a heave compensation and tensioning apparatus for a floating vessel in accordance with the disclosure, wherein the piston rods of said apparatus are shown in mid-stroke; 
         FIG. 2 a    is a schematic representation of the heave compensating and tensioning apparatus for a floating vessel, wherein the apparatus is hydraulically locked; 
         FIG. 3  is a schematic representation of the heave compensating and tensioning apparatus for a floating vessel, wherein the apparatus is mechanically locked for landing; 
         FIG. 4  is a schematic representation of the heave compensating and tensioning apparatus, showing re-direction of fluid flow through a first bypass line on an up heave; 
         FIG. 5  is a schematic representation of the heave compensating and tensioning apparatus, showing re-direction of fluid flow through a second bypass line on a down heave; 
         FIG. 6  is a schematic representation of the heave compensating and tensioning apparatus, showing constriction of fluid flow in the event of a sudden loss of load; 
         FIG. 7  is a perspective view of an upper section of a frame of said apparatus shown in  FIGS. 1-6 ; 
         FIG. 8  is a perspective view of lower section of the frame of said apparatus shown in  FIGS. 1-6 ; 
         FIG. 9 a    is a perspective view of an intermediate section of the frame of said apparatus shown in  FIGS. 1-6 ; 
         FIG. 9 b    is a plan view of the intermediate section of the frame shown in  FIG. 9 a   ; and, 
         FIG. 10  is a partial schematic representation of a derrick and drill floor of a floating vessel showing a heave compensation and tensioning apparatus in accordance with an alternative embodiment configured in-line with various components used in locked-to-bottom operations for oil and gas reserves offshore. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Embodiments of a heave compensation and tensioning apparatus for a floating vessel will now be described by way of example only, and with particular (though not exclusive) reference to drilling for oil and gas reserves offshore. 
     Referring to  FIG. 1  there is shown a partial and schematic view of a derrick  100  and a drill floor  200  of a floating vessel used in locked-to-bottom operations for oil and gas production offshore. The derrick  100  extends upwardly above the drill floor  200  and supports the main hoisting and drilling components used in drilling operations. 
     The derrick  100  may support a hoisting assembly, such as a block and tackle, for raising and lowering a landing string  110  which may be configured to pass through the drill floor  200  and facilitate well testing/well intervention of a subsea production well. A lower end of the landing string  110  may be fixed to the wellhead at the seafloor by means of a tubing hanger in what may be termed ‘locked-to-bottom’ operations. A top drive system  120  may be provided to also facilitate lowering or lifting of the landing string  110  into or out of the wellbore. Fluid produced at the wellhead may be directed through a riser to a flowhead assembly  130  located above the drill floor  200  and thence directed to well testing/processing facilities located elsewhere topside. 
     The hoisting assembly may be provided with a primary heave compensation system. The primary heave compensation system may be an active heave drawworks system or a passive heave compensator mounted on the top of the derrick  100 . As discussed above, if this primary heave compensation system fails or becomes inoperative, the fluctuation in the vertical position of the floating vessel relative to the seafloor due to wave motion will place the landing string  110  under alternating compression and tension. 
     Accordingly, the heave compensating and tensioning apparatus  10 , in the embodiment described herein, provides a back-up or secondary heave compensation system. Alternatively, the apparatus  10  can operate as the primary heave compensation system. The heave compensating and tensioning apparatus  10  may be configured in-line below the rig&#39;s primary heave compensation system. Said apparatus  10  may be operatively associated at one end thereof with the top drive system  120  and fixed at an opposing end thereof to the flowhead assembly  130 . Installed in this way, the heave compensating and tensioning apparatus  10  may be suspended above the drill floor  200  of the floating vessel. 
     As described in the embodiment herein, the heave compensating and tensioning apparatus  10  may be additionally supported by an upper dolly track  140  and a lower guide frame  150  engaged with dolly tracks in the derrick  100 . The lower guide frame  150  may also be adapted to provide a deck  160  on which to house a plurality of winches, including a dedicated man-riding winch to provide operator access to said apparatus  10 , thereby reducing access difficulties due to relative motion between drill floor and said apparatus  10 . It will be appreciated that the derrick  100  may also support a plurality of umbilicals  180  for conveying pneumatic fluid, hydraulic fluid, electrical power, and control signals to said apparatus  10 . 
     In normal inline operation, the heave compensating and tensioning apparatus  10  may be disposed in ‘locked’ mode, as will be described later, so that the primary heave compensation system accounts for the heave of the floating vessel. In the event of failure of the primary heave compensation system, however, the heave compensating and tensioning apparatus  10  may be automatically actuated to provide primary heave compensation for the floating vessel. 
     Referring generally to  FIGS. 2 to 6 , where like reference numerals refer to like parts throughout, there is shown the heave compensating and tensioning apparatus  10  as described herein. 
     The apparatus  10  includes a frame  12 , a pair of cylinders  14  in the form of hydraulic cylinders, and a pair of piston accumulators  16  in operative communication with the pair of cylinders  14  via a primary hydraulic line  18 . 
     The frame  12 , which will be described later in more detail with reference to  FIGS. 7-9 , includes an upper section  20  adapted for attachment to the top drive system  120  and a lower section  22  adapted to interface with the flowhead assembly  130 . The frame  12  also includes an intermediate section  24  for providing rigid structural support for the pair of cylinders  14 . 
     Each cylinder  14  includes a cylinder barrel  26  and a piston rod  28  telescopically translatable within the cylinder barrel  26 . In some embodiments, the piston rod may have a stroke of up to 10 m or more which enables the apparatus  10  to compensate for both vessel heave and tide without the need to adjust the drawworks elevation from the drill floor  200 . 
     A lower end  30  of the cylinder barrel  26  is mounted to the lower section  22 , as will be described in more detail with reference to  FIG. 8 . 
     The piston rod  28  has a free end  32  with a clevis  34  associated therewith. In use, the clevis  34  of each piston rod  28  is operatively associated with the upper section  20  of the frame  12 , as will be described in more detail with reference to  FIG. 7 . 
     An opposing end  36  of the piston rod  28  is associated with a piston head  38 . In use, the piston head  38  is translatable within the cylinder barrel  26 , thereby defining a hydraulic chamber  40  that is filled with hydraulic fluid and a ‘blind’ chamber  42  that is filled with air. The ‘blind’ chamber  42  is commonly referred to as the blind side of the cylinder  14 . 
     In one particular embodiment, the piston rod  28  may be hollow and in fluid communication with the blind chamber  42  of the cylinder  14  via an aperture  44  in the piston head  38 . In this configuration, the hollow piston rod may define a low pressure air accumulator therein to accommodate fluctuation in gas pressure in the blind chamber  42  of the cylinder  14  when the piston rod  28  retracts, or extends in response to, and to compensate for, heave of the floating vessel. Advantageously, this embodiment eliminates the need for low pressure gas accumulators disposed in fluid communication with the blind side of the cylinder  14 , thereby reducing the weight and footprint of the apparatus  10  in comparison with prior art compensators. 
     Each piston accumulator  16  includes a hydraulic chamber  46  and a pneumatic chamber  48 . The hydraulic chamber  46  is filled with hydraulic fluid and is in operative communication with the corresponding hydraulic chamber  40  of the cylinder  14  via the primary hydraulic fluid line  18 . 
     The pneumatic chamber  48  is in communication with a pneumatic pressure vessel  50  via a pneumatic fluid line  52 . 
     In some embodiments the pneumatic pressure vessel  50  may comprise a plurality of air pressure vessels (APV). Each APV may be 1-2 kL, preferably having pressure greater than 100 bar, even more preferably having pressure greater than 200 bar. The pneumatic pressure vessel  50  may operate as a passive ‘spring’ in the heave compensation and tensioning apparatus  10  by storing and dissipating the energy associated with wave motion. 
     The pneumatic pressure vessel  50  may be in fluid communication with an air pressure charging module  54  via line  56 . The air pressure charging module  54  comprises additional high pressure air storage to adjust the pressure of the apparatus  10  when required (e.g. increase pneumatic pressure to compensate for increased hook load). High pressure air fill valve  58  is provided on line  56  and may be opened to increase air pressure to account for increased hook load. Conversely, high pressure air vent valve  60  is also provided on line  56  and may be opened to decrease air pressure to account for decreased hook load. The air pressure charging module  54  is typically at a higher pressure than (up to 150-200%) the pneumatic pressure vessel  50 . For example, if the pneumatic pressure vessel  50  is at 210 bar, the air pressure charging module  54  may be at 345 bar. 
     A compressor  62 , in the form of a high pressure air compressor, may also be provided to supply clean dry high pressure air to initially pressurise the entire apparatus  10  or to charge the air pressure charging module  54  via line  64 . Advantageously, the compressor  62  allows the apparatus  10  to be operated independently from any other high pressure air supply system of the floating vessel. 
     The air pressure charging module  54  is arranged to provide prompt adjustment of air pressure and loads to account for sudden changes in string weight without having to solely rely on the compressor  62 . 
     Advantageously, the pneumatic pressure vessel  50 , the air pressure charging module  54  and the compressor  62  may be disposed on a separate deck, for example in a non-hazardous safe zone. 
     The apparatus  10  further comprises a control valve assembly  65 . The control valve assembly  65  is arranged to control the flow of hydraulic fluid through the primary hydraulic fluid line  18  and thereby retract or extend the piston rods  28  in response to, and to compensate for, heave of the floating vessel. The control valve assembly  65  may be controlled from a control panel  66 . Preferably, the control panel  66  is disposed and operated from the drill floor  200 , thereby enabling the apparatus  10  to be operated remotely from the drill floor  200  without the need to directly access the apparatus  10 . 
     Further the primary hydraulic fluid line  18  may be charged with hydraulic fluid from a hydraulic power unit  68  via fill and drain lines  70  and pilot lines  72 . 
     The control valve assembly  65  comprises an isolation valve  74  capable of preventing fluid flow though the primary hydraulic fluid line  18 . The isolation valve  74  is operable to lock the piston rods  28  at any point along their respective stroke path, thereby forming a rigid link between the upper and lower sections  20 ,  22  of the frame  12 . Hydraulically locking the isolation valve  74  enables the apparatus  10  to be employed in-line with and back-up to a rig&#39;s drill string compensator, such as a crown mounted compensator or an active heave draw works, as described previously. 
     The primary hydraulic fluid line  18  may be further provided with a bypass fluid flowpath comprising a first bypass line  76  and a second bypass line  78 . Each bypass line  76 ,  78  is configured to bypass the isolation valve  74 , thereby allowing flow of hydraulic fluid through the primary hydraulic fluid line  18  between the accumulators  16  and the cylinders  14 . 
     The first bypass line  76  is provided with a bypass relief valve  82 , in the form of a non-reclosing pressure dump valve. As shown in  FIG. 5 , bypass relief valve  82  is configured to redirect fluid flow through the first bypass line  78  when pressure in the cylinders  14  exceeds an upper pressure threshold. The upper pressure threshold may be greater than 200 bar (e.g. 220 bar). The upper pressure threshold is determined and set such that the bypass relief valve  82  will trigger before exceeding the allowable stress in the landing string. 
     The second bypass line  76  is provided with a bypass check valve  80 , in the form of a cartridge style check valve. As shown in  FIG. 4 , bypass check valve  80  is configured to redirect fluid flow through the second bypass line  76  when pressure in the cylinders  14  falls below a lower pressure threshold. The lower pressure threshold may be less than 50 bar (e.g. 15 bar) below the nominal system pressure. The lower pressure threshold is set to limit the amount of compression applied at the well head. 
     In this way, in the event of failure or malfunction of the rig&#39;s drill string compensator, where pressure exceeds the upper pressure threshold or falls below the lower pressure threshold, the control valve assembly is configured to redirect fluid flow through the first or second bypass lines  78 ,  76 , respectively, thereby allowing the apparatus  10  to automatically activate and provide ongoing heave compensation for the floating vessel. The bypass relief valve  82  and bypass check valve  80  are configured to activate rapidly, enabling the apparatus  10  to provide full heave compensation while not exceeding the allowable stress in the landing string  110 . The activation speed may be less than 100 milliseconds. 
     The primary hydraulic fluid line  18  may be further provided with an anti-recoil valve  84 , in the form of a flow shut off valve, incorporating a restricted flowpath  86  therethrough. In an alternative embodiment, the flow shut off valve  84  may have a further bypass line with a flow restricted path (not shown). Under normal operating conditions the anti-recoil valve  84  is open. However, to safeguard against uncontrolled recoil resulting from sudden loss of load (e.g. parting of the drilling string or landing string) below the apparatus  10 , the valve  84  is configured to close to restrict the flow of hydraulic fluid from the accumulators  16  to the cylinders  14 , thereby preventing uncontrolled recoil. Uncontrolled recoil may be indicated by a cylinder velocity exceeding that of the normal operating heave which triggers the flow shut-off valve  84  to close rapidly in less than 100 ms. As shown in  FIG. 6 , closure of the flow shut-off valve  84  redirects flow of hydraulic fluid through an internal restricted flow path  86 , thereby allowing the piston rods  28  to retract in a controlled fashion if there is a loss of load below the apparatus  10 . This feature also ensures that the landing string is not overstressed if the flow shut-off valve  84  is unintentionally closed. 
     In addition, or as an alternative, to hydraulically locking the piston rods  28 , the piston rods may be mechanically locked with a mechanical locking mechanism  88 . In some embodiments the mechanical locking mechanism  88  may comprise a slidable locking pin  90  adapted to engage the clevis  34  of the piston rods  28  when they are in a fully retracted position. The slidable locking pin  90  is actuated by an associated cylinder  92  which urges the locking pin  90  to engage or disengage the clevis  34 . The mechanical locking mechanism  88  may be controlled by the control panel  66 . 
     Referring now to  FIG. 7 , there is shown a detailed perspective view of the upper section  20  of the frame  12  of the heave compensating and tensioning apparatus  10 . The upper section  20  is adapted for attachment to the top drive system  120  via elevator links. 
     The upper section  20  comprises a cross member  200  in the form of a spreader beam. The cross member  200  may be a plate having an upper edge  202 , a lower edge  204 , opposing side edges  206 , a front side  208  and a rear side  210 . 
     The upper section  20  further comprises an attachment member  212  in the form of a lug. In this embodiment, the attachment member upwardly extends from the upper edge  202  of the cross member  200  and is disposed substantially equidistantly from the opposing side edges  206  of the cross member  200 . The attachment member  212  may be integrally formed with the cross member  200  or may be welded to the cross member  200 . 
     The attachment member  212  may be configured to be coupled to the top drive system  120  by various couplers, such as bail arms or elevator links. In this embodiment, the attachment member  212  is provided with a pair of downwardly inclined ears  214  spaced from the upper edge  202  of the cross member  200 . In use, as shown in  FIG. 1 , respective lower ends of the bail arms are engaged with the downwardly inclined ears  214  while respective upper ends of the bail arms are coupled to the top drive system  120 . 
     In one particular embodiment, maintaining engagement of the lower ends of the bail arms with the downwardly inclined ears  214  may be achieved with a retainer  216  in the form of an L-shaped bracket. In use, after engagement of the bail arms with the downwardly inclined ears, the arms of the L-shaped bracket may be connected (such as with bolts, threaded screws, and so forth), respectively, to the upper edge  202  of the cross member  200  and side edge  218  of the downwardly inclined ears  214 . In this way, if there is a recoil event or the load decreases, the lower ends of the bail arms are prevented from disengaging the downwardly inclined ears  214  and, consequently, the heave compensating and tension apparatus  10  is prevented from detaching from the top drive system  120 . 
     The upper section  20  of the frame  12  is also adapted to be operatively associated with the piston rods  28  of the cylinders  14 . The cross member  200  may be provided with a pair of apertures  220 . Each aperture  220  is spaced apart from opposing side edges  206  of the cross member  200 . The apertures  220  are configured, in use, to receive a pin which is inserted through a respective clevis  34  associated with the free end  32  of the piston rod  28  of the cylinder  14 , thereby fixing the free end  32  of the piston rod  28  to the upper section  20  of the frame  12 . 
     The upper section  20  may also comprise a first pair of opposing plates  222  laterally extending from the front and rear sides  208 ,  210  of the cross member  200  and a second pair of opposing plates  224  laterally extending from the front and rear sides  208 ,  210  of the cross member  200 . The first pair of opposing plates  222  is disposed adjacent to the upper edge  202  of the cross member  200 . The second pair of opposing plates  224  is disposed adjacent to the lower edge  204  of the cross member  200 . A plurality of angled brace members  226  may be provided between the first and second pairs of plates  220 ,  222  to provide additional strength and rigidity to the upper section  20 . 
     The upper section  20  of the frame may be further provided with a pair of load bearing lugs  228  in the form of padeyes. The load bearing lugs  228  are spaced apart from opposing side edges  206  of the cross member  200  and extend upwardly from the upper edge  202  of the cross member and the first pairs of laterally extending plates  220 . The load bearing lugs  228  may be disposed in substantially vertical alignment with apertures  220  in the cross member  200 . The load bearing lugs  228  may be capable of bearing the complete self weight of the frame  12  (e.g. a load of up to 10 tonne, preferably up to 50 tonne, and even more preferably greater than 50 tonne). 
     Referring now to  FIG. 8 , there is shown a detailed perspective view of the lower section  22  of the frame  12  of the heave compensating and tensioning apparatus  10 . 
     The lower section  22  comprises a cross member  300  in the form of a spreader beam. The cross member  300  comprises a cylindrical member  302  having an upper edge  304 , a lower edge  306 , an outer cylindrical wall  308 , and an inner cylindrical wall  310 . 
     The lower section  22  also comprises a pair of opposing side plates  312  outwardly extending from respective opposing sides of the outer cylindrical wall  308 . The side plates  312  may be outwardly tapering. 
     The lower section  22  further comprises a pair of split insert members  314  which are locked in place by a collar members  304 . The split insert members  314  comprises a pair of semi-cylindrical members  314   a ,  314   b  which are disposed to abut each other at facing edges  316  thereof. The cylindrical members  314   a ,  314   b  are concentrically disposed to abut the inner cylindrical wall  310  of the cylindrical member  302 . The pair of split inserts is advantageously formed to interface with any one of a plurality of general flowhead assemblies  130 . The collar members  304  are advantageously formed with a wedge type cross section, holding the split inserts  314  securely with the cylindrical member  302  in tension without the need for additional securing bolts. 
     In use, the flowhead assembly  130  is interfaced with the lower section  22  by coupling the flowhead assembly  130  with the split inserts  314  and the collar member  304  proximal to the lower edge  306  of the cylindrical member  302 . In this way, the lower section  22  is capable of locking directly to the flowhead assembly  130  which has the advantage of optimising the stack-up height of the apparatus  10 . 
     The lower section  22  of the frame  12  may be also adapted to engage the cylinders  14 . The cross member  300  may be provided with a pair of opposing shafts  316 . The shafts  316  outwardly extend from the opposing side plates  312  in longitudinal alignment therewith. In use, the shafts  316  are configured to engage the spherical bearings in the lower end  30  of the cylinders  14  in a manner whereby the cylinders  14  are fixed to the lower section  22  of the frame  12 . 
     The lower section  22  may also comprise a first pair of opposing plates  318  laterally extending from the cylindrical member  302  and the side plates  312  of the cross member  300  and a second pair of opposing plates  320  laterally extending from the cylindrical member  302  and the side plates  312  of the cross member  300 . The first pair of opposing plates  318  is disposed adjacent to the upper edge  304  of the cylindrical member  302 . The second pair of opposing plates  320  is disposed adjacent to the lower edge  306  of the cylindrical member  302 . A plurality of substantially vertical brace members  322  may extend between the first and second pairs of plates  318 ,  320  to provide additional strength and rigidity to the lower section  22  and to provide additional handling points. The vertical brace members  322  may be equidistantly spaced with respect to one another. 
     The lower section  22  of the frame  12  may be further provided with a pair of load bearing lugs  324  in the form of padeyes. The load bearing lugs  324  upwardly extend from the first pair of opposing plates  318 . The load bearing lugs  324  may be integrally formed with substantially vertical brace members  326  extending between the first and second pairs of opposing plates  318 ,  320 . The load bearing lugs  324  and the vertical brace members  326  are equidistantly spaced apart from opposing sides of the cross member  300 . Advantageously, the load bearing lugs  324  may be used to lift the frame  12 . 
     The lower section  22  may be further provided with one or more further load bearing lugs  328 , in the form of padeyes, downwardly depending from an underside of the side plates  312 . 
     Referring now to  FIGS. 9 a  and 9 b   , there is shown detailed perspective and plan views of the intermediate section  24  of the frame  12  of the heave compensating and tensioning apparatus  10 . 
     The intermediate section  24  comprises a cross member  400  in the form of a spreader beam. The cross member  400  comprises a pair of spaced apart hollow cylindrical members  402  interconnected by an upper flange plate  404  and a lower flange plate  406 . 
     The cylindrical members  402  are each provided with a flange  408  concentrically disposed at a lower end  410  thereof. The cylindrical members  402  are spaced apart from one another such that the flanges  408  are configured, in use, to receive and couple with respective upper ends of the cylinders  14  so that the piston rods of the cylinders  14  may reciprocally translate concentrically within the hollow cylindrical members  402 . In this way, the intermediate member  400  provides structural rigidity to the frame  12 . 
     The upper flange plate  404  is disposed at respective upper ends  412  of the hollow cylindrical members  402 . In use, when the piston rods are fully retracted, the upper flange plate  404  provides a landing for the upper section  20  of the frame  12 , as shown in  FIG. 3 . 
     The lower flange plate  406  may be spaced from the respective lower ends  410  of the hollow cylindrical members  402 . The lower flange plate  406  may be provided with a pair of load bearing lugs  414 , in the form of padeyes. The load bearing lugs  414  downwardly extend from the lower flange plate  406 . Advantageously, the load bearing lugs  414  may be used to lift the frame  12 . 
     A pair of spaced apart vertical brace members  416  may extend between the upper and lower flange plates  404 ,  406  to provide stiffening. Additionally, a pair of parallel wall members  418  may extend between the upper and lower flange plates  404 ,  406  and the hollow cylindrical members  402 . A mechanical locking mechanism  88  may be mounted to each of the parallel wall members  418 . Co-extensively disposed with respect to the mechanical locking mechanism  88  are respective apertures ( 420 ) in the hollow cylindrical members  402 . The apertures  420  are configured, in use, to receive the locking pin  90  actuated by the hydraulic cylinders  92  of the mechanical locking mechanism  88 . 
     The locking pin  92  is arranged, in use, to engage the clevis  34  of the piston rods  28  when they are fully retracted. It is envisaged that the mechanical locking mechanism  88  may be employed to actuate the locking pin  90  and mechanically lock the piston rods  28  of the cylinders  14  in a fully retracted position when the apparatus  10  is being installed in the derrick  100  and/or when lowering and landing out completions casing/production tubing. Subsequently, the mechanical locking mechanism  88  may be employed to retract the locking pin  90 , thereby allowing the piston rods  28  of the cylinders  14  to telescopically translate within the hollow cylindrical members  402 . 
     In use, the heave compensating and tensioning apparatus  10  may be employed as a back up compensator for a primary compensator in the form of a drill string compensator. The heave compensating and tensioning apparatus  10  may be installed by first fully retracting the piston rods  28  of the cylinders  14  within the cylinder barrels  26  and then employing the mechanical locking mechanism  88  to actuate the locking pin  90  into an extended position so that the piston rods  28  are prevented from translating by said pin  90 . The upper frame section  20  may then be coupled to the top drive system  120  by various couplers, such as bail arms or elevator links. The flowhead assembly  130  may then be interfaced with the lower section  22  by coupling the flowhead assembly  130  with the split inserts  314  and the collar member  304  proximal to the lower edge  306  of the cylindrical member  302 . 
     When the apparatus  10  is located between the top drive system  120  and the flow head assembly  130 , the mechanical locking mechanism  88  may be actuated to withdraw the locking pin  90  so that the piston rod  28  is capable of freely translating within the cylinder barrel  26 . The isolation valve  74  of the control assembly  65  is then engaged to hydraulically lock the piston rods  28  at a desired point along their stroke path, thereby forming a rigid link between the top drive system  120  and the flow head assembly  130 . In this hydraulically locked configuration, the apparatus  10  is primed to rapidly activate if and when the rig&#39;s drill string compensator fails. 
     In the event of the rig&#39;s drill string compensator failing, the pressure in the cylinders  14  may increase in an up heave of the vessel reaching an upper pressure threshold and, conversely, the pressure in the cylinders  14  may decrease in a down heave of the vessel reaching a lower pressure threshold. In response thereto, the control assembly  65  redirects fluid flow to bypass the isolation valve  74  via the first bypass line  78  or the second bypass line  76 , respectively, thereby allowing the piston rods  28  to translate freely in the cylinders  14  to retract and extend in response to, and to compensate for, heave of the vessel. 
     It will also be appreciated that the heave compensating and tensioning apparatus  10  described herein may function as the primary heave compensation system for a floating drilling vessel. 
     Numerous variations and modifications will suggest themselves to persons skilled in the relevant art, in addition to those already described, without departing from the disclosure. All such variations and modifications are to be considered within the scope of the disclosure. 
     For example, an alternative embodiment of the heave compensating and tensioning apparatus  10 ′ is shown in  FIG. 10 . The heave compensating and tensioning apparatus  10 ′ is similar to the heave compensating and tensioning apparatus  10  shown in  FIGS. 1-9  but differs in several respects, in particular the location of the cylinders  14  and gas accumulators  16  relative to the upper and lower frame members  20 ,  22 , respectively. Accordingly, in the following description of apparatus  10 ′ like numerals will refer to like parts throughout. 
     The apparatus  10 ′ includes a rigid frame  12 ′, a cylinder  14  in the form of a hydraulic cylinder, and a piston accumulator  16  in operative communication with the cylinder  14  via a primary hydraulic line  18 . 
     The rigid frame  12 ′ includes an upper section  20 ′ and a lower section  22  adapted to interface with the flowhead assembly  130 , as described previously. The frame  12 ′ also includes a pair of parallel rigid rods  13  interconnecting respective opposing ends  15  of the upper section  20 ′ and the lower section  22 . 
     In contrast to apparatus  10 , the upper section  20 ′ of the frame  12 ′ is not adapted to interface the top drive system  120 . Rather, the upper section  20 ′ of the frame  12 ′ is adapted to support thereabove a cage-like structure  17 . The cage-like structure  17  houses the cylinder  14  and the piston accumulator  16  and auxiliary peripherals associated therewith, such as the air pressure vessel  50 . Advantageously, this particular arrangement reduces the area of deck space occupied by the apparatus  10 ′ 
     The cylinder  14  is mounted on the upper section  20 ′ of the frame  12 ′. The cylinder  14  is as otherwise described in the preceding description except that the free end  32  of the piston rod  28  may be adapted for attachment to the top drive system  120 . Preferably, the adaptation of the free end  32  may take the form of a pair of opposing downwardly inclined ears  19 . In use, as shown in  FIG. 10 , respective lower ends of the bail arms are engaged with the downwardly inclined ears  19  while respective upper ends of the bail arms are coupled to the top drive system  120 . 
     In the claims which follow, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” and variations such as “comprises” or “comprising” are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the apparatus and method as disclosed herein.