TECHNICAL FIELD

A riser tension protection system and an associated backup heave compensation system and method are disclosed. The system and method may find application for example in keeping riser tension within prescribed limits during locked to bottom operations conducted form a floating vessel.

BACKGROUND

As oil and gas offshore exploration and production operations are increasingly established in deeper waters, it has become more common for drilling and well completion 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 testing, 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 completions workover riser, landing string or the tubular (hereinafter referred to collectively and generically as a ‘riser’) is physically connected to the subsea well at the seabed. It will be appreciated that, depending on the nature of the operations, the riser may be connected to a tubing hanger at the well-head, to a subsea tree or other infrastructure at the top of the well. 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 riser to fracture or buckle, resulting in loss of well containment and potential 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 inline passive drill string compensators. Passive heave compensators employ hydraulic cylinders and associated gas accumulators to store and dissipate the energy as the vessel heaves up and down.

Active heave compensation differs from passive heave compensation by having an external control system with external inputs from motion reference units 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-passive compensation systems have a primary passive compensation system with secondary actively driven hydraulic cylinders to reduce tension variations and improve efficiency. Two independent active and passive systems are generally not employed. 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 predictable failures, 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 for normal drilling operations, they fail to a “locked condition”, which is undesirable for well completions, well testing and well intervention operations. Passive compensation systems (e.g. crown mounted compensators) are also not immune to failures. Safe operations and industry standards require additional means of safety to be implemented in the system/equipment configuration. Additional means of safety may include a standard in-line tensioner or traditional compensated coiled tubing lift frame, design of a weak link in the riser/landing string, weak-link bails, 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 a standard inline tensioner or conventional compensated coiled tubing lift frame, there are concerns about the how the system behaves when run in series with the active heave draw works. In the case of the weak link in the riser and weak-link bails, they typically only provide protection in an over-tensioned case and once broken, they provide no support to the riser thereafter. In the case of limiting operating parameters to within the stretch of the riser, this can impose considerable downtime during offshore operations.

AU 20182411088 describes a compensated elevator link that may be rigged up as either a standalone riser tension protector or a backup heave compensator. The described elevator link comprises two hydraulic cylinders each housing a slidable cylinder piston and a rod which is connected at opposite ends to each of the pistons. Each of the cylinders has a rod side for holding a non-compressible fluid. The rod includes an internal passage housing a pair of rod pistons between which a compressible fluid is held. The non-compressible fluid is able to flow into the internal rod passage to fill a region from respective cylinder pistons to the nearest rod pistons. This non-compressible fluid flows between the rod side of the respective cylinder and the internal passage through valved orifices in the cylinder pistons.

The above references to background art does 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 OF THE DISCLOSURE

In broad and general terms, the idea or concept behind the disclosed system and method is to provide a rigid link between a riser and a floating support that automatically triggers should an over tension or under tension event occur but then may enable continuous heave compensation.

In a first aspect there is disclosed a heave compensation system usable as a standalone compensator or as a backup compensator, the system comprising: a housing containing a piston and piston rod connected to the piston and extending from one end of the housing, the housing defining a working fluid region arranged to contain a non-compressible fluid W wherein tension T applied across the piston and an opposite end of the housing pressurises the fluid W to a pressure Pf; an over tension accumulator OTA and an under tension accumulator UTA each having a respective first region in fluid communication with the working fluid region and having respective pistons movable between a topped out position where the respective first regions have a minimum volume and a bottomed out position where the respective first regions have a maximum volume; the OTA piston on a side opposite the first region being under the influence of a compressed gas at a pressure Po, and the UTA piston on a side opposite the first region being under the influence of a compressed gas at a pressure Pu; wherein the system has a dead band where the piston rod is held stationary in an intermediate position within the housing while Tu T To by providing gas pressures Po and Pu such that Pu<Pf<Po and where the OTA piston is topped out and the UTA piston is bottomed out.

In a second aspect there is disclosed a heave compensating system usable as a standalone compensator or as a backup compensator the system comprising: a housing having opposite first and second ends and accommodating a piston and a piston rod coupled to the piston, the piston rod having a free end extending located outside of the housing, wherein the free end is capable of coupling to a first point, and the second end of the housing is capable of coupling to second point, the rod being arranged to stroke between a retracted limit where the free end is at a minimal distance from the first end of the housing and an extension limit where the free end is at a maximal distance from the first end of the housing, wherein the heave compensating system has an intermediate position where the free end of the rod lies inboard of and between the retracted limit and the extension limit; an over tension accumulator (OTA) having a piston and containing or in fluid communication with a gas at a first pressure Po; an under tension accumulator (UTA) having a piston and containing or in fluid communication with a gas in a second pressure Pu; a volume of a non-compressible working fluid in simultaneous fluid communication between the housing, the over tension accumulator and the under tension accumulator; wherein the piston rod is held at the intermediate position when tension T between the first and second points is within a dead-band zone in which Tu T To by action of the gas pressures Po and Pu with the piston in the over tension accumulator being topped out and the piston in the under tension accumulator being bottomed out.

In one embodiment the piston rod is solid.

In one embodiment the OTA is separate from the housing.

In one embodiment the UTA is separate from the housing.

In one embodiment UTA is formed in the rod.

In one embodiment the OTA is coupled to an end of and in line with the housing.

In one embodiment the heave compensating system comprises a gas pressure regulation system operable to equalise gas pressure within the over tension accumulator and under tension accumulator so that Po=Pu after the piston rod has been displaced from its intermediate position by the application of the tension T between the first and second points.

In one embodiment the housing is arranged to hold a maximum volume Vf of the working fluid when the rod is at its maximum retracted position within the housing; the OTA is capable of holding a maximum volume Vo of the working fluid when bottomed out; the UTA is capable of holding a maximum volume Vu of the working fluid when bottomed; and wherein Vf <Vo +Vu.

In one embodiment Vo < Vu.

In a third aspect there is disclosed a heave compensating system comprising: a housing having opposite first and second ends and accommodating a piston and a piston rod coupled to the piston, the piston rod having a free end extending from the first end of the housing and located outside of the housing, wherein the free end is capable of coupling to a first point, and the second end of the housing is capable of coupling to second point, a working fluid region formed between the piston and variable up to a maximum volume of Vf; an over tension accumulator (OTA) having a first region of a volume variable up to a maximum volume Vo; an under tension accumulator (UTA) having a first region of a volume variable up to a maximum volume of Vu; a volume of non-compressible fluid; wherein the working fluid region and each of the first regions are in simultaneous fluid communication with each other enabling the non-compressible fluid to flow there dependent on respective fluid pressure within the OTA, UTA and working fluid region, and wherein Vf <Vo +Vu.

In one embodiment of this aspect Vo < Vu.

In one embodiment the OTA has a volume of compressed gas at a pressure Po, the UTA has a volume of compressed gas at pressures Pu, and wherein the compressed gas in the OTA and UTA are in pressure communication with the non- com pressible fluid; and the gas pressures Pu and Po are arranged to hold the piston rod at mid stroke when tension T is between the first and second points is within a dead-band zone in which Tu < T < To.

In a fourth aspect there is disclosed a heave compensating system usable as a standalone compensator or as a backup compensator comprising: a housing opposite first and second ends and accommodating a piston and a piston rod coupled to the piston, the piston rod having a free end extending from the first end of the housing and located outside of the housing, wherein the free end is capable of coupling to a first point, and the second end of the housing is capable of coupling to second point, and a working fluid region formed within the housing between the piston, the rod and the first end wherein applying tension T across the first and second ends places the working fluid under a pressure Pf; an over tension accumulator (OTA) having floating piston dividing the OTA into a first region in fluid communication with the working fluid region, and a second region under influence of a compressed gas at a pressure Po, the floating piston moving between a topped out position where the first region has a minimal volume and a bottomed out position where the first region has a maximum volume Vo; an under tension accumulator (UTA) having a floating piston dividing the UTA into a first region in fluid communication with the working fluid region and a second region under influence of a compressed gas at a pressure Pu, the floating piston moving between a topped out position where the first region has a minimal volume and a bottomed out position where the first region has a maximum volume Vu; and wherein the system has a dead band where the piston rod is held stationary while TucTcTo by providing gas pressures Po and Pu such that Pu<Pf<Po and where the OTA piston is topped out and the UTA piston is bottomed out.

In a fifth aspect there is disclosed a method of providing tension protection to a riser or other tubular, or backup heave compensation for a riser or other tubular comprising: using tension T in the riser or other tubular to pressurise a non-com pressible working fluid to a pressure Pf; enabling the working fluid to flow between: a rod side of a cylinder having a piston and a connected piston rod; an over tension accumulator; and, an under tension accumulator; applying a pressure Po to the working fluid via a compresses gas acting on an OTA floating piston in the OTA; applying a pressure Pu to the working fluid via a compresses gas acting on an UTA floating piston in the UTA; and arranging the gas pressures Po, Pu such that Pu<Pf<Po to hold the piston and piston rod stationary within the cylinder with the OTA piston topped out and the UTA piston bottomed out while the riser or tubular tension T is in the range TucTcTo, where Tu is an under tension threshold and To is an over tension threshold.

In one embodiment the method comprises equalising the pressure Po with the pressure Pu when the riser or tubular tension T falls outside of the range Tu T To.

In one embodiment the method comprises arranging the OTA and the UTA, so each has substantially the same volume of working fluid when their respective pistons are bottomed out. BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of the system and method as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to becoming drawings in which:

Figure 1 is a schematic showing one embodiment and used to describe the general operating principles of the disclosed method and system;

Figure 2 graphically represents various stages of operation of the embodiments of the disclosed subset and method;

Figure 3 is a schematic representation of a second embodiment of the disclosed system ;

Figure 4 is a schematic representation of a third embodiment of the disclosed system being a modification of the second embodiment shown in Figure 3;

Figure 5 is a schematic representation of a fourth embodiment of the disclosed system being a modification of the second embodiment shown in Figure 1 ;

Figure 6 is a schematic representation of a fifth embodiment of the disclosed system being a modification of the second embodiment shown in Figure 3;

Figures 7a and 7b are representations of a tension frame incorporating two of the system is illustrated in Figure 3.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Figure 1 illustrates general operating principles of embodiments of the disclosed riser tension protection system 10 (“system 10”) and associated backup heave compensation system and methods. The layout is shown in this Figure may also constitute one physical embodiment of the system 10.

The system 10 has a housing/cylinder 12 with opposite first and second ends 14, 16 respectively and accommodating a piston 18. A rod 20 is coupled to the piston 18. A free end 22 of the piston 20 extends from the first end 16, and is located outside, of the housing 12. The free end 22 of the rod 20 is capable of being coupled to a first point (not shown). In the present embodiment this is by way of a pad-eye 24 fixed to the free end 22. The second end 16 of the housing 12 is capable of being coupled to a second point (not shown). In this embodiment this is by way of a pad-eye 26 attached to the end 16. The first and second points could for example be: a top drive of a derrick; and, a flow head attached to a riser, respectively.

A rod side 28 of the housing 12 forms a working fluid region 30 that can hold a volume of a non-compressible working fluid W such as, but not limited to, a water/glycol mixture. The region 30 is the region between the outer surface of the rod 20, and the inner surface of the housing 12 between the first end 14 and the piston 18. The region 30 is of an annular configuration and for this reason may equivalently be refer to in this specification as the annulus 30.

The piston 18 and rod 20 stroke within the operating range of the housing 12. This range can be set so that the rod 20 has a maximum extension from the end 14 where the piston 18 is near but spaced from the first end 14 and the rod end 22 is at a maximum distance from the end 14; and a maximum retraction where the piston 18 is near but spaced from the end 16, with the rod end 22 at a minimum distance from the end 14 but still of course on the outside of the housing 12. In this way it is possible to set the system 10 so that the piston 18/rod 20 do not top out or bottom out (i.e. impact the top 14 or the bottom 16 of the housing 12). This prevents or at least minimises shock loading which may otherwise cause fatigue in a coupled riser.

The volume of the annulus 30 may vary between substantially 0 litres when the rod 20 is at its maximum extension from the end 14, and a maximum Vfmax when the rod 20 is at its minimum extension from the end 14. The pressure Pf of the working fluid W in the annulus 30 is dictated by the tension across the system 10, i.e. in this embodiment across the pad-eyes 24, 26. More particularly the tension acting on the system 10 is transmitted to the working fluid W.

The blind side 31 of the housing 12 (i.e. the side of the housing 12 between the piston 18 in the end 16) contains a gas such as nitrogen at a low pressure Ph.

Ideally the pressure Ph is set a value such that it has negligible effect on the rod 20 when it is being extended or retracted. This may be achieved for example if Ph is close to atmospheric pressure when the rod 20 is fully extended.

The system 10 also includes an over tension accumulator (OTA) 32 and an under tension accumulator (UTA) 34. The OTA 32 and UTA 34 each comprise vessels 36, and 38 respectively which are separate from or not otherwise coupled in line with the housing 12. In this way the tension acting on the system 10 is not applied to the OTA 32 or the UTA 34. The working fluid W is in simultaneous fluid communication with the annulus 30 of the housing 12 and each of the OTA 32 and the UTA 34. This fluid communication is facilitated by a conduit 40 which runs between the housing 12 and the accumulators 32, 34.

The OTA 32 has a sliding piston 42 that can slide between a first end 44 of the vessel 36 and internal stops 46 within the vessel 36. The piston 42 divides the vessel 36 into a first region 48 on a side of the piston 42 facing the end 44, and a second region 50 on a side of the piston 42 facing an opposite end 52 of the vessel 36. The first region 48 is in fluid communication with the working fluid region 30. The first region 48 can hold a maximum volume Vo of the non-compressible working fluid W, being the volume of the vessel 36 between the end 44 and the piston 42 when it is resting against the stop 46. The second region 50 holds a compressed gas at a pressure Po.

The UTA 34 has a sliding piston 54 that can slide between a first end 56 of the vessel 38 and internal stops 58 within the vessel 38. The piston 54 divides the vessel 38 into a first region 60 on a side of the piston 54 facing the end 56, and a second region 62 on a side of the piston 54 facing an opposite end 64 of the vessel 38. The first region 60 can hold a maximum volume Vu of the non-compressible working fluid W, being the volume in the vessel 38 between the end 56 and the piston 54 when it is resting against the stop 58. The second region 62 holds a compressed gas at a pressure Pu. Thus, the pressures Po and Pu are applied to the working fluid W via the respective floating pistons 42 and 54.

In this embodiment Vfmax < Vo + Vu. In one specific example Vo =Vu which means that each of the accumulators 36 and 38 can hold one half of the volume Vfmax. Irrespective of their volumetric relationship, due to the fluid communication between the volumes Vf, Vo and Vu, the pressure of the working fluid Pf is the same in each of these volumes.

A “fill” hose 66 may be provided that extends from the over tension accumulator 32 to an air pressure vessel (APV) or other pressure source on a derrick floor. A one way valve 68 is provided between the pressure source and the over tension accumulator 32. The valve 68 allows gas to flow only in the direction into the over tension accumulator 32. A vent 70 may be installed between the one-way valve 68 and the over tension accumulator 32 to allow controlled venting of gas pressure from the accumulator 32. In this embodiment the vent 70 is controlled by a pilot valve 72 operated by signals from an operator which may be sent either wirelessly or by a conduit 74.

A “fill” hose 76 may be provided that extends from the under tension accumulator 34 to an air pressure vessel (APV) or other pressure source on a derrick floor. A one way valve 78 is provided between the pressure source and the under tension accumulator 34. The valve 78 allows gas to flow only in the direction into the accumulator 34. A vent 80 may be installed between the one-way valve 78 and the accumulator 34 to allow controlled venting of gas pressure from the accumulator 34. In this embodiment the vent 80 is controlled by a pilot valve 82 operated by signals from an operator which may be sent either wirelessly or by a conduit 84.

When the system 10 is used as a standalone riser tension protector, or as a backup heave compensator the piston 18/rod 20 may be held in an intermediate position with reference to the normal operating range of the housing 12. The intermediate position may be a mid stroke position where the end 22 is spaced from the end 14 by a distance which is an average of the distance between these two ends when the rod 20 is at its maximum extension and maximum retraction. In one realisation of the system 10 where the housing 12 is physically separate from the vessels 36 and 38 this mid stroke position coincides with the piston 18 being midway between the ends 14 and 16. This is the position shown in Figure 1. However as will be explained later embodiments of the system enable the intermediate position to be biased from the mid stroke position.

System 10 is arranged so that when used as a standalone riser tension protector, is in its backup mode while used as a backup compensator, the under tension accumulator 34 is bottomed out (i.e. the piston 54 contacts the stop 58) and the over tension accumulator 32 is topped out (i.e. with the piston 42 in contact with the end 44). This is achieved by setting of the gas pressures, Po and Pu to correspond with the desired over tension and under tension setpoints at which the system 10 is activated. The gas pressures Po and Pu can be adjusted as required by filling or venting the accumulators using the respective fill hoses 66, 76 and/or the vents 70, 80. The region between the setpoints is known in the art as the “dead band”. By way of brief explanation when the tension T acting across the system 10, (i.e. forces coupled to or otherwise acting on the rod 20 and the end 16 and pulling in opposite directions away from each other) is either greater than a predetermined minimum threshold tension Tu, or less than a predetermined maximum threshold tension To, the pressures the Po and Pu operate on the working fluid W to hold the piston 18 in the intermediate position. The pressure relationship in the system 10 when in the back up mode is Pu<Pf<Po. If Vo =Vu then, the intermediate position of the piston 18 will coincide with the mid stroke position within the housing 12.

The operation and functionality of the system 10 will now be described with reference to the graph shown in Figure 2. In describing the operation of the system 10 it is assumed that the system 10 is installed in a derrick on a floating vessel between a riser and a top drive/travelling block. In which case the tension T on the riser acts between the pad-eyes 24 and 26.

• The pressure Po of the gas in the OTA 32 is adjustable and governs the over tension setpoint To, which is the point at which the rod 20 will start to stroke out of the housing 12 from its intermediate position during an over tension event on the riser which may occur on an up heave of the floating vessel when the vessel’s primary heave compensator becomes inactive. Thus, when Pf>Po the rod 20 will extend from the intermediate/mid stroke position.

• The pressure Pu of the gas in the UTA 34 is adjustable and governs the under tension setpoint Tu, which is the point at which the rod 20 will start to stroke into the housing 12 from its intermediate position during an under tension event on the riser which may occur on a down heave of the floating vessel when the vessel’s primary heave compensator becomes inactive. Thus, when Pf<Pu the rod 20 will retract from the intermediate/mid stroke position further within the housing 12.

• When the tension on the riser (i.e. the tension between the pad eyes 24 and 26) is between the upper and lower setpoints To, Tu the piston 18/rod 20 is held static in its intermediate (in this case mid stroke) position with the piston 42 in the over tension accumulator 32 being topped out and the piston 54 in the under tension accumulator 34 being bottom out (i.e. against the stop 58), as shown in Figure 1 .

• It is the pistons 42 and 54 being topped out and bottomed out respectively that creates the “dead band”, within which the rod 20 will not stroke. By arranging the max volume Vo of the first region 60 in the under tension accumulator 34 to be 50% of the maximum annulus volume Vf, the dead band occurs at the mid stroke of the piston 20 in the housing 12.

• In an over tension event, the pressure Pf of the working fluid W in the housing 12 will increase until it reaches the pressure Po in the over tension accumulator 32. At this point the floating piston 42 will be forced down, further compressing the gas in the over tension accumulator, allowing the rod 20 to stroke out. This is represented by the over tension stroke curve 92 in Figure 2.

• In an under tension event, the pressure Pf of the non-com pressible fluid W in the housing 12 will decrease until it reaches the pressure Pu in the under tension accumulator 34. At this point the floating piston 54 will be forced up as the compressed gas within the under tension accumulator 34 expands, driving the rod 20 to stroke in. This is represented by the under tension stroke curve 94 in Figure 2.

• When the tension (and corresponding pressure) in the housing 12 returns to within its normal operating range, the system 10 will automatically return to its “backup mode” with the over tension accumulator 32 topped out in the under tension accumulator 34 bottomed out.

It may be desirable however once the system 10 is activated, to equalise the pressure in the UTA 32 and OTA 34 to effectively eliminate the dead band. This corresponds to the pressure relationship Pf=Po=Pu and may be achieved by: (a) venting gas pressure from the OTA 32 reducing it to the pressure Pu in the UTA 34;

(b) increasing the gas pressure in the UTA 34 to the pressure Po in the OTA 32; or

(c) or a combination of both (a) and (b).

This may be performed manually by an operator on a derrick floor supplying and/or venting compressed gas as required through the fill hoses 66, 76 and the vents 70, 80. Thus when an operator sees or is made aware of the activation of the system 10 they can then take action to equalise the gas pressure between the accumulators 32 and 34. To this end a combination of one or more: motion, position and pressure sensors may be used to detect and monitor the position and/or motion of the pistons 18, 42 and 54 as well as the fluid pressure Pf and gas pressures Po and Pu to issue a notification or alarm to an operator to the effect that the system 10 has “triggered” as a result of the tension in the riser being outside of the dead band. On receiving the notification or alarm the operator can then take the appropriate action to equalise the gas pressure in the accumulators 32 and 34.

In an alternate embodiment the equalisation of pressure in the accumulators 32 and 34 can be affected automatically for example by a PLC which can receive and process signals from the above mentioned sensors to fill and/or vent gas pressures required to achieve the equalisation.

In yet a further alternate embodiment equalisation of gas pressure in the accumulators 32 and 34 can be achieved by way of a hose 86 provided with a pilot- controlled valve 88 that extends between the accumulators 32 and 34. The valve 88 can be activated either manually or automatically upon activation of the system 10.

Figure 2 graphically depicts the operation of the system 10 in terms of the load on the system 10 (i.e. the tension on the riser supported by the housing 12/rod 20, which is equivalent to the tension between the pad eyes 24 and 26) and the extension or retraction of the rod 20. The line 89 depicts the system 10 when operated as a standalone riser tension protection system, or in a backup mode when rigged up as a backup heave compensator. For the purposes of this example the rod 20 has a maximum stroke of 24 ft and is set at an intermediate position being the mid stroke position extending 12ft from the end 14 of the housing 12. At the mid stroke position the rod 20 is rigidly held by virtue of the distribution of the non-compressible working fluid W between the housing 12, the bottomed out under tension accumulator 34, top out over tension accumulator 32, and the relative pressures Pu<Pf<Po.

At the mid stroke position, the system 10 has applied to it the “top tension” shown as point 90 on Figure 2 which is the combination of the string weight and overpull. In this example this is nominally shown as 200 kips (1 kip = 1000 pounds-force, which is equivalent to about 4.45kN). The system 10 is arranged to keep the rod 20 rigidly at the mid stroke position until the tension either exceeds the over tension setpoint To (for this example 250kips) or falls below the under tension setpoint Tu (this example 150kips). Thus, while the tension T supported by the system 10 remains within the range Tu T To the rod 20 is held at its mid stroke position. This range of tension is the “dead band”.

If the tension T exceeds the over tension setpoint To then Pf>Po and the rod 20 will commence to stroke out of the housing 12 as shown by the portion 92 of the curve 89 coinciding with the rod extension changing from 12 ft to the maximum 24 ft. If the tension T on the riser falls below the under tension setpoint Tu then Pf<Pu and the rod 20 will commence to stroke (i.e. retract) into the housing 12 as shown by the portion 94 of the curve 89 coinciding with the rod extension falling from 12ft to Oft.

Assuming after either an over tension event or an under tension event, the gas pressure within the accumulators 32 and 34 are equalised with the working fluid pressure Pf (i.e.Pf=Po=Pu), the operating curve of the system 10 will switch to the compensating curve 95 in which the dead band is eliminated and the rod 20 strokes in or out in accordance with variations in tension on the riser.

As a result of the configuration of the system 10 is possible to vary the over tension setpoint and under tension setpoint To, Tu respectively about the top tension 90 independently of each other. In the above-described example the under an over setpoints are symmetrically set at 50 kips either side of the top tension 90. Flowever, because the gas pressures Po and Pu are initially independent of each other they can be arranged to provide a nonuniform spread of the setpoints To and Tu about the top tension 90. For example, the over tension setpoint To may be 60kips above the top tension 90 while the under tension setpoint Tu may be kept at 50kips or say reduced to 45 kips.

Likewise, the system 10 can be arranged so that the dead band does not coincide with the mid stroke position of the rod 20, but rather another position intermediate of the minimum and maximum extensions. For example, with reference to Figure 1 the dead band could coincide with the rod 20 being rigidly held at an intermediate extension of 9 ft by appropriately decreasing the maximum volume Vumax of the first region 60 in the under tension accumulator 34 so that when it is bottomed out against the stop 58 (and with the over tension accumulator 32 being topped out) the volume of non-compressible working fluid W in the annulus 30 retracts the piston 18 to the level 96 in the housing 12.

The ability to independently set the over tension and under tension setpoints To, Tu as well as the degree of extension of the rod 20 at the dead band provides a degree of flexibility and customisation not readily available in prior art systems.

The system 10 may be realised in many different physical configurations. Figure 1 shows one possible configuration where the housing 12 is a cylinder and includes the rod 20, and in which the over tension accumulator 32 and under tension accumulator 34 are physically separate from the housing/cylinder 12 and in particular are not in line between the couplings to the pad eyes 24 and 26.

Figure 3 shows an alternate form of the system designated here as 10a which is functionally identical to the system 10 described above. In describing the system 10a the reference numbers used for describing aspects of the system 10 would be used to denote the same features in the system 10a. The general concept and idea behind the embodiment in Figure 3 is to have the housing/cylinder 12, the UTA 34 and the OTA 32 all in line between the tension application points (24, 26). This can be achieved by forming the piston rod 20 as a hollow structure with an internal floating piston to act as either one of the UTA or OTA. In either case the non- compressible working fluid W would be able to flow between the inside of the hollow rod and the region 30. The floating piston is under the influence on a side opposite the working fluid of a compressed gas at the pressure Pu or Po, dependent on whether this structure is acting as the UTA or the OTA. In the specific embodiment of the system 10a shown in Figure 3 the under tension accumulator (UTA) 34 is incorporated in the piston rod 20, and the over tension accumulator 32 is coupled to the blind side of the cylinder 12. As result of this the pad eye 26 is now coupled to 54 of the over tension accumulator 32. Here, the cylinder 12, UTA 34 and the OTA 32 are in line. So, in this embodiment when the tension acts across the system 10 it also acts across the OTA 32. In any event, as in the first embodiment, the tension T in the riser is transmitted to the working fluid W.

The flow of the non-compressible working fluid W between the cylinder 12, and the over tension accumulator 32 is by the conduit 40. The flow of the non-compressible working fluid W between the under tension accumulator 34 and the cylinder 12 is by way of an orifice 98 in the rod 20 near the piston 18; or, by a port in the piston 18. Due to the arrangement of the conduit 40 and the orifice 98 the non-compressible working fluid W is in simultaneous fluid communication with the annulus 30 of the cylinder 12, the over tension accumulator 32 and the under tension accumulator 34. Figure 3 depicts the system 10a in the mid stroke position in which it will be further observed that the over tension accumulator 32 is topped out with its piston 42 at its closest possible location to the end 44, and the piston 54 being bottomed out against the stop 58.

Although not shown in Figure 3 the system 10a may be further provided with fill hoses, one-way valves, and controlled vents, to provide the same functionality as the hoses 66, 76, one-way valves 68, 78, and vents 70, 80 described in relation to the embodiment shown in Figure 1.

The operation, functionality and ability to vary the under tension and over tension setpoints To, Tu as well as the dead band location in terms of extension of the rod 20 are identical to those described in relation to the system 10.

Figure 4 shows a further system 10b which differs from the system 10a shown in Figure 3 in that the over tension accumulator 32 is now not connected to and in line with the cylinder 12. Rather the over tension accumulator 32 is separate from the cylinder 12 and remains in fluid communication via the conduit 40. Thus, the only difference between the systems 10a and 10b are their physical realisations; their function, operation and performance remain identical.

In each of the described embodiments of the system 10, 10b shown in Figures 1 and 4 the respective accumulators which are separate from the cylinder 12 may be formed with separate air pressure vessels (APVs). This is shown for example in Figures 5 and 6 which depict systems 10' and 10b'.

The system 10' shown in Figure 5 differs from the system 10 shown in Figure 1 in that it comprises an over tension accumulator 32' formed as a combination of a vessel 36' which contains the floating piston 42 and has a total volume being the same as the maximum volume of the first region 48; and an air pressure vessel (APV) 37 in fluid communication with the vessel 36'. The vessel 36' is in fluid communication with the region 30 by the conduit 40 that opens onto one end wall 44 of the vessel 36'. An opposite end of the vessel 36' is in fluid communication with the APV 37 which carries a compressed gas at a pressure Po. This fluid communication may be provided by a rigid or flexible pipe or hose 45. In this way the APV 37 may be mounted in a variety of different positions with respect to the cylinder 12 and/or the vessel 36'. An end wall 46' of the vessel 36' acts in the same way as the stops 46 in the system 10 of Figure 1.

The system 10' shown in Figure 5 further differs from the system 10 shown in Figure 1 in that it comprises an under tension accumulator 34' formed as a combination of a vessel 38' which contains the floating piston 54 and has a total volume being the same as the maximum volume of the first region 60; and an air pressure vessel (APV) 39 in fluid communication with the vessel 38'. The vessel 38' is in fluid communication with the region 30 by the conduit 40 that opens onto one end wall 56 of the vessel 38'. An opposite end of the vessel 38' is in fluid communication with the APV 39 which carries a compressed gas at a pressure Pu. This fluid communication may be provided by a rigid or flexible pipe or hose 47. In this way the APV 39 may be mounted in a variety of different positions with respect to the cylinder 12 and/or the vessel 38'. An end wall 58' of the vessel 38' acts in the same way as the stops 58 in the system 10 of Figure 1. The system 10b' shown in Figure 6 differs from the system 10b shown in Figure 4 in that it comprises an over tension accumulator 32b' formed as a combination of a vessel 36b' which contains the floating piston 42 and has a total volume being the same as the maximum volume of the first region 48; and an air pressure vessel (APV) 37b in fluid communication with the vessel 36b'. The vessel 36b' is in fluid communication with the region 30 by the conduit 40 that opens onto one end wall 44 of the vessel 36b'. An opposite end of the vessel 36b' is in fluid communication with the APV 37b which carries a compressed gas at a pressure Po. This fluid communication may be provided by a rigid or flexible pipe or hose 45b. In this way the APV 37b may be mounted in a variety of different positions with respect to the cylinder 12 and/or the vessel 36b'. An end wall 46' of the vessel 36b' acts in the same way as the stops 46 in the system 10 of Figure 1 .

Figures 7a and 7b show an example where two systems 10a of Figure 3 are used to side-by-side to form a tension frame assembly 100. In the assembly 100 the respective rods 20 of each cylinder 12 are couple to a common upper spread beam 102 while the lower ends of the over tension accumulators 32 are coupled to a lower spread beam 104 by any suitable mechanical coupling. A cylinder cross beam and winch assembly 106 is coupled between the cylinders 12 near their respective rod ends. By this arrangement a fixed working window 108 is created between the rod ends of the cylinders 12 and the ends 54 of the accumulators 32. The fixed work window creates a fixed distance between the lower spreader beam 104 and the cylinder cross beam and winch assembly 106 which does not change length when the piston rods 20 extend or retract within their respective housings 12. A feature which is not readily available in prior art systems. Also in this embodiment preferably, a hose or other conduit provides fluid pressure communication between the working fluid W within the respective regions 30 to ensure both systems 10a are operating with identical pressures and hence both will activate at the same time and stroke uniformly.

Now that an embodiment has been described, it should be appreciated that the system and method maybe embodied in many other forms. For example, mechanical locks may be provided for locking the piston rod 20 in a fully retract position for the purposes of transportation and connecting into a derrick. Additionally, wireless alarms may be provided to give an indication of rod position and gas pressures within various cylinders and accumulators. It should also be readily apparent from the above description that the specific physical form of the system 10 is highly variable, where for example the under tension accumulator may be incorporated within the piston rod 20 as in Figs 3, 4 and 6, or can be provided in a vessel which is separate to the housing 12, with the piston rod 20 being solid or at least not containing any fluid in communication with other fluids of the system 10. When either of the under tension accumulator or over tension accumulator provided as separate vessels that are not subjected to the riser tension that may be located side-by-side with the cylinder 12. Also, as previously mentioned in a variation to the system 10a shown in Figure 3 the OTA 32 may be incorporated within the hollow piston 20 instead of the UTA 34 as currently depicted; with the UTA 34 subsequently coupled in line with the cylinder 12.

In all variations the operating principles stay the same. There is a non-com pressible volume of working fluid that is simultaneously in communication with the main housing/cylinder 12 and the over tension an under tension accumulators 32, 34. The rod 20 is held in the intermediate (which could be mid stroke) position with the under tension accumulator bottomed out and the over tension accumulator topped out when the tension T is within the range Tu T To with the corresponding pressure relationship Pu<Pf<Po. The volumetric relationship Vf < Vomax + Vumax remains the same for all embodiments.

In each of the embodiments is possible to adjust the pressures of the compressed gases acting in the under tension accumulator and over tension accumulator once the tension has exceeded the range TucTcTo, so that Pu=Po=Pf which then enables the piston 18 and rod 20 to smoothly and continuously provide heave compensation without the dead band.

The ability to bias the rod 20 to an intermediate position which is different to the mid stroke position to coincide with the dead band (i.e. effectively move the dead band from the mid stroke position to a different intermediate position) is a same in all embodiments. The ability to vary the over tension setpoint and under tension setpoint To, Tu relative to the top tension point independently of each other is the same in all embodiments. In view of the volumetric relationship Vf < Vomax + Vumax the under tension accumulator may hold more than 50% of the total volume of the non-compressible working fluid W to account for leakage over time.

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 system and method as disclosed herein.