The invention relates to a coiled-tubing injector comprising an injector head provided with driving means arranged to displace a coiled tubing in the axial direction of the coiled tub- ing, and a control unit which is connected to the driving means and which, by a first set of control signals, is arranged to control the axial displacement of the coiled tubing by the driving means. The invention also relates to a method for simultaneous axial displacement of a coiled tubing relative to a subsea well and heave compensation relative to the heave motion of a floating installation. When coiled-tubing operations are performed on subsea wells from a floating installation, the so-called coiled-tubing injector with associated components (blowout preventer, pack ers, etc.) must be heave-compensated by the coiled-tubing injector being placed in a frame which is suspended from a heave-compensation system, for example by said frame being suspended from a heave-compensated top drive or a heave-compensated hoisting block, possibly by said frame being provided with an integrated heave-compensation sys tem which counterbalances the vertical movements of the floating installation.

A drawback in the heave compensation of prior-art coiled-tubing injectors is that relatively tall structures are required to provide vertical guide tracks, for example in a tower, and this results in relatively large structures being in vertical motion relative to the floating installa- tion, and relatively much energy being required for the constantly changing direction of motion and change of speed that must be applied to the coiled-tubing injector in order for the vertical motion of the floating installation not to be transmitted to the part of the coiled tubing that is in or at the wellhead or in the subsea well.

From WO2018094264A1 , a floating installation provided with a heave-compensated upper floor is known, in which several hydraulic cylinders extend between the upper floor and a lower base. The upper floor carries a coiled-tubing system comprising a coiled-tubing reel and an injector head for paying out and retracting a coiled tubing. The upper floor also comprises counterweights for counterbalancing the weight of the coiled-tubing reel. A substantial drawback of the device is that a considerable mass will have to be put into motion during the heave compensation, as both the coiled-tubing reel and the counter- weights are to be accommodated on the upper, heave-compensated floor, which results in an additional mass, both because of a floor of large dimensions and because of the weight of the coiled-tubing reel and the counterweights, in relation to an injector head alone.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art or at least to provide a useful alternative to the prior art.

The object is achieved through the features that are specified in the description below and in the claims that follow.

The invention provides a surface unit for coiled-tubing operations in connection with a subsea well, in which, in a manner known perse, an injector head on a coiled-tubing in- jector comprises several driving means comprising one or more driving motors which are connected to driving devices which are arranged to rest against a coiled tubing to displace the coiled tubing in its axial direction through the coiled-tubing injector. The driving devic es may be wheels, rollers, chains with gripping blocks, etc., according to the prior art known perse. In a manner known perse, the coiled-tubing injector also comprises further equipment required for a coiled-tubing operation to be performed in a manner which is safe and according to regulations, typically by the coiled-tubing injector being provided with a gooseneck directing the coiled tubing from a reel associated with the coiled-tubing injector into the injector head, strippers fitting tightly around the coiled tubing and forming active pressure barriers between the injector head and the well, and one or more blowout preventers. A drive unit is connected to the coiled-tubing injector for the supply of electri cal energy, pressurized fluid, etc., for the operation of engines and actuators. Finally, a control unit is connected to the coiled-tubing injector to monitor and control the coiled- tubing operation.

The coiled-tubing injector is supported on a suitable structure on the floating installation from which the coiled-tubing operation is carried out. Said structure is typically a floor, for example a well floor or a drilling floor, the coiled-tubing injector being so placed that the centre axis of the coiled-tubing injector is arranged above a production tubing, a marine riser or an opening in said floor. To provide an active heave compensation, the control unit is connected to means that register the vertical movements (heave motion) of the floating installation and generate control signals for heave-compensation means connected to the coiled-tubing injector.

The active heave compensation may be combined with passive heave compensation of a suitable type to relieve a tool that is to be landed in or at the well, for example by the heave-compensation means being connected to one or more hydraulic accumulators. Active and passive heave compensations may preferably be operated independently of each other.

In one embodiment, the heave-compensation means are arranged to vertically displace the entire coiled-tubing injector in opposite phase to the heave motion of the floating in stallation, typically in the form of a platform carrying the coiled-tubing injector, the platform hanging from or resting on one or more heave-compensation cylinders.

Alternatively, the heave-compensation means may be made up of the driving motors of the coiled-tubing injector, the driving motors thereby being able to displace the coiled tub- ing in opposite phase to the vertical motion of the floating installation. In this embodiment, these control signals for compensating the heave motion may be superimposed on the control signals for the ordinary vertical displacement of the coiled tubing when the coiled tubing is being run into or out of the well. Thereby the injector head may displace the coiled tubing relative to the well, ensuring at the same time that the coiled tubing is heave compensated in that the vertical motion of the floating installation relative to the well will be counterbalanced.

Alternatively, the active heave compensation may be provided as a constant-tension con trol; that is to say, the heave-compensation means of the coiled-tubing injector are set to hold a constant tension of a prescribed magnitude on the coiled tubing. The invention is defined by the independent claims. The dependent claims define advan tageous embodiments of the invention.

In a first aspect, the invention relates, more specifically, to a coiled-tubing injector, com prising

an injector head provided with driving means arranged to displace a coiled tubing in the axial direction of the coiled tubing, and

a control unit which is connected to the driving means and, by means of a first set of control signals, is arranged to control the axial displacement of the coiled tubing by the driving means,

characterized by

the injector head being supported by a heave compensator which forms a connec tion between a base formed on a floating installation and the injector head, and

the heave compensator being provided with one or more heave-compensation cylinders which are arranged to generate a heave-compensated vertical movement of the injector head.

The heave-compensation cylinders may be connected to a drive unit,

a heave-registration unit which is arranged to register a vertical movement of a floating installation that supports the coiled-tubing injector may be connected to the control unit and may be arranged to generate a second set of control signals, and

the control unit may be connected to the drive unit and is arranged, by means of the second set of control signals, to control the displacement of the heave compensator by the heave-compensation cylinders in order thereby to be able to generate an actively heave-compensated vertical movement of the injector head independently of the first set of control signals.

The heave-registration unit may be a motion reference unit arranged on the floating instal lation.

Alternatively, the heave-registration unit may be a unit arranged to measure an axial ten- sion in the coiled tubing.

A coiled-tubing guide which may be arranged on the injector head may be provided with heave-compensation means.

The coiled-tubing guide may be provided with passive heave-compensation means. Alter natively, the coiled-tubing guide may be provided with active heave-compensation means arranged to be controlled by means of the second set of control signals.

The heave-compensation cylinders of the heave compensator may be connected to a platform arranged to support the injector head.

In a second aspect, the invention relates, more specifically, to a method of heave- compensating a coiled tubing during a coiled-tubing operation in a subsea well performed from a floating installation, the method comprising the following steps:

providing a coiled-tubing injector according to the first aspect of the invention, providing a first set of control signals for generating an axial displacement of the coiled tubing relative to the well by means of the driving means arranged in the injector head,

providing a second set of control signals by means of a heave-registration unit, characterized by the method comprising the further step of:

- controlling the displacement of the heave compensator by the heave-compensator cylinders by means of the second set of control signals to generate a heave-compensated vertical movement of the injector head independently of the first set of control signals.

The heave-registration unit may be a motion reference unit connected to the control sys tems of the floating installation. Alternatively, the heave-registration unit may measure an axial tension in the coiled tubing.

The method may comprise the further step of:

displacing a coiled-tubing guide which is arranged on the injector head, by means of heave-compensation means in step with the heave-compensating vertical displacement of the injector head generated by means of the heave compensator. The displacement of the coiled-tubing guide may be provided as a passive heave com pensation of the coiled-tubing guide. Alternatively, the displacement of the coiled-tubing guide may be provided as an active heave compensation of the coiled-tubing guide, the displacement being controlled by means of the second set of control signals.

In a third aspect, the invention relates, more specifically, to a coiled-tubing injector com- prising

an injector head provided with driving means comprising at least one driving motor arranged to displace a coiled tubing in the axial direction of the coiled tubing, and

a control unit which is connected to the at least one driving motor and, by means of a first set of control signals, is arranged to control the axial displacement of the coiled tub- ing by the at least one driving motor,

characterized by

a heave-registration unit, which is arranged to register a vertical movement of a floating installation that supports the coiled-tubing injector, being connected to the control unit and being arranged to generate a second set of control signals for the at least one driving motor in order thereby to generate a heave-compensated axial displacement of the coiled tubing independently of the first set of control signals.

The heave-registration unit may be a motion reference unit arranged on a floating installa tion supporting the coiled-tubing injector. The heave-registration unit may be a unit arranged to measure an axial tension in the coiled tubing.

A coiled-tubing guide that is arranged on the injector head may be provided with a heave compensator. The heave compensator may be passive, or it may be active, arranged to be controlled by means of the second set of control signals.

In a fourth aspect, the invention relates, more specifically, to a method of heave- compensating a coiled-tubing during a coiled-tubing operation in a subsea well performed from a floating installation, the method comprising the following steps:

providing a coiled-tubing according to the first aspect of the invention,

- providing a first set of control signals for generating an axial displacement of the coiled tubing relative to the subsea well by means of the driving means arranged in the injector head,

by means of a heave-registration unit, providing a second set of control signals for generating an axial displacement of the coiled tubing relative to the floating installation in order thereby to compensate for the heave motion of the floating installation, character ized by the method comprising the following step:

controlling the driving means in the injector head by means of a combination of the first set of control signals and the second set of control signals, the first set of control sig nals for the axial displacement of the coiled tubing relative to the subsea well being super- imposed on the second set of control signals to compensate for the heave motion of the floating installation.

The heave-registration unit may be a motion reference unit connected to the control sys tems of the floating installation.

Alternatively, the heave-registration unit may measure an axial tension in the coiled tub- ing.

The method may comprise the further step of displacing a coiled-tubing guide arranged on the injector head, by means of a heave compensator in step with a heave-compensating axial displacement of the coiled tubing generated by means of the driving means arranged in the injector head. The displacement of the coiled-tubing guide may be provided as a passive heave com pensation of the coiled-tubing guide; that is to say, by the coiled-tubing guide being, for example, springily supported in the injector head. Alternatively, the displacement of the coiled-tubing guide may be provided as an active heave compensation of the coiled-tubing guide, the displacement being controlled by means of one or more actuators that are con trolled by means of the second set of control signals.

In what follows, examples of preferred embodiments are described, which are visualized in the accompanying drawings, in which:

Figure 1 shows, in a partially cutaway side view, a first exemplary embodiment of a coiled-tubing injector according to the invention in a configuration with a ma rine riser;

Figure 2 shows, in a partially cutaway side view, the first exemplary embodiment of the coiled-tubing injector according to the invention in a riserless configura tion;

Figure 3 shows an example of speed curves for a coiled tubing, curve I showing the speed at which the coiled tubing is displaced while being run into and pulled out of a well, curve II showing the speed at which the coiled tubing will have to be displaced in order to cancel out a heave movement of the floating in stallation, and curves l+ll showing a resulting speed for simultaneous run ning of coiled tubing and heave compensation with the coiled-tubing injector according to the first exemplary embodiment by the curve II being superim posed on curve I; and Figure 4 shows a partially cutaway side view of a second exemplary embodiment of a coiled-tubing injector according to the invention in a configuration with a ma rine riser.

Reference is first made to figure 1 , in which the reference numeral 1 indicates a coiled- tubing injector comprising an injector head 13 of a design known per se, in which driving means comprising at least one driving motor 131 connected to driving devices 132 are arranged to rest supportingly against a coiled tubing 4 which, by means of the coiled- tubing injector 1 , is to be run into or pulled out of a subsea well 6. The driving devices 132 are only shown in principle here. On the top of the injector head 13, a gooseneck-shaped coiled-tubing guide 12 is arranged. Below the injector head 13, active pressure barriers are arranged in the form of strippers 14 fitting tightly around the periphery of the coiled tubing 4. The coiled-tubing injector 1 is further provided with a blowout preventer 15 and is connected to a surface wellhead (flow head) 16, where well fluid may flow out of or into a well pipe 31 , for example a production tubing which, in a fluid-communicating manner, connects the well 6 and a floating installation 3, here shown schematically and in section as a supporting structure or a floor. The well pipe 31 is arranged in a marine riser 32 ex tending through a water mass 7 between the well 6 and the floating installation 3. The coiled tubing 4 is stored on a coiled-tubing reel 5 which is arranged on the floating installation 3 near the coiled-tubing injector 1.

The coiled-tubing injector 1 is connected to a drive unit 2 which is arranged to provide the coiled-tubing injector 1 and possibly the coiled-tubing reel 4 with electrical current, pres sure fluids, etc., for operating motors, actuators, activating seals, etc. The coiled-tubing injector 1 is provided with a control unit 133 arranged to generate a first set I of control signals for manoeuvring the coiled-tubing injector 1 so that the coiled tub ing 4 is displaced axially at a prescribed speed and direction relative to the well 6. Con nected to the control unit 133, there is a heave-registration unit 1331 arranged to generate a second set II of control signals in order to, by means of the driving means 131 , 132 of the injector head 13, give the coiled tubing 4 an axial displacement in opposite phase to a vertical movement of the coiled-tubing injector 1 caused by the heave movements of the of the floating installation 3 due to waves, currents in the water masses, changes in the load of the floating installation, etc., in order thereby, by active heave compensation, to eliminate the effect of the heave motion on the axial movement of the coiled tubing 4 rela- tive to the well 6.

To take up the slack in the coiled tubing 4 caused by the heave compensation, the coiled- tubing guide 12 may be provided with separate heave-compensation means 121 formed as a vertically telescoping mount that connects the coiled-tubing guide 12 and the injector head 13 below. The heave-compensation means 121 may be passive, formed as a springy suspension, or active in that they comprise means that, on the basis of said sec ond set II of control signals, displace the coiled-tubing guide 12 relative to the injector head 13 in order thereby to maintain support for the coiled tubing 4 in all phases of run ning the coiled-tubing.

The heave-registration unit 1331 which is connected to the control unit 133 may be a sep- arate unit, or it may be a so-called motion reference unit (MRU) which is connected to the control systems of the floating installation 3.

Figure 2 shows a riserless configuration in which the coiled tubing 4 extends through the water mass 7 between the floating installation 3 and an unconsolidated mass 8 under the water mass 7 in an initial drilling operation, for example the drilling of a pilot hole in the unconsolidated mass 8 by means of drilling equipment (not shown) connected to the coiled tubing 4. In this configuration, the coiled-tubing injector 1 is shown without a blow- out preventer and surface wellhead, but is otherwise provided with or connected to the same elements as described above in connection with the description of the configuration shown in figure 1.

Figure 3 shows an example of an axial displacement speed v as a function of time t for a prescribed speed of a coiled tubing when running in and out of a well (short-dashed curve I), a required axial displacement speed (dash-dotted curve II) in order to cancel out the vertical heave motion, and a resulting speed curve (solid curve l+ll) for the prescribed coiled-tubing running speed to be achieved while the coiled tubing is heave-compensated at the same time.

Reference is now made to figure 4 which shows a second exemplary embodiment of the invention. The injector head 13 is carried by a heave compensator 134 which is supported by the supporting structure 3, the injector head 13 being carried by one arrangement which, here, comprises several heave-compensation cylinders 1342 extending between the injector head 13 and a base 33, shown here as a frame resting on the supporting structure 3. Here, the injector head 13 is shown arranged on a platform 1341 forming a transition between the injector head 13 and the heave-compensation cylinders 1342. This may be advantageous as a technical adaptation of the injector head 13 is not required then for it to be able to cooperate with the heave compensator 134. The heave compensa tor 134 is provided with stabiliser means, shown schematically here as a telescoping guide 1343 extending between the injector head 13 and the base 33. Alternatively, the injector head 13 may hang on the heave-compensation cylinders 1342 extending downwards from a frame 33 which in this embodiment, not shown, rises suffi ciently far up from the supporting structure 3, typically to a level above the top of the injec tor head 13.

In the exemplary embodiment according to figure 4, the control unit 133 is arranged to control heave-compensation cylinders 1342 in such a way that the injector head 13 is heave-compensated by means of the heave compensator 134, as the second set II of control signals that is generated by the heave-registration unit 1331 connected to the con trol unit 133 is used in the control unit 133 to control the heave-compensation cylinders 1342 of the heave compensator 134 by means of the drive unit 2 so that the injector head 13 is given an axial displacement in opposite phase to the heave movements of the float ing installation 3 caused by waves, currents in the water masses, changes in the load of the floating installation, etc., in order thereby, by active heave compensation, to eliminate the effect of the heave motion on the axial motion of the coiled tubing 4 relative to the well 6.

The exemplary embodiment according to figure 4 may also be used in a riserless configu ration corresponding to what is shown in figure 2 where the coiled tubing 4 extends through the water mass 7 between the floating installation 3 and the unconsolidated mass 8 under the water mass 7 in an initial drilling operation, for example the drilling of a pilot hole in the unconsolidated mass 8 by means of drilling equipment (not shown) connected to the coiled tubing 4.

The principle that is shown in figure 4 and described above for the heave compensation of a coiled-tubing injector 1 can also be used in a so-called wireline operation in a well 6 car- ried out from a floating installation 3, as a wireline (cable) that is connected to a tool that is to be lowered into the well 6 can be passed from a drum supported on the floating installa tion, and over a wireline guide (for example a sheave) which is arranged on a heave com pensator 134 of the kind that is shown in figure 4.

It should be noted that all the above-mentioned embodiments illustrate the invention, but do not limit it, and persons skilled in the art may construct many alternative embodiments without departing from the scope of the attached claims. In the claims, reference numbers in brackets are not to be regarded as restrictive.

The use of the verb "to comprise" and its different forms does not exclude the presence of elements or steps that are not mentioned in the claims. The indefinite article "a" or "an" before an element does not exclude the presence of several such elements.

The fact that some features are indicated in mutually different dependent claims does not indicate that a combination of these features cannot be used with advantage.