The invention relates to heave compensated make-up and break-out of drill pipe joints during drilling from a floating mobile offshore drilling unit (MODU), to eliminate or at least reduce bottomhole pressure variations induced by vessel vertical motion transferred to the drill string.

More precisely, the invention relates to a system arranged for active heave compensation of a remotely controlled iron roughneck and power slips assembly, to be suspended underneath a drill floor rotary table of a drilling vessel.

Furthermore, the invention relates to a method of heave compensated make-up and break-out of drill pipe connections during drilling of a subsea well.

Conventional offshore drilling through a marine drilling riser has been the most trusted drilling concept employed for exploration drilling and subsea field production drilling. During drilling and completion of well sections that extend into the oil and gas reservoir, it will be necessary to use a blowout preventer (BOP) connected to the subsea wellhead. The BOP provides safety-critical functions to prevent uncontrolled release of reservoir fluids and gasses during well construction. The BOP may either be hydraulically or electro- mechanically operated. A subsea end of a drilling riser is via a flexible element attached to a top end of a lower marine riser package (LMRP) which is connected to the subsea BOP. The drilling riser extends through the water column and a vessel moonpool to an underside of the drill floor, where it is suspended and connected below the rotary table. The drilling riser has several functions; to serve as a return conduit for cuttings laden drilling mud from the well, to be used for attachment and support of ancillary lines between the subsea BOP and the MODU for well control, and to bring the subsea BOP to/from the subsea wellhead. A riser tensioning system keeps the marine drilling riser stable, while, at the same time, compensating passively for vertical motion of the vessel. The drilling riser can be disconnected from the subsea BOP with a LMRP high-angle release connector.

Many mature fields with production infrastructure that could be utilized for new production are "un-drillable" with conventional drilling techniques. The challenge is narrow margins between pore-pressures and fracture-gradients in the reservoir, because of depleted and/or pressurised reservoir zones. This limits options for new infill wells and increased recovery. Narrow "drilling windows" is also a challenge and limitation for development of many ultra-deep-water assets.

Managed pressure drilling (MPD) is a proven solution to safely drill at narrow margins. These systems are widely accepted in surface installations. MPD from a MODll is typically performed by means of so-called "below riser tension ring RCD" systems. Placement of a rotating control device (RCD) below the tension ring allows full use of a telescopic joint for heave compensation. In combination with the RCD installed on the marine drilling riser, an annular preventer and a flow spool is also installed. This equipment is required to facilitate changing of the RCD sealing elements while circulation continues, and pressure is held in the well.

A revolutionary approach to solving the challenges of narrow "drilling windows" is drilling in open water, known in the industry as dual-gradient drilling (DGD), which will provide a large range of cost and labour savings related to the drilling vessel and equipment. The open water drilling solution eliminates the conventional marine riser, diverter system, bellnipple and riser tensioning system and will enable full simultaneous drilling capabilities for the subsea well construction. A main feature of the solution is a subsea BOP with a wear resistant subsea RCD connected at the top of the BOP, sealing around the drill string. An annular preventer may also be included immediately below the RCD. Another BOP adaptation is an interface with a drilling-mud return system. The BOP control system must also be adapted to the open water drilling mode of operation. The solution will provide improved wellbore pressure management, longer sections can be drilled with the same casing diameter, and wellbore instability can be avoided. Benefits and cost savings with this drilling method will increase with the water depth.

A drill string is a combination of drill pipe joints, a bottom hole assembly (BHA) and any other tools used to make the drill bit turn at the bottom of the wellbore. During drilling mud is pumped through the drill string and exits through drill bit nozzles into the bottom hole, where the drill bit is working, and is circulated back to the vessel. Drilling mud with cut- tings is returned to the surface for treatment and re-use via the well annulus around the drill string, and for conventional drilling, further up through the marine drilling riser and diverter system. With open water drilling, the mud is returned to the surface through a separate mud return system, which may be a flexible hose or a rigid riser, depending on the water depth. At least one subsea mud return pump is included in the system solution.

The drill string is actively heave compensated during drilling operations. For heave compensation of a hanging load, one or more linear actuators, e.g. hydraulic cylinders or winch/wire-rope systems are provided between the crown block and the derrick, or between the travelling block and the hook. During connection of a drill pipe, the drill string is pulled up to clear the drill bit from the bottom hole before it is wedged in the drill floor rotary table with manual slips or power slips. Mud circulation is temporarily stopped during drill pipe connections. For conventional drilling with a marine drilling riser, hydrostatic pressure in the drill string and the well/riser annulus are balanced. For open water drilling, there is a U-tubing effect from unbalanced hydrostatic pressure between the drill string and the annulus, which is discontinued at the seabed equipment. The RCD on the subsea BOP seals around the drill string and counteracts the U-tubing effect together with the mud return pump.

While the upper portion of the well is protected by casing, the open-hole section of the lower portion of the well is exposed to the reservoir formation. Pressure regulation accuracy in MPD control systems is around ±2.5 bar (36.5 psi) to enable safe operation within narrow pressure margins in the reservoir. When a drill string is suspended in slips in the rotary table, the derrick heave compensation system will be temporarily disabled. Drilling vessel vertical motion will thus be transferred to the drill string. When the drill string moves up, pressure is reduced (swab effect), and when the drill string moves down, pressure will increase (surge effect). This induces pressure oscillations in the well that can violate tight pressure margins under rough weather conditions. Vessel heave motion in the North Sea and in areas with similar conditions can be several meters (>3 meters (>10 ft) with amplitudes of 10-20 seconds. Pressure fluctuations due to vessel heave have been of a magnitude higher than the standard limit for pressure regulation accuracy in MPD control systems. A heave of 3.5 meters can generate downhole pressure fluctuations of 30 bar (435 psi).

During surge there is a risk of losing drilling mud into the formation. The hydrostatic pressure of the well will then drop and may cause uncontrolled influx from the reservoir. The same can occur during swabbing because of hydrostatic pressure reduction in the well. During conventional drilling in reservoirs with good operational margins, the risk is low, and operations can proceed up to an allowable level of heave. However, open water drilling will usually be employed in reservoirs with narrow operating windows. It is therefore important to eliminate or reduce pressure fluctuations from drill string surge and swab motion during drill pipe make-up to avoid this to become a major limitation to the utilization of the drilling method.

There have been efforts in the industry to mitigate heave-induced pressure oscillations during MPD with "below riser tension ring RCD" systems. The drilling riser annulus is sealed by the RCD during these operations, and it has been attempted to use sophisticated, active control of the surface MPD choke and a pump to regulate the backpressure, and thus the bottomhole pressure, relative to the heave motion. This approach is challenging, due to the long distances between the bottom hole and the surface equipment, associated fluid dynamics, and demands on the control system response.

Another method being developed to attenuate downhole pressure oscillations is the use of a controllable valve installed in the bottomhole assembly (BHA) in the lower end of the drill string. Installing both the sensor and valve actuator downhole has two main advantages over control from topside. Firstly, the actuator is at, or very close to, where pressure control is desired. Thus, there is practically no delay, and no friction will affect the control input before it reaches the control objective. Secondly, the motion of the BHA of the drill string, which is the main cause of downhole pressure oscillations, is measured directly. Therefore, the system does not rely on being able to accurately predict the movement of the elastic drill string. Since no data exchange is required between the controllable valve and the MODll, except from possible start/stop signals, the system can work autonomously. However, the solution has its own set of challenges related to fluid dynamics. Changing the valve flow creates large differential pressures that can amplify the drill string movement. Downhole pressure oscillations are expected to be reduced by 50% with this approach.

A completely different approach is to remove the very root of the problem. Heave- compensated drilling platforms/floors installed on MODll's are known in the industry as a means of eliminating or at least reducing surge and swab of the drill string BHA during make and break-out of drill pipe connections but are so far very rare in today's drilling vessel fleet. Such systems are large, heavy, and expensive and require extensive structural integration with the vessel. Implementation has to be done as part of a drilling vessel newbuild or could be implemented as part of a major drilling vessel upgrade. WO 2020/067905 A1 discloses a coiled-tubing injector comprising an injector head provided with driving means arranged to displace a coiled tubing, and a control unit which is connected to the driving means and is arranged, by means of a first set of control signals, to control the axial displacement of the coiled tubing by the driving means, wherein the injector head is supported by a heave compensator forming a connection between a base formed on a floating installation and the injector head, and the heave compensator is provided with one or more heave-compensation cylinders which are arranged to generate a heave-compensated vertical movement of the injector head.

US 2015/0315860 A1 discloses an offshore drilling installation comprising a platform selected from the group comprising a vessel, a pontoon, a jack-up, and further comprising a drill string drivingly connected with said platform and optionally provided with a heave motion compensator, wherein a seabed template suspended with lift wires from the platform is placed on a seabed, and wherein clamping means mounted in the seabed template are provided for fixing the drill string relative to the seabed template, which clamping means are embodied as a chuck which is arranged for clamping the drill string whilst enabling the drill string's rotation.

WO 2019/093899 A1 discloses a vessel for performing subsea wellbore related activities, e.g. workover activities, well maintenance, installing an object on a subsea wellbore, etc., while the floating vessel is subjected to heave motion due to waves. A heave compensated hoisting device is mounted on a floating hull and is configured for lowering and lifting an object along a firing line through a moonpool of the vessel to/from a seafloor. A vertically mobile working deck with a hoisting cable opening that is aligned with the firing line and is configured for passing through at least the hoisting cable. A mobile working deck compensator is configured to move the mobile working deck between a lowered position and an elevated position, and to provide a heave compensated motion of the mobile working deck when in the elevated position. An object loading device is configured for introducing the object below the mobile working deck and in the firing line, such that the object can be coupled with a travelling hoist block to enable the winch system to lower the object along the firing line below the mobile working deck and to the seafloor.

Due to the above-mentioned and other challenges related to the elimination or reduction of surge and swab motions of the BHA inside the well, a lightweight solution for heave compensation of an iron roughneck and automatic power slips assembly, is provided. The drilling may be performed in open water or through a heave compensated marine riser arranged without the traditional diverter and telescopic joint arranged at an upper portion of the marine riser.

The object of the invention is to enable a simplified heave compensated make-up and break-out of drill pipe joints during drilling.

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

In one embodiment, a drilling vessel is prepared for open water drilling by removing structures like a marine drilling riser with a telescopic joint (hereinafter also called slip joint), a diverter and a riser tensioning system for conventional drilling. In another embodiment, the drilling may take place through a marine drilling riser, wherein the slip joint and the diverter have been excluded. Thus, there will be space available to suspend an iron roughneck and power slips assembly under a drill floor of the vessel, arranged in line with a rotary table.

The iron roughneck and power slips assembly is mounted in an actively heave- compensated suspension arrangement comprising several linear actuators formed by 1) anchored hydraulic cylinders, or alternatively, 2) wire ropes extending from one or more heave compensated winches, with the wire-ropes looped through anchored, heavy duty snatch blocks at an underside of the drill floor. Benefits are:

• Removes or reduces rig vertical motion induced on the drill string during drill pipe make or break, and thus downhole surge and swab pressure oscillations from a drill string bottom hole assembly.

• Retro-fit friendly solution. Small vessel upgrade required.

• Reduced weight on the vessel compared with heave compensated drilling plat- forms/floors installed on the drilling vessel.

In a first aspect the invention relates more particularly to a system for heave compensated make-up and break-out of drill pipe connections during a drilling operation from a drilling vessel, the system comprising: a blowout preventer locked on a wellhead, a drill string extending from the drilling vessel through the blow-out preventer to a bottom of a subsea well in an oil and gas reservoir, a sealing device above the blowout preventer and arranged to seal around the drill string during the drilling operation, a drilling mud return system connected to the blow-out preventer and provided with a mud-lift pump and a mud return line extending to the drilling vessel, wherein an iron roughneck and power slips are mounted in a vertically displaceable support frame arranged below a drill floor of the drilling vessel, the support frame is suspended in the drill floor by a set of linear actuators arranged for active heave compensation of the support frame, the support frame is vertically guided in a guiding arrangement, and a control system is arranged for remote operation of the iron roughneck and power slips, and for hydraulic or electro-mechanical heave compensation of the support frame.

The drill string may extend through a marine riser suspended in the drilling vessel, with the diverter and slip joint removed

The sealing device may be provided inside the marine riser. An effect of this is that most of the marine riser may provide part of the mud return line. Mud return from below the sealing device is via a separate return line to the drilling vessel.

The drill string may extend through open water with the sealing device connected to an upper portion of the blowout preventer. An effect of this is that the load impact on the drilling vessel is reduced through removal of the marine drilling riser. Mud return from below the sealing device is via a separate return riser to the drilling vessel.

Thus, depending on whether the drill string extends through open water, or through a marine riser, hereinafter also denoted marine drilling riser, the sealing device may be provided directly above the blowout preventer, or in the marine drilling riser.

The set of linear actuators may comprise several hydraulically operated cylinders. An effect of this is that the actuators are easily adapted to the available space.

The set of linear actuators may comprise several wire-ropes extending from one or more winches. An effect of this is that voluminous portions of the linear actuators can be located far away from the support frame.

In a second aspect the invention relates more particularly to a method of heave compensated make-up and break-out of drill pipe connections during drilling of a subsea well, in connection with a need to at least reduce bottom hole pressure variations induced by vertical motion of a drilling vessel and transferred to a drill string during drill pipe connect or disconnect operations, wherein the method comprises the steps of: a) providing a system according to the first aspect of the invention; b) assembling the drill string with a bottom hole assembly and extending the drill string through the iron roughneck and the power slips assembly mounted on the support frame below the drill floor by adding in sequence drill pipes to allow the bottom hole assembly reaching a well bottom, c) extending a wellbore by rotating the drill string bottom hole assembly; d) upon need of extending the drill string, lifting the drill string to provide clearance between the bottom hole assembly and the well bottom; e) wedging the drill string in the power slips; f) heave compensating the iron roughneck, the power slips and the wedged drill string by operating the linear actuators to at least reduce the vertical motion of the drill string in relation to a wellbore; g) making-up the drill string by adding one or more drill pipes to the drill string; h) releasing the extended drill string from the power slips and lowering the drill string to continue the drilling; i) upon need of retracting the drill string from the well, lifting the drill string to provide clearance between the bottom hole assembly and the well bottom; j) wedging the drill string in the power slips; k) heave compensating the iron roughneck, the power slips and the wedged drill string by operating the linear actuators to at least reduce the vertical motion of the drill string in relation to the wellbore; and l) breaking-out the drill string by removing one or more drill pipes from the drill string; m) releasing the shortened drill string from the power slips and lifting the drill string to continue the break-out.

In the following are described examples of preferred embodiments, illustrated in the accompanying drawings, wherein:

Fig. 1 shows an overview of a system for heave compensated make-up and break-out of drill pipe connections during an open water drilling operation from a drilling vessel;

Fig. 2 shows in larger scale an iron roughneck and power slips assembly supported in a first embodiment of a heave-compensated suspension arrangement; Fig. 3 shows the iron roughneck and the power slips assembly supported in a second embodiment of the heave-compensated suspension arrangement;

Fig. 4 shows in smaller scale an overview of the system according to the invention during drilling operation from a drilling vessel through a marine riser; and

Fig. 5 shows in larger scale more details of the arrangement according to figure 4.

Any positional indications refer to the position shown in the figures.

In the figures, same or corresponding elements are indicated by same reference numerals. For clarity reasons, some elements may in some of the figures be without reference numerals.

The figures depict the invention in a simplified manner, and details not relevant to illustrate novel features, may have been excluded from the figures. The different elements in the figures are not necessarily shown in the correct scale in relation to each other.

Figure 1 depicts a first exemplary embodiment of the system according to the invention, wherein a drilling vessel 13 floating in a mass of water 30 is provided with an iron roughneck 1 and power slips 2 assembled on a heave compensated support frame 3. A drill string 7 comprising several drill pipes 7a (also called drill pipe joints) and a bottom hole assembly 7b in the form of a drill bit, etc. is extending from the drilling vessel 13 through the iron roughneck 1 and the power slips 2 to a subsea well 16 extending from a wellhead 15 to a well bottom 16b via a wellbore 16a. The subsea well 16 is provided with i.a. a blowout preventer 14. In this embodiment, a sealing device 17 is provided on top of the blowout preventer 14, and a drilling mud return system 18 comprising a mud-lift pump 18a and a mud return line 18b provides a mud return conduit between the wellhead 15 and the drilling vessel 13. The sealing device 17 is arranged to seal around the drill string 7 during the drilling operation. A control system 19 is arranged to operate the iron roughneck 1, the power slips 2, and the heave compensated support frame 3. A vertically displaceable and heave compensated top drive 12 is provided above a drill floor 4 in line with the drill string 7 and is arranged to rotate the drill string 7.

It is now referred to figure 2. The iron roughneck 1 and the power slips 2 are attached to the support frame 3 which is suspended from the drill floor 4. The support frame 3 is arranged to be vertically displaced along a guiding arrangement 5 attached to a surrounding structure (not shown) of the drilling vessel 13. The iron roughneck 1 and the power slips 2 are lined up with a rotary table 6 forming part of the drill floor 4. The drill string 7 is here shown extending through the open iron roughneck 1 and power slips 2. Drill pipes 7a are assembled and racked at the drill floor 4 in pipe stands (not shown). Normally, each pipe stand consists of three drill pipe joints (7a), and complete pipe stands are used to make up the drill string 7 as the drilling progresses.

In this embodiment the support frame 3 is suspended from several heave compensated linear actuators 8 in the form of hydraulic cylinders powered from a power pack (not shown). Alternatively, the linear actuators 8 may be electro-mechanical devices.

It is now referred to figure 3 showing an alternative suspension arrangement, wherein the support frame 3 is suspended from wire-ropes 9 that are looped through heavy-duty snatch blocks 10 attached under the drill floor 4. Each wire-rope 9 is reeled in and out by means of an active heave compensation winch 11 with electrical or hydraulic drive. The winches 11 are shown installed on the drill floor but may be mounted differently, e.g. at a lower deck level. One winch may handle more wire-ropes 9, e.g. a pair of wire-ropes 9 connected to one side of the support frame 3.

It is now referred to figure 4, depicting an alternative exemplary embodiment of the invention. A marine drilling riser 20 is extending from the wellhead 15 of the subsea well 16 to the floating drilling vessel 13, where the marine riser 20 is connected by a heave compensated tensioning device 20a (see fig. 5) in a way well known in the art. To make room for the heave compensated iron roughneck 1 and power slips 2 below the rotary table 6 at the drill floor 4, an upper portion of the marine riser 20 does not include a slip joint and a diverter.

It is referred to figure 5 for more details. In this embodiment, the sealing device 17 is provided in the upper portion of the marine riser 20. An annulus of the marine riser 20 forms part of the drilling mud return system 18, and a mud return line 18b with a mud-lift pump 18a extends from below the sealing device 17 to the drilling vessel 13. Additional fluid conduits like a choke line 20b and a booster line 20c are arranged along the marine riser, connecting wellhead controls (not shown) of the drilling vessel 13 to the wellhead 15.

For drilling and tripping of the drill string 7 a drill pipe joint 7a or a spacer joint (not shown), is required during the make-up and break-up operations to bridge the gap between the rotary table 6 and the suspended power slips 2. The drill string 7 is suspended from the heave compensated top drive 12 during drilling. Prior to connection of another pipe stand, the drill string 7 is pulled-up to clear the bottom hole assembly 7b from the bottom 16b of the wellbore 16a, before the drill string 7 is wedged in the heave compensated power slips 2. The upper single drill pipe joint 7a, alternatively the spacer joint (not shown) used to bridge the gap between the power slips 2 and the iron roughneck 1 is then disconnected and pulled up through the rotary table 6. The pipe stand to be added is lowered through the rotary table 6 by the top drive 12 and is made up to the drill string 7 by the iron roughneck 1. The power slips is opened, and drilling continues. The sequence is repeated.

During drill string retrieval, a pipe stand is pulled up above the power slips 2, the drill string 7 is wedged into the power slips 2 and the pipe stand is disconnected by the iron roughneck 3 and pulled out through the rotary table 6 and is racked. The top drive 12 is then used to lower the single drill pipe joint 7a, alternatively the spacer joint (not shown) used to bridge the gap between the power slips 2 and the iron roughneck 1 , through the rotary table 6 to the power slips 2. The single drill pipe joint 7a or the spacer joint (not shown) is made up to the top of the drill string 7. The sequence of pulling out and racking a pipe stand is repeated. Once a bottom hole assembly (not shown) is above open wellbore hole, tripping may be done with the ordinary set-up in the rotary table 6.

For tripping into the subsea well 16, the operation will typically be done with the ordinary set-up in the rotary table 6 for the upper, cased part of the subsea well 16.

The iron roughneck 1 and power slips 2 is actively heave compensated during the drill pipe connection operations. The heave-compensation technique is known from heave compensation of the drill string 7 during drilling. This involves measuring the movement of the drilling vessel 13 using a measuring device (for example a motion reference unit ( MRU)) and using a signal representing the motion of the drilling vessel 13 to control a drive to retract or extend the linear actuators 8 (see figure 2), or drive the wire-ropes 9 in or out with the winches 11 (see figure 3), in pace with the heave movement of the drilling vessel 13, to eliminate or at least reduce the vertical pumping movement of the drill string 7 inside the subsea well 16, that would otherwise be a result of heave movement imposed by the drilling vessel 13.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any refer- ence signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise», and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.