NO20210037A1

NO20210037A1 -

Info

Publication number: NO20210037A1
Application number: NO20210037A
Authority: NO
Priority date: 2021-01-12
Filing date: 2021-01-12
Publication date: 2022-06-27
Also published as: GB2616787A; GB202309678D0; NO346362B1; WO2022154666A1

Description

OPEN WATER DRILLING – MUD RETURN SYSTEM

The invention relates to the transport of drilling-mud, with drill-cuttings, from a subsea well to the surface, as part of an open water drilling system for oil and gas drilling, typically from a floating mobile offshore drilling unit (MODU).

More precisely, one main aspect of the invention relates to a reduction of ancillary fluid conduits to be integrated as part of a mud return riser, facilitated through the implementation of a new drilling/well control solution with subsea chokes and circulation of possible hydrocarbon influx from the well via the mud return riser, with kill-mud supply via the drill-string as the primary pathway. Secondary pathways for supply of kill-mud are provided in case the drill-string is sheared. Another aspect of the invention is a mud lift assistance feature, based on arrangements of riser mounted eductors, to reduce the dependence on a subsea pump to provide mud lift.

Conventional offshore drilling through a marine drilling riser has been the most trusted drilling concept employed for exploration drilling and subsea field production drilling. The marine drilling riser conduit extends from the top of the subsea BOP to the underside of the drill-floor of the vessel. The marine 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. The lower end of the drilling riser is attached, via a flexible element, to the top of the Lower Marine Riser Package (LMRP), which is connected on top of the lower BOP. A riser tensioning system keeps the riser stable, while, at the same time, compensating passively for vertical motion of the vessel. The riser can be disconnected from the lower BOP with a LMRP high-angle release connector.

Many mature fields with existing production infrastructure that could be utilized for new production are "undrillable" 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 major challenge and limitation for the development of many ultradeep-water assets.

Managed Pressure Drilling, (MPD) is a proven solution to safely drill narrow margins. While these systems are widely accepted in surface installations, it remains to develop and qualify robust, functional solutions for the realization of MPD from floating drilling units.

Open water drilling, also known as Dual-Gradient Drilling, (DGD) will provide a large range of cost and labour savings related to the drilling vessel and equipment. The benefits will increase with the water depth. Casing set points in wells drilled in deep and ultra-deep water are kick tolerance dependent and pressure control must be maintained within the pore/fracture gradient window. The solution will open the "drilling window" by increasing margins so that the Bottom Hole Pressure (BHP) can be more easily contained between the formation pore pressure and the formation fracture pressure during drilling. This will provide improved well bore pressure management, longer sections can be drilled with the same casing diameter, and wellbore instability can be avoided. The open water drilling solution will eliminate the conventional marine riser and diverter system and will enable full simultaneous drilling capabilities for the well construction.

Current art technology has various shortcomings, which must be overcome to fully realize MPD from floating drilling units. One prominent technology gap is a sufficiently wear resistant and reliable Rotating Control Device, (RCD) for sealing around the drill string. Today's RCD's are mostly equipment intended for surface use and a core technology element in the so called "below riser tension MPD systems".

Another major technology gap is the mud-lift system. This is especially true for drilling in water depths beyond 2,000 ft. (= 610m), due to limited differential pressure capacity of available subsea mud pumps.

The mud return riser system will be based on existing riser and riser tensioner technology, but should be optimised to minimize the weight, complexity, and cost. This is particularly important for operations in deep and ultra-deep water.

During drilling and completion of well sections that extend into the oil and gas reservoir, it will be necessary to use a subsea 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. Subsea BOP`s and their associated control systems have evolved through many decades of use with conventional offshore riser drilling. For open water drilling operations, the BOP must be fitted with a subsea RCD at the top. Another BOP adaptation to be implemented is an interface with the drilling-mud return system. The BOP control system must also be adapted to the open water drilling mode of operation. The BOP may either be hydraulically or electromechanically operated.

The open water drilling-mud return system must be equipped with a mud-lift system, to compensate for pressure loss from friction and directional changes in the mud return flow. Mudlift systems and combinations thereof, can be:

(i) Submersible pump lift

(ii) Gas lift

(iii) Liquid lift

The mud-lift systems have their own unique challenges:

A major challenge is to develop a subsea mud return pump that will have sufficient differential pressure capacity at required flowrate, to operate in deep and ultra-deep water. Economic benefits of open water drilling increase with the water depth. A goal should be to enable open water drilling at water depths of 12-13,000 ft. (3,660m - 3,960m), which will for the most part satisfy current and future demands.

A solution could in principle be to put several pumps in series, connected to, and supported by the mud return riser, but the mass and physical dimensions of a subsea mud-pump will be a major obstacle. Another challenging aspect is the considerable electrical power supply requirement, depending on the capacity of the installed pump, which will require at least one power and signal cable to be clamped to the riser.

Gas lift poses great challenges for 3-phase returns reaching near surface/- topside facilities, with respect to velocity, expansion, and noise characteristics.

Another possibility is liquid lift, based on an arrangement of subsea eductors in the upper part of the riser, to boost the flow of drilling-mud, up through the mud return riser system, for treatment and re-circulation at the drilling vessel. Eductors, which is prior art, require a "propellant" to initiate "lift" and boost of the well fluid. The "driving fluid" will be provided from a surface pump and will typically be a lighter drilling fluid. The solution can be used to overcome the possible differential pressure limitations of a mud return pump at the seabed, as a supplementary solution. It may also be an alternative to the mud-pump.

Document WO 2016/134442 A1 presents a Controlled Mud Level, (CML) system, with a side outlet from a marine drilling riser, connected to a mud return conduit that extends to the surface. Such systems have been in use for some years. They facilitate bottom hole pressure regulation during offshore drilling through level control of the liquid column inside the drilling riser. A subsea pump module, mounted onto the marine drilling riser, is used for mud-lift and level control in the riser. The referenced document describes an alternative mud-lift/-level control solution, where an eductor replaces the mud return pump. A single eductor insert is shown, close to the side outlet of the marine drilling riser, inside the mud return line. It is intended to be retrievable to the surface by means of a wireline tool or a similar intervention method. A separate conduit for supply of eductor "driving fluid" from the surface is included in the solution.

Drilling in shallow formations, prior to the installation of a subsea blowout preventer, is done in open water. This is known as “top-hole” drilling. Drill-cuttings have been deposited at the seabed with conventional top-hole drilling. The release of drilling mud to the environment in this manner is undesirable, both from a cost and environmental perspective. Solutions for top-hole drilling with mud return for treatment and re-circulation at the surface have been successfully employed since the turn of the century, initially for shallow water, and later also for deep water. A subsea pump typically conveys drilling-mud returns from a suction module through a flexible hose to the drilling vessel. The return line is anchored at one end by the pump, while the upper end is connected to equipment on the vessel. In certain applications, such as in deep water and with strong currents, the use of a flexible return line may not be desirable. Document US 7,938,190 B2 describes a mud return system for top-hole drilling in open water, with a mono-bore, rigid riser, typically made up of drill pipe.

After completion of the top-hole drilling operation, with installation of the wellhead assembly, a subsea BOP is installed, for safe drilling into the oil and gas reservoir. If the blowout preventer is hydraulically controlled and actuated, the number of ancillary lines to the BOP will be five: 2 hydraulic conduits, 1 kill line, 1 choke line and 1 mud boost line. The lines are supported by the marine drilling riser for conventional riser drilling. They would contribute significantly to the weight and cost of a mud return riser for open water drilling. This is especially true for drilling in deep and ultra-deep water.

Due to the above-mentioned and other challenges related to conventional riser drilling and well control during reservoir drilling, a solution for open water drilling is developed. The solution will, especially in deep and ultra-deep water, provide several advantages over conventional drilling systems.

The object of the invention is to simplify the drilling-mud return riser configuration for the open water drilling solution, as well as overcoming operational depth constraints, resulting from limited drilling-mud lifting capacity of current art subsea mud pumps and the use of hydraulic BOP.

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

A first aspect of the invention relates to elimination of a choke line in the mud-return riser, facilitated through the integration of at least one drilling choke as part of an electrical subsea BOP. Subsea drilling chokes are known in the patent literature from patent application CA 1054932 A. Document US 9,222,320 B2 describes a subsea pressure control system with subsea chokes.

To mitigate and interrupt a well influx of formation fluids, the Bottom Hole Pressure (BHP) must be increased above pore pressure. The well is shut-in by the activation of pipe-rams on the BOP stack to seal against the drill-string. In a conventional subsea drilling system, well-fluids are then circulated out via the choke line of a marine drilling riser, and through the choke and kill manifold at the surface. The topsides drilling choke will in this case be adjusted to control and vary the drill pipe pressure, casing pressure and BHP during the circulation of hydrocarbons from the well.

By placing drilling chokes closer to the well, preferably integrated with the subsea BOP, remotely controlled choke adjustments will provide a more sensitive response to the well pressure in deep and ultra-deep water, where the choke line will be extremely long. With a subsea drilling choke located at the BOP, circulation to the drilling vessel can instead be done via the mud return riser, which has a larger diameter than a choke line.

Benefits are:

� The larger diameter of the mud return line mitigates issues with pressure drop in a long choke line. This is of particular importance in ultra-deep water, where flow resistance in a narrow choke-line would increase the BHP, possibly beyond the fracture pressure of weak reservoir formations, with drilling fluid loss to the formation as a result.

� Greater well-kick tolerance. Conventionally, frictional losses within the choke line are accounted for within the calculation of maximum allowable annular surface pressure MAASP). Circulation via the mud return system will thus improve the well kick tolerance.

� A possible advantage, if the kick tolerance is limited, is that eliminated choke line pressure drop may allow for a larger bore well casing design programme than would otherwise be employed. This could improve drilling costefficiency and may even provide increased production value from better well design.

� Fast response to downhole conditions. Adjustments of the topside choke valve during a well control situation takes long time to impact the reservoir, due to the long distance between the surface choke and the formation in deep and ultra-deep water. This makes pressure control within a narrow drilling window more difficult.

� Return flow through the riser will eliminate the risk of choke-line plugging from hydrate formation.

� Less weight of the mud return riser string.

� Less cost of the mud return riser system.

� Less engineered complexity of the mud return riser system. � Reduced drilling fluid volume through the well system.

A second aspect of the invention relates to the elimination of the kill line. In the absence of a riser-mounted killline, circulation for influx recovery will primarily be via the drill-string, which will act as a bull-head line.

A complementary and alternative pathway that must be established, mainly in the event of a sheared drill-pipe, is a separate fluid stab-in connection to be installed on the drilling BOP stack to accept a "stand-alone" deployed "emergency-kill" line.

If eductors are going to be used for mud-lift in the riser, it will be possible to utilize the eductor mud-boost-line in an alternative mode of operation, as a kill-fluid conduit. This possible kill-mud pathway will only be available as long as the riser remains connected.

The benefits of eliminating a kill line as part of the riser, and instead use the drill string to circulate the drilling mud, with contingency stab-in of an "emergency-kill" line will increase with the operating water depth. Benefits are:

� Less weight of the mud return riser string.

� Less cost of the mud return riser system.

� Less engineered complexity of the mud return riser system. � A drill-string will have better flow capacity than a riser mounted kill line.

� Mud exits in the bottom of the well, which is an advantage with respect to efficient circulation of hydrocarbons from the well.

A third aspect of the invention is to use an Electrical BOP, thus removing the need for hydraulic supply lines. Advantages will increase with the operational water depth.

Benefits of using an electrical BOP are:

� The need for two hydraulic supply lines is eliminated.

They will be replaced with a slim, high voltage cable for charging of the subsea BOP mounted electrical batteries and for signal communication.

� Less cost of the mud return line string.

� Less engineered complexity of the mud return line string. � Removal of hydraulics:

o Removal of hydraulic surface equipment.

o Removal of hydraulic control pods, accumulators, and hydraulic distribution to actuators, - weight reduction. o Removes water depth limitations of hydraulics. Hydraulic accumulators are less efficient at high ambient pressure, experienced at deep and ultra-deep water.

o Less congested subsea BOP.

o Removal of many potential fault sources and hydraulic leakage points.

o Less wear of sealing elements on well barriers due to controlled operation of actuators.

o No hydraulic BOP fluid, - cost reduction and no fluid release to the environment during test and operation. o Improved controls and monitoring facilities, conditionbased monitoring.

o A full hydro-acoustic system will have secondary control over all remotely operated valving, as well as all safety-critical functionality of the electrically controlled and actuated drilling BOP.

o More available reserve energy from batteries than from hydraulic accumulators.

A fourth aspect of the invention is a mud lift assistance feature, based on arrangements of riser mounted eductors, to reduce the reliance on a subsea pump to provide the mud lift. An advantage with the use of eductors is that mud-lift can be generated without moving parts subsea.

The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

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

Fig. 1 shows a subsea BOP fitted with drilling chokes. An annular preventer and an RCD are connected on top of the BOP. An off-set mud return system with a seabed pump and optional eductors in the upper part of the riser is connected to the BOP. The figure also includes a separate emergency kill, stab-in connection and kill valves on the BOP. An extension of a common eductor drive fluid line has a possible, secondary function as a kill-line.

Fig. 2 illustrates the eductor principle and arrangement.

The figures depict the invention in a simplified manner, and details, not relevant to illustrate what is new with the invention, 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 shows the open water drilling system after completed top-hole drilling, and with the subsea wellhead 1 installed.

A subsea blowout preventer 2, (BOP), preferably fitted with electro-mechanical actuators and "all-electric” subsea controls (not shown) is locked onto the subsea wellhead 1 with a Wellhead Connector 3. One end of a flexible mud return flowline 4 is connected to an outlet from a spool-piece 5 on the BOP 2. The other end of the flowline 4 is connected to an off-set Mud Return Line Base Assembly Frame 6, anchored on the seabed. For shallow water operations, the mud return system may be connected to a Direct Connection spool-piece 7 on the BOP 2. For the off-set connection scenario, shown in figure 1, an isolation valve 8 on the spool-piece 7 will be kept closed and the spool piece 7 outlet will be blanked-off.

The subsea BOP 2 is equipped with well barrier devices, known in the industry, and which can be arranged with different configurations for cutting and sealing around a drill-string 9. The BOP 2 arrangement shown on figure 1 is different from a conventional subsea BOP, in that there is no marine drilling riser and Lower Marine Riser Package (LMRP) connected to the BOP 2. Instead, an assembly of a Rotating Control Device (RCD) housing 10 and an Annular Preventer 11 is locked to a connector receptacle on top of the BOP 2 by means of a connector 12. The spool pieces 5 and 7 are connected to outlets from the BOP 2 Mud Return Line Spool 13, above the well barrier elements, and below the connector 12.

With the subsea well in so-called "pressure over-balance", a drill-string 9 is deployed in open water from a drilling vessel (not shown). The (not shown) bottom-hole assembly of the drill-string 9 is entered into the subsea well via the RCD housing 10, the Annular Preventer 11, the BOP 2 and the wellhead 1. An internal RCD sealing arrangement (not shown) is deployed with the drill-string 9 and is oriented and locked inside the RCD housing 10, prior to the drilling operation.

The RCD seals around the drill-string 9 during drilling. While the drill-bit (not shown) is working in the bottom of the well, drilling mud is pumped from the surface, down through the drill-string 9 and exits through the drill-bit ejector nozzles, into the bottom-hole. The drilling mud provides cooling of the drill-bit and carries drill cuttings out from the bottom hole. Drilling mud, saturated with cuttings, flows back, up through the annulus in the borehole, which encloses the drill string 9. When the cuttings laden mud reaches the Mud Return Line Spool 13 at the upper end of the subsea BOP 2, the mud is directed into the spool-piece 5, where it passes through the open isolation valve 14 and flows further, into the flexible flowline 4 on the seabed.

As the flow exits the flowline 4, it enters a Right-Angled Spool Piece 15, where the flow changes direction and is brought to flow up through the mud return conduit. An electrically driven, subsea mud-lift pump 16 at the base of the rigid mud return riser 17 boosts the mud returns towards the surface. The mud-lift pump 16, will be controlled with a variable speed drive (VSD), which will adjust the pump speed according to output signals from the drilling control system (not shown). Depending on the water depth and the capacity of the pump 16, further boosting of the mud flow to the surface may be required.

Eductors 18 can be integrated with the mud-return riser to provide additional mud-lift along the upper part of the riser 17. Special eductor riser joints (not shown) will be provided, as required. The placement and quantity in the riser 17 will depend on the differential pressure capacity of the seabed located pump 16, the efficiency of each eductor 18 and the total operational water depth of the open water drilling system.

Reference is made to figure 2, which illustrates the eductor 18 principle and arrangement. Eductors are a simple type of pump, which utilize the so-called "venturi effect". The eductor 18 requires a driving fluid for its operation, which will be provided from a designated rig pump, (not shown), via a riser mounted mud-boost line 19. The driving fluid will typically be a treated, lighter mud-mix of a similar type as that which will be pumped into the well via the drill-string 9, during drilling. The eductor 18 is shown with an array of driving fluid nozzles 18A, which are connected to a ringmanifold 18B. The individual nozzles 18A and the ringmanifold 18B are shown fixed onto a support ring 18C. The nozzle pipes 18A penetrate the mud return riser 17 pipe wall and extend towards the suction side of a so-called diffuser, 18D, which will be a reduced diameter flow-section inside the riser 17. When the propellant fluid is injected through the nozzles 18A at the desired capacity, a low-pressure region will be created inside the eductor 18, which induces suction and a boost of the mud return flow and pressure, which compensates limited seabed located pump 16 mud-lift capacity.

If more than one eductor 18 will be needed, the control system design must balance the volumetric flow rates of the driving fluid at multiple elevations throughout the water column. This will be facilitated with remotely adjustable orifices, and associated flowmeters, (not shown), at the output port from the mud-boost line 19 of each eductor riser-joint.

The mud return riser 17 will be fitted with an Emergency Disconnect System, with unlatch of the riser connector 20 and closure of a mud retention valve 21. A flexible joint 22 is included above the mud retention valve 21. A sheared drillstring will, in case of riser 17 disconnect, be pulled out of the BOP 2, with no requirement for unlatch of connector 12.

The configuration of the open water drilling system, with a mud return riser 17, without choke & kill lines, and with subsea chokes, will enable efficient and safe handling of a possible hydrocarbon influx during drilling. Early kick detection functionality will be included on the BOP 2.

The subsea BOP 2 will be fitted with at least one remotely adjustable drilling choke. To provide redundancy, the BOP system is shown in figure 1 with an upper 23 and a lower 24 drilling choke. Both are shown with discharge into the mud return line spool-piece 7, upstream of closed isolation valve 8. The subsea choke 23, 24, discharge will thus always have a flow path to the mud-return system, independent of how it is connected to the BOP 2, (direct or offset alternatives). The upstream side of the subsea drilling chokes 23, 24 is shown connected to the main bore of the subsea BOP, with one connection above the BOP lower pipe ram 25, and one below the shear rams, 26, 27. To provide a double, pressure tight barrier between the high and low-pressure systems, the choke connections to the BOP main bore will be fitted with isolation valves, 28 A/B and 29 A/B, respectively.

In case of hydrocarbon influx, BOP 2 pipe-rams above the lower choke line connection to the BOP main bore will be closed and will seal around the drill-string 9. The lower choke-line isolation valves 29 A/B are opened, and one of the adjustable subsea drilling chokes 23 or 24 will be used to control the bottom hole pressure of the well, while the influx contaminated drilling fluid is circulated out of the well, via the active choke, and will be routed further into the mud return system to the surface, for further disposal.

The surface arrangement (not shown) for the drilling fluids returns will be a directional flow head with an interconnecting flowline to the mud system shale shakers. With no surface installed choke and kill manifold, provision for gas cut mud disposal is accommodated by a surface arrangement, featuring flow diverters and an in-line degasser unit, complete with a gas vent line.

All well condition monitoring in drilling and non-drilling modes of operation will be accomplished by a comprehensive control system, comprising flowmeters, pressure / temperature sensors and level sensors (not shown). The control system will have Early Kick Detection capability built-in, to register well influx with a best possible sensitivity. The control system will incorporate statistical modelling to define the control limits to maintain exact and continuous control of the well pressures during drilling, and when drill pipe connections are made.

The open water drilling system arrangement offers three ways that heavy kill weight mud can be delivered to the well to re-gain control and circulate hydrocarbons out of the wellbore and mud return system. The primary method is to pump mud down into the bottom-hole with the installed drill-string 9.

Two other pathways for the distribution of kill-mud to the well are shown in figure 1.

Kill mud can also be pumped down through a "stand-alone" deployed "emergency kill" line (not shown) with a stab-in for a mating receptable 30 that can be mounted on the BOP 2 top plate 31. The receptacle 30 will be connected to the BOP 2 main bore below the lower pipe-rams 25, via a BOP mounted kill-line 32, with double isolation valves 33 A/B. The use of this pathway would apply if the drill-string 9 is sheared.

If eductors 18 are going to be integrated in the riser 17 for supplemental mud-lift, it will be possible to extend and utilize the mud-boost-line 19 as a kill-fluid conduit. This possible pathway will be available as long that the riser 17 remains connected.

Figure 1 shows the common mud-boost line 19 extended and connected to the BOP 2 main bore, below the lower pipe-rams 25. Eductors 18 will in this case be shut-off with isolation valves 34. Adjustable orifices (not shown) in the mud boostline 19 will be kept open, if used. The associated line 35 on the BOP will be fitted with two isolation valves 36 A/B. A flexible hose 37 will be laid out on the seabed to connect the mud-boost line 19 from the riser system to the BOP 2 via another flexible hose 38 at the riser base and a dis-connect feature 39 to allow line separation if the riser connector 20 is un-latched.

The use of the mud-lift eductor 18 boost-line 19 as a secondary conduit for kill mud supply in a well control event could apply if the drill-string is sheared but will not be available if the riser 17 is dis-connected. If the mud boost line 19 will be used for kill-mud supply, the eductor 18 control system (not shown) will be changed from "auto-control" to manual control.

Claims

P a t e n t c l a i m s

1. A method of circulating drilling fluid, contaminated with hydrocarbons, to a surface system, via a riser system connected to a subsea blowout preventer, BOP (2) in connection with drilling in open water, the system comprising:

- a blowout preventer, BOP (2) locked on a wellhead (1);

- a drill string (9), extending from the surface, down through open water, and further to the bottom of a subsea well in an oil and gas reservoir;

- sealing devices (10, 11), connected on top of the blowout preventer (2), which can be made to seal around the drill-string (9), in connection with the drilling operation;

- instrumentation on the blowout preventer (2), for detection of hydrocarbon influx from the well formation during a drilling operation;

- subsea drilling-chokes (23, 24) which, via a pipe system, are connected to the vertical centerbore in the blowout preventer (2) and with the outlet connected to a drilling fluid return system to the surface;

- subsea drilling chokes (23, 24) which, via a pipe system, have the inlet port connected to the vertical centerbore of the blowout preventer (2), and which have the outlet port connected to a return system for drilling fluid to the surface;

- an upper structural element (13), between the well barriers of the blowout preventer (2) and the sealing devices (10, 11), with outlet to a pipe system, (5, 7) which has two alternative connection possibilities for a drilling fluid return system, outside of the blowout preventer;

- a return line (4), optionally located on the seabed, for flowing of returned drilling fluid to an anchored riser system, which is provided with a pump (16), which lifts the drilling fluid to the surface via the riser (17);

- eductors (18), which may be installed in the riser (17) to compensate for any limitations in the lifting capacity of the pump (16);

- supply line (19) from the surface of a propellant fluid to possible eductors (18) and which is provided with isolation valves (34) for isolation of the connection to each eductor;

- connection point (30), for pumping of drilling fluid into the well from the surface, via a temporary supply line in open water, and a pipe connection (32) from the connection point (30), with isolation valves (33 A/B) to the blow-out preventer (2) vertical centerbore, via a connection port on the underside of the lower pipe-ram (25);

- a secondary pathway for supply of drilling fluid from the surface, to a connection port on the underside of the lower pipe-ram (25) of the blowout preventer (2), through an extension of the riser mounted supply line (19) for drive-fluid to eductors (18), if they are used for supplementary liquid lift;

2. A method according to claim 1, in connection with a need for well control and circulation of drilling fluid after unintentional influx of hydrocarbons into the well, the method comprising the steps of:

- detecting any influx of hydrocarbons by means of instrumentation on the blowout preventer (2);

- closing pipe-rams in the blowout preventer (2) to seal around the drill string (9);

- opening the isolation valves (29 A/B) on the line connecting the blowout preventer (2) vertical centerbore to the inlet of drilling chokes (23, 24);

- opening one of the drilling chokes (23 or 24), while drilling fluid is pumped down through the drill string (9) and hydrocarbon-containing well fluid is circulated out, via the well annulus, through a drilling choke (23 or 24) on the blowout preventer (2) and further through the subsea pipe system, which consists of different components, (7, 13, 5, 4, 15, 20, 21 and 22), up to the pump, (16);

- regulating the pump (16) flowrate, as well as the opening of the active drilling choke, (23 or 24), so that the bottom hole pressure is kept higher than the pore pressure and lower than the fracturing pressure of the formation, while the contaminated drilling fluid is circulated out;

- optionally drive eductors (18) by pumping propellant from the surface through the fluid-drive line (19) and regulate the liquid flow to the individual eductor (18) via a control system.

3. A method according to claim 1, in case of a possible need to pump heavy drilling fluid into the well, after the drill string (9) has been cut, the method comprising the steps of:

- connecting a supply line, in open water, from the surface to a connection point (30) on the top plate (31) of the blowout preventer (2);

- pumping drilling fluid from the surface, via the open water supply line, further through the line (32), with open isolation valves (33 A/B) and into the annulus of the well, via openings in the lower part of the blowout preventer (2).

4. A method of alternative supply of drilling fluid, according to claim 1, in case of a possible need to pump heavy drilling fluid into the well, after the drilling string has been cut, the method comprising the steps of:

- disconnecting any eductors (18), by closing isolation valves (34);

- opening any throttling devices on the eductor (18) propellant supply line (19);

- opening isolation valves (36 A/B) on the blowout preventer (2) and pumping drilling fluid from the surface, down through the line (19), the hose (38), the connector (39), and further through the pipe arrangement and the flexible hose (37), to the well annulus, through openings in the lower part of the blowout preventer (2).