A system and method for circulating drilling fluid in connection with open water drilling
PT 20210037 (346362)
Key information
Title
A system and method for circulating drilling fluid in connection with open water drilling
Application number
20210037
Case type
National
Status
27.06.2022 Patent meddelt (B1)
Filed
12.01.2021
Effective date
12.01.2021
Publicly available
27.06.2022
Next annual fee due
31.01.2027
Applicant
ELECTRICAL SUBSEA & DRILLING AS (NO)
Owner
ELECTRICAL SUBSEA & DRILLING AS (NO)
Inventor
John Dale (NO) et al.
Agent
Rikard Mikalsen (DE)
Granted
27.06.2022
Patent number
346362
Expiry date
12.01.2041
Abstract and drawing
Disclaimer: This text has been machine-scanned and may contain errors – please refer to "Publications" for legally binding content.
The document presents a method for circulation and well control in case of hydrocarbon influx from the reservoir formation during drilling with an open water drilling system. Implementation of subsea chokes (23, 24) with the subsea BOP (2) facilitates circulation back to surface via a mud return riser (17), with kill-mud supply via the drill-string (9) as the primary pathway. Secondary pathways for supply of kill-mud are provided in case the drill-string (9) is sheared. Another aspect of the invention is a mud lift assistance feature, based on arrangements of riser mounted eductors (18), to reduce the dependence on a subsea pump (16) to provide mud lift to the surface.
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27.12.2022
02-01
Endring av fullmektig
2022/19365
UT
GH Forespørsel
Rikard Mikalsen
16.12.2022
01-02
Endring av fullmektig
2022/19365
INN
Power of attorney ESD Mikalsen
Rikard Mikalsen
16.12.2022
01-01
Endring av fullmektig
2022/19365
INN
Generell henvendelse
Rikard Mikalsen
06.12.2022
10-01
Saksbehandling
20210037(346362)
UT
PT Varsel om betaling av første årsavgift (3317) (PT20210037)
Disclaimer: This text has been machine-scanned and may contain errors – please refer to "Publications" for legally binding content.Description
A SYSTEM AND METHOD FOR CIRCULATING DRILLING FLUID IN CONNECTION WITH OPEN WATER DRILLINGThe invention relates to a system and a method of circulating drilling-fluid with drillcuttings 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 fluid 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 fluid return riser, with kill-fluid supply via the drillstring as the primary pathway. Secondary pathways for supply of kill-fluid are provided in case the drill-string is sheared. Another aspect of the invention is a fluid lift assistance feature, based on arrangements of riser mounted eductors, to reduce the dependence on a subsea pump to provide fluid 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-fluid 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 ultra-deep-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 fluid-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 fluid pumps.The fluid 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-fluid 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 electro-mechanically operated.The open water drilling-fluid return system must be equipped with a fluid-lift system, to compensate for pressure loss from friction and directional changes in the fluid return flow. Fluid-lift systems and combinations thereof, can be:(i) Submersible pump lift(ii) Gas lift(III) Liquid liftThe fluid-lift systems have their own unique challenges:A major challenge is to develop a subsea fluid 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 fluid return riser, but the mass and physical dimensions of a subsea fluid-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-fluid, up through the fluid return riser system, for treatment and re-circulation at the drilling vessel. Eductors, which is known in the art,
require a propellant to initiate lift and boost of the well fluid. The propellant 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 fluid return pump at the seabed, as a supplementary solution. It may also be an alternative to the fluid-pump. Document WO 2016/134442 A1, equivalent to US2018073314 A1, presents a Controlled Fluid Level, (CML) system, with a side outlet from a marine drilling riser, connected to a fluid 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 fluid-lift and level control in the riser. The referenced document describes an alternative fluid-lift/-level control solution, where an eductor replaces the fluid return pump. A single eductor insert is shown, close to the side outlet of the marine drilling riser, inside the fluid 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 "propellant fluid" from the surface is included in the solution.US2012067590 A1 describes an arrangement and a method for regulating bottom hole pressures when drilling deep-water offshore wells. The arrangement comprises both a surface and a subsea pressure containment, i.e. BOPs, a high pressure riser and a high pressure drilling pipe. The high-pressure riser and high-pressure drilling pipe may be so arranged between the subsea blowout preventer and the surface blowout preventer that they can be used as separate high-pressure lines as a substitute for choke line and kill line. The subsea BOP has an equalizing loop/bypass that gives the ability to circulate out a large influx past the closed subsea BOP into the high-pressure riser. The arrangement also includes a low riser return system that involves two drilling fluid return pumps.US2012168171 A1 describes a subsea pressure control system with subsea chokes. The system comprises a blowout preventer stack, a subsea annular sealing device, and a choke manifold that variably restricts the flow of the drilling fluid and thereby adjust pressure applied to the annulus at the subsea location. The system also comprises a pressure control system involving several sensors, which can detect well control problems such as an influx in the well.US2014048331 A1 describes a managed pressure drilling system having a well control mode. The system includes a fluid transport system that comprises a booster line coupled to a booster pump, which may be used to compensate for any size discrepancy between
the riser annulus and the casing/wellbore annulus. The drilling fluid pumped down through the booster line may be used to displace any contaminated returns present in the lower marine riser package.All the mentioned prior art documents describe systems and methods for pressure control and fluid return while drilling a subsea well, including means for detecting and circulating out any possible influxes into the well. However, none of the documents describes that the system comprises eductors in addition to a subsea pump to compensate for any lifting capacity limitations of the pump, nor a structural element providing two alternative connection possibilities for a drilling return system.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 fluid to the environment in this manner is undesirable, both from a cost and environmental perspective. Solutions for top-hole drilling with fluid 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-fluid 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 fluid 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 fluid 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 fluid 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-fluid return riser configuration for the open water drilling solution, as well as overcoming operational depth constraints, resulting from limited drilling-fluid lifting capacity of current art subsea fluid 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.The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.A first aspect of the invention relates to elimination of a choke line in the drilling fluidreturn 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 fluid return riser, which has a larger diameter than a choke line.Benefits are: The larger diameter of the fluid 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 drilling fluid 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 cost-efficiency 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 fluid return riser string.● Less cost of the drilling fluid return riser system.● Less engineered complexity of the drilling fluid 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 kill-line, 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 fluid-lift in the riser, it will be possible to utilize the eductor fluid-boost-line in an alternative mode of operation, as a kill-fluid conduit. This possible kill-fluid 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 fluid, with contingency stab-in of an "emergency-kill" line will increase with the operating water depth. Benefits are:● Less weight of the drilling fluid return riser string.● Less cost of the drilling fluid return riser system.● Less engineered complexity of the drilling fluid return riser system.
● A drill-string will have better flow capacity than a riser mounted kill line.● Fluid 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 fluid return line string.● Less engineered complexity of the fluid return line string.● Removal of hydraulics:● Removal of hydraulic surface equipment.● Removal of hydraulic control pods, accumulators, and hydraulic distribution to actuators, - weight reduction.● Removes water depth limitations of hydraulics. Hydraulic accumulators are less efficient at high ambient pressure, experienced at deep and ultra-deep water.● Less congested subsea BOP.● Removal of many potential fault sources and hydraulic leakage points.● Less wear of sealing elements on well barriers due to controlled operation of actuators.● No hydraulic BOP fluid, - cost reduction and no fluid release to the environment during test and operation.● Improved controls and monitoring facilities, condition-based monitoring.● 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.● More available reserve energy from batteries than from hydraulic accumulators. A fourth aspect of the invention is a fluid lift assistance feature, based on arrangements of riser mounted eductors, to reduce the reliance on a subsea pump to provide the fluid lift. An advantage with the use of eductors is that fluid-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 a first aspect the invention relates more particularly to a circulation system for circulating drilling fluid contaminated with hydrocarbons to a surface system via a drilling fluid return riser connected to a subsea wellhead in connection with drilling in open water, the circulation system comprising:a blowout preventer locked on the wellhead;a drill-string 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 connected on top of the blowout preventer, arranged to seal around the drill-string in connection with the drilling operation;a control system connected to the blowout preventer and arranged for detection of hydrocarbon influx from the well formation during the drilling operation;inlet ports of drilling chokes connected to a vertical centre-bore of the blowout preventer and with outlets ports connected to the drilling fluid return riser;a drilling fluid return line spool provided between well barriers of the blowout preventer and the sealing devices forming alternative flow outlet connections to the drilling fluid return riser via a pipe system;a return line arranged for flowing returned drilling fluid to the drilling fluid return riser via a fluid-lift pump arranged to lift the drilling fluid to the surface system via the drilling fluid return riser;whereinone or more eductors are installed in the drilling fluid return riser to compensate for any limitations in the lifting capacity of the fluid-lift pump;a fluid-boost line is arranged to provide a propellant fluid to anyone of the one or more eductors, the fluid-boost line being provided with sixth isolation valves for controlling a propellant fluid flow to each of the one or more eductors;a connection point for a temporary supply line in open water arranged for pumping of drilling fluid into the well from the surface via a BOP mounted well kill-line with fifth isolation valves to the vertical centre-bore of the blow-out preventer through a connection port on the underside of a lower pipe-ram; anda secondary pathway for supply of drilling fluid from the surface to the connection port on the underside of the lower pipe-ram of the blowout preventer, through an extension of the fluid-boost line for propellant fluid to the one or more eductors.
In a second aspect the invention relates more particularly to a method for circulating drilling fluid contaminated with hydrocarbons to a surface system from a subsea well via a circulation system according to the first aspect of the invention in connection with a need for well control and circulation of drilling fluid after unintentional influx of hydrocarbons into the well,wherein the method comprising the steps of:a) detecting any influx of hydrocarbons by the control system connected to the blowout preventer;b) closing pipe-rams in the blowout preventer to seal around the drill string;c) opening fourth isolation valves on the line connecting the vertical centre-bore of the blowout preventer to the inlet of the drilling chokes;d) opening one of the drilling chokes, while drilling fluid is pumped down through the drill string;e) circulating out hydrocarbon-containing well fluid via a well annulus through one of the a drilling chokes on the blowout preventer and further to the surface system via the fluid-lift pump and the drilling fluid return riser;f) regulating the flowrate of the fluid-lift pump as well as the opening of the active drilling choke, 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;g) assisting the fluid-lift pump by operating one or more eductors by pumping propellant fluid from the surface system through the fluid-boost line and regulating the propellant fluid flow to each of the one or more eductors via the control system.In case of a need to pump heavy drilling fluid into the well, after the drill string has been sheared, the method may comprise the further steps of:h) connecting a supply line, in open water, from the surface system to the connection point on the blowout preventer;i) opening the fifth isolation valves; andj) pumping heavy drilling fluid from the surface system via the open water supply line, the BOP mounted well kill-line and openings in the lower part of the blowout preventer to the annulus of the well.Alternatively, in case of the need to pump heavy drilling fluid into the well after the drilling string has been sheared, the method may comprise the further steps of:k) disconnecting anyone of the one or more eductors by closing sixth isolation valves; l) opening any throttling devices at anyone of the one or more of the eductors of the
fluid-boost line;m) opening seventh isolation valves on the blowout preventer; andn) pumping the heavy drilling fluid from the surface system to the well annulus through the fluid-boost line, a second flexible hose, a connector, and further through the pipe arrangement and a first flexible hose, and through openings in the lower part of the blowout preventer.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 offset fluid 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.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 with-out reference numerals.The figures depict the invention in a simplified manner, and details not relevant to illustrate 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 an open water drilling system after completed top-hole drilling, and with a subsea wellhead 1 installed.A subsea blowout preventer 2, hereinafter also called BOP, preferably fitted with electromechanical 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 fluid return flowline 4 is connected to an outlet of a spool piece 5 on the BOP 2. The other end of the flexible fluid return flowline 4 is connected to an offset fluid return line base assembly frame 6, anchored on the seabed. For shallow water operations, the fluid return system may be connected to a direct connection spool piece 7 on the BOP 2. For the offset connection
scenario, shown in figure 1, a first isolation valve 8 on the spool piece 7 will be kept closed and an outlet of the spool piece 7 will be blanked-off. The first isolation valve may also be denoted spool piece isolation valve 8.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 on top of the BOP 2 by means of a connector 12. The spool pieces 5, 7 are connected to outlets of a fluid return line spool 13 of the BOP 2 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). A bottom-hole assembly (not shown) 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 well-head 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 a drill-bit (not shown) is working in a well bottom, drilling fluid is pumped from the surface, down through the drill-string 9 and exits through drill-bit ejector nozzles (not shown) into the bottom-hole. The drilling fluid provides cooling of the drill-bit and carries drill cuttings out from the bottom hole. Drilling fluid saturated with cuttings flows back up through a borehole annulus, which encloses the drill-string 9. When the cuttings laden drilling fluid reaches the fluid return line spool 13 at the upper end of the subsea BOP 2, the drilling fluid is directed into the spool-piece 5, where it passes through an open second isolation valve 14 and flows further into the flexible flowline 4 on the seabed. The second isolation valve may also be denoted flexible flowline isolation valve 14.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 a fluid return conduit. An electrically driven, subsea fluid-lift pump 16 at the base of a rigid fluid return riser 17 boosts the drilling fluid returns towards the surface. The fluid-lift pump 16 is controlled with a variable speed drive, 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 fluid-lift pump 16 further boosting of the drilling fluid flow to the surface may be required.One or more eductors 18 (only one depicted in figure 1) is integrated in the fluid return riser 17 to provide additional fluid-lift along the upper part of the fluid return riser 17. Special eductor riser joints (not shown) will be provided, as required. The placement and numbers of eductors 19 in the fluid return 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 now made to figure 2, which illustrates the principle and arrangement of the eductor 18. An eductor is a simple type of pump, which utilize the so-called "venturi effect". The eductor 18 requires a propellant fluid for its operation, which will be provided from a designated rig pump, (not shown) via a riser mounted fluid-boost line 19. The propellant fluid will typically be a treated, lighter fluid-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 propellant fluid nozzles 18A, which are connected to a ring-manifold 18B. The individual nozzles 18A and the ring-manifold 18B are shown fixed onto a support ring 18C. The nozzles 18A penetrate fluid a pipe wall of the fluid return riser 17 and extend towards a suction side of a so-called diffuser 18D, which provides a reduced diameter flow-section inside the drilling fluid return 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 drilling fluid return flow and pressure, which compensates limited fluid-lift capacity of the subsea fluid-lift pump 16.If more than one eductor 18is required, the control system design must balance the volumetric flow rates of the propellant fluid at multiple elevations throughout the water column. This will be facilitated with remotely adjustable orifices and associated flowmeters (not shown) at output ports of the fluid-boost line 19 of each eductor 18.The drilling fluid return riser 17 is preferably provided with an emergency disconnect system arranged to unlatch a riser connector 20 and closure of a drilling fluid retention valve 21 when required. A flexible joint 22 is arranged above the drilling fluid retention valve 21. The drill-string 9 will, in case of the drilling fluid return riser 17 being disconnected, be pulled out of the BOP 2, with no need of unlatching connector 12.The configuration of the open water drilling system with a drilling fluid 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 is provided with at least one remotely adjustable drilling choke. To provide redundancy, the BOP system shown in figure 1 is provided with an upper drilling choke 23 and a lower drilling choke 24. Both drilling chokes 23, 24 are shown with discharge into the drilling fluid return line spool-piece 7 upstream of closed first isolation valve 8. The upper and lower subsea chokes 23, 24 discharge will thus always have a flow path to the drilling fluid-return system, independent of how it is connected to the BOP 2 (direct or offset alternatives). An upstream side of the drilling chokes 23, 24 is shown connected to a main bore of the BOP 2, with one connection above a lower pipe ram 25 of the BOP 2, and one connection below shear rams 26, 27 the BOP 2. To provide a double, pressure tight barrier between the high and low-pressure systems the choke connections to the main bore of the BOP 2 is provided with third isolation valves 28A, 28B and fourth isolation valves 29A, 29B, respectively. The third isolation valves may also be denoted upstream of choke upper isolation valves 28A, 28B, and the fourth isolation valves may be denoted upstream of choke lower isolation valves 29A, 29B.In case of hydrocarbon influx, pipe-rams above the lower choke line connection to the main bore of the BOP 2 will be closed and will seal around the drill-string 9. The lower choke-line isolation valves 29A, 29B are opened, and one of the adjustable subsea drilling chokes 23, 24 is 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 drilling fluid return system to the surface for further disposal.The surface arrangement (not shown) for the drilling fluid returns is a directional flow head with an interconnecting flowline to fluid system shale shakers. With no surface installed choke and kill manifold, provision for gas cut fluid 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 is accomplished by a comprehensive control system, comprising flowmeters, pressure sensors, temperature sensors and level sensors (not shown). The control system is provided with early kick detection capability built-in, arranged to register well influx with a best possible sensitivity. The control system incorporates 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 of delivering heavy kill fluid to the well to re-gain control and circulate hydrocarbons out of the wellbore and drilling fluid return system. The primary method is to pump drilling fluid down into the bottom-hole with the installed drill-string 9. Two other pathways for the distribution of kill-fluid to the well are shown in figure 1.Kill fluid can also be pumped down through a stand-alone deployed emergency kill line (not shown) connectable to a mating connecting point 30 that can be mounted on a top plate 31 of the BOP 2. A connector 30 is connected to the main bore of the BOP 2 below the lower pipe-rams 25 via a BOP mounted kill-line 32, with two fifth isolation valves 33A, 33B. The use of this pathway would apply if the drill-string 9 is sheared. The fifth isolation valves may also be denoted kill-line isolation valves 33A, 33B.With one or more eductors 18 integrated in the drilling fluid return riser 17 for supplemental fluid-lift, the fluid-boost-line 19 may be utilized as a kill-fluid conduit. This possible pathway will be available as long that the drilling fluid return riser 17 remains connected. Figure 1 shows the common fluid-boost line 19 extended and connected to the main bore of the BOP 2 below the lower pipe-rams 25. The eductor(s) 18 will in this case be shut-off with sixth isolation valves 34. The sixth isolation valve may also be denoted fluid-boost line isolation valves 34. Adjustable orifices (not shown) in the fluid boost-line 19 will be kept open, if used. An associated line 35 on the BOP 2 is fitted with two seventh isolation valves 36A, 36B. The seventh isolation valves may also be denoted blowout preventer isolation valves 36A, 36B. A first flexible hose 37 is laid out on the seabed to connect the fluid-boost line 19 from the drilling fluid return riser system to the BOP 2 via a second flexible hose 38 connecting the base of the drilling fluid return riser 17 and a connector 39 to allow line separation if the riser connector 20 is unlatched.The use of the fluid-boost-line 19 of the fluid-lift eductor 18 as a secondary conduit for kill fluid supply in a well control event could apply if the drill-string is sheared but will not be available if the drilling fluid return riser 17 is disconnected. If the fluid-boost line 19 is used for kill-fluid supply, the eductor control system (not shown) will be changed from "autocontrol" to manual control.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 reference 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.
Claims
P a t e n t c l a i m s1. A circulation system for circulating drilling fluid contaminated with hydrocarbons to a surface system via a drilling fluid return riser (17) connected to a subsea wellhead (1) in connection with drilling in open water, the circulation system comprising:a blowout preventer (2) locked on the 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), arranged to seal around the drill-string (9) in connection with the drilling operation;a control system connected to the blowout preventer (2) and arranged for detection of hydrocarbon influx from the well formation during the drilling operation;inlet ports of drilling chokes (23, 24) connected to a vertical centre-bore of the blowout preventer (2) and with outlets ports connected to the drilling fluid return riser (17);a drilling fluid return line spool (13) provided between well barriers of the blowout preventer (2) and the sealing devices (10, 11) forming alternative flow outlet connections to the drilling fluid return riser (17) via a pipe system (5, 7);a return line (4) arranged for flowing returned drilling fluid to the drilling fluid return riser (17) via a fluid-lift pump (16) arranged to lift the drilling fluid to the surface system via the drilling fluid return riser (17);c h a r a c t e r i z e d i n thatone or more eductors (18) are installed in the drilling fluid return riser (17) to compensate for any limitations in the lifting capacity of the fluid-lift pump (16);a fluid-boost line (19) is arranged to provide a propellant fluid to anyone of the one or more eductors (18), the fluid-boost line being provided with sixth isolation valves (34) for controlling a propellant fluid flow to each of the one or more eductors (18);a connection point (30) for a temporary supply line in open water arranged for pumping of drilling fluid into the well from the surface via a BOP mounted well kill-line (32) with fifth isolation valves (33A, 33B) to the vertical centre-bore of the blow-out preventer (2) through a connection port on the underside of a lower pipe-ram (25); anda secondary pathway for supply of drilling fluid from the surface to the connection port on the underside of the lower pipe-ram (25) of the blowout preventer (2), through an extension of the fluid-boost line (19) for propellant fluid to the one or more eductors (18).2. A method for circulating drilling fluid contaminated with hydrocarbons to a surface system from a subsea well via a system 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,c h a r a c t e r i z e d i n that the method comprising the steps of: a) detecting any influx of hydrocarbons by the control system connected to the blowout preventer (2);b) closing pipe-rams in the blowout preventer (2) to seal around the drill string (9);c) opening fourth isolation valves (29A, 29B) on the line connecting the vertical centre-bore of the blowout preventer (2) to the inlet of the drilling chokes (23, 24);d) opening one of the drilling chokes (23, 24), while drilling fluid is pumped down through the drill string (9);e) circulating out hydrocarbon-containing well fluid via a well annulus through one of the a drilling chokes (23, 24) on the blowout preventer (2) and further to the surface system via the fluid-lift pump (16) and the drilling fluid return riser (17);f) regulating the flowrate of the fluid-lift pump (16) as well as the opening of the active drilling choke (23, 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;g) assisting the fluid-lift pump (16) by operating one or more eductors (18) by pumping propellant fluid from the surface system through the fluid-boost line (19) and regulating the propellant fluid flow to each of the one or more eductors (18) via the control system.3. The method according to claim 2, in case of a need to pump heavy drilling fluid into the well, after the drill string (9) has been sheared, the method comprising the steps of:h) connecting a supply line, in open water, from the surface system to the connection point (30) on the blowout preventer (2);i) opening the fifth isolation valves (33 A/B); andj) pumping heavy drilling fluid from the surface system via the open water supply line, the BOP mounted well kill-line (32) and openings in the lower part of the blowout preventer (2) to the annulus of the well.4. The method according to claim 2 for pumping heavy drilling fluid into a well from a surface system after a drilling string (9) has been sheared, wherein the method comprises the further steps of:k) disconnecting anyone of the one or more eductors (18) by closing sixth isolation valves (34);l) opening any throttling devices at anyone of the one or more of the eductors (18) of the fluid-boost line (19);m) opening seventh isolation valves (36A, 36B) on the blowout preventer (2); andn) pumping the heavy drilling fluid from the surface system to the well annulus through the fluid-boost line (19), a second flexible hose (38), a connector (39), and further through the pipe arrangement and a first flexible hose (37), and through openings in the lower part of the blowout preventer (2).Patentkrav1. Et sirkulasjonssystem for å sirkulere borevæske forurenset med hydrokarboner til et overflatesystem via et borevæskereturstigerør (17) koblet til et undersjøisk brønnhode (1) i forbindelse med boring i åpent vann, hvor sirkulasjonssystemet omfatter:en utblåsningssikring (2) låst på brønnhodet (1);en borestreng (9) som strekker seg fra overflaten ned gjennom åpen sjø og videre til bunnen av en undersjøisk brønn i et olje- og gassreservoar;tetningsanordninger (10, 11) koblet på toppen av utblåsningssikringen (2), anordnet for å tette rundt borestrengen (9) i forbindelse med boreoperasjonen;et kontrollsystem koblet til utblåsningssikringen (2) og anordnet for deteksjon av hydrokarboninnstrømning fra brønnformasjonen under boreoperasjonen;innløpsporter til strupeventiler (23, 24) koblet til en vertikal senterboring av utblåsningssikringen (2) og med utløpsporter koblet til borevæskereturstigerøret (17);et borevæske-retur mellomstykke (13) anordnet mellom brønnbarrierene til utblåsningssikringen (2) og tetningsanordningene (10, 11) som danner alternative strømningsutløpsforbindelser til borevæskereturstigerøret (17) via et rørsystem (5, 7);en returledning (4) anordnet for å strømme returnert borevæske til borevæskereturstigerøret (17) via en væskeløftepumpe (16) anordnet for å løfte borevæsken til overflatesystemet via borevæskereturstigerøret (17);k a r a k t e r i s e r t v e d a tén eller flere eduktorer (18) er installert i borevæskereturstigerøret (17) for å kompensere for eventuelle begrensninger i løftekapasiteten til væskeløftepumpen (16);en væskeløftledning (19) er anordnet for å tilveiebringe et drivmiddelfluid til hvilken som helst av den ene eller de flere eduktorene (18), hvor væskeløftledningen er forsynt med sjette isolasjonsventiler (34) for å styre en drivmiddelstrøm til hver av den ene eller de flere eduktorene (18);et koblingspunkt (30) for en midlertidig tilførselsledning i åpent vann, arrangert for pumping av borevæske inn i brønnen fra overflaten via en BOP-montert brønndrepeledning (32) med femte isolasjonsventiler (33A, 33B) til den
vertikale senterboringen av utblåsningssikringen (2) gjennom en tilkoblingsport på undersiden av et nedre tetningselement (25); ogen sekundær bane for tilførsel av borevæske fra overflaten og til tilkoblingsporten på undersiden av utblåsningssikringens (2) nedre tetningselement (25), gjennom en forlengelse av væskeløftledningen (19) for drivvæske til den ene eller de flere eduktorene (18).2. Fremgangsmåte for å sirkulere borevæske forurenset med hydrokarboner til et overflatesystem fra en undersjøisk brønn via et system ifølge krav 1 i forbindelse med et behov for brønnkontroll og sirkulasjon av borevæske etter utilsiktet tilstrømning av hydrokarboner inn i brønnen,k a r a k t e r i s e r t v e d at metoden som omfatter trinnene: a) å detektere tilstrømning av hydrokarboner ved hjelp av kontrollsystemet koblet til utblåsningssikringen (2);b) lukking av tetningselementene i utblåsningssikringen (2) for å tette rundt borestrengen (9);c) åpning av fjerde isolasjonsventiler (29A, 29B) på linjen som forbinder utblåsningssikringens (2) vertikale senterboringen med innløpet til strupeventilene (23, 24);d) åpning av én av strupeventilene (23, 24), mens borevæske pumpes ned gjennom borestrengen (9);e) å sirkulere ut hydrokarbonholdig brønnfluid via et brønnringrom gjennom én av strupeventilene (23, 24) på utblåsningssikringen (2) og videre til overflatesystemet via væskeløftepumpen (16) og borevæskereturstigerøret (17);f) å regulere strømningsraten til væskeløftepumpen (16) samt åpningen av den aktive, åpne strupeventilen (23, 24), slik at et bunnhullstrykk holdes høyere enn et poretrykk og lavere enn et fraktureringstrykk til formasjonen mens den forurensede borevæsken sirkuleres ut;g) assistere væskeløftepumpen (16) ved å betjene den ene eller de flere eduktorene (18) ved å pumpe drivmiddelvæske fra overflatesystemet gjennom væskeløftledningen (19) og regulere drivmiddelstrømmen til hver av den ene eller flere eduktorene (18) via kontrollsystemet.3. Fremgangsmåten ifølge krav 2, i tilfelle et behov for å pumpe tung borevæske inn i brønnen, etter at borestrengen (9) er kuttet, hvor fremgangsmåten omfatter trinnene:h) å koble en tilførselsledning, i åpent vann, fra overflatesystemet til koblingspunktet (30) på utblåsningssikringen (2);i) å åpne de femte isolasjonsventilene (33 A/B); ogj) pumping av tung borevæske fra overflatesystemet og til brønnens ringrom via forsyningsledningen i åpent vann, den BOP-monterte brønndrepeledningen (32), og åpninger i den nedre delen av utblåsningssikringen (2).4. Fremgangsmåten ifølge krav 2 for å pumpe tung borevæske inn i en brønn fra et overflatesystem etter at en borestreng (9) er kuttet, hvor fremgangsmåten omfatter de ytterligere trinnene:k) å koble fra den eventuelt ene eller de flere eduktorene (18) ved å stenge sjette isolasjonsventiler (34);l) å åpne eventuelle strupeanordninger ved hvilken som helst av den ene eller de flere eduktorene (18) til væskeløftledningen (19);m) åpning av syvende isolasjonsventiler (36A, 36B) på utblåsningssikringen (2); ogn) pumping av den tunge borevæsken fra overflatesystemet og til brønnens ringrom gjennom væskeløftledningen (19), en andre fleksibel slange (38), en kopling (39), og videre gjennom rørarrangementet og en første fleksibel slange (37), og gjennom åpninger i nedre del av utblåsningssikringen (2).