WO2016062314A1

WO2016062314A1 - Apparatus and methods for control of systems for drilling with closed loop mud circulation

Info

Publication number: WO2016062314A1
Application number: PCT/DK2015/000042
Authority: WO
Inventors: William James MCBEATH; David Ewen RITCHIE; John Røn PEDERSEN
Original assignee: Maersk Drilling A/S
Priority date: 2014-10-24
Filing date: 2015-10-26
Publication date: 2016-04-28

Abstract

The present invention relates to a drilling rig comprising a closed loop mud return system 200 comprising a choke manifold 250 adapted to control, during use, a pressure in a mud column in a riser string 100 wherein the system is arranged to be connected to at least one flow line 106 in fluid communication with an annulus of a rotating control device 107, the flow line 106 arranged to capture and direct return flow from said mud column towards said drilling rig.

Description

Apparatus and methods for control of systems for drilling with closed loop mud circulation

Field of the invention

This invention relates to a novel method and apparatus for offshore drilling operations. In particular, this invention relates to a method and apparatus for drilling with closed loop mud circulation.

Background

Traditional drilling has been performed using so-called open loop mud circulation where drilling mud is pumped through the drill string to the distal end of the drill string where it is ejected e.g. through the drill bit. The mud rises again to the drilling rig at the surface via a marine riser (or riser for short), or in the cases where a surface BOP is employed, a conductor pipe. The mud is used for carrying cuttings from the drilling process to the rig as well as ensuring that sufficient pressure is present in the wellbore from the mud column to prevent the bore from collapsing and/or to control influxes of pressurized fluids from the formation into the risers which may result in a kick or even a blowout. When using open loop mud circulation, the mud column is open or substantially open to the atmospheric pressure at the upper end. An exemplary overview of open loop drilling systems can be found is US Patent 6,263,982.

Presently a number of hydrocarbon drilling techniques have been proposed to better manage pressures within or exerted upon a wellbore during drilling activities using a "closed loop" circulating system. Closing the loop means capturing and redirecting what is otherwise the free flow of drilling fluids, cuttings, and hydrocarbons from the drill pipe riser annulus or drill pipe- casing annulus to a choke manifold so that the pressure on the mud column can be controlled. Note, unless otherwise clear, the choke manifold referred to in the present text is distinct from a choke manifold connected to the choke line connected directly to the BOP. The present choke is used when the BOP is closed and the circulation, e.g. to circulate out a kick, is needed.

This is usually accomplished by installing one or more annulus drill string seals which is typically a rotating control device ("RCD") similar to that described in, Williams et al U.S. Pat. No. 5,662,181 , a Riser Drilling Device, or a Drill String Isolation Tool all installed in the riser above a conventional blow-out preventer. In general, the function of these devices is to seal around the drill string so that the upper end of the mud column is capped. Secondly, a fluid return line below the annulus drill string seal redirects the return flow to the choke manifold. The fluid return line is typically mounted to a flow spool. It will be appreciated by the skilled person that the function of the annulus drill string seals and the flow spool can be made as separate elements which can be inserted into the riser string (or on top of the surface BOP e.g. in a jack-up rig) or can be integrated into the string. Summary

It is an object to provide redundancy in relation to a drilling rig according to various aspects as described.

An aspect of the present invention relating to a drilling rig comprising a closed loop mud return system is defined in claim 1 and explained further elsewhere in the present description.

Other aspects of the present invention relating to a drilling rig are defined in claim 16 and explained further elsewhere in the present description.

Certain aspects as described can function independently of each other but may also function together for further effect. Further embodiments are explained elsewhere in the present description and defined in the sub-claims. Brief description of the drawings

Figure 1 schematically illustrates one embodiment of marine riser system arranged for closed loop return;

Figure 2 schematically illustrates a mud return system according to an embodiment of the invention;

Figure 3 schematically illustrates exemplary embodiments of a buffer manifold and a choke/Coriolis manifold; and

Figures 4a, 4b, and 4c schematically illustrate embodiments of a control system. Fig. 1 shows a schematic view of the riser string 100 with the components for a closed loop system. As with an open loop riser, the blow out preventer (BOP) 103 sits on a well head 102 located on the seabed 101. The riser 104 runs from the BOP (typically from the lower marine riser pack (LMRP) of the BOP). In the closed loop system, the flow spool 105 is part of the riser string and located below the annulus drill string seal 107 referred to as the RCD in this text. One or more flow lines may be provided to direct the return flow towards the rest of the closed loop system. In the shown and corresponding embodiments, two flow lines 106a and 106b - instead of traditionally only one - are provided for redundancy and are arranged to direct the return flow towards the rest of the closed loop system. Each flow line 106 is fitted with valves 111a1 , 111a2 and 111 b1 , 111 b2, respectively. These can be used to seal the flow lines when the flow lines are not in use or in the event of a breach in the flow lines. Having two valves on each flow line provides redundancy. The arrows illustrate the mud flow through the drill string and up via the annulus and towards the rig via flow line 106a and/or 106b. See e.g. the connections to a buffer manifold 201 of Figures 2 and 3. The telescopic joint 108, and optionally further riser, connects the riser pipe to the diverter 109 located below the drill floor 110. It is expected that reducing mud weight and compensating by increasing the pressure in the mud column will be increasingly useful for drilling for hydrocarbons. In the event of a failure on the closed loop return system, i.e. the additional components used relative to the conventional open loop drilling method, the pressure in the well bore may drop and allow well fluid to enter the riser/conductor and rise towards the rig with potentially catastrophic consequences. To mitigate this and other issues, the present invention relates to improvements in a closed loop return system.

Fig. 2 shows a schematic illustration of a closed loop mud return system according to the invention implemented in a mud return system 200 of a rig. Throughout this specification, closed loop mud return system is meant to signify one or more parts or components for use in a 'full' closed loop mud return system, i.e. strictly speaking the closed loop mud return system is not closed by itself but may/will together with other parts or systems form a closed loop. The mud return system is in principal not closed until e.g. the drill string is active. This mud return system 200 comprises several

components that could also be present in an open loop system including a kill and choke manifold 202, a mud gas separator 203, mud pits 204, shakers 205, mud pumps 206, diverter overboard lines 207, riser boost line 208, and hot mud fill line 209. A PMCD (pressurized mud-cap drilling) Manifold 210 and a back pressure pump 211 are arranged to provide various operations based on known techniques for performing drilling operations with a closed loop mud return. The control reels 212 and 213 hold the control lines referred to as umbilicals to the RCD 107 and/or flow spool 105. As noted above, the RCD and flow spool are examples of hardware providing the seal and direction of the mud flow as explained above. The control lines 212 and 213 are duplicated for redundancy. The control lines may lead directly to a device to be controlled or to a so-called pod, which is a local device that can receive an instruction to execute a control command and then provide the electrical and/or hydraulic signal to a control function. Examples of functions that can be controlled via the reels 212 and 213 includes the opening and closing of the valves 111 (see Fig. 1), the pressure for the seal around the drill string and an additional annular to close around the drill string etc. Note, that it is a general advantage of some embodiments of the invention to provide redundancy in the control system of the components of the closed loop return system, such as the annulus drill string seal, and one or more flow lines for directing return fluid to a choke manifold arranged so that back pressure on the top on the column of return fluids can be provided. As noted above, this is typically from a position above a BOP, e.g. from a flow spool, located below (such as immediately below) the annulus drill string seal.

Different other variations of the shown mud return system of Figure 2 are possible.

For example, instead of having, as shown, a connection from the PMCD manifold 210 to the buffer manifold 201 , the PMCD manifold 210 may be connected directly to the flow spool 105.

As another example, instead, as shown, of having a connection from the back pressure pump 211 to the buffer manifold 201 , the back pressure pump 211 may be connected directly to the riser boost line 208.

As yet another example, instead of the mud return system comprising the back pressure pump 211 (and its associated connections), an additional back pressure manifold may be located between the mud pumps 206 and the diverter 109.

Other variations are also applicable and at least some of the variations may be combined. The buffer manifold 201 and the choke/Coriolis manifold 250 of the mud return system will be explained in greater detail in connection with Figure 3. Figure 3 schematically illustrates exemplary embodiments of a buffer manifold and a choke/Coriolis manifold.

Illustrated are embodiments of a buffer manifold system 201 and

embodiments of a choke/Coriolis manifold system 250 e.g. as generally shown and explained in Figure 2. The choke/Coriolis manifold system 250 comprises a choke manifold 320 and flow measurement or Coriolis manifold 330.

In Figure 3, only one reference number is given for each element of a given type (unless it has a further meaning within the context) and the same elements will be represented by the same graphical symbol.

The buffer manifold system 201 comprises a number of fluid or mud return connections 305 for transporting mud return fluid, a number of control valves 301 , a pressure relief system 345, a number of target or block elbow elements 303, and a flow and/or pressure meter 304. The buffer manifold system 201 may be a modular unit or system designed for plug-and-play and/or may be integrated in a general manifold area of the rig. The buffer manifold system is preferably fully remotely operated with manual override.

The buffer manifold system 201 is connected via a first 306 and a second 307 flow spool connection, each comprising a control valve 301 , to a flow spool (not shown; see e.g. 105 in Figure 2 and flow lines 106 in Figure 1) and will, during regular operation, receive mud return fluid from the flow spool. The first and second flow spool connection 306, 307 correspond to the other shown fluid or mud return connections 305 but are connected to the flow spool.

The first and the second flow spool connections 306, 307 are, each, connected to a target or block elbow 303. The system 201 is further connected to a PMCD manifold (not shown; see e.g. 210 in Figure 2) and a back pressure pump (not shown; see e.g. 211 in Figure 2) via a PMCD and back pressure pump connection 308, 309, respectively, where each connection also comprises a control valve 301. The PMCD connection 308 connects to the target or block elbow 303 that the first flow spool connection 306 (in the shown embodiment, this is the topmost flow spool connection) is also connected to. This target or block elbow 303 connects, via a control valve 301 to a bypass connection or part of a bypass connection or gut line 310 (forth only referred to as bypass connection). The second flow spool connection 307 (in the shown embodiment, this is the lower flow spool connection) is connected to a further target or block elbow 303 that itself is connected via another control valve 301 to the bypass connection 310. The bypass connection 310 comprises a flow and/or pressure meter 304 or the like measuring a current flow of the downstream mud return fluid.

The bypass connection 310 is connected downstream to another target or block elbow 303. The back pressure pump connection 309 is connected via another control valve 301 also to this target or block elbow 303 further connecting downstream to the choke manifold 320. The bypass connection 310 is connected, via another target or block elbow 303, to a pressure relief system 345.

Traditionally, the pressure relief system only have a single pressure relief valve connected to an overboard relief line for emergency relief.

According to certain aspects, the pressure relief system 345 comprises two pressure relief valves 302 and a control valve 301 where one of the pressure relief valves (designated LP PRV) is a low pressure relief valve while the other pressure relief valve (designated HP PRV) is a high pressure relief valve as will be explained further in the following.

As mentioned, the bypass connection 310 is connected to the pressure relief system 345 and according to these certain aspects, the bypass connection 310 is more specifically connected to both the control valve 301 and the high pressure relief valve HP PRV, where the control valve 301 is connected to the low pressure relief valve LP PRV. The low pressure relief valve LP PRV is connected - bypassing the choke/Coriolis manifold system 250 - via a low pressure (LP) pressure relief line (see e.g. Figure 2) to the shakers (not shown; see e.g. 205 in Figure 2) and the mud gas separator (MGS) (not shown; see e.g. 203 in Figure 2) via various target or block elbows and control valves as shown.

The high pressure relief valve HP PRV is connected to an overboard relief line for emergency relief. In some embodiments, the high pressure relief valve HP PRV is connected to the overboard port/starboard diverter (see e.g. 109 in Figures 1 and 2). Alternatively, the high pressure relief valve HP PRV is connected to a separate overboard relief line, e.g. also port starboard relief lines.

During regular operation, the control valve 301 associated with the pressure relief system 345 will be open and the low and high pressure relief valves LP PRV, HP PRV will be closed. The low and high pressure relief valves LP PRV, HP PRV are automatically controlled as explained in the following.

Each pressure relief valve has a differently set pressure point determining when the respective pressure relief valve should be open or closed. The set pressure point may depend on actual operating conditions but the set pressure point of the low pressure relief valve LP PRV will be lower than that of the high pressure relief valve HP PRV. The pressure of the mud return flow is determined at one or more appropriate locations. When/if the determined pressure exceeds the set pressure point of the low pressure relief valve LP PRV (forth referred to as set low pressure point), the low pressure relief valve LP PRV will open thereby bypassing the choke/Coriolis manifold system 250 and discharge return mud fluid to the shakers and/or the mud gas separator (MGS).

When/if the determined pressure also exceeds the set pressure point of the high pressure relief valve HP PRV (forth referred to as set high pressure point), the high pressure relief valve HP PRV will open (and the control value upstream of the low pressure relief valve LP PRV will close) thereby discharging return mud fluid to the overboard relief line for emergency relief.

According to some embodiments, the high pressure relief valve HP PRV will be opened (and the low pressure relief valve LP PRV be closed) even if the set high pressure point has not been reached. According to these

embodiments, the flow of the return mud fluid is determined and monitored and if the determined flow exceeds a given threshold value and/or it is seen that the flow increase too much and/or too rapidly then the high pressure relief valve HP PRV will be opened (and LP PRV closed) as these indicators may be seen as precursors for a too high pressure. In this way, a dual pressure relief system is provided that readily can address both relatively low volume trapped pressures (that still with an advantage should be handled appropriately) - through the use of the shakers and/or MGS - and more critical relatively high volume trapped pressures - through the use of overboard relief. This adds extra security and redundancy. The low volume trapped pressures may e.g. occur due to self-induced trapped pressures such as a mud pump against a closed choke, which in this way can be recovered using the LP PRV. The high volume trapped pressures may e.g. occur due to a significant gas breakout exceeding the riser rated burst pressure. Please also see the discussion of different embodiments in relation to the LP and HP PRV later in the description. The aspects relating to the pressure relief system 345 and embodiments thereof may function independently of one or more of the other mentioned aspects but may also function together with one or more of these for further effect.

As mentioned, the buffer manifold 201 is connected to the choke manifold 320 via the bypass line 310. During normal operation, the return mud flow will be directed from the flow spool(s) to the choke manifold 320 via the buffer manifold 201.

The choke manifold 320 may be a modular unit or system designed for plug- and-play and/or may be located in the rig high pressure/HP manifold area. The choke manifold 320 is preferably fully remotely operated with manual override.

As shown, the choke manifold 320 comprises a number of chokes 340 being more or less traditional chokes of such systems and comprising a choke element 341 connecting to two control valves 301 where each control valve 301 connected to a respective target or block elbow 303 with one connection further comprising a flow and/or pressure meter 304. One or more choke elements may have one or more choke sensors associated with it.

Traditionally, the choke manifold 320 comprises only two chokes 340 where one is used for redundancy for the other. However, should one choke 340 become non-operational there is no further redundancy if operation is continued. According to certain embodiments of the choke manifold 320, the choke manifold 320 may comprise an additional choke, i.e. a total of three, which will still provide further redundancy even if one choke should become non- operational. According to other embodiments of the choke manifold 320, the choke manifold 320 comprises, as is shown in Figure 3, a four choke configuration configured as four parallel chokes 340 with a central bypass function (via the bypass line 310 and a central control valve 311) where the chokes 340 are arranged as two times two chokes. In such arrangements, two chokes 340 or one choke pair 345 provides redundancy as traditionally, while the other choke pair or two chokes provides further redundancy if one or more chokes becomes non-operational.

According to yet other embodiments of the choke manifold 320, the two choke pairs are arranged and/or used with one choke pair (e.g. the topmost two chokes shown in Figure 3) being used for high flow rates while the other choke pair (e.g. the lower two chokes shown in Figure 3) is being used for low flow rates. The respective flow(s) may be controlled or directed using the control valves.

The chokes 340 when being arranged in pairs or being an equal number are arranged symmetrically, as shown in Figure 3, in relation to the bypass connection or gut line 310.

These embodiments provide further redundancy and flexibility, even if one or more chokes becomes non-operational.

The aspects relating to the choke manifold 320 and embodiments thereof may function independently of one or more of the other mentioned aspects but may also function together with one or more of these for further effect. The choke manifold 320 is connected to a flow measurement or Coriolis manifold 330 that connects more or less traditionally to shakers and the MGS (see e.g. 203 and 205 in Figure 2).

The flow measurement manifold 330 comprises a number of control valves 301 , a number of flow and/or pressure meters 304, a number of target or block elbows 303, and a number of Coriolis meters or the like 350.

The flow measurement manifold 330 may be a modular unit or system so the full manifold may be dismantled and shipped ashore for re-certification (during which a replacement manifold may be installed and used) and/or may be integrated in a high pressure/HP manifold area of the rig. The flow measurement manifold 330 is preferably fully remotely operated with manual override.

The illustrated embodiment of Figure 3 comprises two Coriolis meters 350 for redundancy that are arranged symmetrically in relation to the bypass connection or gut line 310.

The provision of two Coriolis meters 350 provides redundancy and they may be used for kick detection, and/or measurement of fluid flow, fluid density, and/or temperature.

According to some embodiments of the flow measurement manifold 330, one Coriolis meter 350 is used for high flow rates while the other Coriolis meter 350 is used for low flow rates.

A given Coriolis meter 350 may be related or assigned to a given set of chokes and its reading may be used to control the related or assigned set of chokes. In such a case, the Coriolis meter and the set of chokes should be on the same line or path. If a given Coriolis meter 350 is a high pressure

Coriolis meter it could be located prior to the choke(s) being controlled on the basis of it. If the Coriolis meter is not a high pressure Coriolis meter is should preferably be located after the choke(s) being controlled on the basis of it.

The aspects relating to the flow measurement manifold 330 and

embodiments thereof may function independently of one or more of the other mentioned aspects but may also function together with one or more of these for further effect.

In the illustrated embodiments of the buffer manifold 201 , the choke manifold 320, and the flow measurement manifold 330, the control valves 301 provides sufficient dual barriers for dual block and bleed so that operations can continue whilst maintenance is being performed on washed out items. Alternatively, dual block and bleed functionality can be omitted or reduced thereby reducing the necessary number of used or required control valves 301.

Additionally in at least some embodiments of the buffer manifold 201 , the choke manifold 320, and/or the flow measurement manifold 330, electronic signals of all or some of the valve positions and/or gauges may be

communicated to a drilling control network (DCN), the MPD control system, and/or choke control system, i.e. enabling local and remote readings.

Additionally in at least some embodiments of the buffer manifold 201 , the choke manifold 320, and/or the flow measurement manifold 330, one or more of the manifolds comprises one or more local panels showing some or all relevant MPD parameters.

As another alternative, some or all of the target or block elbows 303 may in principal be omitted (but then e.g. causing some disadvantages) and/or be replaced by components providing similar functionality. As an independent aspect, it should be noted (as indicated by the bidirectional arrows to the flow spool in Figure 2) that the buffer manifold 201 may also be adapted to pump mud fluid into the flow spool(s).

As mentioned in connection with Figure 2, the PMCD may be connected directly to the flow spool (see 105 in Figures 1 and 2) instead being connected to the buffer manifold 201. In this case, the buffer manifold 201 will not comprise the PMCD connection 308 and its associated control valve.

As also mentioned in connection with Figure 2, the back pressure pump may (e.g. in combination with the above mentioned variation) be connected directly to the riser boost line (see 208 in Figure 2) instead of to the buffer manifold 201 or it could be omitted or replaced by a back pressure manifold located between the mud pumps and the diverter (see 206 and 109 in Figure 2). In such cases, the buffer manifold 201 will not comprise the back pressure pump connection 309 and its associated control valve. As another variation, an additional flow spool connection may be added connecting the pressure relief system 345 directly to the flow spool. This connection may preferably also comprise at least one control valve and/or a target or block elbow. This additional flow spool connection may be in addition to the first 306 and the second 307 flow spool connection or may be present instead of either or both.

Figures 4a - 4c schematically illustrate embodiments of a sensor control system.

Shown in Figure 4a is an example of a traditional sensor control system 400a comprising at least one control unit or system 401 comprising at least one programmable logic circuit (PLC) and/or the like 403. The PLC(s) 403 is/are connected via one or more electrical communication connections 402 to a plurality of sensors 400a comprising sensors 'A' 404, 'B' 405, 'C 406, etc. The sensors may e.g. be leak sensors, control valves, choke sensors, etc. and combinations thereof.

In a control system as shown in Figure 4a, no redundancy is readily provided in the control system itself to address e.g. a faulty sensor and/or faulty communication from the sensor.

Shown in Figure 4b is a sensor control system 400b according to an independent aspect where a number of sensors 414a, 414b, 414c, 415a, 415b, etc. each are connected to two control units or systems 412, 413, each control unit or system comprising a PLC and/or the like 410, 411. In this way, each control unit/system or PLC will receive the same signals thereby providing redundancy.

Furthermore, according to certain embodiments, further redundancy is provided as will be described in the following.

Further redundancy may be provided by implementing a sensor 'voting' system for important or critical sensors. More specifically, each important or critical sensor has one or more additional sensors providing redundancy on a sensor level. In the illustrated embodiment of Figure 4b, a three sensor voting system is implemented, whereby each relevant or chosen important or critical sensor is 'duplicated' so that three sensors register the same parameter at a given location. As can be seen in Figure 4b and as compared to the embodiment of Figure 4a, sensor 'A' of Figure 4a 404 is now implemented by three sensors 'A1' 414a, 'A2' 414b, and 'A3' 414c instead of only one, the sensor 'B' 405 of Figure 4a is now implemented by three sensors 'B1' 415a, 'B2' 415b, 'B3' (not shown), and so on. This greatly increases redundancy. According to the voting control or mechanism, then if the related sensors provide different signals or values (e.g. within a given threshold to

accommodate for measurement errors or tolerances) then the system will determine the (corresponding) signal or value as being reported by the greatest number of sensors as the correct one that is then used by the given control system. If one (or more sensors) report something different than the other/the majority of sensors that particular sensor may be designated and reported as potentially faulty e.g. prompting that it should manually be checked.

It is to be understood that in principle more than three sensors may be provided preferably as long as the number of sensors is uneven, e.g.

providing a five sensor voting system, seven sensor voting system, etc. but normally a three sensor voting system suffices and already three sensors greatly improve the redundancy level. In principle, the number of sensors may also be even but then giving the possibility of a tie.

It is also to be understood that each sensor need not necessarily be

'duplicated' to the same extent, e.g. one very important sensor may be 'duplicated' more times than other sensors. The important or critical sensors may e.g. be one or more leak sensors, one or more control valves (e.g. one or more of the ones shown and explained in connection with Figures 1 and/or Figure 3), one or more choke sensors (e.g. the ones explained in connection with Figure 3), and/or other relevant sensors. According to some embodiments, the two control units or systems 412, 413 will be located in different and physically apart control centers or control cabinets, e.g. each in a separate well control center. Each control unit or system 412, 413 will preferably be fully equipped and e.g. have its own separate uninterruptable power supply (UPS), etc. According to some embodiments, one control center or cabinet is provided in the 'blue' control room for a Mux control system while the other is provided in the 'yellow' control room, where 'blue' and 'yellow' control rooms are standard well-known terms within the field. Shown in Figure 4c is a sensor control system 400c according to other embodiments of the independent aspect. The sensor control system 400c corresponds to the sensor control system 400b except as explained in the following. The PLCs 410, 411 and sensors 414a - 415b, their function, and how they are connected correspond to as described in connection with Figure 4b except that the two PLCs 410, 411 are not located in two control units or systems 412, 413 as described, e.g. located in different and physically apart control centers or control cabinets. Rather, the PLCs 410, 411 in the embodiment of Figure 4c are both connected to forward the received sensor information (e.g. including one voted sensor signal in case of discrepancy between sensor signal for the same location and potentially an indication that there were a discrepancy) to two networks 420, 421. More specifically, one PLC is connected to a first network, e.g. 420, and the other PLC is connected to another - separate - second network, e.g. 421.

Both networks 420, 421 are connected to a first server A 423 and a second server B 422. In this way, both servers will receive the sensor information from both PLCs 410, 411 over two separate networks. This further increases robustness and redundancy.

The servers are located in different and physically apart control centers or operation stations 424, 425, e.g. each in a separate well control center. Each control unit or operation station 424, 425 will preferably be fully equipped and e.g. have its own separate uninterruptable power supply (UPS), etc. and one, in some embodiments, will be the 'blue' control room for a Mux control system while the other is provided in the 'yellow' control room. The aspects relating to the sensor control system and embodiments thereof may function independently of one or more of the other mentioned aspects but may also function together with one or more of these for further effect.

The following text refers to relevant embodiments throughout the description. In some embodiments, the fluid connection of the closed loop return system is connected to the riser string above (relative to the ground) one or more shear rams arranged to shear drill pipe or riser pipe inside the riser. In some embodiments the fluid connection is connected to the riser a BOP stack at least including one or more (such as two or more, such as three or more) shear rams arranged to shear drill pipe or riser pipe inside the riser. In some embodiments the choke line that is considered connected to the BOP has less than two (such as 1 or 0) annular BOPs and less than two (such as 1 or 0) ram BOPs between its connection to the annulus and the well bore. In general, the term BOP refers to a BOP stack consisting of one of more annular BOPs stacked with one or more ram BOPs and connected to the riser and/or conductor pipe. Most subsea BOP stacks are designed in two units each in a frame namely the Lower Marine Riser Package (LMRP) and the lower BOP stack.

In some embodiments, the choke(s) of the closed loop mud return system (e.g. the choke(s) of the buffer manifold; see e.g. 201 in Figures 2 and 3) is/are connected to return flow lines capturing fluid return at a position in the upper half of the distance from the seabed to the drill floor, such as the upper 3rd, such as the upper 25%, such as such as the upper 10%, such as upper 5%, such as the upper 1%. In one embodiment, annulus drill string seal (or just RCD which is used throughout the text to refer to all types of annulus drill string seals unless explicitly referring to the RCD design), arranged to seal the top of a mud column, is taken to mean that there is substantially no mud above the annulus drill string seal. In some embodiments, the phrase is taken to mean that the annulus drill string seal is arranged to seal the top of a mud column from a further mud column above the seal.

In some embodiments, the annulus drill string seal is arranged to seal around the drill pipe during extended drilling operations, such as drilling more than 50 meters (m), such as drilling more than 100m, such as drilling more than 200m, such as drilling more than 300m, such as drilling more than 500m, such as drilling more than 1 km, such as drilling more than 1.5 km into the ground.

In some embodiments of the invention, it is a general advantage to provide safety in the closed loop mud return system of the rig preferably according to the SIL (safety integrity level) standard such as IEC 61508 which is the international standard for electrical, electronic and programmable electronic safety related systems. This sets out requirements for ensuring that systems are designed, implemented, operated and maintained to provide the required safety integrity level (SIL). Four SILs are defined in accordance with the risks involved in the system application, with SIL 4 being used for the highest risks. In some embodiments of the invention, it is a general advantage to provide safety in the closed loop mud return system of the rig (in particular in relation to such SIL rating), whereby a separate control system is provided to control the LP PRV and/or HP PRV of the return system (see e.g. 302 in Figure 3). This control system is preferably arranged in a separate fire, explosion, and/or water tight cabinet so that it can be monitored, initialized and maintained according to stricter requirement than the remaining control systems and without having to impose such strict requirements on the remaining system. In some embodiments, this remaining system comprises at least the control reel or reels to control the RCD and or flow spool.

In some embodiments, the control system for the LP PRV is connected (preferably following the concept of Figs. 4b or 4c) to a sensor arranged to measure the pressure on the upstream side of the LP PRV which typically corresponds to the pressure from the return flow lines typically from the flow spool. In some embodiments, the control system is arranged to have a set point above which, it opens the LP PRV. In some embodiments, the control system is arranged to measure the flow through the LP PRV, e.g. using one or more Coriolis meters or the like (see e.g. 350 in Figure 3) of a flow measurement or Coriolis manifold (see e.g. 330 in Figure 3). In some embodiments, the event of high flow through the LP PRV (above a set point) causes the control system to open the HP PRV e.g. to vent the flow to an overboard line. In some embodiments, the control system has a pressure set point e.g. measured up stream of the LP PRV and/or HP PRV (in some embodiments, this pressure is the same) which causes the control system to open the HP PRV e.g. to vent the flow to an overboard line. In some embodiments, the system control system comprises all of these set points so that for low flows, a spill is avoided and the flow is handled in the return system, whereas a high flow and/or high pressure flow which may threaten the rig can be diverted outboard. In other words, a low and high pressure PRV system will be integrated allowing for low volume self-induced trapped pressures such as a Mud Pump against a closed choke to be recovered whilst a high volume, such as a significant gas break out exceeding the riser rated burst pressure can be diverted overboard.

In some embodiments, at least one of the control lines (such as two or more), to control the RCD and/or flow spool or similar components providing the same functionality, is in the form of a combined hydraulic and electric and/or optical bundle. In general, it is an advantageous aspect of the invention that all safety critical valves, such as all valves, can be operated locally as an override of the control system, remotely (such as via a computer screen), as a batch control where the control system opens and closes several valves e.g. to guide a flow from one point in the system to another, and finally are controllable valves as a part of an automatic control system e.g. a system which is designed to provide certain operational parameters during an operation.

Throughout the description, the used symbols in the drawings may have a different meaning than what they traditionally may represent. In such cases, the meaning is then the meaning as written in the description.

In the claims enumerating several features, some or all of these features may be embodied by one and the same element, component or item. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, elements, steps or components but does not preclude the presence or addition of one or more other features, elements, steps, components or groups thereof.

Claims

Claims:

1. A drilling rig comprising a closed loop mud return system comprising a choke manifold adapted to control, during use, a pressure in a mud column in a riser string wherein the system is arranged to be connected to at least one flow line in fluid communication with an annulus of a rotating control device, the flow line arranged to capture and direct return flow from said mud column towards said drilling rig.

2. The rig of claim 1 , wherein the system comprises at least two flow lines in fluid communication with the annulus of the rotating control device.

3. The rig of any one of the previous claims, wherein the rig further comprises a back pressure pump in fluid connection with said choke manifold on the upstream side of said choke manifold wherein said back pressure pump is arranged for applying back pressure on said mud column in the riser string.

4. The rig of any one of claims 1 - 2, wherein the rig further comprises a back pressure manifold located between one or more mud pumps and a diverter and being arranged for applying back pressure on said mud column in the riser string.

5. The rig of any one of the previous claims, wherein the rig further comprises a control system for controlling an annulus drill string seal arranged to seal the top of a mud column in a riser string around a drill string during drilling.

6. The rig of any one of the previous claims, wherein the rig further comprises a pressurized mud-cap drilling (PMCD) manifold being in fluid connection with said choke manifold on the upstream side of said choke manifold or being in fluid connection with the at least one flow line.

7. The rig of any one of the previous claims, wherein the rig further comprises a buffer manifold comprising a pressure relief system comprising a low pressure relief valve and a high pressure relief valve, wherein the low pressure relief valve is connected to a low pressure pressure relief line and the high pressure relief valve is connected to an overboard relief line, and wherein the low and high pressure relief valve, each has a set pressure point determining when the given pressure relief valve should discharge return mud fluid via its connected relief line.

8. The rig according to claim 7, wherein the low pressure pressure relief line is connected to a mud gas separator, e.g. via one or more shakers.

9. The rig of any one of the previous claims, wherein the rig further comprises a choke manifold comprising at least two, e.g. three or preferably four, chokes providing redundancy for each other.

10. The rig according to claim 9, wherein the choke manifold comprises four chokes, where two chokes are providing redundancy for each other and is used for high flow rates while the other two chokes of the four are providing redundancy for each other and is being used for low flow rates.

11. The rig of any one of the previous claims, wherein the rig further comprises a flow measurement manifold comprising two Coriolis meters for redundancy and e.g. wherein one Coriolis meter is used for high flow rates while the other Coriolis meter is used for low flow rates.

12. The rig according to claims 11 , wherein the two Coriolis meters are located in parallel.

13. The rig of any one of the previous claims, wherein the rig further comprises redundancy in the control system of components of the closed loop return system, such as a annulus drill string seal, and one or more flow lines for directing return fluid to the choke manifold arranged so that back pressure on the top of the column of return fluids can be provided.

14. The rig of any one of the previous claims, wherein the rig further comprises a sensor control system, wherein a plurality, preferably an uneven plurality, of sensors are provided, each sensor measuring the same one or more parameters at corresponding locations and wherein the sensor control system receives input from each of the plurality of sensors and selects as a sensor result, an input being received by the greatest number of the sensors.

15. The rig according to claim 14, wherein the rig comprises two corresponding sensor control systems, each receiving input from the uneven plurality of sensors.

16. A drilling rig according to any one of claims 1 - 6, wherein the drilling rig further comprises a buffer manifold according to claim 7 or 8 and/or a choke manifold according to claim 9 or 10 and/or a flow

measurement manifold according to claim 11 or 12 and/or redundancy in the control system according to claim 13, and/or at least one sensor control system according to claim 14 or 15.