MULTI-MODE PUMPED RISER ARRANGEMENT AND METHODS
20220389770 · 2022-12-08
Assignee
Inventors
Cpc classification
E21B21/082
FIXED CONSTRUCTIONS
International classification
E21B21/00
FIXED CONSTRUCTIONS
Abstract
The present invention relates to a riser system in the form of a pumped riser (1), i.e. a riser (1) having an outlet (6) from the riser (1) at a depth below the surface (S) of a body of water, where the outlet (6) is coupled to a return pump (7) to return fluid from the riser (1) to the surface (S), and various operational methods to facilitate greater versatility when performing hydrocarbon drilling related operations. The arrangement also comprises a sealing element (15, 16) to seal an annulus (5) of the riser (1), and a by-pass (17) around the sealing element (15, 16). Various methods makes it possible to switch between open mode and closed mode, and vice versa, monitoring leakage across the sealing element (15, 16), as well as performing other operations exploiting the advantages of the two different modes.
Claims
1-83. (canceled)
84. A riser arrangement for performing operations, in a well extending from a bottom of a body of water, the arrangement comprising: a riser, and a choke line coupled to the riser at a BOP in the riser; the riser having a return outlet to a subsea return pump at a location substantially below a surface of the body of water, the return pump being coupled to a return line to pump fluids from the riser to above the surface of the body of water; the return line being a separate line; at least one sealing element being arranged within the riser at a location above the return outlet, wherein a by-pass is arranged to bypass the sealing element, the bypass extending from a port at the riser below the sealing element to a port at the riser above the sealing element, the by-pass being equipped with at least one isolation valve that is selectively controllable; the return line being coupled to a choke downstream of the pump.
85. The arrangement of claim 84, comprising at least one pressure sensor arranged below the sealing element and at least one level sensor or pressure sensor arranged above the sealing element.
86. The arrangement of claim 84, comprising a by-pass to by-pass the return pump.
87. The arrangement of claim 84, comprising a first branch line coupled to the riser substantially above the return outlet but below the sealing element and to the return line downstream of the return pump.
88. The arrangement of claim 84, comprising a second branch line coupled to the riser above the sealing element and to the return line downstream of the return pump.
89. The arrangement of claim 84, comprising a third branch line coupled to the riser above the sealing element and to the return line upstream of the pump.
90. The arrangement of claim 84, wherein the return line is coupled to a boost line and the boost line is coupled to a topside drilling fluid treatment facility, enabling return of drilling fluid from the pump via the boost line.
91. The arrangement of claim 84, wherein a boost line, extending between a lower part of the riser and above the surface of water, is coupled to the riser via a branch line above the sealing element.
92. The arrangement of claim 91, wherein the branch line is coupled to the riser via the bypass.
93. The arrangement of claim 84, wherein a riser insert, with a substantially smaller flow capacity than the riser, is arranged above the sealing element.
94. The arrangement of claim 91, wherein the branch line is coupled to the riser below the riser insert.
95. The arrangement of claim 93, wherein the bypass extends from below the sealing element to above the riser insert.
96. A method of performing operations, in a well extending from a bottom of a body of water, the method comprising: providing a riser having a return outlet to be coupled to a return pump, the return pump being adapted to pump fluid from the riser to above a surface of the body of water, positioning a sealing element in the riser above the return outlet, providing a by-pass around the sealing element, operating in a closed mode where the sealing element and the by-pass are essentially closed to prevent flow therethrough, operating with drilling fluid level reduced to below slip joint in the riser above the sealing element, operating the return pump to reduce the pressure below the sealing element, and switching to an open mode by selectively opening the by-pass to allow flow between the riser below the sealing element and the riser above the sealing element.
97. The method of claim 96, comprising the step of preparing to switch to an open mode by decreasing or increasing the pressure below the sealing element until the pressures above the sealing element and below the sealing element are substantially the same.
98. The method of claim 96, comprising the step of controlling the pressure in the well by adjusting a level of liquid in the riser using the return pump.
99. The method of claim 96, wherein the pressure in the riser below the sealing element is monitored by a pressure sensor.
100. The method of claim 96, wherein a level in the riser above the sealing element is monitored by a level sensor or a pressure sensor.
101. A method of measuring changes in liquid volume in a well extending from a bottom of a body of water, the method comprising: providing a riser having a return outlet to be coupled to a return pump, the return pump being adapted to pump fluid from the riser to above a surface of the body of water, positioning a sealing element in the riser above the return outlet, providing a by-pass around the sealing element, operating in a closed mode where the sealing element and the by-pass are essentially closed to prevent flow therethrough, switching to an open mode by opening the by-pass to allow flow between the riser below the sealing element and the riser above the sealing element, essentially stopping circulation of liquid in the well, measuring the level of liquid in the riser over a period of time, and determining changes in volume of liquid in the well based on the level measurements.
102. The method of claim 101, comprising the step of preparing to switch to an open mode by reducing or increasing a level of liquid in the riser above the sealing element until the pressures above the sealing element and below the sealing element are substantially the same.
Description
BRIEF DESCRIPTION OF DRAWINGS
[0095]
[0096]
[0097]
[0098]
[0099]
[0100]
[0101]
DETAILED DESCRIPTION OF PRIOR ART SYSTEMS
[0102] Some examples of prior art systems that the present invention departs from, will now be explained in order to better understand the subsequent description of the present invention.
[0103]
[0104] For such a system, it could be necessary to use a so-called underbalanced fluid when drilling wells. Particularly if the drilling window is narrow. A drilling fluid is called underbalanced when the pressure in the well with a riser full of drilling fluid is lower than the pore pressure of the formation being drilled. The drilling fluid can be a liquid or a mixture of liquid or gas, such as a foam, depending on the specific gravity needed for the fluid.
[0105]
[0106] A drill string 4 extends along the inside of the drilling riser 1 and into the well (not shown). An annulus 5 is formed between the drill string 4 and the riser 1.
[0107] A drilling fluid, also called mud, is pumped down the drill string 4, out the lower end of the drill string 4 and up the annulus 5. After the mud has exited from the lower end of the drill string 4, it will become mixed with the content of the well, such as oil, gas, water, particles, rocks etc. and flow up the annulus 5. The mud is pumped down the drill string 4 by a rig pump (usually a set of pumps) 40. A pressure sensor 39 is conveniently arranged at the outlet of the rig pump 40, typically on the stand-pipe 70 which is located on, or close to, the drill-floor.
[0108] One or more pressure sensors 29 could be arranged in the riser. However, in practice the sensors are arranged topside. There is also a pressure sensor 51 on the BOP.
[0109] A choke line 47 extends from a BOP 50. The choke line 47 has an isolation valve 48 and a pressure sensor 49. The choke line 47 is coupled to a rig choke 52.
[0110] A kill line 45 is coupled to the BOP 50 via an isolation valve 46. At the upper end of the kill line 45 is a liquid pump 43 and a pressure sensor 44.
[0111] A boost line 23 is coupled between an inlet 24 that is arranged close to the BOP 50. The line 23 has isolation valve 25 and is supplied by a boost pump 41. A pressure sensor 42 is included in the line 23.
[0112] At a depth below the surface there is an outlet 6 from the riser 1. The outlet is coupled to a flow line 108 with an isolation valve 109. The flow line 108 extends to the platform 2, where the line 108 is coupled via a choke 113 and a flow meter 114 to a mud treatment facility (not shown) through line 159. The choke 113 has mounted an upstream pressure sensor 141 and a downstream pressure sensor 140 thereto. A branch line 124 is coupled to a boost pump 123. The line 124, 108 with a valve (such as valve 109) may alternatively be duplicated as a second line from a second outlet, located close to the riser outlet 6. This duplication is to ensure there is an available flow-path to surface in case of any malfunction in the line 108.
[0113] At the top of the riser 1 there is an annular sealing element or diverter 38. The annular sealing element 38 is used to close the riser annulus 5 if gas should rise to the top of the riser 1. Below the diverter 38, is located an outlet 61 commonly known as a bell-nipple. This is connected to a flowline 60 that allows the drilling mud to be routed to the mud treatment facility when operating with a full riser level 245. There is a separate system (not shown) that ensures that the gas is handled in a safe manner when the annular sealing element 38 is used. This is often referred to as the diverter system. The annular sealing element 38 is part of any drilling rig and not specific to the SBP system. The mudline 60 is used to route mud back to a mud treatment facility during conventional drilling operations.
[0114] At a position between the top of the riser 1 and the outlet 6 there is a rotary sealing device (RSD) 15, which in this specification generally is referred to as sealing element. The rotary sealing device 15 is able to seal across the annulus 5 of the riser 1 and at the same time allow rotation of the drill string 4.
[0115] An additional annular seal 16, which is designed to seal around a non-rotating drill string 4. This seal 16 is used when the RSD 15 is to be changed and can also act as a safety measure if the RSD fails.
[0116] When operating the system of
[0117] The pressure below the RSD 15 is greater than atmospheric pressure. If there is a leakage across the RSD, there will be a leakage of well fluids to atmosphere and control methods, such as closing seal 16 and changing the RSD 15, are required.
[0118] The subsea specialized equipment for SBP is monitored and controlled through umbilical 180.
[0119] In
[0120] This known system has a number of advantages but does also have a number of drawbacks, as indicated in the section Background Art above.
[0121]
[0122] As for the system of
[0123] A drill string 4 extends along the inside of the drilling riser 1 and into the well (not shown). An annulus 5 is formed between the drill string 4 and the riser 1.
[0124] A drilling fluid, also often referred to as mud or drilling mud, is pumped down the drill string 4, out the lower end of the drill string 4 and up the annulus 5. After the mud has exited from the lower end of the drill string 4, it will become mixed with the content of the well, such as oil, gas, water, particles, rocks etc. and flow up the annulus 5. The mud is pumped down the drillstring by a rig pump (usually a set of pumps) 40. A pressure sensor 39 is conveniently arranged at the outlet of the rig pump 40.
[0125] A choke line 47 extends from a BOP 50. The choke line 47 has an isolation valve 48 and a pressure sensor 49. The choke line 47 is coupled to a rig choke 52.
[0126] A kill line 45 is coupled to the BOP 50 via an isolation valve 46. At the upper end of the kill line 45 is a kill liquid pump 43 and a pressure sensor 44.
[0127] A boost line 23 is coupled to an inlet 24 that is arranged close to the BOP 50. The line 23 has isolation valve 25 and is supplied by a boost pump 41. A pressure sensor 42 is included in the line 23.
[0128] At a depth below the surface S there is an outlet 6 from the riser 1. The outlet is coupled to a mud return line 8 with an isolation valve 9. The mud return line 8 extends to the platform 2, where there is a flowmeter 14 mounted on the mud return line 8. In normal operations, the mud is routed to the mud treatment system through a line 59 with a valve 53 closed and a valve 55 open.
[0129] As an alternative, the mud may be routed to the mud treatment system through a line 58 and the rig choke 52, with the valve 55 closed and the valve 53 open.
[0130] One or more pressure sensors 29 are arranged in the riser. There is also a pressure sensor 51 on the BOP.
[0131] The outlet 6 and pressure sensor 29 are part of a specialty riser joint 33 that is different to the rest of the riser joints being used. The specialty riser joint 33 is mounted in the riser 1 using regular riser flanges 34 and 35.
[0132] At the top of the riser 1 there is a flowline 60 for return mud and an annular sealing element 38. The annular sealing 38 element is used to close the riser annulus 5 if gas should rise to the top of the riser 1. There is a separate system (not shown) that ensures that the gas is handled in a safe manner when the annular sealing element 38 is used. This is often referred to as the diverter system.
[0133] There is also a fill line 26 that is coupled to the top of the riser 1. A pump 27 may pump mud through the fill line 26. A flow meter 28 or other method of measuring flow is used to keep control of the amount of mud pumped into the riser 1.
[0134] When operating the system of
[0135] There is mounted a pressure sensor upstream 56 and downstream 57 of the pump 7. Sensors 56 and 57 can be used to calculate the pressure generated by the pump 7
[0136] An umbilical 80 extends from the platform 2 down to the pump 7. The umbilical supplies power to pump 7 and also conveys signals and power subsea to CML components such as the riser mounted pressure sensor 29, the isolation valve 9, and the pressure sensors 56 and 57 on either side of the pump 7.
[0137] This system has many advantages but has also some drawbacks. Among the drawbacks are difficulties associated with handling gas influxes that have entered the riser above the BOP, although there exist ways to handle such situations, which will not be described herein.
[0138]
[0139]
[0140] A drill string 4 extends along the inside of the drilling riser 1 and into the well (not shown). An annulus 5 is formed between the drill string 4 and the riser 1.
[0141] Mud is pumped down the drill string 4, out the lower end of the drill string 4 and up the annulus 5. After the mud has exited from the lower end of the drill string 4, it will become mixed with the content of the well, such as oil, gas, water, particles, rocks etc. and flow up the annulus 5. The mud is pumped down the drill string 4 by a rig pump (usually a set of pumps) 40. A pressure sensor 39 is conveniently arranged at the outlet of the rig pump 40.
[0142] At a depth there is a first outlet 6 to which a return pump 7 is coupled. The downstream end of the return pump 7 is coupled to a return line 208 which connects to a line 230 that is connected to the riser 1. The pump 7 has an upstream isolation valve 209 and a downstream isolation valve 210
[0143] A pump bypass line 211 is also included. The pump bypass line 211 has an isolation valve 212.
[0144] There is a riser outlet line isolation valve 209 and a riser inlet isolation valve 222
[0145] In
[0146] From line 208 there is also a branch line 660 to the choke line 47. There is an isolation valve 661 on the branch line 660.
[0147] At a position higher up the riser 1 from the riser outlet 6 and below the inlet line 220, is arranged a Rotating Control Device (RCD) 215. The RCD 15 is typically placed at 900-1600 ft (about 275-50 meters) below Rotary Kelly Bushing (RKB) (not shown).
[0148] Below the RCD 215 could also be located an optional riser annular 216.
[0149] At the top of the riser 1, below the diverter 38, is located an outlet 61 commonly known as a bell-nipple. This is connected to a flowline 60 that allows the drilling mud to be routed to the mud treatment facility when operating with a full riser level 245.
[0150] The riser is equipped with pressure sensors 229 and 299. The pressure sensor 229 is below the RCD 215 and pressure sensor 299 is above the RCD 215.
[0151] The pump 7 receives power through an umbilical 280 from the surface. The umbilical also contains power and signal cables to operate and monitor the subsea valves and sensors.
[0152] In this prior art system, mud is returned to the surface through the riser during drilling. In an upset scenario, where riser gas needs to be handled by the system, the choke line may be used to handle the gas by isolating and bypassing the subsea pump and taking returns up the choke line, whilst pumping down the boost line and regulating the pressure in the riser using the rig choke.
[0153] A boost line 23 is coupled to an inlet 24 that is arranged close to the BOP 50. The line 23 has isolation valve 25 and is supplied by a boost pump 41. There is a branch line 292 from the boost line 23 to the riser 1, with an inlet above the RCD 215.
[0154] There is an isolation valve 291 on the branch line 292.
DETAILED DESCRIPTION OF THE INVENTION
[0155] In the following description it should be noted that whereas only one isolation valve is described to close a particular line, it is common practice to install at least two isolation valves at critical locations. Consequently, “a valve” should be construed as meaning “one of more valves”.
[0156] Moreover, the drawings are not to scale, as the vertical distance will be much larger compared to the diameter of the riser than shown in the drawing.
[0157] Within this description, the term closed riser refers to a system where the annulus is closed off within the riser, and where it is possible to operate with a pressure differential across some sealing device located within the riser. There are several methods by which the pressure differential can be created and maintained known per se to the person of skill.
[0158] Within this description, it is referred to the pump as removing pressure and the choke as adding pressure. From a physical point-of-view, within each of the two components, the opposite is what actually occurs, as the pump adds energy and pressure to the fluid and the choke dissipates, or removes, energy and pressure from the fluid. However, in the context for this invention and as is customary for the driller, we refer to the effect on the wellbore pressure from operating the pump or the choke.
[0159] The terms “mud” and “drilling fluid” are used alternatingly to denote drilling fluid in general. This is meant to cover all types of fluids commonly used for drilling, such as but not limited to liquids, gaseous fluids, gas and liquid mixtures, foam, emulsified water and/or oil, water-based, oil-based, gaseous and synthetic-based drilling fluids. The systems of the present invention may also be used for other purposes than drilling, such as cementing, completions, injection, hydration prevention or fracking, and hence fluids associated with these operations may also be used instead of or in addition to drilling fluid.
[0160] When in this specification, including the claims, the term “coupling” or “coupled” is used, it is to be understood as “fluidly coupling”.
[0161] When in this specification specialized equipment, such as the return pump 7 or return lines, additional to a conventional hardware set-up are described as mounted on the riser, these components could alternatively be mounted adjacent to the riser, e.g. suspended from the drilling rig, or located on the sea-bed.
[0162]
[0163]
[0164] A drill string 4 extends along the inside of the drilling riser 1 and into the well (not shown). An annulus 5 is formed between the drill string 4 and the riser 1.
[0165] A fluid, such as drilling fluid, cement for cementing liners and casings, MEG, water, plug and abandonment cement, glycol, etc. is pumped down the drill string 4. In the following drilling mud will be used as an example. The drilling mud is pumped down the drill string 4, out the lower end of the drill string 4 and up the annulus 5. After the mud has exited from the lower end of the drill string 4, it will become mixed with the content of the well, such as oil, gas, water, particles, rocks etc. and flow up the annulus 5. The mud is pumped down the drill string 4 by a rig pump (usually a set of pumps) 40. A pressure sensor 39 is conveniently arranged at the outlet of the rig pump 40.
[0166] At a depth, that can be between 50-1000 meters, but in most current cases will be around 2-400 meters, below the water surface S there is a first outlet 6 to which a return pump 7 is coupled. The downstream end of the return pump 7 is coupled to a return line 8. The return line 8 extends to the drilling platform or vessel 2 above the surface S. It may contain a flow meter 14. The flow meter 14 can also be arranged another place along the return line 8 than shown.
[0167] Sets of isolation valves 9 and 10 are arranged to facilitate isolation of the return pump 7 at the upstream and downstream end or both.
[0168] In normal operations, the mud is routed to the mud treatment system through line 59 with valve 53 closed and valve 55 open.
[0169] In the figure is also a choke line 47 extending from a BOP 50, shown. The choke line has an isolation valve 48 and a pressure sensor 49. The choke line 47 is coupled to a rig choke 52.
[0170] The return line 8 is also coupled to the rig choke 52 via a line 58. The valves 55 and 53 can be used to determine where the return flow should be directed.
[0171] A pump bypass line 11 is also included. The pump bypass line 11 has an isolation valve 12. Hence the drilling fluid can either be pumped to the surface via the return pump 7 when the valves 9 and 10 are open and the valve 12 is closed, or flow by its own pressure through the pump bypass line 11 when at least one of the set of valves 9, 10 (or preferably both) are closed, and the valve 12 is open.
[0172] A pressure sensor 56 is arranged on the inlet side of the return pump 7 and a pressure sensor 57 is arranged on the outlet side of the pump 7.
[0173] An umbilical 80 provides power to drive the pump, in addition to signal paths and power to operate the sensors, valves and sealing elements. The power supply can be hydraulic or electric, depending on the type of pump used.
[0174] At a position higher up the riser 1 from the return pump 7, but still substantially below the surface, is arranged a sealing device 15, which is of a type that seals around the drill string 4, also when the drill string 4 is rotating. Such closing devices are sometimes called Rotary Closing Device (RCD), In the following we will use the more generic term Rotary Sealing Device (RSD) 15.
[0175] The riser may also have an additional sealing element in the form of an annular seal 16, which is a device with a similar functionality as an RSD, but which is not designed to operate for any length of time with rotation of the drill string 4. It is primarily designed to operate without rotation of the drill string 4. Whereas the RSD 15 is typically installed and retrieved with the drill string 4, the annular seal 16 is installed with the riser 1. More than one RSD can be installed, as well as more than one annular seal. The annular seal 16 is designed to seal around a non-rotating drill string 4. The annular seal 16 may be used for shorter periods instead of the RSD, also with a rotating drill string 4. The RSD 15 and annular seal 16 can be arranged in any order. It is also conceivable to have the RSD located within the annular seal.
[0176] A by-pass line 17 is arranged to bypass the RSD 15 and the annular seal 16. The by-pass line 17 has a valve 18 that can be opened to allow well fluids to flow through the by-pass line 17.
[0177] The return line 8 is also connected to the riser 1 above the RSD 15 and annular seal 16, via an upper branch line 20. The branch line 20 has an isolation valve 22.
[0178] The arrangement may have a conventional kill line 45 that is coupled to the BOP 50 via an isolation valve 46. At the upper end of the kill line 45 is a kill liquid pump 43 and a pressure sensor 44.
[0179] A boost line 23 extends from the surface to an inlet 24 on the riser 1. The inlet 24 is positioned substantially below the pump outlet 6, preferably close to the lower end of the riser 1. The boost line 23 is equipped with one or more isolation valves 25. The boost line 23 is also equipped with a pressure sensor 72 that allows for measuring the level of liquid in the boost line.
[0180] Any suitable line, such as kill, choke, or other existing line on the riser may be used as a fill line instead of the boost line 23. Alternatively, a dedicated fill line may be installed
[0181] A fill pump 41 is arranged to pump liquid down the boost line 23. A pressure sensor 42 is included in the line 23.
[0182] The system of
[0183] At the top of the riser 1, below the diverter 38, is located an outlet 61 commonly known as a bell-nipple. This is connected to a flowline 60 that when operating with a full riser level allows the drilling mud to be routed to the mud treatment facility.
[0184] The system may operate with the riser level 245 at the height of the bell-nipple, or any other location down to the riser outlet 6.
[0185] The riser is equipped with pressure sensors and/or level sensors, such as sensors 29, 30. The sensor 29 is a pressure sensor, while the sensor 30 may be a pressure sensor or a level sensor. Such sensors are well known in the art per se. The pressure sensor 29 is below the RSD 15 and annular seal 16. The sensor 29 may also be arranged on the BOP 50. Alternatively, an additional pressure sensor 51 may be arranged on the BOP 50. Pressure/level sensor 30 is arranged above the RSD 15.
[0186] The pump 7 receives power through an umbilical 80 from the surface. The umbilical also contains power and signal cables to operate and monitor the subsea valves and sensors and annular seals located on riser joints 33 and 36 in addition to the valves and sensors between the outlet 6 and surface S or rig 2.
[0187] In a preferred embodiment the pump outlet 6 is arranged on a first special joint 33, extending between flanges 34 and 35. The RSD 15, annular seal 16, bypass 17 as well as branch lines 19, 20 and 31 are arranged on a second special joint 36 extending between flanges 35 and 37. All these items may alternatively be included in one joint.
[0188] A second embodiment of the invention will now be described in greater detail referring to the schematic set-up of
[0189] The return line 8 extending to the drilling platform or vessel 2 above the surface S contains an additional choke 13 upstream of the rig choke 52. An additional isolation valve 54 is also included. This couples the return line 8 to the rig choke 52 so that flow from the return line can be directed through the additional choke 13 to the rig choke 52. The isolation valves 53, 54, 55 are used to determine where the return flow should be directed, depending on the gas content in the flow. If the gas content is above a certain amount, or if a high gas content is expected, the flow is directed through the chokes 13 and 52. The choke 13 is fitted with an upstream pressure sensor 90 and a downstream pressure sensor 91
[0190] The embodiment of
[0191] The return line 8 is thus connected to the riser 1 both below the RSD 15 and annular seal 16 and above the RSD 15 and annular seal 16, via the lower branch line 19 and the upper branch line 20, respectively. Both branch lines 19, 20 have isolation valves 21, 22.
[0192] The boost line 23, or alternative line used as fill line, is also coupled to the riser 1 at a level above the RSD 15 via a branch line 31, which is equipped with an isolation valve 32 to form a lower fill line.
[0193] The system will normally operate with a riser level 145 below the slip joint 3. The riser level 145 may be operated anywhere between the riser outlet 6 and the bell-nipple 61.
[0194] The system may include all of the additional features shown in
[0195] A third embodiment of the invention will now be described in greater detail referring to the schematic set-up of
[0196]
[0197] A drill string 4 extends along the inside of the drilling riser 1 and into the well (not shown). An annulus 5 is formed between the drill string 4 and the riser 1.
[0198] A fluid, such as drilling fluid, cement for cementing liners and casings, MEG, water, plug and abandonment cement, glycol, etc. is pumped down the drill string 4. In the following drilling mud will be used as an example. The drilling mud is pumped down the drill string 4, out the lower end of the drill string 4 and up the annulus 5. After the mud has exited from the lower end of the drill string 4, it will become mixed with the content of the well, such as oil, gas, water, particles, rocks etc. and flow up the annulus 5. The mud is pumped down the drill string 4 by a rig pump (usually a set of pumps) 40. A pressure sensor 39 is conveniently arranged at the outlet of the rig pump 40.
[0199] At a depth, that can be between 50-1000 meters, but in most current cases around 2-400 meters, below the water surface S there is a first outlet 6 to which a return pump 7 is coupled. The downstream end of the return pump 7 is coupled to a return line 8. The return line 8 extends to the drilling platform or vessel 2 above the surface S. It may contain a flow meter 114. The flow meter 114 can also be arranged another place along the return line 8, for example at the pump 7 outlet, shown as flowmeter 599.
[0200] Sets of isolation valves 9 and 10 are arranged to facilitate isolation of the return pump 7 at the upstream and downstream end or both.
[0201] A pump bypass line 11 is also included. The pump bypass line 11 has an isolation valve 12. Hence the drilling fluid can either be pumped to the surface via the return pump 7 when the valves 9 and 10 are open and the valve 12 is closed, or flow by its own pressure through the pump bypass line 11 when at least one of the set of valves 9, 10 (or preferably both) are closed, and the valve 12 is open.
[0202] There is mounted pressure sensors upstream 56 and downstream 57 of the pump 7. Sensors 56 and 57 can be used to calculate the pressure generated by pump 7
[0203] In normal operations, the mud is routed to the mud treatment system through line 159 with valve 54 closed and valve 155 open. Alternatively, the mud from return line 8 can be routed to the rig choke 52 through line 168 with isolation valve 155 closed and isolation valve 54 open. There is a pressure sensor 92 downstream the rig choke 52. The mud could also be routed to the mud gas separator either through lines 159 or 168 and further on piping routes not shown on
[0204] In
[0205] At a position higher up the riser 1 from the riser outlet 6 but still substantially below the surface, is arranged an RSD 15.
[0206] The riser may also have an additional sealing element in the form of an annular seal 16, which is a device with a similar functionality as an RSD, but which is not designed to operate for any length of time with rotation of the drill string 4. It is primarily designed to operate without rotation of the drill string 4. Whereas the RSD 15 is typically installed and retrieved with the drill string 4, the annular seal 16 is installed with the riser 1. More than one RSD can be installed, as well as more than one annular seal. The annular seal 16 is designed to seal around a non-rotating drill string 4. The annular seal 16 may be used also with a rotating drill string 4 for shorter periods instead of the RSD 15. The RSD 15 and annular seal 16 can be arranged in any order. It is also conceivable to have the RSD 15 located within the annular seal.
[0207] A by-pass line 17 is arranged to bypass the RSD 15 and the annular seal 16. The by-pass line 17 has an isolation valve 18 that can be opened to allow well fluids to flow through the by-pass line 17.
[0208] The return line 8 is also connected to the riser 1 above the RSD 15 and annular seal 16, via an upper branch line 20. The branch line 20 has an isolation valve 22.
[0209] The arrangement may have a conventional kill line 45 that is coupled to the BOP 50 via an isolation valve 46. At the upper end of the kill line 45 is a kill liquid pump 43 and a pressure sensor 44.
[0210] A boost line 23 extends from the surface to an inlet 24 on the riser 1. The inlet 24 is positioned substantially below the pump outlet 6, preferably close to the lower end of the riser 1. The boost line 23 is equipped with one or more isolation valves 25. The boost line 23 is also equipped with a pressure sensor 72 that allows for measuring the level of liquid in the boost line.
[0211] Any suitable line, such as kill, choke, or other existing line on the riser may be used as a fill line instead of the boost line 23. Alternatively, a dedicated fill line may be installed
[0212] A fill pump 41 is arranged to pump liquid down the boost line 23. A pressure sensor 42 is included in the line 23.
[0213] The system of
[0214] At the top of the riser 1, below the diverter 38, is located an outlet 61 commonly known as a bell-nipple. This is connected to a flowline 60 that when operating with a full riser level allows the drilling mud to be routed to the mud treatment facility.
[0215] The riser is equipped with pressure sensors and/or level sensors, such as sensors 29 and 30. Sensor 29 is a pressure sensor, while sensor 30 may be a pressure sensor or a level sensor. Such sensors are well known in the art per se. The pressure sensor 29 is below the RSD 15 and annular seal 16. The sensor 29 may also be arranged on the BOP 50. Alternatively, an additional pressure sensor 51 may be arranged on the BOP 50. Pressure/level sensor 30 is arranged above the RSD 15.
[0216] The pump 7 receives power through an umbilical 80 from the surface. The umbilical also contains power and signal cables to operate and monitor the subsea valves and sensors and annular seals located on riser joints 33 and 36 in addition to the valves, sensors and other equipment for this invention shown between the outlet 6 and surface S on
[0217] In a preferred embodiment the pump outlet 6 is arranged on a first special joint 33, extending between flanges 34 and 35. The RSD 15, annular seal 16 , pressure or level sensor 30, pressure sensor 72 as well as branch lines 17, 20, 31 and 550 with isolation valves 32, 22, 18 and 551 are arranged on a second special joint 36 extending between flanges 35 and 37. All these items may alternatively be included in one joint from flanges 34 to 37.
[0218] The mud level 505 in the riser is typically kept below the slip joint 3 but could for certain operations be raised to the bell-nipple 61.
[0219] A choke 113 is located on the return line 8. Pressure sensors 190 and 191 are located upstream and downstream of choke 113. These can be used to calculate the pressure drop across the choke 113. The system could also include a by-pass arrangement around the choke 113 (not shown on drawing)
[0220] The flowmeter 114 is shown upstream the choke 113. This flowmeter 114 may also be located downstream the choke 113.
[0221] The pressure sensor 29 can be used to measure the pressure in the riser below the location of the RSD 15 or the annular sealing device 16. With the system in open mode this pressure is driven by the riser level and the mud weight. In a closed mode, this pressure can be regulated by the pump, the choke or both in combination.
[0222] The pressure or level sensor 30 measures the mud level above the sealing devices and can be used to monitor the upper riser when operating in closed mode.
[0223] The boost line 23, or alternative line used as fill line, is also coupled to the riser 1 at a level above the RSD 15 via a branch line 31, which is equipped with an isolation valve 32 to form a lower fill line.
[0224] With the system in closed mode, i.e. with the RSD 15 in place and the bypass valve 18 closed, pump 7 can maintain the pressure below the RSD 15 anywhere between that of a return line 8 full of a mud column and the minimum suction pressure allowed by the pump, typically around 1 bara. The pressure at the outlet 6 below the RSD 15 may be higher, equal to or lower than the pressure above the RSD 15. Compared to Dual Gradient concepts described in prior art, it is also significant that pressures at the riser outlet 6 lower than that of a sea-water gradient can be achieved, i.e. a water column from the bell-nipple.
[0225] By operating the choke 113, back-pressure can be added to return line 8. When operating the choke 113, the pump 7 may be used to generate some pressure boost, stopped so that no pressure boost is generated, or isolated by closing isolation valves 9 and 10 and opening isolation valve 12 to allow flow past the pump 7. Closing the isolation valves 9 and 10 to isolate the pump 7, will in some cases increase the maximum pressure rating of the system, as the pump will in some cases be the system component with the lowest pressure rating.
[0226] With this unique combination of the subsea pump 7 and choke 113, pressure can both be added and subtracted from that of a full riser in a seamless manner, giving the system an ability to regulate pressure up or down from that of a full riser, which has hitherto not been possible with prior art solutions. This is important for the driller as it increases the operational window and also gives them more flexibility with regards to choosing mud weight and still being able to keep the bottomhole pressure within the drilling window at all time. The control algorithms to synchronize the choke 113 and the pump 7 to operate in a seamless manner will be familiar to the person skilled in the art.
[0227] Next, will be described how to control the level above the RSD 15, also when operating in closed mode.
[0228] This is done by opening isolation valve 551, keeping isolation valves 9 and 12 closed, open isolation valve 10 and operate the pump 7. Fluid can then be drained from the upper part of the riser 1. By operating the system in such a manner, the driller can reduce the riser Level 505. The driller can also use this feature in conjunction with either the pump 27 or the pump 41, or both, to fill new liquid into the upper part of the riser, above RSD 15, in order raise the level 505, to change mud weight, or alter other properties of the mud in the riser.
[0229] Once the driller has finished circulating the mud above the RSD 15, isolation valve 551 can be closed and isolation valve 9 or isolation valve 12 can be opened to continue operations. There could be a number of reasons why the driller would want to circulate the liquid above RSD 15. Examples include reducing the liquid level above the RSD 15 prior to opening the RSD by-pass line 18, either to reduce the riser level 505 to a lower level than what was the case prior to closing the by-pass line 18 the last time, or to remove liquid that has leaked across RSD 15 during operations.
[0230] With the method described above it is also possible to have a mud above the RSD 15 with properties different to the mud below the RSD 15. Of particular interest to the driller would be the ability to add static pressure to the well without changing out the full mud system, to add pressure above the RSD 15 as additional barrier, or in some cases to operate with a lower density mud than for the rest of the well in the upper portion of the riser.
[0231] Prior art describes how pressure control can be obtained by using a pumped riser system with a sealing device installed where suction is drawn from below the sealing device and discharged into the riser above the sealing device. An example of such a system is shown in the previously described
[0232] In prior art pumped riser systems, as the system shown in
[0233] A fourth embodiment of the invention will now be described in detail, referring to
[0234] In this embodiment of the invention is introduced a unique combination of using an existing auxiliary line, typically the boost line, in combination with a riser insert to create a flushing system that ensures that drill-cuttings will not accumulate on top of the sealing device causing damage and wear.
[0235] in this embodiment is also introduces the possibility of alternatingly using an auxiliary line for three different purposes, either using it for its originally intended purpose, or as an injection line for the flushing system (as described above) or as a return line for mud laden with cuttings. The first two purposes mentioned here may also be used simultaneously.
[0236]
[0237] A drill string 4 extends along the inside of the drilling riser 1 and into the well (not shown). An annulus 5 is formed between the drill string 4 and the riser 1.
[0238] A fluid, such as drilling fluid, cement for cementing liners and casings, MEG, water, plug and abandonment cement, glycol, etc. is pumped down the drill string 4. In the following drilling mud will be used as an example. The drilling mud is pumped down the drill string 4, out the lower end of the drill string 4 and up the annulus 5. After the mud has exited from the lower end of the drill string 4, it will become mixed with the content of the well, such as oil, gas, water, particles, rocks etc. and flow up the annulus 5. The mud is pumped down the drill string 4 by a rig pump (usually a set of pumps) 40. A pressure sensor 39 is conveniently arranged at the outlet of the rig pump 40.
[0239] At a depth, that can be between 500-1000 meters, but in most current cases around 2-400 meters, below the water surface S there is a first outlet 6 to which a return pump 7 is coupled. The downstream end of the return pump 7 is coupled to a return line 608. The return line 608 connects to line 220 which is connected to the riser 1 and through line 223 to an auxiliary line 23. By selectively operating isolation valves 122 and 222, the driller can select which flow path is open. Line 608 may have a flowmeter 599 mounted close to the pump 7.
[0240] Sets of isolation valves 9 and 10 are arranged to facilitate isolation of the return pump 7 at the upstream or downstream end or both.
[0241] A pump bypass line 11 is also included. The pump bypass line 11 has an isolation valve 12. Hence the drilling fluid can either be pumped to the surface via the return pump 7 when the valves 9 and 10 are open and the valve 12 is closed, or flow by its own pressure through the pump bypass line 11 when at least one of the set of valves 9, 10 (or preferably both) are closed, and the valve 12 is open.
[0242] There are mounted pressure sensors upstream 56 and downstream 57 of the pump 7. Sensors 56 and 57 can be used to calculate the pressure generated by pump 7
[0243] In normal operations, the mud is routed to the mud treatment system through the line 220 with isolation valve 122 open and up the riser 1 with a riser mud level 606 at the bell nipple 61.
[0244] In
[0245] At a position higher up the riser 1 from the riser outlet 6 but still substantially below the surface, is arranged an RSD 15.
[0246] The riser may also have an additional sealing element in the form of an annular seal 16, which is a device with a similar functionality as an RSD, but which is not designed to operate with rotation of the drill string 4 for any length of time. It is primarily designed to operate without rotation of the drill string 4. Whereas the RSD 15 is typically installed and retrieved with the drill string 4, the annular seal 16 is installed with the riser 1. More than one RSD can be installed, as well as more than one annular seal. The annular seal 16 is designed to seal around a non-rotating drill string 4. The annular seal 16 may be used for shorter periods instead of the RSD, also with a rotating drill string 4. The RSD 15 and annular seal 16 can be arranged in any order. It is also conceivable to have the RSD located within the annular seal.
[0247] A riser insert 616 is mounted in the riser joint 36 located above the RSD 15. A tie-in line 31 from the auxiliary line 23 is connected to the riser joint 36, above the RSD 15 and below the riser insert 616.
[0248] The riser insert 616 has an axial cross-sectional opening area that is small compared to the internal cross-sectional area of the riser joint 36, typically 5% or less of the riser cross-section.
[0249] A by-pass line 617 is arranged to bypass the RSD 15, the annular seal 16 and the riser insert 616. The by-pass line 617 has an isolation valve 18 that can be opened to allow well fluids to flow through the by-pass line 617.
[0250] The arrangement may have a conventional kill line 45 that is coupled to the BOP 50 via an isolation valve 46. At the upper end of the kill line 45 is a kill liquid pump 43 and a pressure sensor 44.
[0251] A boost line 23 extends from the surface to an inlet 24 on the riser 1. The inlet 24 is positioned substantially below the pump outlet 6, preferably close to the lower end of the riser 1. The boost line 23 is equipped with one or more isolation valves 25. The boost line 23 is also equipped with a pressure sensor 72 that allows for measuring the level of liquid in the boost line.
[0252] Any suitable line, such as kill, choke, or other existing line on the riser may be used as a fill line instead of the boost line 23. Alternatively, a dedicated fill line may be installed
[0253] A fill pump 41 is arranged to pump liquid down the boost line 23. A pressure sensor 42 is included in the line 23.
[0254] The system of
[0255] At the top of the riser 1, below the diverter 38, is located an outlet 61 commonly known as a bell-nipple. This is connected to a flowline 60 that when operating with a full riser level allows the drilling mud to be routed to the mud treatment facility.
[0256] The riser is equipped with pressure sensors and/or level sensors, such as sensors 29 and 130. Sensor 29 is a pressure sensor, while sensor 130 may be a pressure sensor or a level sensor. Such sensors are well known in the art per se. The pressure sensor 29 is below the RSD 15 and annular seal 16. The sensor 29 may also be arranged on the BOP 50. Alternatively, an additional pressure sensor 51 may be arranged on the BOP 50. Pressure/level sensor 130 is arranged above the riser insert 616.
[0257] The pump 7 receives power through an umbilical 80 from the surface. The umbilical also contains power and signal cables to operate and monitor the subsea valves and sensors and annular seals located on the riser joints 33 and 36 in addition to the valves, sensors and other equipment for this embodiment, and which are arranged between the outlet 6 and surface S in
[0258] In a preferred embodiment the pump outlet 6 is arranged on a first special joint 33, extending between flanges 134 and 135. The RSD 15, annular seal 16, pressure or level sensor 30, pressure sensor 72 as well as branch lines 617, 220, 223 with isolation valves 222, 122, 32 and 18 are arranged on a second special joint 36 extending between flanges 135 and 137. All these items may alternatively be included in one joint from flanges 134 to 137
[0259] When routing the return flow up the auxiliary line 23, a new topside flow path is introduced, which will be described below.
[0260] To isolate the pump 41 and ensure that no mud with cuttings enter the pump, isolation valve 233 is closed. Isolation valve 232 is opened and mud is pumped from the pump 7, through lines 608, 220, 223, 23 and 236 to a topside flowmeter 114 and through the choke 113. Here the flow can either be routed to the regular mud treatment system through line 159, with isolation valve 155 open and isolation valve 54 closed, or to the rig choke 52 with isolation valve 54 open and isolation valve 155 closed. The topside choke 113 has upstream and downstream pressure sensors 190 and 191.
[0261] The mud level in the riser 605 is typically kept below the slip joint 3 but could for certain operations be raised to the bell-nipple 61 as shown by reference number 606.
[0262] Pressure sensors 190 and 191 are located upstream and downstream of the choke 113. These can be used to calculate the pressure drop across the choke 113.
[0263] The flowmeter 114 is shown upstream the choke 113. This flowmeter 114 may also be located downstream the choke 113. This may be the case for all embodiments.
[0264] Only one return line 608 is shown. The system may have a second return line to mitigate the risk of blockage.
[0265] As an alternative to having a second return line 8, the bypass line 617, with isolation valve 18, can be used as an instrumented over-pressure protection system. This is achieved by using readings from pressure sensor 29. Isolation valve 18 will open when the pressure from the sensor 29 exceeds a pre-determined set-point.
[0266] In order to ensure that cuttings do not fall down on RSD 15 when pumping cuttings laden mud into the riser above the RSD 15, drilling fluid is pumped down auxiliary line 23 through line 31 and into the cavity in the riser 1 between RSD 15 and riser insert 616. As the RSD 15 is sealing off the annulus, the flow will be forced upwards, through the openings in the riser insert 616 and mix with the drilling fluid above. The upward flowrate through the riser insert 616 will be sufficient to create an upward velocity of drilling fluid through the openings in the riser insert 616 that is higher than the slip velocity of cuttings in the drilling mud. This flowrate will typically be 100-500 litres per minute.
[0267] The driller may want to use the boost line for its originally intended purpose of boosting the riser. This is easily facilitated by opening isolation valve 25 and closing isolation valve 32. For these operations, valve 222 is already closed. If not pumping clean mud into line 31 some cuttings could theoretically enter the cavity between riser insert 616 and RSD 15. In order to avoid this, the driller could partially open isolation valve 25 and isolation valve 32 to create the correct choking effect to allow a controlled flow into the riser through inlet 24 and into the flushing cavity through line 31 at the same time. Alternatively, the driller may opt to accept the risk of cuttings accumulating on top of RSD 15 and not circulate through line 31.
[0268] The system of
[0269] Various possible operational procedures utilizing the above described set-up will now be described. Most of the procedures may be performed using any of the embodiments described herein, while for some procedures a particular embodiment may be necessary. It should be evident from the explanations if a particular embodiment is referred to. Sometimes only one reference number is used for a specific component, while different reference numbers may be used for the same component in the various figures.
Quickly Changing from Closed Riser with Pressure Control and Open Riser with Controlled Mud Level (CML)
[0270] The RSD 15 is kept closed around the drill string 4, With the by-pass valve 18 closed, the pressure in the riser 1 below the RSD 15 can be controlled by adjusting the pump suction pressure.
[0271] If the situation requires or it is beneficial to change the control regime into an open system where the pressure in the well is controlled by the mud level in the riser, this can be quickly done by opening the by-pass valve 18. There is no need to retrieve the RSD 15, as it can be kept closed. As an alternative, if the RSD is designed for it, it can be opened to switch into an open system. In the open mode, the level in the riser 1 can be set at any level between the pump outlet 6 and the top of the riser 1 and be controlled by the pump 7. Thereby the pressure above the RSD 15 can be adjusted to the same as or higher pressure than below the RSD before the valve 18 in the bypass 617, 17 is opened.
[0272] It is of course also possible to go from an open riser mode to a closed riser mode by closing the by-pass valve 18.
Measuring Mud Volume by Switching Between Closed Mode and Open Mode
[0273] When the system is in closed mode, i.e. with the isolation valve 18 of the by-pass 17 closed and under pressure control, it is difficult to measure the volume of mud in the system very accurately. The current measurement methods rely on aggregating flow measurements over time, i.e. flow of mud into the well versus flow of mud out of the well and/or has uncertainties related to effects on topside volume measurement system from factors such as rig motion, heave, poor sensor resolution, pipes that are not completely filled with liquids and so on. The flow measurements have inherent inaccuracies which when combined over time leads to volume estimates that have a significant uncertainty. The pumped riser open mode enables measurements with a higher degree of accuracy.
[0274] By switching from closed mode to open mode, and stopping flow into and out of the riser, any volume change in the well can be accurately determined by the level of mud in the riser 1 in static condition. The switching from closed to open mode can safely be done when the pressure in the riser above the RSD is higher than, lower than, or equal to, below the RSD. When switching between modes, care must be taken to stay within the allowable drilling window, usually given by pore and fracture pressures.
[0275] Also, during circulation, the open mode allows very rapid detection of gain or loss conditions in the well, by observing the riser level.
[0276] When operating in closed mode, the present invention allows for switching into the CML open mode by opening the valve 18 in the by-pass 17 or allowing communication between above and below the RSD 15 directly across the RSD 15. This allows for using the riser 1 as a tank to perform a flow-check or for any other reason use it to measure volume changes in the well in static conditions. The level in the riser may be set so that when the rig pumps are turned off, the pressures above and below the RSD 15 are different. For operational reasons it may not be desirable to open the by-pass unless the pressures above and below the RSD are close to equal. In this case the level in the riser needs to be changed. This change of level will take time.
[0277] As an alternative to the above, and also within the ambit of the invention, in going from closed to open mode and to use the riser as a tank to monitor the well and any change in volume, the mud return line 8 can be used as a tank to monitor the well. To use this line 8, it must be partially evacuated to the correct level to have the desired wellbore pressure. This can be accomplished by allowing the mud return line 8 to drain into the riser above or below the RSD 15 via the branch line 20 by opening the valve 22 or through the pump by-pass 11 by opening the valve 12 (or for the embodiment of
[0278] Since the return line 8 has a smaller diameter than the riser 1 any volume changes in the well will cause a larger change in the return line 8 level than it would have done in the riser 1. Consequently, it should be possible to obtain an even more accurate reading of volume changes using this method than using the riser 1 as a trip tank. Since the level changes more rapidly in the return line 8 than when using the riser 1, the pressure exerted on the well in case of an influx will increase rapidly as the level in the return line 8 increases. Since the diameter of the well in most cases, except when drilling very slim holes is larger than the diameter of the mud return line 8, the system will have a self-regulating effect towards stopping an influx.
[0279] As a second alternative to the above, the boost line 23 can be used as a tank to monitor the well. To use this line, the valve 25 must be opened, and pumping from the pump 41 has to be stopped. The level in boost line 23, and the associated pressure, will now equalize to the pressure in the riser below the RSD. The actual level in the boost line can at any time be verified by the boost line pressure sensor 72. Once the desired level is reached, the boost line can be used to monitor volume in the same manner as the open riser. For this purpose, boost line pressure sensor 72 could be used.
Reducing Wear on RSD by Reducing Differential Pressure
[0280] In closed mode, the pressure above the RSD 15 may be both equal to or lower than the pressure below the RSD, but may in certain operational modes also be kept higher than the pressure below the RSD. This ensures that any leaks across the RSD goes from above to below the RSD, and hence the pressure above the RSD is an additional safety measure against an uncontrolled flow of well fluids to surface.
[0281] However, the higher the differential pressure is across the RSD 15, the greater wear on the RSD. In order to reduce the wear, the differential pressure should be kept low.
[0282] The level/pressure sensors 29, 30 are used to monitor the pressure both below and above the RSD 15. The allowed pressure variation below the RSD 15 is given by the operational parameters of the well, which prescribes that the pressure in the well must be kept between certain limits, such as the fracturing pressure of the formation and the pore pressure of the formation, with associated safety margins. If the pressure difference across the RSD 15 exceeds a predetermined limit, the level of mud above the RSD 15 is reduced, either by opening the by-pass isolation valve 18 in a controlled (gradual) manner, or by adjusting the RSD to increase the leakage rate until the pressure difference is again below the predetermined limit.
[0283] If the pressure difference drops below a predetermined limit, the level of mud above the RSD 15 is raised by filling mud into the riser 1. This can conveniently be done be done through the fill-up line 26 or through the lower fill line 23 and branch line 31.
Monitoring Wear Condition of the RSD
[0284] As the RSD 15 is subject to wear during use, in particular due to the rotation of the drill string relative to the RSD, it must be replaced at intervals. Without any detection of the condition of the RSD it must be replaced at regular intervals based on expected lifetime of the RSD 15.
[0285] With the present invention it is possible to monitor the wear condition of the RSD 15, even when only one RSD 15 is used and without the need to externally supply a liquid. This is based on the fact that the leakage across the RSD increases as the RSD 15 wears. By monitoring the pressure below and pressure or level above the RSD 15 using the level/pressure sensors 29, 30 and keeping track of the flow of mud into and out of the well, as described above, it is possible with the system of the present invention, to monitor the leakage of mud across the RSD 15, and hence the wear of the RSD. The measured leakage rate may also be combined with measurements on the RSD such as e.g. hydraulic pressure or spring load on the RSD to determine wear status.
Reducing Wear on the RSD
[0286] As a further embodiment of the above monitoring of wear of the RSD 15, the system of the invention can also be used to reduce wear on the RSD 15.
[0287] It is known that the wear on the RSD depends on the friction between the drill string and the RSD, the higher the friction, the higher the wear. The friction depends among other factors on the force with which the RSD is set to have against the drill string. The higher this force is, the higher the friction will be. Despite the fact that a higher force and thereby higher friction results in an increased wear, the RSD is set with a relatively high force against the drill string. This is to avoid excessive leakage across the RSD.
[0288] With the present invention, the leakage across the RSD can be monitored. Hence, it is possible to allow a certain leakage as long as the leakage does not exceed a certain predetermined limit. By adjusting setting of the RSD to be near the maximum allowable leakage rate, the wear rate will be reduced, and the lifespan of the RSD will be increased.
Compensate for Increased Leakage Across the RSD
[0289] With the present invention there will be leakage over the RSD 15 in pumped riser closed mode. In at least one operational mode this leakage will be from above to below and will cause the level of drilling fluid in the riser to drop. This can be compensated for by filling mud into the riser to keep the level of mud above the RSD constant. In conventional drilling, filling of the riser will be done through the drill string or a boost line. However, with the set-up of the invention, this is not possible. The filling will therefore be done through the upper fill line 26 or the lower fill line 23 and branch line 31, which both end above the RSD.
[0290] With the present invention it is possible, using the monitoring of leakage described above, to determine the volume of mud that has to be filled into the riser above the RSD.
[0291] Compensation for increased leakage, such as caused by wear of the RSD, by increasing the force with which the RSD presses against the drill string can also be used. However, according to the invention, the level of mud above the RSD and the fill rate of the riser above the RSD is taken into account when determining the pressure with which the RSD presses against the drill string 4. According to the invention leakage can be compensated for both by the above filling of the riser with a controlled rate and by adjusting the pressure of the RSD against the drill string. Thereby the level of mud in the riser 1 above the RSD 15 can be maintained at a constant level. To this end the pump 27 and flow meter 28 are used. This process can be fully automized and controlled by an algorithm.
Stopping Leakage Across RSD
[0292] During certain operations, such as when circulating out a kick, or during connections (static) when operating with a pressure above the sealing element that is close to that below in dynamic conditions, but lower than below in static conditions, leakage across the RSD 15 is often not acceptable. In those cases, the leakage can be stopped or at least brought to within acceptable limits by increasing the force with which the RSD 15 presses against the drill string 4, so that it maintains a tight seal against the drill string. How the force from the RSD against the drill string 4 is increased will depend on the type of RSD and is as such not a part of the present invention. This procedure can be automated by using a controller.
Handling of Influx and Gas in the Mud
[0293] During normal operation, whether this is in closed or open mode, the mud in the well is returned via the return pump 7. However, if there is an influx of gas into the well, it is often not desirable to let the gas go through the pump. In that case the valves 9 and valves 10 are closed. Instead the valve 21 is opened to let the gas flow through the lower branch line 19 and up to the choke 13.
[0294] Alternatively, the influx may also be allowed to flow through the pump by-pass 11 to the choke
[0295] Gas that comes up with the mud can accumulate below the RSD. To get rid of this gas without having it released in an uncontrolled fashion when the RSD 15 is opened or pulled, the bypass 17 is opened to allow a downwards flow of mud from above the RSD with the intention of flushing the gas downwards, through the pump 7 and up the mud return line 8 in a controlled manner. Depending on the conditions this may involve increasing the level above the RSD 15 to allow a higher speed of the pump 7 to create an increased downwards flow. At the same time the riser 1 is filled from the top above the RSD (as explained above). A substantial downward flow through the bypass 17 is thereby generated in the riser 1. The accumulated gas is entrained in the mud flow and flushed through the return pump 7. The flow continues up the return line 8. At the surface it can be routed to a mud/gas separator for safe handling of the gas.
[0296] When increasing the speed of the pump 7, and thereby reducing the pressure below the RSD 15, the pressure on the well will be reduced and the BOP may be closed to ensure the pressure in the well does not drop below acceptable limits. When closing the BOP, known methods for ensuring a high enough pressure below the BOP may be used, such as, e.g., opening the valve to the kill line which is filled with mud. The method by which the well below the BOP is kept above an acceptable level is not part of the present invention.
Preparing the Riser System for Retrofit
[0297] In some cases, the functionality of the RSD 15 and a possible additional closure device, such as an annular seal 16, may be existing in a riser joint intended for other drilling activities such as Surface Back Pressure (SBP) or Riser Gas Handling (RGH). For the system of the invention, the riser joint intended for these other activities could be used and could be modified to be controlled using the control system described above and some or all of the functionality of the system of the invention.
[0298] The riser joint intended for these other well activities could in addition be fitted with additional connections and equipment to facilitate a dual-purpose use as SBP or RGH and also as a portion of the system of the invention. The riser-joint intended for other operations would originally have its own control lines going to surface through an umbilical. In the present invention, it could be equipped with features that enable reconfiguration by adding lines and other hardware required for use as a portion of the system of the invention. Most notably would be re-configuration to allow the existing system to receive controls functionality from surface through umbilical of the added pumped riser equipment. Dual use of the riser joint could extend to including the facilities for mounting the riser pump 7 and associated pressure sensors 29 and outlet 6. These facilities could be optimized between the two uses.
Controlling Wellbore Pressure Using Both Choke and Pump
[0299] In some cases when operating this system, the total pressure, or ECD, in the wellbore with a full riser will be too high when circulating, but too low when not circulating. The pressure therefore needs to be controlled. Other effects such as cuttings loading may also contribute to needing to control the pressure. In such cases, pressure needs to be added when the mud column is static (i.e. there is no circulation) and the excess pressure needs to be removed when circulating. There are many reasons why the driller might need to control the pressure, and this operational mode may be a planned mode, it could be that the mud weight has changed beyond what was expected during operation, it could be that the wellbore conditions have changed, such as when drilling into a pressure ramp, where the pore pressure suddenly is increased, or many other reasons. At any rate, the driller may want to control the downhole pressure in such situations using the choke, the pump or a combination of both.
[0300] Controlling the downhole pressure in closed mode using the choke or the pump alone is known from prior art. For some cases, however, it might be that the driller wants to switch between adding and removing pressure. The terms adding and removing pressure here is relative to the pressure which would have neen the case with an open riser full of mud. In order for this to be operationally efficient and safe, the transition between adding and removing pressure should be seamless. There are many operational scenarios where this could be relevant. Two examples are mentioned below.
[0301] When making a connection, circulation is stopped, and hence the dynamic component of the ECD is removed. The downhole pressure drops, as the there is no flow through the annulus. This pressure drop needs to be compensated for by increasing the riser pressure as the rig pumps are ramped down and reducing the riser pressure as the rig pumps are ramped up. When operating the system in a mode where pressure is removed, the driller may experience a kick that needs to be circulated out. During such a process, additional pressure needs to be applied to the system during the circulation process to account for the low density of gas being circulated out. This concept and the methods by which a kick is dealt with is known to the person skilled in the art. For certain combinations of mud weights and kick size, the driller will need to add pressure at the start of kick circulation and remove pressure at the end of the kick circulation.
[0302] For such cases, the driller can operate the system of
[0303] During this process, at a point where the choke is adding more pressure than the pump is removing, the driller may opt to isolate the pump by closing valves 9 and 10 and opening valve 12. This would be particularly relevant if the system pressure is approaching the pressure rating of the pump, or if it is suspected that gas in quantities that could affect system operation may enter the pump. Although it is possible to control the operation of the system manually by the driller, the system will typically be set up with an automated control system with the choke and pump in a Master-Slave configuration, using sensor readings to maintain the desired wellbore pressure. Such control systems for controlling the wellbore pressure in general are common for Surface Back Pressure operations. Adding the control of the subsea pump and the compressibility of the mud in the return line to such control system, and automating the closing of pump isolation valves and opening of pump by-pass valves is per se well known to a person skilled in the art of control systems.
Changing the Riser Level or Changing Out the Mud Above the RSD
[0304] During operation with a closed system, the driller may want to make changes to the mud in the riser above the closed RSD 15. There are many reasons why the driller may want to do this. In the following a few examples are mentioned. It may be that the driller wants to open the RSD 15 and that the current mud level above is too high. It may be that the driller wants a higher mud weight in the upper part of the riser. It may be that there are cuttings in the upper part of the riser that the driller wants to remove.
[0305] With the system in closed mode and the riser outlet 6 isolated, the upper riser suction line 550 is used to draw suction from the riser. The top-fill pump, or the boost line may be used at the same time to fill into the upper riser. This will obviously facilitate an alteration of the mud level or a replacement of the mud.
[0306] By altering the mud weight, the riser level or a combination of both above the RSD 15, the upper riser pressure can be controlled. In other drilling operations, the concept of a Riser Cap is well-known, where the kill, choke line or boost line is used to alter the mud weight in the riser to create a system with 2 mud weights, typically to add pressure to the wellbore. For such systems, the driller cannot circulate down the drillstring and maintain the Riser Cap intact. In the present system, by using the outlet below the RSD 15, the driller will be able to circulate down the drillstring and up the annulus and at the same time maintain the Upper Riser Cap intact without diluting it with mud from the annulus. With a heavy mud on top, there will be a tendency for the heavier mud to migrate down and mix with the lighter mud below, but this can be managed by only allowing a small opening through or past the RSD 15, which is large enough to give full pressure communication, but low enough to allow only a very limited flow past. A leakage rate from above to below in the order of 1-50 litres per minutes will be achievable, and this can easily be managed by the driller.
[0307] During drilling, gas may have accumulated below the RSD 15. If the downhole pressures allow, the upper riser mud level may be lowered prior to opening the RSD as a safety precaution should any gas migrate up the riser.
[0308] When handling a kick, there will be a period during the kick circulation where there is gas just below the RSD 15. In order to avoid problems if a blockage in the return line should occur at such a time, the system may be fitted with an over-pressure protection system that acts as a by-pass across the RSD and allow the pressurized gas to enter the upper riser. To reduce the risk of negative consequences of such an event, the upper riser suction line 550 can be used to reduce the riser level. The upper riser will then act as a tall separator with distance from the liquid level to surface that is much higher than what is achievable for a conventional separator. This will reduce the risk of a riser unloading event.
Operating in Open Mode—Quickly Reverting to Closed Mode
[0309] It is foreseen that the main operation of this invention, except for the modes described in
[0310] When operating in open mode with the RSD in place, the driller can quickly convert to a closed mode by closing the by-pass isolation valve, or close the RSD itself, depending on the design of the RSD. Closing the by-pass valve can be done in 1 to 5 seconds. The RSD can also be set up to close in 1 to 5 seconds. For the driller, going from open to closed in 1 to 5 seconds is very rapid and will not pose any operational constraints. With this unique feature, the driller can utilize all the benefits of an open CML system while at the same time always having the ability to quickly convert to a closed system to use the efficiency or safety features of a closed system.
Using By-Pass or RSD as Choke Device for Casing Shoe Protection
[0311] When operating in closed mode, the riser pressure, and by continuation the downhole pressure increases due to blockage, wrong operation of equipment or similar. In such a case, the pressure at the casing shoe may get above fracture pressure and the formation may break down, leading to severe losses. To prevent this, the RSD bypass line isolation valve, or the RSD itself if the design allows by reducing pre-load, could be used as a simple choke to allow riser pressure to be released in a controlled fashion prior to breaking down the casing shoe. The accuracy of the choking effect will not be of the quality that can be achieved with a regular drilling choke, but that will be acceptable to the driller as the main objective will be to use the choking effect to avoid breaking down the casing shoe, but to do so in a controlled fashion, rather than quickly releasing pressure, which could have detrimental effects as the pressure could get below pore pressure. If the by-pass isolation valve is a ball-valve, a person skilled in the art will know how to partially open such a valve to act as a choke. Seeing as the normal mode of operation with the present invention is to operate with a reduced riser level with a significant height up to the drilling rig, fluids could in many instances safely be bypassed to the riser above the RSD. Operating a closed riser system intentionally with a reduced riser level is not described in prior art.
Calculating and Compensating for Temperature Induced Density Effects When Not Drilling on Bottom
[0312] When pulling out of hole, particularly on high temperature wells, the drilling mud in the annulus will be heated by the formation. As an effect of the heating, the density will drop and the volume of the mud in the hole will increase. By operating with a reduced riser level in open mode using the riser as a trip tank, the volume change of mud in the wellbore can be constantly monitored. Any equipment being run in or out of the well will have a volume that can be pre-measured and accounted for in the mud volume measurements. Since the volume of each piece of equipment is very accurately known and also the well geometry and dimensions, the change in volume can be used to calculate the change in overall density. The temperature in the formations into which the well is being drilled are either known, can be measured during drilling, or estimated. Based on this, a temperature profile of the mud in the well can be calculated. By combining temperature measurements or estimates with known physical properties of the mud and well geometry, the change in temperature profile over time when not circulating down the drillstring can be calculated. The change in downhole temperatures will cause the density of the mud to drop as it is heated, which in turn will result in a volume expansion of the mud in the well and an associated pressure drop. The expanded volume will expand from the well with a smaller diameter into a riser with a larger diameter, so the net effect will be a drop in wellbore pressure. Based on the known properties of the mud, the volume measurements and observations or predictions on formation temperatures and associated downhole temperature profiles of the mud in the well, the temperature-induced wellbore pressure drop can be calculated. In order to compensate for this drop in wellbore pressure, the level of mud in the riser can be increased to achieve a near-constant wellbore pressure.
[0313] Should a sudden influx occur during this process, other methods described herein can be used to close the riser to handle the influx safely.