GB2378724A

GB2378724A - Retainer valve system for controlling fluid flow through a blowout preventer

Info

Publication number: GB2378724A
Application number: GB0227060A
Authority: GB
Inventors: Vance Nixon; Gary L Rytlewski; Anthony P Vovers
Original assignee: Schlumberger Holdings Ltd
Current assignee: Schlumberger Holdings Ltd
Priority date: 1998-07-29
Filing date: 1999-07-19
Publication date: 2003-02-19
Anticipated expiration: 2019-07-19
Also published as: GB0227060D0; GB2378724B

Abstract

A safety valve system located partially in a wellhead and able to trap pressurised fluid in the pipe string (119, figure 1) has portions above and below the sea bed (110, figure 1). A control valve (314, figure 3) is located in the subsurface part of the well, whilst a retainer valve 200 and a bleed off valve 224 are located above the sea bed in the wellhead (108, figure 1). A latch (126, figure 1) releasably couples the retainer and control valves and excess pressure between these valves can be released via the bleed valve. All three valves and the latch are operated by pressurised control lines 218. Two opposing chambers 267, 268 that can be independently pressurised provide a secondary way of operating the retainer valve. The control lines can give sequential but independent operation of the valves and latch.

Description

CONTROLLING FLUID FLOW BACKGROUND OF THE INVENTION 1. Technical Field This invention relates generally to safety shut-in systems employed during testing or other operations in subsea wells. More particularly, the invention relates to a safety shut-in system having a valve for trapping fluid under pressure in a pipe string.

2. Background Art Offshore systems which are employed in relatively deep water for well operations generally include a riser which connects a surface vessel's equipment to a blowout preventer stack on a subsea wellhead. Offshore systems which are employed for well testing operations also typically include a safety shut-in system which automatically prevents fluid communication between the well and the surface vessel in the event of an emergency, such as when conditions in the well deviate from preset limits. Typically, the safety shut-in system includes a subsea test tree which is landed inside the blowout preventer stack on a pipe string. The subsea test tree generally includes a valve portion which has one or more normally closed valves that can automatically shut-in the well. The subsea test tree also includes a latch portion which enables the portion of the pipe string above the subsea test tree to be disconnected from the subsea test tree.

The subsea test tree may be used in conjunction with a retainer valve and a bleed-off valve. The retainer valve is commonly arranged in the pipe string to prevent fluid from being dumped from the pipe string into the riser when the pipe string is disconnected from the valve portion. The bleed-off valve allows controlled venting of pressure that may be trapped between the closed retainer valve and the closed valve portion of the subsea test tree. Generally, the subsea test tree, the retainer valve, and the bleed-off valve are controlled by fluid pressure in control lines which extend from a pressure source on the vessel to the subsea test tree, the retainer valve, and the bleed-off valve.

The retainer valve may be a normally-open or fail-safe-open retainer valve or may be a normally-closed or fail-safe-close retainer valve. When pressure is lost in the control

line connected to the retainer valve, a fail-safe-open retainer valve defaults to the open 1

position while a fail-safe-close retainer valve defaults to the closed position. For a fail-safe- close retainer, if the retainer-valve control line is inoperable, e. g., if the retainer-valve control line is inadvertently severed, the fail-safe-close retainer valve remains closed.

However, it may be necessary to re-open the retainer valve to permit other operations to be carried out on the well, e. g. , kill the well or retrieve a portion of a tubing or wireline which was severed when the retainer valve was closed. Thus, it would be desirable to provide a secondary means through which the retainer valve can be opened if the retainer-valve control line is inoperable.

Conventionally, three control lines are provided to operate the valve portion of the subsea test tree, the latch portion of the subsea test tree, the retainer valve, and the bleed-off valve. However, conventional systems do not allow for independent control of the valve portion of the subsea test tree, the latch portion of the subsea test tree, the retainer valve, and the bleed-off valve. Typically, the valve portion, the latch portion, and the retainer valve have their own dedicated control lines, and fluid pressure in one of the three control lines operate the bleed-off valve. For example, it is common to connect the control line that operates the latch portion to the bleed-off valve such that fluid pressure in the latch control line opens the bleed-off valve to vent pressure trapped between the retainer valve and the valve portion before the latch portion is disconnected from the valve portion. To allow independent control of the retainer valve, the valve portion of the subsea test tree, the latch portion of the subsea test tree, and the bleed-off valve, an additional control line may be provided to operate the bleed-off valve, but this would generally result in incompatibility with existing equipment. Therefore, it is desirable to provide a method for independently controlling the operation of the valve portion of the subsea test tree, the latch portion of the subsea test tree, the retainer valve, and the bleed-off valve using three control lines.

SUMMARY OF THE INVENTION One aspect of the invention provides a valve system for controlling fluid flow in a pipe extending from a rig through a subsea blowout preventer into a subsea well, the system comprising : a control valve connected to the portion of the pipe in the subsea well; a retainer valve connected to the portion of the pipe above the subsea well and having an outer surface area for engagement with a seal member in the subsea blowout preventer;

a latch for releasably coupling the control valve to the retainer valve; a bleed-off valve for venting pressure trapped between the control valve and the retainer valve; and three control lines operatively connected to independently actuate the control valve, the retainer valve, the bleed-off valve, and the latch using fluid pressure.

Another aspect of the invention provides a method for controlling fluid flow in a pipe extending from a rig through a subsea blowout preventer into a subsea well, the method comprising : applying pressure to a first control line to lock a latch between a control valve connected to the portion of the pipe in the subsea well and a retainer valve connected to the portion of the pipe above the subsea well; using the pressure in the first control line to open the control valve and the retainer valve; releasing pressure from the first control line to close the control valve ; applying pressure to a second control line to close the retainer valve and open a bleed-off valve; bleeding off pressure trapped between the control valve and the retainer valve using the bleed-off valve; and unlocking the latch when pressure in the second control line reaches a predetermined pressure.

Other aspects and advantages of the invention will become apparent from the following description and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic view of a subsea production well testing system.

FIGS. 2A and 2B are cross-sectional views of the retainer valve shown in FIG. 1.

FIG. 3 is a schematic of a control system for the safety shut-in system included in the subsea production well testing system shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a subsea production well testing system 100 which may be employed to test production characteristics of a well. The subsea production well testing system 100 comprises a vessel 102 which is positioned on a water surface 104 and a riser

106 which connects the vessel 102 to a blowout preventer stack 108 on the seafloor 110. A well 112 has been drilled into the seafloor 110, and a tubing string 114 extends from the

vessel 102 through the blowout preventer stack 108 into the well 112. The tubing string 114 ZD z, t) is provided with a bore 116 through which hydrocarbons or other formation fluids can be is provi I t, conducted from the well 112 to the surface during production testing of the well. A test device, such as a pressure/temperature sub, may be provided in the tubing string 114 to monitor the flow of formation fluids into the tubing string 114.

The well testing system 100 includes a safety shut-in system 118 which provides automatic shut-in of the well 112 when conditions on the vessel 102 or in the well 112 deviate from preset limits. The safety shut-in system 118 includes a subsea tree 120 and a retainer valve 200. The subsea tree 120 is landed in the blowout preventer stack 108 on the tubing string 114. A lower portion 119 of the tubing string 114 is supported by a fluted hanger 121. The subsea tree 120 has a valve assembly 124 and a latch 126. The valve assembly 124 acts as a master control valve during testing of the well 112. The valve assembly 124 includes a normally-closed flapper valve 128 and a normally-closed ball valve 130. The flapper valve 128 and the ball valve 130 may be operated in series. The latch 126 allows an upper portion 132 of the tubing string 114 to be disconnected from the subsea tree 120 if desired. It should be clear that the invention is not limited to the particular embodiment of the subsea tree 120 shown, but any other valve system that controls flow of formation fluids through the tubing string 114 may also be used.

The retainer valve 200 is arranged at the lower end of the upper portion 132 of the tubing string 114 to prevent fluid in the upper portion 132 of the tubing string from draining into the riser 106 when disconnected from the subsea tree 120. The retainer valve 200 also allows fluid from the riser 106 to flow into the upper portion 132 of the tubing string 114 so that hydrostatic pressure in the upper portion 132 of the tubing string 114 is balanced with the hydrostatic pressure in the riser 106. An umbilical 136 provides the fluid pressure necessary to operate the valve portion 124, the latch 126, and the retainer valve 200. The umbilical 136 has three control lines which are connected to a pressure source on the vessel 102.

FIGS. 2A and 2B show cross sections of the retainer valve 200. The retainer valve 200 comprises a spanner joint 202 (shown in FIG. 2A) and a valve section 204 (shown in FIG. 2B). The spanner joint 202 and the valve section 204 are connected by a flow tube 206. Referring to FIG. 2A, the spanner joint 202 includes a housing body 208 which is

provided with a bore 210. The bore 210 is aligned with the bore 116 (shown in FIG. 1) of the tubing string 114 when the retainer valve 200 is inline with the tubing string 114. An upper sub 212 is secured to the upper end of the housing body 208 by a threaded connection or other suitable connection. A torque pin 213 prevents the housing body 208 from being over-tightened and makes assembly and disassembly of the housing body 208 and the upper sub 212 easier. The upper sub 212 is provided to couple the housing body 108 to the upper portion 132 of the tubing string 114 (shown in FIG. 1). The flow tube 206 is secured to the lower end of the housing body 208 by a threaded connection or other suitable connection.

A sleeve 214 is mounted at a lower end of the housing body 208. The sleeve 214 is locked to the housing body 208 by lock pins 215 to prevent it from loosening while the spanner joint 202 is in use. A support member 216 is mounted between the sleeve 214 and the housing body 208. The support member 216 centralizes the flow tube 206 within the sleeve 214. The support member 216 also allows passage of flow control lines 218 while preventing damage to the flow control lines 218. The flow control lines 218 connect the control lines in the umbilical 136 (shown in FIG. 1) to various points in the valve section 204 (shown in FIG. 2B). The flow control lines 218 extend through the housing body 208 and apertures in the support member 216. Additional flow lines that are not connected to the control lines in the umbilical 136 also extend through the spanner joint 202 to various points in the valve section 204 (shown in FIG. 2B).

Referring to FIG. 2B, the valve section 204 includes a housing 220 which is provided with a bore 222. The bore 222 is aligned with the bore 116 (shown in FIG. 1) of the tubing string 114 when the retainer valve 200 is inline with the tubing string 114. The lower end of the flow tube 206, which was previously illustrated in FIG. 2A, is secured to the upper end of the housing 220 by a threaded connection or other suitable connection. A lower sub 223 is secured to the lower end of the housing 220. The lower sub 223 allows the housing 220 to be coupled to the tubing string 114 (shown in FIG. 1).

A bleed-off valve 224 is mounted in an outer cavity 225 in the housing 220. A sequencing valve (not shown) is also mounted in an outer cavity (not shown) in the housing 220. The bleed-off valve 224 is controlled by fluid pressure in flow conduit 228 in the housing 220. The sequencing valve is an in-line pressure relief valve which allows transmission of pressure downstream to the latch 126 (shown in FIG. 1) once a minimum specified pressure in a flow conduit (not shown) connected to the sequencing valve has been surpassed. A flow conduit 230 runs through the housing 220 and is connected to the subsea

tree 120 (shown in FIG. 1). The flow conduits 228 and 230 and the flow conduit connected to the sequencing valve are connected to the flow control lines 218 from the spanner joint 202 (shown in FIG. 2A).

A ball valve 232 is arranged inside the housing 220 to control fluid flow through the

housing. The ball valve 232 includes a ball element 234 which is supported by valve seats 236 and 238. The valve seats 236 and 238 are held in place in the housing 220 by valve seat retainers 240 and 242, respectively. The ball element 234 has a bore 246 which is movable between an open position to allow fluid flow through the housing 220 and a closed position g z : l to prevent fluid flow through the housing 220. The orientation of the bore 246 of the ball element 234 is controlled by axial movement of a control sleeve or valve operator 248Although not shown, the ball element 234 is mounted on pins which extend into diametrically opposed apertures in the control sleeve 248 so that when the control sleeve 248 is moved axially, the ball element 234 rotates. A seal (not shown) prevents leakage past the ball element 234 and holds pressure from above when the valve 232 is in the closed position.

The control sleeve 248 and the valve seat retainers 240 and 242 define an annular chamber 252. Fluid leakage from the annular chamber 252 into the bore 222 of the housing is prevented by seals 254. The face 256 of the control sleeve 248 is exposed to fluid pressure in one of the flow control lines 218 from the spanner joint 202 (shown in FIG. 2A).

The face 258 of the control sleeve 248 is exposed to fluid pressure in one of the flow control lines 218 from the spanner joint 202 (shown in FIG. 2A). The control sleeve 248 is normally biased against the valve seat retainer 242 by belleville springs 260 or other suitable spring or biasing device so that the ball valve 232 is normally in the closed position.

However, when fluid pressure that is sufficient to overcome the action of the springs 260 is applied to the face 258 of the control sleeve 248, the control sleeve 248 will move upwardly to open the valve 232. The valve 232 returns to the closed position if the fluid pressure acting on the face 258 is released. Additional pressure may be applied to the face 256 of the control sleeve 248 from one of the flow control lines 218 to assist the spring 260 in fully closing the ball valve 232.

An inner chamber 262 is defined between the valve seat retainer 242 and the housing 220. A piston 264 inside the inner chamber 262 may move axially within the inner chamber 262 in response to pressure differential acting across it. The piston 264 is connected to the control sleeve 248 by piston rods 266. Thus, the motion of the piston 264 is transmitted to

the control sleeve 248 by the piston rods 266. The piston 264 divides the inner chamber 262 into an upper chamber 267 and a lower chamber 268. The upper chamber 267 is vented to the riser 106 (shown in FIG. 1) by a flow line which runs through the housing 220 and the

spanner joint 202 (shown in FIG. 2A) to the annular passage between the riser 106 and the i ZD tubing string 114 (shown in FIG. 1). The lower chamber 268 is also vented to the annular passage between the riser 106 and the tubing string 114 (shown in FIG. 1) through a control z : l line (not shown) that runs from the lower chamber 268 and terminates at the upper end of the valve section 204.

In operation, and with reference to FIG. 1, the subsea tree 120 and the retainer valve 200 are landed in the subsea blowout preventer stack 108 on the tubing string 114. The valves 128 and 130 in the subsea tree 120 and the valve 232 of the retainer valve 200 are open to allow fluid flow from the lower portion 119 of the tubing string 114 to the upper portion 132 of the tubing string 114. In the event of an emergency, the valves 128 and 130 can be automatically closed to prevent fluid from flowing from the lower portion 119 of the

tubing string 114 to the upper portion 132 of the tubing string 114. Once the valves 128 and 130 are closed, the upper portion 132 of the tubing string 114 may be disconnected from the subsea tree 120 and retrieved to the vessel 102 or raised to a level which will permit the vessel 102 to drive off if necessary.

Before disconnecting the upper portion 132 of the tubing string 114 from the subsea tree 120, the retainer valve 200 is closed by moving the ball element 234 (shown in FIG.

2B) to the closed position. The closed retainer valve 200 prevents fluid from being dumped out of the upper portion 132 of the tubing string 114 when the upper portion 132 of the tubing string 114 is disconnected from the subsea tree 120. When the retainer valve 200 is closed, pressure is trapped between the retainer valve 200 and the valve portion 124 of the subsea tree 120. The bleed-off valve 224 is operated to bleed the trapped pressure in a controlled manner. After bleeding the trapped pressure, the latch 126 may be operated to disconnect the upper portion 132 of the tubing string 114 from the subsea tree 120.

The blowout preventer stack 108 includes pipe ram seals 138 and shear ram seal 140. However, other combinations of ram seals may be used. A lower marine riser package 109 is mounted between the blowout preventer stack 108 and the riser 106. The lower marine riser package 109 includes annular preventer seals 142. The lower marine riser package 109 also typically includes control modules (not shown) for operating the annular preventer seals 142, the ram seals 138 and 140 in the blowout preventer stack 108, and other

controls as needed. The ram seals 138 and 140 and the annular preventer seals 142 define a passage 143 for receiving the tubing string 114. The-subsea tree 120 is arranged within the blowout preventer stack 108, and the retainer valve 200 extends from the subsea tree 120 into the annular preventers 142.

Referring now to FIGS. 1 and 2B, the lower chamber 268 in the valve section 204 of the retainer valve 200 is vented to pressure below the annular preventers 142, and the upper chamber 267 is vented to pressure above the annular preventers 142. When one or both of the annular preventers 142 closes around the spanner joint 202, choke/kill lines (not shown) may be used to pressurize the fluid below the annular preventers 142 so that pressure in the lower chamber 268 is higher than the pressure in the upper chamber 267. Thus, when sufficient pressure differential is created between the upper chamber 267 and the lower chamber 268, the piston 264 moves upwardly. The upward motion of the piston 264 is transmitted to the control sleeve 248 through the piston rods 266 to open the ball element 234. This allows the valve 232 to be re-opened if the flow control line that applies fluid pressure to the control sleeve 248 is inoperable. It should be clear that a different type of blowout preventer, e. g. , a pipe ram preventer, or other type of wellhead assembly that includes a sealing member, e. g. , a diverter, may close around the spanner joint 202 to permit the desired pressure differential to be created between the chambers 267 and 268.

Referring to FIG. 3, a control system for the safety shut-in system 118 is shown.

The three control lines in the umbilical 136 are identified as control lines A, B, and C.

Control line A is connected to the ball valve 130, the latch 126, and the bleed-off valve 224 by flow lines 300, 302, and 304, respectively. Pressure in control line A opens the ball valve 130, locks the latch 126, and assists-close the bleed-off valve 224. The flapper valve 128 is connected to the ball valve 130 such that when the ball valve 130 is opened, the flapper valve 128 is also opened. Control line B is connected to the ball valve 130 and the flapper valve 128 by flow lines 306 and 308, respectively. The ball valve 130 and the flapper valve 128 are closed when control line B is pressurized and pressure in control line A is released.

Typically, when pressure is released from the control line A and there is no pressure in control line B, the ball valve 130 and the flapper valve 128 will close because of the action of the springs normally biasing the ball valve 130 and flapper valve 128 to the closed position. However, if there is a blockage from debris or coiled tubing inside the bore of the ball valve 130 and/or the flapper valve 128, then additional force may be required to close

the ball valve 130 and/or flapper valve 128. This additional force is provided by pressure in control line B.

Control lines A and B are connected to a shuttle valve 310. Control line C is connected to a pilot 312 of a control valve 314 by a flow line 316 and to a port of a control valve 318 by a flow line 320. The control valve 312 is connected to the pilot 321 of the control valve 318 by a flow line 322. The control valve 318 is normally open. A flow line 324 connects the shuttle valve 310 to the flow line 322. When there is pressure in control lines A or B, the control valve 318 is closed. Control valve 314 is closed when there is pressure in control line C. Control valve 318 is open when there is no pressure in the flow line 322.

Control valve 314 is connected to the retainer valve 200 by a flow line 326. Pressure in the flow line 326, which is indicative of pressure in control lines A or B, opens the retainer valve 200. The retainer valve 200 is also connected to the flow line 316 by a flow line 327 so that when control line C is pressurized, the retainer valve 200 closes. The control valve 318 is connected to the sequencing valve 226 by a control line 328 and the sequencing valve is connected to the latch 126 by a flow line 330. The control line 328 is also connected to the bleed-off valve 224 by a flow line 332. When the control valve 318 is open, pressure in control line C is communicated to the bleed-off valve 224 and the sequencing valve 226. The bleed-off valve 224 is opened and pressure trapped between the retainer valve 200 and the ball valve 130 and flapper valve 128 is vented off to the riser annulus through the port 288 (shown in FIG. 2B) in the housing 220. When pressure in the control line 328 surpasses a predetermined amount, the sequencing valve 226 allows pressure to be transmitted to control line 330 to unlock the latch 126.

In operation, this control logic allows the ball valve 130 and flapper valve 128, the latch 126, the retainer valve 200, and the bleed-off valve 224 to be independently controlled.

The outcome is sequence dependent. It is important that the latch 126 is not unlocked until all the other valves are closed. This is accomplished by the normally open control valve 318. If there is pressure in control line A or B, then the control valve 318 is in the closed position and the latch 128 cannot be unlocked. By following a predetermined sequence, the

retainer valve 200 or the ball valve 130 and the flapper valve 128 can be closed first. When pressure is applied to control line A, the ball valve 130 and the flapper valve 128 open, the latch 126 locks, and the bleed-off valve 224 has close-assist pressure applied to it. The retainer valve 200 remains open. When pressure is applied to control line B, the ball valve

130 and the flapper valve 128 fail-safe close. Upon bleeding pressure off control line A, the ball valve 130 and the flapper valve 128 close with pressure assist. The retainer valve 200 is then closed by bleeding pressure off control line B.

The retainer valve 200 will remain open by applying pressure to control line A or B.

The retainer valve 200 closes when pressure is applied to control line C and both lines A and B are bled of pressure. If pressure is held on line A and pressure is applied to line C, then the ball valve 130 and the flapper valve 128 will be held open, and the retainer valve 200 will close first. To unlock the latch 126, pressure must be applied to control line C and both control lines A and B must have no pressure. The retainer valve 200 can be reopened

by applying pressure differential across the piston 264 as previously described. p While the invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous variations therefrom without departing from the spirit and scope of the invention. For example, the ball valve 232 in the retainer valve 200 may be replaced with other types of valves, e. g. , flapper valve or gate valve. The subsea tree 120 may have other valves and may have a different configuration.

The pilots 312 and 321 may be replaced with control valves that are electrically controlled with solenoids.

Other means of controlling the opening of the ball valve 232 when the flow control line that supplies pressure to the control sleeve 248 is inoperable may also be provided. For example, the piston rods 266 and the piston 264 could be replaced with a secondary piston that acts directly against the face 258 of the control sleeve 248, and the inner chamber 262 could be connected to the riser annulus via a port (not shown) in the housing body 220. A rupture disc (not shown) may be mounted in the port and configured to burst when a predetermined pressure is applied to the riser annulus, e. g. , when the annular preventer 142 is closed around the spanner joint 202 and choke/kill lines are used to pressurize the lower section of the riser annulus to the predetermined pressure. When the rupture disc bursts, the secondary piston would be exposed to the pressure in the riser annulus and act accordingly on the control sleeve 248. The rupture disc may be selected such that the pressure required to burst the rupture disc is sufficient to overcome the biasing force of the springs 260. In this way, when the rupture disc bursts, the control sleeve 248 moves upwardly and opens the ball valve 232. Using a rupture disc allows the retainer valve to be re-opened only once. With the piston 264, the retainer valve can be re-opened repeatedly. Instead of using a rupture disc, the piston 264 may also be locked to the housing by shear pins that are adapted

to break when pressure in the lower section of the riser annulus is set to a predetermined pressure.

Claims

1. A valve system for controlling fluid flow in a pipe extending from a rig through a subsea blowout preventer into a subsea well, the system comprising: a control valve connected to the portion of the pipe in the subsea well; a retainer valve connected to the portion of the pipe above the subsea well and having an outer surface area for engagement with a seal member in the subsea blowout preventer; a latch for releasably coupling the control valve to the retainer valve; a bleed-off valve for venting pressure trapped between the control valve and the retainer valve; and three control lines operatively connected to independently actuate the control valve, the retainer valve, the bleed-off valve, and the latch using fluid pressure.

2. The valve system of claim 1, wherein : the control valve is responsive to pressure in a first of the three control lines, the first control valve being in an open position when pressure is applied to the first of the three control lines and biased toward a closed position when pressure is released from the first of the three control lines ; the retainer valve is responsive to pressure in all of the three control lines, the retainer valve being in an open position when pressure is applied to the first of the control lines or a second of the three control lines and in a closed position when pressure is applied to a third of the three control lines; the latch is responsive to pressure in the first of the three control lines and the third of the control lines, the latch being in a locked position when pressure is applied to the first of the three control lines and in an unlocked position when pressure is applied to the third of the three control lines; and

the bleed-off valve is responsive to pressure in the first of the three control lines and

pressure in the third of the three control lines, the bleed-off valve being in a t closed position when pressure is applied to the first of the three control lines and in an open position when pressure is applied to the third of the three control lines.

3. The valve system of claim 2, further comprising a second control valve for

preventing pressure from being applied to the third of the three control lines while 0 there is pressure in the first or the second of the three control lines.

4. The valve system of claim 2, further comprising a secondary means for opening the retainer valve, the secondary means comprising two chambers having a pressure differential substantially equal to a pressure differential across the seal member.

5. A method for controlling fluid flow in a pipe extending from a rig through a subsea blowout preventer into a subsea well, the method comprising: applying pressure to a first control line to lock a latch between a control valve connected to the portion of the pipe in the subsea well and a retainer valve connected to the portion of the pipe above the subsea well; using the pressure in the first control line to open the control valve and the retainer valve; releasing pressure from the first control line to close the control valve; applying pressure to a second control line to close the retainer valve and open a bleed-off valve; bleeding off pressure trapped between the control valve and the retainer valve using the bleed-off valve; and unlocking the latch when pressure in the second control line reaches a predetermined pressure.

6. The method of claim 5, further comprising: closing the blowout preventer around the retainer valve such that a seal member in the blowout preventer sealingly engages the retainer valve ;

communicating a first chamber within the retainer valve with pressure above the seal member and communicating a second chamber within the retainer valve with pressure below the seal member; and applying pressure below the seal member to create a pressure differential between the first and second chambers sufficient to move a valve operator to open the retainer valve.

7. The method of claim 5, wherein applying pressure to the second control line

includes preventing pressure from being applied to the second control line until z : 1 pressure is released from the first control line.