Landing string

Abstract

A landing string includes a valve having a valve member mounted within a flow path extending through the landing string, and a valve control system for use in operating the valve to move the valve member between open and closed positions to control flow along the flow path. The valve control system is reconfigurable between a first configuration in which the valve is operated or controlled under a fail-as-is (FAI) mode of operation, and a second configuration in which the valve is operated or controlled under a fail-close (FC) mode of operation.

Claims

1. A landing string, comprising: a valve having a valve member mounted within a flow path extending through the landing string; and a valve control system for use in operating the valve to move the valve member between open and closed positions to control flow along the flow path, wherein the valve control system comprises a closing line for providing communication of pressure to the valve to facilitate closing of the valve member, and a control valve operable between a first control valve configuration in which the closing line is in communication with the valve and a second control valve configuration in which the closing line is isolated from the valve; and a pilot line for communicating a pilot pressure which operates the control valve to move from its first control valve configuration to its second control valve configuration, and wherein relief of said pilot pressure from the pilot line permits the control valve to move from its second control valve configuration to its first control valve configuration; wherein the valve control system is selectively reconfigurable prior to a failure event comprising relief of pilot pressure within the pilot line between: a first configuration in which the valve is operated or controlled under a fail-as-is (FAI) mode of operation; and a second configuration in which the valve is operated or controlled under a fail-close (FC) mode of operation.

2. The landing string according to claim 1, wherein the valve control system is reconfigurable prior to a failure event in accordance with an operator preference.

3. The landing string according to claim 1, wherein the valve control system is reconfigurable between the FAI and FC modes of operation in accordance with operation of a blow out preventer (BOP).

4. The landing string according to claim 1, wherein the valve is a retainer valve.

5. The landing string according to claim 1, wherein the valve is operable to cut an object located within the flow path of the landing string during movement of the valve member from its open position to its closed position.

6. The landing string according to claim 1, wherein the valve comprises: an opening port for facilitating communication with a source of pressure to operate the valve member to move towards an open position; and a closing port for facilitating communication with a source of pressure to operate the valve member to move towards a closed position.

7. The landing string according to claim 1, wherein the valve control system comprises: an opening line for providing communication of pressure to the valve to facilitate opening of the valve member.

8. The landing string according to claim 7, wherein the opening line is operable to communicate pressure to other components or systems of the landing string.

9. The landing string according to claim 7, wherein the pilot line is defined by the valve opening line.

10. The landing string according to claim 1, further comprising a pressure accumulator associated with the closing line.

11. The landing string according to claim 1, wherein when the control valve is in its first control valve configuration, pressure applied within the closing line facilitates operation of the valve member to move to and/or be held within its closed position.

12. The landing string according to claim 1, wherein when the control valve is in its first control valve configuration venting is permitted from the valve via the closing line.

13. The landing string according to claim 1, wherein when the control valve is in its second control valve configuration, venting is permitted from a portion of the valve.

14. The landing string according to claim 1, wherein the control valve is biased towards its first control valve configuration.

15. The landing string according to claim 1, wherein the control valve is a normally open valve.

16. The landing string according to claim 1, wherein the pilot line provides a dedicated function of operating the control valve.

17. The landing string according to claim 1, wherein the failure event comprises severing of the pilot line.

18. The landing string according to claim 1, further comprising a shear sub located below the valve, wherein, in use, the landing string is locatable within a blow out preventer (BOP) such that the shear sub is aligned with a shear ram of the BOP, the pilot line extending along the shear sub such that the pilot line extends from a source of power and along the shear sub via the control valve.

19. A method for controlling a valve within a landing string which includes a flow path, a valve member mounted within the flow path, and a valve control system for use in operating the valve to move the valve member between open and closed positions to control flow along the flow path, wherein the valve control system comprises a closing line for providing communication of pressure to the valve to facilitate closing of the valve member, and a control valve operable between a first control valve configuration and a second control valve configuration, the method comprising: providing a pilot charge in a pilot line to operate the control valve to move from a first control valve configuration in which the closing line is in communication with the valve to a second control valve configuration in which the closing line is isolated from the valve, wherein relief of the pilot pressure permits the control valve to move from its second control valve configuration to its first control valve configuration; and reconfiguring the valve control system prior to a failure event comprising relief of pilot pressure within the pilot line between: a first configuration in which the valve is operated or controlled under a fail-as-is (FAI) mode of operation; and a second configuration in which the valve is operated or controlled under a fail-close (FC) mode of operation.

20. The method according to claim 19, comprising configuring the valve control system prior to the failure event: in the first configuration by not pressurizing or preventing the closing line from being pressurized while the control valve is in its second control valve configuration; and in the second configuration by pressurizing the closing line while the control valve is in its second control valve configuration.

21. A landing string, comprising: a valve having a valve member mounted within a flow path extending through the landing string; and a valve control system for use in operating the valve to move the valve member between open and closed positions to control flow along the flow path, wherein the valve control system comprises a closing line for providing communication of power to the valve to facilitate closing of the valve member, and a control valve operable between a first control valve configuration in which the closing line is in communication with the valve and a second control valve configuration in which the closing line is isolated from the valve, wherein the valve control system is selectively reconfigurable prior to a failure event between: a first configuration in which the valve is operated or controlled under a fail-as-is (FAI) mode of operation by not charging or preventing the closing line from being charged while the control valve is in its second control valve configuration; and a second configuration in which the valve is operated or controlled under a fail-close (FC) mode of operation by charging the closing line while the control valve is in its second control valve configuration.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a diagrammatic illustration of a lower portion of a landing string in accordance with an embodiment of the present invention shown during final stages of deployment into a BOP;

(3) FIG. 2 is a diagrammatic illustration of a valve control system in accordance with an embodiment of the present invention for controlling a retainer valve of the landing string of FIG. 1, wherein the valve is located in an open position and the control system is configured such that the valve is controlled in a FAI mode of operation;

(4) FIGS. 3 and 4 show a sequence of operating the valve control system of FIG. 2 to permit the valve to be closed, while still maintaining the valve in a FAI mode of operation;

(5) FIG. 5 is a diagrammatic illustration of the landing string of FIG. 1 shown during a flow-back operation in which well fluids are flowed upwardly through the landing string;

(6) FIG. 6 shows the valve control system of FIG. 2, reconfigured into a FC mode of operation;

(7) FIG. 7 shown the landing string of FIG. 1 being cut by a shear ram of the BOP which establishes a failure event;

(8) FIG. 8 illustrates the valve control system functioning to cause the valve to close following cutting of the landing string;

(9) FIG. 9 is a diagrammatic illustration of a valve control system in accordance with an alternative embodiment of the present invention, wherein the valve under control is in an open position and the system is configured to operate the valve in a FAI mode of operation;

(10) FIG. 10 illustrates the valve control system of FIG. 9 with the valve in a closed position, while maintaining the FAI mode of operation;

(11) FIG. 11 illustrates the valve control system of FIG. 9 reconfigured to operate the valve in a FC mode of operation;

(12) FIG. 12 illustrates the valve control system of FIG. 9 with the valve closed following a failure event;

(13) FIG. 13 is a diagrammatic illustration of a valve control system in accordance with a further alternative embodiment of the present invention, wherein the valve under control is in an open position and the system is configured to operate the valve in a FAI mode of operation;

(14) FIG. 14 illustrates the valve control system of FIG. 13 with the valve in a closed position, while maintaining the FAI mode of operation;

(15) FIG. 15 illustrates the valve control system of FIG. 13 reconfigured to operate the valve in a FC mode of operation;

(16) FIG. 16 illustrates the valve control system of FIG. 13 with the valve closed following a failure event;

(17) FIG. 17 is a diagrammatic illustration of a valve control system in accordance with a further alternative embodiment of the present invention, wherein the valve under control is in an open position and the system is configured to operate the valve in a FAI mode of operation;

(18) FIG. 18 illustrates the valve control system of FIG. 17 with the valve in a closed position, while maintaining the FAI mode of operation;

(19) FIG. 19 illustrates the valve control system of FIG. 17 reconfigured to operate the valve in a FC mode of operation; and

(20) FIG. 20 illustrates the valve control system of FIG. 13 with the valve closed following a failure event.

DETAILED DESCRIPTION OF THE DRAWINGS

(21) FIG. 1 illustrates the lower portion of a landing string, generally identified by reference numeral 10, in accordance with an embodiment of the present invention, being deployed from a surface vessel (not shown) through a marine riser 12 into a BOP 14 which is mounted on a wellhead 16. The BOP 14 is of a standard configuration and includes a shear ram section 18 including shear rams 19 and a number of pipe ram sections 20 (two in the embodiment illustrated) each including pipe rams 21. As known in the art the shear ram section 18 and pipe ram sections 20 may be operated in accordance with a required well control. In the embodiment shown the marine riser 12 is secured to the BOP 14 via a flex joint connector 15 which provides a degree of permitted relative motion between the riser 12 and the BOP 14. This operates to effectively decouple motion of the surface vessel from the BOP 14.

(22) Landing strings are known in the art and may include any number of required components and architecture.

(23) In the present example embodiment the landing string 10 includes an upper tubing section 22 which extends from the surface vessel and terminates at a retainer valve 24. Although not shown, the upper tubing section 22 may include equipment such as a wireline lubricator valve and the like.

(24) The retainer valve 24 is of a ball valve type, and includes a ball valve member 26, shown in broken outline, which is arranged to be rotated within the landing string 10 to selectively open and close a flow path extending therethrough. In the embodiment shown the retainer valve 24 has a cutting capability, allowing objects positioned within the landing string to be cut during closure of the ball valve member 26. The retainer valve 24 may define a shear and seal valve.

(25) A shear sub 28 is located below the retainer valve 24, and when the landing string 10 is fully deployed the shear sub 28 is aligned with the shear ram section 18 of the BOP 14. The shear sub 28 facilitates cutting of the landing string 10 by actuation of the shear rams 19.

(26) A latch 30 is positioned below the shear sub 28, and in use facilitates parting of the landing string 10 at this section, for example as may be required in certain situations, such as certain emergency situations.

(27) A subsea test tree (SSTT) 32 is located below the latch 30, and in the embodiment shown includes a dual valve barrier, specifically including upper and lower ball valve members 34a, 34b. In use, the ball valve members 34a, 34b are operable to be rotated within the SSTT 32 to selectively open and close the flow path through the landing string 10, thus providing desired well control. The ball valve members 34a, 34b have a shearing capability, allowing objects positioned within the landing string to be cut during closure of said valve members 34a, 34b.

(28) A slick joint section 36 is located below the SSTT 32 and in use provides a suitable engagement surface against which pipe rams 21 of the BOP may be sealingly closed.

(29) A lowermost end of the landing string 10 includes a tubing hanger 38 which is landed within the wellhead 16.

(30) A number of hydraulic control lines extend along the landing string 10, for example from surface, for use in controlling the various systems and components. In the example embodiment shown three control lines 40, 42, 44 are illustrated, in broken outline.

(31) Control line 40 may communicate hydraulic power to the retainer valve 24 to operate the ball valve member 26 to open. Control line 40 may also function to provide hydraulic power to the latch 30 to retain the latch in a locked configuration.

(32) Control line 42 may communicate hydraulic power to the retainer valve 24 to operate the ball valve member 26 to close. A valve control system 50, shown generally in broken outline, provides control to the retainer valve 24. As will be described in more detail below, the control system 50 assists in controlling the retainer valve 24 to operate under a desired safety protocol, such as a fail-as-is (FAI) mode of operation or a fail close (FC) mode of operation.

(33) Control line 46 may provide hydraulic power to any other component or system within the landing string 10. In the exemplary embodiment shown the control line 46 provides a secondary control to the latch 30.

(34) As illustrated, control lines 40 and 44 both extend past the shear sub 28, and as such in the event of activation of the shear ram section 18 will be cut, preventing hydraulic power to be maintained. In such a situation, the control system 50 may provide a degree of necessary control to allow the retainer valve 24 to operate under a desired safety protocol, such as a FC protocol.

(35) In use, the landing string 10 may support a number of wellbore functions, including intervention operations. For example, the landing string 10 may provide a passage, via its flow path, for intervention tooling to be deployed into the wellbore from a surface vessel, for example on wireline or coiled tubing. Further, the landing string 10 may facilitate flow-back operations from the well to the surface vessel, for example as part of a well testing operation, clean-up operation or the like.

(36) In the event of a well control scenario it may be necessary to entirely shut-in the well. In some instances this may be achieved by activating the SSTT 32 to close to contain well pressure, activating the retainer valve 24 to close to contain the fluids within the landing string 10, and then activating the latch 30 to part the landing string 10. The landing string 10 above the latch 30 may then be retrieved, leaving the SSTT 32 in place.

(37) Following this the BOP 14 may be activated to close, for example via the shear ram section 18 and/or pipe ram sections 20.

(38) In other situations, for example where an emergency event causes the BOP 14 to be actuated, the BOP shear rams 19 may close to cut through the shear sub 28 and control lines 40, 44. In such a situation the control system 50 may operate to ensure that the retainer valve 24 closes (fail close mode of operation) to maintain, as quickly as possible, the fluids within the landing string 22.

(39) As described above, the landing string 10 includes a valve control system 50 for use in operating the retainer valve 24, and permitting selection of either a FAI mode of operation or a FC mode of operation. One embodiment of such a valve control system 50 will now be described with reference to FIGS. 2 to 8.

(40) FIG. 2 provides a schematic illustration of the valve control system 50 and the retainer valve 24. As described above, the retainer valve 24 includes a ball valve member 26. The ball valve member 26 includes a bore 52 extending therethrough, wherein the valve member 26 is illustrated in FIG. 2 in a position in which the bore 52 is aligned with a flow path 54 of the landing string permitting flow and passage of objects therethrough. The retainer valve 24 also includes a valve actuator 56, for example in the form of a hydraulic piston which includes an opening port 58 and a closing port 60. In the configuration shown in FIG. 2, hydraulic pressure (represented by a thicker line) is provided via control line 40 to the opening port 58 of the valve actuator, thus positioning the valve member 26 in its open position. As such, control line 40 may be defined as an opening line.

(41) The valve control system 50 includes a normally open control valve 62. As will be described in further detail below, the control valve 62 is operable to move between its normally open position and a closed position (as in FIG. 2) to selectively establish and prevent fluid communication between control line 42 and the closing port 60 of the valve actuator 56. As such, the control line 42 may be defined as a closing line.

(42) The opening line 40 is in pressure communication with the control valve 62 to provide a pilot pressure for effectively operating the control valve 62 to move between its normally open position and its closed position. As such, the opening line 40 may also define a pilot line.

(43) When the control valve 62 is in its closed configuration as shown in FIG. 2, the closing line 42 is isolated by a closed port 64 in the valve 62, while the closing port 60 of the valve actuator 56 is vented via a vent port 66 in the valve 62. Such venting of the closing port 60 prevents hydraulic lock within the valve 24 during opening by pressure applied via the opening line 40.

(44) In the configuration shown in FIG. 2 the valve control system 50 is configured to control the valve 24 in a FAI mode of operation. This is achieved by removing or not applying pressure or significant pressure within the closing line 42. As such, in the event of a failure, such as loss of pressure in the opening/pilot line 40, resulting movement of the control valve 62 to its normally open position will not cause the retainer valve 24 to close, as zero or insufficient pressure will be applied at the closing port 60 of the actuator 56.

(45) When it is desired to operate the valve 24 to close, the opening/pilot line 40 is first vented, as shown in FIG. 3, causing the control valve 62 to move (for example by a bias spring 68) to its normally open position. Following this, pressure is applied within the closing line 42 which is communicated, via the open control valve 62, to the closing port 60 of the valve actuator 56, causing the valve member 24 to close, misaligning the bore 52 from the flow path 54.

(46) As noted above, in the arrangement of FIGS. 2 to 4 the retainer valve 24 is operated under a FAI mode of operation. In this respect the present invention permits an operator to select the particular mode of operation (FAI or FC). For example, certain operations may not necessarily require the retainer valve 24 to close in the event of a possible failure event. As an example, during deployment of the landing string 10 into the BOP 14, as illustrated in FIG. 1, the operator may decide that the safety margins associated with such an operation can permit a FAI mode of operation of the retainer valve 24 to be appropriate.

(47) However, other operations may be such that a FC mode of operation of the retainer valve 24 is most appropriate. One example, as illustrated in FIG. 5, is during flow-back of well fluids to surface, illustrated by arrow 69. In such a circumstance it is desirable to be able to react quickly to close the retainer valve 24 to retain wellbore fluids within the landing string 10 in the event of a failure event, especially caused by closing of the shear rams 19 of the BOP 14. This is particularly true in gas wells where the well fluids within the landing string 10 comprise pressurised gas. As pressurised gas can contain very high levels of energy, a BOP shear event could potentially cause a high energy and rapid escape of the gas from the landing string, with the possible result of ejecting the landing string 10 upwardly through the vessel, with obvious risk to personnel and equipment. Accordingly, it is highly desirable to be able to operate the retainer valve 24 in a FC mode of operation to thus close as quickly as possible in response to such a failure event.

(48) In the present example, the control system 50, as illustrated in FIG. 6, may be configured to permit the valve 24 to be operated as FC. Specifically, the opening/pilot line 40 is charged to firstly hold the valve member 26 in its open position, and secondly to hold the control valve 62 in its closed position. The closing line 42 is also charged, but isolated from the retainer valve 24 by the control valve 62. Accordingly, any failure event which might cause venting of the opening/pilot line 40 will cause the control valve 62 to move to its normally open position, communicating the charged closing line 42 to the retainer valve 24 to thus be closed.

(49) A failure event within the landing string 10 is illustrated in FIG. 7, while the valve control system 50 is configured in the FC mode of operation, as illustrated in FIG. 6. In this respect the shear rams 19 of the BOP 14 have been caused to close, shearing the shear sub 28 and the control lines 40, 42. The pipe rams 21 of the BOP 14 are also indicated in a closed position. FIG. 8 provides an illustration of the reaction of the valve control system 50 to this failure event. In this respect shearing of the opening/pilot line 40 vents the pressure therein, allowing the control valve 62 to move to its normally open portion, communicating the charged closing line 42 with the valve 24, causing the ball valve member 26 to close and block the flow path 54.

(50) In the embodiment of the valve control system 50 first shown in FIG. 2 the closing line 42 may communicate with a source of hydraulic pressure, for example at surface level. In some alternative embodiments the valve control system may also include a pressure accumulator, as illustrated in FIG. 9. In this case the valve control system is identified by reference numeral 150, and is similar in most respects to the system 50 of FIG. 2, and as such like features share like reference numerals. As such, the system 150 includes, at least, an opening/pilot line 140, a closing line 142 and a normally open control valve 162. The system 150 further includes a pressure accumulator 70 which is in pressure communication with the closing line 142.

(51) The system 150 is configured in FIG. 9 in a FAI mode of operation, with the retainer valve 24 open. Specifically, zero or reduced pressure is provided within the closing line 142 and the opening/pilot line 140 is charged to open and hold the retainer valve 24 in its open position, while closing the control valve 162. When the retainer valve 24 is to be closed the pressure within the opening/pilot line 140 is vented, allowing the control valve 162 to move towards its normally open position, following which pressure may be applied in the closing line 142, as shown in FIG. 10, to cause the retainer valve 24 to close.

(52) The system 150 is shown in FIG. 11 reconfigured to operate the retainer valve 24 in a FC mode of operation. This is achieved by providing pressure within the opening/pilot line 140 to open the retainer valve 24 and hold the control valve 162 in its closed position, while also providing pressure within the closing line 142. In this respect the closing line pressure is isolated from the retainer valve 24 by the closed control valve 162. While in this configuration pressure within the closing line 142 may charge the pressure accumulator 70.

(53) In the event of a failure event, such as actuation of the BOP shear rams 19 (see FIG. 7), the opening/pilot line 140 is severed causing pressure to vent and allowing the control valve 162 to move to its normally open position, immediately communicating pressure to the retainer valve 24 to cause this to close, as illustrated in FIG. 12. The pressure accumulator 70 can assist to provide or improve the response time of actuation pressure to close the retainer valve 24, which may have advantages when used in combination with flowing gas wells, for example. Further, the pressure accumulator 70 may provide a degree of safety in that once charged the retainer valve 24 can still be closed even in the event of some failure in the original pressure source.

(54) In some embodiments including a pressure accumulator, a venting arrangement may be provided which permits accumulated pressure to be vented during retrieval of the control system back to surface. This may accommodate changes in hydrostatic pressure during retrieval, and avoid a dangerous pressure differential associated with the pressure accumulator from being established.

(55) In the embodiments described above a single control line 40, 140 is provided to function as both an opening line for the retainer valve 24 and a pilot line for the control valve 62, 162. However, in other embodiments separate individual lines may be utilised, as illustrated in FIG. 13, which provides a diagrammatic illustration of a control system 250 in accordance with an alternative embodiment of the present invention. The control system 250 is largely similar to system 50 first shown in FIG. 2 and as such like features share like reference numerals, incremented by 200.

(56) The system 250 is capable of operating the retainer valve 24 of the landing string 10, and includes an opening line 240a which is in communication with the opening port 58 of the valve actuator 56, such that when pressure is applied within the opening line 240a, as illustrated by a thick line in FIG. 13, the valve member 267 is opened to align the bore 52 with the flow path 54.

(57) The system 250 further includes a normally open control valve 262 and a separate pilot line 240b, which may be in the form of a pigtail, which extends to communicate control pressure to the control valve 262 to selectively control this to move between open and closed positions, as described in more detail below. The pilot line 240b extends past the shear sub 28 of the landing string 10 (see FIG. 1), and as such is aligned with the shear rams 19 of the BOP 14 and thus will be cut and vented in the event of a BOP actuation.

(58) In the arrangement shown in FIG. 13, the retainer valve 24 is open and the system is configured in a FAI mode of operation. In this respect pressure is applied within the opening line 240a to open the retainer valve 24. The pilot line 240b is vented, such that the control valve 262 is configured in its normally open position, thus establishing communication of the closing line 242 with the retainer valve 24, specifically with the closing port 60 of the valve actuator 56. The closing port 60 of the valve actuator 56 may thus be vented directly through the closing line 242, rather than through a separate vent port in the control valve 262.

(59) When it is necessary to operate the retainer valve 24 to close, for example to perform a pressure test, the opening line 240a may be vented and the closing line 242 pressurised, as illustrated in FIG. 14, thus permitting the valve 24 to close. This procedure is reversed to re-open the valve 24. It should be noted that the retainer valve 24 may be operated to open and close under this FAI mode of operation without requiring activation of the control valve 262. Accordingly, the normal operation of the valve 24 may be simplified by the presence of separate opening and pilot lines 240a, 240b.

(60) FIG. 15 illustrates the control system 250 configured to provide a FC mode of operation. This is achieved by pressurising the opening line 240a to open the valve 24, pressuring the pilot line 240b to cause the control valve 262 to close, and pressurising the closing line 242, wherein the closing line pressure is prevented from communicating with the retainer valve 24 by a closed port 264 of the control valve 262. Furthermore, it should be noted that the closing port 60 of the valve actuator 56 is sealed by a closed port 72 provided in the control valve 262, thus preventing risk of drawing any fluids, for example, into the actuator 56.

(61) In the event of a failure event, such as actuation of the BOP shear rams 19 (see FIG. 7), the pilot line 240b is severed causing pressure to vent and allowing the control valve 262 to move to its normally open position, immediately communicating pressure from the closing line 242 to the retainer valve 24 to cause this to close, as illustrated in FIG. 16. Further, during activation of the BOP the opening line 240a will also be severed, causing this to be vented and thus preventing hydraulic locking within the valve 24 during closing.

(62) The control system 250 may be modified to also include a pressure accumulator, in a similar manner to the system 150 of FIG. 9. Such a modified system, in this case generally identified by reference numeral 350, is illustrated in FIG. 17. The system 350 is largely similar to the system 250 first shown in FIG. 13, and as such like features share like reference numerals, incremented by 100. As such, the system 350 includes, at least, an opening line 340a, a pilot line 340b, a closing line 342 and a normally open control valve 362. The system 350 further includes a pressure accumulator 80 which is in pressure communication with the closing line 342.

(63) The system 350 is configured in FIG. 17 in a FAI mode of operation, with the retainer valve 24 open. Specifically, zero or reduced pressure is provided within the closing and pilot lines 342, 340b, and the opening line 340a is charged to open and hold the retainer valve 24 in its open position. When the retainer valve 24 is to be closed the pressure within the opening line 340a is vented and pressure is applied in the closing line 342, as shown in FIG. 18, to cause the retainer valve 24 to close.

(64) The system 350 is shown in FIG. 19 reconfigured to operate the retainer valve 24 in a FC mode of operation. This is achieved by providing pressure within the opening line 340a to open the retainer valve 24, providing pressure within the closing line 342 while also providing pressure within the pilot line 340b which closes the control valve 362. In this respect the closing line pressure is isolated from the retainer valve 24 by the closed control valve 362. While in this configuration pressure within the closing line 342 may charge the pressure accumulator 80.

(65) In the event of a failure event, such as actuation of the BOP shear rams 19 (see FIG. 7), the pilot line 340b is severed causing pressure to vent and allowing the control valve 362 to move to its normally open position, immediately communicating pressure to the retainer valve 24 to cause this to close, as illustrated in FIG. 20. The pressure accumulator 80 can assist to provide or improve the response time of actuation pressure to close the retainer valve 24, which may have advantages when used in combination with flowing gas wells, for example.

(66) It should be understood that the embodiments described herein are merely exemplary and that various modifications may be made thereto without departing form the scope of the invention. For example, is some instances the valve under control may be any other valve within the landing string, such as a lubricator valve, SSTT valve or the like.