Horizontal valve tree leakage isolation plug

Abstract

A dual lock isolation plug for sealing both inside a horizontal valve tree and inside either an internal tree cap or a tubing hanger, the dual lock isolation plug comprising: at least one lower elasto

Description

A valve tree is a main component of an oil and gas production system. A valve tree typically consists of valves, chokes, and other control equipment, enabling operators to control and regulate the flow of hydrocarbons from wells. They play a crucial role in controlling the production process and ensuring the safety and efficiency, particularly in offshore operations. The basic configuration of horizontal valve trees is well known and understood by a person skilled in the art. Traditional dual-bore vertical valve trees (VXT) have the barrier valves located in the vertical annulus and production bores. Horizontal trees (HXT) do not have valves placed in the vertical passage, they are mono-bore, and the valves are arranged in and on the side of the main body of the valve tree. A main difference is that the tubing hanger (TH) is locked into the body of the HXT, while the TH is installed in the wellhead for a VXT. In recent years, an improved type of vertical valve tree has been used. These are known in the industry as mono-bore VXT. They have a central production passage, preferably of a larger dimension, usually 7", which is also typical of a HXT, and this embodiment combines most of the advantages of horizontal and vertical valve trees. Throughout the present disclosure, some of the major components of a horizontal valve tree are referred to, therefore in the interest of clarity they are now briefly explained. A horizontal valve tree comprises a main body with a production wing block, an annulus wing block and a wellhead connector. The main body comprises a lower body and an upper body. Although not shown in the drawings of the present application, in the interest of brevity, the function of the production wing block, annulus wing block and wellhead connector are now explained. While both the production wing block and annulus wing block play crucial roles in subsea production systems, they differ in their functions and the equipment they contain. The production wing block supports and houses equipment necessary for controlling the flow of hydrocarbons from the production tubing. The production wing block typically includes a plurality of valves and other flow control systems. The production wing block ensures the safe and efficient operation of the production process and is designed to withstand high pressures and harsh subsea conditions. The annulus wing block serves a similar purpose however the annulus wing block manages the annular space surrounding the production tubing and casing strings. The annulus wing block houses equipment for regulating the flow of fluids within the annulus, monitoring pressure, and facilitating intervention or maintenance activities. The wellhead connector is also a critical component in that it serves as the interface between the subsea wellhead and the production or drilling equipment. Essentially, it provides a secure connection between the surface and the subsea components, enabling the transfer of fluids, control signals, and power between the subsea well and the surface facilities. The upper body of a horizontal valve tree has an external wellhead profile for connecting the blowout preventer, or a subsea work-over system. A tubing hanger is installed in a centre cavity in the main body of the horizontal valve tree and forms a fluid communication between the well and the production wing block. The horizontal valve tree may be arranged in different ways. In this connection, a relatively short tubing hanger and an internal tree cap may be installed within the horizontal valve tree, the internal tree cap sitting directly above the tubing hanger. In an alternative arrangement, an extended / relatively long tubing hanger may be installed within the horizontal valve tree, without an internal tree cap sitting directly above the tubing hanger. Before oil and gas production is started, the centre passage of the horizontal valve tree will be plugged with a first wireline “crown” plug in the tubing hanger. Where present, i.e. when used in combination with a relatively short tubing hanger, the internal tree cap may either be fitted with a ball valve or a second wireline plug. Where a long tubing hanger is used, without the internal tree cap, a second wireline plug is installed above the first “crown” plug. The tubing hanger or internal tree cap will have an internal, circular locking profile typically used for latching of a running tool into the tubing hanger or internal tree cap. The annulus between the downhole tubing and the casing of the subsea well communicates to a lower annulus port in the horizontal valve tree. The port leads through a passage in the horizontal valve tree main body, i.e. through the lower body and upper body to a bore. Isolation valves are used to open and close the communication with the annulus. The upper end of the bore exits into a void in the horizontal valve tree upper body, above the tubing hanger seals. A leak from the annulus, via the isolation valves to the top of the horizontal valve tree can be blocked with a known method of installing an external pressure-holding cap, which may be locked onto the external profile of the horizontal valve tree. An operational limitation with this approach is that the external cap must first be removed to provide access for a subsequent work-over operation, typically with the use of a marine drilling riser and a subsea blowout preventer above the horizontal valve tree. It is highly desirable to provide a solution wherein removal of the external cap is not required to provide connection of the marine drilling riser I subsea blow out preventer above the horizontal valve tree. Patent document US5,450,905 discloses a method of installing a wellhead component such as a tubing hanger or tree cap utilising hydraulic pressure to move the component downward. A blowout preventer is connected to the production tree. The component has a primary metal seal and an elastomeric installation seal. The component is lowered on drill pipe through the blowout preventer into the tree bore with the installation seal sealingly engaging the bore. The primary seal is in a run-in position prior to setting. The operator closes the blowout preventer and pumps liquid through a kill line into the annular chamber below the blowout preventer. This causes the component to move downward, deforming the primary seal into the sealing engagement with the bore and the component. Patent document US2016 / 0245034A1 discloses a system and method for accessing a well. The system includes a first cap, positionable within a bore of a completion member such as a tree or a spool. The first cap has a sealable interface to accept a second cap, and the second cap when sealed to the first cap is capable of providing a barrier against a release of well fluid external to the well. Patent document US2012 / 0097399A1 discloses a bridging hanger with an expandable anchoring mechanism which lands on a casing hanger in a subsea wellhead and actuates to anchor to the casing hanger. The anchoring mechanism includes a tubular main body, and a tubular sealing sleeve. The tubing sealing sleeve is coaxial with and mounted on an exterior diameter portion of the tubular main body. The body is moveable from an upper run-in position to a lower set position relative to the sealing sleeve. The anchoring mechanism also includes a first and second locking ring, both carried by the sealing sleeve. The first locking ring engages a profile within an interior of the casing hanger in response to movement of the main body from the run-in to the set position. The second locking ring locks the tubular main body in the set position in response to movement of the main body from the run-in to the set position. It will be understood by those skilled in the art that a tubing hanger can be pulled from the horizontal valve tree without removing the tree. Further constructional and operational aspect of horizontal valve trees and associated components are well known to those of ordinary skill in the art and are not described in detail herein in the interest of brevity. The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art. The object is achieved according to the invention as provided in the claims. SUMMARY According to a first aspect of the invention, there is provided a dual lock isolation plug for sealing both inside a horizontal valve tree and inside either an internal tree cap or a tubing hanger, the dual lock isolation plug comprising: at least one lower elastomeric seal ring for forming a seal between the plug and the inside of the internal tree cap or the tubing hanger in use; an upper radially expandable split-ring; an upper elastomeric seal ring; an upper split-ring expansion sleeve and connected actuation ring, a lower radially expandable split-ring; a lower split-ring expansion sleeve and connected latch activation sleeve; a radially expandable lock ring, a lock ring actuation sleeve and a lock-ring holding sleeve; wherein the upper radially expandable split-ring and upper elastomeric seal ring are configured to be radially expanded in use by axial movement of the upper split-ring expansion sleeve, the upper radially expandable split-ring configured to be radially expanded into locking engagement with a first internal locking profile inside a horizontal valve tree upper body and the upper elastomeric seal ring configured to be radially expanded to forming a seal between the plug and the inside of the horizontal valve tree upper body above the upper radially expandable split-ring; the lower radially expandable split-ring is configured to be radially expanded, by axial movement of the lower split-ring expansion sleeve, into locking engagement with a second internal locking profile inside the internal tree cap or the tubing hanger in use; and the radially expandable lock ring is configured to be radially expanded, by axial movement of the lock ring actuation sleeve, into locking engagement with a locking profile inside the upper split-ring expansion sleeve in use; and the lock-ring holding sleeve is configured to selectively hold the lock ring actuation sleeve in use such that the radially expandable lock ring can be held in locking engagement with the upper split-ring expansion sleeve. According to a second aspect of the invention, there is provided a method for installation, seal engagement and locking of a dual lock isolation plug into a horizontal valve tree, the method comprising the following steps: providing a horizontal valve tree comprising an upper body; providing either an internal tree cap or a tubing hanger inside the horizontal valve tree; providing a dual-lock isolation plug according to the first aspect of the invention on an installation tool comprising an actuation plate; latching the installation tool onto the upper body; pushing the plug into the internal tree cap or tubing hanger and sealing the at least one lower elastomeric seal ring on the inside of the internal tree cap or tubing hanger; orienting the actuation plate such that depression of the actuation plate will push against the latch activation sleeve; depressing the actuation plate of the tool to push against the latch activation sleeve, such that the lower split-ring expansion sleeve is axially displaced and wedged-in behind the lower radially expandable split-ring, so that the lower radially expandable split-ring is radially expanded into locking engagement with the second internal locking profile inside the internal tree cap or the tubing hanger; reorienting the actuation plate such that depression of the actuation plate will push against the actuation ring; depressing the actuation plate of the tool to push against the actuation ring such that the upper split-ring expansion sleeve is axially displaced and pushes the upper radially expandable split-ring against an inclined profile of the lower split-ring expansion sleeve, so that the upper radially expandable split-ring is radially expanded into locking engagement with first internal locking profile inside a horizontal valve tree upper body and the upper elastomeric seal ring is radially expanded into a sealing position inside the horizontal valve tree upper body; reorienting the actuation plate such that depression of the actuation plate will push against the lock ring actuation sleeve; and depressing the actuation plate of the tool to push against the lock ring actuation sleeve such that the lock ring actuation sleeve is wedged-in behind the radially expandable lock ring, so that the radially expandable lock ring is radially expanded into locking engagement with the upper split-ring expansion sleeve. According to a third aspect of the invention, there is provided a method of releasing a dual lock leakage isolation plug from a horizontal valve tree with a retrieval tool, the method comprising the steps of: providing a horizontal valve tree comprising an upper body and either an internal tree cap or a tubing hanger inside the horizontal valve tree; providing a dual-lock isolation plug according to the first aspect of the invention sealed and locked within both: the horizontal valve tree upper body; and the internal tree cap or tubing hanger; providing a retrieval tool configured to latch onto the lock ring actuation sleeve in use; latching the retrieval tool onto the lock ring actuation sleeve; and axially displacing the lock ring actuation sleeve upwards thereby carrying the lower split-ring expansion sleeve, latch activation sleeve and upper split-ring expansion sleeve upwards, such that the radially expandable lock ring, upper radially expandable split-ring and lower radially expandable split-ring of the plug are released. According to a fourth aspect of the invention, there is provided a method of releasing a dual lock leakage isolation plug from a horizontal valve tree with a tool, the method comprising the steps of: providing a horizontal valve tree comprising an upper body and either an internal tree cap or a tubing hanger inside the horizontal valve tree; providing a duallock isolation plug according to the first aspect of the invention sealed and locked within both: the horizontal valve tree upper body; and the internal tree cap or tubing hanger; providing a tool configured to screw into a right-hand threaded portion inside the upper portion of the lock-ring holding sleeve; screwing the tool into the right-hand threaded portion inside the upper portion of the lock-ring holding sleeve; screwing the lock-ring holding sleeve upwards, along a left-hand threaded section at the top of the latch activation sleeve such that the radially expandable lock ring is axially displaced while the upper split-ring expansion sleeve is carried upwards by the displacement of the lock-ring holding sleeve, the upwards axial displacement of the upper split-ring expansion sleeve allowing the upper radially expandable split-ring to contract from engagement with the first internal locking profile; and pulling the lock-ring holding sleeve up, thereby carrying the lower split-ring expansion sleeve and latch activation sleeve upwards with the lock-ring holding sleeve such that the lower radially expandable split-ring can contract and release from engagement inside the internal tree cap or tubing hanger. BRIEF DESCRIPTION OF THE DRAWINGS Examples will now be described with reference to the following figures, in which Figure 1A shows a prior art horizontal valve tree with a tubing hanger and internal tree cap installed within the horizontal valve tree; Figure 1B shows a prior art horizontal valve tree with a tubing hanger installed within the horizontal valve tree; Figure 2 shows a dual lock isolation plug; Figure 3A shows the dual lock isolation plug of Figure 2 run into the horizontal valve tree of Figure 1A; Figure 3B shows a detailed view of the dual lock isolation plug of Figure 2 run into the horizontal valve tree of Figure 1A; Figure 4A shows the dual lock isolation plug of Figure 2 with the lower radially expandable split ring in locking engagement with the second locking profile of the internal tree cap; Figure 4B shows a detailed view of the dual lock isolation plug of Figure 2 with the lower radially expandable split ring in locking engagement with the second locking profile of the internal tree cap; Figure 5 shows the dual lock isolation plug of Figure 2 with the lower radially expandable split ring in locking engagement with the second internal locking profile of the internal tree cap and the upper radially expandable split ring in locking engagement with the first internal locking profile of the horizontal valve tree; Figure 6 shows plug in the position shown in Figure 5 additionally with the radially expandable lock ring locked into the internal locking profile of the upper split-ring expansion sleeve; Figures 7A and 7B show a subsea plug installation tool arranged with the plug of Figure 2 prior to the first locking step; Figures 7C and 7D show the end position of the first locking step of the plug of Figure 2; Figures 8A and 8B show the initial position of a second locking step of the plug of Figure 2; Figures 8C and 8D show the end position of the second locking step of the plug of Figure 2; Figures 9A to 9D show the expansion of the radially expandable lock ring in the plug of Figure 2; Figures 10A and 10B show a lower portion of a retrieval tool deposed in the upper portion of the plug of Figure 2; Figures 11A and 11B show a method of releasing the plug of Figure 2; Figure 12 shows a first single lock isolation plug not falling within the scope of the invention; and Figure 13 shows a second single lock isolation plug not falling within the scope of the invention. The drawings are shown in a schematic and simplified manner, and features that are not necessary for explaining the invention may be left out. Identical reference numerals refer to identical or similar features in the drawings. For clarity reasons, some elements may in some of the figures be without reference numerals. The various features shown in the drawings may not necessarily be drawn to scale. A person skilled in the art will understand that the figures are just principal drawings. The relative proportions of individual elements may also be distorted. Any positional indications, such as for example upper and lower, refer to the position shown in the figures. DETAILED DESCRIPTION OF THE DRAWINGS Referring firstly to Figures 1A and 1B, there is provided a horizontal valve tree 1000 according to the prior art. Although not visible in Figures 1A and 1B, the horizontal valve tree 1000 is coupled to a wellhead. The horizontal valve tree 1000 comprises a main body 1100, with a production wing block (not shown), an annulus wing block, and a wellhead connector (not shown). The main body 1100 comprises an upper body 1110 and a lower body 1120. The upper body 1110 comprises an external profile 1111 for connection of a BOP (not shown), - or a subsea work-over system (not shown). The external profile 1111 may be an industry standard H4 wellhead profile. Alternatively, the external profile 1111 may be another profile configured for providing secure connection of a blow-out preventer or subsea work-over system configured with a corresponding profile, such that the two profiles mate to provide secure sealing and attachment in use. The upper body 1110 further comprises a first internal locking profile 1112, as will be explained later. A tubing hanger 2000 is installed in a centre cavity 1101 in the main body 1100 of the horizontal valve tree 1000 and forms a fluid communication channel between the well and the production wing block (not shown). As explained previously, the horizontal valve tree 1000 may be used with different configurations of equipment. Figure 1A shows the main body 1100 of the horizontal valve tree 1000, configured with a tubing hanger in the form of a short tubing hanger 2000A and an internal tree cap 3000 above the tubing hanger 2000A. An alternative horizontal valve tree 1000 configuration with an extended / relatively long tubing hanger 2000B is shown in Figure 1B. The tubing hangers 2000A, 2000B are sealed against the centre cavity 1101 by appropriately configured tubing hanger seals 2001 A, 2001B. Referring to both Figures 1A and 1B, before oil and gas production is started, the centre passage 1101 of the horizontal valve tree 1000 will be plugged with a first wireline “crown” plug (not shown) in the tubing hanger 2000A, 2000B. The internal tree cap 3000 in Figure 1A may either be fitted with a ball valve (not shown) or a second wireline plug (not shown). The horizontal valve tree 1000 in Figure 1B, with the long tubing hanger 2000B, will have a second wireline plug (not shown) installed above the first “crown” plug (not shown). The internal tree cap 3000 in Figure 1A and the tubing hanger 2000B in Figure 1B, each have respective second internal locking profiles 3100, 2100B the location of which is indicated in Figures 1A and 1B, however the details of which cannot be seen clearly in Figures 1A and 1B and will be explained further later. The second internal locking profiles 3100, 2100B in the presently described example are circular locking profiles. The second internal locking profiles 3100, 2100B are normally used for latching of a running tool into the internal tree cap 3000 and tubing hanger 2000B, respectively. It will be understood that the term “second” is not intended to mean that the internal tree cap 3000 or tubing hanger 2000B necessarily comprises a first internal locking profile, but rather to distinguish between the aforementioned first internal locking profile 1112 provided on the inside of the horizontal valve tree 1000. Still referring to Figures 1A and 1B, an annulus (not shown) between the downhole tubing and the casing of the subsea well is fluidly connected (and continues as) an annulus 1102 formed between the tubing hanger 2000A and the horizontal valve tree 1000 in Figure 1A and between the tubing hanger 2000B and the horizontal valve tree 1000 in Figure 1B. The annulus 1102 has an upper termination point at the tubing hanger seals 2001 A, 2001B. Referring to both Figures 1A and 1B, the horizontal valve tree 1000 main body 1100 comprises a fluid communication channel 1130 extending through the horizontal valve tree 1000 main body 1100 from the lower body 1120 to the upper body 1110. The fluid communication channel 1130 provides fluid communication between the annulus 1102, between the tubing hanger 2000A, 2000B and the horizontal valve tree 1000, and a void 1103 above the tubing hanger seals 2001 A, 2001B between the horizontal valve tree 1000 upper body 1110 and the internal tree cap 3000 in Figure 1A and between the horizontal valve tree 1000 upper body 1110 and the tubing hanger 2000B in Figure 1B. The fluid communication channel 1300 comprises first 1131, second 1132 and third 1133 isolation valves which are used to open and close the communication with the annulus 1102. The isolation valves 1131, 1132, 1133 are closed when it is not desired to fluidly communicate with the annulus 1102. Should the isolation valves 1131, 1132, 1133 leak in use, fluid from the annulus 1102 may reach the void 1103 and further leak to the surrounding environment. As mentioned previously, a leak from the annulus 1102, via the isolation valves 1131, 1132, 1133 to the top of the horizontal valve tree 1000, can be blocked with a known method of installing an external pressure-holding cap 4000 as shown in Figures 1A and 1B. In such a procedure, the external pressure-holding cap 4000 may be locked onto the external profile 1111 of the horizontal valve tree 1000. As explained earlier, the external profile 1111 may be a H4 profile. An operational limitation with this approach is that the external pressure-holding cap 4000 must first be removed to provide access for a sub-sequent work-over operation, typically with the use of a marine drilling riser and a subsea blowout preventer above the horizontal valve tree 1000. If present, an external pressure-holding cap 4000 is removed through a known procedure of subsea well intervention prior to the plug 5000 being installed and locked. Referring firstly to Figure 2, a dual-lock leakage isolation plug 5000 is shown in cross-sectional view. The plug 5000 can be used with either of the configurations shown in Figure 1A and Figure 1B. The plug 5000 is referred to herein as a “dual-lock” plug 5000 because, as will now be explained in more detail, the plug 5000 locks at two points in use. When used in the configuration shown in Figure 1A, the plug 5000 locks against the internal tree cap 3000 and the upper body 1110 of the horizontal valve tree 1000. When used in the configuration shown in Figure 1B, the plug 5000 locks against the tubing hanger 2000B and the upper body 1110 of the horizontal valve tree 1000. Still referring to Figure 2, the plug 5000 comprises seal grooves 5110 in the form of circumferential indents in the plug 5000 which are shaped and sized to receive appropriately shaped and sized lower seals 5111. As will be explained, the lower seals 5111 are provided to facilitate sealing between the plug 5000 and either the internal tree cap 3000 or the tubing hanger 2000B, depending on the configuration (Figure 1A or Figure 1B) as previously discussed. The lower seals 5111 are provided as lower elastomeric seal rings. The plug 5000 further comprises a lower radially expandable split-ring 5120, an upper radially expandable split-ring 5130, an upper elastomeric seal ring 5140 and a radially expandable lock ring 5150. The plug 5000 further comprises, a lower split-ring expansion sleeve 5210, a latch activation sleeve 5220, an upper split-ring expansion sleeve 5230, an actuation ring 5240, and a lock ring actuation sleeve 5250. The lower split-ring expansion sleeve 5210 comprises an inclined profile 5211 sloping outwards from the centre of the plug 5000. The upper radially expandable split-ring 5130 has a corresponding profile, as can be seen in Figure 2. The upper split-ring expansion sleeve 5230 comprises an internal locking profile 5231 registered with the radially expandable lock ring 5150, as will be explained further later. The plug 5000 is intended to be inserted into and provide locking against industry standard components. In this regard, the skilled person will be aware of the standard configuration of an internal tree cap 3000, a tubing hanger 2000B and a horizontal valve tree 1000. For the avoidance of doubt, the necessary components of these apparatuses are now explained, with reference to Figures 3A and 3B before the interaction of the plug 5000 with each of these apparatuses in use is explained. It will therefore be understood that although the invention is now explained with reference to internal tree cap 3000, the invention works equally well with the tubing hanger 2000B. For the avoidance of doubt, Figures 3A and 3B provide a schematic representation of the top of the horizontal valve tree 1000 where the tubing hanger 2000A (not visible in Figures 3A and 3B) has been installed and the internal tree cap 3000 has been installed atop the tubing hanger 2000A. Again for the avoidance of doubt it is reiterated that the plug 5000 can be used with either of the configurations of Figure 1A or Figure 1B, and that the plug 5000 will lock into the internal tree cap 3000 and the horizontal valve tree 1000 if the configuration in Figure 1A is provided, and into the tubing hanger 2000B and the horizontal valve tree 1000 if the configuration in Figure 1B is provided. As it can clearly be seen referring briefly again to Figures 1A and 1B, the upper portion of the internal tree cap 3000 and the upper portion of the tubing hanger 2000B, are similar and therefore in the interest of brevity, it is not necessary to explain the locking of the plug 5000 into both the internal surface of the internal tree cap 3000 and the internal surface of the tubing hanger 2000B. However, it will be appreciated by the skilled person that the plug 5000 may equally be used in either of the configurations. Figures 3A and 3B show the plug 5000 described with reference to Figure 2 delivered into the internal tree cap 3000. Features of the internal tree cap 3000 (and so also the tubing hanger 2000B where the tubing hanger 2000B is an extended tubing hanger as in Figure 1B) are now explained before the sealing of the plug 5000 is explained. As previously mentioned, the internal tree cap 3000 comprises a second locking profile 3100 in the form of a circular locking profile. The second locking profile 3100 is provided as standard on the internal tree cap 3000 and is normally used for latching of a running tool into the internal tree cap 3000. The internal tree cap 3000 further comprises a landing surface 3200 such that the plug 5000 can be run into the internal tree cap 3000 and land on the landing surface 3200, as will be explained later. As also previously mentioned, the horizontal valve tree 1000 upper body 1110 comprises a first internal locking profile 1112 in the form of a circular locking profile. The installation of the plug 5000 into the horizontal valve tree 1000 is now described. As shown in Figure 3A, the plug 5000 is run into the horizontal valve tree 1000 and internal tree cap 3000 until the plug 5000 rests against the landing surface 3200 in the internal tree cap 3000. The lower elastomeric seal rings 5111 will seal against the inside of the internal tree cap 3000, as can be seen in Figure 3A. The radially expandable lock ring 5150, the upper radially expandable split-ring 5130, and the lower radially expandable split-ring 5120 are shown as not yet engaged. Figure 3B shows a detailed view of the cross-sectional view shown in Figure 3A. Although the radially expandable lock ring 5150, the upper radially expandable split-ring 5130, and the lower radially expandable split-ring 5120 are shown as not yet engaged, it can be seen in Figure 3B that the radially expandable lock ring 5150 is configured to move into locking engagement with the internal locking profile 5231 of the upper split-ring expansion sleeve 5230. It can also be seen that the upper radially expandable split-ring 5130 is configured to move into locking engagement with the first internal locking profile 1112 of the upper body 1110 and that the lower radially expandable split-ring 5120 is configured to move into locking engagement with the second internal locking profile 3100 of the internal tree cap 3000. The locking of the plug 5000 is now explained by reference to Figures 4A to 6, wherein Figures 4A and 4B show a first locking step wherein the plug 5000 is locked inside the internal tree cap 3000 and Figures 5 and 6 show a second locking step wherein the plug 5000 is locked inside the horizontal valve tree 1000. Referring to Figure 4A and the detail view in Figure 4B, it can be seen that the lower splitring expansion sleeve 5210 is connected to the latch activation sleeve 5220. Axial displacement from depression of the latch activation sleeve 5220 has pushed the lower end of the lower split-ring expansion sleeve 5210 into a wedged position behind the lower radially expandable split-ring 5120, which has been radially expanded into locking engagement with the second locking profile 3100 of the internal tree cap 3000. Axial displacement from depression of the latch activation sleeve 5220 has also moved the inclined profile 5211 of the lower split-ring expansion sleeve 5210 into a position whereby sliding of the upper radially expandable split-ring 5130 on the inclined profile 5211 will move the upper radially expandable split-ring 5130 into locking engagement with the first internal locking profile 1112 as will be explained. In this first locking step presently described, the plug 5000 is locked inside the internal tree cap 3000 but it not yet locked into the horizontal valve tree 1000 (which happens in the second locking step). In this connection, the upper radially expandable split-ring 5130 and radially expandable lock ring 5150 are not yet locked into the respectively locking profiles, i.e. first internal locking profile 1112 of the upper body 1110 for the upper radially expandable split-ring 5130 and the internal locking profile 5231 for the radially expandable lock ring 5150. The second locking step is now described with reference to Figures 5 and 6. Firstly, the upper split-ring expansion sleeve 5230 is pushed downwards by the actuation ring 5240 which the upper split-ring expansion sleeve 5230 is connected to. The upper radially expandable split-ring 5130 is pushed against the inclined profile 5211 of the lower split-ring expansion sleeve 5210 and moves into locking engagement with the first internal locking profile 1112 as can be seen in Figure 5. At the same time, the upper elastomeric seal ring 5140 is displaced to engage and seal against the inside of the upper body 1110 of the horizontal valve tree 1000 above the first internal locking profile 1112. Secondly, referring to Figure 6, the lock ring actuation sleeve 5250 is depressed, so that the lock ring actuation sleeve 5250 is wedged-in behind the radially expandable lock ring 5150. The radially expandable lock ring 5150 is thus radially expanded into a locking engagement with the upper split-ring expansion sleeve 5230. More specifically, the radially expandable lock ring 5150 is radially expanded into a locking engagement with the internal locking profile 5231 of the upper split-ring expansion sleeve 5230. The upper split-ring expansion sleeve 5230 will now be kept in position, thus maintaining a secure dual lock and seal by the plug 5000. The dual-lock and seal engagement sequence is shown with 2D and 3D cross-section views in the figures 7 A-D, 8 A-D, and 9 A-D. The latch activation sleeve 5220, actuation ring 5240 and lock ring actuation sleeve 5250 are depressed with a subsea plug installation tool 6000 that is latched onto the top of the horizontal valve tree 1000 upper body 1110. The tool 6000 comprises an actuation plate 6100, fixed to a tool piston 6200. The actuation plate 6100 is designed to push down the latch activation sleeve 5220, actuation ring 5240 and lock ring actuation sleeve 5250 individually, depending on the rotational orientation for the respective operational steps. The locking sequence can alternatively be performed by pushing down the latch activation sleeve 5220, actuation ring 5240 and lock ring actuation sleeve 5250 directly, with a hydraulic or mechanical tool (not shown). Figures 7A and 7B show the subsea plug installation tool 6000 prior to a first locking step, where it will push down the latch activation sleeve 5220, which in turn will push down the lower split-ring expansion sleeve 5210 into a wedged position behind the lower radially expandable split-ring 5120 thereby radially expanding a lower split-ring expansion sleeve 5120 into locking engagement with the second internal locking profile 3100 in the internal tree cap 3000. Figures 7C and 7D show the end position of the first locking step. Figures 8A and 8B show the initial position, and the Figures 8C and 8D show the end position of a second locking step, where the plug 5000 is locked inside the horizontal valve tree 1000 upper body 1110, while the upper elastomeric seal ring 5140 is pushed into a final sealing position against a circumferential sealing surface inside the horizontal valve tree 1000 upper body 1110 above the first internal locking profile 1112. The actuation plate 6100 has been rotationally re-oriented to push down the actuation ring 5240, which is connected to the upper split-ring expansion sleeve 5230, which again pushes the upper radially expandable split-ring 5130 against the inclined profile 5211 and into locking engagement as previously explained. As also previously explained, the upper split-ring expansion sleeve 5230 has a dual purpose, in that it also displaces the upper elastomeric seal ring 5140 to its sealing position inside the horizontal valve tree 1000 upper body 1110. Referring now to Figures 9A to 9D the expansion of the radially expandable lock ring 5150 is now explained further. From the position shown in Figures 8C and 8D, the actuation plate 6100 is again rotationally re-oriented such that the actuation plate 6100 can depress the lock ring actuation sleeve 5250, so that a radially expandable lock ring 5150 will be radially expanded into locking engagement with the upper split-ring expansion sleeve 5230, which will now be kept securely in place. This is shown in more detail in figure 6. Secondly, referring to Figure 6, the lock ring actuation sleeve 5250 is depressed, so that the lock ring actuation sleeve 5250 is wedged-in behind the radially expandable lock ring 5150. The radially expandable lock ring 5150 is thus radially expanded into a locking engagement with the upper split-ring expansion sleeve 5230. More specifically, the radially expandable lock ring 5150 is radially expanded into a locking engagement with the internal locking profile 5231 of the upper split-ring expansion sleeve 5230. The upper split-ring expansion sleeve 5230 will now be kept in position, thus maintaining a secure dual lock and seal by the plug 5000. Reference is now made to the Figures 10A and 10B, which illustrate a first method for release of the plug 5000 from the horizontal valve tree 1000. Plug release and retrieval may be performed with a drill-string deployed tool through a marine drilling riser and BOP stack (not shown), or as an open water tool operation (not shown). Figure 10A shows a lower portion of a retrieval tool 7000 deposed in the upper portion of the plug 5000. The tool 7000 includes a set of locking dogs 7100, which will be radially extended through operation of the retrieval tool 7000 and will, in a fully extended position, be wedged between two inwards protruding profiles 5251, 5252 of the lock ring actuation sleeve 5250, also shown in 3D in figure 10B. The lock ring actuation sleeve 5250 shown in Figure 10A comprises two inwards protruding profiles 5251, 5252 however it will be understood that the lock ring actuation sleeve 5250 shown in Figures 2, 3A-B, 4A-B, 5, 6, 7A-D, 8A-D, 9A-D has just one inwards protruding profile 5251. Although not necessary, an advantage of providing two inwards protruding profiles 5251, 5252 is that it provides a tool interface for displacement of the sleeve up or down, as required. With the set of locking dogs 7100 of the retrieval tool 7000 fully engaged with the lock ring actuation sleeve 5250, the tool 7000 can be operated to displace the lock ring actuation sleeve 5250 with an upwards axial stroke. The radially expandable lock ring 5150 will thus get room to contract and will release from its engagement with the upper split-ring expansion sleeve 5230. When the lock ring actuation sleeve 5250 is displaced further upwards, it will hit a lock-ring holding sleeve 5260 which again will carry with it the upper split-ring expansion sleeve 5230 and the latch activation sleeve 5220. Upwards displacement of upper split-ring expansion sleeve 5230 will in turn provide room for the upper radially expandable split-ring 5130 to contract from engagement with the first internal locking profile 1112. The lower split-ring expansion sleeve 5210, fixed to the latch activation sleeve 5220, will also be carried upwards, and will be pulled out from behind the lower radially expandable split-ring 5120, providing room for the lower radially expandable split-ring 5120 to contract from its engagement with the second internal locking profile 3100 inside the internal tree cap 3000. The plug 5000 will now be free to be pulled out of the horizontal valve tree 1000 with the retrieval tool 7000. Reference is now made to the Figures 11A and 11B, where a second method of plug 5000 release is illustrated. A plug retrieval tool is not shown in this example. The initial, locked and secured state of the upper portion of the plug 5000 is shown in Figure 11 A. Figure 11B shows a similar image, with the upper radially expandable split-ring 5130 released. Release and retrieval of the plug 5000 may be done with a drill-string deployed tool through a marine drilling riser and BOP stack, (not shown), or as an open water tool operation (not shown). This second method for releasing the plug 5000 is based on a tool (not shown) being screwed into a first, right-hand threaded portion 5261 inside the upper portion of a lockring holding sleeve 5260. This tool interface is shown on the figures 10A-B and 11A-B, but is not included in the examples of the invention shown in Figures 2, 3A-B, 4A-B, 5, 6, 7A-D, 8A-D and 9A-D. The lock-ring holding sleeve 5260 is in threaded engagement with the top end of the latch activation sleeve 5220 through a second, left-hand threaded portion 5262. With the tool fully connected to the top of the lock-ring holding sleeve 5260, it is used to screw the lock-ring holding sleeve 5260 upwards, along the left-hand threaded section at the top of the latch activation sleeve 5220, as illustrated by figure 11B. The upper split-ring expansion sleeve 5230 and the radially expandable lock ring 5150 will then be carried upwards with the axial displacement of the lock-ring holding sleeve 5260, while the lock ring actuation sleeve 5250 is rotated by the screwing action but will not be axially displaced. The upwards axial displacement of the upper split-ring expansion sleeve 5230 will provide room for the upper radially expandable split-ring 5130 to contract from engagement with the first internal locking profile 1112. With the upper radially expandable split-ring 5130 now released, the lock-ring holding sleeve 5260 is pulled upwards with the tool. The lock-ring holding sleeve 5260 is still in partial, threaded engagement with the latch activation sleeve 5220, which is again in threaded engagement with the lower split-ring expansion sleeve 5210, which will thus be pulled out from behind the lower radially expandable split-ring 5120. The lower radially expandable split-ring 5120 will thus have room to contract from its engagement with the second internal locking profile 3100 inside the internal tree cap 3000. With the dual lock of the plug 5000 now fully released, the plug 5000 will now be free to be recovered from the horizontal valve tree 1000 with the tool. The plug 5000 presently described may allow a blow-out preventer and optionally also a marine drilling riser to be installed on top of the horizontal valve tree 1000 without first having to remove an external pressure-holding cap 4000. The plug 5000 may be pulled to the surface through the blow out preventer and, if provided, marine drilling riser, so that subsequent well operations may proceed. It will be appreciated by a person skilled in the art that additional seals may be preferable and are not described here in the interest of brevity as the provision and arrangement of such additional seals will be well known to the skilled person. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. DESCRIPTION OF NON-CLAIM ED EXAMPLES NOT FALLING WITHIN THE SCOPE OF THE INVENTION Figures 12 and 13 show examples of isolation plugs relating to the same technical field but not falling within the scope of the invention. Like reference numerals have been used to indicate like parts compared with the examples described with reference to Figures 2 to 11, with the addition of prime (‘). In the first example, shown in Figure 12, an isolation plug 5000’ has been delivered into a tubing hanger 2000B’ and horizontal valve tree 1000’. Although Figure 12 illustrates the plug 5000’ used with a tubing hanger 2000B’, it will be understood that the plug 5000’ may equally be used with an internal tree cap (not shown), depending on the horizontal valve tree 1000’ configuration. Still referring to Figure 12, the upper radially expandable split-ring and the lower split-ring expansion sleeve are eliminated compared with the previously described plug 5000. The lower radially expandable split-ring 5120’ is radially expanded into engagement with the second internal locking profile 3100’ inside the tubing hanger 2000B’ by means of the upper split-ring expansion sleeve 5230’, which still displaces the upper elastomeric seal ring 5140’, but has in this case been adapted to replace the function of the lower split-ring expansion sleeve 5210, (shown for instance in Figure 5). Installation, locking, release and recovery of a single-lock plug 5000’ will be based on adaptation of the methods that have been described for a dual-lock plug 5000. In a second example, shown in Figure 13, an isolation plug 5000” has been delivered into a tubing hanger 2000B” and horizontal valve tree 1000”. The lower radially expandable split-ring and the lower split-ring expansion sleeve (included for instance in figure 5), are removed. The lower portion of the latch activation sleeve, used to push down the lower split-ring expansion sleeve, is also removed. The upper portion of the former latch activation sleeve is provided as a stationary item in this example and kept in threaded engagement with the body of the plug 5000”. The body of the plug 5000” has in this case been shaped with an inclined profile 5211” sloping outwards from the centre of the plug 5000” and identical with the shape of the lower split-ring expansion sleeve 5210 (shown for instance in figure 5). The function and shape of the upper split-ring expansion sleeve 5230” is identical with what is shown for instance in figure 5. The actuation ring 5240” is depressed and has axially displaced the connected, upper split-ring expansion sleeve 5230”, which again has pushed the upper radially expandable split-ring 5130” against the inclined profile 5211” of the plug 5000” body. The upper radially expandable split-ring 5130” has thus been radially expanded into a locking engagement with the first internal locking profile 1112” inside the upper body 1110” of the horizontal valve tree 1000” above the tubing hanger 2000B”. The upper split-ring expansion sleeve 5230” has also moved the upper elastomeric seal ring 5140” to its sealing position inside the horizontal valve tree 1000” upper body 1110”. Installation, locking, release and recovery of the single-lock plug 5000” will be based on adaptation of the methods described for a dual-lock plug 5000. Depression of the lock ring actuation sleeve 5250” will radially expand the radially expandable lock ring 5150” into a locking engagement with the combined upper body seal and the upper split-ring expansion sleeve 5230” identical with what is shown in Figure 6 for the dual locking of the plug 5000. CLAUSES A selection of numbered clauses is now provided. Clause 1. Dual lock isolation plug, characterized in that the plug comprises; actuation rings / sleeves, for plug locking, sealing, release and; at least one elastomeric seal ring, sealing inside a tubing hanger (TH), or internal tree cap (ITC), and; a split-ring, radially expanded into locking engagement with a locking profile inside a TH or ITC, and; a split-ring, radially expanded into locking engagement with a locking profile inside a HXT upper body, and; an elastomeric seal-ring, sealing inside a HXT upper body, and; a lock-ring, radially expanded into locking engagement with an actuation sleeve. Clause 2. Method for installation, seal engagement and locking of a dual lock isolation plug into a subsea horizontal valve tree, HXT, characterised in that the operation consists of the following steps: a) an installation tool, with an isolation plug is latched onto an upper body of a HXT, whereby the plug is pushed into, and will seal with at least one elastomer seal ring, inside a TH or ITC, depending on the HXT configuration; b)The tool is used to depress an actuation plate of the tool, oriented to push against a latch activation sleeve, which is connected to the top of an expansion sleeve, which will thus be axially displaced and wedged-in behind a split-ring, so that it will be radially expanded into locking engagement with a locking pro-file in a TH or ITC; c) The tool actuation plate is re-oriented to push against an actuation ring, which is connected to the top of an actuation sleeve, which will thus be axially displaced, and push a split-ring against an inclined profile of the expansion sleeve, whereby the split-ring will be radially expanded into locking engagement with a locking groove in the HXT upper body. The actuation sleeve also displaces an elastomer seal ring into a sealing position inside the HXT upper body; d) The tool actuation plate is re-oriented to depress an actuation sleeve, to be wedged-in behind a split-ring, so that it is radially expanded into locking engagement with the actuation sleeve. Clause 3. Single lock isolation plug, characterized in that the plug comprises; actuation rings / sleeves, for plug locking, sealing, release, and; at least one elastomeric seal ring, sealing inside a TH or ITC, and; a latch split-ring, radially expanded into locking engagement with a locking profile inside a TH or ITC, and; an elastomeric seal-ring, sealing inside a HXT upper body, and; a lock-ring, radially expanded into locking engagement with an actuation sleeve. Clause 4. Method for installation, seal engagement and locking of a single lock isolation plug into a subsea horizontal valve tree, HXT, characterised in that the operation consists of the following steps: a) An installation tool, with an isolation plug is latched onto an upper body of a HXT, whereby the plug is pushed into, and will seal with at least one elastomer seal ring, inside a TH or ITC, depending on the HXT configuration; b) The tool is used to depress an actuation plate of the tool, oriented to push against an actuation ring, which is connected to an actuation sleeve, which will be axially displaced and wedged-in behind a split-ring, so that the split-ring will be radially expanded into locking engagement with a locking profile in-side a TH or ITC. The actuation sleeve also displaces an elastomer seal ring into a sealing position inside the HXT upper body; c) The tool actuation plate is re-oriented to depress an actuation sleeve, to be wedged-in behind a split-ring, so that it is radially expanded into locking engagement with the actuation sleeve. Clause 5. Single lock isolation plug, characterized in that the plug comprises; actuation rings / sleeves, for plug locking, sealing, release and; at least one elastomeric seal ring, sealing inside a TH or ITC, and; a latch split-ring, designed to be pushed against an inclined profile, and be radially expanded into locking engagement with a locking profile inside a HXT upper body, and; an elastomeric seal-ring, sealing inside a HXT upper body, and; a lock-ring, radially expanded into locking engagement with an actuation sleeve. Clause 6. Method for installation, seal engagement and locking of a single lock isolation plug into a subsea horizontal valve tree, HXT, characterised in that the operation consists of the following steps: a) An installation tool, with an isolation plug is latched onto an upper body of a HXT, whereby the plug will be pushed into and will seal with at least one elastomer seal ring inside a TH or ITC, depending on the HXT configuration; b) The tool is used to depress an actuation plate of the tool, oriented to push against an actuation ring, which is connected to the top of an actuation sleeve which will thus be axially displaced, and push a split-ring against an inclined profile on the outside of the plug body, whereby the split-ring will be radially expanded into locking engagement with a locking groove in the upper body. The actuation sleeve also displaces an upper elastomer seal ring into a sealing position inside the HXT upper body; c) The tool actuation plate is re-oriented to depress an actuation sleeve, to be wedged-in behind a split-ring, so that it is radially expanded into locking engagement with the actuation sleeve. Clause 7. A first method, characterised in that a leakage isolation plug is released from a HXT upper body with a tool, where the tool is used to latch on-to and upwards displace an actuation sleeve, so that; locking split-rings of a dual lock plug are released when the respective activation sleeves are carried along with the actuation sleeve, or; locking split-rings of a plug with single lock in a TH / ITC are re-leased when the single lock activation sleeve is carried along with the actuation sleeve, or; locking split-rings of a plug with single lock inside an upper HXT body are released when the single lock activation sleeve is carried along with the actuation sleeve. Clause 8. A second method, characterised in that a leakage isolation plug is released from a HXT upper body with a tool, where the tool is first screwed into a right-hand threaded portion inside the upper portion of a lock-ring holding sleeve, and the tool is then used to; screw a lock-ring holding sleeve upwards, along a left-hand threaded section at the top of the TH / ITC latch activation sleeve. The lock-ring will thus be axially displaced while the latch split-ring actuation sleeve is carried upwards by the displacement of the lock-ring holding sleeve; the upwards axial displacement of the latch split-ring actuation sleeve will allow the upper lock-ring to contract from engagement with the locking groove, and; the lock-ring holding sleeve is then pulled up, carrying with it the actuation sleeves for the lower latch split-ring, which will get room to con-tract and release from its engagement inside the TH / ITC, or; if a plug has a single lock into a TH / ITC, the actuation sleeve is carried upwards with the lock-ring holding sleeve, and the lower latch split-ring will get room to contract and release from its engagement inside the TH / ITC, or; if a plug has a single lock into a HXT upper body, the actuation sleeve is carried upwards with the lockring holding sleeve, and the upwards axial displacement will allow the upper lock-ring to contract from engagement with the locking groove.

Claims

1. A dual lock isolation plug (5000) for sealing both inside a horizontal valve tree (1000) and inside either an internal tree cap (3000) or a tubing hanger (2000B), the dual lock isolation plug (5000) comprising:at least one lower elastomeric seal ring (5111) for forming a seal between theplug (5000) and the inside of the internal tree cap (3000) or the tubing hanger (2000B) in use;an upper radially expandable split-ring (5130);an upper elastomeric seal ring (5140);an upper split-ring expansion sleeve (5230) and connected actuation ring (5240),a lower radially expandable split-ring (5120);a lower split-ring expansion sleeve (5210) and connected latch activation sleeve (5220);a radially expandable lock ring (5150), a lock ring actuation sleeve (5250) and alock-ring holding sleeve (5260); whereinthe upper radially expandable split-ring (5130) and upper elastomeric seal ring(5140) are configured to be radially expanded in use by axial movement of the upper split-ring expansion sleeve (5230), the upper radially expandable split-ring (5130) configured to be radially expanded into locking engagement with a first internal locking profile (1112) inside a horizontal valve tree (1000) upper body (1110) and the upper elastomeric seal ring (5140) configured to be radially expanded to forming a seal between the plug (5000) and the inside of the horizontal valve tree (1000) upper body (1110) above the upper radially expandable split-ring (5130);the lower radially expandable split-ring (5120) is configured to be radially expanded, by axial movement of the lower split-ring expansion sleeve (5210), into locking engagement with a second internal locking profile (3100) inside the internal tree cap (3000) or the tubing hanger (2000B) in use; andthe radially expandable lock ring (5150) is configured to be radially expanded, byaxial movement of the lock ring actuation sleeve (5250), into locking engagement with a locking profile (5231) inside the upper split-ring expansion sleeve (5230) in use; andthe lock-ring holding sleeve (5260) is configured to selectively hold the lock ring actuation sleeve (5250) in use such that the radially expandable lock ring (5150) can be held in locking engagement with the upper split-ring expansion sleeve (5230).

2. A method for installation, seal engagement and locking of a dual lock isolation plug (5000) into a horizontal valve tree (1000), the method comprising the following steps:a) providing a horizontal valve tree (1000) comprising an upper body (1110);b) providing either an internal tree cap (3000) or a tubing hanger (2000B) inside the horizontal valve tree (1000);c) providing a dual-lock isolation plug (5000) according to claim 1 on an installation tool (6000) comprising an actuation plate (6100);d) latching the installation tool (6000) onto the upper body (1110);e) pushing the plug (5000) into the internal tree cap (3000) or tubing hanger (2000B) and sealing the at least one lower elastomeric seal ring (5111) on the inside of the internal tree cap (3000) or tubing hanger (2000B);f) orienting the actuation plate (6100) such that depression of the actuation plate (6100) will push against the latch activation sleeve (5220);g) depressing the actuation plate (6100) of the tool (6000) to push against the latch activation sleeve (5220), such that the lower split-ring expansion sleeve (5210) is axially displaced and wedged-in behind the lower radially expandable split-ring (5120), so that the lower radially expandable split-ring (5120) is radially expanded into locking engagement with the second internal locking profile (3100) inside the internal tree cap (3000) or the tubing hanger (2000B);h) reorienting the actuation plate (6100) such that depression of the actuation plate (6100) will push against the actuation ring (5240);i) depressing the actuation plate (6100) of the tool (6000) to push against the actuation ring (5240) such that the upper split-ring expansion sleeve (5230) is axially displaced and pushes the upper radially expandable split-ring (5130) against an inclined profile (5211) of the lower split-ring expansion sleeve 5210, so that the upper radially expandable split-ring (5130) is radially expanded into locking engagement with first internal locking profile (1112) inside a horizontal valve tree (1000)upper body (1110) and the upper elastomeric seal ring (5140) is radially expanded into a sealing position inside the horizontal valve tree (1000) upper body (1110);j) reorienting the actuation plate (6100) such that depression of the actuation plate (6100) will push against the lock ring actuation sleeve (5250); andk) depressing the actuation plate (6100) of the tool (6000) to push against the lock ring actuation sleeve (5250) such that the lock ring actuation sleeve (5250) is wedged-in behind the radially expandable lock ring (5150), so that the radially expandable lock ring (5150) is radially expanded into locking engagement with the upper split-ring expansion sleeve (5230).

3. A method of releasing a dual lock leakage isolation plug (5000) from a horizontal valve tree (1000) with a retrieval tool (7000), the method comprising the steps of:a) providing a horizontal valve tree (1000) comprising an upper body (1110) and either an internal tree cap (3000) or a tubing hanger (2000B) inside the horizontal valve tree (1000);a) providing a dual-lock isolation plug (5000) according to claim 1 sealed and locked within both:the horizontal valve tree (1000) upper body (1110); andthe internal tree cap (3000) or tubing hanger (2000B);b) providing a retrieval tool (7000) configured to latch onto the lock ring actuation sleeve (5250) in use;c) latching the retrieval tool (7000) onto the lock ring actuation sleeve (5250); andd) axially displacing the lock ring actuation sleeve (5250) upwards thereby carrying the lower split-ring expansion sleeve (5210), latch activation sleeve (5220) and upper split-ring expansion sleeve (5230) upwards, such that the radially expandable lock ring (5150), upper radially expandable split-ring (5130) and lower radially expandable split-ring (5120) of the plug (5000) are released.

4. A method of releasing a dual lock leakage isolation plug (5000) from a horizontal valve tree (1000) with a tool, the method comprising the steps of:b) providing a horizontal valve tree (1000) comprising an upper body (1110) and either an internal tree cap (3000) or a tubing hanger (2000B) inside the horizontal valve tree (1000);e) providing a dual-lock isolation plug (5000) according to claim 1 sealed and locked within both:the horizontal valve tree (1000) upper body (1110); andthe internal tree cap (3000) or tubing hanger (2000B);f) providing a tool configured to screw into a right-hand threaded portion (5261) inside the upper portion of the lock-ring holding sleeve (5260);g) screwing the tool into the right-hand threaded portion (5261) inside the upper portion of the lock-ring holding sleeve (5260);h) screwing the lock-ring holding sleeve (5260) upwards, along a left-hand threaded section (5262) at the top of the latch activation sleeve (5220) such that the radially expandable lock ring (5150) is axially displaced while the upper split-ring expansion sleeve (5230) is carried upwards by the displacement of the lock-ring holding sleeve (5260), the upwards axial displacement of the upper split-ring expansion sleeve (5230) allowing the upper radially expandable split-ring (5130) to contract from engagement with the first internal locking profile (1112); andi) pulling the lock-ring holding sleeve (5260) up, thereby carrying the lower split-ring expansion sleeve (5210) and latch activation sleeve (5220) upwards with the lockring holding sleeve (5260) such that the lower radially expandable split-ring (5120) can contract and release from engagement inside the internal tree cap (3000) or tubing hanger (2000B).

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