In the oil and gas industries, petrochemicals and hydrocarbon gases are extracted from deep in the earth through pressure bearing tubulars or"tubing". The tubing forms a conduit from the rock where the petrochemicals reside to the surface where it is terminated at the Wellhead or Christmas Tree. The wellhead is equipped with a number of valves to control and contain the pressure which is present in the tubing. Installed below the wellhead is a Sub Surface Safety Valve (SSSV) which provides a means of closing off the flow and pressure from the well in the event of the wellhead suffering severe damage or some other disastrous event. The operation of a safety valve should provide fail safe closure of the well flow in the event of partial or total failure of surface equipment but still have the ability to remain open following failure of the valve itself provided some surface indication of the failure is apparent. This is to provide the facility to continue producing the well in the event of downhole equipment failure where other contingency measures may be applied.

Wells located on offshore production platforms depend on the safe and reliable operation of the safety valve as many wells are located closely together and any catastrophic incident on one well might easily affect the others. Also, escape routs and abandonment options for personnel are limited.

Most safety valves are installed hundreds of feet below the surface. Where the wells are situated offshore, the SSSV may be situated as much as 1,000 feet below the seabed. Most safety valves are connected to surface by a single hydraulic tube called a control line although some types rely on two control lines. A surface pump generates hydraulic pressure which is applied to the control line and is transmitted to the SSSV. An example of a typical safety valve is shown in Figure 9. The hydraulic pressure acts on a piston inside the SSSV. A large spring inside the valve normally biases the piston and connected valve mechanism to the failsafe closed position. Application of pressure to the piston will compress the spring and move the valve mechanism to the open position. The spring will return the piston to the closed position in response to removal of the hydraulic pressure applied from surface-ie an emergency situation. The spring and SSSV piston must lift and displace the hydraulic fluid for the valve to close properly and a certain response time is needed for this. The response time is proportional to the spring strength, piston surface area, depth of installation and difference between the well pressure at the point of installation and the hydrostatic pressure of the control line fluid at the same point.

The well pressure provides additional closing force acting against the hydrostatic pressure in the control line. These factors all combine to determine the maximum safe setting depth for that valve.

As less oil and gas fields are left to develop on land, more exploration and production is becoming situated offshore. Where accumulations are to be developed in deep water, performance and response times of safety valves becomes more critical.

Obviously, the deeper the safety valve is set (due to the increased water depth) the stronger the spring must become or the less the piston surface area must be to maintain the same performance. Spring design and material selection can provide increased spring loads.

Piston surface area may be reduced but this does require increased surface pressure for operation. Surface pressure is limited by the pressure bearing constraints and reliability of the wellhead feed through, control line pressure limit and SSSV pressure limit.

Both these solutions have constraints and limitations which render present state of the art safety valves suitable for installation in a maximum of around 4-5 thousand feet of water with a single control line.

Response times are also adversely affected with increased depth. Response time is important in the event of a catastrophic incident as by its very nature, instant shut off is desired. The control line is normally a 1/4"O. D. tube and if thousands of feet long, due to frictional forces, will require some hydraulic force to transmit and displace a fluid along its length. For a single line installation, this force must be provided by the operating spring.

Help in closing may be obtained from the well pressure as the closing piston is exposed to this pressure on its bottom surface, high well pressures assisting closure. The operating spring may also loose some strength with time if it is maintained in the compressed position, as would normally be the case.

For the purposes of calculating safe operation, the available well pressure may be ignored as this may change and deplete with time. (The well pressure must be included in calculations for opening the valve as this pressure must be overcome before any movement will be seen. ) A safety factor of 1.5 is often used.

An alternative approach is to equip the SSSV with a Nitrogen gas chamber which has a floating piston. The nitrogen gas on one side of the piston acts as a gas spring. This has the advantage of providing greater forces than are available from a mechanical spring and almost equal force over the whole stroke of the spring whereas a mechanical spring provides most force when fully compressed and least when fully extended (which would be the case just before valve closure). Nitrogen gas chambers are not recommended for long term downhole use as any gas leakage cannot be replaced. When coupled to a safety valve, gas leakage will result in a fail open situation which is very dangerous and may not be apparent until emergency closure is required. The SSSV becomes useless once the gas pressure has leaked off. Although some designs exist to provide an hydraulic circuit which will allow the SV to fail shut in the event of depleted gas pressure, these are overly complicated and generally not favoured.

Another option is the twin control line system. It has the advantage of mitigating all depth sensitivities. The SSSV operating piston has one hydraulic control line acting above the operating piston and one below requiring only a pressure differential between the lines to operate the piston. Unfortunately, in addition to the increased complexity of the installation, damage or loss of integrity to either control line may cause a complete failure of the system and this will quite possibly cause the SV to fail in the open position.

If we consider a SSSV with a twin control line system, when one line (top line) allows pressure to be applied above the piston, the other line (bottom line) must be left open to allow the fluid which is below the piston to be displaced back to surface. Similarly, closing the SSSV would involve pumping down the line which was previously open (bottom line), bleeding off pressure from the previously pressured line (top line) and accepting returns of displaced fluid through that line. The response time for a valve closing in an emergency would be long to allow for bleed down of one line and pressure up of the other and would involve full circulation of the control line fluids.

As the two control lines will automatically be pressure balanced at the depth of the valve installation, the operating pressure for both opening and closing will be that which is required to overcome the frictional forces contained in the downhole valve plus the frictional losses in both control lines. However, if the top line becomes damaged, the valve will remain in the open position until application of pressure on the bottom line which will then move the valve to the closed position permanently. If the hydrostatic pressure of the fluid surrounding the top control line is greater than the hydrostatic pressure within the control line (which is normally the case) and the control line suffers damage at the connection point to the SSS valve, then the SSS valve will be biased open unless pressure is applied to the bottom line to prevent this. This situation may not be known until the operation of the valve is required. In an emergency situation, no guarantees may be made concerning the integrity of one line if the other has sustained some damage as a result of the incident.

If the lower control line becomes damaged in the well near the surface connection or at the surface termination, the SSS valve will remain open regardless. This situation is particularly worrying as damage to the surface equipment (wellhead damage) may cause this to happen so that the emergency situation may also cause the malfunction of the SSSV.

A twin control line system may also feature a spring to bias the Valve to the closed position but as we have seen, various factors may conspire to prevent this.

As most Deep Set Safety Valves are required in offshore deep water installations, most wells will feature a sub-sea wellhead. Wellhead penetration and the number of umbilicals linking the well to the control centre are a major issue for all sub-sea developments. An additional penetration and one more umbilical line would not be greatly welcomed as would be the case for the twin control line system.

The twin control line systems are therefore overly complicated, have a slow response time, are potentially unreliable, do not provide confidence in an emergency and require additional wellhead penetrations and umbilical lines.

Other items which may be installed in a well also suffer from similar difficulties.

These are Smart Well Devices, Underbalanced Drilling Deployment Valves, Circulating Sleeves and Sub Sea Intervention Equipment.

Recent developments in"smart wells"have seen the introduction of a variety of devices which either close off the flow from a particular zone at the bottom of the well or partially choke or regulate the flow from a zone. These devices are located deep in the well at or near the producing formation and are intended to optimise the flow from the different zones in the well. The devices may be at a vertical depth of as much as 15,000 feet or even deeper. Failure of these devices may lead to a partial loss of production but unlike Safety Valves, they are generally not critical to the overall well operation. The simplest of these devices is a two position sleeve usually featuring one position shutting off the flow completely and the other allowing full flow. Very often, because of all the reasons previously discussed, these devices rely on two control lines for operation. Twin control lines offer twice the potential for failure over a single control line system but single control line systems are limited in their deployment for all the previously stated reasons. An example of a single control line, two position sleeve can be seem in Figure 10 which shows one of our existing designs which is known as an Omega SPOT Two Position Tool. This device receives hydraulic pressure from a single control line (1OA) above which acts on an operating piston. The piston (102) is held upwards by a large spring which the piston will compress with applied pressure and will return the piston when pressure is bled off. This movement is transferred to the sleeve (103) allowing it to be positioned either in the open or closed position, allowing or preventing well flow.

"Smart wells"generally feature two or more downhole flow regulators or sleeves (requiring four or more control lines if the twin line type is used) in combination with electronic instruments which provide information as to the wells status. Changing well conditions require alterations to be made to the flow regulators and this is normally done hydraulically. The twin control line system is generally favoured but does create some complexity and subsequent unreliability when coupled with additional electronic and hydraulic devices (including the safety valve which is present in all wells) as all these lines must exit at or near the surface. As has been mentioned previously, the wellhead penetration poses additional problems, not least the initial installation of all the equipment.

Presently, much work is being done to develop all electric systems in recognition of some of these drawbacks. Although multiple wellhead penetrations are greatly reduced with electronic systems, this comes at the expense of increased cost and long term reliability.

These problems are well known with permanently installed electronic well systems over time, especially when coupled with high operating temperatures over 200 deg F. Failure of these systems usually do not provide any contingency unless backed up with an hydraulic system.

Underbalanced Drilling Deployment Valves are a recent addition to the maturing technology of Underbalanced Drilling. Underbalanced Drilling is the term used to describe the drilling process where the oil or gas bearing rock is drilled with less pressure in the wellbore than in the surrounding rock. Traditional mud drilling relies on a column of mud to exert a greater pressure on the oil bearing zone than contained in the zone (to prevent a blow out). Underbalanced drilling features less pressure in the wellbore than in the zone.

This will ensure that no damage is sustained to the reservoir and subsequent production and ultimate recovery are greatly improved as a result. Key to this process is the containment of pressure at surface which is not normally the case with mud drilling. A type of foam is pumped down the drill pipe and back up the annulus to remove drill cuttings, to lubricate the pipe and to cool down the bit. When the drill pipe is removed from the well in order to fit a new drill bit, the pipe must be removed under pressure or"stripped"from the well.

This is a slow process and can be dangerous.

Recent advances in UBD technology have featured a valve which is positioned in the casing above the oil bearing zone but still near the bottom of the well. This is called an Underbalance Drilling Deployment Valve. This valve is hydraulically controlled with two control lines (because of its deep set nature) and is closed once the drill bit and pipe have passed through. The Valve contains the pressure below allowing pressure above to be bled off. This subsequently allows fast removal of the drill pipe in relative safety. Once removed, the pipe is fitted with a new drill bit and run back into the well until just above the valve. The pipe annulus is then sealed at surface, the well is pressured up and the valve opened allowing the pipe to pass through again. This technology suffers from all the previously listed drawbacks in that it requires two control lines, is potentially unreliable, requires two wellhead/casing penetrations and is overly complicated.

Circulating Sleeves are positioned deep in a well and are a type of valve allowing communication between the inside of the pipe and the annulus between the pipe and the casing. These sleeves are sometimes opened and closed by wireline intervention and sometimes from surface by the twin control line system. They may be used to open or close flow from a particular zone, to initiate gas lift or to allow fluid circulation from the pipe to the annulus prior to removal of the pipe from the well.

Surface controlled Circulating Sleeves almost always work on the twin control line principle with both sides of an hydraulic control piston being linked to a control line each. This technology suffers from all the previously listed drawbacks in that it requires two control lines, is potentially unreliable, requires two wellhead/casing penetrations and is overly complicated.

It is the purpose of the invention to remove the need for a second control line, to reduce the number of wellhead penetrations required and thus simplify the installation, to greatly extend the maximum setting depth of a single control line device, to speed up the response time of an hydraulically controlled downhole device and to improve the reliability of installed equipment when used with the invention.

Preferred embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal sectional view of a pilot valve according to the invention, in the form of a"tubing retrievable"pilot valve; Figure 2 is a similar view of another pilot valve according to the invention, and in the form of a three position"tubing retrievable indexing pilot valve" ; Figure 3 is a detail of indexing/valving arrangement of a three position pilot valve; Figure 4 is a view, similar to Figures 1 and 2, of a three position pilot valve in the form of a"wireline retrievable indexing pilot valve", and shown with an insert installed in the nipple; Figure 5 is a longitudinal sectional view of a pilot valve insert for a side pocket mandrel; Figure 6 is an illustration of a side pocket mandrel with the insert installed; Figure 7 illustrates a pilot valve insert for side pocket mandrels; Figure 8 illustrates the side pocket mandrel with indexing insert installed; Figure 9, as referred to above, is a longitudinal sectional view of a typical safety valve with which a pilot valve according to the invention may be used, to control the operation of the safety valve; Figure 10, as also referred to above, illustrates a single control line sleeve to which the invention may be applied; Figure 11 is an enlarged and schematic view of one preferred form of pilot valve according to the invention; Figure 12 is a view, similar to Figure 11, showing in more detail a tubing retrievable pilot valve embodying, in more detail, the schematic features illustrated in Figure 11; and, Figure 13 is a detail view of a wireline retrievable pilot valve, embodying more detailed features of the schematic aspects of the pilot valve shown in Figure 11.

Referring now to Figure 1, the main components of a Pilot Valve according to the invention are described. The figure shows a tubing retrievable Pilot Valve which is run in combination with either a Safety Valve (Figure 9) or a single control line sleeve (Figure 10) below. The main components of the valve are a device body 1 which is threaded top and bottom with a large diameter through passage allowing the device to be installed between sections of production tubing without causing a restriction within the production tubing and includes an entry and exit point 1A, 1B for attachment of a control line, the entry accepting pressure input from surface and the exit permitting low pressure egress to the device to be controlled eg SSSV. A single rod type piston 2 with hollow centre bore allows fluid passage through it and which is connected to, 3 a pressure reduction valve 3 (relief, pop off or other) which is adjustable to contain a certain upstream pressure but will vent any pressure in excess of this point. A flow restricting mechanism 4 is optimal. A low pressure chamber 5 which receives hydraulic fluid from the pressure reduction valve 3 is in communication with both the low pressure exit 1B to the SSSV, a pressure dump mechanism 6 which allows the dumping of hydraulic pressure from the low pressure chamber to the internal bore of the production tubing, a check valve 8 which prevents well pressure or fluids from entering the L. P. chamber and a large operating spring 7 which is situated inside the LP chamber and acts upon the rod piston.

The device may also be built as an integral part of a piece of equipment (Safety Valve or Sleeve) sharing certain common features such as a spring or top connection.

There follows a description of the Pilot Valve used in combination with a SSSV (or other device). The SSSV will be described first followed by the description of the Pilot Valve.

A typical Sub Surface Safety Valve is disclosed in Figure 9. The valve features a top sub 91, an attachment point for a control line 92, an hydraulically actuated operating piston (or pistons) 93, an operating mandrel 94, a power spring 95, a flapper valve and seat 96, an outer housing 97, a bottom sub 98, a flapper hinge pin 99 and flapper return spring 99A. In normal operation, application of hydraulic pressure via a control line and into the top of the safety valve will act on the operating piston 93 to compress the power spring 95.

The flapper 96 pivots around the hinge pin 99 and the flapper will be held open and out of the flow path. When it is required to close the valve, removal of hydraulic pressure will allow the power spring 95to discharge, pushing the operating mandrel 94 and piston 93 back to top stroke. Once the operating mandrel is clear of the flapper, the flapper return spring will bias the flapper to the closed position. The well flow will assist the flapper closure and can provide a slamming shut effect.

The Pilot Valve (Fig. 1) is positioned above the SSSV and features only one control line from surface. A second control line runs from the Pilot Valve to the SSSV. The Pilot Valve system has three main components. A control line pressure isolation/reduction means which may be overcome by increasing control line pressure, a sealed chamber which is referenced to the well pressure and is in communication with both the pressure isolation/ reduction means and the SSSV or other device to be controlled and a means of discharging the chamber pressure in response to control line pressure reduction (following pressure increase as a result of control line pressure increase).

In the relaxed position with the control line pressure bled off at surface, the control line hydrostatic pressure will be isolated from the LP chamber by means of the pressure reduction valve 3 (which due to its design also acts as a check valve). The pressure reduction valve will be specified to"pop"with approximately 1,000 psi more than the hydrostatic pressure at the installed depth. The pressure in the LP chamber 5 in the relaxed position (at this time) will be a similar pressure which is to be found in the tubing.

Any higher pressure will dump via the pressure dump valve 6 which will be open at this time. Two check valves prevent well fluids entering the L. P. chamber. In response to injecting fluid down the control line from surface, the pressure above the rod piston 2 will increase, biasing the piston downwards against the large spring 7. A pressure differential is created across the piston by the Pressure Reduction Valve 3. The piston and Pressure Reduction Valve 3 are both housed in a common mandrel which acts upon a strong spring 7. The mandrel will move downwards in response to a pressure increase above, ie pumping hydraulic fluid from surface. The downwards movement will have two effects. The pressure dump valve (which previously provided a reference of internal tubing pressure) will be closed sealing the LP chamber 5 and the operating spring will become compressed.

The pressure which is being introduced into the LP chamber (in response to a pressure increase from surface) will feed through the L. P. chamber to the exit control line 1B and will act upon the SSSV operating piston and spring. (Prior to this, the SSSV has referenced <BR> <BR> the same pressure as the L. P. chamber both above and below its operating piston. ) As the pressure is increased at surface, the pressure in the L. P. chamber will increase proportionally and will open the SSSV. Once the SSSV spring has become compressed and the valve is fully open and after providing additional over pressure for contingency purposes, the surface pump providing the pressure may be stopped. Locking this surface pressure into the control line will maintain the SSSV in the open position.

A reduction of pressure in the control line will close the valve-but more quickly than would normally be the case. When the pressure above the Pilot Valve piston 2 is reduced, (requiring removal of only the small volume which will represent the compressibility of the fluid contained in the control line) the large operating spring 7, assisted by the existing pressure in the LP chamber 5 will push the rod piston 2 upwards.

Only a small stroke is required to open the pressure dump valve 6. Once the dump valve 6 is opened, the remaining pressure will quickly equalise to the tubing pressure and the large volume of fluid which was previously pumped to open the safety valve will also quickly dump to the tubing. The SSSV will rapidly close as the SSSV operating spring has only to displace the hydraulic fluid a very short distance into the tubing (via the pressure dump valve), not all the way to surface.

Use of the combination of Safety Valve and Pilot Valve will make a standard Safety Valve of the appropriate diameter suitable for Underbalance Drilling applications.

As has previously been described, the present generation of Underbalance Drilling Deployment Valve relies on twin control lines for operation. Not only will development costs be removed by using standard equipment (Safety Valves) but the single control line operation afforded by use of the Pilot Valve will be cheaper to deploy and will be more reliable.

Retrievable Pilot Valve The Pilot Valve mechanism may also be housed inside a Side Pocket Mandrel (SPM). This type of installation will be well known to one skilled in the art. This allows the Pilot Valve mechanism to be returned to surface for re-dress should the need arise by using wireline. This will not involve the removal of the production tubing. The internal mechanism of this variant performs in the same manner as previously described but has fewer components. The design for this application differs slightly from that previously described but the operating principles are the same. A description of a Side Pocket Mandrel Retrievable Device follows with reference to Figure 5, but is for example only.

The Pilot Valve mechanism is housed in a tube around 20 inches long and 1.5" diameter. A latch mechanism situated at the top of the tube locks the mechanism into a "Side Pocket Mandrel" (see figure 6). The SPM is a well known piece of oilfield equipment more normally used for gas lift secondary recovery techniques, but may easily be adapted for this application. The SPM receives a control line from surface (61A) and a second control line (61B) which communicates with the hydraulic device to be operated eg, a SSSV. A sealing arrangement comprising of o-rings exists on the outside of the tube to allow pressure isolation of both lines.

With reference to Fig 5 the Retrievable Pilot Valve consists of the inlet from the control line 51A, an operating rod/piston 52, an operating spring 57 a L. P. Chamber 55, top rod seals 53, bottom rod seals 54, exit port 51B pressure dump port 56 and check valves 58.

The operation of the Retrievable Pilot Valve will now be described by way of example. If operation of a safety valve is desired at 14,000 feet, then the control line hydrostatic pressure at this point will be in the region of 5, 000psi if the control line fluid is hydraulic oil. This 5,000 psi will be present at the inlet (51A). A rod 0. 2" diameter (52) is urged upwards by a pre loaded spring (57) exerting around 170 lbs force. This combination of spring and rod will require around 6,000 psi differential pressure on top of the rod to move it from this point. A seal and a metal tight tolerance ring seal (53) provide a pressure barrier on the rod preventing pressure leakage into the L. P. chamber (55). At the bottom of the L. P. chamber, an open exit point (56) called the pressure dump port exists to allow pressure in the L. P. chamber to vent to the internal well pressure. A check valve (58) prevents fluid or pressure ingress into the L. P. chamber. At the top of the pressure dump port is a duplicate of the top sealing arrangement designed to close off the pressure dump port (56) in response to an increase in control line pressure (51A). As the pressure (51A) is increased, the piston rod (52) will act against the spring and move downwards. Firstly, the rod will seal on the bottom rod seals (54), secondly, with additional pressure, the rod will clear the first top rod seal (53). Once this seal has been passed, pressure may pass through the tight tolerance ring seal (which has been designed not to seal pressure but to slow down pressure egress in order to prevent pressure rush damage to the main seal) into the L. P. chamber (55) which is now sealed closed. This pressure will communicate through the exit point (51B), through the control line and act upon the SSSV. As the surface pressure is increased further, more pressure will be generated in the L. P. chamber which will allow the SSSV to open. Once fully open, no more pressure need be applied at surface.

Relieving the previously applied surface pressure will allow the SSSV to close.

When the control line pressure (51 A) drops below the point where the spring (57) will return, the rod will be urged back upwards. Once the rod (52) has cleared the lower seals (54), the pressure contained inside the L. P. chamber will be allowed to dump into the tubing and equalise with the well pressure via pressure dump port (56). This will have the effect of equalising pressure above and below the SSSV piston and will allow the SSSV spring to push the valve to the closed position.

Indexing Pilot Valve Yet another application of the Pilot Valve is to incorporate an indexing mechanism so that more than one hydraulically controlled piece of well equipment (such as a sleeve) may be operated with only one control line running from surface. As has been stated previously, most hydraulically operated sleeves require two control lines to operate them. A borehole equipped with three sleeves using presently available equipment therefore requires six control lines from the wellhead. This situation has many obvious disadvantages not least, cost. Were these sleeves equipped with Pilot Valves, the number of control lines would be reduced to three running the length of the well. However, a single control line running the length of the well could terminate in a single Pilot Valve which would then selectively distribute to three sleeves via three short control lines. There follows a description of an Indexing Pilot Valve which may control three sleeves but could easily accommodate four or more.

With reference to Figure 2, a Tubing Retrievable Indexing Pilot Valve is shown.

The device shown is a combination of the previously described Pilot Valve in Figure 1 and in the lower portion of the tool, an indexing mechanism which allows output and control of three hydraulically operated devices. The Pilot Valve components may take one of the different forms as previously described but their function will be the same. Other forms are also possible. This diagram is intended for illustration only and the device may feature more or less than three positions.

Figure 2 shows a Tubing Retrievable Pilot Valve with an indexing mechanism fitted. The upper portion of the Pilot Valve is largely the same as previously described. The piston (22) is linked to a cylindrical sleeve (29) which the spring acts upon and also acts as a stroke limiting device. Rotateably connected near the bottom of the sleeve is an indexing mandrel (29A). The Indexing mandrel will rotate a pre determined distance with each cycle of application of pressure down the control line and bleeding off. Each operation of the Pilot Valve will push the piston (22) down and subsequently up advancing the indexing mandrel (29A) on to the next position. Two locating pins (29B) run along an external milled track which allows for seven indexing and seven parking positions see Figure 3.

Each position of the indexing mandrel will mate a protrusion with a corresponding location in the Valve Sub (29C).

The bottom section including Valve Sub (29C) features attachment for three control lines (21 B, 21 C, 21 D) each one running downwards to a sleeve or similar device.

Each control line terminates in the Valve Sub (29C) where it is split into two paths. One path features an injection valve (29D), the other a bleed off valve (29E). Both paths will terminate in an indent profile which will match with the profile of the protrusion on the bottom of the indexing mandrel (29A). As the indexing mandrel (29A) connects with each indent in turn, the option exists to either open or close each valve individually depending on position. If opening is required, then when the indexing mandrel is in the appropriate position, the valve (29D) will be pushed open, pressure will be applied over and above the normal indexing pressure (1,000 psi) and the sleeve will open as previously described. A check valve (29F) is included in this line to lock in pressure to the control line (and sleeve).

Omission of the check valve would allow pressure loss during the subsequent bleed off cycle (and indexing) of the Pilot Valve which might partially close the sleeve.

Closure of the sleeve is achieved by positioning the indexing mandrel (29A) to the appropriate closing position. This will push the bleed off valve (29E) to the open position and allow any pressure which is locked in the control line (21A) or sleeve hydraulics to vent to the L. P. chamber (25). The L. P. chamber will in turn vent to the internal tubing pressure via dump valve (26) when the control line pressure is bled off at surface, thus allowing the sleeve to close. A port allows pressure from below the check valve (29F) to communicate to the bleed off valve (29E). The bleed off valve (29E) will be reset to the closed position when the indexing profile next engages with that sleeves opening indent.

Additionally, an extra position exists which has an injection valve and check valve but is ported to either the internal well pressure or the annular space outside the device.

Engagement with this position will allow a datum or reference position to be established as it will not be possible to"pressure up"in this position, making the position of the indexing mandrel known.

Thus, in summary, the indexing mandrel may be positioned to either open or close any sleeve. The sleeve closing profiles are indexed prior to the opening profiles allowing any sleeve to be opened, closed or maintained in their present position whilst indexing round to address another sleeve. The closing profile, when indexed, will vent trapped pressure which was previously holding that sleeve open and allow it to close; the opening profile will close the bleed off valve and will also open the Injection Valve. The option exists in this position to either open the sleeve by applying more pressure or of leaving the sleeve closed by indexing on to the next position (by bleeding off control line pressure).

Wireline Retrievable Indexing Pilot Valve.

Another variant of the Pilot Valve is a Wireline Retrievable Indexing Pilot Valve as shown in Figure 4. This arrangement allows for the entire Indexing Pilot Valve to be packaged as an insert (41) which is recovered to surface for repair and servicing should the need arise. A nipple (41X) is screwed to the production tubing. The nipple features internal pressure containment for the well fluids, an attachment and communication point to receive a control line from surface (41A), an attachment and communication point to receive three control lines going downwards to three sleeves, and an attachment means inside the nipple for the insert. The nipple is threaded top and bottom to the production tubing. The Insert is locked in place by a Lock Mandrel (41W). This is a common piece of oilfield equipment and is presently used for many similar applications. The Lock (41 W) may be released by using standard wireline techniques which will be well known to one skilled in the art.

Release of the lock will allow removal of the lock and Pilot Valve Insert (41) complete as the two are screwed together. The lock normally engages with a profile in the Nipple (41X). Screwed to the bottom of the lock is the Indexing Pilot Valve Insert (41) which is largely as described and demonstrated in Figure 2 but features external seals (41 Z) which allow transmission of control line pressure (41A) through the nipple to the Pilot Valve. At the bottom end of the nipple (41Y), polished sealing surfaces allow seals (411A, 411B, 411 C, 411 D) to isolate the three outputs from the Pilot Valve. These outputs communicate to the three control lines, (41B, 41C and 41D) which run downwards to three sleeves or other devices.

Retrievable Indexing Pilot Valve for Side Pocket Mandrels Yet another option is a Retrievable Indexing Pilot Valve which may be housed in a Side Pocket Mandrel as can be seen in Figure 8. This is very similar to the Pilot Valve for Side Pocket Mandrels (Figure 6) but includes an extra section below. It is housed in the same Side Pocket Mandrel but an extra two control lines are featured (See Figure 8). With reference to Figure 7, a Retrievable Indexing Pilot Valve Insert for fitment to a Side Pocket Mandrel is shown. The top latch is the same as before and is an industry standard component. The piloting section of the tool is also the same as shown in Figure 5 and operates in the same way. Below this lies the indexing section. The pressure which is generated in the Low Pressure Chamber (77) is allowed to transmit to a lower chamber (751). In the lower section of this chamber, an indexing rod (791), is sealed and will move downwards with pressure application. With each pressure application and bleed off, the Indexing Rod will move down and subsequently up. A key (796) engaging the rod will also engage with a milled track on an outer ratchet sleeve (792) which is fixed to the main body.

As the Indexing rod moves down and up along the track, it will also rotate allowing Injection (793) or bleed off valves (794) to be depressed. Once depressed, the pressure from the L. P. chamber may transmit through the injection valve and out to the relevant control line to the desired sleeve. O ring seals (795) on the outer housing isolate the three output lines from each other. When the Bleed off valves are depressed, the pressure contained in the sleeve and control line is allowed to bleed off through the port to the L. P. chamber (77) and out into the wellbore. A bottom spring allows return of the rod to a datum or relaxed position.

Referring now to Figure 11, this is a schematic illustration of a practical form of deep-set pilot valve according to the invention. The deep-set pilot valve has been designed to improve the safety and reliability of sub-surface hydraulically controlled valve, and to reduce the life cycle costs of devices, such as sub-surface safety valve, remote actuated sliding sleeves and well inflow control devices.

Conventional hydraulically controlled valves require applied hydraulic pressure down the control line to open, and a spring to effect closing of the valve. A point is reached with increasing well depth where the physical and dimensional limitations of the equipment prevent the packaging of an ever more powerful spring to counteract the increased hydrostatic control line pressure. The embodiments of pilot valve disclosed herein (and further examples of which are described below with reference to Figures 11 to 13) remove these limitations, by decoupling the effect of hydrostatic pressure in the control line from the operation of the sub-surface device.

The deep-set pilot valve therefore consists of two primary components. The first component is a sealed pressure chamber which equalises pressure between the hydraulically operated device and the tubing pressure. The second component consists of a piston valve arrangement. Upon application of control line pressure, the piston moves down sealing the pressure chamber. Further pressure/travel allows pressure to build-up in the chamber and act on the hydraulic device. Removing control line pressure opens a dump valve to equalise the pressure between the hydraulically operated device and the tubing.

In Figure 11, reference 101 indicates a single control line from the surface to a downhole location of the pilot valve, to supply input control pressure thereto, whereby the pilot valve responds and thereby controls the operation of the hydraulically controlled device (not shown) set in the wellbore, sub-surface, at a lower location than the pilot valve.

Reference 102 designates a pressure applicator 102 acted upon by the input control pressure and reference 103 is a first valve arrangement which controls the movement of the pressure applicator 102 under the action of the input control pressure. A compression spring 104 applies a constant biasing force to the coupled-together pressure applicator 102 and first valve means 103, to take-up the position shown in Figure 11, when no input control pressure is supplied via control line 101. An intermediate chamber 105 communicates with the pressure applicator 102 via the first valve arrangement 103, such chamber 105 communicating with well pressure when the valve arrangement 103 is closed, and such communication being closed when input control pressure is applied to the pilot valve.

There is also a second valve arrangement 106 and a port 107 which communicate with the sub-surface hydraulically controlled device, when the chamber 105 is pressurised above well pressure in response to input control pressure supplied to the pressure applicator 102 in excess of the pre-set value. However, the hydraulically controlled device (not shown) is allowed to equalise with well pressure in response to removal of input control pressure. There is also a pressure dump outlet 108 to tubing, to dump or equalise the pressure between the hydraulically operated device and the tubing, when input control line pressure to the pilot valve is removed.

Referring to Figure 12, this shows in more detail a tubing retrievable pilot valve, and detailed parts corresponding generally with the schematically illustrated features of Figure 11 are given the same reference numerals.

Figure 13 shows in detail a wireline retrievable pilot valve, and again incorporating detailed features corresponding with the schematic features of Figure 11, and given the same reference numerals.

The preferred embodiments of pilot valve disclosed herein therefore embody the following general features.

A pilot valve (1) for connection to a safety valve to be used in controlling sub- surface flow of pressure fluids to the surface via tubing running to the surface and comprising : a tubular housing which is capable of being installed between sections of production tubing to permit through-flow of pressure fluids; an entry point (1A) on the housing for connection to a control line running to the surface and which supplies a pressure medium to operate the safety valve after undergoing pressure reduction in passing through the pilot valve, whereby the safety valve can close- off flow of pressure fluids to the surface; an exit point (1B) on the housing connectable to the safety valve to supply the pressure medium at reduced pressure; and, a pressure reduction arrangement (2,3, 4, 5,6, 7) in the pilot valve between the entry point and the exit point.

Preferably, the pressure reduction arrangement comprises a pressure reduction valve (3), a flow restricting mechanism (4), and a low pressure chamber (5) for receiving pressure medium from the flow restricting mechanism (4) and communicating with both a low pressure exit to the safety valve and also to a pressure dump mechanism (6) which allows the dumping of hydraulic pressure from the low pressure chamber (5) to the internal bore of the production tubing.

Further, a large operating spring (7) may be situated inside the low pressure chamber (5), and acts upon a rod type piston (2) with a passage running through it and which is connected to said pressure reduction valve (3).

A pilot valve (1) for connection to a safety valve to be used in controlling sub- surface flow of pressure fluids to the surface via tubing running to the surface and comprising: a tubular housing which is capable of being installed between sections of production tubing to permit through-flow of pressure fluids; an entry point (1A) on the housing for connection to a control line running to the surface and which supplies a pressure medium to operate the safety valve after passing through the pilot valve; an exit point (1B) on the housing connectable to the safety valve to supply the pressure medium thereto; and, a rapid response arrangement in the pilot valve to improve the response of the pilot valve to initiation of actuation and de-actuation of the pilot valve.

The pilot valve, as defined above, may be connected to a safety valve, such as a sub-surface type safety valve e. g. as shown in Figure 9, and/or in conjunction with a single control line sleeve, e. g. as shown in Figure 10.

Further, the pilot valve may be incorporated within a tubing retrievable indexing pilot valve e. g. as shown in Figure 2; a wireline retrievable indexing pilot valve, e. g. as shown in Figure 4; a side pocket mandrel, e. g. as shown in Figures 5 to 8.

The invention is also concerned with a method of use of a pilot valve as defined in any one or more of the aspects set out above, in connection with the exploitation of sub-surface fluid reserves.