Description

The invention relates to a method of well intervention. The intervention may be carried out on land or sea based oil and gas wells. Well interventions are remedial operations performed on oil or gas producing wells with the intention of restoring or increasing production. There are four main types of well intervention, namely wireline intervention, coilhose intervention, coiled tubing intervention and finally hydraulic work over intervention. The first three of these are considered to be prior art to the invention. The wireline technique involves running a cable into the well from a platform deck or a vessel. An intervention tool string is attached to the wire and the weight of the tool string, plus additional weighting if necessary, is used to run the wire into the well, where the tool string performs a maintenance or service operation. Wireline intervention is carried out in wells under pressure. The wire is supplied from a drum and passes via two sheaves to a stuffing box which is exposed to well pressure on its well side. Wireline intervention is a light well intervention process. Coilhose intervention, also known as injectorless intervention, is a relatively new well intervention method. Coilhose intervention is similar to wireline intervention, the difference being that the wire is replaced by a flexible hose or steel tubing having a outer diameter similar to that of the wire, enabling a hydraulic communication path into the well with equipment and methodology similar to what is used for wireline intervention, maintaining the same low space and personnel requirements as for wireline interventions. Coilhose intervention is an intervention method, similar to wireline intervention, where a hose or tubing is deployed into a well without the use of a coiled tubing injector, lowered into the well by its own weight and additional weights attached to the end of the hose or tubing while via two sheave wheels from a hose drum. Also, the hose or tubing is pulled out of the well without the use of an injector by pulling the hose or tubing out of the well with the hose drum.The coilhose intervention method is described in WO2012/022987 A2 and related patents.

However, the inventor has recognized that coilhose intervention has a limited pulling force using the hose drum to pull the hose or tubing out of the well, as the hose or tubing will deform, collapse or flatten against the spooled hose or tubing layer on the outermost surface of the drum when the pulling force exceeds the collapse forces of the hose or tubing. This in turn damages the hose or tubing and makes it useless for coilhose intervention beyond a certain pulling force requirement. This limits the operational window for coilhose, as this limits both the achievable depth, as depth increases the weight of the coilhose in the well and as well as limiting the well pressure in which the coilhose intervention can be executed, as higher well pressure requires more weights attached to prevent pressure pushing the hose or tubing out of the well through the pressure sealing stuffing box. To avoid any such deformation, collapse or flattening of the flexible hollow conduit forms the basis and motivation for the invention described in this application. Coiled tubing intervention is a medium well intervention process, requiring the use of a larger space or deck. It has the advantage over wireline intervention that it provides a hydraulic communication path to the well, and it has the advantage over coilhose that it provides a much larger hydraulic communication path. However, coiled tubing intervention uses heavier and more costly equipment and requires more personnel than wireline intervention or coilhose intervention. The coiled tubing is a length of continuous tubing supplied on a reel. The outside diameter of the tubing ranges from small sizes of about 25 mm up to 80 or 90 mm. The tubing is fed from the reel upwardly to a tubing guide, known as a coiled tubing goose neck, and from there via an injector head downwardly towards the well. The coiled tubing injector head is vertically attached to the top of the well and in line with the centre-line of the top of the well and is used to push or "snub" the coiled tubing into the well to overcome the opposing forces created by any pressure differential between the higher pressure inside the well and the lower pressure outside of the well. The pressure inside the well is contained by the coiled tubing stuffing box installed between the well and the coiled tubing injector, which is a circumferential elastomer seal through which the coiled tubing is pushed in or pulled out by the coiled tubing injector. The coiled tubing goose neck typically consists of an arch serving to transfer the direction of the tubing from the inclined direction as it comes off the reel to the required vertical direction as it descends towards the well. The arch is provided with a series of rollers spaced along the length of the tubing and to reduce friction as the tubing passes along the arch. The gooseneck is installed at the top of the injector to guide the tubing into the top of the injector Coiled tubing is usually manufactured from steel alloy and is much heavier and larger than wireline. An injector head is required to push or "snub" the tubing into the well and to pull it out of the well when an intervention job has been completed.

A typical injector consists of a pair of endless chains each mounted on a pair of spaced sprockets and each having a straight run engaging the coiled tubing. The tubing is compressed between the chains which are hydraulically driven to push the tubing downwardly or pull it upwardly.

Viewed from one aspect, the invention provides a method of well intervention in which the flexible hollow conduit extends from a flexible hollow conduit storage spool and into a well, wherein the flexible hollow conduit is deployed into the well by means of its own weight or by weights attached to its outermost end inside the well, creating sufficient gravity forces to pull the tubing downwardly while overcoming any opposing forces created by differential pressure and friction between the outer surface of the flexible hollow conduit and the elastomer of the stuffing box.The stuffing box is installed at the top of the well and is a circumferential elastomer seal through which the flexible hollow conduit passes into or out of the well while maintaining a pressure seal between the pressure inside the well and the outside of the well. Similarly, when pulling out of the well, the required pulling force is achieved by the one-directional puller, which is installed away from the well by letting the flexible hollow conduit pass via one or more sheave wheels after it is pulled out of the well and before entering the one-directional puller installed close to the storage spool. The one- directional puller is similar to a coiled tubing injector, but does not provide any push or "snubbing" force. Also it is much smaller and has a smaller pulling capacity than a coiled tubing injector. The one-directional puller is not designed to provide any pushing force, only pulling forces acting to controllably lower the flexible hollow conduit into the well by weights attached to the lowermost end of the flexible hollow conduit, and to pull the flexible hollow conduit out of the well after the flexible hollow conduit has passed one or more sheave wheels coming out of the well. Once the flexible hollow conduit has been pulled out of the well with the one-directional puller, the flexible hollow conduit is spooled onto the storage spool by its built in tension maintaining system that is designed to only have sufficient pulling force to allow the flexible hollow conduit to be spooled onto the drum in a controlled manner and avoid any "loose" spooling on the storage spool. A high pressure swivel, as known from the coiled tubing drum, is attached to the rotational center of the drum, allowing continuous pumping through the flexible hollow conduit while it is driven into or pulled of the well.

The invention distinguishes itself from coiled tubing intervention as follows. Firstly, the invention distinguishes itself from coiled tubing by driving the flexible hollow conduit into the well by its own weight or by weights added at its outermost end. Secondly, the invention distinguishes itself from coiled tubing intervention by having one or more sheave wheels installed between the storage spool and the well. Thirdly, the invention distinguishes itself from coiled tubing by not having an injector attached to the top of the well and in line with the centerline of the well, but instead having a one- directional puller installed between the storage spool and the lowermost sheave wheel and the one- directional puller is smaller and does not provide any pushing force. Fourthly, the invention distinguishes itself from coiled tubing intervention by using the one-directional puller to controllably slacken the flexible hollow conduit as it is driven downwardly into the well by its own weight or by weights added at its outermost end.

The invention distinguishes itself from coilhose intervention (and consequently from wireline intervention) as follows. Firstly, the invention distinguishes itself from coilhose intervention by adding a one-directional puller in between the storage spool and the lowermost sheave wheel and using the one-directional puller to controllably pull the flexible hollow conduit out of the well.

Secondly, the invention distinguishes itself from coilhose intervention by not using the storage spool to controllably lower the flexible hollow conduit into the well nor to pull it out of the well. Thirdly, the invention distinguishes itself from coilhose intervention by using the storage spool solely to contain the flexible hollow conduit in such manner that it at all times during the intervention maintains the flexible hollow conduit spooled on the storage spool without acting to lower the flexible hollow conduit into the well nor acting to pull it out of the well. Fourthly, the invention distinguishes itself from coilhose intervention as the invention strongly reduces the risk of the flexible hollow conduit being deformed or flattened. In coilhose intervention, where the hose is deployed into the well by means of its own weight from a hose drum and pulled out of the well by a hose drum, there is a risk that the forces applied radially to the cross-sectional area of the flexible hollow conduit will deform or flatten the flexible hollow conduit against the core of the drum or the underlying layers of flexible hollow conduit spooled on the drum as it is lowered into the well or pulled out of the well by the coilhose drum. This is particularly a challenge if the flexible hollow conduit is ductile and has a high degree of metal content, such as a small OD steel tubing. A flattened flexible hollow conduit will block the desired hydraulic communication path to the well. Also, any small deformation of the flexible hollow conduit will create a risk of the stuffing box seal not sealing around the flexible hollow conduit, thus creating a risk of pressure and hydrocarbons being unintentionally released from the well. Such a situation is recognized as non-acceptable by the industry. The inventor has recognized that the use of a one-directional puller between the lowermost sheave wheel and the storage spool which is gripping circumferentially around the outer diameter of the flexible hollow conduit will remove the risk of flattening, collapsing or deforming the flexible hollow conduit from lowering or pulling it into or out of the well with the hose drum. The one directional puller could for instance be made up of two endless rotating chains facing each other and each chain having a half-circle profile matching each other to make up a complete

circumferential grip around the flexible hollow conduit, thus pulling the flexible hollow conduit without the risk of deforming, collapsing or flattening it. The two chains are compressed against each other in order to grip around the flexible hollow conduit. Preferably having the half-circle profiles made from a semi-flexible material such as polyurethane with added Silica Carbide particles to enhance the gripping friction between the half-circle profile and the outer surface of the flexible hollow conduit. For clarification, the sheave wheels have a half-circle profile matching that of the flexible hollow conduit no minimize the risk of deformation of the flexible hollow conduit as it is run over the sheave wheel. For clarification, it should be mentioned that the storage spool provides only sufficient pull on the run of flexible hollow conduit from the one-directional puller only to control the spooling of the flexible hollow conduit and prevent it from becoming relaxed thus creating a risk of uneven spooling and consequent "lock" disabling further un-spooling. The storage spool does not act to pull the flexible hollow conduit out of the well. The inventor has also recognized that there is no need for an injector to provide a downward pushing capacity, as this may be provided by weight attached to the lower end of the flexible hollow conduit to drive the flexible hollow conduit into the well and using the one-directional puller to controllably slacken the flexible hollow conduit as it is driven downwardly into the well by its own weight or by weights added at its outermost end, thus distinguishing the invention from wireline intervention, coilhose intervention and coiled tubing intervention. Furthermore, the inventor has recognized that there is no need for a hose drum nor an injector installed at the top of the well and in line with the center-line of the upper part of the well to pull the flexible hollow conduit out of the well, as this is achieved by the one-directional puller positioned away from and nearly perpendicular to the center-line of the upper part of the well by pulling the flexible hollow conduit out of the well via one or more sheave wheels before the flexible hollow conduit enters into the one-directional puller. This also distinguishes the invention from wireline intervention, coilhose intervention and coiled tubing intervention.

The flexible hollow conduit extends through a seal which seals circumferentially around the outside of the flexible hollow conduit (e.g. a stuffing box or stripper). As flexible hollow conduit, it is preferred to use steel tubing of smaller outer diameters that is more flexible and ductile than larger diameter tubing. For example, a 15 mm OD steel tubing of continuous length tubing may be used. Alternatively, various types of hose with varying metal content may be used.

In the case of low pressure wells, in order to deliver the flexible hollow conduit into the well, the weight of the flexible hollow conduit and the tool attached to the end of the flexible hollow conduit may be sufficient to pull the flexible hollow conduit through the stuffing box. In higher pressure wells there will be an increased resistance to entry of the flexible hollow conduit into the well and additional weight has to be attached to the end of the flexible hollow conduit.

In a preferred method without using an injector installed at the top of the well and in line with the center-line of the upper part of the well to pull the flexible hollow conduit out of the well, a sheave wheel is installed in line with the stuffing box. The sheave wheel transfers the direction of the flexible hollow conduit from the inclined direction as it comes off the lower sheave wheel to the required vertical direction as it descends towards the well through the stuffing box which is installed at the top of the pressure control stack which in turn is installed at the top of the well

The pressure control stack consists of pressure containing and controlling components installed on top of the well to enable controlled containment of the pressure inside the well. Typical components of a pressure control stack from bottom to top are; a blow out preventer able to cut flexible hollow conduit and close the bore in an emergency (Shear-seal), a blow out preventer that comprises of different cutting and sealing components for use as contingency if a leak occurs above the BOP, a lubricator that provides sufficient space and height to install the tool attached to the end of the flexible hollow conduit and enable a subsequent confirmative pressure integrity test of the pressure control stack before opening the well. At the top of the pressure control stack is the stuffing box or stuffing box which provides the continuous circumferential seal around the flexible hollow conduit as it is run into or out of the pressurized well.

The well intervention methods described above may be used on land or on sea based oil or gas rigs. With particular reference to offshore well interventions, it has been proposed to carry these out using coiled tubing which extends from a floating vessel to a subsea intervention stack without being inside a conventional riser. Such a system has been proposed as the Spoolable Compliant Guide System, or SCGS, US2011/198092(A1). In this system a flexible riser is provided by an external coiled tubing, also referred to as a Spoolable Compliant Guide (SGS) and a smaller coiled tubing is inserted through the flexible riser into the well for normal coiled tubing operations. An injector is provided on the vessel to drive the internal coiled tubing downwardly, and the external coiled tubing acts as a guide to prevent buckling of the internal tubing during this process. The injector is also used to pull the internal coiled tubing up out of the well. TheSCGS's flexible riser is exposed to well pressure as it is connected directly to the top of the well and the inner coiled tubing is inserted into the pressurized flexible riser on the vessel through a stuffing box at the top of the flexible riser by being pushed by the coiled tubing injector installed immediately above the stuffing box or stuffing box on the vessel. Thus, the well pressure is transferred all the way up to the vessel through the flexible riser and the coiled tubing is not exposed to the ambient sea between the vessel and the wellhead on the seabed.

In accordance with the first aspect of the present invention, an injector installed at the top of the well and in line with the center-line of the upper part of the well to pull the flexible hollow conduit out of the well is not used to pull the hose out of the well. Further, as discussed above, either no drive system is needed to drive the flexible hollow conduit into the well. This makes it possible to provide, in relation to offshore well interventions, a flexible hollow conduit that extends from the sea surface to the sea floor without being contained in a riser (whether a conventional riser or an external coiled tubing acting as a flexible riser as in the case with SCGS). Therefore, a preferred method of the first aspect of the invention, comprises an offshore well intervention, wherein the flexible hollow conduit is exposed directly to the ambient sea between the sea surface and the top of a subsea intervention stack.

Viewed from a second aspect, the invention provides a method of well intervention in a subsea well having a well head on the sea floor, in which a flexible hollow conduit extends downwardly through the sea from a storage spool installed on a vessel on the sea surface into the well through a subsea intervention stack installed on the well head at the sea floor, and in which the flexible hollow conduit is exposed directly to the ambient sea between the vessel and the top of the subsea intervention stack.

With such a method, the flexible hollow conduit may be lowered into a well by the weight of a tool string attached to the end of the flexible hollow conduit, possibly supplemented by additional weighting. A tractor may be used to pull the flexible hollow conduit tubing into the well. The flexible hollow conduit tubing can be pulled out of the well using the one-directional puller on the vessel which is not attached to the well nor any riser. Thus an injector at the top of the riser and in line with the center-line of the upper part of the riser on the vessel is not required to be installed, nor a riser or an external coiled tubing guide, or a SCG down to the sea floor. There is the significant advantage that the provision of a heavy duty injector (such as of the conventional chain drive type described above) at the sea floor is not needed.

Preferably, the flexible hollow conduit has sufficient flexibility and slack to allow limited movements of the said vessel due to forces from sea and wind without inducing movements to the lower part of the hose adjacent to the subsea intervention stack.

The second aspect of the invention may be used in combination with the first, with or without the various optional features described herein. It will be seen that low cost well interventions may be provided, whether land based or subsea. In preferred arrangements, the use of a heavy duty injector, or the use of a goose neck, or (in the subsea case) the use of a protective riser (whether of the traditional type or consisting of an outer coiled tubing or a SCG), may be avoided in a well intervention. The invention can provide well interventions with hydraulic communication, without the use of a heavy coiled tubing injector and associated equipment, or by use of coilhose intervention with risk of deforming or flattening the flexible hollow conduit, but instead use equipment according to the invention.

The present invention, in its various aspects, also extends to well intervention apparatus and the components of that apparatus as described herein. Certain preferred embodiments of the invention in its various aspects will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is an overview of a conventional wireline intervention system;

Figure 2 is an overview of a conventional coilhose intervention system

Figure 3 is an overview of a conventional coiled tubing intervention system Figure 4 is an overview of an intervention system according to the invention;

Figure 5 is another overview, showing an intervention system according to the invention provided from a floating vessel on a subsea well.

Figure 1 shows a conventional wireline intervention set up for a well on a fixed offshore platform or a land well. The well head is thus "dry" in the sense that it is not underwater and is either above the sea surface or is on land. Figure 1 is amended to describe a conventional wireline intervention set up and the related prior art. Referring to Figure 1, this shows a blow-out preventer (BOP) 2 supported on a deck 4 positioned above a well head 8. The well head 8 is attached to the upper part of the well 36, into which a weight 39 is lowered by the wireline 41 into the well 36. The mass of the weight 39 and the vertical section of the wireline 41 pulls the wireline from the wireline drum 42, via a lower sheave wheel 43 and an upper sheave wheel 44 and through the stuffing box 18 into the well 36. The lower sheave wheel 43 is attached to the lubricator 16 with a tie-down strap 45, enabling the lower sheave wheel 43 to change the direction of the wireline 41 from horizontal to vertical and vice-versa as it passes. The upper sheave wheel 44, is supported by a vertical lifting chain 46. Arrow 47 indicates the upward force provided by the crane or the like. The weight 39 can be integrated with a tool to perform operations inside the well 36. Below the BOP, a riser 6 extends downwardly to the wellhead. The well head 8 supports a tubing hanger and above the well head a production X- mas tree 10 is provided. Between the X-mas tree 10 and the riser 6 a shear-seal blow-out preventer 12 is provided. A support frame 40 is attached to the top of the stuffing box 18. Between the blow out preventer (BOP) 2 and the stuffing box 18, a lubricator riser 16 is provided to accommodate installation of weights and tools 39 before being deployed into the well 36. A lifting chain 31 supports the weight and forces of the components below, i.e. the pressure control stack, comprising components 12, 6, 2, 16, and 18. Arrow 30 indicates the upward force provided by the crane or the like. The wireline 41 is provided on a wireline drum 42 which sits on the deck 4. The wireline drum includes a rotational drive mechanism which provides at least the required force to pull the wireline 41 and the weight 39 out of the well. The wireline drum 42 also includes a spooling mechanism.

Figure 2 shows a conventional coilhose intervention set up for a well on a fixed offshore platform or a land well. The well head is thus "dry" in the sense that it is not underwater and is either above the sea surface or is on land. Figure 2 is amended to describe a conventional coilhose intervention set up and the related prior art. Referring to Figure 2, this shows a blow-out preventer (BOP) 2 supported on a deck 4 positioned above a well head 8. The well head 8 is attached to the upper part of the well 36, into which a weight 39 is lowered by the coilhose 50 into the well 36. The mass of the weight 39 and the vertical section of the coilhose 50 pulls the wireline from the coilhose drum 51, via a lower sheave wheel 43 and an upper sheave wheel 44 and through the stuffing box 18 into the well 36. The lower sheave wheel 43 is attached to the lubricator 16 with a tie-down strap 45, enabling the lower sheave wheel 43 to change the direction of the coilhose 50 from horizontal to vertical and vice-versa as it passes. The upper sheave wheel 44, is supported by a vertical lifting chain 46. Arrow 47 indicates the upward force provided by the crane or the like. The weight 39 can be integrated with a tool to perform operations inside the well 36. Below the BOP, a riser 6 extends downwardly to the wellhead. The well head 8 supports a tubing hanger and above the well head a production X- mas tree 10 is provided. Between the X-mas tree 10 and the riser 6 a shear-seal blow-out preventer 12 is provided. A support frame 40 is attached to the top of the stuffing box 18. Between the blow out preventer (BOP) 2 and the stuffing box 18, a lubricator riser 16 is provided to accommodate installation of weights and tools 39 before being deployed into the well 36. A lifting chain 31 supports the weight and forces of the components below, i.e. the pressure control stack, comprising components 12, 6, 2, 16 and 18. Arrow 30 indicates the upward force provided by the crane or the like. The coilhose 50 is provided on a coilhose drum 51 which sits on the deck 4. The coilhose drum 51 includes a rotational drive mechanism which provides at least the required force to pull the coilhose 50 and the weight 39 out of the well. Additionally, the coilhose drum 51 includes a pressure tight circulation swivel 53, connected to the innermost end of the coilhose 50, enabling continuous circulation of fluid through the coilhose 50 throughout the coilhose intervention operation as the coilhose drum 51 rotates. The coilhose drum 51 also includes a spooling mechanism.

Figure 3 shows a conventional coiled tubing intervention set up for a well on a fixed offshore platform or a land well. The well head is thus "dry" in the sense that it is not underwater and is either above the sea surface or is on land. Figure 3 is amended to describe a conventional coiled tubing intervention set up and the related prior art. Referring to Figure 3, this shows a blow-out preventer (BOP) 2 installed on a deck 4 positioned above a well head 8. The well head 8 is attached to the upper part of the well 36, into which a tool 37 is lowered into the well by the steel coiled tubing 22 to perform operations inside the well 36. Below the BOP, a riser 6 extends downwardly to the wellhead. The well head 8 supports a tubing hanger and above the well head a production X-mas tree 10 is provided. Between the X-mas tree 10 and the riser 6 a shear-seal blow-out preventer 12 is provided. Between the blow out preventer (BOP) 2 and the dual stuffing box 18, a lubricator riser 16 is provided to accommodate installation of a tool 37 before being deployed into the well 36. A coiled tubing injector 32 is attached to the top of the dual stuffing box 18. A coiled tubing gooseneck 33 is attached to the top of the coiled tubing injector 32. A gooseneck support strut 35 supports the gooseneck arch at its end towards the coiled tubing reel. A pair of endless chains 34 are installed inside the injector frame. Each of the endless chains 34 are mounted on a pair of spaced sprockets and each having a straight run engaging the coiled tubing. The tubing is compressed between the chains which are hydraulically driven to push the tubing downwardly or pull it upwardly. The steel coiled tubing 22 is provided on a coiled tubing reel 24 which sits on the deck 4. The coiled tubing reel 24 includes a pulling mechanism, which can provide back-tension to the steel coiled tubing 22 and spool the coiled tubing onto the coiled tubing reel. The coiled tubing reel 24 also includes a spooling mechanism and a high pressure swivel 38, as are known for coiled tubing reels.

Traditionally, the coiled tubing reel 24 provides sufficient pull on the run of tubing from the goose neck only to control the spooling of the tubing and prevent it from becoming a relaxed spring due to residual bending forces in the steel. Traditionally, the reel does not act to pull the tubing out of the well. Arrow 30 indicates the upward force provided by the crane or the like. A lifting chain 31 supports the weight and forces of the components below, i.e. the pressure control stack, injector and gooseneck, comprising components 12, 6, 2, 16, 18 32 and 33. The crane etc. typically pulls more that the weight it is supporting, thus making the complete rig-up more rigid and less responsive to horizontal forces from the coiled tubing reel 24.

Figure 4 shows an intervention set-up according to the invention applied on a well on a fixed offshore platform or a land well. The well head is thus "dry" in the sense that it is not underwater and is either above the sea surface or is on land. Figure 4 is amended to describe an intervention according to the invention. Referring to Figure 4, this shows a blow-out preventer (BOP) 2 supported on a deck 4 positioned above a well head 8. The well head 8 is attached to the upper part of the well 36, into which a weight 39 is lowered by the flexible hollow conduit 60 into the well 36. The mass of the weight 39 and the vertical section of the flexible hollow conduit 60 pulls the flexible hollow conduit 60 from the storage spool 61, via a lower sheave wheel 43 and an upper sheave wheel 44 and through the stuffing box 18 into the well 36. The lower sheave wheel 43 is attached to the lubricator 16 with a tie-down strap 45, enabling the lower sheave wheel 43 to change the direction of the flexible hollow conduit 60 from horizontal to vertical and vice-versa as it passes. The upper sheave wheel 44, is supported by a vertical lifting chain 46. Arrow 47 indicates the upward force provided by the crane or the like. The weight 39 can be integrated with a tool to perform operations inside the well 36. Below the BOP 2, a riser 6 extends downwardly to the wellhead. The well head 8 supports a tubing hanger and above the well head a production X-mas tree 10 is provided. Between the X-mas tree 10 and the riser 6 a shear-seal blow-out preventer 12 is provided. A support frame 40 is attached to the top of the stuffing box 18. Between the blow out preventer (BOP) 2 and the stuffing box 18, a lubricator riser 16 is provided to accommodate installation of weights and tools 39 before being deployed into the well 36. A lifting chain 31 supports the weight and forces of the components below, i.e. the pressure control stack, comprising components 12, 6, 2, 16 and 18. Arrow 30 indicates the upward force provided by the crane or the like. The flexible hollow conduit 60 is provided on a storage spool 61 which sits on the deck 4. The flexible hollow conduit 60 is driven into the well 36 by the weights 39 in a controlled manner by the one-directional puller 64, circumferentially gripping around the outer surface of the flexible hollow conduit and maintaining a varying pull in the direction out of the well on the flexible hollow conduit 60. The one-directional puller 64 provides a pulling force 78 to the flexible hollow conduit only in the direction away from the well 36 and does not provide any pushing force on the flexible hollow conduit 60 towards the well 36. The storage spool 61 includes a mechanism which provides only a limited pull on the run of flexible hollow conduit 60 from the one-directional puller 64 sufficient only to control the spooling of the flexible hollow conduit 60 and prevent it from becoming relaxed thus creating a risk of uneven spooling and un-spooling. This is to prevent any locking or intertwining of the flexible hollow conduit 60, preventing any further un-spooling when lowering the flexible hollow conduit 60 into the well 36. The storage spool 61 does not act to pull the flexible hollow conduit 60 out of the well 36.

Additionally, the storage spool 61 includes a pressure tight circulation swivel 53, connected to the innermost end of the flexible hollow conduit 60, enabling continuous circulation of fluid through the flexible hollow conduit 60 throughout the intervention operation as the storage spool 61 rotates. The storage spool 61 also includes a spooling mechanism. Figure 5 shows the invention applied on an offshore subsea well installed on the sea bed. Figure 5 is amended to describe an sub sea intervention according to the invention. Referring to Figure 5, this shows upwardly from the sea bed 70, there are provided a well head and production X-mas tree 8, a production X-mas tree interface 10, a blow-out preventer 2, a lower lubricator package 71 having an emergency disconnect function, lubricator section 72, a stuffing box 18 for the flexible hollow conduit 60. On the sea surface 80, a floating mono-hull vessel 73 is provided with a moon pool opening 74 through which the flexible hollow conduit 60 extends vertically. The vessel 73 is supplied with a tower mast 76 over the moon pool opening 74. The flexible hollow conduit 60 is supplied from a storage spool 61 on the deck of the vessel 73 via an upper sheave wheel 44 and a lower sheave wheel 43. The lower sheave wheel 43 is attached to the tower mast 76 with a tie-down strap 45, enabling the lower sheave wheel 43 to change the direction of the flexible hollow conduit 60 from horizontal to vertical and vice-versa as it passes. The upper sheave wheel 44, is supported by a vertical lifting chain 46, which is attached to a heave compensating system 75 in the top of the tower mast 76. Arrow 77 indicates the upward force provided by the heave compensating system 75, which varying force compensates for any vertical movements of the vessel induced by heave motions from waves and sea conditions. The flexible hollow conduit is exposed to the ambient sea 79 between the vessel and the stuffing box 18 on the sea bed 70. The flexible hollow conduit 60 is provided on a storage spool 61 which sits on the deck 4 of the vessel 73. The flexible hollow conduit 60 is driven into the well 36 through the ambient sea 79 by the weights 39 in a controlled manner by the one-directional puller 64, circumferentially gripping around the outer surface of the flexible hollow conduit and maintaining a varying pull in the direction out of the well on the flexible hollow conduit 60. The one-directional puller 64 provides a pulling force 78 to the flexible hollow conduit only in the direction away from the well 36 and does not provide any pushing force on the flexible hollow conduit 60 towards the well 36. The storage spool 61 includes a mechanism which provides only a limited pull on the run of flexible hollow conduit 60 from the one-directional puller 64 sufficient only to control the spooling of the flexible hollow conduit 60 and prevent it from becoming relaxed thus creating a risk of uneven spooling and un-spooling. This is to prevent any locking or intertwining of the flexible hollow conduit 60, preventing any further un-spooling when lowering the flexible hollow conduit 60 into the well 36. The storage spool 61 does not act to pull the flexible hollow conduit 60 out of the well 36.