Abstract:
The invention concerns a dual-type plug to be used in a wellhead. The plug has a separate retrievable inner plug ( 50 ). Retrieving only the inner plug allows the use of lighter workover equipment than if the whole plug ( 10 ) must be pulled.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a plug for use in a subsea Christmas tree. The invention also relates to a method of intervention in a subsea well, using a plug according to the invention.  
       BACKGROUND OF THE INVENTION  
       [0002]     When developing subsea oil and gas wells there are stringent demands to the control and containment of the well during all aspects of the work, be it drilling, production or later intervention. The needs for control of well pressure have lead to requirements for safe barriers in the well and/or the Christmas tree, both during production and during intervention work.  
         [0003]     In a horizontal type Christmas tree the production tubing hanger is located within the tree and having a horizontal passage aligned with the horizontal production outlet in the tree. The production control valves are located in the horizontal outlet. The tubing hanger passage extends above the production outlet to gain access to the well and this passage must be closed off during production.  
         [0004]     In a conventional Christmas tree, the tubing hanger is se in the wellhead and the production control valves are located in the vertical passage of the tree. The control valves function as barriers during normal production and the tubing hanger includes means for setting a plug in case the well must be closed down.  
         [0005]     In many countries, rules require at least two barriers between the well and the environment. For example, in a horizontal completion the two barriers are set with one in the tubing hanger above the production passage and the other in the tree bore.  
         [0006]     In U.S. Pat. No. 6,050,339 there is shown a horizontal Christmas tree of a type well known in the art. A first plug is set to close tubing hanger passage above the production outlet. An internal tree cap is set in the vertical bore of the tree above the tubing hanger. The tree cap has an axial bore in which a second plug is set. To gain access to the well, both plugs must be removed.  
         [0007]     In U.S. Pat. No. 5,575,336 there is shown another type of a horizontal Christmas tree. As above a first plug is set in the tubing hanger. A tree cap is set in the tree bore above the tubing hanger. A ball valve is located in the tree cap passage. To gain access to the well a tool is run in to open the valve. Thereafter the plug can be removed. This can be done in one operation, thus saving a run.  
         [0008]     During the lifetime of the well various types of work may be carried out to enhance production or to measure conditions in the well. To gain access to a living well a pressure containment device, containing a number of valves, must be connected to the well before the barriers can be removed. The pressure containment device ensures control over the well during the work. Depending on the type of the work, either a lubricator stack or a riser is connected to the pressure containment device.  
         [0009]     Various parameters dictate the size and complexity of the equipment used during intervention, the main concern being the pressure control valves. One of the principal parameters is well pressure. Higher pressure ratings demand larger equipment. Another parameter is the nominal size of the equipment, related to the tool to be employed during intervention.  
         [0010]     In recent years larger type&#39;s completion has become the norm, with tubing sizes up to 9″. The size of the intervention equipment has therefore also grown bigger, since the internal diameter of the pressure control valves and workover riser must be large enough to pull the correspondingly sized plug(s) through. At the same time, intervention tools have become smaller, down to perhaps 2″ or less. Therefore the need for pulling the plug(s) limits the scaling down of the equipment. This means that smaller equipment cannot be employed even if only small size tools are needed for the work in the well. For example, a standard type completion has a 6″ tubing and therefore a 6″ plug. The equipment must therefore be dimensioned with a nominal 6″ bore even if the tool to be used is only 2″ and could be run in on cable or wireline.  
         [0011]     The size and weight of the equipment also influence the choice of the type of vessel used for well intervention, especially governing the load handling capabilities of the vessel. As an example, a complex operation like pulling the tubing requires a full blow out preventer and drilling riser, resulting in the need for using a large drilling rig that can handle this load. At the other end of the scale, a simple sensor can be run in on slickline or cable requiring only a small boat.  
         [0012]     Reducing the size of the equipment can therefore reduce costs dramatically. As an example, if the equipment can be reduced to 4″ nominal size, the weight can be reduced by more than 30% as compared with 6″ equipment. This again allows smaller size vessels to be used and cut costs dramatically.  
         [0013]     One method to redress this problem is to use smaller size plugs in the tree. However, this solution restricts the choice of intervention methods, and some types of intervention will not be possible or the tubing must be pulled to gain access to the well. Therefore, the preferred choice is to use the full size plugs, to retain the freedom of choice.  
       SUMMARY OF THE INVENTION  
       [0014]     According to the invention, this and other problems are solved by designing a retrievable plug, according to the following claim, that contains at least a smaller plug within the main plug. With that, one can choose to pull only the inner plug or the whole plug assembly, as dictated by the need.  
         [0015]     The plug according to the invention comprises a cylindrical housing, which may be adapted to the bore where the plug is going to be set. It further comprises releasable locking devises for holding the plug in fixed relations to the bore, and sealing devises. The sealing devises may be a part of the main cylindrical houses or separate elements hold in position by the plug. The sort of sealing devises necessary will depend on the working environment for the plug, like pressure, temperature, corrosivity etc.  
         [0016]     By the plug according to the invention there is in the main cylindrical housing at, least one bore comprising at least one inner separate retrievable plug. The inner plug is preferably arranged coaxially with the outer main plug. Alternatively it may be arranged asymmetrically in relation with the main plug. There may also be more than one inner separate retrievable plug, for instance two arranged coaxially within each other.  
         [0017]     The inner plugs have releasable locking devises and are equipped with sealing devises. These locking and sealing devises may be different or similar to the locking devised for the main plug. It might in some cases be favourable to have the same systems to be able to use the same tool for either retrieving the main plug with the inner plugs or only one of the inner plugs. In other cases may it be favourable due to available space to have different locking and sealing devises in the different plugs, since the main plug has a larger dimension than the inner plugs.  
         [0018]     The plug may comprise locking devises that cooperate with grooves in the bore where it is supposed to be set, this may also be the case for the inner plug by shaping the bore in the main cylindrical housing so that is corresponds with the locking devises for the inner plug.  
         [0019]     The plug according to the invention is especially suitable to a part of a Christmas tree for an oil and/or gas well for one or both of the required barriers towards the environment for a well. The plug according to the invention may be set in the tubing hanger and/or in the tree cap as necessary and, while the plug is especially useful for horizontal type trees it can also be used with any conventional trees.  
         [0020]     The plug according to the invention may be used anywhere in the well bore where such functionality is desired. For example the plug can be set in the well tubing in deep wells where several tubing strings are employed and where the topmost tubing have a larger diameter.  
         [0021]     In the method for intervention in a well with a plug according the invention one may employ the tool adapted to retrieve the plug with the inner plug or only the inner plug through the riser, dependent on the size of the tools which should be used to perform the intervention work. This gives benefits both in need for equipment and risk during intervention work.  
         [0022]     The invention will in the following be explained with a preferred embodiment which is one not limiting example of how the invention may be employed, with reference to the drawings. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a vertical section through a Christmas tree of a known type,  
         [0024]      FIG. 2  is a: vertical section through a plug according to the invention, and  
         [0025]      FIG. 3  is a schematic drawing showing various intervention methods. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     In  FIG. 1  there is shown a Christmas tree of a type that is well known in the art. This is meant for illustration purposes only and it should be understood that many types of Christmas trees with various configurations and designs exist and can be used with the invention. Therefore only the main components relating to the invention will be described, since such design must be considered well known for persons skilled in the art.  
         [0027]     Christmas tree  1  includes a main housing  2  with a central bore  3  and a horizontal production outlet  4 . A tubing hanger  5  is locked in bore  3  with tubing  6  extending downwardly therefrom to a production zone in the well. The tubing hanger has a bore  7  axially aligned with the tubing and with the same internal diameter. The tubing hanger includes horizontal passage  8  extending from the bore of the tubing hanger to the outside of the hanger.  
         [0028]     When installing the tubing hanger with the tubing in the well, the tubing hanger is oriented in the tree such that its horizontal passage  8  aligns with the tree production outlet  4 . Production fluid from the well is conveyed through tubing  6  and outlet  4  and to a manifold in the known manner.  
         [0029]     After completion of the well, the part of bore  7  located above passage  8  is closed off with a plug  10 . The plug is retrievably set using a suitable tool (not shown) with the help of latching dogs designed to interface with a groove in the wall of bore  7 , as will be explained in more detail later.  
         [0030]     An internal tree cap  20  is set in the tree bore  3  above the tubing hanger. The tree cap has an internal bore  21  in which is located a ball valve  22 . The ball valve is normally closed but can be opened using a suitable tool (not shown).  
         [0031]     Alternatively, a second plug can be set in tree cap bore  21 , for example as shown in U.S. Pat. No. 6,050,339.  
         [0032]      FIG. 2  shows a plug  30  according to the invention. The plug is a sliding fit within bore  7  in the tubing hanger. The plugs  30  main housing includes outer cylindrical housing  31  and inner cylindrical housing  33 , the inner housing being of shorter length than the outer part. The outer and inner cylindrical housing defines between them an annular space. The sealing devise in the form of a seal  43  is located around the outer surface of the plug  30 , to seal between plug  30  and bore  7  when the plug is set. Outer housing  31  has a number of regular spaced openings  42  radially through its wall. In the openings are located the locking devises, in this embodiment in the form of locking dogs  41  that are held in a normally retracted position but can be made to extend radially to interface with a groove (not shown) in the tubing hanger bore  7 . The inner wall of outer housing  31  has an upwardly facing shoulder  32 . Inner wall of outer housing  31  is, along part of its upper end, machined out to form a circumferential slot  37 .  
         [0033]     Inner cylindrical housing  33  has an inner surface with a lower part machined to form a seal surface  44 , an upward facing shoulder  45  and a ring groove  46 .  
         [0034]     A locking sleeve  35  is mounted for axial movement in the plug in annular space between outer  31  and inner  33  cylindrical housing. A downward facing shoulder  34  on the sleeve engages with shoulder  32  to act as a stop. Shoulder  34  divides locking sleeve in a lower part  39 , middle part  40  with a greater outer diameter and upper part  41 . Upper part has an inward facing flange  36 . Also in upper part  41  but in the outer surface, a radial groove is machined out and a ring  38  located in the groove. The ring  38  protrudes outwards from locking sleeve to slide in slot  37 . This, together with shoulder  34  acts as a limit for upward and downward movement of locking sleeve  35 .  
         [0035]     Alternatively, the slot  37  may be a number of radially displaced axial grooves and ring  38  a number of protruding pins. This will prevent the locking sleeve from rotating but allows axial movement between the lower position shown in  FIG. 2  and an upper position (not shown).  
         [0036]     When locking sleeve is in its lower position as shown in  FIG. 2  the middle part  40  pushes the locking dogs  41  outwards to engage with a ring groove in tubing hanger bore  7 . Using a suitable tool to engage with flange  36 , the locking sleeve can be pulled upwards. Locking dogs  41  will move out of engagement with middle part  40  and into engagement with lower part  39 . The locking dogs  41  are now free to move into their retracted position. The plug  10  can now be pulled out from engagement with the tubing hanger bore  7 .  
         [0037]     An inner plug  50  is shown position co-axially within plug  30 . Inner plug  50  is cup-shaped, with cylindrical part  51  and bottom  52 . Inner plug  50  is a sliding fit within plug  30 . Cylindrical part  51  has an outer wall with a downward facing shoulder  57  and regular spaced radial openings  62 . The inner wall of cylindrical part  51  has an upward facing shoulder  55 . At its upper end there is machined out a slot  56  of the same construction as slot  37 . At the lower end of the plug are located seals  63  to seal against the surface  44  of plug  30 . Locking dogs  53  are located in openings  62  and are held in a normally retracted position but can be made to extend radially to interface with groove  46  in inner wall of plug  30 . A locking sleeve  54  is mounted within cylindrical part  5   1 . Locking sleeve has a downward facing shoulder  60  that abuts against shoulder  55 . At its upper end locking sleeve  54  has an inward facing flange  58 . At its upper end is a circumference groove in which a ring  59  is mounted. The ring  59  slides in slot  56  to limit the locking sleeves axial movement. The locking sleeve  54  is thus movable between a lower position (as shown in  FIG. 2 ) with shoulder  55  acting as a stop, and an upper position (not shown). When locking sleeve  54  is in the position shown in  FIG. 2 , the sleeve will force the dogs  53  outwardly into engagement with groove  46 . The inner plug  50  is now locked within plug  30 . Using a suitable tool acing on flange  36 , the locking sleeve can be moved upwards to allow locking dogs  53  to retract and go out of engagement with groove  46 . The inner plug is now unlocked from plug  30  and can be retrieved.  
         [0038]     Inner plug  50  can be equipped with means for engaging a fishing tool, for example profiles  64  as shown in  FIG. 2 .  
         [0039]     Where there are two plugs in the tree, the upper plug in the tree cap can be designed so that its inner plug is slightly larger than inner plug  50 . After removing inner plug in the upper plug, the inner plug of the lower plug can then be removed through the upper plug without the need for removing the whole upper plug.  
         [0040]     In  FIG. 3  examples of various equipment used for subsea intervention work are shown. The drawing is for illustrating purposes only, and only to show examples of the types of equipment commonly used for intervention and workover operations since there exist many variations of equipment of each type.  
         [0041]     Three types of vessels are shown, a full drilling or intervention rig  70 , an intermediate boat-type vessel  71  and a relatively small boat  72 . The rig is used for heavy intervention type work, having full facilities for all type of intervention work and would include a tower hoist system, heave compensation system, storage space for risers, drillpipe and blowout preventers (BOP) and so on. The intermediate vessel  71  may also be equipped with tower hoist systems capable of running drillpipe but will normally be used for coil tubing operations and smaller workover risers or wireline work, and do not have the large handling capacity of the rig. This type of vessels is normally held in position with dynamic positioning (DP) systems.  
         [0042]     The small vessel  72  have limited handling capacity and are therefore restricted in the type of work and in what weather conditions it can be used and will normally only be used to run equipment on wireline, cable or slickline. This type of vessel are only equipped with cranes.  
         [0043]     Three types of intervention are also illustrated. The equipment shown can be connected to a Christmas tree that can either be a conventional  73  or horizontal  74  Christmas tree. Different designs are talken care of by using adapters between the XT and workover equipment.  
         [0044]     One type of intervention requires a BOP  75  running on riser  76 . The riser can be either 20″ or 14″ as necessary. The tools are normally run in by drillpipe. Since both BOP and risers are very heavy equipment, a heavy rig is normally employed. The second type of intervention uses a pressure control device commonly called a Lower Riser Package (LRP)  77 . Between a workover riser  79  and the LRP  77  is located an Emergency Disconnect Package (EDP)  78 . Well tools are normally run in with coil tubing or wireline. There are several sizes of this type of equipment, dictated by the size of the tubing and the type of work. In the third type the pressure control device is commonly called a Lower Intervention package (LIP)  80  to which is attached a lubricator  81 . Well tools are run in on cable or wireline and the lubricator is used to gain control access to the well.  
         [0045]     As the weight of the equipment increases, the vessel must be able to handle the loads. Especially critical is the passage of the equipment during the splash zone, but limitations are also imposed on the vessels capability of handling the equipment in heavy seas.  
         [0046]     This invention is specially suited to work requiring a workover riser. Several types exist, with nominal sizes 7″, 6″ or 5″. As previously indicated, the nominal size dictates the size of the other equipment, e.g. the pressure control valves in the LRP. A reduction in size from 6″ to 5″ riser may result in a weight saving of more than 30%. It is therefore important to choose the right size of the equipment for the work to be done, since this increases the number and choice of vessels capable of handling the work.  
         [0047]     To gain access to the well, the pressure control assembly (LRP) is fist connected to the Christmas tree. Then the workover riser and EDP are connected to the LRP. A second pressure control assembly (surface BOP) is attached to the top of the workover riser. A tool is run in to remove plug  10  to open the well  10 . If only smaller tools are scheduled to be used during intervention, a smaller LRP and workover riser can be used. A tool is run in to engage with flange  36  to pull the inner plug  30 .  
         [0048]     The method of the invention therefore allows for a wider choice of both equipment and vessels and allows the equipment to be more specifically tailored to the work. When work is planned in a well, it will first be analysed what type of work is necessary. If only small tools are to be used, the choice of workover equipment will be chosen accordingly and a suitable vessel commissioned. After connecting the equipment to the well, a tool is run in to release and retrieve the plug. If the job is light, only the inner plug will be retrieved through the smallbore riser; The work in the well is completed and the equipment disconnected from the well. If it is desired to use larger tools, again a suitable size of workover riser and vessel is chosen. After connecting to the well, a tool is run in, this time to retrieve the main plug, allowing larger tools to be run into the well.