Patent Publication Number: US-8985220-B2

Title: Well intervention system

Description:
FIELD OF THE INVENTION 
     The present invention relates to an improved well intervention system. 
     BACKGROUND TO THE INVENTION 
     Risers are widely used with subsea oil wells. A riser is a tubular attached at one end to a wellhead on the seabed and at the other end to a floating rig or a boat on the surface. The riser passes through a hole in the rig floor or the deck of the boat, and is topped by a riser access device, which may incorporate valves, to allow fluids to enter or leave the well, or plugs or the like. 
     The riser must be maintained in tension to prevent it from collapsing, and tension is applied to the riser by tensioners which extend from the vessel and attach to the riser at a location beneath the vessel deck or floor. The tensioners are generally cables which pay out and reel in as the vessel moves due to the heave of the sea, maintaining a constant tension on the riser, and maintaining the vertical position of the access device fixed relative to the seabed. As the access device is fixed relative to the seabed, and to compensate for the rise and fall of the vessel, the access device is located at a considerable height above the vessel, to prevent the riser access device impacting on the floor or deck of the vessel. 
     However, when is necessary for an operator access to the riser, it is undesirable for the access device to be moving relative to the vessel floor or deck; instead it is preferable for the access device to remain stationary relative to the vessel. To make this possible, a slip joint is provided between the access device and the point at which the tensioners attach to the riser. The slip joint accommodates the heave of the sea permitting the access device to be lowered to the deck or floor. To maintain the riser in tension when access is not required, a compensator applies a lifting force to the access device sufficient to extend the slip joint to the maximum extent of its travel. 
     The compensator can apply the lifting force to the access device through a lifting frame which may be provided above the access device to permit equipment such as injector valves to be attached to the access device or to allow tool strings to be lowered through the access device into the riser. 
     Conventional riser systems have a number of drawbacks. For example, the slip joint can be unreliable and can only be safely utilized when depressurised as they are subject to a pressure end load effect. When the slip joint is depressurised the surface isolation valves cannot be utilised and if there is any leakage past the primary downhole well isolation device both personnel on the vessel and the vessel itself can be exposed to hydrocarbons from the well and the associated consequential risk. 
     Furthermore, in rough seas or in dynamic positioning mode it can be difficult to maintain the position of the vessel directly over the subsea wellhead. Deviation from this position applies a bending moment to the riser, which can result, in some cases, in the vessel having to detach from the wellhead to avoid catastrophic failure. To accommodate the movement of the vessel, the surface equipment has to placed a considerable height above the vessel floor or deck to avoid it impacting with the vessel floor as the vessel moves off location due to the increased distance between the vessel and the well. 
     There are also safety issues associated with accessing the riser from above the flowhead. The elevated positioning of the flowhead and the lifting frame requires high level working on a platform which is moving in relation to the vessel floor or deck to introduce tools or equipment into the riser. 
     A further area of concern is during operation, the compensator system, which applies a tension to the riser through the surface equipment, such as the lifting frame, has been known to over-tension the riser causing the riser to fail with catastrophic consequences. 
     An object of at least one embodiment of the present invention is to obviate or mitigate at least one of the disadvantages of the aforementioned well intervention systems. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a well intervention system for intervening in a subsea well, the well intervention system comprising: 
     a riser access device adapted to be located below a vessel work floor; and 
     a riser adapted to be tensioned and extend from a subsea wellhead to the access device; 
     wherein, in use, the riser access device is attached to the riser at a location below the vessel work floor. 
     In one embodiment of the present invention, the well intervention system provides an arrangement which substantially removes the need for high level working to, for example, introduce a tool string into the riser. As the riser access device is located, in use, below the vessel work floor, operators can assemble the tool and lower the tool into the riser from the level of the work floor. 
     For the avoidance of doubt by “vessel” it is meant any rig, ship, boat, buoy or other vessel suitable for performing drilling or well intervention operations and by “work floor” it is meant the deck or floor of the vessel from where operators execute well intervention operations. 
     Preferably, the riser access device is releasably attachable to the riser. 
     Preferably, the riser access device is attachable by means of a latch. 
     Preferably, the riser access device is sealably attachable to the riser. In one embodiment the riser access device can form a seal sufficient to contain the pressure within the riser. The seal may be sufficient to seal the riser from the external environment. 
     The riser access device may comprise a number of valves including one or more of a flow valve (for removing fluid from the well), a kill valve (for injecting fluid into the well), and a master valve for isolating the well. 
     Preferably, in use, the riser is tensioned from the vessel. 
     Preferably, the well intervention system comprises at least one tensioning device for tensioning the riser. 
     Preferably, in use, the at least one tensioning device extends from the vessel. 
     Preferably, the/each tensioning device is attached to the riser. 
     Preferably, the/each tensioning device is releasably attachable to the riser. 
     Preferably, the riser further comprises at least one attachment point. 
     Preferably, the/each tensioning device is attached to the riser at an attachment point. 
     Preferably, the riser access device is located adjacent the/each attachment point. 
     Preferably, the riser access device is located above the/each attachment point. Locating the riser access device above and adjacent the/each attachment point obviates the need for a slip joint between the attachment point(s) and the riser access device. 
     Alternatively or additionally, the/each tensioning device is attached to the riser access device. 
     In one embodiment, the tensioning devices are tensioning devices found on rigs or boats suitable for performing drilling or well intervention operations. 
     Alternatively or additionally, the riser is tensioned from a second riser. 
     Preferably, the riser is inside the second riser. 
     In another embodiment, a compensator system is attached to the riser access device. 
     Preferably, the compensator system is attached to the riser access device by means of a lifting bridle. 
     Alternatively or additionally, the compensator system is attached to the riser access device by means of a running tool. 
     Preferably, the riser has a fixed length. 
     Preferably, the well intervention system includes at least one flexible member adapted to extend from the riser access device to the vessel. Utilising flexible members to extend from the riser access device to the vessel reduces the effects of movement of the vessel with respect to the wellhead on the seabed. 
     Preferably, at least one of the flexible members is in fluid communication with the riser. 
     Preferably, at least one of the flexible members is adapted to receive fluid from the riser. 
     Alternatively or additionally, at least one of the flexible members is adapted to present fluid to the riser. Utilising flexible members to carry fluids to and from the riser prevents the vessel applying a bending moment to the riser as the vessel moves with respect to the wellhead on the seabed. 
     Alternatively or additionally, at least one of the flexible members comprises a control line. A control line can be provided to send control signals to tools in the riser. 
     Preferably, the riser access device is adapted to be connected, in use, to an apparatus located on or above the vessel work floor. 
     Preferably, in use, the riser access device is connected to the apparatus by means of a variable length tubular. A variable length tubular is useful if the material may have to be contained, such as coiled tubing for example, is to be passed from the apparatus down to the riser access device. It is preferred to contain coiled tubing as a sudden movement of the vessel towards the riser access device could cause coiled tubing between the vessel and the riser access device to buckle. 
     In one embodiment the riser comprises an open water riser. 
     In an alternative embodiment the riser comprises a marine riser and a well entry riser. 
     In this embodiment, the riser access device is attached to the well entry riser. 
     Preferably, at least one tensioning device is attached to the marine riser. 
     Preferably, the well entry riser is tensioned by engagement with the marine riser. 
     Preferably, tensioning apparatus is provided to apply a substantially constant tension to the well entry riser. 
     Preferably, the tensioning apparatus is engaged with the marine riser. 
     Preferably, the tensioning apparatus is as described in the applicant&#39;s co-pending patent application GB 0613393.8. 
     According to a second aspect of the present invention there is provided a method of intervening in a well, the method comprising the steps of: 
     attaching a tensioned riser suspended from a vessel to a subsea wellhead; and 
     accessing the top of the riser through a riser access device located below the vessel work floor. 
     According to a third aspect of the present invention there is provided a well intervention system for intervening in a subsea well, the well intervention system comprising: 
     a riser access device; 
     a tensioned riser adapted to extend from a subsea wellhead to the access device; and 
     a vessel having a vessel work floor; 
     wherein the riser access device is attached to the riser at a location below the vessel work floor. 
     Preferably, the vessel work floor defines an aperture adapted to permit the riser access device to pass therethrough. Such an arrangement permits the riser access device to be raised from or lowered onto the riser. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic view of a well intervention system according to a first embodiment of the present invention; 
         FIG. 2  is a schematic view of the well intervention system of  FIG. 1  shown with a running tool and a wireline pack-off system, with the running tool disconnected from the riser access device; 
         FIG. 3  is a schematic view of the well intervention system of  FIG. 2  with the running tool connected to the riser access device; 
         FIG. 4  is a schematic view of the well intervention system of  FIG. 1  shown with a running tool, a wireline pack-off system, and a tool string with the pack-off system shown above the work floor; 
         FIG. 5  is a schematic view of the well intervention system of  FIG. 1  shown with a coiled tubing feed system; 
         FIG. 6  is a schematic view of the well intervention system of  FIG. 5  with the gooseneck lowered; 
         FIG. 7  is a schematic view of the well intervention system of  FIG. 1  shown with an alternative coiled tubing feed system; 
         FIG. 8  is a schematic view of the well intervention system of  FIG. 7  with the gooseneck lowered; and 
         FIG. 9  is a schematic view of a well intervention system according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Reference is firstly made to  FIG. 1  a schematic view of a well intervention system generally indicated by reference numeral  10 , according to a first embodiment of the present invention. The well intervention system  10  comprises a riser access device  12 , a tensioned open water riser  14  extending between a subsea wellhead  16  and the riser access device  12  and a vessel  18 . The vessel  18  is a floating platform of which only the work floor  20 , from which well intervention operations are performed, is shown for clarity. 
     As can be seen from  FIG. 1 , the riser access device  12  is attached to the top  14   a  of the open water riser  14  at a location below the work floor  20 . Such an arrangement removes the need for high level working to introduce, for example, a tool string into the open water riser  14 , as will be described in detail later. 
     Attached to the riser access device  12  are a flexible flow line  22  and a flexible kill line  24 . The open water riser  14  is kept in tension by tensioning cables  26  are connected between the work floor  20  and the top  14   a  of the riser  14 . As the connections between the riser access device  12  and the work floor  20 , that is the flow and kill lines  22 , 24  and the tensioning cables  26 , are all flexible, then deviation of the vessel  18  from directly above the subsea wellhead  16  results in a limited bending moment being applied to the riser  14 . 
     A flexible lifting bridle  40  is attached to a vessel-mounted compensator system  42 , to raise and lower a running tool  30  into engagement with the riser access device  12 . As the lifting bridle  40  is flexible, movement of the vessel  18  from directly above the subsea wellhead  16  results in a minimal bending moment being applied to the riser  14 . 
     The riser access device  12  further includes a master valve  28  for shutting the riser  14  quickly in the event of an emergency. The master valve  28  seals a production tube (not shown), housed within the open water riser  14 , from the running tool  30  and the flow and kill lines  22 ,  24 . In addition, the master valve  28  is adapted to sever any intervention equipment such as tooling, wire line or coiled tubing passing through the valve  28 . Various well intervention operations can be performed within the open water riser  14  using the system of  FIG. 1 . Examples of these operations will now be discussed with reference to the following Figures. 
     Referring now to  FIG. 2 , there is shown a schematic view of the well intervention system of  FIG. 1  with a running tool  30  and a wireline pack-off system  52 , with the running tool  30  disconnected. The wireline pack-off system  52  is mounted to the running tool  30 , and provides a conduit through which wireline  54  can be fed into the open water riser  14 . The running tool  30  and wireline pack-off system  52  are lowered on to the riser access device  12  by the lifting bridle  40 . A guide  34  is provided to assist in locating the running tool  30  on the riser access device  12 . A flexible umbilical  56  is provided to control the functions of the wireline pack-off system  52  and running tool  30 . An additional umbilical (not shown) controls the functions of the riser access device  12 . 
     Referring to  FIG. 3 , there is shown a schematic view of the well intervention system  10  of  FIG. 1  with the running tool  30  connected to the riser access device  12 . The open water riser  14  is provided with a lubricator valve system  32 . When closed, the section of riser  14  between the riser access device  12  and the lubricator valve system  32  is isolated from well and can be depressurised. The master valve  28  can now be opened permitting access to the open water riser  14 . The wireline tooling (not shown) can then be assembled and lowered into the riser  14 . Once the wireline pack-off system  52  and the running tool  34  are connected to the riser access device  12 , the pressure integrity of the well intervention system  10  can be confirmed by hydrostatically testing the system  10  against the lubricator valve system  32  once the integrity of the upper section of riser  14  and riser access device  12  have been confirmed, the lubricator valve system  32  can be opened and the wireline  54  with associated tooling can be lowered into the lower part of the riser  14  and the well by the wireline pack-off system  52 . 
     Referring now to  FIG. 4 , there is shown a schematic view of the well intervention system of  FIG. 1  shown with a running tool  30 , a wireline pack-off system  52 , and a tool string  60 . As can be seen from  FIG. 4 , a latch connector  62  is provided at the end of the wireline  54  and the pack-off system  52  is raised above the work floor  20  so the tool string  60  can be assembled at the level of the work floor  20  without the need for high level working. A bushing plate  64  mounted to the work floor  20  enables sections of the wireline tool string  60  to be suspended from the work floor  20 , facilitating assembly of the tool string  60 . Once assembled the latch connector  62  is connected to the top of the tool string  60 . The bushing plate  64  is then removed and the running tool  30 , wireline pack-off  52  and the tool string  60  are lowered through the work floor  20  and the tool string  60  is fed into the open water riser  14  as described with reference to  FIGS. 2 and 3 . 
     The well intervention system  10  of the present invention is also suitable for use with coiled tubing. Referring to  FIG. 5 , there is shown a schematic view of the well intervention system of  FIG. 1  shown with a coiled tubing feed system  70 . The coiled tubing feed system  70  comprises a grip/seal BOP  72 , a shear/seal BOP  74 , a coiled tubing injector head  76  and a gooseneck  78 . Coiled tubing  82  is fed to the gooseneck  78  from a drum  84  which is mounted to the vessel deck  20 . The drum  84  pays out or reels in the coiled tubing  82  at a speed which accommodates the sea heave. 
     The BOPs  72 , 74  and the injector head  76  are attached to the running tool  30  and are lowered into engagement with the riser access device  12  by the lifting bridle  40 . Once in position the goose head  78  can be lowered on to the work floor  20  ( FIG. 6 ). When this happens the tension of the lifting bridle  40  and the compensator cables  42  is relaxed and, as they are flexible, they adopt a relaxed or non-tensioned appearance as shown in  FIG. 6 . 
     Referring now to  FIG. 7 , there is shown a schematic view of the well intervention system of  FIG. 1  but with an alternative coiled tubing feed system  70 . In this system  70 , the lifting bridle has been replaced with a variable length telescopic tubular  80 . The variable length tubular  80  comprises a number of telescopically arranged tubing sections  86 . As the sea level rises and falls the variable length tubular accommodates the change by the tubular sections telescoping with respect to each other. The variable length tubular  80  is provided to support the coiled tubing and prevent it from buckling. A tendency for the coiled tubing to buckle may occur if, for example, there is a significant sea heave. In this circumstance, the drum  84  may not be able to accommodate the rapid increase in sea level, and the coiled tubing between the gooseneck  78  and the injector head  76  may be subject to a compressive, buckling force. In this situation, the variable length tubular restricts the movement of the coiled tubing and prevents it from buckling. 
     The telescopic variable length tubular  84  also permits the gooseneck to be lowered to the deck  20  prior to use ( FIG. 8 ). 
     Reference is now made to  FIG. 9 , a schematic view of a well intervention system  110  according to a second embodiment of the present invention. Features in common between the first and second embodiments are given the same numeral in the second embodiment as the first incremented by 100. 
     The well intervention system  110  of  FIG. 9  includes a well entry riser  188  which runs inside a marine riser  190 . As can be seen the riser access device  112  is attached to the well entry riser  188  at a location below the work floor  120 . 
     The marine riser  190  is kept in tension by tensioning cables  126  which extend between the work floor  120  and the top of the riser  190   a . The well entry riser  188  is tensioned off the marine riser  190  by a telescopic tensioning device  192  of the type disclosed in the applicants co-pending United Kingdom application number GB 0613393.8. An example of the tensioning device  192  is sold by Enovate Systems Limited under the brand name EN-TENSE™. 
     The tensioning device  192  has a first portion (not shown) coupled to the well entry riser  188 , a second portion (not shown) coupled to the marine riser  190  and a hydraulic fluid supply (not shown) providing relative movement between the first portion and the second portion to tension the well entry riser  188 . 
     Various modifications and improvements may be made to the above-described embodiments without departing from the scope of the invention. For example, although the vessel of the described embodiments is a floating platform, the system of the present invention could be used with a boat or other vessel suitable for performing well interventions.