Abstract:
An improved valve for sealing a wellbore and method of use, in particular for prevention of well blow outs. In one embodiment, the apparatus is designed to seal a well through rotation of rams. The rams may be actuated to seal the well without having to work against full well pressure as fluid volumes in the apparatus housing, which also contains the rams, are preserved. This functionality allows the apparatus to be constructed with smaller dimensions than existing prior art blow out prevention valves aiding handling and manoeuvrability. The improved valve according to one embodiment also has a mechanism that guides a wireline or a pipeline being run in the wellbore to the centre of the bore and allows a seal to be formed around the wireline or pipeline.

Description:
This application claims priority from United Kingdom (GB) Patent Application Number 0425117.9 filed on 13 Nov. 2004. 
   FIELD OF THE INVENTION 
   The present invention relates to valves, and in particular to an improved valve for use in wireline and wellhead control systems for the oil and gas industry. In one aspect the invention relates to an improved blow out preventer (BOP). 
   BACKGROUND OF THE INVENTION 
   Specialised valves, including blow out preventers (BOPs), have been available for many decades, and provide a means for sealing the wellbore at the wellhead against well pressure if the drilling crew loses control of formation of fluids or if required to seal a wireline or logging cable during well service operations. A conventional ram type BOP is located at the wellhead, and consists of two halves of a cover for the wellbore, located on diametrically opposed sides of the well bore. The covers consist of steel rams and elastomeric ram blocks, and are typically actuated by large diameter hydraulic cylinders located on each side of the wellbore. If a well control event occurs, the hydraulic cylinders will be remotely actuated to force the rams to enter the wellbore from either side, meeting at the centre to seal against well pressure below. Importantly, the BOP must have the ability to seal the wellbore without cutting the wireline, allowing subsequent retrieval of the toolstring. 
   The ram blocks will typically have cooperating, shaped surfaces to ensure alignment. The ram blocks are available in a variety of configurations, and for applications where pipeline or wireline is being run in the well will be provided with a cut-out formation (typically a corresponding pair of semi-circular recesses defining a narrow throughbore) designed to allow the wireline or pipeline to extend through the BOP without damage. The ram blocks will be designed such that a tight seal is formed around the pipeline or wireline and around the aperture through which the rams move. The ram blocks will often be provided with a guide, such that when the rams move from an open to a closed position, the pipeline or wireline is guided into the centre of the wellbore such that it is received in the cut-out formation without damage. 
   The rams in a typical ram BOP are self-energising, in the sense that after they are initially actuated by the hydraulics, well pressure is allowed to get behind a part of the ram, forcing it towards a closed position. The elastomeric seals of the ram blocks are U shaped in longitudinal section through the plane which the rams extend, and extend fully across the opening of the wellbore. The U-shaped seals allow the well pressure to force the seals upward and inward, such that the well pressure energises the seal and tends to keep the rams in a sealed configuration. 
   Other variations on the ram BOP are available, including rams designed to completely shear through pipe or wireline. In addition, some arrangements are manually operated rather than hydraulically operated. In this case, the actuators normally consist of large screws and are rotated by use of a large handle to generate the torque required to drive the rams against the well pressure. 
   Presently available ram type BOPs suffer from a number of deficiencies and drawbacks. In particular, many of the drawbacks arise from the size of the apparatus. Available designs consist of a pair of rams arranged perpendicular to the wellbore, and extending on either side of the wellbore by up to 2.5 meters (around 100 inches). This has significant implications for the construction of well control apparatus at the rig. Ideally, the BOP stack would be located close to the wellhead, to maximise the height above available to accommodate the toolstring. However, in practice, it is difficult to manoeuvre the BOP stack through platforms on the rig structure. A platform on the rig structure would typically have a hole in the grating of around 24 inches diameter (depending on the diameter of pipe passing through). This opening is too small to allow a ram-type BOP to be passed through in its usual upright configuration where the hydraulic actuators are oriented horizontally. This requires the BOP to be manoeuvred carefully such that the cross-axis access is oriented vertically, allowing the BOP to be passed through the opening. This is a difficult and dangerous operation due to the size and weight of the BOP stack. A reduction in weight is limited by the need for large diameter hydraulic rams and high strength steel components. 
   As an alternative, the platform may be provided with a larger diameter slot to allow the passing of the BOP to be conducted more easily. However, this has implications for the health and safety of personnel working at the platform, and may require installation railings or other barriers to prevent personnel from falling through the slots. It is therefore more usual to install the BOP stack at an elevated height above the wellhead, of in excess of 13 meters (40 feet). The consequence is that a greater overall height of well control apparatus is required, or alternatively restrictions are placed on the length of tool string available. This deficiency is particularly important for concurrent operations. 
   Existing BOP designs suffer from other deficiencies. These include problems with the structural strength of the ram. The ram blocks located on either side of the wellbore will typically include a recess and protrusion which cooperate to ensure that the ram blocks are appropriately aligned. The recess formed on a ram will reduce the structural strength in the ram. Moreover, the recess may in fact be formed on an outer surface of the ram, meaning that as the ram is sealing it has a portion that is unsupported. The ram is therefore “floating” rather than being fully supported by the body of the BOP. This can also result in deformation of the ram, impacting on its application to high pressure BOPs. 
   The arrangement of a typical ram BOP can also cause difficulties in guiding the pipeline or wireline to the right part of the ram block formation (i.e. the cut-out formation). These difficulties arise in part due to the problems with keying the rams appropriately. In addition, typically the piston assembly on one side of the wellbore will be slightly looser than the opposing ram, resulting in the loose piston assembly being driven to the full extent of its travel before the opposing piston assembly begins to move. This increases the probability of the wireline or pipeline being snagged or damaged by the guiding profile. This can damage the wireline or pipeline and in severe cases cause it to be sheared. 
   A further deficiency of typical ram-type BOPs is the time taken to actuate the rams, due to the high volume hydraulic cylinders, or worse, the time required to manually close the rams. This has safety implications. 
   The present applicant has identified the need for an improved valve design, and in particular an improved BOP design that differs significantly from a conventional ram type BOP. 
   It is a first aim of the invention to provide an apparatus suitable for sealing a bore that obviates or at least mitigates some of the drawbacks and deficiencies of available valves and/or BOPs. In particular, it is an aim of at least one aspect of the invention to provide an improved valve or BOP for sealing around a wireline, pipeline or conduit in a wellbore. 
   It is a further aim of the invention to provide apparatus for sealing a wellbore of reduced external size, and in particular apparatus of lesser lateral dimension and reduced weight when compared with a typical ram type BOP. 
   A further aim of the invention is to provide apparatus for sealing a wellbore, where the apparatus has a reduced requirement for hydraulic actuation when compared with a typical ram type BOP. 
   Further aims and objects of the invention will become apparent from the following description. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the invention, there is provided apparatus for sealing a wellbore, the apparatus comprising a ram assembly having a pair of rams rotatably mounted in a housing and actuating means, the apparatus having a first open position in which fluid may pass through the wellbore and a second closed position in which the rams abut to seal the wellbore, wherein the apparatus is moved between the open and closed positions by rotating the rams. 
   In the context of this description, the word ram should not be taken to imply a linear movement of components, as with conventional ram-type BOPs. The rams and ram assemblies described herein are analogous to the rams in a conventional ram-type BOP in the sense that they move together to abut and form a seal. References to rams should be taken to mean assemblies, components, members or blocks which are moveable to a position in which they form an obstruction in the wellbore. 
   Preferably, the apparatus has a maximum lateral dimension of 80 cm. 
   Preferably, the maximum lateral dimension is less than 50 cm. 
   Preferably, the maximum lateral dimension is less than 40 cm. 
   According to a second aspect of the invention, there is provided apparatus for sealing a wellbore around a wireline or pipeline being run in the wellbore, the apparatus comprising a ram assembly and actuating means, wherein the apparatus comprises a first position in which the wellbore is open and a second position in which the wellbore is closed, wherein the actuating means causes the ram assembly to rotate from its first open position to its second closed position in which the wellbore is sealed around the wireline or pipeline. 
   According to a third aspect of the invention there is provided a apparatus for sealing a wellbore, the apparatus comprising a pair of rams moveable from a first position in which the wellbore is open and a second position in which the rams seal the wellbore around a wireline or pipeline, characterised in that when the apparatus is in its operating orientation, the apparatus has a maximum lateral dimension which allows it to pass through an aperture in a rig platform through which the wellhead control equipment passes. 
   Preferably, the maximum lateral dimension is less half of the maximum lateral dimension of a ram-type BOP operating on an equivalent diameter wellbore at equivalent well pressure. 
   Preferably, the maximum lateral dimension is less a third of the maximum lateral dimension of a ram-type BOP operating on an equivalent diameter wellbore at equivalent well pressure. 
   Preferably, the maximum lateral dimension is less than 80 cm. 
   Preferably, the maximum lateral dimension is less than 50 cm. 
   Preferably, the maximum lateral dimension is less than 40 cm. 
   According to a fourth aspect of the invention there is provided a apparatus for sealing a wellbore, the apparatus comprising a pair of rams moveable from a first position in which the wellbore is open and a second position in which the rams seal the wellbore around a wireline or pipeline, characterised in that when the apparatus is in its operating orientation, the apparatus has a maximum lateral dimension of less than 80 cm. 
   Preferably, the apparatus comprises a pair of rams rotatably mounted in a housing and actuating means, wherein the apparatus is moved between the open and closed positions by rotating the rams. 
   According to a fifth aspect of the invention, there is provided apparatus for sealing a wellbore, the apparatus comprising a ram assembly having a pair of rams, and actuating means, the apparatus having a first open position in which fluid may pass through the wellbore and a second closed position in which the rams abut to seal the wellbore, wherein the rams are moved from the first and second positions without substantially changing the volume of the actuating means exposed to well pressure. 
   Preferably, the apparatus comprises a pair of rams rotatably mounted in a housing and actuating means, wherein the apparatus is moved between the open and closed positions by rotating the rams. 
   According to a sixth aspect of the invention there is provided a blowout preventer comprising a ram assembly having a pair of rams rotatably mounted in a housing, the blowout preventer having a first open position in which fluid may pass through the wellbore and a second closed position in which the rams abut to seal the wellbore, wherein the blow out preventer is adapted to be moved between the open and closed positions by rotating the rams. 
   The following statements reflect optional features of any of the first to sixth aspects of the invention. 
   Optionally, the rams comprise formations for locating a wireline or pipeline in the closed position. 
   Preferably, the apparatus comprises alignment means for guiding a wireline or pipeline towards the centre of the wellbore during rotation of the rams. 
   More preferably, the apparatus comprises alignment means for guiding a wireline or pipeline towards the formations during rotation of the rams. 
   The alignment means may be a drum adapted to rotate about the axis of rotation of a ram. 
   The drum may comprise a throughbore of a first diameter, through which the wireline or pipeline extends. 
   Preferably, the throughbore defines an aperture which decreases in cross sectional area during rotation of the drum. 
   Preferably, the throughbore is perpendicular to the axis of rotation of the drum. 
   The first diameter is preferably equal to or greater than the inner diameter of the wellbore. 
   Preferably, a first opening of the throughbore is provided with a channel formed outwardly from an edge of the throughbore. 
   More preferably, a second opening of the throughbore is provided with a formation on its edge, diametrically opposed from the channel. 
   The formation may be a second channel formed outwardly from an edge of the throughbore, such that the first an second channels together form an aperture extending through the drum perpendicular to the axis of rotation. 
   The aperture may extend through the axis of rotation of the drum. 
   Preferably, the aperture is aligned with the formations for locating a wireline or pipeline in the closed position. 
   Alternatively, the formation is a cutting edge adapted to shear a wireline or pipeline as the apparatus moves to its closed position. 
   Preferably, each ram comprises a sealing assembly having one or more elastomeric seals. 
   More preferably, the sealing assembly comprises an inner seal adapted to cooperate with an inner seal on the opposing ram to seal against the wireline or pipeline. 
   More preferably, the sealing assembly comprises an outer seal adapted to seal edges of the wellbore against well pressure. 
   Optionally, the inner seal is retained on the ram by the outer seal. 
   Preferably, the apparatus is hydraulically actuated. 
   Optionally, the apparatus is provided with a manual override mechanism. 
   Alternatively, the apparatus is manually actuated. 
   According to a seventh aspect of the invention there is provided a method of configuring pressure control equipment at a rig, the method comprising the steps of:
         Providing a blowout preventer at the rig, the blowout preventer comprising a pair of rams moveable from a first position in which the wellbore is open and a second position in which the rams seal the wellbore around a wireline or pipeline;   Passing the blowout preventer through an aperture in a rig platform while the blowout preventer is in its operating orientation;   Installing the blowout preventer on the pressure control equipment.       

   According to a eighth aspect of the invention there is provided a method of sealing a wellbore, the method comprising the step of rotating a pair of rams in a ram assembly from a first open position in which fluid may pass through the wellbore and a second closed position in which the rams abut to seal the wellbore. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There will now be described, by way of example only, embodiments of the invention with reference to the following drawings, of which: 
       FIG. 1  is a front view of a blow out preventer (BOP) stack in accordance with an embodiment of the invention; 
       FIG. 2  is a view of the BOP stack of  FIG. 1  from a first side, showing longitudinal section through the actuating means; 
       FIG. 3  is a plan view of the apparatus of  FIG. 1 ; 
       FIG. 4  is a perspective view of the embodiment of  FIG. 1 , showing the main body of the BOP being partially transparent to display internal components; 
       FIG. 5A  is a perspective view from one side and above of the ram assembly in accordance with an embodiment of the invention, shown in its open position; 
       FIG. 5B  is a perspective view from above and one side of the ram assembly of  FIG. 5A , shown in its closed position; 
       FIG. 6A  is a perspective view of a ram of the embodiment of  FIGS. 4 and 5 ; 
       FIGS. 6B and 6C  are perspective views of an alignment drum of the embodiments of  FIGS. 4 and 5 ; 
       FIG. 6D  is a perspective view of a ram key plate of the embodiment of  FIGS. 4 and 5 ; 
       FIG. 7A  is a perspective view of an outer seal of a sealing assembly used with an embodiment of the invention; 
       FIG. 7B  is a perspective view of an inner seal of a sealing assembly used with an embodiment of the invention; 
       FIG. 8A  is a plan view of the ram assembly of the embodiments of  FIGS. 4 and 5  in a fully open position; 
       FIG. 8B  is a plan view of the embodiment of  FIG. 5  in a partially closed position; 
       FIG. 8C  is a plan view of the ram assembly of the embodiment of  FIG. 5  in a partially open position; 
       FIG. 8D  is a plan view of the ram assembly of  FIG. 5  in a fully closed position; 
       FIGS. 9A and 9B  are perspective views of an alignment drum in accordance with an alternative embodiment of the invention; 
       FIGS. 10A and 10B  are perspective views of the alignment drum of  FIGS. 9A and 9B  in a ram assembly in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Referring firstly to  FIGS. 1 to 4 , a first embodiment of the apparatus of the invention is shown, implemented as a blow out preventer (BOP) stack, generally depicted at  10 . The BOP stack comprises a pair of BOPS, shown generally at  14 A and  14 B. The BOPs are contained within a main body  12 , which contains a lower tubular  11 B for connecting the apparatus to, for example, a riser, and an upper tubular section  11 A for connection to, for example, a lubricator stack. 
   It will be appreciated that although the drawing shows a dual BOP stack, the BOP can also be used in a single, triple, or other multiple configuration. 
   As most clearly shown in  FIG. 4 , the BOP stack houses an upper ram assembly  15 A and a lower ram assembly  15 B, housed in cylindrical cavities oriented in the Y-axis perpendicular to the main wellbore axis Z. 
   Actuator assemblies  16  and  18  are provided on front and rear sides of the apparatus respectively, each actuator assembly including a pair of hydraulic pistons. In this embodiment, actuator assembly  16  includes one piston  17 A which functions to actuate a ram of assembly  15 A, and one piston  17 B functioning to actuate a ram of assembly  15 B. Similarly, actuating assembly  18  comprises pistons  19 A and  19 B, respectively functioning to actuate opposing rams in ram assemblies  15 A and  15 B. In each case, the pistons are coupled to the rams by scotch yoke type actuators  13 , although it will be appreciated that other arrangements could be adopted. 
   Also shown in  FIGS. 1 to 4  are equalising spool sub-assemblies  20 , located on the diagonal of the body  12 . These allow the controlled release of well pressure prior to moving the ram assembly from a closed back to an open position.  FIGS. 1 to 4  also show  3  injection ports  22  allowing fluid to be injected into the cylindrical sections of the BOP stack. 
     FIG. 4  also shows positioning of the ram assemblies  15 A and  15 B. In  FIG. 4 , both ram assemblies  15 A and  15 B are shown in their fully open positions. The actuation mechanism will be described in more detail below. 
     FIGS. 5A and 5B  show components of the ram assembly  15 A in more detail. The main body and actuators of the BOP have been removed for clarity. The ram assembly comprises a pair of rams  50 ,  51 , an alignment drum  52 , and a ram key plate  54 . The individual components of the ram assembly are shown in  FIGS. 6A to 6D . 
     FIG. 5A  shows ram assembly  15 A in an open position, whereas  FIG. 5B  shows ram assembly  15 A in a closed position. 
     FIG. 6A  shows an individual ram  50 , having a sealing assembly  60  mounted thereon. The ram  50  comprises a main body  62  which is formed from a cylindrical quadrant with main axis Y. In use, the ram rotates about the main cylindrical axis Y. The ram has an interior section removed, to form a part-cylindrical seat  63  for the alignment drum of the ram assembly. The internal radius of the removed section corresponds to the radius of the alignment drum  52 , and is approximately 50% of the total radius of the ram. 
   The part-cylindrical removed section of the ram  50  does not extend along the entire axial length of the ram. The ram comprises an end plate  64 , which is received into a corresponding formation  65  in the alignment drum. 
   Formed in the lower edge  66  of the ram is a channel  67  extending parallel to the main axis Y of the ram assembly. The bore is semi-circular in cross section so that when the lower edge of corresponding ram  51  is placed adjacent, a circular throughbore is provided. The throughbore is arranged to have inner diameter greater or equal to the inner diameter of the wellbore. 
   Along an opposing, sealing edge  68  of the ram, an additional channel is formed in the main body of the ram, in the axis perpendicular to the main axis Y of the ram assembly and the channel  69  in the opposing edge. The channel is semi-circular such that when the sealing edge of the opposing ram is placed adjacent, a circular aperture ( 56 ,  FIG. 5B ) is formed in the ram pair. 
   The ram is provided with a seal assembly  60  comprising an outer seal  71  and an inner seal  73 . The inner and outer seal are elastomeric, and are adapted to seal against a wireline when the rams are in a closed position. It should be noted that no recess is formed in the inner seal corresponding to the aperture  56  formed in the ram pair. 
     FIGS. 6B and 6C  show the alignment drum  52  from two different perspectives. The alignment drum is generally cylindrical, and has a flat end  79  with an axle  80  formed therein. The opposing end of the alignment drum has a section  65  removed. This is a quadrant of axial extent equal to the thickness of the end plate  64  on the ram, and receives the end plate of a ram when the apparatus is assembled. Members  82  are provided for engaging with a spindle of the actuating means. 
   The alignment drum  52  has a cross-axial bore  81  formed therein. The bore  81  is substantially circular in cross section, with diameter corresponding to the inner diameter of the wellbore. In opposing edges  83   a,    83   b  of the cross-axial bore, channels  84   a  and  84   b  are formed, extending outwardly from the centre of the cross-axial bore. The channels  84   a,    84   b  are formed on diametrically opposed sides of the drum, with a diameter less than that of the cross axial bore  81 , but large enough to accommodate a wireline or pipeline being run in the wellbore. The channels extend to a distance such that they define another cross-axial bore or aperture passing through the central axis of the alignment drum. The ends of the channels are rounded to provide a smooth surface for the wireline or pipeline. The edge defined by the rim of the main bore and the rim of the channel is shaped to create a smooth, tapered path from the main bore to the channel. In this example, this is by a pair of tangents inclined to one another at around 90 degrees, with smoothed edges where they intersect the channels. 
     FIG. 6D  shows the ram key plate  54 , which is formed to an outer diameter equal to that of the alignment drum  52 . The ram plate comprises a central bore  86 , which receives the axle  80  of the flat end of the alignment drum  52 . The plate  54  is therefore able to rotate with respect to the alignment drum. Members  87  are provided for engaging with a spindle of the actuating means. The plate is a circular disc with a quadrant removed. This quadrant corresponds to the end plate of a ram, and receives the end plate when in the ram assembly. 
   In its assembled state, the ram assembly is arranged such that the quadrant recess in the alignment drum  52  receives the end plate  64  of one of the rams, ram  50 , such that rotation of the drum effects rotation of the ram, and vice versa. In contrast, the opposing ram  51  is able to rotate with respect to the alignment drum  52 . However, the ram key plate  54 , by virtue of its corresponding quadrant will cause the ram  51  to be rotated along with the ram key plate  54 . 
     FIGS. 7A and 7B  show components of a seal assembly used with an embodiment of the invention. The seal assembly comprises an outer seal  71  and an inner seal  73 , each comprised of elastomeric materials. The outer seal  71  is U-shaped in the plane perpendicular to the sealing surface, and has a curved outer profile  75  shaped to be flush with the outer surface of the ram. Inwardly extending retaining edges  76  are provided at the prongs of the U. The inner seal  73 , which in this example is a different material from the outer seal  71 , is formed in a broad inverted T-shape. The lower surface  77  of the inner seal  73  abuts a supporting member on the main body of the ram, and the edges  76  of the outer seal fit onto the corners  78  of the inner seal. 
   The seal arrangement described above is self retaining in the sense that the outer seal secures the inner seal to the ram body. In use, when the rams are closed to seal the wellbore, the extreme force of the well pressure will tend to cause the elastomeric inner seals to stick together. In the prior art, when moving the ram assembly from a closed position in which the inner seals of opposing rams are in contact to an open position in which the rams are separated, the ram blocks will tend to remain in place in the centre of the wellbore unless they are securely fitted to the rams. It is conventional to provide metal inserts moulded in the seals to provide support for screws or bolts used to attach the seal to the ram. In the embodiment of the present invention, the outer seal provides the retention force for the inner seal, removing the requirement for screws and metal inserts. This simplifies the process for manufacturing the seals, which may be by a simple injection moulding process. 
   In the start position, shown in  FIG. 4 , the pair of rams occupies the lower half of the cylindrical cavity in the BOP body. In this position, the cross axial bore of the alignment cylinder is aligned with wellbore. In addition, the bore defined by the semicircular channels  67  in the lower edges of the rams is in alignment with the main wellbore, and the rams are in an open position. 
   In use, with reference to  FIGS. 1 and 4 , the piston  17 A is actuated, causing rotation of the scotch yoke type actuator, which is coupled to the alignment drum. This effects rotation of the alignment drum 90 degrees in an anti-clockwise sense from the perspective of  FIG. 1 . Simultaneously, the corresponding piston  19   a  on the rear face is actuated and causes rotation of the ram plate. The cooperating quadrant formation engages with the ram causing the ram to be rotated 90 degrees in a clockwise sense from the perspective of  FIG. 1 . 
   After actuation, the two rams occupy the volume of the top half of the cylindrical cavity in the BOP body. In this position, the sealing edges  68  of the ram are located together. The outer seals  71  seals around the edge between the body of the cavity and the main wellbore, and the inner seals  73  seal against the wireline or pipeline extending through the wellbore. The shape of the seal assembly is such that the apparatus is self-energising, as well pressure tends to force the seals upward and inwards, causing the seal to be maintained. 
     FIGS. 8A to 8D  show the operation of the embodiment of  FIGS. 5 and 6  in more detail, and in particular show the function of the alignment drum. 
     FIG. 8A  is a plan view of the ram assembly in its open position. In this position, the rams  50 ,  51  occupy the bottom half of the cylindrical cavity in the BOP body. The Figure shows the throughbore  81  fully aligned with the wellbore. In addition, the semi circular channels  67  formed in the lower edges of the rams provide full clearance of the rams from the wellbore. The wireline  89  extends through the bore defined by the cross axial bore in the alignment drum and the semi-circular recesses in the lower edges of the rams. In practice, the wireline is unlikely to be perfectly central in the bore. 
     FIG. 8B  shows the ram assembly partially through its rotation cycle, approximately 50% through rotation. Here, the alignment drum has rotated through around 45 degrees, and the sealing faces of the rams are inclined at approximately 90 degrees to one another. The wireline extends through the aperture defined by the bore and channels formed either side of the bore, which is now in the shape of a square, rotated though 45 degrees, with an elongate channel extending across the diagonal of the square, formed from the channels  84   a  and  84   b  and aligned with the cut-out formations in the sealing edges of the ram. 
   As the alignment drum continues to rotate in the direction of the arrow, the aperture in which the wireline resides becomes progressively smaller, and the wireline is guided towards the centre of the bore and the locating channels  84   a,    84   b.  The guiding edges of the bore are shaped to provide a smooth path for the wireline. It is notable that the wireline is guided by the upper guiding edge of the bore, and by the lower guiding edge of the bore simultaneously. This is in contrast to a conventional ram-type BOP, in which the one guiding means on one of the rams moves before the other. 
     FIG. 8C  shows the ram assembly almost fully in its closed position. In this position, the rotated square aperture is no longer present, and the aperture is an elongate locating channel aligned with the cut-out formations in the sealing edges of the ram. 
     FIG. 8D  shows the ram assembly in its fully closed position. 
     FIGS. 9A and 9B  show an alternative embodiment of the invention, in which the alignment drum  91  has shape different from that in the embodiment of  FIGS. 5 and 6 . 
   In this example, the invention is implemented as a shear and seal BOP, used when the wireline needs to be cut to release the tool string below. The shearing alignment drum shown in  FIGS. 9A and 9B  differs from the alignment drum of  FIGS. 6A and 6B  in that a locating channel  94  is only provided in one edge of the bore in the alignment drum  91 , which becomes the lower edge as the drum rotates towards the closed position. This lower edge functions to guide the wireline towards the centre of the bore. 
   In place of the upper locating channel on the opposing edge, a rounded cutting edge  92  is formed at the intersection of guiding edges. In this example, the guiding edges are two tangents to the bore edges, inclined to one another at around 90 degrees. 
     FIGS. 10A and 10B  show a ram assembly  100  including the alignment drum  91  at different stages in the rotation cycle. The wireline, guided to the centre of the bore by the lower guiding edges is met by the cutting edge  92  before the drum is fully through its rotation cycle. As the ram assembly proceeds to the fully closed position, the cutting edge shears through the wireline. The wellbore is sealed by the sealing assembly, as described above. 
   The present invention offers numerous improvements over previously proposed valves. 
   Firstly, embodiments of the invention introduce a component of the movement of rams in the vertical, rather than lateral dimensions. In the embodiment described, this is achieved by using a circular movement of the rams used to close a conduit. By “wrapping around” the movement of the rams, the lateral extension of the valve components is significantly reduced. 
   The reduction in lateral dimensions over conventional ram-type BOPs varies according to operating pressure of the BOP and diameter of the wellbore. 
   For 3 inch diameter wellbore, a 10,000 PSI BOP would be expected to have a maximum lateral dimension of around 30 cm, compared with 110 cm for a conventional ram-type BOP. 
   For 6 inch diameter wellbore, a 10,000 PSI BOP would be expected to have a maximum lateral dimension of around 60 cm, compared with 190 cm for a conventional ram-type BOP. 
   In another example, the largest lateral dimension of a BOP stack of the type described is less than 75 cm, compared with around 2.5 meters in an equivalent ram-type BOP. 
   In the example shown in  FIGS. 1 to 3 , the maximum lateral dimension, including hydraulic actuators, is around 40 cm. 
   These dimensions are less than half, and indeed less than a third, of the dimensions of the equivalent conventional ram-type BOP. 
   This improves the manoeuvrability of the apparatus, and allows it to be lowered in, around and through platforms on the rig. Significantly, the BOPs may be installed much closer to the wellhead, having positive effects on the height available for the lubricator stack and the tool string. 
   The present invention also provides apparatus of reduced weight when compared with conventional ram-type BOPs, which further improves manoeuvrability. 
   In addition, the ram assembly design is significant in that the rams move from an open to a closed position without changing the volume of components at well pressure. The cylindrical cavities in which the ram assemblies are located are at well pressure when the apparatus is in its open position. When actuated, there is no requirement to force piston-connecting rods into the well volume, in contrast to the conventional ram-type BOP. This feature of conventional BOPs means that there is a requirement to overcome well pressure, which can only be achieved by significant force. 
   In manually actuated BOPs this requires large screws and large levers in order for the operator to generate enough torque, and this places a huge burden on the operator. In hydraulically actuated systems, there is a requirement for large diameter cylinders, which increases the weight and size of the apparatus. During operation of the present invention, the volume swept by the rams does not change, and the result is that less force is required to actuate the rams. This means that smaller diameter hydraulic actuators can be used. In addition, manually actuated systems are more feasible using the present design. In both manually and hydraulically actuated apparatus, the time taken to close the wellbore is greatly reduced, improving the safety at the rig. 
   The reduced requirement for hydraulics reduces the need to provide screw locking mechanisms which are typically used in conventional ram-type BOPs to manually secure the rams in their closed configuration after actuation by the hydraulics. In the present design, the apparatus could be locked in one position by a simple peg and hole arrangement in the actuating mechanism and the main body of the BOP stack. 
   The manner in which the apparatus is constructed is simple, and allows simple installation and stripping down by removal of two plates on one side of the apparatus. 
   The construction of the apparatus allows convenient location of equalisation channels and injection channels, without conflicting with screws and fittings. 
   The alignment drum offers a number of advantages over conventional ram-type BOPs. 
   In the present embodiment, the rams are fully supported around the entire outer surface of the rams. The alignment of the wireline or pipeline is not done by any component of the rams themselves, and thus compromising the strength of the rams by providing recesses on the supporting surface is not necessary. 
   Alignment of the wireline is by the profiles of the upper and lower edge of the cross axial bore of the drum. The alignment is therefore at two spatially separated locations on the wireline, reducing the likelihood of damage or snagging of the wireline during closing. 
   In addition, the movement of the guiding edges against the wireline is such that the edges move across the wire while moving a small distance along the wireline as the drum rotates. The force is therefore imparted over a short range of points, rather than being a slicing effect at a single point of the wire. This further reduces the tendency of the wireline to snag. 
   The movement of the alignment drum to the closed position provides two guiding edges for the wireline, moving in opposite senses, which by their nature must act in concert, as opposed to the guides in the conventional ram-type BOP which are likely to move separately, reducing their effectiveness. 
   References to wireline applications should not be taken as limiting. The above described apparatus is suitable for sealing a conduit around any wireline, slickline, pipeline, umbilical or cable. Indeed, the apparatus has certain advantages for general valves, even where there is no requirement to seal around an internal line. 
   Various modifications and improvements may be made to the above-described embodiments within the scope of the invention herein intended.