Abstract:
A subsea wellhead ( 10 ) includes annulus pressure monitoring and bleed down ports ( 32, 34, 36 ) whereby excessive pressure may be detected and bled off to a production controls or workover controls system via an electro/hydraulic jumper ( 58 ). A valve block ( 44 ) bolted to the wellhead ( 10 ) includes pressure transducers ( 52, 54, 56 ) and isolation valves ( 46, 48, 50 ). Excessive annulus pressures and hence damage to the completion program may thereby be avoided in HPHT subsea well applications.

Description:
INVENTION BACKGROUND 
     High Pressure High Temperature (HPHT) wells necessitate a requirement to bleed down casing string annuli, to prevent thermal pressure loads from damaging the completion casing program. Thermal expansion of trapped fluid in the casing annuli could otherwise lead to excessive pressure build up causing damage to or failure of the casing completion system. 
     Annulus bleed down can be readily achieved on surface wellhead applications, as the wellhead housing can be provided with annulus outlets. Subsea wellheads do not have annulus outlets. Each casing string is instead suspended and sealed within the wellhead high pressure housing. No provision is made for communication between each casing string annulus and the wellhead exterior. Assuming that it would be possible to extract annulus fluid as and when required, there is the further problem of disposing of the bled off fluid in an environmentally acceptable way. With the introduction of HPHT completions into the subsea environment, there is a need for subsea wellheads that can facilitate annulus bleed downs. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a subsea wellhead comprises a monitoring and/or bleed down port extending laterally through a wall of the wellhead housing and having an interior end connected to a well annulus and an exterior end connectable to a jumper for conveying pressure signals and/or expelled annulus fluid to a controls interface. 
     A preferred embodiment of the invention facilitates the isolation and pressure monitoring of each casing annulus, via a remotely deployable electro/hydraulic control jumper providing a link between the wellhead casing annuli and the subsea production control facility, or a workover control system, as desired. The invention may be used with particular advantage in conjunction with a drill-through horizontal Christmas tree. 
     The preferred embodiment makes use of three primary components. 
     1. A modified subsea wellhead housing containing linked annulus ports. 
     2. A bolt on valve block incorporating independent isolation valves, pressure monitoring equipment and an electro/hydraulic control interface. Alternatively, some or all of these components may be integrated into the wellhead itself. 
     3. An ROV/diver deployable electro/hydraulic control stab plate jumper to facilitate remote connection between the subsea production control system and the wellhead electro/hydraulic control interface. 
     Further preferred features of the invention are in the dependent claims and in the following description of an illustrative embodiment made with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic representation of a wellhead embodying the present invention; 
     FIG. 2 is a more detailed view of the wellhead of FIG. 1; 
     FIG. 3 is a view on arrow III in FIG. 2; 
     FIG. 4 is a front view of an ROV plate of the wellhead; 
     FIG. 5 is a view from behind the ROV plate of FIG.  4  and 
     FIG. 6 shows an ROV deployed jumper. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown a wellhead housing  10  in which is landed a first casing hanger  12 , a second casing hanger  14  and a tubing hanger  16 . The wellhead housing  10  is mounted on an outer casing  18  and the casing hangers  12 ,  14  suspend casing strings  20 ,  22  respectively. Tubing  24  is suspended from the tubing hanger  16 . A first annulus  26  is defined between the tubing string  24  and the casing string  22 ; a second annulus  28  is defined between the casing strings  22 ,  20  and a third annulus  30  is defined between the casing string  20  and the outer casing  18 . A first annulus port  32  is formed extending through the wall of the wellhead housing  10 , having an inner end in communication with the space below the casing hanger  20  and hence in communication with the outermost annulus  30 . A second annulus port  34  is formed extending through the wall of the wellhead housing  10 , having an inner end in communication with the space defined between the casing hangers  12  and  14 , and hence in communication with the production casing annulus  28 . A third annulus port  36  is formed extending through the wall of the wellhead housing  10 , having an inner end in communication with the space defined between the tubing hanger  16  and the production casing hanger  14 , and hence in communication with the tubing annulus  26 . 
     The outer ends of the annulus ports  32 ,  34 ,  36  are connected to hydraulic couplers  38 ,  40 ,  42  contained in a valve block  44  bolted to the wellhead  10 . Each annulus port connection within the valve block  44  is controlled by a respective ROV or diver operable isolation valve  46 ,  48 ,  50  and is equipped with a pressure transducer  52 ,  54 ,  56 . An ROV/diver deployable electro-hydraulic jumper  58  is connectable to the valve block  44  to convey expelled annulus fluid from the hydraulic couplers  38 ,  40 ,  42  to a production controls system or workover controls system (not shown), as appropriate. Electrical couplers  60 ,  62 ,  64  are provided in the valve block  44  and mate with corresponding jumper connectors  66 ,  68 ,  70  for conveying pressure signals to the production or workover controls system. When the pressure reading from one of the transducers  52 ,  54 ,  56  exceeds a critical value, the corresponding valve  46 ,  48 ,  50  can be opened, allowing annulus fluid to be vented or bled off into the production or workover controls system, so reducing the annulus pressure and avoiding damage to the casing completion program. During well drilling operations, the jumper  58  can be disconnected and replaced by a protective cap. 
     FIGS. 2-6 show the wellhead  10 , valve block  44  and jumper  58  in more detail. The wellhead housing  10  is supported in a conductor housing  72  welded to the upper end of a conductor casing  74  surrounding the outer casing  18 . The annulus ports  32 ,  34 ,  36  are drilled vertically downwardly through the wall of the housing  10  from its upper surface  96 , at circumferentially spaced locations. The upper ends of the vertical drillings are then plugged. Radial drillings  76 ,  78 ,  80  provide communication between the wellhead interior and the respective vertical drillings, at the correct vertical locations for communication with the respective casing/tubing annuli. Further horizontal drillings  82 ,  84 ,  86  in the valve block  44  and wellhead housing  10  communicate between the vertical drillings and the valves  46 ,  48 ,  50 . The pressure transducers also communicate with the horizontal drillings  82 ,  84 ,  86 . An ROV plate  98  (FIG. 4) is mounted to one end of the valve block  44  and contains ROV receptacles  100 ,  102 ,  104  for actuation of the valves  46 ,  48 ,  50 . Vertical drillings  88 ,  90 ,  92  lead from the valves  46 ,  48 ,  50  and are connected to the hydraulic couplers  38 ,  40 ,  42  mounted on the ROV panel, by hoses  94 . Electrical wet-mate connectors  62 ,  64 ,  66  on the ROV panel  98  are connected to the pressure transducers  52 ,  54 ,  56  by cables  106 . The electro/hydraulic jumper has corresponding hydraulic and electrical couplers arranged to mate with those in the ROV panel  98  in use.