Patent Publication Number: US-6698520-B2

Title: Light-intervention subsea tree system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Benefit is herein claimed of the filing date under 35 USC §119 and/or §120 and CFR 1.78 to U.S. Provisional Patent Application Serial No. 60/170,061, filed on Dec. 10, 1999, entitled “Light Intervention Subsea Tree System.” This application is a continuation of application Ser. No. 08/732,817, filed Dec. 8, 2000, now U.S. Pat. No. 6,460,621. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to subsea oil and gas production systems and in particular to a subsea tree assembly having certain components that are retrievable by a light-duty workover vessel. 
     2. Description of the Prior Art 
     A conventional subsea wellhead assembly includes a wellhead housing which supports one or more casing hangers located at upper ends of strings of casing extending into the well. A production tree is landed on the wellhead for controlling the production of well fluids. The tree usually carries a choke and valves to control the flow and sensors to monitor the flow. 
     With both conventional and horizontal trees, external chokes and production valves are used to control the flow. If the valves or choke are in need of service, retrieval is difficult and may require the use of a remotely-operated vehicle. Various valves and controls have been located on an apparatus separately retrievable from the tree, but many of the components requiring service may require that the entire tree be removed. 
     SUMMARY OF THE INVENTION 
     A subsea well apparatus is provided for controlling and monitoring production fluid flow from a well. A christmas tree is adapted to land on a subsea wellhead, the tree having a tubular, open upper end. A first flow passage extends from a lower end of the tree to the upper end for communicating fluid with the well. A second flow passage extends downward from the upper end of the tree and has an outlet on a sidewall of the tree for communicating with a flowline. The second flow passage is connected to an annulus access passage and is separated from the annulus access passage by a valve. A production module lands on and is retrievable from the upper end of the tree, the module having a flow loop with one end in communication with the first flow passage and another end in communication with the second flow passage. At least one flow interface device is located in the loop of the production module. The flow interface device may be used to monitor or control the flow and may be a temperature or pressure sensor, a flow or multi-phase flow meter, or a choke. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed to be characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a sectional view illustrating a subsea tree constructed in accordance with this invention shown being landed on a subsea wellhead assembly. 
     FIG. 2 is an enlarged sectional view of a production module that lands on the subsea tree of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, subsea wellhead assembly  11  is conventional. It includes an outer low-pressure wellhead housing  13  that is located at the upper end of a string of a large diameter conductor that extends into the well to a first depth. An inner high-pressure wellhead housing  15  locates within outer wellhead housing  13  and protrudes above. Inner wellhead housing  15  is a tubular member secured to the upper end of large diameter casing that extends to a second depth in the well. The well will have typically two casing hangers  17 . The lower one is secured to a string of casing that extends to a third depth in the well. The uppermost casing hanger  17  is secured to production casing  19  that extends to the total depth of the well. Subsea wellhead  11  has four guide posts  27  extending upward. The upper end of inner wellhead housing  15  is a tubular mandrel  29  having an exterior profile with grooves. 
     A conventional tubing hanger  21  lands in the bore of inner wellhead housing  15  above the uppermost casing hanger  17 . Tubing hanger  21  is secured to a string of tubing (not shown) extending into the well. Tubing hanger  21  has an axially extending production passage  23 . An annulus passage  25  extends through tubing hanger  21  parallel to and offset from production passage  23 . Production passage  23  communicates with the interior of the string of tubing, while annulus passage  25  communicates with an annulus between the string of tubing and production casing  19 . 
     A production tree  31  is adapted to land on subsea wellhead  11  for controlling fluids produced from the well. Tree  31  may alternately be an injection tree for controlling fluids injected into the well. Production tree  31  has guide receptacles  33  that are received over guide posts  27  as tree assembly  31  is being lowered on guidelines  34 . Tree  31  has a wellhead connector  35  on its lower end. Connector  35  is conventional, having dogs  36  that are hydraulically actuated for engaging the grooves on mandrel  29  or having a similar connection device using, for example, collets. 
     An axial first or upward-flow production passage  37  extends through tree  31 . One or more master valves  39 , preferably gate valves, selectively open and close upward-flow production passage  37 . An annulus access passage  41  extends upward to the upper end of tree  31  parallel to and offset from upward-flow production passage  37 . Annulus access passage  41  communicates with annulus passage  25  of tubing hanger  21 , while production passage  37  communicates with production passage  23  of tubing hanger  21 . Annulus access passage  41  has two annulus valves  43 ,  45 . An external cross-over line  48  extends from a port  47  in upward-flow production passage  37  to a port  49  in annulus access passage  41  between annulus valves  43 ,  45  to communicate annulus  25  with upward-flow production passage  37 . A valve (not shown) will also be contained in the cross-over line  48 . Cross-over line  48  enables fluid to be pumped down annulus access passage  41 , through cross-over line  48 , and down production passage  37  to kill the well, if desired. 
     Tree  31  also has a second or downward-flow production passage  51  that extends upward from annulus access passage  41  above annulus valve  45 . Downward-flow production passage  51  is coaxial with annulus access passage  41  and intersects annulus access passage  41  above annulus valve  45 . Downward-flow passage  51  can communicate with the lower portion of annulus access passage  41  by opening annulus valves  43 ,  45 . Downward-flow passage  51  is parallel to and offset from upward-flow production passage  37  and leads to a lateral production passage  53  for controlling flow into an attached flowline. A production valve  55  is located in lateral production passage  53 . 
     The upper end of tree  31  is formed into a configuration of a mandrel  57 , having grooves on the exterior. Tree mandrel  57  has a smaller outer diameter than wellhead housing mandrel  29  in this embodiment. An upward facing funnel  59  surrounds tree mandrel  57  for guidance. 
     A production module  61  is shown in FIG.  2 . Production module  61  is adapted to land on tree mandrel  57 . Production module  61  has a tree connector  63  on its lower end that is of a conventional design. Tree connector  63  has a plurality of dogs  65  that are moved radially inward into engagement with the profile on tree mandrel  57  (FIG. 1) by means of a cam ring  67  or has a similar connection device using, for example, collets. Hydraulic cylinders  69  move cam ring  67  upward and downward. Production module  61  has an upward-flow passage  71  that is positioned to register with upward-flow production passage  37  (FIG.  1 ). Module upward-flow passage  71  leads upward to a cross-over passage  73 . Cross-over passage  73  leads to a downward-flow passage  75  that is parallel to and offset from upward-flow passage  71 . Downward-flow passage  75  is oriented to align and communicate with downward-flow production passage  51  in tree  31  (FIG.  1 ). The set of internal flow passages comprising passages  71 ,  73 , and  75  forms a flow loop within module  61 . If an injection tree is used instead of a production tree, the flow directions in passages  71 ,  73 ,  75  of module  61  will be reversed. 
     One or more Flow interface devices can lie within or adjacent to and in communication with the flow loop of module  61 . The devices may be a variety of types for controlling or measuring, such as a choke, a pressure or temperature sensor, or a flow meter. Shown in FIG. 2 is a choke assembly  77  located in cross-over passage  73 . Choke assembly  77  is of a conventional design and used for variably restricting the flow of production fluid flowing through cross-over passage  73 . An upstream pressure and temperature sensor  79  locates on the upstream side of choke  77 . A downstream pressure and temperature sensor  81  locates on the downstream side of choke assembly  77 . Also, preferably, a multi-phase flow meter is utilized for measuring the flow rate through cross-over passage  73 . Flow meter controls  83 , shown schematically, are located at the upper end of production module  61  for serving the flow-metering hardware located in passage  73 . 
     Hydraulic and electric controls  85  for production module  61  and tree  31  are also located adjacent to flow meter controls  83 . These controls  85  serve the various valves, such as master valve  39 , annulus valves  43 ,  45 , and production valve  55 . An ROV panel  87  may be located on one side of production module  61  for allowing engagement by remote operated vehicles for performing various operations. Production module  61  has a lift wire attachment  89  on its upper end to enable it to be retrieved and re-installed by a light duty workover vessel (not shown) at the surface. Production module  61  may have an annular buoyant tank  91  located near an upper portion of module  61 . Tank  91  may be filled with air or a buoyant material to assist in retrieving module  61 . 
     In operation, the subsea well will be completed conventionally with a subsea wellhead assembly  11  as shown in FIG.  1 . Tree  31  will be lowered on guide wires  34  into engagement with mandrel  29  of wellhead housing  15 . Then, production module  61  is lowered on a lift wire into engagement with mandrel  57  of tree  31  (FIG. 1) with the assistance of upward facing funnel  59  or guideposts. 
     During production, well fluid will flow as indicated by the arrows up tubing hanger production passage  23  and tree production passage  37 . The well fluid flows upward into upward-flow passage  71  of production module  61 , shown in FIG.  2 . As indicated by the arrows, well fluid flows through cross-over passage  73  and then through downward-flow passage  75 . Choke  77  will control the rate of flow. Sensors  79 ,  81  will monitor pressure and temperature. Flow meter controls  83 , if utilized, will monitor the flow rate and water cut. The flow proceeds through downward-flow passage  75  back into tree  31  via downward-flow passage  51  (FIG.  1 ). The production flow proceeds out lateral passage  53  to a flow line. 
     The moveable components on tree  31 , such as valves  39 ,  43 ,  45  and  55  typically require little maintenance. Intervention to change the valves or any other components of tree  31  is not expected to be frequently required. The components of production module  61  are more active and more subject to failure. These components include choke  77 , flow meter controls  83  and the pressure and temperature sensors  79 ,  81 . Production module  61  can be readily retrieved by a small vessel using a lift line to repair or replace any of these components or to allow communication with annulus access passage  41  at the top of the tree  31 . The small vessel need not be large enough to run casing, tubing or to retrieve a tree. 
     While the invention is shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.