Patent Publication Number: US-8991503-B2

Title: Subsea wellhead assembly

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a subsea wellhead assembly. 
     2. Description of Related Art 
     In the event of a problem in the operation of a subsea well, after the well has been secured, it may be necessary to remove to the topside not only a tree, but also a tubing head structure and its associated equipment. Known subsea wellhead assemblies contain an integrated tree and tubing head structure. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, there is provided a subsea wellhead assembly. The subsea wellhead assembly comprises a tubing head structure with a tree above the tubing head structure, and a recoverable module attached to the tubing head structure below the tree, the recoverable module comprising at least one valve and at least one sensor. 
     According to another embodiment of the present invention, there is provided a method of operating a subsea well comprising a subsea wellhead assembly comprising a tubing head structure with a tree above the tubing head structure, and a recoverable module attached to the structure below the tree, the recoverable module comprising at least one valve and at least one sensor. The method comprises removing the tree if a problem occurs in the operation of the subsea well, and, thereafter, removing the recoverable module from the tubing head structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic longitudinal section through a subsea wellhead assembly according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a longitudinal section through a subsea wellhead assembly  1  according to an embodiment of the invention. The wellhead assembly  1  includes a tubing head structure in the form of a tubing head spool  2 , typically in the form of a circular round forging, mounted on a subsea wellhead. Inside the spool  2  is a tubing hanger  3  mounted by suitable means such as by being on a load shoulder  4  formed in the spool  2  as shown or on a retractable load shoulder. Below the tubing hanger  3  there is a casing hanger  5  in the wellhead in a conventional manner, from which is suspended in at least one string of casing  6 . 
     The tubing hanger  3  carries lower production tubing  7  extending into the well and inside the casing string  6 , upper production tubing  8  in the form of a production stab extending to a tree schematically indicated by block  9 , the tree  9  being a so-called “vertical tree”. 
     Attached around the tubing head spool  2  is a recoverable module  10  sitting on a shoulder in the form of a support ring  11  around the tubing head spool  2 , the module  10  being attached to the latter by an attachment arrangement  12 . An upper annulus port  13  opens into the annular space  14  between upper production tubing  8  and the tubing head spool  2  and a lower annulus port  15  opens into the annular space  16  between lower production tubing  7  and the spool  2  and above the casing hanger  5 , so that it is in communication with the annular space  17  between tubing  7  and casing string  6 . Annulus access routing between ports  15  and  13  is provided by the module  10  via an annulus master valve  18  in the module  10 , a sensor in the form of an annulus pressure and temperature transducer  19  in the module  10  and an annulus workover valve  20  in the module  10 , and suitable passageways in the module  10 , typically provided by bores in it. The annulus master valve  18  and the annulus workover valve  20 , which could be gate valves for example, are controlled from a typical subsea control module of the well installation and the transducer  19  is itself coupled to the subsea control module. Control signals to the actuators of valves  18  and  20  and signals to and from transducer  19  are via a disconnectable coupler pair, one half  21  being mounted via the tree  9  and the other hall  22  being mounted on the module  10  (and recoverable with it). The coupling of halves  21 ,  22  may be a retractable one (operable via a diver or a remotely operated vehicle (ROV)) or may be “self-mating”. Pre-installed in the recoverable module  10  is a plug  23 , the module  10  having an access point at  24  for movement of the plug by means of a remotely operated vehicle. 
     The wellhead assembly  1  of  FIG. 1  is installed as follows. The tubing head spool  2  is put on the wellhead and a drilling system including a blowout preventer is installed onto it. A drilling system drills into the subsea formation and the string of casing  6  is installed suspended by casing hanger  5 . Production tubing  7  on tubing hanger  3  is then installed inside the tubing head spool  2 , the well now being completed. The blowout preventer is removed and the vertical tree  9  is installed on the tubing head spool  2  together with the tubing  8 . 
     If it is detected that at least one of the valves  18 ,  20  is faulty and/or if the transducer  19  monitoring pressure and temperature produces an indication that there is a problem in the annular spaces  16  and  17 , due to leaks or transients for example, then a downhole safety valve is operated and an ROV is used to move plug  23  through valve  18  to block the lower annulus port  15 . Thereafter, the tree  9  is removed, for example on a wire, and an ROV is used to detach the module  10  from tubing head spool  2 , by opening the attachment arrangement  12 . Thereafter, the module  10  is recovered, for example on a wire, for repair or replacement. 
     Embodiments of the present invention avoid the necessity to recover the tubing head structure itself and the associated downhole well completion system. More particular, an advantage of the wellhead assembly  1  described above is that, in the event of a problem, there is no need to remove and recover the tubing head spool  2  or the associated downhole well completion system.