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RELATED APPLICATIONS 
       [0001]    This patent application is a non-provisional and claims priority to and the benefit of U.S. Provisional Patent Application No. 61/372,388 filed on Aug. 10, 2010, incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates in general to production of oil and gas wells, and in particular to a protective net system deployed over a subsea wellhead assembly. 
       DESCRIPTION OF RELATED ART 
       [0003]    Typically mounted on any wellbore is a wellhead assembly having valves for controlling the flow of fluids from the wellbore as well as and ports for access to the wellbore. As such, wellhead assemblies normally include actuators associated with the valves and ports. When located subsea, the wellhead assemblies generally include control panels where remotely operated vehicles (ROV) may dock and interface with the wellhead actuators via hot stabs in the control panels. Because to be accessible by an ROV, the control panels are usually on an outer surface of the wellhead assembly. As the control panels are exposed to the surroundings of the wellhead assembly, they are unprotected from impacts that sometimes occur from objects dropped from the sea surface or those that may topple over. While some wellhead assemblies can include a metal pane covering an upper surface, the wellhead assembly may still be susceptible to damage from side or lateral impacts. In some instances subsea wellheads are protected by covers that envelope the wellhead assembly. The covers usually have openings in their sidewalls so an ROV can access the wellhead assembly. These covers though are often large making them unwieldy or heavy and because of the openings, the wellhead assembly within remains vulnerable to damaging impact from a falling and/or toppling object. 
       SUMMARY OF THE INVENTION 
       [0004]    Disclosed herein is an example of a wellhead assembly having a protective netting. In an example embodiment the wellhead assembly includes a wellhead housing, a production tree mounted on the wellhead housing, and a net disposed over the wellhead housing and production tree. The net has an access therethrough than can be selectively opened and closed so the wellhead assembly can be accessed. In an example embodiment, the wellhead assembly also includes a frame member for supporting the net thereon. Optionally, the net can include side panels disposed adjacent a lateral perimeter of the frame. The access can be made up of members that attach to and slide along vertical portions of the frame; thus by coupling lateral ends of the side panels to the sliding members, when the side panels are urged in a direction parallel to the vertical portions of the frame, the side panels move away from a protecting position and into a position so that the wellhead assembly may be accessed. In an example embodiment, the sliding members are collar like and circumscribe a vertical portion of the frame; ends of horizontal net members attached to the sliding members. In an alternate example embodiment, the access is a vertical slit in the net. In an example embodiment, the net is a series of horizontal and vertical net members that are attached to each other and cross over one another. 
         [0005]    Also provided and described herein is a method of protecting a wellhead assembly. In an example embodiment the method includes providing a protective netting having net elements and an access through the net elements that can be opened and closed. The entire wellhead assembly is protected from falling objects by disposing the protective netting over the wellhead assembly. Accessing the wellhead assembly occurs through the access. In an example embodiment, the method further includes providing a frame over the wellhead assembly for supporting the netting. In an example embodiment, the access is a vertical slit in the protective netting. In an example embodiment, a frame is provided that has elongate support posts equipped with sliding attachments on the support posts. Thus by mounting the protective netting on the frame so that at least a portion of the protective netting attaches to the sliding attachments, when the sliding attachments are moved along the support posts, the portion of the protective netting moves away from the wellhead assembly to define the access. In an example embodiment, the attachments are urged along the support posts with an arm attached to a remotely operated vehicle (ROV), this moves the portion of the protective netting to define an opening through the protective netting. With the access in an open configuration, the wellhead assembly can be manipulated with the ROV. In an example embodiment, the protective netting is deployed from a vessel to subsea and onto the wellhead assembly. In an example embodiment, the protective netting and the wellhead assembly are deployed together subsea. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1A  is a side partial sectional view of a subsea wellhead assembly on the seafloor with a protective net. 
           [0007]      FIG. 1B  is a perspective view of the subsea wellhead assembly of  FIG. 1A . 
           [0008]      FIG. 2  is an alternate example of a protective net used with a wellhead assembly on the seafloor. 
           [0009]      FIG. 3A  is a perspective view of a subsea wellhead assembly with a protective net being lowered from a vessel to the seafloor. 
           [0010]      FIG. 3B  is a perspective view of the subsea wellhead assembly and protective net of  FIG. 3A  on the seafloor. 
           [0011]      FIG. 4  is a side perspective view of the subsea wellhead assembly of  FIG. 1  with a protective net assembly having an opening through a protective net. 
           [0012]      FIG. 5A  is an alternate embodiment of a wellhead assembly having a protective netting in accordance with an embodiment of the present invention. 
           [0013]      FIG. 5B  is an embodiment of the wellhead assembly of  FIG. 5A  with a remotely operated vehicle creating an access in the protective netting in accordance with the present invention. 
           [0014]      FIG. 5C  is a detail of an example of an attachment between the protective netting and a support post. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1A  illustrates an example embodiment of a subsea wellhead assembly  10  in a partial side sectional view. The subsea wellhead assembly  10  includes a wellhead housing  12  shown mounted over casing  14  that is inserted into a subsea formation  16 . A coupling mounts a production tree  18  on the wellhead housing  12 . The production tree  18  typically includes laterally projecting fluid flow lines  20  having wing valves  22  disposed within the lines  20 . In one of many possible configurations, a main bore  24  projects vertically along the production tree  18  and in line with the wellhead housing  12 . A swab valve  26  is set in the main bore  24  for controlling the flow through the main bore  24 . Projecting upward from the end of the main bore  24  is a reentry mandrel  28 . 
         [0016]    A protective net  30  is illustrated spanning between and connected to vertically mounted guideposts  32  that are oriented substantially parallel with the main bore  24 . The lower ends of the guidepost  32  are mounted on a support frame  34  shown mounted on the wellhead housing  12  and radially extending outward. The support frame  34  can be made up of individual structural elements or a plate-like planar structure. The subsea well head assembly  10  also includes a service control module (SCM)  36  for local control of the valves  22 ,  26  and other mechanisms provided on the wellhead assembly  10 . A control line (not shown), such as a flying lead, can be attached to the SCM  36  for delivering control commands to and from the SCM  36 .  FIG. 1B  provides a perspective view of the wellhead assembly  10  of  FIG. 1A . In the embodiment of  FIG. 1B , the net  30  includes a series of interconnecting net members  37  that are strategically spaced apart so the net  30  has sufficient strength to protect components of the wellhead assembly  10  from falling debris or other impacts. 
         [0017]      FIG. 2  depicts an alternative embodiment of a net  40  that can be used for protecting a wellhead assembly  10  subsea. In this example the net  40  envelopes the sides and the overhead of the wellhead assembly  10 . The net  40  of  FIG. 2  is made of interconnected net members  42  strategically spaced to provide an adequate barrier for falling and/or toppling objects to prevent impact with the wellhead assembly  10 . The net members  37  and/or  42  can be made from lines, ropes, straps, monofilament, or any other type of elongated member. In one example, the net members  37  and  42  are formed from Kevlar®; optionally, other aramid or carbon fibers can be used for forming the net members  37  and  42 . The dimensions, spacing, and arrangement of the net members  37 ,  42  may vary on the particular application of the net  30 ,  40 ; these details however are within the capabilities of those skilled in the art and may be determined without undue experimentation. 
         [0018]      FIG. 3A  illustrates in a perspective view an example of using a workboat  44  for deploying a net  40 A shown set on a frame  46 . The net  40 A and frame  46  are lowered to the seafloor for protecting the subsea wellhead assembly  10  ( FIG. 3B ). Referring now to  FIG. 3B , the frame  46  includes upper horizontal girders  48  arranged in a rectangle that circumscribes the reentry mandrel  28 . In an example embodiment, the reentry mandrel  28  inserts through the portion of the net  40 A set within the upper horizontal girders  48 . Lower girders  50  are shown horizontally mounted on the seafloor and form a rectangular-like shape having an outer perimeter greater than the upper girders  48 . Vertically mounted posts  52  attach between the upper and lower girders  48 ,  50 . Since the lower girders  50  have a perimeter larger than the upper girders  48 , the vertical posts  52  angle radially outward with distance away from the upper girders  48 . 
         [0019]    Referring back to  FIG. 3A , although the net  40 A and frame  46  are shown being lowered subsea from the workboat  44 , the net  40 A and frame  46  may also be lowered from other types of vessels, including any type of an offshore platform. A remote operated vehicle (ROV)  54 , optionally deployed from the workboat  44 , is adjacent the net  40 A and may be used to aid in laying the net  40 A and frame  46  over the wellhead assembly. 
         [0020]    Referring now to  FIG. 4 , a perspective view is shown of an alternate frame  46 A with a protective net  40 B. In this example, the frame  46 A is largely rectangular with upper and lower rectangularly shaped girders  48 A,  50 A that circumscribe the frame  46 A at vertically spaced apart locations. As such, the girders  48 A,  50 A of  FIG. 4  have largely the same dimensions. Vertical posts  52 A are shown mounted between the respective corners of the upper and lower girders  48 A,  50 A and extending largely parallel to the main bore  24  of the subsea assembly  10 . Included with the upper girder  48 A are horizontal joists  55  shown set between parallel members of the upper girders  48 A. The combination of the upper girders  48 A and the horizontally mounted joists  55  provide an upper surface for supporting the net  40 B. The vertical posts  52 A define a vertical perimeter for the sides of the net  40 B. A vertically extending slit  56  on one side of the net  40 B of  FIG. 4  defines an access by forming an opening by which the wellhead assembly  10  may be accessed. In an example embodiment, the ROV  54  may access the wellhead assembly  10  through the slit  56 . Optionally, the slit  56  may be on another side of the net  40 B, or further optionally, the net  40 B may include more than one slit  56  In the embodiment of  FIG. 4  the ROV  54  includes a mechanical arm  58  and may be controlled via an attached tether line  60 . The arm  58  can be used to slide apart the portions of the net  40 B adjacent the slit  56  to form an opening through the net  40 B for accessing the subsea wellhead assembly  10 . 
         [0021]      FIG. 5A  illustrates an alternate example embodiment of the net  30 A used for protecting the wellhead assembly  10  where the net  30 A further includes side panels  62  that drape along the lateral sides of the wellhead assembly  10 . The panels  62  are supported by optional posts  64 . The panels include horizontal net members  66  that mount to the posts on sliding attachments  68 . Vertical net members  70  may attach across the horizontal members  66  for enhancing the integrity of the panels  62 .  FIG. 5B  depicts an example feature of the net  30 A wherein an arm(s)  58  of the ROV  54  temporarily moves a portion of one of the panels  62  away from a control panel  72  by downwardly sliding the horizontal members  66  to urge the sliding attachments  68  along the posts  64  thereby defining an access by forming an opening. Hot stabs  74  and other attachments on the control panel  72  can be accessed by the ROV  54 , then the panels  62  returned to their configuration of  FIG. 5A  and in a protecting position. An example embodiment of the sliding attachment  68  is shown in expanded detail in  FIG. 5C  where the sliding attachments  68  mount on rods  69  attached to the posts  64  and are slidable over the rods  69 . In an example embodiment, a coefficient of friction between the sliding attachment  68  and rods  69  is maintained at a level to resist sliding movement from gravity thereby maintaining a structural form when left unattended. 
         [0022]    While the invention has been shown or described in only some 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.

Summary:
A subsea wellhead assembly having a net that protects the wellhead assembly from impacts with falling objects. The net can be deployed from a vessel onto a wellhead on the seafloor, or mounted onto a wellhead component above the sea surface and lowered with the wellhead component. The net can be retractable, so that an ROV can access the wellhead assembly by temporarily moving the net into a retracted configuration, then redeploying the net when finished accessing the wellhead assembly.