Abstract:
A system for monitoring strain in a wellbore casing includes one or more gages that are affixed to an outer surface of the wellbore casing. Each gage includes one or more apertures. During operation, variations in the shape and spacing of the apertures are monitored and used to determine a level of strain in the wellbore casing.

Description:
FIELD OF THE INVENTION 
       [0001]    This disclosure relates in general to measuring stress, strain, and fatigue of tubular oil and gas well equipment, and particularly to conduits located within a wellhead housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0002]      FIG. 1  is quarter-sectional view of a portion of a surface wellhead assembly of an offshore well, and shown having a measuring system in accordance with an exemplary embodiment. 
           [0003]      FIG. 2  is a schematic side elevational view of a portion of the casing hanger of the wellhead assembly of  FIG. 1 . 
           [0004]      FIG. 3  is an enlarged view illustrating a measuring gage that is bonded to the casing hanger as illustrated in  FIG. 2 . 
           [0005]      FIG. 4  is a schematic sectional view illustrating inserting a reader under a pressure controlled environment for reading one of the gages of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0006]    In the drawings and description that follows, like parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawings are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. The present invention is susceptible to embodiments of different forms. Specific embodiments are described in detail and are shown in the drawings, with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. The various characteristics mentioned above, as well as other features and characteristics described in more detail below, will be readily apparent to those skilled in the art upon reading the following detailed description of the embodiments, and by referring to the accompanying drawings. 
         [0007]    Referring to  FIG. 1 , a housing  11  is located at the upper end of a riser (not shown) that extends downward to a subsea wellhead assembly. Housing  11  is mounted stationarily on an offshore platform in this example, and the platform has legs that extend to the seafloor. The riser and housing  11  are stationary relative to the platform and not subject to wave and current movements. 
         [0008]    Another component of the surface wellhead assembly includes a casing head  13  mounted on housing  11  by a connector  15 . Casing head  13  is a tubular member having a bore  17  extending through it. Casing head  13  has a plurality of load shoulders  19 , which in this example are retractable. Each load shoulder  19  is retracted by a screw assembly  21  in this embodiment. While in the extended position shown in  FIG. 1 , load shoulders  19  protrude into bore  17 . Alternatively, load shoulders  19  could comprise a single fixed load shoulder. 
         [0009]    A casing hanger  23  is supported on load shoulders  19 . Casing hanger  23  supports a string of casing  25 , which has a lower end that latches or ties back to a subsea casing hanger in the subsea wellhead housing at the seafloor. The operator wishes to apply tension to casing  25  to a desired level and to maintain casing  25  at that amount of tension. Applying and maintaining the tension may be handled in different ways. For example, a ratcheting mechanism may be employed. The ratcheting mechanism allows part of the casing hanger to ratchet upward relative to the casing head, but not downward so as to hold tension. In this embodiment, however, casing hanger  23  has an outer body  27  that secures to external threads  29  on casing hanger  23 . Casing hanger  23  also has a set of interior threads  31  or a profile for securing to a running tool (not shown). While one portion of the running tool pulls tension on casing hanger  23  to provide the desired amount of tension in casing  25 , another portion of the running tool rotates outer body  27  downward into contact with load shoulders  19 . In  FIG. 1 , the upper end of outer body  27  is shown spaced below a downward facing shoulder  32  on the upper portion of casing hanger  23 . While being run-in, the upper end of outer body  27  will be in contact with downward facing shoulder  32 . A stop ring  33  located at the lower end of threads  29  provides a limit to how far downward outer body  27  can be rotated. 
         [0010]    Also, in this embodiment, a mechanism may optionally be included to prevent any upward movement of casing hanger  23  relative to casing head  13  after installation. This mechanism includes a lock ring  35  that is a split ring that is expanded outward into a mating profile  36  by means of a tapered energizing ring  37 . After outer body  27  has been rotated downward into contact with load shoulders  19 , the running tool pushes energizing ring  37  downward to move lock ring  35  into profile  36 . 
         [0011]    After casing  25  has been tensioned and outer body  27  set, the operator would typically remove the running tool, then install a seal  41 . Seal  41  is located on the upper end of a spacer  39  that contacts the upper end of energizing ring  37 . Seal  41  may be of various types, either metal-to-metal or elastomeric. Seal  41  seals between the outer diameter of the upper portion of casing hanger  23  and casing head bore  17 . 
         [0012]    In this example, two additional casing strings  43  are shown extending through casing  25 . Each string  43  may be tensioned and supported similarly in casing heads located above casing head  13 . Also, a string of production tubing  45  is shown extending through inner casing string  43 . Tubing string  45  may also be tensioned and supported in a tubing head in the same manner. 
         [0013]    While installing casing strings  25 ,  43  and tubing  45 , it would be advantageous to be able to know the strain and the amount of tension that exists after the casing hangers or tubing hanger are set. Also, from time to time it would useful to monitor the strain to determine if the initial tension has decreased, such as might occur if the platform settles. Fatigue can occur due to cycles of stress, either from thermal changes or other factors. Although casing head  13  and the various housings for the casing strings  43  and tubing  45  are located on a platform above the sea, casing strings  25 ,  43  and tubing  45  are concealed within the housings and other tubular members. Consequently, conventionally measuring strain in the same manner as one would to accessible conduits is not possible. 
         [0014]    In this exemplary embodiment, a plurality of gages  47  are mounted on casing hanger  23  below threads  29 . Each gage  47  is of a type that will provide an indication of strain without requiring any wires or a battery. As shown schematically in  FIG. 3 , each gage  47  is a thin film of a polymer that is coated with an adhesive for bonding to a metal conduit. Alternately, each gage  47  could be laser etched directly onto the steel body of casing hanger  23 . Each gage  47  has a plurality of apertures  46  that are laser-machined in a geometric pattern. Apertures  46  are spaced evenly apart from each other in a row and are preferably identically sized. In this example, apertures  46  extend axially along one side edge of gage  47  and horizontally along another side edge. When tension is applied, gage  47  stretches slightly, changing the spacing between apertures  46 . This change in spacing is detectable and provides an indication of the stress being applied and the strain occurring. 
         [0015]    Optionally, each gage  47  may have one or two rows of apertures  48  that are spaced apart from each other different amounts and have different widths to define a bar code containing information. In this example, apertures  48  extend along the other axial side edge and other horizontal edge from apertures  46 . Optionally, a central aperture  50  may be cut in the film of gage  47 , but that is not necessary. 
         [0016]    A reader  51  optically reads apertures  46 ,  48  of gage  47  and provides direct measurement of strain and other information. Reader  51  has a lens, a ring light source and strain measurement software. Reader  51  is located within a view port  49  that extends through the sidewall of casing head  13 . Preferably, view port  49  is located on a radial line of the axis of casing head  13 . A flange  53  bolts to the exterior of casing head  13  around view port  47 . An electrical lead  57  extends through a seal assembly  55  of flange  53  and extends to a processor and display  59  that may be located on another level on the platform, such as at the rig floor. Processor  59  contains algorithms that will provide a readout of strain directly based on the optical reading of reader  51 . Gages  47 , reader  51  and processor  59  are commercially available. One manufacturer is Direct Manufacturing, Inc., Columbia, S.C. 
         [0017]    Because the operator will not know in advance exactly how much stretch will exist in casing  25  once tensioned, preferably a plurality of gages  47  are mounted to casing hanger  23  and axially spaced apart from each other.  FIG. 2  shows three rows of gages  47  and they are axially spaced so that with the least amount of stretch expected, the upper row will be visible to reader  51 . With the maximum amount of stretch in casing  25  expected, the lower row of gages  47  would be readable by reader  51 . 
         [0000]    Also, typically while running casing  25 , the operator does not orient casing hanger  23  to any particular rotational position relative to casing head  13 . While orientation can be done, an alternative is to mount a number of gages  47  in horizontal rows extending completely around casing hanger  23 . At least one of the gages  47  will always be aligned with reader  51 , regardless of the orientation of casing hanger  23 . In addition, more than one view port  49  is preferably employed, with the view ports being spaced circumferentially around casing head  13 . The additional view ports  49  allows an operator to insert reader  51  and make readings from different sides of casing hanger  23 . 
         [0018]    In the preferred embodiment, a reader  51  is positioned in casing head  13  while casing  25  is being tensioned. The operator will thus be able to read the strain directly from the display of processor  59  while the tensioning procedure is occurring. The operator will thus know the level of tension that exists in casing  25  after the running tool has been disconnected from casing hanger  23  and outer body  27  landed on load shoulders  19 . Afterward, the operator can remove reader  51  and use it for tensioning inner casing strings  43  and tubing  45 , each of which will contain gages  47  attached to their hangers in a similar manner. 
         [0019]    Also, periodically the operator can insert reader  51  into one of the view ports  49  to monitor the strain in subsequent years. This information allows the operator to determine the tension and fatigue. If pressure control is needed, this can be readily handled by the use of a lubricator assembly  61 , schematically shown in  FIG. 4 . The operator inserts reader  51  into view port  49  on an insertion tool  63 . Insertion tool  63  comprises a tubular rod through which lead  57  will extend. Lubricator assembly has a valve  65 , on its inner end and an injection head  67  on its outer end. The operator closes valve  65  and inserts reader  51  into a chamber located between valve  65  and injection  67 . Injection head  67  is a conventional sealing mechanism that typically employs a pump that pumps grease around a tubular member to form a seal and simultaneously allow the tubular member to be moved along its axis. In this application, injection head  67  is actuated to maintain a seal around insertion tool  63  while valve  65  is opened and insertion tool  63  pushed inward to push reader  51  into close proximity to one of the gages  47 . After taking a reading, the operator reverses the procedure to remove reader  51 . 
         [0020]    It is understood that variations may be made in the above without departing from the scope of the invention. While specific embodiments have been shown and described, modifications can be made by one skilled in the art without departing from the spirit or teaching of this invention. The embodiments as described are exemplary only and are not limiting. Many variations and modifications are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.