Abstract:
A strain gage apparatus for measuring the load applied to a derrick leg mechanically multiplies the deflection or distortion of a strain gage. The apparatus attaches to the derrick leg at two points that are spaced about five feet apart. Accumulated strain along the five-foot section moves one end of an elongate member a significant distance. The distance is much greater than any localized movement or minute strain in the derrick leg. The movement of the elongate member stresses a reaction member upon which the strain gage is mounted.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The subject invention generally pertains to a derrick of a hoist and more specifically to an apparatus for sensing a load applied to the derrick.  
           [0003]    2. Description of Related Art  
           [0004]    Hoist and derrick systems used for drilling or servicing wells often handle loads ranging from about a hundred pounds for lighter well components, such as sucker rods, to a hundred tons or more for a string of well casings.  
           [0005]    The load on the derrick is usually monitored in some way to avoid applying excessive lifting force that may damage well components and to avoid overloading the hoist and derrick.  
           [0006]    The accuracy of the load measurement is preferably sufficient to differentiate 200 pounds of load. Such accuracy, however, can be difficult to achieve for a common derrick having a rated hook load of 200,000 pounds and having a designed ultimate strength of over 400,000 pounds, considering a safety factor of at least two. 200 pounds is a mere 0.05% of a 400,000-pound derrick, so loads varying by 200 pounds may be difficult to differentiate using conventional means. Consequently, a need exists for an improved, more accurate device for sensing the load on a derrick.  
         SUMMARY OF THE INVENTION  
         [0007]    To provide an improved, more accurate device for sensing the load on a derrick, an object of the invention is to provide a device that creates mechanically multiplied strain changes that can be sensed by a strain gage.  
           [0008]    Another object of some embodiments is to attach a strain gage apparatus to a derrick leg, wherein strain changes in the apparatus are greater than corresponding strain changes in the derrick leg.  
           [0009]    Another object of some embodiments is to provide a strain gage apparatus for a derrick leg, wherein the strain in the apparatus is greater than the strain in the derrick leg.  
           [0010]    Another object of some embodiments is to provide a strain gage apparatus for a derrick leg, wherein the strain in the apparatus is less than the strain in the derrick leg.  
           [0011]    Another object of some embodiments is to provide a stain gage apparatus that include two strain gages mounted to opposite faces of a reaction member, wherein the signals from the two gages are combined to provide a combined signal that varies with the load applied to the derrick.  
           [0012]    Another object of some embodiments is to provide a strain gage reaction member that is thinner than a bar that actuates the reaction member.  
           [0013]    Another object of some embodiments is to mechanically multiply the action of a strain gage by applying the strain gage to a reaction member that lies at an angle to a derrick leg.  
           [0014]    Another object of some embodiments is to stress a strain gage with a bar having a distal end that can slide or otherwise move relative to a derrick leg.  
           [0015]    Another object of some embodiments is to provide a strain gage apparatus with an adjustment that can adjust the extent to which a bar can stress a strain gage.  
           [0016]    One or more of these and other objects of the invention are provided by a strain gage apparatus that includes a strain gage attached to a reaction member, which is stressed by a bar attached to a derrick leg, wherein some relative movement may occur between the bar and the derrick leg. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0017]    [0017]FIG. 1 is a schematic view of a strain gage apparatus attached to a derrick leg of a mobile service rig.  
         [0018]    [0018]FIG. 2 is a side view of one embodiment of a strain gage apparatus.  
         [0019]    [0019]FIG. 3 is a cross-sectional view taken along line  3 - 3  of FIG. 2.  
         [0020]    [0020]FIG. 4 is similar to FIG. 4 but with an external load applied to the derrick leg.  
         [0021]    [0021]FIG. 5 is similar to FIG. 2 but showing another embodiment of a strain gage apparatus.  
         [0022]    [0022]FIG. 6 is similar to FIG. 2 but showing another embodiment of a strain gage apparatus.  
         [0023]    [0023]FIG. 7 is similar to FIG. 2 but showing another embodiment of a strain gage apparatus.  
         [0024]    [0024]FIG. 8 is similar to FIG. 2 but showing another embodiment of a strain gage apparatus.  
         [0025]    [0025]FIG. 9 is similar to FIG. 2 but showing another embodiment of a strain gage apparatus.  
         [0026]    [0026]FIG. 10 is similar to FIG. 9 but with a clamp lowered to reduce the effective length of an elongate member.  
         [0027]    [0027]FIG. 11 is similar to FIG. 2 but showing another embodiment of a strain gage apparatus.  
         [0028]    [0028]FIG. 12 is a schematic view showing how a strain gage apparatus can be applied to a wide variety of structures in compression.  
         [0029]    [0029]FIG. 13 is a schematic view showing how a strain gage apparatus can be applied to a wide variety of structures in tension. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0030]    A machine  10  (e.g., a mobile service rig, well drilling rig, crane, etc.), shown in FIG. 1, includes a hoist  12  and a derrick  14  for suspending a load  16  within or above a wellbore  18 . Load  16  is schematically illustrated to represent anything that can create tension in a cable  20  of hoist  12 . Examples of load  16  include, but are not limited to, tubing, suction rods, casings, and friction of such parts within wellbore  18 . Load  16  may vary in magnitude due to acceleration and deceleration of the suspended load or due to frictional changes within the wellbore.  
         [0031]    To monitor the magnitude of load  16 , a strain gage apparatus  22  is attached to a derrick leg  24  of derrick  14 . Load  16  creates a load strain in derrick leg  24 , and strain gage apparatus  22  responds to that load strain by providing a load signal  26  that varies with the load strain or magnitude of load  16 . The term, “load strain” refers to a change in length over a given reference length of derrick leg  24 . Since the actual load strain is not necessarily uniform across the entire length of derrick leg  24 , the term “average load strain” may be used, which is the mean load strain distributed across a major length (e.g., length  88 ) of derrick leg  24 . The term, “derrick leg” refers to any elongate member that provides a derrick with structural support that helps in carrying a suspended load. Examples of a derrick leg include, but are not limited to, a generally upright member  28  under compression, a generally horizontal member  30  in tension, a diagonal member  32 , and various combinations thereof Derrick  14  can be disposed at an angle, as shown, or can be perfectly vertical.  
         [0032]    In addition to load  16 , the weight of derrick  14  itself may contribute to the total load strain in derrick leg  24 , so the derrick&#39;s weight may overshadow small changes in the magnitude of load  16 . Moreover, applying a relatively light load to a derrick designed to withstand much larger loads may produce a load strain that is too small to accurately measure using conventional strain gage techniques. So, strain gage apparatus  22  and various similar embodiments (apparatuses  22   a - g ) provide a mechanically amplified strain that may be easier to detect. The amplified strain in strain gage apparatus  22  may actually be greater or less than the load strain in derrick leg  24 . However, small changes in the magnitude of load  16  preferably creates a change in the amplified strain, i.e., delta amplified strain, that is greater than the corresponding change in the load strain, i.e., delta load strain or average delta load strain. There are various ways of achieving such a response.  
         [0033]    For example, FIGS.  2 - 4 , show a strain gage apparatus  22   a  comprising an elongate member  34  that acts upon a reaction member  36 . FIG. 2 shows apparatus  22   a  when load  16  equals zero, and FIG. 4 shows apparatus  22   a  when load  16  is appreciably greater than zero. Reaction member  36  can be a piece of spring steel approximately 0.5-inches wide, 2.5-inches long, and 0.020-inches thick. An anchor  38  attached to a first point  40  of derrick leg  24  holds one end of reaction member  36  substantially fixed. In this example, anchor  38  comprises an angle  42 , a clamp plate  44 , and two screws  46  that clamp one end of reaction member  36  between clamp plate  44  and angle  42 . An attachment system  48  is schematically illustrated to represent any system that can attach anchor  38  to derrick leg  24 . Examples of attachment system  48  include, but are not limited to, adhesives, welding, threaded fasteners, clamps, magnets, brazing, soldering, etc.  
         [0034]    For apparatus  22   a , elongate member  34  is a round rod threaded at each end, so its effective length can be adjusted. Lengthening elongate member  34  can increase the mechanical amplification or response of apparatus  22   a , which will be explained further with reference to the embodiment of FIGS. 9 and 10. The rod may be less than 1-inch in diameter with a length of about 5-feet. An upper angle  50  connects a proximal end  52  of elongate member  34  to a second point  54  on derrick leg  24 . An attachment system  56  is schematically illustrated to represent any system that can attach angle  50  to derrick leg  24 . Examples of attachment system  56  include, but are not limited to, adhesives, welding, threaded fasteners, clamps, magnets, brazing, soldering, etc.  
         [0035]    A ball joint rod end  58  (sometimes referred to as a rod bearing or a turnbuckle) connects a distal end  60  of elongate member  34  to a clevis  62  that two screws  64  connect to a clamp plate  66 . Another end of reaction member  36  is clamped between clevis  62  and clamp plate  66 . One or more strain gages  68  can be affixed to reaction member  36  to provide a load signal  26  to help determine the value of load  16  by sensing the strain in reaction member  36 . Load signal  26  can be the electrical resistance or impedance of a single strain gage  68 , or signal  26  can be a combined load signal, which is the combined electrical resistance of two strain gages  68 . When two strain gages  68  are used, they are preferably attached to opposite faces of reaction member  36  and wired in series via wires  72 ,  74  and  76  so that their resistance values are added to each other to provide a combined load signal. The term, “strain gage” refers to a part having at least one electrical characteristic (e.g., electrical resistance) that varies upon distorting the part. Strain gage  68  can be a bending beam load cell (e.g., bending-full bridge strain gage SG-6/120-LY11) from Omega Engineering of Stamford Conn. Strain gages  68  can be wired to a conventional Wheatstone Bridge circuit  78 , which in turn can be wired to a common electrical circuit  80 , such as a personal computer, datalogger, digital display circuit, programmable logic controller, or the like. Circuit  80  then converts an output  82  of circuit  78  to a load value (e.g., the weight of load  16 ) that can be recorded or displayed. For greater sensitivity, circuits  78  or  80  may include a strain gage amplifier, such as an SGAMP-2 provided by Industrologic, Inc. of St. Charles, Mo.,  
         [0036]    It should be appreciated by those skilled in the art of strain gage technology that various strain gages can be applied in various ways, and that there is a wide variety of circuitry available for interpreting the response of strain gages. A more important aspect of the invention is how an apparatus, such as apparatus  22   a , can provide a strain gage mounting surface that for a given load change can experience a greater change in strain than an adjacent load bearing surface, such as the surface of derrick leg  24 .  
         [0037]    When derrick leg  24  is completely unloaded (i.e., load  16  is equal to zero and no derrick weight is applied to leg  24 ), the load strain or average load strain in derrick leg  24  may be substantially zero and point  54  may be at a level  82  relative to point  40 . The weight of derrick  14  may create a load strain in derrick leg  24  as indicated by point  54  moving to level  84 , whereby the load strain in leg  24  equals dimension  86  divided by dimension  88 . Since the intent is to determine the value of load  16 , the load strain caused by the weight of derrick  14  can be disregarded by way of an adjustment  90 . In this embodiment, adjustment  90  varies the extent to which elongate member  34  deforms reaction member  36  by adjusting the axial position of nuts  92  on member  34 , which in turn adjusts the effective length of member  34 , so reaction member  36  is substantially unstressed. When reaction member is unstressed, its amplified strain is equal to zero. Thus, adjustment  90  can be used for zeroing strain gage apparatus  22   a . The term, “amplified strain” simply refers to the strain in reaction member  36  as sensed by strain gage  68 .  
         [0038]    When load  16  is applied to derrick  14  (i.e., load  16  is appreciably greater than zero), derrick leg  24  experiences an increase in strain that results in point  54  moving from level  84  to a level  94 , as shown in FIG. 4. This increase in strain is referred to as an average delta load strain, which equals dimension  96  divided by dimension  98 . The movement of point  54  to level  94  pushes elongate member  34  downward, which bends reaction member  36  to create a delta amplified strain therein. The term, “delta amplified strain” refers to a change in the reaction member&#39;s strain in response to a change in the magnitude of load  16 . The separation distance (e.g., distance  98 ) between points  40  and  54  should be sufficient to provide ample movement of distal end  60 . In some cases, the distance between points  40  and  54  is about five feet. When distance  98  is sufficiently long, the delta amplified strain is greater than the average delta load strain, even though derrick leg  24  provides substantially more load support than reaction member  36 . Reaction member  36  lies at an angle (i.e., not parallel) to the length of derrick leg  24  and preferably provides substantially no load support. The term, “substantially no load support” refers to supporting less than 0.001% of load  16 . The delta amplified strain is communicated to Wheatstone Bridge circuit  78 , which enables circuit  80  to record or display the value of load  16 .  
         [0039]    In another embodiment, shown in FIG. 5, a strain gage apparatus  22   b  is similar to apparatus  22   a , except an elongate member  100  replaces member  34  and angle  50 . Also, an angle  102  serves as a combination anchor and reaction member that replaces anchor  38  and reaction member  36 . Attachment system  48  attaches angle  102  to a first point  104  of derrick leg  24 , and one or more strain gages  106  are affixed directly to angle  102  to sense the strain therein. Attachment system  56  attaches a proximal end  108  of member  100  to a second point  110  of derrick leg  24 , and a distal end  112  of member  100  engages angle  102 . A sliding connection  114  (i.e., one member can translate relative to the other) exists between bar elongate member  100  and leg  24 . An increase in load  16  can create a delta load strain between points  104  and  110  in leg  24 , which causes elongate member  100  to push downward against angle  102 . This bends the generally horizontal leg of angle  102  to create a delta amplified strain therein. Strain gage  106  responds to the delta amplified strain to help determine the value of load  16 .  
         [0040]    In another embodiment, shown in FIG. 6, a strain gage apparatus  22   c  is similar to apparatus  22   a , except a cable  116  (an elongate member that is flexible) and an upper angle  118  replaces elongate member  34  and angle  50 . Attachment system  48  attaches a lower angle  120  (combination anchor and reaction member) to a first point  122  on derrick leg  24 , and one or more strain gages  124  are affixed directly to angle  120  to sense the strain therein. Attachment system  56  attaches upper angle  118  to a second point  126  of derrick leg  24 . Eyebolts  128  can be adjusted to maintain tension in cable  116  at all times, so cable  116  applies a continuous upward bending moment on the generally horizontal leg of lower angle  120 . An increase in load  16  can create a delta load strain between points  122  and  126  in leg  24 , which reduces the tension in cable  116 . Reducing the tension in cable  116  reduces the upward bending moment in angle  120 , which in turn creates a delta amplified strain in angle  120 . The delta amplified strain may vary inversely with load  16  but can still be used to help determine the value of load  16 .  
         [0041]    In another embodiment, shown in FIG. 7, a strain gage apparatus  22   d  is similar to apparatus  22   a , except anchor  38  and reaction member  36  are replaced by the combination of an anchor  130 , a reaction member  132  and a spacer  134 . Reaction member  132  is a relatively thin strip of material (e.g., steel) that can be attached to anchor  130  and spacer  134  in any suitable manner, such as gluing, clamping, etc. One or more strain gages  136  can be affixed to reaction member  132  to sense the tensile strain therein. Attachment system  48  attaches anchor  130  to a first point  138  on derrick leg  24 , and attachment system  56  attaches a proximal end  140  of an elongate member  142  to a second point  144  on leg  24 . A distal end  146  of elongate member  142  connects to spacer  134  and the lower end of reaction member  132 . An increase in load  16  can create a delta load strain between points  138  and  144  in leg  24 , which causes elongate member  142  to move distal end  146  downward. This stretches reaction member  132  to create a delta amplified strain therein. Strain gage  136  responds to the delta amplified strain to help determine the value of load  16 .  
         [0042]    In another embodiment, shown in FIG. 8, a strain gage apparatus  22 c includes an anchor  148  that attachment system  48  attaches to a first point  150  on derrick leg  24 . Attachment system  56  attaches a proximal end  152  of an elongate member  154  to a second point  156  on derrick leg  24 . A reaction member  158 , similar to reaction member  36 , has one end attached to a distal end  160  of elongate member  154 . The opposite end of reaction member  158  engages the end of a screw  162  that serves as an adjustment for zeroing apparatus  22   e . One or more strain gages  164  can be affixed to reaction member  158  for the usual purpose. A guide  166  attached to leg  24  helps guide a sliding connection between elongate member  154  and derrick leg  24 . An increase in load  16  can create a delta load strain between points  150  and  156  in leg  24 , which causes elongate member  154  to push reaction member  158  downward against screw  162 . This bends reaction member  158  to create a delta amplified strain therein. Strain gage  164  responds to the delta amplified strain to help determine the value of load  16 .  
         [0043]    In another embodiment, shown in FIG. 9, a strain gage apparatus  22   f  includes a single elongate unit formed to create in combination a reaction member  168 , an anchor  170 , and an elongate member  172 . Attachment system  48 , such as a clamp  174 , attaches anchor  170  to a first point  176  on derrick leg  24 . Attachment system  56 , such as clamp  174 , attaches a proximal end  178  of elongate member  172  to a second point  180  on leg  24 . Reaction member  168  lies between anchor  170  and a distal end  182  of elongate member  172 . One or more strain gages  184  can be affixed to reaction member  168  to sense deflection therein. A sliding connection  186  exists between elongate member  172  and leg  24 . An increase in load  16  creates a delta load strain between points  176  and  180  in leg  24 , which causes elongate member  172  to deflect reaction member  168 . This creates a delta amplified strain in reaction member  168 . Strain gage  184  responds to the delta amplified strain to help determine the value of load  16 .  
         [0044]    The strain in reaction member  168  is generally proportional to the length of elongate member  172  or the distance between points  176  and  180 . Thus, adjusting the separation distance between clamps  174  adjusts the mechanical multiplying effect of apparatus  22   f  In FIG. 10, for example, the upper clamp  174  is moved to a point  180 ′, so the two clamps  174  are closer to each other. For a given change in load, the delta amplified strain in apparatus  22   f ′ of FIG. 10 will be less than the delta amplified strain in apparatus  22   f  of FIG. 9, wherein apparatuses  22   f  and  22   f ′ are structurally the same except for the spacing of clamps  174 . Apparatus  22   f  may be used on a derrick leg whose cross-sectional area is relatively large compared to its applied load whereas apparatus  22   f ′ may be used on thinner derrick legs.  
         [0045]    In another embodiment, shown in FIG. 11, a strain gage apparatus  22   g  includes a single elongate unit formed to create in combination a reaction member  188 , an anchor  190 , and an elongate member  192 . Attachment system  56  attaches anchor  190  to a first point  194  on derrick leg  24 . Attachment system  48  attaches a proximal end  196  of elongate member  192  to a second point  198  on leg  24 . Reaction member  188  extends between anchor  190  and a distal end  200  of elongate member  192 . One or more strain gages  202  can be affixed to reaction member  188  to sense deflection therein. A sliding connection  204  exists between elongate member  192  and leg  24 . An increase in load  16  creates a delta load strain between points  194  and  198  in leg  24 , which causes elongate member  192  to deflect reaction member  188 . This creates a delta amplified strain in reaction member  188 . Strain gage  202  responds to the delta amplified strain to help determine the value of load  16 .  
         [0046]    A strain gage apparatus  22   h  of FIGS. 12 and 13 is schematically illustrated to represent a broader, more generic application of the invention, wherein a load bearing member  24 ′ is schematically illustrated to represent any member subjected to tension or compression. Examples of load bearing member  24 ′ include, but are not limited to, a structural member of a crane, derrick leg, a portion of a road bridge, a hoist cable, guy wire, etc. In FIG. 12, strain gage apparatus  22 h is shown in compression. And in FIG. 13, apparatus  22   h  is shown in tension.  
         [0047]    Apparatus  22   h  includes an anchor  206  that attachment system  48  attaches to a first point  208  on load bearing member  24 ′. Attachment system  56  attaches a proximal end  210  of an elongate member  212  to a second point  214  on load bearing member  24 ′. A reaction member  216  is coupled between anchor  206  and a distal end  218  of elongate member  212 . One or more strain gages  220  can be affixed to reaction member  216  for sensing the strain in reaction member  216  and ultimately determining compressive load  222  or tensile load  224 . A change in load  222  or  224  can create a delta load strain between points  208  and  214  in load bearing member  24 ′, which causes elongate member  212  to exert tension or compression on reaction member  216 . This creates a delta amplified strain in reaction member  216 . In response to the delta amplified strain, strain gage  220  provides a load signal  226  to help determine the value of load  222  or  224 .  
         [0048]    In FIG. 12, a delta load strain is load bearing member  24 ′ is equal to the ratio of dimension  228  to dimension  230 . Dimension  228  is the compression in load bearing member  24 ′ caused by a change in load  222 . Dimension  230  is the length between points  208  and  214  prior to the change in load  222 . A change in strain in elongate member  212  is equal to the ratio of dimension  232  to dimension  234 . Dimension  232  is the insignificantly slight compression that may occur in elongate member  212  caused by a change in load  222 . Dimension  234  is the length of elongate member  212  prior to the change in load  222 . For all practical purposes, elongate member  212  does not experience any appreciable change in length. A delta amplified strain in reaction member  216  is equal to the ratio of dimension  236  to dimension  238 . Dimension  236  is the compression in reaction  216  caused by a change in load  222 . Dimension  238  is the length of reaction member  216  prior to the change in load  222 . In response to load  222  varying, elongate member  212  experiences a change in strain (approximately equal to zero) that is less than the delta load strain in load bearing member  24 ′, and reaction member  216  experiences a delta amplified strain that is greater than the delta load strain.  
         [0049]    In FIG. 13, the delta load strain is load bearing member  24 ′ is equal to the ratio of dimension  240  to dimension  242 . Dimension  240  is the stretch in load bearing member  24 ′ caused by a change in load  224 . Dimension  242  is the length between points  214  and  208  prior to the change in load  224 . A change in strain in elongate member  212  is equal to the ratio of dimension  244  to dimension  246 . Dimension  244  is the insignificantly slight stretch that may occur in elongate member  212  caused by a change in load  224 . Dimension  246  is the length of elongate member  212  prior to the change in load  224 . For practical purposes, elongate member  212  does not experience any appreciable change in length. The delta amplified strain in reaction member  216  is equal to the ratio of dimension  248  to dimension  250 . Dimension  248  is the stretch in reaction  216  caused by a change in load  224 . Dimension  250  is the length of reaction member  216  prior to the change in load  224 . In response to load  224  varying, elongate member  212  experiences a change in strain that is less than the delta load strain in load bearing member  24 ′, and reaction member  216  experiences a delta amplified strain that is greater than the delta load strain.  
         [0050]    Although the invention is described with reference to a preferred embodiment, it should be appreciated by those skilled in the art that various modifications are well within the scope of the invention. For example, the physical orientation of any of the strain gage apparatuses just described can be inverted. It should be noted that although the various reaction members experience strain, the reaction member carries much less load than the load bearing member to which it is coupled. In many cases, the reaction member carries less than one percent or substantially none of the load. Therefore, the scope of the invention is to be determined by reference to the claims that follow.