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RELATED U.S. APPLICATIONS 
   The present application claims priority from U.S. Provisional Patent Application No. 60/420,709, filed on Oct. 23, 2002, and entitled “Riser Tensioner Sensor Assembly”. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   REFERENCE TO MICROFICHE APPENDIX 
   Not applicable. 
   FIELD OF THE INVENTION 
   The present invention relates to riser tensioner systems for use on offshore platforms and more particularly, to riser tensioner systems that absorb oscillatory vertical movements of the platform while supporting the riser. More particularly, the present invention relates to sensors associated with such riser tensioner systems. In particular, the present invention relates to such sensors that measure wear on the cable in the riser tensioner system and for showing the relative movement of the sheave assemblies used in such riser tensioner systems. 
   BACKGROUND OF THE INVENTION 
   Increased oil consumption has lead to exploration and drilling in difficult geographic locations that were previously considered economically unfeasible. As is to be expected, drilling under these difficult conditions leads to problems that are not present under more ideal conditions. For example, an increasing number of exploratory wells are being drilled in deep water, offshore locations in an attempt to locate more oil and gas reservoirs. These exploratory wells are generally drilled from floating platforms, leading to a set of problems peculiar to that environment. 
   As in any drilling operation, offshore drilling requires that drilling fluid must be circulated through the drill bit to cool the bit and to carry away the cuttings. This drilling fluid is normally delivered to the drill bit through the drill string and returned to the floating vessel through an annulus formed between the drill string and a large diameter pipe, commonly known as a riser. The riser typically stands between a subsea wellhead assembly and the floating vessel and is sealed against water intrusion. 
   The lower end of this riser is connected to the wellhead assembly adjacent the ocean floor, and the upper end usually extends through a central located opening in the hull of the floating vessel. The drill string extends longitudinally through the riser and into earth formations defined below the water, and drilling fluids circulates downwardly through the drill string, out through the drill bit, and then upwardly through the annular space between the drill string and the riser, thereby returning to the vessel. 
   As these drilling operations progress into deep waters, the length of the riser and, consequently, its unsupported weight also increases. Riser structural failure may result if compressive stresses is in the elements of the riser exceed the metallurgical limitation of the riser material. Riser tensioner systems are typically used to avoid this type of riser failure. 
   Riser tensioner systems are installed onboard the platform, and apply an upward force to the upper end of the riser, usually by means of cable, sheaves, and pneumatic cylinder mechanisms connected between the vessel and the upper end of the riser. 
     FIG. 1  illustrates one such type of a riser tensioner system as applied to a drilling vessel or drilling platform  1 . The drilling vessel or drilling platform  1  comprises a mast  2 , to which a drilling string  3  is fastened, which drilling string extends in the direction of the borehole (not shown). The drilling string  3  is virtually completely enclosed by a riser  4 . A riser ring  5  is fastened at the top end of the riser  4 . Cables  6 , by means of which a tensile force can be exerted upon the riser  4 , are fastened to the riser ring  5 . Two cables  6  are shown in FIG.  1 . In the prior art, it is customary to connect four, six, eight or twelve cables to the riser ring. The cable  6  extends from the riser ring  5  by way of sheaves  10 ,  11  and  12  in the direction of the cable anchor  13 . When the drilling vessel or drilling platform  1  moves relative to the earth&#39;s surface, for example, as a result of wave action, the drilling vessel or drilling platform  1  will also move upwards relative to the riser  4 . Since the sheaves  11  and  12  are situated on either side of a cylinder  14 , these movements of the drilling vessel or drilling platform relative to the riser  4  can be absorbed. When the drilling vessel or drilling platform  1  moves relative to the riser  4 , the cylinder  14  will be depressed, with the result that the distance between the sheaves  11  and  12  is reduced in the free end of the cable  6  between the sheave  10  and the riser-tensioner  5  will increase. When the drilling vessel or drilling platform  1  moves in the direction of the riser  4 , the opposite will occur. 
   It is important in the use of such riser tensioners that the riser tensioner does not bottom cut during normal operation. In order to avoid this “bottoming out,” the pressure of the pneumatic or hydraulic fluid within the riser tensioner can be increased or decreased, depending upon need. In order to determine the amount of pressure that must be applied to avoid this “bottoming out,” it is important to continually monitor the relative movement between the sheaves  11  and  12  so that movement beyond a desired limit is avoided. For example, in rough sea conditions, it is important to increase the amount of pressure within the cylinder  14  so as to prevent the “bottoming out.” 
   Throughout the motion of the riser tensioner, the cable is subjected to a great deal of wear and tear. Wear and tear is typically measured in terms of “ton-miles.” To avoid cable failure after excessive usage, it is important to have a determination of the amount of wear applied to the cable over time. This measure will typically be based upon the amount of fluid pressure within the cylinder  14 , and the total amount of movement of the cable over the sheaves  11  and  12  over a period of time. 
   It is an object of the present invention to provide a riser tensioner sensor system which can effectively provide an indication of cable wear. 
   It is another object of the present invention to provide a riser tensioner sensor which can continuously monitor relative movement of the sheave assemblies with respect to each other. 
   It is a further object of the present invention to provide a riser tensioner system which provides immediate and reliable feedback to the operator of the offshore platform. 
   It is still a further object of the present invention to provide a riser tensioner sensor system which is very durable, even in the extreme environmental conditions of the offshore oil platform. 
   It is a further object of the present invention to provide a riser tensioner sensor assembly which is easy to use, relatively inexpensive and easy to install. 
   These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is a riser tensioner sensor assembly which can provide an indication of relative movement of the sheave assemblies associated with the riser tensioner, along with an indication of total wear on the cable supported by the sheave assemblies. In the present invention, a sensor target is interconnected to either a movable support or a fixed support of the riser tensioner. A sensor tube is in cooperative relation with the sensor target therein and is connected to the other of the movable support or the fixed portion of the riser tensioner. A sensor means is connected within the sensor tube so as to continually sense the distance that the sensor target moves vertically within the sensor tube. A processing means is connected to the sensor means and to the pressurizing system associated with the riser tensioner so as to provide a humanly perceivable measurement of the wear on the cable in the riser tensioner system. 
   In the present invention, the riser tensioner system includes a first sheave housing supporting a plurality of rotatable sheaves therein. A second sheave housing is also provided which supports a plurality of rotatable sheaves therein. A cable is threaded alternately over the sheaves of the first sheave housing and the sheaves of the second sheave housing. The cable will extend a distance therebetween. A compression means is interconnected between the sheave housings for resiliently connecting the first sheave housing to the second sheave housing. In particular, an inner tube is connected to the first sheave housing and received within an outer tube connected to the second sheave housing. A source of pneumatic pressure hydraulic pressure or pneumatic/hydraulic pressure is interconnected to the interior of these two tubular members so as to provide a suitable resistance to the movement of the first sheave housing with respect to the second sheave housing. A sensor or transducer can be associated with this pressurizing device so as to provide an indication of the amount of pressure within the riser tensioner system. 
   The sensor target of the present invention includes an annular member which is affixed to the first sheave housing so as to move in relation to the movement of the sheave housing. A magnet is received within the annular member. This magnet is a circular magnet having an inner diameter. The annular member has a wear surface positioned adjacent the inner diameter of the magnet. This wear surface is suitably replaceable and will ride in proximity to the exterior surface of the sensor tube. A support rod has one end affixed to the first sheave housing and an opposite end affixed to the annular member. This support rod extends transversely outwardly of the first sheave housing and of the riser tensioner. 
   The sensor tube is a tubular member which extends vertically and generally parallel relationship to the riser tensioner. The sensor tube extends through the interior of the annular member of the sensor target assembly. A spherical member is positioned interior of the sensor tube. This spherical member is made of a magnetically-attractive material, such as chrome-plated steel. The magnet of the sensor target suspends the spherical member within the sensor tube. As a result, the spherical member is movable interior of the sensor tube relative to the movement of the sensor target along the sensor tube. The magnetic attraction of the magnet with respect to the spherical member will continually support the spherical member interior of the sensor tube in a suspended orientation. 
   A base is connected to the bottom of the sensor tube. This base is affixed to the outer tube of the riser tensioner (or some other surface that is fixed relative to the movement of the first sheave housing). In particular, a plate extends outwardly of the outer tube of the riser tensioner. The base is affixed to a top of this plate. A flex coupling is interposed between the base and the top of the plate. 
   A sensor means is connected to the base and is directed upwardly into the interior of the sensor tube. The sensor means is for measuring movement of the spherical member within the sensor tube. The sensor means is an ultrasonic sensor which is interactive with the spherical member. The processing means connected to the sensor means for the purpose of measuring relative movement of the sheave housings with respect to each other. Processing means is also connected to the pressurizing device associated with the riser tensioner. The information is suitably processed so as to provide a humanly perceivable measurement of the wear on the cable. The wear is measured and displayed on a readout instrument. Suitable algorithms are urged to process data so as to determine cable wear. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a diagrammatic illustration of a prior art riser tensioner system. 
       FIG. 2  is a diagrammatic illustration of the riser tensioner sensor assembly of the present invention. 
       FIG. 3  is an exploded view of the riser tensioner sensorassembly of the present invention. 
       FIG. 4  is a cross-sectional view showing the sensor target as positioned over the sensor tube and around the spherical member associated with the sensor assembly of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 2 , there is shown the riser tensioner sensor assembly  20  in accordance with the teachings of the preferred embodiment of the present invention. The assembly includes a riser tensioner system  22 , a sensor target  24 , a sensor tube  26 , and a base  28  supporting the sensor tube  26  in a substantially vertical orientation. A spherical member  30  is illustrated as suspended within an interior of the sensor tube  26  by the action of the sensor target  24 . The sensor  32  is directed upwardly toward the interior of the sensor tube  26  so as to provide an indication of the relative movement of the spherical member  30  within the tube  26  over a period of time. 
   In the present invention, the riser tensioner system  22  includes a first sheave housing  34  supporting a plurality of sheaves  36  and  38  therein. A second sheave housing  40  supports a plurality of sheaves  42  and  44  therein. A cable  46  is threaded alternately over the sheaves  36 ,  42 ,  38 , and  44  of the sheave housings  34  and  40 . A compression system  48  extends between the first sheave housing  34  and the second sheave housing  40  so as to maintain the sheave housings  34  and  40  in a desired position away from each other and to flexibly maintain the sheave housings  34  and  38  in relationship to each other. The compression system  48  is a piston-and-cylinder assembly in which an outer tube  50  forms the cylinder and a second tube  52  acts as a piston rod extending outwardly of the outer tube  50 . A pressurizer  54  is in fluid communication with the compression system  48  so as to suitably pressurize the piston-and-cylinder assembly for the purpose of maintaining the first sheave housing  34  in the desired position with respect to the second sheave housing  40  for the purpose of flexibly connecting the sheave housings  34  and  40  and for preventing any “bottoming out” of the riser tensioner system. The pressurizing device  54  is a source of pneumatic pressure that can adjustably introduce a desired amount of pressure within the interior of the compression system  48 . A transducer line  56  is connected to the pressurizing device  54  for the purpose of activating the pressurizing device  54  so as to increase or decrease pressure therefrom and also for the purpose of providing a numerical reference as to the amount of pressure that has been introduced into the compression system  48 . 
   As can be seen in  FIG. 2 , the second sheave housing  40  has suitable bolts  58  which allow the second sheave housing  40  to be suitably fixedly attached to the offshore platform or to a fixed position on the offshore platform. It can also be seen in  FIG. 2  that the cable  46  extends as a pair of lines between the respective sheaves. As stated earlier, a larger number of sheaves can also be provided for the purposes of the riser tensioner system. 
   The sensor target  24  has a support structure  60  affixed to the first sheave housing  44  and extending transversely outwardly therefrom. Support structure  60  is connected to an annular member  62  positioned over and around the sensor tube  26 . As will be described hereinafter, the annular member  62  supports a circular magnet therein so that the inner diameter of the magnet is adjacent to the outer surface of the sensor tube  26 . A suitable wear surface can be interposed between the magnet and the outer surface of the sensor tube  26 . 
   The base  28  supports the sensor tube  26  in a generally vertically upright orientation in parallel relationship to the riser tensioner  22 . The base  28  also supports the sensor  32  therein. The base  28  has a bottom flange  64  affixed to a plate  66 . A flex coupling  68  is interposed between the bottom of flange  64  and the top surface of plate  66  so as to allow flexibility between the base  28  (and its support of sensor tube  26 ) and angular deflection caused by the movement of the riser tensioner system  22 . The plate  66  is affixed to the outer tube  50  of the compression system  48 . In other words, plate  66  will be in a fixed transverse orientation in generally parallel relationship to the movable support structure  60  and its associated sensor target assembly  24 . 
   The sensor  32  is an ultrasonic sensor that is manufactured by Senix Corporation. The sensor  32  rapidly and accurately measures the distance to target materials without contact. The sensor  32  is directed toward the spherical member  30  so as to measure the distances that the spherical member  30  moves within the sensor tube  26 . 
     FIG. 3  is an exploded view of the riser tensioner sensor assembly  20  of the present invention. Initially, it can be seen that the sensor tube  26  is a stainless steel tube having a threaded bottom  70 . The sensor tube  26  has an inner diameter which is less than the diameter of the spherical member  30 . A plate  72  is welded to the top end of the sensor tube  26  so as to prevent liquid intrusion thereinto. 
   The spherical member  30  is a chrome-plated steel ball. The spherical member  30  is suitably chrome-plated so as to resist corrosion in the harsh environment of the offshore platform. This steel should be of a suitable grade so as to be magnetically attractive to the magnet supported by the annular member of the sensor target. Since the spherical member  30  is properly spherical, it will not “hang up” on any surfaces formed on the interior of the sensor tube  26 . A DELRON™ ring  74  is interposed between the threaded portion  70  of the sensor tube  26  and the externally threaded area  76  of sensor housing  78 . As such, a liquid-tight seal can be properly established between the sensor tube  26  and the sensor housing  78 . A connector  80  is illustrated as positioned within a wall of the sensor housing  78  so as to be connected to the sensor  32 . The sensor  32  is inserted into the interior of the sensor housing  78 . In particular, the handle end  82  of sensor  30  is positioned within a DELRON™ sleeve  84 . The DELRON™ sleeve  84  will establish a liquid-tight containment within the interior of the sensor housing  78 . A retaining ring  86  is then positioned at the end of the DELRON™ sleeve  84 . An in-line term block  88  is then installed with connector  80  onto the end of the sensor  32 . O-ring seal  90  is then positioned interior of the open end of the base  28  so as to establish further a liquid-tight seal between the lower threaded end  92  of the sensor housing  78  and the internally threaded base  28 . Bottom flange  64  extends outwardly of the base  28  at the bottom thereof. Flex coupling  68  is then positioned below the bottom flange  64 . 
     FIG. 4  shows a cross-sectional view of the sensor target  24  of the present invention as positioned to support the spherical member  30  in a desired position within the sensor tube. Initially, a bracket  100  is connected to the support structure  60 . Bracket  100  extends outwardly from the outer wall  102  of the annular member  62 . Annular member  62  has an interior area into which the magnet  104  is positioned. The magnet  104  can be secured within the interior of the annular member  62  in a variety of ways, such as bolting, adhesives, fasteners or other devices. The magnet  104  is positioned between the inner wall  106  of the annular member  62  and the outer wall of the wear surface  108 . Wear surface  108  is an annular member which is interposed between the magnet  104  and the outer surface of the sensor tube  26 . The wear surface  108  can be a TEFLON™ ring which can be replaceably positioned on the interior surface of the magnet  104 . The purpose of the wear surface  108  is to provide a contact surface between the outer surface of the sensor tube  26  and to prevent damage to the magnet  104 . After a desired period of time, the wear surface  108  can be suitably replaced. 
   In  FIG. 4 , it can be seen that the spherical member  30  is supported in suspended relation within the interior of the sensor tube  26 . The strong magnetic forces imparted by the magnet  104  will suspend the steel ball  30  within the sensor tube  26 . Because of the spherical nature of the spherical member  30 , minimal contact will occur between the spherical member  30  and the inner wall of the sensor tube  26 . Additionally, the spherical nature of spherical member  30  will avoid any “hanging up” of the member  30  during its up-and-down movement within the sensor tube  26 . 
   In normal use, the sensor  32  can be connected to a computer system for the processing of information as to the relative movement of the sheave housing  34  of the riser tensioner system  22 . As such, the riser tensioner system  22  can be continually monitored so as to properly set the pressure within the compression system  48  so as to avoid “bottoming out” and to conform the riser tensioner system  22  to environmental conditions. The computer system can calculate the amount of travel of the cable  46  during the up and down movement of the sheave housing  34 . As a result, in combination with the pressure provide from the pressurizing device  54 , the system of the present invention can determine the wear on the cable  46  over time. 
   The foregoing disclosure and description of the invention is illustrative and explanatory thereof. Various changes in the details of the illustrated configuration can be made within the scope of the following claims without departing from the true spirit of the invention. The present invention should only be limited by the following claims and their legal equivalents.

Summary:
An apparatus for providing an indication of relative movement between sheave assemblies of a riser tensioner that has a sensor target connectable to one of the sheave assemblies, a senor tube cooperative with said sensor target and suitable for interconnection to the other of the sheave assemblies, a metallic member positioned within the sensor tube and suspended by the sensor target, and a an ultrasonic sensor cooperative with the sensor tube so as to sense a distance that the metallic member moves within the sensor tube in relation to relative movement between the sensor target and the sensor tube.