You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
This application is a continuation-in-part of U.S. patent application Ser. No. 11/280,118, filed Nov. 16, 2005, and is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates in general to riser tensioners for offshore platform drilling and production floating platforms and, in particular, to tensioner cylinder assemblies having a high pressure external fluid reservoir for lubricating seals between the cylinders. 
     2. Description of the Related Art 
     Risers are used in offshore oil and gas well production for conveying well fluids from a subsea wellhead to a floating production platform. Tensioners are employed at the platform to apply tension to the risers. A typical tensioner comprises a telescoping piston and cylinder arrangement supplied with gas pressure from accumulators. Waves and currents cause the piston and cylinder to extend and retract. Hydraulic fluid within the chamber provides lubrication. 
     In one type of tension design, the piston component comprises a barrel that slidingly engages the cylinder or other barrel. Each barrel has a closed end and an open end, the open ends being in fluid communication with each other. The interiors of the barrels serve as the chamber for receiving gas pressure. By combining the interior of each barrel, the volume of the chamber may be sufficient to eliminate external accumulators or at least reduce the size of external accumulators. A larger internal volume improves the cylinder stiffness versus stroke characteristics. However, provisions must be made for lubrication of the primary seals, and the hydraulic fluid reduces the amount of volume that can be filled with gas. 
     SUMMARY OF THE INVENTION 
     One embodiment of a system, method, and apparatus for a riser tensioner has inner and outer cylindrical barrels that telescopingly engage each other. An external fluid reservoir or tank is mounted outside of the barrels for communicating a small quantity of hydraulic fluid or other media to the working or dynamic seals between the inner and outer cylinder barrels. By using a small external reservoir, the internal volume of the cylinder rod assembly is made available for storage of nitrogen gas or compressed air. By increasing the internal gas volume of the cylinder rod, a “softer” cylinder is employed which has the positive effect of reducing or eliminating the need for external storage tanks or accumulator-type devices. 
     The external reservoir also offers a service improvement in that the cylinder fluid level is easily checked by using a sight glass or calibrated liquid level gage. Another improvement of the invention is that the reservoir is readily isolated from the cylinder assembly by a combination of check and three-way valves. This improvement allows easier access to the working fluid and allows the addition or removal of high pressure fluid to the system without the requirement of a high pressure fluid cart. 
     In one embodiment, the cylinder assembly uses a fluid filled tube or hose that is connected to a radial port and annular groove in the piston end of a cylinder rod. The opposite end of the hose or tube is connected to a top cap of the cylinder assembly and communicates with the gas inside the cylinder. Inlet and outlet ports are located at the top cap to allow gas communication with the interior and fluid communication with the annular groove in the cylinder assembly. A line leading from the gas outlet port of the cylinder is connected to a three-way isolation valve. The other port is connected to a tee and an in-line check valve. Under normal operation, the cylinder gas pressure and the fluid pressure in the piston end of the cylinder are in communication and are under the same pressure, with the exception of the small “head” pressure that is generated by the elevation change between the piston seals and external reservoir via the fluid specific gravity. 
     The small external reservoir is connected to the ports leading from the three-way isolation valve and check valve. One of the valves allows high pressure gas bleed off from the external reservoir. Fill and drain valves allow the addition or removal of fluid from the tank. Filling or draining the tank is accomplished without a high pressure fluid cart by simply positioning the three-way isolation valve such that the pressure is routed back to the cylinder, which will allow the external reservoir to be bled down and vented to atmosphere. The annular groove in the cylinder rod communicates with a radial hole or port and allows the high pressure fluid to access the primary and secondary seals. A zero differential pressure seal below the annular groove prevents fluid leakage due to the effect of gravity. 
     The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features and advantages of the present invention, which will become apparent, are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof that are illustrated in the appended drawings which form a part of this specification. It is to be noted, however, that the drawings illustrate only some embodiments of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
         FIG. 1  is a partial sectional view of one type of floating platform with a riser supported by a tensioning mechanism constructed in accordance with the invention; 
         FIG. 2  is a sectional view of one embodiment of a riser tensioner having a gas and fluid system shown in an operational position and is constructed in accordance with the invention; 
         FIG. 3  is an enlarged sectional view of a portion of the riser tensioner system of  FIG. 2  and is constructed in accordance with the invention; 
         FIG. 4  is a sectional view of the riser tensioner of  FIG. 2  with the gas and fluid system shown in a position for accessing the reservoir and is constructed in accordance with the invention; and 
         FIG. 5  is a high level flow diagram of one embodiment of a method in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , one type of riser tensioning mechanism  10  is depicted. Although mechanism  10  is depicted as a “pull-up” type, one skilled in the art will recognize that the present invention is equally suitable for “push-up” type and other types of tensioning mechanisms. 
     A riser  12  extends downwardly from a platform  14  to a subsea wellhead (not shown). Riser  12  has a longitudinal axis  16  and is surrounded by a plurality of hydraulic cylinders  18 . Each hydraulic cylinder  18  has a cylinder housing  24  having a chamber (not shown). A piston rod  26  has a rod end  28  that extends downward from each cylinder housing  24  and hydraulic cylinder  18 . The piston ends of rods  26  opposite rod ends  28  are disposed within the respective chambers (not shown) of cylinder housings  24 . Hydraulic fluid (not shown) is contained within the housing  24  for pulling piston rods  26  upward. Each hydraulic cylinder  18  also may comprise an accumulator  30  for accumulating hydraulic fluid from hydraulic cylinder  18  and for maintaining high pressure on the hydraulic fluid. A riser collar  32  rigidly connects to riser  12 . The piston rods  26  attach to riser collar  32  at the rod ends  28 . Cylinder shackles  34  rigidly connect cylinder housings  24  to platform  14 . 
     In operation, the riser tensioning mechanism  10  pulls upward on riser  12  to maintain tension therein. Riser collar  32  connects to riser  12  and engages riser  12  below platform  14  and cylinder receiver  18 . Hydraulic fluid pressure is applied to hydraulic cylinders  18  so that riser  12  is maintained in constant tension. Riser collar  32  supports the weight of riser  12  in order to create a tensional force in riser  12 . Hydraulic cylinders  18  automatically adjust to changes in platform  14  position to allow for relative movement between riser  12  and platform  14 . In the event of a failure in one of the four hydraulic cylinders  18 , the remaining hydraulic cylinders  18  will continue to support riser  12  in tension without excessive bending moments being applied to the hydraulic cylinders  18 . 
     Referring to  FIGS. 2-4 , one embodiment of a system and apparatus for a tensioner for applying tension to a subsea member extending to a floating offshore structure is disclosed. In the illustrated embodiment, the cylinder assembly  118  comprises an inner cylindrical barrel  121  (i.e., piston) having an axis  123 , a closed end  125  (e.g., top cap), an open end  127 , an interior  129 , and an exterior surface  131 . An outer cylindrical barrel  133  has a closed end  135 , an open end  137 , and an interior  139  in which the inner barrel  121  is coaxially located to define a chamber  129 ,  139  therebetween. The outer barrel  133  also has an interior surface  141  that closely receives the exterior surface  131  of the inner barrel  121  such that the inner and outer barrels  121 ,  133  telescopingly engage each other at an interface  131 ,  141 . 
     A series of seals, such as high pressure dynamic fluid and gas seals, are located at the interface to seal between the inner and outer barrels  121 ,  133 . In one embodiment, the seals comprise a zero differential pressure seal  143  adjacent the open end  127  of the inner barrel  121 , a primary seal  145  axially spaced apart from the zero differential pressure seal  143 , and a secondary seal  147  axially spaced apart from the primary seal  145  and opposite the zero differential pressure seal  143  relative to the primary seal  145 . 
     The invention also comprises a small, high pressure, external reservoir or tank  151 . In one embodiment, tank  151  comprises a volume of about 4 gallons and is detached from and located remotely relative to the inner and outer barrels  121 ,  133 . A gas conduit  153  extends from tank  151  to the chamber  129 ,  139  for communicating gas therebetween. In addition, a hydraulic fluid conduit  155  extends from the tank  151  to the interface  131 ,  141  for communicating hydraulic fluid therebetween. 
     In one embodiment, the hydraulic fluid conduit  155  extends through the closed end  125  of the inner barrel  121  to a radial fluid passage  157  ( FIG. 3 ) in the inner barrel  121  adjacent the open end  127  of the inner barrel  121 . The radial fluid passage  157  is axially located between the zero differential pressure seal  143  and the primary seal  145 . The inner barrel  121  has a circumferential annular groove  159  in the outer surface  131  that is axially aligned with the radial fluid passage  157 . 
     The hydraulic fluid conduit  155  may further comprise a fluid fitting  161  mounted in the radial fluid passage  157 . An interior fluid tube or hose  163  extends from the fluid fitting  161  to the closed end  125  of the inner barrel  121 . An axial and radial passage  165  extends through the closed end  125  of the inner barrel  121  from the interior fluid tube  163  to an exterior of the closed end  125  of the inner barrel  121 . An exterior fluid tube  167  extends from the axial and radial passage  165  to the external tank  151 . 
     As shown in the illustrated embodiment, the invention further comprises a three-way isolation valve  171  that is located in the gas conduit  153  between the external tank  151  and the chamber  129 ,  139 . A check valve  173  is located in the hydraulic fluid conduit  155  between the external tank  151  and the interface  131 ,  141 . A tee fitting  175  is located between the check valve  173 , the interface  131 ,  141 , and the three-way isolation valve  171  for fluid communication therewith. The gas conduit  153  may further comprise an axial and radial passage  177  extending through the closed end  125  of the inner barrel  121  from the chamber  129 ,  139  to an exterior of the closed end  125  of the inner barrel  121 , and an external gas tube  179  extending between the axial and radial passage  177  and the three-way valve  171 . 
     The tank  151  also may further comprise a liquid level indicator  181 , such as a gage, sight glass, etc., connected to the external tank  151  for visibly indicating a level  183  of hydraulic fluid contained within the external tank  151 . The tank  151  also may incorporate a fluid valve  185  for directly replenishing and draining the external tank  151  with hydraulic fluid, and a gas valve  187  for directly replenishing and draining the external tank  151  with gas. 
     Referring now to  FIG. 5 , one embodiment of a method of applying tension to a subsea member extending to a floating offshore structure is shown. The method begins as indicated at step  201 , and comprises placing an inner barrel into an outer barrel and positioning seals therebetween (step  203 ); mounting a reservoir external to the inner and outer barrels and providing the reservoir with a fluid and a gas (step  205 ); connecting the subsea member to one of the barrels and the offshore structure to the other of the barrels (step  207 ); pressurizing interiors of the inner and outer barrels with gas from the reservoir to urge the barrels to an extended position relative to each other (step  209 ); lubricating the seals with fluid from the reservoir as the inner and outer barrels move relative to each other (step  211 ); before ending as indicated at step  213 . 
     The method may comprise introducing hydraulic fluid axially between the zero differential pressure seal and the primary seal, and providing the inner barrel with a circumferential annular groove in the outer surface for circumferentially dispersing the hydraulic fluid about the inner and outer barrels. The method also may further comprise controlling flow of the gas between the reservoir and the interiors and controlling flow of the fluid between the reservoir and the interiors with a three-way isolation valve between the gas in the reservoir and the interiors, a check valve between the fluid in reservoir and the seals, a tee fitting between the check valve, the three-way isolation valve and the seals for fluid communication therewith. 
     The seals may be lubricated from the reservoir by closing the three-way isolation valve to the tee fitting such that gas pressure from the interiors forces fluid from the reservoir, through the check valve and the tee fitting to the seals, and directly accessing the gas and fluid in the reservoir from an exterior of the reservoir by closing the three-way isolation valve to the reservoir such that gas pressure from the interiors is isolated from the reservoir and limited to the tee fitting and the seals, such that the reservoir is vented to atmosphere to be replenished with or drained of gas and fluid through separate valves. In addition, the method may further comprise visually detecting a fluid level inside the reservoir with a liquid level indicator located outside of reservoir. 
     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 riser tensioner has inner and outer cylindrical barrels that telescopingly engage each other. A small external tank is mounted outside of the barrels for communicating a small quantity of hydraulic fluid the seals between the inner and outer barrels. The external tank allows the internal volume of the cylinder rod assembly to be used for gas storage and reduces the need for accumulators. The fluid level in the external tank is checked via a sight glass, and the tank is isolated from the cylinder assembly by a combination of check and three-way valves, which allows easier access to the working fluid, and allows the addition or removal of high pressure fluid to the system without a high pressure fluid cart.