Abstract:
A load bearing apparatus includes a first container having enclosed ends and at least one expansion segment. The expansion segment includes a first cylinder and a second cylinder of diameter smaller than the first cylinder disposed along a longitudinal axis of the first cylinder. An elastomer seal is connected between the first and second cylinders to form a fluid tight seal between the cylinders. The elastomer seal further permits translation of the first cylinder with respect to the second cylinder in a direction along the longitudinal axis of the first cylinder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Application No. 60/111,705, filed Dec. 10, 1998. 
    
    
     BACKGROUND 
     The invention relates generally to fluid power systems. More particularly, the invention relates to a fluid power system which employs hydraulic pressure to provide load bearing force and extension control. 
     Fluid power systems are employed in a wide variety of small-scale and large-scale industrial applications. For example, fluid power systems are used to generate large compressive forces in plastic molding, multiply input force in braking systems, adjust elevation in jacks, lift weight in cranes, and provide control actuators in steering systems. In the offshore oil industry, hydraulic systems are employed in applications such as subsea template leveling, riser tensioning, and equipment jacking. Many of these applications require long-term service, exposure to corrosive environments, or even immersion in seawater. 
     Fluid power systems typically employ hydraulic cylinders to provide load bearing force and elevation control. As illustrated in FIG. 1A, a hydraulic cylinder  100  conventionally comprise a piston  102  and piston rod  104  disposed within a cylinder  106 . A sliding O-ring seal  110  and a rod wiper  112  are typically positioned about an aperture  114  in the cylinder  106 . The sliding seal  110  and wiper  112  act to seal fluid in the cavity  116  of the cylinder  106  while permitting extension and retraction of piston rod  104  with respect to cylinder  106 . In operation, the piston  102  also comprises one or more sliding seals forming to isolate fluid pressure on either side of the piston  102 . Fluid  118  is generally drawn from a reservoir  120  through a pump  122 . A control assembly  124  directs fluid flow. As illustrated by reference to FIGS. 1B-C, differential pressure (P 1 &gt;P 2 ) provided by the pump  122  acts on the surface areas of the piston  102  to induce a force which extends (F 1 ) or retracts (F 2 ) the piston rod  104 . The magnitude of extension or retraction force is generally described by the relationship of F=PA, where: (F) is the extension or retraction force, (P) is the differential pressure (P 1 -P 2 ) and (A) is the surface area of the piston upon which the pressurized fluid acts. 
     Conventional hydraulics are widely employed. Several aspects of conventional hydraulic cylinders, however, inherently limit their application. The first limiting factor is the difficulty in forming a seal which can contain the high-pressure fluid within the hydraulic cylinder while simultaneously permitting relative movement of the cylinders. Sliding seals are prone to leakage, wear, and failure under high pressure and generally require periodic monitoring and replacement. A second limiting factor relates to the size of the hydraulic cylinder. The maximum size of the seal is generally limited due to difficulties in the manufacturing process. This limited seal size results in smaller-diameter hydraulic cylinders which require higher working fluid pressures to generate loads. 
     In addition to the two limiting factors mentioned above, conventional hydraulic cylinders require strict tolerances on cylinder machining and O-ring fabrication. The fluid employed in hydraulic cylinders is generally an oil derivative. This type of fluid is selected to prevent the leakage or seal corrosion that can occur with other fluids such as seawater or fresh water. The hydraulic fluid can, however, be an environmental contaminant and can be expensive where large quantities are required. 
     It is, therefore, desirable to provide a fluid power system which does not employ sliding seals or moving parts. It is further desirable to provide a seal with less strict manufacturing tolerances, reduced maintenance, and lower failure rates. It is also desirable to provide a system that can employ inexpensive and environmentally friendly fluids, such as fresh or seawater. It is still further desirable to have a system that can be fabricated in large diameters to provide large extension forces with relatively lower working fluid pressures. 
     The principle of nested cylinders having relative displacements is employed in other technical fields, such as bonded rubber shear springs. The offshore oil industry employs bonded rubber shears springs for use as marine shock cells in various applications. The general function of a marine shock cell is to absorb impact loads, such as those induced during the docking operations of a ship to an offshore oil structure. As illustrated in FIGS. 2A-B, a conventional marine shock cell  200  comprises an inner cylinder  202  and a larger diameter outer cylinder  204 . An elastomer annulus  206  spans the gap between the inner  202  and outer  204  cylinders. The elastomer annulus  206  is bonded to the outer surface  208  of the inner cylinder  202  and the inner surface  210  of the outer cylinder  204  during the molding process. The application of a force (F) to the shock cell  200  induces deflection (δX). A designer arranges such variables as cylinder diameter (D), gap length (L), thickness of the elastomer annulus (T), and elastomer mixture in order to produce a desirable reaction force versus deflection characteristic for the shock cell  200 . A generalized reaction force versus deflection characteristic for a shock cell is illustrated in FIG.  2 C. Generally, the elastomer annulus  206  of the shock cell  200  resists deflection with an increasing force (F) as deflection (δX) increases. The area under the curve (A) corresponds to the quantity of impulse energy, or shock, absorbed by the shock cell through full deflection. Upon removal of the external force (F), the shock cell will return to an undeflected condition. 
     A shock cell provides elongation without the use of a sliding seal. Manufacturing tolerances are generally low. Marine shock cells function for decades without maintenance or failure. Shock cells can be manufactured in extremely large diameters. The function of a shock cell or other shear spring, however, is generally the inverse of the function of a fluid power system. A shock cell is a reactive device absorbing external energy input. A fluid power system actuates external energy input to provide power output. The elastomer annulus of a shock cell is designed to impede relative movement between cylinders and is not optimized to form a fluid tight seal. The sliding seal of a fluid power system is designed to enable relative movement between piston rod and cylinder. 
     SUMMARY 
     In general, in one aspect, the invention relates to a load bearing device which comprises an extendable, close-ended container having a first cylinder and a second cylinder, the first cylinder coaxially disposed about the second cylinder and having a diameter which is larger than a diameter of the second cylinder. A first elastomer annulus having an outer circumference bonded to an inner surface of the first cylinder, and an inner circumference bonded to an outer surface of the second cylinder, and first means for pumping a first fluid into and out of the container, wherein the bonds between the elastomer annulus and the first and second cylinders form fluid-tight seal for the container. 
     In general, in another aspect, the invention relates to a load bearing device which comprises at least one expansion segment comprising a first cylinder coaxially disposed about a smaller diameter second cylinder. An elastomer annulus has an inner circumference bonded to an outer surface of the second cylinder and an outer circumference bonded to an inner surface of the first cylinder. End caps enclosing a cavity is formed by the first and second cylinders and the elastomer annulus, and pumping means is included for adjusting a volume of fluid in the cavity, wherein changes in the fluid volume in the cavity induce relative displacements between the first and second cylinders. 
     In general, in another aspect, the invention relates to a method of bearing a load. The method comprises providing an extendable, close-ended container having a first cylinder and a second cylinder, the first cylinder coaxially disposed about the second cylinder and having a diameter which is larger than a diameter of the second cylinder, the first and second cylinder connected together by an elastomer annulus so as to form a fluid-tight seal between the elastomer annulus and the first and second cylinders, and inducing relative movement between the first and second cylinder by adjusting a volume of fluid within the container. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-C illustrate cross-sectional views of a prior art hydraulic cylinder unit in various conditions of extension and retraction. 
     FIGS. 2A-C illustrate cross-sectional views of a prior art marine shock cell in an extended and retracted condition and a graphical representation of the force versus deflection characteristics of the shock cell. 
     FIGS. 3A-C illustrate cross-sectional views of an expandable cylinder unit in a mean and extended position. 
     FIGS. 4A-C show details of the elastomer seal and a comparative force versus deflection characteristic for the expandable cylinder unit of FIGS. 3A-C. 
     FIGS. 5A-B illustrate a cross-sectional and top view of an expandable cylinder unit having a downward angled elastomer seal in a retracted and extended position. 
     FIGS. 6A-B illustrate a cross-section view of an expandable cylinder unit having blanks inserted to reduce fluid volume in a retracted and extended condition. 
     FIG. 7 illustrates a cross-section view of an expandable cylinder unit employing a mixed transmission medium of gas and fluid. 
     FIG. 8 illustrates a cross-sectional view of the expandable cylinder unit having a double action configuration adapted for subsea applications. 
     FIGS. 9A-B show the expandable cylinder unit of FIG. 8 in a fully retracted and fully extended condition. 
     FIGS. 10A-B illustrate cross-sectional views of an expandable cylinder unit having nested expansion segments. 
     FIG. 11 illustrates a cross-sectional view of an expandable cylinder unit configured to provide a tension force. 
     FIGS. 12A-C illustrate cross-sectional views of an expandable cylinder unit forming a sleeve around a tubular member. 
    
    
     DETAILED DESCRIPTION 
     The following embodiments are descriptive only and are not to be considered limiting in any respect. Referring to the drawings wherein like characters are used for like parts throughout the several views, FIG. 3A depicts a load bearing expandable cylinder unit  300  comprising two expansion segments  302 . Each expansion segment comprises an outer cylinder  304 , an inner cylinder  306 , and an elastomer seal  308 . The outer cylinder  304  and the inner cylinder  306  may be made of rolled carbon steel pipe, forged steel ring, or other suitable material that is sturdy enough to withstand fluid pressure. The inner cylinder  306  has a narrower diameter than the outer cylinder  304  and is disposed within the outer cylinder  304  such that a cavity  310  is formed between the cylinders  302  and  304 . The elastomer seal  308  is arranged in the gap formed between the outer cylinder  304  and the inner cylinder  306  and seals the cavity  310 . The elastomer seal  308  includes an inner bond surface  312  and an outer bond surface  314 . The bond surfaces  312  and  314  are bonded to the cylinders  302  and  306 . End caps  316  enclose the ends of the cavity  310 . Although the end caps  316  are shown as convex ellipses in cross-section, it should be clear that the end caps  316  can take on other shapes, e.g., concave, flat, circular. A fluid line  318  through an end cap  316  provides a means of fluid flow to and from the cavity  310 . The fluid line  318  is connected through a valve  320  to a pump assembly  322  by a flexible hydraulic line  324 . The pump assembly draws fluid  326  from and returns fluid  326  to a reservoir  328 . Extension and retraction of the expandable cylinder unit  300  is induced by volumetric control of the fluid  326  within the cavity  310 . In one embodiment, the active fluid  326  is water. As illustrated in FIG. 3B, when water  326  is injected by the pump  322  into the cavity  310 , the expandable cylinder unit  300  elongates (δX) by deflection of the elastomer seals  308 . In general, each seal deflects by a roughly equal fractional amount (δX/number of expansion joints  302 ) of the total elongation (δX). The Elastomers are generally capable of large elongation. A designer may, however, desire to limit the extent of deflection for various reasons such as bond and seal fatigue. In one embodiment, the deflection of an individual elastomer seal  308  from the mean position is held to a 1:1 ratio of the elastomer seal span length (L) such that (δX/2=L). 
     In one embodiment, the elastomer seal construction is similar in fashion to that of an elastomer annulus of a marine shock cell, such as that illustrated previously in FIG.  2 . The elastomer seal comprises a vulcanized mixture of natural rubber. The cylinders comprise sections of rolled carbon steel pipes. The elastomer seal is bonded to the cylinders during the molding process consistent with techniques generally known to the marine shock cell industry. Several variations in the geometry and arrangement of the elastomer annulus may be desirable, however, to be adapted for use as an elastomer seal for a expandable cylinder. 
     Referring now to FIG. 4A, there is shown generalized reaction force versus deflection curves for a shock cell indicated by curve (A). As illustrated the reaction force increases with increasing deflection. The slope of this curve (A) is the stiffness characteristic of the shock cell. The area under the curve represents the quantity of energy required to induce deflection. Where employed as an expandable cylinder, this absorbed energy reduces the quantity of fluid power transmitted by the cylinder. For many applications, efficient power transmission is desirable. This can be achieved by reducing the stiffness characteristic of the expandable cylinder unit  300  to deflection, such as that indicated by curve (B). In a shock cell, the elastomer annulus is designed to provide resistance to deflection. In an expandable cylinder unit  300 , an elastomer seal  308  performs the primary functions of containing fluid pressure and maintaining a fluid tight seal. The thickness of the elastomer seal  308  can, therefore, generally be substantially reduced to that required to perform these functions. 
     As illustrated in FIG. 4B, the elastomer seal thickness need not be uniform. The minimum required thickness at the inner  312  and outer  314  bond surfaces (T 1  and T 2 ) is primarily determined based upon the required bond area to withstand the force of fluid pressure (P) upon the elastomer seal. Bond area is a function of cylinder diameter. A larger seal thickness (T 1 &gt;T 2 ) is therefore generally required on the inner bond surface  312  to achieve an equivalent bond area with respect to the outer bond surface  314 . A secondary factor in determining elastomer seal thickness is shear stress in the elastomer. In general, shear stress is highest at the bond surfaces  312  and  314 . In general, the elastomer seal cross-section is designed to spread shear stress over a large area. Lips and radiuses may therefore be desirable at the edge of the bond surfaces  312  and  314  to further spread out the shear stress. Near the center of the elastomer seal  308 , however, the elastomer material may be subject to almost pure tensile stress. Elastomers, such as rubber, generally have significantly better resistance to tensile stress than to shear stress. This may allow the elastomer seal  308  to be designed with a narrower thickness (T 3 ) near the center of its length (L). For lower fluid pressure applications, the thickness (T 3 ) of the elastomer seal  308  can be reduced such that the stiffness of the expandable cylinder unit  300  to elongation is extremely small. Where higher fluid pressures are employed, the required thickness (T 3 ) may be larger. The overall expandable cylinder unit  300  stiffness, however, may be small relative to the magnitude of extension forces transmitted. The exact placement (X) of the narrowed portion (T 3 ) can be varied dependent upon various factors. 
     As illustrated in FIG. 4C, fluid pressure (P) acting upon the elastomer seal  308  induces a resultant force (F) which, due to the circular cross-section, generally applies nearer the outer edge of the seal  308 . Some portion of this resultant force (F) will be applied to the inner cylinder  306  with the remaining applied to the outer cylinder  304 . This latter portion of the resultant force (F) applied to the outer cylinder  304  acts to reduce the extension force provided by the expandable cylinder unit  300  for a given fluid pressure (P). For this reason, a designer may wish to adjust the cross-section of the elastomer seal  308  to maximize that portion of the resultant force (F) applied to the inner cylinder  306 . For relatively thick seal configurations, the elastomer seal  308  may function in a manner similar to an almost rigid beam for design purposes. Generally, however, an elastomer seal  308  will deform under pressure (P) and permit a designer to adjust the cross-sectional thickness (T 1 , T 2 , and T 3 ) to manipulate the application of the resultant force (F). 
     Additional modifications over conventional shock cell designs may be desirable. The elastomer mixture may be varied to reduce stiffness and increase power transmission efficiency. A designer may also wish to vary the elastomer mixture in order to improve bonding strength and seal fatigue characteristics. Mixtures of natural rubber may generally be preferred due to natural rubber&#39;s elastic properties, resistance to tearing, steel bonding characteristics, and resistance to tensile stress. For certain applications, however, other elastomer mixtures may be preferred. For example, neoprene may be desired where the expandable cylinder unit is employed in the presence of hydrocarbons. For long term use, there may be concern over the absorption of air or fluids into the elastomer. One means for preventing absorption is to have a thin additional layer of absorption resistant material bonded or otherwise applied to the seal surfaces exposed to air or fluids. A designer may also desire to machine the cylinder surfaces to which the elastomer seal is bonded. The machining might be smooth or grooved to improve bond performance. The cylinder surfaces may be prepared in other ways prior to bonding, such as cleaning with acid or other treatments, to further ensure bond quality. 
     In addition to variations in the thickness of the elastomer seal cross-section, a designer may wish to mold the seal in an offset condition. In one embodiment, illustrated in FIG. 5A, an expandable cylinder unit  500  has an offset elastomer seal configuration. The expandable cylinder unit  500  comprises a single expansion segment  502 . The expansion segment further comprises an outer  504  and inner  506  cylinder. An elastomer seal  508  is bonded and molded to the cylinders  504  and  506  with an offset. The outer bond surface is molded offset a distance above the inner bond surface. A top end cap  510  and bottom end cap  512  enclose the fluid cavity  514 . A fluid line  516  through the bottom end cap  512  provides a means of fluid flow to and from the fluid cavity  514 . The fluid line  516  is connected through a valve  518  to a source of fluid power (not shown). The expansion joint  502  is disposed within an external sleeve  520  having an end plate  522  and stiffeners  524  connecting the sleeve  520  to the inner cylinder  506 . A first stopper ring  526  is attached to the inside of the bottom end the sleeve  520 . A second stopper ring  528  is attached to on the outside of the top end of the outer cylinder  504 . When pressurized fluid (P) is injected into the fluid cavity  514 , the expandable cylinder unit  500  will elongate from the retracted position, shown in FIG. 5A, to the extended position, shown in FIG.  5 B. The first stopper ring  526  and second stopper ring  528  are arranged so as to limit the maximum extent of expandable cylinder extension (δX). The offset elastomer seal configuration may be designed so as to defer the higher elastomer tensile stress at the bond surfaces to a point of further deflection. This configuration may allow a designer to permit the deflection of an expansion joint to be elongated (e.g., δX&gt;L) over a non-offset seal configuration such as that shown in FIG.  3 A. The offset seal configuration also has the characteristic of returning to the fully retracted position, shown in FIG. 5A, when fluid power is removed. As illustrated, only one expansion joint  502  is employed. It should be clear, however, that a large number of expansion joints may be employed in parallel to increase the maximum deflection. 
     In one embodiment, illustrated in FIGS. 6A-B, an expandable cylinder unit  600  employs multiple expansion segments  602  in a configuration having a large diameter (D) in comparison to the seal gap length (L). Hydraulic fluids, including seawater, are heavy. In order to reduce the weight of an expandable container unit, some areas of the cavities within the container unit can be voided. In one embodiment, the expandable unit  600  comprises inner cylinders  604  plated on top  606  and bottom  608  to form void convex  610  and concave  612  cavities or blanks. Fluid in the fluid cavity  614  is transmitted through the blanks  610  and  612  by means of fluid passages  616 . In one embodiment, the fluid passages  616  are pipes which connects apertures  618  in the top  606  and the bottom  608  plates. As illustrated in FIG. 6A, the blanks  610  and  612  greatly reduce the volume of fluid in the cavity  614  when the expandable container unit is in the fully retracted position. In other words, the blanks  610  and  612  are arranged so as to almost completely evacuate the expandable container unit in the fully retracted position, shown in FIG. 6A, by filling areas of unnecessary fluid volume. Such a configuration also provides greater contact area and stability in the retracted position. Care should be taken, however, to ensure that the elastomer seals  620 , blanks  610  and  612 , and plates  606  and  608  do not form a vacuum in the retracted position which can impede extension (δX) of the expandable container unit to the extended position, shown in FIG.  6 B. 
     In certain applications, shock loads or oscillator motions may be encountered during expandable cylinder operation. Some amount of shock load or oscillation will be absorbed by the bulging of the elastomer seals. Where shock loads or motions are expected, a designer may wish to add additional capacity to compensate for such service condition. In one embodiment, illustrated in FIG. 7, an expandable cylinder unit  700  employs a mixed medium of air  702  and water  704  as fluid power transmission fluids. An upper fluid line  706  having a valve  708  extends through a top end cap  710  to provide a means of controlling the volume and pressure of air  702  in the expandable cylinder unit  700 . A lower fluid line  712  having a valve  714  extends through a bottom end cap  716  to provide a means of controlling the volume of water  704  in the expandable cylinder unit  700 . The quantity of water  704  is controlled to determine the mean cylinder extension in a load independent manner. The air volume permits some amount of load dependent deflection, providing a spring force to oppose and balance varying external loads. As deflection from shock loads or oscillator motions occurs, the air volume changes increasing or decreasing the air and water pressure to provide the spring force. The quantity of air  702  may be fixed during cylinder extension, with extension provided by injecting water, or quantity of both air  702  and water  704  may be varied. For some applications, however, it may be desirable to use air  702  injection alone as the means of inducing cylinder extension. 
     In certain application, it may be desirable to have an expandable cylinder that provides both extension and retraction forces. In one embodiment, illustrated in FIG. 8, an expandable cylinder unit  800  is designed for under water utilization to provide extension and retraction forces. Elastomer seals  802  are bonded to outer cylinder  804  and inner cylinder  806  without offset. Top end cap  808  and bottom end cap  810  enclose the fluid cavity  812 . A load ring  814  connects the bottom cap  810  to an inner sleeve  816  having a first stopper ring  818 . The upper end cap  808  is connected to an outer sleeve  820  having a second stopper ring  822 . A fluid pipe  824  having a inlet valve  826  passes through the top end cap  808 . An equalization valve  828  is connected to the fluid pipe  824 . In operation, the equalization valve  828  is opened during submergence of the expandable cylinder unit  800  to equalize external water pressure (P 0 ) with internal water pressure (P 1 ). Thereafter, the equalization valve may be closed to permit actuation of the cylinder by a pump assembly  830  drawing or returning water to a fluid reservoir  832 . In one embodiment, fluid is drawn directly from the surrounding water in order to dispense with the need for an external fluid source. As illustrated by reference to FIG. 9A, removal of water from the expandable cylinder unit  800  will induce cylinder retraction (δX 1 ). The magnitude of the retraction force (F 1 ) is determined by the difference between the external (P 0 ) and internal (P 1 ) water pressures, where (P 0 &gt;P 1 ). It should be noted that the magnitude is limited by an minimum internal pressure of 0 psi, at which point a vacuum is formed. The magnitude of the potential maximum retraction force therefore increases with water depth, as external water pressure (P 0 ) increases with depth. As illustrated by reference to FIG. 9B, injection of water will induce cylinder extension (δX 2 ). The magnitude of the extension force (F 2 ) is determined by the difference between the external (P 0 ) and internal (P 2 ) water pressures, where (P 2 &gt;P 0 ). The magnitude of the extension force (F 2 ), however, functions independently of water depth due to the ability to equalize internal and external fluid pressures prior to operation. The magnitude of the extension force (F 2 ) also has no absolute limit. 
     It should be noted that stiffness of the elastomer seals to cylinder extension will generally induce either extension force variation or variation in the internal fluid pressure. Conventional pumps provide fluid flow by inducing head or differential fluid pressure without strict control of the fluid volume pumped. It may therefore be desirable to employ a volume controlled, rather than a pressure controlled, pumping mechanism. In one embodiment, the pump assembly  830  comprises a positive displacement type pumping device. Positive displacement pumps function to pump discrete quantities of fluid to provide precise volumetric control. There are several major categories of positive displacement pumps: piston, rotary, and screw. Piston positive displacement pumps employ pistons to draw a volume of fluid from one source during up stroke and displace that volume to another source on the down stroke. Rotary positive displacement pumps employ chambered disks in which rotating chambers move volumes of water between chambers. Screw positive displacement pumps employ a chambered screw mechanism to perform the same function. While having a maximum design fluid pressure, positive displacement pumps can provide a precision cylinder extension based upon fluid volume control rather than fluid pressure. 
     In certain applications, the amount of cylinder extension as a function of retracted unit height may be of importance to a designer. In one embodiment, illustrated in FIGS. 10A-B an expandable container unit  1000  has a staggered arrangement of two tiers of overlapping expansion segments  1002  and  1004 . There is an outer cylinder  1006  having an elliptical end cap  1008 , and a first inner cylinder  1010  of smaller diameter disposed within the outer cylinder  1006 . The gap between the outer cylinder  1006  and the first inner cylinder  1010  is spanned and sealed by a first elastomer seal  1012 . A second inner cylinder  1014  of still smaller diameter is disposed within the first inner cylinder  1010 . The gap between the first inner  1010  and second inner  1014  cylinder is spanned and sealed by a second elastomer seal  1016  enclosing the fluid cavity  1018 . A fluid pipe  1020  having a valve  1022  passes through the end cap  1008 . In the fully retracted position, as illustrated in FIG. 10A, the first inner cylinder  1004  and second inner cylinder  1014  are nested within the outer cylinder  1006 . The total height (H) of the unit in a fully retracted position may be as low as approximately four times the seal span length (L). As illustrated in FIG. 10B, the total cylinder extension (δX), however, may be as long as eight times the span length (L), where a 1:1 span length versus deflection ratio is used. 
     In certain applications, it may be desirable to configure an expandable cylinder unit to provide a tension force. In one embodiment, as illustrated in FIG. 11A, an expandable unit  1100  is disposed between opposing load blocks to convert cylinder extension force into tension (T). In one embodiment, multiple expansion segments are employed in a symmetric arrangement of nested tiers. The expandable container unit  1100  comprises a top outer cylinder  1102 , a middle outer cylinder  1104 , a bottom outer cylinder  1106 , and two inner cylinders  1108 . The middle outer cylinder  1104  has first guide sleeves  1110 . The inner cylinders  1108  are elongated and have second guide sleeves  1112 . The top outer cylinder  1102  changes a top load block  1114 . The bottom outer cylinder engages a bottom load block  1116 . A first system of extension rods  1118  extend from the top load block  1114  down through the guide sleeves  1110  and  1112  to a bottom end plate  1120  having a padeye  1122 . A second system of extension rods  1124  extend from the bottom load block up through guide sleeves (not shown) to a top end  1126  having a second padeye  1128 . The second system of extension rods  1124  is rotated some offset angle from the first  1118  to avoid interference. In one embodiment fluid is injected into the expandable container unit through a flow line  1130  having a valve  1132 . The extension force of the cylinder presses against the opposing load blocks  1114  and  1116  to produce a tension force in the first and second system of extension rods  1118  and  1120 . The extension force is transmitted to the padeyes  1122  and  1128  to produce tension (T) in slings  1134 . 
     In certain applications, it may be desirable to dispose an expandable cylinder unit about a second body such as a tubular member. In one embodiment, illustrated in FIGS. 12A-C, an expandable cylinder unit  1200  forms a sleeve around a tubular member  1202 . The expandable cylinder unit  1200  comprises four expansion segments  1204  each comprising a first outer cylinder  1206  and a second outer cylinder  1208  of smaller diameter, a first inner cylinder  1210  and a second inner cylinder  1212  of smaller diameter. An outer elastomer seal  1214  is bonded between the first  1206  and second  1208  outer cylinders. An inner elastomer seal  1216  is bonded between the first  1210  and second  1212  inner cylinders. Top  1218  and bottom  1220  end caps enclose the cylindrical shaped fluid cavity  1222 . A fluid line  1224  having a valve  1226  passes through the top end cap  1218  to provide a means of fluid flow to and from the cavity  1222 . A first load ring  1228  connects the bottom end cap  1220  to an inner sleeve  1230  having a first stopper ring  1232 . The upper end cap  1218  is connected to an outer sleeve  1234  having a second stopper ring  1236 . Extension force is applied to the tubular member  1202  through a second load ring  1238  connected at a top end  1240  of the expandable cylinder unit  1200 . Extension and retraction of the unit induce relative displacements (δX) between the tubular member  1202  and bottom end  1242  of the expandable cylinder unit  1200 . 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate the numerous variations therefrom without departing from the spirit and scope of the invention.