CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/598,367, filed on Jul. 25, 2008 now abandoned, which claims the benefit of PCT Application Serial No. PCT/AU2005/000254, filed Feb. 25, 2005, and Australian Patent Application Serial No. 2004900923, filed Feb. 25, 2004. 
    
    
     FIELD OF INVENTION 
     This invention relates to hydraulically assisted fasteners and hydraulic tensioning devices. Such fasteners may comprise a threaded nut or washer and tensioning devices may comprise tensioning rings, particularly those suitable for valves and the like. 
     BACKGROUND OF THE INVENTION 
     The use of fasteners employing hydraulically operated components to apply bolt tension is well established. Examples can be found in U.S. Pat. No. 5,730,569 (Bucknell) (=International Application PCT/AU93/00477=International Publication WO 94/07042). Such fasteners are intended to be installed in a manner allowing their periodic removal for servicing or for maintenance of the equipment on which they are fixed. They are seldom used in situations where they may be permanently applied. Factors which discourage the latter are high initial cost compared to other methods of permanent fixing and deterioration of hydraulic seal material over time especially at elevated temperatures. 
     U.S. Pat. No. 5,730,569 discloses hydraulic fasteners which are simple and therefore inexpensive to produce. However the latter rely on the preservation of seal integrity to maintain the tensile layers in the bolts on which they are applied. Because of the above factors, hydraulically assisted fasteners have not been used in the construction of undersea pipelines nor of conduits for the fixing of flange joins on pipelines, valves and the like. Current methods require divers to operate multiples of specially constructed hydraulic bolt tensioners simultaneously to close underwater flange joints. The cost of such operations is high since they require an entire diving support vessel and crew, as well as divers. 
     Also the risk to personnel and equipment increases significantly with greater depth as well as the cost. As shallower fields are depleted in offshore oil and gas exploration, drilling and production move to deeper waters, and these factors become significant in determining viability. Remote Operated Vehicles (ROVs) are used to perform complex bolting operations at depth where divers&#39; time is severely restricted. These vehicles are limited in their dexterity and even more unreliable in delivering a satisfactory outcome when encumbered by lack of visibility or limited mobility. Currents and other local environmental conditions can also affect operations and cause a blow out in operating expenses. 
     It is an object of the present invention to provide hydraulically assisted fasteners and hydraulic tensioning devices which operate without hydraulic seals to directly tension bolts or members on which they are applied or at least provide a useful alternative to the prior art devices. 
     It is a further object to provide fasteners and devices which do not require locking rings to secure the tensile load produced and which are not subject to the degradation of seal material and which may be used with a charging medium which will not degrade under prevailing operating conditions. 
     SUMMARY OF THE INVENTION 
     According to the present invention an hydraulically assisted fastener comprises 
     a body with a central bore to engage a connector element and with an annular recess opening outwards to an end surface 
     an annular thrust member which fits into and seals the annular recess 
     an annular chamber defined by the recess and the thrust member and 
     a charging medium which is injected into the chamber under pressure and which moves the body relative to the thrust member to tension the connector element and which sets in the chamber to maintain the tension in the connector element. 
     Preferably the connector element is a bolt or a stud. 
     Preferably the body is a nut which screws onto the bolt or stud and the thrust member is a washer with a plain bore. 
     Preferably the recess extends inwards to the bore and the chamber is defined by the recess, the thrust member and the connector element. 
     Preferably the thrust member is a piston ring incorporating an annular flange which extends around and seals the periphery of the body. 
     Preferably the body and/or the thrust member incorporate integral deflecting and/or sealing lips which seal the chamber. 
     Preferably the charging medium is a viscous paste which cures to become solid comprising suspended solids in a self setting compound or particulate solids which behave as fluid media. 
     Preferably the charging medium is a solid like graphite which is injected into the chamber by using a medium exchanger. 
     Preferably the charging medium is a particulate solid of a granular nature such as lead, copper or steel balls. 
     In an alternative form of the invention an hydraulic tensioning device 
     comprises 
     a connector body with a plurality of bores which engage connector elements and which have mating recesses opening outwards to an end surface 
     a plurality of thrust members which seal the recesses 
     a plurality of chambers defined by the recesses and the thrust members 
     at least one distribution gallery interconnecting the chambers and 
     a charging medium which is injected under pressure into the chambers via the distribution gallery or galleries and which moves the connector body relative to the thrust members to tension the connector elements and which sets in the chambers to maintain tension in the connector elements. 
     Preferably the bores are inwardly convergent and receive nut cones which lock the connector body to the connector elements. 
     Preferably the recesses extend inwards to the bores and the chambers are defined by the recesses, the thrust members and the connector elements. 
     Preferably there are additional recesses between adjacent bores. 
     Preferably the connector body and the thrust members are annular discs adapted for use in the flange joints of pipelines, valves and similar apparatus. 
     Preferably the connector body and the thrust member are square, rectangular, hexagonal, polygonal, circular, elliptical or any other shape. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To enable the invention to be fully understood, preferred embodiments will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a plan view of an hydraulically assisted fastener; 
         FIG. 2  is a partial cutaway elevation of the fastener of  FIG. 1 ; 
         FIG. 3  is an isometric view of an hydraulic tensioning assembly fastening a flange joint; 
         FIG. 4  is an exploded view of the assembly of  FIG. 3 ; 
         FIG. 5  is an enlarged partial view of  FIG. 4 ; 
         FIG. 6  is a cross sectional elevation showing an hydraulic charging device; 
         FIG. 7  is a cross sectional elevation showing an alternative to  FIG. 6 ; and 
         FIGS. 8 and 9  are enlarged partial views of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 and 2  show an hydraulic fastener  10  which engages a screw threaded bolt  20 . Fastener  10  has a body  11  with a screw threaded bore  12  which engages threads  21  on bolt  20 . Body  11  has six flat faces  13  to provide purchase for a tensioning tool. The external profile of body  11  can be varied to suit the different types of tensioning tools available. 
     Annular recess  14  is formed in body  11  and opens outwards to end face  15  and inwards to bore  12  of body  11 . The lower portion of body  11  has a peripheral skirt  16  which surrounds annular recess  14 . A thrust member  17  in the form of an annular washer fits into annular recess  14  and has a curved upper face  18 . 
     An annular chamber  30  is defined by annular recess  14  in body  11 , thrust washer  17  and the outer thread of bolt  20 . Nipple  40  has a one-way valve  41  and screws into peripheral skirt  16  of body  11  and is connected to annular chamber  30  by passage  31 . 
     Nipple  40  can be connected to a source of charging medium  50  such as a particulate solid which is injected under pressure through nipple  40  into annular chamber  30  to expand the working volume of annular chamber  30 . Connector body  11  moves in a direction opposite to thrust washer  17  to apply tension to bolt  20 . When the required tension has been applied to bolt  20 , the source of charging medium  50  is disconnected from nipple  40  and backflow is prevented by one-way valve  41 . 
     Charging medium  50  may also be a viscous paste which cures to become solid, a suspended solid in a self-setting compound, or a particulate solid which behaves as a fluid. If the source of the charging medium incorporates a media exchanger, solid injectable media such as graphite may also be used. Particulate solids of a granular nature such as lead, copper or steel balls may also be used as charging materials. Charging medium  50  sets and forms a solid block which prevents movement of body  11  relative to thrust washer  17 , an so prevents any reduction of the tension applied by hydraulic fastener  10  to bolt  20 . By using any of the above charging media, the need for seals between thrust washer  17  and the adjacent contact wall of annular recess  14  in body  11  is removed. Accordingly any reduction of the tension applied to bolt  20  due to seal deterioration is avoided. 
       FIGS. 2 to 6  show a second hydraulic tensioning device  110  used to join pipe flanges PF 1  and PF 2  of respective pipes P 1  and P 2  at a flange joint. For ease of manufacture, hydraulic tensioning device  110  has a device body in the shape of a ring formed of upper and lower annular discs  111 ,  112 . Upper disc  111  has a plurality of downwardly convergent bores  113  through it to receive bolts  120  which extend above pipe flange PF 1 . Each conical bore  113  is shaped to receive a trifurcated nut cone  122  which engages screw threads  121  on bolt  120 . Cone  122  is prevented from escaping from conical bore  113  by spring clip  123 . Lower disc  112  has bores of larger diameter than bore  113  through it which form, with upper disc  111 , annular recess  114  which house thrust washers  117  so that upper  111  and lower  112  discs, thrust washer  117  and bolt  120  form an annular chamber  130  to receive charging medium  150 . 
     Each annular chamber  130  surrounding bolt  120  is interconnected by distribution galleries  151  extending around upper  111  and lower  112  discs. By manufacturing the connector body as two discs, distribution galleries  151  can be machined and upper  111  and lower  112  discs can be locked together by a plurality of joining bolts  119 . 
     As illustrated in  FIG. 6 , charging medium  150  is injected under high pressure using media exchanger  160  which is screwed into passage  131  connecting to distribution gallery  151 . Passage  131  contains non return valve  141  which operates the same way as non return valve  41  in nipple  40  in  FIGS. 1 and 2 . Media exchanger  160  has a body  161  which is connected to a source of hydraulic oil  162  via a hydraulic line  163 . Hydraulic oil  162  is forced into media exchanger  160  under pressure to cause separator piston  164  to move in body  161  of media exchanger  160  thus causing expelling medium  150  from media exchanger  160 . This increases the effective volume of annular chambers  130  and discs 111  and  112  move relative to thrust washers  117  to tension bolts  120  to the required amount. 
     When the required tension has been achieved in bolts  120 , media exchanger  160  is disconnected from passage  131  and non-return valve  141  prevents the release of charging medium  150  from device  110 . As described above charging medium  150  sets to prevent movement of discs  111  and  112  relative to thrust washers  117  thereby preventing any reduction in tension applied to bolts  120 . 
     It will be apparent to the skilled addressee that manufacture of hydraulic tensioning device  110  is relatively simple and inexpensive since no complex machining operations nor tooling is required. The upper and lower discs  111  and  112  of the connector body are bolted together by bolts  119  to enclose distributor gallery  151  and so no intricate drilling operations are required. Each trifurcated nut  122  is inserted in its conical bore  113  and retained with spring clip  123  which provides both retaining and closing forces for the nut  122  assembly. 
     To install, hydraulic tensioning device  110  is fitted over bolts  120  protruding from pipe flange PF 1  as shown in  FIG. 4 . The action of pushing hydraulic tensioning device  110  over bolts  120  allows cone nuts  122  to ratchet over the bolt threads, and eliminates the need to screw the nuts into place. 
     As described above, charging medium  150  flows to each annular chamber  130  via distributor gallery  151  forcing thrust washers  117  to react against adjacent pipe flange PF 1 . This creates tensile forces which are evenly and simultaneously distributed to each bolt  120 . One way valve  141  automatically activates and the pressure pumping apparatus is removed with full pressure remaining in the assembly. Where setting paste is used as the charging medium it will cure rapidly preventing any leakage and subsequent loss of tensile load on bolts  120 . When a particulate solid is used as the charging medium it will retain the tensile load indefinitely as it is already at a high density. 
       FIG. 7  illustrates a third embodiment of the present invention where a standard form of hydraulic nut  210  is charged with charging medium  250  using media exchanger  260  in the same manner described with reference to  FIGS. 3 to 7 . In this case, the pressure of the charging medium is not required to be maintained since the force generated is maintained by locking ring  216  which is screwed into nut  211  and engaged with piston  217  which cooperates with nut  211  to form annular chamber  230 . When this type of hydraulic nut needs to be removed intact at a later time, the charging medium used will be of a fluid nature in order to assist with re-pressurisation for loosening lock ring  216 . 
     Owing to the nature of the charge medium used, the sealing capacity of fasteners  10 ,  210  of tensioning devices  110  need only be rudimentary. As illustrated in  FIGS. 8 and 9  the leading edges of components and sliding engagement may be altered in order to enhance the sealing ability when viscous materials are used as the charging media. 
     The use of charging media  50 ,  150 ,  250 , and in particular, the use of solid injectable media such as graphite and of particulate solids of a granular nature such as steel balls will allow the hydraulic tensioning fasteners and hydraulic tensioning devices of the present invention to be used in high temperature applications. In these situations it may be desirable to use the hydraulic nut of  FIGS. 7 to 9  which has a locking ring  216  to retain the required toad. Removal of fastener  210  would require injection of charging medium  250  to loosen locking ring  216  and to release the pressure to allow device  210  to be unscrewed from bolt  220 . 
     VARIATIONS 
     It will be realized that the foregoing has been given by way of illustrative example only and that all other modifications and variations as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as herein set forth. Throughout the description and claims to this specification the word “comprise” and variation of that word such as “comprises” and “comprising” are not intended to exclude other additives components integers or steps.
Hydraulically assisted fastener ( 10) comprises a body ( 11) with a central bore ( 12) to engage a connector element ( 20) and with an annular recess ( 14) opening outwards to an end surface ( 15). Annular thrust member ( 17) fits into and seals the recess ( 14). Charging medium ( 50) is injected under pressure into annular chamber ( 30) defined by recess ( 14) and member ( 17) and moves body ( 11) relative to member ( 17) to tension element ( 20). Medium ( 50) sets in chamber ( 30) to maintain the tension in element ( 20). Element ( 20) may be a bolt or stud, body ( 11) can be a nut and member ( 17) can be a plan washer. Medium ( 50) can be a curable viscous paste, a suspended solid in a self-setting compound or a particulate solid which behaves as a fluid. Suitable materials include graphite, and lead, copper or steel balls.