Abstract:
A device to provide a seal between a housing for a moving element such as a drill chain, piston or the like and the moving element. The device comprising a storage well to retain seal material, the block having an outlet connecting with an aperture in a housing enabling seal material to flow into the space between a housing and the moving element and form a seal, the device further including an inlet port enabling seal material precursor to be added into the block, and pressurising means to pressurise the seal material precursor and force it out of the aperture.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an improved seal. The seal is particularly for use in the oil industry to provide a seal between a piston and a piston housing; or as part of a slip joint in a marine riser. 
       BACKGROUND ART 
       [0002]    The requirement to provide a seal between separate elements is well known in a wide range of technical fields. Typically this is carried out to prevent passage of a fluid from one region to another, for example due to a requirement to maintain a pressure differential or to prevent the fluid from causing damage outside its use area. 
         [0003]    Where the seal is between two articles undergoing relative motion then the problem becomes more difficult, because if the seal element is to be effective, the seal element needs to engage both articles. The motion then causes wear on the seal element. Additionally, the seal element can impart a force resistive to the motion. 
         [0004]    In the oil industry for example different types of relative motion can be contemplated: both relating to movement of an item of equipment within a generally cylindrical casing. In the case of a casing housing for a drill, then the seal between the drill and the casing needs to be able to withstand the rotatory motion of the drill. In the case of a piston, its longitudinal motion along the axis of the piston and the piston housing causes the difficulty. 
         [0005]    In both of these situations, too soft a seal material can result in rapid wear of the seal element. Too hard a seal material can result in a weak seal as the seal material does not engage well with the various articles. Alternatively, if the seal engages too strongly with the articles then it can inflict wear on these articles and also inhibit their motion. 
         [0006]    Within an oil industry drilling operation it is important that an effective seal be maintained for as long a period as possible, as replacement of the seal element can be both hazardous and expensive. Moreover, some seal elements are in difficult locations many metres underground and so the replacement may not be easy. Further, some seal elements are used in locations where space is at a premium and so the horizontal area taken up by the seal element and the housing therefor should occupy as little space as possible. 
         [0007]    In the particular examples given above, conventional seal elements are sacrificial and comprise a polymeric rubber material, often having a layer of lubricating oils between the seal material and the drill or piston. The material is often housed within a seal block affording access to replace the seal element when required. As indicated however this requires the operation being undertaken to be closed down and for the section containing the seal block to be isolated to prevent backflow from the drill hole. 
         [0008]    It is an object of the present invention therefore to provide a seal to address the above problems. It is a further object of the present invention to provide an improved method of obtaining a seal which addresses the above problems and reduces the frequency with which downtime occurs. 
       SUMMARY OF INVENTION 
       [0009]    According to a first aspect of the invention there is provided; 
         [0010]    a device to provide a seal between a housing for a moving element such as a drill string, piston or the like, and the moving element 
         [0011]    the device comprising a storage well to retain seal material, the well having an outlet connecting with an aperture in the housing enabling seal material to flow into the space between the housing and the moving element and form a seal, an inlet port enabling seal precursor to be added into the well, pressurising means to pressurise the seal material and force it out of the aperture. 
         [0012]    The seal can therefore be continually renewed without the requirement to shut down an operation. 
         [0013]    Preferably, the seal precursor comprises one or more monomers, the or each monomer reacting to form a polymer. Polymeric materials can be selected to have the properties required of the seal material. Moreover, the reaction between monomers can be tailored to suite any change in seal material required. 
         [0014]    Optionally, the seal precursor includes a chemical initiator such as free radical or free radical producer to accelerate the reaction between monomers. Again therefore, further control over the polymer material is obtained. 
         [0015]    Alternatively or further optionally, reaction between monomers is initiated thermally. 
         [0016]    Conveniently, the device includes temperature control means to control the temperature of the seal material. 
         [0017]    Optionally, the seal precursor includes a particulate material distributed throughout the seal precursor distributed throughout its volume. The particulate material further optionally includes polytetrafluoroethylene or a silica glass. 
         [0018]    The pressure exerted on the piston is advantageously provided by hydraulic pressure. 
         [0019]    Optionally, the pressure exerted on the seal material precursor is provided by a piston operated by hydraulic pressure. 
         [0020]    Advantageously, means are included to determine the volume of seal material in the well enabling an operator to determine when replacement seal material needs to be added. 
         [0021]    Conveniently the piston surface in contact with the seal material is profiled to increase the surface area of the piston surface and aid mixing of and integration between seal material precursor and seal material. Further conveniently, the surface is serrated. 
         [0022]    Preferably a pressure indicator is included to determine the pressure in the well. 
         [0023]    The device preferably includes a level indicator to show the level of seal material in the well. The indicator is further preferably a rod whose lower end rests on the surface of the seal material, the rod passing through the housing. 
         [0024]    According to a second aspect of the invention there is provided a method of producing a seal, the method comprising the steps of: 
         [0025]    (i) mounting a seal forming device to a casing, the device including a housing enclosing a portion of a reciprocating or rotating article with which a seal is to be made with the housing, 
         [0026]    (ii) the device having a storage well to retain a seal material, a closeable communicating aperture connecting the storage well with the external surface of an article, an inlet enabling seal material precursor to be introduced into the well, and pressurising means to apply pressure to the seal material; 
         [0027]    (iii) introducing flowable seal material precursor into the storage well, 
         [0028]    (iv) allowing the seal precursor to set into seal material, 
         [0029]    (v) applying pressure via the pressurising means to force the seal material through the aperture into the open volume between the housing and an article to seal the space between the housing and an article, 
         [0030]    (vi) introducing further seal material precursor to retain seal material in the storage well. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0031]    The invention will now be described with reference to the accompanying drawings which show by way of example only, two examples of a seal. In the drawings: 
           [0032]      FIG. 1  is a cut away side view of a sealing block and cylinder; 
           [0033]      FIG. 2  is a top view of a block; 
           [0034]      FIG. 3  is a cut away side view of a double seal assembly; 
           [0035]      FIGS. 4 a, b   , &amp;  c  illustrate the operation of the sealing block of  FIG. 1 ; 
           [0036]      FIG. 5  illustrates the sealant retainer in a block; and 
           [0037]      FIG. 6  illustrates piston profiles; 
           [0038]      FIGS. 7 a -7 c    illustrate an embodiment of an indicator rod; and 
           [0039]      FIG. 8  illustrates a housing usable in an embodiment of a seal. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0040]    The requirement to form a seal between two elements of a machine, to prevent fluid flow, has been known for centuries and a large variety of solutions proposed. The most common of these solutions involves simply interposing a fluid impermeable barrier, which barrier conforms to the shape of each element and so does not allow fluid across. 
         [0041]    Such an arrangement is usually successfully and readily achieved where the two elements are stationary relative to one another. However, where the two elements are in relative motion, then difficulties occur. Such a situation can occur where for example, one of the elements is a piston moving in a cylinder or a drill shaft rotating within a guide housing. Due to the seal element being in close contact with the moving element, wear on the seal element can be rapid. The seal will therefore need to be replaced at frequent intervals to prevent leakage of fluid and also damage to the elements. Replacement can be expensive: partially due to the cost of the seals themselves, but also in terms of the loss of production time due to the machine being switched off. Moreover, especially in the oil industry many seals are not easily accessible and working on their replacement can be dangerous. In addition seals are in locations, such as an offshore drilling rig, where the space occupied by the seal and its housing needs to be kept to a minimum. 
         [0042]    The present invention seeks to prolong the lifetime of a seal by providing a seal which is in effect continually being renewed. This is achieved by feeding through, under pressure, the seal material so that as the leading edge of the seal material is worn away, the material behind the leading edge takes its place in contact with the moving element. In addition replacement seal material is added to the volume of seal material, remote from the seal/element interface, to ensure that the seal does not completely wear away. Such replacement material can be added without the drilling operation ceasing or, where safety considerations render this impracticable, with a minimum downtime to the production or manufacturing process. 
         [0043]    With reference therefore to  FIGS. 1 &amp; 2  the basic elements of the seal are shown. The problem as shown here is to provide a seal about the outside of the reciprocating barrel  10 . The barrel  10  can be a piston moved, either by chemical or mechanical energy (such as motion induced by the waves at sea), in a direction longitudinal to the axis of the barrel  10 . 
         [0044]    The seal material is a solid, resilient material and is retained within a housing  11  made of steel or other suitably strong and corrosion resistant material. To facilitate cleaning of the inner areas of the housing  11 , the housing is provided in two sections, held together by housing bolts  12 . The housing  11  defines a gel seal cavity  13  in which the seal material is retained. The gel seal cavity  13  defines a first portion  13   a  which is vertically arrayed and linked via a connection portion to a second, longitudinally orientated portion  13   b . Said horizontally orientated portion  13   b  opens onto the barrel  10  which therefore enables the seal material within the cavity  13  to engage the barrel  10 . 
         [0045]    In use, pressure is applied to the seal material which forces the seal against the outside of the barrel wall  10   a  of the barrel  10 . In the embodiments, illustrated herein pressure is applied by means of a piston  14 , which is urged against the upper surface of the seal material. To ensure that seal material does not escape around the sides of the piston  14 , piston seals  15  are housed around the piston  14 . It will be readily envisaged that as the piston  14  is designed to pass around the barrel  10  the piston  14  has an annular form. 
         [0046]    At various locations around the piston  14 , means are provided in the form of a through-piston port  16  to enable fresh seal material to be passed into the gel seal cavity  13 . In the embodiment shown this facility is located within a position indicator rod  17 . The rod  17  rises and falls in response to the level of seal material in the cavity  13 . A further seal  18  ensures that material within the cavity  13  does not leak out. 
         [0047]    Pressure can be exerted on the pistons  14  by the conventional means known in the art. For example hydraulic or pneumatic pressure can be applied via the pressure ports  19 . A test or cavity pressure port  20  is provided to indicate to the user the pressure in the seal region. 
         [0048]    The invention described herein is intended for use in conjunction with a seal material which is supplied as a monomeric or partially-polymerised pre-polymer in the form of a freely flowing liquid which can readily flow down the through-piston port  16 , but which polymerises within the gel cavity  13  to form a solid polymeric material. 
         [0049]    The pre-polymer can therefore comprise one or more monomers, together with a polymerisation initiator. Alternatively, the monomers can undergo thermal polymerisation, the housing being equipped in that case with a heating element to ensure that the correct temperature is maintained in the cavity to control the polymerisation rate to that which yields the polymer having the correct properties. 
         [0050]    One of a polymeric material suitable for use are compounds belonging to the polysilicone family. Such materials have a general formula: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0051]    where R is an alkyl group, linear or branched. The catalyst is of a type known in the art and can comprise, —for example, a metal or polymetallic complex having organic ligands. The seal material contemplated can have a Shore Hardness of around 70. 
         [0052]    In addition to the polymeric material a particulate solid can be included to provide a lubricator means which assists in reducing friction between the seal material and the barrel  10  said beads can be a low friction material such as polytetrafluoroethylene (PTFE). Additionally or alternatively an abrasive material can be used either as a replacement for or in addition to the lubricating material. An abrasive material can strip the rust or other irregularities from the surface of the barrel  10 . It can be envisaged that the seal material include regions or slices containing abrasive material to enable periodic cleaning of the barrel  10  to be carried out. 
         [0053]      FIG. 3  is a second embodiment of a seal, functioning in the same fashion as the embodiment in  FIG. 1 , the second embodiment of housing  30  providing however a double seal, each of which is produced as outlined above. 
         [0054]    The process by which a seal is formed and maintained is shown in  FIGS. 4 a -4 c   . In these figures, those parts referred to, which are shown in  FIG. 1  are shown with the same reference numbers. 
         [0055]    In  FIG. 4 a   , the seal material is shown, in solidified form, filling the cavity  13  (this is shown in more detail in  FIG. 5 ). Pressure is exerted on the seal material via the piston pressure ports  19 . Pressure is typically exerted by a hydraulic fluid. The fluid acts to push the piston  14  onto the seal material and cause the plug of seal material to be pushed out of the aperture (not shown) until the material is against the barrel  10  in sufficient amount and with sufficient pressure to cause a seal to be formed. It will be recognised that the solidified seal material should possess sufficient elasticity to enable the seal material to form into the shape of the barrel wall  10   a.    
         [0056]    The piston position indicator  17  at this point is at its maximum extension out of the housing  11  indicating the cavity  13  is full. As the seal material wears away due to the motion of the barrel  10 , and referring now to  FIG. 4 b   , the pressure maintained on the seal material forces the plug of seal material along the cavity  13 . The piston  14  therefore is moved along the cavity  13 , the hydraulic fluid moving into the volume  41  formerly occupied by the piston  13  to maintain the pressure on the piston. As the level of seal material drops this is indicated by the position indicator  17 . 
         [0057]    Once the level of seal material drops below a preset level, un-reacted pre-polymer is added along through the gel injection port  16  to bring the level in the cavity  13  back up to the full mark. The pre-polymer polymerises within the cavity  13  and in sufficient time for it to be solid before it reaches the aperture to the barrel  10 . 
         [0058]    In the above manner a seal is continually maintained and renewed without the need for the action of the barrel  10  to be stopped. Downtime of the machine or process, of which the barrel  10  is a part, is therefore considerably reduced. 
         [0059]    Operation of the device to introduce pre-polymer into the cavity  13  when the level 
         [0060]    of the piston  14  drops, can be done either manually or automatically. For example, when the indicator rod  17  drops to a preset level this can be set up to cause a warning signal to be given off. An operator then adds pre-polymer to the cavity  13 . The pre-polymer is added under sufficient pressure to force the piston ′ 14  upwards, and addition is continued until the level in the cavity  13  is at the required level. 
         [0061]    Alternatively, the drop in the indicator rod  17  can be configured to trigger automatic filling of the cavity  13  with pre-polymer. 
         [0062]    In order to increase the mixing and integration of introduced pre-polymer into the solid material of the set polymeric seal material, the lower edge of the piston which engages the seal material, can be profiled. 
         [0063]    As illustrated in  FIG. 6 , a number of different profiles can be used In  FIG. 6 a   , the lower surface  61  of the piston  60  is concave, causing a discontinuity in the surface of the seal material which enables better bonding. Similarly, the surface  63  of the piston  62  in  FIG. 6 b    produces an indentation in the surface of the material. 
         [0064]    The surfaces  65 ,  67  of pistons  64 ,  66  respectively are serrated, which serrations provide sharp discontinuities and possibly score marks in the seal material which enable the pre-polymer to penetrate the bulk of the seal material and so minimise the discontinuity between the newly introduced seal material and the already present material once the new pre-polymer has set. 
         [0065]    In an additional embodiment, an indicator rod (see  FIGS. 7 a -7 c   ) is removable which particularly facilitates the cleaning out of excess seal material which has hardened within the indicator rod during the introduction of fresh liquid seal precursor. The indicator rod is formed of two main elements, an outer jacket  71  and a removable tubular lining insert  72 . The tubular part  73  of the insert  72  is passed down the central channel of the outer jacket  71  and the insert  72  are retained together by means of a screw threaded fitment, at the proximal end of the rod. The outer surface of the outer jacket  71  has a screw thread at its distal end to allow the rod  70  to be fitted in position. 
         [0066]    The above arrangement enables the insert  72 , once the sealant material has set, to be removed. The removal is facilitated by the narrowing  74  of the insert  72  at its distal end  75 . The narrowing results in the sealant material at that point being weaker, and the twisting action moreover, as the insert  72  is unscrewed causes the sealant material to break away allowing the resultant apertures to be capped off. Alternatively, the insert  72 , once cleaned can be replaced and its end capped off. 
         [0067]    A further means of utilising the housing is shown in  FIG. 8 . In this mode, there is no indicator rod or means of topping up the sealant material. The seal element is provided as a preformed ring which is inserted into the block  11 . The preformed element can include a seal arrangement moulded or fitted to its top edge which prevents hydraulic pressurising fluid from penetrating between the element and the housing. Means are provided to apply pressure to the seal element to fuse it against the original seal already contained within the block  11 . 
         [0068]    In addition to the above embodiment of  FIG. 8  means can also be provided to bond or join together the original seal element and the newly inserted element. Said means can be in the form of an adhesive material or a portion of primer which can partially solubilise the surfaces of the elements, subsequently binding together when the premix sets. 
         [0069]    It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention.