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RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/809,284, filed Apr. 5, 2013, entitled “TESTING AND ISOLATING APPARATUS AND METHOD FOR RISERS,” the entirety of which is incorporated herein by reference as if set forth in full. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a mechanical assembly or “isolation tool” for use in controlling fluid movement through a pipe bore during the course of replacing a section of the pipe. For example, the tool may be used in connection with the process of replacing the worn or damaged upper section of a vertical “riser” associated with an off-shore oil or gas production platform. The invention further relates to a method associated with the pipe section replacement procedure. 
       BACKGROUND OF THE INVENTION 
       [0003]    The invention was developed in connection with providing an isolation tool which could be used ‘downhole’ in connection with the process of replacing the upper section of an upwardly oriented riser associated with an off-shore oil or gas production platform. Such platforms are prevalent in the Gulf coast region of the U.S. It will be described herein in that connection. However, it is contemplated that the tool and method may find application in other fields of use as well and is not limited to downhole applications in upwardly oriented pipes. 
         [0004]    The word “riser” is commonly used to identify a steel pipe extending up from the sea floor to a production platform. Oil or natural gas is produced through the bore of the riser from one or more wells connected to it. 
         [0005]    Many of these risers have been in use for decades in the Gulf coast region. As a consequence, in many cases their upper sections have become weakened and worn by wave action, corrosion and other damaging actions. The well operators pay close attention to this deterioration and will initiate replacement of the upper section of the riser when appropriate. 
         [0006]    Typically the prior art replacement operation has involved:
       closing the valves of the sea floor wellhead(s) to terminate production into the base of the riser;   setting a packer at depth in the riser bore (for example, this might take place at a depth of up to 40′ below the platform), so as to provide a barrier sealing off or isolating the upper section of the riser bore from its lower section;   severing the riser wall at a point above the packer with a mechanical cutting tool;   removing the severed upper section of the riser and lowering a replacement pipe section into its place; and   welding the new riser section end to end to the old section.       
 
         [0012]    It follows that one needs to isolate the pipe wall area, where welding is to occur, from the flammable gas which may still be present in the bore of the riser. 
         [0013]    Heretofore there have been various downhole tools employed to so control the riser bore. In one case, a pig-type device or packer is displaced to the appropriate depth in the bore and then actuated from ground surface by means of an electronic signal. Upon actuation, the packer mechanically expands its seal element radially into sealing engagement with the pipe wall and anchors itself to the wall, thereby providing a barrier to gas movement along the inner surface of the upper pipe segment. 
         [0014]    These prior art tools are closely held and are not publically disclosed in detail in the literature, to our knowledge. However, we understand that they are characterized by several shortcomings, including that:
       the tools are complex and their use entails lengthy set-up times;   the tools are not adapted to monitor and provide a real time indication of the on-going competence or possible leakage of the packer seal element; and   rental of the known tools is expensive.       
 
         [0018]    It is therefore one object of the present invention to provide an isolation tool for use downhole that can be comparatively quickly installed and deployed at the desired depth in a riser bore. 
         [0019]    It is another object to provide a tool adapted to provide a real time indication of seal competency or leakage. 
         [0020]    And it is another object to provide a tool which, due to the simplicity of its structure and operation, can be supplied at less cost than has been common in the past. 
         [0021]    If the tool is to be used at an off-shore platform, it is desirable that it be adapted to be insertable into the riser inlet within the space available. While these platforms are huge structures, the space available to feed an isolation tool into the open upper end of the riser bore is commonly very limited. Typically there might be only about 2½-6′ of “head room” available at the riser inlet. It therefore is a preferred object of the invention to provide a tool which is adapted to be fed into the pipe bore in spite of the usual space limitations at the inlet. 
         [0022]    If the tool is to be used in the context of an off-shore platform riser, it is desirable that it remain cohesive in use. Dropping a steel part down the riser bore is unacceptable to the well operator. It therefore is a preferred object to provide a tool whose components are well tied together to maintain its cohesiveness while still allowing tool components to move to a limited extent in the course of ‘setting’ the tool. 
         [0023]    If the tool is to be used in close proximity to a welding operation, it is desirable to avoid heat damage of the elastomer seals. It therefore is another preferred object to provide a tool adapted to protect the seals against deterioration from heat. 
       SUMMARY OF THE INVENTION 
       [0024]    In one aspect, the present invention provides a primarily mechanical isolation tool adapted for use downhole. The tool is designed to work at depth within the bore of a pipe, such as an upwardly oriented riser associated with an off-shore production platform. 
         [0025]    The tool provides a sealed pressurized barrier internally dividing the pipe bore into upper and lower sections. The barrier functions to block fluid movement along the inner surface of the pipe. But the tool also provides a conduit extending upwardly to the pipe&#39;s open end, for venting rising bore fluid at ground surface. The barrier is actuated or ‘set’ from ground surface by mechanical means extending down through the upper section of the bore. 
         [0026]    In addition, the tool incorporates a system for monitoring, at ground surface, whether the downhole seal elements are leaking and it further preferably incorporates means for tieing tool components together so as to ensure cohesiveness. 
         [0027]    One embodiment of the downhole isolation tool comprises, in combination:
       support means, at ground surface, for engaging the upper end of the pipe and supporting and anchoring downwardly extending segments of the tool;   an isolation assembly for positioning lengthwise in the pipe bore at depth, said assembly comprising a linear assemblage of components including a front plate, a front seal element, a central body, a back seal element and a back plate, said seal elements being axially spaced apart, resilient, deformable and annular, each said seal element being positioned between one end of the body and a plate, whereby, when the plates and body are compressed together, the seal elements may be squeezed and deformed outwardly into sealing engagement with the pipe and may thereby combine with the outer surface of the body and the inner surface of the pipe to form a sealed annular space;   a housing assembly, preferably a string of tubular joints connectable end to end, for lowering the isolation assembly into the pipe bore and positioning it at the desired operational depth, said housing assembly being operative to be anchored at its upper end to the support means and connectable at its lower end with the isolation assembly, whereby the front plate may be held stationary and fixed in place by said housing assembly;   a tubular vent rod means, preferably a string of tubular rods connectable end to end, for connection with the back plate and extending upwardly from the back plate through the isolation assembly to ground surface, said vent rod means being operative to communicate with the lower section of the pipe bore;   means, supportable by the support means, for tensioning the vent rod means to thereby compress the plates and body together and deform the seal elements so that they sealably engage the pipe;   monitoring means, extendable through the upper section of the pipe bore and connectable with the isolation assembly, for circulating pressurized liquid through the sealed space and back to ground surface;   whereby the isolation assembly may provide a sealing barrier having a pair of annular seal elements sealing against the pipe&#39;s inner surface at axially spaced apart intervals, the vent rod means may simultaneously provide a passageway for segregated venting of pipe bore fluid from the pipe bore&#39;s lower section through the barrier to ground surface, said rod means may additionally function to actuate the isolation assembly, the housing assembly may provide a means for lowering, holding and recovering the isolation assembly and the monitoring means may function to provide an indication as to whether the seal elements are leaking       
 
         [0035]    In a preferred optional feature, the back plate is connected by floating locking pins with the back end of the body, said pins being positioned internally of the body in bores which permit limited axial movement of the pins and back plate but the body has shoulders which prevent separation of the back plate and body. In addition, the front plate is connected with the body by locking pins which permit limited axial movement of the body toward the front plate. As a consequence of this arrangement the vent rod means can pull the back plate and body toward the front plate and thereby squeeze the seal elements to actuate the isolation assembly, while the locking pins still ensure cohesiveness of the isolation assembly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    These and other features of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein: 
           [0037]      FIG. 1  is a partial cut-away side view of the isolation assembly; 
           [0038]      FIG. 2  is an exploded perspective view of the components of the isolation assembly of  FIG. 1 ; 
           [0039]      FIG. 3  is a perspective view of the back plate of the isolation assembly; 
           [0040]      FIG. 4  is a partial cut-away side view of the back plate shown in  FIG. 3 ; 
           [0041]      FIG. 5  is a perspective view of the vent rod of the isolation assembly; 
           [0042]      FIG. 6  is a perspective view of the body of the isolation assembly; 
           [0043]      FIG. 7  is a front view of the body shown in  FIG. 6 ; 
           [0044]      FIG. 8  is a cross-sectional view of the body taken at line I-I in  FIG. 7 ; 
           [0045]      FIG. 9  is a cross-sectional view of the body taken at section II-II in  FIG. 7 ; 
           [0046]      FIG. 10  is a cross-sectional view of the body taken at section III-III in  FIG. 7 ; 
           [0047]      FIG. 11  is a perspective view of the front plate assembly of the isolation assembly; 
           [0048]      FIG. 12  is a partial cut-away side view of the front plate assembly shown in  FIG. 11 ; 
           [0049]      FIG. 13  is a cross-sectional partial cut-away view of the back plate, body, vent rod and a seal element in an assembled state, wherein a first set of locking pins are shown as fastening the body with the back plate; 
           [0050]      FIG. 14  is a cross-sectional partial cut-away view of part of the isolation assembly, incorporating a pair of axially spaced apart seal elements, in an assembled state, wherein a second set of locking pins are shown fastening the front plate to the body; 
           [0051]      FIG. 15  is an enlarged partial cut-away side view of part of the isolation assembly positioned within a pipe in an actuated state, wherein a sealed space, defined by the body, pipe and seal elements, is shown; 
           [0052]      FIG. 16  is a side view of a vent rod extension; 
           [0053]      FIG. 17  is a side view of a modular housing string; 
           [0054]      FIG. 18  is a partial cut-away side view of a coupling connecting a vent rod extension to the vent rod; 
           [0055]      FIG. 19  is a side view of the coupling and the vent pipe extension attached to the isolation assembly; 
           [0056]      FIG. 20  is a partial cut-away perspective view showing an embodiment of the fully assembled downhole isolation tool; 
           [0057]      FIG. 21  is a partial cut-away side view of the tool shown in  FIG. 20 ; 
           [0058]      FIG. 22  is a perspective view of the tool, showing a venting tube being connected to the vent rod extension; 
           [0059]      FIG. 23  is an enlarged perspective view of the venting tube connected to the vent rod extension; 
           [0060]      FIG. 24  is a partial cut-away side view of the isolation assembly; 
           [0061]      FIG. 25  is a partial cut-away side view of another embodiment of the isolation assembly; 
           [0062]      FIG. 26A  is a partial cut-away side view of the tool in the process of being lowered into a pipe; 
           [0063]      FIG. 26B  is a partial cut-away side view of the tool positioned within the pipe for isolating a section of the pipe, wherein a cut line has been formed through the pipe side wall; 
           [0064]      FIG. 26C  is a partial cut-away side view of the tool positioned within the pipe for isolating a section of the pipe, wherein a weld has been formed at the cut line; 
           [0065]      FIG. 27  is a partial cut-away side view of the tool positioned within the welded pipe for pressure testing the weld; 
           [0066]      FIG. 28  is a perspective view of a blow out plate; 
           [0067]      FIG. 29  is a front view of the blow out plate shown in  FIG. 28 ; 
           [0068]      FIG. 30  is a cross-sectional view showing an embodiment of the fully assembled downhole isolation tool positioned within a pipe; and 
           [0069]      FIG. 31  is a perspective view of the tool, showing the internal components of the isolation assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0070]    Throughout the description the following terms will be assumed to have the following meanings: 
         [0071]    “Axial”—this term is used to indicate a direction along the longitudinal axis of the pipe and tool. Thus, the term “axially extending” will be understood to mean extending in a direction parallel to the longitudinal axis of the pipe. 
         [0072]    “Front” and “back” and “upper” and “lower”—these terms are used interchangeably to describe the positions of various components of the tool. The terms “front” and “upper” indicate a position closer to the open end of the pipe. The terms “back” and “lower” indicate a position away from the open end of the pipe. 
         [0073]    “Ground surface” is intended to mean the off-shore platform (not shown) or an alternative surface structure adjacent the upper end of the pipe. 
         [0074]    The isolation tool  1  is shown assembled and ready to operate in  FIG. 30 . It is illustrated as inserted in an upwardly oriented pipe  2 , such as an off-shore platform riser whose upper end is to be replaced. The isolation tool  1  will be described herein below in that context, although it is contemplated that it could be applied in a horizontal pipe as well. 
         [0075]    The isolation tool  1  comprises a combination of assemblies which can be deployed into and can work together in the bore  3  of the pipe  2  at depth (usually the depth is less than 40 feet). 
         [0076]    The purpose of the tool is to provide a sealed barrier which prevents flammable downhole fluid from migrating up along the inner surface  4  of the pipe  2  to the point where cutting and welding operations are to take place. Instead the fluid is to be vented through the tool to ground surface in a segregated way. 
         [0077]    It is intended to provide an isolation tool  1  that is primarily mechanical in nature. The tool can be actuated from ground surface and the competence of its seals can be monitored in real time. 
         [0078]    Having reference to  FIGS. 1 and 30 , the isolation tool  1  comprises in general:
       an isolation assembly  100   a  incorporating a linear assemblage of components including: a front plate  112 , a front seal element  31 , a central body  61 , a back seal element  35  and a back plate  11 . When the components are compressed together, the axially spaced apart annular seal elements  31 ,  35  are deformed radially outward and seal against the inner surface  4  of the pipe  2 , thereby combining with the outer surface  62  of the body  61  and the pipe inner surface  4  to define an annular sealed space  63 ;   a housing assembly  7  incorporating a string  8  of tubular pipe joints  9  for lowering the isolation assembly  100   a  into position, holding it stationary during operation and recovering it upon completion of the work;   a vent rod means  10 , comprising a string  17  of tubular vent rods  41 , which is connected with the back plate  11 , communicates with the pipe lower bore  12 , slidably extends through the balance of the isolation assembly  100   a  and up through the housing assembly  7  to ground surface. The vent rod string  17  can be pulled upwardly and tensioned at ground surface to actuate the isolation assembly  100   a  by compressing its components together. It also functions to vent downhole fluid through the isolation assembly  100   a  to ground surface;   actuating means, such as a long nut  13  or hydraulic tensioner, provided at ground surface for pulling and maintaining the vent rod string  17  in a tensioned state so that the seal elements  31 ,  35  engage and continue to seal against the pipe  2 ;   a support means  21  positioned at ground surface for supporting and anchoring the downwardly extending components; and   a monitoring means  24 , comprising conduits  161 ,  163  extending down through the upper pipe bore  23  from ground surface and connecting with the isolation assembly&#39;s sealed annular space  63 . The monitoring means  24  circulates pressurized water down through the annular sealed space  63  and back up to ground surface. The water pressure is monitored and provides an indication in real time of seal leakage.       
 
         [0085]    Now in greater detail and having reference to  FIGS. 2-4 , the back plate  11  is a circular disk having front and back faces  14 ,  16 . At its front end, the plate  11  forms a plurality of threaded holes  17  for receiving the threaded ends of a first set of “floating” locking pins  181 . Each of the plurality of holes  17  extends only part way into the plate  11 . 
         [0086]    The back plate  11  serves as a barrier extending transversely of the pipe bore  3 . It also plays a role in the action of compressing the body  61  and plates  11 ,  112  together to deform the seal elements  31 ,  35  outwardly into sealing engagement with the pipe&#39;s inner surface  4 . 
         [0087]    Referring to  FIGS. 3 and 4 , the back plate  11  is shown as also having a central threaded aperture  19  extending therethrough, for connection with the vent rod string  7 . 
         [0088]    The front face  14  of the back plate  11  is beveled along its perimeter to form an annular bevel  22 . The bevel  22  is rearwardly angled, preferably at about 15°. 
         [0089]    As previously indicated, the vent rod string  17  is connected with the back plate  11  and extends through it. The string&#39;s open lower end communicates with the downhole pipe bore  12 . It has an axial bore  47  for venting downhole fluid through the isolation assembly  101   a  and up to ground surface. As previously indicated, it further functions, when pulled upwardly from ground surface, to bias the back plate  11  and body  61  toward the front plate  112  and thereby compress the seal elements  31 ,  35 . In summary, the vent rod string  17  performs two functions—it provides a means for venting downhole fluid to ground surface and it contributes to compressing the plates  11 ,  112  and body  61  together, to thereby deform the seal elements  31 ,  35 . 
         [0090]    The vent rod string  17  is formed by adding modular extension  311  to the isolation assembly&#39;s vent rod  41 . In the case where the available head room between the deck of the off-shore platform and the riser inlet is only about  3 ′, the extension rods  311  may be only 2′ in length. 
         [0091]    The body  61  is generally cylindrical in configuration and is shown in  FIGS. 6-10 . It has a front face  77 , a back face  78  and an outer side surface  62 . The outer side surface  62  has an outer diameter that is slightly less than the inner diameter of the pipe being worked in, so that an annular clearance or space  63  is defined therebetween. 
         [0092]    The body  61  forms a plurality of through holes  65 , a plurality of threaded recesses  67 , a first port  69  for coupling a first monitoring conduit  161  to the body and a second port  71  for coupling a second monitoring conduit  163  to the body. It further forms a central bore  73  extending therethrough. 
         [0093]    The central bore  73  is provided through the body  61  to allow the vent rod  41  to extend therethrough. The diameter of the body bore  73  is slightly greater than the outer diameter of the vent rod  41 . As a result the body  61  is slidable on the vent rod  41 . 
         [0094]    As shown in  FIG. 8 , the body&#39;s first port  69  connects with a first channel  91  extending radially out to the body&#39;s circumferential surface  62 , at the front end of the body  61 . The second port  71  connects with a second channel  92  that also extends radially out to the body&#39;s surface  62 , at the back end of the body  61 . 
         [0095]    As shown in  FIG. 9 , each of the through holes  65  includes a larger diameter portion  96  on the front end thereof, and a smaller diameter portion  97  on the back end thereof. A ledge  75  is therefore formed at the interface between the portions  96 ,  97 . 
         [0096]    The circumferential edges of both end faces  77 ,  78  of the body  61  are beveled to form surfaces  85 ,  87 . The outwardly flared surfaces  85 ,  87  are adapted to cooperate with the annular seal elements  35 ,  31  to facilitate their outward radial deformation when the plates  11 ,  112  and body  61  are compressed together. 
         [0097]      FIGS. 11 and 12  show the front plate assembly  111 . It includes a front plate  112 , a sleeve  131  attached at its back end  135  to the front plate  112  and extending axially therefrom, and a flange plate  143  attached to its front end  137 . 
         [0098]    The front plate  112  is a circular disk having a front face  113  and a back face  114 . The front plate forms a central aperture  125 , a plurality of through holes  121 , a first opening  115  for the first monitoring conduit  161  to extend through and a second opening  117  for the second monitoring conduit  163  to extend through. The front plate  112  further has an outwardly flared surface or bevel  123  formed at the perimeter of its back face  114  and a circumferential groove  119  formed by its side surface. The bevel  123  is adapted to squeeze the front seal element  35  to radially deform it into sealing engagement with the inner surface of the pipe  2 . 
         [0099]    The sleeve  131  is a tubular member having a back end  135 , a front end  137  and a central bore  133  extending axially therethrough. 
         [0100]    The flange plate  143  forms a threaded central aperture  153  for engaging the sleeve  131 , a plurality of bolt holes  151  and openings  145 ,  147  for enabling the monitoring conduits  161 ,  163  to extend therethrough. 
         [0101]    As shown in  FIG. 14 , the vent rod  41  extends through the back plate  11 , central bore  73  of the body  61  and the front plate  112 . Otherwise stated, the vent rod  41  is connected to the back plate  11  and extends slidably through the isolation assembly  100   a.    
         [0102]    The annular back seal element  31  is positioned between the back face  78  of the body  61  and the front face  14  of the back plate  11 . The annular front seal element  35  is positioned between the front plate&#39;s back face  114  and the body&#39;s front face  77 . 
         [0103]    Referring to  FIG. 13 , a first set of “floating” locking cap screws or pins  181  is inserted into holes  65 . Each locking pin  181  has a head  183  and a threaded reduced diameter shank  185 . The shanks  185  are threaded into the back plate holes  17  and extend through the smaller diameter portions  97  of the body holes  65 . The heads  183  are positioned in the larger diameter portions  96  of the holes  65 . The heads  183  can travel within the hole portions  96  but their downward travel is limited by the ledges  75 . Thus, in the event that the vent rod  41  breaks during operation, separation of the back plate  11  from the body  61  is prevented when the heads  183  contact the ledges  73 . 
         [0104]    The front end of the larger diameter portion  96  of the through hole  65  may be plugged by a cap  190  as shown in  FIG. 1  to protect the locking pin  181  and to prevent foreign objects from accumulating within the through hole  65 . 
         [0105]    Having reference to  FIG. 14 , a second set of “floating” locking pins  191  is shown. Each pin  191  has a reduced diameter threaded shank  195  and a larger diameter head  193 . The shanks  195  extend through the holes  121  formed through the front plate  112  and are threaded into the threaded recesses  67 . The shanks  195  are of sufficient length so that, when threaded into the body recesses  67 , the heads  193  are still spaced from the body front face  113 . This enables the slidable body  61  to be axially displaced toward the front plate  112 . However the locking pins  191  prevent the body  61  and front plate assembly  111  from separating, as the diameter of the plate holes  121  is smaller than the diameter of the heads  193 . 
         [0106]    In summary, the two sets of locking pins  181 ,  191  limit the maximum separation distance between the front plate  112  and the body  61 , and between the body  61  and the back plate  11 , while enabling the back plate and body to move a short distance toward or away from the stationary front plate  112 . 
         [0107]    As previously indicated, the seal elements  31 ,  35  are annular and formed of deformable, resilient material, such as buna rubber. Preferably they have a D-ring configuration with a rounded inner surface profile and a flat outer surface. 
         [0108]    To complete the isolation assembly  100   a,  a lock nut  251  may be screwed onto the front end  53  of the vent rod  41  to abut the flange  143 . The monitoring conduits  161 ,  163  are connected to the body ports  69 ,  71 , as shown in  FIG. 1 . 
         [0109]    The isolation assembly  100   a  may be operated and monitored from ground surface to a depth of up to about 40 feet. 
         [0110]    Operation of the isolation assembly  100   a  involves: pulling the vent rod string  17  upwardly at ground surface with means such as a hydraulic tensioner (not shown) or a long nut  5  and thereby drawing the back plate  11  and body  61  upwardly so that they press against the stationary fixed front plate  112  and squeeze the annular seal elements  31 ,  35  between the beveled surfaces  22 ,  87 ,  85 ,  123 , whereby the seal elements  31 ,  35  deform and extend outwardly to seal effectively against the circular inner surface  4  of the pipe  2 , thereby combining with the body  61  and pipe  2  to define the sealed annular space  63 ; then locking the vent rod string  17  in a tensioned condition so as to maintain the seal; circulating water or the like under pressure from ground surface and back through the conduits  161 ,  163  and sealed space  63 , to provide an indication as to whether seal leakage is occurring; and eventually releasing the vent rod string so as relax it and the seal elements whereby the assembly may be withdrawn. 
         [0111]    The vent rod  41  is extended by adding tubular extension rods  311  to form the string  17  using threaded couplings  341 , as shown in  FIGS. 18 ,  19 . Each coupling contains an O-ring  343  to minimize the escape of flammable fluid passing through the bore  317  of the vent rod string  17 . Similarly, the tubular flanged housing extensions  431  are added and bolted end to end to front plate assembly  111  to form the housing string  8 , as shown in  FIG. 17 . Finally, conduit extensions  471 ,  473  are added and connected with the first and second conduits  161 ,  163 , as shown in  FIG. 20 . 
         [0112]    In summary, the isolation assembly  100   a  is progressively lowered into the riser bore  3  by adding short modular extensions to the vent rod  41 , front plate assembly  111  and conduits  161 ,  163 , thereby forming the vent rod string  17 , housing string  8  and conduit string  6 . Spacer plates  451  may be supplied to centralize the strings  17 ,  8  and  6  and minimize twisting of the strings. 
         [0113]    As shown in  FIGS. 20 ,  20 A,  20 B,  23  and  30 , a support means  21  is positioned at ground surface, for supporting and anchoring the downwardly extending components. Commonly the pipe  2  will have a flange  25  at its upper end. The support means  21  shown comprises an H-shaped lifting bridle  26  adapted to seat on the pipe flange  25 . A flange  401 , forming part of the uppermost housing extension  431 , seats on the bridle  26 . The housing string  8  therefore is suspended from the flange  401  and bridle  26 . 
         [0114]    A long nut  651  is threaded onto the upper end of the vent rod string  17 , which protrudes out of the upper end of the housing string  8 . The nut  651  can be turned to tension the vent rod string  17  to actuate and then lock the isolation assembly  100   a . Alternatively a hydraulic tensioner can be used for the same purpose. 
         [0115]    As shown in  FIG. 26B , the isolation assembly  100   a  will be positioned close to (usually about 1′ below) the cut point  571 . 
         [0116]    As shown in  FIG. 30 , the monitoring means  24  comprises a pump  26  at ground surface, for supplying water under pressure into the supply conduit  161 , for conveyance through the sealed spaced  63 . A pressure gauge  27  shows the pressure of the water returning from the sealed space  63  through the return conduit  163 . Observation of the gauge reading will inform if there is a reduction in pressure, thereby indicating seal leakage. 
         [0117]    The isolation tool described is characterized by several advantages, including:
       it can be relatively quickly deployed—commonly this can be accomplished within a day;   the provision of two axially separated seal elements provides a safety factor—if the primary lower seal begins to leak, there may still be an opportunity to terminate welding;   the spaced double seals and conduit system provide a way to monitor for seal leakage;   the modular design allows the tool to be inserted into the riser bore despite limited head room; and   tieing together of components with the floating locking pins is beneficial in reducing the possibility of losing parts down the riser bore.

Summary:
The isolation tool works at depth in the bore of a pipe whose upper end is to be severed and replaced. The tool isolates the lower pipe bore from the upper bore so that flammable downhole fluid is prevented from reaching a welding site along the pipe wall. The tool incorporates: means for establishing a pair of axially spaced apart annular seals engaging the pipe inner surface; means for venting downhole fluid to ground surface. The tool is primarily mechanical in nature. It finds use in connection with off-shore platform riser renewal operations.