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You are an expert at summarizing long articles. Proceed to summarize the following text: 
The invention relates to a device for carrying a replacement safety valve in a oil or gas well, particularly with a straddle structure for placement inside a damaged “Down Hole Safety Valve” nipple seal area. 
     FIELD OF THE INVENTION 
     Oil and gas well operators have been facing a challenge on how to avoid recompletion of wells where the nipple seal area inside the “Down Hole Safety Valve” (DHSV) is damaged, for instance due to well interventions or corrosion. Various methods have been used including standard insert “Wireline Retrievable Surface Controlled Subsurface Safety Valve” (WRSCSSV), with normal and oversize swell packing elements and honing of the upper and lower seal area prior to installing the insert. 
     To have the DHSV working properly is vital for the security of the well. If the DHSV is damaged the alternatives are to repair it using one of the above mentioned methods, or set a retrievable bridge plug above the DHSV area and leave the well shut in or do a recompletion of the well. To leave the well shut in is not a good option since it cannot be used for production or injection. To recomplete the well is very expensive since a rig is needed for this. 
     The above mentioned methods of repairing a damaged DHSV-nipple seal area&#39;s have failed or been too unsafe. 
     OBJECTS 
     The main object of the invention is to provide a generally improved device for carrying a replacement down hole safety valve. 
     A further object is to provide such a carrier with improved sealing properties. To this end, it is an object to provide improved sealing means to allow the insertion of a DHSV carrier in the most damaged wells. 
     It is also an object to improve the locking dog suspension to enhance the operation of the locking dogs. The locking dog suspension should be easy to install and maintain a reliable tension on the locking dogs. 
     THE INVENTION 
     The invention is defined in claim  1 . Further details are described in the sub-claims. 
     The device for inserting a replacement safety valve in a well tube, is based on a straddle structure for placement inside a damaged “Down Hole Safety Valve” nipple seal area, comprising a pair of sealing assemblies to be activated upon setting of the device to seal said nipple seal area, and a locking dog assembly for locking the device in the well tube, said elements being carried by a tubular mandrel. 
     The main novel features are that at least one of the sealing assemblies comprises an expandable element being activated by a pair of annular press elements. 
     The straddle device is using expandable elements which will allow the sealing elements of the sealing assemblies to be shaped according to the damaged surface inside in the seal area. In addition each element may be equipped with expanding anti-extrusion backup rings on both sides, trapping the element so it has nowhere to go when differential pressure is applied. The straddle device is using the existing nipple profile and no-go for anchoring and is equipped with locking dogs to fit this profile. A commercial safety valve, e.g. a Schlumberger WRDP valve, is made up to the bottom of the straddle, this is then operated by surface control line pressure. 
     By using a sealing assembly according to the invention, the ability of the DHSV carrier device to seal even badly damaged wells is enhanced. 
     The invention also comprises further novel features as stated in claims  2  to  7 , including a novel locking dog suspension. 
     The novel device according to this invention will bring several advantages over prior art technology:
         Uses existing sealing surfaces of old DHSV   Better sealing properties   Anchoring module design maximizes locking contact area   Sealing module construction enhances safety and reliability   Suitable interface for replacement safety valve   Easier mounting       

    
    
     
       LIST OF FIGURES 
         FIG. 1  shows a partly sectioned side view of an embodiment of a safety valve carrier according to the invention, 
         FIG. 2  shows a sectioned perspective view of the lower sealing assembly of the safety valve carrier of  FIG. 1 , while 
         FIG. 3  shows a perspective view of the upper sealing assembly, with the locking dog housing removed to show the arrangement of the locking dogs. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is showing a side view of the main part of a replacement safety valve carrier (straddle device)  11 , the drawing having on the left side an axial and radial cross section of the elements of this carrier. Centrally in the replacement safety valve carrier  11  a mandrel tubing  12  is arranged. The mandrel tubing  12  is connected to the outer part of the carrier  11  by a series of radial shear screws in the upper region. Further, the lower end of the mandrel tubing  12  is connected with threads  14  to a tubular sleeve or nose, which is carrying the replacement safety valve not shown. The replacement safety valve may be of prior art design. 
     The replacement safety valve carrier  11  has a tubular structure carrying
         a lower sealing assembly  15 ,   an upper sealing assembly  16 , and   a locking dog assembly  17  arranged adjacent to the upper sealing means  16  on its upper side.       

     The lower and the upper sealing assemblies  15 ,  16  are providing a straddle structure. 
     According to the invention, the lower sealing assembly  15  and the upper sealing assembly  16 , comprise multiple elements. 
     The Lower Sealing Assembly 
     In  FIG. 2  an embodiment of a lower sealing assembly  15  is shown. The lower sealing assembly  15  comprises a lower and an upper cone ring  19 ,  20  respectively, arranged on a tubular lower sealing mandrel  21 . Between the lower and the upper cone rings, an annular expandable sealing element  22  is arranged. The expandable sealing element  22  has a trapezoidal section with radially diverging sides and being made of a resilient material, such as HNBR. 
     The tubular mandrel  21  is facing the mandrel tubing  12  and being connected to a sleeve  25  connecting it to a corresponding upper sealing mandrel  26  carrying the upper sealing assembly  16 . ( FIG. 1 ) 
     The lower sealing mandrel  21  is comprising a sleeve  21 A carrying a lower nut mandrel  27  which is slidable in an annular recess  28  in a lower sealing backup housing  29 . The upper end of the lower sealing backup housing  29  engages the lower sealing cone ring  19  with an intermediate annular lower sealing backup  30 . 
     Similarly, an annular upper sealing backup  31  is arranged between the upper cone ring  20  and a upper part  21 B of the lower sealing mandrel  21  functioning as an upper sealing backup housing. 
     The sealing backups  30  and  31  each comprises an annular row of overlapping arched sealing backup elements  30 A and  31 A, respectively. Each backup element  30 A and  31 A comprises an outer sealing part  30 B and  31 B and an inner leg  30 C and  31 C. 
     The sealing backups  30  and  31  are attached to the lower sealing backup housing  29  and the upper sealing backup housing  21 B respectively, with ten bolts  32  each. Each spring bolt  32  is carrying a set of dish springs  33 . 
     The head  32 A of each bolt  32  has threads for engaging radial holes in the respective housings, being locked by radial locking screws  328 . The shanks of the bolts  32  are sliding in an opening in the leg  30 C and  31 C respectively, of the associated backup element  30 A and  31 A. 
     The expandable sealing element  22  is activated by the relative axial movement of the lower sealing backup housing  29  and the upper sealing backup housing  21 B at the setting of the device according to the invention. 
     The Upper Sealing Assembly 
     The upper sealing assembly  16  is corresponding generally to the lower sealing assembly  15 , having an expandable sealing element  34  ( FIG. 1 ). 
     The Sealing Backup Assembly 
     Each sealing backup assembly  30  and  31  comprises an annular chain of ten sealing backup elements  30 A and  31 A, respectively. Each sealing backup element comprises an outer, arched sealing backup part  30 B and  31 B, respectively, with a circumferential wing  44  on one side and a corresponding slot  45  on the other to allow overlapping of neighboring sealing backup elements. Thus the sealing backup elements will have a closing annular face against the outer rim of the expandable sealing element  22 ,  34 , to restrict the axial expansion of the sealing element. 
     The Locking Dog Assembly 
     To lock the device according to the invention in the well tube, an annular row of nine locking dogs  61  are arranged over the upper sealing assembly  16 . The locking dogs  61  are arranged in openings  62  in a locking dog housing  63 . The locking dog housing  63  is carrying an axially slideable sleeve  65 . The lower end of the sleeve  65  has a chamfered face  66  facing the upper end of the locking dogs  61  to press them radially outward when setting the device. 
     In  FIG. 3 , the locking dog housing  63  of  FIG. 1  is omitted for clarity. 
     Each of the locking dogs  61  being a block with chamfered lower and upper edges  67 ,  68 , is carried by a pair of axial torsion springs  69 ,  70 . Each torsion spring  69 ,  70  has a central spring rod  73 . The lower end of this rod may have a C-leg  71  engaging a slot  72  in the side of the locking dog  61 . The connection of the torsion springs  69 ,  70  to the locking dogs  61  may have other suitable embodiments. 
     The central rods  73  of the torsion springs  69 ,  70  are extending through a longitudinal slot  74  in the sleeve  65 . 
     At the upper end, each torsion spring  69 ,  70  has a L-leg  75  extending into an annular slot  76  of the locking dog housing  65 . The width of the annular slot  76  is dimensioned to take the movement of the L-legs  75  between the end positions, from setting to locking. 
     The torsion springs  69 ,  70  are pressing the locking dogs  61  inward. In the set position of the device according to the invention, with the no-go  64  engaging a shoulder in the tubular safety valve housing  18 , the locking dogs  61  are forced radially outward by the chamfered lower end  66  of the locking dog cone housing  65 . 
     The arrangement of the locking dogs  61  is making the elements easy to manufacture. The locking dogs according to the invention will have a larger area of contact than prior art locking dogs. Additionally, the torsion springs, acting in pairs, will provide balanced forces acting on the locking dogs. The mounting of the torsion springs  69 ,  70  is easy. 
     The device according to the invention is providing a path for hydraulic fluid to the replacement safety valve through a radial hole  77  in the upper backup housing  21 B and an axial hole  78  in the lower part of the device. Said holes  77  and  78  are interconnected by the sleeve  21 . 
     The upper part of the device comprises a tubular extension  79  of the sleeve  65  with an upper fishing neck  80 . The design of the connection of the extension  79  to the mandrel  12  belongs to prior art technology.

Summary:
Device for carrying a replacement safety valve in a well tube, with a straddle structure for placement inside a damaged “Down Hole Safety Valve” nipple seal area. It is comprising a pair of sealing assemblies ( 15, 16 ) to be activated upon setting of the device to seal said nipple seal area, and a locking dog assembly ( 17 ) for locking the device in the well tube, said elements being carried by a tubular mandrel ( 12 ). At least one of the sealing assemblies ( 15, 16 ) comprises a expandable element ( 22; 34 ) being activated by a pair of annular press elements ( 19, 20 ).