Abstract:
An expander for radially expanding a tubular element by axial movement of the expander through the tubular element is provided. The expander comprising an expander member having a front part of a first cross-sectional size; a rear part of a second cross-sectional size larger than the first cross-sectional size; and an intermediate part arranged between said front part; and, rear part and having a cross-sectional size varying between said first and second cross-sectional sizes. The expander member is provided with fluid supply means for supplying pressurized fluid to the inner surface of the tubular element at a location opposite the intermediate part when the expander member is arranged in the tubular element.

Description:
FIELD OF THE INVENTION 
     The invention relates to an expander for radially expanding a tubular element by axial movement of the expander through the tubular element, and to a method of radially expanding a tubular element. 
     BACKGROUND OF THE INVENTION 
     Radial expansion of tubular elements has been applied, for example, in wellbores whereby a tubular casing is lowered into the wellbore in unexpanded state through one or more previously installed casings. After the casing is set at the required depth, an expander is moved through the casing to radially expand the casing to an inner diameter which is about equal to the inner diameter of the previously installed casing(s). In this manner it is achieved that the inner diameters of subsequent casings are about equal as opposed to conventional casing schemes which have stepwise decreasing casing diameters in downward direction. 
     A problem of expanding such tubular elements is the large force required to move the expander through the tubular element. Furthermore, in case the expander is moved through the tubular by applying fluid pressure at the side of the large diameter part of the expander there is a danger of burst of the tubular element when the high fluid pressure exceeds the burst pressure of the tubular element. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided an expander for radially expanding a tubular element by axial movement of the expander through the tubular element, the expander comprising an expander member having a front part of a first cross-sectional size, a rear part of a second cross-sectional size larger than the first cross-sectional size, and an intermediate part arranged between said front part and rear part and having a cross-sectional size varying between said first and second cross-sectional sizes, wherein the expander member is provided with fluid supply means for supplying pressurised fluid to the inner surface of the tubular element at a location opposite said intermediate part when the expander member is arranged in the tubular element. 
     The method of the invention comprises:
     a) moving an expander in axial direction through the tubular element, the expander including an expander member having a front part of a first cross-sectional size, a rear part of a second cross-sectional size larger than the first cross-sectional size, and an intermediate part arranged between said front part and rear part and having a cross-sectional size varying between said first and second cross-sectional sizes;   b) simultaneously with step a), supplying pressurised fluid to the inner surface of the tubular element at a location opposite said intermediate part.   

     It is thereby achieved that the contact forces exerted by the expander member to the inner surface of the tubular element are supplemented by fluid pressure acting on said inner surface. As a result the required contact forces necessary to expand the tubular element are lowered compared to the situation whereby the contact forces are not supplemented by fluid pressure, and consequently the forces required to move the expander through the tubular element are also lowered. Furthermore, if the expander is moved through the tubular element by the action of fluid pressure in the tubular element, a lower fluid pressure is required to achieve the required movement. 
     Suitably the expander member is arranged to be moved through the tubular element by the action of fluid pressure of a body of fluid acting on said rear part of the expander member, and wherein the fluid supply means includes a fluid passage providing fluid communication between said body of fluid and the inner surface of the tubular element at said location. It was found that the required fluid pressure is lower than in a situation whereby the contact force is not supplemented by fluid pressure, despite the smaller effective area on which the fluid pressure acts to move the expander forward. 
     Suitably the front part of the expander member is provided with sealing means arranged to seal the front part relative to the inner surface of the tubular element. 
     The sealing means can, for example, be applied in case the expander is moved forward by the action of fluid pressure in the tubular element, and whereby the expander includes different elements movable relative to each other between a retracted position in which said rear part has a cross-sectional size smaller than said second cross-sectional size and an expanded position in which the rear end part has said second cross-sectional size. Such expander is sometimes referred to as an expandable cone. Since the clearances between the different elements allow fluid to flow to the inner surface of the tubular element opposite said intermediate part, no other fluid supply means are then required. 
     Suitable sealing means are a ceramic seal, a labyrinth seal or a hard metal seal. 
     The expander member can, optionally, be cone-shaped. Furthermore, the expander member can be provided with rollers arranged to roll along the inner surface of the tubular element during expansion thereof. 
     Typical applications for tubular elements to be expanded are a wellbore tube, a line pipe and a surface pipe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a longitudinal section of an embodiment of an expander according to the invention; 
         FIG. 2  schematically shows a longitudinal section of an alternative embodiment of an expander according to the invention; and 
         FIG. 3  schematically shows a side view of a cone member of the alternative embodiment. 
         FIG. 4  schematically shows a longitudinal section of an alternative embodiment of an expander according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1  is shown a tubular element in the form of a steel casing  1  extending into a wellbore  2  drilled into an earth formation  4 . The casing  1  has an unexpanded section  6  of inner diameter D 1 , a radially expanded section  8  of inner diameter D 2  larger than D 1 , and an intermediate section  10  located between the unexpanded section  6  and the expanded section  10  and having a diameter varying from D 1  to D 2 . 
     A cone-shaped expander  12  is positioned in the casing  1 , the expander having a front part  14  arranged in the unexpanded casing section  6 , an intermediate part  16  arranged in the intermediate casing section  10  and a rear part  18  arranged in the expanded casing section  8 . The outer diameter of the front part  14  is substantially equal to D 1 , and the outer diameter of the rear part is substantially equal to D 2  minus any surplus expansion of the casing  1  (which can be up to 3%). 
     The expanded casing section  8  is filled with a body of wellbore fluid  20 , and the unexpanded casing section  6  is filled with a body of wellbore fluid  22 , whereby the fluid pressure in the body of fluid  20  is significantly larger than the fluid pressure in the body of wellbore fluid  22 . 
     The expander  12  is provided with a number of fluid passages  24  which provide fluid communication between the body of fluid  20  and the inner surface of the intermediate casing section  10  opposite the intermediate part  16 , at regular circumferential intervals. Furthermore, the front part  14  of the expander  12  is provided with an annular seal  26  of ceramic material and of outer diameter substantially equal to D 1 . The seal  26  substantially prevents leakage of fluid from the high pressure body of fluid  20  to the low pressure body of fluid  22 . 
     Referring to  FIG. 2  there is shown a longitudinal section of an alternative expander  30  for expanding the casing  1 , which includes an annular assembly  31  consisting of, in subsequent order, a cone member  32 , a centraliser  34 , a spacer bushing  36  and an annular seal  38 . The annular assembly  31  is held together by a shank  40  having a head  42  at one end thereof and a threaded end portion  44  provided with a nut  46  at the other end thereof. The outer diameter of the annular seal  38  and the centraliser  34  is about equal to the inner diameter of the casing  1  before expansion thereof. 
     Referring further to  FIG. 3 , the cone member  32  is formed of a body  48  tapering from a large diameter end  50  to a small diameter end  52  and provided with a circumferential groove  54  arranged in the tapered surface  55  of the body  48  at an axial position about midway the large diameter end  50  and the small diameter end  52 . The body  48  is furthermore provided with a number of regularly spaced axial grooves  58  arranged in the tapered surface  55 , whereby each axial groove  58  crosses the circumferential groove  54 . A number of fluid passages  60  are provided in the body  48  so as to provide fluid communication between the large diameter end  50  of the body  48  and the circumferential groove  54 . 
     During normal operation of the expander  12  shown in  FIG. 1 , the casing  1  is lowered in unexpanded state into the wellbore  2  whereafter the expander  12  is inserted into the casing  1  at an end thereof, which can be either the upper end or the lower end. Subsequently a relatively high fluid pressure is applied to the body of fluid  20 . As a result the expander is forced to moved in the direction of arrow  30  thereby exerting a radially outward contact force to the inner surface of the intermediate casing section  10 . Said contact force is supplemented by the high fluid pressure which is transmitted from the body of fluid  20  through the passages  22  to the inner surface of the intermediate casing section  10 . The casing  1  is thereby expanded from inner diameter D 1  to inner diameter D 2 . Leakage of fluid from the body of fluid  20  along the expander  12  to the body of fluid  22  is substantially prevented by the seal  26 . 
     It was found that the required fluid pressure in body of fluid  20  necessary to move the expander  12  through the casing  1  is significantly reduced compared to the situation whereby the expander is not provided with the fluid passages  24 . It is believed that this result is due to the contact force from the expander  12  being supplementing by the high fluid pressure acting against the inner surface of the intermediate casing section  1 , and also the lubricating effect of the fluid between the expander  12  and the casing  1 . 
     Normal operation of the expander  30  of  FIGS. 2 and 3  is substantially similar to normal operation of the expander of  FIG. 1 . The expander is moved through the casing  1  by high fluid pressure applied to the expander  30  at the side of the large diameter end  50 . The radially outward contact force exerted to the inner surface of the casing  1  by the cone member  32  is supplemented by the high fluid pressure which is transmitted from the large diameter end  50  to the inner surface of the casing  1  via the fluid passages  60 , circumferential groove  54  and axial grooves  58 . The annular seal  38  substantially prevents leakage of fluid along the expander  30 . 
     Referring to  FIG. 4 , there is shown a longitudinal section of an alternative embodiment of an expander  12  in wellbore  2 . In this embodiment, the expander  12  is provided with rollers  70  arranged to roll along the inner surface of the tubular element (unexpanded casing section  6 , expanded casing section  8 , or intermediate section  10 ) during expansion. Many of the same features from  FIG. 1  are shown. 
     It is to be noted that, in general, the cone member will be in tight contact with the tubular element at two annular contact areas, one near the small diameter end of the cone member and the other near the large diameter end of the cone member. Optimally, the arrangement of the fluid passages should be such that the high fluid pressure is delivered to the inner surface of the tubular element at an axial position inbetween such annular contact areas. Since there will be a tight contact between the cone member and the tubular element at the annular contact areas, the annular contact areas act as seals whereby the contact area near the small diameter end prevents leakage of fluid along the expander. The annular seal at the front end of the expander can therefore optionally be omitted. This is also applicable to cone-shaped expanders in a more general sense. 
     In some applications it can be advantageous to apply an expander member in the form of an expandable cone which can be inserted into the tubular element at a relatively small diameter, and thereafter be expanded to a larger diameter when expansion of the tubular element starts. Since such expandable cone, generally, has separate parts which are movable relative to each other, it is difficult to pump the expandable cone through the tubular element in view of leakage of fluid along such separate parts. By arranging sealing means at the front end part of the expandable cone it is achieved a) that the leakage problem has been overcome and b) that the radially outward contact force between the expander and the inner surface of the tubular element is supplemented by the high fluid pressure acting on said inner surface. In such application, the fluid supply means is simply formed by the clearances between the separate parts of the expander member. 
     Furthermore, the application of a seal at the front part of the expander member allows the application of one or more rollers at the expander member, arranged to roll along the inner surface of the tubular member during the expansion process, in combination with pumping of the expander through the tubular element.