You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
RELATED APPLICATION 
   This Application is the U.S. National Phase Application of PCT International Application No PCT/GB2006/000356 filed Feb. 3, 2006. 
   FIELD OF THE INVENTION 
   The present invention relates to a wellhead assembly and method of installing a wellhead. In particular, the assembly and method concern the positioning and locking of a casing hanger or tubing within a hanger spool while minimising the potential for development of multiple leak paths and/or minimising installation time in the field. 
   DESCRIPTION OF THE RELATED ART 
   Conventional wellhead designs employ multiple tie down bolts to fix the casing hanger in position within the hanger spool. The tie down bolts must be pressure retaining and are commonly utilised in conjunction with packing glands tied around the exterior of the wellhead to provide a more reliable seal. This arrangement gives rise to a number of potential leak paths which may be hazardous, especially in the event that high pressure gas is contained within the wellhead assembly. 
   BRIEF SUMMARY OF THE INVENTION 
   The object of the present invention is to provide an improved wellhead or casing head assembly and method for installing the same which preferably alleviates any one of or all of the problems associated with the prior art. 
   According to a first aspect of the present invention there is provided a wellhead or casing head assembly comprising:
         a first tubular having at least one aperture extending through a sidewall thereof;   a second tubular having a recessed portion in the exterior thereof and arranged to be at least partially inserted within the first tubular;   at least one fastening member wherein the or each fastening member is removably accommodated within the aperture(s) and recessed portion;   securing means for maintaining the fastening member(s) within the aperture(s) and recessed portion; and   seal means arranged to provide a fluid tight seal.       

   According to a second aspect of the present invention there is provided a method for installing a wellhead assembly comprising the steps of:—
         providing a first tubular having at least one aperture extending through a side wall thereof;   providing a second tubular having a corresponding recess in the exterior thereof;   locating the second tubular within the first tubular;   inserting a fastening member in the or each aperture and corresponding recess; and   securing and sealing the fastening member(s) within the respective aperture(s) and recess.       

   Preferably, the first tubular comprises a plurality of apertures having corresponding fastening members, most preferably two such apertures. Preferably, the at least two fastening members are selectively moveable between an engaged configuration in which they are located within both of the apertures and the recessed portion and a disengaged configuration in which they are not located within the recessed portion, but may or may not be located within the apertures. 
   A plurality of apertures can be provided in the first tubular. These can be evenly spaced around the sidewall of the first tubular. Each aperture can be provided with a respective fastening member. Preferably each aperture is arranged with its longitudinal axis substantially perpendicular to the longitudinal axis of the first and/or second tubulars. Preferably the recess is formed around the entire outer circumference of the second tubular. 
   Each fastening member can comprise a dog bolt and/or locking dog arrangement. In the engaged configuration, the or each fastening member is arranged to prevent axial movement of the second tubular with respect to the first tubular. 
   Typically the or each fastening member is/are located within the aperture when in the engaged configuration, with their longitudinal axis substantially perpendicular to the long axis of the first and/or second tubular. In this manner the or each fastening member projects from the first tubular into the recess of the second tubular and thereby presents an impediment to relative longitudinal movement occurring between the first and second tubulars. 
   The securing means can comprise a plate or flange held in place over an outer end of the fastening member. The securing means may further comprise nuts and bolts to hold the flange in place. Securing means are provided to resist movement of the or each fastening member out of the aperture(s) and recessed portion in which they are accommodated in the engaged configuration. Typically the securing means prevent unwanted radially outward movement of the fastening members from occurring whilst the fastening members are in the engaged configuration. The securing means can perform the dual function of allowing the seal means to be incorporated into the apparatus in position between the fastening member and the exterior of the first tubular. 
   Preferably, the seal means are provided to create a fluid tight seal and can seal at least a portion of the area between the securing means and the fastening member. 
   The seal means can surround the periphery of the outer end of the or each aperture in the first tubular. A groove in the exterior of the first tubular surrounding each aperture can be provided. A corresponding groove can be provided on an internal face of the securing means. Each groove can be configured to at least partially accommodate the seal means. Suitable seal means include at least one metal ring joint. In addition or as an alternative, seal means can comprise at least one rubber O-ring seal. One advantage of using annular metal seals is that, in the preferred embodiment utilising two or more fastening members, such an assembly may rely on as few as two metal to metal seals to contain pressure. 
   The first tubular can be a hanger spool and can be at least partially accommodated within a casing or wellhead. The second tubular can be a casing hanger. Annular seals can be provided to seal the exterior of the second tubular within the first tubular. At least two such seals can be provided: at least one positioned above the recesses and at least one other located below the recesses and such annular seals are preferably in the form of ‘O’ ring seals or other suitable elastomeric seals. 
   The apparatus and method of the present invention is suitable for locking and sealing any two or more adjacent tubulars within a wellbore. 
   Embodiments of the method of the present invention have significant benefits over existing methods since the time required for installation of the wellhead assembly is significantly reduced. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     One embodiment of the present invention will now be described, by way of example only, with reference to and as shown in the accompanying drawings, in which: 
       FIG. 1  is a sectional view of an assembled wellhead in accordance with the present invention; 
       FIG. 2  is a top view of a component from which locking dogs are made; 
       FIG. 3  is a perspective view of the component of  FIG. 2 ; 
       FIG. 4  is a sectional side view along the line A-A of the component of  FIG. 2 ; 
       FIG. 5  is in an enlarged view of section B shown in  FIG. 2 ; 
       FIG. 6  is a top view of eight locking dogs, two of which are used in the assembled wellhead of  FIG. 1 ; 
       FIG. 7  is an enlarged perspective view of a portion of one of the locking dogs of  FIG. 6 ; 
       FIG. 8  is an end view of a dog bolt used in the present invention; and 
       FIG. 9  is a sectional view of the dog bolt of  FIG. 8  along the line A-A. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Wellhead completion apparatus according to one embodiment of the present invention is shown generally at  30  in  FIG. 1 . The apparatus  30  includes a hanger spool  20  and a casing hanger  40  with means of securing and sealing these components with respect to one another, which are described hereinafter. 
   The apparatus  30  is shown assembled within a casing head  10  having a throughbore  16  which defines an inner diameter of approximately 30 inches (76.2 cm). The interior of the casing head  10  is provided with an upwardly projecting annular shoulder  12   a . The casing head  10  has twelve apertures  14  (two of which are shown in the sectional view of  FIG. 1 ) extending through the sidewall thereof. 
   The exterior of the hanger spool  20  is provided with a downwardly projecting annular shoulder  12   b  thereby defining a shoulder portion  22 . The angle of the downwardly projecting annular shoulder  12   b  corresponds to the angle of the upwardly projecting annular shoulder  12   a  such that on placement of the hanger spool  20  within the casing head  10 , the annular shoulders  12   a  and  12   b  are in abutment. 
   The shoulder portion  22  of the hanger spool  20  is provided with twelve V-shaped notches  26   a  in the exterior thereof. 
   The hanger spool  20  is also provided with a throughbore  28  which defines a 20 inch (50.8 cm) inner diameter. The inner diameter of the hanger spool  20  has an upwardly projecting shoulder portion  34   a.    
   The upper part of the exterior of the hanger spool  20  is provided with an enlarged portion  32 . The upper enlarged portion  32  of the hanger spool  20  has two apertures  36  (both of which are shown in  FIG. 1 ) extending through the sidewall thereof at diametrically opposite locations such that the apertures  36  are directed substantially perpendicularly along radial axes  15  with respect to the longitudinal axis  18  of the hanger spool  20 . The external opening of each aperture  36  in the upper enlarged portion  32  of the hanger spool  20  is surrounded by an annular groove  38   a.    
   Each groove  38   a  in the exterior of the hanger spool  20  can accommodate a portion of an annular metal ring joint  90 . Two flanges  134  are provided, the inner face of which has an annular groove  38   b  in which a portion of the metal ring joint  90  is accommodated. 
   The casing hanger  40  has an outer diameter which is provided with a downwardly projecting shoulder portion  34   b  for abutment with the corresponding upwardly projecting shoulder  34   a  on the interior of the hanger spool  20 . 
   The casing hanger  40  is provided with a single recess  42  extending for 3600 around the outer circumference of the casing hanger  40 , and is thus in the form of a continuous groove  42 . The recess  42  has a tapered section  102   a . Two annular seals  80 U,  80 L are provided at each upper and lower side of the recess  42  on the exterior of the casing hanger  40 . 
   The apertures  36  and corresponding recess  42  are of sufficient size to accommodate a dog bolt  50  and a locking dog  110  respectively. A component  100  from which locking dogs  110  are obtained is shown in greater detail in  FIGS. 2-5 . 
   As shown in  FIG. 2 , the component  100  is annularly shaped with eight T-shaped slots  114  in the exterior thereof. Section B shown in  FIG. 5  provides a detailed view of a portion of the component  100  having a T-shaped slot  114 . Each T-shaped slot  114  defines a rim  116 . Section A-A of  FIG. 4  shows that one side of the component  100  is provided with a tapered section  102   b . Component  100  is cut into eight separate identical segments as shown in  FIG. 6 . Each segment is a locking dog  110 , a detailed perspective view of which is shown in  FIG. 7 . 
     FIG. 8  is an end view of a dog bolt  50 . The dog bolt  50  has a hexagonal recessed end portion  52 , an annular depression  54  and a projecting rim  56 . 
   Before operational use, the apparatus  30  is assembled within the casing head  10  where the casing head  10  is already located at the opening of a borehole (not shown). The casing head  10  acts as an annular rim to line the mouth of the borehole. The hanger spool  20  is inserted in the throughbore of the casing head  10  until the downwardly projecting shoulder portion  12   b  abuts the upwardly projecting shoulder portion  12   a . Lock down bolts  70  are provided having a V-shaped end portion  26   b  which corresponds with the V-shaped notches  26   a  in the enlarged portion  32  of the hanger spool  20 . Each aperture  14  in the casing head  10  has one lock down bolt  70  inserted therein and extending therethrough. Each bolt  70  is held in place by a corresponding heavy duty nut  140 . 
   The head  56  of each dog bolt  50  is accommodated by T-shaped slots  114  of the locking dog  110 . In this configuration, the rim  116  of the locking dog  110  is positioned in the annular depression  54  of the dog bolt  50  which prevents each locking dog  110  and dog bolt  50  from being pulled apart. The assembled locking dogs  110  and dog bolts  50  are inserted in each aperture  36 . The locking dogs  110  and attached dog bolts  50  are then retracted to allow the casing hanger  40  to be landed within the throughbore  28  of the hanger spool  20 . The hanger spool  20  is now ready to receive the casing hanger  40  whenever the operator requires to run a casing string into the borehole and suspend it from the casing hanger  40 . The operator will lower the casing hanger  40  into the bore  28  of the hanger spool  20  until the upwardly facing annular shoulder  34   a  of the hanger spool  20  abuts the downwardly projecting annular shoulder  34   b  of the casing hanger  40  and thus downwards movement of the casing hanger  40  is arrested. In this position the recessed portion  42  is aligned with the apertures  36  in the hanger spool  20 . 
   Once the casing hanger  40  is in position, the locking dogs  110  are moved radially inwardly such that they are positioned in the recess  42  provided in the casing hanger  40 . The tapered section  102   b  of each locking dog  110  abuts the tapered portion  102   a  of the recess  42 . Locking screws  150  are inserted into the recessed end portions  52  having a hex socket head and are used to maintain the dog bolts  50  in position. 
   Metal ring joints  90  are placed in annular grooves  38   a . Flanges  134  are positioned to cover each aperture  36  having the dog bolts  50  and locking screws  150  inserted therein, such that the groove  38   b  on the inner face of each flange  134  accommodates a portion of metal ring joint  90 . Flanges  134  have holes extending therethrough (not shown). Studs  60  are inserted within the holes in each flange  134  and locked in place by nuts  120 . 
   Accordingly, the flanges  134  provide a means of securing the locking dogs  110  in the engaged position in that they prevent the dog bolts  50  and thus the locking dogs  110  from moving radially outwardly. Furthermore, the locking dogs  110  provide a means of releasably fastening the casing hanger  40  to the hanger spool  20  in that the locking dogs  110  prevent any undesirable upward movement of the casing hanger  40  with respect to the hanger spool  20 . 
   In use hydrocarbons often at high pressure are produced up various conduits in the wellbore. Accordingly the wellhead assembly is required to withstand high pressures without the risk of potentially dangerous leak paths developing. 
   The apparatus  30  employs various means to prevent the development of leak paths. Annular seals  80  provide a seal between the hanger spool  20  and casing hanger  40 . Additionally, annular metal seals  90  are pressure retaining by virtue of their position in the grooves  38   a  and  38   b  in the exterior of the hanger spool  20  and flange  134  respectively. 
   Modifications and improvements may be made to the embodiments hereinbefore described without departing from the scope of the invention.

Summary:
The invention provides a wellhead and a method for installing a wellhead assembly. The wellhead or casing head assembly comprises a first tubular having at least one aperture extending through a side wall thereof. The wellhead or casing head also comprises a second tubular having a recessed portion in the exterior thereof. The second tubular is arranged to be at least partially inserted within the first tubular. The wellhead or casing head assembly also comprises at least one fastening member. The fastening member is removably accommodated within the aperture-(s) and recessed portion. The wellhead also comprises securing means for maintaining the fastening member-(s) within the aperture-(s) and recessed portion. The wellhead further comprises seal means arranged to provide a fluid tight seal.