FIELD OF THE INVENTION 
       [0001]    The present invention in general relates to an improved well head system and in particular to an improved mechanism for locking a valve stack atop a well head, on beam members of the well template. The valve stack may be a Blow Out Preventer (BOP) and according to the invention, by virtue of this locking, the effect of bending moment on well head by the BOP and a riser connected to the BOP is substantially prevented. Particularly, the present invention relates to a well head system according to the preamble of claim  1  and to a locking device according to the preamble of claim  7 . 
       TECHNICAL BACKGROUND OF THE INVENTION 
       [0002]    Well head systems for sub sea exploration are traditionally known to comprise a well head having a well head housing secured to a well casing. It also comprises a valve stack, such as a Blow Out Preventer (hereinafter referred to as BOP), located atop a well head during drilling, work-over operations and some phases of the production. Especially, during drilling operations, the drill bit often penetrates pockets of pressurized formations. Due to this, the well bore experiences rapid increase in pressure and unless prevented may result in disastrous blow outs. Hence locating BOPs atop well heads is now very common and indispensable in sub sea exploration. 
         [0003]    Now, tubular members such as risers are connected on the top of the well head housing along the through bore of a BOP. The well head housing is in turn secured to the well head casing by welding. When a riser is connected and operated on the top of the well head housing, it creates a very high bending moment on the connecting surface of the lower part of the well head housing and the upper part of the casing, i.e. at the welded joint area. As a result, the well head and casing experiences strain causing substantial fatigue and may eventually initiate cracks on the well head, thereby expediting its deterioration. 
         [0004]    In a sub-sea drilling operation the connection of the well head housing and well head casing has to endure stress for 5000 days of the BOP and riser being connected, e.g. during work-over operation and this fairly indicates the amount of strain the well head has to withstand due to bending moment generated during riser operation with a safety factor of 10. 
         [0005]    Now to ensure that the well head does not undergo fatigue and tear by bending moment generated during riser operation, it is essential that the BOP should be firmly locked so that less moment is transferred to the weld zone between the well head housing and the casing. This is also essential to ensure that there is no risk of blow out by virtue of a break in the weld between the well head housing and the casing. 
         [0006]    Attempts are on over the years to appropriately secure BOPs on well heads to prevent well blow outs, but in prior art technology the approach to ensure firm locking of the BOP on the well head components, with a motive to substantially is prevent the effect of bending moment on the lower part of the well head housing and the upper part of the casing during operation of tubular members such as risers, along BOP, is missing. 
         [0007]    To be precise, the prior art does not teach locking of a BOP firmly on the well head components, such as the well template, to prevent the well head from movement due to bending moment generated during riser operation, so that fatigue of the well head is substantially reduced during riser operation. 
         [0008]    Hence, the issue of withstanding heavy bending moment on the welded area of the housing-casing joint during riser operation and fatigue of the welded joint area still remains unresolved. This consequently, leaves the problem of minimising/nullifying fatigue of the well head and a potential risk for cracks in the joint area, unresolved. The worst eventuality of this can of course be that the well head disconnects from the casing and results in an uncontrollable blow-out. 
         [0009]    Accordingly there was a long felt need for a locking technology for locking valve stacks, such as BOPs atop a well head on the well template, so that the effect of bending moment on the well head is substantially reduced. 
         [0010]    The present invention meets this long felt need by locking the BOP on beam members of the well template, by providing specially configured locking devices suitably located on axially movable vertical telescopic arms, the arras being positioned along the vertical supporting columns of the BOP. 
       OBJECTS OF THE INVENTION 
       [0011]    The primary object of the present invention is to provide a well head system which is capable of substantially reducing the effect of bending moment/stress experienced on its welded joint area during riser operation. 
         [0012]    It is yet another object of the invention to provide a BOP atop a well head, which is equipped with a specially configured locking mechanism to substantially prevent the well head from movement due to bending during riser operation through the BOP. 
         [0013]    It is a further object of the present invention to provide a locking mechanism having a plurality of locking devices for locking a BOP on beam members of the well template, so that the effect of high bending moment on the lower part of the well head housing and the upper part of casing is substantially reduced. 
         [0014]    It is a further object of the present invention to minimise/nullify fatigue of the well head and the potential risk for cracks in the well head housing—well casing joint area, during riser operation. 
         [0015]    It is a further object of the present invention to reduce the risk of blow out during riser operation. 
         [0016]    It is yet a further object of the invention to provide a well head system which conforms to the regulatory criteria and safety standard in well drilling processes. 
         [0017]    All through the specification including the claims, the words “BOP”, “riser”, “spindle”, “columns”, “frame”, “beam member”, “clamping arms”, “winching device”, “ROV”, “well template” are to be interpreted in the broadest sense of the respective terms and includes all similar items in the field known by other terms, as may be clear to persons skilled in the art. Restriction/limitation, if any, referred to in the specification, is solely by way of example and understanding the present invention. Furthermore, the description and claim refers to operation of risers and it is hereby clarified that the present invention is equally applicable in respect of operation of other members operated atop sub sea well heads, as will be clear to persons skilled in the art. 
       SUMMARY OF THE INVENTION 
       [0018]    According to a first aspect of the present invention there is provided a well head system for application in sub sea well exploration. It comprises a well head having a well head housing secured to a well casing and at least one valve stack, e.g. a BOP located atop the well head. According to the invention, the valve stack is removably locked on a well template supporting the well head, by a plurality of locking devices. 
         [0019]    According to a preferred embodiment of the first aspect of the present invention each locking device comprises a spindle fixedly attached to a telescopic arm. It is adapted to axially move downward and upward with corresponding axial movement of the telescopic arms for locking and unlocking respectively. 
         [0020]    Preferably, two opposite clamping arms are adapted to grip a beam of he well template. 
         [0021]    More preferably, the lock comprises a securing mechanism acting to lock a main frame carrying the clamping arms to a spindle. 
         [0022]    According to a second aspect of the present invention there is provided a locking device for securing a valve stack atop a well head having a well head housing secured to a well casing. According to the invention, the locking device is adapted to releasably lock the valve stack to a well template supporting the well head. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Having described the main features of the invention above, a more detailed and non-limiting description of some exemplary embodiments will be given in the following with reference to the drawings, in which 
           [0024]      FIG. 1  is a perspective view of a BOP according to a preferred embodiment of the present invention. 
           [0025]      FIG. 2  is an illustrative view of the telescopic arm of the BOP having a winch device according to a preferred embodiment of the present invention. 
           [0026]      FIG. 3  is a front view of the telescopic arm shown in  FIG. 2 . 
           [0027]      FIG. 4  is a sectional view of the telescopic arm shown in  FIG. 3  along the line A-A. 
           [0028]      FIG. 5  is a perspective view of the BOP according to the present invention in operation showing the well head components, including a well template, the well head and the location of the locking apparatus. 
           [0029]      FIG. 6  is a perspective view of a preferred embodiment of the locking apparatus according to the present invention in locked position. 
           [0030]      FIG. 7  is an axial cut section along the vertical axis of the device illustrated in  FIG. 6  for the sake of understanding. 
           [0031]      FIGS. 8 to 10  coherently illustrate the different positions of the locking apparatus during operation. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    The following describes a preferred embodiment of the invention which is exemplary for the sake of understanding the present invention and non-limiting. 
         [0033]    The main aim of the present invention, as stated before, is to substantially reduce the bending moment during riser operation on the lower part of the well head housing (not shown in  FIG. 1 ) and the upper part of the casing (not shown in  FIG. 1 ), where the welding joint between the two is located. This is achieved primarily by firmly locking the BOP on the well template by specially configured locking devices, at several points along the supporting beams of the well template during riser operations, as hereinafter explained with reference to the drawings. This in turn facilitates reducing the effect of bending moment on the well head during riser operation, thereby increasing its longevity. By reducing the effect of such bending moment, fatigue of the well head and the potential risk for cracks in the well head housing—well casing joint area during riser operation, is substantially minimised/nullified. This in turn also reduces the possibility of most unprecedented eventuality of disconnection of the well head from the casing, resulting in an uncontrollable blow-out. 
         [0034]      FIG. 1  illustrates a BOP assembly  1  including a Christmas tree  6  and room for a BOP stack (not shown) within a BOP frame  2  which is located atop a well head  23  (best shown in  FIG. 5 ). It comprises vertical beam members  5  along which are positioned axially movable vertical arms  9 , which are preferably telescopic having one upper portion and a lower portion, the lower portion being slidable through the upper portion. This is clear from  FIG. 1 . The locking devices  7  are located along the slidable lower portion of the arms  9 . The BOP  1  rests on the well head  23  (best shown in  FIG. 5 ). As known to persons skilled in the art, the christmas tree  6 , at the basal portion atop the well head  23  (shown in  FIG. 5 ) may or may not be there. Tubular members such as risers (not shown) are connected to the BOP. The telescopic arms also comprise a suitably located winch device  10  for axial movement of the locking device  7 . As can be seen from  FIG. 1  the locking device  7  locks the BOP on horizontal beams  3 ,  4  of the well template (best shown as item  15  in  FIG. 5 ). These locking devices are effective in firmly locking the BOP along several points on the well template, during riser operation, for achieving the objects of the present invention, as described hereinbefore. 
         [0035]    The axially moving telescopic arms  9  is further illustrated in  FIGS. 2 ,  3  and  4 , showing one such arm. A winch device  10  is suitably located on the telescopic arm  9  for facilitating its axial movement in upward direction by winching action, as will be understood by persons skilled in the art. The winch has a cable arrangement  11 , as shown in the accompanying  FIG. 3 . This arrangement facilitates withdrawal of the lower portion of the telescopic arm in upward direction, along which the locking devices are located. 
         [0036]      FIG. 4  is a sectional view taken along the line A-A in  FIG. 3  which preferably shows several handles  13   a,    13   b  and  13   c.  Each handle is pre-tensioned by a spring  14  and acts against a stop plate  12  on the telescopic arm  9 . The pair of handles  13   a  are pulled preferably by an ROV, so that the lower portion of the telescopic arm  9 , having the locking devices, falls downward, thus employing the locking devices  7 . 
         [0037]    It would be clear from the accompanying  FIG. 1 , that the locking device  7  is located at the lower portion of the telescopic arm  9  and is lowered on the well head components by downward and axial movement of the telescopic arm  9 . How this movement is caused, has been explained in the concluding portion of the preceding paragraph. This mechanism of employing the locking devices works irrespective of the distance between the well template and the initial position of the arms  9 . The locking devices are also adapted to function irrespective of this distance. The handles  13   c  are preferably applied to hold up the lower portion of the telescopic arm  9 , having the locking devices  7 . The handles  13   b  are preferably applied, for parking the telescopic arms, when not in use. 
         [0038]      FIG. 5  illustrates four well heads  23  and a BOP on top of one well head. It also shows a well template  15  which supports the well head and along which the locking devices  7  are connected at different points on the well template  15 . As known to persons skilled in the art, the well template rests on the sea bed in deep sea drilling projects, for supporting the well head. The well template  15  is preferably supported on the supporting columns, such as suction anchors  16 . The locking devices are landed on the well template in the manner as stated before which involves a simple and effective operation irrespective of the distance, but landing them correctly, is very crucial. This may be done, for example, from the deck of an offshore vessel. 
         [0039]    The locking device  7  as shown in  FIG. 6  comprises a spindle  17  partially housed in a hydraulic cylinder  17 ′, as shown in this figure. It also comprises clamping arms  19 , a main frame  21 , two guard members  20  running from end to end of the clamping arms  19  on either side, hinged levers  18  (only one set shown), operable with either of the clamping arms  19 . The spindle  17  is fixed on a column  22  at the lower end of the telescopic arm  9 , which is movable axially with the axial movement of the corresponding telescopic arm  9 . As shown in  FIG. 1  several locking devices  7  are located along several points, near well template  15 . All such locking devices lock the BOP on the well template  15  along several points on the template  15 . Consequently, there is a firm grip which disallows/substantially prevents the BOP from movement due to bending during riser operation. The  FIG. 6  shows the locking device in locked position. As stated before, perfect locking is achieved by this technology, irrespective of the distance between the column  22  and the well template  15 . 
         [0040]    The  FIG. 7  is an axial cut section along the vertical axis of the device illustrated in  FIG. 6  for the sake of understanding. It shows some of the important features by virtue of which, the locking device grips the well template  15  after landing on the same. The spindle  17  is equipped with outer threads  24 . 
         [0041]    An inner wedge portion  26  has inner threads  25  which are adapted to mesh with the threads  24  of the spindle  17 . There also exists outer wedge shaped portion  27  along the outer portion of the inner sleeve  26 . How these portions contribute to effective locking, is explained hereinafter. 
         [0042]    Now the operation of the locking device  7  is explained with reference to  FIGS. 8 to 10 . These figures, as can be seen show different operational positions of the locking device and these figures represent an axial cut section along the vertical axis of the device illustrated in  FIG. 6  for the sake of understanding. 
         [0043]      FIG. 8  shows a position when the locking device is yet to be locked on the template  15 . This figure also clearly shows the different chambers in the hydraulic cylinder  17 ′ and how the spindle  17  is attached to the column  22 . Ideally, the spindle  17  is attached via a spherical ball bearing  22 ′. This allows the spindle to move and allow for taking up any misalignments. The other identical reference numerals represent identical features as in  FIG. 7 . 
         [0044]      FIG. 9  shows a position where the column  22  has come down and landed the locking device  7  on the template beam  15 . The abutment against the template beam presses the supporting frame  21  upwards. Thereby, the hinged levers  18  act to swing the clamping arms  19  downwards so that they come to rest against the template beams and grips around these. The hydraulic cylinder is powered by hydraulic pressure from a hydraulic fluid. As can be seen from the  FIGS. 8 to 10  the cylinder has a bottom chamber  32  and an upper chamber  33 . In the hydraulic cylinder  17 ′ there is also a piston  30 , which is pre-tensioned in the downward direction by a spring  31 . A hydraulic pressure in the upper chamber of the hydraulic cylinder  17 ′ acts against the spring  31 , so that the piston  30  is in its uppermost position when the clamping arms are being actuated for gripping. 
         [0045]    The hinged levers  18  actually act as leaf springs and those act to force the clamping arms  19  inwardly when the distance between the main frame  21  and the column  22  is reduced due to the main frame  21  pressing down on the beam  3 ,  4  and thereby being pushed upward. The leaf spring  18  may have one arm only and having at least two arms is not mandatory. 
         [0046]    In  FIG. 10  the clamping arms  19  have now closed by means of the hinged levers  18  and the grip on the template  15  is completed. As stated in the preceding paragraph, the hinged levers  18  play the role of leaf springs to force the clamping arms  19  inwardly. The guard member  20  ensures that the gripper assumes the correct position on the template beam. When the clamping arms  19  have clamped the beam  3 ,  4  of the template  15 , the hydraulic pressure in the hydraulic cylinder  17 ′ is released and the spring  31  actuates the lock by pushing the piston downward. The piston presses against the outer wedges  27  via pins  34  and thereby forces the outer wedges downward. The outer wedges  27  press radially against and forces the inner wedges  26  inward until their inner threads  25  mesh with the outer threads of the spindle  17 . The inner and outer wedges thereby fixes the spindle  17  relative to the main frame  21 , preventing the main frame  21  from moving. Thereby the spring action from the levers  18  maintains their force on the clamping arms  19  and prevents these from swinging upwards again. 
         [0047]    Similar locking takes place along all points on the beam where respective locking devices are located and so, a firm locking of the BOP on beam  15  supporting the well head is achieved. This ensures substantial prevention of the well head from movement due to bending during riser operation with the BOP, thereby reducing the fatigue and risk of failure of the well head and increasing its lifespan. 
         [0048]    As explained in the preceding paragraphs, the securing of the lock is largely effected by the hydraulic cylinder  17 ′, the spring member  31 , the piston  30 , the inner and outer wedges  26 ,  27  and the spindle  17 . The details of the spring member and the piston arrangement have not be illustrated in detail in the drawings, but a person of skill will have no problem understanding how this works in principle. It should be understood to persons skilled in the art, particularly with reference to the description of  FIGS. 8 ,  9  and  10  that securing of the gripping of the well template  15  by the clamping arms  19  take place by a spindle-cam mechanism. This spindle cam mechanism involves mutual operation of the spindle  17 , the spring member and the piston arrangement of the hydraulic cylinder  17 ′, the spring leaves  18  and the clamping arms  19 . All these coherently facilitate, clamping the BOP  1  firmly on the template  15  by the locking devices  7 . During unlocking of the BOP, the hydraulic pressure is applied to the hydraulic cylinder  17 ′ opposite to the spring member and the locking devices just operate in the opposite way as will be understood to persons skilled in the art. 
         [0049]    The present invention has been described with reference to some preferred embodiments and some drawings for the sake of understanding only and it should be clear to persons skilled in the art that the present invention includes all legitimate &gt;modifications within the ambit of what has been described hereinbefore and claimed in the appended claims.
A well head system for application in sub sea well exploration comprising a well head ( 23) having a well head housing secured to a well casing, at least one valve stack, e.g. A BOP ( 1) located atop said well head ( 23). The valve stack is removably locked on a well template ( 15) supporting said well head by a plurality of locking devices ( 7). Also described is a locking device comprising two opposite clamping arms ( 19) hingedly attached to a main frame ( 21). The Main frame is slidable relative to a spindle ( 17) and can be selectively secured to the spindle ( 17).