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BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates generally to apparatuses for the drilling of oil and gas wells from barges or other vessels, and more particularly to drilling substructures which can be moved to drill two or more wells without relocation of the barge. 
     II. Description of Prior Art 
     In the oil and gas industry, the terrain above the suspected location of hydrocarbon products is largely determinative of the type of machinery used to drill the necessary wells. Onshore drilling operations generally require the least amount of preparation and expense, whereas as deep offshore locations typically require massive underwater frames to support the drilling operations above the water. In most cases, the drilling platform is relatively stationary. However, there are many instances, such as in lakes, swamps and other shallow water areas, in which the terrain allows for the use of barge drilling vessels. The two major classes of drilling barges are bay (or swamp) barges and posted barges. Both types of drilling barges are self-contained drilling systems which can be floated to a desired location by a tugboat or similar means and caused to be stabilized with respect to the ground. In the case of posted barges, three or more vertical posts are jacked downwardly into the mud below the surface of the water until the entire barge and its associated drilling platform are stabilized. In the case of bay barges, the ballasting system of the barge is manipulated until the bottom of the barge rests firmly on the bottom of the river or bay. In either type of drilling barge, once the barge is properly stabilized, the derrick is hingeably raised to its operating position, and the well can then be drilled through a keyway formed into the drilling barge below the drilling platform. After the well is drilled, the barge is moved away from the site so that other activities related to production and completion of the well can be conducted. In this manner, the barge can be moved relatively easily from one drilling site to the next by simply floating it to another location. However, there are distinct disadvantages to the requirement of moving the barge in order to drill another well. 
     Moving the barge requires complete cessation of drilling activities, lowering the derrick, and conducting numerous preparatory tasks prior to moving the vessel. Of course, the entire drilling system must again be placed into its operational configuration once the barge is relocated, consuming even greater time and expense prior to the actual drilling process. In addition to concerns over time and expense, relocation of the barge is sometimes impossible or ill advised, depending upon the underlying terrain or numerous other factors. A particularly poignant example of a situation in which relocation of the barge would be undesirable is in drilling operations in certain parts of Nigeria. Many of the drilling sites in that country are along small rivers and streams having exceedingly soft bottoms. The narrow spaces within which the barge must operate make it difficult to reposition the barge for each well to be drilled. Also, the soft river bottoms require more careful and time-consuming ballasting and deballasting of the vessel than in other environments to ensure a stable platform for the drilling operations. Relocation of the barge under these conditions, therefore, is at best an expensive undertaking, and at worst an impossible task. 
     Finally, the need for frequent relocation of the barge makes it difficult to maintain an adequate security perimeter around the barge. In Nigeria, as well as in other third world countries experiencing political unrest, such drilling operations are a frequent target of vandalism and terrorism. Consequently, it is often necessary to construct fences or barricades around the barge to prevent unauthorized access to the drilling equipment and personnel. Such security measures must often be dismantled and reconstructed during the relocation of the barge, thereby jeopardizing valuable equipment and crew members during these windows of vulnerability. 
     Based upon the aforementioned problems associated with relocation of the barge, there is a distinct need in the industry for a barge drilling system which would permit the drilling of two or more wells while the barge remains stabilized at a single location. In such a proposed system, the barge would remain in a particular location, while the drilling substructure is moved relative to the barge to drill one or more wells in a pattern. Ideally, after a first well is drilled, production and completion operations can be conducted on the first well, simultaneous with the drilling of a second or subsequent well adjacent to the first drilled well. The movable substructure would include a hoisting system suspended from underneath the drill floor, as well as a skate system at the base of the substructure, for easily moving heavy equipment, such as blow out preventers (BOP&#39;s) to and from the wells that have been drilled. Because the barge would remain situated during the maintenance of previously drilled wells, a protective deck would also be employed above the well protect structure for use by crews around those wells undergoing production. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a movable substructure for use in drilling wells which includes the capability to reposition the horizontal location of the substructure relative to a stationary base. 
     It is also an object of the present invention to provide a movable substructure for use in drilling which allows the drilling of two or more wells without requiring a relocation of the stationary base. 
     It is a further object of the present invention to provide a movable substructure for use in drilling which can be used in connection with a drilling barge or other buoyant vessel. 
     Another object of the present invention is to provide a movable substructure for drilling that is sufficient in height to allow for simultaneous drilling of one well during the maintenance of one or more previously drilled wells. 
     A further object of the present invention is to provide a movable substructure for drilling that includes a clamping system for securing the position of the movable substructure relative to the stationary base between repositioning steps. 
     Still another object of the present invention is to provide a drilling barge having reinforcing structural components to accommodate the shifting weight of the movable substructure between the drilling of two or more wells. 
     Another object of the present invention is to provide a drilling barge having a keyway sufficient in structure and dimension to allow the drilling of at least six wells. 
     These and other objects and advantages of the present invention will no doubt become apparent to those skilled in the art after having read the following description of the preferred embodiments which are contained in and illustrated by the various drawing figures. 
     Therefore, in a preferred embodiment, a drilling vessel for selectively drilling two or more wells from a single location of said vessel is provided, comprising a buoyant hull having a deck, including ballasting means in the hull for allowing the vessel to be stabilized on the bottom surface of a body of water; a movable substructure in contact with the deck, wherein the movable substructure includes a drilling platform having a plurality of downwardly extending support legs attached thereto, wherein the height of the drilling platform from the deck is sufficient to allow simultaneous drilling and production activities to occur, and repositioning means, operatively connected between the support members and the deck, for moving the drilling platform in a predetermined direction relative to the deck. In a preferred embodiment, the repositioning means comprises a transverse repositioning device for movement of the drilling platform in a port or starboard direction, and a longitudinal repositioning device for movement of the drilling platform in a bow or stem direction. Furthermore, in a more preferred embodiment, the longitudinal repositioning device is connected to the support legs of the drilling platform, and it resides above and moves relative to the transverse repositioning device. 
     The transverse repositioning device includes at least two horizontal and parallel transverse beams in contact with the deck, while the longitudinal repositioning device includes at least two horizontal and parallel longitudinal beams connected to opposing pairs of the support legs of the drilling platform, wherein the longitudinal beams are parallel to and in contact with the transverse beams. The transverse repositioning device includes a plurality of lift and roll transverse jacking assemblies operatively in contact between the transverse beams and the deck, while the longitudinal repositioning device includes a plurality of lift and roll longitudinal jacking assemblies operatively in contact between the longitudinal beams and the transverse beams. 
     Preferably, each of said transverse jacking assemblies includes a mounting portion in contact with the deck; a jacking element connected to the mounting portion, wherein the jacking element is adapted to lift the transverse beam in a vertical direction away from the deck; a rolling element connected to the jacking element; and a force applying element, such as a hydraulically powered ram, connected to the mounting portion adapted to move the transverse beam over the rolling element relative to the jacking assembly when the transverse beam is lifted by the jacking element. Similarly, each of the longitudinal jacking assemblies includes a mounting portion in contact with an upper surface of the transverse beam; a jacking element connected to the mounting portion, wherein the jacking element is adapted to lift the longitudinal beam in a vertical direction away from the transverse beam; a rolling element connected to the jacking element; and a force applying element, such as a hydraulically powered ram, connected to the mounting portion adapted to move the longitudinal beam over the rolling element relative to the jacking assembly when the longitudinal beam is lifted by the jacking element. 
     The drilling vessel further comprises two or more platens constructed onto the deck beneath the repositioning means, wherein the platens are sized and dimensioned to provide a bearing surface for the drilling platform and the transverse beams, and wherein each of the platens includes a first horizontally extending flange. A second set of horizontally extending flanges are also connected to each of the transverse beams. 
     Also provided are transverse clamping means operatively connected to the transverse beams and matably engageable with the first horizontally extending flanges of the platens for securing the position of the transverse beams relative to the deck between repositioning operations; as well as longitudinal clamping means operatively connected to the longitudinal beams and matably engageable with the second horizontally extending flanges of the transverse beams for securing the position of the longitudinal beams relative to the transverse beams between repositioning operations. 
     For reinforcement purposes, the hull of the drilling vessel further includes a plurality of support members rigidly connected within the hull below the movable substructure, such that the support legs of the drilling platform reside directly above the support members at any position of the movable substructure relative to the deck. As is common, a keyway is also formed into the hull, but is sized and dimensioned to allow the drilling of multiple well locations corresponding to the position of the movable substructure. Finally, in order to move heavy equipment, such as BOP&#39;s and other supplies, the vessel also includes skating means constructed onto the deck and adjacent to the keyway for transporting equipment to and from a desired well. In a preferred embodiment, the skating means comprises a track member fixed relative to the deck and parallel to the keyway; a carrier matably engaged to the track member; and rolling means disposed between the track member and the carrier for enabling smooth movement of the carrier relative to the track member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an overall elevation view of a typical bay barge using the present invention, and generally depicting the movable substructure. 
     FIG. 2A is an overall top view of the bay barge of FIG. 1 depicting the location of the reinforcement columns and platens for the movable substructure. 
     FIG. 2B is a detailed view of the platens which support the movable substructure. 
     FIG. 3A is a more detailed side view of the movable substructure, including the relationship between the port/starboard movement and the bow/stem movement. 
     FIG. 3B is a top view of FIG.  3 A. 
     FIG. 4 is a rear view of the movable substructure shown in FIG. 3A, looking from the stern of the barge. 
     FIG. 5A is top view of a preferred embodiment of the lift and roll jacks used to reposition the movable substructure of the present invention. 
     FIGS. 5B and 5C are side views of the lift and roll jacks of FIG. 5A in position beneath one of the port/starboard walking beams of the movable substructure. 
     FIGS. 5D and 5E are end views of the lift and roll jacks of FIG. 5A in an engaged (jacked up) position and a disengaged (jacked down) position. 
     FIG. 6A is a sectional view of the clamping device used in connection with the movable substructure. 
     FIG. 6B is a top view of the clamping locations for both the transverse and longitudinal walking beams. 
     FIG. 7A is a side view of a proposed well protect structure used to drill multiple wells from a single barge location. 
     FIG. 7B is a top view of the well protect structure of FIG.  7 A. 
     FIG. 8A is a side view of a proposed protective deck used in connection with the well protect structure. 
     FIG. 8B is a top view of the protective deck of FIG.  8 A. 
     FIG. 8C is a detailed view of the connection between the protective deck and the well protect structure. 
     FIG. 9 is a side view of the assembly of the well protect structure and the protective deck in use with a vessel having a movable substructure. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to FIG. 1, a modified bay barge  1  is shown to generally comprise a lower hull  2 , a plurality of sea chests and ballasting chambers  3 , a movable substructure  4  located at the stern of the barge  1 , one or more cranes  5 , a pipe rack area  6 , crew living quarters  7 , a helipad  8 , and a derrick  9 . The movable substructure  4  includes a drill floor  10 , support legs  11 , and a lift and roll jacking system  12  which will be described in greater detail below. 
     FIG. 2A is a top view of the barge  1  with most of the other components omitted for clarity. A keyway  13  through which drilling operations are conducted is located at the stem of the barge  1  and is defined by the space between two parallel extending portions  14 , 15  of the hull. In order to support the shifting weight of the movable substructure  4  in each of its multiple positions, barge  1  is reinforced by a plurality of vertical steel columns  16  connected, such as by welding, between the deck  17  and the bottom  18  of the barge  1 . In the embodiment of the invention shown in the figures, a total of six different wells may be drilled without relocation of the barge  1 . Therefore, a total of twenty-four (24) vertical columns  16  are employed, such that each of the four support legs  11  of the movable substructure  4  are directly above a column  16  when the movable substructure  4  is in a desired position, as will be further explained below. For further reinforcement, and to provide a bearing surface for the jacking system  12  and the movable substructure  4 , three sets of four platens  19 - 22  are also attached to the deck  17  above each pair of columns  16 . For example, platens  19 - 22  serve as the primary bearing surface for the substructure  4  when the drilling is to be accomplished at points  23  or  24  of the drill pattern shown in FIG.  2 A. Similarly, each of the other pairs of drilling points  25 , 26  and  27 , 28  are defined by the presence of the substructure  4  over the next successive groups of platens as the substructure  4  is repositioned toward the stem of the barge  1 . If the substructure  4  is repositioned on the port side  29 , drilling points  23 , 25 , 27  are accessible. If the substructure  4  is repositioned on the starboard side  30 , drilling points  24 , 26 , 28  are likewise accessible. 
     FIG. 2B illustrates the manner in which each of the platens is constructed. A steel horizontal plate  32  is supported by several vertical plates  33  which are preferably welded between horizontal plate  32  and deck  17 . Opposing end plates  34  are also preferably welded to horizontal plate  32 , vertical support plates  33  and deck  17  to create a strong reinforcing support platform over which the movable substructure  4  may rest. 
     FIG. 3A is a side view illustrating the main components of the repositioning means  35  of the movable substructure  4  which permit motion in the bow/stem direction and in the port/starboard direction. FIG. 3B is a top view of the repositioning means  35 , wherein the support legs  11  and other components of the substructure  4  are omitted for clarity. FIG. 4 is a rear view of the invention, looking from the stern of the barge  1 . The movable substructure  4  is always at the stem  38  of the barge  1 . In a preferred embodiment, repositioning means  35  generally comprises a transverse repositioning device  36  for movement of the drilling platform  10  in a port or starboard direction, as well as a longitudinal repositioning device  37  for movement of the drilling platform  10  in a bow or stern direction. In FIG. 3A, the longitudinal repositioning device  37  is connected to the support legs  11  of the drilling platform  10  and resides above and moves relative to the transverse repositioning device  36 . The transverse repositioning device  36  is disposed between the longitudinal repositioning device  37  and the deck  17 . Therefore, when the transverse repositioning device  36  is caused to move, the entire movable substructure  4  and the longitudinal repositioning device  37  are moved in a port or starboard direction. When the longitudinal repositioning device  37  is caused to move, the entire movable substructure  4  is moved in a bow or stem direction, but the transverse repositioning device  36  remains stationary. 
     Referring collectively to FIGS. 3A,  3 B and  4 , transverse repositioning device  36  comprises a pair of horizontal and parallel walking beams  41 , 42  which are in contact with the platens  19 - 22  on deck  17 . A plurality of lift and roll transverse jack assemblies  43  are operatively disposed beneath transverse walking beams  41 , 42  and are used to move transverse walking beams  41 , 42  in a manner to be explained in further detail below. Longitudinal repositioning device  37  also comprises a pair of horizontal and parallel walking beams  44 , 45  which are in contact with the upper surface  46  of transverse walking beams  41 , 42 . A plurality of lift and roll longitudinal jack assemblies  47  are operatively disposed beneath longitudinal walking beams  44 , 45  and are used to move longitudinal walking beams  44 , 45  in a manner to be explained in further detail below. Outboard jacking system supports  48  are also connected to transverse walking beams  41 , 42  and provide sliding or rolling contact with longitudinal walking beams  44 , 45 , thereby serving as a guide for longitudinal walking beams  44 , 45  as they move in a bow or stem direction. Also, shown in FIG. 4 is a removable support strut  49  connected between opposing support legs  11 , which provides additional bracing of the substructure  4  during drilling and maintenance operations. Support strut  49  is removably connected to support legs  11  by a common pin and clevis arrangement or similar fastening hardware known to those in the art. 
     FIGS. 5A,  5 B and  5 C represent top, side and end views, respectively, of the lift and roll jacks  43 , 47  which provide the motive force for both the transverse repositioning device  36  and the longitudinal repositioning device  37 , respectively. Each of the jacks includes a base  50  having a vertical hydraulically controlled jacking device  51  and a horizontal hydraulically controlled jacking device  52 . Vertically controlled jacking device  51  preferably comprises a pair of jacking cylinders  53 , 54  which support a rolling rack  55 . Rolling rack  55  will typically comprise a frame  56  having a plurality of rollers  57  which contact the applicable walking beam during a repositioning operation. As will be further illustrated, rolling rack  55  is raised by jacking cylinders  53 , 54  prior to each move and lowered immediately after each move. Horizontally controlled jacking device  52  preferably comprises a pair of jacking cylinders  58 , 59  pivotally connected to opposite sides of the base  50 , wherein each of the jacking cylinders  58 , 59  includes a ram  60  connectable to a plate  61  extending from the walking beam in question. Suitable hydraulic lines  62  extend from each of jacking cylinders  53 , 54 , 58 , 59  so that the motion in both directions can be controlled in a manner commonly known to those in the industry. 
     As shown more clearly in FIGS. 5B and 5C, horizontal jacking cylinders  58 , 59  are used to push or pull walking beam  41  during each repositioning operation. In the following figures, a transverse walking beam  41  is shown, with understanding that the same arrangement exists for longitudinal walking beams  44 , 45 . In FIG. 5B, a lift and roll jack  43  is shown positioned beneath a walking beam  41  just prior to moving the substructure  4 . Note that the vertical jacking cylinders  53 , 54  have already lifted the rolling rack  55  into contact with the walking beam  41 , such that the walking beam  41  is raised a distance D approximately one inch or less above the platens. When actuated, the horizontal jacking cylinders  58 , 59  push against the plate  61  to which the ram  60  is connected, resulting in the movement of walking beam  41  over the rolling rack  57  as the lift and roll jack  43  remains stationary with respect to the platens. At the completion of the move, shown in FIG. 5C, the walking beam  41  has traveled over a length L, roughly corresponding to the length of the ram  60 . Once the horizontal motion has ceased, the vertical jacking cylinders  53 , 54  are lowered, which allows the walking beam  44  to rest once again on the platens as the rolling rack  55  breaks contact with the walking beam  41 . Now that the lift and roll jacks  43  are no longer supporting the weight of the substructure  4 , the horizontal jacking cylinders  58 , 59  are actuated in an opposite direction, this time pulling the lift and roll jack  43  in a sliding manner against the platens back to its original position with respect to the plate  60 , similar to the position shown in FIG.  5 B. 
     FIGS. 5D and 5E are end views of the lift and roll jack  43  showing the manner in which the vertical jacking cylinders  53 , 54  lift and lower the rollers  57  with respect to the walking beam  41 . Note that when jacking cylinders  53 , 54  are raised, as shown in FIG. 5D, the rollers  57  are in contact with the walking beam  41  such that the weight of the substructure  4  is supported entirely by the jacks  43 . When the jacking cylinders  53 , 54  are lowered, as shown in FIG. 5E, a space S exists between the rollers  57  and the walking beam  41  as the walking beam  41  is supported by the platens. To assist in guiding the walking beam  41  along its path during a move, a plurality of guide members  65  extend from the jacks  43  on each side of walking beam  41 , as shown best in FIGS. 5D and 5E. In a preferred embodiment, such guide members  65  may comprise rollers or similar bearing structures which maintain the walking beam  41  centered on the rolling rack  55 . 
     As can now be appreciated, the entire substructure  4  may be repositioned by conducting several of the foregoing incremental moves until the substructure  4  is over the appropriate well site. Specifically, the transverse walking beams  41 , 42  are moved relative to the platens in accordance with the aforementioned procedure employing a preferred total of eight (8) such lift and roll jacks  43  operated simultaneously, with two such jacks  43  underneath each of the four legs of the substructure  4 , as shown in FIG.  4 . Likewise, the longitudinal walking beams  44 , 45  are similarly moved with respect to the transverse walking beams  41 , 42  by a preferred total of four identical longitudinal lift and roll jacks  47  operated simultaneously, also depicted in FIG.  4 . Thus, to reposition the entire substructure  4  to a different well location, the substructure  4  is first moved in either the transverse or the longitudinal direction, after which it is moved in the perpendicular direction. Although not required, it is preferred that the lift and roll jacks  43 , 47  be alternately oriented, as depicted in FIG. 4, meaning that when the substructure  4  is moved in a particular direction, half of the jacks are pushing while an equal number are pulling. Such an alternating orientation allows for a more uniform movement of the substructure  4  in addition to ensuring that the same forces are applied to move the substructure  4  in either direction. 
     FIG. 6A is a sectional view of the clamping assembly  70  employed with the movable substructure  4  which secures its position after each incremental move. Shown in relation to one of the transverse walking beams  41 , 42 , the clamping system  70  preferably comprises a double-acting hydraulic piston  71  having an externally threaded upper end  72  and a lower end  73 , slidably disposed within an outer cylinder  74  attached to walking beam  41 . Lower end  73  includes a lip  75  which extends underneath the horizontal plate  32  of the platen  19 , while the upper end  72  includes a locking nut  76  threadably attached thereto. Cylinder  74  includes ports  77 , 78  to which hydraulic lines  81 , 82  attach for operation of the clamp  70  using hydraulic controls in a manner understood to those of ordinary skill in the industry. For reasons which will become clearer below, the hydraulic control system for the clamps  70  is interconnected to the hydraulic control system for the jacks  43 , 47 . A nitrogen backup system  80  and relief valve  79  are also fluidically connected to hydraulic line  82  to ensure a secure clamping condition in the event of failure of the usual hydraulic control system. As an additional means of guiding the walking beam  41  with respect to the platens, a roller  83  with appropriate bearings  84  is affixed to lower end  73  which provides rolling contact against the walking beam  41  and the platen  19 . In operation of the clamp  70 , the piston  71  is actuated in a downward direction simultaneously with the operation of the vertical jacking cylinders  53 , 54  to lift the walking beams  41 , 42 , thus releasing the walking beams  41 , 42  for movement relative to the platens by creating a lift space LS slightly greater than the distance D that walking beams  41 , 42  are lifted. As the vertical jacking cylinders  53 , 54  are lowered after the incremental move, the piston  71  is simultaneously raised to secure the walking beams  41 , 42  to the platens once again. Therefore, the walking beams  41 , 42  are always clamped to the platens when there is no motion of the substructure  4 . Once the desired repositioning has taken place through a series of incremental moves as previously explained, the clamp  70  is locked into place by tightening the locking nut  76  against cylinder  74 . Although the foregoing structure and operation of the clamping system  70  has been described with regard to the motion between transverse walking beams  41 , 42  and the platens, an identical arrangement exists between longitudinal walking beams  44 , 45  and transverse walking beams  41 , 42 . Preferably, each of transverse walking beams  41 , 42  includes at least four such clamping systems  70 , with two on either side of each walking beam  41 , 42 , as shown by clamping locations  85  in FIG. 6B. A similar arrangement of two such clamping systems  70  exists for each of the longitudinal walking beams  44 , 45 , shown at locations  86 . 
     FIG. 7A and 7B illustrate a well protect structure  90  for use in connection with the present invention. Because of the ability of the substructure  4  to move from well to well, the single-well protective structures that are normally employed with barge drilling operations are not suitable. Therefore, the well protect structure  90  includes a plurality of hollow, well protective columns  91  through which casing and drill pipe may be passed. Each of the protective columns  91  are connected to one another by an outer frame  92  comprising four vertical support posts  93  and appropriate horizontal support elements  94  and diagonal support elements  95 . The pattern of columns  91  is identical to the pattern of wells that may be drilled by the repositioning of the movable substructure  4 . Each of the columns  91  includes an upper divergent opening  96  in the form of an inverted cone so that drill pipe may be easily guided into the columns  91 . For reasons which will become clearer below, each of the vertical support posts  93  includes an upper opening  97  which allows for the insertion and attachment of a novel protective deck  100 . Finally, a grating deck  98  extends across the upper level of the well protect structure  90  to provide a surface for the movement of crew members. 
     FIG. 8A and 8B illustrate a novel design for a protective deck  100  that can be used with the present invention and the aforedescribed well protect structure  90 . Similar to the well protect structure  90 , the protective deck  100  includes four support posts  101  interconnected to one another by a frame  102 . Support posts  101  are tapered at the bottoms for insertion into the upper openings  97  of the support posts  93  of the well protect structure  90 , as shown in FIG.  8 C. The deck surface  103  comprises steel sheet, preferably ¾ inch in thickness, and is strengthened underneath by a series of side-by-side steel stiffeners  104  extending across the frame  102 . The deck surface  103  extends completely across the protective deck  100  from each side of frame  102  except for apertures  105  in the same pattern as the columns  91  of the well protect structure  90 . Each of the four support posts  101  includes a lifting lug  106  which are used to lower the protective deck  100  onto the well protect structure  90  prior to drilling operations. In FIG. 8C, the illustration depicts the manner in which the support posts  101  of the protective deck  100  are removably attached to the support posts  93  of the well protect structure  90 . An adaptor  107  is welded to the upper opening of the support posts  93  of the well protect structure  90  to allow the connection to the protective deck  100 . The adaptor  107  comprises a sleeve  108  which is welded to a piling  109  driven into the ground through support post  93 . An upper divergent flange  110  is provided to accept the tapered end of support post  101  of the protective deck  100 , while a stop plate  111  welded to the inside of sleeve  108  provides a surface upon which the support post  101  rests. Attachment lugs  112 , 113  are welded to each of support post  93  and support post  101  so that a connection member  114  can be removably attached therebetween. 
     FIG. 9 illustrates the assembly of the well protect structure  90  and the protective deck  100  in use with a vessel  1  having a movable substructure  4 . The well protect structure  90  is first set into the ground in the manner common in barge drilling operations, after which the barge  1  backs against the well protect structure  90  so that it resides within the keyway  13 . Next, the protective deck  100  is lowered by a hoist system  115  located beneath the drill floor  10 , and is then connected in the manner just described. 
     Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.

Summary:
A drilling vessel is provided for selectively drilling two or more wells from a single location of the vessel, comprising a buoyant hull having a deck, including ballasting devices in the hull for allowing the vessel to be stabilized on the bottom surface of a body of water; a movable substructure in contact with the deck, wherein the movable substructure includes a drilling platform having a plurality of downwardly extending support legs attached thereto, wherein the height of the drilling platform from the deck is sufficient to allow simultaneous drilling and production activities to occur; and a repositioning system, operatively connected between the support members and the deck, for moving the drilling platform in a predetermined direction relative to the deck. In a preferred embodiment, a plurality of lift and roll jacks are employed beneath each level of a two-tiered repositioning system such that the substructure may be moved in both a transverse and a longitudinal direction. In this manner, multiple wells may be drilled without relocating the vessel, while allowing maintenance activities to occur on previously drilled wells.