Abstract:
A bottom-supported jack-up rig has a hollow ring mat with a central opening for accommodating a lower portion of a hull nested therein when the rig is in transit. The ring mat has a predetermined surface footprint sufficient to provide stability to the rig when embedded in the sea bottom. The mat buoyancy helps support the hull and the legs in a floating position when the rig is in transit, while allowing lowering of the mat to the sea bottom without assistance of a ballasting/de-ballasting means.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a mat supported jack-up platform that can be used for drilling and production operations offshore. 
   Offshore drilling operations are often conducted using bottom-supported platforms or rigs. One of the most common types of bottom-supported rigs are jack-up rigs that have legs that can be jacked up and down in relation to the platform hull by rack-and-pinion systems. Conventional rigs are towed to a location of prospective drilling operations, while the legs are elevated above the platform, or hull. Once the rig reaches the pre-determined location, the legs are lowered to the bottom and the platform is “jacked up” above the expected wave action to a desired operational draft. The legs are embedded into the sea bottom to support the elevated platform. Some of the rigs have spud cans secured to the bottom of the legs for resting on the sea floor. 
   Another type of jack-up rigs uses a mat that is secured to the bottom of the legs. Such rigs are often employed for muddy, soft bottom conditions. A conventional mat-supported jack-up rig has a base mat, which is a rectangular hollow tank sized to provide stability for the rig when resting on the seafloor. 
   The interior of a typical mat is divided into ballast compartments, which can be flooded to make the mat neutrally buoyant, while the platform is being towed to the drilling site. When a mat supported jack-up is conducting drilling operations, the mat is negatively buoyant and is positioned in contact with the sea floor. The mat distributes the weight of the rig evenly over the bottom and tends to keep it from sinking too far into a soft bottom. 
   The need for ballasting and de-ballasting of the mat structure requires that the rig be equipped with pumps and associated equipment, which increases the cost of the rig and associated drilling operations. 
   The present invention contemplates elimination of drawbacks associated with the prior art and provision of an improved mat-supported offshore platform, wherein the need for the pumping equipment is eliminated. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a mat-supported jack-up rig with a ring mat that is carried by the lower portions of the jack-up legs. 
   It is another object of the present invention to provide a mat for supporting a jack-up rig that does not need to be ballasted or de-ballasted. 
   In the preferred embodiments of the present invention, the jack-up rig has a buoyant hull and a plurality of supporting legs that extend above the hull deck when the rig is in transit. A hollow mat is secured to the bottom of the legs; the mat has a central opening that accommodates the bottom portion of the hull. The mat with a large central opening has sufficient buoyancy to facilitate floating of the hull while the rig is towed and lowering of the mat to the seabed without assistance of conventional ballasting means. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is side schematic view of the mat supported offshore rig of the present invention, wherein the rig is provided with three legs. 
       FIG. 2  is a plan view of the mat of the present invention for a rig with three supporting legs. 
       FIG. 3  is a schematic side view of the mat supported rig of the present invention, with the mat embedded in the seabed. 
       FIG. 4  is side schematic view of the mat supported offshore rig of the present invention, wherein the rig is provided with four legs; and 
       FIG. 5  is a plan view of the mat of the present invention for a rig with four supporting legs. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 through 5  illustrate the preferred overall configuration of the offshore structure of the present invention. The rig  10  comprises a floatable platform, or hull  12 , from which the drilling operations will be performed. A ballastible and deballastible mat  14  is adapted to be embedded into the seabed  16 . A plurality of legs  18  extends from the mat  14  to the hull  12 . 
   The legs  18  are fixedly secured, at their lower portions to the mat  14 . A plurality of leg wells (not shown) is formed in the hull  12  to allow the legs  18  to pass therethrough. The mat  14  is provided with leg wells  20  to receive the lower parts of the legs  18 . A means (not shown) are typically provided on the platform  12  for moving-the legs  18  with respect to the hull  12 . The hull  12  is a floatable hull with sufficient buoyancy to allow the rig  10  to be floated from one location to another. The drilling operations are conducted from the deck  22 . The deck  22  is designed to support a variety of equipment such as a derrick, cranes, storage facilities, and crew living quarters. 
   The mat  14  is hollow in construction, generally rectangular in shape and has four peripheral corners. The mat is comprised of a first elongated member  26 , a second elongated member  28 , a cross member  30  and a connecting member  32 . The elongated members  26 ,  28  are secured in a parallel relationship to each other and transversely to the cross member  30  and the connecting member  32 . If desired, the elongated members  26  and  28  may extend farther than the distal end  38  of the connecting member  32 . Such may be the situation, where the drilling equipment is positioned at the aft of the platform  12 . 
   The mat-forming members, i.e. the elongated members  26 ,  28 , the cross member  30  and the connecting member  32  define a central opening  36 , which occupies a substantial surface area of the mat footprint, or peripheral dimensions. The central opening  36  reduces the buoyancy of the mat  14  and eliminates the need for ballasting and de-ballasting of the mat  14 . 
   The central opening  36  is sized and shaped to accommodate the bottom portion  34  of the platform  12 , such that the bottom portion  34  nests inside the opening  36 . The hull  12  is configured such that the buoyancy lost from the open area  36  of the mat  14  is taken up by the hull  12 . As a result, when the rig  10  is located in shallow waters it has sufficient buoyancy to float, carrying the mat  14  and the supporting legs  18 . 
   Once the rig  10  is towed to the desired location, the legs  18  are lowered to the seabed  16 , pushing the mat  14  into the soft bottom. The mat  14  does not exert an upward force on the platform  12 , as is common with conventional mat supported rigs that require ballasted mats. It is envisioned that the mat  14  will have sufficient weight and less than neutral buoyancy to lower itself to the seabed  16 . When assisted by the legs  18 , forcing the mat  14  downwardly, the mat  14  will move to the bottom and rest on the seabed  16 , supporting the platform  12 . Once the legs are secured, the platform  12  is raised to its operational draft ( FIG. 3 ) by the conventional jack-up apparatus. 
     FIGS. 4 and 5  illustrate a rig  50  that is supported by four legs. Similarly to the embodiment of  FIGS. 1 and 2 , the rig  50  comprises a platform, or hull  52 , a mat  54  and a plurality of supporting legs  58 . The legs  58  extend through leg wells (not shown) formed in the hull  52  and into the leg wells  60  formed in the mat  54 . The mat  54  may have a rectangular or a square configuration and formed by four hollow mat-forming members  62 ,  64 ,  66 , and  68  rigidly connected to each other to form an open “ring” mat. The mat members  62 ,  64 ,  66  and  68  define a large central opening  70  that occupies a substantial surface area in the overall footprint of the mat. 
   Similarly to the opening  36 , the opening  70  helps reduce the buoyancy of the mat  14 , such that the mat  14  will not exert a considerable upward force when it is lowered in the water to the sea bed  16 . The opening  70  is slightly greater than the bottom  54  of the hull  52 . When in towed condition, the bottom  54  nests within the opening  70 , with the hull  52  providing sufficient buoyancy to the rig  50 . When the legs  52  are lowered to the bottom forcing the mat  54  through the water, the mat  54  offers little resistance, facilitating expeditious embedding of the mat  54  into the seabed  16 . 
   The mats  14  and  54  provide sufficient buoyancy when the rigs  10  and  50  are moved through shallow waters. At the same time, the mats  14 ,  54  provide stability to the rigs  10  and  50  when resting on the seabed  16 , while the central open areas  36  and  70  eliminate the need for ballasting of the mats  14 ,  54 . Thus the need for pumps and associated equipment is eliminated. The valuable space on the platform deck may be used for other tasks. 
   Many changes and modifications may be made in the design of the present invention without departing from the spirit thereof. I, therefore, pray that my right to the present invention be limited only by the scope of the appended claims.