Abstract:
A mudmat foundation for supporting an item of subsea equipment on the sea floor comprises a mudmat on which the item of subsea equipment may be supported and a plurality of legs which are pivotably connected to and depend downwardly from the mudmat. The legs are free to pivot outwardly relative to the mudmat upon insertion of the legs into the sea floor to thereby increase the load bearing capacity of the mudmat.

Description:
BACKGROUND OF THE INVENTION 
   The present invention is directed to a mudmat foundation for supporting subsea equipment on the sea floor. More particularly, the invention is directed to a mudmat foundation which comprises a mudmat and a depending umbrella structure that confines the soil beneath the mudmat and thereby increases the bearing capacity of the mudmat foundation. 
   Mudmats, conductor pipes and suction piles are common types of foundations which are used to support items of subsea equipment on the sea floor. The required size of a foundation for a given item of subsea equipment is in part a function of the bearing capacity of the soil at the sea floor, which is given by the following equation:
 
 Q=faL+q   u   A,   1
 
where Q is the bearing capacity of the soil, f is the skin friction factor of the soil, a is the perimeter area of the foundation, L is the penetration depth of the foundation, q u  is the bearing strength of the soil, and A is the bearing footprint area of the foundation.
 
   Often the bearing capacity of the soil is unknown, and in some instances the line of demarcation between the soil and the sea water is very undefined. Under these circumstances, the subsea equipment may sink into the sea floor and become lost. In addition, as shown by equation 1, any attempt to increase the bearing capacity of the soil typically involves increasing either the perimeter area of the foundation or the bearing footprint area of the foundation, or both. However, for poorly consolidated soils, the resulting foundation may be too large to pass through the moonpool of a drilling rig or too heavy to be lifted by the standard equipment on the drilling rig. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, these and other disadvantages are overcome by providing a mudmat foundation for supporting an item of subsea equipment on the sea floor. The mudmat foundation comprises a mudmat on which the item of subsea equipment may be supported, a plurality of legs, and means for pivotably connecting an upper end of each leg to the mudmat. Thus, the legs are free to pivot outwardly relative to the mudmat upon insertion of the legs into the sea floor. Consequently, the legs will trap and consolidate the soil beneath the mudmat and thereby increase the load bearing capacity of the mudmat foundation. 
   The mudmat foundation may also comprise means for limiting the degree to which the legs may pivot. For example, the mudmat foundation may comprise a number of wire rope cables which each engage the legs to prevent the legs from pivoting beyond a predetermined angle. 
   In a preferred embodiment of the invention, the mudmat foundation also comprises a flexible cover which is attached to the legs or the cables, or both. In addition, the cover is ideally water permeable. In conjunction with the legs, the cover functions to further trap and consolidate the soil beneath the mud mat to thereby increase the load bearing capacity of the mudmat foundation. 
   These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front elevation view of the mudmat foundation of the present invention; and 
       FIGS. 2 through 4  are sequential views of the mudmat foundation of  FIG. 1  being installed in the seal floor, wherein the cover component of the invention has been omitted for clarity. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1  the mudmat foundation of the present invention, which is indicated generally by reference number  10 , comprises a mudmat  12  and a plurality of legs  14  which are pivotably connected to the mudmat. The mudmat  12  can comprise any convention mudmat which ideally includes a generally rectangular base  16  and a through bore  18  in which a conductor pipe or the like (not shown) may be secured. 
   The number of legs  14  which the mudmat foundation  10  comprises depends upon the size of the mudmat  12  and the conditions of the soil in which the mudmat foundation will be installed. In the embodiment of the invention shown in  FIG. 1 , for example, the mudmat foundation  10  comprises eight legs  14 , one connected to each corner and the midpoint of each side of the base  16 . For purposes of this description, the mudmat  12  and the legs  14  can be said to define an “interior” volume of the mudmat foundation which is disposed below the mudmat and between the legs. 
   Each leg  14  comprises an elongated post  20  which is comprised of, for example, metal tube stock. In addition, each post  20  includes an upper end  22  which is pivotally connected to a padeye  24  that in turn is attached to the base  16  of the mudmat  12 , such as by welding. Consequently, each leg  14  is free to pivot outwardly relative to the mudmat  12  to thereby enlarge the interior volume of the mudmat foundation. In addition, the upper end  22  of each post  20  is preferably detachably connected to its corresponding padeye  24  by a removable pin or bolt  26 . As a result, the legs  14  may be disassembled from the mudmat  12  by simply removing the pins  26 . 
   Each leg  14  may also include a number of blades  28  which are secured to the post  20  such as by welding. Each blade  28  comprises a relatively broad face  30  which is positioned parallel to the pivot axis of the padeye  24  and ideally at an angle of, for example, between 10° and 30° relative to the longitudinal axis of the post  20 . In this manner, as the legs  14  are inserted into the sea floor, the blades  28  will deflect the soil and urge the bottoms of the legs outwardly relative to the mudmat  12 . 
   In a preferred embodiment of the invention, the mudmat foundation  10  may also comprise a number of lateral restraints  32  to limit the degree to which the legs  14  may pivot relative to the mudmat  12 . In the exemplary embodiment of the invention shown in the Figures, each restraint  32  is shown to comprise a cable which is made of, for example, wire rope. In addition, each cable  32  may be connected to each leg  14  by being threaded through a corresponding hole  34  which is formed in the post  20 . Furthermore, each cable  32  is ideally longer than the adjacent cable closer to the mudmat  12  so as to allow the bottoms of the legs  14  to pivot outwardly relative to the mudmat. 
   The mudmat foundation  10  ideally also comprises a cover  36  which is sized so as to enclose a substantial portion of the interior volume of the mudmat foundation. The cover  36  may be riveted or otherwise secured to the legs  14 , the cables  32 , or both, and may also extend to and be sealed to the mudmat  12 . In a preferred embodiment of the invention, the cover  36  comprises a sturdy yet flexible and water permeable material, such as a plastic mesh type construction fabric. Such a fabric will contain and compress the soil within the interior volume of the mudmat foundation as the mudmat foundation  10  is being installed, but allow any water within the interior volume of the mudmat foundation to pass through into the surrounding environment. 
   When assembled as shown in  FIG. 1 , the legs  14  and any attached cables  32  and cover  36  form a collapsible “umbrella” structure which is preferably removably connected to the mudmat  12  by the pins  26 . As will be described below, this umbrella structure may thus be collapsed during deployment and thereafter expanded during installation of the mudmat foundation in the sea floor. 
   The installation of the mudmat foundation  10  will now be described with reference to  FIGS. 2 through 4 . Prior to installation, the mudmat foundation  10  may be partially disassembled by removing the legs  14  from the mudmat  12  in the manner described above. This will allow the legs  14  and any attached cables  32  and cover  36  to fold into a relatively small package which can be easily stored, transported and manipulated. In preparation for installation, the mudmat  12  is set in the moonpool of a drilling rig or the like and the legs  14  are connected to the padeyes  24  with the pins  26 . In this configuration, which is shown in  FIG. 2 , the legs  14  extend generally vertically from the mudmat  12  and therefore do not add substantially to the lateral dimensions of the mudmat foundation  10 . Consequently, the mudmat foundation  10  may be readily assembled in and lowered through the moonpool. 
   The mudmat foundation  10  may be lowered from the drilling rig on a cable or drill pipe which is connected to the mudmat  12 . As the mudmat foundation  10  is lowered, the legs  14  will remain generally vertical. However, as the legs  14 , and in particular the blades  28 , contact the soil on the sea floor, the legs  14  will deflect the soil and begin to pivot radially outwardly relative to the mudmat  12 , as shown in FIG.  3 . As the mudmat foundation  10  is lowered further, the legs  14  will continue to pivot radially outwardly until the cables  32  become taut. If the cover  36  is employed, further lowering of the mudmat foundation  10  will consolidate the soil by expelling any water from the interior volume of the mudmat foundation. Thus, once the mudmat  12  reaches its final resting position on the sea floor, which is shown in  FIG. 4 , the legs  14  and the cover  36  will have captured a very large volume of soil which will add to the bearing capacity of the mudmat foundation  10  by greatly increasing the total mass which must be moved when the mudmat foundation is subjected to loads. 
   The mudmat foundation  10  may be recovered from the sea floor by simply lifting the mudmat  12  vertically. This will pull the legs  14  out of the sea floor and permit the entire mudmat foundation  10  to be retrieved to the drilling rig. Alternatively, the mudmat  12  can be disconnected-from the legs by removing the pins  26 . In this scenario, the mudmat  12  is retrieved to the drilling rig while the legs  14  and any attached cables  32  and cover  36  are left in the sea floor. 
   It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.