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CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on provisional application No. 60/576,156, filed Jun. 2, 2004. 
    
    
     BACKGROUND 
     This invention relates generally to offshore drilling of wells using floating and non-floating platforms. 
     A floating platform may be moved to a position over a potential well location. The well may be drilled and may be coupled to the rig by a marine riser. Conventionally, there is a substantial distance between the floating platform and the seabed surface. For example, this distance may be several thousand feet. As a result, it is necessary to lower various drill strings, casings, and risers from the surface to the seabed floor. 
     The cost associated with drilling wells with offshore floating platforms may be substantial. For example, rental rates for offshore mobile platforms may run in the range of $300,000.00 a day. Significant time may be spent running various equipment from the rig down to the subsea floor. This time may result in a substantial portion of the cost. 
     Often, it is desirable to drill one well, pick up, and move to another location. The need to connect to a well from the surface involves substantial expenditure of time and, therefore, expense. 
     To address these problems, so-called dual activity rigs have been proposed. In a dual activity rig there may be two derricks, including a main derrick, used for running and hanging off the blowout preventer, and an auxiliary derrick, used to drill the top hole and run surface casing. A blowout preventer riser may be hung off from the rig while the drill string is operating to form the hole. 
     Dual operations may save time from the time a rig arrives on location to the landing of the blowout preventer. This is because the riser and the blowout preventer can be made up and run off line. 
     Thus, there is a need for still better ways to save time during offshore floating platform drilling activities. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, cross-sectional view of one embodiment of the present invention in the course of jetting casing into a seabed; 
         FIG. 2  is a partial, enlarged, cross-sectional view of a portion of the casing in position over the seabed floor in accordance with one embodiment of the present invention; 
         FIG. 3  is a cross-sectional view of the embodiment shown in  FIG. 1  after the casing has been jetted into the seabed floor, during drilling of a smaller diameter hole below that casing in accordance with one embodiment of the present invention; 
         FIG. 4  is a partial, enlarged, cross-sectional view of the embodiment shown in  FIG. 3  in the course of drilling a smaller diameter hole below a larger diameter casing in accordance with one embodiment of the present invention; 
         FIG. 5  is a cross-sectional view showing the smaller diameter casing in one embodiment; 
         FIG. 6  is an enlarged, cross-sectional view showing the portion of the well within the seabed floor, the guide base, and the overlying blowout preventer and riser in accordance with one embodiment of the present invention; 
         FIG. 7  is an enlarged, cross-sectional view taken generally along the line  7 - 7  in  FIG. 1  in accordance with one embodiment of the present invention; and 
         FIG. 8  is a partial, enlarged, perspective view of the lower trolley, riser, and blowout preventer hung off of the rig in accordance with one embodiment of the present invention; and 
         FIG. 9  is a cross-sectional view of the well with the blowout preventer and riser in position in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a floating platform or multiple operation drilling rig  10  is shown in position over a formation. The rig  10  may include a single derrick  12 , which may include multiple levels for different operations. In some embodiments, more than one derrick may be utilized. In the embodiment illustrated, the deck  14  supports an upper trolley  16  and a lower trolley  18 . The upper trolley  16  may ride on the deck  14  and the lower trolley  18  may ride on the track beneath the deck  14 . 
     Also beneath the deck  14  are the floats  80 . Suspended downwardly is a string  82  coupled to a guide base  35 . The guide base  35  is intended to be positioned on the seabed floor (SB) but, in  FIG. 1 , is shown being lowered within the ocean (O). Below the guide base  35  is a larger diameter casing or conductor  22  in accordance with one embodiment of the present invention. The larger diameter casing  22  may be a 36″ casing in one embodiment. The lowermost end of the casing  22  includes a plurality of high pressure jets capable of displacing the seabed floor and forming a hole that will receive the casing  22 . 
     In the position shown in  FIG. 1 , the riser  25  and blowout preventer  24  are hung off of the lower trolley  18  out of position from the drill string  82 . For example, the rig  10  may have just arrived from a prior drilling site. It may have arrived with the riser  25  and blowout preventer  24  hung off of the trolley  18 . As a result, there is no need to spend the time making up of the riser  25  and attaching the blowout preventer  24  because that equipment is already in position, hung off of the rig  10 . 
     Upon arriving at the drilling site, all that would be necessary then is to connect the guide base  35  to the larger diameter casing. In one embodiment, about 4 joints of 36″ casing may be secured to the guide base  34  so that the guide base  35  and casing  22  can be lowered on the string  82  to the seabed floor (SB). 
     Referring to  FIG. 2 , once the casing  22  is at the seabed floor, a plurality of high pressure fluid jets  30  may receive pumped fluid and jet that fluid to form a hole H in the seabed floor ahead of a casing  22 . Thus, as the casing  22  is advanced into the seabed floor, a hole is jetted by an array of three jets  30  in accordance with one embodiment of the present invention. This hole is sufficiently large to enable the casing  22  to be jetted into the seabed floor (SB) and to position the guide base  35  on the seabed floor. Thus, high pressure fluid may be pumped down from the rig  10  and ejected from the jets  30  to remove seabed sediment. 
     Referring to  FIG. 3 , after the casing  22  is in place within the seabed floor (SB) and the guide base  35  is in position on the seabed floor, a smaller diameter bottom hole assembly or drill string  28  may be released ahead of the casing  22 . Then, a smaller diameter hole is drilled using the bottom hole assembly  28  as shown in  FIG. 3 . In effect, the bottom hole assembly  28  simply advances downwardly through the now stationary casing  22 . In one embodiment, the smaller diameter hole may be about either 26″ or 17½″ in diameter. 
     Thus, as shown in  FIG. 4 , an ever-deeper hole  36  is formed as a result of the drilling bit (not shown) operating on the end of the bottom hole assembly  28 . The depth of the hole  36  that is drilled is limited to prevent the bottom hole assembly  28  from reaching a high pressure formation within the seabed floor. Namely, because no blowout preventer is in place at this time, it is desirable to drill a relatively shallow hole  36  without penetrating any high pressure formations. 
     The bottom hole assembly  28  is pulled out of the hole  36 . Then, the smaller diameter casing  20  is run into the hole  36 , which may just be formed by the bottom hole assembly  28 , as shown in  FIG. 5 . A series of sections of casing  20  were first pre-assembled and suspended from the upper trolley  16  as shown in  FIG. 3 . Then, when the bottom hole assembly  28  has been removed, the trolley  16  can be rolled into place within the derrick  12  so that the casing  20  may be lowered into the hole  36 . In some embodiments, it may be desirable to add one more joint of casing to the pre-hung casing before lowering the casing  20  into the hole. The smaller diameter casing  20  is moved with the trolley  16  into a main moon pool and then lowered into the hole as shown in  FIG. 5 . 
     More particularly, the casing  20  is provided through the guide base  35  into the hole  36  to a desired depth. As shown in  FIG. 6 , the casing  20  extends some depth into the seabed floor, passing through the larger diameter casing  22 . The smaller diameter casing  20  may be cemented in position. Then, the string  84  is pulled out of the hole. 
     Referring to  FIG. 7 , the arrangement of the deck  14 , in accordance with one embodiment of the present invention, is illustrated. The position of the derrick  12  is shown. The moon pool  68  has an extension  62  which points towards the upper trolley  16 . The upper trolley  16  includes a slot  90 , arranged to mate with the extension  62  on the moon pool  68 . 
     Thus, the trolley  16  may be advanced to the extension  62  where the made up, hung off, casing  20  has been pre-positioned. The trolley  16  is positioned so that the slot  90  overlays the extension  62 . The tool  72  then lifts the casing  20  from the extension  62  and mounts it on the trolley  16 . The trolley  16  thereafter advances over the moon pool  68  so that the casing  20  may be lowered into the hole  36 . 
     The casing  20  may be stored horizontally on racks  64  and advanced by a conveyor  66 . The casing  20  then may be rotated to a vertical orientation and transferred by a crane  69  into the extension  62 . A larger crane  70  may be utilized to support operations through the moon pool  68 . 
     Referring next to  FIG. 8 , the lower trolley  18  supports the riser  25  and the blowout preventer  24 , which has already been pre-hung from the lower trolley  18 . The lower trolley  18  may ride on bearings  54 , supported by a track  56  on the deck  14 . A split spherical bearing  50  includes portion  50   a  and  50   b  and is openable in the directions indicated by the arrows B. In other words, the bearing  50  includes two portions  50   a  and  50   b  which support the riser  25  on the ring  52  thereof. When the bearing  50  is opened, the riser  25  and blowout preventer  24  may be lifted from the trolley  18  and moved into the moon pool  68 . 
     Finally, referring to  FIG. 9 , the riser  25  and blowout preventer  24 , once in position under the derrick  12 , may be lowered into position and secured to the guide base  35 . After the sequence described herein, conventional completion and production techniques may be used. 
     Once the well is completed, and it is desired to move to another location, the operation may be reversed. The blowout preventer  24  and riser  25  may be removed from the guide base  35 , reattached to the trolley  18 , and moved to the stored position shown in  FIG. 1 . Then, the rig  10  may be moved to a new location with the blowout preventer  24  and riser  25  hung off from the rig  10 . 
     In some embodiments of the present invention, substantial time may be saved in offshore drilling operations. These savings arise by virtue of the fact that multiple operations may be done in a parallel, rather than serially. For example, the smaller diameter casing  20  is pre-made up and, therefore, the time to make up the casing does not add to the overall drilling time. That is, the casing  20  was made up while the casing  22  was being jetted in. Likewise, the cementing and the running in of the smaller diameter casing  20  may be partly done offline, as may be the removal of the landing string from the hole. Finally, the time to run in the blowout preventer  24  may be substantially shortened because the blowout preventer  24  has already been made up and hung off, offline. 
     Thus, in some cases, from 2 to 4 days may be saved over other techniques when drilling in water depths on the order of 5000 feet. Of course, the present invention is in no way limited to any particular drilling depth and may be applicable to any of a variety of well depths. While a floating embodiment is depicted, fixed or stationary platforms may also be used in some cases. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.

Summary:
An offshore platform may include a pair of trolleys. One trolley may be mounted on top of the platform and the other trolley may be suspended below the platform. Each trolley may be positioned on opposed sides of a moon pool in one embodiment of the present invention. The lower trolley may be used to hang off a riser and blowout preventer. The upper trolley may be utilized to hang off smaller diameter casing while larger diameter casing is being made up and positioned within the hole. As a result of the parallel operations made possible by the trolleys, drilling time may be reduced in some embodiments.