Abstract:
A semisubmersible mobile offshore drilling unit (MODU) includes a submersible lower hull comprising a ring pontoon having a trapezoidal shape; an upper hull having the trapezoidal shape; three stability columns supporting the upper hull from the lower hull; and a drilling rig. The MODU may be operated in a tender assist mode or an independent drilling mode. The MODU may be equipped to prepare, drill, and complete a subsea wellbore or to workover an existing subsea wellbore.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the present invention generally relate to a mobile offshore drilling unit. 
         [0003]    2. Description of the Related Art 
         [0004]      FIG. 1A  illustrates a drilling rig  6 . A drilling rig  6  is a portable factory for forming deep wellbores  40  in the ground. When a drill bit  10  is pressed against the ground and rotated, the teeth on the bit  10  grind and gouge the rock into small pieces. These pieces of rock or cuttings must be moved out of the way so the bit teeth can be constantly exposed to fresh, uncut rock. Drilling fluid, such as mud, is used to move the cuttings away from the bit  10 . A mud pump  20  takes mud from mud tanks  22  and pumps it under high pressure up a standpipe  24 , through the swivel  26 , down the kelly  54 , down the drill pipe  30 , through the drill collars  32  and out jets in the bit  10 . Mud, exiting under pressure from jets in the bit  10 , clears the cuttings and moves then up the annulus  42  of the wellbore  40 . The mud and cuttings are then passed over a shale shaker  44  which separates the cuttings from the mud and allows the mud to return to the mud tank  22  for recirculation. The cuttings are sampled periodically for geologic purposes, but most are discarded. 
         [0005]    Since the drill bit  10  is usually rotated to form the wellbore, drilling rig  6  usually includes a rotary system. The rotary system includes drilling floor  50  and rotary table  52 . Located near the center of the rotary table  52  is a kelly bushing and kelly  54 . In many modern drilling units, a top drive motor (not shown) positioned in the derrick on rails replaces the rotary table, kelly bushing, and kelly for imparting rotation to the drill string. Rotational force is transferred from the rotary system to the drill string including drill pipe  30  and drill collars  32 . The drill collars  32  are heavy and stiff and assist in maintaining the bit in a vertical position. The weight of the drill collars  32  applied directly above the bit assists in increasing the cutting ability of the bit  10 . 
         [0006]      FIG. 1B  illustrates the draw works  68  of the drilling rig  6 . Wells are now being drilled to depths in excess of 30,000 feet. The tremendous weight of the drill string must be supported by a substantial derrick  60 . A drilling line  64  is attached to traveling block  66 , sheaved over crown block  62  and attached to draw works  68 . The draw works  68  includes a winch which is used to hoist the drill string, including the drill pipe  30 , drill collars  32  and bit  10 , out of the hole. Any drilling unit needs power to the turn the bit, power to drive the mud pump and power to run all the ancillary machinery. The power system on most offshore rigs is usually diesel/electric. The prime movers, being diesel, are used to drive generators to generate electric power which is used to power the other equipment. 
         [0007]    Mobile offshore drilling units (MODUs) move from one drill site on the water to another. There are two basic types of MODUs used to drill most offshore wells: (1) bottom supported units including submersibles and jack-ups; and (2) floating units including lake barge rigs, drill ships, and semi-submersibles. 
         [0008]      FIG. 1C  illustrates a semi-submersible MODU  70 . Floating units are typically used in water depths greater than where a bottom supported unit is capable. A semi-submersible  70  has a drilling rig  71  mounted in the middle and includes an opening, referred to in the industry as a moon pool (not shown), through which drilling operations are conducted. Semi-submersibles  70  include a lower barge hull  74  which floats below the surface of the sea  72  and is, therefore, not subject to surface wave action. Large stability columns  73  mounted on the lower barge hull  74  support the upper hull  78 , which includes a main deck and machinery deck above the surface of the water  72 . 
       SUMMARY OF THE INVENTION 
       [0009]    Embodiments of the present invention generally relate to a mobile offshore drilling unit. In one embodiment, a semisubmersible mobile offshore drilling unit (MODU) includes a submersible lower hull comprising a ring pontoon having a trapezoidal shape; an upper hull having the trapezoidal shape; three stability columns supporting the upper hull from the lower hull; and a drilling rig. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0011]      FIG. 1A  illustrates a drilling rig.  FIG. 1B  illustrates the draw works of the drilling rig.  FIG. 1C  illustrates a semi-submersible mobile offshore drilling unit (MODU). 
           [0012]      FIG. 2A  is a perspective view of a semi-submersible MODU in a drilling mode, according to one embodiment of the present invention.  FIG. 2B  is a plan view of the semi-submersible MODU in drilling mode.  FIG. 2C  is a profile view of the semi-submersible MODU in drilling mode. 
           [0013]      FIGS. 3A-G  illustrate a drilling rig, a.k.a. derrick equipment set (DES), of the MODU.  FIG. 3A  is a profile view of the DES.  FIG. 3B  is another profile view of the DES.  FIG. 3C  is a profile view of the drill pipe handling system.  FIG. 3D  is a plan view of the drill floor.  FIG. 3E  is a plan view of the process level.  FIG. 3F  is a plan view of the lower level.  FIG. 3G  is a plan view of the base. 
           [0014]      FIG. 4A  is a plan view of a mezzanine deck of the MODU.  FIG. 4B  is a plan view of a machinery deck of the MODU.  FIG. 4C  is a plan view of a tank deck of the MODU.  FIG. 4D  is a plan view of a lower level of the cabin.  FIG. 4E  is a plan view of an upper level of the cabin.  FIG. 4F  is a plan view of the helipad. 
           [0015]      FIGS. 5A-E  illustrate the DES being deployed in tender assist drilling mode.  FIG. 5A  is a plan view of the main deck.  FIG. 5B  is a plan view of the MODU connected to a platform.  FIG. 5C  is a profile view of the DES having been hoisted from the MODU and set on the platform.  FIG. 5D  is a profile view of the DES in place over the platform&#39;s moonpool.  FIG. 5E  is a perspective view of the MODU in tender assist drilling mode. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 2A  is a perspective view of a semi-submersible MODU  100  in a drilling mode, according to one embodiment of the present invention.  FIG. 2B  is a plan view of the semi-submersible MODU  100  in drilling mode.  FIG. 2C  is a profile view of the semi-submersible MODU  100  in drilling mode. 
         [0017]    The MODU  100  may be semi-submersible and include a helipad  101 , an upper hull  105 , one or more stability columns  110 , a submersible lower hull  115 , one or more cranes  120   a - c , one or more life-rafts  125 , a cabin  130 , a remotely operated vehicle (ROV)  135 , a mooring system  140 , a drilling rig, a.k.a. derrick equipment set (DES),  150 , and a moonpool  155 . The helipad  101  may be used for shuttling roughnecks to and from the MODU  100 . The upper hull  105  may have an trapezoidal shape, such as isosceles trapezoidal, and may include one or more decks, such as a main deck  105   p,  a mezzanine deck  105   z,  a machinery deck  105   m,  and a tank deck  105   t.  As discussed above, the upper hull  105  and stability columns  110  may float on the lower hull  115  and three stability columns may support the upper hull from the lower hull. The lower hull  115  may include one or more pontoons and a ballast system. The pontoon may be a ring pontoon having a trapezoidal shape corresponding to the upper hull. The pontoons may float near or on the surface when empty for towing the MODU  100  between locations and may submerge when ballasted for drilling or tender assist drilling mode (discussed below). The ROV  135  may be deployed to perform subsea work. The mooring system  140  may be employed to maintain position of the MODU  100  during drilling or tender assisted drilling. Alternatively or additionally, the MODU  100  may include a dynamic positioning system to maintain position of the MODU  100 . 
         [0018]      FIGS. 3A-G  illustrate the DES  150  in drilling mode.  FIG. 3A  is a profile view of the DES  150 . The DES  150  may include a derrick  205 , a top drive  210 , a drill floor  215 , a process level  220 , a lower level  225 , a base  230 , drawworks  235 , a crown block  240 , a blow out preventer (BOP)  245  (see  FIG. 3B ), a drill pipe handling system  250 , a mud treatment system  255 , a driller&#39;s cabin  265  (see  FIG. 3D ), and a skidding system  275 . The derrick  205 , drill floor  215 , process level  220 , and lower level  225  may be mounted on the base  230 , such as by fasteners. In the drilling mode, the derrick  205  may be located over the moonpool  155  to conduct drilling operations and the base  230  may rest on the main deck  105   p  and/or be fastened to the upper hull  105 . 
         [0019]      FIG. 3B  is another profile view of the DES  150 . The derrick  205  may include an upper mast portion  205   m  and lower housing portion  205   h.  In drilling mode, the mast  205   m  may extend from the housing  205   h.  The portions  205   h,m  may be connected, such as fastened, at an overlap  205   c.  To transport the derrick  205  for tender assist drilling mode or when towing the MODU  100  between locations, one of the cranes  120   a - c  may be fastened to the mast  205   m,  the connection  205   c  disassembled, and the mast  205   m  may then be lowered into the housing. The connection  205   c  may be reassembled, the derrick  205  may be disconnected from the base  230 , and the derrick  205  may be laid down on the main deck  105   p  (see  FIG. 5A ). 
         [0020]      FIG. 3C  is a profile view of the drill pipe handling system  250 .  FIG. 3D  is a plan view of the drill floor  215 . The drill pipe handling system  250  may include a bridge crane, a racking arm, a racking board, an elevator, one or more power tongs (IR), an offline stand building (OSB), and one or more pipe bins. Single joints of drill pipe  260  may be stored in the pipe bins. During and before drilling, the joints  260  may be assembled into stands of two or more joints, such as quad stands. The joints  260  may be assembled by retrieving a joint from the bins using a catwalk machine, raising the joint to a vertical position using the elevator, setting the joint into a mouse hole, and engaging slips of the mouse hole spider. A second joint may then be hoisted over the first joint in a similar fashion. The power tongs may engage the joints and make-up a threaded connection between the two joints. This process may be repeated until a quad stand is assembled. The racking arm may then retrieve the assembled stand and the bridge crane may lift the racking arm to store the stand in the racking board. 
         [0021]    Once the drill string has drilled to an end of a capable depth, the top drive may raise the drill string and set the drill string in a spider of a rotary table in the drill floor. The top drive may then be disconnected from the drill string and raised to a height proximate to the racking arm. The racking arm may retrieve the stand from the racking board and the bridge crane may lift the racking arm and the stand and hold the stand proximate to the top drive. The top drive may then engage the stand with a backup tong, and connect the stand to the quill. The top drive may then lower the stand to an end of the drill string. A second set of power tongs may engage the stand and make-up a threaded joint between the stand and the drill string. While the top drive is drilling with the extended drill string, the handling system  250  may be assembling and storing more stands as needed. 
         [0022]    The drill floor  215  may further include other accessories, such as a hydraulic power unit, an air compressor, a mud logger building, a measurement while drilling (MWD) building, a choke manifold, and a standpipe manifold. 
         [0023]      FIG. 3E  is a plan view of the process level  220 .  FIG. 3F  is a plan view of the lower level  225 . The mud treatment system  255  may include a process tank, one or more shale shakers, a degasser, a desander, a desilter, a centrifuge, one or more feed pumps, an agitator, one or more trip tanks, and a gas separator. The process tank may include a sand compartment, a degas compartment, a desand compartment, a desilt compartment, and a clean mud compartment. 
         [0024]      FIG. 3G  is a plan view of the base  230 . The base  230  may include a frame operably coupled to the skidding system  275 . The skidding system may include one or more linear actuators and one or more skid beams. Each linear actuator may be connected to the base and a respective skid beam. Each linear actuator may be hydraulic, such as a piston and cylinder assembly, or electric, such as a motor and ball-screw assembly. The skid beams may rest on the main deck  105   p  in drilling mode. The base  230  may be slidable on the skid beams by operation of the linear actuators. 
         [0025]    Additionally, the MODU  100  may include riser pipe for assembly and deployment of a riser string (see  FIG. 2C ). Further, the MODU may include production tubing (not shown) for assembly and deployment of a production tubing string. Further, the MODU may include a subsea wellhead and a subsea production tree (a.k.a. Christmas tree), such as a vertical or horizontal tree for deployment to the wellhead. The MODU  100  may prepare, drill, and complete a subsea wellbore. Alternatively, the MODU  100  may be deployed for workover of an existing subsea wellbore. 
         [0026]      FIG. 4A  is a plan view of the mezzanine deck  105   z.  The mezzanine deck  105   z  may include crew quarters and crew facilities, such as a washroom, toilets, locker room, and utility room. 
         [0027]      FIG. 4B  is a plan view of the machinery deck  105   m.  The machinery deck  105   m  may include a diesel/electric power system including diesel drivers and electric generators and a plurality of electrical control panels. The machinery deck may further include one or more mud pumps and one or more mud tanks, one or more reserve mud tanks, pre-load/ballast tanks, dry mud containers, dry cement containers, one or more cement blenders, and one or more cement pumps. The machinery deck may further include additional crew quarters and facilities, such as a galley, mess hall, cinema, and gym. 
         [0028]      FIG. 4C  is a plan view of the tank deck  105   t.  The tank deck  105   t  may include mud pit cellar tanks, drilling water tanks and fuel oil tanks. The tank deck may further include additional pre-load/ballast tanks. Further, electrical cables and piping (not shown) may extend between the decks  105   p,z,m,t  and the DES  150  for transferring fluids and electricity. 
         [0029]      FIG. 4D  is a plan view of a lower level  130   l  of the cabin  130 .  FIG. 4E  is a plan view of an upper level  130   u  of the cabin  130 .  FIG. 4F  is a plan view of the helipad. The cabin  130  may include crew quarters, a hospital, offices, a lounge, an emergency generator, a utility room, a control house, a conference room, a waiting area for the helipad, and an HVAC system. 
         [0030]      FIGS. 5A-D  illustrate the DES  150  being deployed in tender assist drilling mode.  FIG. 5A  is a plan view of the main deck  105   p.  In tender assist drilling mode, the DES  150  may be located on an adjacent production platform  500 . The MODU  100  may be connected to the platform  500  as if the DES were in drilling mode, supplying drilling fluid and electricity to the DES  150 . The MODU  100  may be connected using one or more hawsers and one or more umbilicals. Tender assisted drilling may be useful for workover of existing wells having the existing production platform  500  in place and connected to a subsea production tree (not shown). Otherwise, the existing production platform  500  would have to be disconnected from the subsea production tree and moved so that the MODU  100  could be moved over the production tree and connected to the tree. To move the DES  150  from the MODU  100  to the platform  500 , the DES  150  may be disassembled as discussed above. The DES  150  may be disassembled prior to towing the MODU  100  to the platform  500 . The MODU  100  may further include a telescopic personnel bridge  505  rotatable about the main deck  105   p  for deployment to the platform  500 . 
         [0031]      FIG. 5B  is a plan view of the MODU  100  connected to a platform  500 .  FIG. 5C  is a profile view of the DES  150  having been hoisted from the MODU  100  and set on the platform  500 .  FIG. 5D  is a profile view of the DES  150  in place over the platform&#39;s moonpool.  FIG. 5E  is a perspective view of the MODU  100  in tender assist drilling mode. The MODU  100  may be moored and/or dynamically positioned in place adjacent the platform  500 . The telescopic bridge  505 , hawsers, and umbilicals may be deployed to the platform  500 . The base  230 , drilling floor  215 , process level  220 , and lower level  225  may then be hoisted by the crane  120   a  onto the platform  500 . The crane  120   a  may include an elevator and a tower so that the crane boom may be raised on the tower to accommodate height difference between the MODU  100  and the platform  500 . The derrick  205  may then be hoisted by the crane  120   a  onto the platform  500 . The DES  150  may then be assembled. The DES  150  may be assembled on an edge of the platform  500  and skidded to the platform&#39;s moonpool using the skidding system  275 . The drill pipe  260  may be hoisted from the MODU to the platform and loaded into the rig&#39;s bins. 
         [0032]    Alternatively, instead of the drilling rig, the MODU  100  may include a J-lay tower (not shown) pivotably mounted to the main deck over the moonpool. The J-lay tower may be transported in a horizontal position and then raised to a vertical or near-vertical position for deployment of a subsea pipeline. Alternatively, the MODU  100  with the J-lay tower may be used to lay tendons for a tension leg platform. 
         [0033]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.