Dry Tree Semi-Submersible With Reduced-Height Drilling Floor

Abstract

A floating, semi-submersible offshore drilling vessel has a blowout preventer located on the semi-submersible instead of on the seabed. The mud return housing is attached directly to the blowout preventer without the use of a slip joint. This configuration allows for a significantly reduced drill floor height and, consequently, a lower vertical center of gravity for the rig.

What is claimed is: 
     
         1 . A dry-tree, semi-submersible, offshore drilling vessel comprising:
 a support structure;   a riser tensioner attached to the support structure;   a drilling riser attached to the riser tensioner;   a blowout preventer having a first end attached to the drilling riser and an opposing second end; and,   a control head attached to the second end of the blowout preventer and having a central axial channel and a drilling fluid conduit in fluid communication with the central axial channel.   
     
     
         2 . The semi-submersible vessel recited in  claim 1  wherein the control head is attached to the blowout preventer without the use of a slip joint. 
     
     
         3 . The semi-submersible vessel recited in  claim 1  wherein the control head is attached to the blowout preventer with a fixed-length connector. 
     
     
         4 . The semi-submersible vessel recited in  claim 1  wherein the control head is attached to the blowout preventer with a connector having a fixed axial dimension. 
     
     
         5 . The semi-submersible vessel recited in  claim 1  further comprising a pump attached to the drilling fluid conduit on the rotating head. 
     
     
         6 . The semi-submersible vessel recited in  claim 5  wherein the pump is an in-line, progressive cavity pump. 
     
     
         7 . The dry-tree, semi-submersible, offshore drilling vessel recited in  claim 1  wherein the support structure comprises an upper deck and a lower deck and drilling fluid processing equipment is mounted on the main deck. 
     
     
         8 . The dry-tree, semi-submersible, offshore drilling vessel recited in  claim 7  wherein the drilling fluid processing equipment comprises a shale shaker. 
     
     
         9 . The dry-tree, semi-submersible, offshore drilling vessel recited in  claim 7  wherein the drilling fluid processing equipment comprises a mud tank. 
     
     
         10 . The dry-tree, semi-submersible, offshore drilling vessel recited in  claim 7  wherein the drilling fluid processing equipment is installed at a lower elevation on the vessel than the elevation of the control head relative to the vessel when the tensioner is at the limit of its upward travel. 
     
     
         11 . The dry-tree, semi-submersible, offshore drilling vessel recited in  claim 1  wherein the control head comprises a rotating seal configured to sealing engage a drill string within the central axial channel. 
     
     
         12 . The dry-tree, semi-submersible, offshore drilling vessel recited in  claim 1  further comprising a flexible conduit having a first end that is in fluid communication with drilling fluid processing equipment located on the vessel and an opposing second end attached to the drilling fluid conduit of the control head. 
     
     
         13 . A dry-tree, semi-submersible, offshore drilling vessel as recited in  claim 12  wherein the flexible conduit comprises an elastomer hose. 
     
     
         14 . A dry-tree, semi-submersible, offshore drilling vessel as recited in  claim 12  wherein the flexible conduit comprises flexible steel pipe. 
     
     
         15 . A dry-tree, semi-submersible, offshore drilling vessel as recited in  claim 12  wherein the flexible conduit comprises segments of rigid pipe connected by flexible joints. 
     
     
         16 . A dry-tree, semi-submersible, offshore drilling vessel as recited in  claim 1  wherein the tensioner is a ram-type tensioner. 
     
     
         17 . A dry-tree, semi-submersible, offshore drilling vessel as recited in  claim 1  wherein the support structure comprises an upper deck and a lower deck and the tensioner is mounted to the lower deck. 
     
     
         18 . A dry-tree, semi-submersible, offshore drilling vessel comprising:
 a support structure;   a riser tensioner attached to the support structure;   a drilling riser attached to the riser tensioner;   a blowout preventer having a first end attached to the drilling riser and an opposing second end; and,   a diverter attached to the second end of the blowout preventer and having a central axial channel and a drilling fluid conduit in fluid communication with the central axial channel.   
     
     
         19 . The semi-submersible vessel recited in  claim 18  wherein the diverter is attached to the blowout preventer without the use of a slip joint. 
     
     
         20 . The semi-submersible vessel recited in  claim 18  wherein the diverter is attached to the blowout preventer with a fixed-length connector. 
     
     
         21 . The semi-submersible vessel recited in  claim 18  wherein the diverter is attached to the blowout preventer with a connector having a fixed axial dimension. 
     
     
         22 . A method for lowering the vertical center of gravity of a semi-submersible, offshore drilling vessel having a drilling floor located at a first height, a rotary table on the drilling floor, a drill string engaged by the rotary table, a drilling riser having a first, upper end supported by a riser tensioner, a blowout preventer having a first end and an opposing second end connected to the first end of the drilling riser, a slip joint having a first end connected to the first end of the blowout preventer and an opposing second end connected to a diverter [bell nipple] comprising:
 removing the slip joint;   removing the diverter;   lowering the drill floor to a second, lower height; and, installing a drilling head having a rotating seal and a drilling fluid return conduit on the second end of the blowout preventer.   
     
     
         23 . The method recited in  claim 22  further comprising providing drilling fluid processing equipment on a lower deck of the semi-submersible in fluid communication with the drilling fluid return conduit on the drilling head. 
     
     
         24 . The method recited in  claim 23  wherein the drilling fluid processing equipment comprises a shale shaker. 
     
     
         25 . The method recited in  claim 23  wherein the drilling fluid processing equipment comprises a mud [drilling fluid] tank. 
     
     
         26 . The method recited in  claim 23  wherein the drilling fluid processing equipment comprises a mud [drilling fluid] pump. 
     
     
         27 . The method recited in  claim 23  wherein the drilling fluid processing equipment comprises at least one device from the group consisting of centrifuges, degassers, de-sanders, de-silters and hydrocyclones. 
     
     
         28 . A method for lowering the vertical center of gravity of a semi-submersible, offshore drilling vessel having a drilling floor located at a first height, a rotary table on the drilling floor, a drill string engaged by the rotary table, a drilling riser having a first, upper end supported by a riser tensioner, a blowout preventer having a first end and an opposing second end connected to the first end of the drilling riser, a slip joint having a first end connected to the first end of the blowout preventer and an opposing second end connected to a diverter [bell nipple] comprising:
 removing the slip joint;   removing the diverter;   lowering the drill floor to a second, lower height; and, installing a housing on the second end of the blowout preventer   wherein said housing has a drilling fluid discharge conduit, an inner member rotatable relative to said housing and having a passage through which the drill string may extend, and a seal moving with said inner member to sealingly engage the drill string.