Single column semisubmersible drilling vessel

Abstract

The vessel comprises a pair of laterally spaced elongated hulls having a plurality of upstanding columns spaced along outboard portions of the hulls. In the preferred form, a central column supports a working platform in spaced relation above the hulls. The hulls buoyantly support the vessel in a low draft condition with the hulls having freeboard. The hulls and outboard columns have ballast compartments for ballasting and deballasting the vessel to respectively submerge the hulls, outboard columns, and a portion of the central column to provide a high draft condition with the mean waterline located intermediate the height of the central column and between the platform and the upper ends of the outboard columns, and return the vessel to a low draft condition. The outboard columns provide stability to the vessel during the initial portion of its transition from the low draft condition to the high draft condition and in the final portion of its transition from the high to the low draft condition. Stability in the high draft condition is predominantly provided by locating the center of gravity below the center of buoyancy. Drilling operations in the high draft condition are effected through the central column which also provides decks for mud handling, storage, etc.

What is claimed and desired to be secured by United States Letters Patent is: 
     
       1. A variable draft vessel comprising: a pair of elongated flotation members disposed in spaced side-by-side substantially parallel relation one to the other, said flotation members having a combined displacement sufficient to float the vessel in a low draft condition with the flotation members having freeboard and constituting substantially the entirety of the flotational support for said vessel in said low draft condition;   a working platform;   means connected between said working platform and said flotation members to support said working platform in fixed spaced relation above said flotation members;   a plurality of elements carried by said vessel between said flotation members and said platform with at least one of said elements comprising at least part of said connecting means;   said elements including a central, substantially vertically disposed, column .Iadd.located substantially at the vessel's central vertical axis .Iaddend.and a plurality of stabilizing members spaced laterally outwardly of said central column and one from the other;   the upper portions of said stabilizing members being spaced below said platform;   means for ballasting and deballasting the vessel to respectively submerge the flotation members, stabilizing members and a portion of said central column thereby to provide a high draft floating condition with the water line located intermediate the height of said central column and to return the vessel to the low draft condition, said .Iadd.stabilizing .Iaddend.members being located and having waterplane areas sufficient to provide stability to the vessel during at least a portion of its transition between low and high draft conditions, the displacement of the submerged flotation members, stabilizing members and said portion of the central column being sufficient to maintain the vessel buoyant in the high draft condition;   the vertical spacing between said platform and said flotation members being identical in both said high and low draft conditions of said vessel; and   means carried by said platform for conducting drilling operations external to said vessel when said vessel lies in said high draft floating condition.   
     
     
       2. The vessel according to claim 1 wherein said stabilizing members comprise columns upstanding from said flotation members, the upper end of said central column being connected to said platform with its lower end lying at an elevation lower than the upper ends of said stabilizing columns, the upper ends of said stabilizing columns being fixed a predetermined like distance below said platform in both said high and low draft conditions of said vessel. 
     
     
       3. The vessel according to claim 2 wherein the cross sectional area of the stabilizing columns at an elevation above said flotation members and also .[.containing.]. .Iadd.at an elevation overlapping .Iaddend.a portion of said central column decreases in a direction toward said platform. 
     
     
       4. The vessel according to claim 1 wherein its center of buoyancy is located below its center of gravity in the low draft condition and above the center of gravity in the high draft condition, said stabilizing members comprising columns upstanding from said flotation members a distance sufficient to provide waterplane areas during transition between the low and high draft conditions for vessel drafts between a draft wherein the mean waterline lies at substantially a common elevation with the lower ends of the columns and an intermediate vessel draft whereat the center of buoyancy and the center of gravity lie substantially at a common elevation, the upper end of said central column being connected to said platform and its lower end lying at an elevation lower than the upper ends of said columns. 
     
     
       5. The vessel according to claim 4 wherein the cross sectional area of said columns decreases in a direction toward said platform at an elevation between said platform and said flotation members at least as high as said latter mentioned common elevation. 
     
     
       6. The vessel according to claim 1 wherein said stabilizing members comprise columns upstanding from said flotation members, the upper end of said central column being connected to said platform and its lower end lying at an elevation lower than the upper ends of said columns, said connecting means including supports extending from the upper ends of said columns to said platform, the combined waterplane area of said supports being small in comparison with the waterplane area of said central column when said vessel lies in the high draft condition. 
     
     
       7. The vessel according to claim 1 wherein said central column has a diametrically enlarged portion adjacent its lower end. 
     
     
       8. The vessel according to claim 1 wherein said platform carries a drilling rig and said central column has an opening extending its full length, said rig being superposed over said central column for conducting drilling operations through said opening. 
     
     
       9. The vessel according to claim 2 wherein at least one of said columns on each of said flotation members has a cross section elongated in a direction substantially parallel to the long axes of said flotation members. 
     
     
       10. The vessel according to claim 2 wherein the centroids of said columns are transversely spaced from vertical planes passing through the longitudinal centerlines of said .Iadd.flotation .Iaddend.members. 
     
     
       11. The vessel according to claim 2 wherein said ballasting and deballasting means includes at least two ballast compartments in each of said .Iadd.flotation .Iaddend.members, said two ballast compartments being horizontally spaced one from the other, means for selectively ballasting and deballasting said two compartments in each said .Iadd.flotation .Iaddend.member to alter the natural period of said vessel. 
     
     
       12. The vessel according to claim 1 wherein said central column provides the sole structural connection .[.beteeen.]. .Iadd.between .Iaddend.said .[.base structure.]. .Iadd.flotation members .Iaddend.and said platform. 
     
     
       13. The vessel according to claim 1 wherein said platform extends laterally beyond said central column, and means carried by said vessel for supporting said platform extensions. 
     
     
       14. The vessel according to claim 13 wherein said central column provides the sole structural connection between said base structure and said platform, said platform extensions being cantilevered from said central column. 
     
     
       15. The vessel according to claim 1 wherein the center of gravity of the vessel in the low draft condition is located above its center of buoyancy, the vessel being configured such that in the high draft condition the center of gravity therein is located below its center of buoyancy. 
     
     
       16. The vessel according to claim 1 wherein said flotation members comprise hulls substantially rectangular in cross section and having a greater width than depth, and means connected to said hulls adjacent the upper surfaces thereof and extending transversely of said vessel for maintaining the hulls in spaced substantially parallel relation one to the other. 
     
     
       17. The vessel according to claim 1 wherein the distance between said platform and the upper ends of said .[.column.]. .Iadd.stabilizing members .Iaddend.is equal to or greater than the height of the maximum anticipated wave from crest to trough, the vessel being configured such that the mean waterline in the high draft condition is about medially of the distance between said platform and the upper ends of said .[.columns.]. .Iadd.stabilizing members.Iaddend.. 
     
     
       18. The vessel according to claim 1 wherein said stabilizing members comprise columns upstanding from said flotation members, the cross-sectional area of the stabilizing columns at an elevation above said flotation members and also .[.containing.]. .Iadd.at an elevation overlapping .Iaddend.a portion of said central column decreasing in a direction toward said platform, the upper end of said central column being connected to said platform and its lower end lying at an elevation lower than the upper ends of said stabilizing columns, the center of gravity of the vessel being located above its center of buoyancy in the low draft condition and below the center of buoyancy in the high draft condition, the stabilizing members upstanding from said flotation members a distance sufficient to provide water plane areas during transition between the low and high draft conditions for vessel drafts between a draft wherein the mean waterline lies at substantially a common elevation with the lower ends of the columns and an intermediate vessel draft whereat the center of buoyancy and the center of gravity lie substantially at a common elevation, said central column having a diametrically enlarged portion adjacent its lower end and at an elevation corresponding to the elevation at which the columns decrease in cross-sectional area, said platform carrying a drilling rig and said central column having an opening extending its full length, said drilling rig being superposed over said central column for conducting drilling operations through said opening, the centroids of said columns being transversely spaced from vertical plates passing through the longitudinal centerlines of said flotation members. 
     
     
       19. The vessel according to claim 1 wherein its stability characteristics are characterized by the formula GM = (KG-KG) + BM wherein GM is the metacentric height, KB and KG are the vertical heights of the center of buoyancy and center of gravity respectively above the lower extremity of the vessel and BM is the metacentric radius, the vessel being configured such that in the low draft floating condition a positive valued GM is obtained comprising the sum of a positive valued BM and a negative valued (KB-KG), the vessel being configured such that in the high draft floating condition a positive valued GM is obtained comprising the sum of a positive valued BM and a positive valued (KB-KG). 
     
     
       20. A variable draft vessel comprising: a base structure;   a working platform spaced above said base structure;   means connected between said working platform and  said base structure to support said working platform in fixed spaced relation above said base structure, a plurality of elements carried by said vessel between said base structure and said platform with at least one of said elements comprising at least part of said connecting means;   said elements including a central substantially vertically disposed column .Iadd.located substantially at the vessel's central vertical axis .Iaddend.and a plurality of stabilizing members spaced laterally outwardly of said central column and one from the other;   the upper portions of said stabilizing members being spaced below said platform;   means for ballasting and deballasting the vessel to respectively submerge said base structure, said stabilizing members and a portion of said central column thereby to provide a high draft floating condition with the mean waterline in the high draft condition located intermediate the height of said central column and to return the vessel to a low draft floating condition with the base structure having freeboard, said members being located and having waterplane areas sufficient to provide stability to the vessel during at least a portion of its transition between said low and high draft conditions, the displacement of said base structure, said stabilizing members and said portion of the central column being sufficient to maintain the vessel buoyant in the high draft floating condition;   the vertical spacing between said platform and said base structure being identical in both said high and low draft floating conditions of said vessel;   the vessel being configured such that in the low draft floating condition is center of gravity is located above its center of buoyancy and, in the high draft condition its center of buoyancy is located above its center of gravity; and   means carried by said platform for conducting drilling operations external to said vessel when said vessel lies in said high draft floating condition.   
     
     
       21. The vessel according to claim 20 wherein the contribution to overall vessel stability afforded by the waterplane area of the central column in the high draft condition of the vessel is small in comparison with the much larger contribution to overall vessel stability afforded by locating the center of gravity below the center of buoyancy in the high draft condition. 
     
     
       22. The vessel according to claim 20 wherein said base structure includes a pair of elongated flotation members disposed in spaced side-by-side substantially parallel relation one to the other, said stabilizing means comprising a plurality of columns upstanding from said flotation members. 
     
     
       23. The vessel according to claim 22 wherein said stabilizing columns upstand from said flotation members a distance sufficient to provide waterplane areas during transition between the high and low draft conditions for vessel drafts between a draft wherein the mean waterline lies substantially in a common elevation with the lower ends of said columns and an intermediate vessel draft whereat the center of buoyancy and the center of gravity lie substantially at a common elevation. 
     
     
       24. The vessel according to claim 23 wherein said flotation members have a combined displacement sufficient to float the vessel in the low draft condition with the .Iadd.flotation .Iaddend.members having freeboard, said platform carrying a drilling rig, said central column having an opening extending its full length, said rig being superposed over said central column for conducting drilling operations through said opening. 
     
     
       25. The vessel according to claim 23 wherein at least one of said columns on each of said flotation members has a cross section elongated in a direction substantially parallel to the long axes of said flotation members, the centroids of said columns being transversely spaced from vertical planes passing through the longitudinal centerlines of said members. .Iadd. 26. A variable draft vessel comprising: base means;   a working platform spaced above said base means;   means supporting said working platform above said base means in spaced relationship therewith, said support means including a substantially vertically disposed central column located substantially at the vessel's central vertical axis;   said base means including flotation means providing displacement sufficient to float the vessel in low draft condition with said base means having freeboard;   said base means including means forming a plurality of stabilizing compartments located about said central column so that the waterplane area of each such stabilizing compartment provides righting moment with respect to an axis of the vessel, with each such stabilizing compartment having an upper portion located at a predetermined vertical elevation below and spaced from said platform;   means for ballasting and deballasting the vessel to submerge said base means and stabilizing compartments and a portion of said central column thus disposing the vessel in a high draft semisubmerged operating condition with mean waterline in such high draft semisubmerged operating condition located intermediate the height of said central column above said upper portions of said stabilizing compartments and to return the vessel to a low draft floating condition with said base means having freeboard;   said stabilizing compartments being located and having waterplane areas sufficient to provide stability to the vessel during at least a portion of the vessel transition between said low draft condition and said high draft semisubmerged operating condition; and   the vessel and components thereof including said base means and working platform and central column being configured such that in said low draft floating condition the vessel's center of gravity is located above its center of buoyancy and such that in said high draft semisubmerged operating condition the vessel's center of buoyancy is located above its   
     
     
        center of gravity. .Iaddend. .Iadd. 27.  The vessel according to claim 26 wherein said central column has a diametrically enlarged portion adjacent its lower end. .Iaddend..Iadd. 28. The vessel according to claim 26 wherein said platform carries a drilling rig and said central column has an opening extending its full length, said rig being superposed over said central column for conducting drilling operations through said opening. .Iaddend..Iadd. 29. The vessel according to claim 26 wherein said central column provides the sole structural connection between said base means and said platform. .Iaddend. .Iadd. 30. The vessel according to claim 26 wherein vessel stabilizing characteristics are characterized by the formula GM = (KB- KG) + BM wherein GM is the metacentric height, KB and KG are the vertical heights of the center of buoyancy and center of gravity respectively above the lower extremity of the vessel and BM is the metacentric radius, the vessel being configured such that in the low draft floating condition a positive valued GM is obtained comprising the sum of a positive valued BM and a negative valued (KB- KG), the vessel being configured such that in the high draft floating condition a positive valued GM is obtained comprising the sum of the positive valued BM and a positive valued (KB- KG). .Iaddend. .Iadd. 31. A vessel according to claim 26 wherein the vessel and components thereof are such that the contribution to overall vessel stability afforded by the waterplane area of said central column in said high draft semisubmerged operating condition of the vessel is small in comparison with much larger contribution to overall vessel stability afforded by location of the vessel's center of gravity below the vessel's center of buoyancy in said high draft semisubmerged operating condition. .Iaddend..Iadd. 32. A vessel according to claim 31 further comprising means carried by said platform for conducting operations external to said vessel when said vessel is disposed in said high draft semisubmerged operating condition. .Iaddend..Iadd. 33. The vessel according to claim 26 wherein: said base means comprises at least one hull having hull portions supporting a plurality of columns which provide said stabilizing compartments, such stabilizing columns extending vertically upward from associated hull portions with the upper end portions of such stabilizing columns terminating below and spaced from said platform thereby providing said upper portions of said stabilizing compartments. .Iaddend..Iadd. 34. A vessel according to claim 33 wherein the upper end of said central column is connected to said platform and the lower end of said central column is disposed at an elevation lower than the elevation of the upper ends of said stabilizing columns. .Iaddend..Iadd. 35. The vessel according to claim 34 wherein such stabilizing columns have upper end portions which are at an elevation overlapping the elevation of a portion of said central column and which decrease in cross-sectional area in a direction extending upwardly toward said platform. .Iaddend. .Iadd. 36. The vessel according to claim 26 wherein: said central column has a lower end portion disposed at an elevation below the upper ends of said stabilizing compartments, and said stabilizing compartments extend verticlly upward in relation to said base means and said central column a distance sufficient to provide waterplane areas during transition between said vessel low draft condition and said vessel high draft semisubmerged operating condition. .Iaddend..Iadd. 37. A vessel according to claim 36 wherein: said stabilizing compartments provide such waterplane areas during such transition between a low draft wherein mean waterline is disposed along said base means and an intermediate vessel draft wherein the vessel's center of buoyancy and its center of gravity are both disposed substantially at a like elevation with respect to said central column. .Iaddend..Iadd. 38. The vessel according to claim 26 wherein: said base means comprises a pair of elongated flotation members disposed in spaced side-by-side substantially parallel relation one to the other; and said stabilizing compartments comprise columns vertically upstanding from said flotation members with the lower end of said central column lying at an elevation lower than upper ends of said stabilizing columns. .Iaddend. .Iadd. 39. The vessel according to claim 26 wherein the distance between said platform and upper ends of said stabilizing compartments is equal to or greater than the height of the maximum anticipated wave from crest to trough. .Iaddend..Iadd. 40. A vessel according to claim 39 wherein the vessel and its components are configured such that mean waterline in the high draft semisubmerged operating condition is disposed about medially of the distance between said platform and the upper ends of said stabilizing compartments. .Iaddend..Iadd. 41. A vessel according to claim 26 wherein said central column has a diametrically enlarged portion adjacent its lower end, and vertically located so as to raise the vessel's said center of buoyancy as the vessel draft increases effective after the vessel center of buoyancy passes above the vessel's said center of gravity when the vessel moves from said low draft floating condition to said high draft semisubmerged operating conditions. .Iaddend..Iadd. 42. A vessel according to claim 26, wherein said base means also includes solid material of high specific gravity providing permanent ballast for the vessel. .Iaddend..Iadd. 43. A vessel according to claim 42, wherein said permanent ballast is concrete. .Iaddend..Iadd. 44. A variable draft vessel comprising:   at least one floating body;   a working platform spaced above said body;   support means extending between said platform and said body to support said platform at a fixed distance above said body;   said body having a displacement sufficient to float the vessel in the low draft floating condition with said floating body having freeboard;   means for ballasting and deballasting the vessel to vary the draft thereof between the high draft floating condition with the mean waterline in a position intermediate said platform and said body, and a low draft floating condition with said body having freeboard;   said support means including a central column located at the point of intersection of the pitch and roll axes of said vessel, said column constituting a rigid connection between said platform and said body;   stabilizing members carried by said vessel and located about said central column with at least a pair of said members lying on each of the opposite sides of the vessel pitch and roll axis, said stabilizing members having a vertical extent sufficient to provide waterplane areas during transition between the low and high draft conditions for vessel drafts between a draft wherein the mean waterline lies at substantially a common elevation with the lower ends of said stabilizing members and an intermediate vessel draft wherein the center of buoyancy and the center of gravity lie substantially at a common elevation whereby said stabilizing members provide righting moments about the pitch and roll axes of the vessel;   said vessel in said high draft condition with the mean waterline above the uppermost vertical extent of said members obtaining pendulum type stability with the vessel's center of gravity lying below its center of buoyancy, the contribution to the stability of said vessel in said high draft condition afforded by the waterplane area of said central column being smaller than the contribution to vessel stability afforded by said pendulum type stability. .Iaddend. .Iadd. 45. A variable draft vessel comprising:   at least one floating body;   a working platform spaced above said body;   support means extending between said platform and said body to support said platform at a fixed distance above said body;   said body having a displacement sufficient to float the vessel in low draft floating condition with said floating body having freeboard;   said support means including a central column vertically disposed at a central vertical axis of the vessel located at the intersection of the pitch and roll axes of said vessel;   means for ballasting and deballasting the vessel to vary the draft thereof between high draft floating condition with the mean waterline in a position intermediate said platform and said body, and a low draft floating condition with said body having freeboard;   means carried by said vessel and forming a plurality of stabilizing compartments located at spaced positions about said central column so that waterplane area provided by each stabilizing compartment may provide righting moment with respect to an axis of the vessel, said stabilizing compartments having a vertical extent sufficient to provide such waterplane areas during transition between low and high draft conditions and for vessel drafts between a draft wherein the mean waterline lies at substantially a common elevation with the lower ends of said stabilizing compartments and an intermediate vessel draft wherein the center of buoyancy and the center of gravity lie substantially at a common elevation with said stabilizing compartments providing righting moment about axes of the vessel;   said vessel being configured so that when the vessel is disposed in said high draft condition with mean waterline above the uppermost vertical extent of said stabilizing compartments the vessel obtains pendulum-type stability with the vessel's center of gravity lying below its center of buoyancy, the contribution to the stability of said vessel in said high draft condition afforded by the waterplane area of said central column being smaller than the contribution to vessel stability afforded by said pendulum type stability. .Iaddend.