Semi-submersible offshore vessel

1. A semi-submersible offshore vessel ( 1 ) exhibiting a first end ( 2 ) and a second end ( 4 ), said vessel ( 1 ) comprising:

a substantially rectangular ring-pontoon ( 6 ) including a first transverse pontoon section ( 10 ) located at the first end ( 2 ) of the vessel ( 1 ); a second transverse pontoon section ( 12 ) located at the second end ( 4 ) of the vessel ( 1 ), said second transverse pontoon section ( 12 ) being parallel to the first transverse pontoon section ( 10 ), the ring-pontoon ( 6 ) further including two mutually parallel longitudinal pontoon sections ( 14 ) extending between said first ( 2 ) and second ends ( 4 ) of the vessel ( 1 );

at least four support columns ( 16 , 18 , 20 , 22 ) extending upwardly from respective edge-portions ( 23 ) of said ring-pontoon ( 2 ), said support columns ( 16 , 18 , 20 , 22 ) being arranged in a first column pair ( 24 ) located at the first end ( 2 ) of the vessel ( 1 ) and a second column pair ( 26 ) located at the second end ( 4 ) of the vessel ( 1 ); and

an upper deck structure ( 28 ) positioned upon said support columns ( 16 , 18 , 20 , 22 ), wherein the first transverse pontoon section ( 10 ) has a vertical mean cross-section area (A) which exceeds the corresponding vertical mean cross-section area (B) of the second transverse pontoon section ( 12 ), and the support columns ( 20 , 22 ) in the second column pair ( 26 ) each has a water-plane area (E) which exceeds the water-plane area (D) of each of the support columns ( 16 , 18 ) in the first column pair ( 24 ).

2. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein the square root of the water-plane area (D) of each of the support columns ( 16 , 18 ) in the first column pair ( 24 ) is less than the longitudinal mean width (W 1 ) of the first transverse pontoon section ( 10 ).

3. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein the square root of the water-plane area (E) of each of the support columns ( 20 , 22 ) in the second column pair ( 26 ) exceeds the longitudinal mean width (W 2 ) of the second transverse pontoon section ( 12 ).

4. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein the second transverse pontoon section ( 12 ) has:

an outer side ( 54 ) which at least at pontoon top level for the second transverse pontoon section ( 55 ) is aligned with transverse outer sides ( 56 ) of the support columns ( 20 , 22 ) in the second column pair ( 26 ), and

an inner side ( 58 ) which at least at pontoon top level ( 55 ) is aligned with a transversal internal bulkhead ( 60 ) within said support columns ( 16 , 18 ) in the second column pair ( 26 ).

5. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein the support columns ( 16 , 18 ) in the first column pair ( 24 ) each have:

a transverse outer side ( 62 ) which at least at pontoon top level for the first transverse pontoon section ( 63 ) is aligned with an outer side ( 64 ) of the first transverse pontoon section ( 10 ), and

a transverse inner side ( 66 ) which at least at pontoon top level for the first transverse pontoon section ( 63 ) is aligned with a transverse internal bulkhead ( 67 ) within said first transverse pontoon section ( 10 ).

6. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein the support columns ( 16 , 18 ) in the first column pair ( 24 ) each have:

a transverse outer side ( 62 ) which at least at pontoon top level for the first transverse pontoon section ( 63 ) is aligned with a transverse internal bulkhead ( 67 ) within said first transverse pontoon section ( 10 ), and

a transverse inner side ( 66 ) which at least at pontoon top level for the first transverse pontoon section ( 63 ) is aligned with an inner side ( 65 ) of the first transverse pontoon section ( 10 ).

7. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein the first transverse pontoon section ( 10 ) has a vertical mean cross-section area (A) which exceeds the corresponding vertical mean cross-section area (B) of the second transverse pontoon section ( 12 ) by a factor of between 1.5 and 4.0.

8. A semi-submersible offshore vessel ( 1 ) according to claim 7 , wherein said factor is between 2.0 and 3.0.

9. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein the second transverse pontoon section ( 12 ) has a vertical mean cross-section area (B) which exceeds the corresponding vertical mean cross-section area (C) of each of the two longitudinal pontoon sections ( 14 ).

10. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein the support columns ( 20 , 22 ) in the second column pair ( 26 ) each has a water-plane area (E) which exceeds the water-plane area (D) of each of the support columns ( 16 , 18 ) in said first column pair ( 24 ) by a factor of between 1.3 and 2.5.

11. A semi-submersible offshore vessel ( 1 ) according to claim 10 , wherein said factor is between 1.5 and 2.0.

12. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein the support columns ( 16 , 18 , 20 , 22 ) are inclined upwardly and substantially radially inwardly from the ring-pontoon ( 6 ) to the upper deck-structure ( 28 ) towards a vertical centerline ( 42 ) of the vessel ( 1 ).

13. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein said edge portions ( 23 ) of the ring-pontoon ( 6 ) each has a horizontal mean cross-section area (F) which equals or exceeds the corresponding water-plane area (D, E) of the respective support columns ( 16 , 18 , 20 , 22 ).

14. A semi-submersible offshore vessel ( 1 ) according to claim 13 , wherein said edge portions ( 23 ) include narrowing transition cone elements ( 44 ) adapted to bridge differences in cross sectional areas between pontoon sections ( 10 , 12 , 14 ) and said edge portions ( 23 ).

15. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein said second transverse pontoon section ( 12 ) has a height which exceeds its width (W 2 ).

16. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein one or more steel catenary riser pipes ( 46 ) are attached to said second pontoon section ( 12 ).

17. A semi-submersible offshore vessel ( 1 ) according to claim 1 , further comprising a derrick ( 52 ) for performing offshore drilling operations positioned at said second end ( 4 ) of the vessel ( 1 ).

18. A semi-submersible offshore vessel ( 1 ) according to claim 1 , wherein said first end ( 2 ) is a forward end of the vessel and said second end ( 4 ) is an aft end of the vessel.