Guide frame for riser tower

1. A guide frame for location at one or more points along the length of a riser tower structure of a type having an upper end supported at a depth below the sea surface and comprising a central core and one or more conduits extending from the seabed toward the surface, said conduits being arranged around said central core, wherein said guide frame is adapted to guide the conduits relative to the central core, and wherein said guide frame is attachable to said riser tower structure non-continuously, thereby not becoming an integral part of said riser tower structure, the guide frame being comprised of a plurality of main pieces which are arranged to be assembled together around said central core, and further including apertures for guiding the conduits, each of said apertures being formed from an indent in one of said main pieces of said guide frame, and wherein metallic plates are provided across each joint between the plurality of main pieces, attached to the main piece on either side of said joint.

2. A guide frame as claimed in claim 1 wherein said guide frame is attached to said riser tower by means other than welding.

3. A guide frame as claimed in claim 1 being comprised of two main pieces.

4. A guide frame as claimed in claim 1 wherein said main pieces are arranged such that, when assembled together around said central core, the frame is held in place by bearing pressure and frictional force acting between said central core and said frame.

5. A guide frame as claimed in claim 1 wherein said assembly is effected by bolting together the main pieces to each other.

6. A guide frame as claimed in claim 1 wherein a main structure of the guide frame comprises a non-metallic material.

7. A guide frame as claimed in claim 1 wherein the main structure comprises a plastic material.

8. A guide frame as claimed in claim 7 wherein the main structure comprises an elastomer.

9. A guide frame as claimed in claim 7 wherein the main structure comprises polyurethane.

10. A guide frame as claimed in claim 1 being further provided with structural members arranged around said central core, when in-situ.

11. A guide frame as claimed in claim 10 wherein there is provided one of said structural members at each of the top and bottom of said guide frame.

12. A guide frame as claimed in claim 1 wherein said apertures are completed by a closing piece.

13. A guide frame as claimed in claim 12 wherein said closing piece comprises a metal clamp.

14. A guide frame as claimed in claim 12 wherein said closing piece is comprised of a plastic material.

15. A guide frame as claimed in claim 12 wherein each closing piece is directly fixed to the corresponding main piece.

16. A guide frame as claimed in claim 12 wherein a strap is placed around the cross section of the guide frame to maintain the closing pieces in place.

17. A guide frame as claimed in claim 16 wherein the closing pieces are maintained in place by shear keys.

18. A guide frame as claimed in claim 1 wherein said apertures are designed for the siting therein of riser guides, to guide each conduit.

19. A guide frame as claimed in claim 1 adapted to guide said conduits without holding them, such that they may move axially with respect to one another and the central core.

20. A riser tower of a type having an upper end supported at a depth below the sea surface and comprising a central core and one or more conduits extending from the seabed toward the surface, said conduits being arranged around said central core, wherein said riser tower further comprises one or more guide frames located at corresponding points along the length of the riser tower structure so as to guide the conduits relative to the central core, said guide frames being attached to said riser tower structure non-continuously, thereby not becoming an integral part of said riser tower structure, each of said guide frames being comprised of a plurality of main pieces which are arranged to be assembled together around said central core and further including apertures for guiding the conduits, each of said apertures being formed from an indent in one of said main pieces of said guide frame, wherein metallic plates are provided across each joint between the plurality of main pieces, attached to the main piece on either side of said joint.

21. A riser tower as claimed in claim 20 wherein said riser tower further comprises buoyancy modules which act on the underside of some or all of said guide frames.

22. A riser tower as claimed in claim 21 wherein said buoyancy modules act upon the periphery of said guide frames.

23. A riser tower as claimed in claim 21 wherein said riser tower is arranged such that buoyancy modules act upon different points of some or all of said guide frames.

24. A riser tower as claimed in claim 20 wherein said central core comprises an abutment surface for each of said guide frames, such that, when deployed, the top of said guide frame, or a portion thereof, abuts against said abutment surface.

25. A riser tower as claimed in claim 20 wherein said one or more guide frames are assembled around said central core such that, where there is a longitudinal weld present in the central core, said weld being positioned between two of said main pieces of said guide frame.

26. A riser tower as claimed in claim 20 further comprising umbilical cables, fibre optic cables and other elongate objects, at least some of which are guided or supported by said guide frame.

27. A riser tower as claimed in claim 20 wherein said central core has been treated at the points where said guide frames are attached, prior to their attachment.