Hybrid riser tower and methods of installing same
Disclosed is a riser comprising a plurality of pipelines. In one example there are three such pipelines, extending from the seabed toward the surface and having an upper end supported at a depth below the sea surface, wherein, in one embodiment a first of said pipelines acts as a central structural core, the other pipelines being arranged around said first pipeline. In another embodiment three pipelines are arranged around a structural core. In each case, the first of said pipelines may be a fluid injection line, said other pipelines being production lines. Also disclosed is a riser having buoyancy along at least a part of its length, said buoyancy resulting in said riser having a generally circular cross-section, the circumference of which being non-contiguous. Methods of installing such risers are also are also described.
1. A method of installing a riser, said riser comprising a plurality of conduits extending from the seabed toward the surface and having an upper end supported at a depth below the sea surface by a buoyancy module, said riser being assembled at a place other than the installation site and transported thereto in a substantially horizontal configuration wherein said buoyancy module is attached to said riser by a non-rigid connection prior to said riser being upended to a substantially vertical working orientation, said riser and buoyancy module being transported together by a first, leading, vessel and second, trailing, vessel, the method further comprising the steps of:
connecting said second vessel by a first line to the top end of the riser during transportation and paying in said first line to move toward the riser;
rotating the buoyancy module approximately 90 degrees;
making a permanent connection between said riser and buoyancy module on a service vessel;
disconnecting a second line, which connected the top of the buoyancy module to the top of the riser during transportation, and passing said second line to said second vessel;
disconnecting said first line; and
initiating the upending of the riser.
2. A method of installing a riser as claimed in claim 1 wherein said connection between the buoyancy module and the riser is made at the installation site.
3. A method of installing a riser as claimed in claim 1 wherein said non-rigid connection is made using a chain.
4. A method of installing a riser as claimed in claim 3 wherein said chain is provided in two parts during transportation, with a first part connected, directly or indirectly, to the riser and a second part connected, directly or indirectly, to the buoyancy module while being transported.
5. A method of installing a riser as claimed in claim 4 wherein said parts are of approximately equal length.
6. A method of installing a riser as claimed in claim 4 wherein said parts are each in the region of 10 m to 30 m long.
7. A method of installing a riser as claimed in claim 4 wherein the two parts are connected together on a service vessel.
8. A method of installing a riser as claimed in claim 4 wherein, in order to provide room to make the connection, the buoyancy module tank is rotated prior to connection.
9. A method of installing a riser as claimed in claim 8 wherein said rotation is through approximately 90 degrees.
10. A method of installing a riser as claimed in claim 1 wherein said buoyancy module is towed to the installation site with the riser.
11. A method of installing a riser as claimed in claim 10 wherein said buoyancy module is towed behind said riser by connecting a towing line between the riser and the buoyancy module, independent of any other towing lines.
12. A method of accessing a coil tubing access unit located substantially at the top of a riser structure, said riser structure comprising a plurality of conduits extending from the seabed toward the surface and having an upper end supported at a depth below the sea surface by a buoyancy module, wherein said method comprises
attaching a line to a point substantially near the top of said riser; and
exerting a force on said line to pull said riser, or a top portion thereof, from its normal substantially vertical configuration to a configuration off vertical;
wherein the tension on said line also causes said buoyancy module to be moved a distance laterally away from the vertical axis of said riser, thereby allowing access to the coil tubing access unit from directly above.
13. A method as claimed in claim 12 wherein said buoyancy module is attached, directly or indirectly, to said riser by means of a non-rigid connection.
14. A method as claimed in claim 13 wherein said non-rigid connection comprises a chain.
15. A method as claimed in claim 12 , wherein said line is attached to a lower portion of said buoyancy module.
16. A method as claimed in claim 12 wherein said force is exerted on said line by means of a winch or similar device.
17. A method as claimed in claim 16 wherein said winch is located on a floating production, storage and offloading vessel.