Hybrid riser configuration
AssigneeAker Riser Systems AS
InventorsArne Sele, Magne Nygard
Life of this patent
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Abstract
Hybrid riser configuration comprising a plurality of steel riser pipes substantially inserted in aluminium guide conduits, with buoyancy means and tethering tension means, the guide conduits and riser pipes being rigidly connected to a base anchored to the ocean floor. The guide conduits serve as the tethering tension means and as radial constraint for the respective riser pipe therein to allow the riser pipe to buckle in elastic deformation (Euler) to form a spiral when expanding due to service pressure and temperature. A method for installing the riser configuration is also disclosed.
Claims19
A hybrid riser configuration having a submerged tower comprising a plurality of riser pipes substantially inserted in guide conduits, and also having buoyancy means acting as tower tensioning means, the riser pipes and guide conduits being connected to a base anchored to the ocean floor, wherein a plurality of the guide conduits are acting as multiple tethers, each guide conduit further acting as a radial constraint in elastic spiral deformation of the riser pipe inside, wherein during tow-out and installation, the guide conduits provide necessary buoyancy to make the riser configuration, except the base and buoyancy means, nearly neutrally buoyant.
A method for installing a riser configuration having a submerged tower comprising a plurality of riser pipes substantially inserted in guide conduits and also having a buoyancy tank and gravity base connected by said riser pipes and guide conduits, comprising the steps of:
- fabricating a bundle of guide conduits and riser pipes on a roller bed or rail bed from which it can be launched,
- connecting the buoyancy tank and gravity base to opposite ends of said bundle,
- sealing at least a plurality of the guide conduits and riser pipes of the bundle,
- launching the resultant structure and connecting the buoyancy tank and gravity base ends of the structure to respective towing vessels via towing wires,
- flooding the buoyancy tank to provide it with substantial negative buoyancy so that both the tank and the base will act as clump weights,
- towing the structure to the offshore location for its installation as a sub-surface tow while maintaining sufficient angle and tension in the towing wires to maintain substantial tension in the pipe bundle,
- lowering the base end of the structure by paying out the towing wire connected to the base,
- permitting water to enter the spaces formed between the riser pipes and their respective guide conduit when the base has reached a predetermined depth in order to limit the differential pressure across the wall of the guide conduits,
- continuing to lower the base end of the structure until the structure is perpendicular and suspended from the towing wire connected to the buoyancy tank, and
- lowering the structure to allow the base to penetrate the bottom mud-line and anchoring the base to the ocean floor, and removing the water ballast and towing wire from the buoyancy tank, thus providing tension in the guide conduits.
Dependencies are inferred from the claim text.