Riser System |
 
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Riser System |
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This worked example considers a hybrid riser system – the model is a top tensioned riser concept, based on a free standing hybrid riser arrangement. The system consists of a dual bore vertical rigid steel riser, anchored to the seabed and tensioned by means of a buoyancy tank, with a connection to an FPSO through two flexible jumpers. One of the jumpers takes oil from the wellhead, while the other injects gas into the production stream. A T-frame connects the jumpers to the assembly at the upper end of the riser. The riser system is situated in 700m of water, with the top of the buoyancy tank positioned approximately 50m below the water surface. A schematic of the overall hybrid riser system configuration is shown in the below figure.
Hybrid Riser System Configuration
The central structure is comprised of the following components:
–The lower flex joint, which is 1m in length. The stiffness of the joint is 20kNm/degree.
–The lower frame, which is 9m in length. The frame is effectively rigid, and has a mass of 35 tonnes.
–The main riser, which is 605 m in length. The outer pipe has an external diameter of 364mm, and a wall thickness of 24mm. It is also coated with an additional 80mm wall thickness of insulation material (of density 900 kg/m3). The inner pipe has an external diameter of 296mm, and a wall thickness of 21mm. The outer and inner risers are connected with centralisers at regular intervals (of 25m). The inner pipe is oil filled (of density 800 kg/m3) while the annulus is gas filled (of density 200 kg/m3).
–The upper frame, which is 9m in length. The frame is effectively rigid, and has a mass of 35 tonnes.
–The upper flex joint, which is 1m in length, is located 5m along the upper frame. The stiffness of the joint is 20kNm/degree.
–The buoyancy tank, which is 25m in length. The tank is effectively rigid, and has a mass of 225 tonnes. It has a considerable external diameter of 6m.
The jumpers have the following specifications:
–Both jumpers are 350m in length.
–The oil (of density 800 kg/m3) jumper has an external diameter of 370mm, and a wall thickness of 58mm.
–The gas (of density 200 kg/m3) jumper has an external diameter of 210mm, and a wall thickness of 65mm.
The T-Frame is effectively a rigid massless structure. The below figure shows a close-up of the intersection region between the jumpers and central structure.
Intersection Region