Description

A new concept isproposed by Subsea7: the Tethered Catenary Riser (TCR). The concept consists ofa number of steel catenary risers supported by a subsurface buoy which istethered to the sea-bed by means of an empty single pipe tendon and anchored bya suction pile; flexible jumpers make the connection between the Floatingproduction Unit (FPU) and the buoy. The paper focuses on application of the TCRin deep-water fields of Asia.

Applications

Risers are de-coupledfrom the FPU. The riser system is thus efficient for FPUs with large motions: theflexible jumpers effectively absorb the platform motions, thereby the rigidrisers have very small dynamic motions, which improves both strength and fatigueperformance. The system can be installed before FPU arrival on site, whichimproves the time before first oil. The riser system is best designed for anumber of risers between 4 and 8 in addition to umbilicals, thus convenient forone or two drilling centres. The space used by a TCR on the field lay-out isrelatively small, thus allowing several units to be placed around the FPU inbetween mooring lines.

Results

The TCR system usesonly presently qualified material and technology. The components are all fieldproven as they are used in existing riser systems. Analyses have shown that, with adequate geometry of the buoy, the latter is sufficient stable to induceacceptable tilt when different arrangements of SCRs and flexible jumpers areinstalled, and under variation of the fluid weight distribution in risers. As a result ofinstallation studies, a method very similar to the one commonly used by subea7 for Single Hybrid Risers has been selected for the buoy and tether system. Placement of rigid risers, jumpers and umbilicals is as done by Subsea7 for theBSRs.

Significance

The TCR concept is a credible alternative option for use inincreasingly harsh environments or from host platforms with large motions. Asall components are fully qualified and the installation method well proven, theTCR system is cost effective and ready for project application.

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