Water depths, seabed conditions, high pressures and high temperatures are all factors to be considered when designing systems for the extraction of hydrocarbons in deep and ultra-deepwater fields.
Ultra-deepwater fields have attracted much attention in recent years and have generated a range of riser solutions to transport the produced hydrocarbon fluids to the surface processing facility. In some cases these solutions reflect the particular expertise of the main contractor while project-specific requirements dominate in other scenarios.
HRT assembly ready for launch © by courtesy of Subsea 7
Established solutions include steel catenary risers, flexible flowlines in wave or MWA configuration, single leg top-tension risers and hybrid riser towers. The last-named refers to multiple risers grouped around a single core pipe where the entire assembly is supported in vertical orientation by a massive buoyancy assembly. Due to operational depths going to some 2000msw the majority of solutions require buoyancy to provide hydrodynamic stability and/or riser tension reduction.
With the particular field configurations, depths of reservoirs and relatively benign sea states found offshore West Africa, the hybrid riser tower (HRT) has assumed a dominant position. With the proven success of HRTs in four major West African projects to date the same solution offers significant potential for other deepwater projects both there and elsewhere in the world.
The hybrid riser tower typically comprises 4-12 individual risers grouped around a central core pipe anchored to the seabed and supported by a massive air can at the top. Further to this tower-top air can, buoyancy is routinely provided along the length of the bundle and on occasion at the upper and lower riser tower assemblies – URTA and LRTA respectively.
The primary purpose of the bundle, URTA and LRTA buoyancy systems is to facilitate surface tow-out of the bundle, however the same buoyancy also contributes to the stability of the vertical HRT when in service.