Deepwater Riser Technology

Un-coupled riser systems are increasingly being used in deepwater and ultra-deepwater field developments, where harsh environments predominate.

Hybrid Riser Towers (HRTs) are recognized to have significant benefits for deepwater riser applications in terms of flow assurance, thermal performance and robustness of layout. This latter issue is especially significant when a large number of risers are considered. An HRT provides the required flexibility by avoiding a crowded layout and allowing a progressive deployment. The concept is applicable to deepwater and ultra-deepwater, and to spread-moored and turret-moored FPSO installations.

The Single Hybrid Risers (SHR) in its Single or Pipe-in-Pipe versions, decouples the riser from the vessel dynamic motions and allows developments where the conditions are not suitable for SCR. Previously applied by Saipem on the Kizomba A, Kizomba B, Usan and Kizomba Satellites EPC3 Projects, it is now being utilized also for the Gas Export of the Saphinoá Norte and Cernambi Sul Pre-salt fields in Brazil. The Pipe-in-Pipe configuration of the SHR (PIP- SHR) allows a double service use in the same riser (e.g. production fluid in the inner pipe and gas lift in the annulus).

The Grouped SLOR (Single Line Offset Riser) is a qualified un-coupled riser system developed by Subsea 7 in collaboration with 2H Offshore for deepwater applications and based on the proven single-riser concept. Grouped SLOR is an 'open Bundle' riser solution developed specifically to optimise the riser/vessel interface, production vessel approaches and access for riser inspection and maintenance. It uses a buoyant truss frame to guide the free-standing risers, constraining all risers to move collectively, and thus eliminating the risk of clashing. The Grouped SLOR has great potential for large deepwater developments, which typically have a complex and congested seabed layout immediately adjacent to the production vessel where there are spatial constraints imposed by mooring lines and vessel offsets

The Buoyancy-Supported Riser (BSR) system has been fabricated and is being installed by Subsea 7 for the Sapinhoá (Guará) and the Lula Northeast pre-salt fields in Brazil under contract to Petrobras. The BSR concept consists of submerged buoys each weighing approximately 2,000 tons installed at approximately 250 meters below sea level and anchored to the seabed by eight tethers, two on each corner of the buoy. The buoy supports 27 steel catenary risers of 3.9 km, which are connected to the FPSO by non-bonded flexible jumpers. This BSR system absorbs the dynamics from the FPSO, resulting in almost no dynamic stresses on the SCRs. Since there is very little dynamic response for the SCRs, mechanically-lined pipe is used for the SCR section, thereby optimizing the riser design.

Catenary Offset Buoyant Riser Assembly (COBRA). COBRA is a new technology, which consists of a catenary riser section with a long, slender buoyancy module on top which is tethered down to the sea bed. The top of the catenary riser section is connected to the host platform by a flexible jumper. COBRA is an efficient riser arrangement for host platforms with large motions, e.g. FPSO or Semi submersibles. The flexible jumpers in this riser system effectively absorbs the platform motions, and consequently the steel catenary riser section has almost no dynamic motions, which improves both strength and fatigue performance.

Source: Subsea 7 & Technip