HOUSTON – Subsea 7 is engineering a new concept riser configuration for the Guara-Lula NE field development in the deepwater Santos basin.
Operator Petrobras conceived the idea behind the Buoyancy Supported Riser (BSR) system, which involves deployment of submerged buoys at a midwater level below the FPSO. Following a design competition, Subsea 7 was appointed to take the concept forward as part of a $1-billion SURF contract for Guara-Lula NE.
The project scope includes engineering, procurement, installation and pre-commissioning of four de-coupled BSR systems, to be installed early next year in a water depth of more than 2,100 m (6,890 ft).
Each BSR will comprise a relatively flat, rectangular-shaped buoy structure weighing around 2,000 metric tons (2,205 tons), installed 250 m (820 ft) subsea and anchored to the seabed by eight tethers. The buoys will support a total of 27, 3.9-km (2.4-mi) long steel catenary risers (SCRs) suspended in parallel verticallly to the seafloor, and connected to the FPSO via non-bonded, flexible jumpers.
The BSR system is designed to absorb the dynamics from the FPSO, resulting in minimized dynamic stresses on the SCRs, and making them behave instead like a long, free-spanning pipeline, with the main fatigue response arising from vortex-induced vibration generated by localized currents.
As the SCRs’ dynamic response is so low, BuBi mechanically lined pipe can be used for the SCR section, technology applied since 1995 on towed pipeline bundle projects. However, this will be the first BuBi application involving installation via the reel-lay method, considered more cost-effective for the Petrobras project, and made possible following a recent joint development program by Subsea 7 and stainless steel manufacturer BUTTING.
Also integral to the BSR system is Subsea 7’s Angular Connection Module (ACM) that connects the flexible jumpers to the pre-installed buoys. This device was shortlisted for the American Society of Mechanical Engineers’ (ASME) Best Mechanical Engineering Achievement Award at this year’s OTC.
Because of the large number of risers installed, the buoys can take up different positions, resulting in a large range of angles. The ACM allows connections to be made at misaligned angles of up to 15° reliably and safely, reducing offshore hook-up time.
Although it missed out on the ASME prize, the ACM has been qualified by Petrobras following guidelines recommended by DNV RP-A203 Qualification of New Technology.
Early next year the Seven Oceans vessel will perform the reel lay and hookup to the subsea buoys. Guara-Lula NE’s associated production and injection lines will also be manufactured from BuBi mechanically lined pipe.