Steel catenary risers (SCR) have been a favored choice for recent ultra-deep waters field developments subjected to harsh environments and large Floating Production Units (FPU) motions. The design of SCRs in such conditions is always a great challenge where the key issues are the fatigue near the hang-off and at the touch down point; clashing between lines, especially on fields with a large number of wells and high payloads on the production unit. This paper describes the solution provided by the Buoy Supporting Risers (BSR) concept which has recently been installed in the Santos basin offshore Brazil.
Subsea7 detailed the original concept of BSR system, from the design to fabrication and installation. The BSR concept combines several benefits to overcome the challenges of installing SCRs on ultra-deep waters, such as: allowing decoupling between installation of risers/flowlines and the platform, reducing payload on the production unit, very small dynamics transmitted from production unit to the risers, resulting in significant less fatigue issues. Also, the BSR concept reduces the risks associated with clashing and interference due to the smaller dynamics when compared to traditional coupled solutions. Fabrication is also addressed to highlight main challenges associated to assembling and welding clad and lined pipelines.
In order to install Corrosion Resistant Alloys (CRA) lined pipes with a Reel-Lay vessel, a novel methodology for water pressurized spooling and pipelay was developed to mitigate the risk of liner damage during installation. Full-scale pressurized spooling trials were done ahead of the pipelay campaign for testing and validation of this methodology. In addition, analyses were performed to evaluate the BSR system behaviour during SCRs installation and to minimize buoy de-ballasting interventions.
The SCR itself, due to de-coupling of motions by the buoy, have negligible dynamic response from vessel motions, thereby behaving almost like a static system with robust fatigue performance. The only meaningful fatigue in SCRs is due to current induced VIV and is mitigated using strakes. There was negligible potential clashing with adjacent SCRs.
This paper provides a summary of design and qualification work carried for SCRs installed in the BSR system the faced challenges and solutions engineered during fabrication and installation scopes.