Alternative Method of Buoy Supporting Riser (BSR) Installation

Abstract

Due to the 2200m water depth and harsher environmental conditions, one option that Petrobras is considering for the production of the Pre-Salt fields is the use of a subsurface buoy known as a Buoy Supporting Riser (BSR). It is composed of a subsurface tethered buoy, flexible jumpers connecting the Floating Production Unit (FPU) to the BSR and Steel Catenary Risers (SCRs) connecting the BSR to the flowlines on the sea bottom. The main advantages of this system are that it decouples the FPU motions from the SCRs, reducing fatigue damage in the touch down zone. It may also be installed independently of the FPU, except for the flexible jumpers, which would reduce the risers load on the FPU. Petrobras has been studying this concept since 1997 and has established, as a final stage of the study, a field test with the actual installation of the BSR. This was performed through an alternative method using only Petrobras AHTS boats, in order to avoid critical and expensive resources such as lift barges. With the purpose of validating this new installation procedure, Petrobras performed the referred installation of a 27.2m × 27.2m square ring shaped buoy in Congro Field in the Campos Basin over a water depth of 500m. The buoy was positioned at 80m depth, where the incidence of loads caused by waves is negligible, thus increasing the fatigue life of risers. After the BSR installation, the riser pull-in procedure was also conduced. This paper describes why this technology is necessary for these fields and the model tests made to validate the installation procedures. It also discusses how Petrobras tested the pull-in operations for two flexible risers after the actual buoy was installed. Monitoring systems were designed to check all forces and displacements during the referred installation. These actions will consolidate the BSR technology for Petrobras leading to another riser system option for production in ultra deep waters.