Steel Lazy Wave Risers From Turret Moored FPSO for Deepwater Harsh Environment

Abstract

Steel catenary risers (SCRs) have found greater applications in deep and ultra-deepwater developments. However, the deployment of SCRs in conjunction with a high motion deepwater floater such as the Floating Production Storage and Offloading (FPSO) system faces significant challenges due to their high motion characteristics, especially in harsh environmental conditions. The challenges posed by FPSO’s high motion characteristics include severe dynamic response on the SCRs and poor fatigue performance at the top section and the touchdown point (TDP) area. A number of alternative configurations of the SCR can be employed to decouple the FPSO’s motion from the SCR, thereby improving performance, and this include the steel lazy wave riser (SLWR) configuration. The lazy wave is achieved by introducing buoyancy modules along some lengths of the riser. In this work, a suitable SLWR configuration for deployment in conjunction with a turret moored FPSO was developed for a typical deepwater offshore West of Shetland environmental conditions. The optimum configuration is a low lazy wave configuration; this was achieved after several analyses using ORCAFLEX software program. In determining the optimum configuration, consideration is given to the SLWR sag and hog bend heights, the net buoyancy force, the buoyant section length, and the hang-off angle, among others. The extreme response, considering a combination of 100-year wave with 10-year current was satisfactory; the maximum stress was below the allowable stress level, and the maximum DNV utilization was less than unity, indicating a safe design. The wave-induced fatigue damage was calculated using a total of 216 load cases, resulting from 12-wave directions, and the wave-induced fatigue performance was satisfactory, with the minimum fatigue life observed at the riser’s TPD. Fatigue damage resulting from vortex induced vibration (VIV) was calculated considering currents in the in-plane and the out-of-plane directions to the riser, with a total of 22 load cases. The VIV fatigue performance was not satisfactory, and therefore fairings and strakes will be introduced to some lengths of the SLWR to suppress VIV. Detailed sensitivity studies also showed how the configuration can be further optimized. Overall, the results of this study showed that, the SLWR is a suitable riser concept for deployment from a turret moored FPSO, in deepwater, harsh environmental conditions such as offshore West of Shetland. The riser can be installed using Reeled-Lay installation method. The installation can be performed using pre-lay, abandonment, and recovery, as this offers advantages over the direct transfer approach.