Recently turret-Moored FPSOs have been used in many deep water developments worldwide, with consideration of dis-connectable turrets for harsh environment with iceberg conditions. This trend makes the interactions between FPSO and risers system more important. Further, Steel Lazy Wave Risers (SLWR), which is a compliant variant of the mostly commonly used Steel Catenary Risers (SCR), is becoming an attractive riser option.
The paper provides a review of two riser systems that can be considered for turret-moored FPSOs, and specific emphasis on Steel Lazy Wave Risers. A detailed case study of Steel Lazy Wave Risers for a typical turret moored FPSO with dis-connectable turret is presented. The design drivers are; to reduce the turret pay load, especially in disconnected condition, the selected riser system should have minimum tension variation so that the disconnected turret is stable and an optimum installed overall cost of the system. To achieve the design objectives an improvised Steel Lazy Wave Riser configuration called "Tethered Steel Lazy Wave Risers" (TSLWR) is developed.
Pros and cons for TSLWR are discussed, with consideration of the particular challenges of turretmoored FPSOs with large floater motions, hang-off geometry constraints at turret, hang-off loads, riser interferences, risers pre-installation, and turret disconnection constraints.