Lazy-Wave Steel Rigid Risers for FSO With Spread Mooring Anchoring System
Petrobras developed projects with European companies and Brazilian universities in order to study different configurations of steel risers using flexibilization elements. For the bow turret-moored FPSOs the lazy-wave configuration was considered the most adequate due to its structural behaviour and costs when compared to other configurations. A detailed study was performed by the Petrobras R&D Center to verify the structural integrity of a lazy-wave SCR (SLWR) attached to a turret-moored FPSO at a water depth of 1290 m. The results for the installed riser showed its feasibility. Petrobras continued the studies of the SLWR to verify its behaviour when connected to a FSO with a spread-mooring anchoring. This paper presents the approach and methodology adopted in Petrobras to verify the structural integrity of a SLWR attached to a FSO with spread-mooring anchoring at a water depth of 1800 m. The riser analysis was performed using the Petrobras’s in-house computer codes ANFLEX and POSFAL developed and implemented as part of projects from CENPES with “COPPE/UFRJ - The Engineering Post-Graduating Coordination of the Federal University of Rio de Janeiro”. For VIV (Vortex Induced Vibration) fatigue damage calculation SHEAR7 was used. Maximum stresses were verified through a deterministic non-linear time domain-analysis. The time-domain random nonlinear analysis was considered to be the most appropriate to be used for fatigue damage calculation due to the possibility of representing the existing non-linearities of the model and random characteristic of the environmental loading. For the fatigue damage analysis, a set of load cases that considers the bimodal / bi-directional characteristics of sea-states, probability of occurrence and energy content, was used.