Lazy-Wave Steel Rigid Risers for FSO With Spread Mooring Anchoring System

Abstract

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.

Steel Catenary Riser Design and Analysis for Petrobras Roncador Field Development

“…There was a work carried out at CENPES that focused in considering two peak sea states and with environmental data bringing concomitant occurrence of wave, current and wind , [12,13,14]. .…”

Influence of Fatigue Issues on the Design of SCRs for Deepwater Offshore Brazil

“…Another study performed was checking the structural integrity and feasibility of a 10.75" oil export SLWR connected to the border of a spread-moored FSO at a water depth of 1800 m. The results for the installed riser also showed its feasibility [12]. Another study performed was checking the structural integrity and feasibility of a 10.75" oil export SLWR connected to the border of a spread-moored FSO at a water depth of 1800 m. The results for the installed riser also showed its feasibility [12].…”

“…The main aspect for fatigue is that analysis results showed that buoyancy modules region was more affected by VIV that contributed to almost 90% of fatigue damage in this region, leading to an extension of strakes in top region of about 30% of riser length [12].…”

Dynamic behaviors of conventional SCR and lazy-wave SCR for FPSOs in deepwater

“…The geometric characteristic of the LWSCR was basically suggested to isolate the riser motion from the vessel motions through the factitious arch and sag parts. By the motion isolation effect, LWSCRs may avoid significantly large dynamic amplification (Jacob et al, 1999;Silva et al, 1999;Torres et al, 2001Torres et al, , 2002Torres et al, , 2003Li and Nquyen, 2010). Yue et al (2011) performed analytic study to compare structural performance of conventional SCR, shaped SCR, and LWSCR, which are attached to a turret moored FPSOs in 800 m water depth, West Africa, under assumed 100-yr events.…”