A Frequency Domain Approach for the Random Fatigue Analysis of SCR Considering Bimodal/Bidirectional Characteristic of Campos Basin Sea States

Fatigue verification is an important issue in steel risers design, demanding a good representation of the loading conditions that will occur during the riser entire lifetime. PETROBRAS has carried out a series of measurement and acquisition programs over the past decade, including the Campos Basin simultaneous acquisition of waves, current and wind data. The campaigns are called the PROCAP1 in Marlim Field and PROCAP2 in Barracuda Field. Those programs provided simultaneous environmental data (wave, wind and current) containing multimodal / multidirectional sea-states that occur in Campos Basin, with two main peaks dominating the total energy content [20,21]. As fatigue damage calculation depends on the stresses variations during the lifetime of the structure, the set of loads used in the analysis should be complete enough to represent all possible situations. The high number of loading conditions used in riser fatigue verification associated with the random time-domain analysis that demands a high computer time for processing the analysis, impact the design schedule. The frequency domain approach, based on linearization techniques, is an alternative tool for riser analysis and has been studied mainly for structural fatigue verification applications. For this particular application, due to the low intensity of loadings, the geometric nonlinearity is considered by means of a previous nonlinear static analysis, followed by a dynamic frequency domain analysis on the deformed model. The nonlinearity of the drag part of Morison’s formula has to be conveniently treated by linearization techniques. This work presents a comparative study where the results using a frequency domain analysis are compared to the results of a time domain analysis. Both approaches were used in the analysis of a steel lazy-wave riser (SLWR) model connected to a spread-moored FPSO, submitted to fatigue environmental loadings considering the bimodal/bidirectional characteristic of Campos Basin sea-states. The analyses were performed using the PETROBRAS’s in-house computer codes ANFLEX, ALFREQ and POSFAL developed and implemented as part of projects from CENPES/PETROBRAS with “COPPE/UFRJ - The Engineering Post-Graduating Coordination of the Federal University of Rio de Janeiro”.