2nd Order Pitch and Roll Slow Motions of a Semi-Submersible Platform: Full Scale Measurements and Theoretical Predictions Comparative Study

In October 2007, the semi-submersible platform PETROBRAS 52 (P-52) was installed in Campos Basin (Roncador Field) offshore Brazil. This Unit is moored through 16 lines in taut-leg configuration in a water depth around 1.800m. Its displacement at the operational draft (T = 27.5m) is 80.986t. The maximum production capacity is 180.000bpd. During the design phase of this floating system, a model test campaign was performed in a wave basin and slow drift motions in the vertical plane (heave, roll and pitch) were identified. It is known that resonant responses vary considerably with the damping loads. As these loads are affected by scale effects, by that time, it was a doubt if this phenomenon would happen during the platform operation. Since June 2008, PETROBRAS has been monitoring P-52 motions with the use of accelerometers and rate-gyros. Through spectral analysis of the measured signals, it was possible to verify the presence of slow motions with frequencies around the natural frequencies of roll and pitch during almost the whole monitoring period. Sometimes, the 2nd order amplitudes were even grater than the 1st order ones. Furthermore, the environmental conditions have also been monitored through wave radars, ADCPS (current) and meteorological stations (wind) in the vicinity of P-52 location, making the excitation loads identification possible. A comparative study confronting full-scale measurements and theoretical predictions was performed. First and second-order forces and responses were calculated using WAMIT® second order module. This study permitted to estimate the full scale damping values of this offshore system (hull plus mooring and riser lines) for one of the environmental conditions measured. This work demonstrates the importance of considering the resonant roll and pitch motions in the seakeeping analysis of large-volume semi-submersible platforms, contributing with an important feedback to future designs.