Improved Insight in Roll Motions of Ultradeep Water FPSOs Based on a Comparison of Calculations, Model tests and Full-Scale Measurements on the Girassol FPSO

Full-scale measurements of the motions of the Girassol FPSO are avaitable over a one-year period. During this same period also vessel draft, wave height and wave direction have been recorded. This allows the calculaïion of motion transfer functions asfunction of the FPSO loading condition. A comparison has been made between the measured motion transfer functions and the calculated motion transfer functions based on dijfraction theory, taking into account the measured wave spreading. Viscous roll damping has been added in the calculation to obtain maximum agreement between measured and calculated RAOs. The results show that wave spreading is a very important factor on how the vessel behaves and should be taken into account when evaluating measured full-scale motion RAOs. This paper addresses the full-scale monitoring campaign in some detail and the related issues. Furthermore, it describes the technique how measured wave spreading is taken into account in the calculation of theoretical motion RAOs. It is shown that the agreement between calculated and measured RAOs is greatly improved by the use of measured wave spreading in the calculation. The levels of viscous roll damping found in this tuning process can be described asfunction of loading condition and sea state. The viscous roll damping found in this process also shows good agreement to model test results performed earlier on this same FPSO.