Predicting Wind Loads for FPSO Tandem Offloading Using CFD

To assess the applicability and accuracy of CFD calculations on typical offshore configurations the wind loads for FPSO tandem offloading is investigated. Within the OO1 JIP windtunnel experiments have been carried out for FPSO’s with different deck configurations behind which a shuttle tanker was positioned. For the CFD calculations presented in this paper one specific set-up is chosen, i.e. a fully-loaded, barge-shaped FPSO with 5 square blocks on the deck and a ballast loaded shuttle tanker at distance 450m. The heading with respect to the main flow direction is 30 degrees for the FPSO and 0 degrees for the shuttle tanker, respectively. For the CFD calculations we choose to ‘model the model’, i.e. the same dimensions for the vessels and tunnel walls are chosen as in the wind-tunnel experiments as well as the same velocity, i.e. 25 m/s, which yields a Reynolds number equal to 2 million. Steady-state solutions are calculated for the FPSO alone at 30 degrees, the shuttle tanker alone at 0 degrees and the shuttle tanker at 0 degrees shielded by the FPSO at 30 degrees with a full-scale distance of 450m. CFX and the MARIN in-house code ReFRESCO are used to investigate the applicability of these tools. Furthermore, the use and accuracy of calculations with different mesh types are investigated by using hexahedral h-refined unstructured grids versus block-structured hexahedral grids, which are constructed with the grid generation packages Hexpress and ICEM-CFD, respectively. The comparison between experiments and computations shows that CFD can provide reasonable answers for the wind coefficients on single vessels. For wind shielding, considerations on the improvement of the overall accuracy of CFD calculations for this type of applications are also presented, as it is found to be difficult to obtain the same results as in the wind-tunnel.