Within the OO1 and OO2 “Offloading Operability” JIPs, wind-tunnel measurements were carried out to determine the wind loads on a number of vessels, e.g. Membrane (Prismatic) and Moss (Spherical) type LNG Carriers, Shuttle tanker and FPSO. Furthermore, wind velocity measurements were performed in the wake of the FPSO. For all these vessels CFD calculations are carried out using MARIN’s in-house code ReFRESCO. In [9] the sensitivity of the CFD results was investigated for various parameters, such as grid resolution, grid type, flow solver and geometric simplifications. In this paper it is shown that the calculated forces are very dependent on the atmospheric velocity profile used in the calculations. Therefore, different profiles are first compared with the profile measured in the wind tunnel. Then, the development of the profile in the CFD calculations is investigated. Using the appropriate grid resolution and the correct velocity profile, the wind loads on 5 different vessels are calculated for the angles 0 to 180 degrees in steps of 10 degrees. Overall, the calculated force coefficients agree well with the measurements for all vessels. Furthermore, the wind velocity distribution in the wake of the FPSO shows a good agreement with the velocity measurements. The location and shape of the wake are predicted accurately and the calculated velocities show reasonable agreement with the measured velocity distribution. From the results presented in this paper we conclude that with ReFRESCO the wind loads on typical offshore vessels can be predicted with reasonable accuracy in a cost-efficient manner. The accuracy of the calculated velocity fields is such that these can be used in prediction methods to obtain shielding effects on vessels positioned in the wake.

The effects of hydrodynamic and aerodynamic interactions on a typical shuttle tanker (ST), when offloading a FPSO moored in Spread Mooring System (SMS) configuration are being studied in an extensive research project conducted at the University of Sa˜o Paulo, in collaboration with Petrobras. The numerical models will be incorporated in the simulators in order to evaluate the impact of such effects on the dynamics of the ST and on its DP System power requirement. Present paper focuses the wind shielding effect, since current and wave wake effects were already treated in previous works (Illuminatti et al., 2009; Queiroz Filho et al., 2009). A detailed CFD model of the FPSO and ST were used to evaluate the horizontal forces and yaw moment induced in the ST by the wind when inside the shadow zone defined by the FPSO. The CFD model was calibrated using wind tunnel measurements of a similar system. Typical tandem configurations were considered for the connection and oil-transfer stages of the operation. The power requirements for each DP thruster were then calculated, considering a thrust allocation algorithm. The comparisons with the stand-alone ST configuration indicated that the wind shielding effect is important concerning DP power. Besides, it is proposed a simplified model for wind forces and moment in the ST, considering the wind velocity field in the wake of the FPSO (CFD calculation). The procedure is based on report [7] where the wind force is evaluated through de summation of forces and moment in the stern, middle and bow parts of the ST. Such procedure has the advantage of requiring only one CFD calculation (for obtaining the velocity field downstream FPSO).

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The use of motion compensated equipment has become more frequent in modern DP operations, especially the ones related to the installation, maintenance and decommissioning of offshore wind turbines. In these operations, while low-frequency motions are compensated by DP systems, wave-frequency motions are compensated by gripper frames, heave compensators, hexapods, gangways and other type of motion compensated equipment. A study was performed to investigate if the coupling of a dynamic positioning system with a motion compensation device could improve the accuracy and efficiency of offshore motion compensated operations. The reference case consisted of the positioning of a payload, hanging on a ship-mounted crane, by means of a single horizontal active tugger line and the ship's DP system. Four controllers – consisting of two levels of awareness and two tuning conditions – were designed and implemented in a numerical model. The four strategies were then evaluated in terms of position offsets and actuator usage under a number of sea-states. Results demonstrated that coupled controllers are able to decrease the position deviations of the motion compensated object and decrease the usage of the motion compensation device. The paper describes the control strategies, numerical models and results of the simulations.

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