The directional BC mentioned above was a method in which the velocity-inlet and pressure-outlet conditions were imposed on each side of the computational area according to the azimuth angle of the azimuth thruster. Therefore, from the point of view of the target vessel, it is the same situation as a current flowing at a certain azimuth. An azimuth of 0° corresponds to a situation in which the vessel is going straight, and an azimuth of 90° means a situation in which the current flows from the port side of the vessel to the starboard side. The actual vessels generate load according to the direction of the current, which is called the current load. This study conducted a separate numerical analysis to obtain the load.
Tables 13 and
14 show the
CX,
CY, and the resulting force (
CTotal) for the target vessels, WTIV and FPSO, by classifying the current loads according to each azimuth in the X and Y directions. At this stage, the applied flow rate was the flow rate when the advance ratio was defined as 0.05, and the own current load of the vessel was calculated without the azimuth thruster attached.
Fig. 14 shows the total dimensionless loads calculated for each azimuth based on the value of the resultant force (
CTotal) of the current load when the azimuth is 0° in each target vessel are shown together. As shown in
Tables 13 and
14 and
Fig. 14, the degree of increase in load varied greatly depending on the vessel type. In the case of WTIV, based on the resulting force (
CTotal), when the azimuth was 0°, it increased to approximately four times when the azimuth was 90°. On the other hand, in the case of the FPSO, based on the resulting force (
CTotal) when the azimuth was 0°, the load value was up to approximately 20 times greater when the azimuth was 75°. Moreover, the size of the load value itself according to the direction of the current also showed a large difference depending on the type of vessel. It was shown that FPSO is approximately 10 times larger than WTIV. These results showed that even if the azimuth thruster attached to the target vessel constantly calculates the thrust during numerical analysis. The total thrust (
F_Total) effective for the vessel is greatly reduced as the current load of the vessel becomes excessively large. In particular, the magnitude of the current load may vary greatly according to the vessel characteristics, e.g., the vessel type, hull shape, appendage, and draft.
Fig. 15 shows the distribution of the dimensionless pressure coefficient of the hull calculated under the condition of the current at the 45° azimuth from the port side of each target vessel. The distribution form has different characteristics depending on the target vessel. Therefore, when performing numerical analysis to predict thrust loss due to thruster-hull mutual interference, Fixed_BC can be imposed on the side of the computational area rather than Directional_BC, which unnecessarily causes a large current load of the target vessel to be induced and unrealistically large thrust loss to be estimated. When Fixed_BC is imposed, a corresponding flow rate with a small advance ratio artificially defined for numerical stability was additionally imposed, and the effect of the current load can be minimized because this flow rate was very low. As shown in
Table 10, when the target vessel is WTIV, the loss of thrust estimated through numerical analysis matches well with the model test results. Even when the target vessel is FPSO, the thrust loss according to the azimuth was estimated to be within 10% regardless of the attachment position of the azimuth thruster, as shown in
Table 12. Moreover, when Fixed_BC was imposed as a boundary condition, FPSO, one of the target vessels, was predicted to have relatively less thrust loss than the other target vessel, WTIV. The causes are as follows. The azimuth thruster was installed at the lower part of the head box protruding downward from the hull, so the distance from the hull was relatively far. Second, the duct of the azimuth thruster applied in this study rotated downward (tilt), which can reduce the Coanda effect. Third, the optimal arrangement of azimuth thruster that minimizes interference between the wake direction of the azimuth thruster and the hull, the selection of an azimuth thruster with appropriate capacity, and the advancement of DP control algorithms are fundamentally needed to prevent excessive thrust loss due to thruster-hull mutual interference in offshore facilities or special vessels.