HORIZONTAL OSCILLATION OF A TANKER

In this section, we compare the MOSES predictions for the motions of a tanker on a single point mooring with those of Wichers (1988). The model being simulated consists of a ballasted 200 KDWT tanker with a 90 meter hawser connecting it to the single-point mooring (SPM). The SPM system is exposed to 60 knot head winds and a 1.03 m/s head current. The coordinate system that is used is shown in Figure 37. The global coordinate system is fixed to earth, and head seas are defined as the environment in the positive $x$-direction. Also shown in Figure 37 is the top view of the panel model that is used to define the geometry for the 200 KDWT tanker.

Figure 37: Coordinate System

The MOSES simulation consisted of 1700 seconds of response history that was computed at 2 second intervals. Figures 38 and 39 compare the $x$ and $y$ motions versus time obtained from the MOSES simulation with model tests and simulations that were obtained by Wichers. These results are quite interesting in that we have an unsteady motion of the system in an environment which is constant. In other words, what we see here is the result of an instability. In view of this, the closeness of the three results is quite remarkable. Wichers used experimental results to obtain coefficients in his simulation. For the MOSES simulation, the wind and current forces were computed using the #TANKER load group and nothing else was done.

Perhaps of even greater interest here is the variation of the tanker yaw angle with time. This is shown in Figure 40.

Figure 38: Comparison of MOSES Time Domain Against Wichers Model Test and Simulation – X Motions

Figure 39: Comparison of MOSES Time Domain Against Wichers Model Test and Simulation – Y Motions

Figure 40: MOSES Time Domain – Yaw Motions