# Numerical Modelling of a Floating Wind Turbine Semi-Submersible Platform
## 3 Results and Discussion  
### Decay Tests without Moorings  
The first study was to simulate the decay test of the platform in heave without any external forces, such as moorings, that could affect the behaviour of the tests. In the computational runs, only three DoFs are unlocked, surge, heave and pitch, while the rest are locked. This is done due to the symmetry plane imposed in all the simulations to reduce the computational cost of each of the runs.  
The first step is to compare the results of the experimental and computational runs over time. To do this, the numerical platform is positioned at the highest point recorded by the experimental runs and then it is allowed to fall, and both signals are compared from their highest point recorded.  
In Figure 6, this comparison can be seen clearly. Both signals oscillate around the same equilibrium height, which is a good indication. However, it is clear that the computational run has a lower hydrodynamic damping, as expected. Moreover, the value is similar in the first ones, but starts varying in the latter one. Again, this may be due to the existence of different numbers of DoFs.  
However, to have a better understanding of the run, a damping factor study was undertaken. Because of the difference of DoFs in each of the tests, only a linear fitting was done to compare the damping of each of the tests. The slopes of each of the fittings can be related to the damping of each of the experiments. The slope of the experiment is 0.11, while the slope of the numerical fitting is 0.03. Clearly, the damping in the experimental campaign is much higher.  
Figure 5: Cubic fitting for the time dependency study where all the time-steps showed similar behaviour with increased differences in the latter periods.  
In Figure 7, it can be seen that the damping factor in the latter period is 0, which means that the amplitude increased in the experimental runs.  
This is because the amplitudes are very low and the behaviour of the platform in other DoFs that were not modelled in the simulations affects the response in heave. This is very difficult to simulate in the computational domain, and impossible if some DoFs are locked. These results magnify the effect of the damping in the platform, creating a larger slope. Although in the comparison the computational data seem to follow a linear tendency, it is important to remember that a cubic fitting adapts much better to its results, due to the three DoFs used in the runs. However, the different numbers of DoFs between the experimental and computational runs make this fitting less interesting.  
Figure 8 shows this by deleting the last two damping factors, related to the last period, which show a clearer relation between the slopes, where the experimental test gives a slope  
Figure 6: Signal comparison of the experimental (blue) and computational (orange) decays without the interaction of mooring systems.  
Figure 7: Damping factor study between the experimental and computational tests with linear fitting of both. Larger degree fittings were not studied due to the different DoFs used in both runs.  
of 0.0467 while the numerical one has a slope of the value 0.0287. The difference in height is, again, given by the lesser damping in the numerical models; however, the relation between periods is more logical. This makes it clear that for further research, a much longer time of decay testing will be needed, apart from the study of different DoFs.