Hydrodynamic analysis
In order to gain a better understanding of our platform design characteristics it was important to undertake a hydrodynamic analysis. This was achieved by modelling the platform in DNV GL software SESAM - GeniE, which was meshed and then imported into SESAM - HydroD. The HydroD applications allows for the completion of the hydrodynamic analysis. This application also allows the simulation of mooring lines to be considered in the heave response condition, by understanding the theory behind the software one can input the appropriate values into the restoring matrix of the platform to correctly simulate the mooring lines.
The results obtained show, in the heave response an expectedly low response due to waves and a high frequency of around w=0,725 which is due to the fact that the system is very stiff because of the presence of the mooring lines in the form of the restoring matrix. When the natural frequency is compared to the Pierson-Moskowitz spectrum for various sea states it can be seen that the natural frequency successfully avoids resonance with any of the extreme sea state cases which is a success. However, the natural frequency does coincide with smaller sea states that are perhaps more common everyday waves, although the amplitude is very small. This would suggest that some vibration may occur however this platform is conceptualised so that there are minimal personnel on board therefore the comfort is not a priority in design.
For the response in waves in the pitch direction (which will be identical to roll) the platform has a low natural frequency around 0.4. This would suggest to us that perhaps due to this low frequency and long pitch period that the metacentric height (GM) of the platform is not very large. Now a larger GM does mean that the platform will be more stable so in future iterations of this design it may be more appropriate to try and reduce the water-plane area of the platform (central column) but maintain a large structure further below the surface where waves have less effect and the structure can act as a righting moment thus the GM increased.
However, having said this, no restrictions were placed in the restoring matrix for pitch movement axis so this result does not include the effects of the mooring lines. To do this fully a large and time consuming stability analysis using a software like Orcina Orcaflex would be undertaken to best establish the full stability with all components involved. This platform still maintains a large potential to be used in the North Sea for this purpose because its natural frequency does not coincide with the extreme wave conditions and can meet the needs for energy production and sustainability discussed in other sections of this website.
