Creation of data set (data driven)
The geometry optimization of a mooring system for a FOWT is investigated. The cycles of loads acting on the FOWT arising from wind and waves causes changes in the dynamic characteristics of the floating structure. Thus, the mooring system has to be designed in such a way that it achieves the desired stability requirements, without adversely impacting the reliability of the overall system (Pillai et al., 2019). Good design must, therefore, take into account the characteristics of the floating platform, turbine, and mooring, as well as the interaction between these elements.
For each mooring line, the optimization routine selects the length of the unstretched mooring line, the radius of the anchor from the center-line of the platform, and the z-position of the fairlead from the SWL. The decision variables are shown below.
The length of the line and the radius of the anchor from CL of the platform has been limited to be between 800 m and 900 m, while the z-position of fairlead has been limited between 0 m and 19 m. z = 0 m has been omitted, as it is not desirable to have the fairlead at the water-air interface, while, z = 20 m has also been left out as having the fairlead at the lower-most edge will adversely affect the stability of the structure. These limits have been selected to be in the vicinity of the reference turbine. A simple Pythagoras constraint is used to filter out unfeasible combinations as shown below. A total of 1860 combinations are obtained.
All the decision variables are continuous variables. The number of mooring lines is kept to be 3, the radius of the fairlead is kept to be 58 m, and the angle between the lines is set to be 120 deg. The nominal diameter of the mooring line is also fixed to be 185 mm.
HAWC2 is an aero-elastic code developed by DTU Wind, that is used to calculate the responses of the wind turbine in the time domain.
A complete simulation is run using the model for the reference turbine, platform, and mooring system along with the wind and wave input. A simulation is run for a time of 13 mins. The input files for the mooring system were altered for the changing l, r and z values. 9 output channels were chosen to be evaluated.
Tower top acc was chosen to be evaluated as the large motion of the nacelle can cause degradation of turbine performance and damage to the equipment in extreme conditions. Hence it is desirable to limit TTA.
Surge is the translational motion along the x-axis. It must be constrained so as to limit the motion to not damage the power cable that connects the turbine to the grid.
The critical steady-state offset for FOWT is pitch. A large offset in pitch compromises the efficiency of the turbine and can even cause the turbine to capsize. in extreme cases.
Lastly, the maximum tension at the anchor and the fairlead were chosen so as to make sure that the total tension in the line does not exceed the breaking line strength.
Reference values were obtained for each put channel considered which are later used as constraints in the optimization. Th lengths l, r, and z were set to 850, 840, and 14 m respectively close to the reference values of 850, 837.6, and 14 m.
