What does the support structure "Refine deflections" option in Calculation Outputs do?
PLEASE NOTE: The "Refine Deflections" option is still available in older versions but we do not recommend you use it. It has been replaced by new functionality in 4.9 onwards. See the Knowledge Base article "Refine Deflections superseded by new functionality".
There is an option in the support structure outputs to Refine deflections. With this option disabled, the tower deflection outputs are modal deflections. Modal deflections give a good estimate of overall tower motion, but may not be appropriate to calculate the small deflections at certain support structure stations, such as the foundations. With Refine deflections disabled, the foundation reactions are calculated by combining the modal deflections with the stiffness matrix provided for the foundation station. This can lead to a poor estimate of the foundation reaction loads given the applied loads on the rest of the turbine.
A good illustration of this problem is to consider a monopile tower without a vertical attachment mode. In this case, none of the tower vibration modes have a contribution in the vertical direction, so the tower node vertical displacements will always be zero. Therefore, the foundation reaction force in the vertical direction will always be zero, as the reactions are calculated from the modal deflections. In this case, the Refine deflections option would be needed to obtain correct foundation reaction loads.
With the Refine deflections feature enabled, the tower deflections are re-calculated at each output time step using the underlying finite element model with the external loads applied. The refined deflections give a more accurate estimate of the deflections at the foundation nodes, without being constrained only to fixed mode shapes, hence the foundation reaction forces will correctly balance the applied loads. In the case of non-linear foundations, there is also a static iteration between the refined deflections and the applied foundation load to more accurately calculate the non-linear foundation reaction force. Enabling the refine deflections option will therefore ensure that the applied loads and foundation reactions correspond correctly.
Note that, in all cases, the modal deflections are used to estimate the foundation loads when solving the structural system at each integrator time step. To use the refined deflections at every time step would require iteration on each time step, causing a significant increase in simulation time. The assumption with this modelling choice is that error in foundation load estimate due to using modal deflections doesn’t affect the overall turbine dynamics significantly. It is therefore reasonable to only calculate the foundation reaction loads based on refined deflection on each output time step.