Sensitivity analysis of the first natural frequency of the offshore wind turbines in the Eneco Luchterduinen wind farm

Abstract

In the Dutch coastal waters more and more wind turbines on monopile foundations are built. An important design parameter is the first natural frequency of the wind turbine. The first natural frequency is dependent on environmental conditions and different soil conditions such as scour. This research gives a better insight into the sensitivities of these influences on the first natural frequencies.
The first objective was to determine the sensitivity of different influences on the first natural frequencies in a full scale test environment. The first natural frequencies of four wind turbines (two protected against scour and two not protected against scour) in the Eneco Luchterduinen wind farm are constantly monitored and the natural frequencies are identified. It was found that only the water level has an identifiable influence on the identified first natural frequency and that the design natural frequencies and identified natural frequencies differ significantly.
After the analysis from the full scale test different influences on the first natural frequency in a numerical computer model are investigated. The computer model predictions confirm that the water level has an identifiable effect and only small effects of backfilling of the scour hole on the first natural frequency are found.
The third objective was to compare the sensitivity of different influences on the first natural frequency obtained from the computer model and from the full scale test. The effects of the water level are comparable. And the effect of backfill of the scour hole was found in the model but not in the full scale test. Therefore no conclusion is made on the effect of backfill nor on the presence of backfill. It was found that in order to mitigate the differences in first natural frequency of the test turbines and the model the soil should be modelled twenty times stiffer.
The main influence on the first natural frequency of the offshore wind turbines in the Eneco Luchterduinen wind farm is the soil stiffness. The only other identifiable influence was the water level. Also in the design of the wind turbine the soil stiffness is underestimated significantly, implying overconservative designs.