Effects of acceleration and frequency of input motion on the seismic behavior of offshore wind turbine supported by monopile

Document Type : Original Research

Authors
A. Bateni 1
M. Moradi 2
1 Faculty of Civil Eng., University of Tehran
2 Associate Prof., Faculty of Civil Eng., University of Tehran
10.22034/24.6.95
Abstract
In recent years, the surge in pollutants from fossil fuels has prompted a heightened emphasis on transitioning to clean and renewable energies, with a particular focus on wind power. The deployment of offshore wind turbines stands out as a prominent approach to harnessing wind energy. However, these turbines consistently endure cyclic loading induced by wind, waves, and ocean currents, necessitating foundations that exhibit robust resistance to such repeated stress. Offshore wind turbines are commonly mounted on monopiles, singular tubular structures with diameters ranging from 2 to 8 meters. While these turbines were initially deployed in Europe, their utilization has expanded to seismically active regions such as USA, China and Japan in recent years, owing to their numerous advantages. As a result, their seismic behavior has become a subject of interest. The seismic design of these turbines, similar to other structures, should be based on past earthquakes in the region and adapted to saturated conditions. Until now, a multitude of studies has delved into these turbines, predominantly through numerical research. However, the scarcity of experimental investigations into their seismic behavior has left the impacts of acceleration and frequency of input motion on their design not thoroughly explored. Furthermore, in certain instances, the design of these turbines makes reference to regulations designed for dry conditions. This research investigates the impact of acceleration and frequency of input motion on the seismic response of offshore wind turbines through 9 experiments conducted on samples using a 1g shaking table. Various input motion with different acceleration and frequencies were applied under both dry and saturated conditions, allowing for a comprehensive comparison of turbine behavior. The modeling process included creating a soil environment with specific dimensions through dry deposition and compaction, followed by the embedding of sensors for measuring acceleration and pore water pressure. After these initial steps, the monopile was vertically drove into the soil, and the superstructure was assembled. Displacement sensors were installed to capture the superstructure's displacement at different heights and to measure the settlement of the soil surface on the samples. Then the sample started to be saturated from the bottom of the box and water was placed on the soil surface up to 10 cm to model sea water. Subsequently, harmonic sinusoidal loading was applied, with 9 loadings featuring frequencies of 10 Hz, 5 Hz, and 3 Hz, along with maximum accelerations of 0.2 g, 0.3 g, and 0.4 g, respectively. As indicated by the findings of this research, turbine seismic behavior becomes significantly more critical during resonance phenomena in the most critical state, with the impact of other factors on seismic performance proving negligible in such instances. Moreover, the seismic behavior of these turbines consistently exhibits more critical behavior in saturated conditions compared to dry conditions. In saturated conditions, acceleration amplification in surface soil layers is up to 5 times, profoundly influencing seismic performance, whereas in dry conditions, amplification is limited to 1.2 times. Additionally, as excitation acceleration rises and excitation frequency decreases, the superstructure's maximum acceleration and the turbine's maximum and permanent displacement all increase, signifying a more critical behavior of this structure.

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Modares Civil Engineering journal
Volume 24, Issue 6
November and December 2024
Pages 95-106