Scottish Marine and Freshwater Science Volume 4 Number 5: Modelling of Noise Effects of Operational Offshore Wind Turbines including noise transmission through various foundation types

1 Introduction

Vibration produced by offshore wind turbines during their normal operation transmits through the tower into the foundation where it interacts with the surrounding water and is released as noise. The noise produced by offshore wind turbines can be detected by fish and marine mammals and may lead to alteration of their behaviour. Given that noise is emitted at the interface between the foundation and water, it is likely that the intensity and frequency of the noise will be strongly affected by the nature of the foundation. Factors that may affect the nature of the noise emitted are the surface area of the foundation, the material used to construct the foundation and its internal damping and the nature of the connection of the foundation to the sea floor. There are many designs of foundations including, jackets, monopiles and gravity bases; each of which will have different noise emission characteristics.

The purpose of this study is to determine the relative difference in the underwater noise emitted from different types of foundations. This is modelled using an identical wind turbine and operating conditions, the outcome of which is assessed with regards to the potential impact on marine species.

Finite element methods were used to determine the near-field (<40 m) noise level produced by operational turbines on monopiles, gravity base and jacket foundations. Results from the near-field models were used as source terms in beam trace models to determine the cumulative far-field (up to 20 km) noise level emitted by wind farms consisting of 16 wind turbines mounted on each of the foundation types. The resulting noise fields were compared to audiograms and behaviour parameters to determine the relative effect of jackets, gravity bases and monopiles on marine species likely to interact with offshore wind farms in Scottish waters. The marine species examined were allis shad, eel, salmon, sea trout, harbour seal, harbour porpoise, bottlenose dolphin and minke whale. The range at which each of these species could detect noise from an offshore wind farm is determined, as is the likelihood of a behavioural response.