Underwater Piling Noise

Abstract

This thesis investigates the important physical processes related to underwater sound propagation due to offshore pile driving with the first commercial and numerical model called the Underwater Acoustic Simulator (UAS). To make adequately use of noise mitigation measures during offshore wind projects and to protect marine life the prediction of sound levels in water is important. The aim of this thesis is to assess if the UAS is suitable to give insight in the physics contributing to sound propagation and if the UAS is suitable to quantify sound levels in water for the application in future wind park projects. This is done by comparing the predicted sound levels with the UAS with the measured sound levels obtained during the construction of the Gemini wind park.

The results obtained with the UAS identify the parameters related to the seabed properties as the most important for governing the attenuation of the noise levels as function of distance and frequency. In the UAS the seabed is modelled as a fluid. Especially frequencies < 800 Hz are sensitive to the choice of geo-acoustic parameters. These frequencies show a trend of underestimation of the sound levels compared to the acoustic data. An increase of the predicted sound levels is observed for increasing the coarseness of the sediment of the upper seabed layer and decreasing this layer thickness. The UAS parameters can be tuned to obtain sound levels that agree with the measured sound levels in the Gemini area. However, these parameter values do not represent the properties of a realistic seabed. Additionally, no evident choice of values exist to arrive at sound levels that agree with the acoustic data. It is concluded that the choice of values compensates for the exclusion of the shear effect in the fluid approximation of the seabed in the UAS.

The exclusion of the shear effect in the representation of the seabed in the UAS is regarded as essential. Tsouvalas (2015) shows that the propagation of sound energy at low frequencies is governed by the shear of the soil. These lower frequencies propagate in the deeper parts of the seabed where shear effects play a role. Also, sound energy in the form of Scholte waves propagates at the water-seabed interface, thereby contributing to the sound levels in water. The reason the sound levels propagating at low frequencies is sought in two aspects. In a fluid seabed the geo-acoustic parameters are adjusted and account for the damping associated with shear, while the propagation of sound energy at these wave forms is not modelled. Another possible explanation is the trapping of the low frequency pressure waves in the seabed.

Although the UAS is able to predict sound levels in the Gemini area that agree well with the measured sound levels, this is not due to the correct implementation of the physic regarding the seabed. As a consequence, the model is tweaked to arrive at the measured. The UAS is therefore not considered suitable for application in future wind park projects.