Differentiating between underwater construction noise of monopile and jacket foundation wind turbines: A case study from the Belgian part of the North Sea

Offshore wind farms generate underwater noise during construction, operation and decommissioning. Different foundation designs are in use for offshore wind farms. Steel monopiles, jackets made of four steel pinpiles and gravity-based foundations are applied in the Belgian part of the North Sea. Both monopiles and jacket pinpiles are hammered into the bottom and as such excessive noise is generated during their installation. This paper aims at comparing the emitted underwater noise generated during the piling activities of two wind farms, each using a different type of pile foundation: steel monopiles were used in the Belwind wind farm (Bligh Bank), while jacket pinpiles were chosen for the C-Power project (phases 2 and 3) on the Thorntonbank. Underwater noise was measured from a rigid hull inflatable boat at various distances from the pile driving location during the installation of steel monopiles and pinpiles. Analysis focused on the quantification of the discontinuous impulsive pile driving-generated underwater noise. Zero to peak sound pressure level (Lz-p), unweighted sound exposure level (SEL), cumulative SEL and 1/3 octave spectra were computed in order to quantify and compare the underwater noise. Lz-p and SEL were normalized to 750 m distance from the piling location. Piling of the monopiles generated a Lz-p between 179 and 194 dB re 1µPa. Although piling of the smaller pinpiles was expected to be less noisy, no significant differences with monopiling could be demonstrated (pinpiling Lz-p: 172-189 dB re 1 µPa). Similarly, also SEL, varying between 145 and 168 dB re 1 µPa2s, exerted no statistical difference between mono- and pinpiling and furthermore near identical spectra were measured for both types of piling. The rather wide variability within and broad overlap between both types of piling can possibly be attributed to the (small scale) differences in bottom type as well as in the setup used. However, the radius of discomfort in the harbour porpoise Phocoena phocoena was estimated at 30 km and 19 km for monopiles and pinpiles, respectively. Furthermore, the pinpiling (four pinpiles per foundation) took about 2.5 times more time than the monopiling and will as such have a prolonged impact onto the marine fauna. When standardised to megawatt installed (3 MW turbines on the monopiles and 6 MW turbines on the jacket foundations) both types of piling will score about equally