Real-world application of Danish guidelines for underwater noise to the Horns Rev 3 substation
Benjamin Trimoreau, René Smidt Lützen
Lloyd's Register, Copenhagen, Denmark
As an extension to Horns Rev 1 and 2 offshore wind farms, Horns Rev 3 is currently under construction off Denmark’s western coastline. While installation of the actual wind turbines has not yet commenced, pile driving for a substation jacket foundation (4 piles) was performed in April 2016. The Danish Energy Agency and Energinet.dk previously defined a set of guidelines for underwater noise in the context of Horns Rev 3 project. In accordance with these guidelines, Lloyd’s Register carried out in-situ measurement of underwater noise propagation by means of an air-gun, and measurement of the actual pile driving noise. Furthermore, a prognosis of the cumulative Sound Exposure Level (SELc) was made. The present paper presents the findings from these steps, including comparison to the noise limits stated in the regulation. Practical experience from the measurements as well as the application of the guidelines is presented.
The Danish guidelines for underwater noise prescribe requirements for a noise prognosis, as well as a methodology for control measurements during pile driving. For the substation, in-situ measurements of Transmission Loss (TL) were made along two transects prior to construction, using an air-gun as impulsive sound source. The TL data were fitted to an XˇLog(r)+Aˇr expression. Together with an estimated source level, this was used for a prediction of the cumulative Sound Exposure Level, SELc. Such a prognosis involves the assumption of a hypothetical animal fleeing from a certain initial range r0. This range depends on the use of scaring devices.
During the pile driving, underwater noise measurements were taken and the actual SELc values were determined for each defined pile period.
Noise propagation measurements were taken at practically constant water depth, approximately 17 m. For each transect, a hydrophone was placed at 6 ranges between 375 and 3000 m. Propagation constants were determined as X=14.7 and A=0.00027. A prognosis predicted SELc of 182 dB re1μPa2ˇs at 750 m. Measurement of the actual pile driving noise was performed for all piles at 750 m and two depths. Single-strike SEL values were generally between 155 and 165 dB. SELc was determined for various initial distances of the hypothetical fleeing animal. One case was initial distance 2000 m, corresponding to proper use of scaring devices. This resulted in SELc up to 185 dB, or a 2 dB excess of the 183 dB criteria for the Horns Rev 3 project.
The paper presents a successful, first application of the multiple steps required by the Danish guidelines to a real offshore wind farm case, the Horns Rev III substation. Various issues of uncertainties were experienced, such as fitting of propagation data. Furthermore, certain procedural challenges were identified and will be discussed.
Delegates will experience the application of the Danish guidelines for underwater noise, with its multiple steps. These involve air-gun based noise propagation measurements for use in a simplistic prognosis scheme.