Development of a New Underwater Piling Noise Mitigation System: Using Hydro Sound Dampers (HSD)

Abstract

The aim of the recently published climate control by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) is a reduction of the German greenhouse gas emissions down to 20 % in 2050, compared to the generated greenhouse gas in 1990. To reach the given target a huge growth in renewable energy is necessary. One of the most potential possibilities to produce renewable energy in Germany will be the installation of offshore wind turbines. During the installation of offshore wind foundations such as monopiles, tripods, tripiles and jackets, mostly large tubular steel piles are impact driven to final penetration depth. In the process of impact driving, considerable underwater sound emissions will appear. In recent times, peak sound pressure levels above 190 dB re 1 μPa have been measured at a distance of 750 m away from the installation ground. These peak sound pressure levels, produced during the installation of any kind of driven offshore foundation, is potentially harmful to marine life, in particular to marine mammals such as harbor porpoises, harbor seals or grey seals. To protect the marine life the German Federal Maritime and Hydrographic Agency (BSH) set the maximum allowed underwater sound pressure level to 160 dB (SEL) at a distance of 750 m away from pile driving. To reach the given target by the BSH a new underwater piling noise mitigation system using so called hydro sound dampers (HSD) is presently developed at the Institute for Soil Mechanics and Foundation Engineering at the Technische Universität in Braunschweig (IGB-TUBS). After small scale tests in the so called ‘large wave channel’ in Hannover a full scale test was performed in the Baltic-Sea some weeks ago. The results of the measurements are very promising, as a reduction of 12 up to 20 dB could be generated. Besides existing noise mitigation systems one of the main advantage of the hydro sound dampers is, that the mitigation can be pre-adjusted to a predefined frequency range, as marine mammals are sensitive only for a certain sound frequency range. In this paper, the results of the small and large scale tests and some new research findings concerning the shape and the material of the hydro sound dampers will be presented.

Evaluating effects of dredging-induced underwater sound on aquatic species : a literature review

“…However, in practice, achieving a uniform distribution of free air bubbles in water is challenging due to varying ambient hydrodynamic conditions. Larger air bubbles with lower resonate frequency are more efficient at attenuating lower-frequency sounds (<1 KHz), but are relatively unstable and challenging to produce (Kuhn et al 2012). Therefore, air bubble curtains are typically more efficient at attenuating higher frequency sounds (400-6,400 Hz; Wursig et al 2000).…”

“…Wursig et al (2000) observed a 3-5 dB sound reduction in overall broadband frequencies, and up to 20 dB in the 1.6-6.4 kHz bands. In general, technical limitations of effectively implementing bubble curtains include achieving effective distribution around the sound source, maintaining a consistent distribution of bubble sizes, and the lack of sound mitigation at lower-range frequencies (<1 kHz) (Nehls et al 2007;Kuhn et al 2012).…”

“…HSDs use small gas-filled bladders and robust polyethylene foam elements fixed to a fishnet "curtain" that surrounds a sound source. Measured sound reductions of 20-22 dB (broadband frequencies) HSDs have been achieved in wave flumes testing simulated pile-driving operations (Kuhn et al 2012). Bruns et al (2014) implemented HSDs during off-shore pile-driving construction activities to minimize the potential sound exposures to harbor porpoises and seals.…”