Technical mitigation measures use the physical principles of reflection, absorption, scattering and dissipation. Different applications have become available recently.
3.1.1 Bubble Curtains
A bubble curtain consists of a ring of perforated pipes encircling the pile. Gas bubbles produced by compressed air form a curtain around the pile. The pipe can be either positioned as a large ring at the sea floor (“Big Bubble Curtain”, BBC), or in various arrangements of vertical pipes arranged close to the pile (“Layered Bubble Curtain”, LBC, or “Small Bubble Curtain”, SBC). The curtain may also be confined by casings. However, experiences with “Confined Bubble Curtains”, though demonstrating a high mitigation potential, currently only exist for bridge construction works in shallow waters close to the shore (CALTRANS, 2003).
Bubble curtains have been applied as an effective noise mitigation technique in several experimental and practical setups (WÜRSING et al., 2000; CALTRANS, 2003;
VAGLE, 2003; PETRIE, 2005; GRIEßMANN et al., 2009; ITAP, 2010). Under offshore
conditions in the German North Sea, the effectiveness of the BBC has been proven during the construction of the research platform “FINO 3” (GRIEßMANN et al., 2009) and
at the OWF “Borkum West II” (BIOCONSULT-SH et al., 2012; BELLMANN, 2012). The
LBC has been tested at the first German OFW “alpha ventus” (GRIEßMANN et al., 2010;
ITAP, 2010) and at “BARD OFT1” (KUMBARTZKY, 2012; VERFUSS, 2012). The noise
mitigation potential for both concepts has been demonstrated to be in the order of 12 dB (SEL) and 14 dB (Peak) (Table 1).
117
However, problems encountered during experimental use have to be solved in order to enable the large scale economical use of bubble curtains. E.g. the LBC applied at “alpha ventus” used only a pre-installed lower part of the tube-system. An additional mobile upper system could not be installed due to bad weather (ITAP, 2010). Thus, the tidal current drifted the bubbles away, resulting in sound leakages that greatly reduced the effectiveness. An improved concept of flexible attachment of perforated vertical pipes to the piling frame was tested at “BARD OFT 1” and resulted in sound mitigation of up to 14 dB (SEL) (Table1) (KUMBARTZKY, 2012).
During the installation of the commercial OFT “Borkum West II” a BBC was successfully employed at 31 out of 40 turbines (MENTRUP, 2012). Moreover,
experiments were performed with a double bubble curtain. Preliminary results revealed noise mitigation levels of up to 18 dB (SEL) and 16 dB (peak) when the distance between both pipe half-rings was large enough to form two separate curtains (BELLMANN, 2012). Bubble curtains are currently offered by the German companies
Hydrotechnik Lübeck and Bernhard Weyres Offshore. Their application is planned in several OWFs.
3.1.2 Pile Sleeves
A simple pile sleeve consist of a steel pipe around the pile reflecting a part of the noise back inside. More complex systems use additional layers containing air (foam, composites) making use of the difference of impedance between water and thus absorption, scattering and dissipation effects (ELMER et al., 2007a; NEHLS et al., 2007).
The Noise Mitigation System (NMS) developed by IHC Merwede consists of a double-wall pile sleeve with an air filled inner compartment. A bubble curtain between the NMS and the pile provides an additional noise barrier. A special guiding system keeps the pile and the NMS concentric. In an experimental set-up in shallow water, a damping rate of 20-27 dB in 1/3 octave bands between 150 Hz and 8 kHz was achieved (no broadband mitigation given) (Bob Jung, IHC Hydrohammer BV, pers. comm.). The NMS 6900 will be deployed at the OWF “Riffgat” in summer 2012 (IHC MERWEDE, 2012).
The BEKA-Shells (Bernhard Weyres Offshore) are a combined system based on the principle of a pile sleeve. Two acoustically uncoupled double layered half-shells of steel, filled with a sound absorbing composite material are separated by 10 cm. From the inside, the inner shell is coated with a noise absorbing material. Two bubble curtains are produced between sleeves and between inner sleeve and pile. An additional shield pressed into the ground is supposed to mitigate the sound propagation via the seismic pathway. Due to the combination of several principles the design is very promising. Sound measurements from a representative site are not available yet.
118
Although technically feasible, the concept of a Casing of Fire Hoses with several layers of hoses fixed to frames has not resulted in the development of a commercial application.
3.1.3 Cofferdams
Cofferdams are comparable to pile sleeves, but in contrast to them the space between pile and surrounding cofferdam is completely dewatered. Hence pile driving takes place in air and not in water thus uncoupling the propagation of sound from the surrounding water. Modeling results predicted a noise reduction of about 20 dB for a dewatered cofferdam which was considered to be the most effective mitigation technique of underwater pile driving noise (Applied Physical Sciences 2010). In shallow water, sheet pile walls are often used as cofferdams (CALTRANS, 2009), but this is not feasible in deeper water where steel piles are used.
A pilot test with a Dewatered Cofferdam was performed in Aarhus Bight in December 2011 by Siemens and TenneT with acoustic measurements performed by Rambøll (Kurt Thomsen, Lo-Noise Aps, pers. comm.). Impact pile driving was performed in water depth of about 14 m on a pile of 2,13 m diameter. A comparison of noise immissions with and without cofferdam revealed an average broadband mitigation of 22 dB (SEL) and 18 dB (peak). Best results were achieved for frequencies above 500 Hz. The system is also applicable for jacket foundations. Cofferdams will be deployed for pile driving at the converter platforms BorWin2 in 2012 and HelWin in 2013.
A particular case of a cofferdam is the principle of Pile-in-Pipe Piling which is currently developed by the Hamburg-based company Overdick GmbH & Co KG (E. Overdick, pers. comm.). In this case, four cofferdams are permanently fixed to the legs of the four-legged jacket (“quadjack”). The cofferdams are not reusable as they remain with the foundation and serve as a protective pipe. The piles reach beyond sea level, hence piling occurs only above sea level and the cofferdam acts as a noise barrier over the whole water depth.
3.1.4 Hydro Sound Damper
An innovative noise reducing method is a system of hydro sound dampers (HSD), small gas filled elastic balloons and robust PE-foam elements fixed to nets or frames placed around the pile as developed by the German company OffNoise Solutions. The underlying principle is identical to that of a bubble curtain with the exception that the frequencies at which the maximum damping efficiency is achieved are adjustable by variations in the balloon size. By this, the system allows for the damping of specific frequencies, e.g. in relation to the affected species susceptibilities. In laboratory experiments a broadband reduction of 20-22 dB (SEL) and 19 dB (peak) was achieved
119
(ELMER, 2010). In summer 2012, the HSD will be tested at the OFT “London Array”
(K.H. Elmer, OffNoise Solutions, pers. comm.).
3.2 Alternative Foundation Concepts