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184Chapter 16 • Noise mitigation systems and low-noise installation technologies1234567891011121314151617181920a piling cushion made of aramid fibres can delivernoise reductions of up to 11 dB (SEL) or 13 dB(L peak) (Rustemeier et al. 2012a; . Fig. 16.2). Practicalexperience with the approach was gained duringthe installation of the substructure for the FINO2research platform – a monopile with a diameter of3.3 m – at Kriegers Flak in the Baltic Sea. The pilewas driven into boulder clay beneath a sand layer.A coiled steel rope served as the piling cushion. Anoise reduction of about 6 to 8 dB (SEL) was measuredduring the first few piling strokes. After that,however, the steel rope was too compressed to beeffective (Elmer et al. 2007b). The theoretically derivednoise reduction potential thus still has to befully confirmed empirically. It must be noted thatwith high soil resistance, impulse prolongation cannegatively affect pile drivability. This issue can betackled by increasing the number of pile strikes orthe ramming energy level.16.2.2 Secondary mitigation measuresBubble curtainsA bubble curtain is a layer of air bubbles over the fullheight of the water column. The bubbles are createdby pumping compressed air through a perforatedhose laid on the seabed or around a pile. Air bubblecurtains were first used in connection with harbourworks and bridge building. One of the first full scaletests was conducted in 2000 during the installationof bridge pillars for the San Francisco-Oakland BayBridge (Caltrans 2001).To tailor the bubble curtain approach for offshorewindfarm construction, it had to be adaptedto greater water depths, the conditions of the harshoffshore environment, and serial use. It also had tobe optimised for lean integration into the weatherdependentand time-critical windfarm installationprocess.In principle, two bubble curtain layouts can be-distinguished:‘Big’ bubble curtains, which are laid at a radiusfrom 70 to 150 m from a pile and enclose the-whole construction site‘Small’ bubble curtains, which are placed in thevicinity of a pile.Big bubble curtains (BBCs)A first full-scale test with a BBC was performed inJune 2008 during installation of the substructurefor the FINO3 research platform, a monopile witha diameter of 4.5 m (Grießmann et al. 2010). Thenon-optimised BBC prototype consisted of a hexagonalarray of connected perforated plastic tubes atan average distance of 70 m around the constructionsite. This configuration made for a 12 dB reductionin sound exposure level (SEL) and a 14 dB reductionin peak level (L peak). Based on the experience fromthis first test, an improved prototype BBC was developedand tested at the Trianel Windpark Borkumin winter 2011/2012 (Diederichs et al. 2014). Theimproved system consisted of a durable perforatedhose attached to an anchor chain. The hose and chainwere coiled on a drum on board an auxiliary vesseland deployed at the planned piling site before arrivalof the jack-up barge. This meant that any delays inthe windfarm installation process could be avoided.Several BBC setups, mainly differing in hose design(nozzle size), quantity of supplied air, and systemlayout (linear or circular), were successfully testedon 31 out of 40 tripod foundations. The most effectivesingle configuration (. Fig. 16.3a), deployed in12 instances, delivered an average noise reduction of11 dB (SEL) and 14 dB (L peak). Experiments with adouble linear BBC indicated that greater reductionsof up to 18 dB (SEL) and 22 dB (L peak) may be feasible.The R&D project was the first ever serial test of aBBC during construction of an offshore windfarm.Similar BBC configurations have since been used atseveral other offshore windfarm projects in the GermanBight. Experience has shown, however, that theattainable noise reduction can vary considerably. Thisdepends on the BBC configuration, site-specific parameters(currents and seabed properties), and project-specificinstallation logistics requiring the BBCsystem to be laid at a certain minimum distance fromthe piling site.Small bubble curtains (SBCs)SBCs are positioned in the vicinity of a pile. This hasthe advantage compared with a BBC that no auxiliaryvessel is needed to deploy the noise mitigationsystem.A first full-scale test was conducted with a nonoptimisedprototype at alpha ventus in May 2009.