HydroNAS the future of underwater noise mitigation

HydroNAS™ - the future of underwater noise mitigation Marzena Dziedzicka, Alan West, Charlie Whyte, Geir Olafsson W3G Marine Ltd 23 Rubislaw Den North, AB15 4AL Aberdeen, UK 0044 01224 329 020, Email: [email protected]

Presented at EWEA Offshore Conference and Exhibition 10-12 March 2015 Copenhagen, Denmark

SUMMARY This paper presents the development by W3G Marine (W3GM) of a revolutionary underwater noise mitigation system (NMS), HydroNAS™. The system has gone through a lengthy and robust product development process. Using the impedence mismatch principle, W3GM has identified a unique solution using a versatile, inexpensive durable fabric. Its performance has been assessed through empirical experiment and theoretical analysis. To date, the five year research and development programme has proven a noise reduction of 12-14dB SEL at 750m, with a theoretical maximum of 44dB SEL. The system which can be easily configured to suit exact project specifics and at a fraction of the cost of alternatives.

W3G Marine Ltd HydroNAS™ - the future of underwater noise mitigation

Page 2 of 10 12 March 2015

1.0 INTRODUCTION

Currently most Offshore Wind (OW) turbines are installed onto a fixed substructure which connects to the seabed through either a steel jacket or monopile. Both typically require the use of an impact hammer to drive a pile(s) into the seabed. This generates underwater noise which can be harmful to marine life. To supress the noise an NMS is deployed. During 2010, following demands from the OW industry for a safe, effective, versatile and low cost solution, W3GM initiated an R&D programme develop a solution. First, W3GM identified and partnered with EATEC, a specialist acoustics and vibration engineering firm with a track record of innovation and problem solving. EATEC provided the acoustic ‘brain’ with W3GM developing complementary offshore deployment methodologies which would result in no main installation vessel downtime. The R&D programme has been supported financially by Scottish Enterprise and a UK Round 3 developer and technically by most other European OW developers and industry specialists, such as Dr Michael Bellmann (ITAP).

2.0 TESTING OVERVIEW TO DATE

Trials conducted to date are a proof of concept pool test, scaled up nearshore trial optimisation tests and theortical analysis.

2.1 Principle Trial - Pool test

The pool trial was to determine whether introducing an air barrier in the water column between the noise source and external environment reduced noise.

2.1.1 Materials and Method

The pool trial consisted of a barrier of plastic air pockets (bubble wrap) wrapped around a copper pipe which simulated the pile. The controlled test was performed in a swimming pool. The pile was submerged, excited and measurements were taken screed and unscreened.

2.1.2 General procedure

Figure 1, below, illustrates the experimental set up. The trial was carried out in two phases: - Without the bubble wrap sleeve around the pipe - With the bubble wrap sleeve aroud the pipe

For both arrangements a 22mm diameter copper pile was used in a 2.1m water depth. Five impacts of the hammer were applied with the force peak of 30N. The hammer type was B&K 8202. To measure the noise at 2m distance a DPA 8011 Hydrophone Underwater Microphone with a Sound Devices MP – 1 MIC PRE Battery Microphone Amplifier was used. The signal was captured with a National Instruments data acquisition module driven through the MPI software SO Analyser.



Figure 1- Pool trial setup

2.1.3 Sound sampling and data processing procedure

Data was recorded using a hydrophone. Two sets were collected; One with and one without the barrier. Sampling of the impact force and hydrophone response was at 51,200 Hz for a duration of 650ms. Spectral processing was carried out to give third octave spectra and sound exposure values.

2.1.4 Results

The pool trial proved that by introducing an air barrier between the pile and external environment a noise attenuation is achieved and the level of noise reduction is correlated to the thickness of the barrier. Figure 2 below illustrates that relation.

Figure 2- Pool trial results The pool trial had demonstrated that a significant reduction in underwater noise was possible through the impedence mismatch principle using an unbroken air column. Encouraged by the results of the pool trial W3GM initiated a nearshore prototype trial.

Pressure attenuation with varying wrap lengths

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2.2 12m prototype trial - Tay Estuary

A full scale offshore trial is a major undertaking, requiring significant investment so it was decided to trial a scale up of the principle using drop stitch material.

2.2.1 Materials and Methods

The air sleeve was designed to surround the pile and to provide an unbroken column of air between the entrapped water and the external environment. Material used for the trial was made of drop-stich which assured a constant volume and allowed variable pressure within the system through the entire water column. Restrictions imposed at Dundee harbour prevented piling into the river bed so reactions from the hammer blows were absorbed by a large base plate which rested on the bottom. In an attempt to reduce the vibration from entering the base plate, creating a noise leak path, rubber pads were inserted between the bottom flange of the pile and a matching flange attached to the base plate. The pile had an outside diameter of 406 mm (16 inches), wall thickness of 16 mm and a length of 10.5m. The assembly is shown in Figure 3, below

Figure 3 - System arrangement (a) Schematic (b) Unscreened pile on quay c) Screened pile on quay


The pile was lowered to the river bed at the end of Eastern Wharf in Dundee harbour at geographic coordinates 56°27'53.62"N; 02°56'5.36"W. The dock side was a piled pier and so there was no solid reflecting boundary close to the pile. Water depth recorded was approximately 10m. The hammer used was a BSP Hydraulic Hammer, model SL30da. This had a ram mass of 2,500 kg and a maximum impact energy of 30 kJ. For both trial scenarios, with and without the NMS sleeve, 20 hammer blows per location was applied, resulting with over 360 blows in total, from 9 recording locations.




Hydrophone measurements were taken at distances of 400m, 750m, and 1,200m from the pile in a total of 9 locations as shown in Figure 4, below.

Figure 3- Measurement positions The voltage signal from the hydrophone was digitised by a National Instruments cDAQ module type 9234. This has a 24 bit resolution over ±5 V and automatic anti-alias filters set by the sample rate. The module was controlled by the M and P International application, Smart Office. The capture was set for 20 separate blocks of 65,536 points each sampled at 51,200 S/s. Each block was triggered by detecting a value above the background noise level. A pre-trigger interval of 0.36 seconds was selected to ensure the start of the impulse noise was detected and to give a measure of background noise before piling.

2.2.4 Results

Table 1 summarises the results achieved during the trial. At a distance of 750m Lpeak attenuation was 15dB-18dB and the SEL attenuation was 12dB-14dB.

Table 1- 10m water depth trial results



2.3 Optimisation Trial - NPL

To understand the impact of layering and depth of air barrier a trial was carried out at the UK National Physic Laboratory. Different air barrier arrangements were tested, including different barrier thickness and multiple barriers.

2.3.1 Materials and Methods

Tests were carried out with different barrier configurations. This included:

Blind end (double walled bottom closure with sand infill)

Open end

Single 67 mm air barrier

Single 67 mm air barrier with corner tube inserted to give a noise leak path



Two 67 mm barrier separated by 133 mm water





The tests were performed at the National Physical Laboratory Open Water Facility on the reservoir at Wraysbury, near Staines. Their floating raft is permanently moored in 20m of water, well away from the banks of the 1,000 m x 2,000 m fresh water reservoir. A crane was used to raise and lower the enclosure in order to change the barrier configuration. An LF_T157 projector was used. It is capable of responding in the frequency range 100 Hz to 20 kHz. The waveform was controlled by a signal generator providing an input of 1 Vrms to a power amplifier. The output from the amplifier was used to drive the projector. Some of the tests were carried out with a broadband random signal for which the gain was set to 30dB. Other tests used single frequency sinusoidal inputs for which the gain was set to 40dB.

Figure 4 - NPL optimisation trial


Individual tests were carried out with different barrier configurations and source excitations. For each test 20 records of the source excitation and the response of the hydrophones, each of 1.3 seconds duration, were sampled at a rate of 50k samples per second per channel. The averaged spectra in third octave ranges or narrow bands were used for comparing the performance of the different configurations.



2.3.4 Results

The objective of this trial was to discover the optimum configuration of air barriers to maximise noise attenuation. The following conclusions were drawn for the (high) frequency range tested:

a) Barrier thickness is relatively unimportant. The 67 mm barrier was at least as effective as the 100mm and 200mm barriers.

b) Multiple barriers improve overall noise attenuation, although the magnitude was difficult to quantify because the level of noise reduction with a single barrier was already close to the background noise.

3.0 THEORETICAL CAPABILITY

3.1 Attenuation principle

HydroNAS™ works through creating two impedence mismatchs as the noise travels from water to air then air to water. When a sound wave encounters a material with an acoustic impedance Z that is different from the impedance of the propagation medium, the sound is partly reflected. Hence only part of the sound is transmitted into or through the material. For sound in water the best reflector is air filled material [1]. For the most effective attenuation through impedance mismatch the barrier must be completely unbroken through then entire water column.

3.2 Independent assessment – ISVR Consulting, Southampton University

The theoretical performance of the system was estimated and the significance of the drop stich threads between the walls were determined to evaluate their impact on the noise transmission. ISVR Consulting modelled the fibres as an equivalent fluid with speed of sound given by:

𝑐𝑓𝑖𝑏𝑟𝑒 = √𝐸𝑓𝑖𝑏𝑟𝑒/𝜌𝑓𝑖𝑏𝑟𝑒

The air barriers may be inflated to a pressure of 6.5 Bar so that they can be submerged to a depth of 60m. This increases the density and modulus of the air by a factor of 6.5, but the speed of sound is unchanged. The transmission loss of a fluid layer immersed in another fluid (e.g. water – air – water) at normal incidence is given as [2]:

𝑇𝐿 = 10 log10 (1

1 + 0.25(𝑅2/𝑅1 − 𝑅1/𝑅2)2 sin2 𝑘2𝐿

)

The results demonstrated there is a very little impact on the noise transmission resulting from the threads being present when the system is configured in a horizontal orientation. Figure 6, below, illustrates the results.



Figure 5 - Transmission losses for a 0.2m barrier with fibres

4.0 DEPLOYMENT METHODOLOGIES

4.1 Monopile

W3GM has developed multiple deployment solutions for monopile installation. These are a result of intensive research and consultations with the industry to develop system which would stay completely off the critical path of the main installation vessel without compromising system performance. The ‘Bellows’ and ‘Water Ballast’ solutions are described below.

4.1.1 Bellows

In the Bellows design the HydroNAS™ is mounted underneath the grab arm and has ability to open/close to allow the monopile to be easily position within the arms. It can be deployed to variable water depths throughout a project. Figure 7, below, illustrates the operation.



Figure 6- Offshore monopile deployment solution- Bellow a) Open b) Close c) Deploy

4.1.2 Water Ballast

The ‘Water Ballast’ system is preinstalled on the pile so minimal deck space is necessary. It uses a collar filled with seawater which is positioned above the water line to counteract the buoyancy uplift. Depending on project requirements one or more systems can be preinstalled. Where one system is used, it will be transferred from one monopile to another during other installation works or whilst in transit. Deployment is off the main vessel critical path. The system opens longitudinally. Figure 8, below, illustrates the operation.

Figure 7 - Offshore monopile deployment solution - Water Ballast a) Pre-installed on the pile b) No additional crane lift c) Minimal deck space



4.2 Jacket foundation

For jacket foundations W3GM has developed and patented a piling template solution with the noise mitigation system incorporated in the design. See Figure 9, below. The piling slot indexes between piling locations.

Figure 8 - Piling template with noise mitigation system

5.0 CONCLUSION

Following four years of R&D, W3GM expects to prove HydroNAS™ offshore during 2015. Trials conducted up to date brought highly encouraging results and theoretical assessments give confidence for further development. The system will provide developers and installation contractors a safe, efficient and cost effective NMS alternative which can be tailored to suit exact project requirements. The use of HydroNAS™ can result in no lost main vessel time representing a further cost saving. In addition HydroNAS™ solves other challenges such as seabed bearing strength issues and can even be preinstalled onshore onto the monopile. HydroNAS™ solves all underwater noise challenges faced by the industry at a fraction of the current cost.

6.0 ACKNOWLEDGEMENTS

The development of HydroNAS™ would not have been possible without the input and vison of certain organisations and individuals. Ian Beaton at Business Gateway Aberdeen developed our long lasting relationship with Scottish Enterprise which in turn has financially supported the 10m trial along with SSE. Dr Michael Bellman for his ongoing guidance, advice and validation. BSH for updating W3GM on developments in the German sector. Marine Scotland for assisting with site identification and assessment. Most other European developers and installation contractors through support and input into real case studies.

7.0 REFERENCES

[1] G. Nehls, K. Betke, S. Eckelmann and M. Ros, "Assessment and costs of potential engineering solutions for the mitigation of the impacts of underwater noise arising from the construction of offshore windfarms," COWRIE Ltd., Husum, Germany, 2007.

[2] L. Kinsler, A. Frey, A. Coppens and J. Sanders, Fundamentals of Acoustics, New York: John Wiley & Sons Inc, 2000.

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HydroNAS the future of underwater noise mitigation