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IHC Hydrohammer® decided to be the world’s leading knowledge centre in piling-originated underwater sound. Sound is easily conducted by water and underwater sound can reach distances up to 30km from its source. This enables many sea creatures to ‘see by sound’, either passively or actively. This ability is known as echolocation. For such animals, both fish and mammals, sound is a means to find food or a partner, and to orient themselves or avoid encounters with enemies. Mankind has added several sounds to the natural backdrop, induced by waves, rain and animals: seismic blasting, navigation, echo sounders, drilling platforms, offshore wind farms, dredging and piling (figure 1).
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Due to growing industrialisation, this anthropogenic sound has increased in the last few decades, causing the continuous background noise level in seas and oceans to rise [1]. For example, a common echo sounder sends repeated bursts of sound downwards in the range of 230-245dB in the frequency range of 11-100kHz, and a typical trailing suction hopper dredger generates a continuous omnidirectional sound wave of approximately 186dB in the range of 30Hz-500kHz. Pile driving can reach levels exceeding 175dB, in frequencies of 100Hz-200kHz that are expected to be influential, and in all directions [2].
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| IHC Underwater Piling |
Recently, awareness of the fact that the sounds caused bypiling could disturb the food finding and orientation abilities of sea creatures has grown. This is recognisable in figure 2, where anthropogenic sound almost drowns out natural sound levels except those caused by disasters, so to say, and where piling appears at around the centre of the graph [3]. For that reason, authorities have developed legislation on noise mitigation. In light of this, IHC Merwede business units are in the process of creating solutions for sustainable underwater piling, serving both ongoing economic development and environmental legislation.
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| IHC Underwater Piling |
Synergy for sustainability
IHC Hydrohammer® is the market leader in hydraulic piling knowledge and equipment for both onshore and offshore underwater application. In view of this position, the company decided to be the world’s leading knowledge centre in piling-originated underwater sound. For onshore piling, it had
already developed a bellows solution. This innovation helped it to receive orders for equipment in the Dutch Eemshaven power stations programme, where sound had to be restricted in order not to disturb a nearby seal habitat (figure 3). In light of this ambition, the company organised an international seminar, ‘Sound & Sizes’, for the offshore wind turbine market, where lectures and feedback of users and builders of wind turbines generated a lot of new insights and enthusiasm. However, as IHC Hydrohammer had to focus on its main activity – the building, sales and lease of outstanding hydraulic piling hammers [4] – it found an excellent partner for the design and manufacturing of practical sound-mitigating equipment in IHC Offshore Systems [5].
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| IHC Underwater Piling |
Further synergy was found in joining the innovative Far and Large Offshore Wind (FLOW) consortium, which enabled it to participate in projects and knowledge from a broad spectrum
of Dutch companies and universities. It gave IHC Hydrohammer the opportunity to conduct a series of tests and experiments on several designs and constructions in many conditions. The preliminary results allow them to continue with innovative noise-mitigating systems (NMS). The origins of underwater piling sound The majority of underwater piling sound does not come from the piling hammer but from the pile itself. As it absorbs energy from the hammer, a pulse is generated through the pile wall with a speed of about 5,000m/s, making it ‘sing’. The intensity and composition of the emitted sound primarily depend on the blow energy induced by the hammer, the soil resistance, the pile dimensions and the conditions under which the pile is installed.
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| IHC Underwater Piling |
In a number of tests, conducted during the installation of two meteo masts for RWE in IJmuiden and Nordsee Ost, IHC Hydrohammer found that the maximum noise production tends to flatten out as the blow energy reaches more than 600kJ and asymptotically reaches a value of approximately 175dB against a reference of 1μPa2s. This is illustrated in figure 4. The noise level without NMS is represented in grey and with NMS in dark red. The red line represents the prescribed 160dB limit. The resistance between the soil and the pile also determines the sound levels. Other influential factors include the sea state, soil constitution and rainfall. For example, a rough sea tends to muffle sound, thereby decreasing the perceived far-field noise. The constitution of the surrounding soil and the
application of scour protection has an impact on the acoustic radiation and wave propagation. And rainfall significantly increases measured sound levels.
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| IHC Underwater Piling |
Development of noise-mitigation systems (NMS) The development of NMS types started in 2007 and the first experiments with scaled-down models were carried out in 2009. From basin tests, an actual-size NMS was manufactured and tested at the IHC Merwede yard in Kinderdijk. Offshore
testing in the ESRa Baltic Sea and FLOW projects began in September 2011 and IHC Offshore Systems’ NMS-6900 is currently being deployed and tested during the construction of the Riffgat offshore wind farm near the German isle of Borkum. Simultaneously, scaling-up from 0.9 to 6.5m pile diameters is involved (figure 5). The piling activites for Riffgat are finished and the NMS performed well below 160dB. The NMS basically consists of a thin walled steel tube with a smaller thin walled steel tube inside. The space between the tubes is sealed at the bottom and top, thus creating an air gap. The difference in acoustic impedance between the air and the outer tube causes a high reflection of the acoustic wave travelling from the pile outwards. This results in a reduced excitation of the outer tube, so the amplitude of the structural vibrations is lowered and the acoustic radiation is decreased.
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| IHC Underwater Piling |
In addition, a tube ring with small holes is fitted on the sea floor between the pile and the NMS. When air is pumped into the ring, bubbles ascend to the surface and form a ‘bubble screen’
acting as an extra acoustic barrier. Yard experiments with a 2,200mm diameter prototype showed the potential of this concept. Although the conditions were not yet representative for offshore operation, the screen yielded a massive drop in emitted sound pressure.
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| IHC Underwater Piling |
Current example: NMS-6900
The full-scale IHC Offshore Systems NMS-6900 for the Riffgat project is suitable to enclose pile diameters from 4.9 up to 6.9m and is designed in close cooperation with IHC Hydrohammer
(figures 6-8). This NMS uses the pile for support, for which a diaphragm-shaped gripper with glide pads connects the inner screen to the pile. To avoid the shortcut of vibrations to the outer screen, the connection between the inner and outer screen is isolated with rubber elements. Furthermore, the air cavity between the inner and outer screen is increased significantly. Its width is designed to reflect and dampen the dominant frequencies in the noise spectrum.
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| IHC Underwater Piling |
Between the pile and the inner screen of the NMS-6900, a multi-level air injection system is constructed. This system produces an air-water mixture of increased width by air bubbles of different sizes, having a damping effect over the whole frequency range. Both features are expected to have a
beneficial effect on performance. The NMS-6900 is currently involved in an extensive test programme, in which both the close-range and far-field noise will be measured. The results are expected to provide information on the fluid-structure interaction and noise propagation for further modelling. In the Riffgat project, the NMS-6900 will be applied in close collaboration with the world’s largest monohull offshore lifting vessel, built by IHC Merwede (figure 9), the OLEG STRASHNOV [6].
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| IHC Underwater Piling |
Perspective
Sound mitigation will be an important subject in the future. It is expected that the German regulations will serve as an example for the EU and worldwide treatment of the issue. Within IHC Merwede, several programmes have been designed to serve its development strategy. For example, in the short term, a change in the piling process, increasing the blow frequency at reduced blow energy and a PhD study in cooperation with Delft University of Technology are part of the FLOW programme, facilitating the modelling of sound emission and propagation, which should ultimately enable prediction of the phenomenon and further improvement of the NMS. This is obviously
a long-term project.
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| IHC Underwater Piling |
References
[1] Derived from: Sytske van den Akker. Sound Solutions: Future offshore wind installation techniques without underwater noise. The North Sea Foundation. 2011 and other loci on the
Foundation’s website
[2] Underwater Sound in Relation To Dredging. CEDA position paper. Central Dredging Association. Delft, The Netherlands, November 2011, Table 1. All mentioned dB-values have been referred to the well known 1μPa underwater sound reference level
[3] Frank Thomsen (DHI). CEDA position paper on underwater sound. Presentation at CEDA Dredging Days, Rotterdam, The Netherlands, 2011. Available at http://www.dredging.org
[4] “Piling with a pencil: IHC Hydrohammer®, for superb pile driving”. Ports and Dredging 176. IHC Merwede, Sliedrecht, The Netherlands, 2011. 16-19
[5] “Oil, steel and passion: the story behind two remarkable IHC Merwede companies”. Ports and Dredging 172. IHC Merwede, Sliedrecht, The Netherlands, 2009. 18-21
[6] “Monohull heavy lift vessel: OLEG STRASHNOV, the largest of its kind in the world”. Ports and Dredging 176. IHC Merwede, Sliedrecht, The Netherlands, 2011. 10-15
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