Noise Reduction of UXO Detonations using Passive Acoustic Resonators
1, Kevin Lee2, Andrew McNeese2, Preston Wilson2
1AdBm Technologies, Austin, TX, USA, 2Applied Research Laboratories, The University of Texas at Austin, Austin, TX, USA, 3Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX, USA
The presence of unexploded ordinances (UXO) at offshore wind installation sites in the North Sea is a significant challenge to offshore wind developers, and each UXO needs to be handled by either removing it or detonating it in place. The detonation of these high explosives in the water can create significant amounts of noise in areas where sensitive marine life is known to exist, so reduction of this noise is highly desirable.
In this presentation, a new approach to underwater noise abatement for UXO detonation is introduced and described. The proposed noise abatement system utilizes specially-made air-filled acoustic resonators of prescribed shape and size, based on the expected noise spectrum of the blast, and optimally reduces noise at frequencies of concern for marine life. The acoustic resonators are dispersed around the UXO before detonation and the noise emitted by the blast causes the resonators to go into a volumetric oscillation which takes a significant amount of energy out of the blast wave. It requires no air compressors and is completely passive. This method of reduction has shown up to 50 dB of reduction at the peak frequency in laboratory testing, and is particularly well suited to UXO detonation noise because of the low-frequency nature of the sound.
Laboratory tests of the system will be described in detail, as will the significant advantages of such an approach. In addition, the limitations of noise mitigation of any sort around a UXO sitting on the seafloor will also be covered.
The system can be built in various ways and using a variety of materials, depending on the operational constraints. For commercial offshore use Helmholtz resonators of various sorts have been designed, but this presentation covers proof-of-concept laboratory testing performed at The University of Texas at Austin in which encapsulated bubbles were used as the acoustic resonator. The sound source used is a combustive sound source (CSS) developed at the laboratory, which generates high amplitude sound through the combustion of hydrogen and oxygen gas underwater. Measurements taken in a large test tank demonstrated the effect of various numbers of these acoustic resonators on the sound levels and spectrum of the combustion event.
Testing showed that the void fraction, defined as the percentage of a defined volume which consists of air bubbles, has a direct and predictable effect on the amount of sound attenuated, which agrees with theory. In addition, the frequencies that are maximally attenuated are also directly related to the resonance frequencies of the acoustic elements. Peak reductions of 50 dB at the resonance frequency of the acoustic elements is noted, with significant reductions occurring up to 10 times the resonance frequency of the acoustic elements. Mixing of multiple resonator sizes will be discussed, as will the use of different materials, and the unavoidable limitations around noise mitigation of ground-laying UXO.
A demonstration of using an acoustic resonator-based noise reduction system and its application to UXO is described. The results show that using this approach one can control the amount of attenuation and the frequencies at which there is maximum reduction, unique to any other noise mitigation system. In addition, the operational advantages of this approach to noise mitigation on UXO in the North Sea are significant because of its small footprint and the fact that it's a completely passive system, requiring no air compressors or other source of power.
Attendees will learn about various noise abatement systems and the science behind the relative advantages and disadvantages of different systems. In specific, they will learn about the concepts of bubble acoustics, Helmholtz resonance, and impedance mismatching and how they are applied to noise reduction devices. In addition, the topic of ground-borne vibration and how that affects noise reduction will be addressed. Finally, they will learn about how systems can be tailored to particular applications and the advantages of such an approach.