Underwater acoustical pressure reduction in pile driving at offshore wind farms by the use of air bubble curtain

• Κύριος συγγραφέας: Πολύζος, Ευστράτιος
• Άλλοι συγγραφείς: Πολύζος, Δημοσθένης
• Μορφή: Thesis
• Γλώσσα: English
• Έκδοση: 2019
• Θέματα: Noise mitigation; Pile driving; IEMA; Θαλάσσια αιολικά πάρκα; Απόσβεση ακουστικών κυμάτων; Μέθοδος συνοριακών στοιχείων

An important engineering challenge of the present day, and a vital one for the future, is developing and harvesting alternative sources of energy. Due to that fact, offshore wind farm has a great response as an emerging technology in the wind energy conversion system, by providing large amounts of sustainable energy. However, the most common technique of foundation establishment – the pile driving – seems to influence marine mammals negatively. The noise generated from driving a foundation pile into the seabed using a large impact hammer affects the aquatic life several kilometers away, since lower frequencies (10 – 1000 Hz) with larger wave lengths, do not dissipate in the sea water easily. This is mostly common for piles of large diameter that tend to radiate the acoustic energy in lower frequencies. In an attempt to mitigate the ecological footprint, several techniques have been introduced, main of which is the placement of an air bubble curtain around the pile. Air bubbles in water increase the compressibility several orders of magnitude above that of bubble-free water, thereby greatly reducing the velocity and increasing attenuation of acoustic waves propagating in the water. The purpose of this work, is to understand the mechanism of noise reduction by the use of an air bubble curtain and to compare different cases of curtains in terms of placement and composition for the frequency bandwidth that is of interest. In order to simulate the air bubble curtain, an iterative effective medium approximation (IEMA) method for wave dispersion and attenuation predictions in bubble water, is introduced. Finally, the problem is modeled and solved by the use of the Boundary Element Method, which provide fast and accurate results to semi-infinite, exterior, linear problems.