Parametric study of noise reduction by an air-bubble curtain in offshore pile driving

This paper describes a semi-analytical model that is used for the investigation of the sound reduction during marine piling when an air-bubble curtain is placed around the pile. The model consists of the pile, the surrounding water and soil media, and the air-bubble curtain which is positioned at a certain distance from the pile surface. The aim of the paper is to analyse the principal mechanisms that are responsible for the noise reduction due to the application of the air-bubble curtain in marine piling. The results show that the noise reduction depends strongly on the frequency content of the radiated sound and on the characteristics of the bubbly medium. For piles of large diameter, which are typically used nowadays as foundation piles for offshore wind power generators, the noise reduction is mainly attributed to the reflection of the acoustic waves at the inner surface between the seawater and the bubbly layer. On the contrary, for piles of smaller diameter and when the radiated acoustic energy is concentrated at frequencies closer to, or higher than, the first omnidirectional resonance frequency of the air bubbles, the sound absorption within the bubbly layer and its thickness become critical.