Study of the sound escape with the use of an air bubble curtain in offshore pile driving

Underwater noise pollution generated by offshore pile driving has raised serious concerns over the ecological impact on marine life. To comply with the strict governmental regulations on the threshold levels of underwater noise, bubble curtains are usually applied in practice. This paper examines the effectiveness of an air bubble curtain system in noise reduction for offshore pile driving. The focus is placed on the evaluation of noise transmission paths, which are essential for the effective blockage of sound propagation. A coupled two-step approach for the prediction of underwater noise is adopted, which allows us to treat the waterborne and soilborne noise transmission paths separately. The complete model consists of two modules: a noise prediction module for offshore pile driving aiming at the generation and propagation of the wave field and a noise reduction module for predicting the transmission loss in passing through an air bubble curtain. With the proposed model, underwater noise prognosis is examined in the following cases: (i) free-field noise prediction without the air bubble curtain, (ii) waterborne path fully blocked at the position of the air bubble curtain while the rest of the wave field is propagated at the target distance, (iii) similarly to (ii) but with a non-fully blocked waterborne path close to the seabed, and (iv) air bubble curtain modeled explicitly using an effective medium theory. The results provide a clear indication of the amount of energy that can be channeled through the seabed and through possible gaps in the water column adjacent to the seabed. The model allows for a large number of simulations and for a thorough parametric study of the noise escape when a bubble curtain is applied offshore.