The installation of offshore monopiles using hydraulic impact hammers generates intense impulsive underwater noise, posing risks to marine life and challenges compliance with underwater noise regulations. As monopile sizes continue to increase, associated noise levels also rise , necessitating noise mitigation systems. This study investigates the performance of a double-walled steel noise mitigation screen (NMS) with an integrated air layer and bubble system, designed to reduce underwater noise emissions during pile driving.

A coupled numerical modeling approach was used, combining a finite element model for near-field sound generation with a semi-analytical approach for far-field sound propagation. The model was calibrated and validated against field measurements from an offshore installation campaign. While the unmitigated model showed good agreement with measured sound exposure levels, the numerical representation of the NMS underestimated the observed mitigation performance. While the numerical representation of the NMS captures the primary mitigation mechanism of the NMS rather than its full real-world behavior, it allows for comparing performance across varying soil conditions.

The influence of soil stratification and properties on NMS performance was assessed for uniform and layered sandy soils. Results indicate that soil conditions have a limited influence on unmitigated far-field noise levels (up to 2.2 dB variation) and on mitigated levels when the numerical representation of the NMS is included (up to 1.6 dB variation). However, soil properties significantly affect the distribution of acoustic energy between the seabed and the water column, as well as the amount of energy generated by the pile, thereby influencing the mitigation potential of near-field noise mitigation systems. Dense soils tend to increase absolute mitigation potential due to higher levels of waterborne energy generation, while layered soils facilitate more efficient propagation through upper layers and can promote re-radiation of energy back into the water column.

Due to uncertainties in the numerical representation of the NMS, conclusions regarding optimal soil conditions for its performance remain tentative. Overall, the study demonstrates that soil conditions play a key role in determining the effectiveness of near-field noise mitigation systems and highlights the need for improved modeling approaches to accurately predict NMS performance during offshore pile driving.