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.

The large-scale deployment of offshore wind farms is critical to achieving global renewable energy targets. However, the installation of monopile foundations through impact pile driving generates significant underwater noise, posing risks to marine ecosystems. While noise mitigation measures, such as air-bubble curtains and pulse elongation systems, have been developed to reduce peak sound levels (Lpeak) and sound exposure levels (SEL), less is understood about their effects on cumulative noise exposure (cumSEL) a key metric in regulatory frameworks like that of the United States.
This study investigates the influence of operational parameters, including the target blow count and impulse elongation, on cumulative underwater noise levels during offshore pile driving. Using the SILENCE model, a semi-analytical approach is applied to simulate noise propagation in realistic offshore conditions. The research evaluates the combined effectiveness of mitigation strategies such as pulse elongation and air-bubble curtains, as well as the effect of frequency weighting on sound levels for specific species. The results show that varying the target blow count has negligible influence on cumulative underwater noise, while impulse elongation (PULSE) achieves modest broadband reductions and more substantial reductions when species-specific1 frequency weighting is applied. The combination of PULSE with a Double Big Bubble Curtain (DBBC) offers the most effective mitigation across all marine mammal hearing groups.
This study provides insights into the complex interactions between operational parameters and noise mitigation strategies, supporting the development of effective solutions to comply with cumulative noise regulations. The findings aim to contribute to more sustainable offshore wind farm construction while minimizing impacts on marine ecosystems.