Abstract
This report provides the draft final report for the project entitled “Mitigation of Underwater Pile Driving Noise During Offshore Construction (Phase 1).” This work is sponsored by the Department of the Interior, Minerals Management Service (MMS) Engineering & Research Branch under contract no. M09PC00019. This project addresses high level underwater noise generated by pile driving of large monopiles during construction of offshore wind farms. In particular, this work is an analysis effort to establish the relative importance of the three primary transmission paths and to assess the potential effectiveness of several mitigation options.
In summary, Phase 1 was very successful. The objectives of quantifying acoustic transmission paths, and identifying and ranking noise mitigation concepts, were successfully accomplished. The key to this success was development and application of a physics-based model which enables detailed assessment of the acoustic characteristics of pile hammering, structural response, propagation through water, ground, and air, and treatment performance. The Phase 1 results provide a strong basis for proceeding to the design development and prototyping work envisioned in Phases 2 and 3 of this program. A summary of accomplishments follows.
During this project, APS built and implemented a high fidelity acoustic model of underwater noise produced by driving large piles. A large set of configurations were analyzed, including:
- 15m and 30m water depth (with the corresponding effect on pile dimensions).
- Treatment options including: no treatment; several options of compliant layer on the pile; several options of bubble screen, and a dewatered cofferdam modeled as a thick and very rigid structure considered to be an extreme case of the most effective possible treatment.
- Underwater sound locations near the bottom, in the middle of the water column, near the surface, and an average across depths, all calculated as a function of range.
- Results at frequencies of 100 to 1000 Hz, which evaluation of existing pile driving noise data indicate is the dominant frequency range.
The results clearly support evaluation of the primary transmission paths and mitigation concept options. Findings regarding the transmission paths can be summarized as follows:
- The structureborne radiation path dominates underwater noise for nearly all cases.
- The seismic propagation path is not a significant contributor to underwater noise for the untreated case, where the seismic contribution is 10 to 30 dB below the combination of all paths. The seismic contribution is the limiting factor on the overall effectiveness of treating the structureborne radiation path. With bubble screen or compliant layer treatments, the seismic path becomes a contributing or occasionally controlling path at a few frequencies. With a dewatered cofferdam installed (the most effective treatment) the seismic path is the controlling path at most frequencies.
- The airborne transmission path is not a significant contributor to underwater sound in any case. Even with the cofferdam, the airborne path contribution is 50 dB or more below the combination of all paths.
Findings regarding the underwater sound mitigation concept options can be summarized as follows. Note that these predictions are condensed from a large body of information. Specific actual installations will vary in performance, but these predictions are considered to be an effective quantification of relative performance that can be used to support evaluation of potential pile installations and design concept development.
- A bubble screen is predicted to reduce noise levels by approximately 10 dB. Variation of air volume fraction in the range of 2.5% to 5% does not significantly affect this result.
- A compliant surface treatment is predicted to reduce noise levels by approximately 10 dB. Varying thickness of the treatment in the range of 2 inches to 8 inches does not significantly affect this result.
- A massive dewatered cofferdam is predicted to reduce noise levels by approximately 20 dB. This is considered to be the upper bound on possible noise mitigation treatment performance. Model excursions showed no significant difference between cases with the inside of the pile filled with water, air, or mud.
These modeling results provide a basis for evaluating sound mitigation options with respect to specific requirements, such as frequencies and sound levels which have adverse impacts on specific species of marine life found at specific locations a wind farm is to be installed. These specific requirements will yield mitigation performance metrics which will be used for optimization analysis to identify the most feasible and cost effective design. This optimization analysis is a key element of the design development and prototype implementation and evaluation work proposed for Phases 2 and 3.
Additionally, a side study was conducted whic h indicated that a non-level bottom would not significantly affect these findings.