Innovative WEAP-FE method advances the prediction of underwater noise
With the growing demand for sustainable energy, the number of offshore wind turbines is multiplying at a fast rate. As a consequence, specific underwater noise limits are surfacing in national legislation.
In Germany, for example, regulations include explicit limits and a comprehensive set of mandatory measurement procedures. Reported measurement results show that for unmitigated driving of large diameter piles, these limits are often exceeded. This introduces challenges and uncertainties in the installation process and calls for an accurate, validated prediction tool to quantify the noise emission.
DONG Energy Wind Power and Lloyd’s Register Consulting therefore initiated a pioneering development project to advance the understanding of underwater noise emission and produce a reliable prediction tool.
Combining wave equation analysis for piles and finite element analysis
It was our aim to enable a prediction, in absolute terms, based on technical site-specific input rather than empirical data. To that end, we applied a detailed technique that accounted for the dynamics within the hammer-pile system and the energy dissipation mechanism at the pile-soil interface.
The method combined two techniques: Wave Equation Analysis for Piles (WEAP), a well-established geotechnical tool that calculates the stress wave in the hammer-pile system, and vibro-acoustic Finite Element (FE). Here, the strength of the WEAP was twofold: it represented both the complex pile-soil interaction and the multi-component hammer systems common to offshore piling.
The proposed method relied on WEAP for estimating the related energy dissipation and the loading to the pile head. By introducing these as input to a relatively simple FE model, an operational prediction method was then achieved.
For validation, we tested the WEAP-FE method against empirical data from two experimental cases.
The first case documented the results from a comprehensive measurement campaign at Anholt Offshore Wind Farm. Here, the piling involved a steel monopile in a conical design with varying wall thickness, driven by a hydraulic hammer with no cushion element applied. The comparison between the measured hydrophone data and the computed results revealed an impressive agreement within +/-2 dB for the Sound Exposure Level (SEL).
We then tested the WEAP-FE method against the measurement data from an experimental setup of harbour piling from the Vashon Island ferry terminal. The cylindrical steel piles had a length of 31.9 m, an outer diameter of 0.76 m and were driven by a diesel hammer. No cushion element was applied.
Again, the results in frequency domain were extremely satisfactory, showing a remarkable agreement in overall SEL around +/-1 dB between computed results and measured hydrophone data.
Benefitting the industry
These validation results demonstrated that WEAP-FE was indeed a very promising approach. With the development of this method, prediction of underwater noise emission had taken a leap forward and we had now come remarkably close to full-scale, real-world measurements.
In a broader perspective, this would facilitate the further development of precise noise prediction tools and improved mitigation techniques.