Posters

Abstract

Offshore pile driving represents a unique sound source in that it spans three different media through which acoustic energy can radiate. Generated by an impact or vibratory force at the top of the monopile, vibroacoustic energy travels through the monopile, the air, the water column, and multiple layers of sediment. Traditionally, the potential impacts of sound have been considered separately and/or simplistically within each of these media, with birds and bats being the focus in air and marine mammals and fishes being the focus in the water column. Very little attention has been directed to the sediments of the seafloor or the potential impacts on the marine organisms and their benthic habitats. Acoustic measurements of pile driving show higher levels of intensity near the seafloor than in the water column at ranges of several hundred to several thousand meters, suggesting the influence of elastic interface waves radiating along the seafloor may be more significant than previously thought.   This study demonstrates the importance of including elastic waves in pile driving acoustic models.  A multilayered, three-dimensional spectral element model (SEM) was created to predict the vibroacoustic field generated during offshore pile driving of large, thin-walled, cylindrical monopiles in stratified sediments as is being installed in the North Sea. These vibroacoustic predictions were then coupled to a two-dimensional parabolic equation (PE) model that was adapted to include elastic wave propagation in multiple sediment layers. The models were used to predict generated sound from a real pile driving scenario where both acoustic and ground vibration measurements are available. It is clear that complex models such as the SEM-PE Coupled Model are needed to accurately characterize the pile driving sources and improve predictions of the sound generated by offshore monopile installation, ultimately resulting in more comprehensive modeling to support permitting.