Marine piling - energy conversion factors in underwater radiated sound: review

1. Introduction

Impact piling is a very common occurrence in marine construction projects, such as the installation of turbines for offshore wind farms. The process of impact piling has been shown to generate loud impulsive sounds that propagate into the marine environment. Due to the potential deleterious effects, it is necessary to produce predictions of the radiated sound field from piling operations for impact assessments. Noise modelling is commonly used to generate these sound levels given inputs from the hammer, the pile, and the environment. For most sources, this would typically involve separate calculation of a source function and the propagation loss to determine sound level fields emanating from the source. One method of calculating sound fields has been to predict point source sound levels representing the pile using the ‘Energy Conversion Factor’ (ECF).

The ECF represents the acoustic radiation efficiency of the hammer-pile system and is simply the ratio of the input hammer energy to the radiated sound energy. Using the ECF and a given hammer energy, one can estimate the total acoustic energy radiated into the water column, and consequently the sound exposure levels. Recorded ECFs in literature range from 0.17 % to 1.56 % and take into account every other energy loss mechanism in the piling strike. As such, it represents properties of the hammer, the pile, and the environment in terms of how much sound is radiated from the pile. Given that the range of ECFs span almost 10 dB, the selection of a suitable value will have a large effect on the sound field. It is not fully understood, however, exactly how one arrives at a suitable value of the ECF without prior numerical modelling.

One method for using the ECF has been to generate source levels directly to represent the piling. To separate this from standard uses of the ECF, we have termed this as point-source equivalent ECF throughout this report. The point-source equivalent ECF model provides the source levels of a point source pile which are propagated into the modelled domain using standard underwater acoustic point source propagation models. This method has been used in numerous Environmental Impact Assessments (EIA) to provide sound field predictions from impact piling.

This project was commissioned by Marine Scotland, with the aim to improve the understanding of the point-source equivalent ECF method, and to make recommendations regarding the modelling approaches for impact piling as used in EIAs in Scottish Waters. This report shows the results of the project findings and is structured as follows. Section 2 provides background on derived values of the ECF as applied to impact piling in literature, a review of the use of the point-source equivalent ECF that appear in EIAs and articles, and a brief overview of acoustic propagation models that are commonly used for impact piling. Section 3 shows the results of studies comparing the differences between line source acoustic models, designed for use with piling, and point source models, which includes the point-source equivalent ECF method studied here. Examples are shown for simple geometric models and high-resolution numerical models to show how to different methods of propagation differ and the implications of using the wrong model. Section 4 provides an overview of possible methods for predicting sound levels from piling. These include predictions based on existing measurements and full-scale numerical modelling. Finally, Section 5 provides a summary of the findings and recommendations for future projects.