Significance of dynamic variation in renewable energy device noise to background noise levels under varying conditions

Assessment of the potential impact of underwater acoustic noise from both construction and operation of almost all human marine activities is now commonplace within many international regulatory frameworks. More recent industries like offshore renewables (wind, wave and tidal) have experienced an increasing requirement to understand how their energy conversion devices interact with existing acoustic environments and what their potential for impacts might be. Assessing the potential for impact, however, results in a number of measurement challenges, for example, many of these devices represent highly complex, multi-modal, distributed sources which can be anything from arrays of many small (few meters) devices to larger systems with dimensions of hundreds of meters. The systems can also vary in nature from a floating device, for example a wave energy surface attenuator, or can be distributed throughout the water column, as can be the case for both wave and tidal systems, or wind turbine. In the case of an offshore wind turbine for example, the device is coupled to the seabed, and extends throughout the water column and in to the air. This distribution and configuration can make the measured acoustic characteristics strongly dependent on the environment, with the potential for propagation through the seabed, water and air, with complex interactions, particularly between the seabed and waterborne components of the sound. The environments themselves are also often acoustically complicated. For example, many of the devices are being developed in relatively shallow water where there is often strong bathymetric variation, and other natural and manmade noise sources. The devices themselves often also exhibit time variant characteristics. They can become more energetic with tidal speed, wave height wind speed increase, for example. Furthermore, these properties also alter the `natural' acoustic environment or baseline condition. The relationship between device noise and background or ambient noise is also likely to be fundamental to the effect these devices have on marine species, for example, by causing a change in behaviour or creating a masking effect. It may effect the detection capability of an animal, which may be important for collision avoidance. This paper discusses analysis of some of these challenges in the context of a number of emerging technology types. In particular, the paper considers the dynamically varying nature of the ratio of the noise radiated from the device to the background noise. Data is presented of the operational noise levels against the background noise levels, under different sea-states for a full-scale attenuator wave energy system.