By Air and by Sea: Novel Mobile Platforms Bring Acoustic Monitoring to Offshore Wind Farms

The construction and operation of offshore wind farms carries regulatory requirements for acoustic monitoring of both noise emissions from the activity and marine mammals’ vocalizations. The latter reveal the presence of animals that could be at risk from noise exposure or collision with service ships.

Such requirements call for innovative monitoring technologies capable of rapid deployment, efficient relocation, and wide area coverage.

Autonomous marine vehicles equipped with advanced acoustic receptors, on-board processing, and long-range telemetry are the clear future of this sector. Viable carrier designs include underwater, surface, and even airborne vehicles with water landing capacity; any of these can deploy a single or several hydrophones to sample sound levels in the water.

The sensors may be mounted directly on a vehicle’s hull or suspended from an adjustable cable to achieve optimal configurations for transit and active monitoring. Such positioning is often critical to addressing flow noise, the acoustic disturbance from water rushing past a hydrophone that can potentially mask the sounds of interest. Flow noise is a common hurdle of underwater acoustic measurements from platforms that move through the medium; mitigating solutions include the hydrodynamic shaping of surfaces near the sensors and the adaptive control of the vehicle’s motion during measurements.

MOBILIZING SENSORS

JASCO Applied Sciences (JASCO), a leading developer of underwater systems for acoustic monitoring, has been researching and optimizing the integration of such devices on a range of mobile platforms to meet the expanding requirements of the offshore wind industry.

The evolution of mobile ocean sampling platforms began largely below the water surface, driven by the rapid development and ubiquitous use of tethered Remotely Operated Vehicles (ROVs) to perform diverse subsea tasks in the immediate vicinity—a few hundred meters—of a support vessel or dock. From that technology arose autonomous vehicles in which the physical tether to a control station was replaced by inertial navigation and other onboard intelligence often complemented by periodic confirmation and updating via satellite during surface visits. Running a vehicle mostly at depth does shield it effectively from the adverse impact of surface weather, and for acoustic monitoring it eliminates some significant background noise from waves slapping against the hull.

Traditionally propelled autonomous underwater vehicles (AUVs) are large and complex, and their use remains primarily confined to long-range research missions lasting from weeks to months. The simpler and more agile subsea gliders rely on variable buoyancy and hydrodynamic lift for their forward motion; they have gained popularity as practical platforms for systematic monitoring of small regions such as the surroundings of an offshore wind farm. JASCO adapted its OceanObserver module, a high performance acoustic and oceanographic data acquisition and processing system, to fit as a standard payload in various commercial glider models. The company is developing optimized solutions for equipping these platforms with various configurations of acoustic sensors to enable, for example, the directional localization of sound sources relative to the vehicle’s orientation.

TAKING TO THE SKIES

Perhaps the most intriguing possibilities for mobile underwater acoustic monitoring are opened when unmanned aerial vehicles (UAVs), commonly known as “drones”, are brought to bear. The convergence of a UAV with a body of water, generally dreaded as the terminal outcome of a flight gone awry, has become an intentional and fully recoverable phase of a mission for novel devices designed to land on the waves, float on the surface, and later resume flight to return to base or reposition to a new site.

JASCO is currently engaged in developing a full mission-capable, airborne, acoustic monitoring platform in collaboration with a manufacturer of specialized amphibious UAVs, a variant of which could ultimately swim below the surface as well as flying. An example of the unique capabilities of drone-based monitoring was provided in 2019 by then JASCO scientist Dr. Héloïse Frouin-Mouy and collaborators in Baja California, Mexico. The team collected and published an unprecedented high-quality repertoire of context specific gray whale vocalizations by landing a lightweight hydrophone-equipped UAV near the animals as they engaged in different social behaviours. The ability to combine the scouting power of aerial observation (from the same or an ancillary drone) with the rapid airborne dispatching of an acoustic sensor to the location of spotted animals does also enable the finely targeted monitoring of underwater noise exposure levels for the protection of endangered species.

USVs PROVE SUCCESSFUL

Arguably more mundane in construction than airborne and underwater vehicles, though their apparent simplicity of design is deceptive, unmanned surface vehicles (USVs) are asserting themselves as strong contenders in the contest to the monitoring goal. Indeed, a USV from the start-up company Open Ocean Robotics (OOR) made the winner’s circle in the Offshore Wind Challenge, a six-month technology accelerator program supporting innovations in responsible development of offshore wind energy.

Co-sponsored by offshore wind project developer Vineyard Wind, the Challenge focused on advances in marine mammal monitoring, specifically for data collection and real-time transmission or data analysis. A JASCO OceanObserver payload, installed on the USV to collect and process acoustic data from a towed hydrophone array, detected whale calls in real-life ocean conditions with performance comparable, if not superior, to that of a moored installation. This early success paves the way for USVs to complement other types of acoustic sensor carriers in meeting the challenge of reliable environmental monitoring of the construction and operation of offshore wind farms.

The range of options offered by mobile platform technologies to deliver rapidly targetable monitoring solutions for ocean wind projects is considerable, and still expanding as current and new players bring advances to the field. By designing the OceanObserver system to be compatible with a wide selection of carrier types, JASCO aims at providing a consistent, high-performance data acquisition and processing framework across the technology spectrum. The choice of optimal sensor deployment platforms for a particular project can then be driven primarily, and rightly so, by operational considerations.

This story was originally featured in ON&T May 2021. Click here to read more.