The Internet of Underwater Things for Monitoring Offshore Wind Turbines

Offshore wind farms are multiplying to meet European demands for renewable energy production. A team of scientists from Nantes is working on an underwater acoustic sensor network to continuously monitor the state of offshore wind turbines.

With sixteen offshore wind farm projects along the metropolitan coasts, France is following in the footsteps of its British (44% of the nearly 6,000 offshore wind turbines installed in Europe), Dutch, and German neighbors. The European Commission, for its part, aims to increase production capacity fivefold in ten years, partly thanks to floating wind turbines in deep waters. By 2050, a production of 450 GW is envisaged on European coasts.

To ensure the sustainability of the installations while minimizing human intervention as much as possible (such as divers for submerged structures), it is necessary to accompany these offshore deployments with underwater surveillance technologies. Like the Internet of terrestrial objects, which consists of a large number of sensors communicating via wireless radio links, the Internet of underwater objects consists of a network of wireless sensors, on the seabed or along submerged structures. This allows for continuous surveillance of offshore installations, which can be complemented periodically by mobile robotic devices such as ROVs, AUVs, or USVs.

An offshore wind farm usually consists of 60 to 160 (fixed) turbines organized around a substation to which all the dynamic electrical cables converge. The electrical production is transported from the substation to the land by an export cable buried in the seabed. Floating wind turbines, on the other hand, have critical elements such as the dynamic cable, which must observe a specific curvature to dampen the wave phenomenon, the connection between the dynamic cable and the static cable, and the anchoring system. This type of data is part of what is sought to be transmitted to the surface. In general, these different components are located at depths greater than 40 meters for the Atlantic coasts, and at over 200 meters in the Mediterranean Sea. Due to the movements of the structures (caused by waves and wind) and the envisaged depths, the Internet of underwater objects favors wireless communication.


Underwater Internet of Things for the monitoring of offshore floating wind turbines © Étienne Parrein cc-by-nc-na

Sound for underwater communication

It is possible to rely on different wireless technologies: radio, but this is strongly attenuated in aquatic environments; optics, which requires clear water (without particles) for good direct data transmission; and finally, sound, which has many advantages, including the kilometer range of the signal.

Underwater, acoustic waves have a propagation speed of 1,500 meters per second, five times faster than in air. However, this propagation speed is extremely slow when compared to the speed of light (300,000 km/s): it is 200 thousand times slower than a radio wave in the air. For comparison, acoustic communication at 200 meters below the surface can take as long as a radio communication from Earth to the Moon. But underwater, acoustic waves have a significant advantage: they can be transmitted over several kilometers. Therefore, scientists design their underwater network by making a compromise between range and transmission time.

The equipment that communicates the structure sensor data is an acoustic modem (modulator/demodulator). It is the same equipment found in households for accessing the internet via fiber optics or ADSL, with the difference that it emits sound (like a speaker), usually at a frequency around 30 to 40 kHz (these are ultrasonic, beyond the audible range for humans) and at extremely low rates (50 bits per second). This same equipment can also receive data (like a microphone) to enable two-way communication.

A network to organize data transmission to the surface

With five to ten modems per turbine, or about several hundred modems submerged per wind farm, a networking issue arises. Indeed, it is necessary to coordinate the entire network to reliably transmit structure sensor data and avoid collisions as much as possible, that is, to avoid modems communicating simultaneously.

This is where network engineering comes in, allowing a very large number of devices to communicate in a coordinated manner (similar to the internet). In simple terms, it is about designing a reliable underwater Wi-Fi network. The coordination prerogative falls under layer 2 of the OSI model (Open System Interconnection), commonly known as MAC layer for Medium Access Control. For the past fifteen years, there have been quite a few proposals for MAC protocols for underwater acoustic networks in the literature, which can be summed up in two categories: asynchronous protocols such as T-Lohi, which, like WiFi protocols, use request signals before communicating information, and synchronous protocols such as TDA-MAC (Transmit Delay Allocation), which allocates a specific time interval to each modem to minimize collisions. It is this protocol that the Blue IoT Eolia project team has identified for deploying its sensor network within offshore wind farms.

Virtual experiments in freshwater and sea

Before conducting experimental deployments in natural environments (river, lake, and then maritime domain), the team of scientists works on network simulators to model underwater acoustic propagation, as well as the behavior of communication protocols, down to the packet level. This initial study allows for the correct network parameterization, to guarantee the best performance in terms of quality of service (delivered packet rate in particular), while evaluating the energy consumption of the modems to estimate the network’s lifespan. In fact, the network must function autonomously on battery power for several months without human intervention.

Experiments of the TDA-MAC protocol near the port of La Turballe (Loire-Atlantique) conducted in May 2022. © Blue IoT Eolia Project

After tests in natural environments on the Erdre (near Nantes), in Abbaretz, and in La Turballe, the scientists were able to validate the usefulness of the TDA-MAC protocol for underwater surveillance of offshore wind turbines. Using hydrophone recordings, they also showed that the sound levels of communications between the turbines were well below the disturbance thresholds established in 2019 for cetaceans and pinnipeds.

Maximizing network lifespan (particularly by regulating the acoustic and electrical powers involved), designing the data plan to optimize the transfer of information formatted by the sensors, and interacting with mobile units such as AUVs remain areas for further study. This work can be extended to the surveillance of subaquatic ecosystems or to the security of fixed underwater installations (communication, energy transport, etc.).

Authors
Benoît Parrein, Laboratory of Digital Sciences in Nantes (LS2N), Jean-Marc Rousset Laboratory for Research in Hydrodynamics, Energy, and Environment (LHEEA).

 

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