In shallow waters, offshore wind structures are fixed to the ocean floor, transforming their kinetic energy from wind into electricity that can be used onshore. However, in deeper waters where winds are stronger and wind turbines have the capacity to reap more than double the energy, floating offshore wind systems experience more extreme conditions that make building, operating, and maintaining them more challenging and expensive. California in particular has some of the best conditions for offshore wind energy, with extremely strong winds only 20 miles offshore, but because of the deep waters, only floating is available for much of the Pacific coast. As it begins to develop, offshore wind technology shows great potential– but there is limited information about the optimal solutions for their design, operations and maintenance, and cost reduction.

With funding support from the California Energy Commission, EGD scientist Yuxin Wu and a team are working to address these challenges with fiber-optic sensing, a technology that would allow scientists to monitor the large, floating structures autonomously. A paper he co-authored about one of the first experiments inducing shaking on a turbine tower, the support part of the system that connects the wind turbine at the top of the structure to the base at the bottom, has recently been published in the Journal of Civil Structural Health Monitoring. The shaking the tower was subjected to in their study simulates conditions the system would experience in the deep ocean far offshore. Researchers monitored the turbine’s response to the shaking using fiber-optic sensing to get a close look at how well this technology would monitor the structures in real time. The study was a collaboration between EESA and University of California, Berkeley.

The tower of the offshore wind system that connects the turbine to the base that was subjected to shaking during the experiment. Courtesy of: Yuxin Wu

Fiber optic sensing works by sending light through a thin glass cored cable and recording its backscattered behavior, which provides data about the structure–its temperature, strain, or vibration, for example. The technology can also “hear” ambient signals, such as whale vocalization. Embedded onto offshore wind towers, fiber-optic cables can autonomously provide critical information about the stability and behavior of the system, which could significantly reduce operations and maintenance costs as it reduces the need for frequent visits in these remote, harsh locations in the deep ocean. This technology also allows scientists to catch potential issues with the turbines before repairs become a larger, more expensive problem. 

“Other areas around the country, for example the Atlantic Ocean off the East coast, have been able to develop offshore wind farms on fixed structures because the water is more shallow,” Wu said. “But in California, at the distances offshore that we would need to maximize wind speed and energy potential, our Pacific waters are too deep. Floating offshore systems are the only option, and we demonstrated that fiber optic sensing could be useful to help reduce costs and make them safer and more sustainable.”

Berkeley Lab and UC Berkeley researchers installing fiber optic cables into the turbine tower studied in the experiment. Courtesy of: Yuxin Wu

The study used a shake table at the Pacific Earthquake Engineering Research (PEER) lab at University of California, Berkeley to stress and shake a full-scale, 25-meters-tall tower from the 1980’s, both while it was standing up and while it was laying down, as the ocean would shake the floating system from all directions. They installed fiber optic cables throughout the tower to measure deformations and strain. One major finding was that the sensing technology has enough sensitivity to detect loose bolts of the tower joints, which is a regular maintenance item that can cause serious issues and requires labor for retightening. 

“This is an example of a major benefit from fiber optic sensing,” Wu said. “We don’t have to wait until the structure falls apart to know the bolts are loose. We can detect this early on and guide maintenance activities more effectively.”

By demonstrating the use of this technology embedded on a turbine tower, the team collected some of the first-of-its-kind data as to how fiber optic sensing would work in mimicked ocean conditions. The strain data provided can now help to inform continued research and development to optimize the use of fiber optic cables and help support these turbines in the deep, powerful waters of the offshore open ocean.

However, fiber optic cables are typically used for sensing more stable structures, such as concrete. This study showed that although fiber optic presents an effective monitoring solution, the shaking caused issues with data transmission, which interferes with the quality of the data. 

“This is part of what we need to know to make this technology work for floating offshore wind,” explained Wu. “What are the setbacks, and how can we address them?”

In addition to optimizing fiber optic cables for these large, moving structures, Wu hopes to expand this test to the other parts of the turbine (the blades and base) and research how to embed the technology into the structure design as opposed to attaching it post-construction.

“The ultimate goal is to have fiber optic built into the systems, offering what you could call a ‘nervous system’ for the turbines,” Wu said. 

With the help of fiber optic sensing, Wu and his collaborators are one step closer to increasing our nation’s energy sources and harnessing the immense power of the Earth.