Mar 31 2020
Fiber optic cables have turned out to be remarkably useful scientific sensors. At Lawrence Berkeley National Laboratory (Berkeley Lab), scientists have investigated these fiber optic cables for use in earthquake detection, groundwater mapping, carbon sequestration, and tracking of Arctic permafrost thaw.
Berkeley Lab is working to address barriers to more widespread deployment of offshore wind in California, where floating wind turbines could be a viable option. Image Credit: SarahGower/iStock.
The researchers have currently received new grants to build fiber optics for two innovative applications, such as underground natural gas storage and tracking offshore wind operations.
A fiber cable has a glass core that allows you to send an optical signal down at the speed of light; when there is any vibration, strains, or stresses or changes in temperature of the material that is being monitored, that information will be carried in the light signal that is scattered back.
Yuxin Wu, Study Lead and Scientist, Lawrence Berkeley National Laboratory
Berkeley Lab has received a $2 million from the California Energy Commission for the offshore wind project and has also received $1.5 million for the natural gas project. These two projects will be performed in association with UC Berkeley.
Berkeley Lab will also team up with Schlumberger, PG&E, and C-FER Technologies (a Canadian firm) to conduct the tests for the natural gas project.
From Gearbox Failure to Humpback Whale Movements
Europe is leading the offshore wind development, while the rest of the world is only in the preliminary phases of commercialization, which is increasing rapidly. This also includes the United States, where the Department of Energy (DOE) has been supporting the advancement of the technology.
According to a 2016 DOE report, the United States has abundant offshore wind resources that have the ability to deliver almost twice the overall amount of electricity presently produced in the nation.
For the United States, one of the benefits of offshore wind is that the resource is closer to dense coastal populations. Hence, energy transmission is a major challenge when compared to that of the other renewable energy sources like solar farms and onshore wind, which are generally situated farther away from population centers because of the cost and availability of real estate.
The ocean floor runs steeply off the California coast, rendering floating wind turbines as the only feasible option. Floating wind turbines are secured to the ocean floor by mooring chains, different from traditional “fixed bottom” offshore wind turbines.
However, this technology encounters several barriers, including how to perform operations and maintenance on distant installations in the ocean in a cost-effective way, and how to track if dangers like adverse weather conditions or earthquakes impact operations. Here, fiber optic cables can play a major role.
One of the most expensive components of a wind turbine is the gearbox; they also tend to be the part that’s most vulnerable to failure. Often before they fail they produce abnormal vibrations or excessive heat due to increased or irregular friction. We intend to use fiber optic cables to monitor the vibrational, strain, and temperature signal of the gearbox, in order to pinpoint where problems are happening.
Yuxin Wu, Study Lead and Scientist, Lawrence Berkeley National Laboratory
Wu is also the head of Geophysics Department at Berkeley Lab.
Covering the entire gearbox with fiber optic cables can offer a three-dimensional (3D) map of changes with a millimeter-scale resolution.
“It could help identify problems with the gearbox at an early stage, which would trigger emergency management, before a catastrophic failure causing loss of the whole turbine,” added Wu.
Additionally, the project is aiming to investigate the possibilities of using the fiber optic cables to detect the activity of marine mammals, added Wu. The fiber signal’s sensitivity could facilitate the differentiation between a pod of whales swimming by and the crashing waves, for example.
Environmentally sustainable development of offshore wind is critical. With a large offshore wind farm, there would be many of these mooring lines securing the turbine structures to the ocean floor.
Yuxin Wu, Study Lead and Scientist, Lawrence Berkeley National Laboratory
Wu continued, “If a humpback whale swims by, what are the impacts of these mooring lines on their activities? Will the whales generate unique vibrational signals that can be picked up by the fiber optic sensors? If we can track the signals of a whale swimming by, it will allow us to evaluate whether and how the offshore wind turbine impacts marine mammals.”
Wu further reported that he is trying to find out more various marine mammals, including whales, from marine biologists and is looking for an associate with whom he can work with to investigate the sensors deployed in the ocean.
Making Underground Gas Reservoirs Safer
Likewise, Wu and his fellow researchers are hoping to utilize the fiber optic cables to track the boreholes of natural gas storage reservoirs buried under the ground. The boreholes are used for injecting and extracting gas from large underground storage reservoirs.
The boreholes are just like any other pipe, and they decompose and corrode over time. The large gas leak that occurred at Aliso Canyon in 2016 had forced thousands of families to abandon their homes. It was concluded that this mishap was caused due to corrosion damage to the borehole.
Hence, the integrity of boreholes is very important to store natural gas safely in the subsurface. At present, borehole integrity is largely being tracked by utilizing tools that are costly, intrusive, and incapable of giving real-time, frequent data.
“It is difficult to predict borehole degradation trajectory with the sparse data generated by traditional methods. Having higher frequency datasets covering the entire borehole is key to provide an early warning of potential borehole failures,” added Wu.
In the latest CEC-funded study, Berkeley Lab will collaborate with C-FER, Schlumberger, PG&E, and UC Berkeley to test a unique line of technologies for real-time, autonomous monitoring using a couple of techniques—one using electromagnetic wave reflectometry and the other based on distributed temperature, vibration, and strain sensing in fiber optic cables.
Electromagnetic time domain reflectometry (or EM-TDR) is analogous to the fiber optic technology but it utilizes longer wavelength electromagnetic waves rather than visible light (also an electromagnetic wave but at relatively short wavelength) as signals.
“EM-TDR sends electromagnetic waves into an electronically conductive material, and when there is a change due to damage, such as corrosion, you get an EM signal back which can help you identify corrosion or other degradations,” Wu further added.
Since the borehole is produced from steel, which happens to be electrically conductive, there is no need to install any downhole equipment. Hence, EM-TDR can be easily deployed and utilized under a number of situations that avoid the use of other kinds of sensors. The EM-TDR, on the other hand, is still a nascent technology; the latest research will enable additional testing and improvement.
For the offshore wind project and the natural gas project, the scientific challenge is to improve the design and sensitivity of the technology and develop real-time edge computing technologies.
“In addition to using commercial systems, our team is developing new fiber interrogators that will allow us to not only get to the original raw data but also play with the physics to better design a system that can give us the most sensitive signal we want. In addition, we will be developing machine learning-based edge computing methods to turn raw data into actionable intelligence quickly. This is key for real-time monitoring,” Wu concluded.
Source: https://www.lbl.gov/