Posters

Abstract

Monitoring sea states across the offshore wind farm areas is essential to keep their structures safe, efficiently operate the systems, and assess the environmental effects of wind turbines. Conventional sea state sensors like buoys limit their observable coverage; therefore, installing many sensors across the wide area is necessary to obtain sufficient sea state information. However, such a situation is not practical in terms of cost. Instead, the study proposes utilizing optical fibers for telecommunications embedded in existing power cables on the seabed as a sea state monitoring sensor with distributed acoustic sensing (DAS). DAS is a vibration-sensing technology along optical fibers based on the Rayleigh backscattering of the injected laser. It measures the dynamic strain of the optical fiber in real time at each spatial bin, which is called a "channel" along the fiber. In power cables on the seabed, time-varying water pressure due to waves is expected to exert dynamic strain. This hypothesis motivates us to validate whether the application of DAS to power cables can estimate sea state, such as wave height, period, and the direction of arrival of waves. Hence, the authors carried out a wave tank experiment with a programmable wave generator. An actual power cable is installed under the same condition as the bottom-mounted offshore wind turbines. The experimental results show that (i) the wave period can be accurately estimated from the frequency-domain analysis. (ii) The vibration power of DAS signals depends on only the wave height. In addition, there is a strong linearity between DAS vibration power and the wave height. (iii) The direction of arrival of waves and wavelength can be derived when there are at least two laying angles of the cable. These outcomes promote the feasibility of utilizing the existing power grid across offshore wind farms as sea state sensors.