311 / 2018-09-25 23:56:22

Onshore Simulation Test Setup for Temperature and Strain Sensing of Submarine Cable based on BOTDA

Submarine Cable,BOTDA,Optical Fiber

The 500kV submarine cable route of Hainan Networking System starts from Nanling Village, Jiaowei Township, Xuwen County, Zhanjiang City, Guangdong Province. It crosses the Qiongzhou Strait to Linshi Village, Qiaotou Town, Chengmai County, Hainan Province. Hainan Networking System 500kV AC Submarine Cable is the first ultra-high voltage, large capacity and long-distance cross-sea networking power cable in China. It has the world's longest length and the second largest capacity in the world. It is currently the only communication channel between Hainan Power Grid and China Southern Power Grid. It has got very important political and economic significance. The geological conditions of the seabed are complex, the instability of the muddy seabed, the uneven geomorphology, the slippage of silt landslides, and the combined effects of wave dynamics produce tremendous shear, tensile and torque effects on the power cables buried in the seabed. Due to these conditions the cable got deformed, the armor layer and the insulating layer got damaged as the inside structure is broken, thereby discharging to the ground, and the short circuit causes a malfunction. However, no matter whether the submarine cable is damaged by external force or get deteriorated due to damage of internal insulation or other reasons, it can only be known through online monitoring of the submarine cable body. At present, the most widely used fiber optic sensors are distributed Rayleigh-based distributed fiber optic sensors, distributed Raman scattering-based fiber optic sensors, Brillouin-based distributed fiber optic sensors, etc. Distributed fiber optic sensor with ridge scattering.

In this paper, the submarine cable onshore simulation experiment platform designed mainly consists of three parts. The first part is to use fiber optic cable to bundle the cable, apply gravity and simulate the submarine cable to be anchored. The second part uses multiple constant temperature water bath to simulate temperature fault point. The third part is to simulate the strain failure point by using high-precision displacement platform. The Brillouin coefficient temperature and stress calibration are performed on the bare fiber used in the experiment. The traditional calibration method is to separately calibrate the temperature and strain, which will inevitably bring some errors. In this paper, a new method is used to simultaneously calibrate temperature and strain. Firstly, the bare fiber of 10cm length is fixed in a special metal groove to avoid the slippage of the bare fiber. Then, the metal groove of the bare fiber is placed in a constant temperature water bath, and a certain length of loose bare fiber is placed in the constant temperature water bath to ensure it is not affected. Finally, the constant temperature water bath was maintained at five different temperatures of 30 °C, 40 °C, 50 °C, 60 °C, and 70 °C for 15 min. Since the relaxed bare fiber is not stressed, the Brillouin frequency shift can be obtained by fitting the Brillouin frequency shift at different temperatures of the fiber. For the bare fiber on the metal trough, the pulley is taken out to the external high-precision displacement platform to obtain different strains at various temperatures, and the Brillouin frequency shift strain coefficient is obtained by fitting. After the calibration of the fiber used in the simulation experiment on the submarine cable shore, the onshore simulation experimental platform was designed. The fiber used in the experiment was a bare fiber with a length of 3.5 km. The cable is spliced to the 10m cable at 0.5 km of the bare fiber and the cable is bundled onto the cable. Then select 3 points at 1.5 km of the bare fiber and place them in a constant temperature water bath. Place 50 m bare fiber in each constant temperature water bath. Three locations are selected at 2.5 km of the bare fiber to be placed on the high-precision displacement platform. The length of the bare fiber on the displacement platform should be 50 cm. After the 3.5 km bare fiber is arranged, the first and last ends are connected to the BOTDA fiber Brillouin sensing device and the anchor is simulated at 0.5 km. The constant temperature water bath system at 1.5 km simulates the simulation of the submarine cable at different positions. The temperature fault point passes through a high-precision displacement platform at 2.5 km.

Through the submarine cable onshore simulation experiment, it is first verified that the experimental platform can realize the location and identification of the anchor damage and then verify the minimum space separation rate and temperature resolution of the BOTDA device and then verify the fiber Brillouin sensing. The minimum spatial separation rate and strain resolution of the sensing device. Finally, it shows that not only the whole submarine cable can be monitored in real time but also the failure point of the submarine cable can be quickly reacted and the counter measures can be formulated in time to reduce the cost of loss and troubleshooting.