Application of high-speed fiber bragg grating sensing system for safety monitoring of modern ships

Abstract

Structural health monitoring (SHM) can be understood as the integration of sensing intelligence and possibly also actuation devices to allow the structure loading and damaging conditions to be recorded, analyzed, localized and predicated. SHM sensing requirements are very well suited for fiber optic technology. The fiber Bragg grating (FBG) sensor is a promising sensor of SHM for many applications because of its good optical properties, small size, low insertion loss, antiRF, anti-electromagnetic interference, breakdown voltage, radiation-resistant, light weight, small and large signal bandwidth. There also are reports of the application of FBG sensors for SHM of large industrial and civil works such as dams, bridges [1−2] , aero plane [3] , and tunnels [4,5] . The development of ship’s SHM takes advantage of multidisciplinary work that is being done, and most of the applications reported are about ship hull monitoring systems [6,7] . Previous studies mostly focus on the measurement of static stress [8] . For a modern ship, the thruster is a key part of the propulsion machinery, and the stability of its related equipments has a great influence on the propulsion efficiency of the ship. Electric resistance strain sensors are difficult to be used for the stress measurement for the thruster because of the electromagnetic disturbance and other complicated environment. Low-speed FBG sensors cannot meet the requirements due to the propulsion machinery’s 100 Hz arising vibration frequency. The high-speed FBG sensing system can be used to measure dynamic stress and achieve the goal of safety monitoring in advanced ships. In this letter, the application of a high-speed FBG sensing system for SHM of modern ships is proposed. We apply this sensing system to give on-site tests to an advanced ship. Strain distribution of the bases and supporting frames of its large turbine thrusters are measured both onshore and in the sea when it runs at different work conditions. The maximum natural frequency of this measured structure is 335 Hz. To test all the monitoring points synchronously, an 8-channel senor system is applied, and the data acquisition frequency was 1000 Hz. Most monitoring points are set with single-line sensors for unilateral stress, and the others are set with triple directions sensors to measure shear stress. Temperature compensation is taken with additional FBGs to eliminate the effect of cross-sensitivity between temperature and strain on FBG sensors. As for every single monitoring point, the max and average stress-gas turbine power factor (GTPF): N/Ne curves are analyzed. Then we synthesize all the results from three-time onshore tests and acquire the static stress-GTPF curves. Results from the test at sea are discussed together, thus we can accomplish completely prediction of the dynamic stress-GTPF curve when we did not obtain dynamic data in every work condition because of some objective reason. We examine the vibration amplitude of the wave length of signal light from each specific monitoring point under the same work condition for all tests and find it barely affected by the environment factors, unlike the absolute wave length. Those experimental results are accordant with mechanical theories, and the final reports have obtained clients’ approval. FBGs are periodic alterations in the index of refraction of the fiber core, and only the specified wavelength (Bragg wavelength) related to its grating period is reflected for an input light wave form a broadband source.