| Summary: | 碩士 === 國立臺灣大學 === 工程科學及海洋工程學研究所 === 105 === Ultrasonic detection is a general type of nondestructive testing. It always be used to detect defects in the structure. As offshore wind turbines based structures are located in the ocean, the traditional ultrasonic non-destructive testing can’t be used in offshore wind turbines power system. Guided wave method is a faster and more effective method to detect wind turbine pile surface condition buried in the sea floor. Due to the sand in the sea floor have high viscosity and high water content, guide wave leakage into the sand and lost energy with propagation distance. With the pile buried depth and water content different in the sand, guide wave will have different attenuation. Therefore, we plan to detect different pile surface condition by using guide wave L(0,1) modes. By measuring the attenuation of L(0,1) echo signal from the pile and realize the propagation characteristic, we can monitor the health of pile buried in the sea floor.
The thesis successfully build a double rings transducer array by PZT piezoelectric. We can excite L(0,1) mode propagating over four meter and suppress non-axisymmetric mode on the pipe by using axisymmetric transducer arrays and comb structure. At the same time, we know that sixteen transducers double rings structure is the best method to excite L(0,1) mode from the experiment. We can use this propagating mode excited by transducer array system to know the different pipe surface condition and health situation.
From the experiment result, the guided wave will attenuate by leakage into surround. Attenuation of guide wave will change by the water content and cover force on the pipe surface. When water content increase in the sand, the attenuation will increase. Same as the cover force on the pipe, attenuation will increase with the cover force. The water content and cover force is the health critical point in the offshore wind turbines pile. The thesis provide a reference for the health monitor on the pile.
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