Since January 1997, Osaka Gas Co., Ltd. has utilized ultrasonic flaw detection systems (TOFD method) to externally inspect the inner and outer surfaces of weld seams on spherical storage tanks (spherical gas holders and LPG tanks) while tanks are in use. In these systems, the vehicle mounted ultrasonic flaw detection probe is designed to travel in a straight line only. Hence, the vehicle frequently deviates from the weld seam during inspection. When this happens, inspection must be suspended for manual correction of position. To improve work efficiency by automating inspection, we developed an inspection robot that is propelled by four independently driven magnetic wheels. Since October 1999, this robot has been in experimental operation on an actual spherical tank. Practical introduction of this robot will shorten inspection time by about one week from the conventional 23 days, and will reduce the conventional number of inspectors required from 4 to 2. In other words, it will cut inspection cost for each tank by more than half from the conventional ￥6.40 million to ￥2.75 million.
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