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Development and Field Trials For Seafloor-Mapping AUV

By Dr. Hongwei Zhang • Yan Zheng • Dr. Yuhong Liu

Launch and recovery device in the July 2013 sea trial.

Deepwater hydrocarbon resources, such as oil and gas, are generally distributed in the sea within complicated geological structures, which causes difficulties for deep-sea exploration. In the process of surveying and measuring ocean hydrocarbon resources and installing and operating underwater production systems, it is necessary to make a comprehensive and detailed field survey of relevant seafloor landforms. As an effective platform for payload sensors, AUVs have become a hot technology for ocean observation and engineering, which have certain requirements for mobility, maneuverability and operational accuracy.

Since 2011, Tianjin University has been developing a seafloor-mapping AUV, which is equipped with sensors such as side scan sonar, a sub-bottom profiler and a multibeam echosounder. One of the considerations in the design of the AUV is cost, which, for a geophysical exploration task, mainly revolves around the operation of the mother boat. In order to save cost and minimize sailing days of the mother boat, the AUV's surveying efficiency should be the top priority in the initial design stage. A second concern is that when conducting a mapping task the AUV is supposed to keep a constant altitude above the seafloor. This demands better maneuverability in accordance with the terrain of the seafloor at low speed.

A third concern is the necessity of high-precision navigation and positioning systems to acquire high-quality data. In light of this, the drag force of the vehicle should be reduced as much as possible by optimizing the streamline form via CFD software at a small sacrifice of maneuverability by using a larger length-diameter ratio. To obtain better mobility and maneuverability, especially at low speed, a 2-degree-of-freedom vectored thruster was developed. In addition, the lack of rudders can decrease water resistance. A GPS/INS/DVL integrated navigation system was added to the AUV to enhance the quality of survey data.

In the last year, several sea trials were performed at Fuxian Lake and the Bohai Sea in China to test the navigation and surveying properties of the AUV. Through these trials, the general capacity and compatibility of each system on the AUV was verified and improved.

General Development of the Vehicle
The seafloor-mapping AUV is an integration platform consisting of geophysical exploration sensors, a mechanical structure, a navigation and control system and battery pack. According to the hydrodynamic principle, the vehicle is designed with a slender asymmetric body, with a maximum diameter of 800 millimeters and a length of 7.8 meters. For the contour design, the Granville streamline was adopted, and CFD Fluent software was used to obtain hydrodynamic derivatives and sailing resistance by simulating the flow field of the vehicle. Through dynamic simulation, the contour of the AUV was optimized to minimize resistance. Optimization resulted in a drag coefficient of 0.31, and the diameter of the turning circle is predicted to be 35 meters at a 15 degree rudder angle. According to the value of drag force at various working speeds, the ducted propeller and propulsion motor were designed and selected.

Further, the power consumption of the vehicle should also be determined. Consulting the requirement of voyage and endurance, the battery capacity is determined as 36 kilowatts. To continue this article please click here.

Dr. Hongwei Zhang received a Ph.D. in mechanical engineering at Tianjin University, where he works as an associate professor in mechanical engineering. Since 2010, he has been a part of the development of the seafloor-mapping AUV.

Yan Zheng has been a postgraduate student in the school of mechanical engineering at Tianjin University since 2012. He received a bachelor's at Tianjin University in 2011. He focused on the design of the vectored thruster of the seafloor-mapping AUV.

Dr. Yuhong Liu received a Ph.D. in materials engineering at Northwestern Polytechnical University in China. She works as an associate professor of mechanical engineering at Tianjin University. Her research focuses on the hydrodynamic performance of UUVs.

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