SPiraling Intelligent Robotic Underwater monitoring pLAtform (SPIRULA) - towards repeated, high density and low-cost seafloor monitoring

Repeated multi-modal seafloor observations over an extended period of time have significant applications in marine biology, chemistry and geology. Reducing the logistical effort, cost and complexity in monitoring technology for such tasks, will help to increase accessibility and availability of relevant long-term monitoring data. To this end, a new design concept, the SPiraling Intelligent Robotics Underwater monitoring pLAtform (SPIRULA), is introduced and evaluated. The key idea for SPIRULA is the combination of a static lander and a mobile autonomous vehicle, which is permanently tethered to the lander. Through shared data and energy between the two sub-systems SPIRULA can be made robust, small and with reduced complexity. By unwinding and winding from a passive drum with a taught tether the SPIRULA vehicle is forced on a circle involute path around the SPIRULA lander, which can be analytically described and used during system design and for navigation purposes. Spiraling paths have shown to be efficient in terms of coverage, for instance, a tether length of approximately 20 m allows already for a monitoring coverage of 1200 m2 around the lander. Given a conservative estimate of the energy expenditure of the SPIRULA system and a desired survey area of 1200 m2 at a constant speed along the path of 0.5 m/s it can be shown, that a total energy storage of 10 kWh would provide the system with the ability to conduct between 38 to 82 surveys depending on the sensor suite used. Those surveys can then be spread over days to months to provide a first step towards repeated long-term environmental monitoring on the seafloor with SPIRULA.