Passive pre-tensioning of buoyancy engines for fail-safe and energy efficient depth control

This paper describes the development and testing of a novel pre-tensioned variable buoyancy engine (VBE) that has two major advantages over traditional VBEs, i) fail safe behaviour, where the device expands to achieve maximum buoyancy in the event of power loss, and ii) reduced energy consumption when varying buoyancy at high hydrostatic pressure. The system achieves these properties by using a set of springs to push out a piston that varies the device’s displacement. This paper develops the theory to size these springs for fail-safe and energy efficient operation at different depths. Practical experiments are carried out under simulated hydrostatic pressures using a prototype pre-tensioned VBE designed according to the developed theory. We compare two methods of sealing the piston. Both methods demonstrated fail-safe response when power is lost, and achieved a maximum energy reduction of 25% compared to VBEs with no pre-tension under our experimental conditions. We discuss the implications of our results for different types of autonomous submersible mission profiles and make suggestions for future work.