Deep sea driver

A thruster that is designed to improve the efficiency and control of unmanned underwater vehicles could also be used as a tidal power generator, according to its UK developers.

The electric thruster, which has been developed by researchers at Southampton University, is designed to replace the hydraulic systems on remotely operated underwater vehicles (ROVs) used in the oil and gas industry and for environmental monitoring in the deep ocean or under the ice cap. But the thruster’s propellers can also be reversed, enabling it to generate electricity.

Hydraulic systems used in ROVs are inefficient and less reliable than electric thrusters, said Dr Suleiman Abu-Sharkh, leader of the Southampton research team, whichhas spent the past 10 years developing the device.

The efficiency of the thrusters is affected by power conversion losses, as electricity is needed to supply the hydraulic motors, which are then used to produce mechanical power.

‘Hydraulic thrusters can be less than 40 per cent efficient. But as our thruster is converting electrical power directly into mechanical power, the efficiency can be as high as 85-90 per cent,’ he said.

Hydraulic systems are also prone to leaks, making them less reliable. But the engines used on ROVs as existing electric thrusters are too bulky, due to the separation of the motor from the rest of the device.

In the Southampton design the motor is integrated into the structure of the thruster itself. The motor is fitted within the ring surrounding the propeller, making the unit much more compact, said Abu-Sharkh.

‘You couldn’t have a high-power thruster with normal designs, because the motor would be too big. This competes in size with hydraulic thrusters but is much more efficient.’

Electrical systems also have more accurate thrust control than hydraulic systems, making the ROVs easier to operate and able to perform more delicate tasks.

While testing the device, the team discovered by accident that it could also be used as a water turbine electricity generator.

By driving the propeller backwards, it acts as a turbine, converting energy from the tide into electricity, said Abu-Sharkh. ‘So it has potential applications in converting tidal power into electricity. It can also be used as a pump, or a fan, but at the moment the main market is for ROVs.’

The thruster can work down to ocean depths of 3,000m, and commercial production of the device has now been licensed by TSL Technology. The first commercial units are expected within the next few months to be tested in a new range of all-electric ROVs, which are likely to be used to inspect, repair and maintain oil and gas platforms.

The project was funded by the EPSRC and industrial sponsors including Subsea7, a joint venture between US energy company Halliburton and Norwegian shipping firm DSND.