Cutting the tether but keeping control
Mon, 14 October 2013
Underwater optical communications is an emerging technology, complementing the capabilities of acoustic communications, write Simon Partridge, engineering director, and Darryl Newborough, with subsea product development, at Yately UK based Sonardyne International Ltd.
High bandwidth signals support high data rates which can transmit large amounts of data between underwater assets. As part of the continued technological development, Sonardyne joined Woods Hole Oceanographic Institution (WHOI), trialling a new concept in tetherless AUV/ROV operations using BlueComm wireless video transmission.
A short range, through-water wireless optical communication system capable of broadband speed data transmission, BlueComm provides a high speed optical data uplink with a low data rate acoustic downlink. It extracts large amounts of data and supports the transmission of HD video and imagery in real-time. As water depths increase, conventional ROVs and their associated systems are larger and heavier, requiring bigger, more expensive vessels. However, BlueComm may one day eliminate this need, offering a new degree of freedom in underwater robotics with wireless data transmission at rates of up to 20 Mbit/s.
The trials took place in the Western Pacific onboard the MV Alucia using Nereus, WHOI’s hybrid unmanned vehicle. Depending on its mission, Nereus can swim as an AUV or be transformed into an ROV with a fibre-optic tether. This transmits high quality, real-time imagery but is fragile and needs regular replacement.
These trials eliminate the need for an umbilical altogether, combining the complementary characteristics of wireless acoustic and optical systems. Acoustic methods can transmit data over long distances but only at relatively low speeds; optical signals can send high bandwidth signals for real time control, but only work over a few hundred metres.
Nereus was deployed to 700m and acoustically tracked using Sonardyne’s Ranger 2 GyroUSBL system. A depressor with a BlueComm receiver, depth sensors, AvTrak 6 transceiver for acoustic vehicle and manipulator control and a Wideband Mini Transponder for USBL tracking was then lowered behind. A purpose-built ‘lander’ mimicking a subsea manifold was also deployed. Installed on this was a Compatt 6 transponder acting as a seabed positioning reference and a release to return the lander to the surface.
Multiple dives then took place in depths up to 1,700m where the AUV and depressor were deployed and tracked. Alucia was equipped with a visual output from Ranger 2 for manual manoeuvring, ensuring the depressor remained within 150m of Nereus. With each deployment, Nereus fed real-time video back to the control room with a HD stills camera providing imagery. The AUV was commanded to autonomously ‘fly’ to the lander where low latency acoustic communications and wireless video were used to command a robotic arm to ‘stab’ a connector into the lander, believed to be the world’s first wireless ROV intervention capability. Using BlueComm, multiple colour video streams were simultaneously transmitted in real-time at 15 Mbit/s.
A WIRELESS FUTURE
Wherever there are underwater applications requiring control, monitoring or data transmission, 'going wireless' is a credible alternative. For those involved in ocean monitoring, the technology simplifies and lowers the cost of harvesting data from seabed sensor networks where instruments store logged data until a surface vessel can retrieve it. Using a combination of 6G acoustics and BlueComm optical communications, gigabytes could be uploaded in minutes, making for very efficient surveys. Greater freedom of movement, risk reduction and the receiving of real time video feeds whilst maintaining human control is something that everyone involved can look forward to.