Tethers Redefined: Hybrid ROVs and New Applications in Open Water and Under Ice

Both tethered (ROV) and untethered (AUV) systems have proven to be highly valuable tools for a range of application undersea. Certain enabling technologies coupled with recent advances in robotic systems make it possible to consider supplementing many of the functions performed by these vehicles with appropriately designed semi-autonomous vehicles that are far less expensive operate yet able to perform a range of interventions under direct human control. These systems are effectively a hybrid cross between ROV and AUV vehicles and poised to enable an important new class of undersea vehicle. It is now possible to radically redefine the meaning of the words “tethered vehicle” to include virtual tethering via acoustic and optical means or through the use of small diameter re-useable tethers, providing not power but only high bandwidth communications.  One application of such vehicles is operation under ice.

Overlying ice makes operating conventionally tethered ROVs on the sea floor challenging.  The NUI vehicle is a hybrid ROV intended to provide open-water ROV-like capability beneath ice. The vehicle utilizes a unique hair-thin (250 um) unjacketed fiber optic micro-tether that allows the vehicle to achieve horizontal standoff distances from a support ship of several kilometers, essentially decoupling the motion of the vehicle from the motion of the ship.  NUI is capable of operating both autonomously and, with the addition the micro-tether, as an ROV. We operated NUI in both modes during the expeditions to the Arctic in 2014 and 2016. Upgrades to the vehicle performed in preparation for fieldwork in Sept/Oct 2019 have increased its depth rating to 5000 m and enable in situ conversion between low-drag survey configuration and an expanded-workspace intervention configuration.  We discuss the design and operation of NUI’s unarmored micro-tether, the hybrid control methodology that enables operating the vehicle directly or semi-autonomously and handles forced transitions between modes in case of tether loss, the use of acoustics for communication and navigation, and the recent upgrades. We report field results and lessons-learned.