Challenges in realizing robust systems for deep water submersible ROSUB6000

This paper presents the experiences in realizing robust systems for the 6000 meters depth rated electric work class Remotely Operated Vehicle ROSUB6000, designed and developed by National Institute of Ocean Technology(NIOT), India for applications like carrying out surveys for seabed bathymetry, gas hydrate identification, support vehicle for poly-metallic nodule exploration, and salvage support operations. The ROSUB system comprises Remotely Operable Vehicle (ROV), Tether Management System (TMS), Launching and Recovery System (LARS), Ship Systems and Control console. The electric work class ROV is equipped with two manipulators and an additional pay load capability of 150kg. Robustness for the ROV is a key factor as deep water operations are critical in terms of ship time involved, nature of activities and intervention demands. The system was qualified at a water depth of 5289 meters in Central Indian Ocean Basin. Multiple challenges were faced during system qualifying sea trials in the areas of communication networks, navigation, thrusters, ROV-TMS docking, power system protection, vision systems, control software, and system safety. Problems were addressed by improvised system engineering and by means of introducing redundancies taking into consideration the cost, space and time constraints to attain optimum level of robustness and the availability of component. A super-capacitor aided, pressure-compensated switchgear is designed and implemented to achieve compactness and robustness. The probable loss of navigational information data from the photonic inertial navigation instrument and tether cable twist count data during power outages are managed using a sea battery. The reduced optical performance of the TMS Fiber Optic Rotary Joint in deep waters is analyzed and improved. Water entry in the pressure rated electronic enclosures was managed using water entry detectors and by implementing appropriate control algorithms using distributed controllers in ROV, TMS and the ship. Imaging system performance was improved with enhanced electronics architecture and advanced luminaries. Brushless direct current thruster motor controllers are protected for over voltages using voltage management systems in the ship, TMS and ROV. To reduce the chance of ROV-TMS docking failure in the absence of vision systems, black dock systems incorporated. A ROV-TMS serial data link was introduced to manage critical operations in the ROV during fiber optic network failures. Pilot and Co-Pilot automatic control changeover is implemented by using control software with continuous monitoring.