D. Sathianarayanan

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 150 kg. 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.

Structural Reinforcement of Viewports in Spherical Pressure Hull for Manned Submersibles

Manned submersibles are underwater vehicles. These vehicles are equipped with an atmospheric pressure casing called a spherical pressure hull, which can accommodate up to three people. The spherical pressure hull facilitates safe passage to high-pressure environments. It has circular openings that serve as viewports to enable underwater viewing and intervention. The regions near the openings are the weakest in the pressure hull and must be reinforced. Reinforcement of the viewports is performed using the area replacement method. The amount of material removed from the viewport opening must be replaced along the axis of symmetry of the opening. This is the minimum amount of material that must be placed along the circumference of the viewports. Reinforced viewports in the pressure hull are analyzed using finite element analysis, and the stresses are classified into primary and secondary stresses. The reinforcements of the viewports are carried out in such a way that the calculated primary and secondary stresses are below the permissible limits.

Failure Analysis of Fasteners in a Remotely Operated Vehicle (ROV) System

A failure analysis study was carried out on AISI 204 Cu stainless steel full-threaded fasteners and end-threaded SS 316L fasteners used in a deep water work class ROV system. The AISI 204 Cu stainless steel full-threaded fasteners were used in a Ti–6Al–4V alloy pressure case. The end-threaded SS 316L fasteners were used for attaching the bottom fender in the tether management system (TMS) made of Al 6061-T6 alloy. The mechanical property of both the fasteners was found to be within standard specification. The SEM study of the full-threaded fasteners revealed that the corrosion pit formation in the unfractured interface area is the cause of crack initiation. Subsequently, the crack propagated by stress corrosion cracking and finally, failed by fast overload cleavage fracture. It can be reasoned that a lower wt.% of molybdenum in AISI 204 Cu stainless steel reduced the pitting corrosion resistance of the full-threaded fasteners. The end-threaded fastener failed by stress corrosion cracking, but corrosion pit was not found in the fracture surface, due to the lack of its interface area with the unfractured region. It can be inferred that the chlorine ions of seawater break the oxide film of stainless steel bolts to form corrosion pits. These corrosion pits act as a stress concentration point to initiate a crack, and subsequently the bolts fail by stress corrosion cracking, showing a brittle appearance.

Deep sea qualification of remotely operable vehicle (ROSUB 6000)

A deepwater work class remotely operable vehicle (ROV) namely ROSUB 6000 is developed at National Institute of ocean technology, Chennai, India. ROSUB6000 is an unmanned, free swimming underwater vehicle that has six degrees of freedom. ROUSB 6000 system is controlled from a technology demonstration vessel (TDV) Sagar Nidhi. Launching and retrieval system, power house and control console containers, umbilical cable and direction changing pulley arrangement are parts of the ROSUB 6000 deck system. ROSUB 6000 subsea system comprises Tether management system, propulsion system, communication system, navigation equipments, control system, scientific sensors, robotic manipulators, cameras, lights and sampling devices as its subsystems. Deep sea qualification trials of ROSUB 6000 system were conducted at 12 & 13° N and 80 ° E during October 2009 to qualify the system for deep sea exploration and intervention tasks. Four dives at depths of 2004 (m), 2244 (m), 3044 (m) and 3050 (m) were carried out during the trials. Deep sea qualification of ROSUB 6000 system was completed successfully overcoming various challenges like umbilical cable damage, Termination hose failure etc... up to a maximum water depth of 3050 m. System performance feedback, in-situ oceanological parameters such as dissolved oxygen profiles and subsea images were recorded during trials. Push coring and dropping of Indian flag were successfully done. This paper presents in detail the challenges encountered and outcome of deep-sea qualification trial of ROSUB 6000 system.

Manufacturing Imperfection Sensitivity Analysis of Spherical Pressure Hull for Manned Submersible

Any pressure hull invariably has imperfections as a result of the manufacturing procedure. Imperfections in a spherical pressure hull are the basis for localized buckling and deformation behavior. Numerical analysis and analytical calculations are carried out to predict the buckling behavior and strength of a pressure hull made of titanium alloy (Ti-6Al-4V) for both perfect and imperfect pressure hulls. Finite element analysis is carried out for different imperfection angles to see the effect on strength and buckling. Results of numerical analysis show that there is considerable reduction in both buckling pressure and strength as a result of imperfections. Hence, allowable deviation due to imperfection for a spherical pressure hull has to be considered for thickness calculations.

Qualification of Polar Remotely Operated Vehicle at East Antarctica

National Institute of Ocean Technology (NIOT) under the aegis of Ministry of Earth Sciences, Government of India has developed a Mini work class Polar Remotely Operated Vehicle (PROVe) for polar studies. PROVe is an unmanned, free swimming underwater vehicle. As part of the 34thIndian scientific expedition to Antarctica (ISEA) a team of six members from NIOT participated in the summer expedition in 2015 and carried out system functionality qualification in polar environment at a temperature range from -5° C to -20° C and scientific studies with connected scientific payloads using PROVe at Antarctica. PROVe was mobilized to Antarctica and deployed at Priyadarshini Lake near MAITRI station (Indian Base station at Antarctica) during February 2015. PROVe maneuverability with connected sensor were tested successfully and collected high resolution video images of algal mats covered over the glacial debris. All the operations were tested and the technology use in this epicontinental lake had been proved for further scientific studies in near future. After completion of the exploration at the lake, all the subsystems of PROVe were mobilized to ship by helicopter from Maitri station and reassembled onboard Ivan Papanin to study the Ice shelf at New Indian Barrier. ROV dived up to 62 m water depth successfully and during the dives the ice shelf continuity beyond 62 m depth was recorded. System functionality of all subsystems such as mini deck power converter, control console, umbilical cable, ROV thrusters, electronics and electrical components, poly-propylene frame, buoyancy module, “O” rings designed by NIOT for the low temperature polar environment was tested successfully. During the PROVe trial scientific data such as water temperature, salinity, irradiance, water sampling etc apart from color and black & white camera footings were also successfully collected. This article explains underwater vehicle technology development and its outcome during the exploratory first step in the polar environment of East Antarctic.

Technology development in India for gas hydrates exploration and extraction feasibility studies

Gas hydrates are considered to be one of the promising future sources of energy. The amount of methane sequestered in gas hydrates is enormous. Estimates show that the methane stored in the form of gas-hydrates is in the order of ~1900 trillion cubic meters within the Indian Exclusive Economic Zone. Under the aegis of the Ministry of Earth Sciences, Government of India, the National Institute of Ocean Technology (NIOT), Chennai is involved in developing suitable technologies for the exploration and extraction methodologies of Gas hydrates in the marine settings of India. Studies carried out using the 6000 m depth rated work class Remotely Operated Submersible, ROSUB 6000, designed and developed at NIOT, at a selected site in the Krishna Godavari Basin showed chemosynthetic habitat abundance at a depth of 1019 m. As a part of the ground truth validation of the presence of gas hydrates, an Autonomous Coring System is being developed with a depth rating of 3000 m, with a capability to collect 100 m core below the sea floor. Sea trials are under progress involving the ground truth validation, with an in-situ pressure core sampler. Theoretical studies, laboratory experiments and reservoir simulation activities are done, using the TOUGH+HYDRATE tool, to identify feasible techniques for methane gas extraction from fine clay Indian marine settings. The details of the identified extraction methodologies, and the results are also discussed in this paper.

Failure Analysis of Fiber Optic Communication System in Deep-Water Remotely Operated Vehicle ROSUB 6000

Single-mode fiber optic systems can play vital roles in cabled deep-water vehicle operations at greater depths (>3,000 m). One kind of single-mode fiber optic system, the ROSUB 6000, is used in a deep-water work-class remotely operated vehicle (ROV). Fiber optic link failure of ROV telemetry and sound navigation and ranging were noticed at a water depth of 3,050 m during the ROSUB 6000 system sea trials. A failure analysis of the fiber optic communication system was carried out with the link data logged during different phases of the deep-sea trials. The results from the failure analysis carried out during deep-sea trials showed an increase in the fiber optic link loss from a depth of 900 m onwards. Further analysis of the fiber optic link loss in the laboratory involved pressure and low-temperature testing of all the subsea components in the ROV telemetry link. From the laboratory pressure test results, it was concluded that pressure was not the root cause of the fiber optic link failure. On further analysis, a complete fiber optic link failure was noticed during the low-temperature testing of the subsea components. Furthermore, the low-temperature testing of the individual subsea components revealed that the fiber optic rotary joint (FORJ) insertion loss increased rapidly at low temperatures. This FORJ insertion loss led to complete failure of the fiber optic links in the ROV. The degradation of index-matching fluid in the FORJ was identified to be the root cause of fiber link failure.

Mechanical engineering challenges in the development of deepwater ROV (ROSUB 6000)

A deep water work class ROV (ROSUB 6000) rated for 6000 m subsea depth has been developed at National Institute of Ocean technology, Chennai, India. ROSUB 6000 system consists of a free swimming ROV, tether management system (TMS), power container, control console container, Launching and retrieval system (LARS) and electro optical umbilical cable. Developmental work of ROSUB 6000 system started from March 2003. During the developmental phase and several depth qualification sea trials various kinds of mechanical engineering challenges were encountered. This paper enumerates the challenges encountered. Water entry is one of the major challenges in subsea systems. Water entry through pressure housings may be catastrophic, since pressure housings carries control electronics. Control electronics switches off when water entry alarm is given stopping all the ROV operations. Water entry in the pressure cases of ROV has happened several times. Each instance the reason behind the water entry was different. Failure of components like torque arrester in TMS deep sea winch and thruster frames were analyzed and suitable methodologies to avoid these failures are brought into the practice. Challenges in deck handling of ROV and TMS are weather conditions and tether twist during initial attachment of ROV with TMS. One another big challenge is the launching and retrieval of the system. Challenges due to rough seas, launching and retrieval system hydraulic system faults, operational human errors and their effects on the system components are recorded and reviewed to improve the system reliability. The importance of buoyancy corrections and hydraulic system maintenance were learnt during the various phases of lab and field testing of the deep water ROV.

Design and development of Remotely Operated Vehicle for shallow waters and polar research

This paper details the design, development and qualification of a 500 m depth rated Remotely Operable Vehicle PROVe 500, intended for carrying out scientific research in shallow waters and in challenging Polar Regions. The vehicle with dimensions of 0.96 m × 0.61 m × 0.63 m and weighing 175 kg in air, is designed for a speed of 3 knots at an electric power input of 5 kW. The vehicle which is powered by 300 V DC through the 500 m length of a neutrally buoyant electro-optic umbilical communicates with the surface console through the redundant fiber optic cores of the umbilical. A real time controller on board the vehicle controls the vehicles functions based on the commands from the surface console. The vehicle piloting is done using illuminated cameras, and the support of the navigation system realized using an in-house developed navigation algorithm, with a tuned Kalman filter with inputs from the attitude sensor and acoustic Doppler Velocity Log. The developed vehicle is tested for its hydrodynamic stability, low temperature performance in the in-house test facilities and for navigation at the Idukki Lake in Kerala, where the vehicle is navigated at a depth of 106 m at 2 knots speed with the navigation systems position error of less than 5 % in the dead reckoning mode. The vehicle is being equipped with accessories for carrying out research in Polar Regions.