G.A. Ramadass

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.

Review of Technological Advancements and HSE-Based Safety Model for Deep-Water Human Occupied Vehicles

This paper reviews the latest advancements in subsea technologies associated with the safety of deep-water human occupied vehicles. Human occupied submersible operations are required for deep-water activities, such as high-resolution bathymetry, biological and geological surveys, search activities, salvage operations, and engineering support for underwater operations. As this involves direct human presence, the system has to be extremely safe and reliable. Based on applicable International Electrotechnical Commission (IEC) 61508 Standards for health, safety, and environment (HSE), the safety integrity level requirements for the submersible safety systems are estimated. Safety analyses are done on 10 critical submersible safety systems with the assumption that the submersible is utilized for 10 deep-water missions per year. The results of the analyses are compared with the estimated target HSE requirements, and it is found that, with the present technological maturity and safety-centered design, it is possible to meet the required safety integrity levels. By proper maintenance, it is possible to keep the mean time between failures to more than 9 years. The results presented shall serve as a model for designers to arrive at the required trade-off between the capital expenditure, operating expenditure, and required safety levels.

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.

Design and testing of Control and Positioning System for Underwater mining Machine

As underwater mining machine were relied upon to perform complex operations offshore, in ever increasing water depths, positioning and control of the machine is one of the major issue where it becomes the limiting factor in the performance of the entire system. The control and data acquisition of the mining system must be flexible and capable of being configured for multi tasks with in short time and perform the operations as programmed in real-time basis. This was achieved by designing the data acquisition and control system of the mining machine based on virtual instrumentation concept using compact field point modules from National Instruments, which is an embedded real-time programmable automation controller that runs Lab VIEWTM Real-Time, providing the functionality, connectivity, and flexibility on a small rugged, industrial platform. Acoustic positioning system for the underwater mining machine was based on Super Short Base Line (SSBL) principle which was achieved by having one vessel-mounted transducer with the range of 4000 m and two Sub Sea transponders mounted in the machine. The positioning system provides online position status of the crawler in X, Y and Z directions with reference to the mother vessel. With the help of the acoustic positioning system and its GUI based software, it was possible to track the position of the machine in real time with reference to mother vessel during the sea test. Apart from online position tracking of the mining machine the data were recorded for post analysis. The mother vessel was equipped with Dynamic positioning system and Launching and recovery system to ease the positioning, deployment and retrieval operations of the deep sea crawling machine. The objective of the paper is to present in detail the data acquisition, control and positioning system of the underwater mining machine along with the sea test results.

Development and Performance Validation of a Navigation System for an Underwater Vehicle

Underwater position data is a key requirement for the navigation and control of unmanned underwater vehicles. The proposed navigation scheme can be used in any vessel or boat for any shallow water vehicle. This paper presents the position estimation algorithm developed for shallow water Remotely Operated Vehicles (ROVs) using attitude data and Doppler Velocity Log data with the initial position from the Global Positioning System (GPS). The navigational sensors are identified using the in-house developed simulation tool in MATLAB, based on the requirement of a position accuracy of less than 5%. The navigation system is built using the identified sensors, Kalman filter and navigation algorithm, developed in LabVIEW software. The developed system is tested and validated for position estimation, with an emulator consisting of a GPS-aided fibre optic gyro-based inertial navigation system as a reference, and it is found that the developed navigation system has a position error of less than 5%.

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.

A modeling tool for sensor selection for inertial navigation systems used in underwater vehicles

Underwater navigation system consists of inertial sensors aided by position, velocity and pressure sensors so that the inherent drift in the inertial measurements are minimized. The choice of the selection of sensor is the tradeoff between the cost and accuracy requirements of the mission. The paper presents the development of mathematical model of sensors used in inertial measurement systems and position estimation using MATLAB. This virtual tool takes into consideration the characteristics of user defined sensors, user mission profile with velocity and heading changes as inputs and outputs the estimated position and distance travelled with time. With the proposed mathematical model, it is possible to select accelerometers and gyroscopes which are required to cater to the required position accuracy which in turn helps the designer to select required aiding sensors for the navigation system. An experiment is carried out with a strap down inertial measurement unit consisting of an accelerometer of bias error 4 mg and gyroscope of random walk error of 4°/√hour for a one hour period in a defined profile and it is found that difference between the model and the experiment results is within 20%. Studies are done using this model for estimating the positional accuracies for tethered and un-tethered underwater vehicles with different sensor accuracies. The need for aiding sensors is also explained.

Distributed Real Time Control Systems for Deep Water ROV (ROSUB 6000)

An unmanned Remotely Operable Submersible (ROSUB 6000) capable of working up to 6000 meters water depth is being jointly developed by National Institute of Ocean Technology, India and Experimental Design Bureau of Oceanological Engineering, Russia. In general ROVs are developed to perform complex offshore operations in ever increasing water depths and accordingly reached a high level of technical design. These vehicles are designed to perform multi tasks in real time operation. The distributed real time control system was designed to provide modular architecture with reliable platform for data acquisition, control and logging. This paper aims at presenting the ROV hardware and software design architecture, which has been developed and tested using National Instruments TM Real Time hardware and LabVIEW® software for data acquisition and control.

Integrated Navigation System for Remotely Operable Vehicle for 6000m Water Depth

Many underwater positioning techniques have been developed as response to the increase in exploration and exploitation of ocean resources. Though there are many techniques available most successful ones are based on acoustics. Electromagnetic radiation suffers heavy losses in water and can not travel long distance in oceans. Even with the modern navigation equipment like GPS etc. it is not possible to determine the position and track the underwater objects and submersibles without employing subsurface navigational aids. Though techniques such as dead reckoning and inertial navigation are often employed for this propose the cumulative errors are large and for better results periodic correction of the position is required. As the sound is the only radiation that can travel long ranges in oceans underwater Acoustic Positioning Systems have been employed to fix the position of underwater objects and sites and the advent of modern computers Underwater Acoustic Positioning Systems reliable and versatile. However at longer depths acoustic positioning systems alone can not provide the high accuracy and fast update rates. Hence combinations of acoustic and other navigational systems have been recently developed. National Institute of Ocean Technology (NIOT) and Experimental Design Bureau of Oceanological Engineering (EDBOE), Russia have developed an ROV for 6000m (ROSUB 6000) water depths. To meet the challenge of positioning and tracking of the ROV at 6000m depths an Integrated Navigation System combining inertial navigation system and acoustic positioning system with other aids like Doppler velocity log has been developed.

Assessment of Feasibility of Suction Pile/Anchor Installation and Pullout Testing through Field Tests

Suction pile anchors are large cylindrical (inverted bucket type structure) open at the bottom and closed at the top and largely used for mooring of offshore platforms, exploratory vessels etc. Prediction of the mooring capacity of suction piles is a critical issue faced by the design engineers and rational methods are required to produce reliable designs. Tests have been conducted in an existing natural pond within NIOT campus with the objective of developing methodology of deployment, design and logistics for suction pile installation and testing of mooring capacity under static pullout. Small size suction piles with varying diameters and lengths have been used in the tests. The tests have been carried out in the natural pond with constant water depth of 1.5 m with the top 1.5 m layer of bed comprising soft marine clay. It is found that pile geometry, aspect ratio and angle of pullout have a significant influence on the response to pullout. As angle of mooring load application changes from vertical to horizontal the reaction offered by the suction pile changes from skin friction to passive soil resistance. Resistance offered by the internal plug of soil is found to vary according to dimension of the anchor piles.