10.2 Tether management and umbilical systems

Tether management is crucial for ROV operations, connecting the vehicle to surface control. It provides power, communication, and data transmission, enabling remote control and real-time feedback. Proper management ensures safe and efficient ROV use.

Tethers face challenges like drag and entanglement, impacting ROV performance. Various materials and configurations are used to optimize tether design. Effective management systems help deploy, retrieve, and store tethers, minimizing risks and maximizing ROV capabilities.

Tether Role in ROV Operations

Essential Connection to Surface Control

• The tether is the umbilical cable that connects the ROV to the surface control unit, providing power, communication, and data transmission capabilities
• Tether is essential for controlling the ROV's movements, receiving real-time video and sensor data (sonar, temperature), and transmitting commands from the surface to the vehicle
• Tether length and diameter are determined by factors such as the ROV's operating depth, power requirements, and payload capacity

Key Tether Components

• Power conductors: Copper wires or cables that supply electrical power to the ROV's motors, lights, cameras, and other systems
• Communication lines: Fiber optic or coaxial cables that transmit video, sensor data, and control signals between the ROV and the surface control unit
• Strength member: Kevlar or other high-strength synthetic fibers that provide mechanical support and protect the power and communication lines from damage
• Outer jacket: A protective layer, typically made of polyurethane or polyethylene, that encases the tether components and provides abrasion and chemical resistance
• Tether management is crucial to ensure the safe and efficient operation of the ROV, preventing entanglement, minimizing drag, and maintaining the vehicle's maneuverability

Tether Management Systems

Functions of Tether Management Systems (TMS)

• TMS are designed to control the deployment, retrieval, and storage of the ROV's tether, minimizing the risk of entanglement and ensuring the vehicle's smooth operation
• Tether deployment: Controlling the release of the tether as the ROV descends, maintaining proper tension and preventing slack
• Tether retrieval: Winding the tether back onto the storage drum or spool as the ROV ascends, ensuring a smooth and controlled process
• Tether storage: Providing a secure and organized means of storing the tether when the ROV is not in use, preventing tangles and damage

Types of Tether Management Systems

• Passive systems rely on the ROV's movement and the tether's weight to control deployment and retrieval
  • Examples include tether cages, tether baskets, and free-hanging tethers
• Active systems use powered mechanisms, such as motorized winches or linear cable engines, to manage the tether
  • Provides more precise control and can accommodate longer tether lengths
• Effective tether management is essential for maintaining the ROV's maneuverability and stability, preventing tether entanglement, and extending the tether's lifespan
• TMS should be designed to accommodate the specific requirements of the ROV and its operating environment (depth, current, obstacles), considering factors such as tether length, diameter, and material properties

Tether Challenges

Tether Drag

• Tether drag is the resistance experienced by the ROV due to the tether's weight, diameter, and hydrodynamic properties, which can significantly impact the vehicle's maneuverability and power consumption
• Drag increases with tether length, diameter, and the ROV's speed through the water
• Tether drag can cause the ROV to experience reduced speed, increased power consumption, and difficulty maintaining its desired position or trajectory
  • For example, a longer tether may cause the ROV to drift off course in strong currents

Tether Entanglement

• Tether entanglement occurs when the tether becomes wrapped around underwater structures (pipelines, cables), debris, or the ROV itself, potentially leading to damage, loss of control, or mission failure
• Entanglement risks increase in complex or cluttered environments, such as shipwrecks, offshore structures, or areas with strong currents
• Entangled tethers can cause the ROV to become stuck, experience sudden jolts or tension spikes, or even sever the tether, resulting in the loss of the vehicle

Cable Management Challenges

• Cable management challenges arise from the need to effectively store, deploy, and retrieve the tether while maintaining its integrity and performance
• Improper storage or handling can lead to kinking, crushing, or abrasion of the tether, reducing its strength and electrical or optical performance
• Uncontrolled deployment or retrieval can result in tether slack, loops, or twists, increasing the risk of entanglement or damage
  • For instance, a poorly wound tether on a drum may cause loops to form during deployment, leading to knots or tangles
• Mitigating these challenges requires careful tether design, selection of appropriate materials and configurations, and the implementation of effective tether management systems and operational procedures

Tether Materials and Configurations

Common Tether Materials

• Copper: Offers good electrical conductivity but is heavy and prone to kinking. Suitable for shorter tethers and shallow-water applications
• Fiber optic: Provides high-bandwidth data transmission and is lightweight and flexible. However, it is more expensive and requires specialized connectors and handling
• Kevlar: Used as a strength member, Kevlar offers high tensile strength and low stretch but is sensitive to abrasion and requires careful handling
• Polyurethane or polyethylene: Used for the outer jacket, these materials provide good abrasion and chemical resistance but can be prone to cuts or punctures

Tether Configurations

• Round: A circular cross-section with components arranged concentrically. Round tethers are simple to manufacture and handle but may have higher drag and be more prone to twisting
• Flat: A rectangular or ribbon-like cross-section with components arranged in parallel. Flat tethers have lower drag and are less prone to twisting but may be more susceptible to kinking or damage
• Hybrid: A combination of round and flat sections, designed to optimize the balance between drag, flexibility, and mechanical protection

Factors Influencing Tether Selection

• Operating depth and environment: Deep-water or harsh environments (high pressure, corrosive fluids) may require more robust materials and configurations
• ROV size and power requirements: Larger ROVs or those with higher power demands may need larger-diameter tethers with more conductors
• Data transmission needs: High-bandwidth applications, such as real-time video or sensor data, may require fiber optic communication lines
• Deployment and retrieval methods: The tether design should be compatible with the TMS and winch systems used
• Trade-offs between tether materials and configurations must be carefully evaluated to ensure the optimal balance of performance, reliability, and cost-effectiveness for the specific ROV application