Subsea robotic systems fail. This page documents common failure modes, how operations degrade when failures occur, and recovery procedures. Understanding failure modes is essential for safe operations and realistic planning.
Why This Exists
Failures happen offshore. By understanding failure modes, operators can plan for degraded operations, develop recovery procedures, and make informed decisions about when to continue vs. when to abort.
Who This Is For
- ROV pilots and supervisors
- Operations managers planning missions
- Safety officers assessing operational risk
- Robotics engineers designing systems
- Auditors reviewing operational procedures
Failure Mode Categories
Single Point Failures
Failures that immediately disable the system:
- Power loss — Complete loss of vehicle power
- Tether severance — Complete loss of connection
- Critical sensor failure — Loss of essential situational awareness
- Control system failure — Loss of vehicle control
Operational impact: Mission abort, emergency recovery required.
What can go wrong: No recovery possible, vehicle lost, mission failure.
Degraded Mode Failures
Failures that reduce capability but allow continued operations:
- Thruster failure — Reduced maneuverability
- Sensor degradation — Reduced situational awareness
- Manipulator failure — Loss of work capability
- Communication degradation — Reduced data bandwidth
Operational impact: Reduced capability, mission may continue with limitations.
What can go wrong: Degraded operations exceed safe limits, secondary failures occur, mission objectives not achievable.
Cascading Failures
Initial failure leads to additional failures:
- Tether damage → Power loss → Vehicle loss
- Thruster failure → Loss of control → Collision → Vehicle damage
- Sensor failure → Loss of awareness → Navigation error → Entanglement
Operational impact: Rapid degradation, mission abort likely.
What can go wrong: Cascading failures are difficult to predict and may progress faster than recovery procedures can execute.
Specific Failure Modes
Power System Failures
Primary power loss:
- Causes: Generator failure, tether damage, power distribution failure
- Effect: Immediate loss of vehicle power, unresponsive vehicle
- Recovery: Switch to backup power, or recover via winch if tether intact
- Prevention: Redundant power systems, backup generators, power monitoring
Battery failure (AUV):
- Causes: Battery degradation, over-discharge, thermal issues
- Effect: Loss of propulsion and systems, vehicle may surface or sink
- Recovery: Emergency surface procedures, recovery at surface
- Prevention: Battery monitoring, conservative depth-of-discharge limits
What can go wrong: Backup power not available, backup power insufficient, recovery procedures not executed correctly.
Communication Failures
Tether severance (ROV):
- Causes: Snagging, excessive tension, mechanical damage
- Effect: Complete loss of communication and power
- Recovery: Vehicle may have emergency systems (acoustic release, buoyancy), surface recovery
- Prevention: Tether management, tension monitoring, protective routing
Acoustic link loss (AUV):
- Causes: Range exceeded, acoustic interference, equipment failure
- Effect: Loss of real-time communication, vehicle continues mission or enters failsafe
- Recovery: Vehicle may surface at pre-programmed waypoint, or continue mission autonomously
- Prevention: Range management, acoustic link monitoring, failsafe programming
What can go wrong: Emergency systems not functional, failsafe procedures not executed, vehicle lost.
Sensor Failures
Camera failure:
- Causes: Water ingress, electronics failure, mechanical damage
- Effect: Loss of primary situational awareness
- Recovery: Continue with remaining cameras, or abort if no backup
- Prevention: Redundant cameras, watertight housings, pre-dive testing
Sonar failure:
- Causes: Transducer failure, electronics failure, acoustic interference
- Effect: Loss of long-range awareness, navigation capability reduced
- Recovery: Continue with cameras and other sensors, or abort if navigation critical
- Prevention: Redundant sonar systems, pre-dive testing
INS failure:
- Causes: Sensor failure, calibration drift, initialization error
- Effect: Loss of precise navigation, position uncertainty increases
- Recovery: Continue with dead reckoning, or abort if precision required
- Prevention: Redundant INS systems, regular calibration, initialization checks
What can go wrong: Multiple sensor failures, no backup sensors, operations continue beyond safe limits.
Propulsion Failures
Thruster failure:
- Causes: Mechanical failure, electrical failure, entanglement
- Effect: Reduced maneuverability, inability to maintain position
- Recovery: Continue with remaining thrusters, or abort if insufficient control
- Prevention: Redundant thrusters, pre-dive testing, thruster monitoring
Thruster entanglement:
- Causes: Debris, fishing line, umbilical contact
- Effect: Thruster unable to rotate, reduced or asymmetric thrust
- Recovery: Attempt to clear entanglement, or abort if unable to clear
- Prevention: Thruster guards, operational awareness, careful maneuvering
What can go wrong: Multiple thruster failures, asymmetric failures cause instability, recovery not possible.
Manipulator Failures
Manipulator mechanical failure:
- Causes: Overload, mechanical wear, water ingress
- Effect: Loss of work capability, manipulator may be stuck or uncontrolled
- Recovery: Abort work tasks, continue inspection if possible
- Prevention: Load monitoring, regular maintenance, pre-dive testing
Tool failure:
- Causes: Tool-specific failures (cutting tool, sampling tool, etc.)
- Effect: Specific work task cannot be performed
- Recovery: Switch to backup tool, or abort specific task
- Prevention: Redundant tools, tool testing, proper tool selection
What can go wrong: No backup tools, tool failure causes vehicle damage, mission objectives not achievable.
Recovery Procedures
Emergency Recovery
When immediate recovery is required:
- Abort mission — Cease all work tasks, prepare for recovery
- Assess situation — Determine recovery method based on failure mode
- Execute recovery — Winch recovery (ROV), surface procedures (AUV), or alternative method
- Secure vehicle — Once recovered, secure vehicle and assess damage
- Document incident — Record failure mode, recovery actions, lessons learned
Responsibility: Supervisor directs recovery; pilot executes recovery procedures.
Degraded Operations
When operations can continue with limitations:
- Assess capability — Determine remaining operational capability
- Revise mission — Adjust mission objectives to match capability
- Increase monitoring — Enhanced monitoring for additional failures
- Prepare recovery — Ready recovery procedures in case of further degradation
- Document status — Record degraded status and operational limitations
What can go wrong: Degraded operations exceed safe limits, secondary failures occur, mission objectives not achievable.
Failure Prevention
Design Principles
- Redundancy — Critical systems should have backups
- Monitoring — Systems should monitor their own health
- Failsafes — Systems should fail to safe states
- Isolation — Failures should not cascade to other systems
Operational reality: Redundancy adds cost and complexity. Tradeoffs must be made based on operational requirements and risk tolerance.
Operational Practices
- Pre-dive testing — Verify all systems before deployment
- Regular maintenance — Prevent failures through maintenance
- Operational limits — Operate within system capabilities
- Training — Operators must understand failure modes and recovery
What can go wrong: Testing incomplete, maintenance deferred, limits exceeded, training insufficient.
Data & Records
Failure documentation must include:
- Failure description — What failed, when, under what conditions
- Root cause — Why it failed (if determinable)
- Operational impact — How operations were affected
- Recovery actions — What was done to recover
- Preventive measures — What will prevent recurrence
Audit-worthiness: Failure records must be complete, accurate, and suitable for regulatory review. See Dive Logs & Operational Records for record-keeping requirements.