Autonomous Underwater Vehicle (AUV) is the core equipment for deep-sea exploration, resource prospecting and ocean engineering monitoring. As its power unit, deep-water thruster operates long-term under complex deep-sea conditions featuring high pressure, high humidity and severe corrosion, where the sealing structure is highly susceptible to failure and water leakage. Water leakage is a common and fatal fault of AUVs. Minor leakage will cause corrosion of internal components and lubrication failure, while severe leakage may directly lead to motor short circuit and power outage, and even result in vehicle runaway, sinking and scrapping, bringing huge equipment losses and mission failure. Therefore, mastering scientific and standardized methods for emergency response, maintenance, repair and prevention of water leakage is critical to ensuring the operational safety of AUVs in deep sea and extending equipment service life. This article systematically elaborates the fault causes, graded emergency response procedures, refined repair steps and long-term prevention measures for water leakage of AUV deep-water thrusters, providing technical reference for the operation and maintenance of marine unmanned equipment.
I. Core Hazards and Causes of Water Leakage of AUV Deep-water Thrusters
1. Core Hazards of Water Leakage
AUV deep-water thrusters adopt fully sealed waterproof structure, integrating precision parts such as drive motors, bearing assemblies, sealed oil circuits and electronic control modules. Seawater intrusion will cause cascading damage to equipment. Minor water leakage dilutes lubricating oil and grease inside the thruster, resulting in aggravated bearing wear and transmission mechanism jamming. Meanwhile, salt from seawater adheres to circuit boards and metal components, causing gradual electrochemical corrosion and latent faults. Moderate water leakage dampens motor windings and reduces insulation performance, leading to electric leakage, unstable power output and abnormal rotating speed, which directly affect the navigation attitude and propulsion efficiency of AUVs. Severe water leakage will burn out motors due to short circuit and damage electronic control modules, causing shutdown of one or multiple thrusters. This will lead to power imbalance and attitude loss of AUVs, and may easily trigger major safety accidents such as loss of communication, sinking and collision damage in deep sea.
2. Main Causes of Faults
Based on deep-sea operating conditions and equipment maintenance experience, water leakage faults of AUV deep-water thrusters mainly fall into four categories. First, aging and failure of sealing system. Sealing components including O-rings, end-face mechanical seals and waterproof gaskets endure long-term high-pressure compression, seawater corrosion and alternating high and low temperatures in deep sea, which cause hardening, cracking, deformation and elasticity degradation. This is the primary cause of water leakage. The deeper the operating water depth and the higher the operation frequency, the faster the sealing parts wear out. Second, non-standard installation and maintenance. Misaligned sealing parts, insufficient compression, residual impurities and scratches on sealing surfaces, uneven application of waterproof coating and unbalanced tension of fastening bolts during assembly will destroy sealing integrity and cause gap leakage. Third, off-spec operation. Operating beyond the rated water depth, frequent sudden changes of water pressure or long-term high-load operation will exceed the pressure bearing limit of sealing structures and result in seal failure and water seepage. Fourth, external force damage. Collisions with reefs, scratches by marine debris during navigation, or bumps during hoisting and transportation will deform the thruster shell and damage sealing surfaces, leading to structural water leakage.
II. Graded Emergency Response Procedures for Water Leakage of AUV Deep-water Thrusters
During deep-sea operations, the built-in water leakage sensors of AUVs monitor water inflow inside the cavity in real time. Once an alarm is triggered, the control system will quickly identify the fault level. Maintenance personnel shall follow the principle of "prioritizing equipment safety, containing fault spread and implementing standardized disposal", and conduct graded emergency operations according to the severity of leakage. Continuous operation with water leakage or faults is strictly prohibited.
1. Minor Water Leakage (Warning Level)
Fault characteristics: The sensor issues a minor alarm without obvious power abnormality or abnormal noise from seepage. There is no accumulated water or only trace condensation inside the cavity, and equipment insulation parameters remain normal. This is mostly caused by slight aging of sealing parts and seepage through tiny gaps.
Emergency disposal: The navigation control system immediately records fault data, locks the faulty thruster, automatically reduces its load and maintains stable low-power operation to prevent aggravated seepage caused by high rotating speed. Operators adjust the AUV navigation attitude, avoid areas with violent ocean currents and sudden water pressure changes, work out the optimal return route and return to the surface operating area at low speed. Further diving is forbidden. Keep the thruster in low-power standby before the vehicle surfaces to prevent seawater backflow and worsening water inflow after shutdown.
2. Moderate Water Leakage (Fault Level)
Fault characteristics: Continuous and increasingly frequent alarms, accompanied by rotating speed fluctuation, power attenuation and slight abnormal operating noise. The insulation resistance decreases slightly and obvious water inflow can be observed inside the cavity.
Emergency disposal: The system immediately shuts down the faulty thruster and cuts off the corresponding power circuit to prevent motor burnout caused by short circuit due to water inflow. Meanwhile, activate the AUV power balance control program to compensate power via other normal thrusters, correct navigation attitude and offset yaw and tilt caused by single thruster shutdown. Do not restart the faulty equipment at any time. Navigate to safe waters at full speed, cut off power immediately after returning, avoid pressure-bearing storage, and conduct rapid drainage and dehumidification pretreatment.
3. Severe Water Leakage (Critical Level)
Fault characteristics: Frequent continuous alarms, complete stall of the thruster, severe abnormal noise and abnormal body vibration. Insulation parameters exceed the standard significantly. In most cases, a large amount of water accumulates inside the cavity or the shell is damaged, posing an immediate risk of overall short circuit and vehicle runaway.
Emergency disposal: Activate the highest-level emergency plan, immediately cut off the main power supply and signal links of the faulty thruster to stop the spread of short circuit faults. Start the AUV emergency surfacing system, turn off all non-essential loads, fully ensure the operation of buoyancy adjustment and attitude control modules, and force rapid surfacing. In case of complete power failure and attitude loss, activate the load-dumping self-rescue mechanism to discard redundant counterweights, ensure the main body of AUV surfaces and avoid sinking and damage in deep sea. Isolate the faulty equipment immediately after surfacing and do not conduct power-on inspection.
III. Dismantling Inspection and Standardized Repair Procedures for Leaking Thrusters
After the faulty AUV returns, dismantling and inspection shall be carried out in a dry, dust-free professional maintenance workshop. Operations in open-air or humid environments are prohibited. Conduct comprehensive inspection to locate root causes and implement standardized repairs to eliminate potential risks of secondary water leakage.
1. Pre-treatment Preparation
First, completely disconnect the power supply and signal interfaces of the thruster, mark and protect all circuits to avoid wiring errors and short circuits caused by damp components. Dismantle the connecting bolts of the thruster shell evenly to prevent shell deformation, separate the cavity slowly and drain accumulated water. Record the turbidity and salt residue of the water to preliminarily judge the severity of leakage. Then place internal components, motors, bearings and shell cavities in a constant-temperature dry environment for ventilation and dehumidification. Wipe off seawater residues with anhydrous ethanol to completely remove salt and impurities and prevent continuous corrosion.
2. Comprehensive Fault Inspection
Firstly, inspect the sealing system. Check the integrity of O-rings, mechanical seals and end-face gaskets one by one, focusing on cracks, aging, deformation, wear and installation indentations. Examine sealing surfaces for scratches, corrosion and attached impurities, and verify the tightness of thread seals and cable outlet seals. Secondly, inspect structural appearance. Check the thruster shell, end cover and shaft for deformation, cracks and collision damage to locate structural leakage points caused by external force. Thirdly, test electrical performance. Use an insulation resistance tester to check the insulation of motor windings for dampness, short circuit and inter-turn breakdown, and inspect electronic control interfaces and circuit boards for corrosion, oxidation and short circuit faults. Fourthly, inspect transmission components. Check bearings, gears and other transmission parts for lubrication failure, corrosion, jamming and excessive wear caused by water ingress.
3. Refined Repair and Assembly
Carry out targeted repair for all detected faults. Replace all aged, deformed and damaged sealing parts with brand-new parts of the same model; reusing old sealing parts is forbidden. Polish and apply anti-corrosion treatment to minor scratches and corrosion on sealing surfaces and shells. Replace severely damaged shells and shafts directly. Dry damp motors and electronic control modules at constant temperature and reinforce insulation. They can be reused only after insulation parameters reach the standard. Replace all short-circuited and damaged electrical components. Replace corroded and over-worn bearings and transmission assemblies, and refill with matched waterproof lubricating grease to ensure smooth transmission and reliable sealing.
Follow standardized procedures strictly during assembly. Ensure all sealing surfaces and the inside of the cavity are dust-free, impurity-free and dry before assembly. Apply special waterproof lubricating grease evenly on sealing parts and install them in accurate alignment with uniform compression to avoid misalignment and extrusion deformation. Fasten shell bolts diagonally and evenly to ensure uniform stress on sealing surfaces and prevent leakage through local gaps. Clean excess grease after assembly to keep the equipment tidy.
4. Pressure Sealing Test
Pressure sealing test simulating deep-sea working conditions is mandatory after assembly and repair. Direct put-into-service is prohibited. Adopt immersion pressure test or air tightness test. Set the test pressure according to the rated operating water depth of the equipment and maintain pressure for more than 30 minutes to check for water seepage and pressure drop inside the cavity. Let the equipment stand and dry after passing the test, and re-test electrical insulation performance. The repair is accepted and the equipment can be put into operation only when all parameters meet requirements.
IV. Long-term Prevention & Control: Operation and Maintenance Strategies to Reduce Leakage Failure Rate of Thrusters
Water leakage faults of AUV deep-water thrusters can be effectively avoided through routine maintenance, standardized operation and periodic inspection. Establishing a full-lifecycle prevention and control mechanism is the core to reduce failure rate and ensure stable equipment operation.
1. Standardize Operating Conditions and Prevent Overload Operation
Operate in strict accordance with rated equipment parameters. Operation beyond rated water depth, pressure and rotating speed is prohibited to avoid long-term extreme pressure on sealing structures. Plan navigation routes to avoid areas with violent ocean currents and sudden water pressure changes, reduce frequent start-stop and high-load fluctuating operation of thrusters, and lower fatigue loss of sealing structures. Monitor the temperature, pressure and insulation parameters of thrusters in real time during operation to predict latent faults in advance.
2. Implement Periodic Inspection and Maintenance
Establish a hierarchical inspection system. Conduct basic inspection for frequently operated equipment after every 10 to 15 dives to check the condition of sealing parts, grease volume and shell integrity. Carry out comprehensive maintenance every month, including replacement of aged sealing parts, grease replenishment and cavity cleaning. Perform in-depth disassembly and inspection every year to fully examine the sealing system, transmission structure and electrical performance, and replace over-worn parts to completely eliminate hidden dangers. For long-term idle equipment, conduct regular ventilation and dehumidification, and apply anti-corrosion grease to prevent natural aging of sealing parts and shell corrosion.
3. Standardized Assembly and Protection Management
All inspection and assembly work must be completed in a dust-free, dry and constant-temperature professional site. Operators shall abide by assembly specifications strictly. Rough disassembly and misinstallation of sealing parts are not allowed. Use only original matching sealing parts. Avoid collision, scratch and direct sunlight during storage and transportation to guarantee stable sealing performance. Take full protective measures and install buffer devices during hoisting, transportation and storage to prevent damage to shells and sealing surfaces caused by external impact.
4. Optimize Monitoring and Early Warning Mechanism
Upgrade the AUV water leakage monitoring system by adopting multi-point distributed sensors to improve the identification accuracy of micro seepage, optimize fault grading logic and shorten emergency response time. Improve the equipment operation data recording system. Analyze the wear rule of sealing parts and high-risk operating conditions via big data, adjust inspection cycles and operation plans in a targeted manner, and realize the upgrade from passive rush repair to active predictive maintenance.
Water leakage faults of AUV deep-water thrusters are concealed, sudden and destructive, which directly determine the success of underwater missions and equipment safety. It is impossible to completely eliminate water leakage in harsh deep-sea environments. However, accurate graded emergency response, standardized disassembly & repair and routine maintenance & prevention can minimize fault hazards, reduce equipment loss and extend service life. With the rapid development of marine exploration equipment, standardizing the disposal procedures for thruster water leakage faults and improving the full-lifecycle maintenance system are important foundations to enhance AUV operational reliability and ensure efficient and safe deep-sea engineering operations, as well as providing solid technical support for the stable operation of unmanned underwater equipment.
