A guidance and highlighted best practices and lessons learned to help avoid costly incidents and keep the vessel, crew and cargo safe regarding LNG operations.
Liquefied Natural Gas (LNG) operations require a high level of technical expertise, the implementation of rigorous safety protocol.
Before arriving at any LNG terminal, complete the following checks for safe operations and cargo transfer:
- Secure mooring arrangements
- Safe and accessible ship boarding
- Proper gangway alignment and suitable landing area
- Loading arm compatibility
- Reliable ship-to-shore communication systems, including radio and data links
Additional checks include:
- Emergency communication protocols and Emergency Shutdown (ESD) signal compatibility
- Cargo transfer equipment (manifold alignment, flange compatibility, hose/loading arm specifications, Quick Connect/Disconnect systems)
- ESD interface testing for communication and shutdown activation
- Calibrated instrumentation and control systems, compatible with terminal systems
- Synchronized Custody Transfer Systems (CTS)
- Aligned safety systems (gas detection, fire suppression)
Pre-transfer cargo conference
Before operations, a cargo conference should:
- Review and agree on procedures
- Confirm ramp-up/ramp-down rates
- Align alarm protocols and emergency response plans
- Agree on ESD test sequence (warm/cold)
- Clarify roles and responsibilities
Operational issues in LNG systems / cargo manifold strainer clogging
To mitigate clogging during loading/discharging:
- Perform pre-loading checks for cargo quality
- Inspect and clean strainers and filters regularly
- Monitor Boil Off Gas (BOG) composition for contamination
- Ensure crew is trained in emergency response and system diagnostics
Follow a structured guide for troubleshooting, especially in cryogenic systems and BOG management.
Common troubleshooting areas: Cargo manifold strainers, BOG compressors (PRS/FRS), Cold box strainers, Fuel gas pump strainers, cargo tank cooling.
Preventive measures
- Regular filter and strainer maintenance
- Pre-loading cargo quality checks
- System purging before and after extended idle periods
- Crew training on emergency procedures and warm-up protocols
Documentation and certification
- Prior to any LNG operation, the following documentation and certifications should be reviewed and verified:
- Ship and terminal operational manuals: Ensure procedures are aligned and clearly understood by all parties.
- Certificates of fitness for LNG operations: Confirm validity and compliance with regulatory standards.
- Calibration records for critical equipment: Verify that instruments such as tank level gauges, temperature sensors, and pressure monitors are accurately calibrated.
- Tank inspection reports: Confirm readiness of cargo tanks.
- Cool-down procedures: In order to prevent thermal shock during initial loading.
- Custody transfer system verification: Ensure systems are synchronised and measuring instruments are properly calibrated.
- Cargo documentation: Examine LNG composition, boil-off rates, and any special handling requirements.
These checks are essential to ensure operational safety, regulatory compliance, and accurate cargo accounting during LNG transfer operations.
Rollover
LNG rollover is a hazardous phenomenon that occurs when stratified layers of LNG within a storage tank suddenly mix. This rapid mixing releases large volumes of vapour, which can lead to overpressure, triggering of emergency venting systems or, in extreme cases, a loss of containment and the formation of LNG liquid pools.
- Consolidate the heel into one tank.
- Partially load a second tank to a level that allows room to transfer any remaining heel into it.
- Close the manifold liquid valves, leaving the vapour manifold open.
- Transfer the heel into the partially filled tank, using the ship’s cargo pumps as quickly as safely possible, promoting turbulence to aid mixing.
- Do not load any further LNG into the tank containing the mixture.
- Complete loading of other tanks as per normal procedures.
This procedure should be carefully discussed between ship and shore before loading begins.
Heel is the small volume of cargo retained in the cargo tanks after discharge, used to condition the tanks for the next loading cycle, reducing time in port. After loading, the remaining heel blends with the fresh cargo throughout the voyage.
If stratification occurs despite precautions, indicators may include:
- Reduced boil-off gas flow rate.
- Tank level not decreasing as expected, or even increasing.
- Temperature probes showing lower tank temperatures increasing, while upper temperatures remain constant.
- If stratification is detected, the only solution is to discharge all cargo into a shore receiving tank with proper mixing arrangements.
Stratification in LNG tanks
Contributing factors:
- Slow or uneven filling
- Temperature gradients
- Lack of tank agitation (e.g., in stationary vessels or terminals)
Associated risks:
- Vapour release
- Overpressure
- Activation of safety systems
- Potential damage to containment systems
Preventive measures:
- Monitor density gradients during loading
- Avoid slow fill rates
- Use tank mixing systems or rely on ship motion
- Conduct rollover risk assessments
The most effective stratification and rollover prevention is to avoid the need for mixing LNG and to load only into “empty” cargo tanks.
Carriage of cargo
During transit, LNG naturally evaporates as Boil-off gas (BOG), even with the very efficient insulation systems that are now available. The BOG can be managed either by reliquefication or used as fuel, while tank pressure and temperature are continuously monitored. Specialised cargo containment systems such as membrane or Moss-type tanks preserve the structural integrity of the vessel as carriage involves transporting liquefied natural gas at cryogenic temperatures in vessels designed to maintain product integrity and safety throughout the voyage.
Cargo care and monitoring
Cargo monitoring involves continuous surveillance of tank parameters to ensure safety and operational integrity during LNG transport.
Key Monitoring Areas:
- Tank levels, pressure, and temperature:
- Continuous monitoring using tank radar or float-type level gauges to prevent overfilling and ensure accurate custody transfer.
- Temperature maintained at approximately -162°C; any rise may indicate increased boil-off gas (BOG) generation.
- Pressure monitored to remain within design limits; pressure relief valves and venting systems act as safeguards.
- Gas Detection Systems:
- Integrated systems for early leak identification.
- Insulation and Inter-Barrier Spaces:
- Regular checks for signs of overpressure, leaks, or fire.
- Monitoring of inter-barrier spaces (IBS) and insulation spaces (IS) for anomalies:
- IBS (between primary and secondary barrier): pressure maintained at 0.5–1.0 kPa above atmospheric.
- IS (between secondary barrier and inner hull): pressure maintained at 0.5–1.0 kPa above IBS.
Safety and alarm systems
Robust safety infrastructure ensures immediate response to hazardous conditions.
System components:
Gas and fire detection:
- Continuous monitoring in cargo and void spaces.
- Immediate alarms for methane presence above threshold limits.
- Flame detectors and heat sensors installed in cargo zones.
Fire suppression:
- Systems include dry chemical and water mist technologies.
Data logging and reporting:
Voyage Data Recorders (VDR) and Integrated Automation Systems (IAS) log:
- Tank levels
- BOG rates
- Temperatures
- Alarms and anomalies
Daily reporting ensures traceability and operational awareness.