LNG is transferred from a shore terminal, an LNG bunker barge or an LNG truck-to-ship unit through cryogenic flexible hoses or a loading arm into the vessel's insulated (IGC code type C pressure / membrane / type A/B) LNG tanks, using the pressure differential and an LNG transfer pump. Before transfer the receiving tanks are cooled/conditioned (if warm) by spraying in LNG so they are not thermally shocked, or the tank is first inerted and purged with LNG vapour/boil-off before liquid is introduced. The transfer is carried out at low temperature (-162 C approximately) and modest pressure; vapour generated in the receiving tank is returned to the supplier via a vapour return line/hose to keep the tank pressure stable. Quantity is measured by tank gauges and a custody-transfer mass flow meter, taking account of density at the measured temperature.

Arrival/pre-bunker checks:

• Dedicated bunkering checklists (SIGTTO LNG bunkering guidelines), pre-transfer conference with the supplier.
• Mooring and gangway secured; electrical bonding/earthing lines fitted between ship and barge/shore.
• All bunker valves, tank valves, vapour return, vents and relief valves verified, sea suctions and overboard valves closed in the area; spill containment/booms and drip trays in place.
• Emergency stops and ESD (emergency shutdown) system tested between ship and supply; communications line set up; responsible persons stationed.
• Weather and sea conditions, current, daylight and tide - transfer limited to defined wind/sea-state limits.
• Tank levels, pressures, temperatures, ullage and MAWP (max allowable working pressure) checked; shore and ship tanks have free space; vapour space pressure and temperature within limits.
• Firefighting and PPE readiness; ventilation; exclusion zone for non-essential personnel; no hot work; smoking prohibited.
• Ensure cargo systems in safe mode, and that battery/generator/main engine arrangements are as required by the class for bunkering.

A formal risk assessment is prepared using a structured approach: identify hazards (e.g. cold-contact/burns, frostbite, LNG spill, vapour cloud fire/explosion, overfilling, thermal stress, hose failure, uncontrolled pressure, ignition of flammable vapour); assess each by likelihood and consequence (risk matrix); apply controls/mitigations; and record residual risk. Common tools: HAZID/HAZOP-style brainstorming, bow-tie or fault-tree analysis for the major accidental scenarios (ESD, line rupture, overfill), and a defined emergency response plan. The risk register is reviewed and endorsed at the pre-transfer conference, with emergency shutdown criteria agreed and a clear definition of what terminates the operation.

The LNG is stored in the tanks in the liquid state under its saturation pressure/temperature. For use in the main engine (dual fuel or gas engine) or for fuel-gas supply, the LNG must be vaporised. Vapour is produced by boil-off or by a forced vaporiser: LNG is drawn from the tank, pumped/pressurised, and passed through a vaporiser (often sea-water or a heated glycol/water vaporiser) to convert it to methane gas at the required pressure and temperature. The gas is then handled by a gas supply/valve unit and fuel-gas control system, which filters, regulates and delivers it (typically at 5-6 bar, heated) to the engine's gas admission system. Before starting the engines the fuel lines and gas train are purged/leak tested with nitrogen and/or warmed, the engine is brought to operating mode and switched from pilot diesel to gas once conditions allow. A gas detection and gas pressure/temperature monitoring system protects against leakage, and on gas trip the engine automatically returns to diesel mode.