Handling LNG: Prioritising safety on ships and on shore

Communicating with one another is crucial. This is certainly the case when a fully laden LNG tanker is approaching the terminal. The ship's crew must contact the operations team at the terminal well in advance of their arrival. There are many details to discuss before the tanker docks at the jetty and unloading commences. One of the key details is the estimated time of arrival, which must be disclosed in good time (e.g. 72 hours prior to arrival) and regularly updated. This tells the team at the receiving terminal when they need to start preparing the tanks that are to be filled, be they on shore or on an FSRU (= "floating storage and regasification unit", an LNG terminal ship).

It goes without saying that before LNG tankers can be unloaded, the personnel at the plant terminal need the cargo documents. The pre-operational ship/shore procedures then need to be discussed, and the safety checklists must be completed. LNG comprises around 95% methane. If it mixes with oxygen and the oxygen concentration exceeds a certain level, this can constitute an explosive mixture. To counter this risk and the risks associated with the LNG temperature of -163 °C, certain precautions must be taken in order to guarantee maximum safety during loading and unloading operations.

Before loading and unloading operations can take place, the tanks that will be filled with the cryogenic LNG must be prepared. They must be brought to a sufficiently low temperature to prevent excessive quantities of LNG from violently vaporising when introduced into the tanks. Additionally, the amount of residual oxygen in the tanks must be minimal.

Hours before the supply ship arrives, the team at the unloading terminal must start pre-cooling and inerting the onshore tanks using dry nitrogen. Once the amount of oxygen in the tank drops below 2% and the temperature is around -45 °C, some LNG that is kept in reserve is introduced into the tank. This vaporises, further cooling the inside of the tank to around -130 °C.

Likewise, the loading arms and manifold pipes are purged multiple times with nitrogen until the oxygen content of the purge gas that has been repeatedly discharged is less than 2%. This ensures that explosive methane-oxygen mixtures cannot form during the unloading operation.

Once these preparations are complete, the tanker can dock. As with any large tanker, the LNG carrier must always be securely moored and anchored (or moored to the FSRU, if the terminal is a floating one) before it is unloaded. The next step is one of the most critical: The terminal or terminal ship must be connected to the tanker loading system by means of loading arms, pipes and hoses. This is no easy task. It takes a certain amount of skill to guide the pipes on board using the remote-controlled loading arms and then line them up at the correct angle.

The flanged joints must then be properly sealed and secured. To eliminate the possibility of human error, at least two workers are assigned to this. Electrohydraulic LNG loading arms use game-changing, purpose-built technology that continually compensates for the ship's movements due to the action of the waves. Where they are installed on an FSRU, they can even compensate for the action of the waves between the two ships. In an emergency, the loading arm will automatically disconnect from the ship; ideally, this will happen without much LNG escaping. The use of an emergency release coupling makes this possible. If, however, large quantities of LNG do escape as a result of an operating error, the hydraulic system prevents the plant from being damaged, while the explosion-protected electrical system prevents an explosion.

Before the unloading process can begin, the cargo pipelines and loading arms must also be cooled. This largely prevents deformation as the cargo passes through at -163 °C. If the pipelines are relatively warm when the LNG is introduced, the resultant rapid vaporisation of the natural gas can cause vapour locks to form, which can block the path of any liquefied natural gas that flows along the line subsequently. This may in turn result in damage to pipelines, valves and flanges. One of the things used for cooling is actually LNG, which is sprayed into the arms either from the ship or from the onshore terminal.

The LNG can now be pumped to shore by submersible pumps installed at the bottom of each tank. Pressure differences between the onshore tanks and the ship's cargo tanks facilitate this process. LNG is conducted to the tank filling lines via the loading manifolds; compressors are then used to help deliver the LNG to the LNG tanks on shore or on the FSRU. To maintain the pressure on board the ship, LNG can be vaporised there and conducted in the form of boil-off gas to the cargo tanks. One possibility is to use an onshore fan to force the natural gas into the ship's tanks. Once the unloading process is complete, nitrogen is flushed through the pipeline connections. They must not be reopened until they have returned to atmospheric pressure, no longer contain any liquid, and have undergone the inerting process.

The preparations that must be undertaken at export terminals before loading LNG tankers are much the same as for unloading them. It goes without saying that the supply lines must be free of oxygen. There are some specific procedures that must be performed on board the tanker, however. If the tanker has come directly from the dry dock, the cargo tanks may need to be dried. This can be carried out separately or as part of the inerting process. In the latter case, the inert gas must not be too cold, otherwise it may cause any moisture present to condense. Insulation and inter-barrier spaces must also undergo the inerting process.

Inter-barrier and insulation spaces in membrane-type tankers must also be purged with dry nitrogen when the tankers are loaded or unloaded. Any pressure fluctuations that occur at this point as a result of cooling or warming must be mitigated. The pressure should be maintained slightly above atmospheric pressure to prevent the ingress of gases.

If the ship's LNG tanks have been filled with an inert gas that contains carbon dioxide, they must be injected with LNG vapour before they are filled with LNG. During this process, they are cooled and the heavier inert gas is displaced downwards. The ship's pipework and the inert gas must additionally be free of water and CO₂. To ensure that this is the case, it is purged with nitrogen. Methane is then conducted through the vent mast riser, initially at a concentration of approximately 5%, before finally reaching a concentration of 98% by volume. Ideally – and depending on the local port authority's regulations – this gas will be recovered for reliquefaction rather than being vented into the atmosphere. The pipelines are now successfully oxygen-free, fulfilling a fundamentally important safety requirement.

LNG is now sprayed into the cargo tanks and pipelines to further cool them. Otherwise, LNG would rapidly vaporise, expanding to 600 times its volume in its gaseous state. The loading process must not commence until the temperature in the tanks is approximately equal to that of the LNG that is to be loaded. Once the process is under way, the loading rate is gradually increased. The tank vapour pressure is continuously monitored throughout this process. It should be kept below the value at which the pressure-relief valves would open. Temperature profiles are also recorded at various locations. The predetermined maximum pressure and maximum temperature must not be exceeded. The thermodynamics involved in this process are incredibly complex. Relevant factors include the transfer of heat between the liquid and the tank wall, the compression of the gas phase above the liquid, the vaporisation process, etc. The better the process is understood, the safer and faster the loading operation. Gradually, all tanks are filled in accordance with the cargo plan. The loading rate must be reduced in good time, before the tanks are completely full. Membrane tanks can normally be filled to 98% capacity. Moss-type tanks can be filled to 99.5% capacity.

Multiple processes take place on the ship alongside the loading or unloading operation. For instance, the fill levels in the ballast tanks need to be adjusted as appropriate – when an LNG tanker is not carrying cargo, the ballast tanks will be more or less full; if it is carrying cargo, the ballast tanks will be mostly empty. The flow rates during loading and unloading operations also need to be accurately measured for billing purposes. Special Coriolis mass flowmeters are available for measuring cryogenic liquids in preparation for custody transfer – these flowmeters also measure entrained boil-off gas (two-phase flow) and boil-off loss.

It can take 20 to 30 hours to fully load or unload a large LNG tanker. A multitude of different devices and machines are used over the course of the operation: Temperature gauges, flowmeters, pressure gauges, various pumps (e.g. spray, stripping, ballast, and submersible, motor-driven pumps), compressors and fans, inert-gas systems, heat exchangers, etc. Owing to the increasing price of LNG and the demand for environmentally friendly processes, there is also growing interest in reliquefaction systems, which are now a good option even for smaller LNG tankers and bunker vessels. These also help to cool the LNG tanks, which means that the workers can commence the loading process sooner when the ship arrives at the terminal.