Handling LNG

Loading and unloading tankers requires the utmost care. Product loss is not tolerated; measuring solutions must be reliable and suitable for custody transfer applications. Preparations for the operation are undertaken on board the ship and at the terminal, long before the ship docks. There are many rules that must be observed when it comes to loading and unloading. Ancillary processes such as balancing the ballast tanks must be successfully implemented alongside this operation. The number one rule is safety first, especially when dealing with cryogenic substances like LNG.

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.

Discussing unloading procedures

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.

Inerting and pre-cooling storage tanks and loading apparatus is essential

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.

Skilful control of the loading arms

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.

Careful cooling prevents vapour locks

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 LNG 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.

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