Safety through precision: Equipping LNG tankers

The demand for liquefied natural gas (LNG) is higher than ever. Transporting it on the world's oceans using special tankers reduces importing countries' dependence on individual natural gas suppliers. The shipbuilding industry would be wise to meet the demand for new LNG tankers. These feature sophisticated technology, from the structure of the gas tanks and the devices for handling the boil-off gas, through to the propulsion systems. What matters is ensuring ultimate efficiency and maximum safety for all of this equipment. Explosion protection is especially important in this context. The technical components and systems have to be extremely durable and capable of withstanding harsh conditions at sea, whether in the Arctic or at the equator. The loading process itself gives rise to particular requirements as LNG, like its gaseous counterpart natural gas, is explosive and it is transported at temperatures of -164 °C and -161 °C.

There are two main tanker designs: Those with tanks which are spherical in form and are located half above deck (Moss type). They are like massive Thermos flasks with a stainless steel outer layer and an insulating layer made from polyurethane foam or polystyrene. Nowadays, new vessels tend to be built with multiple prismatic tanks below deck. They get their name "membrane tanker" from the insulating flexible outer plating made up of insulating layers and metal membranes (stainless steel, nickel or Invar). They use the hull shape more efficiently than spherical tanks.

The membrane which gives the tanks their name compensates for tank expansion caused by temperature changes. This is particularly noticeable during loading and unloading. The flexibility of the internal corrugated or grooved metal membrane prevents almost all leaks. However, if they do occur, a barrier made from wood or Invar absorbs any liquefied gas. Pearlite is often used for insulation and is rinsed with nitrogen in some designs. This complex multi-layer system is necessary to prevent heating of the cryogenic gas on the one hand, and to prevent the steel of the vessel wall from becoming too brittle on the other hand. Between the vessel wall and the tanks there is space for the ballast tanks required for empty trips.

Despite the good insulation against cold, minimal quantities of LNG in the form of methane continuously evaporate via a valve. Between two and six per cent of the LNG can be lost over the course of a 20-day journey. This boil-off gas is often use to propel LNG tankers. To do so, it is transported to the boilers of the steam turbines, where it is combusted. As an alternative or in addition, the waste steam can be reliquefied and transported back into the tank. Installing the reliquefaction system required for this is expensive. However, this has the potential to pay off, depending on the size of the ship, the length of the journeys and current LNG prices. Boil-off gas that cannot be reliquefied or used for propulsion is vented via a flare. This ensures that the tank pressure does not increase to an impermissible level and prevents the explosive gas from becoming a hazard for the vessel.

However, the main component of LNG, methane, is a highly polluting greenhouse gas. This is why reliquefaction systems are installed ever more frequently in newly built vessels, for environmental and cost reasons. Or as a minimum, preparations are made to install these systems so that they can be retrofitted if required. For example, the boil-off gas is reliquefied on the largest LNG tankers (14 vessels operated by the Qatar Gas Transport Company with a tank volume of 266,000 m³) that were used in 2022.

The systems used for reliquefaction are becoming increasingly compact. They are now available in the form of intelligent plug-and-play units. Like all of the equipment on board, the compressors used for cooling must be explosion-protected. Remote I/O control units, with intrinsically safe signal transmission and approval for use in Zone 1, are ideal for modern automation strategies. Since space is usually very limited on board vessels, they should have the smallest possible dimensions. The IS1+ system from R. STAHL is ideal, for example, because it can be used to create especially compact stations thanks to its Ex i modules with 8 or 15 channels. It is also robust, resistant to vibrations and has the most important marine certificates.

This is just one example of the stringent safety requirements on board and how these can be met. The standards for designing and equipping LNG tankers are defined in the IMO Gas Code (IGC). The second-safest vessel type, 2G, is used. In accordance with IEC 60092-502, the individual areas of the tanker – similar to hazardous areas on land – are categorised as Zone 0 (in the tank and in the conduits used for loading and discharging the LNG) to Zone 2. Zone 1, for example, covers the area in the immediate vicinity of the pumps and compressors. Areas located further away must be classified as Zone 2.

This is why light fittings and alarm devices on the tanker are explosion-protected (by the Ex d and/or Ex e type of protection) and have the necessary approvals. The operating and monitoring stations and communication equipment are subject to stringent requirements too. Ethernet multimode fibre optics designed with the Ex op is type of protection are suitable for the communication equipment. They are not affected by the electromagnetic fields of motors, etc. The HMIs used alongside the reliquefaction systems for monitoring the inert gas atmosphere, for leak detection and for much more besides are also approved for Zones 1 and 2 and have an extremely robust design.

Other important elements of membrane and Moss-type spherical tanks include the central towers (pipe towers) leading into the tanks. They contain conduits which are connected to the on-deck piping at the top, and pumps for loading and unloading. They also include a stairwell and instrumentation. If the load cools down too quickly or sloshes around, the pipe towers are subjected to stress, which could cause cracks in the material. This is why pipe towers are supported on their underside only in the longitudinal and transverse direction on the tank floor. This enables them to contract in the event of a temperature drop. As early on as the draft stage, the loads impacting the central tower must be taken into careful consideration, just like they are for the loading tanks. To put it simply, LNG tankers require precision work in all regards. This is the only way to ensure the necessary high standard of safety.