Ground tanks and LNG storage systems

As part of an LNG refuelling system, primary storage facilities or simpler satellite systems must be provided. The primary storage plants are generally divided into various sections: storage, unloading, boil-off vapour recovery, bunkering and truck loading (MISE, 2015).

For LNG storage, special tanks are used, specially built under conditions that the LNG must be stored. From this point of view, it is customary to distinguish "onshore" tanks located at ports or in specific fixed refuelling stations (Figure 1) from "on-board" tanks, such as those used by LNG ships or LNG-powered vessels. In this article, considering the tanks on land as those on board ships will be dealt with in report 2. In this article, consider the tank on land (onshore) as those on board ships will be dealt with in the next article.

• UNI EN 14620-1 (2006): "Design and manufacture of vertical, cylindrical, flat-bottomed steel tanks built on-site, for the storage of refrigerated liquefied gases operating at temperatures between 0 ° C and -165 ° C - Part 1: General ", entered into force on December 5, 2006. This standard is the official English version of the European standard EN 14620-1 (September 2006). This defines the general requirements of vertical steel, cylindrical, flat bottom built on-site, above-ground steel tanks for the storage of refrigerated liquefied gases operating at temperatures between 0 ° C and -165 ° C13 (see note1).
• Technical guide and guidance documents for the drafting of fire prevention projects relating to LNG fueling systems with fixed cryogenic tanks serving utilisation systems other than automotive "of the Fire Brigade. 

note1: External tanks can be steel and concrete, combining the two. The standard does not deal with internal tanks made exclusively of prestressed concrete.

Figure 1. LNG storage systems in the port area: graphic example (Oman daily observer 2019)

The choice of the ground tank type to be used depends on a diversity of elements, such as the example below.

• Design pressure
• Volume
• The presence of any additional systems for inerting and secondary barriers
• The loading and filling limits (related to pressure)T
• The cost

For ground tanks, it is usual to use mainly two solutions (DNV, 2015b):

• Flat bottom tanks
• cylindrical tanks ("bullet tanks")

Flat bottom tanks 

Flat bottom tanks are usually used when the volumes to be stored are quite high (over 10,000 m3), and storage occurs at atmospheric pressure. For large tanks, the choice of atmospheric pressure is quite common since it reduces costs for maintaining the pressure itself and for resistance requirements of the materials, which are less stringent (STAVROS, 2016).

Typically this type of tank is equipped with steam management systems, and from an empirical point of view, a daily vaporisation value of about 0.05% is considered acceptable (DNV, 2015b). This type of ground tank presents the problems associated with the "rollover" phenomenon, which is the phenomenon that occurs when layers of liquid are formed at a temperature and, therefore, different densities inside a tank. The heated underlying liquid decreases its density, causing the abrupt mixing of the various layers and the consequent evaporation of a quantity of LNG that could be excessive for release through the safety valves. This can lead to cracks forming in the tanks or other damage due to the pressure developed.

The rollover phenomenon is generally avoided by using liquid handling systems or filling systems at various tank heights. For on-board installations, rollover is much less likely, given the natural shuffling of the cargo as the ship moves. Steam management becomes fundamental in atmospheric pressure tanks such as typically flat bottoms. Practically, it is possible to use four different solutions (Munko, 2007; DNV, 2015b), as shown below:

1.  Combustion of the steam in particular systems and the release of smoke into the atmosphere: this possibility is considered as natural gas is an extremely "clean" fuel. In contrast, the direct introduction of natural gas into the atmosphere can generate pollution, which means the greenhouse effect. Compared with carbon dioxide, it's clear that carbon dioxide is better.
2. The use of gas for propulsion or electricity generation can occur through special turbines following its combustion.
3. Re-liquefaction of the generated steam.
4. It must never increase the pressure inside the tank: this is not possible, if not in a limited form, as flat containment tanks are considered here (typically limited to at most 0.7 bar relative pressure). In the case of tanks under pressure allows avoiding the use of steam disposal systems for prolonged periods.

There are three main containment methods in the context of flat bottom tanks, as shown below.

• Single containment: the storage facility is equipped with an internal tank to contain the LNG and an external shell to support and protect the insulating material (Figure 2).
• Double containment: the storage structure is equipped with an internal tank to contain the LNG and an external containment system both for the support and protection of the insulating material and also to protect from liquid leakage in the event of the internal tank breaking, but not for the steam (Figure 3).
• Total containment: the storage structure is equipped with an internal tank to contain the LNG and an external containment system to support and protect the insulating material while protecting liquid and vapour leaks in tank damage ( Figure 4).

Figure 2. Ground tank for single containment LNG bunkering 

Figure 3. Double containment LNG bunkering tank with dual containment

  Figure 4. Ground tank for full containment LNG bunkering 

Regarding the containment systems, specifications are provided in dimensions to balance the need for control of dispersions in the liquid phase with the opportunity not to limit the vaporisation of the released product. These specifications differ because the containment systems relate to satellite tanks or distribution plants (see note 2)

In the case of large LNG terminals, vast quantities of liquid to be stored may be required. For such situations, it is possible to use multiple flat bottom tanks, possibly also with modular solutions that allow the necessary infrastructural investments to be spread over time.

Note 2: In the case of satellite tanks (ground installations with storage capacities between 5 and 50 t), must provide a containment system with a volume of 2 m³ and a minimum surface area of not less than 2 m² to contain any limited losses of LNG. In this case, the system represents a containment area in the shape of an underground vessel or delimited by walls or by the land's topography. The height of the walls possibly used for the construction of the containment system must be such as not to prevent the intervention of the fire brigade. Must seal any openings for the passage of pipes.

Cylindrical tanks ("bullet tanks")

The second type of ground tank widespread internationally is of bullet tank, used if one wishes to use pressurised tanks.

This type of tank is typically designed to withstand pressures up to 7 bar and is equipped with pressure relief valves. Generally, the capacity of these tanks is between 500 and 6,000 m3 (the typical volume is 1,000 m3), so when the overall storage capacity required is high, and it is intended to store large quantities of LNG, it is necessary to use more tanks. Cylindrical.

The main materials used for this type of tank are steel with a 9% nickel or stainless steel (AISI 304 type). Insulation is obtained through the vacuum solution, perlite, glass wool, and polyurethane (DNV, 2015b).

Factors that influence the capacity of the tanks 

In carrying out the refuelling procedure, it is essential to consider the factors that affect the filling capacity of the tanks to ensure that the activities are carried out in complete safety. As is known, they include:

• Loading temperature: or better, the temperature of the LNG being transferred. As this increases, the vapour pressure increases while the density of the liquid decreases. These factors can cause an increase in the required storage volume.
• Reference temperature: is the temperature at which the natural gas remains in the liquid state at the pressure at which the relief valves are set
• Filling limit: indicates the maximum volume of liquid in the tank, expressed as a percentage of the tank's total volume. Generally, this limit equals 98% at the reference temperature for transport tanks.
• Load limit: indicates the maximum volume of liquid that can be loaded, expressed as a percentage of the tank's total volume. It does not coincide with the filling limit; it also depends on the density of the LNG at the loading temperature and the reference temperature. It can be calculated according to the equation below:

Where: LL is the loading limit, FL is the filling limit, ρR is the density at the reference temperature, ρL is the density at the loading temperature. Typical values range from 85% to 95% depending on the tank type, the pressure value set in the relief valves, etc.

During the completion of refuelling operations, it is essential to evaluate these factors' effects. Generally, inside the tanks, although they can be isolated, there is the formation of a certain amount of vapour, which will be in equilibrium with the liquid. However, as the heat continues to penetrate through the insulation, the density of the liquid tends to decrease due to the temperature increase. Consequently, the space available to steam, especially when the tank is almost fully, decreases further, causing an increase in the vapour pressure. If the rise is out of control, it will reach the relief valve's limit value. At that moment, the LNG temperature will be equal to the reference one.
Consequently, since the density at the reference temperature is lower than that measured at the loading temperature, the load limit will be lower than the fill limit. Increasing the pressure set at the relief valves decreases the time interval that elapses until the valves' opening is more significant. However, the density at the reference temperature will be further low, thus reducing the loading limit. Therefore, finding a compromise between loading capacity and time is necessary until the valves open (see Figures 5 and 6).

 Figure 5. Loading limit for a typical atmospheric pressure tank

It is necessary to consider two other factors:

• The heel or better volume of LNG usually remains in the tanks before the bunkering procedure. This small portion of the liquid is needed to keep the tank cold upstream of the loading of new fuel. The quantity which should remain inside (for safety) must be calculated based on the size and shape of the tanks. Consider also the flow of heat from the outside, the motion of the ship and numerous other parameters. Usually, the first approximation for the initial design stages assumes a value equal to about 5% of the volume.
  
• Usable capacity equals the difference between the load limit and the heel.

Figure 5. Loading limit for a typical pressurised tank