LNG cargo tanks are integral components of liquefied natural gas (LNG) carriers, designed to store and transport LNG in its cryogenic liquid state. These tanks are constructed using specialized materials and insulation techniques to withstand extremely low temperatures and high pressures. Common types of LNG cargo tanks include membrane tanks, spherical tanks, and prismatic tanks, each offering unique advantages in terms of capacity, structural integrity, and thermal insulation. Membrane tanks, for instance, consist of a primary membrane surrounded by insulation layers and a secondary barrier, providing flexibility and excellent thermal performance. Spherical tanks, on the other hand, offer robust structural integrity and are suitable for larger LNG carriers, while prismatic tanks are often used in smaller vessels or as part of hybrid cargo containment systems. Regardless of the type, LNG cargo tanks play a crucial role in ensuring the safe and efficient transportation of LNG across oceans, meeting stringent regulatory requirements and industry standards.
In the context of LNG (Liquefied Natural Gas) cargo tanks, various processes such as aerating, inerting, and cooling are crucial for ensuring safe and efficient operations. Aerating involves the introduction of inert gas, typically nitrogen, into the cargo tanks to displace any remaining LNG vapors and reduce the oxygen content. This process helps to minimize the risk of fire or explosion by creating an inert atmosphere within the tanks. Inerting, on the other hand, involves maintaining a continuous flow of inert gas to prevent the accumulation of flammable gases or vapors during cargo loading, unloading, or storage. Additionally, cooling is essential for maintaining the LNG at its cryogenic temperature, typically around -162 degrees Celsius (-260 degrees Fahrenheit), to prevent it from vaporizing and expanding within the tanks. By carefully managing these processes, operators can ensure the safety, integrity, and stability of LNG cargo tanks throughout the transportation and storage of LNG.
Typical Cargo Operations
Aeration
Once the cargo tanks have been completed and closed for the final time, the next step of operation prior to loading cargo is to aerate the tanks.
Aeration is done for the express purpose of reducing the moisture content of the tank atmosphere to a dew point of at least -45 degrees Celsius. The low dew point prevents any condensation in the tank(s) thus preventing any corrosion of Use of Cargo Pumps on Liquefied Gas Carrierscargo pump bearings and other exposed components.
Inerting
Inerting with CO2 is the second step of cargo tank preparation prior to loading cargo.
Like the preceding step, aeration, inerting also maintains a dew point of -45 °C. However, its main purpose is to lower the oxygen content of the tank to least 1,0 % or less (0,5 % IG Plant Spec.) and thus eliminate any risk of combustion or explosion when the methane is eventually loaded.
Inerting is usually done at an optimum rate that takes advantage of the difference in densities between CO2 and air (O2 and N2).
The efficient displacement of either air or methane with CO2 is commonly referred to as the “piston effect.”
The latest model Inert Gas Generators (IGG’s) typically achieve oxygen contents of even less than 0,1 volume %, without any formation of soot.
This is done with new technology called an “Ultramizing burner” (Smit), which features a patented two-stage oil/air mixing technique.
Another important requirement of the IGC is to supply dry inert gas with very low dew points, mainly to prevent condensation in the tanks. The required dew point is obtained by applying an inert gas refrigeration system (freon system) and an absorption (desiccant) dryer system.
When inerting is done correctly, a “piston effect” is established whereby air or methane vapor is displaced in the tank with a minimum number of tank volume changes.
When inerting is done incorrectly, a “piston effect” is not established and “dilution” takes place, thus air or methane vapor is displaced with a large number of tank volume changes (Fig. 1).
Purging (Gassing-Up)
The next step is to purge or displace the CO2 with methane vapor.
If done correctly, a “piston effect” can also be established in this phase and thus minimize the number of tank volume changes required to remove the CO2.
The CO2 in the tank atmosphere needs to be reduced to 1 % or less in order to prevent any concentration of “dry ice“, which forms at -79 °C and could possibly damage cargo pump windings and bearings as well as plug nozzles.When purging is done correctly, a “piston effect” is established whereby CO2 vapor is displaced in the tank with a minimum number of tank volume changes.
When purging is done incorrectly, a “piston effect” is not established and “dilution” takes place, thus CO2 vapor is displaced with a large number of tank volume changes (Fig. 2).
Cool-Down
After the tanks have been properly purged, it is then time to commence “cooling down” or “cool-down.”
In this phase, LNG liquid from ashore is introduced into a designated tank via the spray-cooling header located in the top of the tank – usually in way of the vapor dome.
The spray-cooling header consists of a length of piping configured with a number of nozzles, which disperse the LNG, in a fine spray pattern, into the upper portion of the tank.
Initially, due to the difference in temperature between the LNG liquid spray and the ambient conditions in the tank, the LNG evaporates before it reaches the bottom of the tank.
Liquid is sprayed into the tank at timed intervals to allow for uniform temperature reduction and minimization of induced thermal stresses. When the mean tank temperature has reached -125 °C, spray cooling can be done continuously and liquid can also be introduced into the tank through the liquid filling line.
During the cool-down process, vapor generated by the LNG absorbing heat and flashing to vapor can either be burned in the ship’s boilers or sent back to the terminal via the HD or Return Vapor Compressor(s).
When the tank temperature has reached -130 °C, spray cooling can be stopped. By this time, liquid will have accumulated on the bottom of the tank thus enabling continuous filling via the liquid fill line.
Once continuous filling has commenced, the tank can be filled to whatever level deemed necessary at that time.
Gas Trials
At gas trials, Cargo Storage System Concepts for Liquid Natural Gas Tankscargo tanks are loaded with LNG for the first time and all cargo and cargo-related equipment and systems are given a thorough testing.
Depending upon time constraints, one tank is cooled down and filled to a designated level.
Following that, the filled tank’s stripping pump is lined up with the spray-cooling header and then used to commence spray cooling the other tank(s).
When the other tank(s)s have been cooled-down, the first tank’s cargo pump(s) discharge line is crossed over to the liquid fill line to the other tanks and then used Heat Transfer into the Tankto transfer LNG to a designated tank.
The first tank is then pumped down and stripped using all pumps, level/temperature sensors, etc., thus testing all systems for that tank.
The tank, which just received the LNG from the first tank, is then tested in the same manner as the first tank.
Following that, the second tank is lined up to discharge to a third tank; thus repeating the testing process until all of the tanks have been completed.
Warming Up
Following gas trials, if the ship is not going into immediate service, it will then commence a reverse order of the preceding processes.
The reverse order is:
- warming-up;
- inerting;
- aeration of the cargo tanks.
Note, if there is no additional work to be done in the cargo tanks, they may remain inerted and thereby omit the aeration stage of the process.
In this phase, the ship’s heavy-duty (HD) vapor compressor is used to take suction from the vapor dome of a cargo tank and then discharge cold vapor to warm-up heaters, which heat up the vapor, and thence lead it back to the cargo tank.
This heating up of vapor and then discharging it back to the tank generates excess vapor, which is usually fed back to the machinery space where it is then burned in the boilers.
Under some circumstances, the vapor has been vented to atmosphere, but this is usually the exception and not the rule.
Inerting (After Warming Up)
Once the cargo tanks have been warmed-up, they can then be inerted in the same manner as described earlier.
The only difference in this process from previous is that the inert gas (CO2) is used to displace and remove methane vapor from the tanks verses dry air.
Once again, it should be stated that the purpose for the inert gas is to prevent a direct methane-to-oxygen interface, which could result in an explosive condition:
- Following the inerting process, the tanks are aerated in the same manner as described earlier in this presentation.
- Once aeration has been completed, the tanks are then safe for personnel to enter.
Aeration (After Warming Up)
- Following the inerting process, the tanks are aerated in the same manner as described earlier.
- Once aeration has been completed, the tanks are then safe for personnel to enter.
Loaded Voyage
Following successful gas trials, if the vessel is to go into service, the cargo tanks are filled with LNG and the vessel departs for its first discharge port. Along the way, the following happens:
- Cargo tank pressure will increase due to slow heat absorption of the LNG cargo. When the pressure reaches a certain limit, the boil off gas compressor(s) will be started and the excess vapor will be sent back to the boilers.
- Any entrained nitrogen will leave the cargo first. This noncombustible vapor is still treated like normal methane boil off and sent to the boilers.
- Following the nitrogen, methane vapor will be generated which is then sent back to the boilers and burned as fuel.
- A heavy sea state will generate more boil-off than a calm one.
In addition to normal underway maintenance, particular attention is given to that equipment which is necessary for discharging cargo – manifold valves, discharge valves, cargo pump controllers, etc.
Upon arrival at the discharge port, authorities, such as United States Coast Guard, may board the vessel for inspection.
Unloading/Discharge
Prior to arrival at the discharge terminal, the vessel commences cooling down its discharge piping.
At the discharge terminal, grounding links, liquid and vapor lines, communication lines and emergency shut down (ESD) lines are connected with the vessel.
Commercially, the most important aspect of the discharge is agreement on the custody transfer figures, i. e. the precise amount of LNG on board the vessel for which the receiving party will pay.
When custody transfer has been approved; communication lines tested; liquid and vapor lines between vessel and terminal cooled down, then actual discharge of cargo can begin.
During discharge, seawater is cascaded over the decks and manifold areas to act as a thermal barrier, should there be a LNG leak.
Typically, as the vessel’s cargo tanks are emptied vapor in the shore tanks that is displaced by incoming LNG is pumped back to the vessel’s tanks. In this manner, shore tanks are not over pressurized and vessel tanks do not go into a vacuum.
Another important aspect of the discharge operation is the vessel’s ballast system. As the vessel is discharged, ballast is taken on at the same rate as that of the cargo discharge. In this manner, the vessel’s draft remains relatively constant and thus there is little strain on the terminal’s manifold arms.
When actual discharge of cargo has been completed, manifold valves are closed and purging of the vessel’s liquid and vapor lines with nitrogen begins.
Also, when discharge of cargo has been completed, the final phase of custody transfer – agreement of the amount of LNG cargo remaining on board – can be completed. This remaining LNG is commonly referred to as “heel“.
Ballast Voyage
Once discharge has been completed, the ballast voyage back to the loading terminal can begin.
On the ballast voyage, several commercial factors come into play:
- Does the vessel have enough heel on board to arrive at the loading terminal with cool tanks?
- Will the vessel go straight to the loading terminal or take its place in a line of other vessels waiting to load?
Both of these factors will have an effect on how warm the tanks will be when the vessel finally begins to load cargo. If the tanks are too warm, then a portion of time at the dock must be spent cooling down again – $$$$$!
Apart from the preceding commercial factors, the ballast voyage is usually uneventful – vapor is burned in the boilers; tank pressures and temperatures are monitored.
In addition to planned maintenance with special attention to loading equipment, one of the most important aspects of the ballast voyage is the ballast water interchange. This is done to minimize the introduction of non-indigenous species into the loading port’s waters.
Inerting Interbarrier Spaces (Membrane Tank Vessels Only)
The air contained in the interbarrier spaces of the membrane tank vessel is to be evacuated and substituted by nitrogen, as inerting medium. The operation may be facilitated by the use of vacuum pumps.
The interbarrier spaces are slightly pressurized to avoid entrance of air in the barrier. Typically the pressure of nitrogen in the space between the primary barrier and the secondary barrier is maintained between 5 and 10 mbar above the atmospheric pressure and the pressure of nitrogen in the space between the secondary barrier and the inner hull is maintained between 5 and 10 mbar above the pressure of the primary barrier interspace. It is, however, important that this pressurization be maintained within low values, generally less than 30 mbar, otherwise the membrane might be seriously damaged.
The inerting medium in the membranes interbarrier spaces has also the other important function to compensate the pressure of the membrane with the actual pressure in the tank, specially in the transient periods of the tank inerting and/or cooling down, when the tank actual pressure may be above or below the atmospheric pressure depending on how the operation is conducted. In this case an equivalent pressurization or depressurization of the nitrogen in the interbarrier spaces will compensate the difference of pressure between the tank and the insulation spaces avoiding the risk of damaging the membranes.