How LNG Carriers (Gas Tankers) Work - Design Types, Loading & Discharge

Summary

This video explores LNG carriers, specialized ships for transporting liquefied natural gas. It covers the liquefaction process, ship layout, and key design types like Moss, Membrane, and SPB. The script delves into the loading procedure, including tank cooling, inert gas purging, and managing boiled-off gas (BOG) during voyages. It also mentions the reliquefaction process for unloading LNG and the use of a VPN for internet privacy.

Takeaways

• 🚢 LNG carriers are specialized ships designed for the long-distance transportation of liquefied natural gas.
• 🌡️ Natural gas is liquefied at -162°C (-260°F), which reduces its volume by approximately 600 times, a process known as liquefaction.
• 🔵 The liquefied natural gas (LNG) is not blue as depicted in animations; it is actually colorless.
• 🛳️ LNG carriers are large vessels, sometimes exceeding the length of an aircraft carrier.
• 🛡️ The industry primarily uses two design types for LNG carriers: Moss-type with spherical tanks, and Membrane-type with more rectangular-like tanks.
• 🛡️ All LNG carriers are required to have double hulls for added protection in case of collision, with water ballast tanks in between.
• 🔩 Moss-type tanks are independent, supported by a high tensile steel skirt, and feature a multi-layer barrier for containment and insulation.
• 🏗️ Membrane-type tanks are integrated into the ship's hull, offering better space utilization and efficiency in cargo capacity.
• 📐 The advanced but less popular SPB (Self-Supporting Prismatic Type B) design resembles oil tankers, with independent tanks inside the hull separated by bulkheads.
• 🌀 LNG loading involves a series of steps including purging with dry air, inert gas, and cold LNG vapor to ensure safety and prevent thermal stress.
• 🔄 During the voyage, boiled-off gas (BOG) is managed by using it as fuel or cooling it down with the help of a reliquefaction plant if necessary.

Q & A

What are LNG carriers and what are they used for?

LNG carriers, also known as liquefied natural gas carriers, are specialized ships designed to transport natural gas over long distances across the world in a liquefied state.

How is natural gas turned into LNG?

Natural gas is turned into LNG through a process called liquefaction, where the gas is cooled down to -162 °C (-260°F), reducing its volume by about 600 times.

What is the significance of the color representation of LNG in animations?

In reality, LNG has no color, unlike the blue representation often seen in animations, which is likely for visual distinction and not an accurate depiction of its appearance.

What are the two most popular design types of LNG carriers mentioned in the script?

The two most popular design types of LNG carriers mentioned are Moss-type and Membrane-type.

What is a double-hull design and why is it used in LNG carriers?

A double-hull design consists of an outer and an inner hull with space in between used for water ballast tanks. It is used in LNG carriers to protect the tanks and cargo inside in case of a collision.

How are the spherical tanks in Moss-type LNG carriers supported?

In Moss-type LNG carriers, the spherical tanks are independent and not part of the ship's hull. They are supported by a supporting skirt made of high tensile steel.

What is the primary barrier in LNG carrier tanks and what is it made of?

The primary barrier in LNG carrier tanks is made of aluminum alloy and serves as the main protective layer for the liquefied natural gas.

What is the advantage of the Membrane-type design over spherical tanks in terms of cargo capacity?

The Membrane-type design has a more rectangular-like tank that integrates directly into the ship's hull, resulting in better space utilization and more efficient fitting to the shape of the hull.

What is the purpose of the reliquefaction plant in an LNG carrier?

The reliquefaction plant is used to cool down and turn the boiled-off gas (BOG) back into a liquid state during the voyage, which can then be sent back to the cargo tank to prevent pressure build-up.

How is LNG loaded into the tanks of an LNG carrier?

LNG is loaded into the tanks using a filling line. Before loading, the tanks are purged of humid air with dry air, and then inert gas is replaced with warm LNG vapor to prepare the tanks for LNG loading.

What is BOG and how is it managed during an LNG carrier's voyage?

BOG stands for Boiled-off Gas, which is the gas that evaporates from the LNG due to external heat. It is managed by using it as a fuel source for the engine or by spraying cold LNG from the bottom of the tank to cool it down and prevent excessive pressure build-up.

Outlines

00:00

🚢 Overview of LNG Carriers and Design Types

This paragraph introduces liquefied natural gas (LNG) carriers, which are specialized ships designed for transporting natural gas in liquid form over long distances. The process of liquefaction is explained, where natural gas is cooled to -162 °C, reducing its volume significantly. The video script outlines the ship's layout, including the engine room, fuel and lubricant oil tanks, water ballast tanks, and other compartments. It also introduces the Moss-type and Membrane-type designs, which are the most popular in the industry, and briefly mentions the less common SPB design. The paragraph concludes with an advertisement for Private Internet Access, a VPN service, which is not directly related to the main topic of LNG carriers.

05:03

🛠 Detailed Analysis of LNG Carrier Designs and Operations

The second paragraph delves deeper into the design and operational aspects of LNG carriers. It discusses the double hull design for safety, the Moss-type design with its spherical tanks and insulation layers, and the Membrane-type design with its integrated hull tanks and better space utilization. The Self-Supporting Prismatic Type B (SPB) design is also mentioned, highlighting its advanced features and the reasons for its limited popularity due to high costs and low demand. The paragraph further explains the process of loading LNG into the tanks, which includes purging the tanks with dry air, replacing humid air with inert gas, cooling the tanks, and managing the boiled-off gas (BOG) during the voyage. It also touches on the unloading process and the use of a reliquefaction plant to manage BOG when the ship is moored. The script ends with a call for viewer comments and a sign-off from the presenter, Lucius.

Keywords

💡LNG carriers

LNG carriers, also known as liquefied natural gas carriers, are specialized ships designed for the transportation of natural gas in its liquefied form over long distances. The script emphasizes their importance in the global energy supply chain, highlighting their size, which can exceed that of an aircraft carrier, and the advanced technology involved in their design and operation.

💡Liquefaction

Liquefaction is the process by which natural gas is cooled down to approximately -162 °C (-260°F), causing it to transition from a gas to a liquid state. This process is crucial for the efficient transportation of natural gas, as it reduces the volume by about 600 times, making it practical for LNG carriers to transport large quantities over long distances, as mentioned in the script.

💡Moss-type

Moss-type is one of the two most popular design types for LNG carriers discussed in the script. It features spherical tanks that are independent of the ship's hull and are supported by a high tensile steel skirt. This design is easily recognizable and plays a critical role in the containment and transportation of LNG.

💡Membrane-type

Membrane-type tanks are another popular design for LNG carriers, offering advantages over spherical tanks in terms of cargo capacity. These tanks are more rectangular and integrated directly into the ship's hull, resulting in better space utilization. The script illustrates this design as an efficient use of space for the transportation of LNG.

💡SPB (Self-Supporting Prismatic Type B)

SPB is an advanced but less commonly used design for LNG carriers, which makes the ship resemble an oil tanker. The tanks are independent and divided into spaces using bulkheads, with girdering for sloshing resistance. The script mentions this design as more advanced but notes its limited popularity due to high construction costs and low demand.

💡Double hull

A double hull design is a safety feature required for all LNG carriers, consisting of an outer and an inner hull with a space in between used for water ballast tanks. This design helps protect the cargo and tanks in the event of a collision, as explained in the script.

💡Boil-off gas (BOG)

BOG, or boiled-off gas, refers to the natural gas that evaporates from the LNG during the voyage due to external heat. The script explains the importance of managing BOG to prevent excessive pressure build-up in the tanks, either by using it as fuel or cooling it down through a reliquefaction plant.

💡Reliquefaction plant

A reliquefaction plant is a system used on LNG carriers to convert BOG back into a liquid state when the ship is moored and needs to maintain the LNG in its cargo tanks. The script describes how this plant helps in cooling and managing the LNG during periods when the ship is not in transit.

💡Dry and Inert gas plant

The dry and inert gas plant is used to prepare the LNG cargo tanks for loading by displacing humid air with dry air and then with inert gas to prevent the formation of flammable mixtures. The script details the process of purging the system and replacing inert gas with warm LNG vapor for safe loading.

💡Cofferdams

Cofferdams are the spaces between tanks on an LNG carrier that prevent liquids from leaking into other tanks. The script mentions cofferdams as part of the ship's design to ensure the integrity and safety of the containment system.

💡Primary and secondary barriers

In the context of LNG carrier design, primary and secondary barriers are layers that serve as containment for the LNG. The script explains that these barriers, particularly in membrane-type tanks, are made of stainless steel or invar and are crucial for the safe transportation of LNG.

Highlights

LNG carriers are specialized ships designed to transport liquefied natural gas over long distances.

Natural gas is liquefied at -162°C, reducing its volume by 600 times for efficient transportation.

LNG carriers are larger than aircraft carriers and come in various designs, such as Moss-type and Membrane-type.

Modern LNG carriers feature dual-fuel systems, utilizing both liquid fuel and boiled-off gas from cargo tanks.

The ship's hull includes water ballast tanks for stabilization and cofferdams to prevent leakage between tanks.

Moss-type LNG carriers are characterized by their spherical tanks supported by a high tensile steel skirt.

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LNG carriers are required to have double hulls for added protection in case of collision.

Membrane-type tanks are integrated into the ship's hull for better space utilization and cargo capacity.

SPB (Self-Supporting Prismatic Type B) tanks offer advanced design but are less popular due to high costs.

LNG loading process involves purging humid air with dry air and inert gas before introducing LNG.

Inert gas in the tanks is replaced with warm LNG vapor to prevent flammable mixtures.

Cargo tanks must be cooled to avoid thermal stress before LNG loading, with a safe cooling speed of 9ºC per hour.

During the voyage, boiled-off gas (BOG) is managed to prevent excessive pressure buildup in the tanks.

Reliquefaction plants convert BOG back to liquid state for storage when the ship is moored.

LNG unloading at the destination involves returning the ship to the LNG production shore to repeat the process.

The video concludes with an invitation for comments and a sign-off from the presenter, Lucius.

Transcripts

LNG carriers, also known as  liquefied natural gas carriers.

This is a type of ship,  

which is designed to transport natural  gas over long distances across the world.

This video is sponsored by Private Internet Access

Natural gas is a fossil fuel energy source that  can be found under the surface of the earth.

When this gas is cooled down to -162 °C (-260°F),

it becomes liquid, and the volume  reduces by about 600 times.

This process is liquefaction.

So if this ship had to carry this  volume of gas in its gas state,

the containment tanks would have been this big.

LNG is not blue like in this animation,  in reality, it actually has no color.

LNG carriers are not some small ships, they  can be longer than an aircraft carrier.

Today we will cover the 2  most popular design types  

in the industry, Moss-type and Membrane-type.

And another very advanced  type but not widely used, SPB

First let’s see the overview of the ship’s layout.

This is the engine room.

Fuel tanks.

Modern LNG carriers have dual-fuel systems,  

in which they use liquid fuel stored  in these tanks to power up the engine,

or they can also use boiled  off gas from its cargo tanks.

Lubricant oil tanks

Gas oil tanks

These tanks are for fresh water,  distilled water, and feed water.

Surrounding the hull are water ballast tanks,

where the ship pumps sea water  inside these tanks to keep the  

ship stabilized when the cargo is not loaded.

Any space between tanks are cofferdams,

they prevent liquids from  leaking into other tanks  

Steering compartment

Reliquefaction plant

Lifeboats

Accommodation for crew members

Ship bridge

Chain locker

Fore peak dry tank

Not all LNG carriers have exact same  design, this is just one example

The Moss-type can be easily  recognized by its spherical tanks.

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All types of LNG carriers are  required to have double hulls.

This is an outer hull, and this is an inner hull.

The space in between is water ballast tanks

The double hull design helps to protect the  tanks and cargo inside, incase of collision.

In moss-type, the spherical tanks are independent,  they are not a part of the ship’s hull.

Instead, they are supported by a supporting  skirt made of high tensile steel.

This is the primary barrier,  made of aluminum alloy.

Insulation layer, made of  Polyurethane or polystyrene Foam

Steel weatherproof cover on top.

Dripping tray, it is used to  collecting and containing liquid leaks

The empty space here is called the void space,  

and is filled with nitrogen for fire  prevention and thermal insulation.

Membrane-type tanks

This design has its advantages over  spherical tanks for cargo capacity.

The tank is more rectangular-like  and consists of several layers.

Primary barrier and secondary barrier  are made of stainless steel or invar.

Inner hull of the ship.

The spaces in between are for insulation  using plywood boxes filled with perlite.

The LNG tanks are directly  integrated into the ship's hull.

This integration results in  better space utilization,  

as the tanks fit the shape of  the ship's hull more efficiently.

This is the picture inside of a membrane tank.

SPB (Self-Supporting Prismatic Type B) tanks

This type of design makes an LNG  carrier look a lot like an oil tanker.  

The cargo tanks are inside the hull,  

but the tanks are independent, they  are not part of the ship’s hull.

SPB tank divides the tank into  4 spaces using tight bulkhead,

and swash bulkhead

The tanks are built with  girders for sloshing resistance  

This design is more advanced  than the moss and membrane type

But not very popular because of the high cost of  construction and low demand by ship operators.

There are more designs of LNG carriers

such as Type-C cylindrical, bilobe,  

and trilobe cargo tanks, but those are  also not widely used in the industry.

Let’s see how LNG is loaded into the tanks

This is the central column.

Inside the column, there are pipes.

Filling line, discharge lines,  

spray line. There are pumps at the bottom  of the discharge lines and spray line.

LNG is loaded into the tank  using this filling line. However,  

LNG can’t just be loaded right away.

Before loading LNG, there might be humid  air inside the tanks. Humid air contains  

water and will form ice. To avoid this,  it needs to be displaced by dry air

This is a dry and Inert gas plant.

Dry air is introduced to the bottom  of the tank through the filling line.

Humid air escapes to the  atmosphere via the vent mast.

All air in general needs to be purged from the  system to prevent formation of flammable mixtures.

Air is replaced with inert gas.

Inert gas contains freezable  gas such as carbon dioxide,  

and needs to be replaced with warm  LNG vapor using the spray nozzles.

The LNG vapor is lighter than the inert gas so  it stays on top and pushes the inert gas down.

The inert gas exhausted back up to the filling  line and then be vented out via the vent mast.

Now the cargo tank has only LNG  vapor and no other mixtures.

The cargo tank needs to be cooled down  before loading to avoid thermal stress,

because right now the tank is warm.

The spray nozzles spray cold  LNG to cool down the tank.

The safe speed of cooling is lower down by 9ºC  

per hour until the equatorial  region reaches at least -115ºC

Now the LNG can enter through the filling line.

LNG spaying continues if needed.

LNG won’t be filled up all the way in the tank.

It is safe to have some space  left for vapor and pressure.

During the voyage, the external heat  passing through the tank insulation

causing LNG to boil off.

This is called BOG (boiled-off gas)

To prevent BOG from building up  too much pressure inside the tank,

It can be utilized as a source  of fuel to power up the engine.

If there are too much BOG, the  spray line can pump cold LNG from  

the bottom of the tank and spray it  from the top to cool down the BOG.  

In cases of the ship being moored and still  have to keep LNG inside, for whatever reason.

BOG will be passed through the reliquefaction  plant, cooled down and turned to liquid state,

then be sent back to the cargo tank.

LNG is unloaded at the destination

The ship goes back to the LNG production  shore and the whole process repeats.

Thank you for watching, if  you have anything to add,  

please leave a comment below. My name is Lucius,  I will see you in the next video.