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How do LNG / LPG ships work?
Where in the value chain do we find LNG ships?
Shipping LNG overseas is by far the most common way to transport natural gas in liquid form. When LNG plants and fields are situated in remote areas, transportation of LNG by ship is often the preferred, or perhaps even the only, alternative for transporting natural gas. Therefore, LNG ships makes a key link in the LNG value chain, enabling transportation of gas between the liquefaction plant and regasification site.
What is an LNG ship?
An LNG ship (or tanker, carrier, vessel) is a ship equipped with containment systems for carrying LNG in bulk. A distinctive feature which separates an LNG ship from other bulk cargo carriers are the heavy insulated, temperature-controlled tanks ensuring the gas is kept in a liquid state at approximately -162 C. The propulsion system of LNG ships is often powered by steam turbines driven by boil-off-gas (BOG) from LNG, boiled liquid fuels such as oil, or a combination. By end 2019, the global LNG fleet consisted of 541 active ships, of which 34 were floating storage regasification units (FSRUs) and 4 were floating storage units (FSUs). The top importing markets of 2019 were Asia Pacific and Asia, while Qatar, Australia and the United States, in that order, were top exporters (IGU, 2020 World LNG Report).
Historically, LNG ships have been operating since the late 1950s, but production first picked up during the 1970s. Since the early 2000s the number of LNG ships delivered per year has drastically accelerated, and so have LNG production scales and vessel sizes. As the LNG industry has been characterized by big scale trades to an increasing extent in recent years, ships with large storage capacities have been a requirement. However, the global LNG fleet ranges from small scale, handy size bulk carriers to large scale carriers, where the world's largest is currently the Mozah, a Q-max ship from Qatargas.
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LNG ships are divided into several different types based on their type of cargo tanks
LNG ships are commonly classified based on their tanks, according to International Maritime Organization (IMO) classification of LNG Vessels. The IMO classification generally separates between
- Independent tanks, where the tank is not a part of the ship hull, i.e. they are not crucial to hull strength, and
- Integral tanks, which form a structural part of the ship's hull.
These main classifications of LNG ship tanks can again be further subdivided into different types.Independent tanks are divided into three types:
- Type A, with design based on classical ship structural design, having quite similar tank arrangement as an oil carrier, with tank design pressure less than 700 mbar. IGC code requires that type A tanks need a full secondary barrier to hold leaks for a minimum of 15 days.
- Type B, either the characteristic spherical Moss-tanks or prismatic IHI SPB tanks. Both require a partial secondary barrier, as opposed to type A, but both operate in pressure below 700 mbar, like type A.
- Type C, based on pressure vessel code and designed to operate above 2000 mbar without any secondary barrier. These tanks can shape as either cylindrical, bilobed or spherical, and due to the lacking secondary barrier, leakage is detected by change in gas composition in hold space, which is equipped with detecting sensors.
Membrane tanks are a type of integral tanks, and there are two common types:
- TGZ Mark III (also referred to as GTT Mark III), designed by Techni Gaz. The layers of this tank type consist of a primary barrier in stainless steel, primary insulation, secondary barrier within a triplex membrane, secondary insulation and finally the hull structure.
- GT 96 (Also referred to as GTT 96), from Gaz transport. This tank model consists of a primary and secondary membrane by an invariable (in its lack of thermal expansion and contraction) nickel-iron alloy called Invar. The insulation of this model is based on plywood boxes with perlite, flushed with nitrogen gas.
How does the cargo cycle for LNG work?
Due to extremely low operating temperatures, LNG cannot be loaded directly into the ship’s storage tanks without adequate preparations. If not done properly, LNG loading can lead to great thermal stress to tanks and pipes.
To ensure safe loading, a sequence of actions must be followed.
- First, the cargo tanks and hold spaces must be completely cleaned and dried. Impurities and humidity in the tanks from dry docking could cause corrosion, condensation and formation of ice.
- Secondly, inserting of the cargo tanks, insulation spaces and inter barrier spaces is initiated. This basically means inserting a low reactive gas, normally nitrogen gas, which for the cargo tanks is done to reduce moisture and oxygen levels. For the other spaces it prevents corrosion, enables detection of LNG leaks if air composition changes, and prevents formation of a flammable air mix.
- Thirdly, the tanks are “gassed up” by piston purging warm LNG vapor, hence removing remaining inert gas and finish the drying process. The tanks are now ready to be cooled down slowly to about -110C to -130C with LNG vapor.
- Finally, the tanks are ready to be loaded with LNG. The LNG carrier can be loaded by utilizing either a traditional jetty or an innovate jetty less transfer system.
When an LNG ship discharge cargo to an LNG terminal, cargo pumps empty the tanks either completely, or leave a small part of the cargo on board, as a “heel” to be used for tank cooling later.
Different type & sizes of Liquefied Natural Gas carriers
Various types of LNG Carrier
General Description
LNG carriers are generally specialized ships transporting LNG at its atmospheric pressure boiling point of approximately -162°C, depending on the cargo grade.
The gas vessels are designed to carry different types of bulk gasses and the vessels are technically advanced and are often classified into two main segments; LPG and LNG.
LNG carrier
LNG(Liquified Natural Gas) Carriers are specifically designed to trade a high volume of LNG. Liquefied natural gas (LNG) is natural gas (predominantly methane, CH4, with some mixture of ethane, C2H6). The LNG ships have a cargo carrying capacity between 125,000 cum to 260,000 cum. The most popular size is up to 180 000 cum.
To facilitate transport, natural gas is cooled down to approximately −163 °C (−261 °F) at atmospheric pressure, at which point the gas condenses to a liquid. The tanks on board an LNG carrier effectively function as giant thermoses to keep the liquid gas cold during storage.
Vessels can be classified into four categories in terms of the cargo containment system:
Moss tanks (Spherical IMO type B LNG tanks)
This system is named after the Norwegian company which designed them (Kvaerner Moss). Most of these vessels have 4-5 tanks. These tanks have a working pressure of up to 22 kPa (3.2 psi), but the working pressure can be raised for an emergency discharge.
IHI (Prismatic IMO type B LNG tanks)
The self-supporting prismatic type B tank is designed by Ishikawajima-Harima Heavy Industries. Due to several incidents in the past, these tanks were designed to avoid damage due to incidents that occurred inside membrane LNG tanks. The design is implemented in very few vessels.
TGZ MARK III
These vessels are designed by Technigas and are a membrane-type design. The membrane consists of stainless steel and the tanks have a 1.2mm waffle pattern' to absorb the thermal contraction when the tank is cooled down.
GT96
This design designed by Gaztransport consists of primary and secondary membranes made up of a material Invar which has no thermal contraction. The insulation is made out of plywood boxes filled with perlite and continuously flushed with nitrogen gas. The integrity of both membranes is permanently monitored by the detection of hydrocarbon in the nitrogen.
Invar, FeNi36 (64FeNi in the US), is a nickel-iron alloy notable for its uniquely low coefficient of thermal expansion (CTE or α).
C-type
C-type tanks are quite common in the small-scale LNG segments. These tanks are designed as cryogenic pressure vessels, using conventional pressure vessel codes (very often vapor pressure). The design pressure for these tanks is in ranges above 2000 bar. The most common shapes for these tanks are cylindrical and bi-lobe. Type ‘C’ tanks are used in both, LPG and LNG carriers.
Membrane (Gaz Transport or Tecnigaz)
Profile of LNG Carrier of the Gaz Transport style
Profile of LNG Carrier of the Gaz Transport style
Profile of LNG Carrier of the Gaz Transport style
Membrane (Gaz Transport or Tecnigaz)
There are two membrane systems in use. In both cases the insulation is fitted directly into the inner hull and the primary barrier consists of a thin metal membrane less than one millimetre thick.
The Gaz Transport system uses two such membranes constructed of ‘Invar’ (36% nickel-iron low expansion alloy
Membrane (Gaz Transport or Tecnigaz)
There are two membrane systems in use. In both cases the insulation is fitted directly into the inner hull and the primary barrier consists of a thin metal membrane less than one millimetre thick.
The Gaz Transport system uses two such membranes constructed of ‘Invar’ (36% nickel-iron low expansion alloy). One acts as the primary barrier and the other the secondary barrier and they are separated by plywood boxes of perlite insulation.
Excalibur – cross section
Profile of LNG Carrier of the Gaz Transport style
Profile of LNG Carrier of the Gaz Transport style
Similar boxes are fitted between the secondary barrier and the inner hull.
Loading is transmitted through the insulation to the ship structure.
No centreline division is possible in this type of tank. The other system, developed by Technigaz, has a stainless steel membrane as the primary barrier while the secondary barrier is included in
Similar boxes are fitted between the secondary barrier and the inner hull.
Loading is transmitted through the insulation to the ship structure.
No centreline division is possible in this type of tank. The other system, developed by Technigaz, has a stainless steel membrane as the primary barrier while the secondary barrier is included in the insulation, which consists of load bearing balsa and mineral woods.
Excalibur – cross section
Cross Section of LNG Carrier of the Gaz Transport style
Cross Section of LNG Carrier of the Gaz Transport style
Cross Section of LNG Carrier of the Gaz Transport style
Cross Section of LNG Carrier of the Gaz Transport style
Cross Section of LNG Carrier of the Gaz Transport style
Moss Tanks
Profile of LNG Carrier of the Moss Rosenberg style
Cross Section of LNG Carrier of the Moss Rosenberg style
Cross Section of LNG Carrier of the Moss Rosenberg style
Spherical tanks are generally produced in aluminium or 9% nickel steel. The sphere is welded to a steel skirt that is connected to the hull of the ship and is then free to expand and contract as necessary.
Cross Section of LNG Carrier of the Moss Rosenberg style
Cross Section of LNG Carrier of the Moss Rosenberg style
Cross Section of LNG Carrier of the Moss Rosenberg style
Insulation is fitted to the outside shell of the sphere but no secondary barrier is regarded as necessary across the upper part of the sphere. However, below the sphere, an aluminium drip tray, together with splash plates, provides secondary protection for the hull.
Different types & sizes of Liquefied Petroleum Gas carriers
LPG Carriers
LPG is Liquid petroleum gas prepared by refining petroleum or "wet" natural gas and is almost entirely derived from fossil fuel sources, being manufactured during the refining of petroleum (crude oil), or extracted from petroleum or natural gas streams as they emerge from the ground.
Large exporters of LPG gasses are; Saudi Arabia, Qatar, the US, and Algeria.
The LPG gasses are used for energy, as petrochemical feedstock, and in the agriculture industry.
LPG Vessels:
LPG vessels are divided into:
· Fully pressurized ships
· Semi-refrigerated ships
· Fully refrigerated LPG
Ethylene ships (semi refrigerated ships able to transport cargoes at -104°C.
The most commonly used sub-segments for the LPG carriers:
Handy Gas Carriers: LPG vessels of about 15 –25,000 cbm. A diverse segment which includes semi-refrigerated, fully-refrigerated and some larger, pressurised ships that carry a wide range of cargoes such as ethylene, petrochemicals, LPG and ammonia on short to medium-haul routes.
The LPG vessels below 15 000 cbm are often referred to as coasters.
Mid-Sizes: LPG vessels of about 25,000–50,000 cbm, typically fully- refrigerated, carrying ammonia or LPG, on intra-regional routes (e.g. within the Americas or Asia) and medium-haul cross-trades (e.g. in the North Sea and Europe).
LGC: LPG vessels of about 50,000–70,000 cbm, mainly carrying LPG and ammonia between ports limitING the VLGC ships to enter.
Very Large Gas Carriers (VLGC): LPG vessels of about 70,000 cbm or above. Many of these vessels are fully-refrigerated and mainly employed on long-haul trade routes, e.g. from Middle East Gulf (MEG) and the United States to Asia.
Liquefied Gas Carriers - Ship Types
Fully Pressurised Gas carrier ‘Lady Hilde’
Fully Pressurised Gas carrier ‘Lady Hilde’
Fully Pressurised Gas carrier ‘Lady Hilde’
Fully Pressurised Ships
These were the first generation of ships to carry liquefied gases.
The ships have a cargo capacity up to ~ 3,500 m³.
These ships carry the cargo in spherical or cylindrical steel tanks, designed for a working pressure of 17.5 kg/cm². This corresponds to the vapour pressure of propane at 45°C, which is the maximum amb
Fully Pressurised Ships
These were the first generation of ships to carry liquefied gases.
The ships have a cargo capacity up to ~ 3,500 m³.
These ships carry the cargo in spherical or cylindrical steel tanks, designed for a working pressure of 17.5 kg/cm². This corresponds to the vapour pressure of propane at 45°C, which is the maximum ambient temperature in which the ship is likely to operate.
No means of temperature or pressure control is necessary.
The tanks are generally Type C spheres and no secondary barrier is required. A double bottom is constructed for ballast water. The hold space around the cargo tanks does not need to be inerted.
Kemira Gas – Side elevation
Fully Pressurised Gas carrier ‘Lady Hilde’
Fully Pressurised Gas carrier ‘Lady Hilde’
Disadvantages
• Due to their shape, the use of underdeck space cannot be optimised
• high design pressure requires considerable tank wall thickness, with consequent increase in displacement weight and cost
• the weight in tons of cargo carried is lower than for a refrigerated ship of similar size, due to cargo density difference
• as the diam
Disadvantages
• Due to their shape, the use of underdeck space cannot be optimised
• high design pressure requires considerable tank wall thickness, with consequent increase in displacement weight and cost
• the weight in tons of cargo carried is lower than for a refrigerated ship of similar size, due to cargo density difference
• as the diameter of the tanks increases, the wall thickness increases to withstand the same pressure. The decreasing ratio of cargo carried to weight of tank makes this solution uneconomical over long haul routes.
Kemira Gas – Side elevation
Fully Pressurised Gas carrier ‘Lady Hilde’
Kemira Gas – Side elevation
Advantages
• They are built with ordinary grades of steel as the cargo is carried at ambient temperature and no insulation is required
• no reliquefaction plant is required
• operations are simpler.
Semi-Pressurised Gas Carrier ‘Kemira Gas'
Semi-Pressurised Gas Carrier ‘Kemira Gas'
Semi-Pressurised Gas Carrier ‘Kemira Gas'
Semi-pressurised / Semi-refrigerated or Semi-pressurised / Fully-refrigerated
General description
These vessels are fitted with a refrigeration plant that provides a fully refrigerated ability while having a high design pressure for the cargo tanks (Pressure vessels), albeit below that required for fully pressurised carriage. The tanks are
Semi-pressurised / Semi-refrigerated or Semi-pressurised / Fully-refrigerated
General description
These vessels are fitted with a refrigeration plant that provides a fully refrigerated ability while having a high design pressure for the cargo tanks (Pressure vessels), albeit below that required for fully pressurised carriage. The tanks are cylindrical in shape and of a thinner construction than the pressurised vessels.
Cargo capacity
Semi-pressurised, semi-refrigerated ships (which are now quite rare) ranged up to 5,000 m³ in size. Their construction is based on carrying propane at a pressure of 8.5 kg/cm², and a temperature of -10°C.
Kemira Gas – Side elevation
Semi-Pressurised Gas Carrier ‘Kemira Gas'
Semi-Pressurised Gas Carrier ‘Kemira Gas'
Semi-pressurised, fully-refrigerated ships generally range up to 15,000 m³. They can be designed to carry the full range of cargoes in cylindrical or spherical tanks and are designed for a minimum service temperature of -48°C and a working pressure of approx. 5 to 8 kg/cm².
Temperature control
The reliquefication plant on these vessels gene
Semi-pressurised, fully-refrigerated ships generally range up to 15,000 m³. They can be designed to carry the full range of cargoes in cylindrical or spherical tanks and are designed for a minimum service temperature of -48°C and a working pressure of approx. 5 to 8 kg/cm².
Temperature control
The reliquefication plant on these vessels generally has a substantial capacity and can, if required, load the cargo as a gas and then reliquefy it onboard.
They are able to heat or cool the cargo during loading operations, or while at sea, and are also able to raise the temperature of the cargo when discharging.
Where a reliquefication plant is fitted it will allow a reduction in the wall thickness of the tanks.
Kemira Gas – Side elevation
Semi-Pressurised Gas Carrier ‘Kemira Gas'
Kemira Gas – Side elevation
Construction
The inner hull volume is used more efficiently than the fully pressurised vessels and the number of tanks varies from 2-6.
A double bottom is constructed for ballast water and the hold space around the cargo tanks does not need to be inerted.
Advantages
Their advantages over fully-pressurised ships are:
· More cargo can be carri
Construction
The inner hull volume is used more efficiently than the fully pressurised vessels and the number of tanks varies from 2-6.
A double bottom is constructed for ballast water and the hold space around the cargo tanks does not need to be inerted.
Advantages
Their advantages over fully-pressurised ships are:
· More cargo can be carried in a tank of the same capacity
· a tank of the same capacity is lighter and cheaper to build
· much larger and more economical ships can be constructed.
Fully Refrigerated Gas Carrier ‘Flanders Harmony’
Fully Refrigerated Gas Carrier ‘Flanders Harmony’
Fully Refrigerated Gas Carrier ‘Flanders Harmony’
General Description
The economic advantages of transporting LPG and ammonia in a fully refrigerated, non-pressurised condition are more evident for longer haul and larger quantity cargoes. The self-supporting prismatic shape of the cargo tanks allows for a better utilisation of the available hold space than the type of ships described prev
General Description
The economic advantages of transporting LPG and ammonia in a fully refrigerated, non-pressurised condition are more evident for longer haul and larger quantity cargoes. The self-supporting prismatic shape of the cargo tanks allows for a better utilisation of the available hold space than the type of ships described previously. The tanks are usually designed for a maximum working pressure of about 0.28 kg/cm2(280 millibars) and a minimum working temperature of -50°C making them suitable for the carriage of butane, butadiene, VCM, ammonia, propane and propylene.
Eeklo – cross section
Fully Refrigerated Gas Carrier ‘Flanders Harmony’
Fully Refrigerated Gas Carrier ‘Flanders Harmony’
Cargo capacity
The ships are typically in the range 15,000 m3 – 85,000 m3, with three common sizes for LPG/Ammonia trades of 30,000 m3, 52,000 m3 and 80,000m3.
Construction
The tanks nearly extend to the full width of the ship, with ballast in the double bottom and upper hopper or wing tanks.
These tanks normally have a centreline bulkhead f
Cargo capacity
The ships are typically in the range 15,000 m3 – 85,000 m3, with three common sizes for LPG/Ammonia trades of 30,000 m3, 52,000 m3 and 80,000m3.
Construction
The tanks nearly extend to the full width of the ship, with ballast in the double bottom and upper hopper or wing tanks.
These tanks normally have a centreline bulkhead fitted with two equalizing valves.
You should be cautious should these vessels develop a list alongside as the tanks carry a large free surface area and if the vessel has problems with the ballast or levelling the cargo during load, they can quickly list over to 2 or 3°.
Eeklo – cross section
Fully Refrigerated Gas Carrier ‘Flanders Harmony’
Eeklo – cross section
Temperature control
The trend for longer voyages has imposed a demand larger ships, and with the increasing size of the ship, the pumping and refrigeration plant capacity has increased proportionally.
Ethylene Carrier can also carry LPG cargoes
Ethylene Carrier can also carry LPG cargoes
Ethylene Carrier can also carry LPG cargoes
General Description
Ethylene carriers are a special type of gas carrier that can transport ethylene fully-refrigerated at its atmospheric pressure boiling point of -104°C. Such ships are often built for specific trades
Many ethylene carriers can also carry LPG cargoes, which increases their flexibility.
Cargo Capacity
Cargo capacitiy depends
General Description
Ethylene carriers are a special type of gas carrier that can transport ethylene fully-refrigerated at its atmospheric pressure boiling point of -104°C. Such ships are often built for specific trades
Many ethylene carriers can also carry LPG cargoes, which increases their flexibility.
Cargo Capacity
Cargo capacitiy depends on the trade for which the vessel was constructed and range from 1,500 - 15,000 cubic metres.
Kemira Gas – Side elevation
Ethylene Carrier can also carry LPG cargoes
Ethylene Carrier can also carry LPG cargoes
Temperature Control
Thermal insulation and a high capacity reliquefaction plant is fitted on this type of vessel.
Construction
Ethylene Carriers have Containment systems can either be Type C, Type B or Type A prismatic free-standing.
If Type C pressure vessel tanks are used, then no secondary barrier is required
If Type B is used a partial sec
Temperature Control
Thermal insulation and a high capacity reliquefaction plant is fitted on this type of vessel.
Construction
Ethylene Carriers have Containment systems can either be Type C, Type B or Type A prismatic free-standing.
If Type C pressure vessel tanks are used, then no secondary barrier is required
If Type B is used a partial secondary barrier is required If Type A is used a full secondary barrier is required.
Because of the cargo temperature of -104°C, the hull cannot be used as a secondary barrier in this case and a separate secondary barrier must be fitted.
For reasons of economy, Type C tanks have predominated in this trade.
Ballast is carried in a full double bottom and wing tank ballast system.
Depends on vessels availability: Single Shipment Quantity
⚠️ LNG - From 125,000 m³ - 90.563 Metric Tons up to 180 000 m³ - 130.410 Metric Tons Vessels
⚠️ BLCO - From 1,000,000 bbl. – 136,425.649 Metric Tons up to 2,000,000 bbl. – 272,851.297 Metric Tons Vessels
⚠️ LNG - From 125,000 m³ - 90.563 Metric Tons up to 180 000 m³ - 130.410 Metric Tons Vessels
Shipping LNG overseas is by far the most common way to transport natural gas in liquid form. When LNG plants and fields are situated in remote areas, transportation of LNG by ship is often the preferred, or perhaps even the only, alternative for transporting natural gas. Therefore, LNG ships makes a key link in the LNG value chain, enabling transportation of gas between the liquefaction plant and regasification site.
⚠️ LPG - From 50,000 m³ - 36.225 Metric Tons up to 70,000 m³ - 50.715 Metric Tons Vessels
⚠️ BLCO - From 1,000,000 bbl. – 136,425.649 Metric Tons up to 2,000,000 bbl. – 272,851.297 Metric Tons Vessels
⚠️ LNG - From 125,000 m³ - 90.563 Metric Tons up to 180 000 m³ - 130.410 Metric Tons Vessels
Shipping LPG overseas is by far the most common way to transport natural gas in liquid form. When LPG plants and fields are situated in remote areas, transportation of LPG by ship is often the preferred, or perhaps even the only, alternative for transporting natural gas. Therefore, LPG ships makes a key link in the LPG value chain, enabling transportation of gas between the liquefaction plant and regasification site and/or Loading ISO Tank Containers Depot.
⚠️ BLCO - From 1,000,000 bbl. – 136,425.649 Metric Tons up to 2,000,000 bbl. – 272,851.297 Metric Tons Vessels
⚠️ BLCO - From 1,000,000 bbl. – 136,425.649 Metric Tons up to 2,000,000 bbl. – 272,851.297 Metric Tons Vessels
⚠️ BLCO - From 1,000,000 bbl. – 136,425.649 Metric Tons up to 2,000,000 bbl. – 272,851.297 Metric Tons Vessels
Shipping BLCO overseas is by far the most common way to transport crude oil in bulk form. When crude oil refinery plants and fields are situated in remote areas, transportation of BLCO by ship is often the preferred, or perhaps even the only, alternative for transporting crude oil. Therefore, BLCO ships makes a key link in the BLCO value chain, enabling transportation of oil between the crude oil refinery plant and final petrochemicals refinery site.
⚠️ LPFO - From 1,000,000 bbl. – 136,425.649 Metric Tons up to 2,000,000 bbl. – 272,851.297 Metric Tons Vessels
⚠️ BLCO - From 1,000,000 bbl. – 136,425.649 Metric Tons up to 2,000,000 bbl. – 272,851.297 Metric Tons Vessels
⚠️ BLCO - From 1,000,000 bbl. – 136,425.649 Metric Tons up to 2,000,000 bbl. – 272,851.297 Metric Tons Vessels
Shipping LPFO overseas is by far the most common way to transport LPFO in bulk form. When crude oil refinery plants and fields are situated in remote areas, transportation of LPFO by ship is often the preferred, or perhaps even the only, alternative for transporting low pour fuel oil. Therefore, LPFO ships makes a key link in the LPFO value chain, enabling transportation of oil between the crude oil refinery plant and heating site.