Technical reference
Clear, practical definitions for the systems, vessels, and processes that shape the LNG value chain.
The temperature at which a gas mixture ignites without an external ignition source.
Autoignition behaviour is one of several combustion properties considered when designing gas-fuelled machinery spaces and hazardous-area arrangements. Values quoted in literature depend on gas composition and test conditions, so design work relies on project-specific data rather than general figures.
A voyage sailed without cargo, typically returning towards a loading port.
On a ballast leg an LNG carrier normally retains a small quantity of cargo, referred to as a heel, to keep tanks cold. Voyage planning considers heel quantity, boil-off, bunker consumption and schedule requirements; the approach depends on the trade and charter terms.
A Type C pressure tank formed from two intersecting cylinders joined by a central bulkhead.
Bi-lobe geometry lets designers fit more pressure-vessel volume into a given hull breadth than a single cylinder of the same height. It is used where hull space is constrained, typically in small-scale and bunkering vessels. Dimensions, design pressure and insulation are project-specific.
Natural gas vapour generated when heat enters LNG in storage or transport.
A portion of LNG vaporises as heat passes through insulation into the cargo. The vapour must be managed to keep tank pressure within design limits, generally by using it as fuel, reliquefying it, or combusting it safely. Boil-off figures are indicative and vary with insulation, filling level, sea state and voyage length.
Redistribution of LNG from a large import terminal into smaller parcels.
Break-bulk services supply feeder vessels, bunker vessels or trucks from a larger terminal, supporting small-scale distribution. Availability depends on terminal capability.
The document recording a fuel delivery to a ship.
A bunker delivery note records the quantity and characteristics of the fuel supplied, together with delivery details. For LNG bunkering the documentation and measurement practices reflect the cryogenic nature of the fuel and applicable industry guidance.
A vessel that supplies LNG as fuel to other ships.
LNG bunker vessels transfer fuel ship to ship, typically using Type C tanks and dedicated transfer equipment. Capacity and equipment vary by vessel and service area.
An assessment carried out before an LNG bunkering operation.
The study covers manifold arrangement, mooring, transfer equipment, safety zones and procedures for the specific vessels involved. It is completed before the first transfer between the parties.
The maximum permitted liquid level in a cargo tank.
Filling limits protect against overpressure and, for some designs, sloshing loads. Limits are set by code requirements and the specific containment design.
The barriers, insulation and supporting structure that hold LNG on board.
A containment system isolates cryogenic cargo from the hull while managing thermal contraction, leak detection and cargo movement. Membrane, self-supporting Type B and Type C pressure-vessel concepts solve the same problem in different ways, and each suits different vessel sizes and services.
The connection point where cargo hoses or arms are coupled to the vessel.
The manifold is where liquid and vapour lines meet the shore or receiving vessel during transfer. Its height, spacing and connection type affect terminal compatibility, which is checked before a transfer through compatibility studies.
A submerged pump used to discharge LNG from a cargo tank.
Cargo pumps are installed inside the tanks and operate at cryogenic temperature. Discharge rate depends on pump arrangement, tank design and terminal receiving capability, so figures are always installation-specific.
The vapour pressure maintained inside a cargo tank.
Tank pressure is managed within design limits using boil-off handling, vapour return and, on some vessels, reliquefaction. Permitted pressure ranges depend on containment type and are set by design and code requirements.
The party that hires a vessel under a charter agreement.
The charterer contracts the use of a ship for a voyage or period. Responsibilities for fuel, boil-off, scheduling and port costs are allocated by the charter terms, which vary between agreements.
An organisation that sets and verifies technical standards for ships.
Classification societies publish rules, review designs and survey vessels in service. Requirements applicable to a specific vessel depend on its class and flag.
A separating space between compartments on board.
Cofferdams isolate cargo spaces from adjacent areas, and on LNG carriers they are typically heated to protect hull steel from cryogenic temperatures. Arrangements follow the vessel design.
Gradually lowering cargo system temperature before loading LNG.
Cooldown reduces thermal shock by introducing LNG slowly into a warm cargo system. Duration and method depend on containment type, the temperature of the system and terminal procedures.
A primary barrier formed from corrugated stainless steel sheets.
Corrugations allow the membrane to absorb thermal movement while remaining liquid-tight. This approach is associated with particular membrane containment systems.
Relating to the very low temperatures at which gases are kept liquid.
LNG operations are cryogenic because natural gas is cooled to approximately −162 °C at near-atmospheric pressure to remain liquid. Cryogenic conditions drive material selection, insulation design and personnel protection across the value chain.
The measured handover of cargo between parties.
Custody transfer establishes the quantity and energy content exchanged, using agreed measurement methods and documentation. Procedures follow contractual terms and recognised industry measurement practice.
Dual-fuel diesel-electric propulsion.
In a dual-fuel diesel-electric plant, several dual-fuel generator sets supply electrical power to propulsion motors and ship services. The arrangement allows flexible load sharing and can accept cargo boil-off gas as fuel.
The operation of transferring LNG from a vessel to a receiving facility.
Discharge uses cargo pumps and a vapour return arrangement to maintain tank conditions. Rates and sequences are agreed with the terminal and depend on the vessel and receiving facility.
An engine able to run on gas or liquid fuel.
Dual-fuel machinery can operate on boil-off gas or conventional marine fuel, depending on availability and operating mode. Specific concepts differ in combustion cycle and gas supply pressure.
A device that allows a transfer connection to separate safely.
Emergency release arrangements are designed to part the transfer connection in a controlled way if vessel movement exceeds limits, normally in conjunction with the emergency shutdown system. Configurations vary by terminal and transfer type.
A sea area with stricter limits on certain ship emissions.
Operating inside such an area affects fuel and machinery choices. Boundaries and applicable limits are defined by the relevant regulatory instruments.
The amount of energy contained in a quantity of LNG.
LNG is commonly traded on an energy basis rather than volume alone, because composition affects heating value. Conversions between volume, mass and energy use agreed measurement methods and cargo-specific composition data.
Emergency shutdown system.
An ESD system brings cargo or bunkering operations to a controlled stop when abnormal conditions are detected. On a transfer interface, both sides are linked so either party can initiate shutdown, and an emergency release arrangement may separate the connection if the vessels move outside agreed limits.
The state in which a vessel is registered.
The flag state administers applicable international and national requirements for the vessel, often through delegated organisations. Obligations depend on the registry and the conventions in force.
The range of gas-in-air concentrations that can support combustion.
Gas concentrations below or above the flammable range will not sustain combustion. Ventilation, inerting and gas detection arrangements are designed with this behaviour in mind; specific values depend on gas composition and conditions.
Floating liquefied natural gas facility.
An FLNG facility processes, liquefies, stores and offloads natural gas offshore, removing the need for an onshore liquefaction plant and long export pipelines. Topsides scope, storage capacity and mooring are project-specific.
Equipment that vaporises LNG to supplement gas supply to consumers.
When natural boil-off is insufficient for the gas demand of engines or boilers, a forcing vaporiser can generate additional vapour from liquid cargo or fuel. Use depends on the propulsion concept and operating mode.
Floating storage and regasification unit.
An FSRU receives and stores LNG and regasifies it on board for delivery into a pipeline network. Units may be purpose-built or converted from trading carriers, and send-out capability varies considerably by project.
Floating storage unit.
An FSU provides moored LNG storage without regasification equipment. It is often paired with onshore or separate regasification capacity, or used to buffer supply at a liquefaction or import location.
Equipment that safely burns surplus boil-off gas that cannot be used as fuel.
When vapour generation exceeds propulsion and auxiliary demand, a gas combustion unit disposes of the surplus under controlled conditions so tank pressure stays within limits. It is a pressure-management device rather than an energy-recovery system.
Systems that monitor for the presence of hydrocarbon gas.
Fixed and portable detection equipment supports safe operation by identifying gas in spaces where it should not be present. Detector types, locations and alarm arrangements follow design and code requirements.
Testing of cargo and gas-fuel systems using gas after construction or conversion.
Gas trials verify that cargo handling, fuel gas supply and safety systems perform as intended before service. Scope depends on the vessel, the project and the approving parties.
Replacing inert gas in a cargo tank with methane vapour.
Gassing up precedes cooldown and loading after a tank has been inerted, removing inert gas so that the tank atmosphere becomes suitable for LNG. Procedures depend on containment type and terminal arrangements.
A space where flammable gas may be present and equipment must be suitably certified.
Hazardous-area classification determines what electrical and mechanical equipment may be installed in a given space and how ventilation is arranged. Classification follows applicable codes and the specific vessel arrangement.
A small quantity of LNG retained on board to keep tanks cold.
Retaining heel avoids a full warm-up of the cargo system between voyages and shortens the cooldown before the next loading. Heel quantity is planned against voyage length, boil-off and charter terms.
The ship structure that supports and protects the cargo containment system.
The interaction between hull and containment differs by system: membrane tanks transfer loads into the hull, while self-supporting tanks carry loads within the tank structure. Arrangements are design-specific.
The IMO code for ships carrying liquefied gases in bulk.
The International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk sets design and equipment requirements for gas carriers, including containment types A, B and C. Always consult the current text and the applicable flag and class interpretations.
The IMO code for ships using gases or other low-flashpoint fuels.
The IGF Code addresses the safety of ships that use gas as fuel, covering fuel storage, bunkering arrangements, gas supply systems and risk assessment. It is central to LNG-fuelled vessel design and to bunkering interfaces.
Replacing a tank atmosphere with inert gas to prevent a flammable mixture.
Inerting is used before gassing up and after discharge operations that require a non-flammable atmosphere. The inert gas source and the required oxygen level depend on the vessel and the operation.
The space within a containment system that holds the insulation layers.
In membrane systems the insulation space sits between the primary barrier and the hull structure and is normally monitored for gas and pressure. Arrangements differ between containment designs.
A low-expansion nickel-iron alloy used in some membrane containment systems.
Low thermal expansion makes the alloy suitable for primary membranes in certain designs. Its use is specific to the containment system concerned.
A marine structure where vessels berth to transfer cargo.
An LNG jetty provides mooring, transfer connections and safety systems for cargo operations. Layout and equipment are specific to the terminal.
A voyage sailed with cargo on board.
On a laden leg, planning focuses on cargo condition, boil-off management and delivery windows. Fuel and boil-off handling depend on the propulsion concept and charter terms.
Cooling natural gas until it becomes a liquid.
Liquefaction reduces the volume of natural gas by roughly 600 times, which is what makes ocean transport practical. It takes place at onshore plants or offshore on FLNG facilities, after the gas has been treated to remove components that would freeze.
A single processing line within a liquefaction plant.
A plant may contain one or more trains, each cooling treated natural gas to liquid. Capacity and process selection are project-specific.
Liquefied natural gas.
Natural gas, predominantly methane, cooled to liquid form so it can be stored and transported efficiently at near-atmospheric pressure. In liquid form it is not stored under high pressure on conventional carriers, which shapes containment and safety design.
Supplying LNG as fuel to a receiving vessel.
Bunkering can be performed ship-to-ship, truck-to-ship or terminal-to-ship. Each method has distinct equipment, procedures, port approvals and compatibility checks, and the choice depends on port infrastructure and the receiving vessel.
A ship built to transport LNG in bulk.
LNG carriers range from small coastal vessels with Type C tanks to very large ocean-going ships with membrane or self-supporting containment. Operational role, containment system, capacity class and propulsion are separate characteristics and should not be collapsed into one label.
Transferring LNG from a facility into a vessel's cargo tanks.
Loading follows cooldown and is carried out at rates agreed with the terminal, with vapour returned ashore to control tank pressure. Sequences and limits are vessel- and terminal-specific.
An articulated rigid arm used to transfer LNG between shore and ship.
Loading arms provide a cryogenic-capable connection able to follow vessel movement within defined limits, usually with an emergency release arrangement. Arm envelopes form part of terminal compatibility checks.
A GTT membrane containment system with a corrugated stainless-steel primary barrier.
Mark III uses a corrugated stainless-steel membrane over reinforced foam insulation panels that also carry cargo loads into the inner hull. Several generations exist with differing insulation performance.
The international convention on prevention of pollution from ships.
MARPOL addresses emissions and discharges from shipping through its annexes. Requirements relevant to a vessel depend on its type, area of operation and applicable annexes.
A two-stroke dual-fuel engine using high-pressure gas injection.
ME-GI engines inject gas at high pressure near the end of compression, which supports diesel-cycle combustion behaviour. The concept requires a high-pressure fuel gas supply system.
A cargo tank whose thin primary barrier is supported by the insulation and inner hull.
Membrane tanks follow the hull form, so they generally use hull volume efficiently. The insulation is structural as well as thermal, and the system is installed as a complete barrier arrangement inside the inner hull.
The principal component of natural gas and LNG.
LNG is predominantly methane, with smaller quantities of other hydrocarbons and nitrogen depending on the source and processing. Composition affects energy content and boil-off behaviour.
Unburned methane passing through an engine into the exhaust.
Methane slip is discussed in the context of gas-engine emissions performance and varies with engine concept, load and operating conditions. Quantifying it requires engine-specific measurement rather than general figures.
A study of vessel mooring behaviour at a berth or during transfer.
Mooring analysis examines environmental conditions, line arrangements and vessel motion to establish safe operating limits. It is performed for the specific vessel, berth and expected conditions.
A self-supporting spherical cargo tank.
A Moss tank carries its own cargo loads and transfers them to the hull through an equatorial skirt. The design is tolerant of partial filling, which suits trades with frequent part-cargo operation.
An inert gas used in cargo systems and present in small amounts in LNG.
Nitrogen is used for purging, barrier spaces and seals, and is also a minor component of LNG depending on source and processing. Its presence influences boil-off composition.
A GTT membrane containment system using invar barriers and insulation boxes.
NO96 employs thin invar primary and secondary barriers supported by insulation boxes. Invar's very low thermal contraction removes the need for corrugations in the membrane.
Transfer of produced LNG from a floating facility to a carrier.
Offshore offloading may use side-by-side or tandem arrangements depending on the facility, environmental conditions and project design. Operating limits are set case by case.
Inspection of foreign vessels by the authorities of a visiting port.
Inspections check compliance with applicable international requirements. Scope and frequency follow the regional inspection regime.
Removal of components from natural gas before liquefaction.
Water, carbon dioxide and other components are removed to prevent freezing and equipment damage during cooling. Process design depends on feed gas composition.
A tank designed to contain contents above atmospheric pressure.
Type C cargo tanks are pressure vessels designed and surveyed against pressure-vessel rules. Their design pressure is set per project, and no single value applies across the segment.
Displacing one gas in a system with another to reach a required atmosphere.
Purging is used before and after maintenance, inerting and gassing-up operations. The gas used and the target concentration depend on the operation and the system being prepared.
A large LNG carrier segment with indicative capacity of approximately 210,000–217,000 m³.
Q-Flex vessels were developed for long-haul Qatari exports and combine large cargo volume with twin propulsion arrangements. Terminal compatibility is a central planning consideration for the segment.
The largest LNG carrier segment, with indicative capacity of approximately 260,000–266,000 m³.
Q-Max is the upper size band developed for Qatari exports. Vessel dimensions restrict the number of terminals that can receive these ships, so the segment is tied to specific trades.
Returning LNG to a gaseous state for pipeline delivery.
At an import terminal or on an FSRU, LNG is pumped to pressure and warmed through vaporisers using sea water, air or another heat source before entering the gas network.
A valve that protects a tank or system from overpressure.
Relief arrangements release vapour in a controlled way if pressure approaches design limits. Set pressures and capacities follow the containment design and applicable code requirements.
Cooling boil-off gas back into liquid and returning it to the cargo tank.
Reliquefaction plant lets a vessel retain cargo instead of consuming or combusting vapour. It adds machinery, energy demand and maintenance scope, so its use depends on the trade and propulsion concept.
A sudden mixing of stratified LNG layers in a tank.
Where LNG of differing density becomes stratified, layers can mix rapidly and release a large volume of vapour. Loading practice and tank monitoring are used to reduce the likelihood; behaviour is specific to cargo composition and tank conditions.
A controlled area established around a transfer operation.
Access and activities within the zone are restricted while transfer is in progress. Size and controls are set by risk assessment and local requirements.
A barrier that contains cargo temporarily if the primary barrier leaks.
Secondary barrier requirements depend on the containment type. Membrane systems generally require a full secondary barrier, while some self-supporting designs may use a partial barrier subject to code conditions.
The rate at which regasified LNG is delivered into a gas network.
Send-out depends on regasification equipment, network requirements and contractual arrangements. Figures are facility-specific and vary with operating conditions.
A generator driven from the propulsion shaft line.
A shaft generator can supply electrical power from the main engine, affecting overall machinery configuration and fuel use. Its use depends on the propulsion concept.
Transferring LNG between two vessels moored alongside each other.
Ship-to-ship transfer is used for bunkering and for cargo transfer to floating units. It requires compatibility checks, mooring analysis, an agreed ESD link and, where relevant, port authority approval.
The party that owns a vessel and normally provides crew and technical management.
Ownership and operational responsibilities can be split across owners, managers and charterers. The allocation is defined by the relevant contracts.
Simultaneous operations taking place alongside a transfer.
SIMOPS covers cases such as bunkering while cargo or passenger operations continue. Each case needs a specific risk assessment and agreement between the parties and the port authority before it proceeds.
A general way of grouping vessels into indicative size bands.
Size-class language is used across shipping to group vessels into broad capacity bands for planning and terminal-compatibility discussions. In LNG shipping the bands are indicative only, and actual dimensions, cargo capacity and terminal acceptance are always vessel-specific.
Movement of liquid cargo inside a partially filled tank.
Sloshing loads are considered in containment design and in filling-level restrictions, particularly for membrane systems. Permitted filling levels and sea-state limits are design-specific.
LNG distribution in volumes below the conventional ocean-trading segment.
Small-scale LNG covers regional distribution, coastal supply, feeder services and bunkering. Vessels in this segment often use Type C tanks, but containment choice still depends on the individual project.
The international convention covering safety of life at sea.
SOLAS sets out broad safety requirements for ships, with gas carriers and gas-fuelled ships also subject to dedicated codes. Applicability depends on vessel type and service.
Self-supporting prismatic Type B containment.
SPB tanks are independent prismatic tanks with internal swash bulkheads that limit cargo movement. The flat tank top leaves usable deck space, which suits floating infrastructure projects.
A propulsion concept using steam raised in boilers able to burn boil-off gas.
Steam propulsion was long associated with LNG carriers because boilers can consume boil-off gas directly. Newer concepts use different machinery arrangements; suitability depends on the project.
Formation of layers of differing density within stored LNG.
Stratification can occur when cargoes of different composition are mixed or when a tank is filled without adequate mixing. It is monitored because of its relationship with rollover.
An assessment of whether a vessel can safely call at a given terminal.
Compatibility studies compare vessel dimensions, manifold arrangement, mooring layout and operational limits against the berth. Acceptance is decided case by case by the terminal operator.
Bunkering or loading directly from shore infrastructure.
A shore terminal supplies LNG through fixed pipework and loading arms or hoses. This method suits ports with established LNG infrastructure and regular calls.
Tri-fuel diesel-electric propulsion.
A tri-fuel diesel-electric plant can operate on gas, marine diesel oil or heavy fuel oil, feeding electrical propulsion motors. It shares the flexible load-sharing characteristics of DFDE arrangements.
Shrinkage of materials when cooled to cryogenic temperature.
Containment systems accommodate contraction through material selection and structural detailing such as corrugations or supports. The approach differs between containment designs.
A charter under which a vessel is hired for a period of time.
Under a time charter the charterer directs commercial employment within agreed limits, while the owner generally retains technical operation. Cost allocation follows the charter terms.
Bunkering from one or more road tankers on the quayside.
Truck-to-ship transfer suits smaller fuel volumes and ports without dedicated LNG infrastructure. Transfer rates are generally limited by the number of trucks and connection cycles.
A mooring arrangement allowing a floating facility to weathervane.
A turret lets a moored facility rotate around a fixed point in response to wind, waves and current. Use depends on site conditions and project design.
A self-supporting prismatic tank designed to conventional ship-structure rules with a full secondary barrier.
Type A tanks are used for some liquefied gas cargoes and require a complete secondary barrier because the design does not rely on leak-before-failure reasoning. They appear less often in LNG service than Type B and Type C concepts.
A self-supporting tank designed with detailed structural analysis and a leak-before-failure philosophy.
Type B covers Moss spheres and SPB prismatic tanks. The detailed analysis behind the design allows a partial rather than full secondary barrier, subject to code requirements.
A self-supporting pressure-vessel cargo or fuel tank.
Type C tanks are self-supporting pressure vessels commonly used in smaller LNG transport and bunkering applications. Their suitability depends on vessel size, cargo volume, operating profile, terminal infrastructure and project-specific design requirements. Pressure, capacity and vapour-handling behaviour differ between installations.
Equipment that converts LNG back into gas.
Regasification facilities use vaporisers that draw heat from seawater, ambient air or a heating medium. Selection depends on site conditions, environmental requirements and project design.
A line returning displaced vapour during cargo transfer.
As liquid enters a tank, vapour is displaced and normally returned to the shore facility or supplying vessel to control pressure. Arrangements depend on the transfer configuration.
A charter for the carriage of cargo on a defined voyage.
Under a voyage charter the owner normally operates the vessel and carries voyage costs, with freight paid for the passage. Terms differ between agreements.
Rotation of a moored floating unit to align with prevailing conditions.
Weathervaning reduces environmental loads on a moored facility and influences transfer arrangements. Behaviour depends on the mooring system and site conditions.
A two-stroke dual-fuel engine using low-pressure gas admission.
X-DF engines admit gas at low pressure and use Otto-cycle combustion in gas mode, which simplifies the fuel gas supply system compared with high-pressure concepts.