LNG Cargo Containment Systems

The cryogenic problem the system has to solve

LNG is natural gas, mostly methane, cooled until it condenses. Methane boils at about $-162$ degrees Celsius at one atmosphere, and a loaded LNG carrier holds its cargo near that point, typically quoted as $-162$ to $-163$ degrees Celsius for the mixture. Liquefaction shrinks the gas roughly 600 to 1 by volume, which is the whole reason the trade exists: you can move 600 cubic metres of methane as one cubic metre of liquid. The price of that density is temperature. Bureau Veritas describes the membrane envelope as keeping LNG at about $-160$ degrees Celsius, and the freezing point of the liquid sits near $-182$ degrees Celsius, so the working window is narrow and entirely below the brittle-transition range of ordinary shipbuilding steel.

That single fact drives every material choice. Ordinary carbon steel goes brittle long before it reaches LNG temperature, so the cold faces of the cargo system are built from materials that stay tough in the cryogenic range: 5083-series marine aluminium for Moss spheres, 9% nickel steel for Type C pressure vessels, austenitic 304L stainless steel and Invar nickel-iron alloy for the GTT membranes. The same cryogenic materials problem reappears in the proposed carriage of other liquefied gases at even lower temperatures, such as hydrogen as a marine fuel at about $-253$ degrees Celsius and ammonia at about $-33$ degrees Celsius, where the containment lessons from LNG carry over with different margins. The hull steel never touches the cold; the insulation system stands between the cargo and the inner hull, and a key design check is keeping the inner-hull plating above its own minimum design temperature so it is not asked to do something its steel cannot.

Two architectures dominate the deep-sea fleet. Moss spherical tanks, developed in Norway by what is now Moss Maritime, are independent self-supporting aluminium spheres carried on the hull through cylindrical skirts. GTT membrane systems from Gaztransport & Technigaz of France use a thin metal membrane backed by a layered insulation that transfers cargo weight into the inner hull. IHI SPB prismatic Type B tanks and small-scale Type C pressure vessels fill specific niches. The choice between them is the single most consequential decision in LNG carrier design, setting the boil-off rate, the cargo-flexibility profile, the sloshing behaviour, and the deck silhouette. The fleet has grown from a few hundred carriers in 2010 to well over 700 in deep-sea service by the mid-2020s, plus a growing population of FSRUs (Floating Storage and Regasification Units), FLNG (Floating Liquefied Natural Gas) units, and small-scale bunker vessels, and that growth has pulled guaranteed boil-off rates down from about 0.15% per day on early ships to 0.07 to 0.085% per day on the newest membrane designs.

The IGC Code and how it classifies tanks

The international framework rests on one instrument: the IGC Code, the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk. The 2016 edition, brought in by Resolution MSC.370(93) adopted on 22 May 2014, is mandatory under SOLAS Chapter VII Part C. It covers ship survival and damage stability, the cargo containment system, process pressure relief through the MARVS (Maximum Allowable Relief Valve Setting), cargo-handling equipment, personnel protection, and fire safety. The containment chapters are the part that decides the shape of the ship.

The Code does not pick a design. It sets a classification keyed to how a tank shares load with the hull and how its safety is demonstrated, then ties a secondary-barrier rule to the cargo’s design temperature. The categories are independent tanks Type A, Type B, and Type C, plus membrane tanks. Get the type right and the rest of the rule follows.

Independent Type A tanks are self-supporting and built to recognised standards for plane-surface vessels, essentially shipbuilding practice with conventional stiffening. They carry low-vapour-pressure cargo and they are common on LPG ships. The catch for LNG is the secondary barrier. Where the cargo temperature at atmospheric pressure is below $-10$ degrees Celsius, a Type A tank needs a complete secondary barrier, a second full liquid-tight envelope around the tank. At LNG temperature that means a complete cryogenic second skin, which is heavy and expensive, so Type A tanks are rare for LNG and the LNT A-BOX is one of the few modern attempts to make a complete-barrier prismatic LNG tank competitive.

Independent Type B tanks earn a lighter barrier by proving they will not fail suddenly. The design uses model tests and refined fatigue and fracture analysis to show crack-propagation behaviour, and it satisfies the leak-before-failure principle: a flaw grows slowly and weeps detectable liquid long before it can run as a fast, brittle fracture. Bureau Veritas describes Type B tanks as relying on that principle, which is why they need only a partial secondary barrier, sized to catch the trickle a leak-before-failure crack would produce rather than the whole cargo. In practice that partial barrier is a drip tray and spray shield under the predicted leak zones. The two LNG examples are the Moss spherical tank and the IHI SPB prismatic tank.

Independent Type C tanks are pressure vessels designed to recognised pressure-vessel codes with defined stress and crack-control criteria. Because the design pressure and material selection keep crack initiation and growth low, the Code requires no secondary barrier, as Bureau Veritas puts it, thanks to the limited risk of leakage or structural failure. Type C tanks are cylindrical, bilobe, or trilobe, hold pressure during the voyage, and dominate LPG, ethylene, and small-scale LNG.

Membrane systems are the fourth category and the dominant one for new deep-sea LNG carriers. The cargo load is not carried by the tank; it passes straight through a thin metal primary membrane into the insulation and on into the inner hull. The membrane is the primary barrier, and a second membrane buried in the insulation is the complete secondary barrier the Code demands for a non-self-supporting cryogenic envelope. The membrane is built to expand and contract without buckling, either through corrugations (Mark III) or through Invar’s near-zero thermal expansion (NO96).

Two numbers anchor the whole secondary-barrier scheme. The $-10$ degrees Celsius design-temperature threshold decides whether a complete or only a partial barrier is required, and the barrier itself must be able to contain a tank leak for 15 days, the window the Code allows for detecting the leak and getting the ship to a place where the cargo can be dealt with. For LNG, well below $-10$ degrees Celsius, the rule resolves to: complete barrier for Type A and membrane, partial barrier for Type B, none for Type C.

IACS Unified Requirements put engineering detail behind the Code. The ones that bear on LNG carriers include IACS UR P3 on pressure-vessel testing, UR L3 on the loading manual and instrument, UR R6 on periodical surveys of liquefied-gas carriers, UR Z23 on hull periodical surveys, and UR Z18 on periodical surveys of equipment in dangerous areas.

Class societies, DNV, Lloyd’s Register, ABS, Bureau Veritas, ClassNK, RINA, and KR, implement the Code through their own rules and approve specific containment systems, including the GTT licences and the Moss design. Each carries gas-carrier notations and the in-house fracture-mechanics and sloshing-assessment capability that Type B and membrane approvals depend on.

SIGTTO, the Society of International Gas Tanker and Terminal Operators, publishes the operational guidance the industry runs on, including its principles for liquefied-gas handling on ships and in terminals, guidance for LNG operations in port areas, LNG bunkering, and custody transfer.

Flag and port states layer jurisdiction-specific approach, escort, and operations rules. Export terminals in Qatar, Australia, the United States, Malaysia, and Algeria and import terminals in Japan, China, Korea, India, and across Europe each set their own conditions. US calls add the Coast Guard’s 33 CFR Part 154 on bulk transfer facilities and 46 CFR Part 154 on liquefied-gas carriers.

Moss spherical tanks

The Moss tank is the design that made the LNG sphere a recognisable shipping silhouette. It is an independent self-supporting sphere, classed Type B, developed in Norway in the 1970s and built in large numbers through the steam-turbine era. The defining feature is geometric: a sphere has no corners and no flat panels, so cargo pressure and thermal stress resolve into uniform membrane stress in the shell rather than into bending at a joint. That is what lets the design pass the leak-before-failure case with only a partial secondary barrier.

The shell is welded from 5083-series marine aluminium, the same family used for cryogenic hulls, in plates that grow thicker toward the equator where stress concentrates. Standard deep-sea spheres run roughly 35 to 45 metres in diameter. The sphere does not sit on the hull. It hangs from a single cylindrical aluminium skirt welded to the equator, and the skirt meets the hull at one horizontal ring. That arrangement lets the whole sphere contract by tens of millimetres as it cools to $-163$ degrees Celsius and expand again on warm-up, without dragging the hull with it. A purpose-designed transition piece in the skirt manages the temperature gradient from cold sphere to warm hull steel.

Insulation is sprayed or laid as polyurethane foam over the outside of the sphere, on the order of 200 to 300 millimetres thick, then sealed under a vapour barrier and weather skin. Because the cargo is held just above atmospheric pressure, the boil-off is what the insulation lets through. Modern Moss insulation packages reach about 0.10 to 0.15% per day; early vessels sat nearer the upper end, and the same range frames why owners ordering new tonnage now look to lower-boil-off membrane systems.

The sphere’s strengths are real and specific. It tolerates any filling level, so a Moss carrier can sail part-loaded or with a large heel without a barred-range restriction, the operational freedom membrane ships give up. The geometry spreads any sloshing impact and gives long fatigue life, and the whole shell is open to visual and dye-penetrant inspection from inside and out, which simplifies the five-yearly survey. The partial-barrier Type B status removes the weight and cost of a full cryogenic second skin.

The penalties are equally specific and they are why the type has lost new-build share. Spheres pack poorly into a ship’s beam, so a Moss carrier needs more hull steel and more length for the same cargo volume than a prismatic membrane ship, and several spheres protrude well above the weather deck. Those domes raise the centre of gravity, add windage that complicates berthing in a beam wind, and block the line of sight from the bridge, which is one reason the navigation bridge sits high and forward on these ships. A typical configuration is four or five spheres for 125,000 to 160,000 cubic metres total. The builders associated with the design are the Japanese yards Mitsubishi Heavy Industries, Kawasaki Heavy Industries, and IHI, plus Korean construction under licence.

GTT membrane systems

GTT, Gaztransport & Technigaz of France, holds the two membrane systems that now win most deep-sea LNG orders. They take opposite engineering routes to the same problem of letting a thin metal skin shrink against a warm hull. Mark III absorbs the contraction with corrugations in stainless steel. NO96 dodges it by using a metal that barely contracts at all. Both put two metal membranes and two insulation layers between the cargo and the inner hull, and both are classed as membrane tanks with a complete secondary barrier.

Mark III

Mark III uses a corrugated stainless-steel primary membrane. GTT specifies it as 304L stainless steel, 1.2 millimetres thick, pressed with two ranks of perpendicular corrugations so the sheet can fold and unfold as it cools and warms instead of tearing or buckling. Behind the membrane sits the primary insulation, reinforced polyurethane foam panels at about 130 kilograms per cubic metre. The secondary barrier is a composite membrane (the Triplex laminate of aluminium foil between glass-fibre cloth) laid within the insulation, and a second foam layer separates it from the inner hull. The whole package is prefabricated as standardised panels and bonded in during construction, which is what makes the system scale across yards.

The boil-off rate falls with insulation thickness across the family, and GTT publishes guaranteed figures for a 170,000 cubic metre ship. The original Mark III ran 0.15 down to 0.125% per day at 270 millimetres of insulation. Mark III Flex thickened the package to 400 millimetres for 0.10 down to 0.085% per day. Mark III Flex+ takes total insulation to 480 millimetres for a guaranteed 0.07% per day. Each step buys a lower daily loss at the cost of a little cargo volume, since the insulation grows inward from the inner hull.

NO96

NO96 is the older GTT line and the one that taught the industry the membrane idea. Its primary and secondary membranes are both made of Invar, a 36% nickel-iron alloy, each only 0.7 millimetres thick. Invar’s near-zero coefficient of thermal expansion is the trick: it shrinks so little on cooling to $-163$ degrees Celsius that the membrane needs no corrugations and can be built flat, with the two identical Invar skins giving full redundancy if the primary leaks. The original NO96 carried the membranes on plywood boxes filled with perlite. Later variants swapped the perlite for better insulants: NO96 GW uses glass wool, NO96 L03 uses foam in the boxes.

GTT’s guaranteed boil-off figures for a 170,000 cubic metre ship track the insulation change. NO96 with perlite was about 0.15% per day, NO96 GW about 0.125%, NO96 L03 about 0.11%, and NO96 L03+ about 0.10%. The most recent step, NO96 Super+, received approval in principle in April 2021 and replaces the plywood boxes with reinforced polyurethane foam panels (R-PUF) in both insulation spaces, with glass-wool flat joints, while keeping the double-Invar membranes. GTT guarantees 0.085% per day for a standard 174,000 cubic metre carrier and as low as 0.08% for a 200,000 cubic metre ship, where the larger surface-to-volume ratio helps.

Mark III against NO96

The two systems now sit close on boil-off, so the choice turns on construction and cargo chemistry. Mark III’s corrugated stainless steel is faster to weld and inspect and tolerates a wider range of yard practice; its membrane is thicker and stiffer. NO96’s flat Invar gives a smooth cargo-facing surface and full membrane redundancy, but Invar is a specialised alloy and the welds are exacting. Across the membrane family, the newest variants of both now guarantee boil-off at or below 0.085% per day, against the 0.15% that framed the early fleet.

IHI SPB tanks

The SPB tank, Self-supporting Prismatic-shape IMO type B, is IHI’s answer to the question membrane and Moss each leave half-answered: can you get prismatic volume efficiency and free filling at the same time? It is a self-supporting prismatic aluminium tank, classed Type B on the same leak-before-failure and fracture-mechanics basis as the Moss sphere, so it too needs only a partial secondary barrier. The difference from Moss is the shape. A prismatic tank fills the hull’s box section the way a membrane tank does, with no domes above deck and a low centre of gravity.

The price of the box shape is internal structure. A flat-sided prismatic tank cannot resolve cargo pressure into pure membrane stress the way a sphere can, so the SPB tank carries internal bulkheads and a centreline swash bulkhead. Those members do double duty. They stiffen the tank against the loads a prismatic shell would otherwise see, and they break up the free surface so the cargo cannot build a large sloshing wave. That is the SPB’s distinctive claim: it can sail at any filling level without a barred range, the same operational freedom as Moss, in a hull-efficient prismatic form. The trade is weight, internal welding, and inspection access around the structure.

Very few deep-sea LNG carriers use SPB tanks, well under a dozen, because the membrane systems undercut them on first cost for full-voyage trades. The design has found firmer ground where unrestricted filling matters most: FLNG and FSRU hulls that load and offload continuously and ride out partial fills in a seaway, and some specialised gas carriers.

Type C pressure vessel tanks

Type C tanks take a different bargain with the Code. Instead of proving leak-before-failure to earn a partial barrier, they are built as pressure vessels to recognised pressure-vessel codes, with defined design-pressure margins, weld categories, and crack-control criteria. Bureau Veritas notes that Type C tanks need no secondary barrier at all, thanks to the limited risk of leakage or structural failure that the pressure-vessel design and material selection deliver. The shapes are cylinders, bilobes, and trilobes with dished heads, made of 9% nickel steel or austenitic stainless for LNG-temperature service and carbon steel for warmer cargo such as fully refrigerated LPG.

Holding pressure is the operational point. A Type C tank can let its cargo warm and the pressure rise during a voyage, soaking up boil-off as a pressure increase rather than venting it, until the tank reaches its MARVS. That makes the system well suited to short trades, bunker vessels, and small carriers where a full reliquefaction or gas-handling plant would not pay. The limit is size: a pressure vessel that has to hold several bar gets heavy fast, and the practical ceiling is on the order of a few thousand cubic metres per tank, well below the tens of thousands a single membrane tank holds. Type C dominates small-scale LNG carriers and LNG bunker vessels in the 1,000 to 30,000 cubic metre range, most LPG and ethylene carriers, and the emerging liquefied-CO2 trade, and it is the standard for inland-waterway LNG bunker barges.

Boil-off gas and how it is managed

No insulation is perfect, so heat leaks into the cargo and a little LNG boils to hold the tank at equilibrium. That vapour is boil-off gas (BOG), and it sets a hard limit on how long a cargo survives and how far a ship can run on its own cargo. The daily rate is quoted as a percentage of cargo volume per day. The newest membrane systems guarantee 0.07 to 0.085% per day, modern Moss tanks sit around 0.10 to 0.15%, and the early fleet ran near 0.15%. On a 170,000 cubic metre cargo, 0.10% per day is 170 cubic metres of liquid LNG, roughly 70 to 80 tonnes of methane vapour every day.

The physics is simple to state. The vapour generated is the heat that leaks in divided by the energy it takes to boil the liquid, $\dot{m} = \dot{Q} / h_{fg}$, where $\dot{Q}$ is the heat-ingress rate through the insulation and $h_{fg}$ is the latent heat of vaporisation of LNG, close to 510 kilojoules per kilogram. Push the insulation thicker and $\dot{Q}$ falls, which is exactly what Mark III Flex+ does at 480 millimetres to reach 0.07% per day. The heat itself comes through the insulation, in through tank attachments and the pump tower, and from warm ambient air and seawater, so the design fights it on every face. The same heat-ingress arithmetic underpins the well-to-wake accounting in well-to-wake intensity, because gas that boils off and is burned or vented is gas that left the supply chain.

What the ship does with the BOG is the operational choice.

Burn it as fuel

The dominant modern arrangement sends BOG to the engines. Gard notes that a modern LNG carrier will normally use boil-off to fuel the ship, which folds the gas straight into the vessel’s speed and bunker warranties. Dual-fuel slow-speed two-strokes (MAN B&W ME-GI on the high-pressure Diesel cycle, WinGD X-DF on the low-pressure Otto cycle), dual-fuel four-strokes, and dual-fuel diesel-electric plants all take BOG as primary fuel. The catch is methane slip, unburned methane through the engine, which matters because methane is a far stronger greenhouse gas than CO2 over a 20-year horizon; the methane slip deep dive covers the engine-cycle differences. Burning cargo as fuel is the same principle that makes LNG as a marine fuel attractive on non-gas ships.

Reliquefy it

Some carriers compress and cool the BOG back to liquid and return it to the tanks, so no cargo is lost. Gard confirms that some LNG carriers can reliquefy that boil-off, fully or partly depending on design. The plant is not free: the early Qatari Q-Flex and Q-Max ships use nitrogen reverse-Brayton cycles drawing on the order of 800 kilowatt-hours per tonne of LNG reliquefied, at capacities around 200 tonnes per day, so reliquefaction trades megawatts of electrical load and capital cost against saved cargo. The gas carrier BOG reliquefy calculator works the energy balance.

Flare it as a last resort

When the engines cannot consume the gas and there is no reliquefaction, excess BOG goes to a gas combustion unit that burns it safely. The GCU is sized to clear several times the normal BOG flow so it can absorb a cargo-conditioning surge, but the gas it burns is cargo lost and methane carbon emitted, so it is the fallback, not the plan.

Manage it with tank pressure

BOG is coupled to pressure: raise the tank pressure and the boiling point rises, so less liquid flashes off. The ceiling is the MARVS, the Maximum Allowable Relief Valve Setting the IGC Code fixes for each tank. Atmospheric membrane and Moss tanks run a low MARVS, on the order of 0.25 bar gauge, and rely on burning or reliquefying the gas. Type C pressure vessels run a high MARVS, several bar, and can ride out a voyage by letting pressure climb. The IGC MARVS check and IGC MARVS example calculators work the pressure case.

Cargo Loading and Discharge

LNG cargo operations are highly specialised due to cryogenic temperature and safety requirements.

Cooling Down Operations

Before loading, empty cargo tanks must be cooled down to LNG temperature (-162°C). Cooling proceeds through stages:

1. Inerting with nitrogen to displace air (reducing oxygen below 5%)
2. Initial cool-down with LNG vapour to about -120°C
3. Final cool-down with LNG liquid spray to -162°C

Cool-down typically takes 12-24 hours depending on tank size and temperature differential. Improper cool-down can cause thermal shock damage to insulation.

Loading

Loading is performed via dedicated cargo manifolds with insulated cryogenic loading arms. Loading rates typically:

• Small carriers (10,000-30,000 m³): 1,500-3,000 m³/h
• Medium carriers (90,000-160,000 m³): 8,000-12,000 m³/h
• Large carriers (170,000-180,000 m³): 12,000-15,000 m³/h

During loading, BOG is returned to shore via vapour-return arms, maintaining tank pressure. The LNG loading rate calculator addresses LNG bunker rate considerations parallel to cargo loading.

Discharge

Discharge is essentially the reverse of loading, with cargo pumps (submerged in cargo tanks) pumping LNG to shore. Modern submerged cargo pumps:

• Capacity: 1,000-2,000 cubic metres per hour each
• Multiple per tank (typically 2-3) for redundancy
• Submerged motor design for cryogenic operation

Heel Management

Most LNG carriers retain a “heel” (small quantity of LNG) at end of cargo voyage to:

• Maintain tank temperature for next loading
• Provide BOG fuel for return voyage
• Avoid full warm-up that would require extensive cool-down at next port

Heel management is a balance between commercial cargo (sold at destination) and operational utility (fuel for return).

Tank Cleaning and Inspection Operations

Periodic operations require tanks to be warmed and inspected:

Warm-up: progressive heating from LNG to ambient via inert gas circulation. Takes 3-7 days depending on tank size.

Gas-freeing: replacement of inert gas with air, allowing personnel entry. Atmosphere monitoring throughout.

Tank entry and inspection: by qualified personnel for class society survey, modification, or repair.

Re-inerting and cool-down: reverse of warm-up, taking another 3-7 days.

Major class society surveys typically occur every 5 years and require tank entry. The total period of tank-out-of-service for class survey can be 2-4 weeks.

Sloshing and the barred filling range

Sloshing is the violent motion of a free liquid surface inside a partly filled tank as the ship rolls and pitches. It is the one phenomenon that separates membrane tanks from every self-supporting design, and it is the reason a membrane LNG carrier cannot sail with its tanks half full. A membrane is a thin metal skin glued to insulation; it has almost no capacity to absorb a sharp pressure spike, and a resonant sloshing wave breaking against the tank top or a corner can deliver exactly that. DNV’s Classification Notes No. 30.9 on sloshing analysis of LNG membrane tanks is the long-standing class basis for assessing those loads, and Korean Register, ABS, and the other societies publish their own membrane-sloshing guidance.

The loads peak at intermediate fills. Studies and class guidance put the worst sloshing impacts in membrane tanks at roughly 20 to 40% of tank height, with the single most severe condition near 30%, where the liquid has enough room to build a travelling wave and enough mass to hit hard. The industry’s response is the barred filling range: membrane LNG carriers are restricted to filling either below about 10% of tank height (a heel, essentially empty) or above about 70% (loaded). The band between 10 and 70% is barred for normal sea passages because that is where the sloshing pressure goes high enough to threaten the membrane. In practice that forces a membrane ship to sail either loaded or near-empty, never half-loaded, which is the operational cost of the volumetric efficiency.

The self-supporting designs do not carry that restriction. A Moss sphere has no flat tank top for a wave to slam and its curved shell spreads any impact, so a Moss carrier can sail at any filling level. An SPB tank breaks the free surface with its internal and swash bulkheads, so it too is cleared for unrestricted filling, which is why SPB shows up on FLNG and FSRU hulls that load and offload continuously. Type C pressure vessels are small and partitioned and ride out sloshing without a barred range. So the sloshing question maps almost directly onto cargo flexibility, and for trades that need part-loaded legs it can decide the containment choice on its own.

Cargo Cooling Operations

LNG cargo operations include various cooling-related procedures.

Sub-cooling

LNG can be sub-cooled below its boiling point to reduce BOG generation. Sub-cooling is achieved by:

• Reliquefaction plant operation during voyage
• Spraying cold LNG over warmer LNG
• Reduced tank pressure operation (lowering boiling point temporarily)

Re-condensing

In some operations, BOG can be re-condensed by spraying it through cold liquid LNG, returning the gas to liquid state. This is done in:

• Onboard reliquefaction systems
• Loading operations with vapour return condensing

The tanker cargo cooling LNG calculator addresses cooling calculations specific to LNG.

Heat Leak Management

Tank insulation maintains low heat leak, but external factors affect actual performance:

• Ambient air temperature (tropics vs polar)
• Sea water temperature (affects double bottom heat leak)
• Wind effects (convection cooling on tank surfaces)
• Thermal short-circuits at tank attachments

Modern operational practice monitors tank temperature distribution, BOG generation rate, and heat leak through-put to identify any developing issues.

Cargo Containment System Survey

Class society surveys of cargo containment systems are mandatory at periodic intervals.

Annual surveys: external visual inspection where accessible.

Intermediate surveys: at 2.5-year intervals, with limited internal inspection.

Special periodical surveys: at 5-year intervals, with full inspection including tank entry. Major activities:

• Visual inspection of primary and secondary barriers
• NDT (non-destructive testing) of welds where indicated
• Insulation condition assessment
• Pump tower inspection
• Cargo handling equipment inspection
• Pressure relief valve testing

Continuous Class Survey schemes allow distributed inspection through the survey cycle, with chief engineer maintaining records and class surveyors verifying.

5-year hydrostatic testing of pressure-relief valves and piping systems.

The scope of LNG carrier surveys reflects the cargo’s hazardous nature and the consequence of containment failures. The total period a tank is out of service for a special survey, counting warm-up, gas-freeing, entry, re-inerting, and cool-down, can run two to four weeks.

Operational Considerations

LNG carrier operations have specific considerations beyond general gas carrier operations.

Manning and Training

LNG carrier crews require specific training:

• IMO Model Course 1.04 (Gas Tanker Familiarisation)
• IMO Model Course 1.06 (Advanced Training for Liquefied Gas Tanker Cargo Operations)
• Gas tanker endorsements per STCW Section A-V/1-2

Crew training typically includes:

• Cargo containment system familiarisation
• Cryogenic handling procedures
• Emergency response (release scenarios)
• Cargo gas hazards (flammability, asphyxiation)
• Specific equipment operation

Senior officers (master, chief mate, chief engineer) need years of progressive gas tanker service, often 3 to 5, before a command-level appointment on an LNG carrier.

Emergency Response

LNG release scenarios require specific response:

Cargo tank release: contained by secondary barrier, controlled venting through pressure-relief system, isolation of cargo equipment.

Manifold leak: emergency release coupling (ERC) automatically separates cargo arms, isolating ship from shore facility.

Fire near LNG: water curtains, dry chemical, foam systems all maintained for emergency use.

Cryogenic exposure: medical kit and procedures for cryogenic burns, evacuation routes from cargo areas.

Cargo Documentation

LNG cargo documentation includes:

• Cargo Operational Plan (specific to voyage)
• Cargo Quantity Survey Report (custody transfer)
• Cargo Quality Certificate (composition analysis)
• Boil-off Gas Calculation Sheet
• Bunker Delivery Note (when LNG used as fuel)

LNG custody transfer is technically complex, with cargo quantity calculated from:

• Tank gauge readings (level, temperature)
• LNG composition analysis
• Pressure-temperature corrections
• Standard reference conditions (typically 15°C, 101.325 kPa)

The tanker calibration UTI tape calculator and tanker calibration portable gas meter calculator address calibration and measurement.

Specific LNG Carrier Configurations

Conventional LNG Carriers (130,000-180,000 m³)

The bulk of the LNG fleet is conventional carriers in the 130,000-180,000 cubic metre range. These vessels typically:

• Length 280-300 metres
• Beam 45-50 metres
• Draft 11-12 metres
• Speed 19-20 knots service
• Crew 25-30
• Two-stroke MAN ME-GI or WinGD X-DF dual-fuel main engine

Q-Flex and Q-Max (210,000-265,000 m³)

Qatari-built ships designed for the Qatar-to-major-import-markets trade. Distinguishing features:

• Increased size driven by economies of scale
• Onboard reliquefaction plant (maintaining cargo as liquid for slow-steaming)
• Larger crew accommodation
• High capital cost per ship

FSRU (Floating Storage and Regasification Units)

FSRUs are LNG carriers modified or purpose-built to provide LNG storage and regasification at the import terminal. Features:

• Permanent or semi-permanent moored at import location
• Onboard regasification plant (heat exchangers, etc.)
• Connected to shore gas grid via subsea or surface pipeline
• Provides flexibility avoiding shore-based regasification investment

FLNG (Floating Liquefied Natural Gas)

FLNG units are floating gas processing facilities producing LNG from offshore gas fields. Features:

• Large process plant onboard
• Cargo tanks for short-term storage before shuttle tanker offload
• Often very large (>100,000 m³ cargo, with 200,000+ m³ on largest)
• Operating in dynamic positioning or moored

LNG Bunker Barges and Vessels

Smaller LNG carriers (1,000-30,000 m³) provide bunker fuel to LNG-fuelled vessels. Type C pressure vessel containment dominates.

Future Developments

LNG cargo containment continues to evolve.

Larger Carriers

The ULNGC (Ultra Large Natural Gas Carriers) concept of 250,000-300,000 m³ is technically feasible and being explored.

Reduced Boil-Off

Continued improvement of insulation systems aims for 0.05% per day BOG, reducing operational costs.

Composite Materials

Investigation of composite materials for some non-pressure-bearing structural elements and insulation systems.

Carbon Capture from Cargo Vapour

Onboard carbon capture and storage from BOG combustion is being investigated for low-carbon LNG transport.

Alternative Containment Systems

Research continues on novel containment concepts:

• Glass-fibre composite outer skin
• Aerogel-enhanced insulation
• Active heat-pumping refrigeration

How the choice gets made

Containment selection is a set of linked trades, not a ranking. Membrane systems win most deep-sea orders because they pack the hull, sit below deck, and now guarantee boil-off at 0.07 to 0.085% per day, but they pay for it with the 10-to-70% barred filling range and a membrane that demands exact welding and sloshing assessment. Moss spheres give up hull efficiency and a clean deck line for unrestricted filling, long fatigue life, and a shell anyone can inspect. SPB chases both at once and lands on the niche where free filling justifies the extra structure. Type C trades capacity for the ability to ride out a voyage on rising pressure with no secondary barrier at all. The IGC Code’s tank-type scheme, the $-10$ degrees Celsius barrier threshold, and the 15-day leak rule sit underneath every one of those choices, and the same cryogenic problem, holding methane at $-163$ degrees Celsius across an ocean, drives them all.

Limitations

This article is an encyclopedic overview, not a design or operating manual. Several caveats matter for anyone applying it.

The figures here are guaranteed or typical values for standard reference ships, mostly 170,000 to 174,000 cubic metre carriers. GTT itself states that the realised boil-off rate depends on tank arrangement, reinforcements, ambient and sea conditions, and cargo composition, so a specific ship’s measured rate will differ from the published guarantee. Do not treat a quoted percentage as a contractual or operating value for any particular vessel; the cargo and bunker warranties for a given ship come from its own documents.

The barred filling range, quoted here as roughly 10 to 70% of tank height with the worst loads near 30%, is a generalisation. The actual barred range for a specific membrane ship is set by a ship-specific sloshing assessment to the relevant class rule (for example DNV Classification Notes No. 30.9 or the equivalent ABS, KR, or BV procedure), and it varies with tank size, internal geometry, and the approved sea states. Always use the vessel’s own loading manual and approved filling limits.

The IGC tank-type definitions and secondary-barrier rules are summarised in plain language. The binding text is Resolution MSC.370(93) as amended and the SOLAS Chapter VII framework, read with the relevant IACS Unified Requirements and the classing society’s rules. Where this article says “about” or “on the order of” for a thickness, diameter, or pressure, the exact value is design-specific and set by the approved drawings. Material grades, weld categories, and inspection scopes follow the approved specification and the surveying society, not a generic figure.

Operational practice, cool-down times, survey intervals, and emergency procedures, varies by operator, flag state, terminal, and the specific containment system, and is governed by the ship’s safety management system, the relevant SIGTTO guidance, and STCW gas-tanker training requirements. This article does not substitute for any of those.

Related Calculators

• Gas Carrier BOG Reliquefy Calculator
• IGC MARVS Check Calculator
• IGC MARVS Example Calculator
• Tanker Cargo Cooling LNG Calculator
• Tanker Bunkering LNG Calculator
• Tanker Calibration UTI Tape Calculator
• Tanker Calibration Portable Gas Meter Calculator
• IGC Argon Liquefied Calculator
• IGC Carbon Dioxide Liquefied Calculator
• IGC Hydrogen Liquefied Calculator
• IGC Nitrogen Liquefied Calculator

Additional calculators:

• IGC Code Tank Types A / B / C / Membrane
• IGC: Methane (LNG)
• LNG Tank - Secondary Barrier Requirement
• IGC: Ammonia

Additional formula references:

• Lng Boil Off Rate Tank

Additional related wiki articles:

• IGC Code: Construction of Gas Carriers

See also

• LNG Carrier
• LNG as Marine Fuel
• LNG Fuel System
• Methane Slip Deep Dive
• Well-to-Wake Intensity
• Hydrogen as Marine Fuel
• Ammonia Marine Engines Overview
• Marine Inert Gas Systems
• Marine Cargo Tank Heating Systems
• Marine Cargo Pumps and Piping
• Marine Tank Gauging Systems
• Chemical Tanker
• IMDG Class 2 Gases

References

• IMO International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code 2014, Resolution MSC.370(93) and amendments)
• SOLAS Chapter VII Part C (Carriage of Liquefied Gases in Bulk)
• IACS Unified Requirement P3, Pressure vessel testing
• IACS Unified Requirement R6, Periodical surveys of liquefied gas carriers
• IACS Unified Requirement Z23, Hull periodical surveys for liquefied gas carriers
• SIGTTO Liquefied Gas Handling Principles on Ships and in Terminals (4th edition, 2016)
• SIGTTO LNG Operations in Port Areas
• SIGTTO LNG Bunkering Operations
• DNV Rules for Classification of Ships, Pt 5 Ch 7 Liquefied Gas Tankers
• ABS Guide for Building and Classing Liquefied Gas Carriers
• Lloyd’s Register Rules and Regulations for the Classification of Ships, Pt 7 Ships of Special Service
• USCG 33 CFR Part 154 (Facilities Transferring Oil or Hazardous Material in Bulk)
• USCG 46 CFR Part 154 (Liquefied Gas Carriers)