Shipping: frequently asked questions

How big is a standard LNG carrier?#

The modern standard is 174,000 cubic metres of cargo, roughly 300 metres long. Older conventional ships are nearer 145,000, and the largest Q-Max vessels hold about 266,000.

What do Q-Flex and Q-Max mean?#

They are Qatari classes: Q-Flex around 210,000 to 217,000 cubic metres, Q-Max around 263,000 to 266,000. Q-Max was sized to the largest ship the loading terminal could handle.

How much LNG is in a full cargo?#

A 174,000 cubic metre cargo is roughly 78,000 tonnes of LNG, using a density near 450 kilograms per cubic metre, which is about 100 million cubic metres of gas once regasified.

What engines do LNG carriers use?#

Older ships use steam turbines, which burn boil-off easily. Modern ships use two-stroke dual-fuel engines such as ME-GI, ME-GA and X-DF, or dual-fuel diesel-electric plants.

How is the cargo pumped off?#

By submerged motor pumps sitting inside the tanks, where pump and motor are one sealed unit in the liquid, so there is no shaft seal for cargo to leak through.

From LNG carrier anatomy.

What is the difference between a membrane and a Moss tank?#

A membrane is a thin metal barrier, around a millimetre thick, that transfers the cargo weight through insulation into the ship's hull. A Moss tank is a self-supporting aluminium sphere that carries its own load and rests on a skirt, independent of the hull.

Why did membrane systems win?#

Because they fill the hull's rectangular space far more efficiently. A membrane ship carries more cargo for the same overall dimensions, which matters for canal fees, berth limits and cost per tonne shipped.

Are Moss tanks obsolete?#

Not obsolete, but rarely ordered for new conventional carriers. Their robustness, tolerance of partial filling and visible tank tops still suit some floating storage and specialised applications.

What is an SPB tank?#

A self-supporting prismatic IMO Type B tank, developed by IHI. It is free-standing like a Moss sphere but box-shaped, so it uses hull space efficiently, and internal bulkheads suppress sloshing.

Who licenses membrane technology?#

GTT of France licenses the Mark III and NO96 families used across most of the modern fleet. KC LNG Tech, formed by KOGAS with three Korean yards, licenses the alternative KC-1 system.

From Membrane versus Moss tanks.

What is a Moss sphere?#

A free-standing spherical aluminium cargo tank supported at its equator by a cylindrical skirt, with the upper half projecting above the weather deck. It carries its own loads, so the hull does not have to support the cargo.

Why did membrane tanks displace Moss spheres?#

Volume and cost. Spheres pack badly into a ship-shaped hull, so a Moss vessel needs a larger, more expensive hull for the same cargo, and the exposed domes add windage and restrict visibility from the bridge.

What advantage do free-standing tanks keep?#

They can be filled to any level. A membrane tank at partial fill risks sloshing damage, while a sphere or a bulkheaded prismatic tank does not, which matters for floating storage, bunkering and any vessel that loads and discharges in parts.

What is an SPB tank?#

A self-supporting prismatic tank of IMO Type B, developed by IHI, with internal bulkheads that suppress sloshing. It fits the hull far more efficiently than a sphere and leaves a flat weather deck.

Are Moss carriers still being built?#

Very few. The design is rare in new conventional carrier orders, though the existing ships continue trading and the concept persists where robustness and partial filling outweigh volume efficiency.

From Containment II: Moss spheres and SPB tanks.

What is Invar and why is it used in LNG tanks?#

Invar is an iron-nickel alloy, roughly 36 per cent nickel, whose thermal expansion is about a tenth that of ordinary steel. In the NO96 containment system it lets the membrane cool by 180 degrees without needing to accommodate much movement at all.

How does the Mark III system handle contraction instead?#

With geometry rather than metallurgy. Its stainless steel membrane is corrugated in two directions, so the corrugations flex as the sheet cools and the material itself can be an ordinary cryogenic stainless.

How thick is the insulation on an LNG carrier?#

Typically somewhere between about 270 and 400 millimetres of layered material, depending on the system and the boil-off rate the owner specified. More insulation means less cargo volume, so the thickness is a commercial decision as much as a technical one.

Why does an LNG tank need a secondary barrier?#

Because a leak through the primary membrane must not reach the hull, where cryogenic liquid would embrittle the steel. The secondary barrier contains any leak long enough for it to be detected and the ship to reach port.

Does better insulation eliminate boil-off?#

No. It reduces the rate but cannot stop heat entering entirely, and the boil-off is what keeps the remaining cargo cold. Modern designs have brought daily rates from around 0.15 per cent to below 0.09, but not to zero.

From Insulation, Invar and holding the cold chain.

What is boil-off gas?#

The vapour formed when a small part of an LNG cargo evaporates because heat leaks through the insulation. It is unavoidable, and it is what holds the remaining liquid at its boiling point.

What is a typical boil-off rate?#

Modern carriers are usually quoted around 0.1 per cent of cargo volume a day, older tonnage nearer 0.15 per cent. Rates rise in warm water and when tanks are partly filled.

Is boil-off wasted?#

Rarely on a modern ship. It is burned as fuel in the main engines or reliquefied and returned to the tanks. Venting is an emergency measure, not routine practice.

What is a reliquefaction plant?#

Machinery aboard the ship that re-chills boil-off vapour into liquid and returns it to the cargo tanks, so the cargo delivered is closer to the cargo loaded.

Why does a ship keep a heel of cargo?#

Because the tanks must stay cold. A small quantity is retained after discharge so the tanks are still cryogenic when the ship reaches the next loading port, avoiding a slow and costly cool-down.

From Boil-off and reliquefaction.

What is a reliquefaction plant on an LNG carrier?#

A refrigeration unit, usually a closed nitrogen cycle, that takes boil-off gas from the tanks, cools it back below its boiling point and returns it as liquid. It preserves cargo at the cost of several megawatts of electrical power.

Why not fit reliquefaction to every ship?#

Because it only pays when the cargo saved is worth more than the fuel burned to save it, and it adds capital cost, weight, complexity and another system to maintain. On ships that burn boil-off usefully in the engines, the calculation frequently goes the other way.

What is a subcooler or partial reliquefaction system?#

A smaller unit that chills boil-off below its boiling point and returns some of it, reducing cargo loss rather than eliminating it. It suits two-stroke ships that generate more boil-off than the engines need at low speed.

What happens to boil-off that is neither burned nor reliquefied?#

It goes to a gas combustion unit and is burned without producing useful work. That is a straight loss, and it exists because venting methane to atmosphere is far worse.

Which ships have full reliquefaction?#

Historically the largest Qatari vessels, which were built with slow-speed diesel engines that could not consume boil-off, so the cargo had to be preserved rather than used. Most conventional carriers use their boil-off as fuel instead.

From Reliquefaction, subcooling and when they pay.

Why did LNG carriers use steam turbines for so long?#

Because a boiler will burn boil-off gas and fuel oil in any ratio without modification, which no other machinery of the era could do. The cargo produced vapour continuously and steam simply absorbed whatever arrived.

How efficient is a steam turbine LNG carrier?#

Around 28 to 30 per cent thermal efficiency, against roughly 42 per cent for dual-fuel diesel electric and about 50 per cent for a modern two-stroke gas engine. It is by far the least efficient option still afloat.

Are steam turbine LNG carriers still being built?#

Essentially no. New orders have used dual-fuel or two-stroke gas machinery for years, though a substantial number of steam ships remain in service, often on long-term charters or as floating storage.

What advantages did steam have besides fuel flexibility?#

Very high reliability, low maintenance demand, a long service life, and no need to dispose of surplus boil-off separately. Steam plants also tolerate poor fuel quality that would damage a diesel engine.

Why did steam finally lose?#

Fuel cost. When boil-off stopped being treated as a nuisance to be consumed and started being valued as cargo, burning it at 30 per cent efficiency became indefensible, and the alternatives had matured.

From Propulsion I: steam turbines and why they endured.

What does DFDE stand for?#

Dual-fuel diesel electric. Medium-speed engines run on gas or liquid fuel and generate electricity, which drives electric motors connected to the propeller. TFDE, tri-fuel, adds heavy fuel oil as a third option.

How much more efficient is DFDE than steam?#

Roughly 42 per cent thermal efficiency against 28 to 30 for steam, so approximately a third less fuel for the same work. That gain is what displaced steam from new orders in the mid-2000s.

Why generate electricity rather than drive the shaft directly?#

Because it decouples engine speed from propeller speed and lets the ship run only as many engines as the load needs, each at its efficient point. It also frees the machinery layout, since the engines need not be in line with the shaft.

What is the main disadvantage of dual-fuel engines?#

Methane slip. These engines burn a lean premixed charge on the Otto cycle, and a small fraction of the gas escapes unburned into the exhaust, which matters disproportionately because methane is a potent greenhouse gas.

Are DFDE ships still being ordered?#

Rarely for conventional carriers, where two-stroke gas engines are now standard. The architecture remains common on floating units and vessels that need large electrical loads for onboard processes.

From Propulsion II: DFDE and TFDE, the dual-fuel era.

What is the difference between ME-GI and X-DF?#

ME-GI injects gas at high pressure late in the compression stroke and burns it on the diesel cycle, which needs a high-pressure fuel gas compressor but produces very little unburned methane. X-DF admits gas at low pressure and burns a premixed lean charge on the Otto cycle, which avoids the compressor but slips more methane.

How efficient are two-stroke gas engines?#

Around 50 per cent thermal efficiency, against roughly 42 per cent for dual-fuel diesel electric and 28 to 30 for steam. They are the most efficient propulsion in commercial marine use.

Which one has lower methane slip?#

ME-GI, by a wide margin, because gas is injected after the air is already compressed and hot, so it burns as a diffusion flame rather than as a premixed charge that can hide in crevices. X-DF designs have narrowed the gap with exhaust recirculation but not closed it.

Why do two-stroke ships need a subcooler?#

Because at low load or in port the engines consume less gas than the tanks produce. The surplus has to be reliquefied, disposed of in a gas combustion unit, or the cargo loses value.

Do these engines still burn liquid fuel?#

Yes. Both run on conventional marine fuel when gas is unavailable, and both use a small pilot injection of liquid fuel to ignite the gas, so a liquid fuel system is always present.

From Propulsion III: ME-GI, X-DF and two-stroke gas injection.

How big is a standard LNG carrier?#

Around 174,000 cubic metres of cargo capacity, which is the current conventional newbuilding size. That is roughly 78,000 tonnes of LNG, or about 100 million cubic metres of gas once regasified.

What is a Q-Max carrier?#

The largest class of LNG carrier, around 266,000 cubic metres, built for Qatar's export projects in the 2000s. Q-Flex vessels are the slightly smaller sibling at roughly 210,000 to 217,000 cubic metres.

Can the largest LNG carriers use the Panama Canal?#

No. Q-Flex and Q-Max vessels are too wide for even the expanded locks, so they trade Qatar to Europe and Asia on routes that avoid Panama entirely. Conventional 174,000 cubic metre ships do fit.

Why did the industry settle on 174,000 cubic metres?#

Because it is close to the largest size that most receiving terminals can accept and that fits the expanded Panama Canal. The constraint is shore infrastructure, not shipbuilding.

What are small-scale LNG carriers used for?#

Serving terminals too small or too shallow for a conventional cargo, breaking a full cargo into parcels for island and coastal markets, and bunkering ships that run on LNG as fuel.

From Ship classes: conventional, Q-Flex, Q-Max and small-scale.

What does Arc7 mean?#

The highest ice class commonly applied to LNG carriers under the Russian Maritime Register system, denoting a vessel able to operate independently in substantial first-year ice rather than needing an icebreaker escort throughout.

What is a double-acting ship?#

A vessel designed to run bow-first in open water for efficiency and stern-first through heavy ice, where the hull form and the propulsion pods are arranged to break it. It avoids compromising open-water performance for ice capability.

Is the Northern Sea Route open all year?#

No. Eastbound transits are seasonal, historically concentrated in the summer and autumn months, though the window has lengthened. Outside it, Arctic cargoes go west or transship, which changes voyage length dramatically.

Why do ice-class carriers cost so much more?#

A strengthened hull, far more installed power, azimuth propulsion pods and winterised systems throughout. The vessels are also built in small series at few yards, so there is little of the cost learning that a long production run brings.

Does ice class affect cargo capacity?#

Yes, indirectly. Structure and machinery take space and weight, so an ice-class vessel typically carries somewhat less cargo than a conventional ship of similar dimensions.

From Ice-class carriers and the Arctic routes.

What is a ballast voyage?#

The return leg with the cargo discharged, carrying only a small heel of LNG and seawater ballast for stability. The ship earns nothing on it, but it is roughly half the round trip, so freight rates have to cover both legs.

What is heel in LNG shipping?#

The quantity of LNG deliberately left in the tanks after discharge, typically a low single-digit percentage of capacity. Its boil-off keeps the tanks cold on the ballast leg so the ship can load immediately on arrival.

What happens if a ship arrives warm?#

It has to be cooled down before loading, by spraying LNG into the tanks over several hours. If the tanks have warmed fully, the ship must also be inerted and gassed up first, which takes days and consumes cargo.

Why not carry more heel to be safe?#

Because heel is cargo that was paid for and is not delivered. Carrying more than needed is a direct loss, so the quantity is calculated for the specific voyage length, weather and the ship's boil-off rate.

How long is a typical LNG round trip?#

Highly route-dependent. Qatar to north-west Europe is roughly two weeks laden; the US Gulf to Japan around three weeks each way via Panama and considerably longer around the Cape. Add loading, discharge and waiting, and a round trip is commonly five to nine weeks.

From The voyage cycle: laden, ballast, heel and the return.

Why can't a warm LNG tank simply be filled with LNG?#

Two reasons. Air in the tank would form a flammable mixture with methane, and thermal shock from liquid at minus 162 hitting warm steel risks damaging the containment. The tank must be inerted, gassed up and chilled gradually first.

What is gassing up?#

Replacing the inert gas in a tank with methane vapour. Inert gas from a shipboard generator contains carbon dioxide, which would freeze solid at cargo temperature and block valves and instruments, so it must be displaced before cooling begins.

How long does the whole sequence take?#

On a ship coming out of dry dock with fully warm tanks, typically one to two days for drying, inerting, gassing up and cooldown combined. A ship arriving with heel aboard needs only a short cooldown of a few hours.

What is thermal shock and why does it matter?#

Cooling one part of a structure much faster than another sets up stresses between them. A membrane is a thin skin attached to insulation and hull, and cooling it too quickly can damage the attachment, so cooldown rates are limited and monitored.

Where does the LNG for cooldown come from?#

Either the ship's own heel, the shore terminal, or another vessel. It is consumed rather than delivered, which is one reason unscheduled warm-ups are expensive.

From Cooldown, gassing up and the first cargo.

What is measured during LNG custody transfer?#

Liquid volume from tank gauges, liquid temperature from sensors at several heights, vapour temperature and pressure, and cargo composition from a gas chromatograph. Energy is then calculated rather than measured directly.

Why is LNG sold by energy rather than volume?#

Because the energy in a cubic metre varies with composition by several per cent. A buyer is purchasing heat, not liquid, so the contract price is per MMBtu and the volume is only an input to the calculation.

How accurate is the measurement?#

Typically within a fraction of a per cent overall. On a cargo worth tens of millions of dollars, even that fraction is a large number, which is why the procedures are so prescriptive and why both parties attend.

Who performs the measurement?#

An independent surveyor, usually appointed jointly, working alongside ship and terminal staff. Both sides witness the gauging and sampling, and disputes are handled under procedures the contract sets out in advance.

What is a custody transfer measurement system?#

The certified instrumentation aboard the vessel — level gauges, temperature sensors and pressure instruments — that has been calibrated and approved for use in commercial measurement rather than merely for operational monitoring.

From Custody transfer: gauging, and who owns the error.

Why transfer LNG between ships instead of using a terminal?#

Because sometimes there is no terminal, or the ship that can reach the cargo cannot make the long voyage. Arctic projects use ice-class vessels for the ice leg and transfer to conventional carriers for the ocean leg.

Is ship-to-ship transfer safe?#

It is a routine operation under established industry guidance, with a compatibility study, a joint plan, linked emergency shutdown systems and defined weather limits. The risks are mooring and manoeuvring rather than the cargo itself.

What is a floating storage unit?#

A moored vessel used purely to hold LNG, with regasification either absent or performed elsewhere. Many are older carriers at the end of their trading lives, kept in service for their tanks rather than their engines.

Can any two LNG ships transfer to each other?#

No. Manifold height and spacing, mooring arrangements, fender requirements and emergency shutdown systems all have to be compatible, and a study confirms it before the vessels ever meet.

What limits when a transfer can happen?#

Weather, principally. Two large vessels moored alongside each other in open water move independently, and wind, swell and current limits are set in advance and enforced, with the operation stopped if conditions approach them.

From Ship-to-ship transfer and floating storage.

What is an LNG loading arm?#

An articulated, insulated steel pipe assembly on the jetty that connects to the ship's manifold. Arms move with the vessel as it loads and the tide changes, and carry couplings designed to part cleanly in an emergency.

What is ESD in LNG transfer?#

Emergency shutdown. The first stage stops the pumps and closes valves on both ship and shore within seconds. The second stage releases the loading arms if the vessel moves beyond safe limits, parting the connection without spilling significant cargo.

Why can't every LNG ship use every LNG terminal?#

Because compatibility is physical. Berth length, water depth, manifold height above the waterline, spacing between connections, mooring arrangements and maximum transfer rate all have to match, and a study confirms it before a first call.

How long does loading a cargo take?#

Roughly twelve hours of pumping for a conventional cargo at typical rates, plus connecting, purging, cooling the arms and completing the measurement. The whole port call is usually closer to a day.

What is the ship-shore link?#

The communication connection between the vessel and the terminal that carries the shutdown signal and operational data. It exists so that either side can stop the transfer instantly rather than by voice over a radio.

From The ship–shore interface: arms, ESD and compatibility.

What is a time charter?#

Hiring a vessel with its crew for a period, from months to twenty years, at an agreed daily rate. The owner supplies and operates the ship; the charterer decides where it goes and pays for the fuel.

What is a spot charter?#

Hiring a vessel for a single voyage at whatever the market rate is at the time. It gives flexibility and costs a premium when ships are scarce, which they frequently are.

Why do LNG freight rates swing so violently?#

Because supply cannot respond quickly. The fleet is small, a new carrier takes years to build at one of a handful of yards, and demand shifts with trade patterns and weather. Rates have moved by an order of magnitude within a single year.

What is tolling, and is it the same as chartering?#

No. Tolling is a liquefaction arrangement where a customer supplies its own gas and pays a fee to have it liquefied, keeping title throughout. It is often paired with a separate charter, which is why the two are discussed together.

Who pays for fuel on a chartered LNG carrier?#

Under a time charter, the charterer. That includes the boil-off consumed as fuel, which is why the vessel's boil-off rate is a negotiated specification and not merely a technical detail.

From Chartering: spot, time charter and tolling.

How are LNG freight rates quoted?#

In US dollars per day, for a defined vessel specification on a defined route. Assessments are published by shipbrokers and price reporting agencies for standard Atlantic and Pacific voyages.

What is time charter equivalent?#

The daily earnings a voyage produces after deducting voyage costs, used to compare a single-voyage fixture with a period charter on the same basis. It is how owners judge whether a spot voyage beat the market.

How far do LNG freight rates actually move?#

A very long way. Rates have run from well under thirty thousand dollars a day in slack periods to several hundred thousand at the tightest, which is a range no other major shipping sector routinely sees.

Why does a canal closure raise rates without any ship leaving the fleet?#

Because a longer route means each ship completes fewer round trips a year, so the same fleet delivers fewer cargoes. Effective capacity falls even though vessel numbers are unchanged.

Are freight rates seasonal?#

Strongly. Northern hemisphere winter demand pulls more cargoes over longer distances, and the tightest rates of the year are usually in the fourth quarter.

From What freight rates mean and why they swing.

Which countries build LNG carriers?#

Overwhelmingly South Korea and China, with Japan retaining a smaller role. Russia has a yard for its own Arctic projects, and European and American yards have essentially left the trade.

Why can so few yards build them?#

Because installing a cargo containment system is specialist work. A membrane tank means hundreds of thousands of precise welds in a cold, confined space, and yards need years and licensor approval to do it reliably.

How long is the queue for a new LNG carrier?#

Slots at the main yards have run several years ahead through the recent ordering waves, so a new order typically means delivery three or more years out.

Do the shipyards make the engines and tanks too?#

Partly. Korean groups build their own marine engines under licence from MAN and WinGD, but the containment system is licensed from GTT or KC LNG Tech, and much cargo machinery is bought in.

Does a change of yard ownership change the ship?#

No. Yards are renamed and sold, which is why a vessel's recorded builder may use a name the yard no longer trades under. We fold those names together so a yard's record stays in one place.

From Who builds LNG ships.

What qualifications do LNG carrier officers need?#

Beyond ordinary certificates of competency, they need basic and then advanced gas tanker training under the STCW convention, plus documented seagoing service on gas carriers. The service requirement is the part that cannot be shortened.

Why is there a shortage of qualified LNG officers?#

Because the advanced endorsement requires time aboard gas carriers, and that time can only be accumulated on the existing fleet. When the fleet grows quickly, demand for officers rises faster than the pool can be trained.

What is a cargo engineer?#

A senior officer responsible for the cargo plant — containment monitoring, boil-off handling, reliquefaction where fitted, and the transfer operations at each end. On LNG carriers this is a specialist role rather than a general engineering duty.

How does simulator training fit in?#

It covers the operations that are rare, slow or dangerous to practise for real, particularly cooldown, gassing up and emergency shutdown. It supplements sea time rather than substituting for the endorsement requirement.

Do the crew handle the cargo differently from an oil tanker crew?#

Substantially. The cargo is cryogenic and its vapour is the ship's fuel, so cargo condition is continuously managed rather than merely contained, and mistakes damage the ship rather than just spilling the cargo.

From Crewing and competency on a gas carrier.

How is LNG delivered to a ship as fuel?#

Three ways. Ship to ship from a dedicated bunker vessel, which is the method for large vessels; truck to ship from road tankers for smaller volumes; and shore to ship by pipeline where a terminal has been built for it.

Why do ships use LNG as fuel at all?#

Because it eliminates sulphur oxide emissions and most particulates, and cuts nitrogen oxides substantially, which matters in emission control areas. Whether it reduces greenhouse gas emissions depends entirely on how much methane escapes unburned.

What is the IGF Code?#

The international code governing ships using gases or other low-flashpoint fuels, covering fuel tanks, piping, machinery spaces and bunkering arrangements. It is to gas-fuelled ships what the IGC Code is to gas carriers.

Is LNG bunkering the same business as LNG trade?#

No. The volumes are far smaller, the customers are shipowners rather than utilities, deliveries are frequent and small, and the infrastructure is bunker vessels and trucks rather than terminals and carriers.

Does LNG as marine fuel reduce emissions?#

It clearly reduces sulphur, particulates and nitrogen oxides. The greenhouse case is contested, because unburned methane escaping through the engine can offset a large part of the carbon dioxide saving, and how much depends on the engine type.

From LNG as marine fuel: bunkering and the supply chain.