LNG carrier operations

LNG carriers are the most technically demanding + commercially valuable segment of the merchant fleet. Cargo is transported at ~-162°C in specialised cryogenic tanks. Every officer + rating on an LNG carrier holds STCW V/1-2 advanced liquefied-gas-tanker training. Ranks command a premium of 40-80% over comparable oil-tanker rates. This reference covers the essential technical + career knowledge for LNG operations.

Tank types

Moss (spherical, Kvaerner-Moss)

Distinctive spherical aluminium tanks visible above deck. Free-standing self-supporting design — no dependence on hull structure. Excellent structural integrity + minimal sloshing. Design was market-leading in the 1980s-90s. Modern Moss ships remain in service but new orders have shifted to membrane. Advantage: partial-loading is allowed at any filling level — makes short-haul LNG bunkering + FSRU operations easier. Cargo capacity ~135,000-155,000 m3 typical.

GTT Membrane (No.96 + Mark III / Mark III Flex)

Dominant modern tank type. GTT (Gaztransport & Technigaz) designs a thin cryogenic membrane liner supported by the hull structure — much higher cargo capacity for the same overall ship dimensions. No.96 uses Invar (36% nickel steel) primary membrane; Mark III / Mark III Flex uses corrugated stainless-steel primary membrane. Recent orders (~90% of new-build LNG carriers 2018-2025) are Mark III Flex. Requires strict partial-loading restrictions (sloshing damage risk between 10-70% filling — must be avoided at sea).

SPB (Self-supporting Prismatic type-B)

Japanese IHI-developed prismatic-shape self-supporting tank. Very small share of the fleet (~10 ships). Advantages: flat deck (better for LNG-bunkering + regasification retrofit) + partial-loading allowed. Disadvantages: higher build cost + shorter market history.

Fleet size classes

Class	Capacity (m3)	Typical LOA	Route archetype
Small-scale / bunkering	1,000 – 30,000	80 – 130 m	Regional distribution + LNG-bunkering
Conventional (post-2000)	125,000 – 180,000	280 – 300 m	Middle East / US / Australia to Asia / Europe
Q-Flex	210,000 – 217,000	315 m	Qatargas dedicated deployment
Q-Max	263,000 – 266,000	345 m	Qatargas dedicated deployment
Ultra-large (post-2025)	271,000	350 m	QatarEnergy 2024-2027 delivery series

Cargo containment + boil-off gas (BOG)

LNG is stored at atmospheric pressure at -162°C. Heat ingress through the tank insulation continuously vaporises a portion of the cargo — the "boil-off gas" (BOG). Typical BOG rate ~0.10-0.15% per day for modern carriers. BOG management is the defining operational challenge:

Fuel-consumption route. Modern dual-fuel propulsion burns the BOG in the main engines — the fleet-wide standard since ~2015. See propulsion section below.
Reliquefaction plant. Some conventional carriers carry a reliquefaction plant that recompresses + cools the BOG back into the cargo tank — preserves cargo mass. Standard on Q-Max + Q-Flex.
Gas combustion unit (GCU). Emergency BOG disposal — burns excess BOG in an inert atmosphere. Every LNG carrier carries a GCU.
Cool-down + gassing-up. Before loading a new cargo, tanks must be cooled from ambient to ~-140°C using LNG spray + circulated vapour. A cool-down error can crack the membrane — highest-risk operation on an LNG ship. Executed by the Cargo Officer + monitored by the Chief Officer + Master.

Propulsion evolution

Steam turbine. Historical standard through the 1990s + 2000s. Steam boilers dual-fired on BOG + fuel oil. Simple + reliable but thermally inefficient (~30%). Only pre-2010 build remaining in service.
DFDE / TFDE (Dual-Fuel Diesel-Electric / Tri-Fuel Diesel-Electric). Medium-speed dual-fuel engines drive generators → electric podded propulsion. Adopted heavily 2005-2015. ~35-40% thermal efficiency.
ME-GI (MAN Energy Solutions Gas Injection). Direct-injection two-stroke slow-speed dual-fuel main engine. High-pressure (~300 bar) natural-gas injection. Very high thermal efficiency (~50%). Standard for modern deliveries.
XDF (WinGD X-DF). Otto-cycle low-pressure (~16 bar) dual-fuel two-stroke. Simpler + less costly than ME-GI. Growing market share. Watch for methane-slip regulation impact from IMO GHG Fuel Standard 2027-2028.

Officer + crew — STCW V/1-2 advanced

Every officer + rating on an LNG carrier holds STCW V/1-2 Advanced Liquefied Gas Tanker training — a specialised 5-day course over + above STCW V/1-1 Basic Tanker training. Master + Chief Officer + Chief Engineer + Cargo Officer additionally hold Ship-type-specific training + typically a company LNG certification. Rank + salary implications:

Master. ~$18-25k USD/month (2025). Advanced STCW V/1-2 + LNG cargo handling course + company-specific LNG certification + typically 3+ years LNG Chief Officer experience.
Chief Officer. ~$13-17k USD/month. Runs cargo operations. Cool-down + gassing-up + partial-loading decisions.
2nd Officer / Cargo Officer. ~$8-11k USD/month. Watchkeeping + cargo tank monitoring.
Chief Engineer. ~$16-22k USD/month. Dual-fuel propulsion + reliquefaction plant + GCU + cryogenic pump maintenance expertise.
Bosun / Pumpman / OS+AB. ~$3-5k USD/month for ratings — 40-80% premium over conventional tanker ratings.

Regulatory framework

IGC Code (International Gas Carrier Code). Mandatory under SOLAS Ch. VII Part C for gas carriers built after 1 July 1986. Detailed tank design + cargo containment + fire safety + electrical + machinery requirements. Substantially amended MSC.370(93) + MSC.441(99) for post-2016 build.
SIGTTO (Society of International Gas Tanker & Terminal Operators). The industry standards-setting body. Publishes the gold-standard best-practice guidance (Liquefied Gas Handling Principles on Ships + in Terminals — the "yellow book").
STCW V/1-2 Advanced Liquefied Gas Tanker. Individual advanced training certificate. Company + flag-state administer.
SOLAS Ch. II-2 (fire safety) enhanced. LNG carriers carry higher-specification fixed-firefighting + hazard-area classification + explosion-proof equipment throughout the cargo area.
IMO GHG Fuel Standard (2027). Methane slip from XDF + ME-GI propulsion will face progressively stricter WtW GHG-intensity limits. See /reference/mepc-83-ghg-fuel-standard.

Notable operators

Concentration is high — the top 5 operators control >50% of the fleet. Major fleets: QatarEnergy(Q-Max + Q-Flex + 2024-2027 ultra-large series — largest single-flag LNG fleet), Nakilat(Qatari fleet manager, ~70 LNG carriers), MOL LNG (Mitsui O.S.K. Lines, ~90 LNG carriers),NYK Line (~80 LNG carriers), K-Line (~50), Maran Gas Maritime(Angelicoussis Group ~40), Dynagas / Golar / Flex LNG / GasLog (Greek + Norwegian independent owners), Shell Shipping & Maritime (Shell in-house fleet + long-term charters). Emerging: COSCO + CMES (China State Shipbuilding-linked) building the Chinese LNG fleet.

Career pathway considerations

Transition from oil / product tanker. The typical LNG conversion path — 3-6 months preparation + STCW V/1-2 advanced course + company-specific familiarisation training. Sponsorship generally required (the training + repositioning cost is not something a seafarer typically self-funds).
Long charter contracts. LNG operators run stable long-term time-charters (10-25 years) — crew rotation + career progression is more predictable than spot-market bulk / container work.
Shore progression. Post-sea LNG expertise commands high shore-side salaries — LNG-project shore-support roles (technical superintendent, cargo consultant, LNG-bunkering operations manager) pay well + open a broad career-map beyond fleet management.
Emerging dual-fuel + alternative-fuel operations. LNG carrier experience is now the transferable skill for ammonia + methanol dual-fuel operations — see /reference/methanol-ammonia-dual-fuel.