Types of Gas Carriers as per IGC Code

The International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code) sets standards for gas carriers. It covers ship classification and lays out essential safety requirements for transporting liquefied gases.

The IGC Code, currently enforced under the 2016 edition with amendments from MSC.370(93), is adopted under Chapter VII of SOLAS and administered by the IMO.

Classification of Gas Carriers

Gas carriers are divided into categories based on their design and the gases they transport. The IGC Code lists ship types like type 1G, type 2G, and type 3G.

Each type is made for specific cargoes and can handle different pressures and temperatures. For example:

• Type 1G ships carry gases with high-risk factors.
• Type 2G ships handle moderate-risk gases.
• Type 3G ships transport lower-risk gases.

Design features, like tank material and insulation, play a big role in meeting these standards.

Carrier Type	Tank Type	Pressure Range	Temperature Range	Common Cargoes
Fully Pressurized	Type C	Up to 17 bar	Ambient	LPG, Propylene
Fully Refrigerated	Type A/B	~1 bar	-45°C to -55°C	LPG, Ammonia
Semi-Pressurized/Refrig.	Bi-lobe, C-type	5–7 bar	-10°C to -48°C	Ethylene, VCM
LNG (Membrane)	GTT No96/Mark III	~1 bar	-160°C	LNG
LNG (Moss)	Type B	~1 bar	-160°C	LNG

Key Requirements of the IGC Code

The IGC Code spells out requirements for gas carriers to keep operations safe and protect the environment. Ships use independent tanks—Type ‘A’, Type ‘B’, and Type ‘C’—depending on the cargo’s pressure and temperature needs.

On top of that, the code calls for safety features: monitoring systems, emergency shutdowns, proper ventilation, temperature control, and fire protection. All these measures help minimize risks during transport.

Cargo handling systems must include vapor return lines, high-level alarms, remote-controlled valves, and ESD (Emergency Shutdown) systems to meet operational and safety requirements defined in Part C of the IGC Code.

Independent Tank Types

Independent tanks in gas carriers are purpose-built to safely move liquefied gases. They follow the International Gas Carrier (IGC) Code, which sets standards for structural integrity and safety. Let’s look at Type A and Type B independent tanks.

Type A Independent Tanks

Type A tanks are prismatic, built with flat surfaces, and usually carry cargoes with low vapor pressure. They’re designed to withstand internal pressures and often have a secondary barrier to catch leaks.

According to the IGC Code and Type A tanks, these tanks need a partial secondary barrier since they’re not the best at preventing leaks. Designers typically add insulation to control temperature and stop heat from getting in. Steel is a common material, and the tanks rest on strong supports.

Type B Independent Tanks

Type B tanks are either spherical or cylindrical and can handle higher pressures than Type A. Moss Rosenberg developed these, and a full secondary barrier is standard for extra safety. This makes them a good fit for carrying LNG and similar cargoes.

The Moss type design uses advanced engineering, like fatigue analysis, to boost durability and reliability. These tanks sit inside the hull and use thick insulation to keep cargo cold. The IGC Code requires strict testing for Type B tanks to make sure they’re up to scratch.

Type C Independent Tanks

Type C independent tanks work as pressure vessels, letting ships safely transport liquefied gases under high pressure. You’ll find these tanks in fully pressurized gas carriers.

Design and Construction Features

Type C tanks stand out for their pressure-resistant design, often spherical or cylindrical. They can handle internal pressures above 2 bar. High-tensile steel gives them the needed strength.

These tanks use a double-hull design, which helps prevent leaks. Their construction follows pressure vessel codes, so they’re reliable for long trips. Advanced insulation keeps the gas cold and stops it from vaporizing. The other tank types in the IGC Code have different builds and uses.

Application in Fully Pressurized Carriers

Fully pressurized carriers choose Type C tanks because they’re great for gases needing higher pressure containment. Their thick walls mean refrigeration isn’t necessary—the pressure alone keeps the gas liquid. This design reduces capital and operational costs by eliminating refrigeration systems.

They’re common for shipping liquefied petroleum gas (LPG) and similar products. Strong pressure capabilities let them transport a broad range of gases, giving operators more options. Their design stays in line with international safety standards.

Membrane Tanks

Membrane tanks are frequently used in gas carriers due to their compactness and efficiency. Thin, flexible membranes supported by insulation attach to the inside of the ship’s hull. GTT Membrane Tank Technology—think GTT No96 and GTT Mark III—dominates this field.

GTT Membrane Tank Technology

GTT membrane systems optimize tank volume and thermal insulation to minimize boil-off losses. Flexible barriers hold the liquefied gas, while insulation or plywood provides structure.

This setup helps minimize thermal loss. Vapor barriers add another layer of safety, enhance leak protection, and maintain structural integrity under dynamic sea conditions.

GTT No96

GTT No96 uses two thin Invar membranes, a nickel-steel alloy, with two insulation layers around each. The primary membrane keeps liquid and vapor inside. The secondary membrane backs it up in case something goes wrong. Provides low boil-off rates, improving containment efficiency and cargo preservation.

The insulation system typically includes perlite or polyurethane foam, with thermal conductivity below 0.025 W/m·K, limiting boil-off gas to less than 0.15% of cargo per day.

GTT Mark III

The GTT Mark III system is a cryogenic membrane containment solution designed for transporting liquefied natural gas (LNG) at atmospheric pressure and -163°C. It uses a primary corrugated stainless steel membrane (0.7 mm thick) to contain the LNG. This membrane is directly supported by a prefabricated insulation panel, which integrates a composite secondary barrier to ensure full redundancy in case of primary failure.

The insulation consists of two layers, combining reinforced polyurethane foam and plywood, reducing thermal conductivity and mechanical stress. The design enables flexible adaptation to different ship sizes while maintaining structural efficiency.

Mark III systems are engineered to limit boil-off gas (BOG) to approximately 0.10–0.15% of cargo volume per day. By minimizing heat ingress and maximizing insulation performance, the system ensures optimal cargo retention and operational safety over long voyages.

GTT’s Mark III Flex and Mark III Flex+ variants further enhance performance by increasing insulation thickness, achieving BOG rates as low as 0.07%/day, contributing to greater fuel efficiency and environmental compliance.

Modern LNG carriers include boil-off gas (BOG) management systems, such as reliquefaction units or gas combustion units (GCUs), to prevent tank overpressure and maintain LNG quality during long voyages.

Moss Type Spherical Tanks

Moss-type spherical tanks pop up often in gas carriers because of their unique design and safety features. People call them MOSS tanks, and their spherical shape gives them serious structural strength under pressure.

Design Features:

• Shape: The sphere spreads stress out evenly, so the risk of failure drops.
• Material: Usually, builders use special alloys or high-strength steel.

Moss tanks don’t depend on the hull for support. This independence boosts safety.

Benefits of Moss Type Tanks:

• Durability: The spherical shape stands up to high stress and pressure swings.
• Safety: Reduces liquid sloshing, enhancing cargo stability and minimizing dynamic load impact during transit.
• Easy Maintenance: Their shape makes maintenance less of a hassle.

Application in Ships:

Moss-type tanks are predominantly installed on LNG carriers with spherical tank designs. They are widely used in LNG carriers due to their structural stability and thermal insulation performance.

Vessels like the “Arctic Princess” class LNG carriers use Moss-type spherical tanks, combining high durability with operational flexibility for global LNG transport.

Drawbacks:

• Space Usage: The shape isn’t the most efficient for packing in cargo.
• Cost: Building these tanks can get pricey compared to other options.

Fully Refrigerated Gas Carriers

Fully refrigerated gas carriers move gases at low temperatures, letting them stay in liquid form. These ships keep pressure close to atmospheric levels. They’re built for substances like liquefied petroleum gas (LPG) and ammonia.

Design and Construction

Most of these vessels use big, prismatic tanks made from materials like special carbon-manganese steel that handle cold and stress. This construction lets them safely move gases at temperatures down to -48°C.

Capacity and Efficiency

These ships can carry a lot—anywhere from 15,000 to 85,000 cubic meters. Loading and unloading are pretty efficient, and transportation costs stay low, so they’re a go-to for bulk transport.

Safety Features

Includes ESD systems, fixed gas detection, and approved fire suppression in accordance with IGC Code Part C.

Operational Areas

They mostly run on long-haul routes, moving gases from production sites to big distribution terminals. These ships play a major role in the global trade of liquefied gases, keeping supply chains moving for industries everywhere.

These vessels must also comply with MARPOL Annex VI emission limits and energy efficiency regulations such as the Energy Efficiency Design Index (EEDI) for gas carriers.

Fully Pressurized Gas Carriers

Fully pressurized gas carriers haul liquefied gases like propane and butane at ambient temperatures. These ships use tanks that come in cylindrical, spherical, or sometimes horseshoe shapes.

The tanks can handle pressures up to 17 bar. Since they’re so sturdy, you don’t need refrigeration systems out at sea.

Each tank stands alone as its structure. Builders usually go with steel that resists the chemicals inside. This material choice ensures corrosion resistance and chemical compatibility.

Key Features:

• Pressure Range: Up to 17 bar
• Cargo Types: Propane, Butane
• Tank Shapes: Cylindrical, Spherical

Fully pressurized gas carriers utilize simpler containment systems suitable for short-haul transport. These ships omit refrigeration and complex insulation systems, relying on tank pressure containment.

Tank materials usually include carbon steel or nickel steel, depending on what gas they’re carrying. That choice helps prevent corrosion and keeps things safe while moving cargo.

For technical specifications and design parameters, refer to the IGC Code and classification society guidelines.

Semi-Pressurized and Refrigerated Gas Carriers

Semi-pressurized and refrigerated gas carriers are pretty adaptable ships. They’re built to move gases that demand specific temperature and pressure conditions.

These ships typically combine pressure and cooling to maintain cargo safety. Maintaining dual control over pressure and refrigeration requires integrated tank and cargo handling systems.

Tank Types:

• Cylindrical Tanks: These tanks fit right inside the ship’s hull. Cylindrical tanks offer efficient hull integration and stable structural support.
• Spherical Tanks: Their geometry supports uniform stress distribution under variable loading conditions.
• Bi-lobe Tanks: These maximize capacity by squeezing into available space. It’s a smart use of every inch.

Liquefied petroleum gas (LPG) and ethylene are under regulated pressure-temperature conditions.

Design Features:

• Pressure Range: Most of these vessels work between 5 and 7 bar. Not too high, not too low.
• Temperature Control: They use built-in refrigeration systems to keep gases at just the right temperature.

Key Advantages:

• Flexibility: These carriers handle all sorts of cargoes. That makes them popular for a reason.
• Safety: Crews rely on advanced safety systems to protect themselves and the environment. Ensures operational safety in compliance with IGC Code requirements.

For more information, refer to the IGC Code and classification society guidelines. Curious about the different tank types? Consult the IGC Code technical appendices or the classification society design catalogs.