Transporting liquefied gases across the sea is a highly technical job. These gases, such as LNG (liquefied natural gas), LPG (liquefied petroleum gas), ammonia, ethylene, and others, must be transported at very low temperatures, at high pressure, or sometimes both. To make long-distance transport safe, ships such as cargo hold tanks or gas carriers are designed with specialised tanks, materials, and advanced cooling systems.
What is a cargo tanker?
A cargo tanker is a large ship specially built to carry liquid goods in bulk across long distances. These liquids can be crude oil, petrol, chemicals, liquefied gas, or even edible oils. The ship has several sealed tanks that keep the liquids safe, stable, and separated during travel.
Different types of cargo tankers
The five most common types of cargo tanks are:
Full-pressurised
Full-pressurised tankers carry cargo in cylindrical tanks that withstand very high internal pressure, usually up to 18 bar, eliminating the need for refrigeration. These vessels use strong steel shells, deep-set supports and safety valves to manage pressure during long voyages.
Semi-pressurised
Semi-pressurised tankers combine moderate pressure capability with refrigeration. They use independent, insulated tanks fitted with compressors, reliquefaction units and accurate temperature gauges. The design supports multiple loading temperatures, making cargo switching easier.
Fully refrigerated
Fully refrigerated tankers transport cargo at very low temperatures in large, prismatic tanks constructed from low-temperature steel. These ships rely on heavy insulation, continuous cooling systems and temperature-monitoring instruments. The box-shaped tanks fit closely within the hull, offering a larger capacity with stable weight distribution.
Ethylene capable
Ethylene-capable tankers can carry extremely low-boiling-point cargo such as ethylene and similar petrochemical gases. They use advanced, dual-purpose tanks made from specialised stainless steel that tolerates cryogenic conditions. Powerful cascade refrigeration plants maintain temperatures as low as minus 104 degrees Celsius.
LNG membrane
LNG membrane tankers utilise thin, flexible membrane systems supported by the inner hull, rather than independent tanks. The membrane, often made of specialised nickel steel alloys, sits behind multilayer insulation that minimises boil-off. This design enhances cargo capacity while maintaining manageable ship dimensions. Sensors track sloshing, temperature and stress patterns, while reliquefaction systems reduce vapour loss.
Understanding cargo tank designs and construction
When it comes to modern cargo oil tanks, they mainly use three designs.
- Type A tanks are prismatic and not built to hold pressure.
- Type B tanks may be spherical or prismatic and are subject to stricter safety assessments.
- Type C tanks are fully pressurised and usually cylindrical or bilobed in shape.
These tanks are made from high-grade steel or aluminium alloys that can handle both strength demands and cryogenic conditions. The insulation materials in tanks, such as polyurethane foam or perlite, help reduce heat entry and maintain the cargo at the optimal temperature. Features such as stiffening rings, swash bulkheads, and support saddles help the tanks remain stable and resist sloshing or stress. All designs must follow the standards set by the International Gas Carrier (IGC) Code.
The reliquefaction process in cargo tankers
The reliquefaction process in a cargo tank ship is detailed below:
- Step 1: Vapour return lines, fitted with pressure-rated valves, collect rising boil-off gas from the tank surface to maintain internal pressure within safe operating limits.
- Step 2: Low-pressure compressors equipped with moisture traps raise the vapour pressure to a controlled level, creating a stable flow for downstream cooling operations.
- Step 3: Dryers fitted with desiccant beds remove water molecules from the vapour stream to protect refrigeration equipment from ice formation during deep cooling.
- Step 4: Heat exchangers using seawater or glycol reduce the vapour temperature to an intermediate level to prepare the gas for efficient high-grade refrigeration.
- Step 5: Refrigeration units fitted with multi-stage compressors and condensers cool the dried vapour until it condenses to allow liquid formation under controlled pressure.
- Step 6: Cold surfaces in the condenser promote stable liquid formation as the gas cools below its saturation temperature, creating cargo-grade liquefied product.
- Step 7: Subcoolers recover residual cold energy from the fresh liquid to improve refrigeration efficiency and reduce the energy load on the main cooling system.
- Step 8: Insulated return lines, fitted with flow-control valves, send the reliquefied cargo back to the storage tank without temperature shocks or pressure disruptions.
- Step 9: Pressure controllers maintain safe tank conditions by adjusting vapour removal and liquid return rates to ensure steady cargo behaviour throughout operations.
- Step 10: Integrated sensors track temperature, pressure, liquid level, and compressor load, allowing the crew to maintain process stability and prevent equipment stress.
Importance of cargo tanker insurance
A cargo tanker marine insurance policy protects shipowners from heavy financial losses caused by accidents, machinery breakdown, cargo damage, pollution, and collisions. Tankers carry high-value cargo that can create huge liabilities if something goes wrong. Insurance supports compliance with global maritime rules, helps cover clean-up costs, and offers protection against third-party claims. It also shields operators from risks linked to storms, operational mistakes, and equipment failure.
Note: This is an indicative list. Please read the policy wordings for the complete list of inclusions/exclusions.
Conclusion
Cargo tankers are essential for global energy transportation. They use specialised tank types, such as Type A, Type B, and Type C, to handle gases stored at distinct pressures and temperatures. Their effective insulation keeps these gases stable during long voyages. At the same time, reliquefaction systems help manage pressure and reduce cargo loss by cooling any vapour back into liquid form, keeping the cargo safe and secure throughout the journey. Since these ships face high operational risks, suitable single transit insurance adds a layer of financial protection for both the vessel and its valuable cargo.
FAQs
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What materials are used to build cargo tanks?
Cargo tanks are usually made of high-grade steel or aluminium alloys. These materials handle very low temperatures and resist cracking. Some tanks also use special welds and insulation to keep cargo safe during long voyages.
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What is boil-off gas (BOG)?
Boil-off gas forms when liquefied gas warms slightly and evaporates. Instead of wasting it, ships either reliquefy it, burn it as fuel, or release it safely, depending on the type of tank and voyage requirements.
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What are independent cargo tanks?
Independent tanks are self-supporting and not part of the ship’s hull. Types A, B, and C all fall under this category. They offer strong containment and are used where pressure or special shapes are needed.
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How is pressure controlled inside cargo tanks?
Pressure is controlled using venting systems, compressors, reliquefaction plants, and temperature management.
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What is an inert gas and why is it used?
An inert gas, such as nitrogen, removes oxygen from cargo tanks, preventing fires or explosions. It forms a safe atmosphere during cargo discharge, cleaning, or when tanks are empty but still contain vapours.
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