Liquefied gas tanker
Since gaseous products occupy considerably less volume in the liquid phase it is far more efficient to carry these cargoes in liquefied form.
Gases can be liquidized by increasing their pressure, reducing their temperature or a combination of both. The requirements for pressurizing and cooling gases are fundamental for the design of gas carriers. For instance, if the cargo is to be carried in a pressurized form at ambient temperature, the containment system must be able to withstand the pressure of the cargo at the highest ambient temperature anticipated.
Liquidizing a substance can be done in two different ways.
- Increase the pressure until the substance becomes liquid at room temperature.
- Decrease the temperature until the substance will become liquid at atmospheric pressure.
Gas liquefaction depends on the properties of the specific gas. Five general types of gas carriers can be distinguished.
- Fully pressurized gas carriers
- Semi-pressurized gas carriers
- Fully refrigerated gas carriers
- Ethylene ships
- LNG ships
Fully pressurized gas carriers tend to be small (cargo capacities up to about 1.000 m3). Gas transported in a fully pressurized gas tanker, is liquefied by pressure only, such pressures ranging up to 17 bars.
Semi-pressurized gas carriers have tanks capable of carrying cargoes at temperatures below ambient. These ships have cargo capacities up to 12.000 m3.
Fully refrigerated gas carriers usually carry LPG at temperatures between -47 °C and -55 °C, depending on the ethane content of the LPG. They may be equipped to carry ammonia and are usually suitable for the full range of hydrocarbons liquefying above -55°C. These are large ships from about 5.000 to 100.000 m3 capacity.
Ethylene ships are designed to carry fully refrigerated cargo at -104 °C. The capacities of these specialized ships tend to be in the range of 1.000 to 12.000 m3.
LNG ships carry their cargo in a fully refrigerated form at -163 °C. These vessels have capacities between 40.000 and 135.000 m3 capacity.
Different tank shapes and sizes are distinguished:
- Membrane tanks
- Semi-membrane tanks
- Independent tanks of type A, B & C.
The hull of the ship supports membrane tanks. The cargo is contained in a primary barrier that is only 0,7 to 1,5 mm thick. A secondary barrier of similar material and thickness is added for security against leaks. Both barriers are separated by insulation. A thicker layer of insulation ‘cradles’ the tank inside the inner hull of the ship, which forms the load bearing structure of the tank.
The semi-membrane tank is a variation on the membrane tank with a thicker primary barrier. Semi-membrane tanks are self-supporting when empty, but when loaded, the dynamic loads are transferred to the hull.
Membrane and semi membrane tanks can only carry substances at (near) atmospheric pressure.
Independent tanks are self-supporting. Pressures inside the tank are not distributed towards the hull and the tanks do not contribute to the hulls strength.
The designation A, B or C indicates the strength of the tank.
Type A tanks are made of materials that are not resistant to ‘crack propagation. To ensure safety, a secondary barrier (outer tank) is added to ensure safety. These types of tanks are used to carry substances at (near) atmospheric pressure.
Type B tanks are made out of materials that have better crack resistance, and design features that make a stronger construction. No full secondary barrier is required, a drip pan being sufficient. These tanks are also designed for (near) atmospheric pressures. They can either be prismatic, cylindrical or spherical.
Type C tanks have design pressure of over 4 bar (g). This tank type is used for semi-pressurized or fully pressurized gas carriers. Type C tanks are always spherical or cylindrical.
Moss tanks are a specific shape of independent tanks. Because these tanks are spherical they are not very space efficient but their shape distributes pressure evenly on the material, making them stronger.
Similar to chemical tankers, the kind of materials that can be carried by a gas tanker is dependent on their type.
Type G1 ships can carry substances with most stringent containment requirements. Distance between the cargo tank and the ship’s side should be at least 1/5 of the vessels breadth (up to 11,5 meters).
Type G2 and Type G3 ships are allowed to carry substances that require less security to prevent leakage of the product to the surroundings.
A liquefied gas carrier is just a ship like any other. The ship’s structure has a double hull like most oil tankers so in the event of a collision or a stranding the ship’s hull is not immediately penetrated. Even if the ship’s hull is breached the tank itself should still be intact.
However unlikely it may be, collision involving a liquefied gas tanker could be very dangerous,, and while there are no special sailing rules for these tankers the crew is, of course, well aware of this danger.
In general, however, refrigerated gas carriers are like any other ship, though the added danger must be recognised
