Toroidal LPG Tanks - Vapor Tanks - 1-Hole Multivale LPG ...

Toroidal LPG Tanks - Vapor Tanks - 1-Hole Multivale LPG ...

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Getting to Know Liquefied Gas Carriers

Previously, we discussed different types of tankers. Here, we'll delve into the types and design of liquified gas carrier vessels.

The need for alternative fuel has increased the significance of ships designed to carry liquefied gas. There are primarily two types of these carriers:

1. LPG (Liquefied Petroleum Gas) Carriers
2. LNG (Liquefied Natural Gas) Carriers

Liquefied Petroleum Gas (LPG)

LPG is comprised of petroleum hydrocarbon products like Propane and Butane, often mixed, and is found extensively both domestically and industrially. The key feature of LPG is its ability to be pressurized into liquid form for transport. However, for safe transport, certain conditions related to pressure and temperature must be met. These include:

• Pressurization at ambient temperature
• Fully refrigerating the gas at its boiling point (-30 to -48 degree Celsius)
• Semi-refrigerating to a reduced temperature and pressurizing

We will examine how these conditions affect the design of different types of LPG tankers.

Gasses like ammonia, ethylene, and propylene are also transported in LPG carriers. Notably, ethylene has a lower boiling point (-104 degree Celsius) and must be carried under semi-refrigerated or fully-refrigerated conditions.

Liquefied Natural Gas (LNG)

LNG is purified natural gas cooled to its boiling point (-165 degree Celsius) just at or near atmospheric pressure. Unlike LPG, LNG isn’t pressurized much above atmospheric levels during transportation. This makes LNG carriers' designs different from LPG carriers.

Design of Various Gas Carriers

Gas carriers are categorized primarily by their cargo containment system, notably into four types:

Integral Tanks

Integral tanks are part of the ship’s structure and influenced by the hull. They're used when LPG is at near-atmospheric conditions. No significant expansion or contraction is required.

Independent Tanks

Self-supporting and not integral to the hull, independent tanks don't affect the hull structure's overall strength. According to IGC Code, they have three types:

Type 'A' Tanks

Designed using traditional ship design methods, these tanks carry LPG under near-atmospheric conditions or LNG. They must have a secondary barrier to contain leaks for at least 15 days.

Type 'B' Tanks

Structured to detect cracks before total failure, these tanks are examined for stress, fatigue, and crack propagation. Most common is the Kvaerner-Moss Spherical tank, usually carrying LNG.

Contact us to discuss your requirements of CNG Tube Skids. Our experienced sales team can help you identify the options that best suit your needs.

• These tanks use space between the inner and outer hull for ballast and cargo protection against collisions.
• The spherical shape evenly distributes stress, minimizing fracture risks.
• The 'Leak before Failure' concept ensures progressive failure, allowing crack detection before ultimate failure.

Type 'C' Tanks

Cryogenic pressure vessels designed with conventional pressure codes, focusing on vapor pressure. They don't require secondary barriers, making them common in both LPG and LNG carriers.

Membrane Tanks

Membrane tanks are non-self-supporting structures with thin membrane barriers supported by hull insulation. They compensate for thermal expansion and contraction via the inner hull. They primarily store LNG.

There are usually two layered systems from Technigaz and Gaz Transport, incorporating stainless steel or Invar membranes.

LPG Containment Systems

LPG containment systems vary from LNG and are thus classified into:

Fully Pressurized Tanks

Transport Propane, Butane, and Anhydrous ammonia in uninsulated cylindrical tanks below the main deck. Capacity is under 2000 cubic meters.

Semi-Pressurized or Semi-Refrigerated Tanks

Carrying similar cargo to fully-pressurized tanks, these larger (5000 cubic meters) ships use insulated, independent Type C tanks with refrigeration plants.

Fully Refrigerated Tanks

These carriers have capacities of 10,000 to 100,000 cubic meters, using Type ‘A’ prismatic tanks for LPG at low temperatures, built from notch ductile steel.

As the demand for alternative fuel grows, so does the need for efficient gas carriers. Research into more efficient hull designs and advanced cargo containment systems is ongoing, offering ample opportunities for innovation in the industry.

The future of gas carrier ships promises numerous advancements in energy efficiency, hull design, and containment systems, suggesting an evolving landscape for shipbuilders and designers.

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