Hazards of Carriage of Liquefied Gas in Bulk and Construction of a Prismatic Cargo Tank

1. Flammability and Explosion
   • Liquefied gases such as LPG vaporise rapidly when released.
   • The vapour can mix with air and form a highly flammable or explosive atmosphere, creating a serious risk of fire or explosion.
2. Toxicity
   • Some liquefied gas cargoes, such as ammonia and vinyl chloride monomer, are highly toxic.
   • Exposure can cause serious poisoning through inhalation, ingestion or skin absorption.
3. Asphyxiation
   • LPG vapours are generally heavier than air.
   • In the event of a leak, the vapour can collect in low-lying areas such as pump rooms and hold spaces, displacing oxygen and creating an asphyxiation hazard.
4. Frostbite and Cold Burns
   • Liquefied gas cargoes are carried at very low temperatures, with fully refrigerated LPG being carried at approximately −50°C.
   • Contact with the liquid cargo or uninsulated pipes and equipment can cause severe frostbite and cold burns.
5. Brittle Fracture
   • Ordinary ship hull steel, such as mild steel, can become brittle at very low temperatures.
   • If cold cargo leaks and comes into contact with unsuitable hull steel, it may cause cracking and catastrophic structural failure.
6. Sloshing
   • Partially filled tanks have a free surface, allowing the liquid cargo to move violently with the ship's motion.
   • This can produce large dynamic impact loads on the tank walls and internal pump towers, potentially causing structural damage.

Fully refrigerated LPG is typically carried in Independent Type A prismatic cargo tanks. These are self-supporting tanks that are independent of the ship's hull structure and do not contribute to the overall structural strength of the vessel.

Sketch – Typical Prismatic Cargo Tank Arrangement

Ship's Hull / Hold Space

┌─────────────────────────────────────┐

│ Secondary Barrier / Hull │

│ ┌─────────────────────────────┐ │

│ │ Thermal Insulation │ │

│ │ ┌───────────────────────┐ │ │

│ │ │ │ │ │

│ │ │ LPG CARGO │ │ │

│ │ │ PRISMATIC TANK │ │ │

│ │ │ │ │ │

│ │ └───────────────────────┘ │ │

│ │ Primary Barrier │ │

│ └─────────────────────────────┘ │

│ Load-bearing supports │

└───────────────┬─────────────────────┘

│

Double Bottom

1. Shape and Structure

• The tank is prismatic, meaning it is generally box-shaped with chamfered/angled top and bottom corners.
• This shape allows the tank to closely follow the contours of the ship's inner hull and therefore maximises the available cargo capacity.
• The tank is internally reinforced with frames and stiffeners to maintain structural integrity.
• A centreline longitudinal bulkhead, together with transverse wash bulkheads, may be provided to strengthen the tank and reduce liquid movement and sloshing.

2. Materials

• LPG is carried at low temperatures, approximately −50°C for fully refrigerated LPG.
• To prevent brittle fracture at these temperatures, the primary barrier/tank is constructed from suitable low-temperature-resistant materials, typically fine-grained carbon-manganese steel.

3. Tank Supports and Chocks

The cargo tank operates at a substantially different temperature from the ship's hull and therefore undergoes thermal expansion and contraction.

• The tank is supported on the double bottom by load-bearing insulation blocks.
• These may be made from specialised hardwood such as Azobé or suitable synthetic materials.
• The supports provide the necessary load-bearing capacity while reducing thermal transfer and structural stresses.
• Anti-roll, anti-pitch and anti-flotation keys/chocks secure the tank against movement caused by the ship's motion.
• At the same time, the arrangement allows the tank to expand and contract freely due to temperature changes.

4. Insulation

• The outside of the primary barrier is provided with high-efficiency thermal insulation.
• Its purpose is to maintain the required low cargo temperature and minimise heat ingress and cargo boil-off.
• Sprayed polyurethane foam (PUF) is commonly used as the insulation material.

5. Secondary Barrier

• Under the IGC Code, Type A tanks are required to have a complete secondary barrier capable of containing leaked cargo for up to 15 days, preventing the cold cargo from coming into contact with the outer hull.
• In LPG carriers, the ship's inner hull is normally used as the secondary barrier.
• The inner hull is also constructed from suitable fine-grained low-temperature steel so that it can withstand the low temperature if the primary cargo tank fails.
• The space between the primary tank and secondary barrier, known as the hold space, is maintained with inert gas or dry air as applicable.