1. Flammability / explosive hazard: the vapours are highly flammable and, mixed with air in the flammable range (between the lower and upper flammable limits), can be ignited by any ignition source causing fire or explosion.
2. Extreme cold / cryogenic hazard: LPG is carried near -50 deg C boiling point; spillage causes severe cold burns (frostbite), embrittlement and fracture of steel structures and deck fittings, and condensation of moisture.
3. Asphyxiation / toxicity: heavier-than-air gas displaces oxygen, causing oxygen deficiency and asphyxiation in enclosed and low-lying spaces; some gases are toxic or anaesthetic.
4. Pressure hazard: liquefied gas develops high vapour pressure as temperature rises; over-pressure can rupture tanks, piping and relief valves; uncontrolled vapour release produces a BLEVE (boiling liquid expanding vapour explosion) risk.
5. Heavy vapour cloud / dispersion risk: because the vapour is denser than air it flows along the deck and water in low patterns, travelling and finding ignition sources at a distance, and can accumulate in enclosed spaces, bilges and accommodation.
6. Equipment/materials failure and corrosion: leakage from cargo piping, valves and flanges due to thermal contraction, corrosion and incompatible materials, plus the risk of liquid carry-over into pipelines and machinery spaces causing damage.

(Also re-condensation, static electricity during loading). These require gas-tight equipment, gas detection, venting, cool-treatment and exclusion of ignition sources.

The tank is a prismatic (rectangular) self-supported tank built from cryogenic nickel-steel or aluminium plates, carried in the hull in a hold. Sketch: - a rectangular tank of flat or corrugated plate, supported on a mild-steel/chock support at the bottom (load-bearing), with vertical side hoppers and roofs, a dome at the top carrying cargo and pressure/vacuum valves, an access hatch and level gauging.

Features:

• It is designed as a self-supporting (type A or B) prismatic tank, adequately insulated externally (polyurethane foam panels) to keep the cargo cold and to reduce boil-off.
• The tank bottom transfers loads through tank-support (wood/mild-steel) to seatings on the hull; the tank top is a dome through which pipes and vent lines pass. A drip tray / insulation and gas-tight secondary barrier is often fitted such that any leakage is contained.
• The cargo is carried at atmospheric or slightly above pressure, and a relief valve (safety valve), pressure-vacuum valve, gas detection, and remote-operated valves are fitted.
• The tank structure uses corrugated internal stiffening to withstand liquid sloshing and cryogenic temperature, with careful welded joints in cryogenic steel of notch-tough grade.
• Venting systems, dry-pipe vent risers and flame arresters prevent accumulation of vapour; the tank is connected to the piping, with high-level and emergency shut-down (ESD) systems.

The description should be accompanied by a sketch showing the tank seatings, insulation, dome, and secondary barrier. (Note: LPG carriers often use semi-pressurised or refrigerated prismatic or cylindrical tanks.)