LPG carrier operations

LPG (propane, butane and their mixtures) can be carried at ambient temperature under pressure, at atmospheric pressure fully refrigerated, or in an intermediate semi-refrigerated condition — the containment system and the cargo-handling procedure both follow from which regime a given ship is built for. The governing instrument is the IGC Code, mandatory under SOLAS Chapter VII Part C, supplemented by SIGTTO (Society of International Gas Tanker and Terminal Operators) operational guidance.

The three containment + handling regimes

Fully pressurised

Cargo is kept at ambient temperature and contained in Type C pressure-vessel tanks strong enough to hold the cargo’s saturated vapour pressure at ambient temperature. No secondary barrier is required because the tank itself is built to full pressure-vessel standards. Typical of smaller coastal LPG carriers and gas-carrier newbuilds serving short-sea trades.

Semi-refrigerated

Cargo is partially cooled and carried at an intermediate pressure below full ambient vapour pressure, usually in Type C tanks rated for a lower design pressure than a fully pressurised ship. This regime gives an operator flexibility to load either refrigerated cargo from a terminal’s refrigerated tanks or pressurised cargo, adjusting onboard reliquefaction plant duty accordingly.

Fully refrigerated

Cargo is cooled to close to its atmospheric boiling point (around −42°C for propane, close to 0°C for butane) and carried at near-atmospheric pressure in large prismatic Type A tanks with a partial secondary barrier. This is the regime used by Very Large Gas Carriers (VLGCs) moving LPG in bulk on long-haul export trades, where atmospheric-pressure tanks give far greater cargo volume per unit of steel than a pressure-vessel design would allow.

Cargo conditioning + cooldown before loading

Before a refrigerated or semi-refrigerated ship can load, cargo tanks and lines that are at ambient temperature must be cooled down gradually using a spray of the cargo itself (or a similar hydrocarbon). Cooling too quickly risks thermal shock — differential contraction between the tank structure, lines and pumps that can crack welds or damage pump seals. The cooldown rate is planned and monitored against temperature limits in the ship’s cargo manual, and cargo cannot be bulk-loaded until the tank has reached a temperature close enough to the incoming cargo’s temperature to avoid excessive flash-off on arrival.

Gassing-up, inerting + purging sequence

Moving a tank from a gas-free, air-filled condition to a cargo-ready condition (or the reverse, before dry-docking or a change of cargo grade) follows a fixed sequence to avoid ever holding a flammable hydrocarbon/air mixture in the tank:

Inerting. Air is displaced from the tank with inert gas (nitrogen, generated on board or supplied from shore) until oxygen content is reduced to the safe operating level.
Gassing-up. The inert atmosphere is then displaced with cargo vapour, again keeping the mixture out of the flammable range throughout, until the tank atmosphere is effectively pure cargo vapour.
Cooldown. With the tank in a cargo-vapour atmosphere, cargo spray cooldown proceeds as above.
Bulk loading. Liquid cargo is loaded once the tank is cold enough and the vapour return line to the terminal is established to manage displaced vapour.

The reverse sequence — purging cargo vapour out with inert gas, then inert gas out with fresh air — is used before opening a tank for inspection or repair. See gas-freeing + atmosphere testing for the enclosed-space entry standard that follows.

Boil-off gas + reliquefaction

Heat ingress through tank insulation continuously vaporises a small fraction of a refrigerated LPG cargo. Most LPG carriers carry a reliquefaction plant that compresses and cools the boil-off gas back into liquid and returns it to the cargo tank, keeping tank pressure within design limits without venting cargo vapour. Where boil-off exceeds what reliquefaction can handle, the vapour is burned in a gas combustion unit rather than vented to atmosphere.

Rollover risk

Rollover is a hazard where two layers of liquefied gas of different density and composition stratify within the same tank — typically after topping up a partially discharged refrigerated tank with cargo of a different composition or temperature. If the layers later reach similar density, they can mix suddenly, releasing a large, rapid burst of vapour that can overwhelm the tank’s venting capacity. Cargo officers avoid rollover conditions by checking compatibility and density/temperature of the incoming cargo against the heel already in the tank before topping up, and by keeping tanks well mixed or fully discharged between dissimilar parcels.

Primary sources

· IGC Code (International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk) — IMO, mandatory under SOLAS Chapter VII Part C
· SIGTTO (Society of International Gas Tanker and Terminal Operators) — Liquefied Gas Handling Principles on Ships and in Terminals