LPG Carriers

For large quantities of gas to be carried over long distances, refrigeration tends to be the most economical method of transportation because of the saving of weight and cost by the reduction in steel needed for the tanks.  The larger LPG carriers are usually fully refrigerated but the smaller ones may be fully refrigerated or part refrigerated and part pressurised.  LPG liquefies at a higher temperature than LNG.  Butane liquefies at minus 14.5o celsius and propane at minus 48.4o celsius.  There is less tendency for LPG carriers to be dedicated ships and there is a much more general market for LPG carriers and they may carry other cargoes besides butane and propane. The gas used for inerting the tanks of both LNG and LPG carriers is nitrogen not fluegas, which would not be pure enough for these cargoes.  The shore may supply the initial nitrogen blanket for the cargo and this is topped up on the voyage by the vessel. With any form of cargo the owner has a claim for deadfreight if the charterer does not supply the full agreed quantity.  All gas carriers have sophisticated instrumentation for measuring the tank contents because there can be no question of opening the tanks to measure the contents and indeed any gas in the tanks prior to loading or after discharge has to be taken into account.  The measurement of cargo on many oil tankers is less automated and not all tankers are equipped so that the ullages can be read in a central control room.  So the deck officer in charge of loading will ensure all tanks are ullaged, i.e. the free space above the cargo is measured.  Ullaging is usually carried out using a steel tape on the end of which is a heavy calibrated brass bob.  A full tank will only be loaded to 98% of its capacity in order to allow room for cargo expansion on voyage.  The tanks will be ullaged through the ullage openings in the deckhead.  The accuracy of such measurements, electronics or manual, may be affected by any movement of the ship and, of course, any rounding off of the measurements.  A small inaccuracy in ullaging a large tank can represent a significant quantity of oil.