Carrying bulk liquefied gases by semi pressurized ships
These vessels are fitted with a refrigeration plant that provides a fully refrigerated ability while having a high design pressure for the cargo tanks (Pressure vessels), albeit below that required for fully pressurised carriage. The tanks are cylindrical in shape and of a thinner construction than the pressurised vessels.
Semi-pressurised, semi-refrigerated ships (which are now quite rare) ranged up to 5,000 m3 in size. Their construction is based on carrying propane at a pressure of 8.5 kg/cm2, and a temperature of -10°C. Semi-pressurised, fully-refrigerated ships generally range up to 15,000 m3. They can be designed to carry the full range of cargoes in cylindrical or spherical tanks and are designed for a minimum service temperature of -48°C and a working pressure of approx 5 to 8 kg/cm².
The reliquefaction plant on these vessels generally has a substantial capacity and can, if required, load the cargo as a gas and then reliquefy it onboard.
They are able to heat or cool the cargo during loading operations, or while at sea, and are also able to raise the temperature of the cargo when discharging. Where a reliquefaction plant is fitted it will allow a reduction in the wall thickness of the tanks.
Fig:Semi pressurized LPG carrier underway
The inner hull volume is used more efficiently than the fully pressurised vessels and the number of tanks varies from 2-6.
A double bottom is constructed for ballast water and the hold space around the cargo tanks does not need to be inerted.
The advantages semi-pressurised ships are:
i) More cargo can be carried in a tank of the same capacity
ii) a tank of the same capacity is lighter and cheaper to build
iii) much larger and more economical ships can be constructed.
The first ships to use this new technology appeared in 1961. they carried gases in a semi-pressurized/semi-refrigerated (SP/SR) state but further advances were quickly made and by the late 1960s semi-pressurized/fully refrigerated (SP/FR) gas carriers had become the Ship owner’s choice by providing high flexibility in cargo handling. These carriers, incorporating tanks either cylindrical, spherical or bi-lobe in shape, are able to load or discharge gas cargoes at both refrigerated and pressurized storage facilities.
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