Inert Gas

An owner’s contractual obligation may be that the vessel should maintain the temperature of the cargo or it may be to raise the temperature if the charterer so requires. If the obligation is to maintain the temperature and the cargo is loaded at a lower temperature, this fact should be protested in writing.  Raising the cargo temperature may be difficult to achieve on a short voyage or in adverse weather conditions and can be costly in terms of bunkers.  With some steamships, the steam required to achieve a high level of heating may necessitate a reduction in steam available for the main propulsion.  Waiting in a river port where the water temperature as well as the air temperature may be very low, may use a large amount of bunkers and this needs to be taken into account in the reserve bunkers carried. In most countries in the world, crude carriers over 20,000 tons SDW and most product carriers over 20,000 tons SDW are not permitted to load or discharge unless their inert gas system is in operation.  The quality of the inert gas being produced by the inert gas generator and the oxygen content of the gas in the tanks must be regularly monitored.  The inert gas in the tanks must be kept topped up during the voyage but the biggest demands on the inert gas plant are when discharging, when air could be sucked into the tanks, and after cleaning/gas freeing.  Cargo surveyors and inspectors may require the inert gas pressure in the tanks to be reduced in order to ullage, dip or inspect tanks. This should be done in accordance with owners’ recommended procedures.  Visual inspection of the inerted tanks of large ships is difficult and not always satisfactory because the inert gas reduces visibility in the tanks.  If further cleaning is required after a vessel’s arrival at the loading port, then in addition to the time lost de-inerting and cleaning, up to 48 hours may be needed for re-inerting. Although the inert gas is passed through a scrubber to remove solid particles, sooty deposits in the tanks can cause discolouration of clean cargoes and a common inert gas venting system presents some risks of contamination by transmission of vapours between different parcels of cargo.  In the event of a collision or any other incident breaching the hull the inert gas will be rapidly lost and the atmosphere in the tank will become potentially explosive.