Safety, practical and legal aspects of handling
By their nature all crude oils and petroleum products are mixtures of a wide range of hydrocarbon compounds with the tendency to produce gas, which tendency also called volatility. One of characteristics of the volatility of crude oils and petroleum products is their ability to continuously vaporise, or, in other words, they liberate gas into the atmosphere continuously while being stored in tank space.
Clear realization of liquid’s ability to continuously vaporise is of vital importance in understanding of the principles of handling of H2S cargoes. Vapour pressure or, simplified, rate of vaporisation of gases liberated from the crudes and petroleum products depends on its temperature, constituents and the ratio of gas to liquid by volume in tank.
Gas Hydrogen Sulphide (H2S) is a product of vaporisation of the constituents mentioned above. It is a very toxic, corrosive and flammable gas. It has a very low odour threshold and a distinctive odour of rotten eggs. H2S is colourless, is heavier than air, has a relative vapour density of 1.189, and is soluble in water. Danger of H2S gas is well known and recognized within shipping industry. Exposure to 700ppm in air rapidly induces unconsciousness (few minutes) and death, when more than 700ppm – immediately fatal.
Presence of H2S is a common occurrence in crude oil, naphthas and fuel oil cargoes, it is imperative therefore that handling of such cargoes require particular attention and special safety measures during loading, carriage and discharging. Due to negative environmental effect of H2S gas vapours its presence in cargo tanks atmosphere is limited to certain levels in many countries and that also places additional legal constraints for all parties involved, i.e. cargo sellers and buyers, charterers and shipowners.
ISGOTT, at sections 188.8.131.52 - 184.108.40.206 describes exposure limits, typical effects of exposure to H2S and enumerates safety procedures for handling cargoes and bunkers with H2S. Notable that sec. 11.1.9. – Loading Cargoes Containing Hydrogen Sulphide (H2S) provides practical guidance on operational measures to minimise "the risks associated with loading cargoes containing H2S" (emphasis mine), while no specific guidance considered necessary for discharging operation.
Naturally, during loading operation H2S vapours displaced by liquid cargo entering the tank space through cargo piping system and escape into the atmosphere via ship’s venting system if no vapour return line connected to ship’s manifolds. Therefore safe conduct of this operation often requires enhanced risk assessment and such additional precautions as use of personal H2S gas monitoring equipment, EEBD and SCABA (see ISGOTT sec 220.127.116.11) to protect personnel working on deck of tanker from poisoning.
Safety guidline given in section 11.1.9 of ISGOTT enumerates the following procedures:
i. Closed loading procedures described in ISGOTT Section 18.104.22.168 (For effective closed loading, cargo should be loaded with the ullage, sounding and sighting ports securely closed. The gas displaced by the incoming cargo should be vented to the atmosphere via the mast riser(s) or through high velocity or constant velocity valves, either of which will ensure that the gases are taken clear of the cargo deck…
In order to undertake closed loading, the vessel should be equipped with ullaging equipment that allows the tank contents to be monitored without opening tank apertures.) should be used.
ii. Venting to the atmosphere at a relatively low tank pressure should be avoided, particularly in calm wind conditions.
iii. Cargo loading should be stopped if there is no wind to disperse the vapours or if the wind direction takes cargo vapours towards the accommodation.
iv. Only personnel actively engaged in ship security and cargo handling should be permitted on open decks. Regular maintenance on deck should be limited or postponed until after the end of cargo operations. Visitors should be escorted to and from the accommodation spaces and briefed on the hazards of the cargo and emergency procedures.
As a part of risk assessment it should be taken into account that if there is no wind to disperse the vapours or if the wind direction takes cargo vapours towards the accommodation the loading operation must be suspended until conditions change .
Risk of H2S poisoning is much less during discharging because no vapour emission goes out from ship’s tanks, i.e. while volumes of cargo diminish in the course of discharging, simultaneously inert gas produced by ship’s IG plant replaces empty volumes and keeps constant positive pressure in tanks and lines.
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Posted by: Hamad S. Balharith, 12 November 2011
I would like to thank you for sharing such useful information.