What is odourising with regards to gas carriers?

Methane, Propane & Butane (& its mixtures) are the most commonly carried hydrocarbon gas cargoes on ships. These gases are also very commonly used at homes & also sold as fuel gases. These gasses being colourless & odourless, it becomes very difficult to detect them in case of a leakage. The process of introducing a compound with a high degree of smell / odour & which does not interfere with the physical & chemical properties of the fuel gas is called odourising or most commonly also as stenching.

Odourising or stenching is mostly carried out at shore terminals, but sometimes due to requirements of a specific trade this is placed as an added responsibility to the shipboard staff. Most common places where these operations are carried out on ships is Indian Sub-continent, Far East Asia, West Africa & The Caribbean Islands.

It is recommended that shipboard odourisation does not take place unless there is no reasonably practical alternative. Wherever possible, the equipment should always be installed and the operation undertaken ashore in the terminal. This is because:

• terminals generally have facilities for disposing of noxious vapours, either by flaring or introducing them to low pressure gasholders.

• The availability of a chemist, experienced in the handling of these compounds, will be more readily available should the need arise.

• One set of injection equipment at a terminal can serve any number of ships.

• It avoids the need to transfer drums from jetty to ship's deck, with the attendant risk of leakage.

• Space for the installation of equipment is normally more readily available at a terminal than on the deck of a- small gas carrier and facilities for the storage of odorant will be better.

• Ship-owners would have to formulate emergency response plan for each transfer that they undertake.

• P & l Clubs may impose conditions on ships cover to undertake this operation.

• Ships may have difficulty in maintaining the conditions of the drums.

• Introduction of stenching agents to ship's tanks may give potential for future contamination liability of cargo.

Organosulphur compounds are the most common odourisers used in the industry & some of them are the most odorous substances known. Two most common compounds currently in use as odorants are Mercaptans and sulphides. Some examples are as below:

• Ethyl Mercaptan (EM), Ethanethiol or Scentinel A

• Diethyl Sulphide (DES), Ethylthioethane

• Tetrahydrothiophene (THT), Scentinel T or Thiophene

Of these EM is the most suitable for the stenching of LPG and is most commonly used. It can be detected at concentrations lower than 1 ppb (parts per billion) in air.