An article by Deck Cadet Diarmid Wilson, onboard British Trader
Crude Oil v LNG
Hydrocarbon cargoes come in many different forms and grades. Two types I've had experience with recently are liquefied natural gas (LNG) and crude oil.
My last trip was onboard British Osprey, a crude oil tanker, which took me across the Atlantic, through the Mediterranean and even briefly into the Pacific.
I'm now half-way through a trip onboard the British Trader, an LNG carrier which has also taken me to many places of interest. My experience with these two types of tanker and cargoes has been varied and I shall explain some of these differences below.
LNG is the potentially more dangerous of the two cargoes. Having said that, there have not been any reported serious incidents regarding LNG tankers to date due to all the extra safety precautions in place and added care taken when handling the cargo. Of course, I do not mean literally ‘handling’ the cargo – we couldn’t do that as it is carried at -160°C. The cargo tanks are heavily insulated, as are the discharge pipelines, to help keep the liquid cold.
However, crude oil is not kept cold - in fact some grades may be heated as the temperature required for its carriage varies from grade to grade. That said, many grades of crude are simply carried at ambient temperature so there is no need to insulate tanks or pipelines.
LNG is loaded at such a low temperature to ensure it remains in liquid form. This is to gain the efficiency in transport – one cubic metre of liquefied natural gas will expand to 1200m3 of vapour when re gasified. For quality control and safety reasons the tank temperature and the pressure are always monitored and controlled onboard.
Crude carrier British Hawthorn
Gas vapour that boils off from the cargo on LNG ships is usually used as fuel in the ship’s boilers.
On crude oil ships the loss of vapour from the cargo is far less, and in any case the vapour would not be used to supplement the fuel for the engines.
LNG is the gas used for heating your home, and used as fuel for power stations, so it therefore needs to be transported all around the world.
Our last voyage was loading a cargo in Trinidad then discharging in the Isle of Grain near London, UK. Now we are heading back to Trinidad to perform the same journey, and after that our next planned load port is, you’ve guessed it, Trinidad! Trinidad is quite warm though, so I really don’t mind!
Until 1990, there were only around 80 LNG ships worldwide. Now there are plans to add more to the world fleet, bringing the eventual total to around 300. But there are far more oil tankers – they comprise around half the world shipping fleet. Until very recently gas ships were built only to serve specific terminals and would stay on fixed runs, but now there is more open trade in LNG and things are changing.
LNG terminals are highly specialized places and there is a very high degree of integration between the ship and the shore. We link into their safety systems, and they link into ours; when we’re loading, should any emergency occur we just have to push an ‘emergency shutdown’ switch (ESD) and this stops the pumps ashore.
Likewise, in the discharge port when we’re pumping out the cargo, if there’s an emergency ashore they can hit their ESD switch and it’ll stop our pumps.
In general, you don’t get this degree of integration at oil terminals, as the trade is not so specialized - oil ships are generally not matched to the terminals to the same degree that gas ships are.
I have enjoyed both my trips on these tankers, and even though they have been quite different they have been equally interesting and it will hopefully always be as interesting throughout my career.