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Qatar relies on a flexible choice of fuels for gas tankers

10th March 2014

New triplex pump drive unit will also power natural gas tankers with LNG in the future

Highly poisonous heavy oil, a waste product of petroleum refineries, is still blown through the funnels of tankers into the air without being filtered. To reduce the burden on the environment, especially near the coast, while being able to react better to price fluctuations, Qatar wants to arrange the future operation of its entire LNG fleet on a flexible basis. 

The dual-fuel engines that MAN Diesel & Turbo developed for this purpose can optionally be operated with heavy oil or with the liquefied natural gas in the hold. To do this, the boil-off gases are reliquefied under high pressure and injected into the ship's engine. Since ships are always operating and only brought into dry dock for maintenance once in five years, the high-performance pumps used for this purpose must be extremely robust and temperature-resistant. 

One of the favourites for the supply of these high-performance pumps is Cryostar, a specialist in cryogenic gases. As a partner of this French manufacturer, Lewa was entrusted with the development of a new drive unit that meets the strict requirements for this application. 

 Heavy oil is easily obtained, difficult to burn, and can be fed into the engine at low pressure. When the large transport ships of the Q-flex and Q-max LNG fleet were built nine years ago, the decision was therefore made to use slow-running diesel engines, since these are also thermally more efficient than steam turbines and burn less fuel. 

But it is foreseeable that tankers like those in Qatar will have to convert to other fuels in the future for environmental reasons – at least near the coast. Transport ships with dual-fuel engines like the ones the Emirate wants to use, however, are new territory for everyone involved. Many ship-owners found the technical risk too high, since LNG can be transported safely in the liquid state only at very low temperatures. However, the option of using on-board natural gas as a fuel is also becoming more attractive because engine efficiency is improved in comparison with heavy oil. 

The challenge for the ships is that liquefied gas warms up during travel, partly converting back into its gaseous state, meaning that its volume grows by a factor of 600 and the pressure in the storage tanks rises. Venting these boil-off gases harms the environment and reduces the profits of both the producers and the charter companies. Instead, in the Q-flex and Q-max LNG fleet, they are therefore reliquefied. This yielded the idea of making the gas available to operate the ship. Additional equipment will therefore be provided to make the fleet flexible in its choice of fuel. This allows a reaction to the current prices in the specific export country: If gas is trading high there, it's better to sell it than to burn  it in the ships’ engine . But in the opposite case, it pays to operate the ship with LNG. But since the gas is extremely explosive, absolutely tight special equipment is needed that can withstand both the temperatures and the high pressures. 

 

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