From article in The Naval Architect – November 1999
A FRESH breeze appears to be blowing through the LNG tanker sector: for the first time in nearly 30 years the prospects for new tonnage powered by diesel engines appear to be approaching reality. The June issue 1999 reported that the leading Korean shipbuilder Hyundai was re-examining diesel propulsion, and since then other leading lights have been studying the potential. The apparently imminent placing of contracts for a new class of LNG tanker for Spanish interests appears to be one of the principal catalysts for all this activity.
In fact, orders for these very expensive ships (largely because of the low cargo temperature and paramount need for efficient insulation and leak-proof gas-handling systems) take a very long time to germinate, since a considerable dose of politics is invariably involved; LNG tankers are normally planned for dedicated routes supplying national power systems over periods of around 20 years or more. It is therefore certain that the new studies currently coming to light have been on the drawing board for some months or even years.
The Naval Architect published one of the latest philosophies for LNG tankers - electric propulsion - in February 1998, when Jari Nurmi from the Finnish consultancy Deltamarin discussed a combined steam and gas turbine electric plant (the steam component being a turbo-alternator utilising waste heat form the gas turbines), which could provide a highly attractive and redundant alternative. Traditionally, LNG tankers have been powered by steam turbines because the necessary boil-off gas from the cargo (normally around 0.15% of the cargo daily) could easily be burned in boilers. However, modern tank insulation technology also means that cargo sometimes has to be forcibly boiled-off, and average rates could now be as low as 0.084% daily.
The new Spanish study (published in the March 1999 edition of Ingenieria Naval) considered five diesel-based possibilities and examined them against a traditional steam turbine plant for a 135,000m3 tanker (the currently popular size) running on a route between Spain and the Caribbean at a speed of 19.5knots. It is interesting to note that twin screws have been especially studied, reflecting - as in the Deltamarin proposals - the modern need for a high degree of inbuilt safety. The study concludes that the best overall option is the twin two-stroke engines, each fitted with a power take-off tunnel gear for an alternator, and driving CP propellers.
Kvaerner Masa plans to use medium-speed engines, ideally in association with ABB/Kvaerner Azipod podded electric drives, in a twin-screw layout. A final novelty is the planned inclusion of the accommodation and machinery forward. Taking all these factors into consideration, it would seem that a most interesting era in the evolution of gas transport could be dawning. The placing of the next LNG tanker contracts is awaited with great interest.