Increasing LNG Carrier Cargo Capacity
The current cargo capacity of 138,000 cubic meter can be increased substantially when the engineroom bulkhead and the aft cofferdam are moved further aft. Changing the overall length or the draft is not recommended, as some major LNG ports have size restrictions. Changing these parameters would impair the flexibility of the vessel. Gas turbine propulsion will allow a rearrangement of the engineroom, since the gas turbine is much smaller than the steam turbine and its steam boilers.
Moving the ER bulkhead aft from frame 71 to frame 45 extends the cargo hold by 20.8 meter. If the gain in gargo hold length is distributed over the four cargo tanks, an increase of 19,000 cubic meter in cargo capacity can be realised. The advantage of this version of the LNG carrier is that it can accommodate both gas turbine electric and gas turbine mechanical drive. The hull form does not have to be changed, so the redesign costs are minimal.
A total rearrangement of the LNG carrier would yield even better results. Cargo capacity would increase by 24,000 cubic meter over the standard design, while the increase thermal efficiency of the combined cycle gas turbine power plant brings fuel cost down by 40%. Increased propulsion efficiency from the podded drive system would bring fuel consumption down even further. Newbuiding cost can be reduced because of the simplified construction of the aft ship, without complex curves around the propeller boss.
In order to show the true revenue making potential of gas turbine driven LNGC alternatives, they have to be compared with the current state-of-the-art conventional LNGC. First of all, on the basis of many contact with the LNG shipping community the most likely LNGC configuration was selected on the basis of technological merits.
Initially, calculations showed the gas turbine electric podded drive LNGC to have the best revenue making capacity, with its high cargo capacity and highly efficient propulsion system. However, in the light of recent events involving podded drive failures, it seems that the reliability of these systems does not yet comply with the requirements of the LNG shipping industry.
The next best alternative, the gas turbine mechanical drive LNGC offers unsurpassed thermal efficiency and high cargo capacity. However, the durability of the reduction gear, clutches and reversing gear for the FPP in commercial marine application is as yet unknown. Some owners have voiced objections to an alternative equipped with a CPP, citing its slightly lower propulsion efficiency.
The gas turbine electric drive LNGC combines excellent thermal efficiency and high cargo capacity, paired with the use of proven technology in the power train. Electric drive systems have gained some acceptance within the LNG shipping community, as illustrated by the order for one 74,000 cubic meter diesel-electric drive LNGC at Chantiers de l'Atlantique last year. Reliability, redundancy and revenue are the key words to this propulsion alternative.
To check the economic viability of the gas turbine electric drive LNGC, a cost calculation model has been designed using a range of input parameters to calculate long run economic performance under differing circumstances and on different trading routes. Three LNG trades are simulated; the short trade (Algeria - France), the medium trade (Trinidad - Spain) and the long trade (Qatar - Korea/Japan). Two different liquid fuel price levels, representing the extremes of the last ten years, have been used to check the survivability of the gas turbine drive alternatives in changing economic circumstances.
Six different aero-derivative gas turbines configuration have been selected to take part in the comparison, making this study the first full-scale performance comparison of all major aero-derivative gas turbine makes for commercial marine propulsion.
There are a number of preliminary conclusion to be drawn:
Additional calculations show that, under certain circumstances, it is economically feasible to re-engine a conventional LNGC with a gas turbine electric drive power plant incorporating gas turbine types GT1, GT2, GT3 or GT6, even if the cargo capacity is not increased. However, the conversion should take place early in the charter for the conversion to be profitable and the vessel will not have the same flexibility and high ROI as LNGCs especially designed to exploit the benefits of gas turbine propulsion to the maximum.
The case for gas turbine electric drive LNGCs is very strong:
With the LNG shipping market heading for a change, operating margins will come under pressure. Gas turbine driven LNG carriers provide a substantial increase in revenue at lower costs. Why not invest in a much more efficient vessel, when it doesn't cost more than a conventional vessel?
MPT Consultancy can assist you in the design, specification and construction of the next generation of LNG carriers.
More information on the gas turbine types and configurations can be obtained from MPT Consultancy. Please contact:
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