Effective turbomachinery for Floating LNG facilities

Onshore sighting of LNG production facilities in places like Australia and Malaysia can often require long and expensive pipelines to bring the fuel to the refrigeration equipment. This has triggered renewed interest in offshore LNG production.

Floating LNG (FLNG) production vessels are being introduced to make smaller and more remote offshore natural gas assets commercially viable. These typically have plant capacities of around one million to five million tons per annum (tpa). A typical LNG plant requires around 35 to 55 MW power per one million tpa of LNG production capacity.

As a result of the lack of space available offshore, the aeroderivative gas turbine is recommended as the best driver option. Other solutions, such as steam turbine drivers, could also be employed.

Aeroderivatives have long been proposed for FLNG and have a number of advantages over their industrial heavy frame counterparts (or other drivers). This includes a smaller footprint and lower weight. An aeroderivative gas turbine’s weight and footprint could be around 40 percent of a comparable heavy-duty gas turbine.

Further, aeroderivatives have modular sections that can be replaced rapidly. Higher thermal efficiency in the 40 to 46 percent range compared with 30 to 35 percent for an industrial frame gas turbine translate directly into savings on fuel and a reduction in carbon emissions.

For some onshore LNG sites, electric motor drivers with a combined-cycle power plant may be an effective, flexible and suitable approach. However, this solution would not be an attractive option for a FLNG vessel. 

It would appear that the justification for electrical motor drives for FLNG may not be as high as anticipated by some vendors.

Variable Speed Drive (VSD) electric motors require large, heavy equipment and auxiliaries (such as transformers and harmonic filters), which renders them overly complex for FLNG. From the perspectives of equipment size, weight, structural steel and overall cost, aeroderivative gas turbines, therefore, have an advantage over large-scale combined cycle power generation and VSD electric motor drivers for FLNG applications.

More in March/April 2013 issue of Turbomachinery International