TURBOMACHINERY FOR FLOATING LNG

Onshore siting of LNG production facilities in places such as 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% 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% range, compared with 30% to 35% 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.

Steam turbines

Traditionally, the propulsion in LNG carriers has been run by steam turbines. The steam turbine uses Boil-off Gas (BOG) as the heat resource for the boiler, with another liquid fuel as a back-up. The technology is well developed, flexible, reliable and simple. However, the overall efficiency is relatively low.

Aeroderivative gas turbines, then, are the best option for an FLNG vessel. In addition to efficiency, weight and space, further operational advantages prioritize the aeroderivative driver. Aerpderivatives are also better designed for a floating or moving environment because they have evolved from aircraft engines. For example, dynamic motions could prevent proper return (or supply) of lubrication oil to the sensitive hydrodynamic bearings in heavy industrial frame gas turbines (or other rotating machinery).

For an aeroderivative gas turbine, on the other hand, rolling-element bearings are generally used and systems have been developed to avoid motion-related problems. They may come with a slightly higher initial price compared to other solutions, but higher efficiencies would equate to payback in less than one year.

In addition, active magnetic bearings could be a reliable approach for FLNG compressors (if successful references become available for the size and service required). Compared with oil-lubricated machines, active magnetic bearings reduce footprint and weight, simplify operation and ensure that the process gas cannot be contaminated with oil.

Safety, of course, impacts FLNG turbomachinery selection, design, development and layout. The effect of machinery weight (and dynamic loads to limit vibration) on the extent and weight of supporting structural steel (and FLNG design) can be significant.

Advanced safety philosophies are needed for the prevention of incidents due to leakage or ignition. Hazard mitigation must encompass fire detection, gas detection and emergency shutdown.

FLNG vessels, after all, have the highest equipment densities compared with other onshore or offshore units. This factor increases the potential severity of a serious failure (or an explosion), which could escalate to a total facility loss.

A catastrophic turbomachinery failure, therefore, could lead to hull damage and even a large-scale discharge of LNG into the sea. This could be followed by a rapid LNG phase transition, which can cause serious structural damage to the vessel, including a stability loss. As a large FLNG vessel would require a substantial workforce (say above 50 people), safety poses a serious challenge as well as an important operating cost.

Author:

Amin Almasi is a Chartered Professional Engineer in Australia, Queensland and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.