42
LNG
INDUSTRY
APRIL
2016
Reduced seasonality in available
power
Conventional refrigeration cycles with compressors driven by
industrial or aeroderivative gas turbines can suffer due to a loss of
gas turbine performance in high ambient temperatures. This is due
to a power decrease resulting from a reduction in air mass flowrate
because the density of the air decreases as the ambient temperature
increases; hence the efficiency of the compressor decreases as the
compressor requires additional power to handle air of lower density.
As a rule of thumb, the output of industrial gas turbines decreases
approximately 0.7% for every 1°C increase in ambient temperature,
and in aeroderivative gas turbines the output decreases
approximately 1.1%per 1°C increase in ambient temperature. In
contrast, an all-electric plant’s production is less affected by ambient
temperatures, and the expected production loss for a given installed
specific power is estimated to be less than 2%of production per 5°C
ambient temperature increase (for the case where the liquefaction
facility uses air cooled heat exchangers) (Figure 2).
Safety and operational benefits
Gas turbines can present an additional safety concern in the
liquefaction process due to the requirement for a fuel gas system.
Electrical motor systems do not have this concern and are less
susceptible to causing an escalating event.
The use of a high voltage switchgear presents its own safety
implications that have been addressed during detailed engineering
design.
With or without HRSGs, large electric motors are safe
compared to gas turbines. Installing a gas turbine with an air intake
unit in a liquefaction area imposes a major safety risk, since intake air
to gas turbines or HRSGs may contain hydrocarbons during an
upset condition, leading to loss of containment. The LNG industry
learned an invaluable lesson in 2004 from the Skikda LNG
explosion, which was caused by ingress of hydrocarbon gas into the
boiler intake. Use of electrical drivers eliminates this safety risk.
1
Project risk factors
LCI drives and 75MWmotors are not new technologies, but simply
extensions of things that already exist. Many LNG plants that are
driven by industrial gas turbines have helper motors to augment gas
turbine power with similar technology, except for the size. Helper
motors are typically limited to approximately 25MW.
LCI technology is well proven, all the way up to 100MW.
NASA’s motor-driven wind tunnel in Virginia, US, has 12-pulse LCIs
on 100MWmotors (fromABB
2
) and has operated them
successfully since 1997 (adjustable speed drive with a single
100MWsynchronous motor, 1998). Another example is the
Hammerfest LNG plant in Norway, where 65MWmotors (from
Siemens) are driven by LCI drivers.
3
The following information demonstrates how Freeport LNG
managed technical risks associated with the electric drives.
Technology risks
The 75MWmotors with 12-pulse LCI VFD from
GE Power Conversionmay be considered new technology
because there is no experience with an exact match in the
service requirements. However, many parameters have passed
the test of proven experience, albeit not in the same package.
GE Power Conversion has experience with synchronous motors
of smaller size at 3600 RPMand with systems of up to 28MW
operating at 5200 RPM. It also has gas turbine helper motor
experience with Trains 4 – 6 at Bonny Island, Nigeria.
The Freeport LNG plant with 9 x 75MWsynchronous electric
motors, driven by LCI VFD technology, will be the world’s largest
all-electric plant built to date.
4
Operating risks
Torsional issues attributed to these large VFD systems are themain
operating risk. These issues include the following:
Interharmonic excitation of mechanical torsional natural
frequencies that can lead to coupling or other mechanical
component failures.
Subsynchronous torsional interaction (SSTI), which is an
excitation of the power supply systemby the VFD that can
ultimately impact operation of the electric generators.
Extensive effort has been applied tomodelling, understanding
and analysing torsional issues. As a result, harmonic filters are
installed with the VFD LCI drive.
Execution risks
Execution risks relate to getting a compressor string delivered and
started up at the site. Themost commonmethod of mitigating
these risks is to carry out testing at themanufacturer’s shop prior to
shipping. Freeport LNG chose to have all three compressor strings
for Train 1 undergo complete full load, full speed string tests to
minimise execution risks. The scope of the string tests includes all
auxiliaries and has an extra step tomeasure string shaft torsional
vibrations.
Conclusion
In summary, the use of electrical motors to drive refrigeration
compressors not only helps reduce air emissions, but also increases
plant efficiency and expected availability. In turn, this substantially
improves project economics when compared to LNG plants that use
gas turbines to drive themain refrigerant compressors.
References
1. DWECK, J. and BOUTILLON, S., ‘Deadly LNG Incident
Holds Key Lessons For Developers, Regulators’,
Pipeline & Gas Journal
,
(May 2004).
2. ‘Adjustable speed drive with a single 100-MW synchronous
motor’, ABB Review, (June 1998), p. 14.
3. Siemens AG, E. S., (2013), Siemens at LNG 17,
4. BRUNO, M., FINNEY, C., and ROTONDO, P., ‘Driving
developments at Freeport LNG’,
LNG Industry
, (April 2015)
pp. 24
–
32.
Figure 3.
3D rendering of the three-train liquefaction project
on Quintana Island.
