Hydrocarbon Processing - October 2021 - GP-17

SPECIAL FOCUS: LNG TECHNOLOGY
gas turbine supplier provides the de-rating
curves for each gas turbine model and version
accordingly. A typical de-rating curve
is shown in FIG. 2 revealing how power output
decreases with the increase in ambient
temperature. Similarly, de-rating curves for
other factors, such as the inlet and exhaust
pressure drops, are used to estimate the actual
power output of a specific gas turbine.
Compressor arrangement. Until recently,
LNG plants had a maximum of
two main refrigerant compressor casings
installed on the same shaftline, with a
variable-speed starter motor usually added
to single-shaft gas turbines. A longer
and more complex shaftline will increase
the complexity of the torsional analysis
study, further to the inter-harmonic excitations
from the variable speed helper
motor, when they exist.
A successful step-out was applied in
the Yamal LNG project (FIG. 3), where
three compressor casings plus a variablespeed
start/helper motor were installed
on the same shaftline, which extended in
length to 50 m (FIG. 4).
The use of a gearbox will also lead to
power losses that must be taken into account.
Almost 2% of transmitted power
could be lost due to the gearbox; however,
it must be noted that technical improvements
are ongoing to reduce such losses.
The addition of a gearbox to the shaftline
will also impact the CAPEX of the project,
noise emissions and the maintenance
of the compressor shaftline, especially if
a gearbox noise hood is added to meet
noise specifications.
Depending on the size of the driver,
one or several compressor casings can
be installed on the same shaftline, and
the temptation can be great to limit the
number of shaftlines for CAPEX purposes.
Selecting a bigger driver with several
compressor casings, instead of selecting
several smaller ones to drive one compressor
casing, might seem to be less
FIG. 3. Novatek's Yamal LNG project in Russia.
expensive from a machine perspective.
However, the CAPEX savings of this selection
is less obvious if the complete installation
of the train is considered.
Applying large gas turbines would
lead to a horizontally-split design for the
middle casing to enable maintenance of
the compressor rotor. This casing will
have a bottom/bottom nozzles configuFIG.
4. Three compressor casings on a common shaftline with a variable-speed helper motor.
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Gas Processing & LNG | SEPTEMBER/OCTOBER 2021 17
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Hydrocarbon Processing - October 2021

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