Hydrocarbon Processing - October 2021 - GP-15

SPECIAL FOCUS: LNG TECHNOLOGY
Turbomachinery configuration
for LNG project concept selection
M. MOSSOLLY, C. BELBOL and V. TIRILLY, TechnipEnergies, Paris, France
Refrigerant compressors are the pumping
heart of every LNG plant. Today,
these compressors are usually centrifugal,
and they vary in size and configuration
according to the context of the project.
Steam turbines, gas turbines (heavy-duty
or aeroderivative) and electric motors-
or a combination of these three-are all
possible options to drive the refrigerant
compressors. It has been demonstrated
that a single shaft can drive one, two or
three compressor casings depending on
the plant capacity, the liquefaction process
technology used and the balancing
needs of power. This is particularly interesting,
as drivers with larger output power
and efficiency can be considered.
This articlea
elaborates on the selection
of an optimum turbomachinery configuration
from a technical perspective,
building on technical experience attained
through working on multiple concept
and early front-end engineering design
(FEED) studies for turbomachinery selection
in LNG projects, some of which
have been executed.
Introduction to compressors. Steam
turbines were used as mechanical drivers
for main refrigerant compressors in
LNG plants until the 1980s, after which
time gas turbines started to replace them.
Electric motors have been used only occasionally
in large-scale plants, but they
have become popular at small- and midscale
facilities.
Steam turbines can be custom-made
to deliver any power level likely to be required
by an LNG refrigerant compressor;
however, the largest referenced electric
motors are capped at around 75 MW.
The largest gas turbine used to date is the
heavy-duty Baker Hughes Frame-9, used
across the LNG mega-trains at Qatargas'
Ras Laffan terminal. These turbines deliver
132 MW at International Organization
for Standardization (ISO) conditions.
They are catalogue machines available in
discrete sizes, which makes the selection
of an optimum gas turbine and shaftline
configuration more challenging.
Compressor selection process. The
production capacity of an LNG plant
is directly linked to the power available
from the refrigeration machinery. Consequently,
the selection of the driver and
the arrangement of the compressors is
performed very early in a project, no later
than at the early FEED phase. The final
selection is made by the owner-operator
based on engineering studies performed
with the assistance of a specialized consultant
(backed by a team of process and
rotating equipment engineers) and in collaboration
with the liquefaction process
licensor. The overall selection study, all
the way up to the recommendation to the
owner, is supervised and led by the project
management team.
At the beginning of a project, awareness
on the general guidelines, characteristics
and key criteria for the selection
process should be determined with the
owner. For example, the standardization
of the gas turbine fleet could be an advantage
where spare parts interchangeability
and familiarity of operators and maintenance
personnel with a specific gas turbine
model are beneficial to operations.
Commercial relations should also be
considered-for example, in a case where
the owner favors one supplier with existing
services agreement or, on the contrary,
where the owner prefers to avoid a
gas turbine supplier with limited service
facilities in the country where the gas turbines
will be installed.
Finally, some owners do not accept
non-proven gas turbines that require
qualification before the FEED or engineering,
procurement and construction
(EPC) phases; whereas other owners are
willing to undertake a qualification process,
including rigorous verification and
extensive testing.
In these times of enhanced sensitivity
to the carbon footprint of liquefaction
facilities, electric motors as prime movers
for the main refrigerant compressors
are increasingly being taken into consideration.
This is a departure from the
situation to date, where electric motors
have been the exception. For example, the
promise of availability of a nearby source
of power from the electric grid at a competitive
price led the Snøhvit LNG plant
in Norway to select a 65-MW electric motor
for each compressor shaftline using
the Linde Mixed Fluid Cascade (MFC)
process. Also, because local regulations
forbid fired equipment in an environmentally
sensitive location, Freeport LNG on
Quintana Island, Texas applied 75-MW
variable-speed electric motors on each
shaftline using the Air Products Propane
Mixed Refrigerant (AP-C3MR) process.
In the coming years, it is anticipated
that electric motor drives may become an
attractive option due to the opportunity
to reduce CO2
emissions by mixing in
low-carbon sources of electricity.
Technical assessment. The first step
in the technical assessment is to determine
the required prime mover power
for each shaftline depending on the LNG
plant design and compressor configuration
(see FIG. 1). This will depend on the
targeted range of LNG plant production
capacity in MMtpy, how many liquefaction
trains are envisaged, which refrigeration
process will be used and how many
refrigeration cycles, and finally how many
refrigeration compressor strings per train.
Gas Processing & LNG | SEPTEMBER/OCTOBER 2021 15
Hydrocarbon Processing - October 2021

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