LNG Terminal Operator's Design Feedbacks and Technical Challenges
LNG Terminal Operator's Design Feedbacks and Technical Challenges
LNG Terminal Operator's Design Feedbacks and Technical Challenges
Gyu-Hyuk Lim
Head of Pyeongtaek LNG Terminal
Lae-Hyun Park
Manager, Technology
Pyeongtaek LNG Terminal
Korea Gas Corporation
ABSTRACT
Pyeong-taek Terminal II (560,000m3 and 720 t/h) is now under construction in order to comply
with the abruptly increasing gas demand in Korea. For the optimum design of the new terminal, the
impacts of the main equipments reliability and availability on both the stream line availability and
construction and operating costs have been carefully evaluated. Other important design parameters,
such as the variation of the imported LNG heating values and the high seasonal gas sendout demand,
have also been optimized in the new terminal design.
This paper presents the practical design improvements that have been directly applied to Pyeong-taek
Terminal II in order to reduce costs and to enhance the terminals availability. These involve
economical vapor handling and re-condensing, zero emission of LNG vaporization, and an optimum
sparing level of the main equipment based on an actual 18-year operating record of reliability and
availability of the main equipment.
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INTRODUCTION
In order to meet the abruptly increased natural gas demand, Pyeongtaek terminal has
continuously been expanded. The terminal added consecutively LNG storage tanks, re-gasification
facilities, and an unloading berth. Presently, 10 units of storage tanks are under operation with a total
capacity of one million cubic meters and 2,020 t/h re-gasification facilities.
During 18 years of operation of the terminal, the existing conventional designs have been
improved upon enhancing the terminal reliability, achieving high controllability, and keeping a high
level of safety. The terminal operators design improvements, as well as experiences in operation and
maintenance, are valuable for the LNG industry. However, the operation feedbacks have been limited
so far even though their role of receiving LNG is important in an LNG chain.
This paper presents the terminal operators practical design improvements against the
conventional design problems occurred unexpectedly during terminal operation and maintenance.
These design improvements, applied to the design of Pyeongtaek Terminal II for reducing costs,
involve economic vapor handling and recondensing, zero emission vaporization, and an optimum
sparing level of the main equipment with their 18 years of operation records. This paper also
discusses the impact of reliability and availability of main vapor handling equipment on revenue losses
and total expenditures based on life cycle costs.
LIC
The mixing of pressurized Boil-Off Gas (BOG) with the sub-cooled LNG in the re-condenser will
increase the temperature of the LNG, resulting in a decrease in liquid density. The temperature and
density of mixed LNG depends on the temperature and flow coming to the re-condenser of both the
ESD
LNG and BOG. Consequently, the
PIC
change in liquid density affects
BOG Compressor
pump performance. One or two
Cold Energy
PSV
boosting pumps are running
Utilization Plant
according to the send-out
demand. The first role of a
Re-condensing
boosting pump is to compensate
Heat
ORV
Exchanger
the low suction pressure of
HP Vaporizer
secondary pumps due to the
operating
pressure
of
rePIC
PIC
PIC
liquefaction system. In order to
save operating costs of boosting
Boost Pump
SMV
2ry Pump
pumps, the variable frequency
HP Vaporizer
drive pumps were installed so LP discharge line
ORV
LP
Vaporizer
that operating costs of the
Legend
LNG Line
boosting pumps can also be
Vapor Line
saved.
The process flow
diagram for vapor recondensing
Fig. 1 Process Flow Diagram of BOG Recondensing System
is shown in Fig. 1.
ES
HP Gas
network
ES
The total BOG rate depends on the terminal operating mode, season, temperature, day/night,
etc. It varies from 20 to 30 t/h in normal holding mode operation and increases by about 10 t/h
during LNG unloading mode. The terminal has high BOG rate because of a high BOG rate of storage
tanks (0.1% per day and 10 storage tanks) and vapor returned from cold energy utilization facility.
Normally, one BOG compressor will be added during the LNG unloading operation (in some cases, it
requires adding two units). The required number of BOG compressors depends on the LNG sources and
ships saturation pressure.
Two different types of BOG compressors have been used: Horizontal ring-type reciprocating
(4sets) and vertical compressors with contactless labyrinth pistons (2 sets). The design capacity of
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each compressor is 12,000 Nm3/h of BOG with a maximum discharge gauge pressure of 10.6 barg
and a discharge temperature range from 14 to 47oC. The vapor of the compressed BOG and returned
re-gasified gas from cold energy utilization plant is used as fuel gas for the tank vacuum breaker,
internal consumption (gas firing absorbed type heating/chilling unit), and neighboring KIA Motors.
The operating pressure range of the storage tank is from 50 170 mbarg. BOG compressors
are started by tank pressure manually or automatically, which can be selected by operators choice.
Combined operation concept - continuous and intermittent operation - has been used for the BOG
compressor operation. When the LNG tank operation pressure reaches 150 mbarg, a unit of compressor
stopped. If tank operating pressure continuously decreases, an additional compressor will be stopped.
Conversely, a BOG compressor will be added when the tank operating pressure reaches 170 mbarg.
Capacity controllers (50, 75, and 100%) are also used to control the compressors operation capacity.
The suction temperatures of the BOG compressors are slightly different in each operation mode
as follows:
Holding mode:
-100 to -110oC
Unloading mode:
-110 to -120oC
During the terminal intermittent operation the suction temperature varies minus 60 70oC after
10 hrs of stopping compressors (no operation of BOG compressors). It requires about 5 to 10 minutes
until the compressor reaches normal operation. The horizontal compressors require suction vapor
desuperheating by using LNG spraying at the BOG knockout drum inlet, while the vertical compressors
can quick start without desuperheating.
The basic principle of the BOG recondensing system is to cool, recondense and send-out the
excess vapor of natural gas by mixing it with a sub-cooled LNG taken from the LP LNG header line. In
order to improve the heat and mass transfer of gas and liquid phases, the condensation takes place
in a packed tower where gas and liquid enter the top of the re-condenser and flow co-currently
through the bed of metal rings, where both phases come in close contact. The re-condenser
operating level is carefully monitored and maintained at half height [1].
Thermally Integrated Vaporization
LNG is stored in storage tanks at cryogenic temperature of approximately minus 160C at
near atmospheric pressure. The LNG is regasified and warmed to temperatures near 0C and desired
send-out pressure. LNG is most commonly vaporized in Open Rack Vaporizers (ORVs) against the
seawater or in Submerged Combustion Vaporizers (SCVs) where the cold energy contained in the
stored LNG is wasted during the regasification process. An LNG terminal has a large storage of
cryogenic energy, which has to be released in order to re-gasify LNG. On the other hand, a power
plant must reject heat to condense steam and to cool equipment.
The terminal integrated thermally with an adjacent Pyeong-taek Thermal Power Plant, of
which has equipped four 350MW steam power generation and
480MW Combined Cycle Gas Turbine (CCGT). The power plant has
Gas Turbine
Exhaust
HRSG
owned and operated seawater lift structure for the power plant
cooling purpose. The seawater that used for steam condensing in the
Steam
Turbine
power plant increases in temperature. This warmed seawater is used
Condenser
Seawater
for LNG vaporization as shown in Fig. 2.
G
Sendout
Gas
LNG
Vaporizer
HRSG: Heat Recovery Steam Generator
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Since the terminal uses the warmed seawater discharged from the power plant, the terminal
eliminates intrinsically the operation of SCVs, which can only be used for back-up or peak shaving. As
a result, this integration leads to no emission of green gas for LNG vaporization.
Combined Cycle Gas
Turbine
Excess
Seawater
Power Plant
LNG Terminal
Pump Station
Vaporizer Pump
Station # 1
Seawater Canal
Seawater Lift
Structure
LNG Vaporizer
Group # 1
Re-Usage
Vaporizer Pump
Station # 2
LNG Vaporizer
Group # 2
Return to Sea
TOT
........................................................................................................................... (1)
TOT + UT f
AT =
MTTF
...................................................................................................................... (2)
MTTF + MTTR
where,
RT
TOT
: Equipment Reliability
: Total operating time of equipment
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UTf
AT
MTTF
MTTR
:
:
:
:
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(6100 hrs) and by GRI (3900 hrs) [5]. The GRI study is based on US plants that are mainly peak shaving
plants, where pumps would largely be used intermittently.
Table 1 Reliability database for key Regasification Equipment
MTTF
MTTR
TOT
UTf
RT
(%)
AT (%)
In-tank Pump
3,221
37
22
698,957
1,085
99.85
98.86
Sendout Pump
4,277
44
20
770,040
1,620
99.79
98.98
5,970
100
95,516
272
99.72
98.35
Process Unit
4,596
147
3*
220,608
1200
99.46
96.90
ORV (U type)
3,288
116
105,248
960
99.10
96.59
ORV (I type)
7,012
61
273,507
390
99.86
99.13
Notes:
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Booster
Pumps
I Type
ORV
U Type
ORV
98.86
Horiz
BOG
Comp.
96.90
98.98
99.13
96.59
Spare Units
Subsystem Availability
1.000
0.9818
0.9946
1.000
0.9985
0.9782
Intank
Pump
Description
Unit Availability (%)
Total Sendout
Availability
with I Type
ORV
Total Sendout
Availability
with U Type
ORV
Horiz
Comp.
Vertical
Comp.
Horiz
Comp.
Vertical
Comp.
BOG
0.9802
BOG
0.9930
BOG
0.9603
BOG
0.9728
Note: Equipment combination is based on 700-900 t/h (750 1,000 MMScfd) sendout
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distributions, i.e., any system/equipment is equally likely to fail at any one time as any other. In an
LNG terminal, a high percentage of this downtime is associated with the type and number of rotating
equipment mainly pumps and compressors [6,7]. The total terminal unavailability is then the sum of
unscheduled terminal shutdown because the scheduled shutdown of system/equipment is either done
during the spare unit operation or during low gas demand seasons.
Figure 5 illustrates the relationship between Availability and unavailability of the BOG
compressor system. BOG system availability increases with an increase in unit availability. Conversely,
the unavailability of the BOG system decreases as unit availability increases. Mathematically,
unavailability can be obtained from deducting the availability from one (1). The unavailability of a
Horizontal compressor system is higher than that of a vertical compressor system.
100.1
0.5
0.4
0.35
99.9
0.3
99.8
0.25
Horizontal
Compressor
Vertical
Compressor
0.2
99.7
0.15
0.1
System
Availability
99.6
100
0.45
System
Unavailability
0.05
99.5
95.5
96
96.5
97
97.5
98
98.5
99
99.5
0
100
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Description
I Type ORV
Horiz Comp
Vert Comp
Horiz Comp
Vert Comp
0.9603
0.9728
0.9802
0.9930
Unavailability of Sendout
0.0397
0.0272
0.0198
0.0070
348.2
238.3
173.5
61.3
63.8
43,7
31.8
11.2
120
100
80
60
40
20
0
2
2.5
3.5
4.5
5.5
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function of maintenance frequency, which is a reciprocated MTTF. Table 4 summarizes the life cycle
costs of maintenance and operation costs.
Table 4 Summary of Operation and Maintenance Costs
Description
(Unit: US$1,000)
Horizontal BOG
Compressors
27
Vertical BOG
Compressors
8
3.7
2.9
258
202
129
88
31,725
31,725
Expense NPV
-62,615
-61,462
The expense NPV of the vertical Compressors is $1.2 million less than that of the horizontal compressors.
This difference does not include difficultness of maintenance work for horizontal compressors against
vertical compressors. The horizontal compressors require more careful efforts in maintenance, which
generally takes more time (about 45% more hours). The actual maintenance records clearly show that
the vertical compressors with contactless labyrinth pistons required less maintenance hours and 20-30%
less spare parts costs.
CONCLUSIONS
1. The terminal has achieved zero emission in LNG regasification and has reduced its costs through
thermal integration with an adjacent power plant.
2. The main equipment reliability and availability have been estimated based on 18 years of actual
terminal operation records. The estimated terminal sendout availability is strongly affected by the
number of spare units or equipment reliability. Low reliability equipment reduces the terminal
sendout availability on stream production.
3. The company decided to replace the existing U type ORVs by the I type ORVs because the
U type ORV shows low reliability and would result in low availability.
4. The vertical compressors with contactless labyrinth pistons show less probable revenue losses
than horizontal compressors because of high availability.
5. The vertical compressors with contactless labyrinth pistons are highly reliable, resulting in less
operation and maintenance costs based on life cycle costs, compared to those of horizontal
compressors.
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REFERENCES CITED
1. Cho, H., Shah, S.N., and Lim, K.H. (2000). Case History for Gas Re-Liquefaction System at the
Pyeongtaek LNG Terminal, South Korea. Proceedings of Gastech 2000.
2. Henley, E.J. and Kumamoto, H. (1981). Reliability Engineering and Risk Assessment. Prentice-Hall,
Englewood Cliffs, NJ.
3. Rao, S.S. (1992). Reliability-Based Design, McGraw Hill, New York.
4. Green, A.E. and Bourne. A.J. (1972): Reliability Technology, Wiley-Interscience, London.
5. Collins, S. (1995). Improving LNG Importation Reliability Proceedings of LNG XI, Birmingham, UK.
6. Vega, F., Hill, D.K., Collins, C. (1998). Plant Reliability Analysis in LNG Plants, Proceedings of LNG
12.
7. Sutton, I.S. (1992). Process Reliability and Risk Management, van Nostrand Reinhold.
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