Hydrocarbon Processing - June 2022 - GP-26

LNG
Given these conditions, some 40-50
operating cases must be evaluated. The
results should be reviewed to determine
the governing case with a different combination
of all equipment and associated
connected lines.
For the normal specification above,
additional scenarios must be studied if
specified in an invitation to bid (ITB)
document, such as:
1. Additional operating cases might
be needed for two unloading jetties
2. Phase II expansion or any
interaction with an existing
LNG terminal.
Traditional
simulation. To meet the
above-mentioned requirement, simulation
is the first priority. A traditional
simulation model requires various input
changes and external calculation assistance
for simulating various cases. LNG
tank and ship boil-off rates are calculated
externally as per client information in the
contract and used as an input for the simulation
model. Some cases only call for a
change in input data; however, for a few
operating cases, the structure of simulation
model must also be modified.
Input for the general simulation
model. The following are the general
simulation input/changes required for
each simulation case, regardless of unloading/holding
mode operation:
* LNG ship and LNG tank
unloading/holding mode boil-off
rate is calculated externally and
used as a simulation input for
concerned operating mode cases.
* LNG composition must be input
into the simulation model manually
based on the considered LNG
composition.
Unloading mode. A change required
Upstream
stream selector
for composition
selection
QF-1
SHIP_LEAN
SHIP_RICH
SHIP_HIN2
US1
SHIP
for an unloading mode case is:
* Generally, the unloading mode
simulation model is prepared first
and used as a basis for preparing
further operating cases, including
the holding mode.
Holding mode. Considering the unloading
mode simulation model is available,
the following changes are required
for the holding mode simulation model:
* Calculate the ship and LNG tank
boil-off based on holding mode
operating conditions.
* Stop boil-off gas (BOG) flow
from LNG tank to ship.
* Divert the required part of flow
from the low-pressure (LP)
LNG pump to the unloading line
to keep cool purpose.
* Change the controller setting in the
simulation model for estimating
keep cool flow.
* In circumstances with more than
two LNG tanks, ensure proper keep
cool LNG flow distribution among
LNG tanks based on hydraulics.
* Change LNG tank operating
pressure.
For the nine steps mentioned above,
> 20 input changes are required besides
the actual structure changes in the simulation
model.
Simplified simulation model. Critical
sections of the LNG terminal simulation
model are discussed here to demonstrate
the usefulness of automation. By adopting
the described approach, users can extensively
reduce the input changes required
for switching between various operating
cases. Users should note that an increase
in automation also increases the complexity
of the model. With these options,
the working time of simulation decreases
significantly. Additional automation can
be communicated, if needed; however,
more automation may increase the convergence
issue, and also occasionally increases
convergence time. This should
be decided case by case. Note: General
simulation means that no optimization
tool or programming are used.
CARGO_PUMP
FIG. 2. LNG composition must be input based
on various operating cases.
26 MAY/JUNE 2022 | GasProcessingNews.com
COMMON SIMULATION
CHANGES FOR UNLOADING/
HOLDING MODE
Ship boil-off. With the help of simple
coding, ship boil-off is calculated in simulation
by linking actual stream properties.
The tutorial codea
is used inside the calculator
block BOG-SHIP in FIG. 1 (SHIP
& BOG RETURN section). Procedure 1
for ship boil-off (for calculator block
" BOG_SHIP " ) is below:
$C1: Ship boil-off: VAP vol%/d
for QMAX ship (0.15 vol%)
$C2: QMAX ship volume
(266,000 m3
)
P(1): Consider LNG stream
actual density (kg/m3
P(2): Consider BOG stream
actual density (kg/m3
)
)
Part 1: R(1)=C(1) × C(2)/100/
24 × P(1)
Part 2: 10 IF (P(3).GT.10000) then,
20 R(2)=R(1)/P(2)
30 else
40 R(1)=14000 × P(2)
50 end if
Return
Code description. In Part 1, LNG
ship boil-off is calculated based on actual
case density. Part 2 is one of the multiple
steps for switching unloading mode to
holding mode. In this step, ship boil-off
was increased to ensure no BOG return
to ship; as during holding mode operation,
SHIP is not present. C represents
the " constant " that can be specified by
the user; P represents the " parameter "
that can be extracted from the simulation
model; and R represents the " calculation
results " that are used in further modules.
LNG tank boil-off. LNG tank boil-off
is governed by many factors. In general,
manual input is required for various cases.
However, with the help of a simple program,
all boil-off calculations are automated
and external calculation assistance is no
longer required. Refer to calculator Block
" TK_HL + PL " in FIG. 1 (LNG TANK section).
Procedure 2 for LNG tank boil-off
(for calculator block " TK_HL + PL " ):
$C1: Tank height
$C2: Tank diameter
$C3: Tank thickness
(50+16)/2 = 33 mm
$C4: Tank material density
$C5: Metal heat capacity
$C6: Five sets of tanks
P(1): LNG density (kg/m3
/hr)
)
P(2): Enthalpy of BOG (Kj/kg)
P(3): Enthalpy of LNG (Kj/kg)
P(4): LNG unloading flowrate
to LNG tank (m3
Part 1: R(1) = P(2)-P(3)
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Hydrocarbon Processing - June 2022

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