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7/29/2019 92010_ Low Temperature Issues
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part of Aker
© 2010 Aker Solutions
Depressurisation of wet gas segment Low temperature issues
Leif Ernstsen
Stavanger
21. October, 2010
7/29/2019 92010_ Low Temperature Issues
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 2 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Background
• AkerSolutions performed
a tie-in
study
for Gudrun to Sleipner.
• Static
simulations
indicated
low
fluid temperatures
during
blowdown
of
inlet
gas segment.
• The
project
wanted
to examine
the
low
temperature
issue
with
dynamic
OLGA simulations.
7/29/2019 92010_ Low Temperature Issues
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 3 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Field Overview
PARTIAL
STABILIZATION
PROCESS
FLARE
14”
WET GAS
PIPELINE
~ 55 km
GUDRUN
SLEIPNER
GAS
SEGMENT
BLOWDOWN LINE
Receiving
conditions:
90 bar
4 °C
92 kg/s
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 4 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Objectives
• Find
locations with
minimum wall
temperatures.
• Examine
mechanism
of
thermal
process.
• Investigate
effect
of
boiling
of
liquid
pockets.
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 5 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Height / Length Geometry from ISOs
Geometry of OLGA network model
-2
-1
0
1
2
3
4
5
6
0 5 10 15 20 25 30 35
X [m]
Y [ m ] Gas segment
Blowdown line
Drain
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 6 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
OLGA Schematics of Simulated Network
12”
2”
3”
8”-12”-20”
14”
7/29/2019 92010_ Low Temperature Issues
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 7 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Simulated Scenarios
General sequence
of
events:
• Normal production• Shut-in
/ cooldown
of
Gas Segment
• Blowdown
Cases presented
here:
Case 1: Blowdown
from 142 bara, -7 °C (immediate
shut-in)
Case 2: Blowdown from 170 bara, -7 °C (shut-in after temp.equilization)
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 8 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Minimum Wall Temperatures Found (worst case)
2”
drain
pipe,
Case 1:
Min. wall
temp.:
-27,8 ºC
Concurrent
fluid temp.:
-28,9 ºC
Inner wall film HTC: 125 W/(m2*K)Location: DRAIN, Pipe 1, Segment 2
3”
BD pipe,
Case 2: Min. wall temp.: -44,7 ºCConcurrent
fluid temp.:
-45,3 ºC
Inner
wall
film HTC:
523 W/(m2*K)
Location: BLOWDOWN, Pipe 4, Segment 1
12”
main
line,
Case 1:
Min. wall
temp.:
-37,8 ºC
Concurrent
fluid temp.:
-39,1 ºC
Inner
wall
film HTC:
137 W/(m2*K)
Location: SEG_2, Pipe 7, Segment 1
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 9 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Time Curves, 2”
Drain
Pipe, Case 1
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 10 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Time Curves, 3”
Blowdown
Pipe, Case 2
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 11 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Time Curves, 12”
Main Line, Case 1
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 12 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Video Clip of 2”
Drain and 12”
Main Line, Case 1
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 13 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
“Findings”: Pipe Spec. and Low Temperatures
• OLGA simulations
have indicated, where
low
temperatures
occur,
and how low they go during Gas Segment blowdown.
• Cost
saving
can
be achieved
by selecting
pipe spec. in line with
simulation results.
In this
actual
case the
client
have chosen
to maintain
the
originally
planned
-100 °C spec.
• Existing
pipe spec. may
be kept
in modification
projects,
where
new
operating conditions
indicate
lower
temperatures
(dynamic
simulations)
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 14 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Additional Info
More simulation
/ modelling
details
in the
following
slides…
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 15 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Modelling Premisses
• Segment consists
of
bare pipes of
duplex
steel
with
properties:
Spec. heat capacity:
460 J/(kg*K)
Therm. conductivity:
21 W/(m*K)
Density:
7850 kg/m3
Inside
roughness:
0,05 mm
• Pipe dimensions, Gas Segment, spec. FS30A (-100 °C, 258,6 barg):2”:
ID=49.22 mm
WT=5.54 mm
3”:
ID=73.66 mm
WT=7.62 mm
12”:
ID=273.1 mm
WT=25.4 mm
14”:
ID=300.02 mm
WT=27.79 mm
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 16 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Modelling Premisses, cont’d
• Blowdown
orifice
ID = 14 mm, simulated
with
Cd=0,84.
• In real life
blowdown
is initiated
by opening
a block
valve
upstream
the
orifice. In the
simulations, however, no
block
valve
was
included,
and the
blowdown
was
started
by ”opening”
the
orifice
over 2 s.
This
approach
is deemed
to be a very
close
approximation
to the
real
life process.
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 17 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Modelling Premisses, cont’d
Fluid: Gudrun gas, year
2013
Component Mole fraction
Methane 0.686732
Ethane 0.104944
Propane 0.056470
i-Butane 0.005997
n-Butane 0.015792
i-Pentane 0.003798
n-Pentane 0.004598
H2O 0.000626
CO2 0.096044
N2 0.005296
C6* 0.007196
C7* 0.005097
C8* 0.003798
C9* 0.001699
C10-C11* 0.001199
C12-C13* 0.000400
C14-C15* 0.000100
Eglycol 0.000163
H2S 0.000051
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 18 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Modelling Premisses, cont’d
• Ambient
air conditions:
Velocity:
0,5 m/s
Temperature:
0 °C and -7 °C
• Pipeline receiving
conditions:
Pressure:
90 bara
Temperature:
4 °C
Rate: 92 kg/sShut-in
pressure:
170 bara
• Simulation
programs:
OLGA 5.3.2
PVTsim
17.0.0
HYSYS 2006.5
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 19 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Simulated Scenarios
Case 1: Normal production
at 4 ºC, ambient
temp. -7 ºC, mass
flow
92,3 kg/s,
outlet
pressure
90 bara
Shut-in: Close
outlet, maintain
mass
flow
until
170 bara, then
close
inlet
Cooldown
until
equilibrium
pressure=142 bara
Blowdown: Open
blowdown
valve
Case 2: Normal production
at 4 ºC, ambient
temp. -7 ºC, mass
flow
92,3 kg/s,
outlet
pressure
90 bara
Set
inlet
pressure
to 90,4247 bara same production
Shut-in: Close
outlet, set
inlet
conditions
to 170 bara / -7 ºC
Cooldown
until
equilibrium
pressure=170,1 bara
Blowdown: Close
inlet, open
blowdown
valve
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 20 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Other Simulated Scenarios
Case 3: Normal production
at -7 ºC, ambient
temp. -7 ºC, mass
flow
92,3 kg/s,
outlet
pressure
90 bara
Shut-in: Close outlet, maintain mass flow until 170 bara, then close inletCooldown
until
equilibrium
pressure=169,7 bara
Blowdown: Open
blowdown
valve
Case 4: Normal production
at 0 ºC, ambient
temp. 0 ºC, mass
flow
92,3 kg/s,
outlet
pressure
90 bara
Shut-in: Close
outlet, maintain
mass
flow
until
170 bara, then
close
inlet
Cooldown
until
equilibrium
pressure=170,5 bara
Blowdown: Open
blowdown
valve
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Flow Assurance and Dynamic Simulation Seminar 2010
Slide 21 © 2010 Aker Solutions part of Aker
Gudrun Gas Segment Blowdown
Simulation Difficulties
The
simulations
turned
out
to be very
prone
to crash. The
P/T trajectory
passes close
to the
critical
point
and through
areas with
steep
gradients
in the
properties. The
following
measures
were
employed
in attempting
to
avoid crash:
• Selecting
small time steps. However, there
turned
out
to be a certain
lower
limit,
below
which
the
algorithm
became
unstable.
• Removing liquid water, but still simulate with 3 phases and WATERFLASH=ON:This
helped
in some
cases. Removal
of
the
small trace of
liquid
water was
deemed
to have only
a marginal effect
on
the
results.
• Changing
the
downstream
boundary
pressure
and temperature
(!!) during the
blowdown: This
clearly
made
the
algorithm
survive
in some
cases.
• Refining
the
P/T grid in the
PVT-table: This
method
had
the
best effect.
With ΔP=2 bar and ΔT=2 °C in the
operating region, crash
were
generally
avoided.
The
PVT-table, however, included
also
some
extreme
high
and low
values
of
P and T to give
room
for algorithm
excursions.
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