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



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.






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






Objectives

• Find

locations with

minimum wall

temperatures.

• Examine

mechanism

of

thermal

process.

• Investigate

effect

of

boiling

of

liquid

pockets.






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






OLGA Schematics of Simulated Network

12”

2”

3”

8”-12”-20”

14”






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)






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






Time Curves, 2”

Drain

Pipe, Case 1






Time Curves, 3”

Blowdown

Pipe, Case 2






Time Curves, 12”

Main Line, Case 1






Video Clip of 2”

Drain and 12”

Main Line, Case 1






“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)






Additional Info

More simulation

/ modelling

details

in the

following

slides…






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






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.







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







• 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






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


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






Other Simulated Scenarios


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


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






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.

Documents

92010_ Low Temperature Issues