Midgard
Kristin
Åsgard A
Åsgard B
Åsgard C
Åsgard BMikkel/Midgard
Flow assurance for Åsgard subsea compression
Mikkel
Magnus Nordsveen, Statoil
NTNU, Trondheim, Norway: 26th Feb 2018
21 februar 20181 Classification: Internal © Statoil ASA
Content
• What is Flow Assurance?
• How do we get multiphase flow – compositions?
• Multiphase flow challenges
• Åsgard subsea compression – surge waves and hydrate incident
21 februar 20182 Classification: Internal © Statoil ASA
Fluid behaviour and control
Flow Assurance: Secure and optimize transport of oil and
gas in multiphase pipeline systems
Multiphase flow
21 februar 20183 Classification: Internal © Statoil ASA
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-50 -25 0 25 50 75 100
Temperature (degC)
Pre
ssur
e (b
ara)
Single-component
Pure C3H6
liquid gas
Multi-component – Oil reservoir
two-phase
Why multiphase flow?
Two-component:
50% C3H6
50% C5H10
liquid gas2 ph
Critical point Critical point
Multipcomponent – Gas reservoir
2 phase
mixture
LiquidGas
Tres
, Pres
21 februar 20184 Classification: Internal © Statoil ASA
Phase envelope and P, T conditions from reservoir to platform (oil field)
2 phase
mixture
21 februar 20185 Classification: Internal © Statoil ASA
Pressure drop from reservoir to platform
Total pressure drop: dP = dPgrav + dPfric
Gravitational pressure drop: dPgrav = reffgH
Fluid Effective density [kg/m3]
Heigth [m]
Dpgrav [bar]
Gas 80 2000 16
50% gas, 50% oil 440 2000 86
Oil 800 2000 157
21 februar 20186 Classification: Internal © Statoil ASA
Multiphase flow challenges (1)
• Pressure drop: How much do we get out of the reservoir?
• Liquid accumulation in Gas Condensate flowlines
21 februar 20187 Classification: Internal © Statoil ASA
Multiphase flow challenges (2)
• Surge and slug flow
− Flooding of separators
− Slugcatchers
Troll
slugcatcher
21 februar 20188 Classification: Internal © Statoil ASA
3 phase surge waves arriving at ÅSGBfrom 20“, 46 km long Y-102 flowline
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17 May 12:00 17 May 18:00 18 May 00:00 18 May 06:00 18 May 12:00
Co
nd
en
sa
te r
ate
[m
3/h
]
Wa
ter/
ME
G r
ate
[m
3/h
]
Time
10.0 MSm3/dWater/MEG rate Condensate rate
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40
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160
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08 Nov 00:00 08 Nov 06:00 08 Nov 12:00 08 Nov 18:00 09 Nov 00:00
Co
nd
en
sate
rate
[m
3/h
]
Wate
r/M
EG
rate
[m
3/h
]
Time
7.5 MSm3/d Water/MEG rate Condensate rate
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40
80
120
160
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5
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14 Sep 22:00 15 Sep 04:00 15 Sep 10:00 15 Sep 16:00 15 Sep 22:00
Co
nd
en
sate
rate
[m
3/h
]
Wate
r/M
EG
rate
[m
3/h
]
Time
6.1MSm3/d Water/MEG rate Condensate rate
Measured surge wave instabilities
0.66NR: 64 % NR: 57.5 %
NR: 39 %NR: 48 %
21 februar 20189 Classification: Internal © Statoil ASA
FT
1D Model (FM) vs data
Measured and predicted liquid rates at 3 phase separator outlets
Predicted liquid rates at 2 phase separator inlet
Damping of surges by
liquid level control
FT
21 februar 201810 Classification: Internal © Statoil ASA
Prediction of surge origin and liquid re-distribution during surging
Liquid content in riser/flowline Holdup in flowline/riser
1 2 3
1
2
3
Y X
Time-trends Profiles - plots
Lazy-S
Riser- 9 km - - 37 km -
21 februar 201811 Classification: Internal © Statoil ASA
Surging details
Pressure
Condensatecontent
Water/MEGcontent
Surge ratesat ÅSGB
21 februar 201812 Classification: Internal © Statoil ASA
Summary - Surge wave instabilities
• FlowManager reproduce surge waves instabilities:
− Water surges are predicted to come from the riser
− Condensate surges are predicted to come from the flowline
• Field data shows:
− No blockage at riser base
− Gas and condensate arrives continuously topside
− Water accumulates and releases
− This is not a “riser slugging” phenomena
• This phenomenon occurs also in inclined flowlines
− ref: Landsverk et al. Multiphase flow behavior at Snøhvit, Cannes 2009
21 februar 201813 Classification: Internal © Statoil ASA
Hydrate formation • Hydrate detection alerts• MEG injection advice
Pipeline Management System
Liquid Surges/Slugs• Liquid accumulation• Ramp-up surges• Water surging
Subsea compressor station• Monitoring• Advise
Dynamic simulator scope
• Wells
• Flowlines
• Subsea compressor station
• Topside separators
• MEG injection
Åsgard Flow Assurance Simulator (FAS)
21 februar 201814 Classification: Internal © Statoil ASA
Discover potential problems applyingdifferent FAS execution modes
Pre
ssu
reSimulationtime2
WI Scenario 1 & 2
1 Current time
• What-If simulator
– Operation and dynamic process understanding
– Planning
– Analyzes
• Real Time for online monitoring
– Current state of the production system
• Look-Ahead early warning system
– Predictions into near future
21 februar 201815 Classification: Internal © Statoil ASA
Minor hydrate incident – slurry at Inlet cooler (1)
• Mikkel B well was restarted after a long shutin period
• MEG injection moved from A-template to B-template
• Next day, a sudden pressure increase over the inlet cooler was experienced
• Handled by reduced compressor speed and MEG injection at the Inlet cooler
• After this incident an alarm based on the FAS prediction of MEG mass fraction has been implemented
A-templateB-template
Predicted MEG mass fraction at A
Switch MEG injection from A to B
21 februar 201816 Classification: Internal © Statoil ASA
FAS predictions vs field meas. during a system restart
ÅSGB gas rate ÅSGB condensate rate ÅSGB water/MEG rate
- FAS - MeasuredCompressor speed Pressure drop: ÅSC-ÅSGB
21 februar 201817 Classification: Internal © Statoil ASA
Summary – FAS for ÅSC
• FAS used for operational planning and online monitoring
• Alarm at ÅSGB on predicted MEG fraction in flowline is implemented
• Good agreement between FAS and measurements are seen for
pressure drop in flow lines and for start-up surges
21 februar 201818 Classification: Internal © Statoil ASA
Content
✓What is Flow Assurance?
✓How do we get multiphase flow – compositions?
✓Multiphase flow challenges
✓Åsgard subsea compression – surge waves and hydrate control
Thank you!
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