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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


Fluid behaviour and control

Flow Assurance: Secure and optimize transport of oil and

gas in multiphase pipeline systems

Multiphase flow


0

5

10

15

20

25

30

35

40

45

-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


Phase envelope and P, T conditions from reservoir to platform (oil field)

2 phase

mixture


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


Multiphase flow challenges (1)

• Pressure drop: How much do we get out of the reservoir?

• Liquid accumulation in Gas Condensate flowlines


Multiphase flow challenges (2)

• Surge and slug flow

− Flooding of separators

− Slugcatchers

Troll

slugcatcher


3 phase surge waves arriving at ÅSGBfrom 20“, 46 km long Y-102 flowline

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120

160

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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

80

120

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

0

40

80

120

160

0

5

10

15

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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 %


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


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 -


Surging details

Pressure

Condensatecontent

Water/MEGcontent

Surge ratesat ÅSGB


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


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)


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


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


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


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


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!

flow assurance for Åsgard subsea compressionstanko/files/courses/tpg4230/2018... · content •...

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