Subsea gas release seminar

Petroleumstilsynet, Stavanger

Asmund Huser, Det Norske Veritas15. November 2006

Version Slide 222 April 2023

Content

Background/Objectives

Approach- Subsea- Surface- Air

Results

Effects- Wind speed- Leak rate- Depth

Conclusions and Recommendations

Version Slide 322 April 2023

Subsea gas releases - Highlites

Increasing risk contributor

Surface model is imortant

DNVS method:

PLUSS SIDE- New surface term model- CFD model of dispersion

MINUS SIDE- Not validated for large releases

+-

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Approach

Olga, Profes

PLUMERISE Dilution, large scale, deep water

Improved surface model

Not validated

CFX -> approx formulas

PHAST Underestimates

Version Slide 522 April 2023

Sea SurfaceGas containing region

The surface model has proven to be one of the most critical parameters for the subsequent gas dispersion analysis

Improved surface model

Outward radial flow gives larger bubble zone

Slip velocity = 0.3 m/s

Normal distribution of vertical gas velocity

Version Slide 622 April 2023

Surface gas velocity profile

00.050.1

0.150.2

0.250.3

0.35

0 10 20 30 40 50 60

Radius (m)

Velo

city

(m/s

)

50kg/s

100kg/s

200kg/s

500kg/s

1000kg/s

Water depth = 300 m

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CFX model

Velocity profile

Turbulence profile

Temperature profile

Surface roughness

Stability

Surface heat flux

Humid air

Open sea turbulence missing

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Validiation of CFD model

Maplin Sands liquid propane release in sea.

Simulation of gas dispersion, heavy gas

0.001

0.01

0.1

1

10 100 1000Distance from release (m)

Mas

s fra

ctio

n ga

s

fine gridcoarse gridTest data

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Database of CFD simulations

50 Open sea simulations:- Leak rates from 10 to 450 kg/s- Water depths from 100 to 300 m- All cases are steady state- Wind speeds from 3 to 8 m/s- Mole weight from 17 to 26.5

Makes approximation formulas

Show physical effects

Version Slide 1022 April 2023

Approximation formulas

Dimensionless groups

L = f(Ug, Uw, Dg) 4 variables

reduced to

L/Dg = f(Ug/Uw) 2 variables

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Results test cases

0200400600800

100012001400160018002000

4600

3600

4600

3600

4600

3600

4600

3600

4600

3600

4600

3600 50 50 50 50 50 50

150 300 150 300 150 300 150 300 150 300 150 300 ss ss ss ss ss ss

2 7 15 2 7 15 2 7 15 2 7 15

300 70 300 70

1 2

Dis

tanc

e (m

)

Downwind extent of LEL (m)

Downwind extent of 50% LEL (m)

Height of plume tip LEL (m)

Height of plume tip 50%LEL (m)

Release rte (kg/s)

Time (s)

Wind speed (m/s)

Water depth (m)

Scenario no.

Version Slide 1222 April 2023

Effect of water depth, 100 kg/s, 6 m/s

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100m depth, 100 kg/s, wind speed 6 m/s

Water Depth 100 m Sea roughness 0.0013m Downwind extent of LEL 140 m

Gas Mole Weight 17 kg/kmol Ambient temperature 10ºC Downwind extent of 50% LEL 195 m

Gas leak rate 100 kg/s Sea temperature 5ºC Height of plume tip LEL 15 m

Wind 6 m/s Bubble zone diameter 40 m Height of plume tip 50%LEL 18 m

LEL 2.8 mass%

50% LEL 1.4 mass%

Version Slide 1422 April 2023

200m depth, 100 kg/s, wind speed 6 m/s

Water Depth 200 m Sea roughness 0.0013m Downwind extent of LEL 180 m

Gas Mole Weight 17 kg/kmol Ambient temperature 10ºC Downwind extent of 50% LEL 260 m

Gas leak rate 100 kg/s Sea temperature 5ºC Height of plume tip LEL 15 m

Wind 6 m/s Bubble zone diameter 60 m Height of plume tip 50%LEL 15 m

LEL 2.8 mass%

50% LEL 1.4 mass%

Version Slide 1522 April 2023

Effect of depth, 50 kg/s, 3 m/s

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100m depth, 50 kg/s, wind speed 3 m/s

Water Depth 100 m Sea roughness 0.0013m Downwind extent of LEL 110 m

Gas Mole Weight 17 kg/kmol Ambient temperature 10ºC Downwind extent of 50% LEL 160 m

Gas leak rate 50 kg/s Sea temperature 5ºC Height of plume tip LEL 38 m

Wind 3 m/s Bubble zone diameter 40 m Height of plume tip 50%LEL 45 m

LEL 2.8 mass%

50% LEL 1.4 mass%

Version Slide 1722 April 2023

200m depth, 50 kg/s, wind speed 3 m/s

Water Depth 200 m Sea roughness 0.0013m Downwind extent of LEL 130 m

Gas Mole Weight 17 kg/kmol Ambient temperature 10ºC Downwind extent of 50% LEL 205 m

Gas leak rate 50 kg/s Sea temperature 5ºC Height of plume tip LEL 40 m

Wind 3 m/s Bubble zone diameter 60 m Height of plume tip 50%LEL 55 m

LEL 2.8 mass%

50% LEL 1.4 mass%

Version Slide 1822 April 2023

300m depth, 50 kg/s, wind speed 3 m/s

Water Depth 300 m Sea roughness 0.013m Downwind extent of LEL 150 m

Gas Mole Weight 17 kg/kmol Ambient temperature 10ºC Downwind extent of 50% LEL 230 m

Gas leak rate 50 kg/s Sea temperature 5ºC Height of plume tip LEL 50 m

Wind 3 m/s Bubble zone diameter 80 m Height of plume tip 50%LEL 65 m

LEL 2.8 mass%

50% LEL 1.4 mass%

Version Slide 1922 April 2023

Effect of leak rate, 300 m, 4 m/s

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300m depth, 50 kg/s, wind speed 4 m/s

Water Depth 300 m Sea roughness 0.013m Downwind extent of LEL 190 m

Gas Mole Weight 17 kg/kmol Ambient temperature 10ºC Downwind extent of 50% LEL 280 m

Gas leak rate 50 kg/s Sea temperature 5ºC Height of plume tip LEL 25 m

Wind 4 m/s Bubble zone diameter 80 m Height of plume tip 50%LEL 30 m

LEL 2.8 mass%

50% LEL 1.4 mass%

Version Slide 2122 April 2023

300m depth, 100 kg/s, wind speed 4 m/s

Water Depth 300 m Sea roughness 0.013m Downwind extent of LEL 170 m

Gas Mole Weight 17 kg/kmol Ambient temperature 10ºC Downwind extent of 50% LEL 260 m

Gas leak rate 100 kg/s Sea temperature 5ºC Height of plume tip LEL 35 m

Wind 4 m/s Bubble zone diameter 80 m Height of plume tip 50%LEL 45 m

LEL 2.8 mass%

50% LEL 1.4 mass%

Version Slide 2222 April 2023

300m depth, 200 kg/s, wind speed 4 m/s

Water Depth 300 m Sea roughness 0.013m Downwind extent of LEL 160 m

Gas Mole Weight 17 kg/kmol Ambient temperature 10ºC Downwind extent of 50% LEL 250 m

Gas leak rate 200 kg/s Sea temperature 5ºC Height of plume tip LEL 50 m

Wind 4 m/s Bubble zone diameter 80 m Height of plume tip 50%LEL 65 m

LEL 2.8 mass%

50% LEL 1.4 mass%

Version Slide 2322 April 2023

300m depth, 450 kg/s, wind speed 4 m/s

Water Depth 300 m Sea roughness 0.013m Downwind extent of LEL 200 m

Gas Mole Weight 17 kg/kmol Ambient temperature 10ºC Downwind extent of 50% LEL 310 m

Gas leak rate 450 kg/s Sea temperature 5ºC Height of plume tip LEL 85 m

Wind 4 m/s Bubble zone diameter 100 m Height of plume tip 50%LEL 110 m

LEL 2.8 mass%

50% LEL 1.4 mass%

Version Slide 2422 April 2023

Effect of leak rate, 70 m, 7 m/s

0

50

100

150

200

250

300

350

400

450

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

leak rate (kg/s)

Dis

tanc

e (m

)

Downwind extent of LEL (m)

Height of plume tip LEL (m)

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Effect of wind speed, 300 m, 450 kg/s

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300m depth, 450 kg/s, wind speed 4 m/s

Water Depth 300 m Sea roughness 0.013m Downwind extent of LEL 200 m

Gas Mole Weight 17 kg/kmol Ambient temperature 10ºC Downwind extent of 50% LEL 310 m

Gas leak rate 450 kg/s Sea temperature 5ºC Height of plume tip LEL 85 m

Wind 4 m/s Bubble zone diameter 100 m Height of plume tip 50%LEL 110 m

LEL 2.8 mass%

50% LEL 1.4 mass%

Version Slide 2722 April 2023

300m depth, 450 kg/s, wind speed 6 m/s

Water Depth 300 m Sea roughness 0.013m Downwind extent of LEL 220 m

Gas Mole Weight 17 kg/kmol Ambient temperature 10ºC Downwind extent of 50% LEL 330 m

Gas leak rate 450 kg/s Sea temperature 5ºC Height of plume tip LEL 40 m

Wind 6 m/s Bubble zone diameter 100 m Height of plume tip 50%LEL 50 m

LEL 2.8 mass%

50% LEL 1.4 mass%

Version Slide 2822 April 2023

300m depth, 450 kg/s, wind speed 8 m/s

Water Depth 300 m Sea roughness 0.013m Downwind extent of LEL 270 m

Gas Mole Weight 17 kg/kmol Ambient temperature 10ºC Downwind extent of 50% LEL 430 m

Gas leak rate 450 kg/s Sea temperature 5ºC Height of plume tip LEL 20 m

Wind 8 m/s Bubble zone diameter 100 m Height of plume tip 50%LEL 25 m

LEL 2.8 mass%

50% LEL 1.4 mass%

Version Slide 2922 April 2023

Effect of wind speed, 50 kg/s, 70 m.

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14

wind speed (m/s)

Dis

tanc

e (m

)

Downwind extent of LEL (m)

Height of plume tip LEL (m)

Version Slide 3022 April 2023

Conclusions and recommendations

Plume rise model in water need updates

Improved surface model suggested, - Needs validation- Needs to be generalized- Use two-phase CFD models to guide experiments

CFD model for air dispersion is flexible and show effects

Approximation formulas developed, - Gives quick estimates- Needs extensions

Effects in air are:- Increasing depth -> increase plume lengths- Increasing Wind speed -> increase plume lengths to a max- Increasing Leak rates -> increase touch down wind

Version Slide 3122 April 2023