1. Subsea gas release seminar
Petroleumstilsynet, Stavanger
Asmund Huser, Det Norske Veritas
15. November 2006

2. Content Background/Objectives Approach Results Effects
Subsea
Surface
Air
Results
Effects
Wind speed
Leak rate
Depth
Conclusions and Recommendations
Version
20 September 2018

3. 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 + -
Version
20 September 2018

4. Approach CFX -> approx formulas PHAST Improved surface model
Underestimates
Improved surface model
Not validated
PLUMERISE
Dilution, large scale, deep water
Olga, Profes
Version
20 September 2018

5. Improved surface model
Outward radial flow gives larger bubble zone
Slip velocity = 0.3 m/s
Normal distribution of vertical gas velocity
The surface model has proven to be one of the most critical parameters for the subsequent gas dispersion analysis
Version
20 September 2018

6. Surface gas velocity profile
Water depth = 300 m
Version
20 September 2018

7. CFX model Velocity profile Turbulence profile Temperature profile
Surface roughness
Stability
Surface heat flux
Humid air
Open sea turbulence missing
Version
20 September 2018

8. Validiation of CFD model
Maplin Sands liquid propane release in sea.
Simulation of gas dispersion, heavy gas
Version
20 September 2018

9. 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
20 September 2018

10. Approximation formulas
Dimensionless groups
L = f(Ug, Uw, Dg) variables
reduced to
L/Dg = f(Ug/Uw) variables
Version
20 September 2018

11. Results test cases Release rte (kg/s) Time (s) Wind speed (m/s)
Water depth (m)
Scenario no.
Version
20 September 2018

12. Effect of water depth, 100 kg/s, 6 m/s
Version
20 September 2018

13. 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
20 September 2018

14. 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
LEL
2.8 mass%
50% LEL
1.4 mass%
Version
20 September 2018

15. Effect of depth, 50 kg/s, 3 m/s
Version
20 September 2018

16. 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
20 September 2018

17. 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
20 September 2018

18. 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
20 September 2018

19. Effect of leak rate, 300 m, 4 m/s
Version
20 September 2018

20. 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
20 September 2018

21. 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
20 September 2018

22. 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
20 September 2018

23. 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
20 September 2018

24. Effect of leak rate, 70 m, 7 m/s
Version
20 September 2018

25. Effect of wind speed, 300 m, 450 kg/s
Version
20 September 2018

26. 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
20 September 2018

27. 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
20 September 2018

28. 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
20 September 2018

29. Effect of wind speed, 50 kg/s, 70 m.
Version
20 September 2018

30. 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
20 September 2018

31. Version
20 September 2018