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

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

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

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

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

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

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

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

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

Approximation formulas Dimensionless groups L = f(Ug, Uw, Dg) 4 variables reduced to L/Dg = f(Ug/Uw) 2 variables Version 20 September 2018

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Version 20 September 2018