DNV GL Headquarters, Veritasveien 1, P.O.Box 300, 1322 Høvik, Norway. Tel: +47 67 57 99 00. www.dnvgl.com
Memo to: Memo No: 113KU04A-4/ HABT_rev00
CENTRICA V/Morten Løkken From: Harald Bjarne Tvedt
Date: 2017-04-07
Prep. By:
QA:
Harald Bjarne Tvedt Helene Østbøll
SUPPLEMENTARY OIL SPILL CONTINGENCY MODELLING FOR THE ODA OIL FIELD (FORMER BUTCH)
SUMMARY
The objective of the project was to review the original Environmental Risk Analysis (ERA) and Oil Spill Contingency Analysis (OSCA) for Oda/Butch oil field (DNV GL, 2015a), due to revised blowout rates (Acona, 2016).
The ERA calculations are based on Operational Environmental Risk Analysis tool (OPERAto), a risk tool developed for the oil field in 2015, while the OSCA evaluations are based on Oil Spill Contingency
Analysis tool (OSCAto), updated OSCAR modelling, stochastic and single simulations and the ERA output.
Revised rate is 9 260 Sm3/d versus the original rate of 8 340 Sm3/d.
Recalculation of Environmental Risk in OPERAto indicates, as percentage of Centrica’s acceptance criteria, limited/negligible output. The result aligns with the findings in the original study.
The findings from the OSCA modelling, both statistics and single simulations, combined with OSCAto calculations indicate that the fraction of recovered oil is significant higher during the summer season while the potential for sedimentation of oil is highest during the winter season. The explanation for these differences are due to weather conditions and oil specific characteristics. The weather’s role, as an energy source during winter, results in increased down-mixing of oil in the water column and reduces the oil available for mechanical recovery.
Based on updated results and the current industry oil spill contingency guideline (NOROG, 2013), addressing environmental risk and oil spill contingency level by applying a cost benefit analysis, the conclusion is that the indicative oil spill response from previous study (DNV GL, 2015a) still is applicable;
9 mechanical recovery systems in barrier 1.
Based on an expected 3-weeks drift time to shore, Centrica has time to monitor a potential oil spill as well as prepare coastal response measures, if required.
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SUMMARY (IN NORWEGIAN)
Hensikten med gjeldende prosjekt var å revidere det opprinnelige miljørisiko- og beredskapsstudie som var utført for oljefeltet Oda/Butch i 2015 (DNV GL, 2015a), grunnet oppdaterte utblåsningsrater for feltet (Acona, 2016).
Miljørisikoberegningene er basert på OPERAto, et risikoverktøy utarbeidet for oljefeltet i 2015.
Beredskapsvurderingene er basert på OSCAto, oppdatert beredskapsmodellering, stokastiske og singel- simuleringer samt miljørisikoresultatene.
Revidert rate er 9 260 Sm3/d sammnelignet med den opprinnelige raten på 8 340 Sm3/d.
Oppdatert miljørisiko ved bruk av OPERAto indikerer, som andel av Centricas akseptkriterier, begrenset/
neglisjerbart utslag. Dette samsvarer med resultatene fra den opprinnelige studien.
Resultatene fra beredskapsmodelleringen, stokastisk så vel som for enkeltsimuleringene, kombinert med OSCAto beregningene indikerer at andelen mekanisk oppsamlet olje er betydelig høyere i sommerhalvåret mens potensialet for sedimentering av olje er høyere i vinterhalvåret. Forklaringen på disse variasjonene er knyttet til værforhold og oljespesifikke karakteristikker. Energitilførsel som følge av værforholdende vinterstid, resulterer i nedblanding av olje i vannsøylen og reduserer oljemengden tilgjengelig for mekanisk oppsamling.
Basert på oppdaterte resultater og gjeldende industriretningslinjer for oljevernberedskap (NOROG, 2013) hvor forholdet mellom miljørisikoanalyse og oljevernberedskapsnivå skal settes i en kost/nytte sammenheng, konkluderes det at det indikative beredskapsnivået fra tidligere studie (DNV GL, 2015a) fremdeles er gjeldende; 9 mekaniske oppsamlingssystemer i barriere 1.
Med en forventet drivtid til land på 3 uker har Centrica tid til å overvåke drivbanen til et potensielt utslipp samt igangsette kystnære beredskapstiltak, om nødvendig.
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1 INTRODUCTION
The present analysis is carried out as a supplement to the full ERA and OSCA analyses performed for the Oda/Butch oil field in 2015 (DNV GL, 2015a), based on a revised and higher 90-percentile blowout rate for development drilling operations (Acona, 2016).
ERA evaluations are based on OPERAto, an environmental risk tool developed for the oil field in 2015 (DNV GL, 2015b). Oil Spill Contingency evaluations are based on OSCAto, updated OSCAR modelling, stochastic and single simulations and the ERA output. The response tool OSCAto was developed for the oil field in 2015/2016 (DNV GL, 2016).
Since the last analyses were performed, an upgraded version of the oil drift model OSCAR has been released, and version 7.0.1 is therefore used for the modelling instead of version 6.2.
Other updated parameters used in the oil drift modelling are:
• Current and wind data (2002-2011)
o Current: daily mean data, 4 x 4 km resolution, SWIM archive (NMI) o Wind: 3-hour-interval, 10 x 10 km resolution, NORA10 (NMI/ Norsk
Dypvannsprogram)
• Number of oil particles; increased from 2 500 to 5 000
• Following time; increase from 15 to 20 days
• Number of simulations/years; from 42 to 48
The consequence of applying the reviewed factors, described above, is higher resolution in the modelling performed in the study.
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2 INPUT DATA Blowout rate
The blowout rate for the prevailing study is 9260 Sm3/d. This equals P90 at a duration of 75 days in a recent well specific study (Acona, 2016).
Duration
For the environmental risk part the duration distribution was the same as in original study (Table 2-1) (DNV GL, 2015a). For the oil spill contingency modelling, weighted duration from Table 2-1 (10 days) is applied. This is in accordance with the NOROG guideline (NOROG, 2013).
Table 2-1 Blowout duration distribution for a topside blowout during the development drilling operation at Oda/ Butch oil field (PL 405) (DNV GL, 2015a).
Release location Duration (days)
2 5 15 35 62
Topside 54.6 % 18.1 % 15.7 % 4.9 % 6.4 %
Topside/subsea probability distribution
Topside/subsea probability distribution is the same as in 2015 with 100 % probability for a topside blowout.
Activity overview
An overview over activities and frequencies for potential blowouts in the first year of operations at the Oda field are presented in Table 2-2.
Table 2-2 Activities and frequencies for a potential oil spill in a development year at the Butch oil field (Lloyd’s, 2016).
Activity Number of activities (peak year)
Frequency Total
Development drilling
2 3.47 x 10-5 6.94 x 10-5
Completion (oil well) 2 1.15 x 10-4 2.30 x 10-4
Water injection well 1 9.46 x 10-6 9.46 x 10-6
Total 3.09 x 10-4
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Acceptance criteria
Centrica Energy has defined acceptance criteria for environmental risk as part of their management system. For the scheduled drilling operation at Butch, Centrica Energy’s installation specific acceptance criteria for environmental risk are used (Table 2-3). The acceptance criteria state the limit for what Centrica Energy has defined as acceptable risk for the company’s activities (probability for a given consequence). These criteria are formulated as a measure of environmental damage to natural resources, expressed by duration and degree of seriousness.
Centrica Energy uses the same acceptance criteria in all regions across the Norwegian Continental Shelf.
Environmental risk analysis captures differences in environmental vulnerability on a regional level based on the presence and vulnerability of environmental resources in each area and through calculation of restitution time for potentially affected resources. This means that the calculated environmental risk is higher for areas where a larger share of a vulnerable population or habitat is affected.
The acceptance criteria express Centrica Energy’s attitude to keep nature as far as possible untouched by the company’s activities. The criteria states maximum tolerated incident frequency which can cause harm to the environment.
Table 2-3 The installation and field specific acceptance criteria for acute pollution according to Centrica Energy.
Severity of environmental
damage
Duration of damage (Recovery/ restitution
time)
Production specific acceptance criteria per installation (per
year of operation)
Production specific acceptance criteria per
field (per year of operation)
Minor < 1 year < 1.0 × 10-2 < 2.0 × 10-2
Moderate 1-3 years < 2.5 × 10-3 < 5.0 × 10-3
Considerable 3-10 years < 1.0 × 10-3 < 2.0 × 10-3
Serious > 10 years < 2.5 × 10-4 < 5.0 × 10-4
Oil type
The oil type applied in the analysis was Ula crude oil, which is the same as in previous study.
Valuable Ecological Component (VEC)
Resource data in OPERAto includes seabirds and shoreline data. The species and habitats are the same as in the original study (DNV GL, 2015a).
Mechanical recovery setup
The supplementary modelling is carried out applying OR and towing vessels, including response times, specified in the original study (Table 2-4).
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Table 2-4 Overview of NOFO systems and response times applied in supplementary OSCAR modelling for development drilling activity at Oda oil field.
* Centrica Energy agreement with operator of oil spill resources - response time (3h).
As previous modelling with Ula oil indicated limited effect of chemical dispersion, mechanical recovery is the focus of the supplementary modelling. The following combinations were set-up:
Response configuration
Number of vessels in barrier 1A
Number of vessels in barrier 1B
Comments
0_0 0 0 Modelling of spill without contingency, to compare
with contingency modelling cases.
6_3 6 3 Normal mechanical setup with 6 systems in
barrier 1A and 3 systems in barrier 1B.
8_4 8 4 Normal mechanical setup with 8 systems in
barrier 1A and 4 systems in barrier 1B.
10_5 10 5 Normal mechanical setup with 10 systems in
barrier 1A and 5 systems in barrier 1B.
NO. Location OR-vessel
Distance (km)
Response time (h)
Location towing
vessel
Distance (km)
Response time (h)
Total response
time (h)
1* Ula 5 3 Stril
Mariner* - 3 3
2 Ekofisk 78 10 RS
Haugesund 291 10 10
3 Sleipner/ Volve 170 11 RS
Kleppestø 390 13 13
4 Balder 249 17 RS Måløy 557 18 18
5 Troll 2 405 18 NOFO pool - 24 24
6 Troll 1 426 19 NOFO pool - 24 24
7 Tampen 471 21 NOFO pool - 24 24
8 Stavanger 1 263 22 NOFO pool - 24 24
9 Gjøa 483 24 NOFO pool - 24 24
10 Mongstad 1 433 28 NOFO pool - 24 28
11 Haltenbanken 932 38 NOFO pool - 24 38
12 Kristiansund 1 724 39 NOFO pool - 24 39
13 Stavanger 2 263 42 NOFO pool - 24 42
14 Mongstad 2 433 48 NOFO pool - 24 48
15 Kristiansund 2 724 59 NOFO pool - 24 59
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Single simulations
To study the potential for sedimentation additional single simulations have been carried out for the different response configuration. Following start dates were used for the examples:
Summer: 2nd of July, 2003.
Winter: 1st of January, 2002.
The scenarios are set-up with weighted released duration (10 days) and rate of 9 260 Sm3/d. The Results are presented for the end of the simulation (day 30).
The results for winter are presented in the memo, while the summer data are presented in Appendix A.
Data presented are sediment/deposition (g/m2) in combination with mass balance and wind speed and direction at end of simulation.
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3 RESULTS
The result section is divided in 3: 1) oil drift results, 2) environmental risk assessment and 3) oil spill contingency analysis.
Oil drift
Seasonal oil drift results, for a topside release, in connection with development drilling at Oda are generated in OPERAto and illustrated in maps (Figure 3-1 and Figure 3-2).
The results indicate that an influence area is potentially larger during the summer months (June-August) compared to the rest of the year. The reason for this is the weather conditions, providing less energy, and thereby limit weathering processes taking place in the water column. The area with > 50 % probability for oil, given a blowout at the Oda field, is similar throughout the year. The variations are primarily taken place in the lower oil hit probability categories.
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Figure 3-1 Seasonal influence area, spring (top) and summer (bottom) in connection with development drilling activity at Oda oil field. Cut-off is 5 % probability for > 1 tonne in a 10 x 10 km grid cell.PS! The area is combined for numerous simulations and does not represent one single simulation. The figure is generated in Google Earth via OPERAto.
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Figure 3-2 Seasonal influence area, autumn (top) and winter (bottom) in connection with development drilling activity at Oda oil field. Cut-off is 5 % probability for > 1 tonne in a 10 x 10 km grid cell.PS! The area is combined for numerous simulations and does not represent one single simulation. The figure is generated in Google Earth via OPERAto.
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Environmental Risk
The environmental risk calculations, based on revised development drilling inputs, are like the 2015 results (Table 3-1 and Table 3-2).
Table 3-1 Annual risk contribution from development drilling activity (2018-2019) as part of Centrica’s’
field specific acceptance criteria. Output generated in OPERAto.
Resource group Minor
< 1 year Moderate
1-3 years Considerable
3-10 years Serious
> 10 years
Pelagic seabirds 0.0 % 0.2 % 0.0 % 0.0 %
Coastal seabirds 0.0 % 0.1 % 0.1 % 0.1 %
Coastal habitats 0.0 % 0.1 % 0.0 % 0.0 %
Table 3-2 Annual risk contribution from development drilling activity (2018-2019) as part of Centrica’s’
installation specific acceptance criteria. Output generated in OPERAto.
Resource group Minor
< 1 year Moderate
1-3 years Considerable
3-10 years Serious
> 10 years
Pelagic seabirds 0.1 % 0.4 % 0.0 % 0.0 %
Coastal seabirds 0.0 % 0.1 % 0.1 % 0.2 %
Coastal habitats 0.1 % 0.1 % 0.0 % 0.0 %
The primary contribution to the risk is the completion operations, due to a higher incident frequency compared to development and water injection drilling operations. The seasonal damage frequencies for seabird and coastal habitats are very low, and in the 10-6 (Figure 3-3).
Figure 3-3 Seasonal environmental damage frequency for a topside oil spill at Oda during Completion operations for seabirds (pelagic and coastal) and coastal habitat. The calculations are generated in OPERAto.
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Environmental risk for marine mammals was ≤ 0.1 % in the original study, independent of damage category. Based on the similar results for the other VECs compared to former analysis, the same tendency is expected for marine mammals.
Oil Spill Contingency Analysis (OSCA)
This section includes the results from the supplementary OSCAR modelling, stochastic and single simulations, and OSCAto.
3.3.1 OSCAR modelling
Oil spill contingency modelling was carried out to identify potential effect of different mechanical oil recovery configurations.
The output from the model indicates that seasonal variations are more profound than the differences observed between different levels of response measures. The fraction of mechanical recovered oil (Collected) is potentially twice as high during the period compared to the winter period, while the opposite trend is observed for sedimented oil fractions (Out of grid) (Figure 3-4). The fraction of Dispersed oil is, by the end of the simulation period, highest during the summer period.
The fraction of Stranded oil is, independent of period, zero at one decimal level.
Oil weathering over time indicates a substantial drop in the Surface oil fraction within the first 5 days after the release ends. This coincides with the moment when the fraction of Collected oil levels. This suggests that limited amount of Surface oil is available for mechanical recovery after this point in time (Figure 3-5).
The Out of grid (Sedimented) oil fraction increases considerably in the post-release period for a winter oil spill scenario. Sedimented oil in the summer is limited, compared to winter, as part of the oil is Collected and remaining is Dispersed in the water column.
Similar trends are observed independent of response configuration.
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Figure 3-4 Mass balance at end of simulation, for different oil spill contingency configurations, summer (top) and winter (bottom).
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Figure 3-5 Mass balance over time for different level of mechanical recovery given a blowout at Oda during development drilling, for summer (March-August) and winter (September-February).
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3.3.2 OSCAto
The output from OSCAto indicates, as expected, more recovered oil during the summer period compared to the winter period (Table 3-3). The seasonal specific differences are due to seasonal variations in weather conditions, which affect the amount of oil available for recovery as well as the effectiveness of the response systems.
Overall, the amount of mechanically recovered oil increases with additional recovery systems; however, the efficiency moving from one configuration to the next configuration declines. The reason for this is linked to less available oil to work on compared to for prior engaged systems.
Table 3-3 Recovered oil (tonnes) by the last systems entered in the configuration for summer and winter (generated in oil field specific OSCAto).
Output from OSCAto indicates limited potential for oil drifting to shore during winter season (Table 3-4).
For the summer period a significant effect is by implementing offshore mitigation measures. Limited volume of oil emulsion is predicted into coastal waters/shoreline given an offshore vessel recovery base of 6-12 systems. Ula crude oil has in studies at 13 °C showed a water uptake of 85 %, which indicates an oil fraction of 15 % (e.g. 8_4 configuration < 10 tonnes of oil) (SINTEF, 1999).
In case of an oil spill during the summer period, the time to prepare emergency preparedness closer to shore is around 3 weeks (independent of mitigation level).
Configuration
Summer Winter
Mechanical recovery (tonnes)
Mechanical recovery (tonnes)
0_0 - -
3_0 622 254
4_2 462 201
6_3 419 189
8_4 319 162
10_5 241 140
12_6 182 111
14_6 44 45
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Table 3-4 Calculated 95-percentil for stranding of oil emulsion and shortest drift time to shore for various response configurations. The data are presented for summer and winter.
Configuration Summer Winter
95-percentil stranding
(tonnes) 95-percentil
shortest drift time to shore (days)
95-percentil stranding (tonnes)
95-percentil shortest drift time to shore (days)
0_0 600 22.4 - -
3_0 172 22.2 - -
4_2 43 24.2 7 -
6_3 71 23.3 - -
8_4 61 19.3 - -
10_5 - - - -
12_6 - - - -
14_6 - - - -
3.3.3 Sedimentation
To link oil spill response level to sedimentation, two single simulations were modelled (Figure 3-6 - Figure 3-9, Figure 3-10 and Appendix A).
It should be emphasized that since single simulations are set-up using a specific start data, the output from the modelling work should be viewed carefully.
The results indicate that seasonal weather variations affect the degree of sedimented oil (out of grid) and recovered oil more than the level of oil spill contingency, from 9-15 systems in the open water barrier (barrier 1a and 1b).
In general, the area potentially affected is similar within the seasons. Intraseasonal variations in potential sediment concentration appears more profound during summer. The influence area presented as concentration data, an effect of 15 mechanical recovery vessels in barrier 1 indicates that oil
potentially in the lower category without use of emergency preparedness is below cut-off level with the response in place. The sedimentation potential is higher during winter, reaching 13-14 g/m2 versus 2-4 g/m2 during summer. The values are gathered based on line transect from blowout location through the grid cell with potentially the highest concentration.
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Figure 3-6 Sedimentation, given different levels of oil spill recovery, for a single simulation during winter season. Following configuration levels are modelled; 0_0, 6_3, 8_4 and 10_5, and 0_0 is shown in this figure. The figure show sediment deposition (g/m2), mass balance at end of simulation (day 30) and main wind direction and velocity in modelling period. Cut-off value 1 g/m2. PS! Outer frame various among illustrations.
Figure 3-7 Sedimentation, given different levels of oil spill recovery, for a single simulation during winter season. Following configuration levels are modelled; 0_0, 6_3, 8_4 and 10_5, and 6_3 is shown in this figure. The figure show sediment deposition (g/m2), mass balance at end of simulation (day 30) and main wind direction and velocity in modelling period. Cut-off value 1 g/m2. PS! Outer frame various among illustrations.
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Figure 3-8 Sedimentation, given different levels of oil spill recovery, for a single simulation during winter season. Following configuration levels are modelled; 0_0, 6_3, 8_4 and 10_5, and 8_4 is shown in this figure. The figure show sediment deposition (g/m2), mass balance at end of simulation (day 30) and main wind direction and velocity in modelling period. Cut-off value 1 g/m2. PS! Outer frame various among illustrations.
Figure 3-9 Sedimentation, given different levels of oil spill recovery, for a single simulation during winter season. Following configuration levels are modelled; 0_0, 6_3, 8_4 and 10_5, and 10_5 is shown in this figure. The figure show sediment deposition (g/m2), mass balance at end of simulation (day 30) and main wind direction and velocity in modelling period. Cut-off value 1 g/m2. PS! Outer frame various among illustrations.
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6_3 winter
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10_5 winter
Figure 3-10 Concentration of sediment contaminant (g/m2) as function of distance (km) from release location. The illustrations represent different level of mitigation measures during winter season; baseline (0_0) to 15 systems (10_5).
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4 CONCLUSION
The objective for this project was to reassess the findings in the original ERA and OSCA for Oda/Butch oil field (DNV GL, 2015a) based on revised and higher blowout rates (Acona, 2016) linked to development drilling operations.
In addition, to a revised rate of 9 260 Sm3/d versus the original rate of 8 340 Sm3/d, the following updates were incorporated:
- Updated version of OSCAR modelling tool (7.0.1. vs. 6.2);
- Updated current and wind data;
- Modelling setup; increased number of particles and simulations and extended following-time.
Recalculation of Environmental Risk (OPERAto) indicates, as percentage of Centrica’s acceptance criteria, limited/negligible output. The result aligns with the findings in the original study.
The results from Oil Spill Contingency modelling, both statistics and single simulations, combined with OSCAto calculations indicate that the fraction of recovered oil is significant higher during the summer period while the potential for sedimentation of oil is highest during winter season. The explanation for these differences are due to weather conditions and oil specific characteristics. The weather acts as an energy during winter resulting in a down-mix of oil as well as reducing the oil available for mechanical recovery.
The potential for oil drifting to shore (95-percentile level) is limited, as is the amount of oil emulsion. In general, the output from OSCAto indicates that limited amount of oil to shore from 6 (4_2) - 12 (8_4) system during summer. For winter the result is negligible. In the original study, no stranding was observed exceeding 9 systems (6_3).
The drift time to shore (95-percentile level) is almost 3 weeks.
Based on recent results and the current industry oil spill contingency guideline (NOROG, 2013), addressing environmental risk and oil spill contingency level by applying a cost benefit analysis, the conclusion is that the indicative oil spill response level at 9 systems in barrier 1 from previous study (DNV GL, 2015a) still is applicable.
The calculated drift time to shore, indicates that Centrica will have time to monitor and prepare coastal response measures, if required.
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5 REFERENCES
Acona, 2016. Memo – Blowout rate evaluations for the 9-5/8” casing design- transient reservoir response evaluations (Oda (Butch) development wells (PL405)). Rev 1- 17th October 2016.
Acona, Akvaplan-niva og DNV GL, 2016. Oljedriftsmodellering for standard miljørisikoanalyser ved bruk av OSCAR – beste praksis.
DNV GL, 2015a. Environmental risk and oil spill contingency analyses for development drilling at Butch field (PL 405). Report no.: 2015-0772, Rev. 00
DNV GL, 2015b. OPERAto Butch. Centrica Energy. Ver. 00, 31.08.2015.
DNV GL, 2016. OSCAto Butch. Centrica Energy. Rev. 01, 06.01.2016.
Lloyd’s, 2016. Blowout and well release frequencies based on SINTEF offshore blowout database 2015.
Report no: 19101001-8/2016/R3. Rev: Final. Date 04 April 2016.
NOROG, 2013. Veiledning for miljørettede beredskapsanalyser, datert 16.08.2013.
SINTEF, 1999. Oppdatert forvitringsstudie for Ula råolje relatert til effektivitet av Foxtail Skimmer.
SINTEF Kjemi, Rapportnr. STF66 F99076.
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APPENDIX A
SINGLE SIMULATIONS RESULTS
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Figure A-5-1 Sedimentation, given different levels of oil spill recovery, for a single simulation during summer season. Following configuration levels are illustrated, from top to bottom: 0_0, 6_3, 8_4 and 10_5. The figure show sediment deposition (g/m2), mass balance at end of simulation (day 30) and main wind direction and velocity in modelling period. Cut-off value 1 g/m2. PS! Outer frame varies among illustrations.
Page 27 of 28 0_0 summer
6_3 summer
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10_5 summer
Figure A-2 Concentration of sediment contaminant (g/m2) as function of distance (km) from release location. The illustrations represent different level of mitigation measures during summer season;
baseline (0_0) to 15 systems (10_5).