Wisting Central crude oil – Properties and behaviour at sea - In relation to oil spill response

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2017:00119- Unrestricted

Report

Wisting Central crude oil – Properties and behaviour at sea

In relation to oil spill response

Authors

Kristin R. Sørheim Oddveig M. Bakken

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Document history

VERSION DATE VERSION DESCRIPTION

1.0 2017-05-22 Draft version

2.0 2017-06-02 Final version approved by OMV Norge

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Executive summary

Spilled oils undergo changes when weathering on the sea surface that affect behaviour and oil spill countermeasures in various ways. Oil weathering varies over time and with different environmental conditions. This summary gives a brief overview of the main changes predicted for Wisting Central crude oil when weathered on the sea surface. The SINTEF Oil Weathering Model (OWM) has been used to predict the oil weathering properties as a function of time.

Wisting Central is categorized as a medium density naphtenic crude oil (0.838 g/mL) with low asphaltene (0.05 wt. %) and wax content (0.72 wt. %) compared with other Norwegian crude oils. The fresh oil and its residues have very low pour points (<-36 °C). The high initial evaporative loss results in an increase of the asphaltene and wax content, which contribute in stabilising the water-in-oil (w/o) emulsions.

Wisting Central formed low viscous water-in-oil (w/o) emulsions with a relatively slow and low maximum water uptake (60 vol. %). The emulsions of Wisting Central effectively released water (45-60 vol.%) when adding emulsion breaker (Alcopol O 60 %) with concentrations of 500 and 2000 ppm by weight. The lowest concentration of 500 ppm was shown to be sufficient to break the emulsions. Emulsion breaker could effectively be used during an oil spill operation to remove or reduce water from the emulsion, normally injected at the skimmer head prior transfer to storage tank to minimize the storage volume.

The OWM prediction of mass balances show that Wisting Central will be removed from the sea surface within 1 day in high wind speeds (15 m/s) at 2 and 5°C, and nearly removed within 5 days at wind speed of 10 m/s due to the high degree of evaporative loss and natural dispersion/entrainment. However, after 5 days with calm weather conditions (2 m/s wind speed) as much as ~65 % of the oil remains on the sea surface.

Oil spilled on the sea surface assumes to reach the ambient water temperature within short time period. The fire and explosion hazard will be high as long as the flash point of the oil is below the sea temperature. For Wisting Central crude oil, the flash point is predicted to exceed the sea temperature within 15 minutes at 2 m/s wind speeds, and faster at higher wind speeds. Some storage tanks in vessels engaged in oil recovery operations may not be classified to carry liquids with flash points lower than 60 °C. Wisting Central will reach this limit within 6 hours after a spill at calm wind speed (2 m/s), and more rapidly at higher wind speeds.

In mechanical recovery operation, low oil/emulsion viscosity < 1000 mPa.s may pose a potential for boom leakage that has to be taken into account with respect to lower vessel speed during recovery. In no breaking wave conditions (wind speeds of 2 and 5 m/s), the oil/emulsion viscosities are below this viscosity limit for >

5 and 2 days, respectively. At higher wind speeds, the emulsions are predicted to surpass 1000 mPa.s within 6- 12 hours. Due to the low emulsion viscosities, reduced flowability towards traditional weir skimmers will not be a problem with typically viscosity limit for flowability < 15-20 000 mPa.s for Wisting Central. In cases when high viscosity of the oil/emulsion is not a limiting factor, high pour point may cause solidification on the sea surface (typically arises 10-15 °C above the sea temperature). However, Wisting Central has low pour points (-36 °C), and solidification of the oil residue is not expected on the sea surface at 2 and 5 °C.

Wisting Central has a good potential for use of oil spill dispersant. The oil was estimated to be easily dispersible with use of dispersant (Dasic NS) for viscosities < 3000 mPa.s and showed some reduced dispersible for viscosities > 3000 mPa.s with dispersant to oil ratio (DOR) 1:25. The predicted time-window estimates that the oil/emulsion is good dispersible within 5 days of weathering at sea temperature of 2 and 5 °C. The upper limit for when the oil/emulsion is considered as not dispersible was not reached with use of the MNS high-energy test (breaking waves conditions, reflecting wind speed > 5 m/s), which also implies a wide time-window for use of dispersant for this crude oil. The MNS-test with no dispersant added also showed that Wisting Central has a potential to easily be naturally dispersed into the water column in in breaking waves conditions.

In general, when the oil expects to have a reduced dispersibility within 0.5-1 hour after the dispersant spraying operation, additional energy or use of a higher dispersant dosage and/or repeated dispersant application, particularly in calm sea conditions, may be required to increase the dispersant efficiency. Providing additional

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energy with use of firefighting (FiFi) systems, thrusters or MOB boats after the dispersant application may enhance the rate of dispersion. Wisting Central was found to be reduced chemically dispersibility with viscosities > 3000 mPa.s.

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1 Introduction

New oil types, from heavy crude oil to light crude oils and condensates with varying properties, are continuously coming into production both in Norway and worldwide. Due to large variations in different crude oils’ physical and chemical properties, their behaviour and fate may vary greatly if spilled at sea. E.g. The

“Braer” accident at the Shetlands (1993) and the “Sea Empress” accident in Wales (1996) have demonstrated how different the fate and behaviour of the crude oils can be when spilled on the sea surface. For that reason, having good knowledge about the expected behaviour of oil at sea in case of an accidental spill is highly valuable.

The Deepwater Horizon incident in the Gulf of Mexico (2010) clearly showed how dispersant application efficiency may change as the oil is weathered and emulsified on the sea surface over a longer period. This may form important support for refining operative strategies in terms of where, when and how dispersants could be effectively applied during a response operation.

According to the Norwegian Environment Agency and the Petroleum Safety Authority Norway regulations for petroleum activities (Aktivitetsforskriften §59), the characterization of oils with respect to their weathering properties and fate in the marine environment should be performed for all oils coming into production.

SINTEF Ocean, Dept. of Environmental Technology has performed a weathering study and dispersibility testing of the Wisting Central crude oil based on a request from OMV Norge. The weathering study was conducted at 5 °C considered as a relevant sea temperature in the Barents Sea. The obtained weathering data were used to predict the weathering properties of the oil by use of the SINTEF Oil Weathering Model (OWM), if spilled at sea under different weather condition at sea temperature of 2 and 5 °C.

Figure 1-1: Location of the Wisting discovery

http://subseaworldnews.com/wp-content/uploads/2013/09/Statoil-OMW-Pleased-with-Wisting-Central-Oil-Discovery- in-Barents-Sea.gif

Wisting Central II in licence PL537 / Wisting (Hoop area), is located 310 km north of Hammerfest and around 180 km away from the nature reserve Bjørnøya (Bear Island), Wisting Central II is the fifth well in the production license PL537, which was awarded in the 20th licensing round in 2009. The Wisting Central II well is the first horizontal appraisal well in the Barents Sea and sets a new drilling record; it is the shallowest horizontal offshore well drilled from a floating drilling facility. Water depth at Wisting is 402 meters.

OMV (Norge) AS as operator has 25% share in the PL537 license. Joint venture partners are Petoro (20%), Idemitsu (20%), Tullow (20%) and Statoil (15%).

https://www.omv.com/portal/01/com/omv/OMV_Group/upstream/norway/activit ies_in_barents_sea

https://www.omv.com/portal/01/com/omv/OMV_Group

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2 Experimental results and discussion

2.1 Small-scale laboratory testing results

The methodology of small-scale testing is described in Appendix B. The small-scale weathering results of Wisting Central crude oil were compared with similar data of other crude oils from the Barents Sea region, which include previously study of Wisting Hanssen, Skrugard from the Johan Castberg oil field, Goliat Realgrunnen, and Alta (Table 2-1). The oils in comparison were selected in agreement with OMV Norge.

Wisting Central crude oil was given the SINTEF ID 2016-320.

Table 2-1: Oils compared with Wisting Central crude oil data in the report

Oil SINTEF-ID Report Number Reference

Wisting Hanssen Akvaplan-niva AS Report: 7419-01 Torske and Wasbotten, 2015

Skrugard 2011-0559 SINTEF:A22589 Øksenvåg, 2012

Goliat Realgrunnen* 2001-0729 SINTEF: STF66 F03104 Moldestad et al., 2003

Alta 2015-0029 SINTEF: A27546 Hellstrøm and Johnsen, 2016

*For simplicity, Goliat Realgrunnen is referred as Realgrunnen throughout this report.

2.1.1 Chemical composition and physical properties

Figure 2-1 shows the chemical composition as the hydrocarbon profile of Wisting Central, whereas the wax and asphaltene content are given in Table 2-3. The physical properties are given in and Table 2-4. Appendix D shows the composition groups derived from the chemical characterization of the fresh crude oil on GC-MS.

Gas chromatographic flame ionization detector (GC-FID) characterization

The hydrocarbon profile of the fresh Wisting Central crude oil was analysed by use of gas chromatography (GC) coupled with Flame Ionization detector (FID) (Figure 2-1). The figure includes the gas chromatographic characterization of the evaporated residues of 150°C+, 200°C+, and 250°C+. Figure 2-2 shows the GC- chromatograms of other fresh crude oils in comparison with Wisting Central.

Based on the GC-FID characterisation, Wisting Central is a highly biodegraded (naphtenic) crude oil that lacks of the typically systematic narrow peaks of the straight-chained n-alkenes (paraffins). Skrugard is also a biodegraded crude oil with low content of the systematic peaks of n-alkanes in the lower range < nC13-nC15).

Wisting Hanssen and Realgrunnen have similarities in their hydrocarbon profiles, where both oils are biodegraded, but also have high peaks of n-alkanes with increasing carbon number, reflecting the wax components (> nC20). Alta is a typically paraffinic crude oil, which exhibits systematic n-alkanes in the area of nC5-nC36.

GC-FID is also a tool in oil spill identification, where common screening parameters are the nC17/pristane and nC18/phytane ratios. These ratios give the relation between the n-alkanes and isoprenoids (pristane and phytane). Thus, biodegradation results in a reduced ratio between the n-alkanes and the isoprenoids. The ratios for Wisting Central and the other crude oils in comparison are given in Table 2-2.

Table 2-2: nC17/Pristane and nC18/Phytane ratios Oil nC17/Pristane nC18/Phytane

Wisting Central - -

Wisting Hanssen 0.9 1.5

Skrugard 0.3 0.6

Realgrunnen 1.5 2.4

Alta 1.1 1.5

-:Ratios not possible tpo obtarin due to lack of n-alkanes of nC17 and nC18

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Figure 2-1: GC/FID chromatograms of fresh sample and evaporated residues of Wisting Central crude oil

Wisting Central Fresh

Wisting Central 150°C+ residue

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Wisting Hanssen

Skrugard SINTEF ID: 2011-02559

Realgrunnen SINTEF ID: 2001-0792 Wisting Central SINTEF ID: 2011-02559

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Figure 2-2: GC/FID chromatograms for fresh oils used for comparison

Asphaltene and wax content fresh and weathered residues

The chemical properties of asphaltene and wax are given in Table 2-3. Wisting Central has low content of both asphaltene (0.05 wt. %) and wax (0.71 wt. %), and is most comparable to the Skrugard crude oil. Wisting Hanssen, Realgrunnen and Alta have all medium content of wax compared to other Norwegian crude oils, and exhibit low contents of asphaltene. Although, Realgrunnen has a slightly higher content of both wax and asphaltenes compared with the other oils.

Table 2-3: Asphaltene ("hard") and wax content for different oils and their residues

Oil type Residue Asph.

"hard"

(wt. %)

Wax

(wt. %)

Fresh 0.05 0.71

Wisting 150°C+ 0.06 0.84

Central 200°C+ 0.07 0.97

250°C+ 0.08 1.2

Fresh 0.03 3.3

Wisting 150°C+ 0.03 3.7

Hanssen 200°C+ 0.03 4.1

250°C+ 0.04 4.6

Fresh 0.14 5.1

Realgrunnen 150°C+ 0.17 5.8

200°C+ 0.19 6.5

250°C+ 0.21 7.4

Skrugard

Fresh 0.05 1.9

150°C+ 0.05 2.0

200°C+ 0.06 2.1

250°C+ 0.06 2.4

Fresh 0.03 3.1

Alta 150°C+ 0.03 3.7

200°C+ 0.04 4.2

250°C+ 0.04 5.1

Physical properties of fresh and weathered residues

Physical properties of Wisting Central and the other crude oils for comparison are listed in Table 2-4. Wisting Central and Alta crude oil exhibit the highest evaporative loss, which also reflect the lowest densities. Wisting Central has very low pour points for both the fresh oil and the residues (<-36°C), which reflects lack of paraffins. The pour points for the other crude oils increase with increasing evaporative loss, and Alta and Realgrunnen reach both high pour points of the 200 and 250°C+ residue (12-18°C). Wisting Central, Wisting

Alta SINTEF ID: 2015-0029

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Hanssen and Skrugard have low viscosities for the fresh oil and their residues, whilst Realgrunnen reaches the highest viscosity particularly for the 250°C+ residue due to the high wax content. The flash points of the oils are more or less comparable within the oils.

Table 2-4: Physical parameters of Wisting Central in comparison with other crude oils Oil type Residue Evap.

(vol. %)

Residue (wt. %)

Density (g/mL)

Flash point (°C)

Pour point (°C)

Visc.

(mPa.s) 5˚C (10 s^-1)

IFT (mN/m)

ini.

Fresh 0 100 0.838 - <-36 10 30

Wisting 150°C+ 17 85 0.859 40 <-36 25 28

Central 200°C+ 29 74 0.871 79 <-36 62 29

250°C+ 42 61 0.883 120 <-36 206 29

Fresh 0 100 0.845 21 <-33 97 24

Wisting 150°C+ 10 90 0.857 61 -12 74 -

Hanssen 200°C+ 18 82 0.862 83 -3 167 -

250°C+ 28 72 0.869 111 3 320 -

Skrugard

Fresh 0 100 0.871 - <-36 32 -

150°C+ 6 95 0.879 61 -33 54 -

200°C+ 11 91 0.884 88 -21 70 -

250°C+ 23 78 0.891 119 6 171 -

Fresh 0 100 0.857 - -12 21 22

Realgrunnen 150°C+ 14 88 0.869 45 9 1411 21

200°C+ 24 78 0.877 84 18 3890 23

250°C+ 34 68 0.884 124 18 10300 23

Alta

Fresh 0 100 0.827 - -15 49 19

150°C+ 19 83 0.853 40 3 277 21

200°C+ 29 74 0.863 76 12 698 22

250°C+ 43 61 0.876 117 15 3606 22

-: No data

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2.1.2 Emulsifying properties of Wisting Central crude oil

Emulsification

In general, emulsification is the mixing of seawater droplets into spilled oil at the water’s surface (water-in-oil emulsion), forming a weathered oil residue that often tends to be relatively resistant to other weathering processes such as evaporation, and also increases the total volume of oil due to the uptake of water into the oil.

Highly stable emulsified oil can be more difficult to recover and may be difficult to chemical disperse.

The emulsifying properties of Wisting Central were studied by use of the rotating cylinders as e.g. described in Hokstad et al., 1993, see Appendix B. Four parallels of the residues of 150°C+, 200°C+ and 250°C+ were made in order to produce data for stability, viscosity, maximum water uptake, kinetics, and the effectiveness of application of emulsion breaker. The emulsions of maximum water content after 24 hours rotation are shown in Figure 2-3, below. The figure shows from left to right emulsions prepared from the residues of 150°C+, 200°C+ and 250°C+, respectively.

Figure 2-3: The rotating cylinders at the start and after 24 hours of rotation at 5 °C

Water uptake and maximum water content

The rate of water uptake (kinetics) was also studied by use of the rotating cylinders, as described above. The water content in the water-in-oil (w/o) emulsions as a function of time is tabulated in Table 2-5. The t1/2 value is defined as the consumed time in hours to incorporate half of the maximum water uptake (vol. %) measured during 24 hours rotating time.

Wisting Central expresses low rate of water uptake for the 150°C+ residue, but reaches a high water content of 78 vol. % after 24 hours rotating time (Table 2-5). The 200 and 250°C+ residues emulsified faster compared to the 150°C+ residue, but reach a lower maximum water content of 62 and 52 vol. %, respectively.

150°C+ 200°C+ 250°C+

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Table 2-5: Water uptake for the evaporated residues of Wisting Central crude oil at 5 °C Mixing

time 150°C+

(Vol. % water) 200°C+

(Vol. % water) 250°C+

(Vol. % water)

Start 0 0 0

5 min 8 12 12

10 min 14 15 19

15 min 14 15 25

30 min 25 31 40

1 hour 32 43 46

2 hours 39 55 50

4 hours 56 63 52

6 hours 69 66 52

24 hours 78 67 52

t 1/2 1.70 0.63 0.25

Stability and efficiency of emulsion breaker

Overall, the findings of stability and the efficiency of emulsion breakers is important in a mechanical recovery situation because separating the oil from water enables optimal use of available storage facilities/tankers. The emulsified oil volume decreases considerably with treatment of an emulsion breaker as water is removed.

The emulsion stability of Wisting Central crude oil was studied by quantifying the amount of water released from the emulsion during 24 hours settling time. In addition, the effectiveness of adding the emulsion breaker Alcopol O 60 % to the emulsion was evaluated. The results are tabulated in Table 2-6. The oil formed stable emulsions, but the emulsions were easily broken when applying emulsion breaker with concentrations of 500 and 2000 ppm (wt.).

Table 2-6: Stability of emulsion and the effectiveness of emulsion breaker at 5 °C of Wisting Central Residue Emulsion breaker Water-in-oil emulsion

(vol. %) at 5 °C

Stability ratio**

Stability ratio after rotation Reference 24 hours *

150ºC+ none 78 77 0.95 0.86

200ºC+ none 67 67 1.00 1.00

250ºC+ none 52 52 1.00 1.00

150ºC+ Alc. O 60 % 500 ppm 78 3 0.01 -

200ºC+ Alc. O 60 % 500 ppm 67 12 0.06 -

250ºC+ Alc. O 60 % 500 ppm 52 9 0.09 -

150ºC+ Alc. O 60 % 2000 ppm 78 6 0.02 -

200ºC+ Alc. O 60 % 2000 ppm 67 6 0.03 -

250ºC+ Alc. O 60 % 2000 ppm 52 6 0.09 -

ppm: parts per million

*: w/o-emulsion after 24 hours rotation and 24 hours settling

** Stability ratio of 0 implies a totally unstable emulsion after 24 hours settling; all the water is settled out during 24 hours settling. Stability ratio of 1 implies a totally stable emulsion

-no data

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Viscosity of emulsified residues

The viscosity expresses the oils ability to resist gradually deformation by increasing shear, where viscosities of so-called Newtonian oils remains constant independent on the applied shear rate (s^-1) at a given temperature.

When shear applies on so-called non-Newtonian oils, the viscosity of such oils decreases.

The viscosities of the water-free residues and emulsified residues of Wisting Central at 5°C is given in Table 2-7. The fresh oil and the water-free residues overall behave more or less as Newtonian fluids at shear rates of 10, 100 and 1000 s^-1, although a slightly decrease in viscosity is observed for the 250°C+ residue. The w/o- emulsions behave more as non-Newtonian fluids, where the viscosities are higher at a lower share rate (10 s^-1) compared to higher shear rate (100 s^-1) due to increasing degree of weathering (evaporating and water uptake).

Overall, the viscosities of the emulsions are low for Wisting Central. The yield stress, the force that must be applied to make the oil to begin to flow, is tabulated in Appendix E. The low yield stress substantiate that the oil has a potential to easily be spread on the sea surface.

Table 2-7: Viscosity of residues / emulsions of Wisting Central at 5 °C Residue Water

content

Viscosity (mPa.s) 5 °C

(vol. %) 10 s^-1 100 s^-1 1000 s^-1

Fresh 0 10 9 10

150°C+ 0 25 25 35

200°C+ 0 62 60 59

250°C+ 0 206 162 157

150°C+ 50 178 162 -

200°C+ 50 439 355 -

250°C+ 50 1229 774 -

150°C+ 75 1060 338 -

200°C+ 75 2058 614 -

250°C+ 75 4734 1125 -

150°C+ 83 810 219 -

200°C+ 70 1857 762 -

250°C+ 56 1622 966 -

*not applicable

2.1.3 Chemical dispersibility

The dispersibility testing of Wisting Central crude oil included:

 Screening of dispersants

 Dosage testing of a relevant dispersant

 Systematic testing on weathered /emulsified samples as a basis to predict the time-window for effective use of dispersant

Screening and dosage testing of dispersants

The screening testing was performed using the low energy test (IFP), reflecting non-breaking waves (< 5 m/s wind speed) according to the Norwegian dispersant testing regulations. The screening testing was performed at a standard temperature of 13°C, using a 200°C+ residue emulsified with 50 vol. % seawater. The results from the screening testing are presented in Table 2-8, where Dasic NS and Corexit 9500 expressed the highest dispersibility effectiveness (86 and 82 %, respectively).

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Table 2-8: Screening testing on Wisting Central crude oil using the IFP-test at standard temperature of 13 °C

Dispersant (DOR/DER =1:25)

Efficiency dispersant (200°C+ /50 vol. %

emulsion)*

Corexit 9500 82

Dasic NS 86

Finasol OSR 52 72

Gamlen OD 4000 69

Radiagreen OSD 60

* The viscosity of the emulsion 289 mPa.s (10s^-1)

The dosage testing was performed at 5°C, using a 200°C+ residue emulsified with 50 vol. % seawater with use of the high energy-test MNS) and low-energy test (IFP). Based on the screening testing, the Dasic NS dispersant was chosen in accordance with the client. The dosage testing gives information about the design conditions for optimal use of dispersant agent with the dispersant-to-oil/emulsion ratios (DOR /DER) of 1:25, 1:50, 1:100 and 1:200, and no dispersant added. The results are presented in Table 2-9, and show an increasing effectiveness for the IFP-test with higher applied DOR. The MNS-results were all high at the different dosages indicating high dispersibility in breaking waves conditions (> 5 m/s wind speed). This was also measured with no dispersant added (> 90 % effectiveness MNS test)) which is not commonly observed for crude oils.

Table 2-9: Dosage testing on Wisting crude oil using the IFP-and MNS-test at 5 °C Dispersant

(DOR/DER)

Efficiency of dispersant on 200°C+/50 vol. % emulsion*

IFP MNS

Dasic NS (1:25) 80 100

Dasic NS (1:50) 68 100

Dasic NS (1:100) 25 90

Dasic NS (1:200) 14 100

No dispersant 0 93

*The viscosity of the emulsion 439 mPa.s (10s^-1)

Systematic dispersant testing of the oil dispersibility at varying weathering

Based on the screening testing, Dasic NS was chosen for the continued systematic testing of the oil dispersibility at varying weathering to determine the time window for dispersant use. Dasic NS is also the dispersant agent in NOFO’s stockpile, and a dosage rate of 1:25 (4 wt.%) is used as the standard procedure used to establish the time window for dispersant application.

Table 2-10 shows the results from this testing as basis to estimate the dispersibility limits expressed as a function of effectiveness and viscosities (see Figure 2-4). The "window of opportunity" was predicted by use of the SINTEF Oil Weathering Model (OWM) based on the input data (dispersibility limits).

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Table 2-10: Effectiveness of dispersant on weathered Wisting Central oil/emulsions at 5 °C Residue Water content

(vol. %)

Viscosity (mPa.s) 10 s^-1

Viscosity (mPa.s) 100 s^-1

Effectiveness (%) Effectiveness (%)

IFP MNS

150°C+ 0 25 25 84 100

200°C+ 0 62 60 81 91

250°C+ 0 206 162 63 100

150°C+ 50 178 162 86 100

200°C+ 50 439 355 80 100

250°C+ 50 1229 774 80 100

150°C+ 75 1060 338 88 100

200°C+ 75 2058 614 62 97

250°C+ 75 4734 1125 16 27

150°C+ 83 810 219 71 100

200°C+ 70 1857 762 72 51

250°C+ 56 1622 966 77 100

Figure 2-4: Dispersant effectiveness of the Wisting Central oil/emulsion at 5 °C

Wisting Central was found to be good dispersible for viscosities lower than 3000 mPa.s reflecting viscosities where the IFP-test produce 50 % effectiveness. Reduced dispersibility is expected with viscosities above 3000 mPa.s. It should be emphasized that reduced dispersibility means that the oil/emulsion is still dispersible, but may require additional energy to enhance effective dispersion. The viscosity limit where Wisting Central is not dispersible, expressed with effectiveness lower than 5 % using the MNS-test, was not reached in the laboratory. This indicates that Wisting Central is expecting to have a wide window of opportunity for dispersant use. The dispersibility limits are also summarised in Table 2-11.

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Table 2-11: Estimated viscosity limit for Wisting Central for use of dispersant and criteria for definition of time window

Dispersibility Criteria (wt. %)

Dispersibility limits based on oil viscosities

(mPa.s) Chemically dispersible IFP efficiency > 50 % 3000 Not chemically dispersible MNS efficiency  5 % - -: The limit was not reached in the laboratory

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3 Predictions with SINTEF Oil Weathering Model (OWM)

3.1 Description of SINTEF OWM

A systematic stepwise laboratory procedure developed at SINTEF (Daling et al., 1990) is used to isolate and map the various weathering processes that take place when oil is spilled on the sea surface. Laboratory study of the weathering properties of Wisting Central crude oil was conducted at 5 °C and the analytical parameters were further used as input to the SINTEF Oil Weathering Model (OWM) (version 4.0 beta). The experimental design for the study of Wisting Central crude oil is described in Appendix B. The input data to the SINTEF OWM is given in Appendix C.

The SINTEF OWM relates oil properties to a chosen set of conditions (oil/emulsion film thickness, wind speeds and sea temperature) and predicts the change rate of the oil’s properties on the sea surface with time.

The SINTEF OWM is schematically shown in Figure 3-1. The predictions obtained from the SINTEF OWM are useful tools in the oil spill contingency planning related to the expected behaviour of oil on the sea surface, and to evaluate the time window for operational response strategies in a spill operation. In this report, the presented predictions span a time period from 15 minutes to 5 days after an oil spill has occurred. The SINTEF OWM is described in more detail in Johansen (1991), and in the user’s guide for the model.

Figure 3-1: Schematic input data to the SINTEF OWM and the predicted output oil properties

Spill scenario

A standard surface release from the OWM was used as the spill scenario.

Oil film thickness

In the OWM, the oils are categorized as condensate, emulsifying crude, low emulsifying crude, heavy bunker fuel or refined distillate based on experimental results obtained in the laboratory. The terminal film thickness varies among these categories based on experimental (field) experience. Wisting is categorized as emulsifying oil.

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Sea temperature

The prevailing weather conditions greatly influence the weathering rate of oil on the sea surface. Two sets of predictions are given in this report. Due to the location of the oil field, the prediction temperatures chosen for Wisting Central were 2 and 5 °C.

Wind speed

The relationship between the wind speed and significant wave heights used in the prediction charts obtained from the SINTEF OWM are shown in Table 3-1.

Table 3-1: Relationship between wind speed and significant wave height used in the SINTEF OWM Wind speed [m/s] Beaufort wind Wind type Wave height [m]

2 2 Light breeze 0.1 - 0.3

5 3 Gentle to moderate breeze 0.5 - 0.8

10 5 Fresh breeze 1.5 - 2.5

15 6 – 7 Strong breeze 3 - 4

3.2 Prediction of Wisting Central weathering properties Input to the OWM

Oil type: Crude oil

Geographical area: Barents Sea Terminal oil film thickness: 1 mm

Release rate: 1.33 metric tonnes/minute for 15 minutes; a total of 20 metric tonnes Sea temperature: 2 °C and 5 °C

Wind speed: 2 m/s, 5 m/s, 10 m/s and 15 m/s

How to use the prediction charts: an example

If the oil has drifted on the sea surface, the following prediction charts can be used to determine the weathering properties of the oil/emulsion. Table 3-2 gives examples for the following scenario:

• Drift time: 12 and 24 hours

• Sea temperature: 2 °C / 5 °C

• Wind speed: 10 m/s

Table 3-2: Example of the weathering properties for Wisting Central obtained from the OWM predictions after 12 and 24 hours of weathering

Weathering property 12 hours 2°C, 10 m/s

12 hours 5 °C, 10 m/s

24 hours 2 °C, 10 m/s

24 hours 5°C, 10 m/s

Evaporation, wt. % 31 32 34 35

Water content, vol. % 51 51 60 60

Flash point, °C 93 96 103 107

Pour Point, °C -36 -36 -36 -36

Viscosity, mPa·s* 850 770 1600 1430

*mPa.s = cP (mPa.s: SI-standard / cP: Industrial denotation)

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Figure 3-2: Evaporative loss of Wisting Central crude oil predicted at sea temperatures of 2°C and 5°C Property: EVAPORATIVE LOSS

Oil Type: WISTING CENTRAL 5C, 2017 Description: SINTEF ID:2016-320

Data Source: Sintef Ocean (2017), Weathering data used

Surface release - Terminal Oil film thickness: 1 mm

Release rate/duration: 1.33 metric tons/minute for 15 minute(s)

OWModel

Pred. date: May. 08, 2017

Wind Speed (m/s): 15 Wind Speed (m/s): 10 Wind Speed (m/s): 5 Wind Speed (m/s): 2

Sea surface temperature: 2 °C

Evaporated (%)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 10 20 30 40 50 60 70 80 90 100

Winter Conditions (5 °C)

Evaporated (%)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 10 20 30 40 50 60 70 80 90 100

(23)

Figure 3-3: Flash point of Wisting Central predicted at sea temperatures of 2°C and 5°C Property: FLASH POINT FOR WATER-FREE OIL

OWModel

No fire hazard

Fire hazard in tankage (<60 °C)

Fire hazard at sea surface (below sea temperature)

Based on flash point measurements of weathered, water-free oil residues.

Flash Point (°C)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 50 100 150

Flash Point (°C)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 50 100 150

(24)

Figure 3-4: Pour point of Wisting Central predicted at sea temperatures of 2°C and 5°C Property: POUR POINT FOR WATER-FREE OIL

OWModel

Chemically dispersible Reduced chemical dispersibility Poorly / slowly chemically dispersible

Based on pour point measurements of weathered, water-free oil residues.

Pour Point (°C)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

-40 -20 0 20

Pour Point (°C)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

-40 -20 0 20 40

(25)

Figure 3-5: Water content of Wisting Central predicted at sea temperatures of 2°C and 5°C Property: WATER CONTENT

OWModel

Water content (%)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 20 40 60 80

Water content (%)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 20 40 60 80

(26)

Figure 3-6: Viscosities of Wisting Central emulsions predicted at sea temperatures of 2°C and 5°C. Predictions are based on measurements of emulsions performed at a shear rate of 10 s^-1

Property: VISCOSITY OF EMULSION Oil Type: WISTING CENTRAL 5C, 2017 Description: SINTEF ID:2016-320

OWModel

Chemically dispersible (<3000 cP) Reduced chemical dispersibility

Poorly / slowly chemically dispersible (>1000000 cP)

Based on viscosity measurements carried out at a shear rate of 10 reciprocal seconds.

Chemical dispersability information based on experiments under standard laboratory conditions.

Viscosity (cP)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

10 100 1000 10000

Viscosity (cP)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

10 100 1000 10000

(27)

Figure 3-7: Predicted mass balance for Wisting Central at 2°C and wind speeds of 2 and 5 m/s Property: MASS BALANCE

OWModel

Evaporated Surface

Naturally dispersed

Temperature: 2 °C Wind speed: 2 m/s

Mass (%)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 20 40 60 80 100

Mass (%)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 20 40 60 80 100

(28)

OWModel

Evaporated Surface

Naturally dispersed

Mass (%)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 20 40 60 80 100

Mass (%)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 20 40 60 80 100

(29)

OWModel

Evaporated Surface

Naturally dispersed

Mass (%)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 20 40 60 80 100

Mass (%)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 20 40 60 80 100

(30)

OWModel

Evaporated Surface

Naturally dispersed

The algorithm for prediction of natural dispersion is preliminary and is currently under improvement.Model predictions have been field-verified up to 4-5 days.

Mass (%)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 20 40 60 80 100

Mass (%)

Hours Days

0.25 0.5 1 2 3 6 9 12 1 2 3 4 5

0 20 40 60 80 100

(31)

4 OWM predictions - Comparison of Wisting Central with other crude oils

Weathering predictions of Wisting Central were compared to a selection of other crude oils from the Barents Sea. The crude oils chosen for comparison are Wisting Hanssen, Skrugard from the Johan Castberg field, Realgrunnen and Alta. The presented comparisons given in the figures below are based on predictions at 5 °C and wind speed of 10 m/s.

4.1 Evaporative loss

The evaporative loss of Wisting Central and the oils used for comparison are presented in Figure 4-1 below.

Wisting Central and Alta have similar and the highest predicted evaporative, while the Skrugard exhibits the lowest evaporative loss due to its high density in comparison with the other oils.

Figure 4-1: Predicted evaporative loss at 5 °C and 10 m/s for Wisting Central compared to other oils

(32)

4.2 Flash point

As oil spilled on the sea surface, the temperature of the oil is cooled to the ambient water temperature within a short period. The fire hazard will be high as long as the flash point of the oil is below the sea temperature. The fire hazard dependent on the proportions of volatile components in the oil is usually over within few minutes due to the rapid evaporation of those components. The flash points of Wisting Central and other oils in comparison are shown in Figure 4-2. None of the oils show any fire or explosion hazard 15 minutes after release where the flash points is far beyond the sea temperature of 5 °C.

Figure 4-2: Predicted flash point at 5 °C and 10 m/s for Wisting Central compared to other oils

(33)

4.3 Pour point

Pour point depends on the oil’s wax content and the amount of light components that are able to keep the waxes dissolved in the oil phase. In addition, the asphaltene content prevents precipitation and lattice formation and hence lowers the pour point. High pour points may prevent the dispersant to soak into the oil slick and influence on the dispersant effectiveness. The pour points of Wisting Central and the other oils for comparison are given in Figure 4-3.

Wisting Central has very low pour points for the fresh oil and its residues, and similar, Skrugard also expresses low pour points and none of this oils will solidify at sea at the selected weather condition. Realgrunnen and Alta have the highest pour points and could therefore potentially pose a challenge with solidification at sea.

Figure 4-3: Predicted pour point at 5 °C and 10 m/s for Wisting Central compared to other oils

(34)

4.3.1 Water content

Water uptake and content for Wisting Central and the oils used for comparison are shown in Figure 4-4. The maximum water uptake of Wisting Central is similar to Wisting Hanssen (60 vol. %), but the Wisting Central has a slower water uptake. Skrugard and Alta have both rapidly and high maximum water uptake of 80 vol. %.

Figure 4-4: Predicted water content at 5 °C and 10 m/s for Wisting Central compared to other oils

(35)

4.3.2 Emulsion viscosity

Figure 4-5 show the predicted emulsion viscosities of Wisting Central and the other oils in comparison.

Wisting Central has low emulsion viscosities compared with the other crude oils. Realgrunnen has the highest emulsion viscosities, whilst Wisting Hanssen, Skrugard and Alta end up with more or less similar viscosities after five days of weathering.

Figure 4-5: Predicted emulsion viscosity at 5 °C and 10 m/s for Wisting Central compared to other oils

(36)

4.3.3 Surface oil

Due to evaporative loss and natural dispersion/entrainment, the amount oil on the sea surface will gradually be reduced. Figure 4-6 illustrates the predicted mass balance of the Wisting Central surface oil compared to the other oils in comparison. Wisting Central has the lowest remaining surface oil during 5 days of weathering due to high evaporative loss and natural dispersion /entrainment. Skrugard expresses the highest remaining surface oil. Wisting Hanssen and Realgrunnen have comparable and similar remaining surface oil.

Figure 4-6: Predicted remaining surface oil at 5 °C and 10 m/s for Wisting Central compared to other oils

(37)

5 Weathering properties of Wisting Central crude oil related to oil spill

5.1 Oil properties

Wisting Central is a medium density crude oil (0.838 g/mL) with low content of wax (0.71 wt. %) and asphaltene (0.05 wt. %), compared with other Norwegian crude oils. Wisting Central is categorised as a naphtenic (biodegraded) crude oil, as shown in Figure 2.1 for chromatographic characterization (GC-FID) of the oil. The hydrocarbon profile shows that the crude oil lacks of the systematic n-alkanes (paraffins). Wisting Central has very low pour point (-<36°C) for the fresh oil and residues. The oil forms water-in-oil (w/o) emulsions maximum water uptake ~ 60 vol. % and with low emulsion viscosities.

Knowledge of the alterations in physical and chemical properties caused by weathering processes are of importance for oil spill response.

5.2 Flash point – Fire/explosion hazard

As oil is spilled on the sea surface, the temperature of the oil will be cooled to the ambient water temperature within a short period. The fire hazard will be at its greatest as long as the flash point of the oil is below the sea temperature. For Wisting Central the flash point will be above the sea temperature within the first 15 minutes after a spill in at 5 and 10 °C, as shown in Figure 5-1 below.

Some vessels/storage tanks engaged in oil recovery operations may not be classified to carry liquids with flash point lower than 60 °C (e.g. towing vessels, smaller cargo or vessels available in the emergency). For Wisting Central, this limit will be reached within approximately 6 hours in low wind conditions (2 m/s) at 2 and 5°C, and more rapidly with higher wind speeds.

Figure 5-1: Flash point at different wind speeds for Wisting Central crude oil at 2°C and 5°C

Wisting Central crude oil – Properties and behaviour at sea - In relation to oil spill response

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