Norwegian Testing of Emulsion Properties at Sea––The Importance

Spill Science & Technology Bulletin, Vol. 8, No. 2, pp. 123–136, 2003

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Norwegian Testing of EmulsionProperties at Sea––The Importanceof Oil Type and Release ConditionsPER S. DALING�*, MERETE ØVERLI MOLDESTAD�, ØISTEIN JOHANSEN�,ALUN LEWIS� & JON RØDAL§

�SINTEF Applica Chemistry, Marine Environmental Technology Department, N-7465 Trondheim,

Norway

�Oil Spill Consultant, Staines, Middlesex TW 18 2EG, UK

§Norwegian Clean Seas Association for Operating Companies (NOFO), N-4002 Stavanger, Norway

This paper is a review of the major findings from laboratory studies and field trials conducted in Norway inrecent years on the emulsification of oils spilled at sea. Controlled bench-scale and meso-scale basin ex-periments using a wide spectrum of oils have revealed that both the physico-chemical properties of the oilsand the release conditions are fundamental determinants of the rate of emulsion formation, for the rhe-ological properties of the emulsion formed and for the rate of natural dispersion at sea.During the last decade, several series of full-scale field trials with experimental releases of various crude

oils have been undertaken in the North Sea and the Norwegian Sea. These have involved both sea surfacereleases, underwater pipeline leak simulations (release of oil under low pressure and no gas) and underwaterblowout simulations (pressurized oil with gas) from 100 and 850 m depth. The field trials have been per-formed in co-operation with NOFO (Norwegian Clean Seas Association for Operating Companies), in-dividual oil companies, the Norwegian Pollution Control Authority (SFT) and Minerals ManagementServices (MMS). SINTEF has been responsible for the scientific design and monitoring during these fieldexperiments. The main objectives of the trials have been to study the behaviour of different crude oils spilledunder various conditions and to identify the operational and logistical factors associated with differentcountermeasure techniques.The paper gives examples of data obtained on the emulsification of spilled oil during these field ex-

periments. The empirical data generated from the experimental field trials have been invaluable for thevalidation and development of numerical models at SINTEF for predicting the spreading, weathering andbehaviour of oil released under various conditions. These models are extensively used in contingencyplanning and contingency analysis of spill scenarios and as operational tools during spill situations andcombat operations.� 2003 Elsevier Science Ltd. All rights reserved.

Introduction

Prior knowledge of the likely behaviour of a spilled

oil and pre-spill analyses of the feasibility of different

*Corresponding author. Tel.: +47-73-59-12-41; fax: +47-73-59-

70-51.

E-mail addresses: [email protected] (P.S. Daling), mer-

[email protected] (M.Ø. Moldestad), oistein.johansen@sin-

tef.no (Ø. Johansen), [email protected] (A.

Lewis), [email protected] (J. Rødal).

response strategies under various environmental andrelease conditions should be an essential part of any

oil spill contingency planning. Predicting the amount

of damage that might occur in various oil spill sce-

narios enables the effectiveness of alternative response

strategies to be assessed. Such a pre-spill analysis has

been formalized as the NEBA process (‘‘Net Envi-

ronmental Benefit Analysis’’) of a combat operation

(e.g. Baker, 1995, 1997).In Norway, the responsible party takes the lead in

responding to an oil spill. This is in accordance with

the ‘‘principle of internal control’’ that is used by the

123

mail to: [email protected]

P.S. DALING et al.

pollution authorities. The Norwegian Pollution Con-

trol Authority requires well-documented contingency

plans for refineries, oil terminals and offshore instal-

lations. The SINTEF Oil Weathering Model (OWM,e.g. Daling et al., 1997; Daling & Strøm, 1999) and theOSCAR (Oil Spill Contingency and Response) modelsystem (Aamo et al., 1996, 1997a,b; Reed et al., 1995–1997, 2001) is now extensively used in such contin-gency planning. Scenario analysis is used to quantifythe fate, weathering, potential environmental conse-quences and the feasibility and effectiveness of variousmitigation methods.

The behaviour of spilled crude oils and refined oil

products depends on:

• the release conditions (the rate and amount of oilspilled, surface release or underwater release, pres-

ence of gas, etc.);

• the prevailing environmental conditions (e.g. tem-

perature, sea-state, currents);

• the physico-chemical properties of the spilled oil

and its propensity to disperse into the water column

or to form stable water-in-oil (w/o) emulsions on the

sea surface.

For example, the Gullfaks crude spilled at the Braer

incident in the Shetlands had a very low content ofwaxes and asphaltenes, which are important com-pounds for stabilizing w/o emulsions formed on thesea surface. This, combined with the exceptionallyviolent weather conditions that prevailed at the time ofthe spill, resulted in almost all of the 84,000 tons of thespilled Gullfaks crude oil being naturally dispersed(Ritchie et al., 1993).

During the Sea Empress spill of Forties Blend crudeoil (another North Sea crude oil) a significant amount

Fig. 1 (a) Mixing of oil and water at sea (from Lewis et al., 1994). (b) Schemrigid skin of waxes, asphaltenes, photo-oxidated compounds (resins).

124

of the surface oil was converted into w/o emulsions(Lunel et al., 1996). The feasibility of various coun-termeasure techniques such as chemical treatment,burning or mechanical recovery, would therefore begreatly influenced both by the release conditions andchanging properties exhibited by the weathered oilresidue or the w/o emulsion that have been formed.

Competition between the dispersion and emulsificationprocesses

Very heavy fuel oils like the industrial heavy fuels

oils spilled by the Erika and the Baltic Carrier formwater-in-oil emulsions slowly. However, many spilledcrude oils will rapidly form w/o emulsions when spil-led at sea (e.g. Lewis et al., 1995a,b). Such emulsionswill initially have low viscosities, will be unstable, andwill tend to revert to the oil residue and water if theyare removed from the mixing action of the sea. Un-stable emulsions are simply mixtures of water dropletsin oil and the w/o emulsion present on the surface willbe the result of the dynamic equilibrium of emulsionformation and emulsion breakdown (see Fig. 1(a)). Asthe viscosity of the oil residue increases due to theevaporative loss of more volatile components and theprecipitation of stabilizing agents (asphaltenes, photo-oxidized compounds (resins) and in some crude oilsprecipitated waxes) the emulsion becomes more stable.The precipitated asphaltenes create an elastic ‘‘skin’’between the water droplets and the oil (see Fig. 1(b)).The stability of the emulsion will increase because thewater droplets cannot coalesce and drain so easilyfrom the emulsion and the equilibrium will tend tofavour emulsion formation. After an extended periodof weathering and mixing at sea the w/o emulsionproduced will have a very high viscosity and be verypersistent.

atic diagram showing how a water emulsion droplet is stabilized by a

Spill Science & Technology Bulletin 8(2)

NORWEGIAN TESTING OF EMULSION PROPERTIES AT SEA

In the early stages of a spill, the rate of natural

dispersion will depend [in a large measure] on the sea

state. Breaking waves provide the energy needed to

disrupt low viscosity (non-emulsified) oils and producethe very small oil droplets with diameters below 50–100

lm that may be considered to be permanently dis-

persed (Delvigne & Sweeney, 1988). One of the factors

that resist the process of natural dispersion is the me-

chanical strength of the oil or w/o emulsion layer.

Weathered oils (Berger et al., 1993) and w/o emulsions(Sherman, 1968) are liquids or semisolids with morecomplex rheological characteristics and will exhibithigh apparent viscosities, an elastic component and/or

Film Thickness of S

1cm

1mm

0.1mm

10µm

0.1µm

0.04µm visibility limit

Sheen formationby droplet resurfacing

Non-emulsifyoil

Emulsifyingoil

EmulsificationSpreading

1µm

SH

EE

NT

HIN

OIL

TH

ICK

OIL

& E

MU

LSIO

N

HOURS

C

Fig. 2 Schematically illustration of the development in film thickness, breversus a non-emulsifying oil at sea.

Evaporation

Em

uls

ific

atio

n w

ith

wat

er

WOR = 1

WOR = 3

WOR = max

S(

(a) (b)

Fig. 3 (a) Flow chart for ‘‘stepwise’’ weathering (evaporation and water-ievaporation residues. (b) Emulsification of oils.


a definite yield stress. W/o emulsification thereforeretards the rate of natural dispersion. Natural disper-sion and w/o emulsification are therefore ‘‘competingprocesses’’ as illustrated schematically in Fig. 1(a).

Thick layers of oil will be able to accommodate

entrained water droplets of greater size more easily

than thin film oil layers. Very thin oil layers will be

disrupted by the presence of water droplets and will

split to release the water. The thickness of the oil layer

therefore has an effect on the ease and rate of w/o

emulsification. The relative rates of natural dispersion

and w/o emulsification will therefore depend on theinitial film thickness (and this will be determined by

urface Released Oil

ing

DAYS

ontinuous slickLarge emulsion

Small emulsion pathes

ak-down and relative lifetime on the sea surface of an emulsifying

Before mixing

Oil

(30 mL)

eawater300 mL)

Axis ofrotation(30 rpm)

24 hoursmixing

WOR0-

2-

4-

6-

8-

24 hours mixing and 24 hours settling

n-oil emulsification) of a crude oil. The four water-free samples are

125

P.S. DALING et al.

release conditions), the sea state (including the pres-

ence or absence of convergence zones) and the phys-

ico-chemical properties of the oil. At low or

intermediate sea states the rate of natural dispersionwill initially be high but will be reduced and eventually

cease as the oil remaining on the sea surface becomes

more viscous due to evaporation of lighter compo-

nents and increased viscosity due to the formation of a

w/o emulsion (Lewis et al., 1994).The turbulence provided by breaking waves is lo-

cally intense, but its effect decreases with weathering of

oil. If the apparent viscosity of the oil or w/o emulsionis very high or if it possess a significant elastic com-

ponent, the prevailing turbulence may be unable to

break up the slick into oil droplets. There will therefore

be no significant amount of droplet formation or nat-

ural dispersion. At higher sea states, the turbulence will

be able to break up viscous emulsions into fragments

which may appear as large lumps which will not nat-

urally disperse but may be washed-over by waves forextended periods. While not being naturally dispersed,

this mechanism will apparently remove the bulk of oil

pollution from localized areas by distributing it across

much larger areas of the sea surface. This is illustrated

schematically in Fig. 2. The life-time on the sea surface

of emulsified oil slicks and fragments of surface

slicks will therefore be relatively longer (under oth-

erwise similar sea conditions) than a light, non-vis-cous, non-emulsifying oil (like a condensate or a diesel

oil).

Emulsification Studies in Laboratoryand Weathering Flume Basins

At SINTEF there has been a continuous R&D ac-

tivity on oil spill weathering and behaviour at sea since

the Ekofisk Bravo accident in 1977. The SINTEF ap-proach for characterization and predictions of the oilweathering properties has involved:

Table 1 Analytical methods used in the determination of the phys-ical and chemical properties

Property Instrument Method

• Bench-scale laboratory weathering studies;

• Meso-scale flume basin weathering;

• Oil Weathering Model (OWM);

• Verification and correlation of laboratory data and

model predictions through ‘‘ground-truth’’ data

from experimental field trials.

Distillation Stiver and Mackay (1984)(a modified ASTMD86/82)

Density DMA 4500 ASTM D4052-81Water content Karl FischerViscosity Physica MCR 300 McDonagh et al. (1995)

(viscosity measured atshear rate 1, 5, 10, 50, 100,

500 and 1000 s�1)Pour point ASTM D97-66

A standardized laboratory investigation of each

specific oil forms the basic input to the SINTEF

OWM. Daling and Strøm (1999) have earlier pre-

sented correlations between oil weathering values

predicted by the model and ground-truth data ob-

tained from field trials.

126

Bench-scale weathering and emulsification studies

A small-scale laboratory procedure to characterize

the weathering and emulsification properties of dif-

ferent oils has been refined and standardized (e.g.

Daling et al., 1990, 1997). This includes ‘‘stepwise’’distillation (topping) of the oils into 150 �C+, 200�C+ and 250 �C+ residues, simulating the evapora-tive loss corresponding to typically 1 h, 1 and 5 daysweathering at sea, respectively. To isolate the influenceof normally simultaneous weathering processes (i.e.evaporative loss, photo-oxidation and water-in-oilemulsification), the oil residues (prepared either bydistillation or photo-oxidation) and the w/o emulsions(altogether 16 weathered samples) are prepared in a‘‘stepwise’’ manner from the fresh oil (schematicallyshown in Fig. 3(a)).

The w/o-emulsification of the oil residues were

carried out based on a modified version of the rotating

cylinder method developed by Mackay and Zagorski

(1982) (see Fig. 3(b)). The methodology used to pro-

duce w/o emulsions and the procedures used are

described in detail by Hokstad et al. (1993). Theemulsification kinetics is mapped by measuring thewater content at fixed rotation times. The maximumwater content is determined after 24 h of rotation. Therotation speed (30 rpm) corresponds to relatively highturbulence at sea, leading to a water uptake rate ap-proximately four times quicker than that observed infield trials with the same oil in wind conditions of 10m/s (Johansen, 1991). That is, 1 h in the Mackay/Zagorski apparatus is roughly giving equivalent watercontent to 4 h of mixing under wind conditions of 10m/s at sea.

The stability of a w/o emulsion is defined by mea-

suring the fraction of water dehydrated from the

emulsion after:

• 1 h settling,

• 4 h settling,

• 24 h settling.


0

20

40

60

80

100

10 15 20 25

Weathering at sea (hours)

Wat

er c

onte

nt (

vol.%

)

UlaEkofiskStatfjordTauGullfaksBalder

0 5

Fig. 4 Water uptake rate for 6 oils at sea temperature: 15 �C andwind speed: 10 m/s. Predictions using SINTEF OWM, based oninput data from bench-scale testing.

1000

100

10

0

Vis

cosi

ty r

atio

Water content (vol%)

Alaskan North Slope

0 20 40 60 80

The Mackay curve150˚C+200˚C+250˚C+Ph.ox

Fig. 6 The Mackay curve and experimental data as input to modelsfor predicting emulsion viscosities for emulsions formed at sea.

0

10

20

30

40

50

60

70

80

90

100

Oil film thickness (mm)

Wat

erco

nte

nt

inem

uls

ion

(vo

l.%

)Troll 200˚C+

Brage 200˚C+

Norne 200˚C+

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Fig. 7 Emulsion formation ability as function of oil type and initialoil film thickness (Melbye et al., 1999).


The instruments and methods used to characterizethe weathered oils are listed in Table 1. Figure 4 pre-

sents examples of the large variation predicted in

water uptake rate and maximum water content ob-

tained for six different North Sea crude oils under

similar environmental conditions. The predictions are

based on input data to the Oil Weathering Model

from standardized laboratory studies of the individual

crude oils.The water droplet size created in the emulsion de-

pends on the shear used during mixing and the mixing

time (see Fig. 5). Both the viscosity and stability

properties of the emulsion will depend, in part, on the

water droplet size distribution in the emulsion. The

water droplet size distribution created by the rotating

flask method is in the same range as that measured in

emulsions formed at sea (e.g. Strøm-Kristiansen et al.,1996). High-intensity mechanical stirring systems tendto produce much smaller water droplets with a tightersize distribution that are unlike those found in emul-sions observed at sea.

Figure 6 illustrates the ‘‘general correlation equa-

tion’’ (black curve) based on the Einstein, Mooney

and Mackay relationships between dispersed water

phase volume and viscosity used in many models topredict the viscosity of emulsion based on the viscosity

Fig. 5 Microscope pictures w/o emulsion using the ro


of parent oil and water content. The model implies a

fixed water droplet size distribution. Specific labora-

tory measurements of the viscosity of oil residues andthe emulsions formed from them in the rotating flask

method, are used to ‘‘tune’’ this correlation equation

for specific oil being considered (see Fig. 6). This will

lead to more reliable predictions of the emulsion vis-

cosity for individual oils, compared to using only a

‘‘general’’ correlation equation, because the variation

in water droplet size distribution is considered for.

The variations in the stability of emulsions formedfrom various crude oils vary greatly with stability in-

creasing with increased oil weathering. These varia-

tions reflect the variability in the availability of

tating flasks, and the influence of mixing time.

127

2

1. Wave generator2. Photolysis (sun-lamp)3. Wind-tunnel4. Sub-surface sampling

4

1

4 m

3

4

Fig. 8 Schematic drawing of the meso-scale flume weathering basin(seen from above).

P.S. DALING et al.

emulsion stabilizing agents from the oil that are im-

portant for the ‘‘skin-formation’’ and elasticity that

are reflected in the rheological properties of the

emulsion.

Table 2 Example of emulsion properties and appearance after weathering

Oils Water content (vol.%)

Norne crude (Waxy, North Sea) 35

Jotun (paraffinic, North Sea) 58Grane (asphaltenic, North Sea) 70IFO-240 Heavy Bunker Fuel (Esso-Refinery) 50

Fig. 9 Difference in w/o emulsion properties and appearance after 3 dayscrude (Waxy, North Sea); (b) Jotun (paraffinic, North Sea); (c) Grane (as

128

The conditions for emulsion formation differ in sub-

sea releases. The oil film thickness at the sea surface is

much smaller. Alternative methods for emulsion for-

mation (tilting flasks, Melbye et al., 1999) have beenused to form emulsions as a function of oil filmthickness. Figure 7 illustrates an example on theemulsion formation as function of the initial oil filmthickness. These data indicates the ability for emulsi-fication depends highly on both the initial film thick-ness and oil type, a minimum film thickness in therange of 50–500 lm (depending on oil type) is neededfor obtaining emulsion formation at sea. Oil concen-trating in windrows or convergence zones associatedwith Langmuir circulation of cells (Spill Science andTechnology Bulletin, vol. 6, No. 3/4, 2000) can reachthese thicknesses, and in situ w/o emulsification undersuch conditions has been observed at sea (Payne &Phillips, 1985; Payne et al., 1987).

in the SINTEF meso-scale weathering basin

Dynamic viscosity at 10 s�1 (cP) Appearance of theemulsion formed

3.000 (5 �C) Solidified oil lumps, turnslowly into brown color

2430 (13 �C) Yellow/light brown11.000 (13 �C) Dark, viscous, sticky, lumps23.000 (15 �C) Dark emulsion. Still dispers-

ible using dispersants!!

weathering in the SINTEF meso-scale weathering basin. (a) Nornephaltenic, North Sea) and (d) IFO-240 Heavy Bunker Fuel.



Meso-scale flume basin weathering and emulsificationexperiments

The meso-scale flume basin (Singsaas et al., 1993) atSINTEF is routinely used to study the weatheringprocesses under controlled conditions as a supplementto the ‘‘stepwise’’ weathering studies. A schematicdrawing of the meso-scale flume basin is given in Fig.8. Approximately 1.7 m3 of seawater circulates in the10 m long flume. The flume is located in a conditioningroom. Two fans (3) placed in a covered wind tunnelallow various wind speeds. The evaporation rate iscalibrated to simulate a wind speed of 5–10 m/s. Theexperimental data obtained from the meso-scale test-

Fig. 10 Map showing the location of the spill site and surroundingareas.

Fig. 11 Formation of surface oil slick by use of the T


ing provides a link between the bench scale testing andfield trials and gives important data input to theSINTEF Oil Weathering Model (OWM). Addition-ally, the flume provides good information about thevariability in appearance (e.g. color, visual behaviourand spreading properties) for various oils. The watercontent and viscosities of four different oils in theflume is shown in Table 2. Figure 9 shows the ap-pearance of the four oils after 3 days weathering in theflume. The colors of the different emulsions are verydifferent from light, yellow to dark brown and black.The water content varies from 35% to 70%. Some ofthe emulsions form a continuous oil slick while othersbreak to form lumps. Figure 9 shows that the ap-pearance of different oil types is very different due tovariations in chemical and rheological properties.

Full-Scale Field Trials in Norway

Unique field data have been obtained on emulsifi-

cation and behaviour of oils spilled during four series

of experimental releases with various oils simulatingvarious release conditions. These studies involved

both surface releases (1994, 1995, and 1996), simulated

sub-sea pipeline leaks (release of oil and no gas, 1994),

and simulated sub-sea blowouts (oil with gas) released

both from 100 m depth (1996) and 850 m depth

(DeepSpill June 2000). Three of the field series were

carried out in the Frigg area in the North Sea, while

the DeepSpill experiment was performed in the Hel-land Hansen area in the Norwegian Sea (Fig. 10). The

field trials have been performed in co-operation be-

tween spill response organisations (NOFO), individual

oil companies, and governmental pollution control

ransrec oil recovery system (NOFO-1994 trial).

129

P.S. DALING et al.

agencies (SFT and MMS). SINTEF was responsible

for the scientific design and monitoring during the

field experiments.

20181614121086420

2nd dispersant spray

Dispersant sprayEvaporation

Treated (Tango)

Control (Charlie)

1st dispersant spray

Amount of oil on sea surface(Tentatively)

Oil

(m³)

The NOFO 1994 field trial––surface release

One of the main objectives of the 1994 field trials

was to verify the results from laboratory studies and

model predictions of oil weathering (evaporation,

natural dispersion and emulsification). Two oil slicks(2� 20 m3 of Sture Blend North Sea crude) were re-

leased gently and controlled on the surface 100 m

behind the release vessel, using the Transrec oil re-

covery system in reverse (Fig. 11). The sea state was

quite rough with wind speed varying between 8 and

12 m/s and about 2.5 m significant wave heights. A

control slick (‘‘Charlie’’) was followed with exten-

sive surface monitoring for 32 h before dispersantstreatment. The other slick (‘‘Tango’’) was treated

with dispersant (Corexit 9500) after 3 and 7 h weath-

ering.

As soon as the oil was released on the surface, the

slicks rapidly formed an elongated shape as some of

the oil was temporarily dispersed and then re-surfaced

away from the thicker areas (in the front of the slick)

and spread out again as sheen. Measurements showeda very wide variation in the slick thickness, with three

main categories observed:

Time from release (hours)0 5 10 15 20 25 30

Fig. 13 Lifetime of surface oil slick; slick treated with dispersant(‘‘Tango’’) versus control slick (‘‘Charlie’’) (NOFO-1994 trial).

• Thick w/o emulsion (2–9 mm) in the front part ofthe slick;

Fig. 12 Schematic drawing of the distribution of oil sl

130

• Black oil (up to about 0.1 mm);

• Sheen (up to about 1 lm).

An estimate of the distribution in area and mass of

these categories based on remote sensing and film

thickness measurement is presented schematically in

Fig. 12. The weathering behaviour and physical

properties of the thick emulsion that formed on thesurface were in accordance with results from previous

laboratory studies. The weathering data were used to

calibrate and verify the SINTEF Oil Weathering

Model (Daling & Strøm, 1999).

Figure 13 gives an estimate of amount of oil on the

surface and the lifetime of the two cohesive slicks

based on aerial monitoring. Further details concern-

ing operational aspects, monitoring, analytical meth-ods and conclusions from this sea trial have been

published in several reports, e.g. Lewis et al.

(1995a,b).

ick thickness after 3 h at sea (NOFO-1994 trial).



NOFO 1995 field trial––surface and underwater ‘‘pipe-line’’ releases

One of the aims of the NOFO-field trial in 1995

(Brandvik et al., 1996) was to study the behaviour of

Fig. 14 Schematic illustration of the sub-sea release arrangement, the plum

Fig. 15 Composite sketch of remote sensing imagery showing the slick di(Uniform) after 4 h of weathering at sea (NOFO-1995 trial).


oil released from sub-sea pipeline leaks. A total of 25m3 of Troll crude was released over a period of 20 minfrom 100 m depth (slick called ‘‘Uniform’’). A refer-ence spill (surface slick ‘‘Sierra’’) was released usingthe same conditions as in the 1994 trials. The release

e created and monitoring strategy (NOFO-1995 trial).

mensions of the surface release (Sierra) and the underwater release

131

P.S. DALING et al.

and monitoring arrangement of the sub-sea release isillustrated in Fig. 14. Due to relatively low exit ve-locities (about 2 m/s), relative large oil droplets wereformed (typically 2–6 mm in diameter) when releasedfrom the sub-sea installation. An oil slick started toform at the surface about 10 min after the start of therelease. Oil samples taken as the oil appeared on thesurface indicated that no w/o emulsion was formed inthe plume, but that the emulsion was subsequentlyformed after the oil appeared on the sea surface. Theweathering properties of the thick emulsion formed onthe surface were similar to the properties of theemulsion generated in the surface release (Sierra).Subsequent analysis of oil samples from both slicksshowed good agreement with predictions using SIN-TEF OWM based on laboratory input data of Troll

Fig. 16 Photo of the surface slick taken during the simulated sub-seablowout (NOFO-1996 trial).

Fig. 17 Schematic drawing of sub-sea blowouts from

132

crude oil (Daling & Strøm, 1999). Observations of thetwo oil slicks 4 h after the releases indicated that the

medium depths (left) and deep waters (right).

Fig. 18 (a) Aerial photo of the surface slick taken after the crude oilrelease. (b) Photo taken during surface sampling from emulsifiedpatches (1–1.5 mm thick) generated during 3 h weathering on thesurface (DeepSpill 2000).



slick formed from the sub-sea release were slightlylarger due to the larger spreading at the start of therelease (Fig. 15).

Further details both concerning operational aspects,monitoring, analytical methods and conclusions from

this sea trial are described in several reports including

Brandvik et al. (1996).

Fig. 19 Comparison between predicted and measured values of viscosity of(DeepSpill 2000).


NOFO 1996 field trial––simulated sub-sea blowout frommoderate depths

One of the aims of the NOFO-field trial in 1996

(Rye et al., 1997) was to increase our knowledge of thebehaviour of the oil in sub-sea blowouts. This wasdone by simulating a blowout released from 100 m

emulsion (top) and water content after surfacing of the oil (bottom)

133

P.S. DALING et al.

depth with a realistic gas-to-oil ratio (GOR¼ 65) anda release rate of 1 m3 oil per minute, with a total of 45m3 Troll crude oil released over 45 min. For safetyreasons, pressurized air was used in place of naturalgas. The release method and monitoring arrangementwere similar to the 1995 sub-sea release (see Fig. 14).However, due to a much higher exit velocity (about 20m/s), very small oil droplets were formed that werecarried to the surface by the gas bubble plume. Thefirst oil appeared on the surface about 2 min after thestart of the release. The high initial spreading of the oilon the surface (see Fig. 16) was in accordance withcalculations using the blowout model developed byFanneløp and Sjøen (1980). The horizontal spreadingof the plume near the surface resulted in a very thinand homogenous oil film on the surface. Some smalltendency to Langmuir cells could be observed by IR-images, resulting in a thickness range of 10–40 lm.

Film Thickness of SReleases

1cm

1mm

0.1mm

10µm

0.1µm

0.04µm visibilitylimit

Sheen formationby droplet resurfacing

Surface release

Emulsifyingoil

Emulsification

1µm

SH

EE

NT

HIN

OIL

TH

ICK

OIL

& E

MU

LSIO

N

HOURS

No emulsification ?

Sub-surface releaceEmulsification

Fig. 21 Schematic illustration of the development in film thickness, break-drelease, versus emulsifying and non-emulsifying underwater releases.

Fraction water dehydrated after 4 and 24 hourssettling

00.10.20.30.40.50.60.70.80.9

1

1.0 2.0 3.0 4.0 5.0 6.0

Fra

ctio

nd

ehyd

rate

d

D4h

D24h

Weathering time at sea

Fig. 20 Stability of emulsions shown as a function of time on thesurface (DeepSpill 2000). Dehydration¼ 1 indicates an unstableemulsion and dehydration¼ 0 a complete stable emulsion.

134

This thickness appeared, however, to be too thin forany emulsion to form on the surface, and the oil slickdissipated naturally within a few hours. Further de-tails concerning operational aspects, monitoring, an-alytical methods and conclusions from this sea trialare described in several reports, e.g. Rye et al. (1996,1997) and Strøm-Kristiansen et al. (1996).

The DeepSpill experiment

The DeepSpill experiment was conducted in the

Norwegian Sea in June 2000, and included releases of

oil and natural gas from 850 m depth. The main ob-

jective was to provide data for verification of numer-ical models for simulating accidental releases in deep

water. Three experiments were performed, one with

gas only, one with gas and marine diesel (simulating a

non-emulsifying oil), and one with gas and crude oil

(Sture Blend). Each release lasted for 1 h, with gas

rates of 1 Sm3/s and oil rates of 1 m3/min. The oil and

gas was pumped from the discharge vessel through

separate coiled steel tubing lines down to a dischargeplatform deployed on the seabed. The gas was trans-

ported to the site in liquefied state (LNG), and was

transformed into gas in a seawater-heated evaporator

during the release (Johansen et al., 2001).Met-ocean data were measured continuously during

the field trial with wind sensors and a downward

facing Acoustic Doppler Current Profiler (ADCP)

mounted under the research vessel, supplemented withan upward facing ADCP mounted on the seabed. The

wind speed during the experiments varied between 9

and 14 m/s. Ocean currents that were measured in the

range of 10–20 cm/s showed a significant variation

urface vs. Sub-surface

DAYS

own and relative lifetime on the sea surface of an emulsifying surface



with depth. Video cameras mounted on Remote Op-

erated Vehicles (ROVs) provided close up pictures of

gas bubbles and oil droplets, and ship mounted echo-

sounders provided images of the plumes of water, gasbubbles and oil droplets that were rising through the

water column.

In both experiments with oil, the first oil appeared

on the surface 1 h after the start of the release, while

the main surfacing took place around 2–4 h after re-

lease. The water in the plume was trapped below the

perennial thermocline (below 500 m depth), while gas

bubbles and oil droplets were then separated from thetrapped water and continued towards the surface with

the rise velocity of individual bubbles and droplets (see

illustration in Fig. 17). Observations during the pure

gas release (without oil) showed no presence of the gas

bubbles above 150 m depth, indicating that the gas

bubbles had been dissolved completely in seawater at

this level. However, the oil droplets continued to the

surface and formed a surface slick that developedgradually over time. In the crude oil experiment, the

oil film thickness in the surface slick was measured in

the range from 200 to 400 lm of non-emulsified oil (see

Fig. 18(a)). This thickness was sufficient for the for-

mation of patches of emulsified oil during the coming

hours on the sea surface (see Fig. 18(b)). The water

uptake rate and the time development of the viscosity

of the thick emulsion was in good agreement withmodel predictions using the SINTEF OWM based in

laboratory input data of the Sture Blend crude oil (see

Fig. 19). The emulsion became stable after about 5 h

weathering on the sea surface (Fig. 20).

The last aerial survey that was made about 8 h after

the start of the crude oil release depicted a slick about

8 km in length and 1 km in width. In the marine diesel

experiment, where no emulsion was formed, themaximum slick size was observed about 5 h after the

start of the release, with a length in the order of 1.5 km

and a width of 500 m. No remains of the slick could be

detected during the last surveillance flight about 3 h

later. The difference in behaviour between emulsifying

and non-emulsifying oils is illustrated schematically in

Fig. 21.

Further details both concerning operational as-pects, monitoring, analytical methods and conclusions

from this sea trial are described in Johansen et al.

(2001).

Conclusions and Recommendations

Laboratory experiments and full scale field experi-

ments have been performed in Norway during the last

decades in order to obtain more knowledge about the

emulsification of crude oils. A broad range of oils has

been investigated in the laboratory both in bench-scale


and meso-scale. Field trials have been performed to

verify laboratory results, for calibration of models and

to study the influence of different release conditions.

The major findings from the work are as follows:Controlled bench-scale and meso-scale basin ex-

periments using a wide spectrum of oils have revealed

that both the physico-chemical properties of the oils

and the release conditions that control the initial film

thickness are fundamental parameters for the rate of

emulsion formation and for the rheological properties

of the emulsion formed.

There is a generally good agreement between anal-ysis of oil samples taken during experimental field

trials and model prediction using the SINTEF OWM

based on oil specific laboratory data.

None of the three underwater releases (1995, 1996

and 2000) showed any emulsion formation in the ris-

ing plumes of oil droplets. The emulsification took

place on the sea surface (when the initial oil film

thickness was sufficient).Simulated underwater pipeline releases indicated

that the oil droplets will surface within a limited area,

forming oil slicks with film thickness sufficient for

emulsion formation.

Simulated underwater blowouts from moderate

depths (<300 m) showed that the gas-bubble plume

will come to the surface, bringing with it entrained

water. The rapid surface spreading of this entrainedwater will cause the surfacing oil to spread into a thin

oil film (see ‘‘surfacing plume’’, Fig. 17). This film may

be too thin for emulsification to take place unless

further concentration of the oil into thicker narrow

bands (windrows) occurs with the development of

wind-driven Langmuir circulation cells.

With blowouts in deep waters (>500 m), the plume

may be trapped in the water column, and the rising oildroplets will surface within a more limited area (see

‘‘trapped plume’’, Fig. 17). This may lead to initial oil

film thicknesses that are sufficient for emulsion for-

mation.

The appearance and texture of oil slicks at the sea

surface can be very different for different oils and de-

grees of weathering. Continuous oil slicks may be

observed at an early stage after release, but weatheringprocesses will soon transform the slicks into a patchy

appearance consisting of broken fragments of highly

emulsified oil. The algorithms used in most oil drift

models are traditionally based on concepts derived for

continuous slicks under calm conditions, and can not

explain properly the spreading behaviour and break-

up of such weathered oil slicks. It is necessary to im-

prove the understanding of the processes causingchange in appearance and texture of oils at sea, and to

develop and further improve existing algorithms for

spreading, break-up and natural dispersion of oil into

the water masses.

135

P.S. DALING et al.

Acknowledgements—The Norwegian Clean Seas Association for

Operating Companies (NOFO) is acknowledged for supporting this

review paper.

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Norwegian Testing of Emulsion Properties at Sea––The Importance