Weathering of Oil Spill - CyberLeninka

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ScienceDirect Aquatic Procedia 4 (2015) 435 – 442

INTERNATIONAL CONFERENCE ON WATER RESOURCES, COASTAL AND OCEAN ENGINEERING (ICWRCOE 2015)

Weathering of Oil Spill: Modeling and Analysis Aditya Kumar Mishra, G Suresh Kumar* Department of Ocean Engineering, IIT Madras, Chennai-600036, India

Abstract Oil spill releases a large amount of crude oil on sea surface. Once the crude oil is spilled, it gradually starts decaying under the influence of concurrent processes collectively termed as oil weathering processes (OWP). Weathering of an oil slick modifies its behavior making it more persistent to marine waters and enduring its lifespan in marine biology. Hence in order to plan an effective response operation, it is vital to have advance knowledge of oil slick behavior. The mathematical representations of OWP are therefore used to predict critical slick properties such as viscosity and density. In addition, an improved understanding of OWP evolved over the years provides an ample opportunity to examine the existing relationships and subsequently to incorporate the required changes. The aim of present paper was to model and analyze effects of initial oil properties on its behavior, immediately after the spill. For this purpose three different crude oils: heavy, intermediate and light were selected. Runge-Kutta fourth order method was used to solve spreading, evaporation, dissolution and emulsification OWP simultaneously. Results indicate that light and intermediate oil uptakes more water than heavy crude oil with time. Hence, the volume for cleanup is expected to be high in case of light and intermediate crude oil spills. Moreover, evaporation and emulsification were found to be extremely sensitive to initial oil viscosity and composition. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-reviewunder under responsibility of organizing committee of ICWRCOE Peer-review responsibility of organizing committee of ICWRCOE 2015 2015. Keywords: Oil spill; weathering; modeling; Runge-Kutta fourth order

1. Introduction In recent years, increased petroleum consumption has promoted oil exploration and oil transport activities in the marine environment. The production and transport of crude oil on the sea surface always holds the risk of spills. In addition, man-made errors and mechanical failures result in incidents like collision of ships, bursting of pipelines,

* Corresponding author. Tel.: +91-44- 2257- 4814; fax: +91-44-2257-4802. E-mail address: [email protected]

2214-241X © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing committee of ICWRCOE 2015 doi:10.1016/j.aqpro.2015.02.058

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Aditya Kumar Mishra and G. Suresh Kumar / Aquatic Procedia 4 (2015) 435 – 442

failure of oil rigs etc., that emit tons of crude oil in the marine environment. In the marine environment crude oil exhibit harmful and long term effects. Oil in the sea water may enter the food chain of marine animal, sink to sea bed affecting marine vegetation, foul the harbour facilities and damage eco-sensitive near shore resources, when washed ashore. Furthermore, oil spills are difficult to recover from the sea surface due to unpredictable nature of the sea surface and weather conditions. Hence, oil spills are undesirable in marine ecology. After the crude oils are spilled, on the sea surface they spreads to form a thin layer called oil slick. The oil slicks are then acted upon by several natural processes together to degrade the oil slick. These processes are referred as oil weathering processes (OWP). The weathering processes significantly alter the slick properties especially density and viscosity of crude oils. Several researchers (Sebastiao and Soares, 1995; ASCE, 1996; Reed et al., 1999; Lehr, 2001; Azevedo et al., 2014; Fingas, 2014) had investigated oil weathering processes. They found that, temporal changes in characteristic slick properties endure slick lifespan on the sea surface. In addition, initial spill conditions and initial oil properties critically affect the evolution of oil slicks. When responding to oil spills or planning a counter measure, prior knowledge of oil properties is of paramount importance. As type and effectiveness of countermeasure selected, highly rely on slick properties. The mathematical models established over the years had been widely used to accomplish the task. Therefore, the focus of the present work was to model and assess the effect of initial oil characteristics on slick properties after spill. For this purpose fourth order accurate Runge-Kutta was found appropriate and used. The set of time dependent equations were solved explicitly with three different crude oils namely light, intermediate and heavy crude oils. 2. Modeling weathering processes Oil weathering processes (OWP) act naturally on oil slicks conceived after oil spills, on the sea surface. It includes spreading, evaporation, dissolution, dispersion, emulsification etc. These processes are complex, selfcompeting and act simultaneously. Although, the processes like evaporation removes major fraction of volatile parts, the residue still thrive on the sea surface as a result of emulsion formation with enhanced oil volume. But then, not all oils emulsify; some even break and separate into oil and water phases. 2.1. Spreading Fay (1971) described spreading to evolve in three stages: inertia-gravity, gravity-viscous and viscous-surface tension. The oil slick was assumed to spread axi-symmetrically, with circular slick before and after spreading independent of wind, wave and currents. The first stage passes rather quickly and third stage is attained when slick gets broken. Hence, an oil slick spends most of lifespan in gravity-viscous regime. In order to model spreading in gravity-viscous stage, the time to end first stage and initial area at the end of the first stage were calculated (Berry et al., 2012). Later area for second stage was estimated using Eq. (4) (Sebastiao and Soares, 1995). 4

t0

§ k2 · § V0 · ¸ ¨ ¸ ¨ © k1 ¹ © Xw g ' ¹

(1)

U sw Uoil U sw

(2)

'

A0

§ k 2 4 · § 'gV05 · ¸ ¨ k12 ¸ ¨© Xw ¸¹ © ¹

S¨

(3)

4

dA dt

V3 k sp A

(4)

where, t0 is time to end stage one of spreading (s), V0 the volume of oil spilled (m3), V is the volume of oil at any time instant t (m3), k2 and k1 are empirical constants (1.14 and 1.45 respectively), g is the gravitational acceleration

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(m2/s), ρsw is the density of sea water (Kg/m3), ρoil is the initial density of oil (Kg/m3), υw is the kinematic viscosity of water(m2/s), A0 is the initial area for stage 2 of spreading(m2) and ksp is the evaporation constant (150 s-1 ). 2.2. Evaporation Evaporation is the primary mechanism of oil removal from the sea surface. It removes most of the volatile fractions of the crude oil within hours of spill. The density and viscosity of the oil slick is significantly modified by evaporation. Therefore, estimating rate of evaporation is crucial. The rate of evaporation was calculated using analytical equation proposed by Stiver and Mackay (1984), assuming oil slick to be single component. The data unavailable on T0 and T g were evaluated using NOAA formulation as in Eq. (7) and Eq. (8) (Berry, 2011). kevp A b (T0 Tg Fe ) ½ ®a ¾ V0 ¯ Toil ¿

dFe dt

(5)

2.5 x10 3 Ws100.78

(6)

T0

532.98 3.125 API

(7)

Tg

985.62 13.597 API

(8)

kevp

Where, Fe is the volume fraction of oil evaporated (%), kevp is the mass transfer coefficient of evaporation (m/s), a and b are evaporation constants (6.3 and 10.3 respectively), T0 is initial boiling point temperature of oil (K), Tg is the gradient of oil distillation curve (K), Ws10 is the wind speed at height of 10 m from sea surface (m/s) and Toil the temperature of the oil spilled (K) (assumed to be equal to sea surface temperature) and API refer to the American Petroleum Institute gravity scale. 2.3. Dissolution Dissolution of crude oil into the sea increases the toxicity of sea water (Riazi and Roomi, 2008). The chemicals present in the oil and capable of dissolution are also easy to evaporate. Since, evaporation is a faster process compared to dissolution most of these chemical evaporate quickly. The amount of crude oil dissolved in sea water, is therefore typically small (in ppm). The rate of dissolution may be estimated using following equations (Shen et al., 1993). The suitable changes were incorporated to obtained volume fraction of oil dissolved as follows:

dFd dt S

§ · S K diss A ¨ ¸ 1000 U oil ¹ © S0 exp(12.0Fe )

(9) (10)

Where, Fd is the volume fraction of oil dissolved in sea water (%), Kdiss is the mass transfer coefficient of dissolution (m/s), S the solubility of oil at time t (g/m3) and S0 is the initial solubility of oil in water (g/m3). 2.4. Emulsification Emulsification results due to wave breaking and sea surface turbulence. The sea surface disturbance entrains sea water into the oil slick. As slick viscosity increases with time, higher amount of water is trapped in the slick. Therefore, the formation of emulsion increases water content of oil. This further impedes the rate of evaporation. In

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addition, it increases density and viscosity of the slick particular. The water content of oil due to emulsion formation can be modeled using Mackay’s formulation Eq. (11) (Sebastiao and Soares, 1995). The impact of emulsification on rate of evaporation was assimilated as per Lehr (1994).

dY dt

§ Y 2 ) ¨1 kemul (1 Ws10 ¨ Y f © kevp _ cor

· ¸¸ ¹

(11)

kevp (1 Y )

(12)

Where, Y is the water content of oil (vol %), Kemul is the mass transfer coefficient of emulsification (2.0 x10 -6, m/s), Yf is maximum water content of water in oil (vol %) and Kevp_corr is the corrected mass transfer coefficient of evaporation considering the effect of emulsion formation (m/s). 2.5. Density and viscosity Density and viscosity of an oil slick on the sea surface are basically influenced by OWP. Evaporation and emulsification are the key processes that improve the viscosity of an oil slick. Effect of evaporation, however was reported more significant than the rate of evaporation. As emulsification causes rapid rise in slick viscosity, the effect of emulsion formation should essentially be encompassed while estimating slick viscosity. The difference in oil slick and sea surface temperature further improves the density and viscosity values. Thus, considering the effect of evaporation, emulsification and temperature difference the density and viscosity may be calculated using Eq. (13) and Eq. (14) (Berry et al., 2012). Owing to, lack of data on reference temperature at which density was measured. The effect of temperature difference on density was not considered.

U net

Y U sw U ref (1 Y )(1 0.18Fe )

Pnet

P0 exp(cevpm Fe ) exp ¨

§ Tref Toil · 2.5 Y ¸ exp ¨¨ 63.16 Y Toil Tref 1.0 0.654 © ¹ ©

§

(13)

· ¸¸ ¹

(14)

Where, ρnet is the resultant density of oil at any time instant t (Kg/m3), ρref is the initial density of oil (Kg/m3), μnet is the resultant density of oil at any time instant t (cP), μ0 is the initial density of oil (cP) and Tref is the temperature at which viscosity was measured before spill (K). 3. Numerical strategy and details The simulations were run for a hypothetical spill of volume 1000 m3, with three different crude oils: Statfjord crude, Kuwait crude and Prudhoe Bay crude oils, respectively. The crude oils were categorized based on API values into light, intermediate or medium and heavy crude oil (Allen and Dale, 1997; Cormack, 1999; Boyed et al., 2001; API, 2011). The set of non-linear equations from Eq. (1) - (14) were solved simultaneously using Runge-Kutta fourth order method for period of 24 hours or one day. The time step of 10 seconds was selected. After each time step, changes in oil properties under the influence of OWP were calculated. The rate of spreading, evaporation, dissolution and emulsification were allowed to vary concurrently. The corresponding behavior of slick was then evaluated based on area of spread, volume remaining and considering the changes in the oil viscosity. The data unavailable in the literature on intermediate crude oil were obtained averaging the information on light and heavy oils.

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4. Results and discussion The set of non-linear equations were solved explicitly using Runge-Kutta time integration technique. Effect of oil weathering processes on light, heavy and intermediate crude oils were examined based on the time dependent model developed. The analysis was carried out assuming a wind speed of 5.0 m/s and sea surface temperature of 298 K. The effects of weathering processes were realized using mathematical models discussed previously in section 2. The oil properties used in the present analysis are presented in Table 1(Howlett, 1998; NOAA, 2000; Aghajanloo and Pirooz, 2011; 2014). The numerical model was first validated against the experimental data and then used for further investigation. Table 1. Characteristic oil properties used in the present work Property

Statfjord crude

Kuwait crude

Prudhoe Bay crude

API

38.4

31.31

25.74

Oil type

Light oil

Intermediate oil

Heavy oil

Density (Kg/m )

832.0

869.0

899.0

Viscosity (cP)

3.03

3.03

35.3

Temperature reference for viscosity (Tref ) (K)

313

313

298.0

Maximum water content (vol %)

90

80

70

T0 (initial boiling point) (K)

301

-

430.6

TG (gradient temperature) (K)

500

-

722.0

Cevpm (evaporation constant for viscosity)

2.0

6.25

8.75

3

4.1. Effect on rate of evaporation and density of oil slick Fig.1. (a) shows the temporal distribution of oil volume evaporated for the selected crude oils. During the simulation period, nature of the plots varies exponentially. It can be observed from figure that light crude oil (Statfjord crude) losses more than 50% percent of its initial volume at the end of 24 hours which was found in a

b

Fig. 1. Comparison of (a) volume fraction evaporated and (b) density variation of light, intermediate and heavy crude oils, respectively, at wind speed of 5.0 m/s for period of 24 hours.

agreement with the experimental data from Sebastiao and Soares (1995). Comparatively, the effect of evaporation

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was less sensed by heavy (Prudhoe Bay crude) and intermediate (Kuwait crude) crude oils. Each losing approximately 25% of oil volume spilled, half the volume of light oil evaporated, at the end of the day. The fact that volatile fractions of oil, with high vapour pressure, evaporate easily and constitute major portion of oil volume evaporated. It proves that, light oils (having low molecular weights) comprise mostly of lighter fractions. On the other hand, heavy oils show lesser rate of evaporation because they mainly comprise of non-volatile, high molecular weight components which are unable to evaporate. Interestingly, the fraction of intermediate crude oil evaporated varies close to heavy oils. One reason for observed behaviour could be the amount of weathering needed for slick to get stabilised on the sea surface. It has been proved that after certain amount of weathering, intermediate crude oil can form stable emulsions and hence could stay on the sea surface for a longer period. Another reason could be the wind speed. The wind of 5.0 m/s in the present work was inadequate to cause the evaporation of intermediate crude oil. The third reason could be the water content. Growth of water content in the oil slick considerably reduces mass transfer from the slick. Hence, it can be understood from the figure that intermediate oils present lesser rates of evaporation, depending on their chemical composition and water content. The lighter oils are easily lost form the sea surface while most of heavier oil remains as residue. Hence, oil slick on the sea surface mainly comprises of high molecular weight compounds. In Fig. 1. (b) density variation of the crude oils with time are presented. The logarithmic behaviour of density curves for the crude oils were evident from the figure. Since crude oils with high initial viscosities do not capture much of sea water, the density of heavy (Prudhoe Bay crude) oil was expected to be low. The light oil however, reflects higher density in comparison to heavy oil, which was due to high evaporation. The high evaporation rate resulted in increased relative concentration of heavy molecules hence rise in density of the slick. Additionally, the improved capacity of oil to trap sea water by weathering, with increase in viscosity could also be the reason. The density of the intermediate oil appears in between heavy and light oil, after 8 hours of evaporation which is justifiable. As intermediate oil evaporate lesser than light crude oil and more than heavy crude oils the density of intermediate crude was expected intermediate of light and heavy oils. Moreover, the water content was expected lesser, hence the plot can be seen nearer to the range of heavy oil. The field observations show that the crude oil remains afloat for most of its life time. Hence, the nature of the density curves closely confirms to the physics perceived in the real time conditions. If the density of oil exceeds that of sea water, it would probably sink. Therefore, the maximum density of oils could be expected to be equal to that of sea water, which is reached at much later period. It was not expected during the simulation period. As light oil was observed exceeding the sea water density which could be an overestimation or as result of very high evaporation rates. Hence, it can be concluded from the plot that density of an oil slick increases mainly because of improved relative concentration of heavier fraction within the oil and partly due to water content. 4.2. Effect on water content and viscosity of oil The water content of the crude oils greatly depends on initial viscosity of the oil slick. An oil slick with lesser initial viscosity will have low water content initially, but with weathering it would increase. However, the crude oils with high initial viscosity would initially show lesser water content and then increase depending on temperature and wind conditions. Therefore, water content can be considered extremely dependent on initial oil properties and weathering conditions. The increased concentration of heavy fraction, typically less susceptible to evaporation, could additionally increase the water residence time of sea water entrained in the oil slick. Hence the water content of oil slick. The water content of an oil slick against weathering duration is illustrated in Fig. 2(a). It shows that water content of the crude oils was quickly reached to maximum within 5 to 8 hours of spill. The plots display exponential behaviour with time. Heavy oil however, achieved the maximum water limit faster than light and intermediate crude. Infact, it properly distinguishes water content behaviour of heavy, intermediate and light crudes. The immediate water uptake by heavy oil appears untrue, because at viscosities as high as 35.3 cP oil would resist entry of water in the slick. Although, it can be considered true if the temperature of slick would be lesser than sea water temperature. The increase of slick temperature to sea water temperature would cause the slick to expand. As a result viscosity of the slick would reduce allowing extra sea water to enter. However, in the present work temperature was maintained constant. Thus, it can be considered overestimating, as emulsification started since the beginning of simulation. The light and intermediate crude shows higher water content which could be true if Fig.


a

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b

Fig. 2. (a) Percentage water content and (b) viscosity variation of crude oils against time, after 24 hours, at wind speed of 5.0 m/s and sea surface temperature of 298 K.

1. (a) and Fig. 1. (b) are observed carefully. Hence, we conclude that in order to accurately estimate water content of oil it is important to be able to correctly predict starting point of emulsion formation or the time where the emulsification actually commences. This is among the current challenges faced by oil spill research community. Since, the exact time or amounts of weathering required was not aware off the present model overestimates. Fig. 2. (b) illustrates the viscosity variation of the three crude oils with time. Viscosity of Prudhoe Bay crude increases sharply to 8000 cP compared to light and intermediate crude. Even though, the water content as seen from Fig. 2(a) was lowest. The increase in surfactant concentration and high viscosity could be the prime reasons for present behaviour. As emulsion formation and water content are immensely dependent on wind condition. The wind speed of 5 m/s was expected to play significant role, whereby wave breaking at sea surface in real field circumstances could increase the water content by water entrainment. Although, the water holding capacity of light oil improves with time as most of oil gets evaporated around 50-60% with time, only 40-50 % of oil volume is expected with low amount of stabilizers. Hence, the low viscosity values are observed from the Fig. 2 (b). The behaviour of intermediate crude on the other hand was slightly unpredicted. The viscosity of the Kuwait crude oil appears to be lower than the light Statfjord crude. At this point the authors are not certain of the reason for the indifferent behaviour, of Kuwait crude. However, one reason that could be thought of would be continuous making and breaking of surface emulsions as a result of balance between evaporation and water entraintment of sea water. As the literature was not rich enough on intermediate crude further investigations are needed before reaching to any conclusion. 5. Conclusions In the present work a time dependent model for oil weathering processes was developed. To understand the effect of initial oil properties on slick evolution due to weathering, three different crude oils: light, intermediate and heavy crude oils were selected. The results from the model shows that time evolution of density and viscosity predominantly depend on the type of oil spilled. Weathering processes such as evaporation and emulsification are extremely sensitive to chemical composition and viscosity of oil slick. Water content of an oil slick was found to be dependent on initial slick viscosity, which eventually reduces with increase in viscosity of initial oil spilled. As light oil uptakes more seawater, the volume for cleanup is expected to be more in case of light and intermediate crude oils spills. The lack of understanding on initiation of emulsification overestimates the water content or may undermine the effect of oil characteristics. Moreover, further investigation on intermediate crude could prove critical from response point of view and may hold the key for complete oil recovery.

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