Oil Erosion in an Annular Flume by Seawater of Varying Turbidities: A Critical Bed Shear Stress Approach

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Abstract

Laboratory experiments were conducted in an annular flume using Hibernia crude oil to determine: (1) the critical shear stress (τc) necessary to remove stranded oil from a surface by resuspension and (2) the effect of suspended sediment concentrations (SSCs) on the oil erosion processes. Two types of erosion were evident: Type I––solution and erosion of soluble aromatics; and Type II––mass erosion of visible droplets. In particulate free seawater at 13 °C, the Type II erosion threshold τcII is 5.0 Pa. This is equivalent to a mean current velocity (Uy) of 0.55 m s−1. At Uy values <0.55 m s−1, Type I erosion occurred as shown by the increase of oil concentrations without visible erosion of the oil surface. Temperature has a strong control on the threshold and rate of oil erosion: the threshold for Type I erosion at 4 °C was higher and erosion rate lower than at 13 °C. No Type II erosion was observed at 4 °C. SSCs also affects the entrainment of oil. Oil erosion was most efficient at moderate SSCs. At very high SSCs, turbulence suppression and drag reduction became effective and oil erosion rate decreased. SSC at 200–250 mg l−1 were observed to give maximum erosion efficiency and is therefore suggested as the optimal concentration for erosion and elimination of heavy crude oil at a water temperature of 13 °C.

Introduction

Low energy coastal settings are considered among the most sensitive of marine ecosystems to oil spills because of their vulnerability to oiling (Fingas et al., 1978; Ritchie, 1993), and because of the difficulty of cleanup (Breuel, 1981; Hayes, 1996). Oil removal from such settings has been aided by several methodologies including removal/disposal, dispersion, sediment cleaning, and mixing (Owens et al., 1987). Where it is possible to use them, these methods have given satisfactory results. However, most of these methods are generally highly destructive to fauna and thus, a decision to leave the coast to natural cleaning processes may sometimes be more appropriate.

In this latter case, the interaction of fine mineral particles with oil stranded on shorelines following spills has been shown to be an effective natural cleaning process, capable of accelerating oil removal in most environments and particularly in low energy, sheltered shorelines, where wave action and abrasion are negligible (Bragg and Owens, 1994, Bragg and Owens, 1995; Bragg & Yang, 1995; Michel et al., 1993; Lee et al., 1997). This process, previously called clay-oil flocculation, (Bragg & Yang, 1995) involves the interaction of oil droplets with fines to form oil mineral aggregates (OMA) which have overall properties that reduce residual oil coalescence at the sea surface and its adherence on solid substrates (Beslier et al., 1980; Gundlach & Reed, 1986). Furthermore, in terms of environmental significance, the dispersion of the oil increases in situ degradation rates (Lee et al., 1997; Weise & Lee, 1997). This significantly reduces the energy required by waves and steady currents to remove the oil from shoreline sediments (Bragg & Owens, 1995).

The effectiveness of natural cleaning by OMA formation depends on a number of factors but is primarily related to the amount of wave energy reaching the shore. Oil erosion and dispersion through the formation of aggregates is facilitated by a combination of wave activity which initiates: (1) rupture of the oil emulsion into small droplets; (2) sediment suspension into the water column; and (3) interaction of fine material with oil droplets. In low energy environments, availability of fine sediment is usually not a limiting factor in the oil/sediment interaction process. It is more likely limited by the energy within the surf zone that is needed to break the oil emulsion into small droplets. Water motion is a critical aspect to this process in that it must impart sufficient momentum to the oil film to rupture it into the droplets that interact with sediments. Some laboratory experiments were conducted on the measurement of vertical dispersion and diffusion of oil droplets and oiled particles (Delvigne et al., 1987). These studies were designed to examine the break-up of a surface oil film into droplets in the sea, and the interaction between oil droplets and suspended particulate matter (SPM). To date, no studies have examined the significance of this process in terms of the bed shear stresses necessary to initiate the rupture of a stranded oil film.

This study examines the steady flow conditions under which a stranded oil film will be eroded. Hibernia crude oil (HCO) was used as the test oil. All experiments were carried out using filtered natural seawater under laboratory conditions. The effect of suspended sediment concentration (SSC) and temperature on the erosion process were examined. It is hypothesized that turbidity associated with suspended mineral fines will reduce the critical shear stress necessary to break the oil film and initiate the process of oil-mineral aggregation due to the presence of a solid-transmitted shear stress (Bagnold, 1966). To test this hypothesis, a study with the Lab Carousel, a laboratory annular flume, was conducted. Lab Carousel is described in detail by Thompson et al. (in press). The characteristics (size and type) of OMA formed were analyzed under various current velocities and levels of SSC and form the subject of a separate publication (Cloutier et al., unpublished data).

This study represents a first step in oil erosion experiments under a range of velocities and temperatures. More studies need to be undertaken in order to provide the optimal conditions of oil elimination for different oil types. This information could be of great value to oil spill responders responsible for the evaluation and selection of spill clean-up techniques including surf-washing facilitated by oil–particle interactions (Lee et al., 1997) and/or the decision to leave the coast to the natural processes of clay-oil flocculation.

Section snippets

Lab Carousel

Lab Carousel is an annular laboratory flume designed to examine the erosion and settling of sediments under controlled, monitored conditions. It is similar in geometry and operation to the Sea Carousel, a benthic annular flume which was developed for field applications (Amos et al., 1992). It consists of an acrylic annular trough, 2 m in diameter, 0.15 m wide, and 0.45 m deep (Fig. 1). The water depth is held constant at 30 cm, which is equivalent to a volume of 0.268 m3. The flow in the

Effect of water temperature on the erosion of Hibernia crude oil

The effect of water temperature on oil erosion was examined by performing replicate experiments in clear seawater (no sediments) under two different temperatures, 4 and 13 °C. The water temperature remained stable during the experiment. Current speed was increased in five steps to 0.75 m s−1 (0.06, 0.125, 0.25, 0.5, and 0.75 m s−1); each velocity interval was held for 1 h, except for the last increment at 13 °C when the current was applied for 2 h (Fig. 4(A)).

For Uy⩽0.50 m s−1, no significant

Discussion

These experiments have revealed several important features of oil erodability under varying SSC. First, there is a clear temperature effect demonstrated by the observation that HCO was not eroded under relatively strong current velocities (up to 0.75 m s−1) at a water temperature of 4 °C. This is attributed to the increase in oil viscosity in cold water (Allen & Nelson, 1983), and to the physical/chemical properties of the waxy crude oil under evaluation, which causes the oil to resist

Conclusions

The results have shown that water temperature is an important parameter influencing the erosion and dissolution of HCO under unidirectional steady flow. Two distinct types of erosion have been identified and are described as following: (1) Type I erosion caused by oil dissolution under an applied current without any visual disturbance of the oil slick surface, and (2) Type II erosion defined as visual erosion of the oil slick and entrainment of oil droplets. Type I erosion is important to

Acknowledgements

The project was financially supported by the Panel of Energy Research and Development (PERD), Canada and by a grant from the Natural Sciences and Engineering Research Council of Canada to P.R. Hill. Special thanks to the Geological Survey of Canada Atlantic (GSCA)––Sediment Dynamic Division, Bedford Institute of Oceanography (BIO), Canada for making the Lab Carousel available. We also thank Sylvie Saint-Pierre, Johanne Gauthier, Jennifer Morissette, Aouregan Terre, and Denis Guay for their

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