Dispersion: Oil Droplet Size Measurements at Sea

794 1993 OIL SPILL CONFERENCE DISPERSION: OIL DROPLET SIZE MEASUREMENTS AT SEA

Tim Lunel Warren Spring Laboratory Gunnels Wood Road Stevenage, Hertfordshire SGI 2BX, U.K.

Dispersion, although one of the most important processes in deter- sprayed with dispersant by aircraft. The premixed oil-dispersant re- mining the fate of oil in the sea, is still poorly understood. This poster leases, as expected, produced the highest oil concentrations in the presents the first successful field measurements of oil droplet size water column (1 to 300 ppm), and these releases also resulted in the Downloaded from http://meridian.allenpress.com/iosc/article-pdf/1993/1/794/2357422/2169-3358-1993-1-794.pdf by guest on 29 September 2021 distribution below oil slicks, a major step in understanding the disper- largest number of subsurface oil droplets. A typical distribution of oil sion process at sea and in designing a dispersant efficiency test that droplet diameters is shown in Figure 1. Figure 2 shows the cumulative relates to conditions at sea. frequency plots for droplet number and droplet volume (the means are If simplified, the process of dispersion can be divided in two stages: respectively 20 |xm and 46 u,m). For the variety of environmental con- the formation of oil droplets in the water column from the surface slick, ditions encountered (wind speeds between 1.0 and 9.0 m/s), for differ- and advection of these droplets by near-surface turbulence down into ent oils and oil-dispersant combinations the number median varied the water column to prevent resurfacing. By Stoke's law, smaller oil over a narrow range of 15 to 25 |xm (volume median 35 to 50 |xm). droplets rise more slowly than large droplets. Thus, conditions that The fact that different oil and oil-dispersant combinations over a result in increased near-surface turbulence and generate a greater range of environmental conditions produce similar droplet size distri- number of smaller oil droplets produce faster dispersion rates. The butions may be a consequence of the fundamental fluid dynamics of the measurement of oil droplet size distributions at sea is, therefore, ocean. Turbulent energy due to wind and waves is introduced to the central to understanding this dispersion process. It also provides a surface ocean at length scales of the order of meters. Such energy is means of quantifying the effects of different environmental conditions converted to turbulence on smaller length scales until the energy is and chemical surfactants on surface slicks, and hence a way of predict- dissipated by the viscosity of seawater. This minimum turbulence ing dispersion rates. length scale is called the microscale. There are no fluid dynamic Currently available dispersant efficiency tests, such as the Warren mechanisms for producing large numbers of droplets with a smaller Spring Laboratory (WSL), Mackay-Nadeau-Steelman (MNS), and diameter than the microscale. Milgram and colleagues have estimated Institut Français de Pétrole (IFP) tests, produce different efficiency by a theoretical approach that the microscale associated with a break- values for the same combinations of oil and dispersant. These different ing wave is typically of the order of 6 |xm.3 This estimate of minimum efficiency values have been shown to be related in part to the different droplet size is in good agreement with the observed oil droplet size amounts of energy each test imparts to the system and hence the distribution in Figure 1. 1 different sizes of oil droplet sizes produced. These tests have not been Despite the fact that the oil droplet size distributions at sea have designed to relate to oil droplet sizes observed at sea. been found to be broadly similar, the number density of the droplets is very different for different releases. For example, for releases of 200 liters the number of droplets counted during the 5-minute sampling time were, for Flotta crude, 150 counts; medium fuel oil (MFO) Instrumentation premixed (1:20) with a Type I dispersant, 1,200 counts; MFO premixed (1:20) with a Type III dispersant, 17,500 counts; and Flotta Previous unsuccessful attempts to make field measurements of oil crude premixed (1:20) with a Type III dispersant, 16,000 counts. droplet sizes have used laser diffraction techniques (e.g., Malvern These differences in droplet number density for a given droplet size instruments), which interpret the diffraction pattern formed by a laser distribution form the basis for determining the efficiency of the disper- beam passing through a diffraction that formed by a number of drop- sion process. lets.2 However, this technique is not very selective in the particles that are sized. Air bubbles, suspended sediment, and plankton in the surface sea water have resulted in a high and variable background and have made measurement of oil droplet sizes at sea impossible. The Phase Doppler Particle Size Analyzer (PDPA) technique used in this work is based on single oil droplets passing through the intersec- tion of two laser beams and scattering light to produce an interference fringe pattern. The instrument is optimized for the refractive index of interest, so that air bubbles, for example, are not measured. The symmetry of the interference pattern produced can be used to reject nonspherical particles such as suspended sediment and plankton. By rejecting air bubbles, suspended sediment, and plankton, the signal- to-noise ratio is increased dramatically, and oil droplet sizes can be measured at sea. Another problem in measuring droplet sizes at sea is how to intro- duce the sample to the instrument. Pumping the seawater sample from the sea through an instrument volume would create shear forces that could have a significant effect on the oil droplet distribution. The PDPA has been modified so that that the probe head is submerged at a depth of 0 to 5 m, to make in-situ measurements of oil droplet sizes.

10 1 25 32 - 39 46 53 60 67 74 81 95 102 109 117 Measurements at sea Diameter (um) Figure 1. Oil droplet size distribution of premixed oil-dispersant com- During the summer of 1992, WSL carried out several trials using the bination (medium fuel oil and Slickgone NS) measured at sea using PDPA in conjunction with measurements of oil concentration by fluo- a Phase Doppler Particle Analyzer (PDPA), with background counts rometry. The trials involved oil, oil-dispersant mixtures, and oil (a maximum of 15 counts in any category) subtracted POSTER SESSION B 795

no | 1 rank dispersants and give efficiency values for different oil-dispersant 100 I- _ ^^^■■■■■■■■■■■M|IM""" combinations. Initial work indicates that currently available tests create larger droplet sizes than those observed at sea. This result will be discussed in more detail in the poster presented at the conference.

Conclusions

Oil droplet size measurements are critical in understanding natural and chemically enhanced dispersion at sea and in designing laboratory tests for dispersant efficiency which relate to conditions at sea. This poster outlines the first successful measurements of oil droplet size at sea and shows that for a variety of test oils and dispersants the range of

mean diameters lies between 15 and 25 |xm (volume distribution 35 to Downloaded from http://meridian.allenpress.com/iosc/article-pdf/1993/1/794/2357422/2169-3358-1993-1-794.pdf by guest on 29 September 2021 0 I ►►►►^rt-^ 1 1 ! j 1 1 50 |xm). 0 20 40 60 80 100 120 140 Diameter (um) , number + volume References Figure 2. Cumulative frequency distribution for premixed oil-dispersant combination (medium fuel oil and Slickgone NS) in terms of number and volume of droplets, indicating that the number median is 1. Daling, P. S., D. Mackay, N. Mackay, and P. J. Brandvik, 1990. 20 |xm and the volume median is 46 u,m Droplet size distributions in chemical dispersion of oil spills: To- wards a mathematical model. Oil & Chemical Pollution, v7, ppl73-198 2. Delvigne, G. A. L., 1985. Experiments on natural and chemical dispersion of oil in laboratory and field circumstances. Proceedings Laboratory measurements of the 1985 Oil Spill Conference, American Petroleum Institute, Washington D.C., pp507-514 Work is under way to use the information on oil concentration and 3. Milgram, J. H., R. G. Donnelly, R. J. Van Houten, and J. M. oil droplet size measured at sea to design a laboratory test that recre- Camperman, 1978. Effects of Oil Slick Properties on the Disper- ates the microscale effects observed at sea. In this way we will be able sion of Floating Oil into the Sea. U.S. Coast Guard Report to produce a laboratory test that relates to conditions at sea, in order to CG-D-64-78. NTIS No. ADA062693