Tim Nedwed

The primary biodegradation of dispersed crude oil in the sea.

Dispersants are important tools for stimulating the biodegradation of large oil spills. They are essentially a bioremediation tool - aiming to stimulate the natural process of aerobic oil biodegradation by dispersing oil into micron-sized droplets that become so dilute in the water column that the natural levels of biologically available nitrogen, phosphorus and oxygen are sufficient for microbial growth. Many studies demonstrate the efficacy of dispersants in getting oil off the water surface. Here we show that biodegradation of dispersed oil is prompt and extensive when oil is present at the ppm levels expected from a successful application of dispersants - more than 80% of the hydrocarbons of lightly weathered Alaska North Slope crude oil were degraded in 60 d at 8 °C in unamended New Jersey (USA) seawater when the oil was present at 2.5 ppm by volume. The apparent halftime of the biodegradation of the hydrocarbons was 13.8 d in the absence of dispersant, and 11 d in the presence of Corexit 9500 - similar to rates extrapolated from the field in the Deepwater Horizon response.

Lab tests on the biodegradation of chemically dispersed oil should consider the rapid dilution that occurs at sea.

Most crude oils spread on open water to an average thickness as low as 0.1 mm. The application of dispersants enhances the transport of oil as small droplets into the water column, and when combined with the turbulence of 1 m waves will quickly entrain oil into the top 1 m of the water column, where it rapidly dilutes to concentrations less than 100 ppm. In less than 24 h, the dispersed oil is expected to mix into the top 10 m of the water column and be diluted to concentrations well below 10 ppm, with dilution continuing as time proceeds. Over the multiple weeks that biodegradation takes place, dispersed oil concentrations are expected to be below 1 ppm. Measurements from spills and wave basin studies support these calculations. Published laboratory studies focused on the quantification of contaminant biodegradation rates have used concentrations orders of magnitude greater than this, as it was necessary to ensure the concentrations of hydrocarbons and other chemicals were higher than the detection limits of chemical analysis. However, current analytical methods can quantify individual alkanes and PAHs (and their alkyl homologues) at ppb and ppm levels. To simulate marine biodegradation of dispersed oil at dilute concentrations commonly encountered in the field, laboratory studies should be conducted at similarly low hydrocarbon concentrations.

Intercomparison of oil spill prediction models for accidental blowout scenarios with and without subsea chemical dispersant injection.

We compare oil spill model predictions for a prototype subsea blowout with and without subsea injection of chemical dispersants in deep and shallow water, for high and low gas-oil ratio, and in weak to strong crossflows. Model results are compared for initial oil droplet size distribution, the nearfield plume, and the farfield Lagrangian particle tracking stage of hydrocarbon transport. For the conditions tested (a blowout with oil flow rate of 20,000 bbl/d, about 1/3 of the Deepwater Horizon), the models predict the volume median droplet diameter at the source to range from 0.3 to 6mm without dispersant and 0.01 to 0.8 mm with dispersant. This reduced droplet size owing to reduced interfacial tension results in a one to two order of magnitude increase in the downstream displacement of the initial oil surfacing zone and may lead to a significant fraction of the spilled oil not reaching the sea surface.

Detecting Arctic oil spills with NMR: a feasibility study

To meet the world’s growing energy needs, the oil industry is pursuing oil resources in ice-prone regions. These activities will require robust oil spill contingency plans. One area of need is a method to remotely detect oil that is trapped beneath or within ice. The current operational method for oil detection within or under ice requires placing personnel on the ice to take measurements. A primary challenge with these measurements is the speed at which they can be collected. Presented here is a scaled-down prototype of an Earth’s field nuclear magnetic resonance device that can be moved from one spot to another on the ice by a helicopter to quickly survey large areas. This smallscale version has been built and tested. It successfully differentiates an oil surrogate from the bulk water signal by using an adiabatic inversion, followed by a delay to suppress the otherwise overwhelming water signal before acquiring the signal after an adiabatic half passage. The device will be scaled up, and further testing will be conducted. Initial proof-of-principle results show great promise for the development of a remote oil detector.

Use of passive samplers for improving oil toxicity and spill effects assessment.

Methods that quantify dissolved hydrocarbons are needed to link oil exposures to toxicity. Solid phase microextraction (SPME) fibers can serve this purpose. If fibers are equilibrated with oiled water, dissolved hydrocarbons partition to and are concentrated on the fiber. The absorbed concentration (Cpolymer) can be quantified by thermal desorption using GC/FID. Further, given that the site of toxic action is hypothesized as biota lipid and partitioning of hydrocarbons to lipid and fibers is well correlated, Cpolymer is hypothesized to be a surrogate for toxicity prediction. To test this method, toxicity data for physically and chemically dispersed oils were generated for shrimp, Americamysis bahia, and compared to test exposures characterized by Cpolymer. Results indicated that Cpolymer reliably predicted toxicity across oils and dispersions. To illustrate field application, SPME results are reported for oil spills at the Ohmsett facility. SPME fibers provide a practical tool to improve characterization of oil exposures and predict effects in future lab and field studies.

Lab Tests on the Biodegradation Rates of Chemically Dispersed Oil Must Consider Natural Dilution

ABSTRACT Many light-to-medium crude and fuel oils will spread rapidly on open water to an average thickness < 1 mm and perhaps < 0.1 mm. Effective application of dispersants results in the rapid transport of oil as small droplets into the water column. Simple calculations predict that the turbulence of 1 m waves will rapidly entrain dispersed oil into the top 1 m of the water column, diluting a 1 mm thick slick to a concentration of 1,000 ppm and a 0.1 mm slick to 100 ppm. Within a short period of time (likely less than 1 day), a dispersed oil plume in the open sea will mix into the top 10 m (or more) of the water column to give average oil concentrations of 100 ppm for the 1 mm thick slick and 10 ppm for the 0.1 mm thick slick, and dilution will continue as time proceeds. Measurements conducted during actual spill events support these calculations. Average concentrations measured in dispersed oil plumes 1 m below the surface are in the range of 100 ppm oil or less. Dilution to concentrations below 5 ppm ...

HERDING AGENTS THICKEN OIL SPILLS IN DRIFT ICE TO FACILITATE IN SITU BURNING: A NEW TRICK FOR AN OLD DOG1

ABSTRACT In situ burning is an oil spill response option particularly suited to remote ice-covered waters. The key to effective in situ burning is thick oil slicks. In loose drift ice conditions oi...

Field verification of the Offshore Operators Committee (OOC) Mud and Produced Water Discharge Model

Abstract This paper describes the use of field data on drilling mud and produced water dispersion to verify the Offshore Operators Committee (OOC) Mud and Produced Water Discharge Model (the “OOC Model”). Field studies for produced water and water-based drilling mud discharges provided data on effluent properties, discharge rates and pipe diameters, ocean currents, water column salinity and temperature, and measured effluent concentration at distances up to 103 m downcurrent from the discharge point. Data on effluent properties and field conditions were used as input data for OOC Model predictions of water column concentrations. This paper compares concentrations predicted by the OOC Model with field observations and describes the field data in sufficient detail to support their use to check the performance of other related models. The measured concentrations exhibited high variability due to turbulence and ambient current variations. The field data nonetheless showed clear trends in dilution with distance from the discharge point and demonstrated that both produced water and drilling mud discharges are rapidly diluted after discharge into the marine environment. There was good agreement between field observations and OOC model predictions. The results of these studies document the ability of the OOC Model to predict concentrations from field discharges of drilling mud and produced water.

Using Herders for Rapid In Situ Burning Of Oil Spills on Open Water

ABSTRACT Since 2004, the main goal of R&D on herding agents (also called oil collecting agents) has been to determine their ability to enhance in situ burning of oil in ice concentrations too low for natural containment of the oil slick by the ice itself (i.e., ice concentrations between <10 and 60%). Unexpectedly, the results also indicate that the concept of in situ burning enhanced by herders has the potential of being extended to open water conditions. Herders were studied in the 1970s as an open water oil spill response technique but the goal was to provide containment for mechanical recovery. In this application herders were limited to relatively calm conditions because the herder itself dissipated quickly in higher seas allowing the slick to respread. This dissipation occurred over periods of tens of minutes: not enough time to allow skimming of the herded slick. In situ burning is a process that requires only minutes to implement using minimum logistics and equipment. Thus, the potential exists th...

Verification of the OOC Mud and Produced Water Discharge Model using lab-scale plume behaviour experiments

Abstract This paper describes comparisons of predictions by the Offshore Operators Committee (OOC) Mud and Produced Water Discharge Model with published data on experimental laboratory-scale observations of plume behavior in flumes, towing tanks, and wind tunnels. The ability of the OOC Model to predict the plume characteristics observed in numerous individual tests of plume behavior covering a wide range of discharge conditions was examined. Data are provided summarizing the predictive ability of the model for all tests. The dimensionless parameters describing the physical characteristics of the plumes were within the range expected for typical Gulf of Mexico produced water and drilling fluid discharges for most of the comparisons. The model was in good to excellent agreement with measurements of plume behavior under most discharge conditions. The model was less accurate for those experimental cases where extreme plume bending occurred. It is believed that turbulence caused by the discharge pipe itself influenced the behavior of these extreme-bending plumes. The predicted density of solids accumulation on the bottom of a towing tank, a lab-scale analogy of the seabed accumulation of drilling solids, was within a factor of two of experimental observations. Excluding the three simulations of extreme bending plumes, the model showed no systematic trend towards either under- or over-estimation of any parameter examined. This suggests that experimental errors or biases may have contributed to observed differences between predictions and observations. The results of the validation testing described in this report increases confidence in the use of OOC Model predictions as an alternative to the difficult and expensive process of performing field measurements for every practical discharge situation.