ABSTRACT 2017-330

Oil spill response is a challenging task, especially for a deep water well release. The efficiency of mechanical recovery equipment deployed at the sea surface may be reduced due to challenges with detecting oil and then deploying equipment while slicks are continuing to move, weather, break apart, and spread. In-situ burning may have similar limitations. The ability to use dispersants could provide significant benefits when other response techniques have reduced efficiency. Subsea injection of dispersants offers some significant benefits for oil spill response including access to the freshest and non-emulsified oil in a high turbulence environment, ability to reduce the amount of dispersant by injecting it directly into the oil stream, ability to operate day and night under a wider range of weather conditions, and availability of a large water volume to rapidly decrease the concentration of a dispersed oil plume.

The American Petroleum Institute has sponsored research on subsea dispersant injection for over 5 years. Project teams are looking into subsea dispersant injection effectiveness, fate and effects, subsea plume monitoring, and numerical modeling. The information from these projects along with information in the literature are being used to conduct a comparative risk assessment on a hypothetical blowout assuming different response strategies. This effort is ongoing.

As a result of this research, the body of knowledge regarding subsea dispersant injection has grown. Laboratory studies have demonstrated the utility of directly injecting dispersants at the source of a blowout. Work has begun to understand the biodegradation potential and toxicity of a subsea plume to deep-water organisms. Additionally, progress has been made on enhancing numerical models to predict the fate of oil dispersed subsea. This paper provides an update of the recent progress made on this research.

INTRODUCTION

The American Petroleum Institute (API) formed a Joint Industry Task Force (JITF) on Oil Spill Preparedness and Response (OSPR) in 2010 to evaluate the response to the Macondo incident. The JITF released a report describing its evaluation on September 3, 2010 (OSPR JITF, 2010) that recommended additional study or enhanced communication for multiple areas of oil spill response. The recommendations formed the basis for research projects on various aspects of oil spill response technology, including the Subsea Dispersant Injection Program.

Subsea dispersant injection (SSDI) is the direct application of dispersants into a highly turbulent jet of oil potentially released during a subsea blowout (Figure 1). Injecting dispersants at the release point allows contact with the oil before it has a chance to weather and emulsify. Further, the turbulence in the release jet provides the energy needed to break the treated oil into small droplets. This is important because the rise velocities of oil droplets are proportional to the square of the droplet diameter. Reducing the rise velocity of oil droplets keeps some of the oil from surfacing and moves the surfacing location. This has the benefits of increasing the amount of biodegradation, mitigating impacts to surface dwelling wildlife, reducing the persistence of the oil, reducing stranding of oil on shorelines, and also protecting personnel attempting to control the well from exposure to volatile organic compounds.

Figure 1.

Diagram illustrating SSDI and the subsequent biodegradation process that begins when dispersants are applied to a deep water release.

Figure 1.

Diagram illustrating SSDI and the subsequent biodegradation process that begins when dispersants are applied to a deep water release.

In 2011, the commercial fish catch was higher than any year with the exception of 2000 (Figure 2). This is very likely because of the fishing moratorium placed over much of the GOM in 2010. That is, this data indicates the impacts to the commercial fishery from the spill may have been less than the benefits of the moratorium. The use of SSDI may have been at least a partial reason for this. The catch after 2011 was within the 15-year range even when excluding 2010. The value of the catch after 2010 is also well within the 15-year range.

Figure 2.

Yearly National Marine Fisheries Service Gulf of Mexico commercial fishery landing estimates. The catch value is in 2000 dollars.

Figure 2.

Yearly National Marine Fisheries Service Gulf of Mexico commercial fishery landing estimates. The catch value is in 2000 dollars.

In addition, GOM seafood has been very rigorously tested. Test results since May 2010 show no evidence of contamination from either oil or dispersants. Levels of contamination have consistently tested 100 to 1,000 times lower than safe thresholds established by the US Food and Drug Administration (BP, 2015).

By reducing the droplet size of the rising oil, SSDI also had the benefit of keeping fresh, volatile oil from surfacing near the well site. This reduced the release of volatile organic compounds to the atmosphere near the well and subsequent exposure to personnel on vessels attempting to control the well.

The photos in Figure 3 illustrate the reduction in oil surfacing near the well site caused by SSDI during the Macondo incident. The images provide an aerial view of the scene immediately over the well including vessels used for well-control operations. May 9 was 1 day before a 24-hour test of SSDI was initiated. Prior to SSDI oil rose to the surface and formed a significant slick directly over the well. SSDI was initiated at 5:30 AM on May 10. We estimate that the sea surface had over 90% less oil 11-hours later based on these images. The metocean conditions (currents and winds) do not account for the reduced surface expression of the oil while dispersants were used. SSDI was stopped on the morning of May 11 and a new slick appeared at the surface 5 hours later. These photos provide evidence that SSDI significantly altered the fate of oil released during the incident.

Figure 3.

Aerial photos taken over the Macondo well site before SSDI started (upper left image; May 9, 2010), after 11 hours of injection (upper right image; May 10, 2010), and 5 hours after injection ceased (bottom image; May 11, 2010).

Figure 3.

Aerial photos taken over the Macondo well site before SSDI started (upper left image; May 9, 2010), after 11 hours of injection (upper right image; May 10, 2010), and 5 hours after injection ceased (bottom image; May 11, 2010).

Additional evidence supports this conclusion. During the Macondo event, each vessel responsible for well control collected hourly data on the VOC concentration with monitors on their decks. Figure 5 shows this data compared to the hourly SSDI rates and the hourly wind speed collected from the NOAA meteorological buoy closest to the Macondo well.

Instead of showing VOC data from individual monitors, Figure 4 presents the simple sum of all the VOC data from all 10 vessels for the period May 30 – June 10, 2010. Of the available data (which spanned May 30 – August 4), the May 30 – June 12 period had many more instances where SSDI was stopped and these periods lasted longer than other periods. The sum VOC data is presented because there is too much scatter in the VOC data from any single vessel because there was no information on the location of any vessel relative to the well site, whether or not a slick happened to be near a vessel, or if a vessel was upwind or downwind of a surface slick. In addition, there is no information describing operations on the individual vessels that could have contributed to a high VOC reading irrespective of nearby oil slicks. The sum VOC data for all the vessels allows a smoothing of the scatter from data for individual vessels. The wind speed is provided in Figure 5 because it has an important effect on VOC dilution, i.e., high winds could have kept minimized VOC spikes.

Figure 4.

Volatile organic compound measurements collected from May 30 to June 10 on 10 vessels located near the Macondo well site along with data on the hourly subsea dispersant injection rates and local wind speed. (Data on VOCs and SSDI rates provided by USEPA; wind speed collected from the nearest NOAA meteorological buoy)

Figure 4.

Volatile organic compound measurements collected from May 30 to June 10 on 10 vessels located near the Macondo well site along with data on the hourly subsea dispersant injection rates and local wind speed. (Data on VOCs and SSDI rates provided by USEPA; wind speed collected from the nearest NOAA meteorological buoy)

As seen in Figure 5, there appears to be a correlation between no or low SSDI rates and spikes in sum VOC concentrations. Periods with SSDI injection rates at or above 10 gpm did not have spikes. This data provides additional evidence beyond the 1 day test on May 10 that SSDI significantly influenced the fate of oil.

Although the data above provides credible evidence that SSDI did significantly alter the fate of oil during the Macondo oil spill, the incident wasn’t a controlled experiment so the available data does not provide fully conclusive evidence on the effectiveness. It is for this reason and the fact that the technique had not been evaluated with controlled experiments prior to 2010 that API funded the research summarized in this paper.

Industry now incorporates SSDI into contingency plans and equipment stockpiles in strategic locations worldwide in order to implement this tool during an emergency. Industry has developed a large-scale, multiple-year project to conduct controlled testing of the method to support these contingency plans. The project is divided into five work areas: Effectiveness, Fate & Effects, Modeling, Monitoring, and Communications. Recent efforts have been to complete a comparative risk assessment of SSDI using data from the API-sponsored research and the literature. Studies supporting each of these project areas are in progress. This paper summarizes the API Subsea Dispersant Project.

RESULTS/DISCUSSION

Effectiveness Team Project

The effectiveness project was designed to (1) evaluate the effectiveness of SSDI, (2) recommend dispersant injection methods, and (3) provide data that can be used to construct numerical models that simulate blowouts. Scaled testing of SSDI using an unpressurized tower tank, a pressurized tower tank, and a large wave basin has been completed.

The work consisted of six phases of testing. More detailed descriptions of the various phases of these experiments are described elsewhere (Brandvik, et al. 2017 and Brandvik et al. 2016). Here we will summarize the important findings and conclusions from the six phases.

Phase I

Phase I studied injection location and dispersant to oil ratios (DORs) at low pressure. Injection far upstream of the release point, injection just upstream of the release point, and injection into the jet of oil above the release point were studied. All three produced significant reductions in droplet sizes as long as the injection wasn’t into the jet of oil >6 pipe diameters from the release point.

DORs were varied from 1:1000 to 1:25 and compared to control tests with no dispersant. DORs from 1:1000 – 1:250 provided limited reduction in droplet size compared to the control. At a DOR of 100:1, there was a significant decrease in droplet size compared to the control. DORs of 1:50 and 1:25 provided an additional reduction in droplet size as expected.

Phase II

Phase II testing was designed to evaluate SSDI effectiveness with various oil types and three dispersants at low pressure. SSDI effectiveness somewhat depended on the oil type when dispersed with Corexit® 9500. Corexit® 9500 at a 1:100 DOR reduced all four oils to a median droplet size of roughly 86 microns. The absolute reduction in droplet size varied with the different oils as control tests without dispersant had droplet sizes that varied from 280 microns for the Oseberg (paraffinic) oil to 144 microns for Norne.

Comparing SSDI for all three dispersants across the four oils, Corexit® 9500 produced the smallest dropelts most of the time followed by Finasol® OSR 52, and Dasic® Slickgone NS.

Phase III

Phase III testing was high pressure experiments with “dead oil.” These experiments were performed with oil that had been stabilized at 1 atm (“dead oil”) to remove any dissolved gas prior to these tests. They were completed to determine if pressure effected droplet sizes of oil in an energetic jet with and without SSDI. The experiments repeated the Phase I experiments but were performed at the equivalent pressure of 1750 m of seawater. The results showed that the droplet size of the oil was not influenced by pressure.

Phase IV

Phase IV testing evaluated the potential for droplet coalescence just outside the energetic discharge jet. Coalescence was studied by placing instruments to measure droplet sizes (LISST 100X manufactured by Sequoia Instruments) at 2 m and 5 m above the release point of jets of oil in the tower basin. This allowed a determination of any changes in the droplets size distribution as the plume of oil rose between the instruments. Tests found that there was no significant difference between the distributions indicating that there was no net coalescence. Coalescence above 5 m is possible but less likely because the plume of oil will further dilute and thereby reduce droplet collision frequency that is necessary for coalescence to occur.

Coalescence and droplet breakup far outside the energetic discharge jet was also studied. These experiments used an inverted cone system that had a down flow of seawater that allowed individual droplets to be suspended and observed at a location in the system depending on their terminal velocity. This system was designed to simulate droplets rising through the water column at their terminal velocity in order to observe droplets outside an energetic jet. Droplets can further reduce in size either by shearing into two or more droplets or undergo tip streaming.

Tip streaming results when surfactants concentrate at a trailing edge of a droplet because the flow of water across a droplet as they rise forces them there. This causes a trailing edge to have very low surface tension and this combined with high local shear can result in small oil droplets separating from the parent droplet. Tip-streaming was also observed previously in a study by Gopalan and Katz (2010).

In the experiments, the occurrence and magnitude of tip streaming was a function of the oil viscosity and dispersant loading with more viscous oils having greater tip streaming. Tip streaming did not occur for the low viscosity oils tested. These results are consistent with the use of the modified capillary number predictions of tip streaming described by Gopalan and Katz (2010). Droplets breaking into two or more were not observed in these tests because they were generated from a highly turbulent jet to only produce droplets that were stable when rising at their terminal velocity.

Phase V

Phase V testing included high pressure experiments with “live oil.” These experiments were performed with oil that had not been stabilized and was saturated with gas. In addition, some tests included additional free gas at a gas-to-oil ratio of 1:1 by volume. Live oil” plus gas represents the hydrocarbons that would be released during a blowout. These experiments used the same Oseberg as in Phase III but it was re-saturated with natural gas at the pressure of the test (either the equivalent of 580 m, 1200 m, or 1750 m of seawater). The seawater temperature was maintained at 4°C and oil temperatures ranged from 20°–50°C. These results showed SSDI reduced droplet sizes of both “live oil” alone and “live oil” with gas consistent with observations for both “dead oil” at high and low pressure. In addition, SSDI reduced the gas bubble sizes but not to the same extent as the oil droplets.

Phase VI

Phase VI testing was conducted to evaluate larger-scale releases. Larger-scale releases with release nozzles between 2.5 – 5 cm were completed at the large wave basin facility. The release nozzles were roughly ten times larger than was possible in the tower tanks. “Dead oil” was used in these tests by necessity. In these tests, SSDI at 1% reduced droplets sizes by over an order of magnitude. Another important finding from these larger-scale experiments was that dispersants mixed into the oil and caused unimodal droplet-size distributions. This indicates that dispersants mixed into the jet of oil to uniformly treat all of it. We would expect a bimodal distribution with a peak of larger and smaller droplets if the dispersant treatment wasn’t uniform. This indicates that the injection techniques we used to simulate a simple injection wand can be adequate; however, larger scale tests are needed to fully verify.

Fate and Effects Team Project

The subsea dispersants project team also coordinated research to study the fate and effects of dispersed oil in a deep-water environment. This project area included evaluation of the biodegradation and toxicity of dispersants and dispersed oil on deep-water communities. The effort began with a workshop held in October 2012 to develop a framework for protocols to be used during the biodegradation and toxicity testing. Attendees included subject matter experts in chemistry, deep-water ecology, microbiology, and toxicity from academia, government, and industry. Recommendations from the workshop were used to develop requests for proposals (RFP) to conduct biodegradation and toxicity testing.

The initial biodegradation research project was a review of recent literature on deep-water petroleum biodegradation. The findings from this review were used to determine what additional work may be needed. This resulted in a feature article in Environmental Science & Technology that was published in December 2015 (Hazen and Prince, 2015). A key finding described in this paper are that oil degrades in both shallow and deep marine waters with half-lives of days to months. In contrast, oil that reaches shorelines and anaerobic sediments is likely to persist far longer. Further, microbial communities can rapidly shift to hydrocarbon degraders. Because of the significant amount of work done by others, no additional research funded by the API is planned in the near term to study oil or dispersed oil biodegradation.

The toxicity research was organized into three inter-related potential phases. Phase I consists of a literature review and toxicity modeling; Phase II includes toxicity testing at 1 atmosphere pressure; and Phase III includes toxicity testing under pressure representative of the deep-sea environment. Much of this work is still underway.

In Phase 1, we evaluated the toxicity of dissolved gases to determine their influence on crude oil toxicity and the relation between toxicity of hydrocarbons and pressure. One way that a surface oil spill differs from oil released during a blowout is that the latter includes gases dissolved in the oil and free gas. Almost all possible scenarios for a surface release are oil that was stabilized near the well site to remove volatile components. Oil released during a blowout would not be stabilized and would include both dissolved and free gases.

Aquatic toxicity data for dissolved gases was not available in the literature so a toxicity model (McGrath and Di Toro, 2009) was adapted to assess the toxicity of C1 to C4 gases and their relative contribution to overall hydrocarbon aquatic toxicity. An oil spill fate model was used to predict the concentration of dissolved components for several deep water blowout scenarios. These concentrations were then used with a toxicity model to predict the toxicity of all components of crude oil. The model indicates that dissolved gases have a limited role (maximum 1.4% of the total toxicity across all spill scenarios) in contributing to the toxicity of the whole crude oil. This is due to the rapid dilution and degradation of dissolved gases.

Literature also indicates that toxicity testing of baro-tolerant species at ambient pressures is likely conservative because pressure appears to be antagonistic to the effects of acute toxicity (Johnson et al., 1942a,b; Johnson and Eyring, 1948; Johnson and Flagler, 1950, 1951; Johnson et al., 1954). A theory for this reversal is that acute toxicity (narcosis) is caused when the total volume of a non-aqueous compound in a cell exceeds a critical level. High pressure reduces this volume and therefore reduces the toxicity threshold (Mullins, 1954). Thus, we have only conducted toxicity testing (summarized below) at ambient pressures to avoid safety concerns associated with toxicity tests at elevated pressure.

The objective of Phase II of the toxicity study is to conduct acute, lethal toxicity tests of barotolerant, diel migrating species at ambient laboratory pressure (i.e., 1 atmosphere) using both single test chemicals and water accommodated fractions (WAFs) / chemically enhanced water accommodated fractions (CEWAFs) of a widely studied crude oil (Alaska North Slope, ANS) to allow comparison with existing Species Sensitivity Distributions (SSDs) for shallow water organisms. Three test species – a deep-water coral (Lophelia pertusa), sablefish (Anoplopoma fimbria), and a shrimp (Pandalus borealis). Test compounds include individual high purity aromatic compounds (e.g., naphthalene, 2-methyl naphthalene (2-MN), and phenanthrene). This testing is ongoing and only preliminary findings are summarized here. The goal of these tests is to determine if deep-water organisms are more or less sensitive than shallow-water organisms.

SSD is a tool developed three decades ago to support ecological risk assessments (Klapow and Lewis, 1979, Mount, 1982, Blanck, 1984, Posthuma et al., 2001). SSD is a statistical distribution describing the variation in toxicity of a set of species to a single compound or a mixture. SSDs exist for species that reside in surface marine waters but there is limited data on the toxicity of species that resides in deep water.

In addition, Phase II testing includes toxicity tests at 1 atm using ANS crude oil that already has an SSD associated with it. Test organisms are the same barotolerant species used in the single chemical tests. The toxicity test results for the reference oil and the barotolerant species were compared to the existing SSD to determine if the tested deep-water organisms are more or less sensitive than shallow-water species.

Data from our study is available for the toxicity of 2-MN on the coral and shrimp and phenanthrene on the shrimp and sablefish. Only effects concentrations were determined for corals because of challenges in determining lethality in these tests. The 96-hr EC50 for coral and 2-MN was 0.72 mg/L, and the 96-hr LC50 for shrimp and 2-MN was 0.39 mg/L. These data were compared to LC50 of 2-MN for nematodes, Dungeness crab, grass shrimp, and brown shrimp (CAFÉ, 2016). The corals appeared more resilient than these organisms based on the EC50 while the shrimp in our study appeared slightly more sensitive to all species but the brown shrimp.

The 96-hr LC50 for shrimp and phenanthrene was >1 mg/L and 0.2 mg/L for sablefish. This compares to phenanthrene 96-hr LC50 for shallow water organisms that range from 0.31 – 0.89 mg/L (Unger et al., 2007, Vergauwen et al., 2015, Stinger et al., 2012, Eastmond et al, 1984, Abernethy et al., 1986, and Millemann et al., 1984). Thus, sablefish were slightly more sensitive to phenanthrene than shallow-water species while deep-water shrimp were less sensitive.

We have collected CEWAF LC50 data on sablefish but not WAF at this point. The CEWAF LC50 was 56.4 mg TPH/L. The sablefish fall in the middle of SSDs for a range of species (CAFÉ, 2016).

As mentioned, toxicity testing of other species with all the single compounds and the WAF/CEWAF are ongoing. We expect to have results in early 2017. The data we’ve collected to this point, however, indicates that deep-sea barotolerant species may not be more sensitive to oil or individual oil components than species dedicated to shallow water.

Modeling Team Project

The goal of the Subsea Injection Program’s Modeling Project is to enhance existing numerical tools to estimate the fate of dispersed oil plumes resulting from subsea injection. Models that predict the fate of deep-water oil discharges have been available for more than 10 years. These models, however, were not designed to include the change in droplet sizes caused by injection of dispersants. The focus of the modeling team project has been to develop droplet-size predictions algorithms capable of predicting droplets produced with SSDI.

The research was divided into two components: the first was an evaluation of existing oil droplet size models and integrated plume models, and the second was development of new models to predict oil droplet and gas bubble sizes.

Work to identify and evaluate existing droplet size models and integrated models is complete. This work included identification of oil droplet size data sets from lab and field tests. A peer-reviewed publication describing the model inter-comparison study is available (Socolofsky et al., 2015).

The second step in the modeling research was to develop droplet-size prediction models. Two droplet size prediction models have been developed and validated using data from the efficacy testing described earlier. One model, the modified Weber algorithm, is a static model that predicts the droplet size distribution at the end of a static jet (Johansen et al., 2013). The other is a dynamic model that predicts the time varying droplet size distribution within an energetic release jet and beyond, V-Drop J (Zhao et al., 2014).

Monitoring Team Project

The Monitoring Team’s focus was to evaluate, develop and recommend plans and technologies for SSDI monitoring. The effort consisted of two tasks:

  • Preparation of an API report “Industry Recommended Subsea Dispersant Monitoring Plan” API Technical Report 1152,

  • Preparation of an API bulletin “Industry Guidelines on Requesting Regulatory Concurrence for Subsea Dispersant Use” API Bulletin 4719

Industry Recommended Subsea Dispersant Monitoring Plan

In May 2013, the U.S. National Response Team (NRT) published a document describing their suggestions for monitoring dispersant use during atypical spills of subsea releases and prolonged surface releases (NRT, 2013). These suggestions were intended for use by Regional Response Teams. The API’s Subsea Dispersant Monitoring project team developed an industry recommended subsea dispersant monitoring plan as well (API, 2013). Both the Industry and the NRT plans have a similar goal of providing response teams with information on the effectiveness of dispersant operations during an event. The Industry plan only describes monitoring tools for SSDI and does not describe a surface dispersant monitoring protocol. Further, the Industry plan is focused on collecting information about the effectiveness of SSDI and the fate of subsea dispersed oil that can provide information for operational decision making. Monitoring protocols that require days for processing and evaluation are not a focus of the Industry plan because this information won’t be useful for supporting daily operational decision making. Another key aspect of the Industry plan is to stage the monitoring requirements to allow rapid implementation of easily-deployable tools followed by placement of more complex monitoring tools as the event proceeds. Further, the Industry monitoring plan does not recommend “shut down” criteria in the event that a defined performance parameter (e.g., reduction in dissolved oxygen concentration) may be exceeded. Instead the Industry plan recommends that exceeding a performance criteria triggers re-evaluation of the benefits of subsea injection when compared to other response options before recommending shut down or altering operations. In October 2013, the Industry plan was completed and was made available online at http://www.spillprevention.org/documents/API%201152-Industry-Recommended-Subsea-Dispersant-Monitoring-Plan.pdf.

Industry Guidelines on Requesting Regulatory Concurrence for Subsea Dispersant Use

During the Deepwater Horizon incident, the Region VI Response Team (RRT VI) had pre-authorization plans for surface dispersant use, but concluded that those plans were not applicable to a subsea, relatively continuous application of dispersant. As a result, incident specific implementation policies were developed during the course of the response. Since 2010, several oil spill response drills sponsored by industry have indicated that existing policies and guidance on SSDI use can be enhanced by developing a standard document to collect information that may be required by RRTs to support SSDI decisions. Such a document is currently in preparation as an API bulletin (API Bulletin 4719). These guidelines are based upon lessons learned from exercises with input from RRT VI agencies.

The guidelines provide forms and checklists recommended for use by industry. They describe the RRT concurrence request process, proposed information submission recommendations specific to subsea dispersant injection and the use of tradeoff analyses as decision support tools. Also included are practical flowcharts and checklists specific to Incident Management Team (IMT) positions that are integral to subsea dispersant use, and guidance on the preparation of subsea dispersant operations and monitoring plans. This document provides operational guidelines intended for actual events or exercises and provides a basis for engagement from a range of relevant stakeholders.

In addition, the document provides guidelines for regulatory approval of SSDI in the United States in accordance with Subpart J. The lessons learned captured from numerous companies during drills in addition to input from members of IPIECA and IOGP serve as a baseline for initial guidance to share with other countries and organizations to assist to help developing their own guidelines.

As mentioned, this document is undergoing final review and should be available through the API by 2017. When issued, it will provide response teams with information and checklists needed to put SSDI in place, provide them with checklists needed to collect information that regulators and stakeholders may require to provide their concurrence for SSDI, and provide RRTs with a document that is consistent across industry to allow efficient decision making.

Comparative Risk Assessment

One of the final major tasks of the API subsea dispersant injection project was to complete a comparative risk assessment (CRA) of the technology on a hypothetical blowout from a deep-water well in the Gulf Of Mexico. Details of this ongoing work are described by French-McCay (2017). The goal of this effort was to compare the trade-offs of different oil spill response options relative to a no intervention option. It was not a NRDA as we were not quantifying impacts but performing a comparative analysis.

The hypothetical blowout was a 45,000 bbl/day release in 1400 m in De Soto Canyon. The oil was 34.2 API with a viscosity of 8.4 cP at 20°C, and a gas-to-oil ratio of 2000. The event lasted 21 days before a capping stack was installed.

The approach we used for the comparative analysis was as follows:

  1. Conduct hypothetical oil spill scenario modeling assuming different response strategies: no response, mechanical recovery-in situ burning-surface dispersants (MBSD), and MBSD plus SSDI

  2. Define environmental compartments (ECs; e.g., shoreline, sea surface, water column, sea floor, etc.) within a predefined study area

  3. Define valued ecosystem components (VECs)

  4. Conduct an exposure analysis

    • Define fraction of area days (for surface slicks) or volume days (for the water column) in each EC that had oil above pre-defined thresholds

    • Use the area days / volume days to determine the fraction of VECs in each EC that were potentially exposed

    • Apply a factor for VEC recovery and resilience based on their reproductive rates

    • Score each VEC in each EC as a function of percent VEC exposed and recovery time

  5. Complete a comparative Analysis

    • Analysis of Each Response Option

    • Compare / Contrast Results Between Response Options

This process provided a quantitative approach for comparing the exposure potential of each VEC in each EC for each response strategy modeled. It also allowed a sensitivity value to be applied to individual VECs scores based on a concern that certain VECs (e.g., threatened and endangered species) should get higher scoring.

After the above was complete, a workshop was held with external stakeholders to evaluate the results. The results of this workshop are being analyzed. The preliminary results indicate that protecting long-lived, surface dwelling VECs provided the greatest reduction in VEC scoring. That is, marine turtles and mammals consistently had the highest scores – indicated greater exposure and slower recovery. The response strategy that kept the most oil off the surface (MBSD & SSDI) provided the greatest reduction in these scores. Additionally, the technique showed in a quantitative way that a significant reduction in exposure scores could be achieved by dispersing the oil in the water column because this allowed dilution / degradation to happen in three dimensions. Allowing oil to surface caused this oil to concentrate in two dimensions. Further, allowing the oil to hit shorelines allowed it to concentrate in one dimension.

Communications Team Project

One of the most important components of the SSDI project is communications. This is to both inform external groups of findings and to receive input from experts to guide research plans. A communications plan was developed that includes formation of external technical advisory committees that are staffed by appropriate experts, holding workshops to develop research plans, developing fact sheets that describe the various project objectives, ongoing efforts, and accomplishments-to-date, and writing project newsletters as important research data is generated. Newsletters can be read online at http://www.api.org/environment-health-and-safety/clean-water/oil-spill-prevention-and-response/api-jitf-subsea-dispersant-injection-newsletter.aspx.

Additionally, the Subsea Dispersants team continues to engage in outreach efforts with broader OSPR and research communities. Specifically, team members have given presentations at multiple conferences and provide yearly updates to the U.S. Interagency Coordinating Committee for Oil Pollution Research. These presentations describe preliminary research findings and future plans.

SUMMARY & CONCLUSIONS

The American Petroleum Institute (API) is coordinating research to better understand SSDI operations and the potential effects of dispersed oil on deep-water environments. Research is nearing completion and final results should be available in 2017 and communicated through a series of peer-reviewed publications.

Efficacy Studies

Research included performing scaled tests of subsea dispersant injection to determine effectiveness under various conditions and to identify optimal injection methods. This research supports the use of SSDI as a primary contingency method. The main conclusions from the efficacy work are as follows:

  • SSDI significantly reduced oil droplet sizes in scaled testing,

  • Effectiveness depended on oil types, dispersant & dosage,

  • Dispersant provided the highest effectiveness when injected close to the release point to allow the dispersant to act on the energetic jet of oil,

  • Dispersant-to-oil ratios (DORs) as low as 1:100 were effective at significantly reducing droplets sizes,

  • The scaled testing indicates that SSDI effectiveness is not dependent on pressure,

  • Any of the three dispersant products stockpiled worldwide were effective at dispersing the crude oils, although effectiveness varied and higher or lower DORs may be needed depending on the oil – dispersant combination,

  • Break up oil droplets outside the energetic jet did not appear significant for low viscosity oils at the DORs studied,

  • Larger-scale tests (from 1 – 3 mm release nozzles to 2.5 – 5 cm release nozzles) indicate that the effectiveness of SSDI is not dependent on scale as all the oil was treated at both scales, i.e., droplet size distributions at both scales were similar and unimodal with no bimodality,

Fate & Effects Studies

The subsea dispersants project team also coordinated research to study the fate and effects of dispersed oil in a deep-water environment. This project area included evaluation of the biodegradation and toxicity of dispersants and dispersed oil on deep-water communities. This work is continuing but the main conclusions thus far are as follows:

  • Oil degrading microbes exist in all marine environments including deep water,

  • Oil degradation studies available in the literature indicate that crude oil has a half-life of days to a few months if aerobic conditions can be maintained,

  • Oil that reaches shorelines and becomes anaerobic is likely to persist far longer,

  • Aquatic toxicity modeling of dissolved gases indicates they have a limited role (< 1.4%) in overall crude oil toxicity,

  • Toxicity testing of baro-tolerant deep-sea species at ambient pressure is likely conservative because pressure appears to be antagonistic to acute toxicity from narcosis, and this avoids safety concerns / costs associated with toxicity tests at elevated pressure,

    • Deep-sea species were not more sensitive to whole oil or individual oil components than species that live at ambient pressure.

Modeling Studies

The modeling project area evaluated enhancements needs for existing numerical tools to estimate the fate of dispersed oil plumes resulting from subsea injection. A summary of the accomplishments from the modeling project area are as follows:

  • A peer-reviewed publication describing the model intercomparison study is available (Socolofsky et al., 2015).

  • Two droplet size prediction models have been developed and validated using data from the efficacy testing

    • ○ The modified Weber algorithm (Johansen et al., 2013), is a static model that predicts the droplet size distribution at the end of a static jet.

    • ○ The V-Drop J (Zhao et al., 2014) is a dynamic model that predicts the time varying droplet size distribution through an energetic release jet and beyond.

Monitoring Projects

The Monitoring Team’s focus was to evaluate, develop and recommend plans and technologies for SSDI monitoring. A summary of the accomplishments from the monitoring project area are as follows:

  • Preparation of an API report “Industry Recommended Subsea Dispersant Monitoring Plan” API Technical Report 1152,

  • Preparation of an API bulletin “Industry Guidelines on Requesting Regulatory Concurrence for Subsea Dispersant Use” API Bulletin 4719.

Comparative Risk Assessment

One of the final major tasks of the API subsea dispersant injection project was to complete a comparative risk assessment (CRA) of the technology on a hypothetical blowout from a deep-water well in the Gulf Of Mexico. This work is still ongoing but preliminary findings are as follows:

  • Preliminary findings are that surface dwelling marine turtles and mammals have the greatest exposure potential during a blowout,

  • SSDI may significantly reduce this exposure by reducing the amount of surface oil.

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