National Academies Press: OpenBook
« Previous: REFERENCES
Suggested Citation:"APPENDIX A--ROC Model Discussion." National Research Council. 2013. A Review of Genwest's Final Report on Effective Daily Recovery Capacity (EDRC): A Letter Report. Washington, DC: The National Academies Press. doi: 10.17226/18579.
×

APPENDIX A

ROC Model Discussion

Genwest (2012) uses the ROC model to estimate the thickness of the heaviest concentrations of oil, which would be targeted and encountered by skimmers. ROC incorporates weathering (Galt, 2011) and spreading (Galt and Overstreet, 2011) of oil based on spill volume, oil properties, and environmental conditions. On the basis of the committee’s discussions with the report authors, weathering was included in the ROC model runs that were used to develop the nominal average oil thickness by day in the Genwest (2012) report. Weathering includes evaporation, emulsification (formation of mousse by entrainment of seawater into the oil), dissolution, and degradation (both biodegradation and photo-oxidation). The floating oil volume of light crude oils, which contain substantial fractions of volatiles, would decrease rapidly in the first hours to days after release, so the average oil thickness should also decrease rapidly. Evaporative losses even for medium and heavy crudes can also be substantial (up to 30% of the mass/volume) over the first 3 days after release. Dissolution of the slick and degradation would not change oil volume or thickness significantly over hours to a few days. On the other hand, emulsification would increase oil thickness. ROC appropriately includes these changes with weathering algorithms used in state-of-the-art oil spill models.

ROC considers the bounding area of a contiguous oil slick, which initially spreads radially by gravitational slumping controlled by viscous forces. To account for wind and wave motion and oil droplet formation and entrainment due to wave motion, the circular area of spread oil is extended in the downwind axis by 3% of wind speed (a common rule of thumb for oil drift rate, range 2-6%; ASCE Task Committee on Modeling Oil Spills, 1996) plus an additional 0.5% of the wind speed if the wind is greater than 6 m/s (to represent the additional downwind velocity expected in windrows associated with Langmuir cells). This approximates for the two-dimensional model in ROC the three-dimensional process (described based on field data by Elliott, 1986), where oil entrained into underlying water is left behind the leading edge of the floating oil because the oil floating on the water surface moves at roughly 3% of wind speed, whereas droplets in underlying water move slower. The smallest droplets remain under water the longest and undergo little wind transport, whereas progressively larger droplets resurface behind the leading edge sooner (due to higher buoyancy). This results in a comet-shaped slick with thick oil at the downwind end and sheen trailing behind. This was noted first by Elliott et al. (1986) and more recently through numerical simulations by the Boufadel group (Boufadel et al., 2006, 2007). These spreading processes are modeled explicitly in three-dimensional oil spill models such as SIMAP (French-McCay, 2004) and presumably in OSCAR (Reed et al., 1995; Aamo et al., 1997).

The correction of the contiguous oil slick thickness to account for the leading edge of thicker oil does appear to be included in the ROC model runs used to develop the nominal average oil thickness. In committee discussions with the Genwest report authors, they confirmed that this spreading correction was in fact included. Details of those calculations (i.e., what factor or factors were used) are not available.

Figures 16-18 in Galt and Overstreet (2011) present estimated oil thickness at Langmuir convergences relative to initial spill thickness for a range of wind conditions and oil densities. Their conclusion was “that under most conditions of wind- and windrow-spacing, spilled oil could collect in convergences comprising only 20%, or less, of the original spills’ area.” In ROC

Suggested Citation:"APPENDIX A--ROC Model Discussion." National Research Council. 2013. A Review of Genwest's Final Report on Effective Daily Recovery Capacity (EDRC): A Letter Report. Washington, DC: The National Academies Press. doi: 10.17226/18579.
×

(based on the data in Figures 16-18 of Galt and Overstreet, 2011), the inclusion of Langmuir cell convergences (windrows) amounts to a factor of 5 or more (up to 100 times) increase in thickness compared with a mean over the area encompassing one or more cells. The scale of Langmuir cells is taken as three times the mixed-layer depth (Galt and Overstreet, 2011), which is typically 10 to 30 m. Thus, Langmuir cell windrows would be about 30 to 90 m apart (about 100 to 300 ft).

If a skimmer’s swath width is order 100-1,000 ft and its speed is 0.75 kt based on ERSP, the skimmer would sweep 0.2 to 2 square miles in a 12-hour operational period. Thus, skimmers would be crossing both windrows and open water, and large swath widths are much larger than the windrow spacing. In addition, Langmuir cells are not always present; they will appear after wind blows in one direction for a relatively long time (many hours or more). Thus, this analysis indicates that inclusion in ROC of a factor 5-100 increase in thickness due to Langmuir cells would overestimate the average oil thickness that would be encountered by a skimmer. However, based on discussions with the Genwest report authors, the ROC model runs used to develop the nominal average or representative oil thickness did not include consideration of thickening of oil in windrows due to Langmuir circulation. This is appropriate for the reasons described above. Unfortunately, the Genwest report is not clear on this point; it merely cites Galt and Overstreet (2011), which describes the basis and inclusion of such correction factors in ROC.

Suggested Citation:"APPENDIX A--ROC Model Discussion." National Research Council. 2013. A Review of Genwest's Final Report on Effective Daily Recovery Capacity (EDRC): A Letter Report. Washington, DC: The National Academies Press. doi: 10.17226/18579.
×

REFERENCES

Aamo, O., M. Reed, and A. Lewis. 1997. Regional Contingency Planning Using the OSCAR Oil Spill Contingency and Response Model. Trondheim, Norway: SINTEF Applied Chemistry, Environmental Engineering. Available online at www.itk.ntnu.no/ansatte/Aamo_Ole.Morten/pdf/region.pdf. Accessed August 28, 2013.

ASCE Task Committee on Modeling Oil Spills. 1996. State-of-the-art review of modeling transport and fate of oil spills. Journal of Hydraulic Engineering 122:594-609.

Boufadel, M.C., R. Bechtel, J. Weaver. 2006. The movement of oil under non-breaking waves. Marine Pollution Bulletin 52:1056-1065.

Boufadel, M.C., K. Du, V.J. Kaku, and J. Weaver. 2007. Lagrangian simulation of oil droplets transport due to regular waves. Environmental Modelling & Software 22:978-986.

Elliott, A.J. 1986. Shear diffusion and the spread of oil in the surface layers of the North Sea. Deutsche Hydrografische Zeitschrift 39:113-137.

Elliott, A.J., N. Hurford, and C.J. Penn. 1986. Shear diffusion and the spreading of oil slicks. Marine Pollution Bulletin 17:308-313.

French-McCay, D.P. 2004. Oil spill impact modeling: Development and validation. Environmental Toxicology and Chemistry 23:2441-2456.

Galt, J.A. 2011. Oil Weathering Technical Documentation and Recommended Use Strategies. Edmonds, WA: Genwest Systems, Inc. Available online at http://www.genwest.com/DRAFT%20ROC%20OilWx%20TechDoc.pdf. Accessed August 26. 2013.

Galt, J.A. and R. Overstreet. 2011. Development of Spreading Algorithms for the ROC. Edmonds, WA: Genwest Systems, Inc. Available online at http://www.genwest.com/DRAFT%20ROC%20Spreading%20TechDoc.pdf. Accessed August 26, 2013.

Genwest (Genwest Systems, Inc.). 2012. EDRC Project Final Report. Edmonds, WA: Genwest Systems, Inc. Available online at http://www.bsee.gov/uploadedFiles/BSEE/Research_and_Training/Technology_Assessment_and_Research/tarprojects/600-699/Genwest_EDRC-Project_Final_Report-1.pdf. Accessed August 26, 2013.

Reed, M., O.M. Aamo, and P.S. Daling. 1995. Quantitative analysis of alternate oil spill response strategies using OSCAR. Spill Science & Technology 2:67-74.

Suggested Citation:"APPENDIX A--ROC Model Discussion." National Research Council. 2013. A Review of Genwest's Final Report on Effective Daily Recovery Capacity (EDRC): A Letter Report. Washington, DC: The National Academies Press. doi: 10.17226/18579.
×
Page 26
Suggested Citation:"APPENDIX A--ROC Model Discussion." National Research Council. 2013. A Review of Genwest's Final Report on Effective Daily Recovery Capacity (EDRC): A Letter Report. Washington, DC: The National Academies Press. doi: 10.17226/18579.
×
Page 27
Suggested Citation:"APPENDIX A--ROC Model Discussion." National Research Council. 2013. A Review of Genwest's Final Report on Effective Daily Recovery Capacity (EDRC): A Letter Report. Washington, DC: The National Academies Press. doi: 10.17226/18579.
×
Page 28
Next: APPENDIX B--A Review of the EDRC Project Final Report »
A Review of Genwest's Final Report on Effective Daily Recovery Capacity (EDRC): A Letter Report Get This Book
×
Buy Ebook | $9.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

The Bureau of Safety and Environmental Enforcement (BSEE) enforces compliance with oil spill response requirements for facilities located seaward of the coast line. It commissioned Genwest Systems, Inc., to assess the existing planning standard for response to offshore oil spills with mechanical oil skimming systems, known as the Effective Daily Recovery Capacity (EDRC), and to consider improvements to that standard. In its report, Genwest proposed an Estimated Recovery System Potential (ERSP) calculation as an alternative to EDRC. BSEE specifically asked the National Research Council committee to consider three aspects of the ERSP approach in the Genwest report: the proposed methodology of ERSP, its applicability, and the computer model behind ERSP.

A Review of Genwest Effective Daily Recovery Capacity (EDRC) Project Final Report is an objective technical evaluation of the report produced by Genwest Systems, Inc., on the Effective Daily Recovery Capacity (EDRC). This report evaluates the scientific basis of the methodology, applicability, and modeling approach used in the Genwest report. In response to its statement of task, the authoring committee found the new approach for estimating the efficiency of oil skimmers presented by Genwest, (ERSP), to be basically sound and a substantial improvement over methods currently employed by BSEE in its rule-making. However, there are a number of simple improvements that can and should be made to the ERSP approach that would be extremely useful. This report examines the methodology and applicability of the ERSP calculation and discusses Genwests’s computer model.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!