APPENDIX B: ESTIMATING FORCES ON OIL SPILL CONTAINMENT BOOMS
Appendix B
1.0 INTRODUCTION
There has been a continuing requirement to predict the drag force on a towed containment boom without conducting tank tests. Formulae have been developed, but until recently, they have not been accurate over extended ranges of conditions and boom types. There has been general agreement that the drag force on a towed boom is proportional to the square of the tow speed; however, much more is needed to compute this force with any degree of precision.
Until recently, two formulae were commonly used to estimate forces on booms: a relatively simple formula contained in the Technical Information Papers published by the International Tanker Owners Pollution Federation (ITOPF) and a more complicated formula developed by Exxon Production Research Company and published since 1986 in the World Catalog of Oil Spill Response Products. The former is referred to the ITOPF formula and, for the purposes of this Appendix, the latter is referred to as the Exxon formula. Comparing the loads predicted by these formulae with the loads measured in at-sea tests has shown that both formula underestimate the actual drag force on booms in many cases.
Recently, controlled tests were performed with the specific purpose of comparing measured drag force with computed force using existing formulae, and further, developing a more exact means of computation using the new formulae developed from these measured results. The U.S. Department of the Interior, Minerals Management Service (MMS) 1999 test program, "Estimation of Towing Forces on Oil Spill Containment Booms (1)," was specifically designed to develop a means of computing drag force that follows measured results more exactly. The tests were performed in the Ohmsett test tank and included:
· Measuring forces on seven different booms towed at a range of speeds.
· Comparing the results with loads predicted by ITOPF and Exxon formulae.
· Developing a new set of empirical equations in the ITOPF format that match measured results.
Although these tests were an important step in developing a computational method of estimating forces on booms, this work had several limitations. First, the force of the wind was not considered in determining total force. Second, the empirical formulae developed require a separate constant for each boom type and each of three operating environments. Third, and most important, the only wave heights tested were 7.3 inches and 12.3 inches, or roughly ½ foot and 1 foot. A set of constants for existing equations were proposed for larger waves, but these could not be confirmed with measured results.
This Appendix summarizes the work that was performed at Ohmsett, including the recommendations for calculating drag force. This is followed by a brief discussion of the ITOPF and Exxon equations. Finally, a recommended set of formulae and constants are proposed for Calm, Protected, and Open Water classifications.
(Five pages of text and references)
