This study compared dispersant performance at the U.S. Minerals Management Service facility, Ohmsett, with dispersant performance at sea. In 2003, at-sea dispersant tests were conducted in the United Kingdom with Intermediate Fuel Oils (IFO) of differing viscosities aimed at determining the viscosity of oil that limits chemical dispersion. These tests were repeated at Ohmsett using identical combinations of oils, dispersants and DORs. The at-sea tests showed that the oil viscosity limit for dispersion at relatively low wave energies (winds = 7 to 14 knots) lay in the range between the viscosities of IFO 180 (viscosity = 2075 cP at 16°C) and IFO 380 (viscosity = 7100 cP at 16°C). Tests at Ohmsett at a wave paddle frequency of 33.3 cpm were consistent with this finding. These tests also suggested that “limiting viscosity” is not a single value, but is a variable that is influenced by wave energy and dispersant type. Results also showed that Ohmsett tests at a wave paddle frequency of 33.3 cycles per minute (cpm) produced levels of effectiveness somewhat higher than at sea while tests 30 cpm waves produced results that were lower than at sea. Tests in 33.3-cpm waves showed effects of dispersant type on dispersant performance that were consistent with those observed at sea.

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