Offshore Oily-Water Separation

Abstract

Summary Various oily-water separation devices were evaluated for deoiling drilling-rig deck drainage. A loose-media coalescer selected for the application averaged 92% removal of free oil over a 1-year period of operation when it was fitted for solids removal and media cleaning. Dispersed-air flotation cells were found to remove the same amount of oil from water that could be removed by filtration with a filter retaining particles with diameters greater than 8 microns. Filtration is a simple and accurate method for determining the oil-removal efficiency that can be expected from a flotation separator for a particular application. Introduction Many types of equipment are available for the separation of oil from waste water, and the selection of a process to attain a prescribed separation efficiency for a particular application can be very difficult. It is not possible in many cases to predict the effectiveness of an oily-water separator, even if the prediction is based on case histories of similar applications. The applicability of an oily-water separator usually can be judged by the performance of laboratory or field tests however, the tests require the availability of the actual water to be treated. Laboratory tests that simulate field conditions have demonstrated the parameters involved in oily-water separation and have been useful in the design of separators and the selection of chemical aids for separation. A thorough investigation of oily-water separation by gas flotation has shown that separation efficiency is a function of oil drop size, oil concentration, gas concentration, gas bubble size, electrolyte concentration, and type of oil. These results demonstrate that a given separation process cannot achieve consistent results for different waste waters. This paper gives results obtained for the application of several types of separators for offshore oil waters. These projects resulted in some effective modifications to commercial separators and also resulted in a simple effective test for the anticipated effectiveness of dispersed gas flotation devices. Experimental Setups Experimental setups of equipment are described for each type of equipment tested under the Results section. The oil content of waste waters was measured by extraction and infrared spectrophotometry. A 100-mL sample was acidified with hydrochloric acid to pH 3.0 or less and was shaken vigorously with 10 mL of carbon tetrachloride. The solvent was allowed to settle, and an aliquot sufficient to fill an absorption cell (3 mL) was withdrawn with a syringe. If the extract was cloudy, it was filtered before the cell was filled. The absorption at 3.3 microns was measured by a portable infrared spectrophotometer. The spectrophotometer was calibrated with dehydrated samples of the actual oil present in the samples whenever possible. Results Drilling-Rig Deck Drainage The separation of oil from drilling-rig deck drainage is an imposing problem because of the variety of oils that might enter the system, the presence of mud solids and cuttings fines, and the possible entry of surface-active agents into the system. Fortunately, the average volume of drainage to be treated is small (about 1,000 BWPD), and the input of oil is small but variable. The input of oil can be held to very low volumes by good housekeeping and the use of strategically placed drip pans. Water to be treated by the deck-drainage system is usually fresh (rain and wash water), but it can be salty from sea spray and from the fire system. JPT P. 234^