Design Concepts for Offshore Produced-Water Treating and Disposal Systems

Abstract

Summary The general theories governing, produced-water treating equipment design are presented and qualitative conclusions drawn from the resultant equations. The design and use of various common components used in offshore produced-water treating systems are discussed. Introduction The engineer charged with designing an offshore produced-water treating and disposal system is faced with an almost insurmountable problem. While the endpoint is fixed for the design, usually little is known about the quality of the water that must be treated. The basic data set forming the basis of the current U.S. discharge guidelines is contained in a 1974 study conducted for the Offshore Operators Committee (OOC). Each type of final treating device was analyzed separately with specific conclusions drawn for pits, tanks, flotation systems, plate coalescers, fibrous media coalescers, and loose media coalescers. The report concluded: "Flotation units and loose media coalescers were found to produce the lowest mean effluent oil concentrations of the processes studied. Flotation is a widely used process and is applicable almost universally. Loose media coalescers were not selected to represent exemplary performance because of operating problems at many locations." The validity of this conclusion concerning loose media coalescers subsequently was verified by an in-depth study conducted by the OOC. The U.S. EPA analyzed these data, plus additional data furnished from industry and from EPA-conducted field verification studies, and arrived at their interim guidelines of 48 mg/L. The derivation of discharge quality based on exemplary performance of flotation systems does not imply that the installation of a flotation system will necessarily meet this criterion. In determining "exemplary" flotation systems the EPA eliminated many data. For example, in the original OOC study, about 25 % of the flotation systems analyzed had mean effluent-oil concentrations in excess of 48 mg/L. Thus, any water treating system that includes a flotation unit must be designed carefully for the conditions expected at that particular facility. Similarly, it is not necessary to install a flotation unit in each water treating system. The EPA-specified effluent limitations can be achieved with less sophisticated equipment under certain circumstances. W.L. Berry states: " Not all treatment processes are applicable at all locations. Performance of water treatment systems is not only a function of retention time, design parameters, use of chemical coagulants…but is also influenced by a number of factors most of which the operator has little or no control over including: water characteristics, crude type and solubility, degree of entrained oil emulsification, sizes of the entrained oil droplets, and suspended solids concentrations and characteristics. [Because of these factors,] if identical treatment systems are installed at two different locations, the average produced-water effluent oil content may be quite different." JPT P. 276^