Hydrocyclones: A Solution to Produced-Water Treatment

Abstract

Summary Progressing into deeper water and more hostile environments in the search for new oil and gas reserves has placed an increasing demand on the industry to develop lighter, more compact, and more efficient process equipment to replace their traditional counterparts. A recent application of cyclone technology for liquid/liquid separation of oil from produced water has shown considerable promise during extensive field testing. This paper outlines the basic construction and principle of operation of the deoiling hydrocyclone and discusses system design, early operational experiences, and test results from the first full-scale commercial application of the four-in-one hydrocyclone concept on the Murchison platform. In addition, and of perhaps more significance, early results from field tests of the larger 60-mm [2.4-in.] cyclones on the Hutton tension leg platform (TLP), where conventional equipment was adversely affected by platform motion, are discussed. Introduction Produced water represents an increasing portion of the total produced fluids during the life of a field. Although the treatment of produced water offers no direct economic incentive, a considerable portion of both engineering and operations time is spent modifying and maintaining produced-water cleanup systems to ensure satisfactory operation. In the Murchison field, although the existing system performed adequately, albeit through labor-intensive operation, optimum reservoir development required water-injection capacity beyond the original design basis. Consequently, this created the need to upgrade the capacity of the produced-water treatment system. A similar upgrade in water-handling capacity was required for the Hutton field, but more important, the platform could not achieve the U.K. Dept. of Energy specification for overboard effluent of 40 ppm with the originally installed equipment. Investigations into alternatives to the traditional flotation cells/plate separators for oily-water cleanup revealed that the hydrocyclone separator warranted serious consideration. Our first experience with hydrocyclones was with pilot plant trials of the Serck-Baker Oilspin developed as a result of an extensive research program conducted at Southampton U. While these tests, carried out on the Murchison and Hutton platforms in early 1985, were not an unqualified success, they demonstrated the equipment's potential and enough encouragement for us to commit to the first full-scale commercial application of the Vortoil trademark four-in-one 35-mm [1.4-in.] hydrocyclone later that year. The Vortoil hydrocyclone, developed by an Australian company but based on the original Southampton U. design, was considered to be a more refined and advanced product capable not only of achieving the required performance but also of reducing the nursemaid attention associated with more traditional equipment. The many operational advantages of Vortoil make it ideally suitable for a retrofit it situation. However, the true worth of hydrocyclone technology should be realized on future marginal developments where space and weight are a premium and on floating structures like the Hutton TLP where motion sensitivity is a problem.

Operational Control of Hydrocyclones During Variable Produced Water Flow Rates—Frøy Case Study

“…In a de-oiling hydrocyclone, the lighter oil phase migrates toward the center of the vortex, and the heavier water phase is forced toward the cyclone wall. By means of the characteristic twofold vortex structure of hydrocyclones, the purified water flow exits the hydrocyclone through the underflow, while the oil-enriched center core exits the hydrocyclone through the overflow, as described by several authors Meldrum 1988;Ditria and Hoyack 1994).…”

Application of H∞ Robust Control on a Scaled Offshore Oil and Gas De-Oiling Facility

“…The PDR controller operates on the principle that the PDR is approximately linearly proportional to the flow split ratio (R f ), defined as (R f = F u /F i ), which is the ratio of the hydrocyclone's inlet volumetric flow rate (F i ) to the underflow volumetric flow-rate (F u ) [15,19,25,36,37]. If it is assumed that the flow inside of the hydrocyclone is sufficient to create an strong centripetal force, thus forcing all of the oil into an oil core, then in theory controlling the flow through the overflow will control the flow of oil through the hydrocyclone.…”

“…) [15,25,34,36], where C i is the concentration of oil in the inlet to the hydrocyclone and C u is the concentration of oil in the underflow of the hydrocyclone. Due to the common use of the PDR as the controlled variable, and the influence of the underflow and overflow valves on the hydrocyclone's dynamics, they were chosen as the controlled and manipulative variables in our considered model.…”

“…In the opposite case when the overflow valve is fully open, excess water will be sent through the overflow, which, again, necessitates recirculation in order to purify the overflow. It was shown in [14,25] that a hydrocyclone operates at a high efficiency as long as the pressure drop ratio (PDR) and the inlet flow rate to the hydrocyclone are kept within certain boundaries-1.5-3 with respect to the PDR [26]-and the boundary of the inlet flow rate to the hydrocyclone is specific to each individual hydrocyclone set-up [25]. Furthermore, it was shown that the PDR has an insignificant effect on the separation efficiency as long as it is kept above 1.7 PDR [27], and that the inlet flow rate to the hydrocyclone must be kept above a system specific threshold in order to ensure high separation efficiency [27], which in some cases has been reported to be as high as 99% [25].…”

“…It was shown in [14,25] that a hydrocyclone operates at a high efficiency as long as the pressure drop ratio (PDR) and the inlet flow rate to the hydrocyclone are kept within certain boundaries-1.5-3 with respect to the PDR [26]-and the boundary of the inlet flow rate to the hydrocyclone is specific to each individual hydrocyclone set-up [25]. Furthermore, it was shown that the PDR has an insignificant effect on the separation efficiency as long as it is kept above 1.7 PDR [27], and that the inlet flow rate to the hydrocyclone must be kept above a system specific threshold in order to ensure high separation efficiency [27], which in some cases has been reported to be as high as 99% [25]. It is also noteworthy that the inlet flow rate to the hydrocyclone has no influence on the PDR, as was shown in [27], and in most cases only the PDR is controlled, and the inlet flow rate, neither to the gravity separator nor to the hydrocyclone, is controlled.…”