Research and Application of CDFU Cyclonic Dissolved Air Flotation Technology in Offshore Platform Produced Fluid Treatment
Abstract: This paper explores in depth the application and research of cyclonic dissolved air flotation technology in the advanced treatment of produced fluid on offshore oil platforms. With the spread of tertiary oil recovery and the ever-deeper development of offshore production, offshore oilfields - constrained by large produced-fluid volumes and limited space - face severe challenges with traditional treatment processes: declining oil-water separation efficiency, deteriorating reinjection water quality, rising chemical dosing and injection difficulties, which can no longer meet complex production needs. Cyclonic dissolved air flotation (CDFU) with its excellent oil-water separation efficiency and compact structure provides an innovative solution to these bottlenecks, showing highly promising application prospects in offshore platform produced fluid treatment.
Keywords: offshore platform; produced fluid treatment; cyclonic dissolved air flotation (CDFU)
1. Current status of produced fluid treatment on offshore platforms
Offshore oilfields, as strategic pillars of national energy supply, have surpassed the 50-million-ton mark, contributing nearly 60% of domestic crude production growth. Heavy oil accounts for more than half of offshore crude output, so sustained, efficient heavy oil production has become key to energy security. Against this backdrop, enhanced oil recovery technologies such as steam huff-and-puff thermal recovery and chemical flooding, well suited to offshore heavy oil reservoirs, have been widely applied.
However, as production continues, problems with the extraction technologies have become prominent. In polymer-flooded oilfields, long-term polymer injection has significantly changed the physicochemical properties of produced fluid, causing a series of treatment bottlenecks:
Intensified emulsification, heavy separation load: Polymers make oil and water form an abnormally stable emulsion, sharply increasing separation difficulty; the oil content of separated water rises steeply and the water content of treated crude can hardly meet targets stably.
Reduced chemical effectiveness, rising operating costs: Traditional demulsifiers decline markedly in effectiveness on polymer-flooded produced fluid. To meet treatment requirements, sites have to greatly increase chemical dosing, directly driving up operating costs.
Equipment challenges, heavier maintenance burden: Existing treatment units (such as heat exchangers) tend to coke and clog when handling polymer-flooded produced fluid, requiring frequent shutdowns for cleaning - increasing maintenance workload and affecting the continuous, stable operation of the whole treatment system.
Difficult byproduct disposal, affecting downstream processes: The large amounts of sticky oily sludge generated bring trouble to site operations and continuously affect the normal operation of downstream treatment facilities.
In summary, as offshore oilfield development advances, produced fluid composition grows ever more complex; under the combined effects above, traditional processes can no longer meet current demands for efficient, intensive production - process upgrading and technical innovation are urgent.
2. Current status of produced fluid on an offshore platform in the Bohai Sea
An offshore production platform in the eastern Bohai Sea is rich in oil and gas resources; exploration began in 1996, the first well was drilled in 2000, and system production assessment was completed in 2005. The field's proven petroleum in place is 5222.07x10^4 m3, of which heavy viscous oil is 4187.87x10^4 m3 - 85% - making heavy oil development highly valuable. The produced fluid handled by the platform's jet pump power-fluid separator ranges from 35-60 C with oil content up to 20%; the medium contains large amounts of foam and is severely emulsified, with poor overall water quality. Current produced fluid separation is unsatisfactory, affecting the stable operation of downstream processes and constraining the efficiency and compliance of the platform's overall production system.
3. New produced fluid treatment process
To address these difficulties, the project team conducted a systematic investigation and comprehensive comparison of mainstream treatment processes on the market. After fully evaluating applicability, maturity and performance, they ultimately selected CDFU (high-efficiency cyclonic flotation unit) treatment technology. The technology has accumulated reliable operating data in industrial applications, successfully deployed in multiple projects including PetroChina Dushanzi Petrochemical electro-desalting wastewater treatment, Tuha oilfield oily wastewater treatment, and CNOOC Bohai offshore platform oily water treatment.
CDFU, whose full name is Cyclonic Dissolved gas Flotation Unit - a high-efficiency cyclonic flotation device. Its technical core is the innovative integration of two high-efficiency units - cyclonic centrifugal separation and dissolved air flotation - into one synergistic system. Through the combined action of the cyclonic field and dissolved micro-nano bubbles, the capture and removal of oil droplets and suspended solids are significantly enhanced, greatly improving flotation efficiency and shortening hydraulic residence time, achieving efficient, rapid purification of oily wastewater - especially suitable for all kinds of difficult-to-treat industrial oily (and suspended-solids-laden) wastewater.
The process schematic is shown in Figure 1.
Figure 1 Process schematic
3.1 Produced fluid treatment process principles
Flotation is a mature physical-chemical separation process. Its core mechanism is to inject a large number of highly dispersed micro-nano bubbles into the water, which act as carriers to effectively adsorb suspended oil droplets or flocculated matter, forming 'bubble-particle' complexes. Being less dense than water, these complexes rise rapidly to the surface under buoyancy, gathering into a scum layer that is easy to remove mechanically, thereby achieving efficient separation of oil from water and solids from liquid, and the engineering goal of water purification.
Flotation introduces highly dispersed micro-nano bubbles into the water as mediation carriers; they adhere to suspended oil droplets, particles or flocs, forming composite floats less dense than water that rise quickly to the surface and enrich as scum, thus achieving efficient separation of the oil-water-solid multiphase system - a physical purification method.
As an efficient physical separation method, flotation is widely used in wastewater treatment processes, mainly including dissolved air flotation, electrolytic flotation, dispersed air flotation and induced air flotation. In recent years, with deepening multi-technology integration, hybrid processes such as coalescence-flotation, flotation-magnetic separation and cyclone-flotation have emerged, significantly improving performance and adaptability. Among them, cyclonic dissolved air flotation (CDFU) integrates advanced German EDUR gas-liquid two-phase pumps with a high-precision gas flow control system, achieving 100% saturated dissolution of gas. The system is highly compact, stable and reliable, with excellent dissolved-air efficiency and energy utilization. The dissolved air performance is shown in Figure 2.
Figure 2 Hydrodynamic model of the dissolved air pump and dissolved air performance
Dissolved water passes through a specially designed pipeline release device that efficiently generates large numbers of micro-nano bubbles. The device uses a large-diameter pipe release structure that both produces uniform, fine ultra-micro bubbles (5~30 μm) and avoids internal clogging, significantly improving release efficiency. Figure 3 shows the release performance of the high-efficiency release device.
Figure 3 Dissolved air microbubble performance
Water carrying micro-nano bubbles (5~30 microns) enters the inlet pipe of the high-efficiency cyclonic dissolved air flotation unit, mixes with the feed, and enters the vessel tangentially from the bottom, forming a weak cyclonic field. This field drives oil droplets and bubbles toward the axis while enhancing their collision and adhesion probability. Flotation then lifts the multiphase mixture of oil, bubbles and suspended solids to the surface, where it is discharged via the pressurized oil removal system; the clarified water exits from the bottom outlet. Figure 4 shows the cyclonic separation structure.
Figure 4 Cyclonic dissolved air flotation separation schematic
To further enhance the synergy of cyclonic separation and flotation, a dedicated cyclone tube inside the vessel precisely controls the centrifugal field strength. Cyclonic separation and flotation form a significant synergistic effect, enabling the CDFU unit to achieve pollutant removal far higher than either single technology within a very short residence time.
3.2 Produced fluid treatment process flow diagram overview
The produced fluid, pre-treated by the three-phase separator, enters two stages of high-efficiency cyclonic dissolved air flotation for advanced treatment. In this process, emulsified oil is captured by ultra-micro bubbles and then continuously aggregates, coalesces and demulsifies under the cyclonic action, forming oil clusters that separate rapidly from the wastewater. The demulsified, coalesced oil collects and flows back to the upstream three-phase separator for secondary treatment; the flotation-treated wastewater goes to downstream facilities for further treatment.
4. Analysis of produced fluid treatment performance
The newly added CDFU (high-efficiency cyclonic flotation unit) significantly improved oil-water separation of the produced fluid, with greatly improved effluent appearance. The comparison of main influent and effluent quality indicators is shown in Table 1 and Figure 2.
Table 1 CDFU operating test data
Sample No. | CDFU inlet | CDFU water outlet | CDFU separation efficiency (%) |
1 | 1201.5 | 116.5 | 90.30% |
2 | 1187.2 | 115.6 | 90.26% |
3 | 1355.4 | 122.5 | 90.96% |
4 | 1295.8 | 114.9 | 91.13% |
5 | 1305.7 | 121.8 | 90.67% |
6 | 1365.5 | 118.6 | 91.31% |
7 | 1351.6 | 107.8 | 92.02% |
8 | 1232.5 | 106.8 | 91.33% |
9 | 1246.3 | 117.2 | 90.60% |
10 | 1220.9 | 106.8 | 91.25% |
11 | 1287.9 | 113.9 | 91.16% |
12 | 1310.3 | 115.8 | 91.16% |
13 | 1301.8 | 121.2 | 90.69% |
14 | 1268.7 | 109.8 | 91.35% |
15 | 1197.5 | 111.8 | 90.66% |
Figure 2 CDFU operating test data chart
5. Conclusion
The newly added CDFU (high-efficiency cyclonic flotation unit) delivers stable produced fluid treatment with remarkable oil-water separation: average oil content of treated fluid dropped from about 1275.2 mg/L at the inlet to about 114.7 mg/L - an average reduction of 1160.5 mg/L - with oil removal efficiency as high as 90.99%, far exceeding the 80% design requirement.
CDFU The commissioning of the (high-efficiency cyclonic flotation unit) successfully solved the platform's poor power-fluid separation problem and effectively reduced the operating load of downstream treatment facilities.CDFU The (high-efficiency cyclonic flotation unit) has broad application prospects in offshore platform produced fluid treatment.
