When Gravity Separation Runs Out of Gravity: Adding CFU and CDFU to an Offshore Platform

Offshore produced water treatment rarely fails suddenly. It degrades, quietly, as the reservoir ages. A platform designed around hydrocyclones and an oil-water separation tank performs well for years, then the oil content in the discharge starts creeping up on the 18 mg/L line and the operators begin looking for margin.

The process as it stands

The water treatment system on this oilfield production platform combines hydrocyclones with an oil-water separation tank. The hydrocyclone performs preliminary oil-water separation by centrifugal action, exploiting the density difference between oil and water. The separation tank in series downstream removes gas and residual oil further by thermochemical settling, so that the oil content of the effluent meets the discharge-to-sea standard, below 18 mg/L.

Both stages depend on the same physical assumption: that oil is enough lighter than water, and the droplets large enough, for the density difference to do the work in the time available. That assumption holds early in field life and weakens steadily afterwards.

Why the assumption expires

As the properties of the produced fluids change in future, in particular as crude API gravity falls and viscosity rises, gravity and centrifugal separation become markedly less effective. Heavier oil rises more slowly. Higher viscosity slows droplet migration and damps coalescence. The residence time that was adequate at commissioning is no longer adequate, but the vessel volume cannot grow. Introducing new water treatment technology and integrating it with the existing process therefore becomes a necessity rather than an improvement.

The CFU route: three mechanisms in one vessel

The CFU process combines flotation, centrifugation and coalescence to separate oil and water efficiently. Gas — usually natural gas or nitrogen — is injected into the inlet pipe, forming large numbers of fine bubbles in the water and creating a three-phase mixture of water, gas and the substances to be removed. Under the combined action of interfacial tension, bubble buoyancy and static pressure difference, the micro-bubbles attach to the surface of fine oil droplets and form aggregates of lower density than water, which rise quickly to the surface. The separated media are then discharged separately through internal guide pipes set at different heights.

Four process features follow from that arrangement, and each one matters on a platform. There are no internal moving parts and the footprint is small, which saves operating space and supports a high degree of automation. Combining flotation with hydrocyclone technology raises the treatment efficiency of oily wastewater compared with either alone. The unit accommodates a fairly wide range of flow fluctuation and operates stably. And under most operating conditions it meets the overboard discharge limit for oily wastewater without chemical agents — no reagent supply chain to the platform, no storage, no dosing maintenance.

What the CDFU adds

The SINOKLE CDFU new technology takes the same idea further. The CDFU cyclonic dissolved gas flotation system integrates three technologies: cyclonic centrifugal separation, ultra-fine bubble generation and dissolved gas flotation. Their combined action delivers fast, efficient separation of free oil, emulsified oil and suspended solids from wastewater.

The key hardware is a high-efficiency dissolved gas pump and a dedicated micro-bubble generation and release device, producing bubbles as fine as 5 to 30 microns. That size range is chosen for a reason: emulsified oil droplets are mostly between 0.5 and 25 microns, so micro-bubbles can capture them more effectively at a comparable scale, and that is what makes purely physical demulsification possible. Combined with cyclonic centrifugation, the technology not only promotes collision between bubbles and oil droplets but, because of the density difference between media, accelerates the migration of bubbles and droplets towards the cyclone core under centrifugal force, strengthening flotation separation.

Process advantages in offshore terms

• High oil removal efficiency, over 90% in a single stage, removing droplets as small as 2 microns.
• Skid-mounted design with a short hydraulic retention time of only 1 to 5 minutes and a footprint of 10% to 20% of a conventional dissolved gas flotation system; small in volume and light in weight.
• Excellent operating stability: there are no internal screens, coalescing plates or other components prone to clogging, and because the unit does not rely on internal packing it resists blockage and keeps treatment performance stable.
• Strong adaptability to changes in hydraulic load, crude oil viscosity and oil content.
• Few moving parts, high reliability, simple installation and low operating and maintenance costs, with automatic control that runs smoothly and safely.

How to integrate with what is already on deck

The temptation is to insert a new unit upstream and let the old train act as polishing. A more productive arrangement usually places the new technology where it exploits the existing pumps and pressure envelope, and demotes the hydrocyclone to a sand and solids guard rather than an oil-removal stage. Three checks decide the layout: whether the existing dissolved gas supply can serve the new unit; whether the weight and centre of gravity change fits the platform's structural margin; and whether the retention time reduction shifts the bottleneck to the separation tank's gas handling.

The requirement that does not change is the 18 mg/L overboard limit, and it is worth specifying against the worst crude slate expected rather than the current blend. A flotation stage chosen for today's API gravity will face the same ageing problem as the hydrocyclone it was meant to protect.