Case Study: Modifying a Desander Design To Handle Low Flow Rates

The Challenge A major North Sea operator had faced issues with sand filling up the production separator on a late-life well on the Norwegian Continental Shelf. The well was previously shut in due to sand production. A jetting operation was required, with solids to be separated on the water line upstream of the reinjection pumps in order to protect the pumps from erosion damage. The expected solids sizes were fines of 20–150 microns and sand in the 150–800-micron range. Flow rate was estimated at a rate of 240 m3/day. The service provider, Expro, was the main contractor and requested assistance from FourPhase to supply the desanding solution. The Solution When presented with the operational data, the project team immediately understood that the low flow rate would be one of our main challenges. The given rate was in the lower end of the capacity window for our DualFlow cyclone designs. Low inflow pressure causes lower differential pressure in the cyclones, resulting in reduced separation efficiency that we were keen to avoid. Throughout more than 10 years of operation, we have built a solids management database, recording all parameters and separation rates during more than 150,000 operational hours. Where we previously relied on extensive computer simulations with computational fluid dynamics modeling, our database now allows us to calculate the sweet spots of solids separation from existing well parameters without any further expensive and time-consuming modeling. This benefits our customers, as our data increases our ability to quickly determine if a given scenario will work or not. In the described case, our database would serve us well in understanding how we could solve the customer's challenge. Plugging the flow data into a calculation tool coupled to the solids database, we calculated which range of differential pressures we could expect in the cyclones. Another calculation tool projected the separation efficiency of solids as a function of pressure conditions. We quickly found that our regular cyclone design with 100-mm-diameter liners were too large, creating a pressure drop that was not beneficial for separation efficiency. Experience, along with calculations based on database figures drawn from previous operations with low flow rates, pointed toward a smaller cyclone liner insert. We had previously manufactured reduced-size liner inserts to meet separation requirements on operations with lower flow rates. Now, we started to consider even smaller-sized inserts. Our calculations showed that a 50-mm-diameter liner insert would provide the best results under the given circumstances (Fig. 1). Seeking internal quality assurance, the project team aligned with our engineering and operations departments, verifying the calculations. The engineering department then quickly got to work creating 3D-printed prototypes for testing. FourPhase has developed and installed a complete test facility at our headquarters in Bergen, Norway (Fig. 2). The test loop gives us an opportunity to test a variety of equipment in a controlled and safe environment.