Wax Deposition Prediction and Control in Waxy Crude Oil Tieback Flowlines for South China Sea Deepwater Oil Development

Crude oils are generally very complex chemical systems consisting predominantly of hundreds to thousands of hydrocarbon compounds from simple low-molecular-weight paraffin to high-molecular-weight waxes. There are many factors effecting wax deposition such as the fluid composition, pressure and temperature and so on. Wax deposition is complex dynamic process related to transient heat and mass transfer, therefore it is very difficult to predict wax deposition velocity accumulation amounts on the wall in waxy crude oil tieback flowlines. There exist many waxy crude oil tieback flowlines for South China Sea deepwater oil development, which can cause many flow assurance problems. It is very important to carry out wax deposition prediction calculation and effective measures are adopted to solve flow assurance problems. In this study, based on certain waxy crude oil tieback flowlines for South China Sea deepwater oil development, OLGA software is used to simulate transient flow characteristics in waxy crude oil tieback flowlines such as wax precipitation rate, thickness of wax layer deposited at wall, pigging frequency and so on. OLGA software simulation results is coincident with field pigging operation results, finally obtaining a good simulation methods to predict transient flow characteristics in waxy crude oil tieback flowlines and giving good suggestions to determine pigging periods and frequency. The study results show two 10 inch non-insulation steel flowlines are recommended to develop deepwater waxy crude oil field and pigging periods should comprehensive consider thickness of wax layer deposited at wall and the increase of pressure caused by wax deposition. It is reasonable to 35 days pigging periods during 2 year after put into production, 60 days pigging periods after 2 years. The results show maximum liquid flow rate for a 10 inch flowline is 7000m3/d, and it is reasonable that one flowline is used to transport when flow rate is lower than 7000m3/d, and two flowlines are used to transport when flow rate is more than 7000m3/d. The pump pressure at FPSO is kept at 6000kPaA, and required maximum liquid flow rate of pigging at FPSO is 4200m3/d (175m3/h).