Experimental and numerical study of methods to displace oil and water in complex pipe geometries for subsea engineering

The purpose of the thesis is to study fluid displacement operations in complex pipe geometries utilized in offshore petroleum industry. Typically, MEG or Methanol is circulated through specific sections of the subsea production systems to lower the hydrocarbon content. This is often done at the beginning of production after a prolonged production shut-in, to avoid formation of hydrates or to minimize the emissions of chemicals to the environment. Experimental and numerical analyses have been conducted modifying a previously built pipe system formed a U-shaped jumper, adding a fluid recirculation line, a jet pump, a centrifugal pump, some new valves and sensors. During the experiments the volume fraction in the U-shaped jumper of the displacing fluid was estimated versus time by measuring the level of the oil-water interface in each pipe segment. The system was filled and displaced with both distilled water with 3% water content of salt and Exxsol D60. Numerical simulations were performed using the one-dimensional transient multi-phase flow simulator LedaFlow. It has been investigated the necessary displacing time required to achieve target hydrocarbon concentration in the domain, optimal displacement rate for efficiently removal of hydrocarbons, and how these variables depend on two different fluids (oil and water) and their properties. The displacement has been also modelled including or removing the recirculation line. After carrying out the simulations and performing the experiments, the results were compared, also against a new simplified mathematical model based on uniform mixing in a tank with the same volume as the pipe geometry. The results show that there is a fair agreement between the experimental results, and the results of the simplified model and the LedaFlow simulations. When including the recirculation line it took longer time to reach the target volume fraction, but the displacing rate can be lower than when the recirculation line is not present.