Subsea Chemical Storage and Injection Station - Single line batch re-supply of chemicals - An experimental investigation into displacement mechanisms and separation methods
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This thesis seeks to simplify the re-supply process of a subsea chemical storage and injection (SCS\&I) station by identifying possible alternative transport solutions. Specifically, this thesis explores the technical solution of sending several liquids in succession through a single line without mixing occurring between the different liquids. The thesis explores using a polymer gel as a spacer liquid between the chemicals. To decide which polymers might be a good fit, several experts were contacted. The possible spacer fluid selected for testing was Xanthan gum (XG). This is a non-toxic polymer already in use in the oil and gas industry. Three experiments were initially designed. Experiment 1 explores how the spacer liquid's viscosity evolves over time. When used in the transport system, the spacer fluid might spend some time in the pipe. If its properties were to change, this would have to be accounted for. Apart from one sample, the XG gel seemed stable both at room temperature storage and cold storage. However, mold developed on the samples with the lowest XG concentration after a month. The reason could be exposure to air or contamination during testing. So, another experiment, experiment 3, was designed to counter this effect. Experiment 2 investigates how the XG gel reacts with the production chemicals that will be transported with this system in a static environment. This is to ensure compatibility of the chemicals during transport. Testing against glycol showed some initial dissolving of the XG gel, but the mix stabilized after a few days and seemed to not mix any further. Testing against other chemicals was not achieved during the work on this thesis. Experiment 3 is a continuation of experiment 1, but it seeks to eliminate the effect of exposing the sample to air. When in use in the system, the XG gel will not be exposed to air, and if this exposure is the cause of the mold, this problem is not relevant for the final system. As a further improvement on experiment 1, temperature measurements are conducted simultaneously as viscosity measurements. In experiment 1 the temperature is only measured once before the viscosity measurements. The results of experiment 3 were much the same as experiment 1, the sample seemed stable over time, and a significant correlation was found between viscosity and temperature. Experiment 4 investigates how the spacer liquid reacts with production chemicals in a dynamic setting, as well as the ability of the spacer liquid to keep the production chemicals separate during transport. A rig was designed and built in the hall laboratory. It consists of a tube shaped as a U with pressure applied on each end. The spacer liquid is put into the tube and placed in the middle of the tube, then the injection chemicals are placed on each side of the spacer liquid. Pressure is applied alternately at the ends of the tube, resulting in a back and forth fluid flow. Experiment 4 showed that it was hard to keep the spacer liquid from mixing with the other chemicals in a dynamic environment. Higher concentration gels were tried to see if the higher viscosity would help, and a longer tube was tried in order to see if a less frequent change of direction would result in less mixing. Keeping the chemicals separate using a Xanthan gum gel was not successfully achieved in this thesis. However, the matter is not closed, and further work is needed before a final conclusion is reached. Future work should further investigate Xanthan gum gel as a spacer liquid. New experiments should be designed where the liquids are propelled by a pump rather than pressurized air to make the test conditions more realistic. Other candidates for spacer liquids should also be investigated, especially cross-linked polymers.