(403h) Flow Assurance Issues Associated with Ice Deposition Under Freezing Conditions
AIChE Annual Meeting
Tuesday, October 31, 2017 - 3:15pm to 4:45pm
With the ever increasing global energy needs, oil companies have taken aggressive measures to ensure continuity in oil supplies. This has led to the construction of oil pipelines in harsh environments including extremely low temperatures. Under these temperature conditions, water even in trace amounts may form ice and lead to pipeline blockages and associated risks. A pipeline blockage due to an ice plug was reported to delay the restart of the Poplar pipeline system gathering crude oil from Montana and North Dakota. Ice formation in oil pipeline has been considered a high risk threat by Trans-Alyeska Pipeline System leading the company to invest millions of dollars into ice formation research. The declining throughput makes the oil cool to lower temperatures. In oil temperature falls below the freezing point, ice forms which may lead to valve failure, flow restriction due to ice accumulation, and even plugging of pipelines.
This paper presents low water cut (<2 %) ice formation studies and its effects on the oil transportation system. A 2-inch diameter carbon steel flow loop was instrumented to measure droplet size, pressure, temperature, and differential pressure. The water cut and wax content changes with time were monitored. Three test fluids were used, including kerosene (40o API, 1.6cp at 70 oF), a light crude oil (40o API, 2% wax content, 11cp at 70 oF), and a blend of the two oils. The effect of pipeline geometry was studied, including low spots, straight pipe section with an inserted rod, and straight pipe section with a perforated plate. Particle interactions, crystallization, and ice melting process were observed using Particle Video Measurement (PVM).
Results indicated that, like wax, ice deposition requires a negative temperature gradient. Visual observations from PVM showed that ice deposition resulted from either freezing water in low spots or from ice particles migrating from the bulk to the wall. Flowline plugging resulted from excessive accumulation of ice particles or the wall and/or from severe aggregation of ice particles that eventually get stuck in flow restrictions such as perforated plates or inserted metal rod. The morphology of the deposited ice strongly depends on oil composition; the oil with 2% wax resulted in glazed ice whereas the kerosene, no wax, resulted in rime ice. The morphology of ice is very important since it defines the hardness and other properties of ice. Additionally, amount of the deposited ice increased with increasing wax content. This suggests that the porosity of ice was affected by presence of wax, i.e., the interaction of ice-wax affects the amount of the occluded hydrocarbons. Furthermore, the hydrocarbons in deposit contained a higher wax fraction than the test oil. The oil with 2% wax resulted in a delayed onset but quick plugging behavior whereas kerosene resulted in quick onset but gradual plugging behavior. Other test conditions and fluid properties affecting ice deposition were also studied such as flow geometry, watercut and flow rate. Lastly, CFD simulations were performed for the different geometries and results showed qualitatively agreement with experimental results.