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Vacuum insulated tubing (VIT) is a specialized tubular designed to minimize heat loss from production or injection fluids to the environment in oil, gas and geothermal wells. VIT strings are used in deepwater wells for flow assurance or to mitigate annular pressure buildup. VIT use requires accurate knowledge of its insulating performance. Although VIT performance can be estimated from analytical tools, such as finite element analysis (FEA), an experimental approach provides a more direct measurement and can be used to validate analytical tools. We have developed a new experimental method to address this need. In this method, one or two VIT joints are placed in an ice-water bath. A precisely measured flow of heated air flows inside the VIT. The temperature change of the flowing air is measured between the inlet and outlet of the VIT test specimen. The insulating performance of the VIT is then calculated from this temperature difference using heat exchanger theory with effectiveness-number of transfer units (&#ξ03B5;-NTU) approach. A proportional-integral-derivative (PID) controller is used to control the air temperature at the VIT inlet by regulating power to the heater. This paper illustrates the data reduction method and uncertainty analysis using sample test data. The method allows for rapid measurement of VIT performance at many different temperatures, with the air flow rate being used to optimize the test sensitivity and to reduce experimental uncertainty. As currently designed, the apparatus is able to test single- and double-joint VITs with effective body conductivities between 0.002-0.1 W/m/°C (0.001-0.06 Btu/hr/ft/°F) and temperatures up to 400°C (750°F); however, the design allows the apparatus to be modified easily for higher or lower conductivities. Although designed for VIT, this method may be applied to other types of tubulars. Currently, there is no widely accepted standard method for experimental testing of VIT performance, and it is hoped that this new method may evolve to an industry standard. |
