Cooldown of Subsea Deadleg With a Cold Spot: Experimental and Numerical Heat-Transfer Analysis

Abstract

and showed that large eddy simulation (LES) was better suited than k–ε based models for predicting the temperature profile. However, the experimental temperature data were obtained solely for the pipe wall, and the velocity field and the thermal field inside the pipe were not investigated. In a study by Jensen and Grafsrønningen (2014), a 3-hour-long cooldown experiment was conducted on a water-filled T-shaped acrylic-glass pipe, representing a production header with a vertical deadleg. The header was insulated, while the deadleg was kept uninsulated. The T-shaped pipe dimensions were representative of a subsea production pipe, but unlike a subsea pipeline, the experiment was set up with air at room temperature as the surrounding medium. Temperatures in the T-shaped pipe were measured internally with RTDs and externally with pipe-wall-mounted thermocouples, while velocity data were obtained in the deadleg by use of PIV. These measurements were used as benchmark data to establish a suitable numerical model. The study scrutinized the accuracy of standard RANS turbulence models in predicting the flow kinematics inside the vertical deadleg when the flow was both turbulent along the pipe wall and laminar closer to the center of the pipe at the same time. Mesh independent results were obtained by running a series of mesh convergence tests. It was shown that cooldown simulations were more sensitive to mesh design than the choice of turbulence model. Mean velocities in the deadleg compared well with experimental PIV data during the first 60 minutes, but the RANS model was not able to predict the laminar-flow kinematics that occurred after this time. The thermal field was correctly predicted with a RANS model for 3 hours of cooldown, even though the flow was laminar in the entire deadleg after 60 minutes. The heat loss in the experiment was limited by the heat-transfer rate to the surrounding air, and not by the internal natural convection. Thus, the accuracy of the RANS model for predicting the internal flow kinematics was not essential for calculating the cooldown times. Rayleigh-Benard convection in enclosures, where a fluid is heated from the bottom and cooled from above, has been the topic of many research papers. Recent experimental, numerical, and theoretical advances in Rayleigh-Benard convection were presented in Chillà and Schumacher (2012). The paper scrutinized experimental and numerical data from a series of publications on RayleighBenard convection in cylindrical enclosures. The underlying studies differed in terms of the temperature gradient between the top and bottom plate, the fluid inside the enclosure, and the aspect ratio Γ = L/H of the enclosure, where L is the characteristic length and H is the height of the enclosure. For 107 ≤ Ra ≤ 1012, the authors showed how large-scale convection (LSC) inside the enclosure influences the overall heat loss in the system. LSC refers to the tendency for thermal plumes of the same type to cluster together and form a large-scale flow. The number of convection cells in the large-scale flow depends on the aspect ratio of the enclosure. By comparing various experimental and numerical studies with the same Ra but with different aspect ratios, it was shown how the structure of the LSC has an influence on the overall heat loss in the system, and that Copyright © 2016 Society of Petroleum Engineers