Experimental and numerical heat-transfer analysis was conducted on a T-shaped acrylic-glass pipe, representing a production header in a subsea production system with a vertical deadleg. The header was insulated, while the deadleg was not insulated and carried a cold spot on the top. The experimental conditions were set to mimic those of steady-state production, followed by a 3-hr shutdown (cooldown). The internal fluid temperature and the wall temperature were measured by use of resistance temperature detectors and thermocouples, respectively, while particle image velocimetry was used to measure the velocities in the deadleg. It was shown that the mean velocity field during both steady state and cooldown was periodic, with a clockwise and counterclockwise rotation along the deadleg wall. By use of a k–ω shear-stress transport Reynolds-averaged Navier-Stokes (RANS) model in ANSYS CFX, the thermal field was correctly predicted for 3 hr of cooldown by modeling the cold spot as an isothermal wall. The RANS model was unable to recreate the periodic velocity field observed in the experiment.