Development of a Pipe Heat Transfer Correlation for use in Sea Bed Proximity

Accurate prediction of Overall Heat Transfer Coefficient (U Value) is of vital importance to predict potential for flow assurance problems in subsea pipelines and design programs to mitigate such problems. In gas dominant wells it is important to predict the onset of condensation of water. In oil dominant wells producing waxy crudes the onset of wax deposition is of interest. An error in U value prediction would lead to incorrect prediction of these phenomena and hence incorrect design of flow assurance strategies. Several correlations based on shape functions have been developed to predict U Value for different burial depths. Accuracy of these correlations depends upon accurate input for convection heat transfer coefficient by sea water, hext.

External Heat transfer coefficient over pipe depends on the flow properties and velocity distribution around the pipe. Existing correlations assume symmetrical flow pattern across the pipe. However when the pipe is in close proximity to the sea bed, flow pattern above and beneath the pipe is not identical. Flow pattern on the bottom side of the pipe depends upon the gap between the pipe and the sea bed. During partial pipe burial, contribution to heat transfer from the bottom side of pipe is significantly less compared to top side. Several CFD simulations were performed for different burial depths and Reynolds number. Correlation for hext was modified by including burial depth as new parameter. When the pipe is far away from the sea bed, the correlation predicts hext as that of freely suspended pipe. On the other hand when the pipe is completely buried, the correlation predicts hext close to that of flow over a flat plate. The correlation was validated against data generated from CFD simulations.