Overall heat transfer coefficient (OHTC) of the buried pipe.
In offshore developments pipeline burial may be required for protection and on-bottom stability by means of placement of rock, grit or seabed material on the pipe. Acc. to a publication from Bai2005, burial can result from the gradual infill of sediments on a trenched pipeline; it can also develop from seabed mobility or pipeline movement.
When a pipe is fully buried a term, $h_{soil}$, is added to the definition of the overall heat transfer coefficient to account for the heat transfer by conduction through the asymmetric soil layer surrounding the pipe. The outside film coefficient $h_{ext}$ is replaced with a pseudo film coefficient, $h_{amb}$, modeling the heat transfer by convection at the sea/soil interface.
Case when $D_{ext}/2$ < $H$.
| $\frac{1}{\frac{D_{ref}}{D_{in} h_{in}}+\frac{D_{ref}}{D_{wall} U_{wall}}+\frac{1}{h_{buried}}}$ | Carslaw & Jaeger |
| $\frac{1}{\frac{1}{h_{buried}}+\frac{1}{h_{amb}}}$ | Morud & Simonsen / OTC 23033 |
| $\frac{1}{\frac{D_{ref}}{D_{in} h_{in}}+\frac{D_{ref}}{D_{wall} U_{wall}}+\frac{1}{h_{buried}}+\frac{1}{h_{amb}}}$ | Ovuworie |
| $\frac{1}{\frac{D_{ref}}{D_{in} h_{in}}+\frac{D_{ref}}{D_{wall} U_{wall}}+\frac{D_{ref}}{D_{ext} h_{soil}}+\frac{1}{h_{amb}}}$ | fully buried |
