I have found that when i convert my model to buried pipe model all pipe density for buried pipe are set to zero. Is that configuration for buried pipe model or something?

TCK

Read this thread , here in this forum.

But when running the buried model (without any density), under sustained case, vertical displacement at buried nodes are zero as shown in the desplacement report, even for a very soft soil model. This seems not realistic. Is it necessary to reinput pipe density and fluid density back in again? Please suggest.

As stated in the referenced thread, the assumption is that the soil provides continuous support for the pipe. There is no deflection or weight stress.

Even in a soft soil, the meshing algorithm will properly consider the soil properties in determining the restraint stiffnesses. With the resulting generated model, the densities are set to zero. This assumes that the weight of the pipe has deformed the soil during construction, and there will be no displacements in the Sustained Load Case.

If you disagree, or want to see the effect of the pipe weight, then yes, you can redefine the appropriate densities. Typically, this is only necessary on the first pipe element.