If the system rests in a properly prepared and properly filled-in trench what loads are you expecting on the weld? The weight is supported by the trench bottom and the system should see little distortion (and load) from weight. In CAESAR II, buried pipe is analyzed to evaluate the effects of (thermal) strain. Even though the temperature change may be low, buried pipe runs can be quite long and you can accumulate a lot of bending at your changes in direction. It's (thermal) strain that loads your weld.
No, do not re-define your densities.The soil stiffnesses that the buried pipe processor adds to your model are not pre-loaded to carry deadweight so your added loads will deflect the entire system downward based on x=F/k.
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Dave Diehl