Generally we tend to avoid using beam-modeling software like CAESAR for D/t ratios greater than 100, as the failure mechanisms often consist of thin wall buckling.
If your cooling water line is feeding equipment, you run the risk of feeding mud to them due to failures CAESAR isn't design to catch.
With that said, are you sure you should be using the temperature you're using, whatever it is, for the line underground? You may wish to push back for verification on that number. It may be likely that the temperature of the line should be really close to that of the ground anyways, rendering expansion stresses trivial.