Coking in the stripper bottoms to the debutanzer is very unlikely the cause of the line pluggage. It is more likely that the pluggage is due to Iron Sulfide scale that is entrained in sour water trapped in the stripper. At typical stripper operating pressure and temperatures any sour water carried into the stripper from the HP separator upstream will be trapped in the stripper. Here are my recommendations:
First, check the sour water level in the HP separator and make sure it’s in the boot and not above the draw nozzle to the stripper. Ensure that the HP separator sour water boot is functioning and that sour water is flowing out of the boot.
Second, if line pluggage becomes a chronic issue consider installing a sour water settler in the stripper reboiler circuit to remove any trapped water.