To repair coil pipes of furnace used for pyrolysis of hydrocarbon, we sometime should cut off the bad sections and insert special repairing shells. Consequently, it is necessary to join together new steel pipes and the pipes whose walls became thinner due to operating conditions. As a result, additional stress concentrators may appear. We study stress-strain state of compound tubes at the zones of junction (weld seams). We employ a finite element method to estimate strength and bearing ability of the structure. We study weld seams using various models for geometrical inhomogeneity under various boundary conditions.