The welded joints are often dimensioned over one given lifetime while following an approach of the type S-N on the basis of acceptable probability of failure which should not be exceeded over the base period. The cracks induced by the welding process can be propagated and become critical during the useful lifetime of the structure, unless they are detected and repaired in time. Consequently, prevention of fractures becomes a necessity and is carried out by inspection and analysis of the harmfulness of defects. In this paper, we propose a mechanical model of steel tube HLE X70 at the weld region using the Paris law and where some parameter effects are discussed. To determine the optimum inspection cycle, it is necessary to know the lifetime of the welded joints. The reliability analysis is performed through specific algorithms to elucidate the most likely failure configuration. The economic inspection cycle is determined using the reliability index and the total cost of the structure.