Inspection of the natural gas transmission system Zhidilovo–Skopje and distribution pipeline Skopje South using the in-line method, and inspection of Deve Bair–Zhidilovo (0–4 km) and distribution pipelines Skopje North 1.8 km, Bunardzhik 5.4 km, Kumanovo 7.7 km, Kratovo 4.6 km and Kriva Palanka 1.5 km using NDT methods