Damaged welded pipes for oil and gas rigs exposed to internal pressure - failure estimation Marko Rakin  1 , Miodrag Arsić 2 , Živče Šarkoćević 3 , Bojan Medjo 1 , Nataša Tomović 1 and Aleksandar Sedmak 4 1 University of Belgrade, Faculty of Technology and Metallurgy, Karnegijeva 4, 11120 Belgrade, Serbia 2 Institute for Testing of Materials (IMS), Bulevar vojvode Mišića 43, 11000 Belgrade, Serbia 3 High Technical School of Professional Studies, Nušićeva 6, 38227 Zvečan, Serbia 4 University of Belgrade, Faculty of Mechanical Engineering, Kraljice Marije 16, 11120 Belgrade, Serbia  presenting author’s email: marko@tmf.bg.ac.rs Keywords: Casing pipe, Surface defect, Maximum pressure, Finite element analysis Abstract. The paper deals with the integrity assessment of API J55 steel casing pipes for drilling rigs, manufactured by high frequency contact welding procedure. The influence of corrosion defects on the pipeline load carrying capacity is determined through pressure test of a pipe with corrosion damages simulated by machining the circular holes. Finite element analysis of the damaged pipe subjected to internal pressure is used for determining the stress/strain conditions in the damaged area of the pipe. Also, numerical model was used for establishing the load carrying capacity of the pipe with different damage levels (i.e. defect depth and length). Several expressions from the literature are used for estimation of the maximum pressure in the damaged section of the pipe, and the solutions are compared with the predictions of finite element models and experimental results. Introduction During the exploitation, pipes in the oil and gas drilling rigs are typically subjected to a corrosive atmosphere, high pressures and elevated temperatures. Having in mind that reliability of drilling rig systems is crucial for efficient production and that their failure can cause environmental hazard, a large amount of research activities has been conducted on the pipes with corroded areas [1-8]. One of the solutions for assessment of the corrosion defect influence to the pipe integrity is ASME B31G code [1]. Since it is often regarded as too conservative, other procedures have been derived in order to improve its predictions, e.g. a modification by Kiefner and Vieth [2,3] (often denoted as modified ASME B31G). Det Norske Veritas (DNV) developed procedure [4] for estimating the load carrying capacity of corroded pipelines which takes into account the internal pressure and axial compressive loading. European FITNET project [5] also dealt with such problems, and the pipes with corrosion defects are included in the resulting integrity assessment procedures. Adib-Ramezani et al. [6] derived an approach for determining the remaining strength of the corrosion damaged pipes through modification of SINTAP procedure [9] (typically used for analysis of cracked geometries, including cracked pipes [10]). In this work, criteria for estimation of maximum pressure for casing pipes used in the oil and gas drilling rigs are discussed. The pipes were manufactured by high frequency contact welding (HF) of API J55 steel, and the influence of local corrosion is examined on a pipe with different damage