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Abstract: Pipelines are considered one of the most efficient ways of transferring oil and gas from the extraction sites to refineries, and finally to consumers. However, leakage or loss of containment of pipelines in the transmitting grid can be caused by several factors such as overpressure, cracking, and corrosion defects. Goals are set to ensure the pipelines safety while managing to minimize the maintenance expenses. To achieve these goals, this thesis focuses on the reliability analysis on different aspects of the design and defect assessment of pipelines. Assessment of reliability levels associated with the design and safety factors (DFs and SFs) by evaluating the probabilities of failure (POFs) for different pipe configurations, corrosion defects, and DF/SFs in conducted in this thesis. Two limit states are considered: yielding of intact pipes based on the Barlow’s equation and failure of corroded pipes based on the RSTRENG model under internal pressure. The POFs associated with different DF/SFs are calculated and discussed; the safety levels (e.g., safety class) of pipes based on the different DF/SFs are also determined. The DFs commonly used for the yielding of intact pipes are found to be in the highest safety class. The SFs used for corroded pipes based on the RSTRENG model is also found to be in the highest safety class for long corrosion defects, but in the lowest safety class for short corrosion defects. Dent defects can decrease the life span of oil and gas pipelines. Subsequently, this thesis performs a strain-based reliability analysis on pipe dent defects using a response surface method (RSM) and the first order reliability method (FORM). Two different limit states are used for the reliability analysis, which are the exceedance of the strain capacity of the pipe material by the maximum equivalent plastic strain (MEPS) generated in the pipe and the exceedance of the unity by the ductile strain damage generated in the dented region. The ductile strain damage in the dented region is calculated using the ductile failure damage indicator (DFDI) and strain limit damage (SLD) damage models. Different pipe configurations, pipe lengths, indenter sizes, and dent depths are considered. A suitable finite element (FE) model for the reliability analysis was developed for this study using the FE analysis software ABAQUS. The uncertainties in the pipe wall thickness, the dent depth, the yield strength of the pipe material, and the strain capacity are considered for the reliability analysis. The POFs of several dent defects were calculated. For the strain capacity limit state, it has been found that the POF, which is highly related to the nominal value of the MEPSs generated in the dent defect, is not only related to the indentation depth or the size of the indenter. Thus, the dent depth criterion used in the engineering practice can lead to inconsistent reliability levels in dented pipes. For the ductile strain damage limit state, The SLD and DFDI damage model-based POF were also found to increase with the increase of the dent depth and the decrease of the indenter size. Also, the SLD and DFDI strain damage criterion are found to be more sensitive to the change of indentation depth and the indenter size than the MEPS criterion. The interaction of the dent and corrosion defects is also investigated at the final part of this thesis using strain-based reliability analyses based on the MEPS. A finite element (FE) model is developed and validated to calculate the strains in the DCCD region. The same reliability technique used in the plain dent problem is used to calculate the PoF for the DCCD. A combination of two pipes with different outside diameter to wall thickness ratios, different dent depths, and different corrosion depths and lengths within the dent depth are considered to investigate the effect of the corrosion defect on the pipe dent defect. From the obtained results, it is found that in the case of the overlap of the corrosion edge with the plastically strained region under the indenter, the probability of failure increases significantly. Otherwise, from a strain-based reliability analysis approach, for corrosion depth up to 60% of the pipe wall thickness defects within dent defects, the corrosion defect does not influence the probability of failure of the DCCD.