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It is essential to inspect the pressurized equipment such as vessels, pipes, heat exchangers, boilers, etc., which are under the influence of variable load periodically to minimize the possibility of damage occurring or early disclosure of existing damage. These inspections may be carried out at fixed time intervals or can be carried out at determined intervals depending on a risk assessment that considers settlement of the equipment, operating conditions, and the potential danger of the equipment's chemical contained. Within the scope of this study, we evaluate the thin-walled pressurized equipment under variable internal pressure load. Special attention is crucial to the hot points where the stress is relatively high for inspection of fatigue-related damage on the equipment. We know that stress concentration factors are critical in welded zones in thin-walled pressure vessels. Therefore, the fatigue crack formation in the welded joints is more likely than the equipment's base metal. As a result of the study, we present the probability of time-dependent damage under the effect of fatigue caused by variable internal pressure for butt welded joints. Also, we propose a new approach for periodic control planning. As a case study, damage probabilities of the fuel or gas pipelines operating under variable pressure are calculated based on the diversity of the mentioned parameters, and a new approach is provided to determine the most suitable periodic inspection interval. [ABSTRACT FROM AUTHOR]

The pipelines should be inspected periodically to minimize the probability of failure. The timing of these periodic inspections can be determined by a risk assessment that considers the probability of failure of the pipeline and the potential undesirable effects of the chemical transferred. These are referred to as risk-based inspection methods in the literature. We evaluate pipelines with internal surface defects and under the effect of variable pressure in this study. Internal surface defects were considered production-related, and there is a lack of studies in the literature examining such kinds of defects. These defects generate the concentration of stress on the pipeline section. The reliability variation of the pipeline depends on some factors, namely, surface defect geometry, the pipe section geometry, and the mechanical properties of the pipe. Fatigue is accepted as the crucial damage mechanism. The cause of variable load is considered as the number of operations. As a result of the study, the probability of fatigue failure for pipelines with internal surface defects was evaluated. Then, a new approach was proposed for risk-based inspection planning of the pipelines. As a case study, the probability of fatigue failure on a fuel pipeline established between a refinery and an oil terminal operating in Turkey was calculated. Also, we tried to determine the most appropriate inspection time. The data used in the planning phase was collected from a measurement process performed on the pipeline using PIG (Pipe Inspection Gauge). Finite element analysis was used to calculate stress concentration factors. Finally, we used the Monte Carlo simulation method to obtain the probability of failure. • We evaluate pipelines with internal surface defects and under the effect of variable pressure in this study. • These defects generate the concentration of stress on the pipeline section. • The probability of fatigue failure for pipelines with internal surface defects was evaluated. • A new approach was proposed for risk-based inspection planning of the pipelines. [ABSTRACT FROM AUTHOR]