Predicting Remaining Useful Life of Well Tubulars: A Reliability-Based Approach

Abstract This work estimates the Remaining Useful Life (RUL) of tubulars throughout the lifecycle of oil and gas wells. The RUL evaluation is supported by probabilistic analysis that addresses uncertainties associated with geometrical and material properties, regarding the failure modes defined in API/TR 5C3 code. The proposed framework is applied to tubular design and monitoring analyses, considering degradation effects due to corrosion and extreme scenarios, such as Worst Case Discharge (WCD). The proposed solution comprises a mechanical model to provide the safety factors and the probability of occurrence associated to failure modes involving internal/external pressure and axial force, according to API/TR 5C3 models. By applying structural reliability concepts and the Monte Carlo simulation, the uncertainties are taken into account and the load scenarios can be revisited over time, evaluating the probability of occurrence of unexpected events. This framework allows estimating integrity indicators for RUL analysis of tubulars, such as the failure rate and the mean time to failure (MTTF). The case study presented aims to assess RUL for a 20-year lifetime offshore production well, leveraging statistical information about tubular geometry, material and degradation effects. The simulation covers two operational phases: drilling for the first 2 months and production for the following 238 months. The probability of failure grows over time as corrosion progressively reduces the wall thickness of tubulars, weakening their structural integrity. As a result, the survival function declines, while the failure rate rises with time. This deterioration in strength leads to a greater likelihood of failure throughout the well's lifecycle, potentially affecting its remaining useful life (RUL). Such a trend directly impacts the mean time to failure (MTTF) of tubulars, influencing the longevity of oil and gas wells. The proposed methodology assesses the integrity of well tubulars in both serviceability and survival load scenarios, enabling advanced features such as the consideration of corrosion in WCD scenarios. Accounting for the variability inherent to mechanical and geometrical variables throughout the service life of casing and tubing strings allows evaluating their mechanical condition and failure probability over time. These findings contribute to the decision-making process in RUL analysis, enhancing well design and monitoring practices.