Structural integrity of casings is critical for safe and economic operation of offshore wells and it can be compromised by external corrosion due to exposure to environment such as seawater. Assessing the impact of severe casing corrosion on the structural integrity of the well can be challenging due to complexities arising from local wall loss, non-uniform wall thickness, presence of holes due to severe corrosion, and non-bonded cement. Such complexities are not adequately addressed by existing codes/standards and available analytical equations.
This paper presents a study that utilized a combination of a thermo-mechanical well simulator and threedimensional (3D) finite element analysis (FEA) to model severely corroded offshore well casing strings towards assessing structural integrity of the wells. The goal of the study was to determine the extent of allowable corrosion in conductor and surface casing beyond which the wells would be at risk for failure, and if topping off the cement in the annulus would be beneficial for structural integrity.
A thermo-mechanical well simulator was utilized to model how corrosion wall loss evolved over time for both the conductor and surface casing, and the corresponding impact on their load capacity. In conjunction with the well simulator, full 3D FEA was conducted to model various complexities, such as the effect of corrosion holes and non-uniform corrosion. The 3D FEA helped assess the impact of remedial cementing on casing integrity and refine the critical wall thickness limit needed to withstand loads predicted by the well simulator. Compression and buckling were identified as governing failure modes and FEA results were compared with a buckled conductor in the field. The systematic, mechanics-based approach used in this study provided a basis for risk assessment where at-risk wells can be prioritized for remediation and/or abandonment.