Corrosion Under Insulation (CUI) – Risk Based Inspection, Non-Destructive Inspection and Fitness for Service API-579
Authors: Largura Luiz C. , Araujo Mauro D. , Silva Arilson R. 
Published: October 2023

This paper presents a Risk-Based Inspection (RBI) analysis of a pressure vessel (distillation column type) with a "Fitness-for-Service" (FFS) assessment of the equipment according to API-579 level 3. The evaluation of the resistance and stability of equipment corroded by Corrosion Under Insulation (CUI) is considered when subjected to design loads.

The Risk Based Inspection evaluation was performed according to API 581 3rd edition, using the latest version of the GIEnd-RBI specialized software. The analysis showed a high probability level for Corrosion Under Insulation, which represents a Medium-High risk for the equipment, as the calculated Consequence of Failure is Medium. The highly effective "A" Inspection required by RBI Analysis, was done after thermal insulation removal, surface preparation and visual inspection, which revealed an advanced local metal loss of the equipment component areas as predicted by the RBI analysis. With the inspection results and 3D scan of the corroded areas, an FFS assessment was performed according to API-579. The conclusion is that the equipment has a remaining strength factor of 1.0, more than the RSF of 0.9 required by the standard.

Risk-Based Inspection (RBI) is an approach used in various industries, including oil and gas, petrochemicals, and chemical processing, to optimize inspection and maintenance activities. It prioritizes inspection efforts based on the risk associated with equipment or assets, focusing on areas most susceptible to damage and degradation. Corrosion under insulation (CUI) is a significant concern in industries where insulated equipment or piping is exposed to corrosive environments. RBI can be effectively employed to manage CUI-related risks.

Benefits of Risk-Based Inspection for Corrosion Under Insulation:

Cost Optimization: RBI allows organizations to allocate inspection resources more effectively, focusing efforts on areas with the highest risk, which can lead to cost savings in the long run.

Safety Improvement: By identifying and addressing high-risk areas prone to CUI, the likelihood of unexpected failures and potential safety incidents is reduced.

Enhanced Reliability: RBI ensures that assets vulnerable to CUI are appropriately maintained, which can extend their operational life and increase overall system reliability.

Compliance and Integrity Assurance: Employing RBI can help organizations comply with regulatory requirements and demonstrate a commitment to maintaining asset integrity.

It's essential to note that RBI is a risk management tool and should be used as part of a comprehensive asset integrity management program, incorporating other elements such as corrosion monitoring, material selection, and proactive maintenance practices to effectively mitigate the risks associated with corrosion under insulation.

In summary, the main advantage of the methodology presented in this paper is the use of RBI to define the areas to be inspected in a more accurate way, as well as the use of a 3D scan technique to map the corroded regions with more refinement than other techniques like C-Scan, providing a more precise data for the FFS evaluation level 3 using finite element analysis.