Maintaining the mechanical integrity of above ground storage tanks (AST’s) is the focal point of tank inspection programs. Performing internal inspections is an integral part of a tank integrity program, however, deciding when to take a tank out of service to perform an internal inspection is not an easy determination to make. Tank operators are faced with the question, “do I really have to take this tank out of service right now? And, if so, what kind of damage and how extensive is the damage inside the tank, especially to the floor?”
There are industry standards such as the API Standard 653, Tank Inspection, Repair, Alteration and Reconstruction, which provide guidelines for tank openings. These recommendations are calendar based and may not always be the optimum interval for a tank opening. In some cases, the interval may be too short, thereby causing an unneeded expenditure of money and downtime for the tank. In other cases, the interval may be too long and excessive damage could result because of the overdue internal inspection. Therefore, the ongoing problem for the tank operator is “how can I optimize my on-stream inspection program while not significantly altering my risk of tank failure? What can I do to help ensure the tanks I open are those that are in most need of attention? Moreover, can I extend tanks that are in good condition?” For answers to these questions, tank operators are more frequently turning to Risk Based Inspection (RBI) programs. RBI programs help develop an inspection program based on the risk ranking (probability of failure vs. consequences of failure) of the tank.
Why Non-Intrusive Inspections
Most RBI programs use historical data and “what if” scenarios to assess risk and develop inspection programs. This includes original fabrication documentation and inspections, enhancement and modification records, past use and past inspection results. From this and other assumptions, a risk assessment is developed with an inspection program that is intended to maintain or reduce current risk based on a reduction in the probability of failure. Because the assessments are made utilizing historical and environmental information, they may lack information on the current operating condition of the tank. Recent changes in environment or usage may not be taken into account in the assessment or the inspection program. Most often the inspection recommendations are based on calendar information, and as a result, the intervals may not be optimized. By using inspections that assess the in-service condition of the tank, current degradation conditions can be identified and more effectively addressed in the assessment and the corresponding inspection program. Non-Intrusive inspections have become a priority for many companies. There are several reasons for this, some of which include:
Safety – eliminating entry into a confined space without any significant loss of information represents an improvement in overall plant operational safety.
No interruptions of service – The non-intrusive inspections are performed with the tank in service and product in the tank. There is little or no loss of availability of the tank due to the inspection.
Cost – It is very expensive to prepare a tank for internal inspection.
No disturbance to tank – For most inspections there is little or no disturbances to the tank. There is no paint removal, insulation stripping, no scaffolding, etc.
Quick Results – The results of the inspections are available during or shortly after the inspections are completed. Should a significant problem be detected, it can be identified and a corrective plan of action formulated. This can prevent an unexpected tank failure.
Planning Work Scope for Internals – When planning a tank opening, the condition of the tank floor, etc. are unknowns. Until an entry is made, the planners generally have minimal ideas as to what they will find, so they plan for a worst-case scenario and hope for the best. Non-intrusive inspections can provide the planners with information about the condition of the floor, as well as other areas of the tank. With this information, a better work scope can be planned, a more accurate determination can be made of what the material and labor needs will be, and the estimate of the turnaround of the tank will be more precise.