Equipment inspection and maintenance at Koch Industries’ Pine Bend (Minnesota) refinery has hitherto been based primarily on inspection history, with little consideration for consequences of failure or business interruption. Recently, however, risk-based inspection (RBI) has been recognised as a technology that could offer cost, risk and safety benefits for the refinery. By identifying where the risks were and how to control them, plant operators would save money and reduce risk at the same time.

• API-RBI software files populated with the required data per unit.
• Inspection plan guidelines based on projected risk five years after the next turnaround. This consisted of tables and reports indicating the overall risk, likelihood and conse- quence factors, the recommended inspection methods and coverage for each equipment item, and each of the damage mechanisms identified.
• Three sets of Process Flow Diagrams showing the basic information used in the study. One set included the operating temperatures and pressures. The second set included representative fluids, phases, and isolation devices for inventory grouping. The third identified damage mechanisms, susceptibility (high, medium or low), corrosion rates, and materials of construction.
• Detailed inspection plans for all process equipment.
• Active damage mechanisms and process constraints/limits identified for pressurised equipment.
• A cost benefit and risk reduction page summary.
• Top ten risk items identified per unit.
• Prioritised risk ranking of pressurised equipment.
• Equipment database for management of equipment risk in the future.

The data gathering and analysis phase included on-site meetings with inspectors to interpret the RBI results, turnaround planning meetings to prepare inspection plans, and training inspectors on how to interpret RBI results when preparing inspection plans.

Cost-effective maintenance
The total Maintenance and Inspection Turnaround cost of the refinery was reduced by 10%. The projected $11,800,000 total savings were achieved by removing low-risk equipment from the turnaround list, or opting for a non-intrusive inspection technique (no vessel entry) where appropriate. Non-intrusive inspections offered higher risk reduction per dollar spent than the traditional intrusive inspection techniques. The savings associated with non-intrusive inspections were primarily the result of reduced maintenance costs as a result of avoiding vessel entry.

In addition to the turnaround cost savings shown in the diagram, the refinery achieved risk reductions by applying the proper inspection techniques to the correct damage mechanisms.

At the end of the project, the inspection intervals for 627 pressure vessels were extended from five to 10 years. Potential wet H2S damage was found on 212 pressure vessels, not previously identified, and 359 new items were found to be susceptible to localised corrosion. The refinery is now planning future work over several turnarounds. Equipment not inspected at the next turnaround will be checked to see if its risk profile becomes unacceptable before the following one. The refinery is now moving to intervals of 10 to 20 years.

Simpler inspection procedures
Previous inspection practices were internal visual and ultrasonic thickness (UT) measurements for general and localised thinning, and wet fluorescent magnetic particle testing (internal WFMPT) for stress-corrosion cracking (SCC) mechanisms. Now, in place of these practices, the refinery uses a combination of external manual and automated shear wave UT for SCC detection, a combination of external straight beam manual UT and automated shear wave UT for the detection of hydrogen-induced cracking and stress oriented hydrogen-induced cracking, and UT thickness measurements for general thinning. Vessel entry is only considered if the use of an external technique is cost prohibitive or not effective in finding the damage mechanism.

Other benefits of the implementation phase included the quantification of the risk of operating parameter changes, reduction in risk of leaks to the atmosphere due to material degradation, identification of opportunities for consequence mitigation, development of universal risk standards for Koch Industries as a whole, and consistent work processes to allow franchising to other Koch processing sites.

An evergreen future
After completing the RBI implementation phase, DNV has now begun what is known as the Evergreen Process. This phase of the project will include the preparation of Management of Change Work Processes to ensure that the RBI software files are kept updated at all times, and the training of inspectors in the use of the software for analysis and updating. The refinery plans to keep the RBI process evergreen by:

• Putting new data into the software,
• Re-running the analysis and inspection plans, and
• Re-running the cost-benefit analyses to justify their time and efforts.

Now the refinery operators know where the risks are and what they must do to reduce them; and what the savings will be.