Inspection planning is the very core of why we complete RBI or condition based inspection plans. Condition based planning is very cut and dry; based on the condition of the equipment upon inspection, you determine the next interval. But what about when you’re using an RBI assessment to schedule your inspections. Do you know how your planning methodology affects your intervals? Let’s look at the pros and cons of the two most used methods:
- Risk Target Method
- Bucket Method
PROS:
Risk Target
In the risk target method you multiply your POF by the COF to get a risk in ft2/yr or $/yr. You then compare this number with your company’s risk target or risk threshold. Items that are above the risk threshold are due for inspection, and items that are below the target will become due once they cross that line. This method is especially useful when you have a fully quantitative assessment, since you must have numerical values for both the POF and the COF. With this method, hard numbers can be compared and presented to management, with a level of granularity that allows you to specifically determine the risk ranking of each component.
Bucket
In the bucket Method you determine your POF and COF (qualitatively or quantitatively) and then plot it on a risk matrix. As the POF increases, you move vertical on the matrix (assuming POF is on the vertical axis) and once the POF hits a threshold, an inspection is required. In many cases this threshold changes based on the COF column in which the equipment is located. Using this method helps visually see large numbers of equipment at one time. It also allows groups of equipment (High COF) to be identified for further analysis or placed in special programs for risk management.
CONS:
Risk target
The risk target method requires a quantitative analysis so that a probability and consequence can be determined. Using a qualitative ranking system will only work to a point, but you will find that many of your equipment items will have similar risk numbers, thus defeating the precision of the Risk Target Method. Also, how do you determine your acceptable risk threshold? It also adds a layer of complexity by not specifically utilizing the risk matrix; which can be daunting to inexperienced personnel. When a corporation is used to adhering to a risk matrix for all decisions, one example being deferral sign-offs, and inspections are now being planned based on a risk target, “Management Education” becomes critical to achieving buy-in and success.
Bucket
The bucket method treats equipment in the same risk matrix box equally. For example, take two towers in your Crude Unit, the Kerosene Stripper and the Heavy Gas Oil Stripper (HGO Stripper). In this example, the kerosene stripper has a COF of $10MM and the HGO Stripper with $1M. Both fall in the same box on the risk matrix, which means they both have a POF threshold of 0.0001 (using my theoretical Risk Matrix). But when you determine the risk number for these, at that threshold, you realize the kerosene stripper is allowed to rise to 10x the risk of the HGO. Does this properly risk rank the equipment?
HGO Stripper Risk = 0.0001 X $1MM = $100
The bucket method also does not allow for improvements in safety without creating a bureaucratic nightmare. Let’s say you have implemented the risk target method and have had a mature RBI program for 15 years. You have limited inspections each turnaround to just the essentials, because you have created long term asset strategies, and you are happy with how your program is running. Suddenly, corporate sends out a mandate that due to a new law, safety and risk in each plant must be reduced by 25% within the next 5 years. Using the risk target method, your risk target can be reduced by 25% and immediately you will be able to identify critical equipment that will need to be added to turnaround scopes. With the bucket method, a new risk matrix will need to be created, typically by corporate, and signed-off up the ladder, which can take a lot of time that you don’t have.
Conclusion
Both methods, risk target and bucket, are acceptable for determining inspection intervals. But fully understanding the impact of your inspection planning Method will allow you to get the most out of your RBI program and make the appropriate decisions to help keep your plant safe and operating.
Which Method do you like the most? Is there a reason you like one over the other? Let me know in the comments below.