What part plays RBI in tank inspections

By Carlos Molina

This time I will make a break in the API 653 series and talk you about Risk Based Inspection. This change in the course of the blog is according to 2 reasons

1. I don´t want to overload google with the “api 653 question” keyword
2. I need to study for my API 580 certification examination on December, 19^th.
3. I need to make my posts more appealing to comments, questions and answers.

Those are enough reasons to make this break. However, the subject of Risk Based Inspection (RBI) is not alien to static equipment inspection and API certifications, so any thing I write about RBI will be helpful for API certificated personnel. That´s why (besides some writer´s block) I want to try this subject this time.

If you continue reading, you will have a basic understanding in the relationship between RBI and the API 653 standard. And RBI is important for the following reasons:

1. Environmental concerns regarding tank leaks are rising. Cost of cleaning an penalties are prohibitive.
2. Frequent unnecessary inspections can be costly and risky.
3. A more exact approach to maintenance, possible only with more accurate instruments and a broader inspection knowledge.

So let´s go on.

RISK BASED INSPECTION

According to API 580,

Risk is the combination of the probability (likelihood) of some event occurring during a time period of interest and the consequences, (generally negative) associated with the event.

And according to Wikipedia,

Risk Based Inspection (RBI) is an Optimal maintenance business process used to examine equipment such as pressure vessels, heat exchangers and piping in industrial plants. It examines the Health, Safety and Environment (HSE) and business risk of ‘active’ and ‘potential’ Damage Mechanisms (DMs) to assess and rank failure probability and consequence. This ranking is used to optimize inspection intervals based on site-acceptable risk levels and operating limits, while mitigating risks as appropriate

Risk based inspections is then a systematic analysis to determine a plan for inspection of assets based on their associated risk instead of predetermined fixed intervals of time. This way you can optimize maintenance costs and hopefully prolong asset life.

Risk Assessment Matrix

The RBI acronym is used 18 times in the API 653 standard. It stands for Risk Based Inspection. In the standard, RBI appears as a mean to shorten or lengthen inspection intervals, which would be useful, having in mind that scheduled inspections of a tank can be a costly and risky task. Consider what 6.4.2.4 has to say:

API 6.4.2.4 As an alternative to the procedures in 6.4.2.3, an owner/operator may establish the internal inspection interval using RBI procedures in accordance with this section

In any way, an RBI analysis shall be carried on by a group of individuals form a range of disciplines and with the skills to conduct the assessment. If different teams make RBI assessment fot he same tank, inspection recommendations may vary.

And about the principles of RBI, you can find  them in API 580 RP, intended to supplement API 510, API 570, and API 653. A good summary of reasons for tank deterioration can be found in API 571 RP “Damage Mechanisms Affecting Fixed Equipment in the Refining Industry”.

API 6.4.2.4. RBI assessment shall be performed by an individual or team of individuals knowledgeable in the proper application of API 580 principles to aboveground storage tanks, and experienced in tank design, construction details, and reasons for tank deterioration, and shall be reviewed and approved by an authorized inspector and a storage tank engineer. The initial RBI assessment shall be re-assessed at intervals not to exceed 10 years, at the time of a premature failure, and at the time of proposed changes in service or other significant changes in conditions.

API 653 also enlists a set of minimum likelihood factors and consequence factors that should be taken into account for the analysis. In a tank, likelihood factors you would consider would be those related to shell, and bottom thickness and weld quality, and things like soil resistivity, quality of lining, etc. Regarding consequence factors, you should consider Safety, Health, and Environmental Consequences.

API 6.4.2.4. RBI assessment shall consist of a systematic evaluation of both the likelihood of failure and the associated consequence of failure, utilizing the principles of API 580. RBI assessment shall be thoroughly documented, clearly defining all factors contributing to both likelihood and consequence of tank leakage or failure.

Requirements of the standards are clearly risk based driven, and represent a minimum consensus between members of the API committees. For example, the requirement for hydrostatic testing of tanks can be avoided only when there is difficult access to water for the test, the standard asking for other means of testing when this is the case. However, making the hydrostatic test to detect leaks before initial operation, although expensive, is way much less expensive than having to do it after operation. The members of the committee surely consider the other testing alternatives to be too costly or too unreliable for both all leak, settlement and weld strength testing

Another example is that when you make a hydrostatic test, according to the design, the standard asks the test to be made to design level. If you test a tank that will contain water, you will surely do it until design level, but if it contains oil, you better try to make the test above the roof-to-shell joint even if the tank doesn´t ask, because you consider the risk of failure too high and the cost of testing relatively low.

Besides, RBI is useful for tank repair or construction in another terms. Well, have you been around one of those tank projects were there are endless discussion about some little non-compliance with the standards? RBI can help you decide more easily on any subject.

FOR THE EXAM

In the examination, rather one or two questions are based in inspections intervals related to API 580. Don´t forget to study numerals 6.4.2.1 through 6.4.2.5.

I hope this is a good introduction to the importance of RBI in tank inspection. Of course I intend to go deeper in the subject in the future.

As always, please let me know if there is anything wrong or missing in this article.