Inspectioneering Journal

Fitness for Service from an RBI Point of View

By Luciano Narcisi, Senior Consultant at Stork AMT. This article appears in the May/June 2014 issue of Inspectioneering Journal.

As an RBI consultant, I frequently get involved in discussions spawned from a misunderstanding of the relation between Risk Based Inspection (RBI) approaches and Fitness for Service (FFS) assessments.  Questions like: If thickness is below the minimum required by design, why does the risk stay so low?  If we have more than 10 years of remaining life, why is inspection recommended in 4 years?  If we are measuring high corrosion rates, why does the risk stay stable?  Does this mean we do not have to inspect? And so on and so forth.  My intention with this article is to answer these questions and more by exploring the relation between FFS and RBI, through the review of a case study discussed herein.  I will use thickness analysis as an example because I think it is the most common and intuitive metric.  Similar principles could be used for other damage mechanisms.

This case study involves a column in a topping unit installed in 1998 (Figure 1), which was first inspected in 2006 and then again in 2012.  Both inspections were external and with similar scope.

Figure 1. Topping Column.
Figure 1. Topping Column.

Because of budget restrictions and the impossibility to get inside, in both cases inspectors performed an external inspection focused on reachable places, mainly near platform zones.

Thickness measurements were limited to insulation inspection windows.  Inspectors found generalized internal corrosion in top head, with remaining thickness below the minimum required by design.  The corroded area was analyzed according to API 579 Part 4 requirements.  Once the 2012 inspection was finished, FFS and RBI assessments were performed to verify fitness for service and remaining life, and define a strategy and timeline for future inspections.

This content is available to registered users and subscribers

Register today to unlock this article for free.

Create your free account and get access to:

  • Unlock one premium article of your choosing per month
  • Exclusive online content, videos, and downloads
  • Insightful and actionable webinars
Interested in unlimited access? VIEW OUR SUBSCRIPTION OPTIONS

Current subscribers and registered users can log in now.

Comments and Discussion

Posted by George Addison on June 30, 2014
I trust they have a new top head being fabricated... Log in or register to read the rest of this comment.

Add a Comment

Please log in or register to participate in comments and discussions.

Inspectioneering Journal

Explore over 20 years of articles written by our team of subject matter experts.

Company Directory

Find relevant products, services, and technologies.

Training Solutions

Improve your skills in key mechanical integrity subjects.

Case Studies

Learn from the experience of others in the industry.


Inspectioneering's index of mechanical integrity topics – built by you.

Industry News

Stay up-to-date with the latest inspection and asset integrity management news.


Read short articles and insights authored by industry experts.

Expert Interviews

Inspectioneering's archive of interviews with industry subject matter experts.

Event Calendar

Find upcoming conferences, training sessions, online events, and more.


Downloadable eBooks, Asset Intelligence Reports, checklists, white papers, and more.

Videos & Webinars

Watch educational and informative videos directly related to your profession.


Commonly used asset integrity management and inspection acronyms.