NDE Input to Fitness for Service Decision Making

Introduction

We have discussed various factors that can affect the reliability of NDE techniques (i.e., probability of detection - POD and sizing accuracy) in Part 1. In general, it's difficult to quantify these uncertainties. In fact, it's impossible to fully quantify the uncertainties in NDE results. However, one can achieve a higher level of reliability by reducing or minimizing the uncertainties. We will discuss some of the steps one can take to minimize the uncertainties in NDE results. By minimizing or reducing the uncertainties in NDE results, fitness for service (FFS) assessments can be less conservative and thus, provide more margin to the serviceability of the equipment.

Steps to Minimize the Uncertainties

1. Do your basic economic analysis first

We know that we cannot eliminate the uncertainties in NDE results completely. However, we can minimize these uncertainties or improve the quantification of uncertainties if it’s economically justifiable. For example, it makes no sense to spend $100K to better quantify the uncertainties in NDE results if the cost of equipment replacement is only $10K or the consequence of failure is insignificant. In many cases, this decision can be made by using common sense, and you don’t need to conduct a formal/detailed decision analysis to decide if extra efforts are needed.

2. Develop application and operator specific data

Using results from generic round-robin tests only gives you a starting point, but it puts you in the right ballpark. Applying results from these generic tests directly to your application is often too optimistic. It's optimistic because these tests were conducted in a laboratory environment and often involved highly qualified operators. Obviously, they do not resemble the actual field condition and the operators you use. As such, you will need to apply a debit to the results you are getting.

The more you know about the applications and the operators, the better handle you have on the uncertainties in applying round robin-results or the penalty you apply to the results.