Piping CML Optimization: Optimizing the Definition

Introduction

“Condition monitoring location (CML) optimization” is a term we frequently hear, particularly for piping circuits. Everyone wants their circuits to be “CML optimized,” but what does this truly mean? Borrowing a term from API RP 581, Risk-based Inspection Methodology, the broad definition of CML optimization as it relates to internal thinning is easy: we wish to establish the minimum CML coverage to correctly identify the true damage state.^[1] Note that the true damage state is related to remaining life in traditional, time, or condition-based inspection programs. Although we sometimes focus on having sufficient CML coverage to correctly define the corrosion environment, corrosion rates are only part of the picture. Projected wall thickness is the ultimate goal when it comes to inspection, repair, and replacement planning. 

While the broad definition for CML optimization is easy, breaking this definition down to specific methods, algorithms, and business logic is not so straightforward. What is the basis for a given “optimization?” How precisely do we need to characterize the corrosion environment so we can quantify the amount of wall loss? Will CML quantities and selections be based on expert opinion, statistical methods, or a blend of both? How is the inspector, facility, company, and industry experience incorporated into the analysis? Are we optimized for a snapshot in time or is there an allowance for future degradation? There are as many answers to these questions as there are methods presently employed.

Setting the Ground Rules

For the purpose of this discussion, we will be focusing on wall thickness data taken at prescribed locations to monitor internal corrosion. In this context, “CMLs” are actually thickness monitoring locations. We normally generate this data using ultrasonic thickness (spot or scanning) and profile radiographic techniques. 

At a minimum, we expect CML optimization to provide guidance in proper CML quantities and placement. It must be consistent with the assigned, internal thinning damage mechanisms, while also considering site and industry experience. Additional benefits resulting from this dedicated, focused review of historical thickness data and inspection practices will often include the following:

• Identification of and follow up inspection for potential localized corrosion areas
• Recommendations to establish new CMLs or archive / inactivate unneeded CMLs
• Identification and resolution of data anomalies, with resulting database “cleanup”
• Opportunities to reduce measurement error and improve repeatability
• Adjustments to circuit boundaries: combining, splitting, or moving boundaries, as appropriate
• Prioritization for circuits that are candidates for 100% component inspection

As will be discussed later, the specific benefits will depend on many factors, including the quality and quantity of historical data, the corrosion environments, and the capabilities of the data analysis process.