API RP 579-1. FITNESS FOR SERVICE EVALUATION

API RP 579-1 is a practical recommendation developed by the American Petroleum Institute (API) that provides a comprehensive methodology for Fitness for Service Evaluation. This guide provides a structured approach to evaluating the ability of equipment to continue operating safely, specifying under which conditions once some type of damage, deterioration or defect has been detected, whether caused by factors such as mechanical damage, overload, overheating, corrosion, erosion, fatigue, creep, or presence of hydrogen, among others.

Evaluation procedure

The primary objective of the evaluation process is to determine the suitability of the equipment to continue operating under current operating conditions. If it is determined to be suitable, the equipment can continue to operate. Otherwise, a requalification will be required.

The standard is made up of different parts, where the evaluation procedures are organized according to the type of defect and/or the damage mechanism. In certain cases, when the type of damage is not evident, it may be necessary to jointly apply evaluation procedures from different parts.

The general evaluation procedure consists of eight steps. Each part of this standard is based on this methodology to determine the procedure for each specific defect type.

·??????? STEP 1 – Identification of failure and damage mechanism

·??????? STEP 2 – Applicability and limitations of FFS evaluation procedures

·??????? STEP 3 – Data requirements

·??????? STEP 4 – Evaluation Techniques and Acceptance Criteria

·??????? STEP 5 – Remaining life evaluation

·??????? STEP 6 – Remediation

·??????? STEP 7 – In-Service Monitoring

·??????? STEP 8 – Documentation

EVALUATION LEVELS

Each part provides three levels of evaluation. In general terms, each level of evaluation provides a balance between a conservative criterion, the amount of information required for the evaluation, the skills of the personnel who carry it out, and the complexity of the analysis required. Ideally, practitioners typically progress sequentially from a Level 1 analysis to a Level 3 analysis, whenever feasible.

Level 1

Procedures at this level are intended to provide conservative selection criteria that can be applied with the least amount of component inspection or information possible.

Level 2

These procedures involve a more detailed evaluation. Although the inspection information is similar to that of Level 1, more extensive calculations are performed during the evaluation at this level.

Level 3

These procedures are designed to provide a more detailed evaluation and more accurate results. They require more detailed inspection information and typically involve the use of numerical analysis (FEA).

EVALUATION TECHNIQUES

When carrying out the evaluation, various techniques are used, including:

·??????? Allowable stress

This method is based on the calculation of stresses generated by different loading conditions, as well as the classification and superposition of the results of said stresses. The calculated stresses in an assigned category or class are then compared to an allowable stress value. It is important to note that this method has limited applicability due to the inherent complexity of establishing adequate stress ratings on components that exhibit defects.

·??????? Remaining resistance factor

Through non-linear stress analysis, it is possible to obtain a more accurate estimate of the safe load capacity of a component. The concept of Remaining Strength Factor (RSF) is used to evaluate the acceptability of a component for continued service.

The RSF is defined as the ratio between the limit load of the damaged component and the limit load of the undamaged component.

REMAINING LIFE EVALUATION

Once it has been determined that the defective component meets the acceptable conditions to continue operating, its remaining life is calculated.

It’s important to note that remaining life calculation is not intended to provide an accurate estimate of actual time to failure, but rather is used to determine an appropriate inspection interval, develop an in-service monitoring plan, or evaluate the need for remediation.

Each part of this guide provides guidance on how to calculate remaining life. In general, the remaining life is established by stating the time required to reach a specific operating condition, such as the maximum working pressure (MAWP) or a reduced operating condition such as the maximum allowable working pressure MAWPr.

There are three possible scenarios:

a)????? The remaining life can be calculated with a reasonable degree of certainty, such as in the case of a uniform corrosion situation, where it is possible to estimate a future corrosion rate. However, these estimates must be conservative to account for inherent uncertainty, such as variations in material properties, stress levels, and actual damage.

b)???? The remaining life cannot be established with reasonable certainty, such as in the case of a stress corrosion cracking mechanism, where reliable data on the crack growth rate are not available. In these cases, remediation methods should be considered, such as plating to isolate the fault.

c)????? There is minimal or no remaining life. In this case, a repair is required. The damaged component must be repaired, a coating applied to isolate it from the environment, and/or frequent monitoring must be performed before the equipment can return to normal operation.

?

For more information: Fitness for Service Evaluation & Repair of Piping Systems