The fitness for service assessment methods are used to assess the critical pressurized components and welded elements for identification of the mitigation needs to safely use the assets. Various industries such as power generation, process plants, aerospace, oil and gas, marine industry, etc make use of fitness for service methods throughout different stages of the asset’s lifecycle.

The General Procedure of FFS

• Identification of flaw and damage mechanism
• Determination of applicability and limitations of FFS assessment procedures
• Gathering of information
• Assessment technique and acceptance criteria
• Remaining life assessment
• Remediation
• In-service monitoring
• Documentation

FFS Standards

Widely used standards or procedures for FFS include API 579-1 /ASME FFS-1, BS 7910 and DNV-RP-101 for generic applications. In oil and gas industry, FFS of girth weld flaw acceptance for pipeline construction and operation maintainess is normally performed using fracture mechanics based application specific procedures such as API 1104 Appendix A and CSA Z662 Annex K.

General Method

The API-579 FFS assessment procedures are detailed in the standard based on damage type and mechanism. Even though the fitness for service assessment for each type of flaw varies significantly the general FFS approach is somewhat similar. A single FFS assessment may involve multiple related parts of the code. Three levels of assessment are defined, with increased data requirements and analitical complexity for higher requirement of analysis accuracy. The following figure provides a high level discription of assessments.

Level 1 Assessment

This is a preliminary screening level that involves simplified calculations and conservative assumptions to quickly determine if the structure is fit for service. It is suitable for straightforward cases with minimal uncertainty.

• The most conservative, easiest to perform and normally a start point.
• Intended to provide conservative screening criteria that can be utilized with a minimum amount of inspection or component information.
• May be performed by either an inspector or engineering personnel.
• Assessment technique and acceptance criteria.

Level 2 Assessment

This level provides a more detailed analysis using refined calculations using specific data of the equipment and its operating conditions. It is used when a Level 1 assessment indicates that further evaluation is necessary.

• Intended to provide a more detailed evaluation and more precise results than a level 1 assessment.
• Uses inspection information similar to that required for a level 1 assessment.
• Typically performed by engineers, or engineering specialists experienced and knowledgeable in FFS assessments.

Level 3 Assessment

The most detailed and comprehensive level, involving advanced techniques such as finite element analysis (FEA). Level 3 assessments are used for complex situations where precise evaluation of the equipment's condition is required.

• Typically require building FEA models to address complicated geometries, non-linear material properties, etc.
• If the results pass the requirements, operation plans can be made.
• If the results fail, plans need to be made for repair, re-rate or replace.

API 579 Assessment Examples

Below are examples of API 579 Level 3 FEA analysis.

General Procedure

ECA methods involve various processes, including:

• Procedure Qualification: Verifying that the welding procedures meet the necessary standards.
• Material Properties: Lab test data for the base pipe and the weld including fracture toughness to ensure the data meets the ECA procedure requirements.
• Inspection Technology: Utilizing advanced techniques like automated.
• Ultrasonic Testing: To detect flaw.
• Fracture Mechanics Calculations: Calculating the critical flaw sizes using the given fracture toughness and the loading level.

Industry Standards

The most widely used ECA code in the oil and gas industry is BS 7910, and in North America API 1104 Appendix and CSA Z662 Annex K are regulartory acceptable.

• BS7910: BS 7910 provides generic procedures for structural integrity assessment, and particularly can be used for girth weld ECA for welding of pipelines and related facilities. The standard includes comprehensive techniques for determining loading types, residual stress and flaw interactions.
• API 1104 Appendix A: API 1104 Appendix A provides a procedure for girth weld ECA for welding of pipelines and related facilities. The ECA procedure is associated with the main body of API 1104 that offers guidelines for various aspects of pipeline welding, including welding process, welding procedure development, qualification of welders and, and criteria for the acceptance of welds based on visual inspection and non-destructive testing methods.
• CSA Z662 Annex K: CSA Z662 Annex K provides procedures for girth weld ECA for welding of pipelines. Two options are available for use depending on the user’s needs. Option 1 procedure is generally more conservative than the option 2 procedure.

The figure below shows the ECA output for flaw acceptance for girth welds made in a NPS 48 Grade 483 pipeline.