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Engineering Critical Assessment

Oil & Gas Industries

Introduction

Most pipeline codes include flaw acceptance criteria to ensure that flaws introduced during fabrication do not impact the performance of the pipeline during installation or operation.

Historically the flaw acceptance criteria in pipeline codes have been based on a combination of good workmanship principles in conjunction with RT (radiography) inspection.

Over the last 20 years there has been an increasing trend to adopt alternative fitness-for-service concepts for critical pipeline applications. The concept of fitness-for-service has led to the development of alternative flaw acceptance criteria that are determined by performing an Engineering Critical Assessment (ECA).

It is now standard practice to perform an ECA for all fracture and fatigue critical pipelines. The adoption of ECA methods is ideally suited to modern construction methods that use mechanized welding in combination with automated ultrasonic testing (AUT).

The adoption of ECA concepts for critical pipelines will lead to improved integrity since the emphasis is placed on the most important aspects of overall quality:
  • Material Selection
  • Procedure Qualification
  • Improved Inspection Technology

The major features of the standard workmanship and ECA based approaches can be summarized as follow:

The adoption of ECA based flaw acceptance criteria can also result in significant economic benefits through reduced repair rates.

ECA in Cross Country Pipeline

The two most widely used cross country pipeline construction codes in North America are API 1104(2) and CSA Z662(3). Both of these codes include appendices that present procedures for deriving alternative flaw acceptance criteria for pipeline girth welds based on ECA methods.

The ECA method included in API 1104 Appendix A is based on the CTOD Design Curve and only considers failure by fracture. The user is required to perform a detailed pipeline stress analysis and qualify the pipeline girth welds to one of two specified CTOD toughness values (0.005 inch or 0.01 inch). The CTOD Design Curve method in API 1104 Appendix A includes an allowance for welding residual stresses but does not consider stress concentration factors (SCFs) associated with H-Lo misalignment or local weld toe geometry.

The ECA method included in CSA Z662 Appendix K requires the user to perform separate assessments for fracture and plastic collapse. The fracture assessment is performed using the CTOD Design curve. Unlike API 1104 Appendix A, the fracture assessment method in CSA Z662 Appendix A does not consider welding residual stresses since full scale tests performed to validate the ECA method demonstrated that the assessment method is still conservative, even if residual stresses are ignored. In addition, unlike API 1104 Appendix A, CSA Z662 Appendix K allows the user to qualify the girth welding procedure to the minimum measured CTOD toughness rather than a specific value. The CSA Z662 Appendix K fracture assessment procedures do not consider SCFs arising from Hi-Lo misalignment or local weld toe geometry.

ECA in Offshore Pipeline Codes

DNV OS F101(4) is probably the most comprehensive offshore pipeline code and covers design, construction and operation of offshore pipelines. DNV OS F101 allows users to develop ECA based flaw acceptance criteria for pipeline girth welds using the methods presented in BS 7910.

The BS 7910 assessment procedures for pipeline girth welds are more comprehensive than the procedures presented in API 1104 Appendix A and CSA Z662 Appendix A. As stated earlier, BS 7910 uses a FAD approach and consequently fracture and plastic collapse are assessed simultaneously. In addition it is standard practice to take full account of welding residual stresses and SCFs arising from Hi-Lo misalignment and local weld toe geometry effects.

DNV has recently published DNV OS F108 "Fracture Control for Pipeline Installation Methods that Introduce Cyclic Plastic Strains". DNV OS F108(5) uses a modified BS 7910 approach in which the installation strain is converted to an equivalent stress using a Neuber Analysis and the material stress - strain curve. In addition DNV OS F108 requires that the pipe and girth weld material toughness is characterized as a crack growth resistance curve. Moreover, since the shape of an R-curve is dependent on the mode of loading and the associated level of constraint, it is recommended that the toughness tests are performed on SENT specimens, rather than the traditional highly constrained deeply notched SENB specimens, to ensure that the specimens more accurately represent the level of constraint experienced in a pipeline. Rcurve testing will not only provide a more representative measure of material toughness but it will also increase the flaw tolerance in cases where failure is fracture controlled. Finally DNV OS F108 allows the Plastic Collapse cut-off (Lrmax) in BS 7910 to be extended out to the ratio of Tensile Strength to Yield Strength since the loading experienced during reeling is displacement controlled.

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