Brian D. Newbury

Tensile Strain Capacity Equations for Strain-Based Design of Welded Pipelines

Various industry efforts are underway to improve or develop new methods to address the design of pipelines in harsh arctic or seismically active regions. Reliable characterization of tensile strain capacity of welded pipelines is a key issue in development of strain-based design methodologies. Recently, improved FEA-based approaches for prediction of tensile strain capacity have been developed. However, these FEA-based approaches require complex, computationally intensive modeling and analyses. Parametric studies can provide an approach towards developing practical, efficient methods for strain capacity prediction. This paper presents closed-form, simplified strain capacity equations developed through a large-scale 3D FEA-based parametric study for welded pipelines. A non-dimensional parameter is presented to relate the influence of flaw and pipe geometry parameters to tensile strain capacity. The required input parameters, their limits of applicability and simplified equations for tensile strain capacity are presented. The equations are validated through a comprehensive full-scale test program to measure the strain capacity of pressurized pipelines spanning a range of pipe grades, thickness, weld overmatch and misalignment levels. It is shown that the current simplified equations can be used for appropriate specification of weld and pipe materials properties, design concept selection and the design of full-scale tests for strain-based design qualification. The equations can also provide the basis for codified strain-based design engineering critical assessment procedures for welded pipelines.Copyright

Microstructure and Hydrogen-Induced Failure Mechanisms in Fe and Ni Alloy Weldments

The microstructure and fracture morphology of AISI 8630-IN625 and ASTM A182-F22-IN625 dissimilar metal weld interfaces were compared and contrasted as a function of postweld heat treatment (PWHT) duration. For both systems, the microstructure along the weld interface consisted of a coarse grain heat-affected zone in the Fe-base metal followed by discontinuous martensitic partially mixed zones and a continuous partially mixed zone on the Ni side of the fusion line. Within the partially mixed zone on the Ni side, there exists a 200-nm-wide transition zone within a 20-μm-wide planar solidification region followed by a cellular dendritic region with Nb-Mo–rich carbides decorating the dendrite boundaries. Although there were differences in the volume of the partially mixed zones, the major difference in the metal weld interfaces was the presence of M7C3 precipitates in the planar solidification region, which had formed in AISI 8630-IN625 but not in ASTM A182-F22-IN625. These precipitates make the weldment more susceptible to hydrogen embrittlement and provide a low energy fracture path between the discontinuous partially mixed zones.

Hydrogen Induced Mechanical Property Behavior of Dissimilar Weld Metal Interfaces

A series of catastrophic failures between alloy 625 weld metal (referred to as ‘A625’) and AISI 8630 low alloy steel forgings (referred to as ‘8630’) on cathodically-charged subsea equipment demonstrate the need to gain a better understanding of the hydrogen-induced tearing resistance of these interfaces as well as similar types of interfaces also currently used in the field. Other dissimilar metal weld interfaces in use include ASTM A182 F22 (referred to as ‘F22’) welded with A625. Similar metal alternatives are also in use, including F22 welded with low alloy steel (referred to as ‘LAS’). Slow strain rate single-edge notched bend tests under hydrogen charging conditions were used to establish ‘R-curve’ crack growth resistance trends for the F22-A625 and F22-LAS weld metal interfaces. Differences in ‘R-curve’ crack growth behavior between the different weld metal interfaces have been observed and compared to R-curve results from 8630-A625 interfaces. The F22-LAS interface demonstrates the most tearing resistance under slow strain rate after hydrogen charging followed by the F22-A625 and then the 8630-A625 interface. Subtle differences between the weld metal microstructures are described and provide a possible explanation regarding the difference in ‘R-curve’ behavior.Copyright

Development and Qualification of Pipeline Welding Procedures for Strain Based Design

This paper reviews the specific testing methodologies implemented for the qualification of mechanized pulsed gas metal arc welding (PGMAW) procedures for strain based design applications. The qualified welding procedures were used during recent construction of an offshore pipeline subject to potential ice scour with an initial design target of 4% tensile strain capacity. This paper addresses the integrated development of linepipe specifications, large scale validation testing, weld procedure development, and finally, the verification of robustness through full scale pressurized testing of actual girth welds on project pipe material. The qualification sequence, from linepipe specification development through final full scale girth weld proof test is described.Copyright

Approaches to Qualify Strain-Based Design Pipelines

The importance of using strain-based design pipelines is growing due to the increasing number of projects in challenging environments such as permafrost, offshore ice hazards, active seismic areas, and in high temperature/high pressure operations. To ensure pipeline integrity in environmentally sensitive areas and overall cost effectiveness, a strain-based design approach needs to consider all key interrelated design aspects including strain demand, design methods, material selection, strain capacity validation, and impact on construction and operation. To that end, significant research and development efforts have been made by the industry to facilitate the qualification of strain-based designed pipelines. This paper describes methods developed for the qualification of strain-based design pipelines, and demonstrates how recently developed strain capacity prediction tools, calibrated by full scale testing, can facilitate concept selection, material qualification and integrity verification of such pipelines.Copyright

Recent Concepts for the Welding of High Strain Pipelines

Standard allowable stress-based pipeline designs (strain demand ≤ 0.5%) are now giving way to more complex strain-based designs (strain demand higher than 0.5%) as the locations of future pipelines move into regions of increased strain demand. The increase in required levels of strain demand are attributed to seismic activity, soil movement, soil liquefaction, frost heave, thaw settlement, ice scour or a combination thereof. Pipelines in high strain demand regions are typically limited by the strain capacity of the girth weld. As strain-based pipeline design has matured, it has become evident that specific material properties (both weld metal and line pipe), defect size, defect location, misalignment, and operating pressure each affect the strain capacity of the pipeline. This paper reviews proposed design and testing methodologies for the qualification of strain-based design welding procedures. These methods have been applied in the development and qualification of welding procedures for the construction of pipelines subject to longitudinal tensile strains in excess of 2%. Strain-based design requires considerably more effort than traditional design in terms of girth weld qualification and testing. To ensure adequate girth weld strain capacity for strain-based design the testing includes large scale and full-scale pressurized testing for design validation.Copyright

Effects of Aging on the Mechanical Properties of Pipeline Steels

The intelligent design of a given pipeline system intended for operation beyond the elastic limit should incorporate specific features into both the base material (line pipe) and girth weld that enable the affected system to deform safely into the plastic regime within the intended strain demand limits. The current paper focuses on the mechanical properties known to influence the strain capacity of the base material (i.e., line pipe steel independent of the girth weld). Line pipe mechanical properties of interest include: longitudinal yield strength, tensile strength, yield to tensile strength ratio, reduction of area, elongation and uniform elongation. Of particular interest (in consideration of the conventional thermally applied corrosion protection coating systems to be employed), are the longitudinal mechanical properties in the “aged” condition. The present study investigates six (6) different pipeline steels encompassing grades X60 (415 MPa) to X100 (690 MPa), and includes both UOE Submerged Arc Welded - Longitudinal (SAW-L) and seamless (SMLS) forming methods.Copyright