Doug P. Fairchild

Development of the SENT Test for Strain-Based Design of Welded Pipelines

Strain-based design (SBD) pipelines are being considered to develop hydrocarbon resources in severe environments. As part of a research program to develop a SBD methodology, work was conducted to develop a suitable fracture mechanics test that can be used as part of a strain capacity prediction technique. The single edge notched tensile (SENT) specimen geometry has been chosen due to the similarity in crack-tip constraint conditions with that of defects in pipeline girth welds. This paper describes a single-specimen compliance method suitable for measuring ductile fracture resistance in terms of crack tip opening displacement resistance (CTOD-R) curves. The development work included investigation of the following items: specimen geometry, crack geometry and orientation (including crack depth effects), direct measurement of CTOD. The results demonstrate that toughness measurements obtained using a B = W configuration (B = specimen thickness, W = specimen width) with side grooves are similar to those using a B = 2W configuration without side grooves; however, specimens with side grooves and B = W geometry facilitates even crack growth. Studies of crack depth have shown that ductile fracture resistance decreases with increasing ratio of the initial crack depth to specimen width, a0 /W. Studies of notch location and orientation (outer diameter (OD) and inner diameter (ID) surface notches and through-thickness notches) have shown an effect of this variable on the CTOD-R curves. This has been partly attributed to crack progression (tearing direction) with respect to weld geometry and this effect is consistent with damage modeling predictions. However the experimentally observed difference of CTOD-R curves between ID and OD notches is believed to be primarily due to the material variability through the pipe thickness.Copyright

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

Continued Advancements Regarding Capacity Prediction of Strain-Based Pipelines

In areas of large ground movements, pipelines may be subjected to large longitudinal strains. It is imperative that strain-based design methods are developed for such pipelines. As reported previously, a comprehensive experimental and numerical program to characterize the tensile strain capacity of welded pipelines was undertaken. Models were developed that are capable of predicting strain capacity based on input parameters such as pipe geometry and properties, internal pressure, weld flaw geometry, weld properties, and high-low misalignment. These models (equations) have been validated against a data base of about 50 full-scale pipe strain tests that included a broad range of geometries and pipe grades (8–42″, 13–25mm, X60–X80). In the current paper, further developments are described. A pressure factor has been incorporated into the models. Whereas the previous models assumed that the circumferential stress from internal pressure was 80% of the specified minimum yield strength (SMYS) of the pipe, the pressure factor allows the calculation of strain capacity as a function of pressure that results in hoop stresses from zero to 80% of SMYS. Additionally, ranges for pipe yield-to-tensile ratio and weld tearing resistance curves (R-curves) have been expanded. New equations and associated flaw assessment diagrams for example cases are provided.Copyright

Strain Capacity Prediction of Strain-Based Pipelines

Strain-based design (SBD) is used to complement conventional allowable stress design for pipelines operated in environments with potentially large ground movements such as those found in permafrost and seismically active regions. Reliable and accurate prediction of tensile strain capacity (TSC) plays a critical role in strain-based design. As reported previously over the past 6+ years, a comprehensive experimental and numerical program was undertaken to characterize the TSC of welded pipelines, develop a finite element analysis (FEA) approach and equations capable of predicting TSC, and establish a strain-based engineering critical assessment (SBECA) methodology. The previous FEA model and TSC equations were validated against about 50 full-scale pipe strain capacity tests and are accurate within the validated variable ranges. In the current paper, enhancements of the previous model and equations are described. The enhancements include incorporation of advanced damage mechanics modeling into TSC prediction, development of a new TSC equation, expansion of variable ranges and functionality upgrades. The new model and equation are applicable over larger ranges of material properties and flaw sizes. The new FEA model is also used to establish surface flaw interaction rules for SBD. The new FEA model is validated against more than 40 full-scale non-pressurized and pressurized tests and underpins the development of the new TSC equation. The equation is validated against a total of 93 full-scale tests (FST). In addition to the enhancements, sample applications of the TSC model and equation are presented in the paper, for example, an investigation of the effects on strain capacity of Luders strain and ductile tearing. Challenges in predicting TSC are also addressed.Copyright

Flexible Friction Stir Joining Technology

Reported herein is the final report on a U.S. Department of Energy (DOE) Advanced Manufacturing Office (AMO) project with industry cost-share that was jointly carried out by Oak Ridge National Laboratory (ORNL), ExxonMobil Upstream Research Company (ExxonMobil), and MegaStir Technologies (MegaStir). The project was aimed to advance the state of the art of friction stir welding (FSW) technology, a highly energy-efficient solid-state joining process, for field deployable, on-site fabrications of large, complex and thick-sectioned structures of high-performance and high-temperature materials. The technology innovations developed herein attempted to address two fundamental shortcomings of FSW: 1) the inability for on-site welding and 2) the inability to weld thick section steels, both of which have impeded widespread use of FSW in manufacturing. Through this work, major advance has been made toward transforming FSW technology from a “specialty” process to a mainstream materials joining technology to realize its pervasive energy, environmental, and economic benefits across industry.

Study of Mechanical Properties and Characterization of Pipe Steel welded by Hybrid (Friction Stir Weld + Root Arc Weld) Approach

Friction stir welding (FSW) has recently attracted attention as an alternative construction process for gas/oil transportation applications due to advantages compared to fusion welding techniques. A significant advantage is the ability of FSW to weld the entire or nearly the entire wall thickness in a single pass, while fusion welding requires multiple passes. However, when FSW is applied to a pipe or tube geometry, an internal back support anvil is required to resist the plunging forces exerted during FSW. Unfortunately, it may not be convenient or economical to use internal backing support due to limited access for some applications. To overcome this issue, ExxonMobil recently developed a new concept, combining root arc welding and FSW. That is, a root arc weld is made prior to FSW that supports the normal loads associated with FSW. In the present work, mechanical properties of a FSW + root arc welded pipe steel are reported including microstructure and microhardness.

A Case Study in High Strain Capacity Pipeline Qualification: PNG LNG Project

The onshore pipeline portion of the Papua New Guinea Liquefied Natural Gas (PNG LNG) project traverses terrain with seismically active faults with potential ground displacements up to four meters. The resulting longitudinal strain demand exceeds 0.5% strain, thereby requiring use of strain-based pipeline design (SBD) technology. This paper discusses the application of previously developed strain-based design methodologies to successfully qualify the PNG LNG pipeline system for a design tensile strain demand up to 3%, and flexibility to increase the design strain demand with additional restrictions on key variables impacting strain capacity at select locations. Key SBD pipeline qualification activities are discussed along with the required project timeline. The first activity is specifying, evaluating and procuring line pipe suitable for strain-based design. SBD line pipe must be strain-age resistant, have excellent longitudinal uniform elongation, and have tightly controlled ultimate tensile strength (UTS) limits to ensure robust girth weld overmatch. The girth welds must exhibit upper shelf fracture toughness, excellent tearing resistance, and have sufficient tensile strength to ensure adequate girth weld strength overmatch. The pipeline qualification effort culminates in full scale pipe strain testing as proof of performance. The specimens for these tests are fabricated with project-specific pipe, girth welds, and pipe fit-up (hi-lo misalignment). The girth welds contain machined flaws in both weld metals and heat affected zones, these flaws being sized consistent with acceptable flaw sizes predicted from analytical models and prior experience. The results of these tests and their significance are described. Efforts to reduce capacity through lowering strain demand are outlined, along with examples of construction challenges the project has successfully faced. Key engineering and project decisions, and lessons learned from this qualification effort are also detailed.Copyright

Advanced Strain-Based Design Pipeline Welding Technologies

To meet the increasing worldwide demand for natural gas, there is a need to safely and economically develop remotely located resources. Pipeline construction is a major activity required to connect these remote resources to markets. Such pipeline routes may cross areas containing geohazards such as discontinuous permafrost, active seismicity and offshore ice gouging. These pipelines may be subjected to longitudinal strains above 0.5%. To safely design pipelines for such conditions, a strain-based design (SBD) approach can be used in addition to conventional allowable stress designs (ASD).Significant pipeline construction cost savings can be achieved with the use of higher strength steels (X70+) due to reduced pipe wall thicknesses (less steel) and faster girth welding. However, a robust welding technology for higher strength SBD pipelines is often a technology gap depending on the target level of longitudinal strain that needs to be accommodated, since such applications often demand excellent weld toughness at low temperatures (−15°C) and high tensile strength (>120ksi). This paper discusses the development of an enabling welding technology that offers a superior combination of strength and toughness compared to commercially available technologies.Acicular ferrite interspersed in martensite (AFIM) has been previously identified as a useful high strength weld metal microstructure that can be applied in field pipeline construction. This paper describes how this microstructure has been used to create welds with excellent strength overmatch and good ductile tearing resistance for X80 SBD pipelines. This approach has been implemented for mainline, double-joining and repair welding applications. This paper describes the welding procedures, mechanical properties achieved, estimated strain capacities, and the results of a full-scale pipe strain capacity test.Copyright

Godin Lake Trial: X120 Field Welding

In February 2004, ExxonMobil and TransCanada PipeLines, Limited (TCPL) cooperated to construct a 1.6 km long segment of X120 pipeline near Wabasca, Alberta. The line included numerous mainline and tie-in welds, and an extra section of pipe was constructed in the field for later destructive testing to assess the characteristics of field welds. This line represented the first field application of welding procedures and consumables developed specifically for X120. The technologies used performed well; productivity and weld repair rates were better than expected, and the properties of these welds made in challenging conditions compare well with development welds made under controlled “shop” conditions.Copyright