S. Kibey

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

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