TC PipeLines

This study presents a detailed assessment for the response of elbow elements in ABAQUS under internal pressure. Two main cases are considered: (1) a standalone 90 deg pipe bend, which was analyzed using both elastic and elasto-plastic material characterizations using shell and elbow elements; and (2) a geometrically nonlinear analysis with a materially elastic characterization of a piping system was conducting using shell, elbow, and pipe elements. Although the results show the capabilities of elbow elements to simulate the response of pipe bends in the elastic regime, the elements do not provide reliable predictions beyond the elastic range. Additionally, an assessment of the ASME B16.49 2017 elbow thickness equation and previously published stress estimate equations was carried out by comparisons with elbow and shell elements predictions.

The cohesive zone model (CZM) is one of the most widely used damage models to describe the fracture processes of brittle and ductile materials, and has been usually combined with the conventional finite-element method (FEM). CZM in the context of the more effective extended finite-element method (XFEM) has recently been implemented in many applications, but it has not been widely used for crack propagation of pipelines. This paper aims to investigate the capability of the XFEM-based cohesive segment approach implemented in Abaqus to predict crack propagation of pipelines by calibrating a linearly decreasing traction-separation law with two damage parameters, the maximum principal stress and the fracture energy. The damage parameters for vintage pipeline steel (API 5L Grade X52) were systematically calibrated and verified by comparing the numerical results with eight full-scale experiments of pressurized and circumferentially surface-cracked pipe specimens. A correlation between the damage parameters and material yield strength and fracture toughness is discussed and an investigation of mesh size sensitivity included.

The effects of mechanical factors on crack growth behavior during the second stage of high pH stress corrosion cracking in pipeline steel were investigated by applying several loading scenarios on compact tension (CT) specimens. The main mechanism for stage 2 of intergranular crack propagation is anodic dissolution ahead of the crack tip which is highly dependent on crack-tip strain rate. The maximum and minimum crack growth rates were 3 × 10−7 mm/s and 1 × 10−7 mm/s, respectively. It was observed that several factors such as mean stress intensity factor, amplitude, and frequency of loading cycles determine the crack-tip strain rate. Low R-ratio cycles, particularly high-frequency ones, enhance secondary crack initiation, and crack coalescence on the free surface. This mechanism accelerates crack advance on the free surface which is accompanied with an increase in mechanical driving force for crack propagation in the thickness direction. These findings have implications for pipeline operators and could be used to increase the lifespan of the cracked pipelines at stage 2. For those pipelines, any loading condition that increases the strain rate ahead of the crack tip enhances anodic dissolution and is detrimental. Additionally, secondary crack initiation and coalescence could be minimized by avoiding internal pressure fluctuation, particularly rapid large pressure fluctuations.

The RT61 is a three-stage industrial power turbine which couples with the SGT-A35 aeroderivative gas generator (formerly named Industrial RB211). It was designed for improved efficiency and modular construction for ease in maintainability. The aeroderivative SGT-A35 (GT61) product serves both oil & gas and power generation market with a fleet size of greater than 90 units. In recent years, there has been increased emphasis on clean energy, only complemented by the regulatory changes and market conditions. The power generation and oil & gas customers (upstream, midstream, and downstream) are continuously looking for opportunities to decrease their greenhouse gas emissions and reduce fuel consumption by improving the gas turbine cycle efficiency. The SGT-A35 (GT61) power turbine has >93% isentropic efficiency and industry standard overhaul schedule of 100,000 hours. However the potential for further cycle efficiency improvements and reduction in emissions exist by optimizing the power turbine capacity to a specific load range. This served as the main motivation for this technical work. This paper discusses the engineering efforts taken in implementing the above stated improvement and further optimizing the product for reduced emissions. The improvements are discussed on the product level and on the TransCanada Pipelines fleet level. A new power turbine variant was developed on a demanding timeline driven by the customer project. A detailed development project was undertaken to establish the new operating point, aerodynamic design, and the new geometry. It was optimized to the customer project-specific load range. During the manufacturing phase, novel rapid prototyping methods were used to achieve desired lead times. Flow path change was limited to the first stage vane to minimize the introduction of new risks and uncertainties.

Elbow fittings are manufactured using quenching and tempering heat treatment processes. Such fittings can occasionally exhibit localized regions with lower yield strength than the design target, potentially due to non-uniform heat treatment. This paper presents an analytical methodology to examine the influence of these localized lower yield zones on the load capacity of the affected pipe fitting. In parallel, full-scale testing has been performed to quantify the actual response of the elbows under a combination of different loading conditions. The experimental data is used to validate the analytical approach. Details of the analytical method include a two-fold criterion: a global failure based on elastic plastic stress analysis and a local failure based on the tri-axial strain limit per ASME Boiler and Pressure Vessel Code Section VIII, Division 2. This paper presents the details of the finite element model development, assessment procedure, validation and parametric analysis of the size and location of the low yield zones in the elbow fittings. The fittings are analyzed for three possible operating scenarios: internal pressure, internal pressure with opening moment and internal pressure with closing moment. To characterize the influence of the low yield zone on the strength of the pipe, a parameter termed as "effective yield strength" is introduced. This approach is further demonstrated and found suitable for predicting burst pressures of components with lower yield zones of various diameters and thicknesses. This assessment method can be further extended to assess other pipeline components that exhibit similar behavior.