Finite Element Analysis of Local Strength in Elbows Under External Loads

Literature Overview

The paper by Lei Junjie, Wang Ruiying, Wang Xuecui, Jia Jinyin, Chu Wenshan, Zhang Weiwei, Shao Yanhong, and Li Shuocun from the Design Company of Offshore Oil Engineering Co., Ltd., published in Liaoning Chemical Industry (2013, Vol. 42, No. 5, pp. 506-509), presents a finite element method (FEM) study of the local strength of a DN100 SCH160 90-degree long-radius elbow under combined loading conditions. The study is particularly relevant to offshore platform piping systems where elbows are subjected to complex multi-axial loading from internal pressure, axial force, bending moment, and torque.

Technical Approach and Methodology

The authors developed a computational model using APDL (ANSYS Programming Language) command streams to automate the local strength assessment of the elbow. The study object is a DN100 (nominal diameter 100 mm) SCH160 (wall thickness schedule 160, approximately 14.27 mm for DN100) 90-degree long-radius elbow. The combined loading conditions include:

The APDL command stream approach is noteworthy because it enables parametric studies and batch processing of multiple load cases, which is essential for comprehensive design verification in offshore piping where numerous elbows must be assessed under various load combinations.

Stress Distribution and Local Strength Assessment

The FEM analysis revealed several important stress distribution characteristics:

1. Stress concentration at the inner arc: The inner arc of the elbow, where the pipe wall is subjected to compressive stresses under bending, showed elevated stress levels. This is consistent with the well-known behavior of curved pipe elements under bending loads.
2. Local thinning effects: Areas of the elbow wall that were locally thinned (due to manufacturing tolerances, corrosion, or forming-induced thinning) exhibited significantly higher stress levels than the nominal wall thickness regions.
3. Elbow-support interaction: The study specifically examined the stress distribution at the interface between the elbow and its support structure. The constraint imposed by the support created additional stress concentrations that could compromise the local strength.
4. Combined loading effects: The superposition of multiple load types created complex stress states that could not be adequately assessed by simple hand calculations or standard code methods.

Code Compliance and Design Considerations

The local strength assessment methodology presented in this study complements the requirements of offshore piping design codes such as API RP 2A, DNV-RP-F304, and ASME B31.4. These codes generally employ stress intensity factors and stress reduction factors to account for the stress concentration effects of elbows. However, the FEM approach provides a more detailed and accurate assessment, particularly for:

• Thick-walled elbows (SCH160 and above) where code formulas may be less accurate
• Complex support configurations that create non-standard boundary conditions
• Combined loading scenarios that exceed the scope of simplified code methods
• Local thinning due to corrosion or manufacturing defects

Engineering Practice Integration

For offshore platform piping design, the following practices are recommended based on the insights from this study:

1. Parametric FEM analysis: Use APDL or similar scripting capabilities to systematically evaluate elbows under various load combinations, including extreme loads (seismic, storm, fire) and fatigue loads.
2. Support optimization: Design elbow supports to minimize stress concentrations while providing adequate restraint. The study highlights the importance of proper support placement and design.
3. Corrosion allowance: For thick-walled elbows in corrosive service, the FEM model should include corrosion thinning scenarios to verify that adequate strength remains after corrosion allowance is applied.
4. Fatigue assessment: The stress distribution data from the FEM analysis can be used as input for fatigue life assessment, particularly at locations of high stress concentration.

Key Questions and Reflections

The study raises an important question about the applicability of code-based stress assessment methods for thick-walled elbows. For SCH160 elbows, the wall thickness to diameter ratio is relatively high, and the assumptions underlying code formulas (which are typically derived for thin-walled pipe) may not be fully applicable. The FEM approach provides a more rigorous assessment, but the question remains of how to integrate FEM results into the code compliance process.

Additionally, the study focuses on elastic stress analysis. For elbows subjected to extreme loads that may cause plastic deformation, elastic FEM results may overestimate the actual strength. A follow-up study incorporating material nonlinearity (elastic-plastic analysis) would provide a more complete picture of the elbow's structural capacity.

Study Insights and Implications

This paper demonstrates the value of FEM-based local strength assessment for offshore piping elbows, particularly for thick-walled fittings under complex multi-axial loading. The use of APDL command streams for automated analysis is a practical approach that can be readily adopted by design engineers. The study reinforces the importance of considering elbow-specific stress concentrations in design and highlights the limitations of simplified code methods for thick-walled elbows. Engineers should consider FEM analysis as a supplement to, rather than a replacement for, code-based design methods, using it to verify critical elbows and to optimize support design.