Finite Element Analysis of Plastic Limit Internal Pressure for Partially Thinned Buried Tees

Literature Overview

This paper, published in Pipeline Technology and Equipment (2010, Issue 1, pp. 34–36), presents a finite element analysis of the plastic limit internal pressure for buried equal tees with local wall thinning defects on their outer surface. The authors from Wuhan Institute of Technology and China University of Petroleum (Beijing) used ANSYS elastic-plastic FEA software to investigate how defect size and position affect the plastic limit load and failure mode of buried tees. This work is directly relevant to pipeline integrity assessment, where local corrosion or mechanical damage leads to wall thinning that must be evaluated for remaining strength.

Background and Motivation

Buried pipelines are subject to various degradation mechanisms including external corrosion, cathodic protection interference, and mechanical damage from excavation or settlement. Local wall thinning is one of the most common forms of degradation, and its effect on structural integrity must be quantified to determine whether a component can continue in service, requires repair, or must be replaced. Tees, as pressure-containing components, are particularly vulnerable because the geometric discontinuity at the branch intersection creates stress concentrations that interact with wall thinning defects.

The plastic limit load represents the maximum internal pressure a component can sustain before undergoing irreversible plastic deformation. Determining this value for components with local thinning is essential for fitness-for-service assessments.

Methodology

The authors used ANSYS with an elastic-plastic material model that incorporated both geometric and material nonlinearities. The analysis considered a buried equal tee subjected to internal pressure only, with local wall thinning applied to the outer surface of the main body.

Defect Parameters Varied

Failure Assessment Approach

The plastic limit load was predicted using the load-strain diagram obtained from the corrosion zone. This approach is consistent with the limit load method used in fitness-for-service assessments, where the transition from elastic to fully plastic behavior defines the limit load.

Key Findings

Effect of Defect Size

The plastic limit load decreases as the defect size increases. This is intuitively expected, as larger defects remove more material and reduce the load-bearing cross-section. However, the relationship is nonlinear: small increases in defect size can lead to disproportionate decreases in plastic limit load, particularly when the defect approaches a critical size relative to the wall thickness.

Effect of Defect Position

The position of the defect along the main body bottom significantly affects the plastic limit load. Defects located at or near the branch intersection experience higher stress concentrations and therefore have a more detrimental effect on the plastic limit load than defects located farther from the intersection. This finding is consistent with the stress distribution in tees, where the branch intersection is the region of highest stress concentration.

Failure Modes

The analysis identified different failure modes depending on defect size and position. For small defects, the failure mode is characterized by localized yielding at the defect site. For larger defects, the failure mode transitions to a more global plastic collapse, where the entire cross-section yields and the component loses load-bearing capacity.

Engineering Practice Integration

The results of this study have direct applicability to pipeline integrity assessment programs. In practice, operators use inline inspection (ILI) tools to detect and characterize wall thinning on buried pipelines. The detected defects are then assessed using methods such as the B31G or modified B31G methods, or more sophisticated methods such as the ASME B31G-1991 standard or the API 579 fitness-for-service framework.

The finite element approach presented in this paper provides a more accurate assessment than simplified analytical methods, particularly for defects located near geometric discontinuities such as tees. The interaction between the stress concentration from the tee geometry and the local wall thinning can lead to a more severe degradation of remaining strength than would be predicted by methods that consider the defect in isolation.

Practical Assessment Workflow

1. Defect characterization: Use UT or ILI data to determine defect depth, width, and position relative to the tee geometry.
2. Finite element modeling: Create a detailed FEA model of the tee with the actual defect geometry applied.
3. Load-strain analysis: Apply internal pressure and extract the load-strain curve from the defect region.
4. Limit load determination: Identify the plastic limit load from the load-strain curve.
5. Safety factor evaluation: Compare the plastic limit load with the operating pressure and apply an appropriate safety factor.

Key Reflections

This study demonstrates the value of finite element analysis in pipeline integrity assessment, particularly for complex geometries where simplified analytical methods may be inadequate. The tee geometry introduces stress concentrations that interact with wall thinning defects in ways that are difficult to capture with simplified methods.

One limitation of the study is that it considers only internal pressure loading. In practice, buried pipelines are also subject to external soil loads, traffic loads, and bending moments from pipeline deflection. A more comprehensive assessment would include these additional load cases. Furthermore, the study does not address the effect of material properties degradation due to aging or corrosion, which can further reduce the plastic limit load.

The approach of using the load-strain diagram from the corrosion zone to predict the plastic limit load is a practical and effective method, but it requires careful interpretation. The definition of the plastic limit load must be clearly stated, and the safety factor applied should account for uncertainties in defect characterization, material properties, and loading conditions.