6. What Is A Pipe Stress Analysis And Why Is It Important?
- Ovolifts
- Mar 19
- 5 min read
A pipe stress analysis is an engineering assessment that evaluates the forces, loads, and deformations experienced by a piping system during its operation, ensuring the pipes can safely handle the stresses imposed by various factors such as internal pressure, temperature fluctuations, external loads and in our case, displacements caused by lifting.
This analysis is conducted following established codes like ASME B31.3 or B31.4, which provide the necessary safety limits to prevent material failure, leaks, or excessive deflection in pipelines.
When lifting in-service piping, the importance of a pipe stress analysis becomes paramount. In these situations, the line is carrying fluid or gas, and the piping system is subject to operational stresses. Lifting the pipe to access corroded touchpoints and to perform repairs introduces additional stresses, particularly at the lift points and adjacent areas. Without proper analysis, these added stresses can lead to failures, including cracks or leaks at damaged locations, nearby flanges, fixed points or branching connections.
6.1 Key Elements of Pipe Stress Analysis
Pipe stress analysis considers various factors to assess how a pipeline responds to different types of loads and stress conditions. Some of the most critical elements include:
6.1.1 Nominal Pipe Size And Wall Thickness
Piping comes in various sizes and wall thicknesses, each suited for specific applications. The pipe's size, or bore, is often chosen based on the volume of product that needs to be transported. Larger bore pipes can transport more product but are more rigid and vulnerable to bending stresses during lifting operations. They also require stronger support structures due to their weight.
Smaller bore pipes are more flexible, can better resist external loads, and are suitable for high-pressure systems.
Wall thickness impacts a pipe's strength and must meet industry standards, such as those set by API 570, to safely handle internal pressure and external forces. Additionally, the wall thickness includes a corrosion allowance to account for a nominal amount of wall loss over a defined useful life of the system. This ensures long-term durability, especially in harsh environments. A proper stress analysis ensures safe lifting and handling of pipes, minimizing the risk of deformation or failure during operations.
6.1.2 Internal Pressure:
The pressure of the fluid or gas inside the pipe affects how the pipe wall behaves under load. Higher internal pressures increase hoop stress and will sooner reach the stress limit allowable for a given lift height compared to a lower pressure system.
6.1.3 SG of Contents
The Specific Gravity (SG) of the contents refers to the density of the product inside the pipe using water density as a baseline. Water has an SG = 1.0, oils are around SG = 0.9 and gasses are in the range of SG = 0.05
Accounting for the SG of contents is important for accurately calculating the pipe weights and corresponding reaction loads during lifting. This is especially important for larger bore piping where the weight of the contents can more than double the weight of the pipe.
6.1.4 External Loads:
In addition to the weight of the pipe and the contained fluid, there can be external loads applied to the piping such as those created by thermal expansion / contraction. Loads are applied to the piping when it’s being lifted and some less common factors can include wind or seismic activity.
6.1.5 Guides and Fixed Points
Piping systems expand and contract throughout their life span. This occurs as the piping heats and cools either by the changing ambient temperatures over daily and seasonal periods but also from changes in temperature of fluid contained in the piping, especially during start/stop operations.
Piping systems are designed to accommodate this expansion/contraction by building in sections of flexibility such as expansion loops, but in other areas the piping must be fixed to the support or joining equipment to ensure the piping has a controlled range of movement.
Various types of guides and anchors are used to achieve this level of control such as longitudinal guides, lateral guides, fixed points and spring hangers etc. All of which exert external loads onto the piping and must be considered in a piping stress analysis.
6.1.6 Lifting Heights and Lift Locations
Where a pipe is lifting in relation to the surrounding geometry, together with the amount of displacement that is applied are both crucial elements of a pipe stress analysis.
When lifting a pipe in-service, selecting the appropriate lift points is crucial. Incorrect placement of lifting jacks can create localized stress concentrations, increasing the risk of pipe failure. Pipe stress analysis allows engineers to model the lift and identify points of concern before the actual lifting operation.
6.2 Corrosion Allowance
A corrosion allowance refers to the additional thickness added to the pipe wall during the design phase to account for material loss due to Corrosion Under Pipe Supports (CUPS), also known as Touchpoint Corrosion or Contact Point Corrosion. This allowance ensures that the pipe maintains its structural integrity over the period of the design life and meets safety requirements even as material is lost due to corrosion.
The nominal wall thickness of the pipe includes the corrosion allowance in addition to the thickness required for withstanding internal pressure and other stresses.
The minimum wall thickness for analysis excludes the corrosion allowance since it represents the pipe's condition after the expected material loss.
During a stress analysis, the reduced thickness (nominal thickness minus corrosion allowance) is used for:
Pressure-induced stresses: Thinner walls result in higher hoop stresses, longitudinal stresses, and axial stresses.
Flexibility and displacement analysis: Reduced wall thickness affects the pipe's stiffness and flexibility.
Example:
If a pipe operates in a corrosive environment where the expected corrosion rate is 0.1 mm/year, and the design life is 20 years, the corrosion allowance might be set at 2 mm (0.1 mm/year × 20 years). This allowance ensures that after 20 years, the pipe can still safely withstand its operational loads.
6.3 Codes And Standards
Industry standards, such as ASME B31.3 for process piping and ASME B31.4 for liquid transportation systems, govern the safe design and operation of pipelines, including during maintenance activities like lifting. These codes provide guidelines for allowable stress limits based on material properties, pipe thickness, and operating conditions. A pipe stress analysis ensures that the lifting operation remains within these allowable limits, safeguarding both the pipeline and personnel.
Moreover, these codes outline the procedures for modeling complex scenarios, such as multi-point lifting, where the line is supported at several points simultaneously. This approach, informed by stress analysis, reduces the risk of overstressing specific sections of the pipe, particularly near corroded touchpoints or areas with significant wall loss.
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