Pipe Stress

From Design to Root Cause Failure Analysis 

A pipe stress analysis checks for compliance with the relevant design codes and standards. A code compliant design is important to reduce the chance of potential failures in the future. A pipe stress analysis is therefore an essential part of the design for any industrial system that contains piping.
If a failure did occur, we can help figure out why it happened, how to solve it, and how to prevent similar failures in the future. Failures in a piping system have many different causes, ranging from a lack of flexibility, to transient forces due to water hammer. It can be difficult to identify the cause of a particular failure and to know what needs to be taken into consideration for your system. Especially when dynamic effects are at play, the cause may not always be obvious.
DRG has expertise in both: 

1. Static Analysis: Piping Flexibility Study

DRG conducts piping flexibility assessments using the software Caesar II from Hexagon. We have a thorough understanding of the software and are also an official Caesar II training provider for Hexagon.
The main objective of a piping flexibility study is to keep code stresses within the allowable limits set by the relevant codes and standards. DRG conducts assessments in conformance with industry standard American ASME B31 and European EN13480 design codes. When required we can also perform the analysis for wide range of alternative design codes, such as RToD Stoomwezen (Dutch code) or PD 8010. Fiberglass systems are assessed for conformance with ISO 14692, please see our fiberglass service for more details.
The code stress is of course not the only criterion that should be considered. Loads on supports, nozzles, and flanges are also assessed. Support and nozzle loads will be compared with provided allowable loads, while the equivalent pressure on the flanges is compared with the pressure rating tables in AMSE B16.5 or EN1092. Special attention will be paid to the support configuration and supporting structure should large loads be unavoidable. When necessary, supports, nozzles and flanges can be analyzed in more detail using FEA.
Our engineers are adept at taking special conditions into account. Whether the piping is buried, on an offshore plant or FPSO, or when cold service means that the risk for brittle fracture needs to be evaluated; we will make sure that all the relevant aspects of the analysis are considered. The recent code improvements in Stress Intensifications Factors (SIFs) and flexibilities for tees and bends as per ASME B31 J (2017) are also included in our approach, which is especially applicable for thin-walled piping.

2. Dynamic Analysis

Engineering does not always stop after the static analysis of your piping system. If there is a risk of dynamic forces on the piping, then these should also be considered at the design stage. Dynamic excitation can be either mechanical, such as direct forcing by vibrating equipment or seismic activity, or fluid driven such as pulsations or water hammer within the pipe.
If excitations are expected, or if vibrations are observed in the field, DRG can be your partner in achieving a robust system, by conducting a dynamic analysis. A dynamic stress analysis is often an extension to a static analysis. However, running a dynamic analysis is not just a case of adding dynamic loads to your static model. Factors such as support stiffness, gaps and grid resolution have a significant impact on the results.
There are three approaches for a dynamic piping analysis, which we would perform using Caesar II or FEA software such as ANSYS or ABAQUS:
Option 1: Modal analysis
A modal analysis is the starting point for a dynamic analysis, and can determine the mechanical eigenmodes (resonance modes) of the system. This gives an insight into how susceptible a system is to excitation by a dynamic loading.
Option 2: Cyclic Loading, fatigue failure is a risk
When a periodic loading is present, for example when a forced excitation by equipment is present, the possibility of fatigue failure needs to be evaluated.
For this analysis, the loads will be applied periodically at a given frequency. It will then be calculated how much a given mechanical resonance frequency will be excited. The resulting code stresses will be checked for conformance with the relevant design fatigue curve such as those in ASME B31.3 section 302.3.5 (d).
Option 3: Single event worst case loading, pipe rupture is a risk
In the case of water hammer/pressure surges (view our surge analysis services), the excitation force is not periodic (i.e. the frequency of events is much lower that typical Eigenfrequency of piping system) and instead the risk to the piping is a large magnitude single event. In this case DRG will apply the dynamic loading as a function of time in a time history analysis. The arising stresses, and flange and nozzle loadings will be evaluated against occasional loading limits from the appropriate design code.