Fatigue Life of a Pipeline Subjected to Cyclic Internal Pressure.
Year: Sept. 2009- Oct. 2011.
Internal pressure in gas pipes are fluctuating during their service lifes and it can start fatigue in these pipes. In this thesis, fatigue crack growth rate in a gas pipeline of API 5L-X70 with the weld regions are calculated. Two regions in the gas pipe are welded. First region is a longitudinal welding across the centerline of pipe witch has made when the pipe was producing. Second region is a circumferential welding region between two pipes witch merges two pipes in a pipeline. The residual stresses produced in the process of welding, strongly affect the crack growth rate. If residual stresses are tensile, the crack growth rate increases and fatigue life will be decrease.
The design formulations and methods for fatigue of the structures are based on S-N curves determined experimentally. This analysis method, determinate the total fatigue life of a pipe with any defect. But various events such as accident, earthquake and explosion may cause a crack on the body of pipes. So in this thesis, a semi elliptical crack with a predefined dimensions are applied to the welded regions of gas pipe, and the crack growth are determined by Linear Elastic Fracture Mechanics method. Previous researches show that stress intensity factor is the most important parameter to describe fatigue crack growth rate in Linear Elastic Fracture Mechanics. In this research ABAQUS software is used to simulate the crack in the pipe and calculate stress intensity factor in the crack front. Crack surfaces impelimented to the FE model of pipe, are in radius plane of pipe and one of elliptic diameters is in the internal surfaces of pipe. Calculated SIF's are compared to an analytical method results. Paris and Forman equations with suitable constant material parameters are used to calculate fatigue crack growth rate due to residual stresses and internal cyclic pressures in the pipe. These equations strongly depend on stress ratio and range of stress intensity factors. a step by step method is used to simulate fatigue crack growth and calculate fatigue life.
In this research a 2-D thermal elastic-plastic analysis in ABAQUS has been done to calculate the residual stresses magnitude and distribution along circumferential weld region. Goldak function is used to apply heat flux to the FE model. A DEFLUX subroutine has performed for applying heat flux to the finite element model and a FILM subroutine has been used to simulate thermal boundary conditions. This analysis shows that residual stresses along weld direction, near the center line of weld are comparable with yield stress. This tensile stresses can strongly increase crack growth rate in this region.
Fatigue life of two welded regions is compared. The results of the comparison show that circumferential welded regions are the most critical region in the pipe in fatigue crack growth. The number of load cycle is showed versus crack dimensions in suitable charts, and fatigue life of the pipes with a crack can be estimated by using these charts. The result of simulating fatigue crack growth shows that 2-D crack modeling for investigating fatigue life may causes an error in the results and it suggests to model 3-D crack for assessment of fatigue life.
Keywords: fatigue life, welding residual stress, stress intensity factor, pipelines.