Fatigue performance of over-stressed beadweld due to pulling on the bead

Abstract

The pipeline production rate and the pipeline’s superior mechanical properties are the crucial parameters for the offshore pipe laying company. Pulling on the bead, i.e. the action of moving the pipe on the firing line to the subsequent weld stall as the vessel moves forwards allows a fast and reliable pipe welding process. During this movement, the ends of the pipes are intermittently not supported and the bending moment in the weld is created as a consequence. Such operation imposes tensile loads in the welds, that are especially high for the 3 mm thick beadweld and the subsequent 3 mm thick hot pass (6mm thick in total). An important question arises whether imposing loads above the beadweld material yield capacity affects the overall fatigue performance of the girth weld. It is in the best interest of the offshore industry to assure that the pipeline welds properties during their 20-30 years operation lifetime are not adversely affected during its installation. This study investigates the effect of pulling on the bead on the weld’s fatigue behaviour.
The overstressing simulation of the test coupons, Finite Element Analysis (FEA) of the loaded beadweld and the small-scale specimens comparative fatigue study were conducted to investigate the effect of straining history on the full girth weld fatigue performance. Additionally, the holistic approach was implemented to investigate the consolidated effects of other factors: local weld root misalignment (Hi-Lo), weld porosity and microstructure quality. Four test coupons were welded together and overstressed to simulate the pulling on the bead action. To assess the stress-strain state in the beadweld and hot pass during the overstressing simulation the FEA analysis was done. The FEA results allowed to quantitatively determine the exact locations of the elastically and plastically strained beadweld material. Full welds were checked against the porosity and subsequently, 46 fatigue specimens were fabricated and tested in the 4-point-bending fatigue loading setup. Additionally, the fatigue crack observation techniques were implemented in order to separate fatigue crack initiation and growth phases out of the total fatigue life. The optical observations (with the optical-digital microscope) and the Potential Drop techniques were used. Tensile and hardness tests were done to compare the strength properties of specimens with different straining and porosity histories. Finally, the microstructure and fracture surfaces of the fatigue broken specimens were studied.
The research done in this study gives a good overview of the fatigue behaviour of the girth welds and provides insights into the straining history effects. Plastic straining was found not to affect the fatigue performance more than currently acceptable elastic straining does. Moreover, plasticity was found to reduce the negative effect of porosity. Additionally, the prevalent influence of Hi-Lo on the crack initiation phase was confirmed, making it the driving negative factor for the fatigue resistance.