RESIDUAL STRESSES IN PIPELINES

The structural integrity and cost of pipelines are of major concern in oil, chemical, and other industries. Pipelines can be subjected to severe thermal, seismic, pressure, and other mechanical loads, and for this reason an increasing amount of attention has been given to their analysis.

Residual stress has got the attention due to the reason it has great effect on the performance and life of the component. It has got more significant importance when considered for pipelines as it plays an important role in issues as Stress Corrosion Cracking (SCC), Hydrogen Induced Cracking (HIC), Fatigue Cracking, Welding Stresses, Heat Treatment Effectiveness, Surface Enhancements due to Cold Work, Bending due to Seismic Activity and Installation Stresses. For a perfect design beneficial stresses should be induced into the component whereas the effect of harmful stresses should be minimized.

Welding being the most important industrial process mostly being used in ship building, pipeline fabrication etc. Residual stresses arising after welding exert a considerable influence on the service characteristics of welded equipment and their control allows to avoid failure of welded joint. The influence of residual stresses on service characteristics of welded equipment has been analyzed by many scientists.

J.T. Assis, V Monin_, J.R.Teodosio, T. Gurova  presented the experimental results of residual stress measurements by X-ray tensometry method. He concluded that the that stress measurements on the outer surface of the welded joint are not enough to examine residual stress states in weld regions and it confirms the need to study stress distribution along the depth. This can be accomplished by means of stress measurements after surface layer removing. It is clear that in the case of layer removal by machining or grinding it is necessary to undertake electropolishing to remove residual stresses introduced by machining.

Analysis of equilibrium equations for circular weld seam is more complicated but the importance of knowing the stress distribution along the depth is obvious. Stress measurements after surface removal are presented in figure 2. The stress distributions for the samples with a linear circular weld seams show that the compressive residual stresses at the centre of the weld seams reverse to tensile stresses after removing a surface layers approximately equal to 0,5 mm.