Life Extension of Welded Structures Using HFMI Techniques - Potential Application to Offshore Structures

Fatigue damage development in welded structures is a local phenomenon and if one need to achieve an extension of the life for the structure local post weld improvements need to be use in order to reduce/remove local features which contribute to the fatigue damage. In order to enhance the life time of load carrying welded structures without large amount of cost investments, e.g. redesign and replacement of existing structures, post weld improvement techniques need to be more applied. New High Frequency Mechanical Impact (HFMI) technologies have been developed in the last 10 years which enables cost-effective life extension and reparation of welded structures. The use improvement techniques for technical life enhancement upgrade and repair of welded structures within various industries, e.g. oil and gas, have been an accepted practice. HFMI treatment techniques are based on localized peening process of the welded joints and the devices are portable. The impacting results in a local cold plastic deformation which remove weld defects reduce stress concentration and induce compressive residual stresses which eventually will enhance the fatigue life of the welded structure. An overview of existing improvement techniques for welded structures is given followed by description of new technologies (HFMI). A brief description of the new international guideline and design recommendations within the International Institute of Welding (IIW) is given. Several validation studies on the fatigue performance of HFMI techniques are presented and onsite potential applications of the techniques for joints in fixed offshore structures are outlined.