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Tubular repair without welding

The results of a TWI study of fatigue behaviour of tubular joints show that, by grinding out shallow fatigue cracks, fatigue strength can be improved without any making good by welding.

With many offshore structures in North Sea fields reaching the middle years of their design lifetime it is not surprising that an increasing number of fatigue cracks are being reported. In tubular lattice constructions, cracking invariably occurs at the welded connections or nodes between tubular members, usually initiating at the weld toe and propagating through the tube wall.

A Group Sponsored Project investigated alternative methods of repairing such cracks, paying special attention to the fatigue life of the joint after repair.

Repair welding in which the crack is removed by gouging or grinding and the excavation made good by welding  was found to give an adequate fatigue strength. However, the high cost of welding offshore, especially if the casualty joint is under water so that a hyperbaric chamber has to be employed, makes it unattractive. Hence, repair solutions avoiding welding were also investigated. For part wall flaws the method showing most promise was 'remedial grinding' in which the crack is removed with high speed rotary burrs, and the resulting excavation is left unrepaired. The technique is similar to that widely used to treat weld toes to improve fatigue strength. The results demonstrated that, for shallow excavations extending around only a small percentage of the joint circumference, the fatigue strength after treatment was substantially better than that of the original joint. In other words, the removal of a shallow crack is analogous to the use of weld toe grinding as a life improvement technique.

As excavation length and depth increase, the fatigue strength after repair decreases and eventually drops below that of the original joint. However, it was found that for short excavations good fatigue performance could be obtained for depths as great as 60 per cent of the wall thickness.

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