A method developed by TWI for assessing the fitness-for-purpose of root defects in single-sided welds has been shown to produce results in agreement with the observed life of a service failure offshore.
Jacket structures using pre-fabricated nodes inevitably carry single-sided butt welds made from the outside. These closure welds are required either as circumferential girth welds, or to close access windows cut in the braces. When made without backing bars, these welds have a low fatigue classification in current design rules because of uncertainty about the effects of misalignment and poor root profile.
It is not possible to use conventional backing bars when making a girth weld from the outside only. Backings can be used when closing an access window, but at an increased cost.
There is thus great economic incentive to assess closure-weld defects using fitness-for-purpose criteria.
TWI has developed a method for assessing isolated defects that have been detected by inspection. It also provides guidance on inspection intervals, based on fracture mechanics, using integration of the fatigue crack growth law to predict the growth of a defect. Growth is predicted in two stages: from the initial defect to breakthrough; and then growth around the circumference.
A practical solution
The method described was applied to the failure by fatigue/ductile bearing of a brace in a North Sea jacket structure operated by Chevron UK Ltd. A crack had initiated at a root defect in a single-sided weld closing an access window. After penetrating the brace wall, the crack had grown around the circumference until final separation was by ductile tearing.
The initiating defect could be represented as a semi-elliptical flaw 15mm deep and 100mm long. Based on this, predictions for the two stages of crack growth (growth to a through-thickness flaw, followed by growth around the brace circumference) were made which agreed well with the observed behaviour of the joint.
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