Single sided welding (welding from the outside only) is used as the principal method for connecting braces to chords for tubular joints in offshore structures in areas of the world other than the North Sea. Single sided welding presupposes that the fatigue crack always initiates at the weld toe. However, if the stress concentration factor (SCF) at the internal weld root of a single sided weld is relatively high compared to the SCF at the external weld toe (say SCFin = 0.8 *SCFex, then the crack may initiate at the weld root due to the more onerous S-N curves relevant for the root detail. To investigate the acceptability of considering only the outside of single sided welds in rather stiff tubular joints, an analysis program of K- joints has been performed. The program included FEM analysis of 9 K-joints and comparison of the results to other known data. The investigations showed that there were joint geometries that indeed gave relatively high SCFs on the inside compared to the outside such that fatigue initiating from the inside should be considered for certain joint geometries. This paper gives a description of the analysis program and summarises the results. Guidelines for determining when the weld root stresses is a real design consideration will be given.
Tubular joints have traditionally been made by welding the braces directly onto the chord without using a brace stub. The weld was therefore single sided, i.e. welded from the outside only. The root of the single-sided weld would contain notches and smaller defects, but this was not seen to be a problem since the joints nearly always failed from a crack initiated and growing from the outside. As the tubular dimensions got bigger and the joints were subjected to significant fatigue loading, as for the jackets in the North Sea, the tubular joints were made as separate structures.
