| dc.description.abstract | Fatigue-induced cracks in tubular joints represent one of the major threats to the structural
integrity of offshore infrastructure. Consequently, developing and researching efficient
temporary and permanent repair solutions is essential. Investigation of various technics and
methods is ongoing, and the recent advancement in the exploration of crack deflection holes in
combination with weld-toe grinding has made a foundation for further investigation of the
method.
This thesis investigates the effectiveness of delaying crack propagation in tubular joints
subjected to high cycle fatigue with crack-deflecting holes combined with weld-toe grinding.
Furthermore, a numerical analysis has been conducted to facilitate a better comprehension of
the stress field and stress evolution inherent in the experimental work.
One tubular double T joint was tested experimentally through a three-phased testing scheme.
In the initial phase, the specimen underwent cyclic axial loading yielding stresses within the
high cycle fatigue regime to the point of achieving a through-thickness crack. The load range
throughout the pre-cracking phase was maintained constant with an R-ratio of 0.1. During this
phase, an investigation of the stress concentration factor of the intact specimen was also carried
out. The next phase of the testing program comprised repairing the cracked specimen by the
use of crack-deflecting holes. These crack-deflecting holes were drilled in the crack tip
proximity, and the weld toe behind the drilled hole was ground according to DNV-RP-C203
and BS 7608 specifications. In the final testing phase, the performances of the repaired
specimen were investigated by subjecting the specimen to further cyclic loading. As the crack
induces higher stress concentrations, the load range was reduced to keep the stress levels within
the parameters of high cycle fatigue. Upon attaining a clear indication of enhanced fatigue
endurance post-repair, the load range was significantly increased to facilitate the examination
of subsequent crack initiation and propagation.
In this study, the crack was successfully arrested in the crack-deflecting holes. Furthermore, the
repair method indicated an apparent enhancement of the fatigue endurance of cracked tubular
joints subjected to stress ranges within the high-cycle fatigue regime. Finally, the recent
discovery of reverse coalescence in conjunction with crack initiation after repair with crack
deflecting holes was confirmed for both hole reparations in the experimental work. Although a
single test is not a proof of anything in fatigue, the results are promising and may justify further
investigation. | |
