Issue 9

C. M. Sonsino, Frattura ed Integrità Strutturale, 9 (2009) 3-12 ; DOI: 10.3221/IGF-ESIS.09.01 5 Load Sequence, COLOS) was derived from sea-state observations in the North Sea. One sequence with the size L s = 4.95 x 10 5 cycles corresponds to one year of service, Fig. 7. Figure 6 : Geometry of welded K-nodes. Figure 7 : Colos spectrum. All of the tested tubulars were instrumented in the critical areas with strain gauges and two failure criteria were defined for presentation of the test results: Fatigue life to the initiation of a crack with a depth of a  1.00 mm, detected by DC potential drop technique, and the fatigue life to break-through, Fig. 8 . Crack propagation between these two incidents was also registered. The nominal stress range on the ordinate of Fig. 8 is valid only for the braces of the K-nodes and not related to the chord side of the weld seams. As the failures occure on the chord side, for which a nominal stress cannot be defined, the nominal stress concept is not applicable for the assessment of the investigated K-nodes. The calculations of the hot-spot and notch stress ranges will be explained later. Fig. 8 includes, as well as the mean curves with the probability of survival P s = 50% for the break-through criterion, also the fatigue life curve for variable amplitude loading with P s = 97.7 %, which is the assigned probability to fatigue life curves in the IIW-recommendations [4] . This curve is obtained by reducing the mean curve by the factor of j = 1.37 resulting from the assumed scatter T  = 1:1.50 and a Gaussian log-normal distribution. For assessment of the accuracy of the hot-spot and local concepts, the IIW-design curves (allowable stresses) FAT 90 and FAT 225 (P s = 97.7%) were used. The curves were corrected for thickness and the sea-water corrosion was considered according to GL-design rules [5]. In the case of the local stress concept, as the load dependent notch stresses were