Fatigue at welded connections is a potential failure mode which may compromise the integrity of offshore structures. In order to gain a better understanding of this phenomenon cyclic stressing experiments have been performed upon 25.4 mm thick butt welded ABS-DH 32 steel plate specimens, both freely corroding and with various levels of cathodic protection, in flowing sea water and employing the C/12/20 loading spectrum. The objective of this program was to investigate the influence of a realistic loading spectrum upon fatigue of welded steel connections in sea water in perspective to more available, constant amplitude data. The results of these tests are compared to historically available constant deflection data, where specimens were fatigued under otherwise similar conditions, and also to existing design criteria. Various techniques are discussed whereby constant and spectrum deflection fatigue results can be compared directly, and it is concluded that for the present test results equivalent stress as determined by the Miles equation yields the best correlation between the two.
Fatigue has been judged to be of major importance with regard to integrity of offshore structures (1-3). This failure mode may be particularly critical at welded tubular connections where both macro- and micro-geometrical irregularities contribute to a concentration of stress. Figure 1 illustrates this schematically for the case of a "V" connection.
Present design procedure for fixed offshore structures of the jacket type typically employs the following sequential steps (4):
Definition of the long-term wave climate based upon the best available information.
Global analysis of the structure to determine its response to (1). In the case of deep water structures (period greater than three seconds) dynamic amplification effects are included.
Evaluation of local stresses at tubular connections in terms of sea state. These stresses are subsequently assembled to develop a long-term distribution. From this and the appropriate design S-N curve the cummulative damage, 0, can be calculated from the expression
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A critical assumption upon which accuracy of this design technique depends is that the S-N curve incorporated into the analysis properly reflect the fatigue response of a particular connection in question. Historically, these curves have been based upon data from constant amplitude fatigue experiments, and it is not known how accurately this represents actual welded connections which experience stressing in sea water from wind, wave, tidal, duty loadings and, perhaps, dynamic amplification. Similarly, this design procedure can be no more correct than the linear damage summation technique which is generally employed in the analysis. This method presumes that the fatigue damage resulting from a particular cycle is independent of that from pre ceding cycles or that interactive effect are not significant. Some justification for this arises from the large amount of historically developed fatigue data for ferrous alloys which shows general correlation between multi amplitude cycles-to-failure and life predictions based upon linear damage summation.
