Speakers
Description
Wind induced fatigue damage affects most slender structures such as flare booms and bridges in offshore environment. Even though a structure never experience wind loading over the ultimate limit, vibrations can cause fatigue-induced failure of structural details. A widely used approach in the offshore industry for calculating fatigue induced damage on complex structures is the use of FEM models and annual probability of wind speed and directional data. Two methods for estimating fatigue damage are used: the spectral density approach and time history non-linear dynamic analysis. Both methods are based on the hot spot stress assessment approach. FRAMEWORK and WINDPACK are the software which uses the spectral density approach, and USFOS can be used to perform a non-linear time history dynamic analysis. Time history analysis offers more control, since each load case can be analyzed and verified separately, whereas the spectral transforms all load history into spectral diagrams. The spectral equations however are well documented and based on site collected wind turbulence data. There is research demand for further development of both methods, mainly for confirmation with real structural behavior. However, since spectral approaches are generally less demanding, comparing the two methods is also of interest for the practicing industry.
The main objective of this thesis is to compare the results and parametric sensitivity of the three different software using the two methods of fatigue calculation, due to wind buffeting. The effect of vortex induced vibrations on individual members are not included in this thesis. Multiple fatigue analyses of a typical flare tower in the North Sea are done with the same FEM model, with as similar input as possible, for all three software. Parametric studies have been executed for weight factor, drag coefficient, wind block combination and relative velocity. The main findings of this thesis are that USFOS predicts on average 66% the fatigue life that FRAMEWORK finds. For weight factor 1.0 cases, the difference becomes 49%. FRAMEWORK predicts on average 41% of the fatigue life that WINDPACK predicts. FRAMEWORK seems to be rather sensitive to weight factor change in the range of 1.0 to 1.1, which does not correlate with the change in the same range in the other two software.
