Mohammadi, Said FawadSaid FawadMohammadiGalgoul, Nelson SzilardNelson SzilardGalgoulStarossek, UweUweStarossek2020-03-252020-03-252016Proceedings of the Annual Offshore Technology Conference (1): 506-518 (2016)http://hdl.handle.net/11420/5472Industry codes require inspections of critical joints within certain intervals to ensure the structural integrity of fixed offshore structures and suggest the use of spectral fatigue analyses to identify them. Inspections of fixed offshore platforms routinely show very few signs of degradation at the joints identified by the recommended computational methods, however. This illustrates the low degree of accuracy of these methods. The uncertainties involved in the fatigue assessment of fixed offshore structures have stimulated a continuous improvement of the techniques used to assess fatigue resistance. Industry codes have replaced early deterministic fatigue analyses with spectral techniques. These are considered less reliable but more efficient than approaches in the time domain, which are generally considered highly accurate but computationally unfeasible. This paper evaluates both the time and the frequency domain approach using a large platform as an example. The results obtained from a computationally excessive full-scale time domain fatigue assessment quantify the errors resulting from the assumptions and simplifications made in spectral fatigue analyses. These findings also indicate that the simplifications involved lead not only to the well-known inaccuracy but also to a lower fatigue resistance prediction. Our results confirm that spectral methods are very conservative for offshore structures, especially those that are sensitive to dynamic excitation. This excessive conservatism can lead to unnecessary and costly inspections. The analysis in the time domain, by comparison, is more accurate and recent developments show that methods based on approximation of fatigue damage exist that make fatigue assessments in the time domain computationally feasible. This paper shows the main causes of the inaccuracies of the spectral method and quantifies them. It also shows the advantage of using the approximated time domain fatigue analysis, which makes this method feasible for use during design projects. Performing fatigue analyses in the time rather than frequency domain increases the accuracy and the reliability of the results and extends the fatigue life of the joints of the structure since several conservative assumptions are eliminated.enTechnikIngenieurwissenschaftenConference PaperOther