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Item type:Publication, Access status: without files , An efficient time domain fatigue analysis and its comparison to spectral fatigue assessment for an offshore jacket structure(Elsevier, 2016-06-07); ; ; Industry 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. Spectral methods 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 approaches using a large platform as an example. The results obtained from a, computationally demanding, full-scale time domain fatigue assessment are used as the standard to quantify the errors resulting from the assumptions and simplifications made in the spectral fatigue analysis. These results indicate, also, that the simplifications involved in spectral fatigue lead not only to the well-known inaccuracy but also to consistently lower fatigue lives. This excessive conservatism leads to unnecessary and costly inspections. This paper contributes to the literature in two important ways. First, it shows the main causes of the inaccuracies of the spectral method and quantifies them. Second, it proposes a novel approach for the time domain fatigue analysis which drastically reduces the computational burden, while maintaining a high degree of accuracy. Performing fatigue analyses in 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.Publicationtype: Journal Article78 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Comparison of time domain and spectral fatigue analyses of an offshore jacket structure(International Society of Offshore and Polar Engineers (ISOPE), 2016); ; The results obtained from a computationally excessive full-scale time domain analysis of an offshore jacket structure quantifies the errors resulting from the assumptions and simplifications made in spectral fatigue analysis. These findings also indicate that the simplifications involved lead not only to the well-known inaccuracy but also to a lower fatigue resistance. This paper shows the main causes of the inaccuracies of the spectral fatigue analysis and quantifies them. In addition, this paper verifies the efficacy of an approximation method suggested by Mohammadi et al. (2015) that drastically reduces the computational burden necessary to perform fatigue analyses in time domain.Publicationtype: Conference Paper66 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Efficient fatigue assessment in the time domain in comparison to spectral fatigue analysis of an offshore jacket structureIndustry 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.Publicationtype: Conference Paper109