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Offshore structures, such as offshore oil platforms and wind turbines, are continually experiencing environmental loads that can cause fatigue, reduce the strength, and increase the risk of failure. There is a high demand for better evaluating the fatigue lifetime of offshore structures to preserve the structural integrity of existing platforms and to be aligned with the energy growth from renewable sources exposed to an offshore environment. Current fatigue models associate the fatigue life with a large number of uncertainties, mainly due to the unpredictability of the wave loads. In contrast with models based on the estimation of the loads, the methodology covered in this thesis is based on measured responses through the implementation of a structural health monitoring system. Operational Modal Analysis (OMA) is performed on the monitoring data for determining the dynamic properties of the system, such as natural frequencies, mode shapes, and damping ratios. This thesis presents research on the impact of this methodology on the fatigue life of offshore structures. One approach intends to reduce the uncertainties on the standard deviation of the fatigue life. Being the fatigue damage mostly dependent on the stress ranges, a virtual sensing technique is applied for estimating stresses, referred to as actual stresses. For this purpose, the identified mode shapes are expanded into a finite element model allowing the responses to be known at any point in the structure. The actual fatigue life is assessed through the calculation of the equivalent stress range. The described procedure has been applied to two distinct models in the lab and a real offshore structure operating in the North Sea. Stresses were estimated with high precision when using OMA assisted virtual sensing methodology. The equivalent stress range based on the actual strains resulted in a significantly lower coefficient of variance, which leads to cost reduction in terms of inspection planning and maintenance of the offshore structure.A second approach aims to investigate the wave-structure interaction, which can further reduce the uncertainties of fatigue damage’s mean value. A wave simulation tool has been created based on linear wave theory. Besides making a high amount of simulated data available, this code provides wave loads to be applied to a numerical model. With OMA, it can be used to compare estimated responses with the measured ones. Besides, the simulations are the basis for evaluating a correlation between wave loads and structural responses that can be later used to estimate the fatigue damage before or after the monitoring period. Wave flume experiments have been performed using a scale model for providing data to this research. Monitoring data from the previously mentioned real offshore structure are also evaluated.Last, the fatigue life extension of offshore structures is discussed, given the outcome from the present research and in comparison with the design fatigue assessment. Limitations and outlook for future work are also presented. |
