S-N Curves and fracture mechanics based fatigue assessment of offshore floating wind turbines

Abstract

With increasing water depth in offshore applications, the traditional fixed wind turbines have a higher cost in design safety and energy production. Therefore, several concepts of offshore floating wind turbines (FWTs) have been proposed. However, these kinds of structures are prone to fatigue damage due to harsh environmental conditions. The objective of this thesis is not only to quantify the fatigue damage predicted by different methods but also to provide a basis for the currently immature procedure of fatigue assessment of FWTs. The traditional S-N curves and fracture mechanics (FM) based fatigue assessment approaches are widely used in offshore fixed wind turbine systems. However, hydro-elastic loads and coupled loads between floating platform and mooring system cannot be ignored in FWT system. The wave and wind induced loads are highly related to the structural motions and responses and the instantaneous position should be updated with the changes of hydrodynamic and aerodynamic forces all the time. Moreover, the structure response amplitude increases at the nature frequency and nature eigen-frequency. These factors bring a lot of challenges in the fatigue assessment of offshore FWTs. Due to a lot of non-linearities in FWT systems, the fully coupled aero-hydro-servo-elastic analysis is conducted with use of FAST software to obtain the time history of structural dynamic response. Then the fatigue life is calculated by Rainflow counting method with related S-N curves and Miner’s rule. A simplified lumping approach combined with joint probability of wind and waves is utilized to reduce large amount of computational time. Only 498 load cases have a probability of 0.1 ‰ and higher which makes the time-domain analysis more efficient and accurate. The total probability of occurrence is 98.2% and it is feasible to use these load cases to conduct fatigue calculation. The results calculated by the simplified lumping approach show a good agreement with only 0.91% discrepancy. Although the non-linearities for the entire FWT structure and dynamic control system are taken into account by time-domain analysis, the analysis may end up due to the complicated and time-consuming process. Thus, the spectral fatigue analysis can be regarded as an alternative to the time-domain analysis for the quick fatigue assessment of FWTs. A narrow-band solution and six wide-band solutions are presented and compared. The results show the big discrepancy in fatigue lives compared to the results predicted by time-domain analysis when use different spectral fatigue models. However, the solution proposed by Tunna has only two parameters which can be considered as the best quick spectral prediction model for FWTs even though the result is not superior. FM based analysis is an alternative to predict remaining fatigue lives. It is advanced compared to S-N curves based approaches since it can provide detailed crack growth description. Parametric studies of initial crack sizes, critical crack depths, stress concentration factors (SCFs) and mathematical load sequences are investigated with use of Paris’ equation. Nevertheless, it is found that FWTs are more prone to physical load sequence effect due to the noticeable increases and decreases in mean stress. Therefore, two overload (OL) retardation models are explained and compared based on experimental data. Afterwards, a modified Space-state model is proposed with consideration of threshold stress intensity factor range and fracture toughness which has been applied in the fatigue assessment of a spar-type FWT successfully. As a summary, S-N curves based approaches can be applied in the design phase of FTWs with consideration of SCFs. But it is not reliable for the reassessment after some years in operation e.g. due to corrosion effect, change of geometry or the variations of material constants which are not described in S-N curves. FM based approach with Paris’ equation gives the details of crack propagation process but does not take load sequence effect into account which results in a more conservative fatigue life. Thus, the modified Space-state model with consideration of threshold stress intensity factor range and fracture toughness is recommended herein to perform fatigue assessment for offshore FWTs. Keywords: Offshore floating wind turbine, fatigue assessment, coupled aero-hydro-servo-elastic analysis, S-N curve, fracture mechanics, load sequence