The overall competitiveness of offshore wind turbine towers is significantly influenced by the selection of the connection. The following three types of connections: a conventional bolted ring flange (RF) connection, ring flange connection with defined contact surfaces (RFD), and C1 wedge connection (C1-WC) are considered. A quantitative comparison is made to enhance performance in a specific condition and enable further optimization of these connections in engineering practices. The study compares the tensile behaviour and fatigue performance of these connections by validated finite element (FE) simulation and analysis. The proposed FE modelling is based on a realistic geometry including all contacts present in the connections, steel full-range stress–strain relationship and ductile damage model. The efficiency and accuracy of the FE models are validated through the comparison with the performed tests. Then, a series of parametric FE analyses are carried out to examine the impact of the applied boundary conditions, bolt pretension level, and steel grade on the behaviour of connections. Load-displacement curves, bolt evolution curves, and stress responses are analysed to compare their tensile behaviour. The effectiveness of conventional segment specimen testing is evaluated thoroughly. For the fatigue performance of connections, the results indicate that the segment specimen testing substantially underestimates the fatigue performance of C1-WCs. This discrepancy is essential to be considered in the tower design. It is also noted that C1-WCs are rather insensitive to the variation of pretension force level, which show superiority to avoiding the difficulties associated with typically bolted joints. This research provides in-depth knowledge for the practical application of such connections and further optimization.

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Fixed offshore wind turbines are increasingly developed for high latitude areas where not only wind and wave loads need to be considered, but also moving sea ice. Current structural design rules do not adequately consider the effect of ice loading on fatigue life, due to missing studies on fatigue strength of welded joints under combined wind, wave, and ice action. Thus, a methodology to determine combined variable-amplitude loading (VAL) spectra was developed in a previous study. The stress state time-history at an exemplarily selected point in the support structure of an offshore wind energy monopile was translated into a VAL sequence. This sequence is used as an input for fatigue tests of butt-welded joints in the current study. The current study presents the VAL spectrum and the corresponding VAL time series, the results of the fatigue tests and compares them to typical fatigue damage sums for other stress spectra.@en

Fixed offshore wind turbines continue to be developed for high latitude areas where not only wind and wave loads need to be considered but also moving sea ice. Current rules and regulations for the design of fixed offshore structures in ice-covered waters do not adequately consider the effects of ice loading and its stochastic nature on the fatigue life of the structure. Ice crushing on such structures results in ice-induced vibrations, which can be represented by loading the structure using a variable-amplitude loading (VAL) sequence. Typical offshore load spectra are developed for wave and wind loading. Thus, a combined VAL spectrum is developed for wind, wave, and ice action. To this goal, numerical models are used to simulate the dynamic ice-, wind-, and wave-structure interaction. The stress time-history at an exemplarily selected critical point in an offshore wind energy monopile support structure is extracted from the model and translated into a VAL sequence, which can then be used as a loading sequence for the fatigue assessment or fatigue testing of welded joints of offshore wind turbine support structures. This study presents the approach to determine combined load spectra and standardized time series for wind, wave, and ice action.

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In this paper, an analytical method is presented to assess FLI (Frequency lock-in) of monopile support structures for offshore wind turbines, subjected to loading by floating sea ice. The method is introduced, justified and presented with an example. The main idea of the method is to use existing knowledge regarding structural response velocities at which FLI can occur. Using such velocities, the structural response can be determined easily when single mode response is assumed, which is a conservative choice when analyzing FLI. The method is particularly useful for early stages of the design process, when the implication of ice loads on the design needs to be assessed without time-consuming simulations. Results from the method can also be used to identify structural configurations which are most critical for ice loading.

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Ice-induced vibrations have to be considered in design of vertically sided offshore structures subjected to loading by sea ice, such as offshore wind turbines and oil- and gas platforms. The interaction between ice and structure may result in high global peak loads and the occurring structural vibrations can contribute significantly to the overall fatigue of the structure. A regime of particular interest is the frequency lock-in regime in which the interaction causes the structure to oscillate at high amplitude with a frequency close to one of its natural frequencies. Assessment of frequency lock-in in the design phase can be done based on simple approaches once for given ice conditions the natural modes to experience frequency lock-in and the range of ice drift velocities for which lock-in develops are known. Determining those modes and velocities is however challenging due to the nonlinear nature of the interaction between ice and structure and limited available reference data. In this paper two methods are applied to determine the structural modes and ice drift velocities required as an input for simplified design approaches. The first method is the application of design standards and estimation formulas available from literature. The second method is the application of a recently developed numerical model for simulation of the interaction. The methods are applied to two existing structures which have experienced frequency lock-in and an offshore wind turbine designed to be employed at a location with mild ice conditions. Results show that the estimation formulas do not match with full-scale observations of the existing structures and can therefore not be applied to obtain input for the simplified design approaches. The second method shows to give simulation results consistent with the full-scale observations. Application to the offshore wind turbine reveals that it is most susceptible to frequency lock-in in the second mode.@en