Development of Combined Load Spectra for Offshore Structures Subjected to Wind, Wave, and Ice Loading

Abstract

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 effects of ice loading and its stochastic nature on 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.

Fatigue Strength of Fixed Offshore Structures Under Variable Amplitude Loading Due to Wind, Wave, and Ice Action

“…The variable amplitude loading sequence is based on a novel combined load spectra and standardized stress-time sequences for OWTs that account for wind, wave, and ice action. To this goal, the load interaction of an exemplary monopile support structure in the Baltic Sea was determined in a previous study, see [20]. In the current study, butt joints representative for monopile support structures were subjected to one stress-time sequence derived in [20].…”

“…In the first pre-processing step, a critical load case assumption for the fatigue limit state must be defined. This has been done according to IEC61400-3-1 [25] for an exemplary monopile support structure in the southern Baltic Sea with sitespecific wind and wave data, and estimated ice event conditions in Braun et al [20]. These loads have been applied in two different modes of the turbine (idling and in operation).…”

“…Due to different probabilities of load scenarios (for instance different direction of wind and waves), the structural response around the circumference of the butt joint varies significantly. Thus, the methodology proposed by Milaković et al [2] and Braun et al [20], and presented in Figure 1, was adopted to determine combined load spectra and time series. The stress-time series obtained from the numerical simulations are assessed (in 30° steps around the circumference) using rainflow counting.…”

“…Fatigue design of OWTs is typically performed by means of stress-based fatigue assessment methods calibrated to constant amplitude loading (CAL) data, linear damage accumulation hypothesis, and representative load spectra or time series [12,[16][17][18][19][20]. For scenarios that differ from the usual loading scenarios, calibration using variable-amplitude loading (VAL) testing is required to verify applicable damage sums or similar quantities.…”

“…Thus, this study presents fatigue test results of an exemplary butt joint in an OWT under combined VAL due to wind, wave, and ice action. This is achieved by determination of a standardized load sequence from a numerical model that represents a case study for an OWT subjected to a number of drifting ice events during its lifetime, see [20]. To this goal, the occurrence of different load cases on an OWT in the southern Baltic Sea was simulated to obtain the structural response of an exemplary monopile support structure.…”