A comparative study of different methods for predicting the long-term extreme structural responses of the combined wind and wave energy concept semisubmersible wind energy and flap-type wave energy converter

Abstract

The combined wind and wave concept semisubmersible wind energy and flap-type wave energy converter was developed in the EU FP7 project MARINA Platform. It consists of a four-column semisubmersible with a 5-MW wind turbine placed on top of the central column and three flap-type wave energy converters on top of three pontoons that connect the four columns. Numerical and experimental studies have been performed to demonstrate the functionality and the survivability of the combined concept. In extreme conditions, both wind turbine and wave energy converters are set in a protection mode which reduces the dynamic loads and responses. In this article, different methods for predicting longterm (50-year) extreme responses considering the wind and wave conditions at two given European offshore sites are carried out, and structural response quantities are calculated, compared and presented. The full long-term analysis was performed and regarded as the reference method, and the corresponding results are compared with the modified environmental contour method and the environmental contour method. The response quantities studied here are the axial forces and bending moments of the semisubmersible wind energy and flap-type wave energy converter, including those of the wind turbine (blade, shaft and tower), arms of the flap-type wave energy converters and mooring lines, as well as the platform motion in 6 degrees of freedom. The extreme responses that are dominated by the aerodynamic loadings are effectively calculated either by the full long-term analysis or the modified environmental contour method. Compared to the full long-term analysis, the environmental contour method gives an under-prediction of the long-term extreme responses of quantities related to the wind turbine (e.g. internal loads of blades and tower). For the extreme responses that are dominated by the hydrodynamic loadings, all the three methods provide similar results.

Structural Safety Assessment of Marine Operations From a Long-Term Perspective: A Case Study of Offshore Wind Turbine Blade Installation

“…The proposed methodology in this paper is a modified form of the 'all sea state' [11,12] approach, and therefore a brief overview of the original method is given here. This method was first introduced in late 1950s and is used extensively today in the design of an offshore structure to estimate extreme responses under environmental loads.…”

Sensitivity Analysis of a Bottom Fixed Offshore Wind Turbine Using the Environmental Contour Method

“…An iterative process, where inner contours are tested, is required to find this equivalent return period. After this process, the largest value of all the short-term extremes for all the environmental conditions on all the inner contours is then considered for the long-term response prediction [8].…”

“…Finally, the values are used to find a sensitivity index (S) as indicated by Eq. (8). This indicator helps us to better reflect and quantify the sensitivity (uncertainty propagation) in the short-term response (simulated response) when an uncertainty in the sea state parameters is introduced, which is the main purpose of this work.…”

Wave power absorption by an oscillating water column (OWC) device of annular cross-section in a combined wind-wave energy system

“…Due to the various possible advantages as a result of the combination of the wind and wave energy converters, several concepts of the combined system have been proposed, and the concept feasibility of different integrated platforms has been assessed. Examples include the integration of a point-absorber-type wave energy converter with a semisubmersible type or a spar type wind turbine [21,[23][24][25]; integration of an OWC with a floating or bottom-mounted offshore wind turbine [26][27][28][29][30][31], and semisubmersible flap concept [22,[32][33].…”