Dynamic response and reliability analysis of an offshore wind turbine supported by a semi-submersible platform

Wind Energy is the fastest growing renewable energy source in the world. The trend is expected to continue with falling costs of technology, energy security concerns and the need to address environmental issues. Offshore wind turbines have a few important advantages over land-based turbines; offshore sites experience stronger and less turbulent winds, there are fewer negative aesthetic impacts in an offshore location, there is greater ease in the transport of wind turbine components over sea than on land, etc. Large offshore wind turbines mounted atop floating platforms offer a viable solution for deepwater sites. Of the various floating platform concepts that are being considered, a moored semi-submersible platform is considered in this study.

The dynamic response and reliability analysis of a 13.2~MW offshore wind turbine supported by a moored semi-submersible platform is the subject of this study. A model for this integrated system has been developed and its various physical, geometric, and dynamic properties have been studied in this and another associated study. Loads data for the extreme and fatigue analysis of such systems are generally attained by running time-domain simulations for a range of sea states that are representative of the expected site-specific metocean conditions. The selected site of interest in the North Sea has a water depth of 200 m. The Environmental Contour (EC) method is used to identify sea states of interest that are associated with a target return period (50 years). These sea states are considered in short-term (1-hour) simulations of the integrated turbine-platform-mooring system. The dynamic behavior of the integrated wind turbine system is studied. Critical sea states for the various response loads are identified and the sensitivity of the system to the metocean conditions is discussed. Estimation of 50-year response levels (for turbine loads, platform motions, and the mooring line tension at the fairlead) associated with the target probability is subsequently carried out using 2D and 3D Inverse First-Order Reliability Method (FORM) approaches.