Dynamic Analysis of Offshore Floating Wind Turbine Combined with Wave Energy Converter

Abstract

The combined offshore wind and wave energy on an integrated platform is an economical solution for the offshore energy industry as they share the infrastructure and ocean space. The study presents the dynamic analysis of the Submerged Tension-Leg Platform (STLP) and Frustum Tension-Leg Platform (FTLP) combined with a heaving-type point absorber wave energy converter (WEC). The feasibility study of the hybrid concept is performed using the aero-servo-hydro-elastic simulation. The study analyses the responses of the combined system to understand the influence of the WECs on the STLP and FTLP platforms for various operating conditions of the wind turbine under regular and irregular waves. The platform responses are analysed for the North Atlantic wave region. A positive synergy is observed between the platform and the WECs, and the study focuses on the forces and moments developed at the interface of the tower and platform to understand the effect of wind energy on the turbine tower and the importance of motion amplitudes on the performance of the combined platform system. Further, the hydrodynamic performance of circular and concentric arrangements of cone-cylinder-type heaving WECs around STLP and FTLP is analysed. The influence of the hydrodynamic coefficients is analysed by determining the ratio of the hydrodynamic coefficients for a single WEC system to those for a hybrid system. The study analyses the instantaneous wave power absorbed and the wave power under the influence of PTO for the WECs arranged around the TLP floaters. The rigid body analysis observed reduced motion response for the STLP+6WECs and FTLP+8WECs configurations. The dynamic responses of the hybrid platforms for different mooring layouts are studied for different met ocean conditions. The time history and spectrum of the generator power are analysed to observe the effect of second-order wave load and turbulent wind loads on the power production of the hybrid floater under different mooring configurations. Further, the most probable values of the motion amplitudes are calculated using long-term response analysis for the hybrid wave and wind energy system. The long-term distribution is performed using the short-term responses based on Rayleigh distribution and North Atlantic wave data. The transfer function for the long-term analysis of the floater is obtained using the numerical simulation tool FAST. The analysis is performed for zero-degree wave heading angle and different operational conditions of the wind turbine. Thereafter, the reliability of hybrid floating wind turbine platforms against extreme loads is established using the Inverse First Order Reliability Method (IFORM) which includes the randomness in the gross wind environment and the extreme response given wind conditions. The maximum values of the responses for both 1-D and 2-D models are studied and compared. The probability of the exceedance of the responses (Surge, sway, and yaw) for the platforms is studied for different return periods. The study suggests the best possible arrangement pattern for wave power absorption and power uniformity among the floaters in the array. The study performed will be helpful in the design and analysis of the combined wave and wind energy device for wave power absorption.