Floating wind turbines : the transport phase

Abstract

The worldwide demand of renewable energy is increasing rapidly because of the climate problem. Wind energy appears as a clean and good solution to cope with a great part of this energy demand. Therefore, floating wind turbines have been investigated as a possible solution to increase the efficiency from the wind, as a renewable energy source. A critical phase for the floating wind turbines is the transport phase. Economically, the floating wind turbines should be transported in an upraised position and assembled from the construction site to the actual offshore installation site. Sway has been given generic data for a possible floating wind turbine. A finite element program is developed in CALFEM for the wind turbine to detect the response from hydrodynamic forces during tow out in upraised and assembled position. To introduce hydrodynamic forces, liearized wave theory and Morison equation have been used. Due to relatively large transport velocity, the relative difference between water particle velocity and structural velocity will introduce hydrodynamic damping. This is a non linear problem and a routine in CALFEM has been developed with theory for constant average acceleration. The tower model has been built up with 2 dimensional dynamical beam elements. Briefly summarized theory for harmonic response, hydrodynamics and Morison equation have been given. A suggestion of maximum transport conditions during tow out has been carried out. The suggestion is based on the response analysis and parameters like different wave conditions and different chain connections for the tow line are considered. Summarized, the suggested maximum transport conditions are : - Maximum wave height: 5 meters, - Maximum transport velocity: 2.5m/s, - Chain support in node: 10 - 90 meters from the bottom of the tower. In addition, a sea depth study of parts of the Norwegian cost line is given, and maps have been generated using “Norge Digitalt”. From this investigation we can see that because of the many deep fjords that the Norwegian coast line has, it is fit to transport floating wind turbines in upraised position. Also, huge areas in the ocean are available for floating wind turbines farms in the future.