Stress concentration at the door opening of steel towers for wind turbines

The increasing interest in wind energy leads to higher demands of wind turbines. Higher production rates caused by the demand make it essential to develop tower for wind turbines in a way that cost savings in the whole production and assembly are achieved. An optimization of the tower and tower details which lead to material or assembly time reductions is the main objective of a European research project, HISTWIN. The project is focusing on competitiveness of steel towers for wind turbines. Steel towers for multi megawatt turbines usually consist of several conical steel segments which are welded together to sections. These sections are connected by bolted flange connections or by the innovative friction connection developed within the project. This report investigates the lower tower section which includes the door opening. The door is used for service and maintenance to come into the tower. Around the door opening there is a stiffener. Design of the door opening and the stiffener is govern by a stress distribution. These stresses were analyzed using the FEM software Abaqus v.6.7. The tower’s shell thickness and the thickness of the stiffener around the door opening were investigated in the parametric study done by the Abaqus. The aim was to investigate the influence of thickness and material changes around the door opening and its influence on the stress level. Another criterion of thin walled structures is the resistance against buckling. The tower’s structure was also investigated concerning the possibility to reduce resistance due to buckling. The reduction of the bearing resistance of the lower segment of the tower varies with the amplitude of imperfections. The state of the art tower uses steel S355 with yield strength of 355MPa. A change of the material to steel with higher strength was studied. It is concluded that savings of the amount of steel used are possible.Validerat; 20101217 (root