Numerical consideration of local joint flexibilities

The influence of local joint flexibilities on both structural behavior as well as fatigue loads was studied. Two different approaches to include the joint flexibilities in structural analyses were presented, i.e., the Buitrago approach, which uses parametric formulae depending on certain joint types and geometries, and the substructuring approach, which deals with arbitrary geometries. The effect on the dynamic structural behavior was negligible in the region of low global eigenfrequencies that were excited by wind and wave loads on offshore wind turbines. However, for higher local modes that could be excited by the operation of the turbines, the effect could be much more significant. Due to the change in distribution of member forces, especially of the bending moments, within the structures caused by the local joint flexibilities, this had a significant influence on the fatigue loading of the joints. At the example of the research platform FINO 1, the damage equivalent moments at the joints could be reduced by up to 40% if local joint flexibilities were incorporated in the structural model. This is an abstract of a paper presented at the Eighteenth International Offshore and Polar Engineering Conference (Vancouver, Canada 7/6-11/2008).