Comparison of stiffening types of the cutout in tubular wind turbine towers

A very efficient type of wind turbine tower is the tubular steel tower configured as a cantilever cylindrical or conical shell, which is used considerably nowadays. A typical feature of such towers is the presence of a manhole cutout near the bottom. This cutout is of considerable dimensions and lowers significantly the tower strength including stress concentrations and increased danger of local buckling. Therefore, it is common to stiffen the region around the cutout aiming at compensating for this loss of strength. In this paper the most common types of stiffening are investigated by means of nonlinear finite element analyses calibrated via experimental testing and the efficiency of each type is highlighted. It is found that simple stiffening types consisting of either a peripheral frame or two vertical stringers and a ring are particularly efficient and can be used instead of more complex ones. Rules for dimensioning of the stiffeners are also proposed.