Wind Turbines near Flood Defences

Abstract

Building wind turbines near flood defences has numerous advantages over other locations on land. The water authorities, which own the land of the flood defence, will only permit the turbines if the flood protection assessment satisfies the safety standard after the wind turbines have been constructed. There are three types of failure of a wind turbine: falling over of the turbine, nacelle falling off the tower and blades falling of the rotor. The first two, with their relation to the flood defence, are analyzed in this report. An assessment is made, in which the additional failure probability of the flood defence is determined.

The main white spots in the flood protection assessments are: First, the assessment method of wind turbines near flood defences is a gray area, which leaves room for interpretation. Secondly, the effects of the constant vibrations of wind turbines are unknown, this is currently being researched. Thirdly, the size and depth of the crater which develops after an impact of a nacelle hitting the flood defence. This third white spot is researched in this report.

The Material Point Method turns out to be a good method to determine the size and depth of a crater. The nacelle of the Enercon E-126 turbine can cause at maximum a penetration depth of 3.8 meters on a dry sea dike consisting of sand. There are two impact locations on the dike which can be distinguished: an impact on the crest and an impact on the slope. If the impact is on the crest, both the crest is and large parts of both slopes are affected. The soil below the slopes is lifted up, this causes the revetment to lose its coherence. An impact on the slope will mainly affect the slope and the crest is only limited affected. However, the size and depth of the crater depend largely on the soil characteristics and the potential energy of the nacelle. Those are taken conservative in the model. So, the penetration depth of 3.8 meters is the maximum.

The Expertise Network for Flood Protection gave requirements for wind turbines near flood defences. The case study is again an Enercon E-126 turbine built directly at the inner side of a sea dike in Groningen. Overtopping & overflow, inner slope-instability and instability of the revetment are considered as the governing failure mechanisms for the additional failure probability. When using only conservative assumptions, the flood protection requirements are not satisfied. However, with using more realistic assumptions, the additional failure probability can be determined more precise and therefore be lowered. In the end, the wind turbine can be moved further inland to lower the hitting probability of the flood defence, this will decrease the additional failure probability.