Teeter end impacts: analysis and classification of most unfavourable events

Abstract Two‐bladed wind turbines are recently being discussed more often as the question arises for the most suitable offshore turbine concept. Regarding this turbine concept, a solution is required for the more challenging dynamics. A teetered hub has often been a load reduction concept of two‐bladed turbines. During normal operation, a teetered hub eliminates the hub bending moment coming from unequal blade loading. But looking at extreme load cases, the teeter end impact is a major problem. The teeter end impact is quite often described as the occasion that destroys the load‐reducing advantage of the teeter mechanism. The turbine must be designed to withstand the loads from the teeter impacts leading to additional weight, which is actually supposed to be reduced by using the teeter hinge. Although the teeter end impact is often described as a kind of ‘killer aspect’, a more detailed analysis and quantification of its nature is not given in open literature. This paper will do an analysis and quantification of the loads coming from teeter end impacts using an existing teetered turbine, the Controls Advanced Research Turbine 2 (CART2). First, there will be a look at analytical teeter equations to get an overview of the basic parameters leading to teeter movement. Then, teeter end impact behaviour will be analysed using aeroelastic load simulations of the CART2 according to International Electrotechnical Commission (IEC) 61400‐1 edition 3. For each design load case, the most significant teeter response will be examined. A classification of teeter end impacts will be extracted from the simulation data. Results will be compared with a rigid turbine in order to get an evaluation of how severe teeter end impacts are, compared with extreme loads of a rigid turbine. Additionally, these results will be compared with modified teeter parameters of the CART2. These are the introduction of pitch–teeter coupling, a reduced free teeter angle and a different Lock number. It will be shown to what extent these parameters may reduce the intensity of teeter end impacts. Results show that it is worth discussing teetered turbines as an alternative to today's three‐bladed turbines. According to this study, teeter end impacts need not be regarded as completely intolerable, and there are several turbine parameters that have a significant influence on them. Copyright © 2015 John Wiley & Sons, Ltd.