Meaning
Vibration phenomena occur when two distinct structural natural frequencies merge, leading to destructive resonance under constant aerodynamic load. Design agreements for wind turbine blades require modeling to ensure that mode coalescence does not happen within the operating speed range of the rotor. This prevention secures the longevity of the generator.
Structural Failure
Procurement specifications define the required frequency separation between the bending and twisting modes of the blade. If mode coalescence is observed during wind tunnel testing, the supplier must redesign the internal spar. This requirement avoids structural failure in the field.
Warranty Exclusion
Warranties do not cover damage caused by running the turbine outside the specified environmental conditions. If the operator changes the control software to bypass speed limits, causing the blades to experience mode coalescence, the supplier is released from all repair obligations. This provision protects the manufacturer from unauthorized software modifications.
Remedial Engineering
Remedial actions for turbines showing signs of early structural wear include retrofitting the blades with tuned mass dampers. The cost of these retrofits is split between the parties depending on whether the issue stems from a manufacturing defect or an incorrect wind resource assessment by the buyer. This cost-sharing agreement reduces litigation risks by establishing a clear path to return the turbine to service without waiting for a legal judgment on liability.
