Abstract
Floating offshore wind turbines (FOWTs) are susceptible to instability which has come to be called negative damping. Conventional land-based wind turbine controllers when used with FOWTs may cause large-amplitude platform pitch oscillations. Most controllers have since been improved to reduce motions due to this phenomenon. In this paper, the motions induced using one of the original controllers are studied. The current study is performed using the coupled time-domain program FAST-SIMDYN that was developed in Marine Dynamics Laboratory (MDL) at Texas A&M University. It can study large-amplitude motions of floating offshore wind turbines. FOWTs use various controller algorithms of operation based on the available wind speed depending on various power output objectives i.e., to either maximize or level out power absorption. It is observed that the transition region for controllers is often chaotic. So, most studies focus on operations away from the transition region below and above the transition wind speeds. Here, we study the transition region using the theoretical insight of nonlinear motion response of structures. This study reveals the presence of a very interesting and potentially hazardous nonlinear phenomenon, bifurcation. This finding could help explain the chaotic motion response that is observed in the transition region of controllers. Understanding the nature and cause of bifurcation could prove very useful for the future design of FOWT controllers.
