  The kinematics of offshore wind turbines are of great importance when
installing the turbines, as the motions of the components during craning
operations are a limiting factor. Most critical is the installation of the
blades: the blade's bolts need to be inserted into the rotor flange, an
operation that requires great precision. Both the blade and the turbine undergo
environmental loading, leading to relative motions between the blade root and
the hub during installation. Results from an offshore wind farm installation
measurement campaign showed, that the partially installed turbines show
intricate patterns of motion (orbits) in the horizontal plane. The mechanism
behind the formation of these orbits remains elusive so far.
  In this paper, we present a novel torsional coupling mechanism linking
motions in the fore-aft and side-side direction. It can explain the formation
of orbits that change direction.
