It has long been known that in-line (IL) response will influence cross-flow (CF) vortex shedding forces and vice versa. However, empirical codes for prediction of vortex induced vibrations (VIV) of slender marine structures have so far been limited to handle CF or IL response separately without taking into account the interaction between the two response modes. The motion phase angle between IL and CF displacement is a key parameter to be included in the empirical codes in order to model such interaction. The present study uses the data from Shell’s High mode VIV experiments that were performed at the MARINTEK Offshore Basin in March 2011. This extensive test program provides a rich dataset for measuring the motion phase angle and hydrodynamic force coefficients under different flow conditions. It is found that the energy transfer from the fluid to the pipe is related to counter-clockwise trajectories inside the excitation region; while clockwise trajectories are associated with hydrodynamic damping forces. The influence of the travelling wave behavior on motion phase angle and hydrodynamic force coefficients are also studied. It was found that the spatial variation of the motion phase angle of the beam is different when travelling waves dominate the response.
