Objectives  Long-lasting Vortex Induced Motion(VIM)may cause the Spar platform mooring system fatigue and even the structure damage. Therefore, the adverse effects of VIM must be fully taken into account.

  Methods  In this paper, the CFD solver naoe-FOAM-SJTU and dynamic grid method developed by ourselves are used to make numerical simulation of the characteristics of the vortex induced flow field around the Spar platform. The time Delayed Detached-Eddy Simulation(DDES) turbulence model based on shear-transport-stress equation is used to simulate the three-dimensional fine wake vortex structure of the Spar platform with helical strakes. The longitudinal motion, transverse motion and yawing of the Spar platform with helical strakes at different reduced velocities are studied. The platform mooring system is stimulated by using the linear spring system. The research also takes into account the effects that the platform coupling effect at different degrees has on VIM. Then the numerical simulation results are compared with the model test results, to analyze the time history, spectral characteristics and locking of longitudinal and transverse VIM of Spar platform with helical strakes and to reveal the intrinsic mechanism of VIM.

  Results  The results show that the helical strakes can effectively reduce the response amplitude of Spar platform's VIM; the CFD solver naoe-FOAM-SJTU has good accuracy and reliability for the calculation and simulation of the offshore platforms' VIM.

  Conclusions  The numerical simulation and analysis of the VIM of Spar platform with helical strakes is of great significance to the practical design of Spar platform.