In　the　present　paper,　the　effects　of　external　disturbances　on　cross-flow　vortex-induced　vibrations　(VIVs)　of　a　top　tensioned　riser　are　investigated　based　on　a　wake　oscillator　model.　Two　different　flow　conditions,　namely　uniform　and　linearly　sheared　profiles,　and　different　combinations　of　disturbance　frequency　and　amplitude　were　considered.　Simulation　results　showed　that　synchronization　between　VIV　and　disturbance　can　occur　when　the　disturbance　frequency　approaches　the　Strouhal　frequency　or　when　the　disturbance　amplitude　is　significant.　This　synchronization　leads　to　a　significant　increase　in　riser　response　and　the　commonly　used　industry　practice　of　simulating　VIV　and　disturbance　independently　and　summing　both　could　underestimate　the　actual　fatigue　rate　in　such　condition.　However,　in　certain　cases　where　VIV　did　not　lock　onto　the　disturbance,　it　was　found　that　the　disturbance　could　suppress　the　VIV　and　the　industry　practice　tends　to　overestimate　the　fatigue　rate.