Vortex-induced　vibrations　(VIVs)　of　a　top　tensioned　riser　(TTR)　subjected　to　flows　with　spanwise　varying　directions　have　been　simulated　and　studied　in　this　paper.　The　riser　is　simplified　as　an　Euler-Bernoulli　beam　and　its　dynamics　is　described　using　the　finite　element　method　with　the　Absolute　Nodal　Coordinate　Formulation　(ANCF).　The　hydrodynamic　forces　due　to　vortex　shedding　are　modelled　with　wake　oscillators　attached　to　nodes　along　the　riser.　The　model　has　been　firstly　used　to　simulate　an　experiment　and　acceptable　model　results　have　been　obtained　in　comparison　with　the　measurements.　Then,　the　VIVs　of　the　TTR　under　two　flow　conditions,　one　with　uniform　and　the　other　with　linearly　sheared　velocity,　have　been　simulated.　In　both　cases,　the　direction　of　the　flow　varies　linearly　along　the　riser.　Simulation　results　have　shown　different　response　characteristics　at　small　and　high　flow　velocities　with　respect　to　the　frequency,　amplitude　and　vibrating　direction　of　the　riser.　In　general,　at　a　small　flow　velocity　where　the　response　of　the　riser　is　dominated　by　the　standing　wave,　the　variation　in　the　flow　direction　has　a　major　influence　on　the　VIV　of　the　riser.　However,　when　the　travelling　wave　is　predominant　at　a　high　flow　velocity,　the　effect　of　the　spanwise　varying　flow　direction　is　insignificant.　The　energy　transfer　between　the　structure　and　flow　has　been　studied　to　reveal　the　possible　mechanism　that　leads　to　these　differences　in　the　responses　of　the　riser　at　small　and　high　flow　velocities.　The　results　of　the　present　paper　suggest　that　the　engineering　simplification　of　ignoring　the　influence　of　flow　direction　variation　on　VIV　may　only　be　applicable　at　high　flow　velocities　when　the　response　of　the　riser　is　dominated　by　the　travelling　wave.