Compared　to　fixed-bottom　wind　turbines,　offshore　floating　wind　turbines　(OFWTs)　are　subject　to　more　prominent　forced　vibrations　that　require　control　measures.　Considering　the　limited　space　within　the　nacelle,　this　paper　proposed　a　novel　methodology　for　designing　a　tuned　mass　dampers　(TMD)　installed　inside　the　tower.　To　achieve　an　optimal　design,　a　4-degree-of-freedom　simplify　dynamic　model　is　established　to　predict　the　vibration　characteristics　of　spar　OFWTs　quickly　and　accurately.　A　new　TMD　configuration　installed　inside　the　tower　is　proposed　to　reduce　the　effect　of　space　limitation　and　to　determine　the　stroke　limitation　Based　on　the　proposed　methodology,　the　stiffness　and　damping　of　the　TMD　are　designed　and　optimized,　and　then　the　detailed　structural　parameters　are　calculated.　The　comparison　results　show　that,　the　proposed　TMD　achieves　a　maximum　peak　suppression　rate　of　90.67%　in　tower　top　fore-aft　deformation　power　spectral　density,　which　is　19.92%　higher　than　that　of　the　TMD　designed　by　the　formula　method.　The　proposed　TMD　can　effectively　address　the　problem　caused　by　the　space　limitation　within　nacelle,　and　provide　superior　vibration　suppression　for　spar　OFWTs.　©　2024　Elsevier　Ltd