Freak　waves　are　an　extreme　marine　environment　factor　in　offshore　structure　design　and　become　a　potential　risk,　particularly　for　laying　oil-gas　pipelines　in　deep　waters.　The　objective　of　this　study　was　to　reveal　the　freak　wave　effects　on　dynamic　behaviors　of　offshore　pipelines　for　deepwater　installation.　Thus,　a　dedicated　finite　element　model　(FEM)　for　deepwater　pipeline　installation　by　the　S-lay　method　was　developed　with　special　consideration　of　freak　waves.　The　FEM　also　took　pipelay　vessel　motions,　pipe-stinger　roller　interactions,　and　the　cyclic　contacts　between　the　pipeline　and　seabed　soil　into　account.　Real　vessel　and　stinger　data　from　an　actual　engineering　project　in　the　South　China　Sea　were　collected　to　obtain　an　accurate　simulation.　Moreover,　an　effective　superposition　approach　of　combined　transient　wave　trains　and　random　wave　trains　was　introduced,　and　various　types　of　freak　wave　trains　were　simulated.　Extensive　numerical　analyses　of　a　12　inch　gas　pipeline　being　installed　into　a　water　depth　of　1500　m　were　implemented　under　various　freak　wave　conditions.　The　noticeable　influences　of　freak　waves　on　the　pipeline　and　seabed　responses　were　identified,　which　provides　significant　awareness　of　offshore　pipelines　for　deepwater　installation　design　and　field　operation　monitoring.