Rony, J.S.Karmakar, D.2026-02-032025Journal of Ocean Engineering and Marine Energy, 2025, , , pp. -21986444https://doi.org/10.1007/s40722-025-00433-6https://idr.nitk.ac.in/handle/123456789/20623The environmental contour (EC) method is one of the popular modelling approaches to predict the long-term responses of the Floating Offshore Wind Turbine (FOWT) platforms. The method is recommended in the design guidelines and standards as, it emerged as a practical method to estimate the extreme dynamic responses for relatively minimal number of environmental conditions. The EC method has the advantage of separating the probabilistic description of the environment from the structural design. In the present study the 1-D and 2-D EC models are estimated based on the Inverse First Order Reliability Method (IFORM). The models estimated were used to predict the extreme long-term responses of the single Submerged Tension Leg Platform (STLP) and the STLP combined with heaving cone-cylinder wave energy converters (STLP-WEC). The aero-servo-hydro-elastic simulation tool FAST is used to simulate the extreme responses for the five particular return periods (1-Year, 10-Year, 20-Year, 50-Year and 100-Year) considering the HornsRev site. The wind load conditions for the FAST platform were simulated using the tool TURBSIM. The study further analysed the long-term extreme moments developed at the base of the turbine tower to analyse the influence of the wind and wave load on the wind power absorption. The maximum value of the mooring line tension developed on the mooring cables of the platform for different return period conditions are also studied to understand the reliability of the floating system. The study observed to be useful for predicting the long-term design loads of STLP wind turbine. © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2025.Dynamic responseInverse problemsMooringOffshore oil well productionOffshore wind turbinesReliability analysisSimulation platformTension-leg platformsWind powerDesign loadEnvironmental contour methodsFirst order reliability methodsInverse first order reliability methodLeg-typeLong term analysisReliability based designReturn periodsSubmerged tension-leg platformWind loadMooring cables