Faculty Publications
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Item Long term response analysis of TLP-type offshore wind turbine(Taylor and Francis Ltd. michael.wagreich@univie.ac.at, 2020) Vijay, K.G.; Karmakar, D.; Guedes Soares, C.The performance of offshore wind turbine supported with different configurations of Tension-leg-platform (TLP) are studied for vertical plane motion responses (surge, heave, and pitch) along with the side-to-side, fore–aft, and yaw tower base bending moments. The long-term distribution is carried out using the short-term floating wind turbine responses based on Rayleigh distributions and North Atlantic wave data. The long-term response analysis is performed for the 5 MW TLP-type offshore wind turbine. The study aims at predicting the most probable maximum values of motion amplitudes that can be used for design purposes. The transfer functions for surge, heave and pitch motions of the floater are obtained using the FAST code. The performance of floating structure in the long-term analysis not only depends on the transfer functions but also on the careful selection of design wave spectrum model. Among different theoretical design wave spectrum models, three models are chosen that closely represents the sea states and the response spectrums are computed for these models. As the nature of the response spectrum of the floating structure is analogous with the input wave spectrum model, it can be assumed to have the same probabilistic properties and modeled as a stationary stochastic process. The long-term probability distributions for TLP-type floater configuration for surge, heave and pitch motion amplitudes along with the tower base bending moments are used for design purposes, so as to guarantee the safety of the floating wind turbines against overturning/capsizing in high waves and wind speed. The calculation of the long-term distribution using FAST will help in the preliminary analysis of the performance of floaters in the study of wave-induced response of floaters. © 2018, © 2018 Indian Society for Hydraulics.Item Coupled dynamic analysis of spar-type floating wind turbine under different wind and wave loading(Springer Science and Business Media Deutschland GmbH, 2021) Rony, J.S.; Karmakar, D.; Guedes Soares, C.G.In the present study, the coupled dynamic modelling of three different configurations of spar platform is performed using time-domain aero-servo-hydro-elastic simulation. The spar platforms are coupled with 5 MW NREL floating wind turbine and mooring sub-models. The coupled aero-servo-hydro-elastic simulation is performed using the simulation tool FAST with WAMIT as the sub module to obtain frequency domain hydrodynamic characteristics. The major emphasis is given to analyse the Response Amplitude Operators (RAOs) to understand the stability of the structures. The responses are calculated for surge, sway, heave, roll, pitch and yaw motions. The study determines the performance of the structure under the wind load developed for the turbine support structure on analysing the tower base forces and moments. The analysis for three different configurations of spar platform is performed for various environmental conditions of North Sea. The studies observed that the responses of the platforms tend to increase with increase in wind speed and wave height. Further, it is observed that surge and pitch motion is dominant for all the three configurations of spar platform. The present study provides an insight into the power performance, structural integrity and dynamic motions of the floating wind turbine under various operational and survival conditions which help the designers to develop better design standards. © 2021, Sociedade Brasileira de Engenharia Naval.Item Coupled Dynamic Analysis of Hybrid Offshore Wind Turbine and Wave Energy Converter(American Society of Mechanical Engineers (ASME), 2022) Rony, J.S.; Karmakar, D.The combined offshore wind and wave energy on an integrated platform is an economical solution for the offshore energy industry as they share the infrastructure and ocean space. The study presents the dynamic analysis of the Submerged Tension-Leg Platform (STLP) combined with a heaving-type point absorber wave energy converter (WEC). The feasibility study of the hybrid concept is performed using the aero-servo-hydro-elastic simulation tool FAST. The study analyzes the responses of the combined system to understand the influence of the WECs on the STLP platform for various operating conditions of the wind turbine under regular and irregular waves. Positive synergy is observed between the platform and the WECs, and the study also focuses on the forces and moments developed at the interface of the tower and platform to understand the effect of wind energy on the turbine tower and the importance of motion amplitudes on the performance of the combined platform system. The mean and standard deviation for the translation and rotational motions of combined wind and wave energy converters are determined for different sea states under both regular and irregular waves to analyze the change in responses of the structure. The study observed a reduction in motion amplitudes of the hybrid floating system with the addition of the wave energy converters around the STLP floater to improve the energy efficiency of the hybrid system. The study helps in understanding the best possible arrangement of point absorber-type wave energy converters at the conceptual stage of the design process. © © 2021 by ASMEItem Numerical investigation of offshore wind turbine combined with wave energy converter(Springer Nature, 2023) Rony, J.S.; Sai, K.C.; Karmakar, D.The coupled dynamic analysis is performed for three different types of offshore floating platforms combined with a wave energy converter (WEC) mounting a 5-MW NREL (National Renewable Energy Laboratory) wind turbine. The Response Amplitude Operators (RAOs) are analysed for the three concepts of combined wind and wave energy platforms for different wind and wave conditions. The hydrodynamic performance for the three different platforms is conducted considering different load cases. The time domain aero-servo-hydro-elastic tool is used to study the motion responses of the combined system under real operational conditions. The platform’s responses are observed to increase with the increase in the wind speed. In the case of floating hybrid platform, surge responses are minimal for the hybrid spar-tours combination for any load case condition. Minimum surge and sway ensure higher wind power absorption. The study further focuses on the tower base forces and moments to study the impact of wind and waves on the combined floater. Fore-aft shear forces and fore-aft bending moments are higher for the platforms indicating the importance of wind-wave loading. The time domain responses are further used as the transfer function to predict the most probable maximum values of motion amplitude expected to occur during the life-time of the structure which can be used for designing a floating wind turbine (FWT) against overturning in high waves. The long-term models are constructed using various short-term situations expected to occur during the structure’s life-time and weighing them appropriately. The long-term distribution uses North Atlantic wave data, and short-term responses are calculated considering Rayleigh distribution. A brief comparative study of the three combined offshore floaters is performed to understand the structural integrity, power performance and dynamic motions of the floating wind energy platform combined with WECs. © 2023, The Author(s), under exclusive licence to Sociedade Brasileira de Engenharia Naval.Item Coupled dynamic analysis of semi-submersible floating wind turbine integrated with oscillating water column WEC(Springer Science and Business Media Deutschland GmbH, 2024) Sebastian, B.; Karmakar, D.; Rao, M.The present study envisages to investigate the coupled dynamic behaviour of three configurations of a hybrid wind-wave energy system integrating Oscillating Water Column (OWC) wave energy converters to DeepCwind semi-submersible supporting an NREL (National Renewable Energy Laboratory) 5 MW wind turbine. DeepCwind semi-submersible is a platform designed specifically for the purpose of supporting floating offshore wind turbines and the stability of the platform has been well confirmed by scaled-down experiments and numerical studies. The numerical simulation for the present study is performed using the aero-hydro-servo-elastic tool OpenFAST. The dynamic responses of the hybrid platforms are determined for different operational and parked wind speed conditions of the wind turbine in irregular waves. The motion responses, tower base forces and moments, mooring tensions and power absorption of the hybrid configurations have been characterized. Furthermore, the effect of coupling between the semi-submersible platform and the OWCs is studied by comparing the results of the combined platforms with that of the uncoupled wind energy platform. The coupled dynamic analysis in the time domain shows that increasing the number of OWC helps to reduce the motion responses in heave and pitch. The capture width ratio of the system is observed to be highest for hybrid configuration with a single OWC device. The present study will be helpful in the design and analysis of hybrid floating wave-wind energy platform. © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2024.Item Dynamic analysis of a semi-submersible offshore floating wind turbine combined with wave energy converters(Taylor and Francis Ltd., 2025) Sebastian, B.; Karmakar, D.; Rao, M.Hybrid wind–wave energy systems harness both offshore wind and wave energy resources using a shared floating platform, reducing capital and operational costs through common infrastructure. The present study numerically investigates the dynamic performance and power absorption of three hybrid concepts combining the DeepCwind Semi-submersible Platform (SSP) with (i) Oscillating Water Columns (OWC), (ii) Torus Wave Energy Converter (WEC), and (iii) Flap-type WEC. Frequency-domain analyses using WAMIT and time-domain simulations using OpenFAST are performed to assess platform motions, tower base moments, mooring tensions, and WEC power output for different sea states. The integration of WECs significantly improves the hydrodynamic behaviour of the DeepCwind SSP. Flap-type WECs demonstrate the best dynamic performance, reducing heave and pitch by up to 68% and 58%, and mooring tension by 54%. The OWC system achieves the highest power absorption and a 55% capture width ratio, but increases surge and pitch motions by 6% and 27%, respectively, on introducing additional loads on the system. © 2025 Informa UK Limited, trading as Taylor & Francis Group.Item Reliability based design loads of hybrid submerged tension leg-type offshore wind turbine platform(Springer Science and Business Media Deutschland GmbH, 2025) Rony, J.S.; Karmakar, D.The 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.Item Parametric study on the effect of mooring configurations on the dynamic responses of the Septon semi-submersible 5 MW floating wind turbine(Springer Science and Business Media Deutschland GmbH, 2025) Sebastian, B.; Karmakar, D.; Rao, M.Offshore floating wind turbines (FWTs) offer a promising solution for harnessing wind energy in deep waters, where fixed-bottom turbines become impractical. Over the past decade, consistent advancements in technology have significantly reduced the levelized cost of energy, making large-scale deployment of FWTs increasingly feasible. The key factors influencing both cost and performance include the design and optimization of the substructure, mooring system, and power grid. The mooring system plays a pivotal role in ensuring platform stability and minimizing excessive motions that could impact the energy production efficiency and structural integrity of the FWT. The present study investigates the effects of different mooring configurations on the dynamic response of a novel semi-submersible wind turbine platform. This study analyzes two distinct mooring arrangements, spread mooring and cross-mooring, to determine the optimal configuration. The numerical investigation takes into account multiple parametric variations, including spread angle, cross angle, mooring line diameter, and line length, assessing their effects on platform motions and mooring line tensions. Numerical simulations are performed using an aero-hydro-servo-elastic simulation, which considers the coupled interactions of wind, waves, and structural components under various irregular sea states. This study reveals that the choice of mooring configuration significantly affects both platform stability and mooring line loads. A spread mooring system with a 30–60° divergence angle is identified as the optimal configuration for minimizing platform motions while keeping mooring tensions within safe operational limits. Conversely, cross-mooring configurations tend to exhibit higher tensions, particularly at larger cross angles. The cross-moorings require a minimum of 15–35 m additional mooring length compared to spread moorings for line tension to be within safe limits. The findings from the present study offer valuable insights into the optimal design of mooring systems for floating wind turbines, contributing to enhanced performance and reliability in deep water offshore wind farms. © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2025.Item Long-term response analysis of hybrid STLP-WEC offshore floating wind turbine(Taylor and Francis Ltd., 2025) Rony, J.S.; Karmakar, D.In the present study, time-domain response analysis of different configurations of a hybrid Submerged Tension Leg Platform (STLP) combined with a Point Absorber-type Wave Energy Converter (WEC) (STLPWEC) is performed using the aero-servo-hydro-elastic simulation. The study employs long-term analysis technique to predict the most probable values of motion amplitudes, and the forces and moments developed at the tower base for the hybrid STLP-WEC system under operational wind speed conditions of the 5 MW wind turbine. The long-term distribution is performed using short-term responses based on Rayleigh distribution and North Atlantic wave data. The performance of the offshore STLP-WEC system depends on both the transfer functions (translational and rotational motions) and the wave spectrum model. A comparative study of the long-term responses using JONSWAP spectrum model for different configurations of the hybrid STLP-WEC systems is performed, providing valuable insights for designing floating hybrid systems. © 2025 Informa UK Limited, trading as Taylor & Francis Group.Item Effect of the wind turbine floater geometry on the uncertainty associated with the hydrodynamic loading(Elsevier Ltd, 2025) Raed, K.; Karmakar, D.; Guedes Soares, C.The study aims to contribute to the establishment of the reliability-based design for floating offshore wind turbines by quantifying the uncertainty in Morison's wave force in the extreme conditions for two floating wind turbine platforms, namely, the Spar and the OC4 DeepCwind semi-submersible. Numerical models are developed to estimate the wave forces on cylindrical members with different configurations and then to quantify the uncertainty in the output using the propagation law of uncertainty. Morison's coefficients are extracted from Sarpkaya's data as a function of relative roughness, Keulegan-Carpenter number, Reynolds number and the member inclination angle. The combined uncertainty for each input is investigated based on the gathered data from different sources of uncertainties. The First-Order Second-Moment method is then adopted to quantify the output uncertainty based on the uncertainty in the input variables. Furthermore, the contribution of each random variable to the total uncertainty is analysed. The study reveals that wave height is the most significant contributing random variable to the total uncertainty. © 2025