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 Performance of a hybrid TLP floating wind turbine combined with arrays of heaving point absorbers(Elsevier Ltd, 2023) Rony, J.S.; Karmakar, D.In the present study, the hydrodynamic performance of circular and concentric arrangements of cone-cylinder-type heaving point absorbers around a Submerged Tension-Leg Platform (STLP) is analysed using the numerical model in the frequency domain based on the potential flow theory. The presence of the Wave Energy Converters (WECs) around the STLP floating wind turbine platform affects the hydrodynamic performance of the hybrid floating platform. So to illustrate the effects of WECs on the platform, the ratio of hydrodynamic coefficients for a single WEC system to that for a hybrid system is analysed. An array of heaving point absorbers is placed in circular and concentric patterns to understand the performance of heaving point absorbers in the absorption of wave energy. The cone-cylinder type heaving point absorber is selected for the present study as they yield more power as compared to other shaped point absorbers. The study compares the wave power absorption of each point absorber around the platform for irregular wave conditions of the North Sea. The effect of incoming waves is illustrated by analysing four different wave heading angles. To quantify the performance of the WECs in an array, the q-factor and coefficient of variation are studied for each array at different sea states. The study suggested the best possible arrangement pattern for wave power absorption and power uniformity among the floaters in the array. The study performed will be helpful in the design and analysis of the possible arrangement of point absorbers around the floating wind turbine platform for wave power absorption. © 2023 Elsevier LtdItem 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 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 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. © 2025Item Dynamic analysis of a TLP-type floating wind turbine combined with OWC wave energy converter(Springer Nature, 2025) Sebastian, B.; Joju, A.; Karmakar, D.The present study examines the dynamic effects of integrating oscillating water column wave energy converters on the offset columns of a tension leg floating wind turbine platform in an asymmetric and symmetric configuration. Two configurations are considered, featuring two and four oscillating water columns combined with the tension leg platform supporting a 5 MW wind turbine. The hydrodynamic analysis of the combined wind-wave energy system uses a linear diffraction-radiation tool to compute hydrodynamic coefficients and wave excitation forces in the frequency domain. The coupled dynamic responses of the hybrid platforms are evaluated in the time domain under various irregular sea states, using an aero-hydro-servo-elastic simulation tool. The performance of the hybrid systems is compared with a baseline floating wind turbine platform to quantify changes in dynamic responses. Power absorption of the oscillating water columns is computed using a linear power take-off system. The findings indicate that adding oscillating water columns leads to a slight increase in the heave and pitch motions of the platform. The system with a diagonally placed two-oscillating water column configuration demonstrates higher efficiency, achieving a maximum capture width ratio of 57%. This study provides valuable insights into the feasibility of hybrid offshore renewable energy concepts. It supports the design and implementation of integrated wind-wave systems to deliver clean and sustainable energy. © The Author(s), under exclusive licence to Sociedade Brasileira de Engenharia Naval 2025.