• Journal of Ocean Engineering and Technology
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• v.34 no.6
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• pp.454-460
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• 2020

In 2019, the Korean government announced the 3rd Basic Plan for Energy, which included expanding the rate of renewable energy generation by 30-40% by 2040. Hence, offshore wind power generation, which is relatively easy to construct in large areas, should be considered. The East Sea coast of Korea is a sea area where the depth reaches 50 m, which is deeper than the west coast, even though it is only 2.5 km away from the coastline. Therefore, for offshore wind power projects on the East Sea coast, a floating offshore wind power should be considered instead of a fixed one. In this study, a response analysis was performed by applying the analytical conditions of IEC61400-3-2 for the design of floating offshore wind power generation systems. In the newly revised IEC61400-3-2 international standard, design load cases to be considered in floating offshore wind power systems are specified. The upper structure applied to the numerical analysis was a 5-MW-class wind generator developed by the National Renewable Energy Laboratory (NREL), and the marine environment conditions required for the analysis were based on the Ulsan Meteorological Buoy data from the Korea Meteorological Administration. The FAST v8 developed by NREL was used in the coupled analysis. From the simulation, the maximum response of the six degrees-of-freedom motion and the maximum load response of the joint part were compared. Additionally, redundancy was verified under abnormal conditions. The results indicate that the platform has a maximum displacement radius of approximately 40 m under an extreme sea state, and when one mooring line is broken, this distance increased to approximately 565 m. In conclusion, redundancy should be verified to determine the design of floating offshore wind farms or the arrangement of mooring systems.

• Ocean Systems Engineering
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• v.6 no.2
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• pp.203-216
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• 2016

Over recent years the offshore wind turbines are becoming more feasible solution to the energy problem, which is crucial for Egypt. In this article a three floating support structure, tension leg platform types (TLP), for 5-MW wind turbine have been considered. The dynamic behavior of a triangular, square, and pentagon TLP configurations under multi-directional regular and random waves have been investigated. The environmental loads have been considered according to the Egyptian Metrological Authority records in northern Red sea zone. The dynamic analysis were carried out using ANSYS-AQWA a finite element analysis software, FAST a wind turbine dynamic software, and MATLAB software. Investigation results give a better understanding of dynamical behavior and stability of the floating wind turbines. Results include time history, Power Spectrum densities (PSD's), and plan stability for all configurations.

• Journal of Advanced Research in Ocean Engineering
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• v.2 no.4
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• pp.179-191
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• 2016

As the demand for renewable energy has increased following the worldwide agreement to act against global climate change and disaster, large-scale floating offshore wind systems have become a more viable solution. However, the cost of the whole system is still too high for practical realization. To make the cost of a floating wind system be more economical, a new concept of tension leg platform (TLP) type ocean floating wind system has been developed. To verify the performance of a 5-MW TLP type ocean floating wind power system designed by the Korea Advanced Institute of Science and Technology, the FAST simulation developed by the National Renewable Energy Laboratory is used. Further, 1/50 scale model tests have been carried out in the ocean engineering tank of the Research Institute of Medium and Small Shipbuilding, Korea. This paper compares the simulation and ocean engineering tank test results on motion prediction and tension assessment of the TLP anchor.

• Ocean Systems Engineering
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• v.11 no.1
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• pp.17-42
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• 2021

Articulated loading platforms (ALPs) belongs to a class of offshore structures known as compliant. ALP motions have time periods falling in the wind excitation frequency range due to their compliant behaviour. This paper deals with the dynamic behavior of a double hinged ALP subjected to low-frequency wind forces with random waves. Nonlinear effects due to variable submergence, fluctuating buoyancy, variable added mass, and hydrodynamic forces are considered in the analysis. The random sea state is characterized by the Pierson-Moskowitz (P-M) spectrum. The wave forces on the submerged elements of the platform's shaft are calculated using Morison's Equation with Airy's linear wave theory ignoring diffraction effects. The fluctuating wind load has been estimated using Ochi and Shin wind velocity spectrum for offshore structures. The nonlinear dynamic equation of motion is solved in the time domain by the Wilson-θ method. The wind-structure interactions, along with the effect of various other parameters on the platform response, are investigated. The effect of offset of aerodynamic center (A.C.) with the center of gravity (C.G.) of platform superstructure has also been investigated. The outcome of the analyses indicates that low-frequency wind forces affect the response of ALP to a large extent, which otherwise is not enhanced in the presence of only waves. The mean wind modifies the mean position of the platform surge response to the positive side, causing an offset. Various power spectral densities (PSDs) under high and moderate sea states show that apart from the significant peak occurring at the two natural frequencies, other prominent peaks also appear at very low frequencies showing the influence of wind on the response.

• Journal of Ocean Engineering and Technology
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• v.28 no.4
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• pp.288-293
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• 2014

The IEC standard for onshore or offshore wind turbines requires additional dummy simulations (at least 5 s) for the transient responses due to initial conditions. An increase in the dummy time causes a considerable increase in the computational cost considering multiple design spirals with several thousand design load analysis cases. A time of 30 s is typically used in practical simulations for a wind turbine design with a fixed platform. However, 30 s may be insufficient for floating offshore wind turbines (FOWT) because the platforms have lower natural frequencies, and the transient responses will last much longer. In this paper, an initial condition application algorithm is implemented for WindHydro, and the appropriate dummy simulation time is investigated based on a series of dynamic simulations of a FOWT. As a result, it is found that more than 300 s is required for the platform to have stationary motion after the initial transient responses for the FOWT under the conditions considered.

• Journal of Ocean Engineering and Technology
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• v.32 no.1
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• pp.9-20
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• 2018

The present study deals with the conceptual design of a motion reduction device for a floating wave-offshore wind hybrid power generation platform. A damping plate attached to the bottom of a column of a large semi-submersible is introduced to reduce the motion of the platform. Performance analyses on various shapes and configurations of damping plates were performed using the potential flow solver, and the appropriate configuration and size of the damping plate were selected based on the numerical results. In order to see the effect of viscous damping, a small scale model test was performed in a 2D wave flume. The performances of five different damping plates were measured and discussed based on the results of free decay tests and regular wave tests.

• Journal of the Society of Naval Architects of Korea
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• v.51 no.4
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• pp.300-310
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• 2014

This paper presents a mooring design procedure of a floating offshore wind turbine. The environment data of south offshore area of Jeju collected from Korea Hydrographic and Oceanographic Administration(KHOA) are used for hydrodynamic analyses as environmental conditions. We considered a semi-submersible type floating wind turbine based on Offshore Code Comparison Collaborative Continuation(OC4) DeepCWind platform and National Renewable Energy Laboratory(NREL) 5 MW class wind turbine. Catenary mooring with studless chain is chosen as the mooring system. Important design decisions such as how large the nomial sizes are, how long the mooring lines are, how far the anchor points are located, are demonstrated in detail. Considering ultimate limit state and fatigue limit state based on 100-year return period and 50-year design life, respectively, longterm predictions of breaking strength and fatigue are proposed.

• Smart Structures and Systems
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• v.24 no.3
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• pp.403-414
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• 2019

Offshore drilling has become a key process for obtaining oil. Offshore platforms have many applications, including oil exploration and production, navigation, ship loading and unloading, and bridge and causeway support. However, vibration problems caused by severe environmental loads, such as ice, wave, wind, and seismic loads, threaten the functionality of platform facilities and the comfort of workers. These concerns may result in piping failures, unsatisfactory equipment reliability, and safety concerns. Therefore, the vibration control of offshore platforms is essential for assuring structural safety, equipment functionality, and human comfort. In this study, an optimal multiple tuned mass damper (MTMD) system was proposed to mitigate the excessive vibration of a three-dimensional offshore platform under ice and earthquake loadings. The MTMD system was designed to control the first few dominant coupled modes. The optimal placement and system parameters of the MTMD are determined based on controlled modal properties. Numerical simulation results show that the proposed MTMD system can effectively reduce the displacement and acceleration responses of the offshore platform, thus improving safety and serviceability. Moreover, this study proposes an optimal design procedure for the MTMD system to determine the optimal location, moving direction, and system parameters of each unit of the tuned mass damper.

• International Journal of Naval Architecture and Ocean Engineering
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• v.11 no.1
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• pp.1-10
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• 2019

Nowadays, the study on Floating Offshore Wind Turbines (FOWTs) is being performed globally. Dozens of numerical simulation tools have been developed for designing FOWTs and simulating their performances in combined wave and wind environments. On the other hand, model tests are still required to verify the results obtained from numerical simulation tools. To predict seakeeping performance of the OC3-Hywind platform, a OC3 spar model moored by a 3-leg catenary spread mooring system with a delta connection was built with a 1/128 scale ratio. The model tests were carried out for various sea states, including rotating rotor effect with wind in the Ocean Engineering Wide Tank, University Of Ulsan (UOU). The model test results are compared with the numerical simulations by UOU in-house code and FAST.

• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.367-375
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• 2020

This paper presents an integrated analysis about dynamic performance of a Floating Offshore Wind Turbine (FOWT) OC4 DeepCwind with semi-submersible platform under real sea environment. The emphasis of this paper is to investigate how the wave mean drift force and slow-drift wave excitation load (Quadratic transfer function, namely QTF) influence the platform motions, mooring line tension and tower base bending moments. Second order potential theory is being used for computing linear and nonlinear wave effects, including first order wave force, mean drift force and slow-drift excitation loads. Morison model is utilized to account the viscous effect from fluid. This approach considers floating wind turbine as an integrated coupled system. Two time-domain solvers, SIMA (SIMO/RIFLEX/AERODYN) and FAST are being chosen to analyze the global response of the integrated coupled system under small, moderate and severe sea condition. Results show that second order mean drift force and slow-drift force will drift the floater away along wave propagation direction. At the same time, slow-drift force has larger effect than mean drift force. Also tension of the mooring line at fairlead and tower base loads are increased accordingly in all sea conditions under investigation.