In recent years, the social demands for the introduction of renewable energy are increasing, demonstration projects of floating offshore wind turbine are being implemented and planned around the world. In Japan, a demonstration test named Fukushima FORWARD (Fukushima Floating Offshore Wind Farm Demonstration Project) has been conducted since 2011. Fukushima FORWARD is a project carried out by the Ministry of Economy, Trade and Industry, the world’s first floating offshore windfarm with a total capacity of 14 MW, including three floating offshore wind facilities and one floating offshore substation. In Fukushima FORWARD, Japan Marine United Corporation is in charge of floater part EPCI (Engineering, Procurement, Construction and Installation) of one floating offshore wind facility and one floating offshore substation. The floating offshore wind turbine (Ship Name: Fukushima Hamakaze) designed and built by Japan Marine United Corporation is equipped with a downwind 5 MW wind turbine. The floating structure adopts the advanced spar shape in order to reduce wave frequency motions and is moored by six spread catenary mooring lines. In the design of floating offshore wind turbines, it is important to estimate motions with high accuracy. Especially floating offshore wind turbine equipped with horizontal axis wind turbine requires heavy RNA (Rotor Nacelle Assembly) to be installed on the tower, and the floater motion greatly influences the design of the tower base. The tower base is required to have sufficient reliability because it directly leads to collapse of the wind turbine if it is damaged. On the other hand, the tower base is generally constructed with a cylinder made of extremely thick steel plate which is difficult to bend and weld, and if it has excessive safety factor the cost has increased greatly. Also, estimating the motion is the basis for estimating the load on the floating structure. In this paper, statistical analysis of long-term data measured by Fukushima FORWARD floating offshore wind turbine focusing on the motion and its features are introduced. In addition, we compare the motion obtained by potential theory and coupled analysis with actually measured motion using the measured wave and wind data and evaluate the validity of the analysis method.
We are developing cross-shaped semi-submersible floating structure composed of simple and lean stiffened plate structure. In this paper, we introduce the features and development studies of our 12MW semi-submersible floating offshore wind turbine. First, we introduce the main features of it. Second, we introduce the tank test of it, the purpose of which is verifying the behavior under the condition only wave load is applied. As a result of tank test, we verified motion response which is calculated by numerical calculation tool based on potential theory is well reasonable. Third, we introduce the initial structural analysis, the purpose of it is to decide initial scantling. Finally, we introduce the coupled analysis of our 12MW semi-submersible floating offshore wind turbine. Concerning the model of coupled analysis, we verified it from the several points of view. We verified the motion response of it under the condition that only wave load is applied. Compared to the result of numerical calculation and tank test, we confirmed motion response of coupled analysis model is valid. We also confirmed the natural frequency of the whole structure and the displacement due to both rigid motion and elastic deformation are reasonable compared to the result of FEM analysis. Then, we performed the coupled analysis under the condition that both wave load and wind load are applied considering the effect of turbine’s operation.
In recent years, the social demands for the introduction of renewable energy are increasing, demonstration projects of floating offshore wind power generation are being implemented and planned around the world. In Japan, a demonstration project named Fukushima FORWARD (Fukushima Floating Offshore Wind Farm Demonstration Project) has been conducted since 2011. Fukushima FORWARD is carried out by the Ministry of Economy, Trade and Industry. The project is the world’s first floating offshore windfarm with a total capacity of 14 MW, including three floating offshore wind facilities and one floating offshore substation. In Fukushima FORWARD, Japan Marine United Corporation is in charge of floater part EPCI (Engineering, Procurement, Construction and Installation) of one floating offshore wind facility and one floating offshore substation. This floating offshore substation is installed in order to observe meteorological and oceanographic data and motion data as well as boosting the generated electric power. Since the installation in 2013 it continues to record various kinds of continuous data. The substation is an advanced spar type floater moored by four spread catenary mooring lines. In the design of the mooring system for offshore structure, the motion of the structure under environmental external force is very important. The motion of the moored floating structure is divided into wave frequency motion, which is a motion of a relatively short period, and low frequency motion caused by mooring restoring force and variable external force, both of which are important elements in the design. Among them, wave frequency motion is known to be accurately estimated by potential theory as a result of research on various types of structures. On the other hand, in addition to the existence of various calculation methods including time domain analysis, its statistical characteristic and applicability are entirely depending on the target structure. Also, observation data of low frequency motion have been very few. In this paper, long-term data observed at the floating offshore substation in Fukushima FORWARD was analyzed with focusing on low frequency motion and its statistical properties were clarified. Furthermore, we analyzed the low frequency wave force spectrum and motion by conventional low frequency motion theory using the wave drifting force calculated by the potential theory. And, we compared the calculated value with the analysis result of the observation data and validated the applicability of the simplified low frequency motion theory.
Global population growth and climate change are driving a need for increased clean renewable energy generation. One such resource is wind energy and while the onshore and fixed offshore wind energy industries are mature, the floating offshore wind energy industry is still at a demonstration phase. Floating wind turbine platforms are generally of a much smaller displacement than the typical offshore structures that have been used in the oil and gas industry. This difference results in changes to the platform dynamics, especially those resulting from second order wave forces. Existing research into low frequency drift motions of small body platforms has been mainly confined to numerical modelling with some experimental work. This work expands on this knowledge by validating numerical modelling with full scale observational data. In this paper, a numerical time-domain model of a relatively small displacement platform is developed. The platform is installed in a relatively shallow water depth of about 110 m and station keeping is provided by four equally spaced catenary mooring chains. The required fidelity for the low frequency response is compared using first order forces only and either a full QTF (quadratic transfer equation) or Newman’s approximation. The model is compared with observation data from the Fukushima FORWARD project’s floating substation, an advanced spar type, which is composed of measurements of multidirectional wave spectra, wind and current as model inputs and six DOF platform motions as outputs. In addition to this the model computational expense is reduced by decreasing the number of wave directions simulated. The accuracy of such reductions is then described. Observation data is grouped according to sea-state data. An empirical drag coefficient formula is proposed. The 50 year return period design sea-state is also modelled using a JONSWAP spectrum and the various numerical models.
A shallow draft cylindrical barge type floater with footing close to the water surface was experimentally evaluated in waves to investigate non-linear motion characteristics. The floater was designed to be used as an option for FOWT—floating offshore wind turbines. The non-linear mechanism can be promoted due to the viscous force that acts on the footing edges and the footing interaction with the free surface. In general, the observed non-linear viscous damping is modeled as a force proportional to the square of the relative velocity between the floater and the water. Therefore, the viscous damping levels is expected to increase, and the response in waves, to decrease. However, an increase in motion responses was observed for a broad range of wave periods. An attempt was made to clarify the hydrodynamic mechanism by comparing wave tank experiments, numerical calculations by CFD—computational fluid dynamics codes, and linear potential theory codes. Regular wave tests for three different wave height conditions were carried out, including free decay tests in still waters. For CFD simulations, the OpenFOAM code was selected. For potential theory simulations, the WAMIT code was chosen. As a result of the research, three points could be highlighted and discussed: first, the hydrodynamic phenomenon that contributed to the non-linear motion of the floater was identified; second, the increase and coupling of the motions response of heave and pitch motions; and finally, the phenomenon that the footing held water mass and lifted it to the water surface. The CFD calculations were able to get good qualitative results compared with the experiments and confirmed the use of CFD as a useful tool to capture the non-linear hydrodynamic phenomenon. The linear potential theory was not able to capture the phenomenon discussed herein.
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