• Wind and Structures
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• v.37 no.6
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• pp.461-471
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• 2023

The main objective of this study was to establish design guidelines for three key design variables (spar thickness, spar diameter, and total draft) by examining their impact on the stress distribution and resonant frequency of a 2.5-MW spar-type floating offshore wind turbine substructure under extreme marine conditions, such as during Typhoon Bolaven. The current findings revealed that the substructure experienced maximum stress at wave frequencies of either 0.199 Hz or 0.294 Hz, consistent with previously reported experimental findings. These results indicated that the novel simulation method proposed in this study, which simultaneously combines hydrodynamic diffraction analysis, computational dynamics analysis, and structural analysis, was successfully validated. It also demonstrated that our proposed simulation method precisely quantified the stress distribution of the substructure. The novel findings, which reveal that the maximum stress of the substructure increases with an increase in total draft and a decrease in spar thickness and spar diameter, offer valuable insights for optimizing the design of spar-type floating offshore wind turbine substructures operating in various harsh marine environments.

• Wind and Structures
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• v.11 no.1
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• pp.19-33
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• 2008

This research investigates the adaptive input estimation method applied to the multilayer shearing stress structure. This method is to estimate the values of wind load inputs by analyzing the active reaction of the system. The Kalman filter without the input term and the adaptive weighted recursive least square estimator are two main portions of this method. The innovation vector can be produced by the Kalman filter, and be applied to the adaptive weighted recursive least square estimator to estimate the wind load input over time. This combined method can effectively estimate the wind loads to the structure system to enhance the reliability of the system active performance analysis. The forms of the simulated inputs (loads) in this paper include the periodic sinusoidal wave, the decaying exponent, the random combination of the sinusoidal wave and the decaying exponent, etc. The active reaction computed plus the simulation error is regard as the simulated measurement and is applied to the input estimation algorithm to implement the numerical simulation of the inverse input estimation process. The availability and the precision of the input estimation method proposed in this research can be verified by comparing the actual value and the one obtained by numerical simulation.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.21 no.6
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• pp.938-945
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• 2012

This research proposes FEM analysis for Tandem welding process used in wind tower and predicts optimal welding process to improve the stability of welded structures. Three dimensional elasto-plastic analyses are employed to evaluate thermo-mechanical behavior of residual stress and deformation during Tandem welding for different distance between two touches. To confirm the thermal distribution, Goldak's ellipse heat source model and the real size wind tower pipe model are utilized. Four different analyses are being performed, where in each case the distance between two electrode torches is being changed and residual stress and welding deformation are predicted. Depending on base material state, each case is divided into: Liquid (100mm), Austenite+Liquid (200mm), Austenite+Cementite (400mm), Pearlite+Cementite (800mm).

• 한국신재생에너지학회:학술대회논문집
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• 2011.11a
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• pp.43.2-43.2
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• 2011

Wind turbine pitch/yaw bearings are relatively big and have different operating conditions like very heavy load to support compared with widely used industrial bearings. Once pitch/yaw bearings failed, according to their special surroundings, serious damages like higher repair costs and additional costs by stopped electricity generation are occur. Therefore, pitch/yaw bearings must be designed to have enough strength and fatigue life under actual operating conditions. In this study, with finite element analysis, it was investigated that stress distribution between rolling elements and raceway and comparatively analyzed using widely used guideline (NREL DG03). Design parameters of wind turbine pitch/yaw bearings are also analyzed, and it could be used as reference for the large bearing design field.

• Wind and Structures
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• v.31 no.1
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• pp.59-73
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• 2020

Wind fragility analysis (WFA) of concrete chimney is often executed disregarding temperature effects. But combined wind and temperature effect is the most critical limit state to define the safety of a chimney. Hence, in this study, WFA of a 70 m tall RC chimney for combined wind and temperature effects is explored. The wind force time-history is generated by spectral representation method. The safety of chimney is assessed considering limit states of stress failure in concrete and steel. A moving-least-squares method based dual response surface method (DRSM) procedure is proposed in WFA to alleviate huge computational time requirement by the conventional direct Monte Carlo simulation (MCS) approach. The DRSM captures the record-to-record variation of wind force time-histories and uncertainty in system parameters. The proposed DRSM approach yields fragility curves which are in close conformity with the most accurate direct MCS approach within substantially less computational time. In this regard, the error by the single-level RSM and least-squares method based DRSM can be easily noted. The WFA results indicate that over temperature difference of 150℃, the temperature stress is so pronounced that the probability of failure is very high even at 30 m/s wind speed. However, below 100℃, wind governs the design.

• Proceedings of the KSRS Conference
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• 2002.10a
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• pp.621-626
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• 2002

A satellite scatterometer is a microwave radar sensor used to measure the backscattering at a sea surface. This instrument transmits radar pulses to the sea surface and measure the radar energy reflected back towards the source. Changes in wind velocity make sea surface roughness change and then affect on backscattered power. This gives us information of sea surface wind speed. Directions of wind vectors are acquired by multiple, collocated, and nearly simultaneous measurements. It should be noted that the scatterometer observes not the wind directly but the wind stress vector relative to the surface current. This suggests the possibility that the satellite scatterometer winds can include the effect of the surface current. This study shows the evidence that scatterometer measure surface wind stress, not surface winds and presents the velocity structure of oceanic warm and cold eddies.

• Advances in Computational Design
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• v.8 no.3
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• pp.191-217
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• 2023

The objective of this study was to evaluate the foundation structure of a 3.6-MW wind turbine generator (WTG) installed offshore in Western Korea. The ultimate limit state (ULS) and fatigue limit state (FLS) of the multi-pile steel foundation (MSF) installed at the Saemangeum offshore wind farm were structurally investigated using the finite element (FE) software, ANSYS Workbench 19.0. According to the ULS analysis, no plastic deformation was found in any of the components constituting the substructure. At the same time, the maximal stress value reached the calculation limit of 335 MPa. According to the FLS results, the stress concentration factor (SCF) ranged from 1.00 to 1.88 in all components. The results of this study can be applied to determine the optimal design for MSFs.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.7 no.3
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• pp.265-276
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• 1995

In this Paper, the Performance of the convolution method has been investigated as an effort to develop a simple system of predicting wind-driven surface current on a real time basis. In this approach wind stress is assumed to be spatially uniform and the effect of atmospheric pressure is neglected. The discrete convolution weights are determined in advance at each point using a linear three-dimensional Galerkin model with linear shape functions(Galerkin-FEM model). Four directions of wind stress(e.g. NE, SW, NW, SE) with unit magnitude are imposed in the model calculation for the construction of data base for convolution weights. Given the time history of wind stress, it is then possible to predict with-driven currents promptly using the convolution product of finite length. An unsteady wind stress of arbitrary form can be approximated by a series of wind pulses with magnitude of 6 hour averaged value. A total of 12 pulses are involved in the convolution product To examine the accuracy of the convolution method a series of numerical experiments has been carried out in the idealized basin representing the scale of the Yellow Sea and the East China Sea. The wind stress imposed varies sinusoidally in time. It was found that the predicted surface currents and elevation fields were in good agreement with the results computed by the direct integration of the Galerkin model. A model with grid 1/8$^{\circ}$ in latitude, l/6$^{\circ}$ in longitude was established which covers the entire region of the Yellow Sea and the East China Sea. The numerical prediction in terms of the convolution product has been carried out with particular attention on the formation of upwind flow in the middle of the Yellow Sea by northerly wind.

• Journal of the Korean Society of Safety
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• v.29 no.2
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• pp.53-61
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• 2014

This study evaluated wind loads considering a local basic wind velocity and construction periods to define the level of applied wind loads for system supports. Structural responses of system supports were examined and compared to those of system supports with the level of wind loads following various standards and specifications for permanent and temporary structures. And, the maximum combined stress ratios were estimated to evaluate the structural safety of a considered system support. From results, it was found that the wind load level should be applied in accordance with construction periods when estimating the safety of system supports. Looking into the response by change of the basic wind velocity according to local regions, it is no need to consider wind loads in regions with the basic wind velocity of 30 m/s. However, it was analyzed that wind loads should be considered in the regions with the basic wind velocity of 40 m/s or above. In addition, wind loads should be considered in designing system supports located at the region with the basic wind velocity of 35 m/s starting from construction period of 1.5 years. The standard specification for temporary work was analyzed as an incorrect standard in evaluating wind loads, since it underestimated the response of system supports in accordance with the local basic wind velocity and construction periods.

• Proceedings of the KSRS Conference
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• 2008.10a
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• pp.309-312
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• 2008

The advent of high resolution products of surface wind and temperature derived by satellite data has permitted us to investigate ocean and atmosphere interaction studies in detail. Especially the Kuroshio extension region of the western North Pacific is considered to be a key area for such studies. We have constructed gridded products of surface wind/wind stress over the world ocean using satellite scatterometer (Qscat/SeaWinds), available as the Japanese Ocean Flux data sets with Use of Remote sensing Observation (J-OFURO). Using new data based on improved algorithm which have been recently delivered, we are reconstructing gridded product with higher spatial resolution. Intercomparison of this product with the previous one reveals that there are some discrepancies between them in short-period and high wind-speed ranges especially in the westerly wind region. The products are validated by not only comparisons with in-situ measurement data by mooring buoys such as TAO/TRITON in the tropical Pacific and the Kuroshio Extension Observation (KEO) buoys, but also intercomparison with numerical weather prediction model (NWPM) products (the NRA-1 and 2). Our products have much smaller mean difference in the study areas than the NWPM ones, meaning higher reliability compared with the NWPM products. Using the high resolution products together with sea surface temperature (SST) data, we examine a new type of relationship between the lower atmosphere and upper ocean in the Kuroshio Extension region. It is suggested that the spatial relation between the wind speed and SST depends upon, more or less, the surrounding oceanic condition.