Techno Press
Tp_Editing System.E (TES.E)
Login Search


was
 
CONTENTS
Volume 21, Number 6, December 2015
 

Abstract
.

Key Words
.

Address
.

Abstract
With an increasing demand of a renewable energy, new offshore wind turbine farms are being planned in some parts of the world. Foundation installation asks a significant cost of the total budget of offshore wind turbine (OWT) projects. Hence, a cost reduction from foundation parts is a key element when a cost-efficient designing of OWT budget. Mono-piles have been largely used, accounting about 78% of existing OWT foundations, because they are considered as a most economical alternative with a relatively shallow-water, less than 30 m of seawater depth. OWT design standards such as IEC, GL, DNV, API, and Eurocode are being developed in a form of reliability based limit state design method. In this paper, reliability analysis using the response surface method (RSM) and numerical simulation technique for an OWT mono-pile foundation were performed to investigate the sensitivities of mono-pile design parameters, and to find practical implications of RSM reliability analysis.

Key Words
pile foundation; mono-pile; reliability analysis; response surface method; soil-pile interaction

Address
Sun B. Kim: Coastal Engineering Division, Korea Institute of Ocean Science and Technology, 787 Haean-ro, Sangnok-gu, Ansan 15627, Korea
Gil L. Yoon and Jin H. Yi: Coastal Engineering Division, Korea Institute of Ocean Science and Technology, 787 Haean-ro, Sangnok-gu, Ansan 15627, Korea;
Department of Convergence Study on the Ocean Science and Technology, Ocean Science and Technology School, Korea Maritime and Ocean University, Busan, Korea
Jun H. Lee: School of Civil and Environmental Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu,
Seoul 03722, Korea



Abstract
Seismic reliability analysis of a jacket-type support structure for an offshore wind turbine was performed. When defining the limit state function by using the dynamic response of the support structure, a number of dynamic calculations must be performed in a First-Order Reliability Method (FORM). That means analysis costs become too high. In this paper, a new reliability analysis approach using a static response is used. The dynamic effect of the response is considered by introducing a new parameter called the Peak Response Factor (PRF). The probability distribution of PRF can be estimated by using the peak value in the dynamic response. The probability distribution of the PRF was obtained by analyzing dynamic responses during a set of ground motions. A numerical example is presented to compare the proposed approach with the conventional static response-based approach.

Key Words
offshore wind turbine; support structures; reliability; earthquake; dynamic response; peak response

Address
Dong-Hyawn Kim and Gee-Nam Lee: Department of Ocean Science and Engineering, Kunsan National University, Kunsan, 54150, Korea
Yongjei Lee: Chang Minwoo Structural Consultants, Seoul, 135-907, Korea
Il-Keun Lee: Korea Expressway Corporation Research Institure, Gyeonggi, 18489, Korea

Abstract
Monopiles have been most widely used for supporting offshore wind turbines (OWTs) in shallow water areas. However, multi-member lattice-type structures such as jackets and tripods are also considered good alternatives to monopile foundations for relatively deep water areas with depth ranging from 25–50 m owing to their technical and economic feasibility. Moreover, jacket structures have been popular in the oil and gas industry for a long time. However, several unsolved technical issues still persist in the utilization of multi-member lattice-type supporting structures for OWTs; these problems include pile-soil-interaction (PSI) effects, realization of dynamically stable designs to avoid resonances, and quick and safe installation in remote areas. In this study, the effects of PSI on the dynamic properties of bottom-fixed OWTs, including monopile-, tripod- and jacket-supported OWTs, were investigated intensively. The tower and substructure were modeled using conventional beam elements with added mass, and pile foundations were modeled with beam and nonlinear spring elements. The effects of PSI on the dynamic properties of the structure were evaluated using Monte Carlo simulation considering the load amplitude, scouring depth, and the uncertainties in soil properties.

Key Words
pile-soil-interaction (PSI); bottom-fixed offshore wind turbine; random sampling; natural frequency; scouring

Address
Jin-Hak Yi and Gil-Lim Yoon: Coastal and Environmental Engineering Division, Korea Institute of Ocean Science and Technology, Gyeonggi 426-744, Korea;
Department of Convergence Study on the Ocean Science and Technology, Ocean Science and Technology School, Korea Maritime and Ocean University, Busan, Korea
Sun-Bin Kim: Coastal and Environmental Engineering Division, Korea Institute of Ocean Science and Technology, Gyeonggi 426-744, Korea
Lars Vabbersgaard Andersen: Department of Civil Engineering, Aalborg University, Aalborg, Denmark


Abstract
Using large monopod bucket foundations as an alternative to monopiles for offshore wind turbines offers the potential for large cost savings compared to typical piled foundations. In this paper, numerical simulations are carried out to assess the risk of structural buckling during installation of large-diameter bucket foundations. Since shell structures are generally sensitive to initially imperfect geometries, eigenmode-affine imperfections are introduced in a nonlinear finite-element analysis. The influence of modelling the real lid structure compared to classic boundary conditions is investigated. The effects of including soil restraint and soil–structure interaction on the buckling analysis are also addressed.

Key Words
monopod; bucket foundation; buckling; instability

Address
Soren Madsen and Lars V. Andersen: Department of Civil Engineering, Aalborg University, Sofiendalsvej 9-11, DK-9000 Aalborg, Denmark
Rodney Pinna: AkerSolutions Pty. Ltd., Perth, Western Australia, Australia
Mark Randolph: Centre for Offshore Foundation Systems, The University of Western Australia, 35 Stirling Highway, CRAWLEY WA 6009, Australia


Abstract
In this study, the feasibility of vibration-based damage detection methods for the wind turbine tower (WTT) structure is evaluated. First, a frequency-based damage detection (FBDD) is outlined. A damage-localization algorithm is visited to locate damage from changes in natural frequencies. Second, a mode-shape-based damage detection (MBDD) method is outlined. A damage index algorithm is utilized to localize damage from estimating changes in modal strain energies. Third, a finite element (FE) model based on a real WTT is established by using commercial software, Midas FEA. Several damage scenarios are numerically simulated in the FE model of the WTT. Finally, both FBDD and MBDD methods are employed to identify the damage scenarios simulated in the WTT. Damage regions are chosen close to the bolt connection of WTT segments; from there, the stiffness of damage elements are reduced.

Key Words
frequency-based damage detection, mode-shape-based damage detection, wind turbine tower structure, Midas FEA

Address
Tuan-Cuong Nguyen, Thanh-Canh Huynh and Jeong-Tae Kim: Department of Ocean Engineering, Pukyong National University, 45 Yongso-ro, Nam-gu, Busan, Korea

Abstract
In this study, the geometrical setup of a turbine blade is tracked. A research-scale rotating turbine blade system is setup with a single 3-axes accelerometer mounted on one of the blades. The turbine system is rotated by a controlled motor. The tilt and rolling angles of the rotating blade under operating conditions are determined from the response measurement of the single accelerometer. Data acquisition is achieved using a prototype wireless sensing system. First, the Rodrigues\' rotation formula and an optimization algorithm are used to track the blade rolling angle and pitching angles of the turbine blade system. In addition, the blade flapwise natural frequency is identified by removing the rotation-related response induced by gravity and centrifuge force. To verify the result of calculations, a covariance-driven stochastic subspace identification method (SSI-COV) is applied to the vibration measurements of the blades to determine the system natural frequencies. It is thus proven that by using a single sensor and through a series of coordinate transformations and the Rodrigues\' rotation formula, the geometrical setup of the blade can be tracked and the blade flapwise vibration frequency can be determined successfully.

Key Words
signal processing; stochastic subspace identification; Rodrigues\' rotation formula; wireless sensing system; a research-scale turbine blade system

Address
Chin-Hsiung Loh, Yu-Ting Huang and Wan-Ying Hsiung: Department of Civil Engineering, National Taiwan University, Taipei 10617, Taiwan
Yuan-Sen Yang: Department of Civil Engineering, National Taipei University of Technology, Taipei, Taiwan
Kenneth J. Loh: Department of Structural Engineering, University of California at San Diego, La Jolla, CA 92093, USA

Abstract
The efficiency of wind turbines (WT) is primarily reflected in their ability to generate electricity at any time. Downtimes of WTs due to \"conventional\" inspections are cost-intensive and undesirable for investors. For this reason, there is a need for structural health monitoring (SHM) systems, to enable service and maintenance on demand and to increase the inspection intervals. In general, monitoring increases the cost effectiveness of WTs. This publication concentrates on the application of two vibration-based SHM algorithms for stability and structural change monitoring of offshore WTs. Only data driven, output-only algorithms based on stochastic subspace identification (SSI) in time domain are considered. The centerpiece of this paper deals with the rough mathematical description of the dynamic behavior of offshore WTs and with the basic presentation of stochastic subspace-based algorithms and their application to these structures. Due to the early stage of the industrial application of SHM on offshore WT on the one side and the required confidentiality to the plant manufacturer and operator on the other side, up to now it is not possible to analyze different isolated structural damages resp. changes in a systematic manner, directly by means of in-situ measurement and to make these \"acknowledgements\" publicly available. For this reason, the sensitivity of the methods for monitoring purposes are demonstrated through their application on long time measurements from a 1:10 large scale test rig of an offshore WT under different conditions: undamaged, different levels of loosened bolt connections between tower parts, different levels of fouling, scouring and structure inclination. The limitation and further requirements for the approaches and their applicability on real foundations are discussed along the paper.

Key Words
offshore wind turbine; structural health monitoring; stochastic subspace identification

Address
Peter Kraemer and Herbert Friedmann: Wölfel Beratende Ingenieure GmbH + Co. KG, Max-Planck-Strasse 15, 97204 Höchberg, Germany

Abstract
In this study, a novel vision-based bolt-loosening monitoring technique is proposed for bolted joints connecting tubular steel segments of the wind turbine tower (WTT) structure. Firstly, a bolt-loosening detection algorithm based on image processing techniques is developed. The algorithm consists of five steps: image acquisition, segmentation of each nut, line detection of each nut, nut angle estimation, and bolt-loosening detection. Secondly, experimental tests are conducted on a lab-scale bolted joint model under various bolt-loosening scenarios. The bolted joint model, which is consisted of a ring flange and 32 sets of bolt and nut, is used for simulating the real bolted joint connecting steel tower segments in the WTT. Finally, the feasibility of the proposed vision-based technique is evaluated by bolt-loosening monitoring in the lab-scale bolted joint model.

Key Words
bolted joint; bolt loosening; vision; image processing; steel structure; wind turbine tower

Address
Jae-Hyung Park, Thanh-Canh Huynh, Sang-Hoon Choi and Jeong-Tae Kim: Department of Ocean Engineering, Pukyong National University, 45 Yongso-ro, Nam-gu, Busan, Korea

Abstract
A double-skinned composite tubular (DSCT) wind power tower was suggested and automatic section design software was developed. The developed software adopted the nonlinear material model and the nonlinear column model. If the outer diameter, material properties and design capacities of a DSCT wind power tower are given, the developed software performs axial force-bending moment interaction analyses for hundreds of sections of the tower and suggests ten optimized cross-sectional designs. In this study, 80 sections of DSCT wind power towers were designed for 3.6 MW and 5.0 MW turbines. Moreover, the performances of the 80 designed sections were analyzed with and without considerations of large displacement effect. In designing and analyzing them, the material nonlinearity and the confining effect of concrete were considered. The comparison of the analysis results showed the moment capacity loss of the wind power tower by the mass of the turbine is significant and the large displacement effect should be considered for the safe design of the wind power tower.

Key Words
wind tower; column; DSCT; composite; large displacement effect

Address
Taek Hee Han and Young Hyun Park: Coastal Development Research Center, Korea Institute of Ocean Science and Technology, 787 Haeanro, Ansan 15627, Korea
Deokhee Won: Coastal Disaster Prevention Research Center, Korea Institute of Ocean Science and Technology,
787 Haeanro, Ansan 15627, Korea
Joo-Ha Lee: Department of Civil Engineering, The University of Suwon, 17 Wauan-gil, Hwaseong, 18323, Korea



Techno-Press: Publishers of international journals and conference proceedings.       Copyright © 2017 Techno-Press
P.O. Box 33, Yuseong, Daejeon 34186 Korea, Tel: +82-42-828-7996, Fax : +82-42-828-7997, Email: info@techno-press.com