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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Whole-life foundation and structure integrity' taking place on Wednesday, 12 March 2014 at 14:15-15:45. The meet-the-authors will take place in the poster area.

Young-Jun You Korea Institute of Construction Technology, Korea, Republic of
Co-authors:
Youn-Ju Jeong (1) F P Young-Jun You (1) Min-Su Park (1) Du-Ho Lee (1) Byeong-Cheol Kim (1) Yoon-Koog Hwang (1)
(1) Korea Institute of Construction Technology, Goyang, Korea, Republic of

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Mr. You has been working in the structural engineering field for almost 15 years. He is currently a senior researcher at the Korea Institute of Construction Technology (KICT) in South Korea. He studied structural engineering at the Yonsei University in Seoul. After his studies he spent 15 years at KICT and has been involved in projects Bridge Management System, Structural Health Monitoring, and Development of GFRP rebar. His research is focused on the substructure of wind power.

Abstract

CFMP based hybrid substructure system for offshore wind and modular installation method

Introduction

According to the increasing of offshore wind turbine capacity from 2.0~3.0MW to 5.0~10.0MW, substructure system also should be large-sized. However, the increasing of the size of substructure system has disadvantages to the wave-induced hydrodynamic forces subjected to the substructure system because of the larger cross-sectional area, and to the installation cost because of the heavier total weight of the substructure, respectively. Therefore, in order to corresponding to the large capacity of offshore wind turbine, it needs to optimally design substructure system so as to satisfy low-hydrodynamic force, high-structural capacity, and low-installation cost.

Approach

In order to reduce hydrodynamic forces subjected to the substructure, it needs to optimally arrange cross-sectional area. In order to improve structural capacities, it is important to improve strength, stiffness and vibration resistance of substructure system. Also, in order to reduce installation cost, it needs to optimally control the self-weight of substructure system and to install without large capacity offshore crane during the installation.
In this study, CFMP (Concrete Filled Multi-Piles) based offshore wind substructure system was introduced, as presented in Figure 1 (). CFMP based substructure system was designed to reduce hydrodynamic force, to improve structural safety against to the harsh offshore condition, and to reduce installation cost. In order to reduce wave-induced hydrodynamic force subjected to the substructure system, multi-piles system was adapted. Also, in order to improve structural strength, stiffness, and vibration resistance of substructure system, multi-piles system filled with the concrete. Therefore, CFMP system was designed so as to satisfy both low hydrodynamic force and high structural capacity of strength, stiffness, and vibration resistance.
In addition, in order to improve overturning stability of substructure system, the bottom part of substructure system consists of gravity based the concrete cone. The gravitational restoring force of the concrete cone resist to the overturning combined with the foundation piles. Namely, combining force of the concrete cone and foundation piles resist to the overturning moment. This concept has an advantage that two components of gravitational restoring force and foundation piles resistance can be apply alternatively according to the seabed soil condition from the soft to the rock. The CFMP and the concrete cone connect with the grouting at the sea condition. This modular installation method can become possible substructure installation by using only small capacity’s offshore crane, instead of large capacity’s offshore crane which is very expensive and too difficult to rent, and resulting in reducing total construction cost.


Main body of abstract

In order to verify hydrodynamic force reduction effect, diffraction analysis were carried out for the three cases arrangement of multi-piles, mono, five-multi piles, and nine multi-piles, and hydrodynamic forces subjected to the three substructure system were analyzed. Then, in order to investigate ultimate structural safety and serviceability, integrated static structural analysis was carried out and structural behaviors of the strength and deformation were evaluated. Also, in order to investigate dynamic modal safety, modal analysis was carried out and natural frequency and resonance possibility were evaluated.
In this study, NREL 5.0MW turbine model was selected for the integrated structural analysis of CFMP based substructure system, and tower model which was designed corresponding to the NREL 5.0 MW turbine model was applied. The total weights of turbine and tower models were about 350 and 220 ton, respectively.
As the results of diffraction analysis for the three cases arrangement of multi-piles, wave force of multi-piles cases indicated about 32% lower level than that of mono-pile. Therefore, the reducing of wave force subjected to the substructure system should be contributed to reduce bending moment and to improve overturning stability of substructure system. As the results of the integrated analysis for the total system of the offshore wind including the blades, turbine, tower, and substructure system, reduced wave-induced hydrodynamic forces subjected to the substructure system resulted in about 3% reducing of total displacement at the Transition Piece (TP) and about 5% reducing of maximum bending moment of the substructure system. Therefore, as the results of this study, it was found that the CFMP based offshore wind substructure system of this study satisfied structural safeties in respect of ULS (ultimate limit state) and SLS (serviceability limit state). Also, this system satisfied vibration safety, namely resonance was not occurred.
In order to evaluate installation cost, the logistic for the modular installation was defined, as presented in Figure 2 () and 3 (). At first, some components of the foundation piles and CFMP member move to the installation site onto the towing barge, and the heavy concrete cone can be moved by towing in self- floated condition. Then, some foundation piles are imbedded into the seabed by hammering, and the concrete cone sinks onto the foundation piles position by sea water ballasting into the concrete cone. After connecting foundation piles with the concrete cone by the grouting, CFMP member on the barge is lifted up and sunk on the concrete cone by offshore crane. Finally, the CFMP member connect with and the concrete cone by the grouting at the sea condition.
Based on the modular installation logistic for the substructure system, required offshore equipments and capacities of crane, towing tug boat, and barge during the installation were evaluated and summarized in Figure 4 (). As the results of the evaluation of installation equipments and capacities, it was found that the CFMP based hybrid substructure system of this study has an advantage of the reducing of installation period about 20% than the typical steel substructure such as the jacket. Therefore, it can be possible to reduce installation cost of the substructure system about 5~10%. In addition, considering low-priced material cost of the concrete, material and fabrication costs also can be reduced rather than the typical steel substructure system.
Based on the modular installation logistic for the substructure system, required offshore equipments and capacities of crane, towing tug boat, and barge during the installation were evaluated and summarized in Figure 4. As the results of the evaluation of installation equipments and capacities, it was found that the CFMP based hybrid substructure system of this study has an advantage of the reducing of installation period about 20% than the typical steel substructure. Therefore, it can be possible to reduce installation cost of the substructure system about 5~10%. In addition, considering low-priced material cost of the concrete, material and fabrication costs also can be reduced rather than the typical steel substructure system such as jacket.


Conclusion

In this study, CFMP (Concrete Filled Multi-Piles) based offshore wind substructure system was introduced. CFMP based substructure system was designed to reduce hydrodynamic force, to improve structural safety against to the harsh offshore condition, and to reduce installation cost. In order to verify hydrodynamic force reduction effect, diffraction analyses were carried out and hydrodynamic forces subjected to the substructure were analyzed. Then, in order to investigate ultimate structural safety and serviceability, integrated static structural analysis was carried out and structural behaviors were evaluated. Also, In order to evaluate installation cost, the modular installation logistic was defined and required offshore equipments and capacities during the installation were evaluated.
As the results of diffraction analysis for the three cases arrangement of multi-piles, wave force of multi-piles cases indicated about 32% lower level than that of mono-pile. Also, as the results of the integrated structural analysis, reducing of hydrodynamic forces subjected to the substructure system resulted in about 3% reducing of total displacement at the Transition Piece (TP) and about 5% reducing of maximum bending moment of the substructure system.
As the results of the evaluation of installation equipments and capacities, it was found that the CFMP based hybrid substructure system of this study has an advantage of the reducing of installation period about 20%. Therefore, it can be possible to reduce installation cost of the substructure system about 5~10%. In addition, considering low-priced material cost of the concrete, material and fabrication costs also can be reduced rather than the typical steel substructure system such as jacket. Therefore, it is respected that the CFMP based offshore wind substructure system of this study contributes to improve structural safety and to reduce installation cost of the substructure system.



Learning objectives
Korea goverment plan to construct large offshore wind farm at southern-western Korean sea.
However, this area has disadvantages of harsh wave, typhoon, and soft soil conditioms.
Therefore, it is very important to disign cost effective substructure system in respect of the structural safety and installation cost.
This study was performed as a sub-part of National Project to develop new substructure system to apply Korean sea.



References
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[3] W. Brook-Hart, P.A. Jakson, M. Meyts, P. Gifford: Competitive Concrete Foundations for Offshore Wind Turbines, International Foundation (2010).
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