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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Floating wind turbines' taking place on Wednesday, 12 March 2014 at 16:30 -18:00. The meet-the-authors will take place in the poster area.

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

Motion analysis of pontoon system for offshore wind power

Introduction

Many wind power generators have been constructed and operated in land but the lack of earth space, economical issue, generating efficiency, etc. drive them to shift site offshore. As known well, the farther system is installed from land, the higher wind power is efficient and the more expensive as seen in Figure 1 [1]. The size of power generator including blades is getting huge. From these comprehensive situations and causes, many researches are predicting that floating wind power will be the solution. Therefore this is the time to need approaching efforts to floating substructure for wind power.

Approach

The advantages of floating wind farms are as follows [2]:
• Vast offshore wind resources with higher and steadier wind speeds
• Over 75% of worldwide power demand from coastal areas
• Power increases with cube of wind speed ~ 50% higher offshore
• Lower offshore wind turbulence – longer farm life ~ 25-30 years
• Experience of oil industry essential for the development of safe and cost effective Spar, TLP and hybrid wind turbine floaters
Floating wind power has already tried. The world's first operational floating wind turbine, Hywind, was assembled in Norway in 2009 and WindFloat and Bule H were also constructed [3].
It is said that larger turbines could allow for lower operation and maintenance costs, installation and foundation costs per unit of capacity [4]. Its evidence is to be found from the trend of rotor diameter in Figure 2 [5].
More than 5 megawatt (MW) turbines [6] are being adapting to the fixed type offshore wind power while generating capacities of installed floating wind power are less than 3 MW.
Therefore, it is be predicted that 5 MW turbine will be adapted to the floating wind power system.
One of required techniques for floating substructure of wind power is to make it more stable. The motion by wind and wave could give undesirable affect one the electrical and mechanical wind power system. Various floating substructures have been proposed and researched as seen in Figure 3 [7].
It is known that the mooring type of the first economically adaptive floating substructure would be Tension Leg Platform (TLP) as seen in Figure 4 [8].
This study is about floating pontoon system moored by TLP to support super structure of large wind power. A new conceptual platform was proposed and its principle was introduced. Hydrodynamic analysis was performed to observe its motion behavior and the result was compared with that of platform with cylinder shape.


Main body of abstract

CONCEPT OF HIGH STABLE PONTOON SYSTEM
In Figure 5 (a), it is certain that right state is more stable than left one. Comparing the mid and right state with same lengths of left and right arm and only a mass w exists, the mid would keep changed state in an ideal condition after an external force is load on a side and disappeared, while the right would go to former state. This results from changed arm length as seen in Figure 5 (b). If the lever is not straight like the right figure in Figure 5 (a), the arm length of one side becomes longer than original one and the opposite side shorter when a force loaded on a side. After loaded force disappeared, restoring moment occurs due to the difference of arm lengths and finally system would go to original equilibrium state.
The case that acting forces on this explained system are applied oppositely is seen in Figure 5 (c). B means buoyancy and T means dragging force. This is same with the explained except for only different kind of acting forces and acting directions. The layout of wind power system by this principle is like Figure 5 (d).
If pontoons are dragged down by some force from a natural floating state, additional buoyancy occurs and this force will act as B in Figure 5 (c). It is considered in this paper that all pontoons are arranged with same angle from a top view.
For the example system with four pontoons having same rectangular section, it would be balanced by buoyancy of pontoons and submerged part of tower and weight of them as seen in Figure 6 (a). If the system is to be dragged down under water surface with some force like TLP mooring, submerged parts in Figure 6 (b) will contribute to the total system as buoyancy, exactly upward dragging force.

MOTION OF PONTOON SYSTEM BY ANALYSIS
The conditions of wind generating system for target were decided as referring those of 5 MW wind power and all members were considered as steel. Superstructure consists of a tower with 120 m in height, 10 m in diameter and a hub which weighs 3100 kN including blades. Substructure was considered for two types. One is circular mono pontoon and another is circular four pontoons. Both has same weight and shape ratio.
AQWA was used for analysis of hydrodynamic motion in time domain. Wave conditions of 5.37 seconds in period and 5.4 m in height are applied.
Figure 7 shows the difference between mono-pontoon and multi array pontoon system. Maximum pitch values were 0.601 for mono-pontoon and 0.277 so that multi-pontoon system showed more stabilized motion. Moreover, mono-pontoon showed irregular period while multi-pontoon systems kept constant period. This means structure swung back and forth irregularly and this could give a negative effect to mechanical system. Therefore it could be said that pontoon system type of multi array is more advantageous than mono pontoon because the former showed reduced pitch motion of about 54% than that of the latter.
The reduced pitch effect by submergence is presented in Figure 8. Pitch value was reduced to about 95% for mono-pontoon and 60% for multi-pontoon. From this result, it is certain that submerging substructure for wind power could give good stability.
There was an unexpected result for submerged mono- and multi-pontoon. As seen in Figure 9, submerged mono-pontoon showed more stable motion than that of multi-pontoon system. Therefore it could be said that mono-pontoon is more stable than multi-pontoon in pitch. However, it could not be right that submerged mono-pontoon is always more advantageous than submerged multi-pontoon. As seen in Figure 9, the period of mono-pontoon according to time was not kept constantly while multi-pontoon systems kept constant period. As described previously this could give a negative effect to mechanical system. The reduced pitch value of submerged mono-pontoon could result from no floating force by dragging and further study is needed.


Conclusion

In this study, a concept for making large wind power system feasible and more stable was proposed. Principle and method of it using buoyancy were explained and equations were driven for acquiring needed geometry of pontoon. Hydrodynamic analysis was performed to check the behavior of the proposed and to investigate its feasibility for the specification of 5 MW wind power system. Wave conditions of 5.37 seconds in period and 5.4 m in height are applied samely. The effectiveness of newly proposed platform type was verified about pitch motion by comparing with the motion of cylindrical platform with same ratio of height, weight, and thickness. Through this analysis, results as follows were found.
• In state that the buoyancy of floating platform and the weight of superstructure make equilibrium, multi arrayed pontoon system showed pitch motion more less 50% than that of mono-pontoon. Consequently multi arrayed pontoons system is to be considered more effective than mono pontoon system.
• For both mono and multi pontoon system, in state that pontoons were dragged down under water level, in other words, when submerged, the pitch of mono pontoon system was reduced to about 95% and 60% for multi-pontoon. From this result, it is certain that submerging substructure for wind power could give good stability.
• Comparing submerged mono and multi pontoon system, mono pontoon system showed less pitch motion than that of multi system. This could be from that additional buoyancy by dragging down was not considered in this analysis.
• Therefore, extended detail analysis is needed for circular type, different shape and ratio and so on.



Learning objectives
This study is for checking the feasibility of submerged multi arrayed pontoon system. The effectiveness of submerged state of pontoon was checked first and then the motion of multi arrayed pontoon system was compared with that of mono pontoon.


References
1. Alla Weinstein, Floating offshore wind system: The WindFloat, http://www.nmsfocean.org/files/5.5_AllaWeinstein.pdf)
2. http://web.mit.edu/windenergy/windweek/Presentations/P6%20-%20Sclavounos.pdf
3. http://en.wikipedia.org/wiki/Floating_wind_turbine
4. Snyder B, Kaiser MJ. A comparison of offshore wind power development in Europe and the US: patterns and drivers of development. Applied Energy 2009;86:1845–56.
5. EWEA, Wind Energy - The Facts, a Guide to the Technology, Economics and Future of Wind Power, Brussels, Belgium, 2009.
6. REpower: C-Power’s offshore wind farm Thornton Bank II installed, http://www.repower.de/ fileadmin/press_release/2012_07_27_Thornton_Bank_II_installed.pdf
7. http://offshorewind.net/Other_Pages/Turbine Foundations
8. http://www.principlepowerinc.com/principle power wind float brochure.pdf