Keywords: offshore monopile, internal stresses, total displacements, offshore wind farms.

Due to the limitation of the earth resource, renewable enery such as wind, solar and wave energy has attracted great attentions among governments, industries and academics in the world. The application of wind energy throughout the world is growing fast. Over the past two decades, on-shore wind energy technology has been intensively studied for reduction in costs and is now competitive with fossil and nuclear fuels for electric power generation in many areas worldwide [4].

While on-shore wind technology becomes mature, offshore wind energy is at the beginning stage. Off-shore wind farms are different from on-shore installations for several reasons: (i) the wind turbine generators have, on average, larger diameters and rated power, (ii) the installation and the maintenance are more expensive, (iii) the submarine electrical connection to shore increases the investment costs. Despite of the high costs compared with on-shore wind farms, off-shore applications allow a increased energy efficiency, due to the higher average wind speeds and the reduction of the sitting and environmental issues, particularly with regards to noise, visual constraints and space limitations, since off-shore wind farm are commonly built some km away from the coast.

The monopile is one of three types of substructures, which have been used for most offshore wind farms. The external loadings of the offshore monopile consist of numerous components, hydrodynamic loadings, static water pressures, lateral earth pressure, overturning force from wind turbine and wind loadings. As a result of reacting with these loadings, the internal stresses and deformation of the monopile can be significantly affected. The wave-induced loadings around a vertical cylinder have been investigated by Zhu [2] in the past. In that approach, analytical solutions for wave-induced forces and momentum acting on the pile were presented, which can be used as the external loading for offshore monopiles. Moreover, offshore engineers have also carried out the structure capacity by considering a fatigue damage of offshore structure under a long period wave loading acting [3] and the distribution of internal stresses within a concrete pile under a combination of vertical structure loads and lateral wave loads [1]. However, Eicher et al. [1] only investigated the wave-induced loading without considering the soil response and wind action. To date, combined wind, wave and soil loadings on offshore monopiles have not been investigated, even it is important for the design of monopiles.

The study of the deformation and the stress of an offshore monopile under a combination loading have been examined by an analysis from FEMLAB. Six major parameters are used to complete this study; they are wind velocity, cylinder radius, wave period, wave height, water depth and wave incident angle.

1

J.A. Eicher, H. Guan and D.S. Jeng, "Stress and deformation of offshore piles under structural and wave loading", Ocean Engineering 30, 369-385, 2003. doi:10.1016/S0029-8018(02)00031-8

2

S.P. Zhu, "Diffraction of short-crested waves around a circular cylinder", Ocean Engng, Vol. 20, No 4, pp.389-407, 1993. doi:10.1016/0029-8018(93)90003-Z

3

T. Yankova, Wave Loading on Offshore Wind Turbines. Course Project, Technicsche Universität Braunschweig Institut für Wissenschaftliches Rechnen, Germany, 2002.

4

A.D. Owen, "Renewable energy: externally costs as market barriers", Energy Policy, Vol. 34, pp. 632-642, 2003. doi:10.1016/j.enpol.2005.11.017

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