The Behavior of Monopile Foundation in Sandy Seabed under Horizontal Cyclic Loading

The European Commission's ‘Action Plan for Energy Efficiency’ envisages a 20% reduction in energy consumption by 2020. Energy generation using offshore wind turbines will contribute to this. In Germany, offshore wind farms are planned in the North Sea and Baltic Sea, whereby the North Sea has particularly challenging meteorological conditions. One of the foundation options being discussed for offshore wind turbines is the large-diameter monopile foundation. The cyclic wind and wave load acting on the monopile foundation can be distributed in 10E6 cycles over the lifetime of the structure. The deformations of large-diameter monopiles resulting from this highly cyclical load cannot be modelled using the calculation methods currently available. Although the ‘Standard Baugrunderkundung des Bundesamts für Seeschifffahrt und Hydrographie’ (BSH, 2003) calls for cyclic laboratory tests to predict the cyclic load-bearing behaviour of monopile foundations, the performance of such tests is not described in detail in this paper. The purpose of this paper is to develop a computational method for determining the displacement of a monopile in a non-cohesive seabed under highly cyclic swell loading by means of cyclic triaxial test results.

Description:

A computational approach consisting of a stiffness reduction and a finite element calculation was developed in order to be able to map the behaviour of a monopile foundation in a non-cohesive soil under cyclic loading. The accumulation of displacement due to cyclic loading is interpreted as a specific decrease of the secant stiffness modulus in each element of the pile-soil FE model. If the elastic strain is negligibly small, the secant modulus after the Nth load cycle EsN of each element can be formulated with a permanent axial strain after the first load cycle εap,N=1 and after the Nth load cycle εap,N. Combined with the empirical approaches according to Huurman (1996), the reduction of the secant stiffness modulus of each element of the finite element model can be determined with the stress-dependent variable X and two regression parameters b1 and b2 from cyclic triaxial tests.

Equation 1

The regression parameters depend on the soil mechanical properties of the non-cohesive subsoil and can be derived from cyclic triaxial test results. The stress-dependent parameter X can be expressed by the principal stress at failure in the static triaxial test σ1,f and by the cyclic axial stress σ1,cyc.

Equation 2

In order to determine the cyclic displacement response, the associated stiffness reduction is assigned to the elastoplastic material in a finite element model. With the presented stiffness reduction method, the displacement accumulations of complicated and highly cyclically loaded pile-soil systems can be simulated on the basis of currently limited available cyclic triaxial test results. This method is currently being used to analyse the influences of geometry, pile stiffness, loading conditions and soil density in parameter studies.

Publications:

[1] Achmus, M., Abdel-Rahman, K., Kuo, Y.-S. and Peralta, P. (2007). "Untersuchungen zum Tragverhalten von Monopilegründungengründungen unter zyklischer Belastung," Pfahlsymposium 2007, Braunschweig, pp. 95-115.

[2] Achmus, M., Abdel-Rahman, K. and Kuo, Y.-S. (2007). "Numerical Modeling of Large Diameter Steel Piles under Monotonic and Cyclic Horizontal Loading," International Symposium on Numerical Models in Geomechanics (NUMOGX), Rhodes, Greece, pp. 453-459.

[3] Kuo, Y.-S. (2007). "Numerical Modeling of Monopile Foundations under Cyclic Horizontal Loading," 2. nationalen PhD-Seminar der EAWE (Deutsche Sektion), Hannover, Germany, pp. 17-18.

[4] Achmus, M., Abdel-Rahman, K. and Kuo, Y.-S. (2007). "Zur Bemessung von Pfahlen fur Offshore-Windenergieanlagen unter statischen und zyklischen Lasten," Gigawind Symposium 2007, Hannover.

[5] Achmus, M., Abdel-Rahman, K. and Kuo, Y.-S. (2007). "Behavior of large diameter monopiles under cyclic horizontal loading," International Colloquium on Structural and Geotechnical Engineering (ICSGE), Cairo, Egypt, pp. 860-870.

[6] Kuo, Y.-S. and Achmus, M. (2008). "A numerical model to simulate the performance of foundation elements under cyclic loading," Proceeding of the BGA International Conference on Foundations, Dundee, Scotland, Vol.2, pp. 1247-1258.

[7] Achmus, M., Abdel-Rahman, K. and Kuo, Y.-S. (2008). "Design of Monopile Foundations for Offshore Wind Energy Plants," 11th Baltic Geotechnical conference -Geotechnics in maritime engineering, Gdansk, Poland, Vol. 1, pp. 463-470.

[8] Kuo, Y.-S. (2008). On the behavior of large-diameter piles under cyclic lateral load, Mitteilungsheft 65, Institut fur Grundbau, Bodenmechanik und Energiewasserbau, Leibniz Universitat Hannover.

[9] Achmus, M., Kuo, Y.-S. and Abdel-Rahman, K. (2008). "Zur Bemessung von Monopiles für zyklische Lasten," Bauingenieur, Vol.83, pp. 303-311.

[10] Achmus, M., Kuo, Y.-S. and Abdel-Rahman, K. (2008). "Behavior of monopile foundations under cycliclateral load," Computers & Geotechnics. (revised)

[11] Kuo, Y.-S, Achmus, M. and Abdel-Rahman, K. (2008). "Application of cyclic triaxial test results on estimation of lateral deforamtion of monopiles foundations," Chinese Journal of Geotechnical Engineering. (revised)