Behavior of monopile foundations under cyclic lateral load

Design of monopiles for offshore wind turbines in 10 steps

“…The authors suggest caution when predicting the effects of very high numbers of load cycles. The numerical investigations and laboratory tests carried out by Achmus et al (2009) and Kuo et al (2012) go up to 10 000 cycles, and tests by Byrne et al (2010) and Leblanc et al (2010) have been carried out for up to 65000 cycles. However, these are still orders of magnitudes below the expected number of load cycles of an OWT.…”

“…Long and Vanneste (1994) suggest that one-way loading is the critical load scenario and two-way loading causes less accumulated strain. Achmus et al (2009) and Kuo et al (2012) also focus on one-way loading in their analysis. However, Byrne et al (2010); Leblanc et al (2010) found that the most critical scenario is two-way loading with / = −0.5.…”

“…In other words, loosely speaking the depth to which the lateral loads from the pile head are being transferred. Kuo et al (2012) and Achmus et al (2009) consider approach (1) and (2) above impractical and unsuitable for monopile design. They also found that zero toe-kick criterion was overly conservative and that the vertical tangent criterion produced counter-intuitive embedded length requirements.…”

“…1 , 2 -model parameters for Achmus et al (2009) -eccentricity of loading (= / ) -zero shear force point location below mudline 0 -first natural frequency 1 , -upper limit of the 1P frequency range -fixed base (cantilever beam) natural frequency of the tower -characteristic yield strength -gravitational constant -distance from zero shear force location to pile toe -air gap between the highest expected wave crest level and the platform -wave number 0 -equivalent stiffness of first tower mode ℎ -horizontal modulus of subgrade reaction -total structural mass, also strain accumulation exponent 0 -equivalent mass of first tower mode -mass of the pile -mass of the rotor-nacelle assembly -mass of the tower -mass of the transition piece ℎ -horizontal coefficient of subgrade reaction ℎ1 -coefficient of subgrade reaction at the first load cycle ℎ -coefficient of subgrade reaction after N load cycles -shape parameter of Weibull distribution -undrained shear strength of soil , , -time, also degradation parameters degradation model -grout thickness -pile wall thickness -tower wall thickness -wall thickness of the transition piece -transition piece wall thickness -turbulent wind speed component -extreme gust speed , -Extreme Operating Gust (EOG) wind speed at cut-out wind speed -turbulent wind speed component for ETM ( , , ) -horizontal water particle velocity ̇( , , ) -horizontal water particle acceleration -characteristicc cyclic stress ratio ℎ -hub height -Rotor swept area , -lateral and rotational foundation flexibility coefficients -inertia coefficient -substructure flexibility coefficient -Thrust coefficient -rotor diameter -bottom diameter of the tower -pile diameter -top diameter of the tower -transition piece diameter -equivalent Young's modulus -pile Young's modulus ( ) -vertical distribution of soil's Young's modulus 0 -initial (small deformation) Young's modulus of soil at 1 depth -Young's modulus of the tower material -equivalent bending stiffness for horizontal tower top loading -Extreme Operating Gust -Extreme Sea State -Extreme Turbulence Model -Extreme Wave Height -soil density parameter -horizontal load carrying capacity of the foundation -pile installation parameter -maximum horizontal force at the mudline expected in the lifetime of the turbine -cyclic load ratio parameter -total horizontal wave force , -horizontal force due to the extreme operating gust at rated wind speed , -horizontal force due to extreme turbulence model at rated wind speed -fore-aft (along-wind) force on the turbine -drag force due to waves -inertia force due to waves -horizontal load capacity of the foundation (assuming soil failure) -Fatigue Limit State * -modified shear modulus -soil's shear modulus -maximum wave height (for a given significant wave height )…”

Observed dynamic soil–structure interaction in scale testing of offshore wind turbine foundations

“…Bearing capacity in vertical direction The design and construction of foundations for offshore turbines are challenging because of the harsh environmental conditions and as a result provide a focus of major research in Europe, see for example Achmus et al (2009), Kuo et al (2012), Cuellar et al (2012). The UK has also embarked on a massive scheme of investment in offshore wind power development to meet future energy challenges.…”