Observed variations of monopile foundation stiffness

Kallehave, Dan; Thilsted, C; Diaz, Alberto Troya

doi:10.1201/b18442-98

Cited by 15 publications

(15 citation statements)

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“…Based on the evolution of the OWT dynamics from pre-storm to post-consolidation stages, it is concluded that the monopile stiffness degradation induced by even strong storm is not expected to be permanent. This inference confirms the observations from previous fields measurements, for instance from those reported by (Kallehave et al 2015).…”

Section: Discussionsupporting

confidence: 93%

3D FE dynamic modelling of offshore wind turbines in sand: Natural frequency evolution in the pre-to after-storm transition

Kementzetzidis¹,

Versteijlen²,

Nernheim³

et al. 2018

Numerical Methods in Geotechnical Engineering IX

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3D non-linear finite element analyses are proving increasingly beneficial to analyse the foundations of offshore wind turbines (OWTs) in combination with advanced soil modelling. For this purpose, the well-known SANISAND04 bounding surface plasticity model (Dafalias & Manzari 2004) is adopted in this work to incorporate key aspects of critical state soil mechanics into the analysis of monopile foundations in sand. The final 3D soil-foundation-OWT model is exploited to simulate the response of an 8 MW OWT to a long loading history of approximately 2 hours duration. The scope is to investigate/explain the drops in natural frequency observed in the field during storms, as well as its subsequent recovery. The numerical results point out a strong connection between transient frequency drops and pore pressure accumulation, whereas the original OWT natural frequency seems to be restored as a consequence of post-storm re-consolidation.

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Section: Discussionsupporting

confidence: 93%

3D FE dynamic modelling of offshore wind turbines in sand: Natural frequency evolution in the pre-to after-storm transition

Kementzetzidis¹,

Versteijlen²,

Nernheim³

et al. 2018

Numerical Methods in Geotechnical Engineering IX

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“…The first natural frequency f 0 was estimated by post-processing acceleration signals recorded at the rotor-nacelle-assembly (RNA). Despite the differences in OWT size, soil conditions and environmental loads, it is still interesting to compare the f 0 -drops recorded by Kallehave et al (2015b) to the corresponding drops predicted in this study for an 8 MW wind turbine in stiff sand (D r = 80%). In particular, the experimental data associated with wind speeds of about 24 m/s and 47 m/s (see red and blue arrows in Figure 19a) are considered in the comparison.…”

Section: Transient Drops In Natural Frequencymentioning

confidence: 84%

“…Only a few literature studies investigated the dynamics of OWTs based on field monitoring and observations related to the geotechnical performance of the foundation. A notable example is the work by Kallehave et al (2015b), in which the performance of a 2.3 MW OWT in dense to very dense sand is discussed after a monitoring period of 2.5 years. The OWT considered was founded on a monopile having length 18.4 m and diameter 3.9 m, and experienced only one strong storm during the monitoring period -see gray-shaded are in Figure 19a.…”

Section: Transient Drops In Natural Frequencymentioning

confidence: 99%

Geotechnical aspects of offshore wind turbine dynamics from 3D non-linear soil-structure simulations

Kementzetzidis

Corciulo

Versteijlen

et al. 2019

Soil Dynamics and Earthquake Engineering

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The massive development of the offshore wind industry is motivating substantial research efforts worldwide, where offshore wind turbines (OWTs) of increasing size are being installed in deeper water depths. Foundation design is a major factor affecting the structural performance of OWTs, with most installations founded to date on large-diameter monopiles.This work promotes advanced 3D finite element (FE) modelling for the dynamic analysis of OWT-monopile-soil systems. A detailed FE model of a state-of-the-art 8 MW OWT is analysed by accounting for dynamic soil-monopile interaction in presence of pore pressure effects. For this purpose, the critical-state, bounding surface SANISAND model is adopted to reproduce the hydro-mechanical cyclic response of the sand deposit. The response to realistic environmental loading histories (10 minutes duration) are simulated, then followed by numerical rotor-stop tests for global damping estimation.While linking to existing literature, all FE results are critically inspected to gain insight relevant to geotechnical design. The modelling tools adopted (i) support the robustness of 'soft-stiff' foundation design with respect to natural frequency shifts, even during severe storm events; (ii) provide values of foundation damping in line with field measurements; (iii) suggest that pore pressure effects may more likely affect soil-monopile interaction under weak-tomoderate environmental loading.

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“…Full-scale measurements of monopile-based OWTs also confirm the non-linear hysteretic foundation response. Kallehave et al (2015) observed that the measured natural frequency of monopile-based OWTs decreased with increasing wind speeds and related it to increasing displacement levels. The same conclusion was reached by Damgaard et al (2013) when analysing the reduction in natural frequency with increasing acceleration levels.…”

Section: Observed Foundation Behaviourmentioning

confidence: 95%

response to referee 1

Nygaard¹

2017

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Several studies have emphasized the importance of modelling foundation response with representative damping and stiffness characteristics in integrated analyses of offshore wind turbines (OWTs). For the monopile foundation, the industry standard for pile analysis has shown to be inaccurate, and alternative models that simulate foundation behaviour more accurately are needed. As fatigue damage is a critical factor in the design phase, this study investigates how four different soil-foundation models affect the fatigue damage of an OWT with a monopile foundation. This study shows how both stiffness and damping properties have a noticeable effect on the fatigue damage, in particular for idling cases. At mud-line, accumulated fatigue damage varied up to 22 % depending on the foundation model used.Published by Copernicus Publications on behalf of the European Academy of Wind Energy e.V.

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Observed variations of monopile foundation stiffness

Cited by 15 publications

References 0 publications

3D FE dynamic modelling of offshore wind turbines in sand: Natural frequency evolution in the pre-to after-storm transition

3D FE dynamic modelling of offshore wind turbines in sand: Natural frequency evolution in the pre-to after-storm transition

Geotechnical aspects of offshore wind turbine dynamics from 3D non-linear soil-structure simulations

response to referee 1

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