Cyclic behaviour of laterally loaded (mono)piles in sand: With emphasis on pile driving effects

At the end of 2019, the European Union (EU) put forward the European Green Deal to facilitate the technological progress necessary to achieve CO2-neutrality by 2050. Such a monumental achievement would require massive investments in infrastructure for the harvesting, storage and the transnational transportation of green energy. To date, the more mature of the scalable (cf. to hydroelectric) green-energy resources is offshore wind, with joint academic and industry efforts allocated to reduce its capital expenditure. Approximately 13-37% of the required investment for offshore wind farms is currently expended on the design, manufacturing, and installation of the substructure. Further reduction in the cost of offshore wind can be achieved by addressing the main technical challenges associated with the predominant offshore wind foundation, i.e., the monopile. The main challenges typically relate to its lifetime operations, namely, (i) the identification of the wind turbine's fundamental frequencies, which are strongly dependent on the monopile-soil interaction, (ii) and the prediction of the lifetime foundation tilt, but also the current installation technology (impact driving); the current norm in the offshore industry. In particular, impact driving is associated with (i) long installation times, especially in the presence of competent soils, (ii) excessive use of construction material (steel) to avoid pile damage under many hammer blows, and (iii) costly underwater noise mitigation measures to reduce noise the levels of installation-borne noise emissions harmful to marine life.In an attempt to accelerate the growth of offshore wind, the Netherlands, country of origin of this study, has supported several research initiatives to reduce the engineering and manufacturing costs for the prevalent offshore wind foundation in the country (the monopile). This study elaborates upon the experimental findings of two major research projects, namely the DISSTINCT (2014-2018) and the Gentle Driving of Piles (2018-2022) projects, each designed to address specific technical uncertainties associated with the foundation concept. The DISSTINCT project (launched in 2014) aimed to improve the understanding of the natural frequency of installed monopiles as well as the engineering procedures used in the identification thereof. By conducting experiments at full scale on a monopile installed in the IJsselmeer lake in the Netherlands, the experimental campaign produced invaluable data on the dynamic response of monopiles during small amplitude lateral vibrations. Later, the GDP project (launched in 2018) was designed to propose, engineer, and demonstrate a novel monopile installation procedure, foreseen to alleviate most of the aforementioned installation-related challenges; the Gentle Driving of Piles (GDP) method. Moreover, the project would provide answers to questions concerning the long-term response of (mono)piles in sandy soils, relative to the installation method. For these reasons, an extensive experimental campaign was conducted in the port of Rotterdam (Maasvlakte II), where a total of 9 piles were driven into the sandy Maasvlakte soil via different driving procedures, namely with the established impact hammering, the traditional axial vibro-driving, and the new GDP method. Subsequently, the cyclic lateral performance for four of these piles (which were heavily instrumented), was evaluated via an elaborate 82.000 load cycle (≈42 hours) loading programme of slow (0.1 Hz) high amplitude, and fast (0.1 - 4 Hz) low amplitude cyclic force applied to the (mono)piles' head. This study elaborates and builds upon experimental findings from the above-mentioned test campaigns. These measurements were first carefully examined, and later interpreted using a variety of modelling tools (both 1D and 3D FE modelling) formulated and adapted to meet the particular geotechnical and loading challenges of the examined fieldwork. Enabled by the diversity of the field and numerical work performed, this study addresses a number of engineering challenges and knowledge gaps related to the design of monopiles, namely i) their post-installation resonance frequency, ii) the long-term response to environmental loading, and iii) the impact of the installation method on the long-term operations. In particular, 3D FE modelling was adopted to successfully simulate the dynamic response of the examined monopile in the DISSTINCT project. The modelling efforts enabled the interpretation of the field test measurements, and in turn, inspired confidence in the suitability of available simulation tools to identify the resonance frequencies of monopile foundations, and accurately calculate dynamic soil-monopile interactions. For the interpretation of the GDP field test data, 1D FE modelling was employed. In the field, the elaborate lateral loading programme returned a fairly complex cyclic pile response, with pronounced differences in the performance of piles installed by different installation methods. The particular geotechnical conditions at the GDP site, i.e., site inhomogeneity and the 4 m deep unsaturated topsoil, prevented the direct comparison of the installation methods. This was later achieved through the formulation of a cyclic soil reaction p-y model able to simulate soil ratcheting and gapping effects. The results provided rich insights into the impact of relevant installation effects on the cyclic pile response on many loading cycles and indicated that the GDP-installed piles performed excellent overall in lateral cyclic loading.Offshore Engineerin

3D FE seismic analysis of a monopile-supported offshore wind turbine in a non-liquefiable soil deposit

Japan is one of the most seismically active countries in the world, and is currently planning to invest on offshore wind energy. In support of this relevant energy transition, this work presents a numerical study regarding a monopile-supported offshore wind turbine (OWT) under seismic loading conditions. For this purpose, a realistic design of an 8 MW OWT is considered, wished-in-place in a layered, non-liquefiable Japanese site. The OWT seismic response is analysed via 3D FE modelling incorporating for the soil the well-established SANISAND bounding surface plasticity model, and thus enabling realistic simulation of the cyclic hydro-mechanical response of the coarse-grained materials at the considered site. Site data regarding soil stratigraphy and past earthquake records were obtained from the well-known Japanese KiK-net database. From these data, the 15 parameters of the SANISAND model were calibrated by combining back-analysis of seismic records and available linear elastic soil properties.The influence of different intensity earthquake records on the response of the OTW-monopile-soil system is examined both for horizontal and vertical seismic components. Even in a non-liquefiable site, pore pressure effects and their impact on the structural response are clearly visible in the simulation results. The likely coexistence of seismic and SLS wind/wave loading is also considered for completeness.Geo-engineeringOffshore Engineerin

Frequency effects in the dynamic lateral stiffness of monopiles in sand: insight from field tests and 3D FE modelling

With the offshore wind industry rapidly expanding worldwide, geotechnical research is being devoted to foundation optimisation - most intensively for large-diameter monopiles. The analysis and design of monopiles still suffers from significant uncertainties in relation to cyclic/dynamic loading conditions. The aim of this work is to shed new light on dynamic soil-monopile interaction, based on the results of unique full-scale experiments performed at the Westermeerwind wind park (Netherlands). The response of a 24 m long, 5 m diameter monopile to harmonic lateral loading of varying amplitude and frequency is inspected. The analysis of original field measurements (soil accelerations and pore pressures) enables the lateral stiffness observed at the monopile head to be linked to dynamic effects occurring in the surrounding soil. The interpretation of measured data is supported by three-dimensional finite-element studies, also looking at the influence of drainage conditions and monopile size. The set of results presented supports the need for dynamics-based monopile design, as higher frequencies gain relevance in the most recent offshore wind developments.Accepted Author ManuscriptOffshore EngineeringEngineering StructuresGeo-engineerin

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

The 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 min 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 might more likely affect soil-monopile interaction under weak-to-moderate environmental loading.Accepted Author ManuscriptOffshore EngineeringGeo-engineerin

From cyclic sand ratcheting to tilt accumulation of offshore monopiles: 3D FE modelling using SANISAND-MS

Serviceability criteria for offshore monopiles include the estimation of long-term, permanent tilt under repeated operational loads. In the lack of well-established analysis methods, experimental and numerical research has been carried out in the last decade to support the fundamental understanding of monopile-soil interaction mechanisms, and the conception of engineering methods for monopile tilt predictions. With a focus on the case of monopiles in sand, this work shows how step-by-step/implicit, three-dimensional (3D) finite-element (FE) modelling can be fruitfully applied to the analysis of cyclic monopile-soil interaction and related soil deformation mechanisms. To achieve adequate simulation of cyclic sand ratcheting and densification around the pile, the recently proposed SANISAND-MS model is adopted. The link between local soil behaviour and global monopile response to cyclic loading is discussed through detailed analysis of model prediction. Overall, the results of numerical parametric studies confirm that the proposed 3D FE modelling framework can reproduce relevant experimental evidence about monopile-soil interaction, and support future improvement of engineering design methods.Accepted author manuscriptOffshore EngineeringGeo-engineerin

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

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.Offshore EngineeringGeo-engineerin

Experimental identification of the dynamic behaviour of pile-soil system installed by means of three different pile-driving techniques

A novel pile-driving technique, named Gentle Driving of Piles (GDP), that combines axial low-frequency and torsional high-frequency vibrations has been developed and tested recently. During the experimental campaign, several piles were installed onshore, making use of the GDP shaker. Besides those, a number of additional piles were installed using conventional pile-driving techniques, i.e. impact piling and axial vibratory driving. After the completion of the installation phase, the installed piles have been subjected to impact hammer tests with the following goals. First, the in-situ dynamic properties of the pile-soil system have been identified. Second, the post-installation soil state has been investigated, along with its evolution in time for each pile driving scenario. Preliminary analyses, of the data collected during the impact tests show dissimilar trends in the overall dynamic response between the piles installed with impact hammer and those installed with the axial and the GDP shakers.This observation suggests a difference in the post-installation dynamic behaviour of the pile-soil systems related to different pile-driving techniques. In this paper, a first attempt is made to identify the differences in the overall pile-soil dynamic behaviour of the piles installed by means of the three different pile-driving techniques.Dynamics of StructuresOffshore EngineeringEMSD CE&GGeo-engineeringBusiness RelationsEngineering Structure