A systematic study into the response of monopiles to lateral cyclic loading in medium dense and dense sand was performed in beam and drum centrifuge tests. The centrifuge tests were carried out at different cyclic load and magnitude ratios, while the cyclic load sequence was also varied. The instrumentation on the piles provides fresh insights into the ongoing development of net stresses, bending moments and deflections as cycling progresses. Parallels between the test results and corresponding cyclic triaxial tests are drawn. The paper combines the results from this study with those from previous experimental investigations to provide empirical design recommendations for monopiles subjected to unidirectional cyclic loading.
Summary For foundation design the geotechnical analyses and interpretations often rely on isolated 1D boreholes and the geophysical data is only used to confirm horizontal layering. The great amount of information capture in the geophysical data, not only related to layering but also related to soil parameters, are therefore not used. Geophysical data are collected in 2D lines and/or 3D volumes and therefore provides the natural link to re-populate geotechnical properties found in the 1D boreholes onto a larger area and thereby build a consistent and robust ground model. There is therefore a great potential in using this data in a quantitative way during all phases of a project.
This paper deals with three different and important issues for centrifuge testing on monopiles for offshore wind turbines. A series of centrifuge model tests has been conducted on cylindrical stiff model monopiles that were installed at 1g and in-flight before being loaded laterally as well as on conical model piles installed in-flight. All model tests were performed in normally consolidated dense dry sand, simulating drained conditions. The tests confirmed three important issues for centrifuge modelling of monopiles for offshore wind turbines. First, to avoid any noticeable grain-size effect, the geometric ratio between average grain-size in poorly graded soils and pile diameter has to be larger than 88. Second, the non-linear stress distribution with depth, which is often neglected, has to be taken into account in the analysis of the lateral response. Finally, the pile lateral load–displacement tests confirm that both stiffness and strength increase following in-flight installation and that in-flight installation is needed to avoid any scale effects. This paper illustrates how these three issues are important factors in achieving reliable centrifuge modelling, which is capable of scaling model observations to prototype.
Summary The in-depth integration of sparse 1D geotechnical data with 2D UHR seismic reflection in a consistent geological framework forms the back-bone of the data-driven ground model approach. In order to predict CPT or geotechnical parameters (and their uncertainties) across the entire development area, one typically relies on geostatistical methods, like 3D kriging, Considering that the 2D line spacing is often larger than key geological phenomena, this interpolation will lead to uncertainty. In this paper, we investigate the effect of line spacing and geological complexity on the model prediction, using the TNW site (offshore the Netherlands) as a case study. We focus on an area with ultra-high-resolution 3D data, and decimate the volumes of 4 sub-sets with distinct geological features and complexity in order to assess the uncertainty on the interpolation, using both geostatistical and machine learning methods.
Suction caissons have been used for numerous oil and gas installations and are increasingly considered as a foundation solution for offshore wind turbines (OWTs). There can be significant differences between the two offshore energy applications in the load paths and magnitudes, soil type and caisson aspect ratio (skirt length to diameter). This paper investigates the response of suction caissons in dense sand to a range of cyclic vertical loading histories relevant to a jacket-supported OWT, with an emphasis on cyclic tensile loading. The findings are based on a series of experiments performed in a centrifuge, such that soil stresses reflect those in the prototype. The caisson was installed using suction at enhanced gravity, followed by cyclic loading and then caisson extraction. The installation and extraction results are discussed in a companion paper. This paper focuses on the caisson load–displacement response under vertical cyclic loading. The centrifuge experimental results reinforce findings from previous work, add insights into the load transfer mechanisms and provide confidence in their applicability to the prototype, both qualitatively and quantitatively. The results highlight the complexity of the caisson response, particularly under tensile loading, with the influences of average load, cyclic load amplitude and drainage discussed in detail.
The exploration and development of offshore renewable energy sources necessitates the handling of vast amounts of multi-disciplinary data across extensive areas. An efficient method of linking and integrating these data into a coherent ground model has become a theme of great interest. Such a ground model should assist developers in determining subsequent site investigations, optimizing field layout and foundation design, identifying geohazards and implementing suitable risk mitigation strategies. However, current practices often result in a qualitative integration, where geological formations are identified without assigning geotechnical properties to them, rendering the ground model unsuitable for site development. Moreover, the complexity of geological histories, particularly in relation to glacial and interglacial cycles and sea-level fluctuations, poses significant challenges. Despite the scarcity of geotechnical data, efficient integration of multi-disciplinary data is essential for creating reliable ground models that capture uncertainty accurately. Geophysical data provides information on the 3D geological and structural framework, while unit properties can be derived from in situ tests and laboratory data consistently. Our goal is to develop a fully integrated ground model, applicable to geotechnical applications, such as the Ten Noorden van de Waddeneilanden Wind Farm Zone (TNW), located off the Dutch coast. We propose a site-specific, data-driven integrated ground model that facilitates further development of the TNW. This quantitative ground model merges all available geotechnical and geological Ground Investigation data with geophysical data into a comprehensive, consistent 3D ground model, highlighting stratigraphic and spatial variations in ground conditions across the site.
Abstract Quantitative integrated ground models are a requirement for proper cost optimal site characterization, for offshore renewables, coastal activities and O&G projects. Geotechnical analyses and interpretations often rely on isolated 1D boreholes. On the other hand, geophysical data are collected in 2D lines and/or 3D volumes. Geophysical data therefore provides the natural link to re-populate geotechnical properties found in the 1D boreholes onto a larger area and thereby build a consistent and robust ground model. The geophysical data can be used to estimate geotechnical data and, as of today, there are a few methods available that can reliably map the dynamic properties from the seismic data (stratigraphic information, P-wave velocities, amplitudes, and their attributes) into geotechnical or geomechanical properties, particularly for shallow sub-surface depth. Being able to predict soil properties away from boreholes is important, as often the field layout changes during the development phase, and hence, information at the specific foundation locations may not be readily available. We have developed a workflow to build quantitative ground models following three approaches: (i) a geometric model in which the seismic data interpretations guide the prediction of geotechnical properties; (ii) a geostatistical approach in which in addition to the structural constraints, we used the seismic velocities to guide the prediction; and (iii) a multi-attribute regression using an artificial neural network (ANN). We apply it to a set of publically available data from the Holland Kust Zuid wind farm site in the Dutch sector of the North Sea. The result of the workflow yields maps or sub-volumes of geotechnical or geomechanical properties across the development site that can be used in further planning or engineering design. In this study, we use the tip resistance from a CPT as an example. The tip resistance derived using all methods generally give good results. Validation against randomly selected CPT shows good correlation between predicted and measured tip resistance. The ANN performs better than the geostatistical approach. However, these two approaches require good data quality and a rather large dataset to be effective. Therefore, using a global dataset not restricted to the Holland Kust Zuid site may improve the prediction. Moreover, using existing empirical correlation and calibration through laboratory testing or by training another ANN model, the geotechnical stiffness/strength parameters such as angle of friction or undrained shear strength could be derived. The next step is to use the results and their uncertainty into a cost assessment for the given foundation concepts.
Currently monopiles are the most common foundation solution for offshore wind turbines. The design of monopiles relies on empirical data from tests performed on long, slender, small-diameter piles loaded predominantly in shear. In contrast, a monopile is a large-diameter, relatively short pile on which load is applied with a large eccentricity. With centrifuge tests as the basis, this paper investigates the behaviour of a rigid pile loaded with a high eccentricity. A test series was carried out to simulate idealized monotonic load cases for monopiles supporting an offshore wind turbine. Centrifuge tests were performed on model monopiles subjected to stress distributions equal to prototype monopiles with pile diameters ranging from 1–5 m and eccentricities ranging from 8.25–17.75 pile diameters. It was possible to identify a unified response of all of these tests by using dimensional analysis and Rankine’s passive earth pressure coefficient as a normalization parameter. The normalized ultimate soil resistance was unaffected by acceleration level and load eccentricity, indicating that the failure mechanism was the same for all tests. Based on the centrifuge tests, a reformulation of soil–pile interaction curves is presented. The normalized initial stiffness of the soil–pile resistance curves was seen to increase linearly with depth in the centrifuge tests. The reformulation differs from current guidelines in terms of the shape of the interaction curve and magnitude of ultimate resistance.
Summary As part of the energy transition, there is a significant growth in offshore wind energy developments on a global scale. These projects require the integration of large volumes of geotechnical and geophysical data, which need to be put in a consistent geological context to understand the geological complexity of the site, and its implications for engineering design. In this paper, we present the state-of-the-art development of a data-driven, integrated ground model for the TNW (Ten noorden van de Waddeneilanden) offshore wind site, the Netherlands. The model starts from establishing a detailed seismostratigraphic or structural model, and culminates in a predictive 3D ground model, which contains parameters – as well as their uncertainty – relevant for geotechnical engineering and design applications (e.g., foundation design, geohazard assessment, layout design, etc.). The models combine seismic inversion and machine learning techniques to optimize the workflow.
One of the geotechnical challenges for a monopile-supported offshore wind turbine is to create a foundation design procedure that incorporates the effects of cyclic loading from wind and waves in a safe and easy way. Improved procedures may enable the use of monopiles on deeper waters, but still secure a robust and cost-beneficial foundation design. In order to develop new design procedures it is essential to understand the pile–soil interaction. With centrifuge tests as the basis, this paper discusses the effects of the soil–pile interaction, with the focus on accumulation of displacements and change in secant stiffness in dense sand. Hence a centrifuge test series simulating idealised cyclic loads on a monopile supporting an offshore wind turbine was carried out. The validity of these centrifuge tests is discussed, and a simple design procedure is presented for prediction of the accumulation of displacements and change in secant stiffness based on the results from the centrifuge tests.
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