TY - BOOK

T1 - On the displacement accumulation of monopiles under lateral cyclic loading

AU - Frick, Dennis

PY - 2024/10/1

Y1 - 2024/10/1

N2 - Up to now, the monopile is the most popular foundation type for supporting offshore wind turbines (OWTs). However, the current design guidance for monopile foundations is largely based on experience from the oil and gas industry, where both the pile dimensions as well as the design load conditions are different from offshore wind. As OWTs are quite sensitive to tilting, a key concern in the monopile foundation design is the accurate prediction of accumulated deformations due to long-term cyclic horizontal loads. Although the offshore guidelines (OGLs) and certification authorities require compliance with certain deformation criteria, no generally accepted method for the calculative verification currently is given by the OGLs. Therefore, in the past decades, several researchers investigated the displacement accumulation of monopiles to develop calculation methods for the estimation of permanent displacements due to lateral cyclic loading. However, a problem is the complexity of the pile-soil interaction and the multitude of influencing factors. This leads to the fact that different proposed methods partly provide very different or even contradictory results. As a consequence, the design of monopile foundations is often very conservative, which prevents the desired cost optimization. This thesis presents experimental and theoretical research, aimed at improving the knowledge concerning the behaviour of large diameter monopiles subjected to long-term lateral cyclic loading in cohesionless soils. Based on a literature review including the topics of current design regulations as well as different empirical and numerical investigations on the prediction of the monopile bearing and deformation behaviour with a special focus on lateral cyclic loading, selected methods for the determination of the displacement accumulation were examined and their results were compared using example calculations. The findings of this review were used to identify relevant parameters affecting the displacement accumulation of monopiles and to develop a comprehensive 1 g small-scale model test campaign for a systematic investigation of the key mechanisms driving the pile response. The parameters that have been assessed include the cyclic load magnitude, cyclic load ratio, load eccentricity, grain size distribution and soil relative density as well as the pile embedment length. In order to get additional insights into the reasons for different accumulation rates due to varying system or load configurations, further experimental tests involving the visualization of soil redistribution or densification effects resulting from lateral cyclic loading using digital image correlation (DIC) were carried out and evaluated. In addition to the experimental tests, the present thesis includes numerical finite element (FE) investigations in which the stiffness degradation method (SDM) was applied to calculate the monopile response to one-way lateral cyclic loading. Within the FE studies, the focus was mainly on scale effects resulting from different stress states in small and large scale as well as the influence of the relative pile-soil stiffness. The physical model tests revealed the cyclic load ratio to be a decisive parameter influencing the pile head displacement accumulation. For nearly rigid piles, it was found that asymmetrical two-way loading results in approximately 40% higher accumulation rates compared to simple one-way loads with complete unloading. The load magnitude influences the absolute value of the pile head displacement, but it changes the accumulation rate only marginally, at least for the load range studied experimentally. For a constant relative load magnitude also load eccentricity, type of sand or soil relative density seem to have a minor influence on the rate of accumulation within the investigated range. From the DIC experiments and the numerical studies, it is concluded that the relative pile-soil stiffness is a significant factor. The numerical parameter study shows the stiffer the pile responds to loading in relation to the soil, the greater the rate of displacement accumulation. Since the stiffness of a non-cohesive soil is stress-dependent, a different relative pile-soil stiffness can result for a defined system when calculated in different scales. Furthermore, also the load magnitude may influence the relative pile-soil stiffness and therefore accumulation rate when being very low and below a certain threshold. In the end, the observations made from the experimental and numerical investigations were used to derive a simple procedure for a first estimation of the global pile head displacement accumulation rate considering load characteristics as well as the relative pile-soil stiffness without large computational effort.

AB - Up to now, the monopile is the most popular foundation type for supporting offshore wind turbines (OWTs). However, the current design guidance for monopile foundations is largely based on experience from the oil and gas industry, where both the pile dimensions as well as the design load conditions are different from offshore wind. As OWTs are quite sensitive to tilting, a key concern in the monopile foundation design is the accurate prediction of accumulated deformations due to long-term cyclic horizontal loads. Although the offshore guidelines (OGLs) and certification authorities require compliance with certain deformation criteria, no generally accepted method for the calculative verification currently is given by the OGLs. Therefore, in the past decades, several researchers investigated the displacement accumulation of monopiles to develop calculation methods for the estimation of permanent displacements due to lateral cyclic loading. However, a problem is the complexity of the pile-soil interaction and the multitude of influencing factors. This leads to the fact that different proposed methods partly provide very different or even contradictory results. As a consequence, the design of monopile foundations is often very conservative, which prevents the desired cost optimization. This thesis presents experimental and theoretical research, aimed at improving the knowledge concerning the behaviour of large diameter monopiles subjected to long-term lateral cyclic loading in cohesionless soils. Based on a literature review including the topics of current design regulations as well as different empirical and numerical investigations on the prediction of the monopile bearing and deformation behaviour with a special focus on lateral cyclic loading, selected methods for the determination of the displacement accumulation were examined and their results were compared using example calculations. The findings of this review were used to identify relevant parameters affecting the displacement accumulation of monopiles and to develop a comprehensive 1 g small-scale model test campaign for a systematic investigation of the key mechanisms driving the pile response. The parameters that have been assessed include the cyclic load magnitude, cyclic load ratio, load eccentricity, grain size distribution and soil relative density as well as the pile embedment length. In order to get additional insights into the reasons for different accumulation rates due to varying system or load configurations, further experimental tests involving the visualization of soil redistribution or densification effects resulting from lateral cyclic loading using digital image correlation (DIC) were carried out and evaluated. In addition to the experimental tests, the present thesis includes numerical finite element (FE) investigations in which the stiffness degradation method (SDM) was applied to calculate the monopile response to one-way lateral cyclic loading. Within the FE studies, the focus was mainly on scale effects resulting from different stress states in small and large scale as well as the influence of the relative pile-soil stiffness. The physical model tests revealed the cyclic load ratio to be a decisive parameter influencing the pile head displacement accumulation. For nearly rigid piles, it was found that asymmetrical two-way loading results in approximately 40% higher accumulation rates compared to simple one-way loads with complete unloading. The load magnitude influences the absolute value of the pile head displacement, but it changes the accumulation rate only marginally, at least for the load range studied experimentally. For a constant relative load magnitude also load eccentricity, type of sand or soil relative density seem to have a minor influence on the rate of accumulation within the investigated range. From the DIC experiments and the numerical studies, it is concluded that the relative pile-soil stiffness is a significant factor. The numerical parameter study shows the stiffer the pile responds to loading in relation to the soil, the greater the rate of displacement accumulation. Since the stiffness of a non-cohesive soil is stress-dependent, a different relative pile-soil stiffness can result for a defined system when calculated in different scales. Furthermore, also the load magnitude may influence the relative pile-soil stiffness and therefore accumulation rate when being very low and below a certain threshold. In the end, the observations made from the experimental and numerical investigations were used to derive a simple procedure for a first estimation of the global pile head displacement accumulation rate considering load characteristics as well as the relative pile-soil stiffness without large computational effort.

U2 - 10.15488/17993

DO - 10.15488/17993

M3 - Doctoral thesis

T3 - Mitteilungen/Institut für Geotechnik (IGtH), Leibniz-Universität Hannover

CY - Hannover

ER -