It is well known that piles embedded in sand accumulate lateral deformation (displacement and rotation) when subjected to horizontal cyclic loading. The rate of accumulation depends on various parameters, such as loading conditions and properties of the pile-soil system. For nearly rigid piles, such as monopile foundations for offshore wind turbines, an essential aspect is the type of loading, which is determined by the ratio of the cyclic minimum load to cyclic maximum load. Several studies have shown that asymmetric two-way loading generally results in larger accumulated pile deformation compared with other types of loading, especially oneway loading with complete unloading in each cycle. This article presents the planning, execution, and evaluation of physical 1g small-scale model tests on the deformation accumulation of laterally loaded rigid piles due to cyclic loading focusing on soil deformations resulting from various cyclic load ratios. To visualize soil deformation fields and rearrangement processes within the soil profiles, particle image velocimetry (PIV) was applied in the tests. The evaluation of the model test results provides insights into varying accumulation rates and highlights the capabilities as well as limitations of PIV. The observations are summarized under the of findings, which may assist in planning future PIV experiments.

The reliability of monopiles is paramount for the uninterrupted operation of offshore wind turbines. However, this reliability is often challenged by environmental factors, such as scour and corrosion, as well as the inherent uncertainty in loads, soil properties, and environmental variables. Therefore, this study proposes a time-variant non-probabilistic reliability assessment strategy for laterally loaded offshore monopiles under scour and corrosion. This strategy is grounded in non-probabilistic interval and interval process models, along with a lateral response analysis of soil-pile system that accounts for the effects of scour and corrosion. The time-variant reliability index is determined using an enhanced HL-RF algorithm. The application of this method to specific cases of laterally loaded monopiles under scour and corrosion demonstrates its remarkable feasibility and adaptability, even for high-dimensional uncertainty scenarios. Furthermore, a sensitivity analysis, based on the proposed strategy, enables the identification of uncertain parameters that significantly impact monopile reliability, providing valuable insights for practical engineering applications. Additionally, this framework has the potential to be extended for a more comprehensive evaluation of the fatigue characteristics of offshore monopiles, an area that merits further exploration.

Microbially Induced Calcium Carbonate Precipitation (MICP) provides an environmentally friendly solution for reinforcing large diameter monopiles for offshore wind turbines (OWTs). This study presents an investigation into the lateral responses of monopiles with precast microbial reinforcement using a low-pH one-phase method. Both static and cyclic loading tests were carried out. The results of static loading tests show that the failure mode of the bio-reinforced monopile was an overall overturn failure. The lateral bearing capacity was increased by 50% and the bending moment was reduced by about 25% with the bio-reinforcement. Further investigation was conducted on the secant stiffness, damping ratio, and accumulated deformation of the bio-reinforced monopile under various cyclic loadings. With the bio-reinforcement, the accumulated deformation under one-way cyclic loading can be reduced by 30%-60%. The influences of cyclic loading parameters and loading sequence on pile stiffness were clarified. The growth ratio of pile stiffness due to bio-reinforcement under one-way loading was between 1.65 and 2.82, and decreased with increasing load amplitude. The ratio was smaller under two-way loading. Three competing factors on pile stiffness were identified: cyclic compaction of the unreinforced sandy soil, weakening of the bio-reinforced soil and soil subsidence around the bio-reinforced soil.

The paper presents the results of 3D coupled cyclic time history numerical analyses of a monopile supporting a 12 MW Offshore Wind Turbine, installed in dense cohesionless soils and subjected to a 600-s load history corresponding to the high phase of a 35-h design storm. The goal of the study is to investigate the governing mechanisms and gauge potential conservatisms or uncertainties in approaches for monopile analysis used in practice. The Ta-Ger constitutive model, implemented in FLAC3D and calibrated against site-specific cyclic tests, is used to model the complex soil response. Emphasis is placed on the effect of drainage conditions, an aspect typically overlooked in practice, although often stated as critical. Analyses show that the drainage of the system can substantially affect the response. In low-permeability soils (e.g., cohesionless soils with low-plasticity fines) widespread liquefaction may occur inducing high rotations above allowable limits. On the contrary, systems that can drain effectively within each cycle, develop moderate excess pore pressures which do not jeopardize performance. Current design procedures are often unable to accurately capture these effects possibly leading to either conservative or unconservative outcomes. Suitably validated advanced numerical analyses can be used as complementary tools to standard methods to assess these uncertainties.

The sustained growth of the offshore wind sector is leading to the construction of offshore wind farms in highly seismic regions of the world. Hence, a large proportion of the potential sites may be exposed to liquefaction risk. Monopiles are directly affected by this phenomenon given their preference as a support system for offshore wind turbines over other foundation types. This paper includes a comprehensive study of the lateral response of monopiles against the combined action of earthquake induced liquefaction and environmental loading using centrifuge modelling. The experimental setup was designed to compare the amount of excess pore pressure generated within the soil adjacent to the monopile with and without operational wind/wave loading. The results revealed higher accumulation of excess pore pressure in the non-laterally loaded case, and significant differences in the amount of excess pore pressure recorded in the windward and leeward sides of the monopile in the laterally loaded scenario. In addition, the study provides data on the rotation experienced by monopile supported offshore wind turbines in liquefiable soils.