This paper discusses the challenges of installing monopile offshore wind foundations in bedrock and introduces a new hybrid-monopile design as an alternative to rock-socketed monopiles. The performance of the hybridmonopile is evaluated through 1-dimension beam-spring theory and 3-dimension Finite Element analyses, with a focus on soil-foundation interaction and cyclic loading behaviour. The hybrid-monopile design is optimized and validated at two offshore sites in Korea. It is shown that optimized design can reduce required monopile penetration to avoid rock-socketed monopiles. The hybrid-monopile design shows a positive impact on reducing lateral pile displacement and rotation, particularly in soft ground conditions. The suggested 3D FE (Finite element) design approach and optimization with an additional seabed-level support structure can reliably avoid the need for rock-socketed monopiles.

Offshore wind turbines are subjected to long-term cyclic loads, and the seabed materials surrounding the foundation are susceptible to failure, which affects the safe construction and normal operation of offshore wind turbines. The existing studies of the cyclic mechanical properties of submarine soils focus on the accumulation strain and liquefaction, and few targeted studies are conducted on the hysteresis loop under cyclic loads. Therefore, 78 representative submarine soil samples from four offshore wind farms are tested in the study, and the cyclic behaviors under different confining pressures and CSR are investigated. The experiments reveal two unique development modes and specify the critical CSR of five submarine soil martials under different testing conductions. Based on the dynamic triaxial test results, the machine learning-based partition models for cyclic development mode were established, and the discrimination accuracy of the hysteresis loop were discussed. This study found that the RF model has a better generalization ability and higher accuracy than the GBDT model in discriminating the hysteresis loop of submarine soil, the RF model has achieved a prediction accuracy of 0.96 and a recall of 0.95 on the test dataset, which provides an important theoretical basis and technical support for the design and construction of offshore wind turbines.

The construction process of offshore wind farms in China is rapidly accelerating and seabed geological conditions vary significantly across different sea regions of China, which provides huge challenges for the construction of offshore wind farms. Foundation construction accounts for 30% of the total investment of offshore wind turbines (OWT), and ensuring the stability of pile-foundations is crucial for ensuring safe construction and normal safe operation of the OWT, which necessitates a comprehensive understanding of mechanical characteristics of seabed construction material under complex cyclic loadings. Therefore, this article focuses on studying the cyclic behavior of mucky clay (MC) and weathered granite residual soil (GRS) through serial cyclic triaxial tests. The stress-strain and stress path behaviors under various cyclic loads are analyzed for both types of submarine soils. Furthermore, the mechanical properties and load-bearing characteristics are thoroughly investigated by examining cumulative strain development, pore pressure behavior, and stiffness attenuation modes. More critical, a critical failure prediction model is established, for plotting the critical failure surface for both MC and weathered GRS, and the study reveals the cyclic failure modes of typical submarine soil under cyclic loading, which provides valuable insights for the design and construction of offshore wind farms.

Offshore wind power is a hot spot in the field of new energy, with foundation construction costs representing approximately 30% of the total investment in wind farm construction. Offshore wind turbines are subjected to long-term cyclic loads, and seabed materials are prone to causing stiffness degradation. The accurate disclosure of the mechanical properties of marine soil is critical to the safety and stability of the foundation structure of offshore wind turbines. The stiffness degradation laws of mucky clay and silt clay from offshore wind turbines were firstly investigated in the study. Experiments found that the variations in the elastic modulus presented L-type attenuation under small cyclic loads, and the degradation coefficient fleetingly decayed to the strength progressive line under large cyclic loads. Based on the experimental results, a random forest prediction model for the elastic modulus of the submarine soil was established, which had high prediction accuracy. The influence of testing the loading parameters of the submarine soil on the prediction results was greater than that of the soil's physical property parameters. In criticality, the CSR had the greatest impact on the prediction results. This study provides a more efficient method for the stiffness degradation assessment of submarine soil materials in offshore wind farms.

The rock-based sea area has great prospect of development and construction of offshore wind farms (OWFs), and the mainstream construction sites of OWFs in China have shifted from the soil-based seabed to the rock-based seabed area. Previous studies about mechanical properties of seabed materials and bearing characteristics of pile foundation in OWF mainly focus on the submarine soil-based seabed, resulting in lack of direct reference for the construction of offshore wind power in the rock seabed. Therefore, the study concentrates on the investigation of failure criterion of submarine completely weathered granite (CWG) of offshore wind farms in rockbased sea area under cyclic loads. Firstly, dynamic triaxial tests are carried out, and two unique development modes of CWG are revealed under different cyclic loads. The experiments analyze insight stiffness attenuation law and establish the prediction model of stiffness attenuation based on the logarithm formula. More critical, a unique development law of damping ratio of submarine seabed materials is discovered and discussed, and two cyclic failure criteria based on cumulative strain and dissipated energy are put forward to divide the critical CSR under cyclic loads, which gives helpful reference for the construction of offshore wind farms in rock-based sea area.

The southeastern rock base sea area is the most abundant wind resource area, and it is also the mainstream construction site of offshore wind farms (OWFs) in China. The weathered residual soil is the main seabed component in the rock base area, which is the important bearing stratum of the offshore wind turbine foundation. Previous studies on the mechanical properties of seabed materials and bearing characteristics of the pile foundations in OWFs have mainly focused on the submarine soil-based seabed, resulting in a lack of direct reference for the construction of offshore wind power in the rocky seabed. Therefore, the mechanical properties of weathered residual soil and the bearing behaviors of monopile foundations are mainly investigated in this study. Firstly, dynamic triaxial tests are conducted on the weathered residual soil, and experiments analyze insight into the evolution law of the hysteresis curve, cumulative strain, and stiffness attenuation. Then, the horizontal loading behaviors of monopile foundations in residual soil are analyzed by numerical simulations; more critically, the service performances under wind and wave coupling loads are evaluated, which provide a direct theoretical basis for the construction and design of offshore wind turbine foundations in rock base seabeds.

The principal construction-area of offshore wind farm (OWF) in China has shifted from the soil-based seabed area to the weathered rock-based area. The current studies about construction materials of OWF mainly focus on the submarine soft-clay or sandy soil seabed, resulting in the design and construction of OWF in rock-based area cannot directly refer to the existing research conclusions and results recommended in standards. Therefore, this paper concentrates on the investigation of cyclic behavior of submarine completely weathered granite of offshore wind farms in rock-based sea area under cyclic loadings. The experiments analyze the cyclic stress-strain and stress path behavior, revealing two unique development modes under different dynamic loadings, i.e., the stable development mode and the destructive development mode. A unified forecasting model of pore water pressure with different development modes is established, and further research investigates insight critical failure stage by concentrating on pore water pressure. More critical, the unique mechanical properties and microscopic characteristics of completely weathered granite are clearly expounded by comparing with submarine soft clay and sandy soil. This gives the engineering suggestion of design value of OWT foundation in completely weathered granite seabed, and is a helpful reference for the construction of OWF in rock-based sea area.

The offshore wind farm industry has recently experienced significant global growth. This study presents a thorough site investigation and analysis of the cyclic resistance of marine clay for offshore foundation design, using the Shaba wind farm in southern China as a case study. In-situ cone penetrometer (CPTu) tests and borehole sampling are conducted to explore the geotechnical characteristics of the soils. However, the soil conditions are characterized by multiple layers and complex sedimentary components. The classification and mechanical properties, such as water content and cyclic resistances, are compared through CPTu interpretation and laboratory tests. The findings indicated that a single physical indicator cannot determine cyclic resistance. In addition, the well-established method in existing literature proved unsuitable for marine clay. Consequently, multiple regression analysis shows that a linear relationship exist between cyclic resistance and depth-corrected CPTu index [EXP(qE/fs)0.3/H], hence a new evaluation method is developed to predict the cyclic resistance of marine clay based on CPTu data. This research aims to provide more reliable guidance for geotechnical investigations, supporting the rapid expansion of offshore wind farms.