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The strong nonlinearity of shallow-water waves significantly affects the dynamic response of floating offshore wind turbines (FOWTs), introducing additional complexity in motion behavior. This study presents a series of 1:80-scale experiments conducted on a 5 MW FOWT at a 50 m water depth, under regular, irregular, and focused wave conditions. The tests were conducted under regular, irregular, and focused wave conditions. The results show that, under both regular and irregular wave conditions, the platform’s motion and mooring tension increased as the wave period became longer, indicating a greater energy transfer and stronger coupling effects at lower wave frequencies. Specifically, in irregular seas, mooring tension increased by 16% between moderate and high sea states, with pronounced surge–pitch coupling near the natural frequency. Under focused wave conditions, the platform experienced significant surge displacement due to the impact of large wave crests, followed by free-decay behavior. Meanwhile, the pitch amplitude increased by up to 27%, and mooring line tension rose by 16% as the wave steepness intensified. These findings provide valuable insights for the design and optimization of FOWTs in complex marine environments, particularly under extreme wave conditions. Additionally, they contribute to the refinement of relevant numerical simulation methods.

During the service period, the offshore wind turbine foundation mainly bears the wind load from the upper structure and the periodic loads such as wave load and sea currents from the lower structure. Long-term cyclic loads have an important impact on the cumulative deformation, foundation stiffness changes, and horizontal ultimate bearing capacity of the offshore wind turbine bucket foundation. This paper conducts cyclic loading tests on mono-bucket foundations under unidirectional and multidirectional cyclic loading conditions based on the multidirectional intelligent cyclic loading system of offshore wind turbine foundations and analyzes the cumulative effects of the loading direction, vertical load, and drainage status on the mono-bucket foundation during the cyclic loading process, effects of rotation angle, cyclic stiffness, and monotonic bearing capacity of foundation after cycles. The research results show that multidirectional cyclic loading significantly reduces the cyclic cumulative rotation angle of the mono-bucket foundation, and the maximum reduction rate can reach more than 50%. After unidirectional and multidirectional cyclic loading, the bearing capacity of the bucket foundation can be increased by up to 30% and 20%, respectively. At the same time, this paper proposes calculation formulas for vertical load, number of cyclic loading, and normalized cumulative rotation and establishes a calculation method for vertical load and bearing capacity of the foundation after cycles.

This study investigates the influence of inside-trapped water and remedial grouting on the vertical bearing behaviour of suction bucket foundations in sand through 1 g laboratory model tests. The tests were designed to compare the relative responses of different trapped-water and grouting conditions under the same model scale, sand preparation procedure, and loading protocol. Two target trapped-water conditions were considered: a condition without an observable continuous water layer beneath the bucket lid and a condition with an initial trapped-water thickness of approximately 2 cm. These conditions were controlled and verified before loading using the scale attached to the transparent bucket wall and the underwater camera monitoring system. The results show that inside-trapped water modifies the vertical load-transfer path between the bucket lid and the internal soil plug. When a water layer exists beneath the lid, direct lid–soil plug contact is weakened, and the foundation resistance relies more strongly on skirt-side resistance and the resistance mobilized near the bucket rim. Under cyclic vertical loading, the trapped-water case exhibited larger cumulative displacement and a lower post-cyclic bearing response than the no-trapped-water case. The secant cyclic stiffness showed a continuous increase in the no-trapped-water case, whereas a rise-then-fall trend was observed in the trapped-water case, which may be associated with cyclic densification, soil plug disturbance, changes in lid–soil plug contact, and possible local pore pressure development. Remedial grouting filled the trapped-water space beneath the bucket lid and partially restored the lid–soil plug load-transfer path. Under the present model test conditions, the post-cyclic dimensionless bearing capacity of the grouted cases increased by approximately 13–16% relative to the ungrouted trapped-water case. The grouting cases with different bentonite contents showed similar recovery trends within the limited dataset, suggesting that the improvement was mainly related to filling and sealing the trapped-water space rather than to the intrinsic strength of the grout material.

Offshore wind turbine foundations are commonly subjected to large horizontal, vertical, and bending moment loads. Marine soils have high moisture content, high compressibility, high sensitivity, and low strength, resulting in insufficient foundation bearing capacity. In order to improve the bearing capacity of wind turbine foundations and reduce foundation settlement, an internal vacuum preloading method combined with electroosmosis reinforcement is used to reinforce the soil within bucket foundations. The pore water pressure, vertical settlement, pumping quality of the soil during the reinforcement process, soil moisture content before and after the reinforcement, and undrained shear strength were analyzed. Horizontal and vertical bearing capacity model tests were carried out on the reinforced and nonreinforced soil inside the bucket foundation. Results show that vacuum preloading combined with electroosmosis reinforcement reduces soil moisture content inside the bucket foundation by approximately 20%, and the undrained shear strength of the internal soil increases by approximately 20 times. Soil reinforcement has high spatial uniformity. Results of the bucket foundation bearing capacity model show that when the soil inside the bucket foundation is strengthened, horizontal bearing capacity increased by 2.9 times and vertical bearing capacity increased by 2.1 times. Vacuum preloading combined with electroosmosis reinforcement can effectively improve the shear strength of soft soil and enhance the bearing capacity and stability of bucket foundations.

Driven by the rapid development of offshore wind farms, bucket foundations have come to constitute a very promising form of foundation for offshore wind turbines, mainly owing to their efficient construction. However, the penetration resistance of the suction penetration of a bucket foundation, when calculated inaccurately, may lead to installation failure of the foundation. In this study, model tests were performed on the suction penetration of a mono-bucket mono-compartment foundation and a mono-bucket multi-compartment foundation in saturated fine marine sand, aiming to compare their penetration resistances and critical suctions, and the development of a soil plug in the two models was analyzed. The results will provide a design reference for the penetration construction of bucket foundations for offshore wind turbines.

A modified suction caisson (MSC), which was reported by the authors of this paper previously, comprises an external short-skirted structure that is added to a regular suction caisson (RSC). It has been proved that MSCs can improve the lateral bearing capacity and limit the deflection of the caisson compared with RSCs. A series of model tests were conducted to investigate responses of MSCs subject to uplift loading in saturated sand. The effects of external skirt dimensions on the uplift bearing capacity of MSCs were considered. In addition, the influences of the sealed top lid of the skirted structure on the uplift bearing capacity and the resulting passive suction of MSCs were also studied. It was found that the uplift bearing capacities of MSCs are 1.4-1.7 times that of RSCs. Moreover, test results in serviceable conditions show that the sealed external skirted structure of perspex-made suction caissons significantly contributed to the uplift bearing capacity as a result of passive suction.

The composite bucket foundation (CBF) with seven honeycomb subdivisions is a new foundation for offshore wind turbine structures. The bearing capacity of CBF can be improved by consolidation of soil inside the CBF, which is caused by the vacuum preloading method after installation. A three-dimensional numerical model is established to simulate the consolidation process of soil for CBF with and without subdivisions in terms of vertical settlement, pore water pressure and void ratio of the soil. This analysis investigates the reinforcement effect of the two foundation types to assess the influence of the bulkheads. The results obtained show that there are obvious reinforcement effects for both foundation types. In the early stage of consolidation, vertical settlement is rapid, and this becomes stable with time. The depth at which the pore water pressure becomes negative is the depth showing the main reinforcement. Vacuum pressure decreases continuously with increase in soil depth and time. In addition, the excess pore water pressure in the soil dissipates, which turns into the soil effective stress. Bulkheads provide vertical drainage channels in the soil and shorten the seepage path, allowing the extraction of more pore water. This is conducive to the improvement of shallow soil, while also decreasing the extraction of pore water in deep soil and the region of the soil that can be reinforced.

Jack-up units are extensively utilized in offshore geotechnical investigations and the recent installation of wind turbine foundations. Engineers require accurate predictions of spudcan penetration resistance to assess its final penetration depth and identify potential risks, e.g., punch-through, ensuring safety during the installation of jack-up units. Notably, for seabed stratigraphy comprising sand with interbedded clay layers, a significant discrepancy exists between the penetration resistance calculated using current industry guidelines and actual measured results. In this study, three-dimensional (3D) large deformation finite-element (LDFE) analyses were performed to model the spudcan penetration response in a seabed consisting of a sand–clay–sand soil profile. A comprehensive parametric study was conducted to examine the influence of soil strength and layer thickness on penetration resistance. The variations of the soil failure mechanisms are demonstrated and associated with the varying penetration resistance, which form the basis of the new calculation method developed for the sand seabed with interbedded clay.

Driven by the rapid development of offshore wind farms, bucket foundations have come to constitute a very promising form of foundation for offshore wind turbines, mainly owing to their efficient construction. However, the penetration resistance of the suction penetration of a bucket foundation, when calculated inaccurately, may lead to installation failure of the foundation. In this study, model tests were performed on the suction penetration of a mono-bucket mono-compartment foundation and a mono-bucket multi-compartment foundation in saturated fine marine sand, aiming to compare their penetration resistances and critical suctions, and the development of a soil plug in the two models was analyzed. The results will provide a design reference for the penetration construction of bucket foundations for offshore wind turbines. Keywords: composite bucket foundation; penetration resistance; critical suction; bulkhead

As the preferred architecture for high-speed and high-resolution analog-to-digital converters (ADC), the accuracy of pipelined ADC is limited mainly by various errors arising from multiple digital-to-analog converters (MDAC). This paper presents a multi-dimensional (M-D) MDAC calibration based on a genetic algorithm (GA) in a 12-bit 750 MS/s pipelined ADC. The proposed M-D MDAC compensation model enables capacitor mismatch and static interstage gain error (IGE) compensation on the chip and prepares for subsequent background calibration based on a pseudo-random number (PN) injection to achieve accurate compensation for dynamic IGE. An M-D coefficient extraction scheme based on GA is also proposed to extract the required compensation coefficients of the foreground calibration, which avoids falling into local traps through MATLAB. The above calibration scheme has been verified in a prototype 12-bit 750 MS/s pipelined ADC. The measurement results show that the signal-to-noise and distortion ratio (SNDR) and spurious-free dynamic range (SFDR) are increased from 49.9 dB/66.7 dB to 59.6 dB/77.5 dB with the proposed calibration at 25 °C. With the help of background calibration at 85 °C, the SNDR and SFDR are improved by 3.4 dB and 8.8 dB, respectively.