Search Results (5)

Large-diameter rock-socketed monopiles supporting offshore wind turbines in soft clay strata face significant geotechnical risks from negative skin friction (NFS) induced by construction surcharges. While the effects of NFS on axial drag loads are documented, the critical interaction between horizontal pile loading and NFS development remains poorly understood. This research bridges this gap using a rigorously validated 3D finite element model that simulates the complex coupling of vertical substructure loads (5 MN), horizontal loading, and surcharge-induced consolidation. The model’s accuracy was confirmed through comprehensive verification against field data for both NFS evolution under surcharge and horizontal load–displacement behavior. The initial analysis under representative conditions (10 MN horizontal load, 100 kPa surcharge, 3600 days consolidation) revealed that horizontal loading fundamentally distorts NFS distribution in the upper pile segment (0 to −24 m), transforming smooth profiles into distinct dual-peak morphologies while increasing the maximum NFS magnitude by 57% (from −45.4 kPa to −71.5 kPa) and relocating its position 21 m upward. This redistribution was mechanistically linked to horizontal soil displacement patterns. Crucially, the NFS neutral plane remained invariant at the clay–rock interface (−39 m), demonstrating complete independence from horizontal loading effects. A systematic parametric study evaluated key operational factors: (1) consolidation time progressively increased NFS magnitude throughout the clay layer, evolving from near-linear to dual-peaked distributions in the upper clay (0 to −18 m); NFS stabilized in the upper clay after 720 days while continuing to increase in the lower clay (−18 to −39 m) due to downward surcharge transfer, accompanied by neutral plane deepening (from −36.5 m to −39.5 m) and 84% maximum axial force escalation (12.5 MN to 23 MN); (2) horizontal load magnitude amplified upper clay NFS peaks at −3.2 m and −9.3 m, with the shallow peak magnitude increasing linearly with load intensity, though it neither altered lower clay NFS nor neutral plane position; (3) surcharge magnitude increased overall NFS, but upper clay NFS (0 to −18 m) stabilized beyond 100 kPa, while lower clay NFS continued rising with higher surcharges, and the neutral plane descended progressively (from −38 m to −39.5 m). These findings demonstrate that horizontal loading critically exacerbates peak NFS values and redistributes friction in upper pile segments without influencing the neutral plane, whereas surcharge magnitude and consolidation time govern neutral plane depth, total NFS magnitude, and maximum drag load. This research delivers essential theoretical insights and practical guidelines for predicting NFS-induced drag loads and ensuring the long-term safety of offshore wind foundations in soft clays under complex multi-directional loading scenarios. Full article

Offshore wind power is a new trend in renewable energy development. However, during the operation of offshore wind turbines, the rock-socketed monopile foundation is subjected to long-term cyclic loads, which will cause the seawater to erode the rock around the monopile foundation and reduce the ultimate end-bearing capacity. There is no suitable rock mass classification for evaluating the quality of marine bedrock and no theoretical method for accurately calculating the ultimate end-bearing capacity of the monopile foundation. Therefore, based on the existing rock mass classification, an ocean rock mass classification (OMR) that is applicable to marine bedrock is proposed. The ratings of four geological indices (R₁, R₂, R₃, and R₄) in the OMR classification are reset by the analysis hierarchy process and modified according to the geological conditions of marine bedrock. Then, an accelerated test of seawater erosion is used over 60 days to simulate seawater erosion for up to 12 years to determine the adjustment factor for the effect of time, F_t, in the OMR classification. Based on the OMR classification, a theoretical calculation method of the ultimate end-bearing capacity of the offshore wind monopile foundation under the overall sliding failure mode of rock mass is proposed. The theoretical calculation method was employed for offshore wind engineering, and the reliability of the theoretical calculation and three-dimensional numerical simulation was validated. The results show that the theoretical and numerical results for the ultimate end-bearing capacity without seawater erosion are similar to the measured results, with a relative error of less than 9%. The theoretical results are always larger than the numerical results, with a relative error of less than 7%. Finally, the theoretical and numerical results were used to guide the design and construction of an offshore wind turbine. The offshore wind turbine has been operating for 8 years, and its displacement is 15.3 mm, which is less than the numerical result of 16.94 mm. Full article

Monopiles are commonly used in the construction of offshore wind turbines. However, implementing drive-drill-drive construction techniques in decomposed granite seabed may lead to borehole instability during the window period between drilling and pile driving, resulting in significant project losses. This study provides a comprehensive understanding and approach to address the causes of borehole instability in rock-socketed monopiles in decomposed granite seabed. Using the Pinghai Bay offshore wind farm project in Fujian, China as an example, the details of drive-drill-drive and reverse-circulation drilling techniques employed in monopile construction were introduced. An improved sampling method was utilized to obtain decomposed granite samples, and a series of in situ and laboratory tests were conducted to analyze the physical and mechanical properties of marine-decomposed granite. By examining three cases of monopile construction, the factors contributing to borehole instability during rock-socketed monopile construction in decomposed granite seabed were identified, and corresponding recommendations were proposed. The results indicated that construction technology and unfavorable geological characteristics of decomposed granite are the primary causes of borehole instability. Collapses occurred mainly in highly and moderately decomposed granite layers. Employing smaller boreholes can reduce the likelihood and impact of borehole instability. Full article

A monopile is the most conventional structure foundation for offshore wind turbines (OWTs) in the world. However, the Korean offshore wind industry has mostly been using the jacket type of foundation. The main reason for the current situation in Korea is that most of the marine soil consists of weak layers of sand and clay. Thus, the monopile foundation depth has to be deep enough to satisfy the intended serviceability design requirement of the monopile and the rotation limit at the seabed; a conventional monopile design concept alone might be insufficient in Korean offshore conditions, or otherwise could be very expensive, e.g., resulting in a rock socket installation at the tip of the monopile. The main objective of this paper is to introduce a novel hybrid monopile that is composed of a monopile and a supplemental support with three buckets, followed by assessing the lateral resistance of the hybrid system through physical experiments and finite element (FE) simulations. Namely, 1/64.5 small-scaled monopile and hybrid physical models with a monopile diameter of 7 m for a 5.5 MW OWT were loaded monotonically. The results show that the hybrid monopile improves the lateral bearing capacity regarding the initial lateral stiffness and ultimate load. The FE analyses of the corresponding physical models were also implemented to support the results from the physical model test. The numerical results, such as the structural member forces and soil deformation, were analyzed in detail. Additionally, a case study using FE analysis was conducted for the 5.5 MW OWT hybrid monopile support installed in a representative Korean weak soil area. The results show that the hybrid monopile foundation has a larger lateral resistance and stiffness than the monopile. Full article

With the development of offshore wind energy in China, more and more offshore wind turbines are being constructed in rock-based sea areas. However, the large diameter and thin-walled steel rock-socketed monopiles are very scarce at present, and both the construction and design are very difficult. For the design, the dynamic safety during the whole lifetime of the wind turbine is difficult to guarantee. Dynamic safety of a turbine is mostly controlled by the dynamic impedances of the rock-socketed monopile, which are still not well understood. How to choose the appropriate impedances of the socketed monopiles so that the wind turbines will neither resonant nor be too conservative is the main problem. Based on a numerical model in this study, the accurate impedances are obtained for different frequencies of excitation, different soil and rock parameters, and different rock-socketed lengths. The dynamic stiffness of monopile increases, while the radiative damping decreases as rock-socketed depth increases. When the weathering degree of rock increases, the dynamic stiffness of the monopile decreases, while the radiative damping increases. Full article