• Geomechanics and Engineering
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• v.19 no.1
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• pp.79-91
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• 2019

Renewed interest in the long-term pile foundations has been driven by the increase in offshore wind turbine installation to generate renewable energy. A monopile subjected to repetitive loads experiences an evolution of displacements, pile rotation, and stress redistribution along the embedded portion of the pile. However, it is not fully understood how the embedded pile interacts with the surrounding soil elements based on different pile geometries. This study investigates the long-term soil response around offshore monopiles using finite element method. The semi-empirical numerical approach is adopted to account for the fundamental features of volumetric strain (terminal void ratio) and shear strain (shakedown and ratcheting), the strain accumulation rate, and stress obliquity. The model is tested with different strain boundary conditions and stress obliquity by relaxing four model parameters. The parametric study includes pile diameter, embedded length, and moment arm distance from the surface. Numerical results indicate that different pile geometries produce a distinct evolution of lateral displacement and stress. In particular, the repetitive lateral load increases the global lateral load resistance. Further analysis provides insight into the propagation of the shear localization from the pile tip to the ground surface.

• Geomechanics and Engineering
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• v.28 no.6
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• pp.585-598
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• 2022

The monopile-friction wheel hybrid foundation is an innovative solution for offshore structures which are mainly subjected to large lateral eccentric load induced by winds, waves, and currents during their service life. This paper presents an extensive numerical analysis to investigate the lateral load and moment bearing performances of hybrid foundation, considering various potential influencing factors in sand-overlaying-clay soil deposits, with the complex lateral loads being simplified into a resultant lateral load acting at a certain height above the mudline. Finite element models are generated and validated against experimental data where very good agreements are obtained. The failure mechanisms of hybrid foundations under lateral loading are illustrated to demonstrate the effect of the friction wheel in the hybrid system. Parametric study shows that the load bearing performances of the hybrid foundation is significantly dependent of wheel diameter, pile embedment depth, internal friction angle of sand, loading eccentricity (distance from the load application point to the ground level), and the thickness of upper sandy layer. Simplified empirical formulae is proposed based on the numerical results to predict the corresponding lateral load and moment bearing capacities of the hybrid foundation for design application.

• Journal of Ocean Engineering and Technology
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• v.38 no.2
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• pp.74-85
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• 2024

Offshore wind turbines are supported by various foundations, each with its considerations in design and construction. Gravity, monopile, and suction bucket foundations encounter geotechnical issues, while jacket and tripod foundations face fatigue problems. Considering this, a gravity foundation based on a steel skirt was developed, and a monopile foundation was analyzed for Pile-Soil Interaction using the p-y curve and 3D finite element method (3D FEM). In addition, for suction bucket foundations, the effects of lateral and vertical loads were analyzed using 3D FEM and centrifuge tests. Fatigue analysis for jacket and tripod foundations was conducted using a hotspot stress approach. Some hybrid foundations and shape optimization techniques that change the shape to complement the problems of each foundation described above were assessed. Hybrid foundations could increase lateral resistance compared to existing foundations because of the combined appendages, and optimization techniques could reduce costs by maximizing the efficiency of the structure or by reducing costs and weight. This paper presents the characteristics and research directions of the foundation through various studies on the foundation. In addition, the optimal design method is presented by explaining the problems of the foundation and suggesting ways to supplement them.

• Journal of the Korean GEO-environmental Society
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• v.15 no.9
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• pp.47-58
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• 2014

This study explores methods for modeling the foundation-seabed interaction needed for the load analysis of an offshore wind energy system. It comprises the comparison study of foundation design load analyses for NREL 5 MW turbine according to various soil-foundation interaction models by conducting the load analysis with GH-Bladed, analysis software for offshore wind energy systems. Furthermore, the results of the aforementioned load analysis were applied to foundation analysis software called L-Pile to conduct a safety review of the foundation cross-section design. Differences in the cross-section of a monopile foundation were observed based on the results of the fixed model, winkler spring and coupled spring models, and the analysis of design load cases, including DLC 1.3, DLC 6.1a, and DLC 6.2a. Consequently, under all design load conditions, the diameter and thickness of the monopile foundation cross-section were found to be 7 m and 80 mm, respectively, using the fixed and coupled spring models; the results of the analysis conducted using the winkler spring model showed that the diameter and thickness of the monopile foundation cross-section were 5 m and 60 mm, respectively. The study found that the soil-foundation interaction modeling method had a significant impact on the load analysis results, which determined the cross-section of a foundation. Based on this study, it is anticipated that designing an offshore wind energy system foundation taking the above impact into account would reduce the possibility of a conservative or unconservative design of the foundation.

• Ocean Systems Engineering
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• v.12 no.4
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• pp.439-459
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• 2022

This paper describes experimental studies of impact pressure generated by breaking regular waves in shallow water on a vertical cylinder. Experimental work was carried out in a shallow water flume using a 1:30 - scale model of a vertical rigid circular hollow cylinder with a diameter 0.2 m. This represents a monopile for shallow water offshore wind turbines, subjected to depth induced breaking regular waves of frequencies of 0.8 Hz. The experimental setup included a 1 in 10 sloping bed followed by horizontal bed with a constant 0.8 m water depth. To determine the breaking characteristics, plunging breaking waves were generated. Free surface elevations were recorded at different locations between the wave paddle to the cylinder. Wave impact pressures on the cylinder at a number of elevations along its height were measured under breaking regular waves. The depth-induced wave breaking characteristics, impact pressures, and wave run-up during impact for various cylinder locations are presented and discussed.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.33 no.4
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• pp.245-253
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• 2020

In this study, natural frequencies are compared using several pile-soil interaction (PSI) models to evaluate the effects of each model on resonance safety checks for a monopile type of wind turbine structure. Base spring, distributed spring, and three-dimensional brick-shell models represented the PSIs in the finite element model. To analyze the effects of the PSI models on a natural frequency, after a stiffness matrix calculation and Winkler-based beam model for base spring and distributed spring models were presented, respectively; natural frequencies from these models were investigated for monopiles with different geometries and soil properties. These results were compared with those from the brick-shell model. The results show that differences in the first natural frequency of the monopiles from each model are small when the small diameter of monopile penetrates hard soil and rock, while the distributed spring model can over-estimate the natural frequency for large monopiles installed in weak soil. Thus, an appropriate PSI model for natural frequency analyses should be adopted by considering soil conditions and structure scale.

• Journal of the Korean Geotechnical Society
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• v.36 no.2
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• pp.29-42
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• 2020

In this study, centrifuge tests were performed to investigate the effect of the lateral load-moment combination on the p-y curves for 7 m-diameter monopiles installed in sand for offshore wind turbine. For the objectives, a centrifuge testing system was developed and tests were conducted at an acceleration of 68.83 g using well-instrumented model monopiles under two different lateral load-moment combinations simulated by different loading heights: 1 and 5 times monopile diameter from the ground surface. The sand was prepared as medium loose sand. Based on the centrifuge test results, the experimental p-y curves were evaluated and compared with previous literatures including API codes. The experimental results reveal that the p-y curves were little influenced by the combination of lateral load and moment. It was also found that the embedded length affects p-y curves.

• Journal of the Korean GEO-environmental Society
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• v.13 no.12
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• pp.91-98
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• 2012

In this paper, the behavior of offshore wind turbine(OWT) foundation which is modeled by using existing design method and FEM is compared. When the same type of foundation is designed under the same sea and ground condition, the behavior characteristics with each model are compared. As a result, the member forces between apparent fixity and distributed spring type foundation which consider the ground stiffness are not different markedly, while fixed-base type foundation shows relatively lower member forces, which results in smaller safety margin. In other words, considering ground stiffness is reasonable because soil-pile interaction affects significantly on the analysis result. A case study with a monopile shows significant errors between p-y and FEM model at the head and tip of the pile. Also, it shows that the errors at the tip with diameter increase of the pile is larger. Thus, considering ground characteristics and engineering judgment are necessary in practice. A comparison of reliability analysis between tripod and monopile type foundation on the same condition shows larger probability of failure in monopile type and it indicates that the safety margin of monopile type can be lower.

• Journal of Korean Society of Steel Construction
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• v.25 no.4
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• pp.421-430
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• 2013

To predict behaviors of offshore wind turbines which are highly laterally loaded structures and to design them rationally, evaluating the soil-foundation interaction is important. Nowadays, there are many soil modeling methods for structural analysis of general structures subjected to vertical loads, but using the methods without any consideration for design of a monopile foundation is eschewed because it might cause wrong structural design due to the deferent loading state. In this paper, we identify the differences of the member forces and displacements by design methods. The results show that fixed end method is barely suitable for monopile design in terms of checking the serviceability because it underestimate the lateral displacement. Fixed end method and stiffness matrix method underestimate the member forces, whereas virtual fixed end method overestimates them. The results of p-y curve method and coefficient of subgrade reaction method are similar to the results of 3D soil modeling method, and 2D soil modeling method overestimates the displacement and member forces as compared with other methods.

• Journal of the Korean Geotechnical Society
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• v.40 no.4
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• pp.33-47
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• 2024

Offshore wind structures are subject to long-term repeated horizontal loads from wind, waves, and currents, making it essential to consider these loads in the design of offshore foundations. In this respect, monopiles are large-diameter hollow steel pipes that are relatively simple to construct compared with piles used on onshore sites. They can provide stable support for wind structures and have well-established design codes, leading to their widespread use globally. The behavior of monopiles under lateral static loads is typically assessed using the p-y method proposed by the American Petroleum Institute (API). However, the applicability of p-y curves to large-diameter monopiles exposed to repeated cyclic horizontal loads, such as those experienced in offshore wind applications, must yet be evaluated. Thus, this study evaluated the behavior of monopiles under two-way cyclic horizontal loads in loose silty sand, a representative soil type of the southwestern coast of Korea, using centrifuge model tests. The results demostrated that the behavior of monopiles varied depending on the loading level, number of cycles, and direction of the cyclic loads. Furthermore, the p-y curve method proposed by the API overestimated the behavior of a large-diameter monopile installed in silty sand under two-way cyclic loads.