• Wind and Structures
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• v.21 no.6
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• pp.597-607
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• 2015

With an increasing demand of a renewable energy, new offshore wind turbine farms are being planned in some parts of the world. Foundation installation asks a significant cost of the total budget of offshore wind turbine (OWT) projects. Hence, a cost reduction from foundation parts is a key element when a cost-efficient designing of OWT budget. Mono-piles have been largely used, accounting about 78% of existing OWT foundations, because they are considered as a most economical alternative with a relatively shallow-water, less than 30 m of seawater depth. OWT design standards such as IEC, GL, DNV, API, and Eurocode are being developed in a form of reliability based limit state design method. In this paper, reliability analysis using the response surface method (RSM) and numerical simulation technique for an OWT mono-pile foundation were performed to investigate the sensitivities of mono-pile design parameters, and to find practical implications of RSM reliability analysis.

• Land and Housing Review
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• v.11 no.2
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• pp.87-94
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• 2020

Offshore renewable energy resources are attractive alternatives in addressing the nation's clean energy policies because of the high demand for electricity in the coastal region. As a large portion of potential resources is in deep and farther water, economically competitive floating systems have been developed. Despite the advancement of floating technologies, the high capital cost remains a primary barrier to go ahead offshore renewable energy projects. The dynamically installed piles (DIPs) have been considered one of the most economical pile concepts due to their simple installation method, resulting in cost and time-saving. Nevertheless, applications to real fields are limited because of uncertainties and underestimated load capacity. Thus, this study suggests the appropriate analytical approach to estimate the inclined load capacity of the DIPs by using the upper bound plastic limit analysis (PLA) method. The validity of the PLA under several conditions is demonstrated through comparison to the finite element (FE) method. The PLA was performed to understand how flukes, soil profiles, and load inclinations can affect the inclined load capacity and to provide reliable evaluations of the total resistance of the DIPs. The studies show that PLA can be a useful framework for evaluating the inclined load capacity of the DIPs under undrained conditions.

• 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.

• Journal of the Korean GEO-environmental Society
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• v.20 no.12
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• pp.41-47
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• 2019

This paper deals with the reliability analysis of foundation for an offshore wind turbine system. Reliability analyses were carried out for suction bucket foundation considering the uncertainties in soil and structural parameters. In reliability analysis, the vertical and lateral resistances are defined as base limit states. The case studies were carried out using the preliminarily designed foundations at western-south mainland sea of Korea. From reliability analyses, vertical resistance for free-slip condition has overall lower reliability index, and submerged unit weight and internal friction angle of seabed soil are governing factors in vertical and lateral resistance in this case.

• 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 Geotechnical Society
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• v.39 no.12
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• pp.61-73
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• 2023

Suction buckets are feasible options for offshore foundations to support subsea structures in deep water, enabling suction-induced installation by pumps. Recently, hybrid suction bucket foundations that combine single or multiple suction buckets with a mat foundation have been considered. The foundations effectively increase the load capacity while reducing construction costs. However, there is still insufficient experimental validation of hybrid suction bucket foundations regarding their bearing capacity. Furthermore, research on the horizontal load capacity under low vertical and moment loads is inadequate. In this study, we investigate the feasibility of using a hybrid suction bucket foundation for subsea installations in clay. We considered two types of hybrid suction bucket foundations: a circular mat with a single suction bucket and a square mat with multiple buckets. Centrifuge tests were performed to understand the hybrid suction bucket foundation characteristics under horizontal loads and their corresponding bearing capacity. Particularly, we verified the effect of the mat foundation and bucket embedment depth on the horizontal bearing mechanism and capacities. Results confirmed that the hybrid suction bucket foundation outperforms the single suction bucket.

• Geomechanics and Engineering
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• v.16 no.5
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• pp.463-469
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• 2018

In offshore engineering, lateral cyclic loading may induce excessive lateral movement and bending strain in pile foundations. Previous studies mainly focused on deformation mechanisms of single piles due to lateral cyclic loading. In this paper, centrifuge model tests were conducted to investigate the response of a $2{\times}2$ pile group due to lateral cyclic loading in clay. After applying each loading-unloading cycle, the pile group cannot move back to its original location. It implies that residual movement and bending strain are induced in the pile group. This is because cyclic loading induces plastic deformation in the soil surrounding the piles. As the cyclic load increases from 62.5 to 375 kN, the ratio of the residual to the maximum pile head movements varies from 0.30 to 0.84. Moreover, the ratio of the residual to the maximum bending strains induced in the piles is in a range of 0.23 to 0.82. The bending strain induced in the front pile is up to 3.2 times as large as that in the rear pile. Thus, much more protection measures should be applied to the front piles to ensure the serviceability and safety of pile foundations.

• Journal of the Korean Geotechnical Society
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• v.32 no.12
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• pp.69-78
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• 2016

A suction bucket foundation has been considered to be a potential foundation type for offshore wind turbines. A suction bucket foundation is usually installed in soft soil, so the accumulated displacement of the foundation may occur under long-term cyclic loads. In this study, a series of 1-g model tests were performed to analyze the accumulated rotation of suction bucket foundations under long-term cyclic horizontal loads. The dry model ground was prepared to have two different soil densities by air-pluviation method. The model tests were performed varying the embedment depth of the suction bucket, the soil density, and the amplitude of cyclic load. A one-way horizontal cyclic load was applied over $10^4$ cycles. Test results showed that the accumulated rotation of the suction bucket foundation increased with the increase in the number of cycles and load magnitudes. Based on the model test results, a new equation was proposed to evaluate the accumulated rotation of the suction bucket foundations in dry sandy ground under long-term cyclic horizontal loads.

• Geomechanics and Engineering
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• v.38 no.2
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• pp.139-155
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• 2024

The piled raft foundations are subjected to lateral loading under the action of wind and earthquake loads. Their bearing behavior and flexural responses under these loadings are of prime concern for researchers and practitioners. The insufficient experimental studies on piled rafts subjected to lateral loading lead to a limited understanding of this foundation system. Lateral load sharing between pile and raft in a laterally loaded piled raft is scarce in literature. In the present study, lateral load-displacement, load sharing, bending moment distribution, and raft inclinations of the piled raft foundations have been discussed through an instrumented scaled down model test in 1 g condition. The contribution of raft in a laterally loaded piled raft has been evaluated from the responses of pile group and piled raft foundations attributing a variety of influential system parameters such as pile spacing, slenderness ratio, group area ratio, and raft embedment. The study shows that the raft contributes 28-49% to the overall lateral capacity of the piled raft foundation. The results show that the front pile experiences 20-66% higher bending moments in comparison to the back pile under different conditions in the pile group and piled raft. The piles in the piled raft exhibit lower bending moments in the range of 45-50% as compared to piles in the pile group. The raft inclination in the piled raft is 30-70% less as compared to the pile group foundation. The lateral load-displacement and bending moment distribution in piles of the single pile, pile group, and piled raft has been presented to compare their bearing behavior and flexural responses subjected to lateral loading conditions. This study provides substantial technical aid for the understanding of piled rafts in onshore and offshore structures to withstand lateral loadings, such as those induced by wind and earthquake loads.

• Journal of the Korean Solar Energy Society
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• v.35 no.3
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• pp.9-16
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• 2015

In order to support various studies for assessment of onshore and offshore wind turbine system including foundations, the land-based version of 2MW PMSG direct drive wind turbine has been analyzed using HAWC2 that account for the coupled dynamics of the wind inflow, elasticity, and controls of the turbine. this work presents the steady-state response of the system and natural frequency of the first thirteen structure turbine modes as a function of wind speed. Rotor, generator speeds, pitch angle, power production, thrust force, deflections of tower and blade are compared for one case below and one case above the rated wind speed.