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  • Öge
    Numerical analysis of monopile foundations for offshore wind turbines
    (Graduate School, 2021-10-05) Massah Fard, Maryam ; Erken, Ayfer ; Erkmen, Bülent ; 802142009 ; Earthquake Engineering
    The high worldwide concern for renewable energy resources and the great privilege of seas and oceans for the wind power supply is the point that is no longer hidden among the new generation research. The EU (European Union) capacity is predicted to grow to 150 GW by 2030. Fast growth of offshore wind farms all around the world specifically among EU countries and also newly quake-prone areas of Adriatic, Spain, US (Pacific Coast, Gulf of Mexico), and Japan, makes it significant to learn more about both superstructure and substructure parts of these complex systems. To properly evaluate the effects of environmental loads on the performance of offshore wind turbines (OWTs), soil-pile-structure coupled effects need to be considered. Monopiles are the most common type of foundations for these structures, and as OWT monopile foundations are large diameter piles, it seems that current design methods that are defined for slender and small diameter piles are not appropriate for them. The other important factor is that these methods are mostly applicable for offshore oil and gas industry projects with limited number of load cycles and are not capable of considering liquefaction possibility. While OWTs are the structures that sustain extreme cycles of loading, liquefaction is a phenomenon that should be investigated in detail as it is so critical for the performance of the pile foundations under cyclic sea-wave loads. In the current research, numerical dynamic analysis of an offshore wind turbine based on a monopile foundation is carried out during cyclic sea-wave actions and seismic motion that may lead to liquefaction in the soil domain surrounding the pile foundation due to the cyclic stresses induced by the pile displacements by using the open-source program, OpenSees. The effects of foundation properties such as monopile diameter and its embedment depth and sea-wave characteristics like period, sea-water depth, duration, and level of dynamic loads on the performance of the system are investigated. The outcomes in terms of deformations, excess pore water pressure, and internal forces are presented and discussed. Findings give a more reliable evaluation of the dynamic performance of the offshore wind turbines monopile foundations and liquefaction susceptibility in the surrounding soil medium under cyclic sea-wave loads. In this regard, for the investigation of the problem in more detail, six chapters are considered for the explanation of the system specifications, the OWT system, its foundation, loading procedure, and equations for the environmental loads. Chapter 1 contains the introduction, purpose of the study, and explanation about the categorization of the problem within the chapters. General information about the OWT system, OWT foundation, loading procedure, environmental loads, and methodology followed by specified equations, design guidelines and standards, liquefaction phenomenon, and literature review studies of the previous researchers close to the subject of the thesis is explained in Chapter 2. In the present study, the effects of different parameters on the performance of the OWT system by considering the soil-monopile-OWT structure interaction by using the Finite Element (FE) open-source program, OpenSees, are investigated. In this regard, modeling of the surrounding soil and the obtained soil constitutive model in the analysis is one of the main steps of FE modeling. For this purpose, Chapter 3 is mainly about the soil modeling definition. Elastic-plastic modeling of the soil is explained in more detail in this chapter. First, general information about different soil constitutive models and the equations for the dynamic motion, are defined after the general specification explanation. UCSD soil constitutive model (developed by University of California, San Diego) considered in the present study is defined in more detail by the presentation of the equations, and calibration of the soil constitutive model through laboratory tests. At the end of this chapter, verification of this soil model is done by using cyclic simple shear test data of different researchers and simulation of the cyclic simple shear test through the FE program. The results of the numerical modeling of a problem are more reliable while verification studies of the numerical modeling are done. In this regard, Chapter 4 presents verification studies of the OWT system by considering both superstructure and substructure sections, and the results are compared with the available ones from the literature review studies in specified graphs. Chapter 5 demonstrates the main OWT system selected for the present research after doing all the preliminary analyses and getting certain and reliable results from the verification studies. Effects of different properties for the monopile foundation, seismic motion, and sea-wave cyclic loads are investigated in this chapter and the results are categorized in tables and graphs. As the duration of the cyclic sea-wave loads (number of cycles) is observed as one of the main factors in liquefaction possibility, it is investigated in more detail by performing some detailed simulations of the problem. In this regard, back analysis of the OWT problem is performed by using the results from the FE modeling of the OWT problem and cyclic triaxial test simulation for the soil element in different depth values from the seabed surface. The summary of the conclusions for all the analyses and the future studies related to this subject are explained in Chapter 6. According to the presented results, a large diameter monopile foundation rotates about the tip more like a rigid body with limited values for the pile lateral deformation. Increasing the diameter of the foundation under the same sea-wave loading duration makes monopile stiffer, leading to less lateral displacement for the pile, and lower excess PWP ratio and shear strain in the surrounding soil. The soil response as the excess pore water pressure ratio and shear strain values decrease for larger diameter foundations while they behave in an increasing trend over time. This performance makes it essential to study the effect of the duration of the cyclic load for investigation of the OWT system. This factor is evaluated and presented in the current study in more detail. The duration of the cyclic load is concluded as an important factor for the evaluation of the performance of the OWT system during its life span mainly under large numbers of cyclic sea-wave loads. After the duration, the sea-water depth is a significant factor especially more effective than the duration for the monopile foundation behavior. The maximum values for the deformation of the monopile foundation are achieved during the case with higher sea-water depth. By comparing the results through the sole application of the sea-wave load and the coupled application of the sea-wave and earthquake loads, the response of the surrounding soil is more affected by the seismic motion. The shear strain approaches high rates during the coupled application of loads, and the pore water pressure approaches high amounts by applying the earthquake loads. Based on the numerical analyses, liquefaction is possible in the soil surrounding the offshore wind turbine by applying the prescribed earthquake and the sea-wave load simultaneously. This performance confirms the significant role of the earthquake motion for the performance of the soil surrounding the offshore wind turbine in seismic-prone areas.
  • Öge
    Dynamic response of partially saturated sands under foundations
    (Graduate School, 2021-09-09) Viand Seyedi, Seyed Mohsen ; Bayat Eseller, Esra ; 802142003 ; Earthquake Engineering
    The liquefaction or softening of the foundation soils and the potential vulnerability of existing buildings on liquefiable soils continues to be of major concern to the public because it has repeatedly caused severe damages to buildings with shallow or heavy foundations during earthquakes, such as 1967 Niigata (Japan), 1999 Adapazari (Turkey), 2010 Maule (Chile) and the 2011 Christchurch (New Zealand) earthquakes. The liquefaction of soils leads to a decrease in the effective stress and a consequent degradation in the associated shear strength and stiffness. When the excess pore pressures are built up to initiate full liquefaction (near-zero effective stress state) beneath the foundations, significant deformations such as excessive settlements, tilting and shifting of overlying buildings can occur. An urgent need exists to develop costeffective liquefaction mitigation measures that can be easily and widely used for buildings founded on a liquefiable soil. In recent years, some researchers have been investigating liquefaction mitigation techniques, through series of laboratory tests that involve the artificial introduction of air/gas bubbles and creating partially saturated zones in the liquefiable soil deposits. These techniques intend to prevent the occurrence of liquefaction by increasing the compressibility of the pore fluid with the generation of some amount of air/gas in the fully saturated sand, thus basically, by transforming the sand into partially saturated state. Also, some physical model tests have been conducted to study the liquefaction behavior of partially saturated soils in free field and under foundations. The research, particularly investigating the response of shallow foundations rested on these soils, has suggested that the liquefaction potential of liquefiable soils and relevant building settlements significantly decrease by the reduction of degree of saturation. Although there are many studies in the literature for liquefaction response analysis of fully or partially saturated sands and building movements resting on these sands, performance-based earthquake engineering requires improved procedures to give a deep and more realistic insight into the field. This thesis includes the initial research tasks of a TÜBİTAK funded project (No: 213M367) titled ''Dynamic Response of Sands Mitigated by IPS (Induced Partial Saturation) under New and Existing Structures''. The primary goal of this thesis is numerically modelling of partially saturated sands in free field and under foundations and to determine how much partial saturation should be induced in liquefiable areas. It uses a novel technique to reduce degree of saturation of fully saturated sands that is called Induced Partial Saturation, IPS. This thesis also will indicate how accurate the RuPSS (a novel model to predict excess pore water pressure ratio in partially saturated sands) empirical model is in predicting the liquefaction response of remediated sites by IPS. In this due, the application of IPS technique in tackling the liquefaction damages in free field and under foundations will be examined and modelled in this thesis. The main goals of this thesis can be listed as below: 1) An exhaustive literature review was done to collect almost all of the studies that involve reduction in the degree of saturation and creation of partially saturated zones in the liquefiable soil deposits. 2) A hypothesis has been made in the proposal of the thesis regarding the excess pore pressure generation (during the shaking phase) and its dissipation (during the post shaking phase) along with the settlement of free field and foundations. Its accuracy will be evaluated in the thesis through the numerical models and validations with studies available in the literature. 3) Analysis of the behaviour of fully and partially saturated sands under cyclic loadings considering different soil parameters through numerical modelling and comparison of the numerical analysis results in FLAC3D software with the experimental data available in the literature. 4) Analysis of the behaviour of fully and partially saturated sands under earthquake records in free fields and under foundations considering different soil parameters through numerical modellings in FLAC3D software. 5) There is a key parameter required in numerically modelling the partially saturated sands that is called air-water mixture bulk modulus, Kaw. According to the literature review, there is a well known equation for predicting Kaw. It will be shown that this equation equation is not very accurate in predicting a proper value for Kaw for soils with degrees of saturation below 80% or for partially saturated sands mitigated by IPS against liquefaction. Proposing an alternative empirical function to predict Kaw of partially saturated sands for using in numerical modellings will be the fourth task. The proposed function will be examined through comparison of numerical analysis results with those obtained from experimental test results in the literature. 6) A parametric study on the liquefaction analysis of fully and partially saturated sands in free field and under foundations in PLAXIS2D software. Results were presented in terms of excess pore pressure generations and dissipations and also the settlement responses both in free field and under foundations. 7) Finally, according to the obtained results and comprehensive literature review performed in this thesis, it is was concluded that there are some gaps in the field of partial saturation application in both experimental and numerical studies. Those gaps were listed and discussed in detail to pave the way of finding topics of research for the future studies. The researchers and engineers will benefit by using the outcomes and proposed models of this thesis in their research or in real case projects to analyze the dynamic response of partially saturated sands naturally existing in nature. Also, the mathematical predictive models that will be developed in this thesis will not only be used in the implementation of IPS, but also in the estimation of liquefaction responses in partially saturated sands existing under buildings. Besides contributions in science and engineering, the ultimate goal of this thesis is to help the safety of our society. The best outcome of this thesis will be to provide opportunities for the IPS technique to be implemented in liquefiable areas selected in Turkey and determination of partial saturation needed to improve the liquefiable zones.