Browsing by Author "Haldar S."

Now showing 1 - 20 of 27

3D Modeling of Long-Term Dynamic Behavior of Monopile-Supported Offshore Wind Turbine in Clay
(2019) Bisoi S.; Haldar S.
This article outlines the long-term dynamic behavior of the monopile-supported offshore wind turbine (OWT) in clay. A three-dimensional (3D) finite-element (FE) model was developed that uses viscoelastic material constitutive models for soils in conjunction with stiffness-degradation functions to examine the long-term behavior of monopile-supported OWTs subjected to transient loading. The proposed numerical model was validated by experimental results and recorded data from real offshore wind turbines. Effects of amplitudes of wind and wave loads and their frequency, monopile length and diameter, and rotor-nacelle assemble (RNA) mass on the long-term dynamic behavior of a 5-MW OWT due to different load cycles were studied. The results show that the effect of wind load governs the design and that monopile diameter has a significant role in the long-term behavior of the structure. � 2019 American Society of Civil Engineers.
Analysis of beams on heterogeneous and nonlinear soil
(2016) Haldar S.; Basu D.
Anewmethod for nonlinear analysis of Euler-Bernoulli beamsresting on heterogeneous, multilayered soil is presented. The governing differential equations for beamand soil displacements are obtained using the principle of virtual work, and these equations are solved using one-dimensional finite-element andfinite-differencemethods. Using the analysis, beamdisplacements can be obtained as a nonlinear function of applied loads if the beam and soil geometry and properties are known. The distinct feature of the analysis is that beam responses with accuracy comparable with those obtained from equivalent two-dimensional finite-element analysis are obtained within seconds. Examples illustrate how the proposed method can be applied in practice to obtain beam displacements in soil deposits that are characterized with nonlinear stress-strain relationships, havemultiple layers, and have amodulus varying spatially within each soil layer. Parametric studies are performed that highlight the effects of soil layering and nonlinearity on the beamresponse. � 2016 American Society of CivilEngineers.
Appraisal of the in Situ Variability and Modeling Uncertainty of Dynamic Soil-Piled Raft-Structure Interaction on Seismic Response: A Probabilistic Approach
(2016) Saha R.; Pal A.; Haldar S.
The effect of in-situ variability of undrained shear strength of clay and different soil structure interaction (SSI) modeling approaches on the seismic response of piled-raft supported structure is studied. SSI leads to increased or decreased responses of structure as compared to fixed base design philosophy. The distribution of forces in column and pile head may also depend on inherent variability of soil. This results in an unsafe or conservative design of column and pile due to seismic loading. The soil-piled raft-structure interaction is modeled as beam on non-linear Winkler foundation. Response statistics of the system are estimated using Monte Carlo simulation. Finally, design implications are suggested for seismic design of piled raft supported structure with an emphasis on probabilistic SSI based sustainable design approach.
Beam on spatially random elastic foundation
(2011) Haldar S.; Basu D.
Beams on elastic foundations have been mostly analyzed using the Winkler model, in which a series of disjointed elastic springs are used to represent the foundation soil. However the two-parameter Vlasov's foundation model gives better representation of beam-soil interaction since it incorporates normal and shear resistance of soil. Previous studies examined response of beams on elastic foundation model considering model parameters as deterministic quantity. In contrast to previous studies, this paper examines the response of beams resting on elastic soil media with spatial random variations of soil Young's modulus. The randomness in the soil properties is incorporated in the two-parameter Vlasov's foundation model and, subsequently, the beam responses are obtained. Statistics of beam response are obtained by Monte Carlo simulation approach. It is observed that the soil inherent variability alters the beam response significantly. � 2011 ASCE.
Design of monopile supported offshore wind turbine in clay considering dynamic soil-structure-interaction
(2015) Bisoi S.; Haldar S.
This paper addresses the feasibility of soft-soft and soft-stiff design approaches considering a 2. MW and 5. MW monopile supported three bladed offshore wind turbine (OWT) founded in clay. The serviceability limit state and fatigue life of the structure are checked in order to assess the safety. Resonance condition is also avoided keeping the fundamental frequency of the system away from the rotor frequencies. The OWT system is modeled using linear beam and soil-structure interaction is accounted for incorporating American Petroleum Institute based cyclic p-y springs attached to the monopile. Aerodynamic and hydrodynamic loads are applied on the structure and dynamic analysis is carried out using a finite element method in time domain. Overall mass of the structure is examined considering two design approaches in order to obtain an economical design solution. The study shows that soft-soft design is possible for 2. MW OWT subjected to rated wind speed for long tower. Rotor nacelle assembly mass and tower height is found to have governing role in soft-stiff design and on the material consumption. Embedded depth of monopile beyond critical depth has marginal impact on design. Fatigue life is observed to be governing design criteria for OWT at stiff clay. � 2015 Elsevier Ltd.
Dynamic analysis of offshore wind turbine in clay considering soil-monopile-tower interaction
(2014) Bisoi S.; Haldar S.
A comprehensive study is performed on the dynamic behavior of offshore wind turbine (OWT) structure supported on monopile foundation in clay. The system is modeled using a beam on nonlinear Winkler foundation model. Soil resistance is modeled using American Petroleum Institute based cyclic p-y and t-z curves. Dynamic analysis is carried out in time domain using finite element method considering wind and wave loads. Several parameters, such as soil-monopile-tower interaction, rotor and wave frequencies, wind and wave loading parameters, and length, diameter and thickness of monopile affecting the dynamic characteristics of OWT system and the responses are investigated. The study shows soil-monopile-tower interaction increases response of tower and monopile. Soil nonlinearity increases the system response at higher wind speed. Rotor frequency is found to have dominant role than blade passing frequency and wave frequency. Magnitude of wave load is important for design rather than resonance from wave frequency. � 2014 Elsevier Ltd.
Dynamic behavior of monopile supported offshore wind turbine system
(2015) Bisoi S.; Haldar S.
Monopiles are designed such that the natural frequency of offshore wind turbine structure shall be kept away from rotor excitation frequency to avoid resonance. Tower-monopile system responses, such as rotation is essential design parameter to ensure safety. The output of the turbine can be enhanced with larger rotors and installed at higher water depth. However, dynamic characteristics of tower-foundation system alters significantly due to change in water depth, hub height, overall mass, wind and wave load characteristics. This paper critically assesses the dynamic characteristics of tower-monopile system considering feasible variation of rotor speed, tower height and wind speed. It is observed that soft-stiff is a suitable design approach. Decrease in Rotor Nacelle Assembly (RNA) mass is found to be most effective in design than that of the reduction in tower height. The effect of thickness of tower and monopile is insignificant in design of OWT structure. � 2015 Taylor & Francis Group, London.
Effect of climate change on dynamic behavior of monopile supported offshore wind turbine structure
(2015) Bisoi S.; Haldar S.
Dynamic behavior of monopile supported offshore wind turbine is challenging due to complex long term wind and wave loading. Design of offshore wind turbine (OWT) structure primarily requires estimation of the fundamental frequency which needs to be kept away from excitation frequencies of wind and wave loading to avoid dynamic amplification of response and early fatigue damage. Global warming changes the wind and wave pattern due to pressure changes over the earth. The effect of climate change is having significant effect on wind speed, significant wave height and wave period, which in turn changes the dynamic behavior of OWT system. Fluctuating wind speed and wave height due to the effect of climate change may result in increased response and early fatigue damage. This study focuses on dynamic behavior of the monopile supported offshore wind turbine structure due to the climate change variability corresponding to 50 years future wind speed values. Historic wind data is utilized to project the future wind speed. The system is modeled using a beam on nonlinear Winkler foundation model. Soil resistance is modeled using American Petroleum Institute based cyclic p-y and t-z curves. The dynamic response and change in fatigue life of OWT structure is examined due to the effect of climate change and design implications are also suggested.
Effect of Climate Change on the Reliability of Offshore Wind Turbine Foundations
(2016) Haldar S.; Basu D.
The reliability of monopile supported offshore wind turbine (OWT) is assessed considering the uncertainties in aerodynamic and hydrodynamic loads and spatial variability of soil. Traditional designs assume that applied loads are constant during the design period of OWTs. However, fluctuations in climatic conditions alter the future wind and wave loads because of which time-dependent reliability assessment of OWTs is performed in this study. Statistical downscaling method using the general circulation model corresponding to the A2 emission scenario is used to predict the future wind and wave responses. The impact of climate change on the probability of failure for the serviceability limit state of maximum allowable mudline rotation is studied. The uncertainties in soil properties, and wind and wave loads are incorporated in the analysis. The reliability of an OWT structure founded in sand is investigated for the period 2015-2050. The OWT system is modeled using a beam supported laterally by a nonlinear Winkler foundation characterized by the American Petroleum Institute recommended cyclic p-y curve for sand. This study shows that the reliability of OWT is changed significantly because of the effects of climate change. � ASCE.
Effect of in-situ variability of soil on seismic design of piled raft supported structure incorporating dynamic soil-structure-interaction
(2016) Das B.; Saha R.; Haldar S.
Inherent variability of soil considerably affects the seismic design of piled raft supported structures. Conventional design of such structure adopts fixity at base level of superstructure and pile head. However, soil-pile-superstructure interaction largely affects the fundamental frequency and design forces in columns and piles. In contrast, fixed base assumption cannot capture soil structure interaction (SSI) effect. In addition, uncertainty in soil may further leads to a change in the dynamic behavior of the system. This study examines the effect of inherent variability of undrained shear strength of soil in seismic design of structures supported by piled raft foundation embedded in soft clay. Superstructure is modeled as lumped mass stick model and piled raft slab is modeled as rigid plate. Pile is modeled as Euler-Bernoulli beam element and soil resistance is modeled using linear Winkler springs attached to the pile. Dynamic analysis is carried out in time domain to estimate the responses. Monte Carlo simulation technique is used for probabilistic assessment of the fundamental frequency and forces at column and pile attributing a wide range of parametric variation of a representative soil-piled raft-superstructure system. The study shows that the fundamental frequency and forces in column and pile changes significantly due to soil variability. � 2016 Elsevier Ltd.
Effect of raft and pile stiffness on seismic response of soil-piled raft-structure system
(2015) Saha R.; Dutta S.C.; Haldar S.
Soil-pile raft-structure interaction is recognized as a significant phenomenon which influences the seismic behaviour of structures. Soil structure interaction (SSI) has been extensively used to analyze the response of superstructure and piled raft through various modelling and analysis techniques. Major drawback of previous study is that overall interaction among entire soil-pile raft-superstructure system considering highlighting the change in design forces of various components in structure has not been explicitly addressed. A recent study addressed this issue in a broad sense, exhibiting the possibility of increase in pile shear due to SSI. However, in this context, relative stiffness of raft and that of pile with respect to soil and length of pile plays an important role in regulating this effect. In this paper, effect of relative stiffness of piled raft and soil along with other parameters is studied using a simplified model incorporating pile-soil raft and superstructure interaction in very soft, soft and moderately stiff soil. It is observed that pile head shear may significantly increase if the relative stiffness of raft and pile increases and furthermore stiffer pile group has a stronger effect. Outcome of this study may provide insight towards the rational seismic design of piles. � 2015 Techno-Press, Ltd.
Experimental and numerical studies on the dynamic and long-term behavior of offshore wind turbines in clay
(2018) Bisoi S.; Haldar S.
Monopile foundations for offshore wind turbines (OWTs) are exposed to long-term dynamic loads from wind and waves. Hence, the long-term dynamic behavior of a monopile supported OWT is necessary for the sake of stability during the serviceable period. To assess the safety of the system, the serviceability limit state regarding the allowable tilt of the monopile at mudline is satisfied, and the fundamental frequency of the system is kept away from the rotor and blade passing frequencies. The present study investigates the long-term performance of monopile supported OWT in clay using a series of scaled model tests. The fundamental frequency and damping of an OWT system and the rotation and lateral deflection of a monopile head are examined for various load cycles with different amplitudes. The effect of long-term loading cycles and amplitude on the soil-pile stiffness is evaluated. It is observed that the fundamental frequency of the system decreases with the number of load cycles, whereas damping of the whole system is increased. The responses regarding accumulated rotation and deflection at monopile head are found to be increasing with the number of load cycles. For the numerical simulation, soil is modeled as viscoelastic material with a stiffness degradation function in three-dimensional finite element analysis. Finally, the fundamental frequency and response of the OWT obtained from the model test are validated with numerical results. A good agreement is observed between experimental and numerical results. Copyright � 2018 by ASTM International.
Impact of climate change on design of offshore wind turbine considering dynamic soil-structure interaction
(2017) Bisoi S.; Haldar S.
This study assesses the serviceability and fatigue limit states of the offshore wind turbine (OWT) founded in clay incorporating the impact of climate change. Two different offshore locations at east and west coasts in India are chosen. The ensemble of future time series of wind speed, wave height, and period is forecasted using statistical downscaling model (SDSM) at the regional level using the general circulation model (GCM) corresponding to the A1B, A2, and B1 emission scenarios. The downscaling model is calibrated by comparing simulations driven by the National Centers for Environmental Prediction (NCEP) high-resolution data and station data. Responses of OWT are obtained from dynamic analysis in a time domain using finite element (FE). The tower and monopile are modeled as Euler-Bernoulli beam, and soil resistance is modeled as American Petroleum Institute (API)-based p-y springs. The study shows future wind and wave loads are site specific, and it increases in the west coast and decreases in the east coast of India due to climate change. The simulation shows a substantial increase in future wind energy production at west coast compared to that of the east coast; however, safety margin considering serviceability and fatigue life decreases which requires modification in the design. � 2017 by ASME.
Impact of climate change on dynamic behavior of offshore wind turbine
(2017) Bisoi S.; Haldar S.
Global warming is expected to change the wind and wave patterns at a significant level, which may lead to conditions outside current design criteria of monopile supported offshore wind turbine (OWT). This study examines the impact of climate change on the dynamic behavior and future safety of an OWT founded in clay incorporating dynamic soil�structure interaction. A statistical downscaling model is used to generate the time series of future wind speed and wave height at local level. The responses and fatigue life of OWT are estimated for present and future periods and extent of change in design is investigated at offshore location along the west coast of India. Wind speed, wave height, and wave period data are collected from the buoy deployed by Indian National Centre for Ocean Information Services and the future climate for the next 30 years is simulated using the general circulation model corresponding to Special Report on Emission Scenarios A1B scenario. The OWT is modeled as Euler�Bernoulli beam and soil�structure interaction is incorporated using nonlinear p-y springs. This study shows that changes in design of OWT are needed due to increased responses owing to the effect of climate change. Fatigue life is found to be decreased because of climate change. � 2017 Taylor & Francis.
Improvement of machine foundations using reinforcement
(2009) Haldar S.; Sivakumar Babu G.L.
The main objective in the design of a machine foundation is to restrict the maximum amplitude of the foundation motion to within the safety limits. When the underlying soil is poor
Inelastic seismic behavior of soil-pile raft-structure system under bi-directional ground motion
(2014) Dutta S.C.; Saha R.; Haldar S.
Performance based design of structure requires a reasonably accurate prediction of displacement or ductility demand. Generally, displacement demand of structure is estimated assuming fixity at base and considering base motion in one direction. In reality, ground motions occur in two orthogonal directions simultaneously resulting in bidirectional interaction in inelastic range, and soil-structure interaction (SSI) may change structural response too. Present study is an attempt to develop insight on the influence of bi-directional interaction and soil-pile raft-structure interaction for predicting the inelastic response of soil-pile raft-structure system in a more reasonably accurate manner. A recently developed hysteresis model capable to simulate biaxial interaction between deformations in two principal directions of any structural member under two orthogonal components of ground motion has been used. This study primarily shows that a considerable change may occur in inelastic demand of structures due to the combined effect of such phenomena. � 2014 Elsevier Ltd.
Influence of dynamic soil-pile raft-structure interaction: an experimental approach
(2015) Saha R.; Haldar S.; Dutta S.C.
Traditionally seismic design of structures supported on piled raft foundation is performed by considering fixed base conditions, while the pile head is also considered to be fixed for the design of the pile foundation. Major drawback of this assumption is that it cannot capture soil-foundation-structure interaction due to flexibility of soil or the inertial interaction involving heavy foundation masses. Previous studies on this subject addressed mainly the intricacy in modelling of dynamic soil structure interaction (DSSI) but not the implication of such interaction on the distribution of forces at various elements of the pile foundation and supported structure. A recent numerical study by the authors showed significant change in response at different elements of the piled raft supported structure when DSSI effects are considered. The present study is a limited attempt in this direction, and it examines such observations through shake table tests. The effect of DSSI is examined by comparing dynamic responses from fixed base scaled down model structures and the overall systems. This study indicates the possibility of significant underestimation in design forces for both the column and pile if designed under fixed base assumption. Such underestimation in the design forces may have serious implication in the design of a foundation or structural element. � 2015, Institute of Engineering Mechanics, China Earthquake Administration and Springer-Verlag Berlin Heidelberg.
Influence of Inherent Soil Variability on Seismic Response of Structure Supported on Pile Foundation
(2019) Chanda D.; Saha R.; Haldar S.
Traditional seismic design is limited to fixed base assumption of superstructure. Such perception proved to be misleading from the post facto analysis of different failure case studies. In addition, inherent variability of soil parameters may lead to a considerable effect on seismic response of foundation and superstructure elements. Incorporation of shear strength variability of soil along with dynamic soil structure interaction phenomenon may result in increased or decreased transmitted shear to the pile as compared to fixed base shear which may lead to unsafe or over-safe pile design. The present study assesses the influence of shear strength variability of soil on seismic response of pile foundation considering soil nonlinearity. Probabilistic analysis is performed by Monte Carlo simulation technique. The study infers that variability of shear strength parameters of soil has significant effect on response of pile embedded in clay, while such effect is marginal in sandy soil. However, it is also observed that adoption of different pile�soil interaction modeling contributes variability in probabilistic response of pile. Finally, a case study shows the importance of shear strength variability of soil on increase in percentage of steel requirement in pile foundation. � 2019, King Fahd University of Petroleum & Minerals.
Probabilistic analysis of monopile-supported offshore wind turbine in clay
(2018) Haldar S.; Sharma J.; Basu D.
The dynamic behaviour of monopile supported offshore wind turbines (OWTs) is considerably influenced by inherent variabilities of soil properties and loading. This study investigates the effect of uncertainties in soil shear strength properties, and wind and wave loads on the reliability of OWT structures founded in clay. The OWT system is modeled as an Euler-Bernoulli beam and soil-structure interaction is incorporated using the American Petroleum Institute based cyclic p-y relationships. The uncertainties in soil undrained shear strength, in the parameters and equations of the p-y method, and in wind velocity are considered. Random wave loads are estimated from the code specified power spectral density function of the vertical sea surface elevation. Uncertainties in OWT responses are quantified using Monte Carlo simulations. The effects of length and diameter of the monopile, vertical sea surface displacement spectrum, and the probability distribution of wind speed on the dynamic responses of OWT are investigated. The study shows that the various power spectral density functions of wave surface displacement and various probability density functions of wind speed have a marginal effect on the response and fatigue life of OWTs. The mean fundamental frequency of the OWT system is significantly affected by the variability of the undrained shear strength of clayey soil. � 2017 Elsevier Ltd
Probabilistic seismic design of soil-pile raft-superstructure system
(2015) Das B.; Saha R.; Haldar S.
Sustainable design of foundations and structural members requires safe and environmental friendly structure with minimum material use. Piled-raft foundations are commonly used to support heavy structures in soft soil. Traditionally, seismic design of such structure is carried out considering fixity at base level in a soil-pile raft-structure system while pile head is considered to be fixed for design of pile. Major drawback of this fixed base assumption is that it cannot capture soil structure interaction (SSI) effect. Incorporation of SSI may result in increased or decreased transmitted shear to the pile and column as compared to fixed base shear which may lead to unsafe or over-safe pile design. Since natural soil properties vary at different points within geologically distinct layers. As a result, transmitted shear to the pile and column depends on in situ variability of design soil parameters, thus leads to transformation of the whole problem to reliability based SSI design problem. Thus, precise estimation of safety margin is needed for rational and sustainable design of pile. Hence, a probabilistic analysis incorporating SSI is performed in present study to develop insight into the problem for refining the existing design guidelines. � ASCE 2015.