Subhamoy Bhattacharya

Dynamic analysis of wind turbine towers on flexible foundations

Offshore wind turbines are considered as an essential part to develop sustainable, alternative energy sources. The structures themselves are both slender and highly flexible, with a subsea foundation typically consisting of a single large diameter monopile. They are subject to intense wind and wave loadings, with the result that significant movement of both the exposed structure and the upper part of the monopile can occur. Although the structures are intended for design life of 25 to 30 years, very little is known about the long term behaviour of these structures. This paper characterizes the dynamic behaviour of these structures. A simplified approach has been proposed for the free vibration analysis of wind turbines taking the effect of foundation into account. The method is based on an Euler-Bernoulli beam-column with elastic end supports. The elastic end-supports are considered to model the flexible nature of the interaction of these systems with the foundation. A closed-form expression of the characteristic equation governing all the natural frequencies of the system has been derived. Theoretical developments are explained by practical numerical examples. Analytical as well as a new experimental approach has been proposed to determine the parameters for the foundation. Some design issues of wind turbine towers are discussed from the point of view of the foundation parameters.

Model Container Design for Soil-Structure Interaction Studies

Physical modelling of scaled models is an established method for understanding failure mechanisms and verifying design hypothesis in earthquake geotechnical engineering practice. One of the requirements of physical modelling for these classes of problems is the replication of semi-infinite extent of the ground in a finite dimension model soil container. This chapter is aimed at summarizing the requirements for a model container for carrying out seismic soil-structure interactions (SSI) at 1-g (shaking table) and N-g (geotechnical centrifuge at N times earth’s gravity). A literature review has identified six types of soil container which are summarised and critically reviewed herein. The specialised modelling techniques entailed by the application of these containers are also discussed.

Liquefaction of soil in the Emilia-Romagna region after the 2012 Northern Italy earthquake sequence

At the end of May 2012, the Po plain region in northern Italy was shaken by a long sequence of seismic events. The 2012 Northern Italy earthquake sequence counted two mainshocks, about 1,600 aftershocks and lasted for several weeks. Although the mainshocks, which occurred on May 20 and May 29, 2012, registered a moment magnitude of 5.9 and 5.8, respectively, these two events caused widespread soil liquefaction and substantial damages to the built environment. This paper reports lessons learnt from a field investigation conducted in the areas affected by the earthquake sequence. Based on the field observations, it was concluded that despite the relatively low magnitudes of the shocks, most of the damages occurred as a consequence of liquefaction phenomena and/or absence of retrofitting of historical structures. The latter comprise churches, tower bells, towers, castles and fortresses. It was found that the occurrence of liquefaction was mainly associated with the presence of saturated alluvial soil deposits which were characterised by high liquefaction susceptibility. It was noted that these highly liquefiable soils were mainly located in proximity of ancient river courses that were artificially diverted in the eighteenth century to mitigate flooding and other hydrological risks.

CPT-based probabilistic evaluation of seismic soil liquefaction potential using multi-gene genetic programming

In this paper, liquefaction potential of soil is evaluated within a probabilistic framework based on the post-liquefaction cone penetration test (CPT) data using an evolutionary artificial intelligence technique, multi-gene genetic programming (MGGP). Based on the developed limit state function using MGGP, a relationship is given between probability of liquefaction (PL) and factor of safety against liquefaction using Bayesian theory. This Bayesian mapping function is further used to develop a PL-based design chart for evaluation of liquefaction potential of soil. Using an independent database of 200 cases, the efficacy of the present MGGP-based probabilistic method is compared with that of the available probabilistic methods based on artificial neural network (ANN) and statistical methods. The proposed method is found to be more efficient in terms of rate of successful prediction of liquefaction and non-liquefaction cases, in three different ranges of PL values compared to ANN and statistical methods.

Model Tests on the Long-Term Dynamic Performance of Offshore Wind Turbines Founded on Monopiles in Sand

© 2015 by ASME.The dynamic response of the supporting structure is critical for the in-service stability and safety of offshore wind turbines (OWTs). The aim of this paper is to first illustrate the complexity of environmental loads acting on an OWT and reveal the significance of its structural dynamic response for the OWT safety. Second, it is aimed to investigate the long-term performance of the OWT founded on a monopile in dense sand. Therefore, a series of well-scaled model tests have been carried out, in which an innovative balance gear system was proposed and used to apply different types of dynamic loadings on a model OWT. Test results indicated that the natural frequency of the OWT in sand would increase as the number of applied cyclic loading went up, but the increasing rate of the frequency gradually decreases with the strain accumulation of soil around the monopile. This kind of the frequency change of OWT is thought to be dependent on the way how the OWT is cyclically loaded and the shear strain level of soil in the area adjacent to the pile foundation. In this paper, all test results were plotted in a nondimensional manner in order to be scaled up to predict the consequences for prototype OWT in sandy seabed.

Experimental Investigation of Dynamic Behavior of Cantilever Retaining Walls

The dynamic behaviour of cantilever retaining walls under earthquake action is explored by means of 1-g shaking table testing, carried out on scaled models at the Bristol Laboratory for Advanced Dynamics Engineering (BLADE), University of Bristol, UK. The experimental program encompasses different combinations of retaining wall geometries, soil configurations and input ground motions. The response analysis of the systems at hand aimed at shedding light onto the salient features of the problem, such as: (1) the magnitude of the soil thrust and its point of application; (2) the relative sliding as opposed to rocking of the wall base and the corresponding failure mode; (3) the importance/interplay between soil stiffness, wall dimensions, and excitation characteristics, as affecting the above. The results of the experimental investigations were in good agreement with the theoretical models used for the analysis and are expected to be useful for the better understanding and the optimization of earthquake design of this particular type of retaining structure.

Design of FPSO Piles Against Storm Loading

FPSO [Floating Production Storage and Offloading] structures have been accepted as a sustainable economic solution for deepwater development projects. Short to medium length (typically 15 to 25m) large diameter driven piles are often used to anchor FPSOs. The loading in such piles during a storm can be resolved into two components: (a) Lateral load, which is one-way cyclic; (b) Tensile (upward) load, which is typically only a few percentage of the lateral load. The greatest uncertainty in the analysis is the load carrying capacity of the pile, since the cyclic storm loading results in progressive degradation of the soil (sand or clay) supporting the pile. Thus understanding the degradation of the supporting oil is critical, for a safe, economic design. This paper thus hastwo aims: (a) to propose criteria and considerations for design of such piles; (b) to set out simple modifications in the p-y formulation that will provide a safe working envelope for the full range of ground conditions likely to be encountered at different sites. A parallel is also drawn to the approach routinely used by the geotechnical earthquake engineering profession, and reported centrifuge tests have been used to validate the proposed modification.

Buckling and bending response of slender piles in liquefiable soils during earthquakes

Design of pile foundations in seismically liquefiable soils involves identifying the appropriate failure mechanisms. Piles in liquefiable soils are conventionally designed against bending failure due to lateral loads arising from inertia and/or lateral spreading. This is strong evidence that there is another mechanism, which the code does not consider, that may govern the failure of these foundations. In this paper, the response of a single end bearing pile in liquefied soil with and without the effect of axial load has been presented. The effect of liquefaction is incorporated in the pile–soil interaction through nonlinear analysis using the finite difference program Fast Lagrangian Analysis of Continua (FLAC). The method of analysis is carried out using the well documented failure of Showa Bridge piles which failed during the 1964 Niigata earthquake. The response of the pile is also evaluated using dynamic analysis. The need for proper identification of failure mechanisms as well as design guidelines is highlighted.

Experimental Assessment of Seismic Pile-Soil Interaction

Physical modeling has long been established as a powerful tool for studying seismic pile-soil-superstructure interaction. This chapter presents a series of 1-g shaking table tests aiming at clarifying fundamental aspects of kinematic and inertial interaction effects on pile-supported systems. Pile models in layered sand deposits were built in the laboratory and subjected to a wide set of earthquake motions. The piles were densely instrumented with accelerometers and strain gauges; therefore, earthquake response, including bending strains along their length, could be measured directly. Certain broad conclusions on kinematic and inertial SSI effects on this type of systems are drawn.

Monopile head stiffness for servicibility limit state calculations in assessing the natural frequency of offshore wind turbines

Monopiles are used as foundations for offshore wind energy towers. These large diameter monopiles (4–6 m) which result in extremely stiff short monopiles do not follow the same elastic deformation patterns as those exhibited by small diameter (0.5–1.5 m) monopiles usually used for supporting structures in offshore oil and gas industry. Design recommendations (API and DNV among others) which have been developed on the basis of full-scale load tests on long, slender and flexible piles are not suitable for designing foundations for offshore wind turbine structures. Furthermore, as these facilities are very sensitive to rotations and dynamic changes in the soil-pile system, the precise prediction of monopile head displacement and rotation constitutes a design criterion of great importance. In this article, the semi-analytical finite element analysis which combines the conventional finite element method with Fourier series representation of displacements is used for the study of monopiles for offshore wind turbines under lateral loading in homogeneous soils. In order to develop functional forms of design equations for large diameter monopiles like the ones used for supporting offshore wind turbines, an extensive parametric study has been carried out. The analyzed parameters including pile slenderness ratio and soil stiffness allow to obtain simple and approximate closed-form solutions for static head stiffnesses of both rough and smooth short monopiles embedded in homogeneous soils. It has been found that lateral stiffness KL, rotational stiffness KR and cross coupling stiffness KLR for both rough and smooth interfaces depend only on the value of the monopile slenderness Lp/Dp rather than the relative soil/monopile rigidity Ep/Es usually found in the offshore platforms designing codes (DNV code for example). These analytical expressions have been incorporated into the expressions of the Offshore Wind Turbine (OWT) natural frequency of two wind farm sites. Excellent agreement has been found between the computed and the measured natural frequencies.