Dr Georgios Nikitas


Geotechnics Experimental Officer
+44 (0)1483 689618
09 AB 01

Academic and research departments

Department of Civil and Environmental Engineering.

My qualifications

2020
PhD in Geotechnics (Thesis Title: A Study on Soil-Structure Interaction of Offshore Wind Turbine Foundations)
University of Surrey, UK
2019
Postgraduate Certificate in Higher Education
University of Surrey, UK
2016
MSc by Research in Earthquake Engineering (Thesis Title: Experimental proof of concept of a low-cost liquefaction mitigation technique for residential buildings)
University of Bristol, UK
2011
MEng in Civil Engineering (Dissertation Title: Investigation of the supply and water usage in the prefecture of Drama)
Democritus University of Thrace, GR

My publications

Publications

Dammala PK, Adapa MK, Bhattacharya Subhamoy, Nikitas Georgios, Rouholamin M (2017) Dynamic Soil Properties for Seismic Ground Response Studies in Northeastern India, Soil Dynamics and Earthquake Engineering 100 pp. 357-370 Elsevier
Stiffness and damping properties of soil are essential parameters for any dynamic soil structure interaction analysis. Often the required stiffness and damping properties are obtained from the empirical curves. This paper presents the stiffness and damping properties of two naturally occurring sandy soils collected from a river bed in a highly active seismic zone in the Himalayan belt. A series of resonant column tests are performed on the soil specimens with relative densities representative of the field and with varying confining pressures. The results are compared with the available empirical curves. Furthermore, a ground response analysis study is also carried out for a bridge site in the region using both empirical curves and experimentally obtained curves. It has been observed that the application of empirical modulus and damping curves in ground response prediction often leads to underestimation of the seismic demands on the structures.
Nikitas G, Arany L, Aingaran S, Vimalan J, Bhattacharya S (2016) Predicting long term performance of Offshore Wind Turbines using Cyclic Simple Shear apparatus, Soil Dynamics and Earthquake Engineering 92 pp. 678-683 Elsevier
Offshore wind turbine (OWT) foundations are subjected to a combination of cyclic and dynamic loading arising from wind, wave, 1P (rotor frequency) and 2P/3P (blade passing frequency) loads. Under cyclic/dynamic loading, most soils change their characteristics. Cyclic behaviour (in terms of change of shear modulus change and accumulation of strain) of a typical silica sand (RedHill 110) was investigated by a series of cyclic simple shear tests. The effects of application of 50,000 cycles of shear loading having different shear strain amplitude, cyclic stress ratio (ratio of shear to vertical stress), and vertical stress were investigated. Test results were reported in terms of change in shear modulus against the number of loading cycles. The results correlated quite well with the observations from scaled model tests of different types of offshore wind turbine foundations and limited field observations. Specifically, the test results showed that; (a) Vertical and permanent strain (accumulated strain) is proportional to shear strain amplitude but inversely proportional to the vertical stress and relative density; (b) Shear modulus increases rapidly in the initial cycles of loading and then the rate of increase diminishes and the shear modulus remains below an asymptote. Discussion is carried out on the use of these results for long term performance prediction of OWT foundations.
Cui L, Bhattacharya S, Nikitas G (2017) Micromechanics of soil responses in cyclic simple shear tests, EPJ Web of Conferences 140 02008 EDP Sciences
Offshore wind turbine (OWT) foundations are subjected to a combination of cyclic and dynamic
loading arising from wind, wave, rotor and blade shadowing. Under cyclic loading, most soils change their
characteristics including stiffness, which may cause the system natural frequency to approach the loading
frequency and lead to unplanned resonance and system damage or even collapse. To investigate such
changes and the underlying micromechanics, a series of cyclic simple shear tests were performed on the
RedHill 110 sand with different shear strain amplitudes, vertical stresses and initial relative densities of soil.
The test results showed that: (a) Vertical accumulated strain is proportional to the shear strain amplitude but
inversely proportional to relative density of soil; (b) Shear modulus increases rapidly in the initial loading
cycles and then the rate of increase diminishes and the shear modulus remains below an asymptote; (c)
Shear modulus increases with increasing vertical stress and relative density, but decreasing with increasing
strain amplitude. Coupled DEM simulations were performed using PFC2D to analyse the micromechanics
underlying the cyclic behaviour of soils. Micromechanical parameters (e.g. fabric tensor, coordination
number) were examined to explore the reasons for the various cyclic responses to different shear strain
amplitudes or vertical stresses. Both coordination number and magnitude of fabric anisotropy contribute to
the increasing shear modulus.
Cui Liang, Bhattacharya Subhamoy, Nikitas Georgios, Vimalan JN (2017) Predicting Long Term Performance of OWT Foundation using Cyclic Simple Shear Apparatus and DEM Simulations, Offshore Site Investigation Geotechnics 8th International Conference Proceeding pp. 1132-1139 Society for Underwater Technology
Under cyclic loading, most soils change their characteristics. Cyclic behaviour (change of shear modulus and accumulated strain) of the RedHill 110 sand was investigated by a series of cyclic simple shear tests. The effects of application of 50,000 cycles of shear loading with different shear strain amplitudes and vertical stresses were investigated. The results correlated quite well with the observations from scaled model tests of different types of offshore wind turbine foundations and limited field observations. Specifically, the test results showed that shear modulus increases rapidly in the initial loading cycles and then the rate of increase diminishes; the rate of increase depends on strain amplitude, initial relative density and vertical pressure. Complementary DEM simulations were performed using PFC2D to analyse the micromechanics underlying the cyclic behaviour of soils. It shows that the change of soil behaviour strongly related to the rotation of principle axes of fabric and degree of fabric anisotropy.
Bhattacharya S, Nikitas G, Arany L, Nikitas N (2017) Soil-Structure Interactions (SSI) for Offshore Wind Turbines, IET Engineering and Technology Reference 24 (16) The Institution of Engineering and Technology
Soil-Structure-Interaction (SSI) for offshore wind turbine supporting structures is essentially the interaction of the foundation/foundations with the supporting soil due to the complex set of loading. This paper reviews the different aspects of SSI for different types of foundations used or proposed to support offshore wind turbines. Due to cyclic and dynamic nature of the loading that acts on the wind turbine structure, the dominant SSI will depend to a large extent on the global modes of vibration of the overall structure. This paper summarises the modes of vibration of offshore wind turbines structures supported on different types of foundations based on observations from scaled model tests and numerical analysis. As these are new structures with limited monitoring data, field records are scarce. Field records available in the public domain are also used to compare with the experimental findings.
Bhattacharya Subhamoy, Hyodo M., Nikitas Georgios, Ismail B., Suzuki H., Lombardi D., Egami S., Watanabe G., Goda K. (2017) Geotechnical and infrastructural damage due to the 2016 Kumamoto earthquake sequence, Soil Dynamics and Earthquake Engineering 104 pp. 390-394 Elsevier
An active sequence of earthquakes (foreshock, main-shock, and aftershocks) hit the Kumamoto area (Japan) in April 2016, resulting in 69 deaths and considerable economic loss. The earthquakes induced numerous ground failures and cascading geo-hazards, causing major damage to important infrastructures. The main damage patterns include: (a) surface rupture with widespread subsidence of the surface ground, resulting in damage and disruption to transport infrastructure; (b) landslide and slope failure of mountains causing severe damage, collapse and near-collapse of bridges; and (c) liquefaction in some areas of Kumamoto City. Following the earthquakes, field surveys were conducted to study the damages and to understand the main cause of the observed failures. This technical note provides a summary of the geotechnical and infrastructural damage in Kumamoto and the lessons learnt and future research needs are also highlighted.
Jalbi Saleh, Nikitas Georgios, Bhattacharya Subhamoy, Alexander Nicholas (2019) Dynamic design considerations for offshore wind turbine jackets supported on multiple foundations, Marine Structures Elsevier
To support large wind turbines in deeper waters (30-60 m) jacket structures are currently being considered. As offshore wind turbines (OWT?s) are effectively a slender tower carrying a heavy rotating mass subjected to cyclic/dynamic loads, dynamic performance plays an important role in the overall design of the system. Dynamic performance dictates at least two limit states: Fatigue Limit State (FLS) and overall deformation in the Serviceability Limit State (SLS). It has been observed through scaled model tests that the first eigen frequency of vibration for OWTs supported on multiple shallow foundations (such as jackets on 3 or 4 suction caissons) corresponds to low frequency rocking modes of vibration. In the absence of adequate damping, if the forcing frequency of the rotor (so called 1P) is in close proximity to the natural frequency of the system, resonance may occur affecting the fatigue design life. A similar phenomenon commonly known as ?ground resonance? is widely observed in helicopters (without dampers) where the rotor frequency can be very close to the overall frequency causing the helicopter to a possible collapse. This paper suggests that designers need to optimise the configuration of the jacket and choose the vertical stiffness of the foundation such that rocking modes of vibration are prevented. It is advisable to steer the jacket solution towards sway-bending mode as the first mode of vibration. Analytical solutions are developed to predict the eigen frequencies of jacket supported offshore wind turbines and validated using the finite element method. Effectively, two parameters govern the rocking frequency of a jacket: (a) ratio of super structure stiffness (essentially lateral stiffness of the tower and the jacket) to vertical stiffness of the foundation; (b) aspect ratio (ratio of base dimension to the tower dimension) of the jacket. A practical example considering a jacket supporting a 5MW turbine is considered to demonstrate the calculation procedure which can allow a designer to choose a foundation. It is anticipated that the results will have an impact in choosing foundations for jackets.
Cui Liang, Bhattacharya Suby, Nikitas George, Bhat Ankit (2019) Macro- and micro-mechanics of granular soil in asymmetric cyclic loadings encountered by offshore wind turbine foundations, GRANULAR MATTER SPRINGER-VERLAG
Offshore wind turbine foundations are subject to 107 to 108 cycles of loadings in their designed service life. Recent research shows that under cyclic loading, most soils change their properties. Discrete Element Modelling of cyclic simple shear tests was performed using PFC2D to analyse the micromechanics underlying the cyclic behaviours of soils. The DEM simulation were first compared with previous experimental results. Then asymmetric one-way and two-way cyclic loading pattern attained from real offshore wind farms were considered in the detailed parametric study. The simulation results show that the shear modulus increases rapidly in the initial loading cycles and then the rate of increase diminishes; the rate of increase depends on the strain amplitude, initial relative density and vertical stress. It shows that the change of soil behaviour is strongly related to the variation of coordination number, rotation of principal stress direction and evolution of degree of fabric anisotropy. Loading asymmetry only affects soil behaviours in the first few hundred of cycles. In the long term, the magnitude of (³max - ³min) rather than loading asymmetry dominates the soil responses. Cyclic loading history may change the stress-strain behaviour of a soil to an extent dependent on its initial relative density.
Xu Ying, Nikitas George, Zhang Tong, Han Qinghua, Chryssanthopoulos Marios, Bhattacharya Subhamoy, Wang Ying (2019) Support Condition Monitoring of Offshore Wind Turbines Using Model Updating Techniques, Structural Health Monitoring SAGE Publications
The offshore wind turbines (OWTs) are dynamically sensitive, whose fundamental frequency can be very close to the forcing frequencies activated by the environmental and turbine loads. Minor changes of support conditions may lead to the shift of natural frequencies, and this could be disastrous if resonance happens. To monitor the support conditions and thus to enhance the safety of OWTs, a model updating method is developed in this study. A hybrid sensing system was fabricated and set up in the laboratory to investigate the long-term dynamic behaviour of the OWT system with monopile foundation in sandy deposits. A finite element (FE) model was constructed to simulate structural behaviours of the OWT system. Distributed nonlinear springs and a roller boundary condition are used to model the soil-structure-interaction (SSI) properties. The FE model and the test results were used to analyze the variation of the support condition of the monopile, through an FE model updating process using Estimation of Distribution Algorithms (EDAs). The results show that the fundamental frequency of the test model increases after a period under cyclic loading, which is attributed to the compaction of the surrounding sand instead of local damage of the structure. The hybrid sensing system is reliable to detect both the acceleration and strain responses of the OWT model and can be potentially applied to the remote monitoring of real OWTs. The EDAs based model updating technique is demonstrated to be successful for the support condition monitoring of the OWT system, which is potentially useful for other model updating and condition monitoring applications.