Prendergast, Luke J.
Prendergast, Luke J.
Prendergast, Luke J.
Now showing 1 - 10 of 17
- PublicationDesign Tools Available For Monopile EngineeringMonopiles have been by far the most common support structure used for offshore turbines, with approximately 75% of existing wind farms founded on these large diameter steel tubes EWEA(2014). However, despite the widespread prevalence of monopiles across the wind sector, the design tools commonly used by industry have typically evolved from those developed by the oil and gas sector, which apply to significantly different design conditions. This paper introduces some of the design approaches available in practice and identifies some of the limitations of current offshore codes. Finite Element Methods (FEM) are suggested as a means of more accurately considering offshore soil behaviour, although the importance of accurate calibration of these models against real data (lab and/or field data) is stressed. Novel means of determining the in-situ frequency response are also discussed and the potential implications for monopile design at different sites. Finally, some design aspects of XL monopiles are considered that suggest monopiles may be pushed into ever increasing water depths.
- PublicationDevelopment of a Vehicle-Bridge-Soil Dynamic Interaction Model for Scour Damage ModellingDamage detection in bridges using vibration-based methods is an area of growing research interest. Improved assessment methodologies combined with state-of-the-art sensor technology are rapidly making these approaches applicable for real-world structures. Applying these techniques to the detection and monitoring of scour around bridge foundations has remained challenging; however this area has gained attraction in recent years. Several authors have investigated a range of methods but there is still significant work required to achieve a rounded and widely applicable methodology to detect and monitor scour. This paper presents a novel Vehicle-Bridge-Soil Dynamic Interaction (VBSDI) model which can be used to simulate the effect of scour on an integral bridge. The model outputs dynamic signals which can be analysed to determine modal parameters and the variation of these parameters with respect to scour can be examined. The key novelty of this model is that it is the first numerical model for simulating scour that combines a realistic vehicle loading model with a robust foundation soil response model.This paper provides a description of the model development and explains the mathematical theory underlying the model. Finally a case study application of the model using typical bridge, soil, and vehicle properties is provided.
349Scopus© Citations 30
- PublicationDynamic soil-structure interaction modeling using stiffness derived from in-situ Cone Penetration TestsThis paper presents the results of an experimental and numerical investigation into the natural frequency of a pile driven into dense sand. The experimental arrangement involves fitting accelerometers along the pile shaft and using a modal hammer to induce lateral vibration. The natural frequency is obtained by performing Fourier analysis on the acceleration signals. A numerical model is developed that models the pile as a beam supported by lateral springs. The natural frequency is obtained by performing an eigenvalue analysis in the numerical model. The spring stiffness is derived by first obtaining the G0 value for the sand at the installation location. This is achieved using the rigidity index, a correlation between the cone tip resistance qc value and the small-strain shear modulus G0. The G0 value is converted to lateral spring stiffness values using an equation derived analytically from the beam on an elastic foundation case. Good agreement is observed between the experimentally measured natural frequency and that which is calculated from the numerical model. This research paves the way for more accurate assessments of dynamic soil-structure interaction, and can be particularly useful in the design of structures that are dynamically sensitive such as wind turbines.
- PublicationNon-Intrusive Bridge Scour Analysis Technique using Laboratory Test ApparatusLarger and more frequent flood flows expose foundation soils to stronger erosive forces, increasing the likelihood that scour of piers (and abutments) will compromise the structural integrity of some bridges. The development of low-cost, low maintenance, non-destructive methods of bridge scour analysis is therefore becoming ever more important in light of the current economic climate. The use of embedded sensors that measure vibration responses of a structure, due to train loading, may offer potential to track changes in the foundation soil stiffness matrix caused by scour and may inform engineers in implementing appropriate protection schemes. This paper presents a laboratory investigation in which the dynamic response of a scaled pier, installed in a bed of sand and instrumented with an accelerometer, is recorded for a constant and repeatable excitation. Sand stiffness properties were manually altered by increasing the scour depth in progressive experiments. For each experiment, a vibration response was recorded and this was converted to a frequency response using a fast Fourier transform (FFT). Differences between the dynamic signatures of the pier for the different scour conditions investigated were analysed to explore whether this type of non-destructive testing could provide a viable method of detecting scour before the structural integrity of the bridge reaches a critical stage. Results indicate that significantly different frequency responses are recorded for decreasing elevations of bed material around the model pier, indicating that the method may provide the basis for a simple and effective means of monitoring scour around bridge piers.
- PublicationLaboratory investigation of a bridge scour monitoring method using decentralized modal analysisScour is a significant issue for bridges worldwide that influences the global stiffness of bridge structures and hence alters the dynamic behaviour of these systems. For the first time, this article presents a new approach to detect bridge scour at shallow pad foundations, using a decentralized modal analysis approach through re-deployable accelerometers to extract modal information. A numerical model of a bridge with four simply supported spans on piers is created to test the approach. Scour is modelled as a reduction in foundation stiffness under a given pier. A passing half-car vehicle model is simulated to excite the bridge in phases of measurement to obtain segments of the mode shape using output-only modal analysis. Two points of the bridge are used to obtain modal amplitudes in each phase, which are combined to estimate the global mode shape. A damage indicator is postulated based on fitting curves to the mode shapes, using maximum likelihood, which can locate scour damage. The root mean square difference between the healthy and scoured mode shape curves exhibits an almost linear increase with increasing foundation stiffness loss under scour. Experimental tests have been carried out on a scaled model bridge to validate the approach presented in this article.
Scopus© Citations 18 59
- PublicationA review of bridge scour monitoring techniquesThe high profile failure of the Malahide Viaduct, in Dublin, Ireland which is a part of the EU TEN-T network of critical transport links was caused by foundation scour. In a study of five hundred bridge failures that occurred in the United States between 1989 and 2000, flooding and scour were the cause of 53% of the recorded failures (Wardhana and Hadipriono 2003). Scour is a common soil-structure interaction problem. In light of current changes in climate, increased frequency of flooding, coupled with the increased magnitude of these flood events, will lead to a higher risk of bridge failure occurring. Monitoring scour is of paramount importance to ensure the continued safe operation of the aging bridge asset network. Most monitoring regimes are based on using expensive underwater instrumentation that can often be subject to damage during times of flooding, when scour risk is at its highest. This paper presents a critical review of existing scour monitoring equipment and methodologies with a particular focus on those that use the dynamic response of the structure to indicate the existence and severity of the scour phenomenon affecting the structure. A sensitivity study on a recently developed monitoring method is also undertaken.
Scopus© Citations 201 545
- PublicationThe effect of scour on the dynamic response of an offshore wind turbineWind turbines are dynamically sensitive structures, with excitation forces arising from the rotor spinning at a specific angular velocity and the blade passing the turbine tower at a set frequency. It is critical that the structural design of the turbine is undertaken in such a way that the system natural frequency resides away from the resonant excitation bands. This will become increasingly important over the next decade as turbines evolve and the operational frequency bands change. The system natural frequency is governed by the structural properties of the turbine tower and the nacelle weight, combined with the stiffness of the soil-foundation elements. An accurate estimate of the soil stiffness is crucial to ensure a realistic model of the overall turbine behaviour. Over 75% of offshore wind turbines currently have monopile foundations, which are designed as a soft-stiff system, with the functional design frequency of the turbine structure falling between the upper and lower excitation frequency bands. The dynamic stability of the foundation is provided by the interaction between the monopile shaft and the adjacent soil strata. In this paper, the effect of scour on the frequency response of an offshore wind turbine is investigated numerically for a range of different soil densities. The turbine system is modelled using simple numerical modelling techniques. Euler-Bernoulli beam elements are used to model the tower. Altered versions of these elements are used to model the monopile, with an extra node and degree of freedom to allow the input of lateral soil stiffness into the model. The nacelle and rotor system is modelled as a lumped mass at the top of the turbine tower. In-situ CPT-based approaches are used for modelling the soil stiffness. The effect of scour is investigated for each design case.
- PublicationThe effect of variations in soil stiffness on the dynamic response of an offshore wind turbineOffshore wind is a growing energy sector that has potential to provide vast quantities of the energy required to meet renewable energy targets. The recent growth in this sector has stimulated renewed interest in the behaviour of large diameter monopiles. Currently, over 75% of offshore wind turbine generators are founded on monopile foundations. The future predictions for offshore wind indicate th at these systems will be constructed in deeper waters further from the coast. This increase in water depth will lead to longer monopile free lengths, a factor that could negatively impact on their dynamic stability. This paper presents the results of a num erical investigation in which a range of monopile diameters in different water depths are analysed with respect to the overall wind turbine system natural frequency. Three different sand stiffness profiles are generated corresponding to loose, medium dense and dense sand using the industry standard American Petroleum Institute lateral loading design code. These sand profiles are indicative of offshore conditions found at many sites. Results indicate that significant care is required to ensure that the globa l stiffness response of the structure is such that the system natural frequency does not coincide with resonant excitation bands from the rotor’s motion. The stiffness of the soil governs the diameter of the monopile that can be used in different design water depths.
- PublicationA comparison of initial stiffness formulations for small-strain soil-pile dynamic Winkler modellingDynamic Soil-Structure Interaction (DSSI) is an area of much ongoing research and has wide and varied applications from seismic response analysis to offshore wind foundation response. DSSI covers a wide range of load regimes from small-strain vibrations to large-strain cyclic loading. One of the most common ways to model DSSI uses the Winkler model, which considers the soil as a series of mutually independent springs. The difficulty with modelling DSSI arises with the inelastic and nonlinear load–displacement response of soil with increasing strain, therefore modelling of large-strain DSSI relies on the specification of many interrelated parameters. The relative magnitude of these parameters can have a significant effect on the modelled response. In this paper, the specification of an initial stiffness coefficient to model the elastic (small-strain) response of a soil–pile system is investigated. The coefficient of subgrade reaction method can be used to generate spring stiffness moduli for Winkler type models. A number of subgrade reaction theories have been proposed and their application to the problem of static loading has been widely studied. However, relatively little research concerning the application of these models for small-strain dynamic loading has been undertaken. This paper describes a sensitivity study in which a number of subgrade reaction models were used to estimate the frequency response at small-strain levels for a range of pile geometries and ground conditions. A field investigation was undertaken on two piles with different slenderness ratios to estimate the frequency response and damping ratios. The experimental results were compared to predictions of damped natural frequency obtained from numerical models using the force input and measured damping ratio from each experiment. The ability of each subgrade reaction formulation to model the response at small-strain levels is evaluated.
Scopus© Citations 56 470
- PublicationPerformance Testing of a Novel Gravity Base Foundation for Offshore WindIn recent years, the international demand to produce green energy has been growing to address the issues of energy security and climate change. To date, the wind sector has probably advanced the most due to high availability of wind resources. Erecting wind turbines offshore, however, presents significant new engineering challenges. Offshore foundations for these energy converters must be able to resist large overturning moments as well as numerous cycles of lateral loading caused by wave and wind. Thus, the need for an efficient cost-effective foundation to support the turbines is becoming more important. In this paper, a specific design of a gravity base foundation system developed for offshore wind turbines is considered. The foundation is a conical hollow concrete gravity type structure which rests on the seabed and utilises its self-weight to support the turbine. A scale-model of the proposed foundation has been experimentally tested at the University College Dublin test site in Blessington, Ireland. This paper presents the findings of this research.