Now showing 1 - 10 of 41
  • Publication
    The effect of variations in soil stiffness on the dynamic response of an offshore wind turbine
    (European Wind Energy Association, 2013-11-21) ; ;
    Offshore 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.
  • Publication
    The impact of soil autocorrelation on pile load displacement behaviour
    (Pan-Am CGS 2011 Organizing Committee, 2011-10-02) ; ;
    Foundation design is often controlled by the serviceability limit state and the mobilised settlement under operational conditions is often the governing design condition. Accurate predictions of pile displacements are often hampered by the inherent soil variability. This paper describes an analysis which incorporates the uncertainty in soil properties directly into the pile settlement calculations through a monte-carlo simulation. A t-z analysis is performed which assumes the axial load in a pile is resisted by non-linear uncoupled spring elements, which are dependent on the properties of the surrounding soil. The input soil parameters are modelled by log normally distributed variables. The ultimate friction mobilised by the soil springs is calculated using the Cone Penetration Test based Imperial College pile design approach. CPT data from an Irish dense sand test site is used in the analysis. The springs are assumed to be auto-correlated with depth in a similar manner to the CPT profile, with the degree of correlation defined by the scale of fluctuation. In the final section, the results are discussed in light of previous research which assumed uncorrelated soil properties.
  • Publication
    Effect of vehicle velocity on exciting the lateral dynamic response of two-span integral bridges
    (Civil Engineering Research Association of Ireland, 2016-08-30) ; ;
    Vibration-based Structural Health Monitoring (SHM) is an area of ongoing research and has received much attention from researchers in recent years. Online damage detection methods for bridges rely on placing sensors on the structure to detect anomalies in measured parameters such as acceleration, frequency or displacement among others. Changes in these parameters can be used to infer the presence of damage such as cracking in bridge beams, foundation scour etc. These methods mostly rely on using the signals arising on a bridge from ambient traffic or environmental loading. For foundation scour detection purposes, the lateral response of a bridge is of particular interest in that this has been shown to be particularly sensitive to the scour phenomenon. Vehicle-Bridge Interaction (VBI) effects can have a significant influence on the condition of output vibrations from a bridge element. In this paper, the effect of vehicle travelling velocity on the lateral response of a typical highway two-span integral bridge is investigated. It is shown that depending on the velocity of the vehicle relative to the oscillatory period of the bridge it traverses, the bridge's dynamic response is either amplified or diminished by varying degrees.This phenomenon could influence the accuracy of a particular damage detection method relying on output system vibrations to infer damage.
  • Publication
    Design Tools Available For Monopile Engineering
    (European Wind Energy Association, 2014-03-13) ; ; ;
    Monopiles 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.
  • Publication
    Bridge scour monitoring using accelerometers placed on bridge piers - a numerical investigation
    Bridge scour has been identified as one of the leading causes of bridge failures in recent years, and caused the collapse of the Malahide Viaduct, a mainline railway bridge on the TEN-T network, in North Dublin in 2009. Scour is the term given to the removal of supporting soils from around foundations as a result of increased hydraulic stresses, a process exacerbated in times of heavy flooding. Scour is notoriously difficult to predict due to its dependence on many uncertain hydraulic parameters and difficulties in determining the exact condition of underwater foundations. This is a particular problem for rail bridges which may have been built more than 150 years ago. Protecting structures against scour generally involves relatively expensive installations that may not prove economically viable for widespread usage. Therefore, monitoring of scour-critical infrastructure is an attractive solution for infrastructure managers. The most common method for scour monitoring is to undertake visual inspections using divers to obtain the depth of scour around a foundation. This can prove time consuming and be particularly dangerous in times of heavy flooding, when scour is most critical. This paper charts the development of a new method of scour monitoring that uses the bridge structure itself to indicate the severity of the scour problem. Scour causes a reduction in foundation stiffness by removing soil from around foundation elements. This can manifest itself as a change in the natural frequency of bridge piers, due to the increase in effective length and reduction in soil stiffness. This paper presents a numerical examination of the effect of scour on the frequency response of a range of bridge pier geometries. The purpose is to show the effectiveness of the technique at remotely monitoring scour using accelerometers placed above the waterline.
  • Publication
    Shaft Capacity of Open-Ended Piles in Clay
    This paper describes an experimental investigation designed to assess the impact of pile end condition on the capacity of piles installed in soft clay. A series of field tests are described in which instrumented open-ended and closed-ended model piles were jacked into soft clay. The radial stresses, pore pressures, and load distribution were recorded throughout installation, equalization, and load-testing. Although the total stress and pore pressure developed during installation were related to the degree of soil plugging, the radial effective stress that controls the shaft resistance was shown to be independent of the mode of penetration. The long-term shaft capacity of the open-ended pile was closely comparable to that developed by closed-ended piles, suggesting a limited influence of end condition on the fully equalized shaft resistance. In contrast to the shaft resistance, the base capacity was highly dependent on the degree of plugging.
      1621Scopus© Citations 66
  • Publication
    Performance Testing of a Novel Gravity Base Foundation for Offshore Wind
    (Civil Engineering Research Association of Ireland, 2016-08-30) ; ;
    In 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.
  • Publication
    The effect of scour on the dynamic response of an offshore wind turbine
    (Civil Engineering Research Association of Ireland, 2014-08-29) ; ;
    Wind 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.
  • Publication
    Modelling the Cone Penetration Test in sand using Cavity Expansion and Arbitrary Lagrangian Eulerian Finite Element Methods
    (Elsevier, 2011-06) ;
    The paper considers two techniques to model the Cone Penetration Test (CPT) end resistance, qc in a dense sand deposit using commercial finite element programmes. In the first approach, Plaxis was used to perform spherical cavity expansion analyses at multiple depths. Two soil models, namely; the Mohr–Coulomb (MC) and Hardening Soil (HS) models were utilized. When calibrated using simple laboratory element tests, the HS model was found to provide good estimates of qc. However, at shallow depths, where the over-consolidation ratio of the sand was highest, the relatively large horizontal stresses developed prevented the full development of the failure zone resulting in under-estimation of the qc value. The second approach involved direct simulation of cone penetration using a large-strain analysis implemented in Abaqus/Explicit. The Arbitrary Lagrangian Eulerian (ALE) technique was used to prevent excessive mesh deformation. Although the Druker–Prager soil model used was not as sophisticated as the HS model, excellent agreement was achieved between the predicted and measured qc profiles.
      3081Scopus© Citations 91
  • Publication
    Soil properties at the UCD geotechnical research site at Blessington
    Over the past ten years, the Geotechnical Research Group (GRG) at University College Dublin have developed a research site at Blessington, County Wicklow, for the purpose of testing foundation systems. This paper presents the results of field and laboratory tests conducted to obtain the geotechnical parameters of Blessington sand. The in-situ tests included cone penetration and dilatometer tests. Sonic coring was performed in three boreholes at the site and complete recovery was obtained in boreholes up to 14 m deep. Additional disturbed samples were taken from trial pits which were up to 6 m deep. The classification tests performed on samples compared favourably with those inferred from correlations with in-situ test data. The strength, stiffness and mineralogy were also determined by a suite of laboratory tests including SEM imagery, triaxial tests and ring shear testing. The accuracy of conventional correlations in predicting the laboratory measured parameters is discussed.