Now showing 1 - 5 of 5
  • 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.
      222
  • Publication
    An investigation into the effect of scour on the natural frequency of an Offshore Wind Turbine
    Rapid expansion of the offshore wind industry has stimulated a renewed interest in the behaviour of offshore piles. There is widespread acceptance in practice that pile design methods developed for the offshore oil and gas industry may not be appropriate for designing wind turbine foundations. To date, the majority of offshore wind turbines are supported by large diameter monopiles. These foundations are sensitive to scour which can reduce their ultimate capacity and alter their dynamic response. In this paper, the use of a vibration-based method to monitor scour is investigated. The effect of scour on the natural frequency of a model monopile was measured in a scale model test. A spring-beam finite element numerical model was developed to examine the foundation response. The model, which used springs tuned to the small-strain stiffness of the sand, was shown to be capable of capturing the change in frequency observed in the scale test. This numerical procedure was extended to investigate the response of a full-scale wind turbine over a range of soil densities, which might be experienced at offshore development sites. Results suggest that wind turbines founded in loose sand would exhibit the largest relative reductions in natural frequency resulting from scour.
    Scopus© Citations 137  790
  • Publication
    A review of bridge scour monitoring techniques
    The 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 208  550
  • Publication
    An investigation of the changes in the natural frequency of a pile affected by scour
    Scour around bridge foundations is one of the leading causes of bridge failure. Up until recently, the monitoring of this phenomenon was primarily based around using underwater instrumentation to monitor the progression of scour holes as they develop around foundation systems. Vibration¿based damage detection techniques have been used to detect damage in bridge beams. The application of these vibration based methods to the detection of scour has come to the fore in research in recent years. This paper examines the effect that scour has on the frequency response of a driven pile foundation system, similar to those used to support road and rail bridges. The effect of scour on the vibration characteristics of the pile is examined using laboratory and field testing. It is clear that there is a very clear reduction in the natural frequency of the pile as the severity of scour increases. It is shown that by combining state-of-the-art geotechnical techniques with relatively simple finite element modelling approaches, it is possible to accurately predict the natural frequency of the pile for a given scour depth. Therefore, the paper proposes a method that would allow the estimation of scour depth for a given observed pile frequency.
    Scopus© Citations 130  522
  • 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.
      202