Graduation of Joachim Kleiberg

Fatigue in floodgates due to dynamic wave loading

• Professor of graduation: Dr. ir. B. Hofland  (TU Delft)

• Supervisors of graduation:  Ir. O.C. Tieleman (TU Delft), Ir. M. Versluis (Witteveen+Bos), Ir. E. ten Oever (BAM Infraconsult), Dr. ir. P van der Male (TU Delft)

The ability to predict the fatigue of flood gates due to dynamic wave loading is becoming increasingly important as coasts and waterways worldwide are being reinforced to suit a changing climate. There are few comprehensive ways to do so however, which often leads to conservative estimates and therefore cost-inefficient designs. This thesis presents an integral framework with which the fatigue of flood gates due to dynamic wave loading can be described in much more detail, while also being efficient and adaptable to different circumstances. It considers the entire wave spectrum rather than individual waves, takes higher response modes into account, models the full fluid-structure interaction, and models the lifetime probabilistically rather than as a set of normative events.

The problem is split into smaller self-contained modules, which can be combined to answer the main question. The first set of modules aims to solve the fatigue experienced by a single load event based on readily available data. This was done by creating a wave spectrum from the site properties, average wind velocity, and average water level, and using that spectrum to generate random realisations of wave load spectra on the gate by employing the random phase-amplitude model. These wave loads can then be transformed to pressure spectra which will dynamically excite the fluid-structure system. The pressures were split up into a quasi-static and an impulsive part. The quasi-static pressures were solved with linear wave theory, while the impulsive pressures due to the overhang were found using Wood & Peregrine pressure-impulse theory.

The gate is then parametrically modelled in a FEM software package in order to export the in-vacuo response modes. These are combined with the fluid-structure interaction model from Tieleman et al. to derive the response of the fluid-structure system to external pressures. Discretising and multiplying the previously derived pressure spectra then gives the response of the system for a particular load event.

The fatigue was evaluated with both the standard Eurocode method and a common spectral method for comparison, and results in a fatigue damage factor which shows what fraction of the materials fatigue capacity is depleted by the imposed load. This concludes the section on the fatigue to a single load event.

Next, the load events imposed on the gate over its lifetime were probabilistically defined. This was done by fitting two independent probability distributions to historical wind- and water level data, the latter of which was also adjusted for climate change. These probability distributions are then split into segments which are integrated to obtain discrete load cases with representative values for the average wind velocity, water level, and probability of occurrence. Two filters were also applied to remove the load cases which cause a negligible amount of fatigue over the lifetime of the gate.

Computing the fatigue damage caused by the average values of each of these load case bins then gives a set of fatigue damage factors with an associated probability of occurrence, which can be used to perform a Monte Carlo analysis (or direct integration). From this analysis an expected lifetime can be determined.

A case study for a hypothetical parametrically defined gate with an overhang located in the Afsluitdijk was performed, where the response for different gate configurations and environmental conditions was evaluated. The fatigue response was evaluated for fatigue across the gate, at the critical coordinate, and per mode. A ULS check was also done for the chosen design. Results following from the model were also compared to those of methods commonly used in practice.

The method gives insight into the relative importance of different modes and load events, can compute fatigue for the entire gate or just a critical coordinate, and does so in a much more time-efficient manner than current numerical models. It gives a more comprehensive view of the fatigue over the lifetime of the gate by modelling its entire lifespan rather than a set of design load events. The modular structure of the integral framework allows for easy adaptation to other use cases in hydraulic engineering where fatigue due to hydrodynamic loading is of interest.