The dynamic behaviour of pump gates in the Afsluitdijk: Application of semi-analytical fluid-structure interaction models

As part of the Afsluitdijk project the discharge capacity, which is currently facilitated by two sluice complexes, is increased to cope with future sea level rise. It was decided to realise this by an innovative solution: the pump gate. The steel lifting gates, each containing three pumps, will be implemented in the existing sluice complex in phases, expecting a total of thirteen by the year 2050. This concept provides the possibility of pumping when necessary without hampering the discharge capacity during gravity flow, in which case the gates are lifted. The gates were designed to withstand quasi-static loads. Mainly due to the presence of the pumps, which have a high weight and require limited vibrations, the dynamic behaviour of the gate may lead to more strict design requirements. Two reference designs are investigated: the regular, and flood defence pump gate. The latter is designed to act as part of the primary flood defence, and is therefore significantly more robust. The analysis of the pump gate is limited to three components: the gate structure with its supports, pumps, and fluid. Standard expressions for the hydrodynamic pressures do not apply to the pump gate and surrounding fluid, mainly due to the three-dimensional vibration shape of the gate and the presence of the pumps. General methods or numeric models to quantify vibrations are not readily available for a continuous system with interacting gate, pumps, and fluid. In this thesis, a method is developed to determine the dynamic behaviour of gate-fluid systems confined by sluices. This method is based on a frequency domain semi-analytical coupled modal analysis, able to directly solve the behaviour of gate and fluid for the linearized equations. Several fluid schematizations are found in literature taking surface waves, compressibility, or neither into consideration. The validity of these schematizations was investigated for a wide range of water depths and excitation frequencies. Distinct regions were found in which these physical processes do or do not have an effect on the hydrodynamic mass. The so-called `transition region' is characterised by the absence of both compressibility and surface wave effects. The hydrodynamic mass is therefore frequency-independent in this region and no hydrodynamic damping is present. The response of the pump gate reference designs is quantified by a three-dimensional plate model, based on previously described method. Both designs have considerably higher eigenfrequencies than those corresponding to regular wave excitation. For the Den Oever case, the quasi-static approach therefore suffices when considering wave loads. This is not the case for excitations originating from the pumps, which relate to a wider and higher frequency range. As a consequence of the preliminary design phase, exact pump specifications are not available. Results are therefore based on a pump envelope of possible excitations and presented as risks. These apply to the Den Oever gate designs, but are also relevant to the pump gate concept in general. Three response amplitudes were quantified: the gate's deflection, pump vibration velocity and the resulting fluid pressures. Furthermore, based on the amplification of the static deflection an estimation of the maximum stresses is made. For both designs maximum deflections were less than a millimetre, which is negligible considering the dimensions of the gate. This is explained by the relatively small amplitude of the pump excitation forces compared to the total of static loads. Nevertheless, a significant deflection amplification (dynamic/static) is found. When internal stresses are amplified similarly, a risk of fatigue and even direct failure exists for the regular pump gate design. The more robust flood defence pump gate design reduces stresses to acceptable levels. Vibration velocities of the pumps were compared to ISO limits for non-rotating pump parts. For the regular design at several frequencies these limits were exceeded. The robust design is able to limit these velocities considerably. Most concerning is the magnitude of the pressure fluctuations in the fluid. For the regular design, pressure head amplitudes at the suction side up to 0.5 metre were found. This leads to an increased risk of cavitation, and therewith damage and a reduced efficiency. Furthermore, total head fluctuations over both sides of the pumps can be in the same order as the static pump head. This is expected to lead to an unacceptable reduction in efficiency. The robust flood defence pump gate does reduce these fluctuations over the first range of the investigated excitation frequencies (< 250 rad/s), but for several frequencies leads to a larger response at higher frequencies. %This is likely due to the closer overlap of the structural and fluid eigenfrequencies in that case. %High pump excitation frequencies can therefore be very harmful. The dynamic behaviour of gate and fluid should therefore be considered in further design of the pump gate. Concluding, the dynamic behaviour of the pump gate designs should be considered in further design phases, since several risks were identified. The combination of a robust gate design and limited high frequency pump excitations may lead to an acceptable design.Hydraulic Structures and Flood RiskHydraulic EngineeringCivil Engineering and Geoscience

Wave-induced vibrations of flood gates: modelling, experimentation and design

Flood gates form an essential part of many flood defence systems in coastal areas. During storm events, these gates are subjected to extreme loads from various sources. This dissertation addresses the dynamic behaviour of flood gates induced by wave impacts. A semi-analytical fluid-structure interaction model is developed to predict vertical flood gate vibrations. This model aims to overcome both the lack of accuracy involved with existing engineering methods and the high computational costs of numerical finite element methods. Scale experiments have been performed of wave impacts on flexible gate structures with an overhang to validate this model. The resulting dataset is made available for further research on the involved physical phenomena. Methods are then presented to design and assess the safety of flood gates subjected to wave impacts. At the Afsluitdijk in the Netherlands, wave impacts on the flood gates have played a major role in the design. Several case studies are performed in this dissertation based on that situation.Hydraulic Structures and Flood Ris

Bending Vibrations of the Afsluitdijk Gates Subjected to Wave Impacts: A Comparison of two Design Methods

This paper describes a first case study of the application of a newly developed fluid-structure interaction model to the design of flood gates. The gates in the Afsluitdijk, that will be replaced in the coming years, are considered. Due to the presence of a concrete beam in front of the gates breaking wave can occur, leading to high impact pressures acting on the gate. For this case both a quasi-static approach and a more detailed semi-analytical model representing the dynamic behaviour including fluid-structure interac-tion are applied to determine the maximum deflection of the gate. Results show the capability of the model to efficiently quantify flood gate vibrations while considering the involved fluid-structure interaction. For the Afsluitdijk case this leads to a slightly lower maximum deflection of the gate, and therefore potentially al-lows a more economical design.Hydraulic Structures and Flood Ris

Project Hue: Report and field study on the water related problems and solutions in and around the Cau Hai lagoon and the Tu Hien inlet, Vietnam

The Tam Giang-Cau Hai lagoon system, lying in the Thua Thien-Hue province in central Vietnam, is affected by a tropical monsoon climate. This among others is the reason the Cau Hai Lagoon area has a long history of floods and other water related problems. Inhabitants are very dependent on the lagoon, as the main sources of income of people living in the region are fishing, agri- and aquaculture. The project goal has been formulated as follows: Finding an economic as well as technical feasible solution to reduce the water related problems, specifically navigability, salt intrusion and floods, in and around the Cau Hai Lagoon and the Tu Hien inlet and thereby improving the economic development of the region. Concerning flood risk, navigability and salt intrusion the inlet stability and size are important aspects. Using an echo sounder the bathymetry of the inlet has been measured. The measured size of the inlet was one of the input parameters for the hydraulic model that has been set up. This basic model of the Cau Hai basin system was made to test some alternative solutions for the Tu Hien inlet. The different solutions were simulated for five different scenarios. These scenarios include average dry season conditions, average wet season conditions and multiple extreme events. The output of the model for the different alternatives was used to rate the alternatives for a couple of criteria in a Multi Criteria Analysis. Other criteria of the MCA are qualitatively rated. The most promising alternative proved to be the one including a jetty at the northern side of the Tu Hien inlet in combination with a bank protection at the other side. In this way a large part of the littoral drift is blocked, enlarging the equilibrium cross-section of the inlet. This in turn results in a better flood evacuation capacity, navigability and water quality in the lagoon. For both mentioned elements a preliminary technical design is made, resulting in the stone class needed for the armour layers, dimensions of the toe and characteristics of the filter.Hydraulic EngineeringCivil Engineering and Geoscience

A three dimensional semi-analytical model for the prediction of gate vibrations immersed in fluid

A model is developed to predict bending vibrations of flood gates with fluid on both sides. The liquid flow is three-dimensional and the gate is represented as a thin plate. The fluid response is considered within the linear potential flow theory including the effect of compressibility and the generation of free surface waves. This way, the hydrodynamic fluid pressure exerted on the gate is predicted accurately in both low and high-frequency regimes. Both the structural and fluid responses are expressed in the modal domain as a superposition of modes. A semi-analytical solution of the fluid-interaction problem is obtained by describing the complete system in terms of in vacuo gate modes, which is computationally efficient compared to existing numerical methods. This allows for the accurate prediction of flood gate vibrations for a large number of simulations, making it possible to perform fatigue calculations and probabilistic evaluations. The case of a typical flat flood gate subjected to an impulsive wave impact is studied with the developed model. Results show the capability of the model to efficiently quantify flood gate vibrations considering the involved fluid-structure interaction, which can lead to more economical designs compared to common engineering practice.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Hydraulic Structures and Flood RiskOffshore EngineeringDynamics of Structure

A fluid–structure interaction model for assessing the safety of flood gate vibrations due to wave impacts

This paper establishes a computationally efficient model to predict flood gate vibrations due to wave impacts including fluid–structure interaction. In contrast to earlier models, composite fluid domains are included to represent the situation of a flood gate in a dewatering sluice with the presence of an overhang that causes the confined-wave impacts. The dynamic response of the gate-fluid system is derived in the frequency domain using a substructuring mode matching technique, in which the gate vibrations are first expressed in terms of in-vacuo modes while the liquid motion is described as a superposition of linear potentials. Pressure impulse theory is employed to predict the impulsive wave impact loads, which are superposed on the quasi-steady wave loads. The computational efficiency of the developed model allows for a large number of simulations. This makes it possible for the first time to perform probabilistic evaluations for this type of problems without doing concessions on the accuracy of the physical modelling of the involved fluid–structure interaction processes. This is demonstrated by application of the developed models within a probabilistic framework resulting in the explicit quantification of the failure probability of flood gates subjected to wave impacts.Hydraulic Structures and Flood RiskDynamics of StructuresOffshore Engineerin

A novel design method for wave-induced fatigue of flood gates

This paper presents a novel design method to predict fatigue of flood gates due to dynamic wave loading. The accumulation of fatigue damage is predicted probabilistically over the entire lifetime of the structure rather than with a set of normative events. Load events are defined using a joint probability distribution of historical wind and water level data. The random phase-amplitude model is employed to obtain realisations of the wave state for every combination of environmental conditions. Linear wave theory and pressure-impulse theory are used to predict both quasi-steady and highly dynamic wave pressures. The stress response of the structure is predicted with a hybrid semi-analytical and finite element model. By applying a mode matching technique the fluid-structure interaction is solved in a computationally efficient manner. This facilitates the large number of simulations required for a comprehensive fatigue analysis without making concessions in the physical modelling. The fatigue damage is then evaluated with the linear Palmgren-Miner method by applying a rainflow algorithm. A Monte Carlo analysis is performed to estimate the expected fatigue lifetime of the structure. The modular structure of the model routine allows for easy adaptation to other situations where fatigue due to hydrodynamic loading is of interest. The design method is applied to a case study of a flood gate with an overhang inspired by the situation at the Afsluitdijk. Non-fundamental modes are taken into account without simplification of the fluid-structure interaction process and found to be governing for the fatigue damage for the studied case. Moreover, the interference of vibrations due to consecutive wave impacts is shown to have a significant influence on the outcome of the fatigue assessment. For the case study, the design method leads to a 10-20% reduction of the governing fatigue damage compared to a method commonly used in practice. At specific locations on the flood gate fatigue damage is found to be underestimated by current design methods. The presented design method is therefore found to be a significant improvement.Hydraulic Structures and Flood Ris