Peter Frigaard

A time-domain method for separating incident and reflected irregular waves

Abstract A new method for separating an irregular, 2D wave field into incident waves propagating towards a structure and reflected waves propagating away from the structure is presented. The method is based on the use of digital filters and can separate the wave fields in real time. The efficiency of the method is demonstrated through numerical and physical tests. The method is very useful for comparisons of incident waves and structural response in the time domain. Furthermore, the method will be very useful in design of active wave absorbers.

AquaBuOY-the offshore wave energy converter numerical modeling and optimization

Ocean energy and offshore wave energy conversion in the United States is at a significant milestone. During the next couple of years, ocean energy technology developers and energy officials have a potential to deploy pilot scale ocean energy conversion installations. This capability comes at a time of increased interest in ocean energy worldwide. The paper outlines AquaEnergys experience in developing the Makah Bay pilot offshore power plant and results of the projected performance data developed using E2I EPRI guidelines. The Makah Bay, WA offshore pilot power plant uses AquaEnergys point absorber wave energy conversion device - AquaBuOY. The device is the next generation of the technology that combines the Swedish Hose-Pump and the IPS Buoy technologies to generate clean energy from ocean waves. Currently the project is undergoing environmental permitting in support of FERC & NOAA licenses. In early 2004 AquaEnergy, US, and RAMBOLL, DK, performed output projections to substantiate the expected performance of the Makah Bay pilot plant in support of the E2I EPRI Offshore Wave Energy Feasibility Demonstration Project. AquaEnergy will conclude its presentation with a brief overview of current legislation affecting the industry. In 2004, ocean scientists, engineers, and developers can continue to lay the groundwork for government spending and interest in ocean energiesThis paper describes development of the mathematical model simulating ocean performance of an offshore wave energy point absorber device-AquaBuOY. The AquaBuOY is the next generation of the technology, based on the IPS point absorber system and the hose pump, both of Sweden. AquaEnergy Group Ltd., USA, is developing the system in cooperation with RAMBOLL, Denmark. In March 2003 the Danish Energy Authority awarded a grant for a design study that includes development of the numerical model for the AquaBuOY operation, experimental testing and design optimisation. The scale model tests will be carried out at Aalborg University, Denmark in order to optimise the device design, operation and installation configuration with the goal of minimising system footprint. The paper provides an overview of the numerical modelling used in establishing system operating characteristics. The experimental results from the model tests on mooring forces under survival conditions will be presented during the conference in conjunction with different footprint configurations and different mooring systems. Finally the performance data based on theoretical and experimental results will be presented for the AquaBuOY in five representative generic sea states. Ocean energy and offshore wave energy conversion in the United States is at a significant milestone. During the next year, ocean energy technology developers and energy officials have the potential to deploy pilot scale ocean power plants and transition to commercial plants in the US. This capability comes at a time of increased interest in ocean energies at the National Academy of Sciences and the US Department of Energy. AquaEnergy will conclude its presentation with a brief overview of current legislation affecting the industry. In 2004, ocean scientists, engineers, and developers can continue to lay the groundwork for government spending and interest in ocean energies.

Experimental Study of the Development of Scour and Backfilling

This paper deals with the development of scour holes in time and space around individual offshore monopiles. It is based on physical flume tests of a model-scale pile subjected to current and/or irregular water waves. The main focus is on backfilling, i.e. the wave-induced or current-wave-induced deposition of sediment into holes that have been previously scoured. The development of the scour depth, scour volume and a so-called scour shape factor is quantified which may be useful to understand and benchmark the development of scour holes.

On 3-Dimensional Stability of Reshaping Breakwaters

The Labrador Sea Extreme Waves Experiment (LEWEX), is an international basic research programme concerned with full-scale measurements, analysis, modelling and simulation in test basins of 3-dimensional seas. The research is carried out in order to assess the significance of 3-dimensional sea states in engineering applications. The first phase of the programme full scale wave measurements in the North Atlantic Ocean was performed at a site and time that had a high probability of encountering severe sea states. The present publication shows examples of measured bi-modal directional sea spectra obtained with the WAVESCAN buoy and directional sea spectra measured with an airborne Synthetic Aperture Radar (SAR). Directional spectra of gravity waves are obtained with the SAR both in open waters and below an ice cover. Further work is needed in order to verify SAR-measurements with in-situ observations. In-situ measured directional spectra are also compared with hindcast spectra from the 3G-WAM model. Hindcast significant wave heights were found to be lower than the in-situ measurements.The littoral drift model developed at DHI and ISVA, see Deigaard et al. (1986b) has been extended to include the effects of the irregularity of the waves, of a coastal current and a wind acting on the surf zone. Further, a mathematical model to simulate the near-shore current pattern along a barred coast with rip channels has been developed. The influence on the littoral drift of the irregularity of waves, wind, coastal current, and rip channels is discussed. It is concluded that irregularity of waves and presence of rip channels must be considered while coastal current and wind action are of minor importance.At present, the Prodhoe Bay oil field in Alaska contributes a substantial amount of the domestic oil production of the United States. Oil is also expected to be present on the continental shelf of Alaska, and it is estimated that approximatedly 28 percent of the total U. S. reserve is located beneath the shallow ice covered seas of the Alaskan continental shelf. To expolre and to exploit these oil rich resources, engineers are confronted with hostile oceanographic conditions such as high tides, waves, strong currents and sea ice. The same area is also rich in fishery resources. Being one of the most productive fishing grounds in the nothern Pacific, the potential ecological impact due to an oil spill is of a major concern. This paper describes the methologies used for the development of a modeling system for the oil risk analysis. The system is designed with generality in mind so it can be used for other coastal areas. The development of three dimensional models used in the modeling system described here have been published in the earlier International Coastal Engineering Conferences (Liu and Leendertse, 1982, 1984, 1986) and a report published recently by RAND (Liu and Leendertse, 1987). In the oil-spill risk analysis, these three dimensional hydrodynamic models are coupled to a two-dimensional stochastic weather model and an oil weathering model.The two projects (LUBIATOWO 79 and LUBIATOWO 86) were aimed at study pore pressure behaviour in natural sand bed in the coastal zone of the Baltic sea under the action of storm waves. During both projects, the wave induced pore pressures at the various levels in the sea bed were measured. The collected data were used to verify the applicability of the various theoretical approaches. In the conclusion, the range of the application of the particular analytical method is given.The rapid recession of the shingle bank of Hurst Beach (up to 3.5m/yr) makes it an excellent natural laboratory for the study of the factors which influence the stability of shingle beaches. Studies have included: the significance of long period, high energy, swell waves the classification and quantification of overwash processes run-up and seepage characteristics the effect of settlement of the underlying strata and the implications for practices in shingle nourishment. The studies have revealed the distinctive character of shingle beaches as compared with the more fully researched sand beaches. More detailed research on shingle beaches is justified particularly in relation to (i) the run-up characteristics including its interaction with swash cusps and (ii) the influence of the subsidiary sand fraction on the beach characteristics.Environmental assessment, engineering studies and designs were completed for a new 26.5 m3/s seawater intake system in the Persian Gulf. The original intake facility consisted of a curved, 60m breakwater with one end attached to the shoreline, a settling basin immediately adjacent to the shoreline and dredged to a maximum depth of approximately 5m, and a pumphouse structure located on shore such that the seaward wall formed one side of the settling basin. The facility located on an island in the Gulf, which served multiple seawater uses, had experienced both structural and operational problems, the latter consisting principally of excessive ingestion of sediment and seaweed. These factors plus the requirement for additional demands for seawater beyond plant capacity caused the owner to initiate a study of alternative intake systems, produce a design for the most effective solution and construct the new intake system.A two-dimensional wave prediction model suitable for use on personal computers is described. The model requires the twodimensional time-dependent wind field as input. Output consists of wave height, wave period, and wave direction estimates at all grid points on a computational grid representing an enclosed or semi-closed basin. Model predictions compare favorably with observations from a wave research tower in Lake Erie. A formula is provided to estimate how long a model simulation would take on a personal computer given the surface area of the computational domain, the grid size, and the computer clock speed.Cullera Bay is a neritic ecosystem placed on the Spanish Mediterranean Littoral largely influenced by the Jucar River, that brings about lower salinities than surrounding waters, and broad variations of its values. An extensive research, with 9 samplings throughout the year, was carried out, measuring both physical and chemical parameters, and the planktonic communities. The trophic status of the ecosystem, the spatial and temporal variations of the nutrients and the planktonic communities were studied, evaluating the influence of the river loads and the littoral dynamics. Some essential basis to allow a suitable emplacement of waste waters disposals along the Valencian littoral are set up in order to minimize the gradual eutrophication of this coast.In the last two years a whole of studies was realized in order to determine precise solutions to the regeneration of Villajoyosas beach, in the Spanish mediterranean coast. Investigations were carried out to the surrounding coastal areas based in field investigations and laboratory analyses of the beaches materials.

Absorbing Wave-Maker Based on Digital Filters

An absorbing wave maker operated by means of on-line signals from digital FIR filters is presented. Surface elevations are measured in two positions in front of the wave maker. The reflected wave train is separated from the sum of the incident and rereflected wave trains by means of digital filtering and subsequent superposition of the measured surface elevations. The motion of the wave paddle required to absorb reflected waves is determined and added to the original wave paddle control signal. Irregular wave tests involving test structures with different degrees of reflection show that excellent absorption characteristics have been achieved with the system. INTRODUCTION Coastal engineering problems are often solved by means of physical models. Physical modelling of coastal engineering phenomena requires the capability of reproducing natural conditions in the laboratory environment. One of the problems associated with the physical modelling of water waves in laboratory wave channels is the presence of rereflected waves. In nature the sea constitutes an open boundary which absorbs waves reflected by the coastal system. A wave channel is a closed system: waves reflected from a model structure will be rereflected at the wave paddle, thus altering the characteristics of the wave train incident to the model structure. Consequently, the reproduction of a specified

Spatial Distribution of Wave Pressures on Seawave Slot-Cone Generator

acceptance number 166 SPATIAL DISTRIBUTION OF WAVE PRESSURES ON SEAWAVE SLOT- CONE GENERATOR 1 Author: Vicinanza, Diego 2 Author: Margheritini, Lucia 3 Author: Frigaard, Peter Breakwaters Coastal structures Dynamic response Inlets Long waves Overtopping Wave generation

Wave Run-Up on the Zeebrugge Rubble Mound Breakwater: Full-Scale Measurement Results Versus Laboratory Results

Abstract A clear difference between full-scale wave run-up measurements and small-scale model test results had been noticed during a MAST II project. This finding initiated a thorough study of wave run-up through the European MAST III OPTICREST project. Full-scale measurements have been carried out on the Zeebrugge rubble mound breakwater. This breakwater has been modeled in three laboratories: two 2D models at a scale of 1 : 30 and one 3D model at a scale of 1 : 40 have been built at Flanders Hydraulics (Belgium), at Universidad Politécnica de Valencia (Spain), and at Aalborg University (Denmark). Wave run-up has been measured by a digital run-up gauge. This gauge has proven to measure wave run-up more accurately than the traditional wire gauge. Wave spectra measured in Zeebrugge have been reproduced in the laboratories. Results of small-scale model tests and full-scale measurement results have been compared. This comparison confirmed the MAST II outcome: wave run-up is underestimated by small-scale model tests. The difference between full-scale measurement results and small-scale model test results is the result of model effects. The porosity of the armor layer has a significant influence on wave run-up and may explain the dependency of wave run-up on the water level observed in Zeebrugge. An influence of the spectral shape has also been noticed.

Effect of Short-Crestedness and Obliquity on Non-Breaking and Breaking Wave Forces Applied to Vertical Caisson Breakwaters

This paper addresses wave forces applied to vertical caisson breakwaters. Design diagrams are proposed to evaluate the reduction of the wave force induced by breaking and non-breaking short-crested oblique waves with increasing horizontal length of the caisson units. Results are derived by testing a model of a typical Italian vertical breakwater with a moderately high rubble mound in the CRF-LSF (Wallingford, UK) wave basin equipped with a multidirectional wave generator. Seven adjacent caisson modules were instrumented with synchronized force transducers. Non-breaking and breaking wave loads are distinguished on the basis of the frequency content. The reduction factor of the load per unit span is derived by the measured spatial correlation, following the argument developed by G. I. Taylor for random movements.

Wave Run-Up on the Zeebrugge Rubble Mound Breakwater: Full-Scale Measurement Results

Abstract Full-scale wave run-up measurements have been carried out on the Zeebrugge rubble mound breakwater in the frame of the EU-funded OPTICREST project. Wave run-up has been measured by a run-up gauge and by a so-called spi-derweb system. The dimensionless wave run-up value Ru2%/Hm0 measured in Zeebrugge equals 1.77 for Iribarren number ξom = 3.63. An (apparent) influence of the water level on wave run-up has been observed. The porosity of the armor layer has a significant influence on wave run-up and may explain the dependency of wave run-up on the water level observed in Zeebrugge. Full-scale measurement results have been compared to literature data. The full-scale wave run-up data are in good agreement with the existing results of wave run-up tests with rip-rap slopes but are clearly higher than other wave run-up data resulting from tests with rubble mound breakwaters.

Horizontal Coherence of Wave Forces on Vertical Wall Breakwaters

Spatial coherence of wave impact pressures at a vertical breakwater in multidirectional seas is studied as part of an EU project under the LSF-TMR programme. The lay out and programme of tests are shortly described. A method for the identification of breaking waves is presented. The percentage of breaking waves for increasing wave height are estimated and compared with existing empirical formulae. The horizontal dimension of the breaker is investigated using two different methodologies: the first analyses the decreasing of the highest 1/250 force with increasing horizontal dimension of the caisson, with special attention to the effects of wave spreading, and the second analyses the spatial correlation of impulsive force per unit length along the breakwater.