Peter Frigaard

• Professor (Associate) at Aalborg University

The influence of directional spreading of waves is significant for wave-induced loads, wave breaking and nonlinearity of the waves. For physical model testing performed at test facilities such as the Ocean and Coastal Engineering Laboratory at Aalborg University, it is crucial to validate if the test conditions match the target sea states by measur...

The influence of directional spreading of waves is significant for wave-induced loads, wave breaking and nonlinearity of the waves. For physical model testing performed at test facilities such as the Ocean and Coastal Engineering Laboratory at Aalborg University, it is crucial to validate if the test conditions match the target sea states by measur...

With increasing demand for renewable energy resources, the development of alternative concepts is still ongoing. The wave energy sector is still in vast develop- ment on the way to contribute to the energy production world wide. The present study presents the development of the Exowave wave energy converter made so far. A numerical model has been e...

For design validation of offshore structures and conceptualisation of wave energy converters, physical model testing performed in wave basin laboratories is often applied. In such cases, knowledge about the wave conditions is of great significance. For validation of the wave condition in such tests, different methods for estimation of the direc...

A spatio-temporal noise robust filtering method is proposed to separate 2D linear wave fields into incident and reflected irregular waves using water surface elevation gauges. The method is based on phased array methodologies, using delay-and-sum beamforming techniques with optimal noise attenuation. The method is developed for an arbitrary number...

This paper provides an in-depth analysis of the erosion at Nørlev Strand located on the Danish west coast. This location suffers from severe erosion; the result of a combined chronic, acute and, at least partially, man-induced erosion. The analysis is done by studying the average climatic conditions in the vicinity, and the effects of storms. Analy...

In the present paper, the experimental data on wave run-up on slender monopiles from recently published small and large scale tests are reanalyzed using different methods for the wave analysis. The hypothesis is that the post processing has an impact on the results, due to depth limited and highly nonlinear waves in many of the tests. Thus, the ide...

Existing active absorption systems do not take into account the spurious waves caused by the segmentation of the wavemaker. Thus, the theoretical estimated performance curves for oblique waves are only valid for infinitely narrow segments. In the present paper, it is demonstrated that by ignoring the spurious waves, an unstable system might be desi...

In the present paper, the performance of active absorption systems based on nearfield surface elevation measurements is studied. Based on linear wavemaker theory the performance of such systems can easily be calculated for linear waves. A recent study demonstrated that bound superharmonics in regular waves is also well absorbed by such system and h...

As part of the Commission's internal Low Carbon Energy Observatory (LCEO) project, the Joint Research Centre (JRC) is developing an inventory of Future Emerging Technologies (FET) relevant to energy supply. A key part of the LCEO initiative is the consultation of external experts, addressing both those with in-depth experience in specific fields an...

As part of the Commission's internal Low Carbon Energy Observatory (LCEO) project, the Joint Research Centre (JRC) is developing an inventory of Future Emerging Technologies (FET) relevant to energy supply. A key part of the LCEO initiative is the consultation of external experts, addressing both those with in-depth experience in specific fields an...

Knowledge of the incident and reflected waves present in laboratory experiments is a key issue in order to correctly assess the behaviour of the tested structure. Usual applied reflection separation algorithms are based on linear wave theory. These linear methods might result in unreliable estimates of the incident and reflected waves in case the w...

The present paper presents a new active absorption method for wavemakers in physical models based on digital filtering of signals from wave gauges in the nearfield. Such system is needed to maintain control of the waves when performing model tests where reflections from the models are significant. Similar systems have been described earlier, but in...

A new benchmark problem is proposed and evaluated targeting fluid related shape optimization problems, motivated by design related ocean engineering tasks. The analyzed test geometry is a bottom mounted, polygonal structure in a channel flow. The aim of the study is to analyze the effect of shape variations of the structure on the resulting horizon...

Most of today’s renewable energy systems rely heavily on investments as well as public financial support. This support is often given by means of a higher sales price for each kWh produced, i.e. feed-in tariffs (FITs), green certificates or Renewable Obligation Certificates (ROCs), or by a fixed amount, being a percentage of the construction costs...

Laboratory testing of structures placed in combined wave–current flows is a valuable source of information for the fulfillment of offshore engineering related tasks and the development of ocean energy devices. In recirculating wave–current flumes, one of the problems encountered during such experimental studies is the occurrence of undesirable curr...

This note describes the influence of wave spreading, directionality and local bathymetry on the efficiency of the Sea Slot Cone Generator (SSG) wave energy converter pilot plant in Kvitsøy, Norway. This is an overtopping device i.e. its efficiency is directly proportional to the overtopping flows into the three reservoirs the device has one on top...

An experimental large scale study on wave run-up generated loads on entrance platforms for offshore wind turbines was performed. The experiments were performed at Grosser Wellenkanal (GWK), Forschungszentrum Küste (FZK) in Hannover, Germany. The present paper deals with the run-up heights determined from high speed video recordings. Based on the me...

An extensive model test program has been carried out in order to assess the behavior of a tension leg moored substructure as support of a floating offshore wind turbine (FOWT). The floater was inspired by an industrial design. The tests focused on the ultimate limit state (ULS) behavior, therefore no aerodynamic or gyroscopic effects were included,...

CFD models are promising in predicting non-linear wave loads on fixed and floating offshore structures. The NS3 model is described in this paper and it has been validated by means of model test such as wave run-up on monopiles in regular waves. The goal for the use of the NS3 model is to make a detailed investigation of the effect of 2D waves on th...

During the first decade of the 21st Century the World faces spread concern for global warming caused by rise of green house gasses produced mainly by combustion of fossil fuels. Under this latest spin all renewable energies run parallel in order to achieve sustainable development. Among them wave energy has an unequivocal potential and technology i...

Short-term wave forecasting plays a crucial role for the control of a wave energy converter (WEC), in order to increase the energy harvest from the waves, as well as to increase its life time. In the paper it is shown how the surface elevation of the waves and the force acting on the WEC can be predicted using FIR filter. The predictors have been v...

Extreme sea states, which the IEC 61400-3 (2008) standard requires for the ultimate limit state (ULS) analysis of offshore wind turbines are derived to establish the design basis for the conceptual layout of deep water floating offshore wind turbine foundations in hurricane affected areas. Especially in the initial phase of floating foundation conc...

The relative density of backfilled soil material around offshore monopiles is assessed through experimental testing in the Large Wave Channel (GWK) of the Coastal Research Centre (FZK) in Hannover. The relative density of the backfill material was found to vary between 65 and 80 %. The dependency of the relative density of backfill on the maximum p...

To generate a substantial amount of power, Wave Energy Converters (WECs) are arranged in several rows or in a 'farm'. Both the power production and cost of a farm are lay-out dependent. In this paper, the wave power redistribution in and around three farm lay-outs in a near shore North Sea wave climate, is assessed numerically using a time-dependen...

The modelling of swash zone (SZ) sediment transport and the resulting morphodynamics have been areas of active research over the last decade. However, many details are still to be understood, whose knowledge will be greatly advanced by the collection of high-quality data under the controlled large-scale laboratory conditions. The research describes...

1 lm@civil.aau.dk 1 pf@civil.aau.dk 3 Vicky.Stratigaki@UGent.be The wave energy sector is in need of showing positive experience from the real sea trial in order to prove its feasibility. For this purpose, an accurate knowledge of wave conditions at the selected location of installation is fundamental. A design challenge for wave energy devices is...

Together with new opportunities, offshore wind farms raise new engineering challenges. An important aspect relates to the erosion of bottom material around the foundation of the wind turbines, caused by the local increase of the wave and current induced flow velocities by the pile's presence. Typically, the expected scour has a considerable impact...

Wave run-up on foundations is a very important factor in the design of entrance platforms for offshore wind turbines. When the Horns Reef 1 wind turbine park in Denmark was designed the vertical wave run-up phenomenon was not well known in the industry, hence not sufficiently considered in the design of Horns Reef 1. As a consequence damage was obs...

Aim of this note is to analyse the possible application of a Wave Energy Converter (WEC) as a combined tool to protect the coast and harvest energy. Physical model tests are used to evaluate wave transmission past a near-shore floating WEC of the wave activated body type, named DEXA. Efficiency and transmission characteristics are approximated to f...

The present paper deals with loads on wind turbine access platforms. The many planned new wind turbine parks together with the observed damages on platforms in several existing parks make the topic very important. The paper gives an overview of recently developed design formulae for different types of entrance platforms. Moreover, the paper present...

When building offshore wind turbines, a static scour protection is typically designed, allowing no movement of top layer stones under design conditions, leading to an often conservative design. Further more, little design guidelines exist for a scour protection around a monopile foundation under combined wave and current loading.In part 1 (De Vos e...

Wave loads are important in problems related to offshore structure, such as wave run-up, slamming. The computation of such wave problems are carried out by CFD models. This paper presents one model, NS3, which solve 3D Navier-Stokes equations and use Volume of Fluid (VOF) method to treat the free surface. NS3 is used to simulate the wave run-up due...

The performance of an array of closely spaced point absorbers is numerically assessed in a frequency domain model. Each point absorber is restricted to the heave mode and is assumed to have its own linear power take-off (PTO) system. Unidirectional irregular incident waves are considered, representing the wave climate at Westhinder on the Belgian C...

Oscillating point absorber buoys may rise out of the water and be subjected to bottom slamming upon re-entering the water. Numerical simulations are performed to estimate the power absorption, the impact velocities and the corresponding slamming forces for various slamming constraints. Three buoy shapes are considered: a hemisphere and two conical...

Wave Star is a Wave Energy device developed and installed in Denmark. The paper describes the scale 1:40 model installed in Nissum Bredning in 2006, and gives production figures from this installation. Further the Hanstholm Scale 1:2 model is described.

Wave power presents significant advantages with regard to other CO2-free energy sources, among which the predictability, high load factor and low visual and environmental impact stand out. Galicia, facing the Atlantic on the north-western corner of the Iberian Peninsula, is subjected to a very harsh wave climate; in this work its potential for ener...

The present study deals with numerical modelling of the Wave Star Energy WSE device. Hereby, linear potential theory is applied via a BEM code on the wave hydrodynamics exciting the floaters. Time and frequency domain solutions of the floater response are determined for regular and irregular seas. Furthermore, these results are used to estimate the...

The SSG (Sea Slot-cone Generator) is a wave energy converter of the overtopping type. The structure consists of a number of reservoirs one on the top of each other above the mean water level in which the water of incoming waves is stored temporary. In each reservoir, expressively designed low head hydro-turbines are converting the potential energy...

This paper discusses a new type of Wave Energy Converter (WEC) named Sea wave Slot-Cone Generator (SSG). The SSG is a WEC of the overtopping type. The structure consists of a number of reservoirs one on the top of each others above the mean water level in which the water of incoming waves is stored temporary. Using this method practically all waves...

The case described in this article regards 4.7 km of beach in Ferring, situated approx. 5 km south to the mouth of the Limfjord strait on the west coast of Denmark. In 2005, this section of the beach was nourished with 721,000 m 3 of material in order to protect backshore properties, natural heritages and the recreational value of the beach. After...

Any kind of Wave Energy Converter (WEC) requires information on how optimize the device in terms of hydraulic performances and structural responses. This paper presents results on wave loading acting on an innovative caisson breakwater for electricity production. The Seawave Slot-Cone Generator (SSG) concept is based on the known principle of overt...

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 spiderweb system. The dimensionless wave run-up value Ru2%Hm0 measured in Zeebrugge equals 1.77 for Iribarren number xi(om) = 3.63. An...

A clear difference between full-scale wave run-up measurements and small-scale model test results had been noticed during a MAST 11 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 h...

During the last decade, several offshore wind-farms were built and offshore wind energy promises to be a suitable alternative to provide green energy. However, there are still some engineering challenges in placing the foundations of offshore wind turbines. For example, wave run-up and wave impacts cause unexpected damage to boat landing facilities...

The Wave Dragon is an offshore wave energy converter of the overtopping type. It consists of two wave reflectors focusing the incoming waves towards a ramp, a reservoir for collecting the overtopping water and a number of hydro turbines for converting the pressure head into power.In the period from 1998 to 2001 extensive wave tank testing on a scal...

The study is reported by a number of independent reports. A brief description is given in the following. REPORT 1 - Prediction of scour - recommendations Candidate analytic equations are chosen and tested to available data. The objective is to provide recommendations for prediction of scour depth. REPORT 2 - Review of monopile scour This is a l...

Consider the use then of the authors’ equation (9) in predicting relative runup levels (Ru2%/Hm0) for a number of different armoured slopes subject to prototype waves varying from Hm0=1·0 m (as on a small inland reservoir), to Hm0=12 m (as on a rubble breakwater facing an open ocean). For the purpose of this analysis, wave steepness values between...

Wave Dragon is a floating wave energy converter working by extracting energy principally by means of overtopping of waves into a reservoir. A 1:4.5 scale prototype has been sea tested for 20 months. This paper presents results from testing, experiences gained and developments made during this extended period. The prototype is highly instrumented. T...

Seven sets of data for wave run-up on a rubble mound breakwater were combined and re-analysed, with full-scale, large-scale and small-scale model test results being taken into account. The dimensionless wave run-up value Ru2%/Hm0 was considered, where Ru2% is the wave run-up height exceeded by 2% of the wave run-up events and Hm0 is the significant...

The paper describes the methods used to estimate (calculated from some indirect measurements) the overtopping of the wave energy converter Wave Dragon placed in a real sea environment. The wave energy converter in quistion is the 237-tonne heavy Wave Dragon Nissum Bredning Prototype. Comparisons are made with laboratory measurements of the overtopp...

This 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 R...

Full scale wave run-up measurements have been performed on the Zeebrugge rubble mound breakwater. Wave run-up also has been investigated on various small scale models of the Zeebrugge breakwater. A significant difference between the results has been noticed. Additional small scale model testing has been carried out on a slightly modified scale mode...

The implementation and performance of a new active wave generating-absorbing boundary condition for a numerical model based on the Volume Of Fluid (VOF) method for tracking free surfaces is presented. This numerical boundary condition AWAVOF is based on an active wave absorption system that was first developed in the context of physical wave flume...

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 estima...

The Wave Dragon is an offshore wave energy converter of the overtopping type, utilizing a patented wave reflector design to focus the waves towards a ramp, and the overtopping is used for electricity production through a set of Kaplan/propeller hydro turbines. During the last 2 years, excessive design and testing has been performed on a scale 1:50...

The paper concerns horizontal wave forces on caisson breakwaters in multidirectional breaking seas. It is based on model tests performed in a 3D wave tank at Hydraulics and Coastal Engineering Laboratory, Aalborg University. The measured horizontal wave forces are compared to the Goda force. Good agreement with the Goda formula is found for waves n...

The paper concerns horizontal wave forces on caisson breakwaters in multidirectional breaking seas. It is based on model tests performed in a 3D wave tank at Hydraulics and Coastal Engineering Laboratory, Aalborg University. The measured horizontal wave forces are compared to the Goda force. Good agreement with the Goda formula is found for waves n...

A model for calculating the accumulated travelled stone distance is given based on the easy accessible parameters such Hs, Tp, θ, number of waves and Dn50. A comparison with selected prototype structures is presented showing the relation between breakwater damage and armour stone motion. The comparison also signifies that only little transport shou...

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...

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 sur...

The boundary element method is applied to the computation of irregular gravity waves. The boundary conditions at the open boundaries are obtained by a digital filtering technique, where surface elevations in front of the open boundary are filtered numerically yielding the velocity to be prescribed at the boundary. By numerical examples it is shown...

The paper deals with the 3-dimensional stability of the type of rubble mound breakwaters where reshaping of the mound due to wave action is foreseen in the design. Such breakwaters are commonly named sacrificial types and berm types. The latter is due to the relatively large volume of armour stones placed in a seaward berm. However, as also convent...