Jorgen Ranum Krokstad

• Master, PhD, Professor
• Norwegian University of Science and Technology

This paper presents a comparison between experimental data of a model-scale 4 MW monopile offshore wind turbine subjected to extreme irregular sea states in finite water and the numerical models suggested in offshore wind energy standards to assess ULS conditions. The model is fully flexible with its 1st and 2nd eigenfrequencies and 1st mode shape...

In this paper, a joint distribution of all relevant environmental parameters used in design of offshore structures including directional components is presented, along with a novel procedure for dependency modelling between wind and wind sea. Probabilistic directional models are rarely used for response calculation and reliability assessments of st...

The fatigue damage sensitivity to wave directionality on large-diameter monopile foundations for use in the offshore wind turbine industry is investigated. A frequency-domain approach with the Dirlik method is used to estimate fatigue damage and to evaluate the effect of wave spreading and swell separation compared to a total sea representation. To...

We present experimental data from MARIN on a bottom-fixed offshore wind turbine mounted on a monopile in intermediate water depth subjected to severe irregular wave conditions. Two models are analysed: the first model is fully flexible and its 1st and 2nd eigenfrequencies and 1st mode shape are representative of those of a full-scale turbine. This...

The design process for offshore wind turbines includes a fatigue life evaluation of the structure with the relevant environmental conditions at the specified wind farm location. Such analyses require long-term distributions of the environmental parameters including their correlation. In general, the significant wave height, wave peak period and mea...

This paper presents the results of an experiment carried out in a wave flume aiming at reproducing a 50-year wave condition on an extra-large bottom-fixed offshore wind turbine mounted on a monopile. The model is a stiff cylinder mounted on a spring allowing rotation of the system around its base only in the wave propagation direction. Under these...

Loads from extreme waves can be dimensioning for the substructures of offshore wind turbines. The DeRisk project (2015-2019) aims at an improved load evaluation procedure for extreme waves through application of advanced wave models, laboratory tests of load effects, development of hydrodynamic load models, aero-elastic response calculations and st...

The present work identifies the qualitative hydro-elastic contributions to fatigue damage on large volume monopiles intended for use in the offshore wind energy industry. Although aerodynamic effects cannot be neglected in a complete dynamic analysis of the structure, the scope of this work is limited to wave loads and soil effects on a turbine in...

This work reviews some of the main tools used for designing a measurement campaign on a multi megawatt scale wind turbine. The aim of the measurements is to contribute to the qualification of new large turbine technology, assess the fatigue life of wind turbine components, as well as to validate simulation tools. Due to the size of the wind turbine...

In order to provide a benchmark for the income potential of wave energy converters, the project described in this document was carried out in order to estimate and analyse the energy conversion for a representative selection of converter designs. Eight different wave energy converter designs and their operation in five different locations in Europe...

This paper describes an efficient Monte Carlo based method for prediction of extreme response statistics of fixed offshore structures subjected to random seas. The nonlinear structural response known as "ringing" is studied, which is caused by the wave impact force on structural support units. Common challenge for design of such structures is a sou...

The aim of this study is to estimate the mean annual power absorption of a selection of eight Wave Energy Converters (WECs) with different working principles. Based on these estimates a set of power performance measures that can be related to costs are derived. These are the absorbed energy per characteristic mass [kWh/kg], per characteristic surfa...

In this study, a selection of Wave Energy Converters (WECs) with different working principle is considered. It comprises a heaving device reacting against the seabed, a heaving self-reacting two-bodies device, a pitching device, and a floating OWC device. They are inspired by concepts which are currently under development. For each of these concept...

The monopile is the dominating substructure concept used for offshore wind turbines. Offshore wind farms have so far been built at a depth of up to 24m. As the importance of wave loads increase when deeper waters are considered for wind farms, it becomes increasingly important to correctly and efficiently include wave loading in structural analysis...

Floating production systems are often exposed to combined waves and current. Normally, the effects from a steady current are neglected in the calculation methods for dynamic loads and motions. In some parts of the world the current velocities can be so large that neglecting wave-current interaction effects is questionable. Computer programs that ha...

Floating production systems are often exposed to combined waves and current. Normally, the effects from a steady current are neglected in the calculation methods for dynamic loads and motions. In some parts of the world the current velocities can be so large that neglecting wave-current interaction effects is questionable. Computer programs that ha...

Introduction The fish farming and aquaculture industry are expanding and the demand for suitable locations for fish farms is increasing. In the future more of the fish farms will be located offshore, as the number of suitable near-shore locations are limited. A move towards the use of offshore locations are also motivated from environmental and aes...

The roll motion of a yacht at zero Froude number is investigated, motivated by limitations of existing theoretical models of roll motion when applied to bodies with large appendages. A time domain single degree of freedom roll model has been developed in order to identify the dominant excitation and damping sources. The canoe body forces were deter...

The fish farming and aquaculture industry are expanding and the demand for suitable locations for fish farms is increasing. In Norway, as well as in other countries, this calls for new technological challenges, as fish farms are being installed at locations that are more exposed to waves, wind and current. In the future more of the fish farms will...

investigated, motivated by limitations of existing theoretical models of roll motion when applied to bodies with large appendages. A time domain single degree of freedom roll model has been developed in order to identify the dominant excitation and damping sources. The canoe body forces were determined from a wave diffraction program whilst the app...

This paper details an experimental investigation of wave run-up on a fixed vertical surface piercing circular cylinder. The study focuses on two important parameters, wave steepness and body slenderness, which are shown to influence the wave run-up. The wave steepness, kA, is varied from 0.041 - 0.284, and the body slenderness parameter, ka, is var...

In this paper, we study the interaction of gravitational water waves with stationary ocean structures near a vertical boundary or in a wave channel. The water depth is assumed finite implying that the interference effect of not only the vertical boundaries but also the horizontal bottom is considered. Presented here are a potential flow theory and...

In this paper, we study the interaction of gravitational water waves with stationary ocean structures near a vertical boundary or in a wave channel. The water depth is assumed finite implying that the interference effect of not only the vertical boundaries but also the horizontal bottom is considered. Presented here are a potential flow theory and...

New technical challenges arise when moving to deeper water locations for oil exploration. In order to meet some of these challenges, Det Norske Veritas and Marintek are jointly carrying out a three year research project on Deep Water Analysis Tools - DEEPER under the auspices of the Royal Norwegian Council for Scientific and Industrial Research. Th...

New results from the most recent work within the Norwegian Joint Industry Project (JIP) "Higher Order Wave Load Effects on Large Volume Structures" are presented. A nonslender theoretical model is validated from experiments for two fixed, vertical cylinders with different diameter/peak wavelength ratios. A combination of complete diffraction first-...