Tag Archives: Foundation and Mooring

Computationally efficient modelling of dynamic soil-structure interaction of offshore wind turbines on gravity footings


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M. Damgaard, L.V. Andersen, and L.B. Ibsen – Renewable Energy, August 2014

Abstract

The formulation and quality of a computationally efficient model of offshore wind turbine surface foundations are examined. The aim is to establish a model, workable in the frequency and time domain, that can be applied in aeroelastic codes for fast and reliable evaluation of the dynamic structural response of wind turbines, in which the geometrical dissipation related to wave propagation into the subsoil is included. Based on the optimal order of a consistent lumped-parameter model obtained by the domain-transformation method and a weighted least-squares technique, the dynamic vibration response of a 5.0 MW offshore wind turbine is evaluated for different stratifications, environmental conditions and foundation geometries by the aeroelastic nonlinear multi-body code HAWC2. Analyses show that a consistent lumped-parameter model with three to five internal degrees of freedom per displacement or rotation of the foundation is necessary in order to obtain an accurate prediction of the foundation response in the frequency and time domain. In addition, the required static bearing capacity of surface foundations leads to fore–aft vibrations during normal operation of a wind turbine that are insensitive to wave propagating in the subsoil—even for soil stratifications with low cut-in frequencies. In this regard, utilising discrete second-order models for the physical interpretation of a rational filter puts special demands on the Newmark β-scheme, where the time integration in most cases only provides a causal response for constant acceleration within each time step.

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Filed under Modeling, Wind

Study on the marine growth effect on the dynamic response of offshore wind turbines


W. Shi, H-C. Park, J-H. Baek, C-W. Kim, Y-C. Kim, and H-K. Shin – International Journal or Precision Engineering and Manufacturing, 2012

Abstract

Jacket foundation is regarded as a suitable solution for wind turbines in the intermediate water depths ranging from 30 to 80 m. Numerous types of marine fouling organisms may be found on the submerged members of jacket. Marine growth is found to influence loading of offshore structures by increasing tube diameters, drag coefficient, mass and hydrodynamic added mass and structural weight. In this paper, the types and distribution of marine growth are mentioned. We aimed to investigate the effects of marine growth with different thicknesses, densities and hydrodynamic coefficients values on characteristics of an offshore wind turbine with jacket foundation. The eigen analysis shows that the marine growth has a little effect on the first order natural frequencies while it has higher effect on second and third order natural frequencies of the support structure. Thickness and density have a strong effect on hydrodynamic loads. To obtain the properties of marine growth and hydrodynamic coefficients in the design of offshore wind turbines, full-scale measurements are needed.

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Fatigue life prediction of mooring chains for a floating tidal current power station


F. Jing, L. Zhang, and Z. Yang, Journal of Marine Science and Application, 2012

Abstract

As a kind of clean and renewable energy, tidal current energy is becoming increasingly popular all over the world with the shortage of energy and environmental problems becoming more and more severe. A floating tidal current power station is a typical type of tidal current power transformers which can sustain the loads of wind, waves, and current, and even the extreme situation of a typhoon. Therefore, the mooring system must be reliable enough to keep the station operating normally and to survive in extreme situations. The power station examined in this paper was installed at a depth of 40 m. A 44 mm-diameter R4-RQ4 chain was chosen, with a 2 147 kN minimum break strength and 50 kN pretension. Common studless link chain was used in this paper. Based on the Miner fatigue cumulative damage rule, S-N curves of chains, and MOSES software, a highly reliable mooring system was designed and analyzed. The calculation results show that the mooring system designed is reliable throughout a 10-year period. It can completely meet the design requirements of American Petroleum institution (API). Therefore, the presented research is significant for advancing the design of this kind of power station.

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Filed under Component Development

FSI analysis of deformation along offshore pile structure for tidal current power


CH Jo, DY Kim, YH Rho, KH Lee, C Johnstone – Renewable Energy, August 2012

Abstract
Due to global warming, the need to secure an alternative clean energy resource has become an international issue. Tidal current power is now recognized as one of the clean power resources in Korea, where there are many strong current regions on the west and south coasts. Recently, large scale tidal devices have been deployed with a maximum rotor diameter of 18 m. These devices impose significant loading on supporting structures. In many cases, a pile fixed foundation is used to secure the structure. However, due to the high density of seawater, the drag and lift forces are much larger than in air, causing extensive stress and deflection to the pile tower structure. In this study, a numerical analysis of the hydro-forces from a rotating tidal current turbine to a tower was conducted to determine the deformation distribution along the pile tower.

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Analysis and optimization of a tethered wave energy converter in irregular waves


EE Bachynski, YL Young, RW Yeung- Renewable Energy, December 2012

Highlights
► Fundamental dynamic response of moored wave energy converters in 3 degrees of freedom. ► Semi-analytical method to maximize energy extraction and ensure structural safety. ► Important to tune resonance frequencies to site-specific probabilistic wave climate. ► Mooring lines introduce additional low-frequency resonance. ► Low center of gravity and low radius of gyration minimize coupled pitch-surge motion.

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Filed under Modeling

Analysis and optimization of a tethered wave energy converter in irregular waves


EE Bachynski, YL Young, RW Yeung- Renewable Energy, December 2012

Highlights
► Fundamental dynamic response of moored wave energy converters in 3 degrees of freedom. ► Semi-analytical method to maximize energy extraction and ensure structural safety. ► Important to tune resonance frequencies to site-specific probabilistic wave climate. ► Mooring lines introduce additional low-frequency resonance. ► Low center of gravity and low radius of gyration minimize coupled pitch-surge motion.

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Filed under Component Development, Modeling

Disruption to benthic habitats by moorings of wave energy installations: A modelling case study and implications for overall ecosystem functioning


V Krivtsov, B Linfoot – Ecological Modeling, April 2012

Highlights
► OrcaFlex and a Matlab script was used to simulate a WEC and the resulting disturbance of bottom sediments. ► Simulations were performed for regular and irregular wave regimes. ► This is the first published estimate of the affected area of benthic habitats. ► Sediment erosion by mooring lines will effect a whole range of ecosystem processes. ► These issues are important in calculations of ecological risks of any moored objects.

 

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Filed under Modeling, Standards and Protocols