- Reducing cost in Offshore Wind by integrated structural and geotechnical design.
Development of offshore wind is crucial for a future without fossil fuels. The production of this renewable energy increases every year. However, overall costs need to be reduced to make offshore wind competitive. The REDWIN-project was initiated to investigate improvements in economical offshore wind energy solutions.
Offshore wind turbines are designed and constructed in several different ways according to wind and wave conditions, sea currents, depths and soil conditions. The dynamics of structures is heavily influenced by the stiffness of the foundation. This stiffness is not constant but influenced by the movement of the structure. However, the behaviour of foundations could not be included in structural analyses in a realistic way, no matter how detailed and accurate it's description by geotechnical engineers. No comprehensive computational tools that could integrate the two areas existed.
REDWIN brought together experts from the two engineering fields, to jointly develop better computational methods. The aim was to design new models describing soils and foundations that will be integrated with the computational tools used by structural engineers today. This will improve the reliability in the analyses and design, and make it possible to optimize the structures. In turn, this optimization gives less costly design thus reducing the cost of energy. Hence the project title, REDWIN - REDucing cost in offshore WINd by integrated structural and geotechnical design. The models developed in REDWIN capture almost all foundation types and foundation behaviour such as non-linear stiffness, coupling between load components, material damping, and the effects of cyclic loading.
REDWIN was headed by NGI (Norwegian Geotechnical Institute). Other partners were NTNU (Norwegian University of Science and Technology), IFE (Institute for Energy Technology), Dr.Techn. Olav Olsen, Statoil and Vattenfall. The project was launched in 2015 and completed at the end of 2018.
- Redwin has contributed to a significant improvement of state-of-the-art geotechnical modelling capabilities for the soil and foundations for OWTs.
- Redwin has helped remove barriers between structural and geotechnical engineers, and represents an important step towards integrating the geotechnical discipline into a streamlined OWT design process.
- The project results will enable lowering costs for future OWT foundations, and will be an important tool for evaluating potential for lifetime extension of OWT farms.
- The most significant output is a library of new soil-foundation models for time-domain dynamic analysis. These overcome several of the limitations in existing design tools, and allow designers to adopt advanced foundation models in OWT design.
- Another important output is the improved understanding of soil and foundation damping, and the contribution from foundation to the overall energy dissipation in the OWT-foundation system.
Offshore wind turbine (OWT) has grown to be a significant renewable energy source during the last decade. However, there is a consensus among the operators that for the offshore wind energy to be long-term sustainable, the cost of wind farms has to be reduced. Cost reduction can be achieved in several ways. Optimal foundation solutions and steel tonnage are two aspects with significant potential.
Current practice of accounting for soil-structure interaction in the analyses of OWT is based on conservative approaches and assumptions. Most of these approaches have been borrowed from other disciplines and are often not validated against actual measurement of OWTs. A key issue is damping which is still unresolved. Recent measurements of an OWT project have indicated large errors in estimation of the soil stiffness. Both these provide examples of parameters that have significant impact on the dynamic response of the structure. A major weakness of most of the existing soil/foundation models is that they are decoupled from the structural model which prevent effective and optimal analyses of the complete system.
The objective of this research is to develop a library of soil-interaction models which will account for the key parameters mentioned above and integrate them in the state-of art hydrodynamic-structural numerical model code 3DFloat. The combined code will be validated and calibrated against actual and model test data of OWTs. In addition, simpler models based on the practical concepts used in the industry will be developed and tested against the advanced models. A final outcome of the project is to conclude on possible cost savings from design using the new soil interaction models developed within the project coupled with integrated analyses tools.
The project will involve research institutes (NGI and IFE), university (NTNU), engineering firm (Olav Olsen) and two major international industrial partners (Statoil and Statkraft).