Offshore Engineering - Tidal Stream Generators

Project Partners

The offshore engineering research group at the Chair of Structural Analysis is supported by the following project partners:

The long-term goal of establishing a worldwide economy based on renewable energy sources has led to a wide variety of innovative technologies in the past decades. Recently, the enormous energy potential of the world's oceans has been recognized as a promising addition to the traditional land based approaches in the renewable energy sector. The search to find efficient ways to harvest the oceans' energy has led to a particularly promising new technology: tidal stream generators. Similar to wind turbines, these structures convert the kinetic energy of a fluid (in this case streaming ocean water) into electrical energy. Particularly, their independence of weather conditions for power generation, as well as the possibility to precisely predict the oceans' tidal streams, have given this technology a special status amongst renewable energy technologies.

The realization of the structures is currently limited to a few prototypes. However, these have shown that tidal stream generators can contribute significantly to the world's power supply. Based on these experiences, efforts are being made to further optimize the structure in order to push the technology towards commercial use in sustainable large-scale projects. In particular, considerable progress concerning the stream turbine has already been made. Furthermore, innovative solutions for the foundations of the structures need to be developed, which take into account the unique environmental conditions and severe weather scenarios that can occur during the construction and operation of the generators. Using modern computational methods, it is possible to exploit the full optimization potential of the foundation, therefore significantly contributing to the efficiency of the overall structure. The challenge lies in finding new and creative solutions for the complex problems associated with offshore engineering, flow simulations, and shape optimization.

Project Overview

The Chair of Structural Analysis is working together with the Technology Center for Renewable Energy (Ed. Züblin AG) in order to research and develop modern turbine foundation concepts for ocean stream generators. As part of this project, a detailed evaluation of a prototype structure that has recently been constructed is carried out. The aim is to optimize and enhance the structure, making it suitable for large-scale offshore parks in the near future. One of the key elements of the research project is the analysis of the hydrodynamic behaviour of the structure. Using Computational Fluid Dynamics (CFD), detailed models are developed that incorporate the environmental conditions and build the basis of the structural evaluation. These simulations require location specific data, which are analyzed and incorporated into the numerical model as boundary conditions. The result is a numerical flow and wave channel that is specifically developed at the Chair of Structural Analysis in order to research the foundation structures. Using these tools, it is possible to carry out detailed structural analysis and to further develop the foundations using CFD-optimization techniques. Overall, the aim is to provide the necessary means to design an efficient structure that can be used in an economical realization of tidal stream generators.

Flow Simulation

In this project, modern CFD modeling methods are used in order to carry out realistic simulations of the highly turbulent fluid flow around the foundation. This allows for a detailed analysis of the effects of the flow on the structure. In the process, the incompressible Navier Stokes equations are solved using a Finite Volume discretization. The resulting numerical models require large inlet and outlet distances to the object, leading to a relatively large domain. Furthermore, a fine mesh resolution is needed in order to incorporate the turbulent characteristic of the flow, with Reynolds numbers of the order 10 million. This is of particular importance in the near wall regions of the model. As a result, extremely large computational costs ensue, which are reduced to a practical range using a Reynolds Averaged Navier Stokes approach (RANS). The boundary conditions for the velocity field and the turbulence parameters in the numerical flow channel are based on local data measured at the offshore site.

Wave Simulation

In order to incorporate the environmental conditions at an offshore site realistically into a numerical model, the wave loading on the structure needs to be considered in addition to the previously described tidal flow field. The structure is located well below the water surface even during neap tide conditions (the lowest tide). However, it has been found that the forces on the foundation caused by waves are nevertheless quite significant. This is due to the elliptical fluid particle paths that develop in the water column directly underneath the surface wave. The ensuing orbitals result in high velocity gradients, which in turn lead to high forces on the structure. In the numerical model, the wave kinematics is produced using a wave generator at the inlet, which generates a nonlinear wave profile using a stream function approach. The varying free surface resulting from the wave motion is realized using a multiphase model based on the Volume of Fluid method. A site specific wave height, wave period and the local water depth are used as input parameters for the simulation.

Shape Optimization

The optimization of structures subjected to a fluid flow is a common and widely researched topic in the automotive and aeronautics industry. However, the resulting streamlined bodies are generated under the assumption that the structure encounters a fluid flow with a one sided main direction. At typical locations of tidal stream generators, the periodic behaviour of the tides results in a complete reversal of the flow field direction twice a day. The result is a new optimization problem, which leads to geometries with form parameters that are not documented in standardized structural codes or tables. Using modern CFD optimization techniques, it is possible to find geometries that are specifically designed for tidal conditions, therefore unfolding the full potential of the structure. In this task, the Chair of Structural Analysis relies on a diverse and long wealth of experience.

Project Implementation

  • Daniel Markus