Offshore Engineering - Tidal Stream Generators

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.
 

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.
 

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.