Scour phenomenon

Simulation of scour processes at the founding structures of FINO3 and FINO1

Forschungs- und Entwicklungszentrum Fachhochschule Kiel GmbH
Prof. Dr.-Ing. Kai Graf

The project deals with the establishment of offshore wind turbines and the important phenomenon of scour in this context. The basis for a safe but also economic design of every offshore wind turbine is the correct dimensioning of the foundation and the structure. An important factor is the scour to be taken into account. This refers to the leaching of sediment at the bottom of the water by the flow around the structure. Scour significantly weakens the founding of an offshore structure.

The aim of the project is to implement forecasting methods from recent research carried out by internationally leading institutes and thus to model and simulate the scour process on the research platforms FINO1 and FINO3. For these research platforms already extensive measurements of scour phenomenon are available.

Schleswig-Holstein's Ministry for Energy Transition, Agriculture, Environment and Rural Areas is funding the project with state funding in accordance with the Directive for the Granting of Research, Development and Technology Transfer (FIT) by the end of July 2017.

The expected scour can hardly be predicted realistic. Therefore, scour prevention has been the subject of numerous studies due to its relevance in recent years. However, this has not yet led to generally accepted formulas or calculation methods. Even more elaborate prognosis methods are in part not insignificant orders of magnitude in addition to the scour determined in measurements.

The method to be implemented requires as input variables quantitative information on the sea conditions, usually described by the sea state energy spectrum. In order to validate the procedure, the sea state spectrum should be measured. A comparison of sea state and scour measurement on the one hand and simulation results on the other hand thus allows to examine the modeling of the process and, where appropriate, to improve and thus come to an accurate forecasting method. Approaches to improving the digested models are improved flow modeling and improved sediment convection. By generalizing the method, it can then also be used for newly designed offshore structures and generalized sea state spectra, which will lead to a significantly improved prognosis of the scour process and thus a more economical foundation design. In this context increasing the computational efficiency is of particular importance, since flow simulations taking into account free surface flow are generally computational intensive. This goal has been achieved by implementing numerical schemes which allow larger time steps in a time-stepping transient calculation method.

Within the project a flow simulation method has been developed which is based on the simulation of viscous turbulent flow, which allows to take into account the free water surface as well as the sediment motion. The method employs three flow faces, water, air and sediment. For the sediment a particular approach has been chosen taking into account the properties of the sediment. The efficiency of the method is enhanced by implementing some high resolution discretization schemes for the convection of the sediment, air and water concentration. These schemes allow larger time step sizes than possible so far in the transient time marching solution method.