36 / 2025-04-24 11:27:43

Performance of a collar structure as scour protection: a comparison of CFD and physical model test results

Scour protection around offshore foundations typically consists of quarry rock. The flourishing offshore wind sector is leading to an increased demand in scour protections. However, loose rock scour protections are becoming more costly due to limited availability of both rock installation vessel and good quality rock in some parts of the world. Therefore, the need for alternative scour protection solutions is increasing.

Flow-altering structures, which are widely applied in bridge engineering, are a form of scour mitigation measure. These can lead to scour depth reduction by decreasing the bed shear stress in the pile amplification zone via breaking the horseshoe vortex and protecting the seabed against erosion due to the downflow at the upstream pile face. Flow-altering structures may be manufactured from materials other than rock (such as steel, concrete or composites), and can be installed with a wide range of offshore installation vessels, which makes them a suitable alternative scour protection solution to rock.

In this study, the effectiveness of a novel flow-altering collar design for scour reduction around monopiles is assessed by means of CFD modelling (model coupled with sediment transport), and physical model testing. The physical model tests provide observations into the scour development around the collar structure and data for validation, while the numerical simulations create detailed insights into local flow characteristics and resulting shear stresses. The CFD simulations have been performed using the 3D CFD flow solver TUDflow3D. The model uses a structured grid and an immersed boundary method for modelling the influence of the structure. Particular attention has been paid to ensure that the flow around the collar is captured accurately. Furthermore, the model uses morphological acceleration which allows the use of the CFD model for engineering investigations at a reasonable computational time (De Wit et al., 2023).

A comparison between first simulation results and preliminary physical model tests shows that the scour simulation predictions are in good agreement with the scour development around a monopile foundation in the presence of the collar (see Figure 1). Additional physical model tests are conducted to investigate the effects of design characteristics on the collar performance and to enable further validation of the CFD results. This combined approach allows us to optimize the flow-altering collar design and, by using the validated numerical model, to make scour predictions on the field-scale situation prior to installation.
De Wit, L., Plenker, D., & Broekema, Y. (2023). 3D CFD LES process-based scour simulations with morphological acceleration. Paper presented at the 11th International Conference on Scour and Erosion (ICSE-11), Copenhagen, Denmark.