Offshore wind turbine foundations are often subject to scour erosion processes. If the risk of scour is considered high, a layer of rocks is placed on the seafloor around the monopile. In the North Sea, many scour protections for monopile foundations are two-layer configurations consisting of a protective armour layer and an underlying filter layer.
The current practice is to place the protection layers in two separate operations. The so-called 3-step scour protection construction consists of:
The Rock Dumping Vessel (RDV) must therefore visit each wind turbine location twice to install the scour protection.
For reasons of time, cost, efficiency and logistics, it is very advantageous to use a 2-step approach, i.e. to place the entire scour protection on the seabed during one visit of the RDV and install the monopile afterwards. This ensures significant cost savings, fewer transport movements and a smaller carbon footprint of the entire operation.
To exploit these benefits, it is essential to verify whether the pile will successfully penetrate the scour protection into the underlying seabed under ‘self-weight penetration’, i.e. without applying an external force to the monopile. At present, it is still difficult to accurately predict the technical feasibility and required force of such penetration. The main reason is the lack of knowledge about the behaviour of the rocks during self-weight penetration. We need to understand better the influence of rock grading, shape and strength, and the interaction with the underlying seabed. Furthermore, local point-load reactions to the monopile can cause deformation of the penetrating monopile, such as buckling, ovalisation, and damage or failure of the pile liner.
In OPIS, we aim to better understand pile penetration through scour protection. This enables an optimised and less risky installation of monopiles. Therefore, we need to improve our understanding of the driving mechanism and the response of the scour protection layer under a range of different conditions, such as subsurface conditions, rock size and shape, dimensions of the monopile, and scrour protection properties.
In OPIS, we perform a series of small-scale laboratory tests to improve the understanding of an object penetrating a granular medium under different conditions. In the second step, we will execute medium-scale laboratory tests to calibrate and verify numerical models based on the discrete element method (DEM) and finite element method (FEM). The combination of laboratory-scale testing and numerical modelling will allow for a better understanding of the pile penetration process and investigation of possible optimisation measures.
If self-weight penetration fails, impact or vibrating pile driving may be required to install the monopile finally. Therefore, part of the research is focussed on improving better understand of the impact of such technologies on scour protection and pile integrity.
In OPIS, the datasets from small and medium size experiments up to scale 1:4 and information from the partners’ field experience on pile penetration through scour protection are one of the main results. In addition, we will place selected data from the lab and field in a structured database.
The project will also deliver an empirical/analytical model. The model predicts the maximum penetration force required for self-weight pile penetration for a given scour protection layout, e.g. grading of the scour protection and monopile dimensions. This is delivered as an Excel spreadsheet or as a (Python) script.
In addition, OPIS will provide a validated simulation environment to explore Innovative solutions that: