Perforation of Monopiles to Reduce Hydrodynamic Loads and Enable use in Deep Waters

Abstract

The offshore wind industry in Europe has experienced significant growth in the past decade, with wind farm development mostly focusing on the shallow area's in the North Sea. Naturally, the market is driven to reduce the Levelised Cost of Electricity (LCoE) to become more competitive with fossil fuels and less dependent on government subsidies. A transition in wind farm development towards deeper waters is expected and already observed in the market, driven by decreasing availability of shallow area's and higher wind resource at far offshore locations. The majority of the northern part of the North Sea is between 60 - 120 meter deep, currently the jacket foundation is deemed as the foundation of choice for this water depth range. Despite several technological advantages of the jacket, the main downsides are the large engineering effort and welds required to produce such a foundation resulting in difficult series production and high costs. This does not align with the industry's ambition to lower the LCoE. As such, the need for a technologically viable and economically attractive foundation concept for waters between 60 - 120 meter deep arises.

The goal of this research can be divided in to two parts. The first part is to determine the potential of conventional monopiles in this water range and identify the main limiting factors. To do so, a monopile is dimensioned at a selected reference location for three turbines representing the current, near future and future outlook of the market. The designed monopiles are tested for manufacturabiliy, Ultimate Limit State (ULS) and Fatigue Limit State (FLS) to identify the technical showstoppers. Next, in the second part, a novel monopile design is introduced and analysed to work around the identified limits.

To dimension the monopiles for the three reference turbines a parametric dimensioning script is developed. The monopile geometry is dimensioned to have a selected first natural frequency of 0.20 Hz, based on the relevant frequency diagrams. Next, these geometries are tested against mudline ULS failure for the power production and parked condition load cases. Hereafter, an FLS check for the B1, C1 and D S-N curves is conducted based on the obtained scatter tables for site conditions. It was found that D-curve fatigue damage for non grinded butt welds is the main limiting factor for all dimensioned monopiles in deep water. However, industry experts believe that all welds can be grinded in the production process, eradicating the need to assess the D-curve. When assuming this statement to be true the newly obtained limits become ULS failure during parked conditions for the 15 MW reference turbine and manufacturability constraints for the 20 MW reference turbine. The Haliade X showed no limits within the specified water depth range when neglecting the D curve fatigue damage.

A perforated monopile concept with reduced available area for wave loading is introduced. A Computational Fluid Dynamics (CFD) model based on the 2003 Menter Shear Stress Transport turbulence model is constructed for a perforated monopile to gain insights into how waves propagate through the structure and the forces associated with this. The CFD model is verified against experimental wave flume data before being used for further analysis showing a root mean square error of 0.0192 between model results and experiments. The CFD model is used to assess three geometries with different perforations and levels of porosity. No increased drag around the first natural frequency caused by the perforations, hinting to favourable dynamics, was found in any of the test cases. As such, the dynamic response of the three perforated monopiles was found to be unchanged when compared to a reference pile without perforations. Despite this, a significant reduction of lifetime fatigue damage was observed caused by the reduced forces acting on the structure resulting from the smaller frontal surface. Next, the mudline stresses are recalculated and a structural finite element model to assess the stress concentrations around the perforations is set up to verify the maximum allowable stress level threshold is not exceeded. A geometry was found which shows a 35.5% reduction of lifetime fatigue damage whilst stresses remain below the maximum threshold, hereby showing the potential of the perforated monopile. Implementing this perforation allows the use of monopiles up to 87 meter deep, limited by D curve fatigue. Reference piles without perforations were found to be infeasible for all assessed water depths, also limited by D curve fatigue. It is shown that the perforation concept can either be implemented to push the monopile foundation to deeper waters, or can be used to realise steel reduction at current water depths.