Overdick

Design of large diameter monopile wind turbine foundation structures

Introduction

Decreasing subsidies and increasing competition in the offshore wind sector require cost and time efficient engineering approaches. The ability to combine several engineering disciplines and create an integrated design of wind turbine foundation and transition piece structures in every design stage is considered as a major benefit for project schedule, cost monitoring and quality management.

Large Diameter Monopiles

The latest developments in offshore wind turbines support and foundations call for large diameter monopile structures. These could be used in water depths up to 40 meters, depending on metocean and soil properties. The efficient design of these structures requires the highest engineering standards while keeping the project costs on an economically low level.

The foundation pile diameters are currently reaching up to 8.5 m. With these large diameters, the traditional design tool box is reaching its limits. This requires a re-thinking of the applicable tools to create a safe and reliable, yet economical and efficient monopile design.

A variety of advanced and problem-dedicated design and calculation tools enable Overdick’s engineers to address challenges linked with hydrodynamic loads in extreme waves, cyclic loading effects, fatigue assessment, pile driving and pile handling. Experience has taught us that, some design details such as flange connections and the loadings on the monopile appendices, have a significant influence on the overall performance of monopiles. OV provides all in-house know-how to properly include these design details at an early project stage and throughout the project by employing advanced simulation techniques.

Load Assessment on Monopile appendices

Due to maintenance and access requirements, the monopiles are equipped with several appendages like boat landings or service platforms. Service platforms are usually located above the highest expected wave crest. However, very large waves approaching the monopile will run-up the pile and create comparatively high loads on the appendages. These loads may further become the governing criteria for the structural design of the appendages.

Usually these wave run-up loads are determined based on empiric formulations which, due to their approximate nature, yield rather high loads. As an alternative, loads on these structures can also be determined using advanced methods, in this case viscous 3D multiphase CFD simulations (see figures below).
Due to the constantly-developing computational power and software tools, the effort for such a simulation today has become rather small and should be considered as standard load determination for deriving optimized design loads. Experience at Overdick has shown that by the use of such methods the resulting design can be significantly optimized.

Advanced Engineering as Key for Optimization and Cost Savings

Large diameter monopile design may require new approaches on engineering tools and verification methods. Advanced simulation technologies allow a direct load simulation and offer optimization and associated cost saving potential for this new type of offshore foundation structures.

Overdick is capable of covering all relevant disciplines for the design of large monopile foundations, using the latest and advanced design methods in combination with more than 17 years of offshore structural design experience.

Figure 1: CFD Simulation of large MP in design wave with wave run-up on to service platform
Figure 2: RANSE CFD Simulation of WTG foundation in design waves
Figure 3: Pressure distribution on MP in design wave
Figure 4: FEM Detailed Analysis of MP appendices

See also: Offshore Wind Projects