Towards the Dynamic PressureMeter (T-DPM)

WHY

Monopile foundations are widely used to support offshore wind turbines. To design these foundations safely and efficiently, engineers must accurately determine the soil properties. Soil stiffness and damping strongly affect how a wind turbine vibrates in response to wind, waves, and currents.

In current design practice, the resonance frequency of wind turbines is poorly predicted. Measurements from wind farms show that the resonance frequency is often higher than predicted. A primary reason is uncertainty in the soil shear stiffness used in the design models. Because of this uncertainty, designers frequently use conservative assumptions, which can lead to using more steel than necessary. At the same time, incorrect predictions can also lead to unexpected vibration levels, which increase fatigue damage over the turbine’s lifetime.

Another issue is that resonance frequencies sometimes drop during storms and even during normal operating conditions. These changes are likely related to soil behaviour at larger strains (larger deformations). When loads are higher, the turbine moves more, and the soil experiences larger strains, reducing the stiffness. Current design methods often do not capture this behaviour well. Uncertainty in soil damping adds to the problem because damping controls vibration amplitudes and therefore fatigue loading.

Soil stiffness and damping can be studied in laboratory tests, but lab results often differ from real in-situ behaviour. This is because it is challenging to reproduce real field conditions, including stress levels and drainage. For sandy soils, samples may need to be rebuilt in the lab, which changes the original soil structure. Standard in-situ tests like CPT and SCPT can estimate stiffness, but they often focus on small-strain stiffness or depend on correlations, leading to significant uncertainty.

A pressure meter could be a better tool because it measures soil behaviour in the ground. However, existing pressure meters primarily apply static loading and cannot test the soil dynamically at frequencies similar to those of offshore wind turbine vibrations. The Dynamic PressureMeter (DPM) aims to solve this by measuring in situ soil stiffness and damping at the appropriate vibration frequencies and strain levels. This can lead to less conservative monopile designs, steel savings, and reduced fatigue risk. It may also reduce the need for additional in situ tests and extensive laboratory testing, saving time and cost.

WHAT

Future R&D in in-situ soil testing should focus on a self-boring Dynamic PressureMeter (DPM) rather than a cone-based tool, as self-boring operations minimise soil disturbance. This project is a first step towards a self-boring DPM by developing and proving the dynamic excitation unit.

The project will develop and validate a laboratory-scale prototype DPM unit capable of exciting soil at strain levels and oscillation frequencies representative of offshore wind turbine behaviour (target TRL 5). Development of the self-boring mechanism itself is out of scope, as this already exists and can be integrated at a later stage.

First, the DPM unit will be designed and constructed in-house at TU Delft, with careful selection of the excitation mechanism to achieve stable harmonic loading and with sensors chosen to measure both small and large displacements accurately. Second, the unit will be calibrated using a pipe with well-known elastic properties. Controlled radial deformation will be imposed at different excitation frequencies, measured directly, and compared with DPM readings to calibrate the device and quantify membrane effects.

Third, the unit will be validated through acrylic tank tests in well-characterised soils under disturbance-free “in-place” conditions. Using existing interpretation tools, strain-dependent shear stiffness and damping will be back-calculated from measured responses and compared with reference values.

EXPECTED RESULTS

The project will deliver three main results.

A lab-scale DPM unit will be designed and built at TU Delft, with a harmonic excitation mechanism suitable for future offshore operation (from a vessel or the seabed). Sensors will be selected to measure both small and large displacements, enabling soil properties to be determined across relevant strain levels.

The DPM unit will be calibrated using a pipe with known elastic properties. Radial deformation will be applied at different excitation frequencies and measured directly to calibrate the DPM response, while also quantifying membrane effects and verifying results with model predictions.

The DPM unit will be validated in acrylic tank tests using well-characterised soils. Existing interpretation tools will be used to back-calculate strain-dependent shear stiffness and damping and compare these with reference values, providing proof of concept for accurate dynamic soil characterisation.