GEOLAB - Punch-Through Modelling in Monopile Installation

The global urgency to address climate change has accelerated the shift to renewable energy sources, with wind energy emerging as a sustainable alternative to fossil fuels. Offshore wind farms, supported by monopile foundations, are essential to meeting carbon reduction targets. However, as monopiles scale up to support larger turbines, they present engineering challenges, particularly in ensuring foundation stability during installation. A critical risk is "punch-through" failure, where soil bearing capacity is compromised at stratigraphic boundaries, potentially causing rapid monopile acceleration and structural damage. The objective of the GEOLAB project "Punch-Through Modelling in Monopile Installation" (PTM2I), based at the Deltares Geo-Centrifuge, is to advance the knowledge in wind farm construction. In particular, the research focuses on the installation of monopiles in soil profiles characterized by a shallow layer of stiff, coarse-grained soil underlain by a layer of soft clay. These ground conditions pose significant challenges due to the abrupt decrease in soil bearing capacity with depth, which can lead to the uncontrolled pile acceleration (i.e., pile-run).

The PTM2I project is based on a series of 15-g centrifuge tests, aimed at simulating the installation of a monopile in layered soils, with a very dense soil layer overlaying a soft or medium stiff clay layer. Given the low curvature of large monopiles, in the experiments, a metal plate was used to represent a small angular section of the pile. The experimental campaign contains several original aspects: (1) it assesses the effect of clay strength in the punch-through mechanism; (2) it provides insights into this failure mechanism, not only when penetration rate is imposed but also when the experiment is force-controlled, mimicking self-weight penetration; (3) it gives indication about potential phenomenological differences when the penetration is either monotonic or cyclic

Each centrifuge test included the in-flight execution of a CPTu test and the subsequent installation of two plates, one after the other. Two different soil materials were employed to simulate the ground: a densely packed sand layer (the Barskarp B15 sand, with Dr ~ 80%) overlying a clay layer (Vingerling K147clay). The clay layer in the strongbox was prepared by placing tightly packed “bricks” of clay, previously consolidated in oedometric conditions at two different preconsolidation stresses (40 and 200 kPa). This made possible to run the tests considering either a very soft clay layer or a medium-stiff clay layer. During the tests, displacement and velocity of each actuator (CPTu and plates) were monitored during penetration. Load cells were installed at each plate head.

Pore pressures in the clay layer were monitored by pore water pressure probes installed on the back wall of the strongbox, and in the permeable layer at its bottom. Both plates were equipped with total pressure and pore water pressure sensors at their tips, and on either lateral side, to monitor total stress changes during penetration.

The results of the experimental campaign provided original and significant insights into the mechanics of the pile/soil interaction during installation, under both displacement- and load-controlled jacking, monotonic and cyclic conditions and different stratigraphic conditions. These tests represent an invaluable source of reliable data to validate some advanced numerical methods such as MPM, Coupled Eulerian-Lagrangian (CEL) method and PFEM.

These advanced simulation tools could then be used in the quantitative assessment and mitigation of the risk of pile run, supporting industry in managing this important aspect of offshore wind turbines design.

The project results represent a very important contribution towards the achievement of the larger objective of advancing the knowledge of wind farm construction, particularly in addressing the challenges posed by the increasing size and weight of monopile foundations.

As industry continues to push the boundaries with larger and heavier monopiles, the insights acquired through this research not only ensure the safety of these energy projects but also strengthen their position as reliable, sustainable, and resilient components of our infrastructure system.