WP5 – Design framework for offshore foundations in weak rock

Work Package 5 develops a simplified but robust design framework for offshore foundations in weak rock. Building on the results from site investigation, laboratory testing and numerical modelling, the aim is to create a practical workflow that links ground characterisation more directly to engineering design and foundation sizing. The ambition is to improve transparency, consistency and efficiency in how offshore weak rock conditions are handled in design, while still retaining the most important physical mechanisms.

A central part of WP5 is to account for how weak rock behaviour may change during installation and under repeated environmental loading. This includes cyclic effects, post-installation response, stiffness and strength development, and the impact of installation method on later foundation behaviour. The work package also explores whether detailed rock behaviour can be represented in a simpler way through spring-based models (e.g. Figure 1) or macro-element formulations, so that the essential soil–structure interaction mechanisms can be captured in a format better suited for engineering design workflows.

Cyclic behaviour is incorporated using the NGI cyclic methodology, where contour diagrams relate load amplitude, number of cycles, and mean load to degradation in stiffness and strength.

The design methodology is formulated to address both ultimate and serviceability limit states. At the ultimate limit state (ULS), the focus is on ensuring adequate capacity to resist extreme loading without failure. At the serviceability limit state (SLS), the emphasis is on controlling displacements and rotations to maintain acceptable performance throughout the operational life of the structure.

 The framework builds directly on the outcomes of WP4, including numerical modelling of laboratory and field tests, CPT-based simulations for ground characterisation, and parametric studies used to generalise behaviour across a range of conditions. These inputs are synthesised into simplified design curves and spring formulations, enabling practical and efficient application in engineering design.

WP5 also supports the development of guidance for weak rock grading, cyclic design assessment and installation effects. The final goal is a streamlined, engineering-ready design methodology that connects site investigation, interpretation, modelling assumptions and foundation sizing into one coherent framework. This ensures a consistent and efficient approach to the design of monopile foundations in weak rock, balancing accuracy with practical usability. This is intended to support both early-stage project screening and more detailed design of offshore foundations in shallow weak rock conditions.