Probabilistic modelling of wave Run-Up loads on offshore monopile platforms (Prob-RunUp)

WHY

In the North Sea, monopiles remain the preferred foundation type and are typically equipped with secondary steel structures such as access platforms and ladders. These platforms, often located above still water level, are exposed to extreme wave impacts that pose a significant risk to structural integrity and operational continuity.

This risk is particularly critical for monopile structures with large diameters, which obstruct wave propagation and induce significant vertical wave run-up. As a result, water levels can rise along the pile and impact horizontal platforms from below, even when the incident wave crest remains below platform level. Such impacts can generate severe loading conditions, potentially leading to structural damage, costly repairs, and increased downtime.

Despite its importance, the physical behaviour of wave run-up and its associated loads remain insufficiently understood. Current design practices rely heavily on conservative assumptions to account for this uncertainty. While this approach ensures safety, it also results in oversized structures, increased material usage, and higher capital and operational expenditure for offshore wind projects. As monopile diameters continue to increase for larger turbines, these uncertainties—and their cost implications—are expected to grow.

Existing models and empirical formulations struggle to predict wave run-up accurately. The phenomenon is highly non-linear and influenced by multiple interacting factors, including wave characteristics, monopile geometry, and the location of wave breaking. Observations show that waves with similar parameters can produce significantly different run-up heights and impact forces, highlighting the limitations of deterministic design approaches currently used in industry standards.

Ongoing research efforts, such as the Run-Up project initiated by Deltares, Van Oord, and Vattenfall, are addressing these challenges by improving deterministic modelling through laboratory experiments and numerical simulations. However, there is an increasing need to move beyond deterministic methods towards probabilistic approaches that explicitly account for uncertainty and variability.

In this context, innovation is essential. Developing probabilistic methods to relate environmental conditions to the likelihood of extreme run-up loads enables more accurate, risk-based design. This shift will allow engineers to optimise structures, reduce unnecessary conservatism, and ensure safe, certifiable designs. Ultimately, advancing the understanding and modelling of wave run-up is crucial for enabling efficient and scalable offshore wind deployment in increasingly challenging environments.

WHAT

The project aims to develop and validate a probabilistic framework that links wave conditions to vertical wave run-up loads on monopile-mounted platforms. This framework will enable a more realistic representation of uncertainty and variability in extreme wave impacts, supporting safer and more efficient offshore wind design.

To achieve this, the project will focus on creating generic building blocks rather than a complete design tool. These building blocks will allow engineers to develop site-specific probabilistic models in future applications.

The work will include developing simplified probabilistic transfer functions that relate key wave parameters—such as significant wave height, peak period, and steepness—to run-up heights and resulting loads. In addition, the project will characterise uncertainty by identifying probability distributions and recurrence behaviour of extreme run-up events, determining which variables most strongly influence outcomes.

Validation is a key part of the project. The probabilistic concepts and models will be tested using experimental data from the ongoing JIP Run-Up project, ensuring that the approach is grounded in realistic observations.

Finally, the project will outline the next steps for further development, including integration into design workflows, additional validation needs, and opportunities for scaling the approach toward industry-standard applications.

EXPECTED RESULTS

The project will deliver a practical probabilistic framework to better predict wave run-up loads on monopile-mounted platforms, supporting improved offshore wind design. Its main result will be a structured approach that links environmental conditions, such as wave height, period, and steepness, to the likelihood of extreme run-up loads, while explicitly accounting for uncertainty

A key outcome is a set of simplified probabilistic transfer functions that describe how sea state characteristics translate into run-up heights and associated loads. In addition, the project will characterise the variability of extreme events by defining probability distributions and identifying the most influential factors affecting run-up behaviour.

The framework will be tested and compared to experimental data from the Run-Up project to demonstrate how probabilistic methods can be applied in practice.

Further results include clear guidelines that enable engineers to apply the developed building blocks to create site-specific probabilistic models. The project will also produce a summary report and supporting documentation to share knowledge with industry partners.

Finally, the outcomes will include concrete recommendations for future work, outlining how the approach can be extended towards full design methods, integrated into industry standards, and further validated in larger-scale applications.