title: Characterisation of wave run-up on floating offshore wind turbine substructures
Creators: name: Foster, Tegan
Publisher: name: The University of Edinburgh
Year: 2025
Subjects: subject: wave run-up
subject: offshore wind
subject: Semi-submersible
subject: floating wind turbine
subject: regular waves
subject: semi-empirical model

description: Wave run-up is the amplification of an incident wave as it collides with a surface piecing object. This interaction between waves and structures can cause damage to man-made offshore platforms and secondary steel components including boat landing, railings, ladders, and access equipment. Consequently, predicting and demonstrating this behaviour within different sea states during platform design stages, is an essential requirement for complying with standards and achieving system certification. However, run-up is a complex phenomenon, and it is difficult to estimate. Existing methods for predicting run-up tend towards semi-empirical methods developed for bottom-fixed structures with simple geometries rather than complex floating structures. Therefore, design practices today focus on experimental testing to observe and measure wave run-up, and if possible, combine this approach with empirical and numerical modelling methods. In the context of floating offshore wind turbine substructures, these dynamic platforms interact with the local wave field differently, potentially leading to greater amplification of the incident wave. This results in greater risk of platform damage and increased project costs. Furthermore, existing empirical modelling methods have not been applied to floating systems and their suitability for predicting local run-up has not been reviewed. This work addresses this accordingly. In this work, a scale model of the Stiesdal Offshore A/S TetraSub platform is tested at the FloWave Ocean Energy Research Facility; a circular wave-current test basin. The floating semi-submersible platform is combined with a representative 15 MW wind turbine, and moored using a semi-taut mooring system. The model system is subjected to 34 regular waves and two irregular sea states, with significant emphasis on the measurement devices, and motion capture system to aid characterisation of the floater response. The bespoke wave run-up device is positioned directly into the oncoming waves and is used to record wave run-up for the duration of each test. The empirical data collected within this test campaign is used to validate the existing analytical methods for wave run-up on floating structures, with careful attention to the post-processing of the position data. The suitability of each method is then assessed for global run-up measurements. It is found that methods based on the velocity head stagnation theory perform well, with RMSE values as low as 0.016 m; but wave run-up error in larger waves for individual regular wave test cases ranges between -25% and 40% depending on the model. Similar behaviour has been noted on fixed structures. Through comparison of the existing analytical methods, opportunities to integrate the platform response into new analytical run-up modelling methods for floating structures and improve run-up estimation, are investigated. This work predominantly explores the inclusion of heave and surge velocity components. New methods are presented, and proved less effective with RMSE values ranging between 0.017 m and 0.020 m; this is due to the over simplified mechanisms including the wave parameters, platform geometry, and wave-platform phasing. This initial work highlights the complexity of including platform motion within analytical methods. Other observations within this work note that the relationship between measured local and global run-up is not linear, and that existing methods cannot be used to predict local run-up. This is a notable limitation with current methods, since the local run-up measurement is required for platform design. Therefore, it is recommended that further work focuses on validating these observations using larger data sets. A simplified run-up specific test campaign using a floating cylinder, in steeper waves, would be advantageous. This could then be extended to include multiple floating cylinders and irregular waves.