This paper presents an investigation on the interaction between extreme waves and monopile support structures. A time-domain nonlinear potential flow model is developed based on the weak-scatterer approximation. On the free surface, the 4th-order Runge-Kutta scheme with an arbitrary Lagrangian-Eulerian (ALE) approach is adopted in the time marching process. The corresponding boundary value problem is solved by the higher-order boundary element method (HOBEM) at each time step. A comparative study has been performed to investigate the interaction of steep focused wave groups with a fixed vertical cylinder. The present numerical results of wave elevations and dynamic pressures recorded at different positions are provided and compared with the experimental data. Then the wave run-up and inline force are analysed in the time-frequency domain based on wavelet transforms.

INTRODUCTION

In ocean and coastal engineering, the safe and economic design of offshore structures needs an accurate description of hydrodynamic loads particularly in the extreme events. High-frequency wave loads occurring in extreme wave events have been identified in recent years. The high-frequency wave loads are typically excited on tension-leg platforms (TLPs) or gravity-based platforms (GBSs), and they may generate vibrations at the resonance period of the structure. The highfrequency resonant phenomenon is so-called ‘ringing’, due to its sudden appearance. Unlike springing, ringing usually occurs during the passage of steep wave crests, and can generate fairly high levels of stress within a burst of only a few oscillations. Ringing may also occur in offshore wind turbines, which has recently been a topic of particular interest since ringing induced loads may be a concern with regard to extreme loads and fatigue damage. However, proper evidence and analysis models of these forces and responses are generally lacking.

The phenomenon of ringing has motivated several theoretical and experimental studies. The experimental studies have been mainly carried out in wave tanks with incoming focused wave groups and steep random waves (e.g. Chaplin et al., 1997; Bachynski et al., 2017; Riise et al., 2018). The primary focus of the theoretical studies has been to capture the wave loads up to the third-harmonic component in regular waves (Faltinsen et al., 1995; Malenica and Molin, 1995). In view of the simplification of these approaches, neither of these theoretical studies involves the condition of steep transient waves or breaking waves, nor is the region of validity of the approximation clear even in the case of regular waves. Some fully nonlinear potential flow models have also been developed (Ferrant, 1998; Shao and Faltinsen, 2014) as well as the CFD models (e.g. Paulsen et al., 2014; Chen et al., 2014). However, both the fully nonlinear models and CFD models remain time consuming for practical applications and present some weaknesses in terms of robustness.

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