Application of high-order hydrodynamic models to floating offshore wind TLP: numerical and experimental analysis

Abstract

With the large-scale development in the last decades of fixed offshore wind across Europe and the ever-more-present threat of climate change dominating national and global agenda, the exploitation of wind power in deep-water using floating wind turbines is gathering a significant amount of interest. Compared to other types of floating wind platforms, Tension-leg-platforms (TLPs) are less compliant systems resisting dynamic forces through their pre-tensioned tendons. Whilst this reduces the weight of the platform hull, understanding extreme loading cycles in the mooring system becomes an important design issue.

Recent research has revealed the importance of considering sum-frequency second-order and third-order loads to capture extreme events, such as slack-line events and ringing events. These events occur when high-frequency wave loads excite the resonant vertical modes of large and stiff floating systems. For the offshore wind industry, it is essential to ascertain whether such events can be stochastically predicted across numerous random sea-states, addressing both ultimate limit state and fatigue design scenarios.

This thesis presents a comprehensive review of existing numerical methods available to engineers for calculating second and third-order forces in aero-hydro-servo-elastic time domain solvers, commonly used in the offshore wind industry for assessing multiple design load cases. These models encompass potential (semi-analytical and BEM) and strip-theory approaches (Rainey and FNV). Subsequently, these approaches are applied to a complete academic floating offshore wind TLP platform, considering both fixed and fully dynamic conditions. The nonlinear hydrodynamic loading on the platform in the fixed condition is compared against high-fidelity simulations obtained using a Navier-Stokes CFD numerical wave tank. Furthermore, an experimental campaign is designed to investigate the application of these numerical models to dynamic conditions, encompassing wave and coupled wind-wave excitation.

Both the CFD and experimental results indicate that, while previous literature has primarily focused on inertial loads, viscous effects, particularly vertical drag, exert a more significant influence on the third-order response of the studied TLP system. This finding emphasises the necessity for further research into modelling viscous drag on complex floating structures. Additionally, the experimental campaign underscores the importance of characterizing both structural and viscous damping as significant parameters that strongly affect the resonant response of the floating offshore wind TLP system.

Finally, a time-frequency analysis of the response is undertaken which serves to identify the ringing events. This analysis shows that whilst numerical models fail to capture ringing events adequately, they can still predict their occurrence, albeit with lower amplitudes. This thesis thus presents a comparative review of the numerical wave loading approach available for the stochastic treatment of ringing and slack-line events in floating wind TLP systems, providing valuable insights for the industry.