Modelling Extreme Wave-Current Conditions and their Interaction with Offshore Renewable Energy Systems

  • Yong Yang

Student thesis: Phd

Abstract

Waves and currents coexist in a wide range of natural locations for the deployment of offshore renewable energy (ORE) systems. However, the effect of currents on wave-structure interaction has received little attention to date but is known to affect wave characteristics and associated loading. It is necessary to be able to model these conditions and their interaction with ORE systems. In this project, a numerical flume for waves on variable sheared currents based on the state-of-the-art Smoothed Particle Hydrodynamics (SPH) modelling technique is developed, which can be applied for modelling extreme wave-alone and wave-current conditions (including breaking waves and sheared currents) and their interaction with offshore and ocean structures (including dynamic response). Numerical wave-current flumes based on SPH are rare and the generation of waves on sheared currents has received no attention to date. Therefore, a numerical flume for waves on variable sheared currents using SPH with open boundaries is first proposed. Open boundaries are applied for the generation of combined wave-current conditions whilst a modified damping zone acting on the vertical velocity component is used for wave absorption. A general numerical flume is achieved by testing wave-alone cases (regular, irregular and focused waves) and current-alone cases (uniform, linearly sheared and arbitrary sheared currents). Tests of focused waves interacting with arbitrary sheared currents demonstrate a powerful and flexible numerical wave-current flume is achieved. Modelling complex and extreme events of combined waves and sheared currents interacting with a vertical cylinder is of great significance for practical applications in the design of ORE systems (cylinders are common substructures) which has received no attention to date based on SPH (rare using other numerical approaches). Therefore, the SPH model's capability for modelling this is firstly validated. The SPH model's ability at capturing the harmonic structure of the elevations and loading in combined wave-current conditions is also validated. Taking the advantage of the SPH method, wave amplitudes are increased up to, and beyond, the breaking threshold highlighting a significantly increased breaking threshold steepness with negatively sheared currents and a complex relationship between peak force and wave phase. These tests demonstrate the capability of the present numerical wave-current flume for modelling highly nonlinear fluid-structure interaction problems. Accurate force prediction on offshore and ocean structures due to near-breaking and spilling breaking waves based on numerical approaches remains a problem of great practical importance for the ORE industry as these extreme waves are common in ocean environments. The numerical model's ability at capturing the secondary load cycle and obtaining the higher-mode vibrations in an object being impacted by waves have received no attention to date using the SPH method. Therefore, numerical results are validated against experimental measurements of wave force on the vertical cylinder using a rigid model. Then, a flexible model is used to study the dynamic response to extreme wave events showing that the secondary load cycle phenomenon appears in highly nonlinear fluid-structure interaction and becomes larger with dynamic response. This thesis presents a reliable numerical wave-current flume which can be used for the research on fluid-structure interaction, design of ORE systems and de-risking ORE systems in realistic, complex, and extreme marine environmental conditions.
Date of Award1 Aug 2024
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorPeter Stansby (Supervisor), Benedict D. Rogers (Supervisor) & Samuel Draycott (Supervisor)

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