CFD Modelling of Fluid Responses Between Side-by-side Structures in Waves and Currents

Ship-to-ship (STS) transfer of liquid fuels at sea is being commonly used to ensure high offloading volumes and a stable energy supply to the global market, without a need to upgrade existing infrastructures in the ports. Side-by-side (SBS) offloading is a preferred fuel-transfer solution as it hides the offloading work in between the vessels. However, in this setup, strong hydrodynamic interaction in the gap between the adjacent vessels may arise. Fluid resonances, characterized by amplified water oscillations in the gap and violent hydrodynamic forces on the adjacent vessels when the wave frequencies approach the natural frequencies of the gap flow, present a critical challenge in designing and operating those offshore structures.

A large number of real-life marine operations are carried out in coastal areas where appreciable water-depth effects on the wave nonlinearities and the wave-structure interactions are expected. In this thesis, nonlinear piston-mode fluid resonance in the gap formed by two identical fixed barges in close proximity is investigated using a two-dimensional (2D) fully nonlinear numerical wave tank based on a state-of-the-art computational fluid dynamic (CFD) solver. To delve into the effect of water depth on higher-order resonances in the gap, consistent models are employed to describe the incident waves and wave-structure interactions in finite and shallow water depths. Besides, coupled piston-mode fluid response and the heave motion of the barges are studied between two identical barges in SBS configuration under finite-depth and shallow-water waves. To understand possible critical responses of the gap flow and the floating barges, regular-wave conditions that can excite up to 5th-order nonlinear gap resonance and also the resonant heave motion of the barges are considered.

Another crucial factor is the ocean current which can significantly alter both the encounter frequencies and the wavelengths. The interaction between waves and currents becomes more pronounced at higher current speeds, potentially leading to gap resonances differing from those observed in wave-only scenarios. To address this, 2D fluid resonance within the narrow gap formed by two identical SBS fixed barges is investigated under various wave-current conditions, covering both uniform and shear currents. Building upon this, an investigation of three-dimensional (3D) gap resonance between two SBS fixed barges under ‘wave+uniform-current’ conditions is further carried out in the CFD-based numerical wave tank. It delves into how wavecurrent interactions affect the 3D response of the gap flow to head- and beam-sea waves, emphasizing the significance of considering both piston and sloshing modes of gap response.

After conducting extensive studies on fluid responses and wave-current-structure interactions, this thesis emphasizes the importance of consistently considering the effects of water depth and ship motion in the design and analysis of SBS marine operations involving piston-mode gap resonances. Additionally, it recommends placing smaller offloading ships behind larger receiving ships as a preferred arrangement during offshore and coastal offloading operations. Furthermore, neglecting the influence of currents in SBS marine operations is deemed non-conservative, as it can significantly impact resonant gap responses and the spatial structures of free-surface elevation along the gap. Through high-fidelity CFD modeling, this thesis aims to advance the understanding of the intricate hydrodynamic interactions between SBS structures in waves and currents, offering valuable insights for enhancing the safety of relevant marine operations in real sea environments.