A CFD Study of Focused Extreme Wave Impact on Decks of Offshore Structures

Abstract

The design of offshore structures for extreme/abnormal waves assumes that there is sufficient air gap such that waves will not hit the platform deck. Due to inaccuracies in the predictions of extreme wave crests in addition to settlement or sea-level increases, the required air gap between the crest of the extreme wave and the deck is often inadequate in existing platforms and therefore wave-in-deck loads need to be considered when assessing the integrity of such platforms. The problem of wave-in-deck loading involves very complex physics and demands intensive study. In the Computational Fluid Mechanics (CFD) approach, two critical issues must be addressed, namely the efficient, realistic numerical wave maker and the accurate free surface capturing methodology. Most reported CFD research on wave-in-deck loads consider regular waves only, for instance the Stokes fifth-order waves. They are, however, recognized by designers as approximate approaches since “real world” sea states consist of random irregular waves. In our work, we report a recently developed focused extreme wave maker based on the NewWave theory. This model can better approximate the “real world” conditions, and is more efficient than conventional random wave makers. It is able to efficiently generate targeted waves at a prescribed time and location. The work is implemented and integrated with OpenFOAM, an open source platform that receives more and more attention in a wide range of industrial applications. We will describe the developed numerical method of predicting highly non-linear wave-in-deck loads in the time domain. The model’s capability is firstly demonstrated against 3D model testing experiments on a fixed block with various deck orientations under random waves. A detailed loading analysis is conducted and compared with available numerical and measurement data. It is then applied to an extreme wave loading test on a selected bridge with multiple under-deck girders. The waves are focused extreme irregular waves derived from NewWave theory and JONSWAP spectra.

On the Assessment of Numerical Wave Makers in CFD Simulations

“…Although CNWTs are more computationally costly than their lower fidelity counterparts, CNWT experiments can deliver accurate results in high resolution, which is particularly useful for the investigation of specific flow phenomena around coastal and offshore structures. Numerous studies are reported and reviewed in [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22], where CNWTs are employed for the analysis of different marine engineering problems.…”

Coupling of viscous and potential flow models with free surface: Implementation and application to offshore engineering

“…The prescribed velocities are imposed on the external part of the boundaries to represent the wavemaker and wall boundaries [16]. This can be translated to a Neumann condition in…”

“…where U * is the boundary data which may be acquired from a wave theory [16] and n the unit normal pointing outward to the domain. Furthermore, the following dynamic and kinematic free-surface boundary conditions are imposed on Γ f PF D , [17] ∂ ∂…”

Hydrodynamic Performance of Submerged Plates During Focused Waves

“…By specifying complex structures such as plate breakwaters, jetties, and platforms as horizontal plates, Suchithra and Koola [20] found that the impact force generated by focused waves was larger than that of regular waves. The effect of beams under plates on wave impact was numerically addressed in another study [21]. Furthermore, the influence of structural stiffness [22], green water [23], and accompanying current [24] on freak wave action was investigated.…”