Investigation into the effects of air entrapment on the wave-in-deck loads on offshore structures in extreme weather events

Background. Slamming is described as an impact between a solid structure and an oncoming wave in which highly concentrated forces and pressures are induced on the structure over a short duration of time, causing significant strain and inducing vibrations throughout members of an offshore structure. These loads are a key contributor to instability for an offshore structure, and it is critical during the design that a proper analysis is conducted to estimate the envelope values of the forces and pressure distribution. The structural members under the deck plating are exposed to wave action form air pockets in which the air is entrapped during the wave impact. The presence of such air pockets makes the determination of wave slamming loads extremely challenging by either analytical predictions (due to the three-phase interaction problem) or by model testing (due to difficulties with scaling of air interaction effects).

Scope and Methods. This thesis reports on the effect of air entrapment on the magnitude and distribution of impact loads of wave-in-deck events on fixed offshore structures through experimental and numerical investigations. It proposes a methodology for incorporating the effect of air entrapment into the scaling of impact pressure from the model scale to full scale. Experimental investigations were conducted at the Australian Maritime College (AMC) model test basin at two different levels of complexity, namely preliminary and main studies, each consisting of two different stages (four different experimental studies in total) to examine extreme wave events associated with a representative 100-year return period sea state in the North Sea wave climate. The scope of the preliminary experimental part started with the examination of the constraints and limitations of the testing facility in order to find the most suitable model scales for the main study. This part also focused on the generation of scalable waves at the chosen three scales such that the scaling effects on wave generation could be identified. Using collected data from the preliminary experiments, the three models of a smooth deck (i.e., a flat deck structure of a solid underside without internal or external pockets) at different scales were subjected to regular and focused wave conditions to investigate the scaling effects in directional forces and impact pressure. Both regular and focused wave conditions were investigated at three different model scales (1:75, 1:100 and 1:125 scales) of a simplified deck structure with full-scale dimensions of 76 m x 76 x 15 m. Following the study of scaling effects on a smooth deck structure, nine different geometries of the air pocket were arranged in the middle of the underside of the 1:75 scale model in order to investigate the air entrapment during regular and focused wave-in-deck slamming events. In parallel with the model test investigation, a CFD analysis of the same wave-in-deck problems was performed using the Volume of Fluid (VoF) model implemented in the commercial CFD code STAR-CCM+ ver 12.04. The CFD models were thoroughly validated using measured experimental data. Subsequently, this numerical study extended to the full-scale prototype. On this basis, a scaling methodology for the impact pressure with entrapped air has been developed, which could be incorporated into the design cycle of offshore deck structures.

Findings. The experimental study on three model scales confirms that scaling effects can impact the forces and pressures measured in the model testing. These effects can exhibit themselves in different forms, broadly categorized as fluid-structure related. If the wave scaling is not perfectly followed in the fluid-related effects, it can significantly diverge from the actual results. For example, an 11% difference in wave crest between theoretically scaled and experimentally generated waves can change the resultant directional forces up to 50% for a model scale of 1:100, and this difference may increase even further for models of small scale. On the other hand, the structure-related scaling effects can appear in the form of dynamic vibration after impacts which can alter the peak magnitude of forces and change the value of slamming drag coefficients in horizontal and vertical directions. The entrapped air significantly affected the impact pressure for a model scale with an air pocket. This point was first theoretically discussed using a non-dimensional analysis to relate the impact pressure to the energy in the pocket using a generalized formulation and then its simplified version. Experimentally, the measured data support the theory by following the same trend between the impact pressure and volume of the pocket as well as the non-dimensional impact velocity and geometrical parameters of the pocket and model. Using the numerical models and extended theoretical interpretation from the air pocket model testing, an improvement to the scaling of impact pressure with air entrapment was proposed using the Bagnold number.

Conclusion. Overall, air entrapment effects during wave-in-deck impact events were identified, and a better understanding of the problem for fixed offshore structures was acquired. It can be concluded that the slamming pressure is affected by the entrapped air and is related to the volume of entrapped air. If the volume of entrapped air increases, the magnitude of the pressure reduces. Correlating this relationship to the Bagnold number reveals that Froude's similitude law can lead to extreme overestimating in the full-scale prediction of impact pressure when air entrapment occurs. The comparison between the CFD results and theory suggested that the proposed methodology can function relatively accurately for scaling impact pressure at local points inside an air pocket, mainly the central area of the pocket that thoroughly assimilated air entrapment effects.