Numerical modelling of aeration and hydroelasticity in slamming loads and responses of marine structures

Abstract

Slamming plays a significant role in the ultimate and fatigue limit state design of marine structures. Despite a relatively long history of investigations, there are still gaps in knowledge and open questions in understanding the slamming phenomenon and the approach it needs in the design phase due to its complex nature and limitations of research tools. The so-called hydroelasticity effect, which is the coupled interaction of structural responses with the body of fluid on both global and local scales, is one of the main complex aspects of slamming. Variation of fluid compressibility due to the mixing of air bubbles with the fluid, called aeration, alters the slamming loads and could also affect the hydroelastic coupling. The possible interaction of the two mentioned processes affecting the slamming physics and how to approach it in the analysis of slamming is still not well understood and is the focus of investigation in this thesis.

The research methodology of this work is based on studying the details of pressure and the flow field around the slamming area and the evolution of slamming force and structural response employing numerical modeling. A numerical tool for this purpose was developed and validated against benchmark experimental data available in the literature.

In the study of hydroelasticity, local shell deformations, as well as global deformations of the structure, were studied. The interlinked effects of local hydroelasticity and aeration were investigated by performing two sets of numerical simulation campaigns on the water entry of elastic flat plates and cylindrical shell sections. Both studies revealed that local flexibility has a noticeable reducing effect on peak values of slamming pressure and forces. This reduction effect of flexibility disappears for plates in the presence of aeration, which shows a significant indication of interdependence in the roles of aeration and hydroelasticity in slamming dynamics. In plate entry and cylinder entry simulations, aeration shows a damping effect on the response strain oscillations, strengthening with increasing aeration.

Both water entry studies present new insights with valuable details on slamming load's major characteristics and local structural response of plates and cylinders. Noticeable differences between plate and cylinder entries were observed; for instance, aeration causes a substantial extension of slamming load duration in plate entries, but no meaningful change is observed in cylinder entries. Extensive parameter studies led to new functional relations to determine peak slamming pressure/force in pure and aerated water entries in terms of relatively simple power-law approximations, which have been derived for plates and cylinders. The study shows that hydroelasticity may not be an essential issue for locally stiff structures, but considering air entrainment and entrapment processes is important to determine local loading characteristics.

This thesis also presents a novel simplified model of wave slamming on an SDOF cylindrical structure. The model could reproduce the experimental slamming force and pressure time series of the large-scale wave slamming on a vertical monopile with a reasonable accuracy level. The validation study shows that the introduced simplified model could present valuable data on the physics of the interaction of a flexible cylindrical structure with impacting body of water. The simplified model was applied in a parameter study to investigate the effect of global structural vibration characteristics and aeration on wave slamming loads and structural response characteristics. The parameter study indicates that processes related to compressibility, such as aeration and air entrapment, are far more important than the structure's global flexibility. Since wave impact events in natural conditions may incorporate variable aeration levels in the water, which is shown to alter the structural response and duration of vibration, in both deterministic and stochastic studies of wave impact dynamics, the compressibility parameter is important and should be considered in the analysis.