Fluid-structure interaction analysis of impact-induced loads and hydroelastic responses of ship structures

Hosseinzadeh, Saeed (2023) Fluid-structure interaction analysis of impact-induced loads and hydroelastic responses of ship structures. Tallinn University of Technology, Doctoral Thesis, 124pp.

Record type: Thesis (Doctoral)

Abstract

Water entry impact, also known as slamming impact, is a phenomenon that occurs on ships and offshore structures over a short period of time in rough sea conditions. This highly non-linear fluid-structure interaction problem causes large hydrodynamic loads on the structure that can lead to significant structural damage. Although slamming impacts have been extensively studied, the elasticity contribution to structural behaviour has rarely been taken into consideration. Since the dynamics of slamming involve air-water-structure interactions, a severe impact can easily induce elastic deformations that affect the fluid flow and the pressure field, i.e., the response is hydroelastic. Therefore, it may not always
be accurate to simplify the problem to a rigid body impact. To properly address this phenomenon, it is essential to comprehend the effect of the hydrodynamic loads and the simultaneous structural responses of the ship’s hull.
This thesis studies impact-induced loads acting on a 3D non-prismatic wedge section and its dynamic responses, both experimentally and computationally. A series of systematic free-fall drop tests were conducted on a 3D complex V-shaped section at various drop heights. A drop test tower was constructed to allow experiments to be carried out at various heights ranging from 25 cm to 2 metres. Two wedges with different masses were tested to investigate the effects of wedge mass on slamming loads and responses. To study the effect of flexural rigidity on structural responses, the bottom plates of the wedge were designed with two different bending stiffnesses. The importance of hydroelastic analysis is explained based on experimental observations.
Experiments provide the insight into the hydroelastic phenomenon. The experimental findings are exploited in the development of a coupled fluid and structure simulation model. The findings indicate that a two-way coupling numerical model is needed to accurately simulate slamming loads and structural responses at high impact velocities.
Consequently, the simultaneous interaction between the fluid and structural dynamics is considered in the proposed numerical simulations.
In this thesis, both 2D and 3D flexible structures are simulated to get a deeper insight into the dynamics of hydroelastic slamming. Firstly, the water entry problem is numerically analysed by implementing a two-way coupling approach on 2D steel and aluminium structures. A comparison of constant velocity and freefall impact is presented to examine the effect of freefall motions. It is found that hydroelasticity depends on the deadrise angle and impact velocity, and the elastic behaviour increases with smaller deadrise angles and higher drop heights. Secondly, a 3D aluminium wedge with varying deadrise angles is simulated using two different numerical models. The slamming problem is modelled using an explicit nonlinear finite element method (MMALE) and an implicit CFD-FEM coupling approach. The results of the two numerical methods are validated and
compared with the experimental data. The numerical computations obtained from different methods are found to be in satisfactory agreement with the experimental measurements, indicating their reliability and accuracy. The importance of simulating the fluid-structure interaction problems is evaluated by considering a hydroelasticity factor that exhibits a noteworthy influence on the unstiffened bottom plate for all examined impact velocities. A detailed analysis is performed to compare different numerical approaches and thoroughly discuss the advantages and disadvantages associated with each method.

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