A stone beak hammer is one of dredging equipment and it has occasionally utilized to crush the rock of the seabed. There are two types depending on a way how to use: a central type that utilizes impact force generated by free falling of the hammer at an appropriate height. Otherly, an impact type that repeatedly hits the rock with compressed air. It is efficient to understand the behavior of the hammer in the water and calculate the impact force when the hammer hits the seabed to determine the size of hammer and its strength in the early stage of design.This study focuses on developing a numerical model for underwater drop of a stone breaking hammer using Fluid Structure Interaction (FSI) analysis. Nowadays, there are lots of studies on FSI analysis being conducted in various fields such as marine, aviation and medical. It is attributed to an advantage of FSI analysis which can contemplate the effect of the surrounding fluid of the structure. Generally, structural and hydrodynamic analysis were performed independently, but the necessary of the FSI analysis was emphasized as higher demands for the precise study on the moving structure in fluids.The 15 scaled-down test was conducted by a Research Institute of Medium &amp; Small Shipbuilding (RIMS). Every three tests were conducted to measure the impact force using load cells. The hammer dropped at the sea level, four water depths were considered, changing by a jig: 0.779m, 1.112m, 1.445m, 1.579m. However, the impact force was observed differently, though they were the same water depth. It is due to contact angle of hammer was slightly different when it hit the ground. In addition, the impact force has increasing tendency by having higher water depth.ANSYS/LS-DYNA was adopted to simulate the hammer and two fluids (air-water) interaction for underwater drop.To describe the experimental condition, same values in hammer dimension, ground and hammer’s material properties, four water depth were used. Two equations of state of air and water were used to determine the hydrostatic behavior by calculating pressure as a function of density and energy. Symmetric conditions of fluids were considered to illustrate the infinite fluid’s fields and the hammer fell free in the air. It was assumed that angle attack remained perpendicularly at the ground, when the hammer vertically dropped into the water. The behavior of hammer was captured depending on change in depth. The contact force between the hammer and ground was compared with the underwater drop characteristic of the stone break hammer in experiment.Several numerical models were considered to assure the accuracy. First, it was observed the effect of material property of ground, regardless of used ground material in the test. Second, it is examined the influence of element type of ground: shell vs. solid element. Third, a fluid modelling extent was varied to find a certain ratio to water depth and water width which gives converged impact force. The specific ratio was used to estimate the impact force in deeper depth by reducing simulation time. Numerical results have a slight difference compared to each three experimental data in one depth. However, a similar tendency was obtained that impact force was increased in the higher depth. And it was confirmed that the numerical results have a good agreement with average values in the experiment.

Development of Numerical Model for Underwater Drop of a Stone Breaking Hammer Using Fluid Structure Interaction Analysis

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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