Virtual Mass of Submerged Structures

A structure submerged in water exhibits different dynamic behavior due to the interaction of the surrounding water than when it vibrates in air. The submerged structure is subjected to additional hydrodynamic pressures and its dynamic characteristics alter when it vibrates in water. Virtual mass concept is increasingly being used in design to account for the influence of surrounding water and this is used indirectly to represent the hydrodynamic pressure. A simple experimental technique for the determination of virtual mass for partially and fully submerged structures is presented herein. The virtual mass depends on the geometry of the structure and its dynamic properties in air. Due to the surrounding water, the natural period of vibration of a structure elongates and the damping increases. Due to the added mass of the surrounding water, the stresses and strains in the structure under dynamic conditions are increased but some relief is provided by the increased damping.