Internal Waves in a Large Offshore Storage Tank

Abstract

The internal wave in a partially filled tank, or sloshing'' of liquid as it is commonly known, involves the transfer of liquid from one side of the storage tank to the other in the form of a wave. Sloshing is a natural period phenomenon. There are many natural periods in which the sloshing may be excited. Usually, the fundamental mode of motion is the most critical one. Simple calculations provide these natural periods for a rigid rectangular or circular cylindrical tank. If the storage tank is in motion, such as a liquid vessel on road or on rail, the force generated by this motion and the associated run-up may be severe. For an offshore storage tank, the liquid inside the tank is excited by the motion of the tank wall or bottom for an external agent, e.g., waves or earthquakes. In this case, the tank walls are deflected from the dynamic pressure distribution around its outside. Similarly, the waves induce a motion of the foundation of the structure. The purpose of this paper is to examine the sloshing phenomenon in waves due to the displacement of the wall as well as the displacement of the bottom of a gravity-based offshoremore » oil storage tank. Simple analysis was made to predict the wave heights generated by the internal waves due to the translational and rotational motion of the cylindrical storage tank. A wave tank test was performed with the model of a shallow water oil storage tank. The storage tank wall was modeled for the stiffness following Froude-Cauchy similitude. In addition, the actual foundation on the seafloor was modeled in the test with springs. No attempt was, however, made to introduce the foundation (soil) damping into the model. The test data from regular waves generated in the wave tank were used to correlate the theory.« less