Sloshing of Partially Filled LNG Carriers

The motion of the LNG fluid inside gas carriers is normally restricted by the loading condition of the vessel, ie either the vessel is operated at near empty condition or at fully loaded condition. In this way resonance or sloshing effects of the fluid on the ship's hull are limited. However nowadays the LNG carriers are considered to be operating at intermediate loading conditions. LNG tanks are also foreseen to be used for anchored FPSO's, with their filling ratio varying during the production. Subsequently LNG carrier will be sailing with partially filled LNG tanks and FPSO-LNG will be sometimes in conditions with partially filled tanks. In this condition the LNG fluid is more likely to be induced into resonance due to wave action and roll motions. This resonance or sloshing behaviour of the LNG fluid will lead to high impact pressures on the thermal insulation. Due to the different physical properties of the LNG fluid with respect to water in terms of density and viscosity little is known on the behaviour of the LNG fluid in resonance condition. In this paper we will describe a study of model test experiments and numerical procedures based on VoF techniques for the behaviour of LNG in resonance condition. In the model test experiments special care is taken into the effect of air bubbles and viscosity on the pressure impacts. Using High speed video the wave front formed by the bore of the LNG in resonance is observed and the impact to the tank hull is measured. Introduction The problem of sloshing in the offshore and shipping industry is already an old one. Sloshing can occur in storage tanks. Especially in LNG tanks the effects of sloshing can be quite detrimental.