Finite Element Analysis of Von-Mises Stress Distribution in a Spherical Shell of Liquified Natural Gas (Lng) Pressure Vessels
This research work investigated the modeling of Von Mises stress in LNG Spherical Carbon Steel Storage tank using assumed displacement Finite Element analysis based on shallow shell triangular elements. Using equations of elasticity, constant thickness carbon steel spherical storage tanks were subjected to different loading conditions. This paper stresses the need for proper definition of shallow element using sector angles to obtain the shallowness. The shallow spherical triangular element has five degrees of freedom at each of its corner node, which are the essential external degrees of freedom. The assumed displacement fields of these shallow triangular elements satisfied the exact requirement of rigid body modes of motion. The FORTRAN 90 programming language was used for the programme coding to solve finite element equations resulting from the model while Von Mises stresses distribution within the spherical storage tank shell subjected to different internal pressures were determined. The results showed that the use of non-shallow elements due to improper sector angles resulted in unreliable results while real shallow elements produced results that tallied with ASME Section VIII Div 1, Part UG values.
Evaluation of the pressure-resisting capability of membrane-type corrugated sheet under hydrodynamic load
Abstract Fluid impact load-induced collapse of the corrugated sheet in the primary barrier of liquefied natural gas carrier (LNGC) cargo containment systems is recognized as one of the most dangerous deformation phenomena because it leads to catastrophic failure of the containment system. Thus, significant efforts have been dedicated to precisely evaluating the load-carrying capacity of the primary barrier. However, due to the lack of testing methods and the complex geometry of the corrugated primary barrier sheet, the structural behavior of the sheet under impact-induced pressure has rarely been investigated. The aim of this study is to develop design support methodology ensuring the integrity of the primary barriers in LNG cargo containment systems. A custom-built experimental facility was developed to create uniform pressurization on the primary barrier sheet. The out-of-plane deformations of the sheet were observed over time during the experiment. To create a reliable numerical method applicable in the design stage, a series of parametric studies using finite element analyses (FEA) were conducted. Based on the comparison between the numerical solutions and experimental results, a practical method to estimate the typical deflection patterns based on the corrugation type was developed. Thus, we demonstrate that the insights from experimental observations as well as FEA techniques can be used in the practical design support methodology.
Bifurcation Buckling Analysis of Conical Roof Shell Subjected to Dynamic Internal Pressure by the Finite Element Method
Cylindrical tanks with conical roof shells are utilized as oil storage tanks and for some containment vessels. It is known that conical roof shells and torispherical shells subjected to static internal pressure buckle into a displaced shape with circumferential waves caused by an instability condition commonly called bifurcation buckling. It can be important to obtain the dynamic bifurcation buckling load in designing conical roof shells. In this paper, the bifurcation buckling pressure is calculated for dynamic pressure during accident conditions as characterized by step pressure loading, ramp pressure loading and pulse pressure loading. The minimum bifurcation buckling pressure is shown to be a linear function of radius-to-thickness ratio R/h of the shell in a linear fashion on a logarithmic scale. The minimum bifurcation buckling pressure is minimum for conical roof shells subjected to the step loading. The minimum dynamic bifurcation buckling pressure for step loading is about half of the static bifurcation buckling pressure.