Tomohiko Tsuchida

Simulation for a Floating Roof Behavior of Cylindrical Storage Tank due to Wind Load: Part 2 — Sloshing Response Analysis

The floating roofs are used in large cylindrical storage tanks to prevent evaporation of oil. The single-deck floating roof, considered herein, consists of a thin circular plate called “deck” attached to a buoyant ring of box-shaped cross section called “pontoon”. The deck plates are deformed to create waves and they are subjected to cyclic bending due to wind load. This phenomenon may lead to initiate fatigue cracks at the welding joints. It is important to know the vibration characteristics of the waves in the deck plate. The authors have reported a CFD analysis of a cylindrical storage tank due to uniform wind flow in another paper. This paper presents the axisymmetric finite element analysis for the sloshing response of the single-deck floating roofs in a cylindrical storage tank subjected to wind load obtained by the CFD analysis. It is assumed that the liquid is incompressible and inviscid, and the roof is linear elastic while the sidewall and the bottom are rigid. The basic vibration characteristics of the waves in the deck plate, such as frequency and amplitude, are investigated.Copyright

Seismic Response Analysis for Sloshing of a Single-Deck Floating Roof With Center Pontoon in Oil Storage Tank

Floating roofs are widely used to prevent evaporation of contents of large cylindrical oil storage tanks. The 2003 Tokachi-Oki earthquake caused severe damage to floating roofs due to liquid sloshing. Seven single-deck floating roofs deformed to leak oil on them, and they lost buoyancy to sink. Two of them were the single-deck type with center pontoon in large diameter tanks. The present paper deals with an axisymmetric finite element analysis for the sloshing response of a floating roof with center pontoon. The hydrodynamic coupling of the fluid and the floating roof under seismic excitation is taken into consideration in the analysis. The fluid is assumed to be incompressible and inviscid, and the roof is assumed to be linear elastic. In addition, the sidewall and the bottom are assumed to be rigid. In the finite element analysis, the behavior of the fluid is formulated in terms of dynamic pressure using the Eulerian approach. The basic vibration characteristics of the single-deck floating roof with center pontoon, such as the natural periods and vibration modes, can be obtained from this analysis. These characteristics are shown comparing with those of the single-deck floating roof without center pontoon. The seismic response analysis for the input of an earthquake wave is also performed.Copyright

Assessment of Fatigue Crack Growth in Imperfect Lap Joint: Part 2 — Crack Propagation Analysis Using “SCANP”

The deck plates of single-deck-type floating roofs for large oil storage tanks are joined by single-welded Full-fillet lap joints. In areas with frequent strong winds, fatigue cracks sometimes occur in the welds of the deck plate. The aim of the present study is to investigate the effect of the gap imperfection of the lap joints on the fatigue life. In the case that tensile load acted on Full-fillet lap joints, the stress at the crack face becomes larger by gap imperfection of the lap joint. The authors have developed a software system called “SCANP (Surface Crack Analysis Program)”, a software system to evaluate the stress intensity factor, K, and to simulate fatigue crack propagation for surface cracks for arbitrarily distributed surface stresses. The fatigue life of a lap joint was predicted by the “SCANP” using the calculated stresses at the root crack face and the toe crack face. The number of loading cycles to penetration for arbitrary bending load amplitudes and tensile load amplitudes are obtained.Copyright

Assessment of Fatigue Crack Growth in Imperfect Lap Joint: Part 1 — Analysis of Stress Distribution on Virtual Crack Plane

The deck plates of a single-deck-type floating roof for large oil storage tanks are joined by single-welded full-fillet lap joints. In areas with frequent strong winds, fatigue cracks sometimes occur in the welds of the deck plates. The aim of the present study is to investigate the effect of the gap imperfection of the lap joint on the fatigue life. Generally, weld imperfections significantly reduce the fatigue crack propagation life of the welded joints. A stress analysis using the finite element method was performed to provide the basic data for the fatigue crack growth analysis.Copyright

Simulation for a Floating Roof Behavior of Cylindrical Storage Tank due to Wind Load: Part 1 — CFD Analysis

Floating roofs are used in large cylindrical storage tanks to prevent evaporation of oil. The floating roof is said to vibrate in high winds like undulation of the sea surface. The wind induced, sea-surface-undulation-like vibration may initiate fatigue cracks at welded joints in the floating roof deck. In this two-part study, the authors attempted to simulate the vibration. In Part1 wind flow over an isolated cylindrical oil-storage tank was simulated without considering the motion of the roof. Computed unsteady pressure load data were transferred to structural analyses. Response analyses of the floating roof under the wind load are dealt with in Part2. The present paper describes the wind flow simulation. The computed pressure fluctuation over the roof exhibits broadband spectra and no remarkable dominant frequency. To gain some insights into characteristics of the roof pressure fluctuation and its association with global flow structures, the Snapshot Proper Orthogonal Decomposition (POD), the Dynamic Mode Decomposition (DMD), and the Complex POD were applied.Copyright

Lagrangian Finite Element Formulation to Axisymmetric Liquid Sloshing

The sloshing analysis of liquid storage tanks by the finite element method is basically categorized into two approaches, Lagrangian approach and Eulerian approach. In the Lagragian approach, the behavior of the fluid is expressed in terms of the displacements at nodal points. The advantage of the Lagragian method is that the computer code can be easily developed to modify an existing structural analysis code. The disadvantage is that some spurious modes are included in the vibration modes. The Lagrangian method is widely used in two- and three-dimensional problems. On the other hand, it has not been reported its applicability to the axisymmetric problem. This paper presents the applicability of the Lagragian method to the axisymmetric sloshing problem. The eigenvalue of an elemental stiffness matrix is analyzed in order to investigate the characteristics of the rotational stiffness to the compressibility of the fluid. As a result, this method is found to be difficult to apply to the axisymmetric problem if the equation of motion is directly solved using time integration. However, it gives the highly precise response solutions if the only sloshing modes are taken out and the modal analysis technique is used.Copyright