Seismic Qualification of Piping Systems by Detailed Inelastic Response Analysis: Part 3 — Variation in Elastic-Plastic Analysis Results on Carbon Steel Pipes From the Benchmark Analyses and the Parametric Analysis

It is recognized that piping systems used in nuclear power plants have a significant amount of the safety margin, up to the point of boundary failure, even when the input seismic load exceeds the allowable design level. The reason is attributed to the large strength capacity of the piping systems in the plastic region. In order to establish an evaluation procedure, in which the inelastic behavior of piping systems is considered in a rational way, a task group activity under the Japan Society of Mechanical Engineers (JSME) has been conducted. As a deliverable of this activity, a Code Case in the framework of the JSME Nuclear Codes and Standards is now being developed. The Code Case provides the strain-based criteria, an evaluation procedure using the response-spectrum based inelastic analysis, and detailed inelastic response analysis based on a finite element model. For developing the Code Case, inelastic benchmark and parametric analyses of the tests of a pipe element and piping system made of carbon steel were conducted to investigate the variation of the elastic-plastic analyses results. Based on these analytical results, it is assumed that setting the yield stress has a significant influence on the inelastic analytical results, while the work hardening modulus in the bi-linear approximation of the stress-strain curve has little influence. From the results of the parametric analyses, it is confirmed that the variation in the analytical results among the analysts would be reduced by having a unifying analysis procedure. In this paper, the results of the parametric analyses and the variation in the elastic-plastic analysis are discussed.

Laminated Type Isolation Device for Light Weight Structure Using Urethane Elastomer

At the present, base isolation system has been recognized by general earthquake resistant technique since the Great Hanshin Earthquake 1995. The seismic isolation will be aggressively applied to not only architectural and civil structures but also various structures, because the effectiveness on seismic safety had been demonstrated again in the Great East Japan Earthquake. In generally, although the base isolation system is divided into laminated rubber bearing type and friction sliding bearing type. In the case of former type, shape factor, maximum or minimum outer shapes and so on are restricted by the material characteristics in visco-elastic material. In general, the isolation structure is used in high damping rubber. However, we pay attention to base isolation using urethane elastomer. Urethane elastomer has excellent elasticity, mechanical strength, abrasion resistance, weather resistance, oil resistance, impact resistance the absorbent, anti-vibration and excellent low-temperature properties. Furthermore, it is possible to impart various characteristics by a combination of isocyanate and polyol and chain extender, requires no large-scale apparatus, it has the advantage molecular design is easy. In previous study, the research and development of laminated type base isolation device using urethane elastomer was carried out to upgrade a seismic safety for various structures. The fundamental characteristics was investigated from several loading test by using various experimental devices, and the design formula for the stiffness and equivalent damping coefficient is formulated as an approximate expression of mechanical characteristics until now. It was confirmed that urethane elastomer is not hardening up to 500% shear strain. Moreover, the experimental examination for aged deterioration in the urethane material has been continuously carried out. As the results, it was confirmed that the laminated type seismic isolation device using urethane elastomer is possible to develop as a practicable device from the stable mechanical properties as considering in design step. In this study, the small-scale laminated type base isolation device using urethane elastomer is advanced to the direction of further technical upgrading and of scale down for light-weight structure as a sever rack. The first stage, basic properties of the urethane elastomer has been investigated by loading test. Furthermore, the design equation is created by loading test using urethane elastomer. The validity of the design equation has been confirmed. The second stage, the compression creep test with laminated type base isolation device has been investigated to confirm an effect on light-weight mechanical devices.

Seismic Performance Evaluation for Steel-Frame-Structure Considering Member Fracture

Through the 2011 Tohoku Earthquake or the 2016 Kumamoto Earthquake, much larger earthquakes are considered recently in the seismic designs of large steel-frame structures. When structures are exposed by these severe ground motions, partial destructions in the structures, such as damage or fracture of members could happen. Especially, the low cycle fatigue of steel structures because of the repeated load from these long-term ground motions is a serious problem. However, current seismic performance evaluation method based on nonlinear dynamic analysis considers only elastic and plastic deformation of each member, excluding the fracture of members. If this member fracture happens during earthquakes, there is considered to be many effects on the seismic performance, like the changes of the vibration property, the dynamic response and the energy absorbance capacity of structures. Therefore, the fracture of members is preferably taken into account in the seismic performance evaluation for these large earthquakes. This paper proposes the dynamic analysis method for steel-frame structures which can express the member fracture. Dynamic analyses considering and not considering member fracture under the repeated loads supposing the long-term earthquake are conducted to the FEM model of full-scale structure. By comparing each result, the effects of considering member fracture to the seismic performance such as the dynamic response and the energy absorbance capacity are discussed.

Seismic Qualification of Piping System by Detailed Inelastic Response Analysis: Part 2 — A Guideline for Piping Seismic Inelastic Response Analysis

A Code Case in the framework of JSME Nuclear Codes and Standards is currently being developed to incorporate seismic design evaluation of piping by detailed elastic-plastic response analysis and strain-based fatigue criteria as an alternative design rule to the current rule, in order to provide a more rational seismic design evaluation. The Code Case provides two strain-based criteria; one is a limit to maximum amplitude of equivalent strain amplitude derived from detailed analysis and the other is a limit to the fatigue usage factor also based on the equivalent strain amplitude. A guideline for piping seismic analysis based on inelastic response analysis is also being developed as a mandatory appendix for the code case. The guideline provides the methodology to obtain the elastic and plastic strains in seismic response and contains descriptions for analysis code, FE modeling including material property definition, time history analysis method, damping, seismic input condition and verification and validation method. This paper introduces the outlines of them.

Seismic Response Analysis of Flexible Drain System Into External Floating Roof Storage Tanks

Floating roofs are commonly used worldwide on top of cylindrical oil storage tanks as a primary means to prevent formation of vapor above stored products into the storage tanks and should provide a safe and efficient storage of products with minimal risk for the environment. However, aboveground storage reservoirs built in seismic zones are prone to earthquake damage. Extensive research has been done to enhance performance of the floating roof tanks against damage to ground foundations, fixed and floating roof, tank shells as well as adjacent piping. Indeed, the stored oil sloshing in a cylindrical storage tank is known to have caused damage to the tank shell, tank roof and as well to anti-rotation columns. One of the possible dangers of liquid sloshing is the resultant damage to in-situ roof drain systems within external floating roof tanks. Indeed, roof drain systems are designed for continuous withdrawal of rainwater from external floating roofs, and if damaged, would result in dysfunction of the systems and irreversible discharge of oil products into the containment dyke. In this regard, a reliable roof drain system should have the capability to withstand liquid sloshing effects, and to a certain degree, ensure resistance in events of displacement of the floating roof. The aim of this document is to use knowledge of flexible pipe technology and industry recognized dynamic analysis software to analyze the effects of earthquakes on the integrity of a flexible drain pipe system. Analysis of liquid sloshing effects on flexible drain pipe systems using dynamic analysis software will be presented and the effects of structural damages such as loss of anti-rotation columns on the integrity of flexible drain pipe systems will be assessed. In the end, the document will propose recommendations on how industry can further enhance roof drain systems within external floating roof tanks to ensure performance and functionality after occurrence of earthquakes.

Nonlinear Finite Element Analysis of Unanchored Steel Liquid Storage Tanks Subjected to Seismic Loadings

Steel liquid storage tanks are widely used in industries and nuclear power plants. Damage in tanks may cause a loss of containment, which could result in serious economic and environmental consequences. For the purpose of the earthquake-resistant design of tanks, it is important to use a rational and reliable nonlinear dynamic analysis procedure. The analysis procedure should be capable of evaluating not only the comprehensive seismic responses but also the damage states of tank components under artificial or real earthquakes. The present paper deals with the nonlinear finite element modeling of steel liquid storage tanks subjected to seismic loadings. A reduce-scale unanchored steel liquid storage tank with the broad configuration from a shaking stable test (i.e., the INDUSE-2-safety project) is selected for this study. The fluid-structure interaction problem of the tank-liquid system is analyzed using the Abaqus software with an explicit time integration approach. In particular, the steel tank is modeled based on a Lagrangian formulation, while an Arbitrary Lagrangian-Eulerian adaptive mesh is used in the liquid domain to permit large deformations of the free surface sloshing. The finite element results in terms of the sloshing of the liquid free surface and the uplift response of the base plate are evaluated and compared with the experimental data that is obtained from the shaking table test for the tank under the INDUSE-2-safety project.

Seismic Design of Free Standing Racks in Japanese Nuclear Power Plants

The free standing racks are spent fuel storage racks with self-sustained structure without fixation to the pit floor or pit walls. If a free standing rack receives a force to move it due to an earthquake, the force acting on each member of the rack is reduced in compared to the floor-anchored racks owing to sliding of the free standing rack. Now it is planned to exchange the existing floor-anchored racks with the free standing racks to secure higher seismic resistance. In previous studies, efforts were made to establish a behavior analysis model that allows for evaluation of sliding and rocking behaviors of free standing racks and to make out a seismic design method based on an evaluation technique to evaluate, in a conservative manner, vibration test results of full-scale free standing racks. The free standing racks which consist of connected eight racks are designed with this seismic design method. It was confirmed that the free standing racks have enough seismic resistance by performing evaluation using the basic seismic motion and making an analysis on beyond the design event.

Core Seismic Experiment and Analysis of Full Scale Single Model for Fast Reactor

To design fast reactor (FR) core components, seismic response must be evaluated in order to ensure structural integrity. Generally, the fast reactor core is made of several hundred core elements in hexagonal arrangement. When a big earthquake occurs, large horizontal displacement, vertical displacement (raising) and impact force of each core element may cause a trouble for control rod insertability, reactivity insertion and core element intensity. Therefore, a seismic analysis method of a fast reactor core considering three-dimensional nonlinear behavior, such as bouncing, impact, fluid-structure interaction, etc. was developed. This paper presents a validation of the core element vibration analysis code in three dimensions (REVIAN-3D) for a full scale model. In this validation, the vertical behavior (rising displacement) and horizontal behavior (Impact force, horizontal response) as a single core element of the analysis result agreed very well with the experiments.

Univariate Fragility Models for Seismic Vulnerability Assessment of Refinery Piping Systems

Piping systems of energy industries in oil & gas play a critical role in meeting the increasing global energy demand. A great portion of these pipelines is located in high seismic-prone areas. Such systems have been found to be quite vulnerable to seismic events. Current seismic design approaches to piping systems are mainly based on the allowable stress method, even though more modern design methods are currently available for buildings or nuclear power plants; for example, the Performance-Based Earthquake Engineering (PBEE) framework has not been applied yet to piping systems and relevant structures. In this respect, both information about the quantification of limit states for pipes and adequate non-linear structural models for seismic analysis of piping systems and relevant structures are very limited. One of the key ingredients of PBEE approach for the assessment of the seismic vulnerability of existing structures is the evaluation of fragility curves, namely the probability of exceeding a certain level of damage for a given seismic intensity measure (IM). However, the contributions in the literature on this delicate aspect are very limited. This paper deals with such a problem by using a very popular method, namely the Cloud Analysis, originally developed as a method for probabilistic seismic demand analysis of civil structures. This method is here applied to a typical piping system for process plants. For this purpose, the structure is properly modelled, especially support structure and pipe, including pipe fittings like elbows and bolted flange joints. Using natural accelerograms selected from the PEER database and in accordance with given hazard conditions, the probabilistic seismic demand analysis is performed adopting different engineering demand parameters (EDP) consistent with the damage states expected in the pipes and fittings and in the support structure. According to the results of experimental tests campaign performed in the past by some of the authors on flanged joints, and elbows, different damage states (leakage, yielding, rupture) have been identified and related to the corresponding EDP and the corresponding probability of exceeding has been determined by assuming a lognormal distribution of the response. The analysis intends to recognise the most probable damage condition in a refinery piping system subjected to a seismic input.

Research and Development of Viscous Fluid Dampers for Improvement of Seismic Resistance of Thermal Power Plants: Part 1 — Fundamental Analysis and Component Test

In 2011, Great East Japan Earthquake that is the largest earthquake ever observed occurred. The earthquake had large energy, huge tsunami, long duration time and many aftershocks. Devastated area needed retrieval and revival. However, when electric power was lost, the retrieval delayed. It is necessary to improve seismic proof construction for power plants. In this paper, authors proposed vibration control by adding dampers for coal-fired power plant and developed it. Fundamental analysis and component test of the damper were conducted. As the analytical results, characteristic of the damper was searched what kind of performance is more effective for the coal-fired power plants. In the component test, actual scaled prototype of the developed damper was produced and its performance was checked by loading test. As the test results, suitable characteristic of the damper was searched by the analysis. Finally, authors proposed two analytical methods of the developed damper.