Development of Piping Stress Analysis Guideline for Neste Jacobs' Projects

The aim of this research was to develop a piping stress analysis guideline to be widely used in Neste Jacobs Oy’s domestic and foreign projects. The company’s former guideline to performing stress analysis was partial and lacked important features, which were to be fixed through this research. The development of the guideline was based on literature research and gathering of existing knowledge from the experts in piping engineering. Case study method was utilized by performing stress analysis on an existing project with help of the new guideline. Piping components, piping engineering in process industry, and piping stress analysis were studied in the theory section of this research. Also, the existing piping standards were studied and compared with one another. By utilizing the theory found in literature and the vast experience and know-how collected from the company’s employees, a new guideline for stress analysis was developed. The guideline would be widely used in various projects. The purpose of the guideline was to clarify certain issues such as which of the piping would have to be analyzed, how are different material values determined and how will the results be reported. As a result, an extensive and comprehensive guideline for stress analysis was created. The new guideline more clearly defines formerly unclear points and creates clear parameters to performing calculations. The guideline is meant to be used by both new and experienced analysts and with its aid, the calculation process was unified throughout the whole company’s organization. Case study was used to exhibit how the guideline is utilized in practice, and how it benefits the calculation process.

A modified structural stress method for fatigue assessment of welded structures

Fatigue life assessment of weldedstructures is commonly based on the nominal stress method, but more flexible and accurate methods have been introduced. In general, the assessment accuracy is improved as more localized information about the weld is incorporated. The structural hot spot stress method includes the influence of macro geometric effects and structural discontinuities on the design stress but excludes the local features of the weld. In this thesis, the limitations of the structural hot spot stress method are discussed and a modified structural stress method with improved accuracy is developed and verified for selected welded details. The fatigue life of structures in the as-welded state consists mainly of crack growth from pre-existing cracks or defects. Crack growth rate depends on crack geometry and the stress state on the crack face plane. This means that the stress level and shape of the stress distribution in the assumed crack path governs thetotal fatigue life. In many structural details the stress distribution is similar and adequate fatigue life estimates can be obtained just by adjusting the stress level based on a single stress value, i.e., the structural hot spot stress. There are, however, cases for which the structural stress approach is less appropriate because the stress distribution differs significantly from the more common cases. Plate edge attachments and plates on elastic foundations are some examples of structures with this type of stress distribution. The importance of fillet weld size and weld load variation on the stress distribution is another central topic in this thesis. Structural hot spot stress determination is generally based on a procedure that involves extrapolation of plate surface stresses. Other possibilities for determining the structural hot spot stress is to extrapolate stresses through the thickness at the weld toe or to use Dong's method which includes through-thickness extrapolation at some distance from the weld toe. Both of these latter methods are less sensitive to the FE mesh used. Structural stress based on surface extrapolation is sensitive to the extrapolation points selected and to the FE mesh used near these points. Rules for proper meshing, however, are well defined and not difficult to apply. To improve the accuracy of the traditional structural hot spot stress, a multi-linear stress distribution is introduced. The magnitude of the weld toe stress after linearization is dependent on the weld size, weld load and plate thickness. Simple equations have been derived by comparing assessment results based on the local linear stress distribution and LEFM based calculations. The proposed method is called the modified structural stress method (MSHS) since the structural hot spot stress (SHS) value is corrected using information on weld size andweld load. The correction procedure is verified using fatigue test results found in the literature. Also, a test case was conducted comparing the proposed method with other local fatigue assessment methods.

On the Application of Beam on Elastic Foundation Theory to the Analysis of Stiffened Plate Strips

The main objective of this thesis is to show that plate strips subjected to transverse line loads can be analysed by using the beam on elastic foundation (BEF) approach. It is shown that the elastic behaviour of both the centre line section of a semi infinite plate supported along two edges, and the free edge of a cantilever plate strip can be accurately predicted by calculations based on the two parameter BEF theory. The transverse bending stiffness of the plate strip forms the foundation. The foundation modulus is shown, mathematically and physically, to be the zero order term of the fourth order differential equation governing the behaviour of BEF, whereas the torsion rigidity of the plate acts like pre tension in the second order term. Direct equivalence is obtained for harmonic line loading by comparing the differential equations of Levy's method (a simply supported plate) with the BEF method. By equating the second and zero order terms of the semi infinite BEF model for each harmonic component, two parameters are obtained for a simply supported plate of width B: the characteristic length, 1/ λ, and the normalized sum, n, being the effect of axial loading and stiffening resulting from the torsion stiffness, nlin. This procedure gives the following result for the first mode when a uniaxial stress field was assumed (ν = 0): 1/λ = √2B/π and nlin = 1. For constant line loading, which is the superimposition of harmonic components, slightly differing foundation parameters are obtained when the maximum deflection and bending moment values of the theoretical plate, with v = 0, and BEF analysis solutions are equated: 1 /λ= 1.47B/π and nlin. = 0.59 for a simply supported plate; and 1/λ = 0.99B/π and nlin = 0.25 for a fixed plate. The BEF parameters of the plate strip with a free edge are determined based solely on finite element analysis (FEA) results: 1/λ = 1.29B/π and nlin. = 0.65, where B is the double width of the cantilever plate strip. The stress biaxial, v > 0, is shown not to affect the values of the BEF parameters significantly the result of the geometric nonlinearity caused by in plane, axial and biaxial loading is studied theoretically by comparing the differential equations of Levy's method with the BEF approach. The BEF model is generalised to take into account the elastic rotation stiffness of the longitudinal edges. Finally, formulae are presented that take into account the effect of Poisson's ratio, and geometric non linearity, on bending behaviour resulting from axial and transverse inplane loading. It is also shown that the BEF parameters of the semi infinite model are valid for linear elastic analysis of a plate strip of finite length. The BEF model was verified by applying it to the analysis of bending stresses caused by misalignments in a laboratory test panel. In summary, it can be concluded that the advantages of the BEF theory are that it is a simple tool, and that it is accurate enough for specific stress analysis of semi infinite and finite plate bending problems.

Fatigue study of the fillet weld waving on weld toe and the penetration in the weld root based on the effective notch stress approach

The fatigue failure of structures under fluctuating loads in fillet weld joints raises a demand to determine the parameters related to this type of loading. In this study, the stress distribution in the susceptible area of weld toe and weld root in fillet welded models analyzed by finite element method applying FEMAP software. To avoid the geometrical singularity on the path of analytical stress analysis in the toe and root area of a weld model the effective notch stress approach applied by which a proper fictitious rounding that mostly depend on the material of structure is applied. The models with different weld toe waving width and radius are analyzed while the flank angle of weld varied in 45 and 30 degrees. The processed results shows that the waving compare to the straight weld toe makes differences in the value of stress and consequently the stress concentration factor between the tip and depth of the waves in the weld toe which helps to protect the crack of propagation and gives enough time and tools to be informed of the crack initiation in the structure during the periodical observation of structure. In the weld root study the analyses among the models with the welding penetration percentage from non-penetration to the full-penetration shows a slightly increase in the root area stress value which comparing with the stiffening effect of penetration conclude that the half-penetration can make an optimization between the stress increase and stiffening effect of deep penetration.

A modified nominal stress method for fatigue assessment of steel plates with thermally cut edges

Thermal cutting methods, are commonly used in the manufacture of metal parts. Thermal cutting processes separate materials by using heat. The process can be done with or without a stream of cutting oxygen. Common processes are Oxygen, plasma and laser cutting. It depends on the application and material which cutting method is used. Numerically-controlled thermal cutting is a cost-effective way of prefabricating components. One design aim is to minimize the number of work steps in order to increase competitiveness. This has resulted in the holes and openings in plate parts manufactured today being made using thermal cutting methods. This is a problem from the fatigue life perspective because there is local detail in the as-welded state that causes a rise in stress in a local area of the plate. In a case where the static utilization of a net section is full used, the calculated linear local stresses and stress ranges are often over 2 times the material yield strength. The shakedown criteria are exceeded. Fatigue life assessment of flame-cut details is commonly based on the nominal stress method. For welded details, design standards and instructions provide more accurate and flexible methods, e.g. a hot-spot method, but these methods are not universally applied to flame cut edges. Some of the fatigue tests of flame cut edges in the laboratory indicated that fatigue life estimations based on the standard nominal stress method can give quite a conservative fatigue life estimate in cases where a high notch factor was present. This is an undesirable phenomenon and it limits the potential for minimizing structure size and total costs. A new calculation method is introduced to improve the accuracy of the theoretical fatigue life prediction method of a flame cut edge with a high stress concentration factor. Simple equations were derived by using laboratory fatigue test results, which are published in this work. The proposed method is called the modified FAT method (FATmod). The method takes into account the residual stress state, surface quality, material strength class and true stress ratio in the critical place.

Ydinreaktorin painesäiliön ulkoisen jäähdytyksen lämmönsiirtokokeita

The purpose of this work was to design and carry out thermal-hydraulic experiments dealing with overcooling transients of a VVER-440-type nuclear reactor pressure vessel. Sudden overcooling accident could have negative effect on the mechanical strength of the pressure vessel. If part of the pressure vessel is compromised, the intense pressure inside a pressurized water reactor could cause the wall to fracture. Information on the heat transfer along the outside of the pressure vessel wall is necessary for stress analysis. Basic knowledge of the overcooling accident and heat transfer types on the outside of the pressure vessel is presented as background information. Test facility was designed and built based to study and measure heat transfer during specific overcooling scenarios. Two test series were conducted with the first one concentrating on the very beginning of the transient and the second one concentrating on steady state heat transfer. Heat transfer coefficients are calculated from the test data using an inverse method, which yields better results in fast transients than direct calculation from the measurement results. The results show that heat transfer rate varies considerably during the transient, being very high in the beginning and dropping to steady state in a few minutes. The test results show that appropriate correlations can be used in future analysis.

Multivariable regression analysis and hot-spot stress approach into fatigue life estimations of welded joints

Kolmen eri hitsausliitoksen väsymisikä arvio on analysoitu monimuuttuja regressio analyysin avulla. Regression perustana on laaja S-N tietokanta joka on kerätty kirjallisuudesta. Tarkastellut liitokset ovat tasalevy liitos, krusiformi liitos ja pitkittäisripa levyssä. Muuttujina ovat jännitysvaihtelu, kuormitetun levyn paksuus ja kuormitus tapa. Paksuus effekti on käsitelty uudelleen kaikkia kolmea liitosta ajatellen. Uudelleen käsittelyn avulla on varmistettu paksuus effektin olemassa olo ennen monimuuttuja regressioon siirtymistä. Lineaariset väsymisikä yhtalöt on ajettu kolmelle hitsausliitokselle ottaen huomioon kuormitetun levyn paksuus sekä kuormitus tapa. Väsymisikä yhtalöitä on verrattu ja keskusteltu testitulosten valossa, jotka on kerätty kirjallisuudesta. Neljä tutkimustaon tehty kerättyjen väsymistestien joukosta ja erilaisia väsymisikä arvio metodeja on käytetty väsymisiän arviointiin. Tuloksia on tarkasteltu ja niistä keskusteltu oikeiden testien valossa. Tutkimuksissa on katsottu 2mm ja 6mm symmetristäpitkittäisripaa levyssä, 12.7mm epäsymmetristä pitkittäisripaa, 38mm symmetristä pitkittäisripaa vääntökuormituksessa ja 25mm/38mm kuorman kantavaa krusiformi liitosta vääntökuormituksessa. Mallinnus on tehty niin lähelle testi liitosta kuin mahdollista. Väsymisikä arviointi metodit sisältävät hot-spot metodin jossa hot-spot jännitys on laskettu kahta lineaarista ja epälineaarista ekstrapolointiakäyttäen sekä paksuuden läpi integrointia käyttäen. Lovijännitys ja murtumismekaniikka metodeja on käytetty krusiformi liitosta laskiessa.

Fatigue analysis of brackets with and without scallop for bridge and other deck structure using effective notch stress.

Due to functional requirement of a structural detail brackets with and without scallop are frequently used in bridges, decks, ships and offshore structure. Scallops are designed to serve as passage way for fluids, to reduce weld length and plate distortions. Moreover, scallops are used to avoid intersection of two or more welds for the fact that there is the presence of inventible inherent initial crack except for full penetrated weld and the formation of multi-axial stress state at the weld intersection. Welding all around the scallop corner increase the possibility of brittle fracture even for the case the bracket is not loaded by primary load. Avoiding of scallop will establish an initial crack in the corner if bracket is welded by fillet welds. If the two weld run pass had crossed, this would have given a 3D residual stress situation. Therefore the presences and absence of scallop necessitates the 3D FEA fatigue resistance of both types of brackets using effective notch stress approach ( ). FEMAP 10.1 with NX NASTRAN was used for the 3D FEA. The first and main objective of this research was to investigate and compare the fatigue resistance of brackets with and without scallop. The secondary goal was the fatigue design of scallops in case they cannot be avoided for some reason. The fatigue resistance for both types of brackets was determined based on approach using 1 mm fictitiously rounded radius based on IIW recommendation. Identical geometrical, boundary and loading conditions were used for the determination and comparison of fatigue resistance of both types of brackets using linear 3D FEA. Moreover the size effect of bracket length was also studied using 2D SHELL element FEA. In the case of brackets with scallop the flange plate weld toe at the corner of the scallop was found to exhibit the highest and made the flange plate weld toe critical for fatigue failure. Whereas weld root and weld toe at the weld intersections were the highly stressed location for brackets without scallop. Thus weld toe for brackets with scallop, and weld root and weld toe for brackets without scallop were found to be the critical area for fatigue failure. Employing identical parameters on both types of brackets, brackets without scallop had the highest except for full penetrated weld. Furthermore the fatigue resistance of brackets without scallop was highly affected by the lack of weld penetration length and it was found out that decreased as the weld penetration was increased. Despite the fact that the very presence of scallop reduces the stiffness and also same time induce stress concentration, based on the 3D FEA it is worth concluding that using scallop provided better fatigue resistance when both types of brackets were fillet welded. However brackets without scallop had the highest fatigue resistance when full penetration weld was used. This thesis also showed that weld toe for brackets with scallop was the only highly stressed area unlike brackets without scallop in which both weld toe and weld root were the critical locations for fatigue failure when different types of boundary conditions were used. Weld throat thickness, plate thickness, scallop radius, lack of weld penetration length, boundary condition and weld quality affected the fatigue resistance of both types of brackets. And as a result, bracket design procedure, especially welding quality and post weld treatment techniques significantly affect the fatigue resistance of both type of brackets.

Simulation of structural stress history based on dynamic analysis

Modern machine structures are often fabricated by welding. From a fatigue point of view, the structural details and especially, the welded details are the most prone to fatigue damage and failure. Design against fatigue requires information on the fatigue resistance of a structure’s critical details and the stress loads that act on each detail. Even though, dynamic simulation of flexible bodies is already current method for analyzing structures, obtaining the stress history of a structural detail during dynamic simulation is a challenging task; especially when the detail has a complex geometry. In particular, analyzing the stress history of every structural detail within a single finite element model can be overwhelming since the amount of nodal degrees of freedom needed in the model may require an impractical amount of computational effort. The purpose of computer simulation is to reduce amount of prototypes and speed up the product development process. Also, to take operator influence into account, real time models, i.e. simplified and computationally efficient models are required. This in turn, requires stress computation to be efficient if it will be performed during dynamic simulation. The research looks back at the theoretical background of multibody dynamic simulation and finite element method to find suitable parts to form a new approach for efficient stress calculation. This study proposes that, the problem of stress calculation during dynamic simulation can be greatly simplified by using a combination of floating frame of reference formulation with modal superposition and a sub-modeling approach. In practice, the proposed approach can be used to efficiently generate the relevant fatigue assessment stress history for a structural detail during or after dynamic simulation. In this work numerical examples are presented to demonstrate the proposed approach in practice. The results show that approach is applicable and can be used as proposed.

Survival-time analysis of white spruce during spruce budworm defoliation

Abstract

Fatigue Analysis of Welded Cruciform Joints with Cold Laps

Työssä on tutkittu vetojännityskuormituksen alaisena olevien hitsattujen kuormaa kantamattomien X-liitosten hitsin paikallisen geometrian variaation vaikutusta väsymislujuuteen. Muuttujina olivat reunan pyöristyssäde, kylmäjuoksun suuruus ja kylkikulma. Geometristen muuttujien parametrinen riippuvuussuhde on analysoitu usealla elementtimallilla. Väsymistarkastelu on suoritettu käyttämällä lineaaris-elastista murtumismekaniikkaa (LEFM) tasovenymätilassa ja materiaalina terästä. Särönkasvun suunnan ennustamisessaon käytetty maksimipääjännityskriteeriä sekä jännitysintensiteettikertoimet on määritetty J-integraalilla. Särön ydintymisvaihetta ei ole otettu huomioon. Rakenteen on oletettu olevan hitsatussa tilassa ja jännitysheilahdus on kokonaan tehollinen. Särön kasvunopeuden ennustamiseen on käytetty Paris'n lakia. Väsymislujuustulokset on esitetty karakteristisina väsymisluokkina (FAT) ja sovitettu parametriseksi yhtälöksi. Lopuksi väsymisanalyysin ennustamia tuloksia on verrattu saatavilla oleviin väsytystestituloksiin.

Theoretical Analysis and Simulations of Vertically Vibrated Granular Materials

The dynamical properties ofshaken granular materials are important in many industrial applications where the shaking is used to mix, segregate and transport them. In this work asystematic, large scale simulation study has been performed to investigate the rheology of dense granular media, in the presence of gas, in a three dimensional vertical cylinder filled with glass balls. The base wall of the cylinder is subjected to sinusoidal oscillation in the vertical direction. The viscoelastic behavior of glass balls during a collision, have been studied experimentally using a modified Newton's Cradle device. By analyzing the results of the measurements, using numerical model based on finite element method, the viscous damping coefficient was determinedfor the glass balls. To obtain detailed information about the interparticle interactions in a shaker, a simplified model for collision between particles of a granular material was proposed. In order to simulate the flow of surrounding gas, a formulation of the equations for fluid flow in a porous medium including particle forces was proposed. These equations are solved with Large Eddy Simulation (LES) technique using a subgrid-model originally proposed for compressible turbulent flows. For a pentagonal prism-shaped container under vertical vibrations, the results show that oscillon type structures were formed. Oscillons are highly localized particle-like excitations of the granular layer. This self-sustaining state was named by analogy with its closest large-scale analogy, the soliton, which was first documented by J.S. Russell in 1834. The results which has been reportedbyBordbar and Zamankhan(2005b)also show that slightly revised fluctuation-dissipation theorem might apply to shaken sand, which appears to be asystem far from equilibrium and could exhibit strong spatial and temporal variations in quantities such as density and local particle velocity. In this light, hydrodynamic type continuum equations were presented for describing the deformation and flow of dense gas-particle mixtures. The constitutive equation used for the stress tensor provides an effective viscosity with a liquid-like character at low shear rates and a gaseous-like behavior at high shear rates. The numerical solutions were obtained for the aforementioned hydrodynamic equations for predicting the flow dynamics ofdense mixture of gas and particles in vertical cylindrical containers. For a heptagonal prism shaped container under vertical vibrations, the model results were found to predict bubbling behavior analogous to those observed experimentally. This bubbling behavior may be explained by the unusual gas pressure distribution found in the bed. In addition, oscillon type structures were found to be formed using a vertically vibrated, pentagonal prism shaped container in agreement with computer simulation results. These observations suggest that the pressure distribution plays a key rolein deformation and flow of dense mixtures of gas and particles under vertical vibrations. The present models provide greater insight toward the explanation of poorly understood hydrodynamic phenomena in the field of granular flows and dense gas-particle mixtures. The models can be generalized to investigate the granular material-container wall interactions which would be an issue of high interests in the industrial applications. By following this approach ideal processing conditions and powder transport can be created in industrial systems.

Assessment of meshing techniques for fatigue analysis of load carrying welded joints using the effective notch approach

The effective notch stress approach for the fatigue strength assessment of welded structures as included in the Fatigue Design Recommendation of the IIW requires the numerical analysis of the elastic notch stress in the weld toe and weld root which is fictitiously rounded with a radius of 1mm. The goal of this thesis work was to consider alternate meshing strategies when using the effective notch stress approach to assess the fatigue strength of load carrying partial penetration fillet-welded cruciform joints. In order to establish guidelines for modeling the joint and evaluating the results, various two-dimensional (2D) finite element analyses were carried out by systematically varying the thickness of the plates, the weld throat thickness, the degree of bending, and the shape and location of the modeled effective notch. To extend the scope of this work, studies were also carried out on the influence of

Parallel-Operating Three-Phase Voltage Source Inverters – Circulating CurrentModeling, Analysis and Mitigation

The maximum realizable power throughput of power electronic converters may be limited or constrained by technical or economical considerations. One solution to this problemis to connect several power converter units in parallel. The parallel connection can be used to increase the current carrying capacity of the overall system beyond the ratings of individual power converter units. Thus, it is possible to use several lower-power converter units, produced in large quantities, as building blocks to construct high-power converters in a modular manner. High-power converters realized by using parallel connection are needed for example in multimegawatt wind power generation systems. Parallel connection of power converter units is also required in emerging applications such as photovoltaic and fuel cell power conversion. The parallel operation of power converter units is not, however, problem free. This is because parallel-operating units are subject to overcurrent stresses, which are caused by unequal load current sharing or currents that flow between the units. Commonly, the term ’circulatingcurrent’ is used to describe both the unequal load current sharing and the currents flowing between the units. Circulating currents, again, are caused by component tolerances and asynchronous operation of the parallel units. Parallel-operating units are also subject to stresses caused by unequal thermal stress distribution. Both of these problemscan, nevertheless, be handled with a proper circulating current control. To design an effective circulating current control system, we need information about circulating current dynamics. The dynamics of the circulating currents can be investigated by developing appropriate mathematical models. In this dissertation, circulating current models aredeveloped for two different types of parallel two-level three-phase inverter configurations. Themodels, which are developed for an arbitrary number of parallel units, provide a framework for analyzing circulating current generation mechanisms and developing circulating current control systems. In addition to developing circulating current models, modulation of parallel inverters is considered. It is illustrated that depending on the parallel inverter configuration and the modulation method applied, common-mode circulating currents may be excited as a consequence of the differential-mode circulating current control. To prevent the common-mode circulating currents that are caused by the modulation, a dual modulator method is introduced. The dual modulator basically consists of two independently operating modulators, the outputs of which eventually constitute the switching commands of the inverter. The two independently operating modulators are referred to as primary and secondary modulators. In its intended usage, the same voltage vector is fed to the primary modulators of each parallel unit, and the inputs of the secondary modulators are obtained from the circulating current controllers. To ensure that voltage commands obtained from the circulating current controllers are realizable, it must be guaranteed that the inverter is not driven into saturation by the primary modulator. The inverter saturation can be prevented by limiting the inputs of the primary and secondary modulators. Because of this, also a limitation algorithm is proposed. The operation of both the proposed dual modulator and the limitation algorithm is verified experimentally.