Academic literature on the topic 'Lüder FMR model'

The countries in the Central American region, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua and Panama, have carried out a reform process of their public financial management systems, seeking to guarantee higher-quality, reliable and timely information, using mainly the International Public Sector Accounting Standards (IPSAS). In this context, the chapter has a double objective. On the one hand, valuing from the Model of Contingency of the Professor Lüder, the influence of the environment in the implementation of processes of innovation in systems of government financial management in the Central American region. On the other hand analyze the main implementation strategies in the process of adoption or adaptation of IPSAS in these countries. For this purpose, interviews have been made with those responsible for the process of implementation of IPSAS in the countries of the region, as well as an in-depth review of the documents and legislation issued in government financial administration in these countries.

This study examined the fracture behaviour of pipes containing surface flaws oriented circumferentially and made from a material that exhibits yield discontinuity (known as Lüders plateau) with the view of making recommendations for the assessment of pipes subject to high level of plasticity. Starting with the fundamental and first principles, uniaxial tensile tests were carried out with the use of digital image correlation (DIC) to observe the formation and propagation of Lüders bands quantitatively. Finite element (FE) analyses were then carried out to simulate the Lüders banding phenomenon in uniaxial tensile specimens and consequently cracked pipes. Different material models were adopted in FE analyses, including the stress-strain curve with a flat stress plateau neglecting upper yield stress, and the so-called ‘up-down-up’ (UPU) stress-strain curve for refining crack driving force predictions. The numerical analysis of tensile tests demonstrated that UPU stress-strain model satisfactorily simulated the main macroscopic features of Lüders band observed in the experiment. FE analysis of flawed pipes using both flat and UPU stress-strain curves produced a similar trend in the crack tip opening displacement (CTOD)-strain trajectory as that obtained from large-scale testing. It was seen that the shape of the UPU stress-strain curve, particularly the magnitude of softening, considerably affects the magnitude of crack driving force in the flawed pipe. However, the strain localisation associated with Lüders banding was not observed in the circumferentially flawed pipe in the case of using the flat stress-strain curve. The CTOD crack driving force obtained from simulations was lower than the CTOD obtained from experiments in the Lüders plateau regime, even with the consideration of ductile tearing. Finally, as a result of this study, recommendations on the optimum choice of material parameters were made for more accurate predictions of crack driving force in the presence of yield discontinuity.

A study has been performed to better understand ultimate bending moment and strain capacities of pipelines in relation to criteria defined in the design codes. An 18″ HPHT flowline was designed to undergo global buckling on uneven seabed and to resist trawl gear interference. The high temperature (155 degC) and pressure (300 bar) posed considerable design challenges for material selection and design criteria. A CRA-lined X60 CMn pipeline was selected for the project. The pipeline was of seamless manufacture for which the stress/strain characteristics are subject to the effect of Lüders bands. The DNV-OS-F101 code covers a wide range of D/t but does not specifically address Lüder’s material behaviour which could significantly reduce the bending moment capacity of pipe. The global buckling and trawl pull-over FE analysis results indicated the pipe was highly utilized, requiring excessive amounts of seabed intervention at great cost to meet the DNV LCC criteria. Detailed FE simulation of limit states for local buckling and strain localization of a 3D solid element pipe model was performed, with both Roundhouse and Lüders material properties, to investigate pipe capacity in relation to that stipulated by the design codes. The pipe moment capacity was established by obtaining the moment curvature relationship by bending the local pipe section subject to internal pressure until the maximum resistance was reached. Imperfections were introduced to initiate local buckling at the desired location. To determine strain concentration factors and strain localization, the effects of thickness changes and weld misalignment were also studied. The DNV OS-F101 LCC moment criterion formulation computes a decreasing moment capacity for increasing internal pressure. It has been suggested in the literature that this is correct for higher D/t but the criterion may be conservative for pipes with lower D/t. The combination of Lüders material with low D/t is not specifically addressed by any design code. Clarification of these aspects will provide a better understanding of the risk of failure for highly utilized seamless pipelines and allow for modified design criteria that will reduce seabed intervention costs. The results of the study showed that a higher bending moment criterion and associated strain criterion could be adopted for the design that allows for the higher initial strain caused by Lüder’s plateau. The ultimate bending moment capacity of low D/t pipe with Lüder’s material was found to be similar to that of Roundhouse material due to work hardening. In addition, it was demonstrated that the potential strength of the CRA liner could enhance the moment capacity of the seamless pipe.

Abstract For small to medium size rigid pipelines, reel-laying is an efficient installation method. Higher lay rates are achieved thanks to unreeling rigid pipeline down to seabed. However, the process is mechanically onerous. Pipelines go through successive large bending strains up to 2% or 3%. This is the primary degradation mechanism and affects the mechanical in-service performance of the subsea line. Assessment of material response to the installation sequence is a key point for pipeline engineering. Conventional monotonic material stress-strain relation or cyclic Ramberg-Osgood models are not suitable for repeated large bending sequences or reel-lay installation FE simulations. A parameter-based material model has been tailored for carbon steel seamless pipe under reeling. The evolution of the material behaviour from the first load to stabilised cycle is addressed, by introducing an Abaqus User Subroutine written in FORTRAN. The material properties are programmed in each straining cycle, then active in their corresponding loading cycle. Specimens from DNV450 grades pipe have been tested in an extensive small-scale test and bending trial campaign in order to calibrate finite element model. DIC is used during tensile test and LCF (Low Cycle Fatigue) test to allow accurate measurement on large straining ranges. The parameter-based model can provide response to the first load, including Lüders plateau, up to stabilized hysteresis loop; it is fully parametric by Python. Comparison is made with the more conventional material model or full-scale bending trials demonstrating improved accuracy and agreement with testing. The proposed material model is part of an in-house material model library integrated in a numerical tool package dedicated to rigid pipeline design.

Strain-based design (SBD) is used to complement conventional allowable stress design for pipelines operated in environments with potentially large ground movements such as those found in permafrost and seismically active regions. Reliable and accurate prediction of tensile strain capacity (TSC) plays a critical role in strain-based design. As reported previously over the past 6+ years, a comprehensive experimental and numerical program was undertaken to characterize the TSC of welded pipelines, develop a finite element analysis (FEA) approach and equations capable of predicting TSC, and establish a strain-based engineering critical assessment (SBECA) methodology. The previous FEA model and TSC equations were validated against about 50 full-scale pipe strain capacity tests and are accurate within the validated variable ranges. In the current paper, enhancements of the previous model and equations are described. The enhancements include incorporation of advanced damage mechanics modeling into TSC prediction, development of a new TSC equation, expansion of variable ranges and functionality upgrades. The new model and equation are applicable over larger ranges of material properties and flaw sizes. The new FEA model is also used to establish surface flaw interaction rules for SBD. The new FEA model is validated against more than 40 full-scale non-pressurized and pressurized tests and underpins the development of the new TSC equation. The equation is validated against a total of 93 full-scale tests (FST). In addition to the enhancements, sample applications of the TSC model and equation are presented in the paper, for example, an investigation of the effects on strain capacity of Lüders strain and ductile tearing. Challenges in predicting TSC are also addressed.