Nodal Analysis Research Articles

A simplified model to predict transient liquid loading in gas wells

This paper presents a simplified model to predict the inception of liquid loading in gas wells and its subsequent transient phenomenon. This model enables the estimation of erratic or cease of production due to liquid loading using a simple but robust technique. This approach is validated with field data. The model described in this paper proposes the use of the so-called “nodal analysis technique” to predict liquid loading in gas wells. The approach proposed modifies the tubing performance relationship instead of using the common critical velocity or minimum pressure point concept. This modification enables the simple use of nodal analysis to accurately predict liquid loading initiation, including the amount of time required to cease production after the inception of liquid loading. The model shows good agreement field data on the prediction of liquid loading.From the modeling results and comparison with field data, it is possible to conclude that this model can provide a reasonable prediction of the liquid loading phenomena. For instance, one of the main objectives of using models to predict liquid loading is to anticipate when a gas well would start suffering from liquid loading problems, and potentially stop flowing. The use of conventional models showed a significant mismatch in the critical flow for liquid loading initiation when compared to field data while the use of the model proposed would reduce this mismatch significantly. In addition to that, the use of this simplified model also enables understanding of the main field symptoms related to liquid loading in gas wells.

An automated FPGA real-time simulator for power electronics and power systems electromagnetic transient applications

The paper presents an automated FPGA-based real-time electromagnetic transients simulator for power electronics and power systems applications. The simulator features an automated procedure enabling its straightforward applications to different topologies by avoiding the need of complex FPGA programming. The proposed solver integrates: (i) the Modified Augmented Nodal Analysis (MANA) method, (ii) the Fixed Admittance Matrix Nodal Method (FAMNM), (iii) the optimal selection of the switch conductance parameter, and (iv) efficient sparse matrix-to-vector multiplier. The simulator is able to accurately reproduce, in real-time, electromagnetic transients taking place in power electronics devices together with electromagnetic waves propagating in transmission lines. The peculiar structure of the MANA-FAMNM solver enables to reach extremely low integration time steps and avoids the need to redesign the FPGA code. The results of the proposed simulator are validated by dedicated comparisons with off-line EMTP-RV simulations and a hardware-in-the-loop (HIL) test for a three-phase two-level inverter and a three-phase power network.

Decision making by using a computer algorithm to analyze hydrocarbon production systems

This paper shows an algorithm that allows to automate the procedures of nodal analysis and flow optimization in a hydrocarbon production system. The procedure of nodal analysis is highly useful in flow wells, intermittent wells or in wells with artificial production systems. The nodal analysis evaluates a production system divided into two basic components: flow through vertical piping or production piping, and flow through horizontal piping or discharge line. For the prediction of each component's behavior, the pressure drop in each component is obtained. In order to obtain the pressure drops, nodes in different important points within the production system must be assigned; therefore, production expenses can vary and, by using a suitable calculation method, the pressure drop between two nodes is calculated. Then, a node is selected and the pressure drops are added to or subtracted from the initial pressure point or departure node, until obtaining the solution node. The results obtained when using the algorithm have allowed to update both procedures, obtaining advantages such as improvement in response time, among others. This analysis is a crucial point when making decisions related to production costs in any oil company.

Speed-up hyperspheres homotopic path tracking algorithm for PWL circuits simulations.

In the present work, we introduce an improved version of the hyperspheres path tracking method adapted for piecewise linear (PWL) circuits. This enhanced version takes advantage of the PWL characteristics from the homotopic curve, achieving faster path tracking and improving the performance of the homotopy continuation method (HCM). Faster computing time allows the study of complex circuits with higher complexity; the proposed method also decrease, significantly, the probability of having a diverging problem when using the Newton–Raphson method because it is applied just twice per linear region on the homotopic path. Equilibrium equations of the studied circuits are obtained applying the modified nodal analysis; this method allows to propose an algorithm for nonlinear circuit analysis. Besides, a starting point criteria is proposed to obtain better performance of the HCM and a technique for avoiding the reversion phenomenon is also proposed. To prove the efficiency of the path tracking method, several cases study with bipolar (BJT) and CMOS transistors are provided. Simulation results show that the proposed approach can be up to twelve times faster than the original path tracking method and also helps to avoid several reversion cases that appears when original hyperspheres path tracking scheme was employed.

Transient Analysis of Lumped Circuit Networks-Loaded Thin Wires By DGTD Method

With the purpose of avoiding very fine mesh cells in the proximity of a thin wire, the modified telegrapher’s equations (MTEs) are employed to describe the thin wire voltage and current distributions, which consequently results in reduced number of unknowns and augmented Courant–Friedrichs–Lewy (CFL) number. As hyperbolic systems, both the MTEs and the Maxwell’s equations are solved by the discontinuous Galerkin time-domain (DGTD) method. In realistic situations, the thin wires could be either driven or loaded by circuit networks. The thin wire–circuit interface performs as a boundary condition for the thin wire solver, where the thin wire voltage and current used for the incoming flux evaluation involved in the DGTD-analyzed MTEs are not available. To obtain this voltage and current, an auxiliary current flowing through the thin wire–circuit interface is introduced at each interface. Corresponding auxiliary equations derived from the invariable property of characteristic variable for hyperbolic systems are developed and solved together with the circuit equations established by the modified nodal analysis (MNA) modality. Furthermore, in order to characterize the field and thin wire interactions, a weighted electric field and a volume current density are added into the MTEs and Maxwell–Ampere’s law equation, respectively. To validate the proposed algorithm, three representative examples are presented.

New Authorities: Relating State and Non-State Security Auspices in South African Improvement Districts

Disciplines engaged in governance studies have increasingly recognised the pluralistic nature of governance practices, involving both the state and non-state operating as auspices and providers in local and global settings. Security governance developments, in particular, have resulted in security provision being undertaken outside of state processes by the non-state in new power formations. Drawing on the work of the Ostroms and adopting a nodal analysis, this paper locates, within this context, the widespread emergence of ‘Improvement Districts’ as a new authority in hybrid, polycentric security governance arrangements. The paper seeks to highlight the process by which Improvement Districts are created and organised; how they assert authority on public spaces and how they impact on security networks. The main argument is that Improvement Districts constitute a site in which there are multiple, shifting sites of authority and that this has implications for security provision in terms of effectiveness, regulation and power.

Current and voltage dependent sources modelling in MATE–multi-area Thévenin equivalent concept

The objective of this paper is to provide a clear derivation of the Multi-Area Thévenin Equivalent Concept (MATE) including current- and voltage-dependent sources. The links concept in MATE is advantageous in representing branches connecting subsystems. MATE deviates from Diakoptics and from the Modified Nodal Analysis (MNA) methods in the way it is solved, by manipulating the submatrices in a form that preserves the individuality of the internal subsystems while solving their interdependences at the level of Thévenin Equivalents. The generalization presented in this paper expands the link branch equations to dependent, coupled, linear or nonlinear relations, thus resulting in unsymmetrical matrices. Its significance occurs when complex control systems and power system equations are simultaneously solved in an Electromagnetic Transients Program (EMTP). In this case, exact results can be achieved with less computational effort for power system dynamics studies. A test case with simulation results illustrates the main modelling concepts.

Plane-Pair PEEC Model for Power Distribution Networks With Sub-Meshing Techniques

We present an improved plane-pair partial element equivalent circuit (PEEC) model for power distribution network modeling based on the PEEC formulation. The model can include via connections, decoupling capacitor macro-models, and discontinuities such as holes in the plane-pairs. An efficient approximate inductance sub-meshing model is described for large printed circuit plane-pairs with complex geometries and numerous vias. The modified nodal analysis (MNA) used leads to a flexible circuit solution where we can compute inductances, resistances, impedances, or other circuit models, including dc solutions. The MNA equations include effective optimizations such as the placement of capacitors. Today, a large class of methods are available based on numerous formulations including finite-difference time-domain, finite-element method, integral equation model, and cavity models. Each of the approaches has its own type of problems for which it is most suitable.

Nodal Analysis by Use of ESP Intake and Discharge Pressure Gauges

Summary Electrical submersible pumps (ESPs) are often not only equipped with bottomhole-pressure gauges, but more frequently contain discharge pressure gauges as well. This combination of gauges can be used to extend the classical nodal-analysis approach with an extra component. Inclusion of an additional discharge pressure gauge allows for the construction of two sets of inflow and outflow equations: one set where the selected node is the pump intake and one set where the selected node is the pump discharge. A graphical method demonstrates how to evaluate these two sets of equations simultaneously. This graphical method is then further condensed into a few simple and easy to use guidelines. Six possible situations can be faced during operation of an ESP: improving/decreasing performance upstream of the intake pressure gauge, improving/decreasing performance between the intake and the discharge pressure gauges, and improving/decreasing performance downstream of the discharge pressure gauge. Analysis of the changes in intake and discharge pressures can quickly identify which one of these six scenarios is affecting the production performance, thus enabling the breakdown of the production system into the reservoir performance, ESP performance, and tubing performance. Several field examples of oil wells completed with ESPs that contain intake and discharge pressure gauges illustrate the benefit of this procedure. This method is applicable for any downhole pump with intake and discharge pressure gauges. Even if pump maps are not available, or when the well's liquid-flow rates are not (accurately) known, it is a reliable method to localize downhole production problems. This approach makes full use of the advantages of multiple downhole gauges; nodal analysis in combination with intake and discharge pressure gauges is a quick, inexpensive, and powerful method to determine the location of production problems, so appropriate actions can be taken to maximize well performance.

Nodal Analysis for Unconventional Reservoirs—Principles and Application

Summary Nodal analysis is the standard technique used to evaluate the performance of integrated production systems. Two curves represent the capacities of the inflow and of the outflow, and the intersection of the two curves gives the solution operating point. Limitations of traditional nodal analysis include: Results are offered only at a snapshot, not as a function of time. Inflow-performance-relationship (IPR) models are limited, with none available for shale gas wells. Analysis is performed on a well-by-well basis, with no account of multiwell interference. We propose a new nodal-analysis method that enables the study of transient production systems, such as unconventional reservoirs, with IPR models generated from a high-speed semianalytical reservoir simulator and outflow curves generated from a steady-state pipeline simulator. The use of analytical reservoir simulation allows accurate, reliable modeling of the real inflow system. The new approach studies the time-lapse behavior of the system, with consideration of production history and neighboring-well interference. This new method enables the study of transient deliverability at the wellhead, where the measurement is usually available, and shows the time-lapse relationship between wellhead pressure and production rate. We provide examples of wellhead deliverability and choke management and explain advantages of the method with case studies involving tight and shale wells. The method is also applied to design and optimize artificial lift in unconventional wells and to study the method's validity over time. In addition, we discuss an example of operational well dynamics with time-lapse nodal analysis. Furthermore, this new method generates discussion about some concepts that are often taken for granted—What should be the definition of IPR in a transient production system? On the IPR curve, is the zero-rate-pressure the reservoir pressure? Can IPR curves at two different timesteps cross each other? Finding the answers to these questions will help us better understand production systems. The commonly used productivity-index (PI) method is reviewed and compared with the new method. Results show that one should not use the PI method when well operational conditions change.

Sentinel node biopsy using blue dye and technetium99 in advanced gastric cancer: anatomical drainage and clinical application.

Lymph node metastases are an independent prognosis factor in gastric carcinoma (GC) patients. Radical lymphadenectomy can improve survival but it can also increase surgical morbidity. As a principle, sentinel node (SN) navigation surgery can avoid unnecessary lymphadenectomy without compromising prognosis. In this pilot study, 24 patients with untreated GC were initially screened for SN navigation surgery, of which 12 were eligible. Five patients had T2 tumors, 5 had T3 tumors and 2 had T1 tumors. In 33% of cases, tumor diameter was greater than 5.0 cm. Three hundred and eighty-seven lymph nodes were excised with a median of 32.3 per patient. The SN navigation surgery was feasible in all patients, with a median of 4.5 SNs per patient. The detection success rate was 100%. All the SNs were located in N1 and N2 nodal level. In 70.9% of cases, the SNs were located at lymphatic chains 6 and 7. The SN sensitivity for nodal staging was 91.6%, with 8.3% of false negative. In 4 patients who were initially staged as N0, the SNs were submitted to multisection analyses and immunohistochemistry, confirming the N0 stage, without micrometastases. In one case initially staged as negative for nodal metastases based on SN analyses, metastases in lymph nodes other than SN were found, resulting in a 20% skip metastases incidence. This surgery is a reproducible procedure with 100% detection rate of SN. Tumor size, GC location and obesity were factors that imposed some limitations regarding SN identification. Results from nodal multisection histology and immunohistochemistry analysis did not change initial nodal staging.

Dynamic Circuit Model for Spintronic Devices

In this work we propose a finite-difference scheme based circuit model of a general spintronic device and benchmark it with other models proposed for spintronic switching devices. Our model is based on the four-component spin circuit theory and utilizes the widely used coupled stochastic magnetization dynamics/spin transport framework. In addition to the steady-state analysis, this work offers a transient analysis of carrier transport. By discretizing the temporal and spatial derivatives to generate a linear system of equations, we derive new and simple finite-difference conductance matrices that can, to the first order, capture both static and dynamic behaviors of a spintronic device. We also discuss an extension of the spin modified nodal analysis (SMNA) for time-dependent situations based on the proposed scheme.

Voltage Stability Assessment Using Equivalent Nodal Analysis

This paper presents a simple method for voltage stability assessment by reducing the bus impedance matrix of a network into its two-bus equivalent model at a referred bus. Closed-form equations of the equivalent model could then be derived to facilitate an equivalent nodal analysis at the referred bus to determine its voltage stability limit. The method was also shown to have parallels with the well-accepted modal analysis of the Jacobian matrix. These include defining geometric indices to identify critical buses and participation factors to quantify contribution of network elements to their criticality. The method however does not require post power flow solution processing of the Jacobian matrix; such computational advantage may make it suitable for online applications. The IEEE 300-bus system was tested with the method which was shown to be valid and computationally robust and efficient.

Recursive partitioning identifies greater than 4 U of packed red blood cells per hour as an improved massive transfusion definition.

Massive transfusion (MT) is classically defined as greater than 10 U of packed red blood cells (PRBCs) in 24 hours. This fails to capture the most severely injured patients. Extending the previous work of Savage and Rahbar, a rolling hourly rate-based definition of MT may more accurately define critically injured patients requiring early, aggressive resuscitation. The Prospective Observational Multicenter Major Trauma Transfusion (PROMMTT) trial collected data from 10 Level 1 trauma centers. Patients were placed into rate-based transfusion groups by maximal number of PRBCs transfused in any hour within the first 6 hours. A nonparametric analysis using classification trees partitioned data according to mortality at 24 hours using a predictor variable of maximum number PRBC units transfused in an hour. Dichotomous variables significant in previous scores and models as predictors of MT were used to identify critically ill patients: a positive finding on Focused Assessment with Sonography in Trauma (FAST) examination, Glasgow Coma Scale (GCS) score less than 8, heart rate greater than 120 beats/min, systolic blood pressure less than 90 mm Hg, penetrating mechanism of injury, international normalized ratio greater than 1.5, hemoglobin less than 11, and base deficit greater than 5. These critical indicators were then compared among the nodes of the classification tree. Patients omitted included those who did not receive PRBCs (n = 24) and those who did not have all eight critical indicators reported (n = 449). In a population of 1,245 patients, the classification tree included 772 patients. Analysis by recursive partitioning showed increased mortality among patients receiving greater than 13 U/h (73.9%, p < 0.01). In those patients receiving less than or equal to 13 U/h, mortality was greater in patients who received more than 4 U/h (16.7% vs. 6.0%, p < 0.01) (Fig. 1). Nodal analysis showed that the median number of critical indicators for each node was 3 (2-4) (≤4 U/h), 4 (3-5) (>4 U/h and ≤13 U/h), and 5 (4-5.5) (>13 U/h). A rate-based transfusion definition identifies a difference in mortality in patients who receive greater than 4 U/h of PRBCs. Redefining MT to greater than 4 U/h allows early identification of patients with a significant mortality risk who may be missed by the current definition. Prognostic/epidemiologic study, level III.

Magnetic Circuit Analysis for a Magnetless Double-Rotor Flux Switching Motor

In this paper, an efficient and effective magnetic circuit model is developed for the analysis of a magnetless double-rotor flux switching motor. First, a magnetic circuit network for the proposed machine is built, and formulas for the calculation of the magnetic components in the circuit, including the permeances of teeth, yokes, slots, leakage permeances, and air-gap permeances, are derived. Then, by applying Kirchhoff’s current law to each node in the network, nodal analysis is used to solve the matrix equation. All the circuit variables, and hence the complete motor magnetic characteristics are being considered. When the rotor rotates, the air-gap permeances are adjusted accordingly, and the above calculations are repeated, so as to find the complete static performance of the motor. Last, the validity and the effectiveness of the proposed method are verified using both the experimental results and the simulation results obtained from the time-stepping finite-element method.

A thermodynamic approach to compare the performance of rhombic-drive and crank-drive mechanisms for a beta-type Stirling engine

In this study, the effect of rhombic drive and crank drive mechanisms on the performance of a beta-type Stirling engine was investigated by nodal analysis. Kinematic and thermodynamic relations for both drive mechanisms were introduced and a Fortran code was written for the solution. Piston strokes, cylinder and displacer diameters, hot and cold end temperatures, regenerator volumes and heat transfer surface areas were taken equal for both engines with two different drive mechanisms. In the analysis, air was used as the working gas. Engine power and efficiency were compared for different charge pressure values, working gas mass values, heat transfer coefficients and hot end temperatures. Maximum specific engine power was 1410 W/L for the engine with rhombic drive mechanism and 1200 W/L for the engine with crank drive mechanism at 4 bars of charge pressure and 500 W/m2K heat transfer coefficient. Rhombic drive mechanism was relatively advantageous at low working gas mass values and high hot end temperatures. In comparison with the engine having rhombic drive mechanism, the relatively poor kinematic behaviour of the engine having crank drive mechanism caused lower engine efficiency and performance. Heat transfer coefficient was also predicted by using an experimental pressure trace.

Modeling Percolation in Polymer Nanocomposites by Stochastic Microstructuring.

A methodology was developed for the prediction of the electrical properties of carbon nanotube-polymer nanocomposites via Monte Carlo computational simulations. A two-dimensional microstructure that takes into account waviness, fiber length and diameter distributions is used as a representative volume element. Fiber interactions in the microstructure are identified and then modeled as an equivalent electrical circuit, assuming one-third metallic and two-thirds semiconductor nanotubes. Tunneling paths in the microstructure are also modeled as electrical resistors, and crossing fibers are accounted for by assuming a contact resistance associated with them. The equivalent resistor network is then converted into a set of linear equations using nodal voltage analysis, which is then solved by means of the Gauss–Jordan elimination method. Nodal voltages are obtained for the microstructure, from which the percolation probability, equivalent resistance and conductivity are calculated. Percolation probability curves and electrical conductivity values are compared to those found in the literature.

Analysis of Trackside Flywheel Energy Storage in Light Rail Systems

The objective of this paper is to analyze the potential benefits of flywheel energy storage for dc light rail networks, primarily in terms of supply energy reduction, and to present the methods used. The method of analysis is based on train movement and electrical-network load-flow simulation. The results of the analysis indicate potential energy saving of up to 21.6% due to the introduction of the flywheel energy storage. The energy saving effects of receptivity (or energy transfer from one train to another) are also considered. Additional benefits of the flywheel energy storage in terms of voltage drop improvements of 29.8% and a reduction in peak substation power loading of 30.1% are demonstrated in a test case scenario. A novel traction electrical-network load-flow algorithm using modified nodal analysis (MNA) is described in detail. This allows an intuitive representation of network elements such as trains and substations and a direct solution of substation currents.

Numerical simulation of a grounding grid buried in horizontal multilayered earth based on ‘T’ typical basic elements in the frequency domain

In this study, the authors propose a new mathematical model that allows the accurate computation of currents flowing through a substation's grounding grid buried in horizontal multilayered earth, which is a hybrid of Galerkin's method of moment and the conventional nodal analysis method based on ‘T’ typical basic elements. The dynamic state complex image method and the closed form of Green's function of a horizontal dipole or monopole in the earth model are introduced into this mathematical model to enhance its efficiency. In addition, an analytical formula for mutual induction and impedance coefficient calculations are used to accelerate the application of this method. Finally, some numerical results related to a practical grounding problem are discussed and some conclusions are given.

Whole Body MRI in the Staging of Esophageal Cancer - A Prospective Comparison with Whole Body 18F-FDG PET-CT

Background: Positron emission tomography and computed tomography (PET-CT) is established in the staging of esophageal cancer. In this study, an MRI protocol was designed to emulate the anatomical (T1-weighed (T1W) and T2W imaging) and functional information (diffusion-weighted imaging) provided by PET-CT. Methods: In all, 49 patients with carcinoma of the esophagus underwent PET-CT and whole-body MRI (WBMRI). WBMRI was carried out using dedicated sequences tailored to detect metastatic disease at each area corresponding to the anatomical coverage of PET-CT. Nodal status was determined from histopathology and endoscopic ultrasound biopsy (EUS). Results: PET-CT and WBMRI identified the primary tumor in 46/49 (94%) and 48/49 (98%) patients, respectively. Nodal analysis in patients undergoing surgery (n = 18) yielded sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy of 27, 100, 100, 47 and 56% for PET-CT, compared with 30, 100, 100, 53 and 61% for WBMRI. When nodal analysis included both surgical specimens and EUS criteria (n = 39), sensitivity, specificity, PPV, NPV and accuracy were 46, 91, 93, 40 and 59% for PET-CT compared with 59, 92, 94, 50 and 67% for WBMRI. Both imaging modalities identified distant metastases in 2 patients. Conclusion: WBMRI has similar accuracy to PET-CT in detecting the primary tumor, nodal deposits and for exclusion of systemic metastatic disease.