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IN THIS ISSUE

### RESEARCH PAPERS

J. Energy Resour. Technol. 2007;129(4):281-288. doi:10.1115/1.2790979.

Drill strings are used in oil and gas production as well as geothermal wells. They experience destructive vibrations, many of which are highly dependent on drill string modes. In this paper, we show that the lowest frequency modes are not necessarily the most critical and we delineate a methodology for reducing the number of modes representing the drill string. The frequency response function and stability diagram are used as measures of dynamic similarity between the proposed model and the drill string. We also introduce a novel approach to represent a drill string in laboratory test rigs. This approach not only represents the drill string dynamics but also offers flexibility to modify, remove, or augment the modes representing the system. The underlying principle is that in a multi-degree-of-freedom in-series spring-mass system with Rayleigh damping, dynamic modes can be decoupled. Applying the force to the end node (bit), the modes can then be configured separately in a parallel arrangement where their contributions to bit displacement are added algebraically. A practical arrangement for this purpose is proposed in this paper. Construction of a test rig that accurately represents the drill string dynamics is critical to validation of any test data on bits, bottom hole assemblies, instrument subs, and so on.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(4):289-299. doi:10.1115/1.2790980.

This work presents the mathematical method to design complex trajectories for three-dimensional (3D) wells using spline in tension as coordinate functions. 3D spline-in-tension trajectories are obtained for various end conditions: free end, set end, free inclination/set azimuth, and set inclination/free azimuth. The resulting trajectories are smooth continuous functions which better suit the expected performance of modern rotary steerable deviation tools, in particular, point-the-bit and push-the-bit systems. A continuous and gradual change in path curvature and tool face results in the smoothest trajectory for 3D wells, which, in turn, results in lower torque, drag, and equipment wear. The degree of freedom and the associated parameters of the 3D curves express the commitment between the average curvature to the final length of the path that can be adjusted to fit the design requirements and to optimize the trajectory. Several numerical examples illustrate the various end conditions. This paper also presents the full mathematical expressions for the 3D path for four end conditions. The method is directly applicable to the well planning cycle as well as to automatic and manual hole steering’s. Spline-in-tension functions differ from the cubic functions in the extent that an additional parameter, which represent the “tension” of the curve, can be controlled. A totally “relaxed” curve is identical to a cubic curve, and as the tension increases a shorter curve length is obtained with a consequent effect in the curvature profile along the curve. In the limit, as the tension increases to infinite, the spline-in-tension approaches to a straight line. The tension offers an additional degree of freedom, which can be used to further optimize the final trajectory. The 3D spline-in-tension model provides the most versatile model to plan a 3D well trajectory to date. Suitable manipulation of the curve parameters, namely, $L0$, $L1$, and the three tensions, allows to give to the planned trajectory any desired behavior.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(4):300-306. doi:10.1115/1.2790981.

A generalized computational method for planar kinematic analysis of pumping units is presented in this study. In this method, a local coordinate system is assigned to each body with respect to a fixed global coordinate system. The position of each point in a body is determined by specifying the global translational coordinates of the local coordinate system origin and its rotational angle relative to the global coordinate system. Constraint equations of motion are developed using the vector of coordinates of the connected bodies. These equations are solved to yield the position, velocity, and acceleration of the individual linkages at each instance of time. Both rotational and translational types of joints are considered in the analysis. The translational joint analysis is not discussed in this paper as they are not applicable for beam pumping units. This method can be used as an effective tool for pumping unit design and optimization. An example is provided to show the application of this method.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(4):307-313. doi:10.1115/1.2790982.

This paper presents the potential of composting oil wet drill cuttings as a drilling waste disposal option. The potential is substantiated by results from several laboratory and field experiments. Artificially oil wetted drill cuttings were prepared by adding commonly used base oils from Norwegian offshore operations to a representative clay. Degradation of the hydrocarbon components in the oily wet cuttings by vermicomposting was successfully accomplished. The composts were beneficially used as part of growing media for landscape plants; ryegrass, coniferous, and deciduous trees, and the fertilization effect was compared with commercial NPK fertilizers. The plant growth studies showed that the composts produced by treating artificial oily drill cuttings by vermicomposting had considerable fertilizing effect on ryegrass and trees.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(4):314-324. doi:10.1115/1.2790983.

One of the key factors in the operation of a natural gas pipeline network is the linepack in the network. The desired operation of the network as derived from estimated receipts and deliveries is expressed in terms of the desired linepack profile that must be maintained. The compressor stations in the pipeline network are then operated in a manner that generates this linepack profile. Generally, the operating points selected for the units in the compressor stations are based on experience and experimentation and are therefore not optimal. In this paper, we present a systematic approach for operating the units of a compressor station to meet a specified linepack profile. The first step in developing this approach is the derivation of a numerical method for analyzing the flow through the pipeline under transient nonisothermal conditions. We have developed and verified a fully implicit finite difference formulation that provides this analysis capability. Next, the optimization of the compressor stations is formulated as a standard nonlinear programing problem in the following form: Find the values in the design variable vector denoted by $b=[b1,b2,…,bn]T$, to minimize a given objective function $F(b)$, subject to the constraints $gj(b)⩽0$, $j=1,…,m$. Here, $n$ is the number of operational parameters whose optimal value is to be determined, while $m$ is the number of operational constraints that must be enforced. In our formulation, the design variables are chosen to be the operating speeds of the units in the compressor stations, while the objective function is taken to be the average fuel consumption rate over the interval of interest, summed over all units. The constraint functions $gj(b)$ are formulated suitably to ensure that operational limits are met at the final solution that is obtained. The optimization problem is then solved using a sequential unconstrained minimization technique (SUMT), in conjunction with a directed grid search method for solving the unconstrained subproblems that are encountered in the SUMT formulation. The evaluation of the objective function and constraint functions at each step of the optimization is done by using the fully implicit analysis method mentioned above. A representative numerical example has been solved by the proposed approach. The results obtained indicate that the method is very effective in finding operating points that are optimal with respect to fuel consumption. The optimization can be done at the level of a single unit, a single compressor station, a set of compressor stations, or an entire network. It should also be noted that the proposed solution approach is fully automated and requires no user involvement in the solution process.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(4):325-331. doi:10.1115/1.2790994.

In this present work, the various emulsified fuel ratios of 50D:50E (50% Diesel No. 2: 50% ethanol 100% proof), 60D:40E, and 70D:30E have been prepared. Performance and emission tests are carried out for the emulsified fuel ratios and they have been compared with diesel fuel. The test results show that 50D:50E has given the best result based on the performance and less emission than the other fuel ratios. By keeping the selected fuel 50D:50E, the same performance and emission tests are conducted by varying their injection angles at $18deg$, $20deg$, $23deg$, and $24deg$. The outcome shows better performance and less emission by the fuel 50D:50E at $24deg$ injection angle (IA). Further, ignition delay, maximum heat release, and peak combustion pressure tests have been conducted. These results show that increase in IA decreases the delay period, thus increasing the pressure obtained at the maximum output. Also, $P‐θ$ diagram is drawn between crank angle and cylinder pressure. The maximum value is attained by the fuel 50D:50E at $24deg$ IA. All the tests have been conducted by maintaining the engine speed at $1500rpm$. The result shows that 50D:50E ratio fuel has been identified as a good emulsified fuel and its better operation is obtained at $24deg$ IA based on its best performance and less emissions.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(4):332-337. doi:10.1115/1.2794768.

Homogeneous charge compression ignition (HCCI) is a new engine technology with fundamental differences over conventional engines. HCCI engines are intrinsically fuel flexible and can run on low-grade fuels as long as the fuel can be heated to the point of ignition. In particular, HCCI engines can run on “wet ethanol:” ethanol-in-water mixtures with high concentration of water. Considering that much of the energy required for processing fermented ethanol is spent in distillation and dehydration, direct use of wet ethanol in HCCI engines considerably shifts the energy balance in favor of ethanol. The results of the paper show that a HCCI engine with efficient heat recovery can operate on a mixture of 35% ethanol and 65% water by volume while achieving a high brake thermal efficiency (38.7%) and very low $NOx$ ($1.6ppm$, clean enough to meet any existing or oncoming emissions standards). Direct utilization of ethanol at a 35% volume fraction reduces water separation cost to only 3% of the energy of ethanol and coproducts (versus 37% for producing pure ethanol) and improves the net energy gain from 21% to 55% of the energy of ethanol and coproducts. Wet ethanol utilization is a promising concept that merits more detailed analysis and experimental evaluation.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(4):338-347. doi:10.1115/1.2794769.

Direct injection (DI) diesel engines emit a far more disagreeable exhaust odor than gasoline engines, especially at low temperatures and at idling. There is no proper system of odor reduction in these conditions in DI diesel engines. This study investigated a charcoal-adsorption system to reduce exhaust emissions including odor in a DI diesel engine at idling under no load operations, where exhaust temperatures are low. Low temperature exhaust gas is passed through a charcoal adsorber. Charcoal has the property of adsorbing odorous gas components. Here odor is reduced more than 0.5 points, a significant odor reduction depending on the engine and adsorber conditions. Exhaust noise, nitrogen oxides $(NOx)$, and eye irritation are also significantly reduced with the system. This study further investigated water-washing system for odor reduction in DI diesel engines at low exhaust temperature conditions. Exhaust gas is passed through the water in the water tank of the system. Aldehydes, organic acids, and other oxygenated components, which are the main odorous components in exhaust gases, are dissolved in water reducing exhaust odor significantly. Eye irritation of exhaust gases is also significantly reduced. The water-washing system not only reduces the odor and eye irritation but also carbon dioxide $(CO2)$, carbon monoxide (CO), $NOx$, and smoke are reduced more than 20–30%. The sound level of exhaust gases is also reduced $10–15dB$ with this system. Air dilution is also attempted in this study for odor reduction where a large amount of fresh air is mixed with exhaust gases. Here dilution ratio of 5 is used. Air dilution alone can reduce odor about 0.5 points. However, odor about 1.5–1.6 points (about 60–65%) can be reduced when air dilution is used in combination with charcoal-adsorber and water-washing system, and odor level is lowered below level 2, which is acceptable for all human beings.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(4):348-354. doi:10.1115/1.2794770.

An irreversible cycle model of the Otto heat engine is established, in which the temperature-dependent heat capacities of the working fluid, the irreversibilities resulting from the nonisentropic compression and expansion processes, and heat leak losses through the cylinder wall are taken into account. The adiabatic equation of ideal gases with the temperature-dependent heat capacity is strictly deduced without using the additional approximation condition in the relevant literature and used to analyze the performance of the Otto heat engine. Expressions for the work output and efficiency of the cycle are derived by introducing the compression ratio of two isochoric processes. The performance characteristic curves of the Otto heat engine are presented for a set of given parameters. The optimum criteria of some important parameters such as the work output, efficiency, compression ratio, and temperatures of the working fluid are given. Moreover, the influence of the compression and expansion efficiencies, the variable heat capacities, the heat leak, and other parameters on the performance of the cycle is discussed in detail. The results obtained are novel and general, from which some relevant conclusions in literature may be directly derived. This work may provide a significant guidance for the performance improvement and optimal design of the Otto heat engine.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2007;129(4):355-359. doi:10.1115/1.2794771.

Methanol utilization in a compression ignition engine has held tentative promise for a number of years, and, in fact, the concept has seen large scale field trials intended to demonstrate this option as a precursor to commercial implementation. However, results from those tests have identified some of the practical problems encountered with this fuel, namely, (1) its difficulty of vaporization and (2) its high autoignition temperature. Luminosity promoting additives, which facilitate radiative transport as a component of flame spread (because pure alcohol burns with little luminosity, continuum radiation as a reaction transport mechanism is essentially absent), intake air heating, active and passive heat sources, etc., represent some of the attempts to overcome limitations of these two factors. Except for intake air preheat, these augmentation methods have been noted to result in poor off-load thermal cycle efficiency. Focusing on the case of intake air preheat (which can be achieved by elevated compression ratio), and to model the chemical reaction kinetics, the partially stirred reactor model in CHEMKIN was used. This approach provided examination of the chemistry and reaction rates associated with an actual trial in which methanol was the fuel under study. To initiate this simulation, literature available reaction mechanisms were obtained, and then the experimental cylinder pressure history was matched by control of heat release rate via the partially stirred reactor model. This is represented within the reactor model by changing the turbulent mixing intensity factor. The overall reaction sequence, which models cylinder pressure, and attendant extent of reaction were the major focus. The minor focus included production of emission gases, e.g., the aldehydes and unburned fuel. Not only are the model results consistent with actual findings, they also support a method for addressing causes of off-load inefficiency and engine failures due to engine oil dilution with fuel.

Commentary by Dr. Valentin Fuster

### TECHNICAL BRIEF

J. Energy Resour. Technol. 2007;129(4):360-363. doi:10.1115/1.2794773.

Drilling operations in deep and ultradeep water are increasing around the world. The development of these substantial prospects provides many challenges and requires the integration of knowledge with prudent designs at different stages of the well development. Also, more wells are drilled in rotary steerable mode and other instrumented bottom-hole assemblies. All of these tools need power, which is stored in batteries. Because the downhole power supply is limited for these tools, an alternate mode of energy supply is needed so that these tools can remain downhole for long hours. This paper examines a new energy source extraction method using the hydraulic energy obtained from the circulated drilling fluid when downhole motors are used.

Commentary by Dr. Valentin Fuster

### DISCUSSION

J. Energy Resour. Technol. 2007;129(4):364-365. doi:10.1115/1.2794775.
FREE TO VIEW

First, the current author would like to thank everyone who provided comments in Refs. 1-3 regarding the results published in Ref. 4. The aim of the paper was to demonstrate capabilities of the 3D Computational Fluid Dynamics (CFD) technique for modelling a Stirling engine.

Commentary by Dr. Valentin Fuster