J. Energy Resour. Technol. 2000;122(3):105-109. doi:10.1115/1.1289384.

Terrain slugging is extremely likely to occur sometime during the life of a deep subsea pipeline/riser system. But, can the terrain slugs be mitigated? In this study, we performed transient multiphase flow simulations to explore the feasibility of sea floor separation to mitigate terrain slugging and to optimize fluid transport from wellhead to downstream processing. We considered the effects of separator location and separation efficiency on production system behavior. We found that there is an optimal amount of gas underflow into the liquid line and there is an optimum location for subsea separator. Our study also covered how the operational transient processes, such as startup and shutdown, would affect the performance of the pipeline/separator system. It is necessary to have a good understanding of the performance of subsea separation system before a reliable one can be designed. The results of this study are aimed toward improving the design of sea floor processing for ultra-deepwater production. [S0195-0738(00)00403-9]

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2000;122(3):110-114. doi:10.1115/1.1289382.

Detailed observations were made for slug flow in an upward to downward pipeline configuration with 13 capacitance sensors. The results presented in this study are based on four typical tests, which demonstrate the phenomena encountered in slug dissipation. In downward flow, slug frequency decreases with different extent at different flow rates. There is a reduction of slug length after the turning over from upward to downward flow. However, the slug tends to recover its stable length in further downstream development, except for flow that quickly becomes stratified. Growth of the slug length is accomplished by picking up liquid left by the downstream dissipating slug before being fully absorbed by the liquid film. [S0195-0738(00)00503-3]

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2000;122(3):115-122. doi:10.1115/1.1288209.

One commonly used method for determining oil and gas production velocities is to limit production rates based on the American Petroleum Institute Recommended Practice 14E (API RP 14E). This guideline contains an equation to calculate an “erosional” or a threshold velocity, presumably a flow velocity that is safe to operate. The equation only considers one factor, the density of the medium, and does not consider many other factors that can contribute to erosion in multiphase flow pipelines. Thus, factors such as fluid properties, flow geometry, type of metal, sand production rate and size distribution, and flow composition are not accounted for. In the present paper, a method is presented that has been developed with the goal of improving the procedure by accounting for many of the physical variables including fluid properties, sand production rate and size, and flowstream composition that affect sand erosion. The results from the model are compared with several experimental results provided in the literature. Additionally, the method is applied to calculate threshold flowstream velocities for sand erosion and the results are compared with API RP 14E. The results indicate that the form of the equation that is provided by the API RP 14E is not suitable for predicting a production flowstream velocity when sand is present. [S0195-0738(00)00203-X]

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2000;122(3):123-128. doi:10.1115/1.1289392.

In horizontal and extended reach drilling, a large frictional drag may occur. If the pipe buckles laterally or into a helical shape, additional lateral contact force, LCF, is developed between the pipe and the wellbore wall, increasing the drag force. This paper presents the results of an experimental study of the lateral contact force between the drill pipe and the wellbore wall, for helical pipe configuration. Comparison of the experimental results with the current analytical models is also presented. A horizontal well was simulated using a 2-in-dia hole, 86-ft long, and three different sizes of pipe. Two different techniques were used to measure the lateral contact force. Results from both techniques seem to be in good agreement. The comparison with the current analytical models shows that higher values are predicted. The results will find application in directional drilling, horizontal drilling, and coiled tubing operations. [S0195-0738(00)00603-8]

Topics: Force , Gages , Pipes
Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2000;122(3):129-135. doi:10.1115/1.1289767.

An experimental setup was built at the University of Tulsa to study buckling and post-buckling behavior of pipes constrained in straight horizontal and curved wellbores. Experiments were conducted to investigate the axial force transfer with and without static internal pressure. Different stages of buckling phenomena and their relation to the axial force, the pipe diameter (1/4 and 3/8 in.) and the pipe end-support conditions have also been investigated. Experimental results have shown that the buckling load is a strong function of the pipe diameter and the pipe end-support conditions. Static internal pressure appears to have insignificant influence on the buckling behavior of pipes. A brief review of recently developed mathematical models to predict buckling behavior of pipes in inclined, curved, and horizontal sections of wellbore is also presented. Applications of the current theory are presented by using recently developed computer simulator. Results of the theoretical analysis have confirmed the versatility and effectiveness of computer simulator for better understanding and solving buckling related problems in the field. [S0195-0738(00)00903-1]

Topics: Force , Pipes , Buckling
Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2000;122(3):136-141. doi:10.1115/1.1289638.

A shroud is commonly used around the motor of an electrical submersible pump (ESP) to accelerate reservoir fluids past the motor for cooling. Standard practice has been to design the shroud/motor configuration relative to the casing using a minimum fluid velocity of 0.3048 m/s (1 ft/s) rule of thumb as a production strategy. The increase in the use of ESPs to exploit heavy oil reservoirs has brought up the necessity of revising this rule in order to prevent motor burnouts. A parametric study has been conducted using the computational fluid dynamics software CFX4.2 to examine the heat transfer behavior of the shroud motor configuration as a function of motor/shroud standoff. The objective of this effort is to examine the validity of the historical rule of thumb for heavy oils. Results for a case study on an oil with a viscosity of 78 cp @ 320 K are presented. Further, to explore the possibility of enhancing the heat transfer characteristics, the flow configuration was modified by incorporating several openings on the shroud. Based on the obtained results, it can be concluded that fluid velocity should be kept around 0.85 m/s (2.8 ft/s) as opposed to 1 ft/s to assure proper cooling of the motor. Also, flow redistribution by proper placement of the slots on the shroud may produce better heat transfer between the oil and the motor wall. [S0195-0738(00)00703-2]

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2000;122(3):142-146. doi:10.1115/1.1289391.

A model has been developed to simulate multiphase flow in the wellbore and heat transfer processes between the well and formations. The model is capable of handling dynamic well depth during drilling, varying flow regimes in multiphase flow, phase change between liquid and gas, and kicks or lost circulation depending on the pressure difference between the wellbore annulus and formation. The model requires simple data input and is able to handle complicated drilling cases such as casing installation, changing drilling fluids, and drillpipe/coiled tubing connections during drilling operations. [S0195-0738(00)00303-4]

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2000;122(3):147-152. doi:10.1115/1.1286123.

A simple model has been developed to estimate the sensible thermodynamic properties such as Gibbs free energy, enthalpy, heat capacity, and entropy of hydrocarbons over a wide range of temperatures with special attention to the branched molecules. The model is based on statistical thermodynamic expressions incorporating translational, rotational and vibrational motions of the atoms. A method to determine the number of degrees of freedom for different motion modes (bending and torsion) has been established. Branched rotational groups, such as CH3 and OH, have been considered. A modification of the characteristic temperatures for different motion mode has been made which improves the agreement with the exact values for simple cases. The properties of branched alkanes up to 2,3,4,-trimthylpentane have been calculated and the results are in good agreement with the experimental data. A relatively small number of parameters are needed in this model to estimate the sensible thermodynamic properties of a wide range of species. The model may also be used to estimate the properties of molecules and their isomers, which have not been measured, and is simple enough to be easily programmed as a subroutine for on-line kinetic calculations. [S0195-0738(00)00902-X]

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2000;122(3):153-160. doi:10.1115/1.1289766.

Preliminary feasibility studies based on breakeven refrigeration thermodynamics, have been conducted for candidate power conditioning components in a transportable radar power system (Donovan, B.D. et al., 1995 “Effects of Refrigeration in a Transportable Cryogenic Aerospace Application,” Proc. 30th IECEC, Vol. 1, pp. 473–478; Ramalingam, M. et al., 1996 “Systems Analysis for a Cryogenic Aerospace Terrestrial Radar Power System,” Proc. 31st IECEC, Vol. 1). The analysis based on breakeven refrigeration thermodynamics revealed that in the case of a general switching device such as a power MOSFET, it would be more beneficial to operate it at 150 to 220 K, using a Stirling cycle-based cryocooler. The overall system efficiency was jeopardized by way of large input power requirements to cool small heat loads at lower temperatures, while the performance of the device itself suffered at higher temperatures. The break-even refrigeration thermodynamic analysis was also applied to multilayer ceramic capacitors at cryogenic temperatures. It was found that in order to avoid a power penalty for cooling the capacitor to 77 K, the cryocooled equivalent series resistance (ESR) value would have to be a factor of 40 lower than that of a conventional capacitor ESR value if using a Gifford-McMahon (GM) cooling cycle. A factor of 12 better improvement in ESR is required for a yet-to-be-developed more efficient Stirling cycle. In this paper, this break-even thermodynamic analytical concept was then partially extended from the component level to the radar power system level. The entire power system was sized based on several combinations of cryocooled generators, power conditioning, and antenna equipment. The analysis revealed that even though the radar output could potentially be increased two-to threefold by the introduction of cryocooled technologies, the sizes of the coolers begin to negate these advantages. Several power systems were evaluated with reference to a common figure-of-merit to arrive at an optimum configuration. [S0195-0738(00)00803-7]

Commentary by Dr. Valentin Fuster

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