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RESEARCH PAPERS

J. Energy Resour. Technol. 1986;108(1):1-7. doi:10.1115/1.3231235.

This paper presents a numerical method for analyzing the effect of clearance on closely conforming rotary rock bit journal bearings. The modified Boussinesq point-force displacement influence function and the modified profile function are introduced in conjunction with the discretization of the integral equation. Automatic mesh generation is employed to redefine a new pressure area boundary and therefore round-off errors normally found while solving a large-scale linear system of equations can be avoided. The numerical method has been implemented in a computer program and has been applied to the problems of misaligned and perfectly aligned conformal contact. There is a close agreement with the Persson’s analytical solution at the center of the aligned bearing length. However, owing to a high stress concentration at the bearing edge, the edge pressure distribution will differ significantly from the Persson’s plane stress model. The conformal contact will have a comparatively higher peak pressure and lower contact-angle than the Hertzian line contact prediction. The results of the analysis provides a design tool for improving drill bit life.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):8-14. doi:10.1115/1.3231247.

The hydrodynamics of isothermal, one-dimensional gas-solids suspension is theoretically analyzed. A computational model is developed. The model is applied in predicting the pressure drop distribution in air-sandstone mixture flows through a vertical annular space (simulating the flow stream between a bore hole and a drill pipe). The model can be applied to any isothermal, one-dimensional flow of gas-solid suspension. The numerical results are in satisfactory agreement with the experimental data collected from studies done on drilled cutting carrying capacity of air in air-drilling operations.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):15-19. doi:10.1115/1.3231234.

The design of threaded connectors based on an elastic analysis appears overly conservative. This, in turn, will result in unnecessary material and manufacturing costs. To improve cost effectiveness, the design of connectors from the elastic-plastic viewpoint is warranted. This paper presents a simplified approach on the elastic-plastic finite element analysis of connectors. This approach would save tremendous computer costs which may be incurred in conducting a regular elastic-plastic analysis of threaded connectors.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):20-24. doi:10.1115/1.3231236.

This paper describes the problems and solutions raised in an experimental approach for a shotcrete underground system design. The system consists of a retrofit dry-wet process dual purpose shotcrete machine and an industrial robot.

Topics: Robots , Gunite , Design , Machinery
Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):25-28. doi:10.1115/1.3231237.

The force exerted by a moving ice sheet on an offshore structure is known to vary strongly with the geometrical conditions at the contact with the structure and with the rate of ice movement, resulting in a wide spectrum of failure modes including both in-plane and out-of-plane failures of either brittle or plastic type. In this paper attention is concentrated to only one limiting case, in which the ice sheet moves so slowly that no fracture occurs at the contact with the structure, but the ice undergoes only in-plane creep deformations.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):29-34. doi:10.1115/1.3231238.

Two full-scale ductile fracture propagation experiments on segments of line pipe pressurized with nitrogen gas have been conducted underwater at a depth of 40 ft (12 m) to evaluate the ductile fracture phenomenon in underwater pipelines. The pipes were 22-in. (559-mm) diameter and 42-in. (1067-mm) diameter. Fracture velocities were measured and arrest conditions were observed. The overpressure in the water surrounding the pipe resulting from the release of the compressed nitrogen gas contained in the pipe was measured in both experiments. The overpressure in the water reduces the stress in the pipe wall and thus slows down the fracture. In addition, the water surrounding the pipe appears to be more effective than soil backfill in producing a slower fracture velocity. Both of these effects suggest a greater tendency toward arrest for a pipeline underwater than would be the case for the same pipeline buried in soil onshore. Further verification of this effect is planned and a modified version of the existing model for predicting ductile fracture in buried pipelines will be developed for underwater pipelines.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):35-47. doi:10.1115/1.3231239.

The collapse of long thick-walled tubes under external pressure is studied both experimentally and analytically. A two-dimensional nonlinear formulation of the problem is presented. The formulation is general enough to include initial geometric imperfections of the tube cross section such as initial ovality and wall thickness variation. In addition the effects of residual stresses and of initial inelastic anisotropy are considered. Experiments on tubes with D/t values between 10 and 40 were carried out. Good agreement between experiments and theory is shown to occur provided all parameters are modeled correctly. A study of the effect of the various parameters of the problem on the collapse pressure is also presented.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):48-51. doi:10.1115/1.3231240.

Cable is normally recovered by hauling in from ahead, with the cable guiding the ship. In an early paper, Zajac presented a method for recovery, proposed by Shea, in which the cable is overrun, and hauled in tending aft. The potential advantage is lower tension in recovery. Cable was recovered with overrun in two recent exercises. The tensions and cable angles observed at the ship in these exercises are used to work out cable configurations and bottom tensions. The calculations show that the bottom tension was considerably less than it is for normal recovery in both exercises. As the overrun is increased, the tension decreases, and tends to a limiting value defined by the ship speed and the slack rate at which the cable was originally laid. Recovery with overrun is practical and advantageous.

Topics: Cables , Ships , Tension
Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):52-58. doi:10.1115/1.3231241.

An anchoring system for an offshore structure must meet certain prescribed requirements controlled by factors such as the site environment, operational constraints and the vessel employed. Its adequacy, survival and ability to stay on site must, therefore, be checked out with proper methods of analysis. The inclusion of cable dynamics is an important consideration in the dynamic analysis of a moored vessel. In this paper, mooring line equations of motion are derived for a multi-component, n -segment model using Lagrange’s modified equation, permitting anchor motion, and then numerically solved to yield time histories of cable displacements and cable tensions for the various cable configurations that can occur. Initial conditions can be provided through the mooring line static catenary equations. The nonlinear restoring force terms in the vessel equations of motion are generated through the dynamic tension- displacement characteristics of individual lines. The equations of motion of the moored vessel subjected to an open ocean environment are then numerically solved to yield time histories of vessel motions and cable tensions. An example involving a moored production barge is examined and results are compared with those of previous work in which a quasi-static cable configuration is employed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):59-64. doi:10.1115/1.3231242.

In this paper we outline a method for the approximate prediction of the static and lift responses of a flexible cylinder in a unidirectional variable stream. Our prediction is based on information derived from experimental results involving rigid cylinders forced to oscillate sinusoidally orthogonally to a uniform stream. Our approach represents the multifrequency lift response of a flexible cylinder in a sheared current by predicting a number of independently determined, monochromatic, multimode dynamic solutions. For each such multimode solution, our procedure allows the simultaneous evaluation of lift response frequency, amplitude, and phase between modes. A numerical example assuming bimodal solutions is included to illustrate our method for the geometry of a single-tube marine riser.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):65-71. doi:10.1115/1.3231243.

When structures having inclined surfaces, such as cone-type and inclined-pile structures, are constructed in coastal and offshore cold regions, sea ice forces must be considered in their design. In order to estimate these forces, the relationships of the coefficients of static and kinetic friction between sea ice and construction materials must be evaluated. The authors have been conducting, for four years, experiments on the coefficients of friction between sea ice and various commonly used offshore construction materials such as concrete and steel. This paper summarizes the results of this study. The coefficients of friction have been found to be affected by the following: (i ) relative velocity (i.e., velocity of construction material relative to sea ice); (ii ) sea ice temperature; and (iii ) surface roughness of construction material. They have been found to be relatively unaffected by the following: (i ) contact area, (ii ) normal stress, (iii ) growth direction of sea ice, and (iv ) water at the sea ice-material interface.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):72-76. doi:10.1115/1.3231244.

Flexibility monitoring is a vibration-based method for simplifying the detection of major underwater damage on offshore jacket platforms. Ambient vibrations are detected at each of the underwater framing levels relative to abovewater vibration in the fundamental sway and torsional modes. Derived are flexibility parameters which relate to the shear flexibilities of each framing bay and of the foundation. Great promise has been shown by laboratory and field testing. This paper presents a comprehensive sensitivity assessment for severance of diagonal members over a wide range of structural redundancy for generic platform configurations.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):77-83. doi:10.1115/1.3231245.

The vortex-induced vibration response of long cables subjected to vertically sheared flow was investigated in two field experiments. In a typical experiment, a weight was hung over the side of the research vessel by a cable that was instrumented with accelerometers. A typical experiment measured the acceleration response of the cable, the current profile, the tension, and angle of inclination at the top of the cable. Total drag force was computed from the tension and angle measurements. Two braided Kevlar cables were tested at various lengths from 100 to 9,050 ft. As a result of these experiments, several important conclusions can be drawn: (i ) the wave propagation along the length of the cable was damped, and therefore, under most conditions the cable behaved like an infinite string; (ii ) response spectra were quite broad-band, with center frequencies determined by the flow speed in the region of the accelerometer; (iii ) single mode lock-in was not observed for long cables in the sheared current profile; (iv ) the average drag coefficient of long cables subjected to sheared flow was considerably lower than observed on short cables in uniform flows; (v ) the r.m.s. response was higher in regions of higher current speed. A new dimensionless parameter is proposed that incorporates the properties of the cable as well as the sheared flow. This parameter is useful in establishing the likelihood that lock-in may occur, as well as in estimating the number of modes likely to respond.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):84-90. doi:10.1115/1.3231246.

Considering the effects of oil compressibility, leakage, hydrodynamic damping and friction, in addition to other nonlinearities, such as bang-bang relay, pump saturation and limiters of oil pressure and pump flow, the performance of five different types of ship steering gear control systems are examined using digital computer simulation techniques. Three of the controllers examined are “bang-bang” controllers, while the remaining two are “analog”. The behavior and performance of the various systems on three ship configurations are compared. It is shown that the analog steering gear controllers can be expected to demonstrate superior performance of the ship/steering system in calm water operations.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1986;108(1):91-94. doi:10.1115/1.3231248.

Conformal mapping may be used to approximate the two-dimensional pore pressure distribution around a cylinder with a single longitudinal slot. A nonsymmetric flow field can significantly affect the stress distribution around a slotted casing. In order to investigate analytically the importance of such asymmetry on incipient rock yielding and casing collapse in producing wells, an approximate solution is obtained for the pore pressure distribution in the extreme case of a cylinder with a single slot. The derived solution does not employ the Schwartz-Christoffel transformation used in previous work. Hence the resulting expression gives the pore pressure as an explicit function of position. Comparison with the exact solution for typical reservoir radius to casing radius ratios gives predicted flow rates correct to within one percent.

Commentary by Dr. Valentin Fuster

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