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EDITORIAL

J. Energy Resour. Technol. 1982;104(1):1. doi:10.1115/1.3230374.
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Abstract
Commentary by Dr. Valentin Fuster

RESEARCH PAPERS

J. Energy Resour. Technol. 1982;104(1):2-13. doi:10.1115/1.3230377.

Many types of marine structures are susceptible to vortex-excited oscillations. These include the risers and conductor tubes that are employed in offshore drilling and production, deep water pipelines, and members of jacketed structures. Deepwater pile installation and driving operations also have been hampered by problems arising from vortex shedding. A discussion is given in this paper of the problems caused by vortex shedding from flexible, bluff cylinders in steady current flows. In particular, recent measurements of the steady and unsteady deflections caused by the vortex-excited drag and lift forces are discussed. Various approaches that have been developed for the suppression of vortex-excited oscillations are reviewed. A classification and a comparison are made of the effectiveness of several suppression devices, and some practical examples of their application are presented.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1982;104(1):14-19. doi:10.1115/1.3230373.

The basic equations for nonplanar transverse vibrations of marine risers are derived from the theory of elastic rods. A numerical method is developed for solution of the equations by time integration. Spatial discretization is accomplished by a hybrid finite element method. Vortex excitation is modeled by the coupled wake oscillator proposed by Iwan and Blevins. The vortex oscillator equations are integrated numerically in time along with the riser equations. By way of example, several typical riser problems are analyzed including forced vibration and vortex-induced vibration.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1982;104(1):20-28. doi:10.1115/1.3230375.

The dynamic response characteristics of the taut-moored platform or the so-called Tension Leg Platform in regular waves are studied both by several series of tank tests and by some simplified methods of linear and nonlinear analyses. By comparison of the results it is shown that the analytical methods are valid and practical. In this paper the nonlinear responses observed in the model tests, as well as the characteristics of the linear frequency responses, are discussed in detail and clarified. In addition, statistical analysis is carried out on a full-scale TLP model in a real sea state.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1982;104(1):29-38. doi:10.1115/1.3230376.

The forces acting on a three-dimensional cylinder with arbitrary symmetrical cross section are derived taking into account viscous effect and applying linear-processing techniques. Then general expressions for the hydrodynamic forces, motion equation and its solution for a semisubmersible platform in regular waves are obtained. Based on linear theory of statistical analysis, it is proposed to employ the concept of “equivalent wave height” for the calculation of transfer functions with which both the short-term and long-term distribution and statistical characteristics of the motion of a semisubmersible may be estimated. A computer program has been developed. Comparison between model experimental and theoretical data has shown satisfactory agreement.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1982;104(1):39-46. doi:10.1115/1.3230378.

With random sea simulation, application of linear spectral analysis method to offshore structures with moderate drag force has been assessed. Findings indicate overprediction of response for short natural periods and underprediction for very long periods. Tentative corrective measures are recommended. Significant force and response reductions have been calculated for flexible structures which are not adequately predicted by the linear spectral method.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1982;104(1):47-52. doi:10.1115/1.3230379.

A new concept is presented which is aimed at improving the methodology for determining the wave forces on offshore structures. The Inertial Pressure Concept (IPC) is based on a direct, empirical approach to calculating forces. The resulting method can be formulated to include realistic sea state wave kinematics while not being dependent on a particular kinematic representation. Futhermore, the method should be as easy to apply as the Morison equation, but will allow diffraction and three-dimensional aspects to be considered.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1982;104(1):53-57. doi:10.1115/1.3230380.

A linear equation is mathematically derived for hydrodynamic forces on a marine riser under effects of free surface and floating-vessel motion using a velocity-potential method. It accounts for inertia and wave damping forces, including the force caused by riser motion, and empirically includes the drag force caused by viscosity. The equation, when reduced to a simpler form, is basically identical to the semi-empirical Morison equation for the inertia and drag forces. Theoretical validity of the simpler equation and the Morison equation is discussed. Previously, practical, semi-empirical force equations on the riser have been suggested, ignoring the effects of the free surface and the wave damping. The equations in current practice are compared with the present simpler equation.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1982;104(1):58-62. doi:10.1115/1.3230381.

Several three-dimensional pipeline problems are used to illustrate a general pipeline and riser model with twist. A continuation technique is developed to solve convergence problems. Suitable stiff numerical methods are reviewed for general applications.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1982;104(1):63-72. doi:10.1115/1.3230382.

The study reported in this paper deals with the development of design data and strategies for optimizing the performance of wave energy generators. Rectangular buoys are considered for illustration. The buoy dimensions necessary for maximizing the conversion efficiency while minimizing its size for a particular ocean condition are evaluated. A control scheme based on changing the load damping coefficient provides an effective means for maximizing the conversion efficiency at different wave frequencies.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1982;104(1):73-77. doi:10.1115/1.3230383.

If a pipeline buckles in the presence of sufficiently large external pressure, a propagating buckle is initiated. The buckle propagates along the pipeline until it encounters a region of low pressure or an arresting device. This paper suggests a new buckle arrestor design. It is comprised of a rod tightly wound around the pipe to form a number of turns. Its main characteristic is that it can be used in continuous pipelaying methods (reel pipelaying). It can also be used in the case of pipes coated with concrete where it can be inserted into the concrete. Due to its design, it restricts the stress concentration at the discontinuity when bending is applied to the pipe arrestor assembly. Experiments have shown that its efficiency is comparable to other arrestors. An empirical expression for the arrestor efficiency is presented.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1982;104(1):78-83. doi:10.1115/1.3230384.

The design of a spread mooring system of given pattern with single segment lines on a flat seafloor is formulated as a mathematical programming (optimization) problem. Design variables are the outboard length and initial tension in each line. Constraints limit maximum tension, anchor pull and anchor uplift for each line under each loading condition. Maximum vessel excursion is limited to a circle of specified radius. The optimal design is that which minimizes total weight of outboard line and is obtained by computer solution of the optimization problem. Three examples are presented to demonstrate the method.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1982;104(1):84-90. doi:10.1115/1.3230385.

This paper presents a procedure on the finite element method for analyzing a bolted flange connector and compares this method with three traditional approaches. The finite element method considers such effects as flange interface separation, nonlinear and nonconstant flange stiffness, and bolt bending. A comparison of the finite element model with the three traditional methods (each employs a formula for flange stiffness) shows a fairly close correlation for total bolt force versus applied load, but a wide discrepancy for maximum bolt stress versus applied load. The discrepancy between the finite element model analysis and the three other methods (empirical by Weiss and Wallner, truncated conical area by Roetscher, and the classical hollow cylinder approach) can be attributed to the change in flange stiffness during separation and the occurrence of bolt bending. The selected method of analysis was shown to significantly affect the results of a bolt fatigue analysis, but was shown to have little effect on a static analysis. In offshore applications, the environment creates dynamic stresses which make a fatigue analysis essential for long-term safety.

Commentary by Dr. Valentin Fuster

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