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RESEARCH PAPERS: Offshore Mechanics

J. Energy Resour. Technol. 1985;107(3):297-314. doi:10.1115/1.3231193.

Over the recent years, following the very rapid increase in the construction and installation of offshore structures, there has been a considerable growth of interest in the assessment of the probabilities and consequences of collision and damage of such structures. This is reflected by the very large number of papers published over the last 15 yr and the multitude of conferences and meetings held on the subject. Many research programs have been completed or are in progress at many centers and institutions over the world. Accidental loading and damage are now accepted design parameters recommended for consideration in a number of Codes for the design in offshore structures. This paper reviews the state-of-the-art with respect to the probabilities and consequences of collisions and accidental loading in general, and methods for the assessment of the design of steel offshore structures against damage. Most of the available information in the field of offshore collisions and accidental loading emanates from research and experience related to ship safety. However, in this paper emphasis is placed on research activity and available information concerned with offshore structures, such as platforms, semisubmersibles, etc. There is a considerable amount of information available on methods for evaluating the extent and effects on damage of these structures and in estimating their residual strength in the damaged condition. As this is an area currently of major interest in the offshore industry, the paper presents comprehensive information and some new results relating to all major structural components. The state-of-the-art with regards to methods and principles for design against damage is also reviewed and commented upon. The paper concludes with general recommendations and indications of areas where future research could be most usefully directed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1985;107(3):315-328. doi:10.1115/1.3231194.

The computation of wave forces is one of the most vital tasks in the design of offshore structures. Many analytical tools are available for the determination of wave effects on offshore structures. These methods may be divided into two major categories: one for small members of an offshore structure and one for large members. A hybrid method is used for structures that have both types of members. The advances made in the last few years in the specific area of computing the high-frequency forces are reviewed here.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1985;107(3):329-334. doi:10.1115/1.3231195.

This paper presents the results of an application of a strip technique for the prediction of the lateral drift forces on a semisubmersible platform floating in oblique regular waves. The method employs Maruo’s formula and source distribution technique, without taking account of the hydrodynamic interaction between the twin hulls and columns of the semisubmersible. Overall the strip technique shows a more favorable correlation with the experimental data than the three-dimensional theory. It is, however, premature to conclude that the technique has been fully validated.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1985;107(3):335-341. doi:10.1115/1.3231196.

Almost all submarine pipelines are currently laid on the sea floor. This practice is inherently expensive and the system is difficult to maintain and has a high possibility of excessive bending stress. The concept of suspending a pipeline under the ocean surface is investigated in the reported study. An optimum design algorithm is developed which can minimize the dynamic oscillation and the tensile stress of the pipeline resulting from the wave excitation. Numerical examples are given to illustrate the optimization procedure.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1985;107(3):342-349. doi:10.1115/1.3231197.

An analytical model of dented tubular members is constructed. Using this model, an ultimate strength interaction relationship between axial force and biaxial bending moments is derived. The implementation of this interaction relationship to take account of denting and bending damage in the analysis of nonlinear behavior of tubular space frames by ISUM (the Idealized Structural Unit Method) is described. An experimental study of the behavior of dented members subjected to pure bending is reported. Comparison with theoretical predictions is presented.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1985;107(3):350-355. doi:10.1115/1.3231198.

A procedure, developed for predicting the elastic-plastic general axisymmetric collapse of stiffened and unstiffened cylinders, is presented. This permits the formulation of reliable lower bounds to collapse loads for any combination of radial pressure and axial loadings, with any prescribed forms and levels of initial geometric imperfection, in a simple, “column-type,” analysis. The approach provides a unified format for the treatment of isotropic and orthotropic cylinder buckling, which is compact and design orientated.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Arctic Engineering

J. Energy Resour. Technol. 1985;107(3):356-361. doi:10.1115/1.3231199.

A decision-theoretic approach to the problem of optimizing strategies for the protection of submarine pipelines against ice scour is presented. The model simultaneously considers twinning, deep trenching, and valve segmentation. The opitmal solution is dependent on the uncertainty regarding the frequency of pipeline damage and the associated consequences. Two sources of uncertainty are considered, namely the rate and the depth of scour. Consequences of pipeline damage are represented by their impact on economy and enviornmental safety. An example is presented and the model is used to check the sensitivity of the optimal solution to different input parameters.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1985;107(3):363-368. doi:10.1115/1.3231202.

A one-dimensional stress-strain relationship is developed for pure polycrystalline ice subjected to uniaxial compression. The model is based on the continuous damage theories and includes the effects of elastic, plastic and brittle deformation mechanisms. A damage law for ice is established based on a statistical model for internal microfracture. Quantitative results are presented by directly relating the formation of internal cracks to published acoustic emission response of ice samples subjected to compression.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1985;107(3):369-374. doi:10.1115/1.3231203.

This work presents the results of uniaxial compression tests on freshwater polycrystalline ice. Grain size of the test material ranged from 1.5 to 5 mm, strain rate ranged from 10−6 to 10−2 s−1 and the temperature was −5°C. The grain size effect emerged clearly as the strain rate increased to 10−5 s−1 and persisted to the highest applied strain rates. On average, the stated increase in grain size brought about a decrease in peak stress of approximately 31 percent. The occurrence of the grain size effect coincided with the onset of visible cracking. The strength of the material increased to a maximum at a strain rate of 10−3 s−1 , and then dropped somewhat as the strain rate increased further to 10−2 s−1 . Strain at peak stress generally tended to decrease with both increasing grain size and increasing strain rate. The results are discussed in terms of the deformational mechanisms which lead to the observed behavior.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1985;107(3):375-380. doi:10.1115/1.3231204.

Thirty-six constant strain-rate uniaxial tension tests were performed on vertically oriented multi-year pressure ridge samples from the Beaufort Sea. The tests were performed on a closed-loop electro-hydraulic testing machine at two strain rates (10−5 and 10−3 s−1 ) and two temperatures (−20° and −5°C). This paper summarizes the sample preparation and testing techniques used in the investigation and presents data on the tensile strength, initial tangent modulus, and failure strain of the ice.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1985;107(3):381-387. doi:10.1115/1.3231205.

The main force components on inclined offshore structures in the Arctic include the flexural and crushing behavior of the ice sheet, rotation and translation of broken ice, gravity and buoyancy in lifting and moving ice, and friction between ice and structure. Tests were carried out to isolate these components and analytical expressions were derived for them. Measured and predicted components compare favorably, the analytical model overpredicting by about 20 percent.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Ocean Engineering

J. Energy Resour. Technol. 1985;107(3):388-393. doi:10.1115/1.3231206.

A theory has been developed for the analysis of fiber-core wire rope with multilayered strands. The rope is subjected to both an axial force and an axial twisting moment. The previously developed linear theory for helically shaped wires is used and the equations governing compliance of the fiber core are formulated in a linear fashion. The resultant linear equations are easily solved. The theory is applied to a 6 × 19 Seale fiber-core wire rope and dimensionless results are presented. A load-deformation curve for a Seale fiber-core wire rope is obtained experimentally. Both the theoretically predicted effective modulus of elasticity and the predicted effective Poisson’s ratio of the rope compare favorably with the experimental results.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Rock Mechanics

J. Energy Resour. Technol. 1985;107(3):394-396. doi:10.1115/1.3231207.

The heat-transfer characteristics for free convection associated with isothermal vertical cylinder with surface mass transfer (blowing or suction) embedded in a saturated porous medium are analyzed. The nonsimilar equations are solved by using a suitable variable transformation and employing an implicit finite difference method. The numerical results for Nusselt number are expressed as functions of the parameters, ξ and γ , which represent the effects of the cylinder curvature and the surface mass transfer, respectively. It is found that the local Nusselt numbers for a vertical cylinder are less sensitive to the surface mass transfer than those for a vertical plate. Blowing of mass decreases the heat transfer coefficient but the suction of mass increases the heat transfer coefficient.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Petroleum Fluid Mechanics

J. Energy Resour. Technol. 1985;107(3):397-401. doi:10.1115/1.3231208.

An engineering design study was performed on a gas-gathering system in a major offshore oilfield in Saudi Arabia. The dew point gas from separators will undergo retrograde condensation in the pipelines with reduction of pressure and temperature. Calculations presented for sizing the pipelines include phase behavior, pressure drop, temperature profiles, liquid volumes, flow pattern predictions, liquid flow rates and the minimum gas velocity required to continuously remove liquid from the risers.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Solid Waste Processing

J. Energy Resour. Technol. 1985;107(3):402-405. doi:10.1115/1.3231209.

This paper presents the development of a novel solid-phase methane fermentation process involving acidic bioleachate production from an organic bed and biomethanation of the bed-liquefaction products in an external methane digester. Process operation with municipal solid waste showed that about 81 percent of the biodegradable volatile solids (VS) could be stabilized during three months of batch operation to afford a methane yield of 0.21 std m3 / kg VS added under ambient (∼25°C) conditions; this compares favorably with an ultimate mesophilic (35°C) methane yield of 0.26 std m3 / kg VS added.

Commentary by Dr. Valentin Fuster

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