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RESEARCH PAPERS: Offshore Mechanics/Ocean Engineering

J. Energy Resour. Technol. 1984;106(1):2-9. doi:10.1115/1.3231015.

Ocean and arctic engineers typically play three roles. Through research, they contribute new understandings of basic scientific principles applied in a strenuous and hostile environment. Through design, they apply that knowledge to such crucial functions as construction, transportation, communication, extraction of natural resources, and environmental conservancy. Through serving as a systems analyst or administrator, they are concerned with both the industrial management of marine technology and with its social management. In this latter concept, ocean and arctic engineers bond their technical talents with deeper perceptions of their client’s dilemmas in a concern for ends as well as means. This exercise is the least familiar to the engineering professional, yet it entails the most trenchant contribution that engineers make to human affairs and thus the noblest expression of social responsibility. This paper traces the connections of ocean and arctic engineering to their broadest context—the economic, social, legal, cultural and political ramifications that increasingly season intrinsic elements of research, design and application. With a series of historical and anecdotal vignettes, this theme is elaborated to dramatize the sharp changes that have accompanied the interaction of technology and society generally in the last four decades. Moving from examples of the excitment of personal enlightment through discovery to the satisfaction of successful operating performance, this review concludes with four evolving challenges: updating tools of practice to incorporate notions of risk assessment and social impact; major reforms in engineering education to emphasize breadth instead of specialization; active engagement in technology-policy analysis to facilitate citizen enlightenment and foresight; and recognition that engineering ethics must parallel technical proficiency and virtuosity in carrying to the future the unique torch of noblesse oblige .

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):10-17. doi:10.1115/1.3231005.

A linear response analysis method of the tension leg platform (TLP) subjected to regular waves is proposed. In this analysis method, flexibility of the superstructure can be taken into account in the equations of motion; response motions, tension variations of tendons and structural member forces are solved simultaneously. The applicability of this method is confirmed by comparison with the test results on two kinds of small-scale TLP models. The structural responses obtained from these calculations and their effects on tension variation of tendons are studied. Finally, several kinds of structural response characteristics are conclusively discussed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):18-23. doi:10.1115/1.3231013.

A simple model for a complex system is presented that contains the essential properties of the guyed tower. The dynamic response to wave load of a guyed tower is represented by a single degree of freedom model. Tower response, with different wave heights and hydrodynamic coefficients, using statically and dynamically determined mooring properties were calculated for a specific tower in 1100 ft (335 m) of water. The importance of this study lies in a good modeling of the cable forces, including the hysteretic effects of line damping, and in identifying cases where the quasi-static model may fail.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):24-31. doi:10.1115/1.3231014.

On March 27, 1980, the semi-submersible platform Alexander L. Kielland broke down in a storm in the North sea, resulting in a loss of 123 lives. The investigation subsequently performed by the inquiry commission showed that one of the lower tubular bracings had failed by fatigue. As a result, the vertical leg attached to it was torn off, and the platform capsized. The fatigue fracture had started from a double fillet weld joining a 0.325-m tubular attachment to the bracing. The fillet welds were partially cracked in the early history of the platform due to lammelar tearing. Cumulative damage calculations indicated that the design fatigue life of the bracing was inadequate.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):32-37. doi:10.1115/1.3231020.

The development of the tension leg platform (TLP) for the Hutton Field in the North Sea represents the first application of this deepwater concept. The use of vertical tension legs as the mooring system is the most novel part of the TLP design. One of the unique features of the tension legs is the use of 92.5-mm (3.64-in.) thick tubulars forged from 3 1/2 percent Ni-Cr-Mo-V high-strength steel (795 MPa (115.3 ksi) minimum yield strength) as mooring elements. Because of the importance of these elements to the survival of the TLP, the chemical composition and the steelmaking procedure were critically examined and optimized. In addition, extensive evaluation of the corrosion fatigue behavior of the steel was undertaken. This paper discusses the basis for the steel selection and specification, the results of the rigorous corrosion fatigue test program, and the assessment of the variation of properties along the length and through the thickness of full-size components. These results illustrate the suitability of the proposed high-strength steel for the mooring system application and establish confidence in both steelmaking and quality control procedures.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):38-42. doi:10.1115/1.3231021.

The Random Decrement Technique has shown promise as an inspection technique for offshore structures. The major advantage of this technique is that it requires only measurements of the dynamic response of the structure and not the input excitation causing the response. On offshore platforms, such random input forces occur from wind, waves, and currents. The Random Decrement Technique was evaluated together with a number of other NDE techniques under the NDE round robin testing program sponsored by the United States Geological Survey and the Office of Naval Research. A series of tests, damage scenarios, were conducted on a model of an offshore structure in a blind-mode by an independent neutral agent. Test data were given to the corresponding advocates to be analyzed and interpreted to predict the damages. Initial results indicated that the Random Decrement Technique was able to identify all the damage and non-damage situations with the usage of only four accelerometers mounted on each of the legs of the structure.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):43-45. doi:10.1115/1.3231022.

An analytical method for the stress analysis of tubular joints of T, Y, K type is presented in this paper. The stress distribution and stress concentration factor of the joints are calculated. Numerical results are in good agreement with the experimental results.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):46-54. doi:10.1115/1.3231023.

The influence of tension, pressure, and weight on different aspects of pipe and riser behavior has already been the subject of many articles (see references). In spite of this it has frequently been misunderstood, sometimes with serious consequences. The object of this paper is to explain the subject clearly and the approach is therefore deliberately very elementary. The principal problems of bending, buckling, yielding and strains, usually treated in separated papers, are here treated homogeneously. The widely used concepts of “effective tension” and “effective stress” are given interpretations, easy to visualise, which are related to analogous concepts in other engineering fields. Finally, eight diverse particular examples of the influence of tension, pressure and weight on pipe and riser behavior, are discussed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):55-62. doi:10.1115/1.3231024.

The High Strength Line Pipe Research Committee organized by the Iron and Steel Institute of Japan has conducted five full-scale burst tests on line pipe of 48 in. o.d. × 0.720 in. w.t. (wall thickness) and grade X70 under pressure of 80 percent SMYS with air: 1) to study the influence of separation on the arrestability of shear fracture, and 2) to obtain the material criterion for arresting the propagating shear fracture. Test pipes of Charpy V notch energy from 80 to 290J with different amount of separation, were produced from both controlled rolled steels and quenched and tempered steels. These research projects clarified that the separation of material itself did not influence the crack propagation behavior and its arrestability. Furthermore, the material criterion for arresting the shear fracture was analyzed by the pressure-velocity relationship counterbalancing the crack velocity curve and gas decompression curve.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):63-64. doi:10.1115/1.3231025.

Two classes of submarine pipeline handling problems are considered, namely the lifting of a pipe with a free end for tie-in purposes, and the raising of part of a continuous line for hot-tapping or repair. If pipeline slopes are small (as is usual in seabed work), engineers’ beam theory may be used in combination with a Newton-Raphson solution to locate the separation points between pipe and seabed. Key results are presented.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):65-69. doi:10.1115/1.3231027.

This paper describes a dynamic model of a pipeline portion exposed to wave and current forces for the purpose of assessing on bottom stability. When an exposed span is marginally stable, and trenching not possible, some motion in the presence of an extreme wave may be tolerable. Analysis of a particular case shows that a few feet of movement would occur with a 100-yr wave. The final configuration after passage of one wave is given by the analytical model and it is predicted that a moderate residual stress would be induced. It is concluded that this kind of analysis—on a case by case basis—could be used to assess risk of pipeline damage due to storm-induced movement.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):70-78. doi:10.1115/1.3231028.

The paper summarizes the results of a laboratory study of the separate and combined effects of bed proximity and large velocity gradients on the frequency of vortex shedding from pipeline spans immersed in the thick boundary layers of tidal currents. This investigation forms part of a wider project concerned with the assessment of span stability. The measurements show that in the case of both sheared and uniform approach flows, with and without velocity gradients, respectively, the Strouhal number defining the vortex shedding frequency progressively increases as the gap between the pipe base and the bed is reduced below two pipe diameters. The maximum increase in vortex shedding Strouhal number, recorded close to the bed in an approach flow with large velocity gradients, was of the order of 25 percent.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):79-87. doi:10.1115/1.3231029.

The paper deals with the subject of initiation of a propagating buckle in an offshore pipeline. If the external pressure is high enough, then a propagating buckle can be initiated by locally denting the pipe. Such a buckle will propagate at any pressure above the propagation pressure of the pipe. The pressure at which a local geometric imperfection transforms itself into a propagating buckle is known as the initiation pressure. This pressure depends on the geometric characteristics of the damage. The paper restricts itself to a parametric study of damages produced by point, knife and plate indentors. It is found that the geometry of these types of damages can be well represented by the ratio of minimum diameter: maximum diameter of the most damaged section.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):88-95. doi:10.1115/1.3231030.

The directional stability of three modern ship types with conventional nonadaptive autopilots is examined. It is shown that while attempting to improve the course-keeping ability of these ships, the potential exists for the operator to unknowingly cause a loss of directional stability with the controls available to him. Stable controller settings are established to prevent such an occurrence. The results of a design method allowing optimization of the autopilot to achieve a desired degree of stability controllability versus steering-induced added resistance is also presented.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):96-102. doi:10.1115/1.3231031.

A reconnaissance level study of the Pilgrim Springs geothermal system was conducted to determine the near-surface thermal regime and to obtain information on the ground water flow regime within the thawed ground area. Measurements included soil temperatures, apparent electrical conductivity of the soil, electrical conductivity and temperature in the Pilgrim River, saturated hydraulic conductivity of the soil and ground water flow characteristics (direction and velocity). In addition the size, number and characteristics (geometry, direction of flow) of near-surface convective plumes were investigated. Measured temperature profiles were used to estimate ground water flow velocities. There are approximately 2–3 km2 of thawed ground surrounded by permafrost on the order of 100 m in thickness. The highest temperatures were found in the southwest quadrant of the thawed area where a pool of ground water at ≈ 92°C exists at 14–32 m below the ground surface. Temperature measurements suggest that water in the pool is flowing laterally and vertically. Temperature and electrical conductivity measurements suggest that this pool of water underlies most of the thawed ground area although the possibility of several, unconnected sources of hot water and multiple pools has not been ruled out. Conductivity measurements suggest that hot and/or saline water rises in convective plumes from the pool at about 40–60 sites. The Pilgrim River appears to be heated by heat transfer from the geothermal area. Saline ground water enters the Pilgrim River, probably through its bed, increasing the conductivity of the river water.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):103-106. doi:10.1115/1.3231003.

Exact solutions to problems of conductive heat transfer with solidification are rare due to the nonlinearity of the equations. The heat balance integral technique is used to obtain an approximate solution to the freezing of a semi-infinite region with a linear, initial temperature distribution. The results indicate that the constant temperature Neumann solution is acceptable for soil systems with a geothermal gradient unless extremely long freezing times are considered. The heat balance integral will yield good solutions, with simple numerical work, even for non-constant initial temperatures.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):107-112. doi:10.1115/1.3231004.

Ice action on two cylindrical structures, located side by side, has been investigated in a small-scale experimental study to determine the interference effects on the ice forces generated during ice structure interaction. The proximity of the two structures changes the mode of ice failure, the magnitude and direction of ice forces on the individual structure, and the dominant frequency of ice force variations. Interference effects were determined by comparing the experimental results of tests at different structure spacings.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Ocean Engineering

J. Energy Resour. Technol. 1984;106(1):113-119. doi:10.1115/1.3231006.

The behavior of waves interacting with islands has gained renewed interest with the construction of exploratory drilling islands in the Arctic. This paper focuses upon the behavior of waves incident upon axisymmetric islands characterized by circular contours which vary with water depth. The island profiles of Arthur and Pocinki, which have closed form solutions, and a single tier conical island are examined. A new dimensionless formulation of Arthur’s ray theory and an extremely accurate numerical procedure to evaluate the ray integrals are presented. It is shown that each island profile leads to a distinct wave pattern about the island. These wave patterns are presented in figures which portray the wave capture and wave breaking about circular islands. It is intended that the methodology presented be used to initially assess trends and to evaluate the need for more refined analyses.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):120-129. doi:10.1115/1.3231007.

The northern margin of St. Croix, V.I. is characterized by three morphological features: a narrow shallow water carbonate shelf; a steep slope; and a deep-sea basin. Shallow water reef material and carbonate debris occurs along the shelf. A thin carbonate ooze overlies the steep slope where occasionally rock outcrops occur. Trough sediments are clayey silts and turbidite deposits. Average shear strengths range between 3.1–10.0 kPa for the slope and 5.8–28.3 kPa for the trough (1-m cores). Values range as high as 69.1 kPa at sediment depths greater than one meter. Average sensitivities range between 3.2–6.1 for the slope and 4.1–12.1 for the trough (upper 1 m). Higher sensitivities [8–12] indicate the presence of metastable deposits and considerable strength loss upon disturbance. Variation in the sand, silt, and clay sized fractions, the nature of the carbonate particles, and bioturbation strongly affect the variability in the geotechnical properties. Quantitative studies of local morphology, processes and sediment properties reveal the need for detailed data in specific areas of engineering interest.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Drilling/Manufacturing

J. Energy Resour. Technol. 1984;106(1):130-136. doi:10.1115/1.3231008.

The purpose of this investigation was to gain a better understanding into the mechanical behavior of the API 8 Round casing connection, when subjected to service loads of assembly interference, tension and internal pressure. The connection must provide both structural and sealing functions and these functions were evaluated by several methods. Part I discusses the methods of analysis, which include hand calculations using strength of materials, finite element method via unthreaded and threaded models, and experimental analysis using strain gages. Comparisons of all three methods are made for stresses and show that the finite element method accurately models connection behavior.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):137-143. doi:10.1115/1.3231009.

Comparison of stresses in Part I showed that the APILT&C 8 Round threaded casing connection could be accurately analyzed with finite element methods, with models that both included and excluded the threads. Part II further investigates the effects of thread tapers and external loads on connection stresses. It also assumes a criteria for determining maximum assembly interference using the finite element models. Tension loading is shown to affect pin hoop stresses, while being insignificant to coupling hoop stresses. Hand-calculated stresses are shown to give reasonable hoop stress values.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(1):144-152. doi:10.1115/1.3231010.

The performance of the API LT&C 8 Round casing connection is investigated using 7-in.-29-lb/ft pipe size. Part I showed that the finite element method was applicable for analysis using both a threaded and an unthreaded model. Part II investigated the effects of various thread tapers and external loads on connection hoop stresses. It also evaluated maximum assembly interference based on an assumed criteria. Part III addresses connection sealability based on contact pressure. Geometric end effects are found to be significant in the unthreaded model. Sealability in the middle portion predicted by hand equations is shown to be reasonable. The threaded model shows that tension reduces sealability by disengaging thread stab flanks. Assembly forces are analyzed and, along with the test results, are used to find an equation for torque. The effects of thread taper variations, tension, and pressure on torque are shown.

Commentary by Dr. Valentin Fuster

DISCUSSIONS

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