RESEARCH PAPERS: Offshore Mechanics and Arctic Engineering

J. Energy Resour. Technol. 1984;106(4):387-392. doi:10.1115/1.3231096.

The study reported in this paper deals with the development of a dynamic model for the analysis of elastically supported gyroscopic absorber systems for ship stabilization. The gryoscopes are mounted on elastically supported platforms at the fore and aft ends of the ship to minimize both the roll and pitch movements. Springs and dampers are also utilized between the gyroscope gimbal and the platform. Several design configurations of the absorber are considered. Optimal design procedures are utilized to find the system parameters for best performance in each case. The performance of the resulting optimum absorber shows that introducing the elastic spring and damper between the gimbal and platform has a significant effect on reducing the ship-roll action.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):393-398. doi:10.1115/1.3231097.

The kinematics of a gyrocompass configuration whereby the binnacle is pendulously mounted inside a set of gimbals, commonly installed on commercial marine vessels, is examined under vessel pitch and roll motions. The gimballing errors of this gyrocompass resulting from such motions are determined as a function of its geometry. It is shown that this design results in significantly greater errors than those resulting from the same configuration with one degree of freedom removed. This anomalous behavior is explained in terms of the compass geometry using approximate analytical techniques, the results of which compare well with experimental full-scale tests.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):399-404. doi:10.1115/1.3231098.

Active control of the motion of marine structures may produce structural and operational savings over conventional passive designs. It is shown how equations of motion of typical marine structures, including those with frequency-dependent coefficients, may be rendered into a form compatible with modern controller design methods. Linear quadratic Gaussian control theory is summarized and applied to a simple example. The limitations of the theory are discussed and the effects of a force-limited actuator are quantified.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):405-411. doi:10.1115/1.3231099.

A method allowing a priori optimization of fixed structure steering controllers is presented. The technique is based on the second method of Lyapunov, and allows optimization of the controller parameter values to a predefined quadratic index of steering quality. The design methodology involved is applied to different controller structures using a 250,000-dwt tanker as an example. The resulting performance is compared to that achievable for a standard setting conventional autopilot on the same ship, using frequency domain analyses. The results presented suggest that the method has the potential to result in improved steering performance.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):412-418. doi:10.1115/1.3231100.

Field data from structural, geotechnical and wave measurements made on the concrete gravity platform TCP2 of the Norwegian FRIGG field are disclosed in this paper. Data recorded during 3 yr are interpreted and compared to design values. The stability of invariant parameters has been followed and demonstrated in view of monitoring the long-term behavior stability of the platform. The measured behavior differs from the design behavior and exhibits that design assumptions were conservative. Particularly during this 3-yr period the platform experienced very severe storms with waves up to 25.7 m, a value close to the maximum design wave (100-yr wave: 29 m). It is shown from field data how a bridge connection to a neighboring platform may influence the actual behavior compared to the design one and must be taken into account to understand the measured behavior. It is finally shown that supposed invariant parameters like natural frequencies and others are stable on a long-term basis, and how these invariants are intended to be used for the long-term stability monitoring.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):419-425. doi:10.1115/1.3231101.

A semisubmersible moored in waves experiences a steady offset and two types of motion—a first-order motion at frequencies corresponding to the incident wave frequencies and a slowly oscillating motion near the natural frequency of the semisubmersible/mooring system. An extensive wave tank testing of a semisubmersible model was undertaken in which the motions of the semisubmersible and the loads in the mooring lines were measured. The semisubmersible was tested in the tank in a head sea as well as a beam sea heading in a series of regular waves, regular wave groups and irregular waves. The test results of the steady offset and first-order and slowly oscillating motions are presented for each heading and for each of these wave series as functions of the wave period. The experimental results are correlated with theoretical results based on a 3-D diffraction theory which takes into account the appropriate first and second-order terms. It is found that the nonlinear damping terms are quite important in explaining the behavior of the moored semisubmersible in waves and that the steady drift loads in wave groups can be determined from results based on regular waves.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):426-436. doi:10.1115/1.3231102.

This paper presents comparisons between the results of hydrodynamic analysis and two sets of model test data for the wave-induced motion response of tensioned buoyant platforms. The comparisons are presented with emphasis on the measured and predicted behavior of the tether elements. The two sets of data used are from (i) tests performed jointly by Heriot-Watt University and University College London at the National Maritime Institute (NMI) Ltd., and (ii) published data from tests at the Norwegian Hydrodynamics Laboratory. The hydrodynamic analysis used in the comparisons is described, together with the assumptions underlying its formulation and the manner in which the lateral dynamics of the tethers are accounted for. The analysis and test data show good agreement for surge motions although discrepancies are observed for the tether tension amplitude response at certain wave frequencies. The paper also presents detailed tether tension time histories from tests in regular long-crested waves at NMI. These data demostrate the problem of high-frequency tension oscillations (often called ringing) in tethers. Although this feature is not modeled by the hydrodynamic analysis, time history data are presented to enable interpretation of the underlying physical mechanism of this phenomenon.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):437-443. doi:10.1115/1.3231103.

A simple single-degree-of-freedom model of a tension leg platform is used to assess the reliability of the common practice of calculating wave-induced forces at the undisplaced position of offshore structures. This assessment is conducted in conjunction with the Morison equation based modeling of the wave-induced forces on slender structural members. It is shown by numerically integrating the equation of motion that the calculation of wave forces on the displaced position of the structure introduces a steady offset component in the structural response. This is valid for either deterministically or stochastically described wave fields. Several parameter studies are conducted. Furthermore, reliable approximate analytical deterministic and stochastic solution techniques are developed which conform to and, in fact, predict the conclusions drawn from the results of the numerical studies.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):444-450. doi:10.1115/1.3231104.

An efficient method for the analysis of the linear and nonlinear static and dynamic motions of offshore systems such as risers and single-leg mooring towers is presented. The technique is based on the finite element approach using connected coordinates for arbitrary large rotations and includes terms due to loads such as buoyancy, gravity, random waves, currents, ship motions and Morison’s equation. Practical features include the addition of intermediate articulations and modeling of the loading arm between the riser and associated tanker. Parametric studies are presented to show that stable and accurate results are obtained using relatively large time step increments leading to efficient design studies.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):451-457. doi:10.1115/1.3231105.

The paper describes the results of a model test series with the purpose of determining the hydroelastic vibrations of a nearbed pipeline span exposed to flow conditions created by steady current, waves and waves superimposed on steady current. The study has been conducted using a model composed of a spring-mounted rigid pipe segment and a flat plate simulating the sea bed. The hydroelastic cross-flow vibrations of the pipe segment are presented as function of the flow velocity, flow condition (waves and/or steady current) and the relative distance of the pipe to the seabed. A simple approach to analyze the vibrations caused by an irregular wave train is presented.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):458-465. doi:10.1115/1.3231106.

Twenty full-scale test runs were conducted during the cable strumming experiments reported in this paper. These consisted of ten pairs of equivalent tests conducted in air and in water with a cable fitted with arrays of attached masses. The measured in-air natural frequencies are in good agreement with computed code predictions for the second and higher (up to n = 5) cable modes. The first mode frequency apparently was influenced by the sag of the cable. The measured mode shapes of the cable vibrations in water also are in agreement with the computed mode shapes. For the experiments in water the computed and measured natural frequencies are in good agreement. Drag coefficients in the range CD = 2.4 to 3.2 were commonly observed when the cable-attached mass system was strumming due to the water flow past it.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):466-470. doi:10.1115/1.3231107.

A time-series simulation method, based on the principle of time series modeling for dynamic systems, is used to reproduce a wide-band stress history from a prescribed stress spectral model for fatigue testing of offshore structures. The optimization procedures and stability of the time series model for the prescribed spectrum are presented and discussed. The optimization procedures are developed on the basis of the Levison-Durbin algorithm, which usually produces a stable time series model if the order of the time series model is even. An example is presented to demonstrate the applicability of the proposed method to long-time, high-cycle fatigue testing.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):473-479. doi:10.1115/1.3231110.

In order to investigate the mechanisms and the factors to govern the brittle fracture initiation during the fatigue crack propagation at low temperatures, fracture toughness tests under wide range of loading rates, fatigue tests at low temperatures and fracture toughness tests after having been given pre-loading were performed on steels and weld junctions. The fatigue fracture toughness K fc was estimated as equivalent as the fracture toughness K c under the monotonic tensile loading if they were compared at the same loading rate, since the residual stress due to the cyclic loading was less effective on the brittle fracture initiation. The calculated fatigue life based on Paris’s formula taken into consideration of the crack closure phenomena showed a good one-by-one agreement with the observed fatigue life up to the brittle fracture initiation. Therefore, a design curve was preliminarily drawn to determine the fatigue life at low temperatures.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):480-488. doi:10.1115/1.3231111.

Combined strip and rosette gage measurements and results from three-dimensional, finite element calculations are in excellent agreement with frozen stress photoelastic results for an efficient shape of cast-steel node under axial, brace loading. Three different meshes showed that two layers of elements through the thickness are needed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):489-495. doi:10.1115/1.3231112.

Exxon’s Mississippi Canyon 280-A (Lena) platform represents the first commercial application of the guyed tower concept for offshore drilling and production platforms. Unlike a conventional offshore platform, the guyed tower is held upright by an array of guylines attached near the upper end of the structure and radiating outward to anchor piles driven into the seabed. This paper describes the functional requirements of the various components of this unique guying system and shows how each component was designed to meet those requirements. Among the design parameters discussed are guying system stiffness and strength, fatigue and wear life, corrosion protection, and assembly. The major length of the guyline consists of spiral-wound bridge strand, most of which is sheathed in polyethylene. The cable constructions and terminations are described. Near the tower, the sheathed cable is connected to a length of bare cable which passes through a special fairlead arrangement to direct the cable to the upper attachment above the water line. The fairlead arrangement and the upper attachment are described as well as the means provided to protect the bare cable from corrosion. Unique pinned connections in the system are designed to ease assembly while still providing the required load capacity and service life. A special anchor pile attachment eye design allows for large tolerances in anchor pile orientation. A description of the clump weight shows how this component is designed to achieve the desired system stiffness while also providing stability against overturning or excessive settlement into the seabed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):496-501. doi:10.1115/1.3231113.

In this paper, the pile-soil interaction behavior of a large-sized steel pipe pile in soft silty clay along the downstream of the Yangtze River is analyzed and discussed through an in-situ experiment under lateral static, cyclic and dynamic loadings, along with the static and dynamic triaxial tests on soil samples in laboratory. In the analysis, the finite element method and other simplified methods are compared using different models of lateral soil stiffness. From the results of the tests as well as the analysis, a possible formulation of the p-y curves of the pile in soft clay under lateral loadings is suggested, and a simplified method of dynamic analysis taking into account of the pile-soil interaction is also proposed.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):502-508. doi:10.1115/1.3231114.

In this study, the problem of frost penetration in high-clay content soils is examined with the view to assessment of cyclic freeze-thaw effects on initial virgin unfrozen natural clays. The requirements for control of the frost-heave problem in many construction projects in the northern regions of Eastern Canada demand that proper prediction be made for both frost-heave pressures and magnitudes of frost heaving in the high-clay content soils. Laboratory cyclic freeze-thaw and frost-heave tests conducted have shown that the mechanical properties of the clay soils, which are virgin unfrozen natural soil samples obtained from Northern Quebec, are sensitive to cyclic freeze-thaw. In the cyclic freeze-thaw tests conducted, scanning electron micrographs taken from samples after various cycles show that the original soil fabric becomes more disturbed after the first few cycles. Particles are re-arranged, and a regrouping of particles into larger stable (or apparently stable) soil fabric units are formed after a certain number of cycles. In essence, this study provides the experimental information concerning the cyclic freeze-thaw effects on mechanical properties for some high-clay content soils, not initially subject to freezing.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):509-514. doi:10.1115/1.3231115.

The mechanical behavior of sea ice has been found to be strongly dependent on the deformation rate. Consequently, prediction of ice load on arctic structures must include an adequate consideration of the deformation rate and its effects on the ice behavior. Analytical work for ice load prediction has to date been concentrated on obtaining stress fields around the structure, satisfying an “average” strain-rate dependent strength of ice. In other words, the stress field and the strain-rate field are not directly related. Consequently, there is a gap in understanding the relationship between ice loads and ice deformation. This shortcoming has been recognized, and in the last two years, large-scale field experimental programs have been conducted to measure the deformation pattern and rate around large indenters and an actual offshore island. This paper illustrates a procedure by which the strain-rate dependent mechanical properties of ice can be directly integrated into the force prediction method. This method assumes that the strain-rate field around the structure can be estimated from field measurements and relies on the bound theorems of creep analysis. This paper concentrates on a nonimpact condition in a uniform landfast ice field. The load scenario considered is one of initial breaking out. The effects of nonhomogeneities in the ice field are not included.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):515-520. doi:10.1115/1.3231116.

For slowly moving freezing fronts in soil, the heat-transport equation may be approximated by the Laplacian of temperature. Consequently, potential theory may be assumed to apply and the temperature state can be approximated by an analytic function. The movement of freezing fronts may be approximated by a time-stepped solution of the phase-change problem, thus solving directly for heat flow across a freezing or thawing front. Moisture transport may approximated by using an exact solution of the moisture-transport equation assuming quasi-steady-state conditions, appropriate boundary conditions, and an exponential function relating unsaturated hydraulic conductivity (defined within the thawed zones) to pore water pressure (tension). This approach is used to develop a single model of ice segregation (frost-heave) in freezing soils. Applications to published and experimental one-dimension soil column freezing data show promising results.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Ocean Engineering

J. Energy Resour. Technol. 1984;106(4):521-526. doi:10.1115/1.3231117.

The object of this paper is to discuss the use of empirical factors in the prediction of armored cable axial stiffness. The stiffness of the armor wires and core that compose an armored cable are obtained from empirical data. To predict the net axial stiffness of a cable from the stiffnesses of its components a model of how the pitch diameter of an armor wire decreases as the cable is tensioned is required. The pitch diameter contraction is modeled in this paper by an experimentally determined Poisson’s ratio for the cable. The accuracy of cable axial stiffness predictions using these empirical factors is illustrated by comparing the results of mathematical models of armored cable to tension-elongation test data for ten different cables.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):527-532. doi:10.1115/1.3231118.

On designing optical fiber cables, it is necessary to deal with the weakness of optical fibers, such as a small breaking elongation compared with metals and excess optical loss under both lateral and hydraulic pressure. This paper presents a structural design method for submarine optical fiber cables, based on the study of both lateral and hydraulic pressure characteristics. This paper also clarifies that a composite tension member increases the lateral and hydraulic pressure strength of the cable and can protect optical fibers from extremely large force during laying and recovery.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):533-538. doi:10.1115/1.3231119.

For burying submarine cables under a seabed by a water jet cable burier, it is important to improve digging depth of water jet. This paper discusses digging properties with air-bubble water jet, and clarifies the relation between digging depth and water jet parameters such as traverse speed of nozzle V n (m/s), air content ratio C , and jet momentum per unit time M (N). It is found from experiment results that the digging depth H* (m) is represented by

H* = (1 + 0.26C0.65) (10.31 − 4.54Vn)

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Rock Mechanics/Drilling Mechanics

J. Energy Resour. Technol. 1984;106(4):539-542. doi:10.1115/1.3231120.

Exact solutions associated with the problem of a compressible or an incompressible fluid displacing a compressible reservoir fluid are derived. Simplified forms for the fluid leak-off coefficients, available in the literature, are deduced as special cases. Nondimensionalized expressions suitable for parameter sensitivity calculations and assumption validation are also presented.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):543-553. doi:10.1115/1.3231121.

This is Part II of a two-part paper which reviews in depth the modeling of the propagation of large hydraulic fractures in underground rock formations. Based on experimental observations reviewed in Part I the need for models of the growth of general-shaped planar hydraulic fractures has been demonstrated. Part II contains reviews of both coupled fluid/solid three-dimensional fracture models and some more basic two and three-dimensional problems in linear elastic fracture mechanics applicable to the growth of fractures in the vicinity of in-situ discontinuities (interfaces) in rock-type and/or tectonic stress. The latter is referred to as the vertical growth problem since hydraulic fractures are quite often oriented vertically with respect to horizontal layering or stratification. The state of current research in both overall modeling and basic vertical growth studies is assessed and recommendations for future research are made.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 1984;106(4):554-561. doi:10.1115/1.3231122.

A mathematical formulation has been developed for calculating the cross-sectional shape of hydraulic fractures. This formulation treats the problem as a free-boundary-type problem and is modeled after mathematical formulations developed for contact and lubrication problems. Numerical solution of the resulting equations has been used to address problems involving particularly difficult in-situ stress distributions, including problems in which the fracture breaks through high-stress barriers. The technique is illustrated on two example problems.

Commentary by Dr. Valentin Fuster


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