Showing papers in "Journal of Offshore Mechanics and Arctic Engineering-transactions of The Asme in 1987"
226 citations
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TL;DR: In this article, the important mechanisms affecting the deterioration of Arctic icebergs are modeled based on parameterizations developed for the International Ice Patrol, and the model simulations of three icebergs in the Grand Banks/Labrador Sea area off the Canadian east coast are compared with observations.
Abstract: In this paper the important mechanisms affecting the deterioration of Arctic icebergs are modeled based on parameterizations developed for the International Ice Patrol. The model simulations of the deterioration of three icebergs in the Grand Banks/Labrador Sea area off the Canadian east coast are compared with observations. The model parameterizations provide a reasonable approximation of iceberg deterioration with best results being obtained where detailed observations of the above-water and underwater portions of the iceberg are available enabling accurate estimations of iceberg mass and other parameters needed for the model. A quantitative assessment of the deterioration of icebergs in the Grand Banks/Labrador Sea area is also carried out for each month of the year when the sea surface temperature was above zero. Wave erosion is found to be the major deterioration mechanism. Wave erosion and the resulting calving of overhanging slabs together account for more than 80 percent of the deterioration rate. A small iceberg drifting on the Grand Banks could lose up to one third of its mass in one day.
58 citations
TL;DR: In this article, the effects of wave directionality on the loads and motions of long structures were investigated and a numerical procedure based on Green's theorem was developed to compute the exciting forces and hydrodynamic coefficients due to the interaction of a regular oblique wave train with an infinitely long, semi-immersed floating cylinder of arbitrary shape.
Abstract: The present paper deals with the effects of wave directionality on the loads and motions of long structures. A numerical procedure based on Green's theorem is developed to compute the exciting forces and hydrodynamic coefficients due to the interaction of a regular oblique wave train with an infinitely long, semi-immersed floating cylinder of arbitrary shape. The linear transfer function approach is used to determine the wave loads and motions of a structure of finite length in short-crested seas. The effect of wave directionality is expressed as a frequency-dependent, directionally averaged reduction factor for the wave loads and a response ratio for the body motions. Numerical results are presented for the force reduction factor and response ratio of a long floating box subject to a directional wave spectrum with a cosine-power-type energy spreading function.
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TL;DR: In this article, a coupled ice-water equation is derived for the response of partially ice-covered continental shelf waters to atmospheric and tidal forcing, and the assumptions underlying these equations are discussed, with emphasis on the physical processes represented by the air-ice and icewater drag coefficients.
Abstract: Governing equations are derived for the response of partially ice-covered continental shelf waters to atmospheric and tidal forcing. The assumptions underlying these equations are discussed, with emphasis on the physical processes represented by the air-ice and ice-water drag coefficients. Combining the equations that separately govern the water and ice floe motions, a coupled ice-water equation is obtained. The coupled equation reduces, under certain simplifying assumptions, to an equation involving only the air and water velocities relative to the ice velocity. This suggests an approximate but extremely simple methodology for the determination of drag coefficients for ice floes. The methodology is applied to data obtained during BASICS and MIZEX, and shown, in the former application, to yield drag coefficients comparable to those obtained from considerably more laborious methods.
13 citations
TL;DR: In this article, Stroboscopic Poincare mapping techniques based on discrete time sampling are used to give information about the approach to instability of an articulated mooring tower and a vessel in regular beam seas.
Abstract: When monitoring the wave-driven motions of a compliant offshore facility, be it an articulated mooring tower or a vessel, the engineer would like to be able to predict, in real time, any incipient jump to resonance that might be imminent due to the slowly evolving sea-state. We explore in this paper a study of some new possible prediction techniques for both a jump to a main fundamental resonance leading to capsize and a flip bifurcation to a subharmonic resonance. Stroboscopic Poincare mapping techniques based on discrete time sampling are used to give information about the approach to instability. The first application of these techniques is in the prediction of the jump in resonance and consequent capsize at a cyclic fold in the roll response of a vessel in regular beam seas. Secondly, the techniques are shown to work extremely well in a variety of computational situations when applied to the simulation of an articulated mooring tower during the approach to the potentially dangerous oscillations produced by the onset of subharmonic resonance at a flip bifurcation, in both regular and irregular ocean waves.
TL;DR: In this paper, the coefficients of static and kinetic friction of sea ice on unfrozen beach gravel were calculated from measurements made while dragging two large blocks of ice (approximate weights 23,300 and 26,400 lb (10,600 and 12,000 kg) up a sloping beach surface.
Abstract: The coefficients of static and kinetic friction of sea ice on unfrozen beach gravel were calculated from measurements made while dragging two large blocks of sea ice (approximate weights 23,300 and 26,400 lb (10,600 and 12,000 kg)) in short increments up a sloping beach surface The results of 36 separate experiments yielded values of the coefficients in static and kinetic friction of μs = 050 and μk = 039, respectively These values should be applicable to calculations of the forces required to drive an ice sheet on shore during an ice push event
TL;DR: In this article, the spectral analysis of a tension leg platform is studied using spectral analysis techniques and the fatigue life for resonant vibration is found to be adequate, including linear damping from wave radiation and energy dissipation in the tethers.
Abstract: The dynamic response of a tension leg platform is studied using spectral analysis techniques. Equations of motion are derived for the rigid platform and its legs which are idealized as cables with longitudinal inertia. Attention is focused on the high-frequency resonant response of the platform in heave, pitch and roll modes with typical periods of 3 to 5 s. These modes are lightly damped, but the wave excitation at the high natural frequencies is small. Thus the response may turn out to be small. Formulas are derived for expected fatigue damage in the tethers due to high-frequency resonant vibration. The analysis includes linear damping from wave radiation and from energy dissipation in the tethers. By way of example, the results of the analysis are applied to a typical tension leg platform for petroleum production in 640 m (2100 ft) of water. The fatigue life for resonant vibration is found to be adequate.
TL;DR: In this article, the authors consider the application of the limit state philosophy of structural analysis to pipeline design and present a review of the state of current knowledge on various aspects of geometry and material characteristics, loading and structural behavior.
Abstract: This paper considers the application of the limit state philosophy of structural analysis to pipeline design. General aspects of the philosophy are discussed and the approach to the evaluation of safety factors is reviewed. The paper further considers the various limit and serviceability states which would be relevant to a pipeline and reviews the various factors which may require consideration, before a code embodying the limit state philosophy could be formulated. A review of the state of current knowledge on various aspects of geometry and material characteristics, loading and structural behavior is presented. It is intended that such a review can be used as the basis for a larger study to provide guidance and data for the evaluation of rational levels of safety factor. The major conclusion reached by the authors is that a limit state philosophy would be valuable in providing a suitable framework, which may highlight the significant aspects of pipeline design and which can most easily accommodate new requirements and results obtained from research.
TL;DR: In this article, a method for probabilistic collapse analysis of an offshore structure is proposed, where wave loads are estimated by using Stokes third-order theory and Morison's formula.
Abstract: This paper proposes a method for probabilistic collapse analysis of an offshore structure. Wave loads are estimated by using Stokes third-order theory and Morison’s formula. Plastic collapsing is evaluated by taking account of the combined load effect to generate the safety margins, using a matrix method. Probabilistically dominant collapse modes are selected through a branch-and-bound method. The proposed method is successfully applied to a jacket-type offshore platform.
TL;DR: In this article, an electrohydraulic pump in conjunction with simultaneous recording of pressure and diametral displacement as a function of time was used to improve the performance of a borehole jack test system under operational conditions in the High Arctic in first-year and multi-year sea ice.
Abstract: As the working temperatures in ice are very close to its melting point, it behaves viscoelastically and experiences what is commonly known as high temperature embrittlement. Its mechanical properties are rate sensitive and analysis must include load and displacement history. Borehole jack tests can be improved by the use of an electrohydraulic pump in conjunction with simultaneous recording of pressure and diametral displacement as a function of time. These small but significant modifications in the test procedures permit analysis of the response of a borehole jack test system under operational conditions in the High Arctic in first-year and multi-year sea ice.
TL;DR: In this paper, the authors outline a methodology for determining propeller ice milling loads as a function of propeller characteristics, propeller speed, ship speed, ice conditions and the depth of the milling depth.
Abstract: In designing ice transiting ships, a major concern is the design of the propeller to provide adequate strength to resist ice loads due to propeller ice milling while still providing good propeller efficiency for open water observations as well as high icebreaking thrust at slow advance speeds. As a result, propeller design is a compromise between strength and efficiency. This is especially true for ice transiting ships that must transit long distances on ice-free routes and then perform difficult ice-breaking operations. The geometric properties of a propeller blade that provide good strength are blade width and thickness. Unfortunately, increasing these properties does not provide the best efficiency. Propeller design for ice transiting ships in general has tended to favor strength and reliability over efficiency in design compromises. The purpose of this paper is to outline a methodology for determining propeller ice milling loads as a function of propeller characteristics, propeller speed, ship speed, ice conditions and depth of ice milling to help in the propeller design process.
TL;DR: In this article, the authors analyzed the transition zone in an unconfined propagating buckle using a simple model of the pipe consisting of rings and string-like generators, and derived an expression for the length of the transition interval in terms of the material and geometric parameters of pipe.
Abstract: When a buckle propagates down an underwater pipeline due to excessive hydrostatic pressure, the permanent plastic deformations and sectional collapse of the tube are restricted to a finite length transition zone. The existing theoretical predictions of the propagation pressure Pp are based on ring models neglecting the axial effects in the transition zone. An analysis of the transition phenomenon has not been reported so far. The present paper analyzes the transition zone in an unconfined propagating buckle using a simple model of the pipe consisting of rings and stringlike generators. The thrust of this paper is toward establishing a basic understanding of the transition phenomenon, necessary to improve the theoretical predictions of the propagation pressure further. Expressions for the length of the transition zone have been derived in terms of the material and geometric parameters of the pipe. The results are compared with limited experimental data kindly provided to us by Kyriakides.
TL;DR: In this paper, the mean and variance of the spatial coherence estimate of the directional spectrum were studied and a directional sea estimation procedure based on Fourier series expansion was proposed, combined with a Maximum Entropy condition.
Abstract: The statistical variability of directional sea estimates is considered. A general, but brief study in the mean and variance of the spatial coherence estimate is first presented. This function is found to be relatively unstable when it is not close to 1. A directional sea estimation procedure based on Fourier Series Expansion of the directional spectrum combined with a Maximum Entropy Condition is then described. This method is used in a numerical directional analysis test in order to demonstrate the effect on sea state estimates from the variability in the spatial coherence. Numerical sea states for this test are generated with 2049 frequencies and 100 directions per frequency. Significant statistical scatter is observed in the resulting estimated directional spectra and their parameters, in qualitative agreement with the spatial coherence statistics considered theoretically.
TL;DR: In this paper, the authors describe a highly effective method for computing the dynamics of the catenary-shaped suspension of flexible hose systems, which accounts for a number of nonlinearities, it is 3D and it is performed in the time domain.
Abstract: The paper describes a highly effective method for computing the dynamics of the catenary-shaped suspension of flexible hose systems. The method accounts for a number of nonlinearities, it is 3-dimensional and it is performed in the time domain. The paper addresses the analysis method and demonstrates its effectiveness on a sample flexible riser analysis.
TL;DR: In this article, both the method of averaging and a multi-frequency incremental harmonic balance approach are used in understanding the primary and secondary resonances in the response of a structure exhibiting nonlinear stiffness characteristics and excited by a nonzero mean oscillatory fluid flow.
Abstract: The dynamic response of a structure exhibiting nonlinear stiffness characteristics and excited by a nonzero mean oscillatory fluid flow is investigated. Both the method of averaging and a multi-frequency incremental harmonic balance approach are used in understanding the primary and secondary resonances in the response. Several parameter studies are presented where the results of these two methods are compared with those obtained by numerical integration and are discussed in detail. The results point to the need for a multi-frequency solution formulation for accurate representation of the response offset and to the difficulties of using the standard method of averaging formulation for investigation of secondary resonances in the response of this system.
TL;DR: An orthotopic elastic power law creep model for sea ice is presented and then a finite element method of analysis is developed and applied to study the effect of sea ice anisotropy on indentation in the creeping mode as discussed by the authors.
Abstract: An orthotopic elastic-power law creep model for sea ice is presented and then a finite element method of analysis is developed and applied to study the effect of sea ice anisotropy on indentation in the creeping mode. Numerical simulations are performed under plane stress conditions to predict the influence of interface adfreeze and friction, variability in parameters of a transversely isotropic material model for sea ice, rubble pile or grounded ice foot, and ice sheet velocity on global forces and local pressures generated on a rigid cylindrical indenter. The results are compared with those from an approximate method of analysis based on the upper bound theorem. Interface pressure distributions are obtained in addition to contours of stress and strain.
TL;DR: In this article, the numerical evaluation of wave flow past these complex multibody systems is performed using a coupled finite element/boundary element model, together with the near field (direct integration) calculation of drift forces.
Abstract: Hydrodynamic interactions between the component bodies of a multi-body offshore system can cause substantial enhancement of certain load effects, such as mean horizontal drift forces and radiation damping. It is found, for example, that for a group of N similar members these forces can be of the order of N2 times the force on one member in isolation, over a range of low frequencies; and at higher frequencies interaction effects continue to have important consequences on design parameters. This paper discusses some theoretical background which sheds light on this phenomenon, and presents numerical results for semisubmersible, TLP and other configurations which provide further insight. The numerical evaluation of wave flow past these complex multibody systems is performed using a coupled finite element/boundary element model, together with the near field (direct integration) calculation of drift forces. Results are compared with an existing approximate method, and with idealizations based on the columns alone, with a view to the development of simple procedures for hydrodynamic synthesis.