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Showing papers in "Journal of Offshore Mechanics and Arctic Engineering-transactions of The Asme in 1999"


Journal ArticleDOI
TL;DR: In this article, it is shown that for an axisymmetric system utilizing heave modes, it is possible to absorb an energy amounting to the incident wave power on a crest length which equals the wavelength divided by 2π, even though the power take-off is applied to the relative motion only.
Abstract: Wave-energy converters (WECs) need a reaction source against which the wave forces can react. As with shore-based WECs, sometimes also floating WECs react against a fixed point on the seabed. Alternatively, for a floating WEC, force reaction may be obtained by utilizing the relative motion between two bodies. A load force for energy conversion is assumed to be applied only to this relative motion. It is assumed that either body oscillates in one mode only (mostly, the heave mode is considered here). The system, if assumed to be linear, is proved to be phenomenologically equivalent to a one-mode, one-body system, for which the wave excitation force equals the force which is necessary to apply between the two bodies in order to ensure that they are oscillating with zero relative motion. It is discussed how this equivalent excitation force and also the intrinsic mechanical impedance of the equivalent system depend on the mechanical impedances for the two separate bodies, including the radiation impedance matrix (which combines radiation resistances and added masses). The equivalent system is applied for discussing optimum performance for maximizing the absorbed wave energy. It is shown that, for an axisymmetric system utilizing heave modes, it is possible to absorb an energy amounting to the incident wave power on a crest length which equals the wavelength divided by 2π, even though the power take-off is applied to the relative motion only. Moreover, it is shown that it is possible to obtain an equivalent excitation force which exceeds the wave excitation force on either body.

150 citations





Journal ArticleDOI
TL;DR: In this article, Xu et al. compared theoretical simulations of vessel free-fall impact on calm water, with zero and nonzero initial heel angles, with drop test experiments of ship freefall impact, and showed varying degrees of qualitative and quantitative agreement.
Abstract: Theoretical simulations are compared with drop test experiments of vessel free-fall impact on calm water, with zero and nonzero initial heel angles. The impact force and moment are predicted by using a theory described in Xu et al. (1998). The resulting motions of vessels at asymmetric impact include vertical water entry and transverse rolling. The downward deceleration equation combines the hydrostatic and hydrodynamic forces and the weight of hull, while the rolling equation takes into account the restoring moments due to asymmetry. Another simplified approach of a water entry dynamics and injury model (Gollwitzer and Peterson, 1996) is also applied to evaluate the impact acceleration for symmetric cases. The experiments were conducted at the Coastal Systems Station, Panama City, Florida, using a 2 ft × 8 ft (0.61 m × 2.44 m) prismatic hull model with 20 deg deadrise. The comparisons between the asymmetric impact theory and experiment shows varying degrees of qualitative and quantitative agreement.

38 citations


Journal ArticleDOI
TL;DR: In this article, a methodology for computing vortex-induced vibrations (VIV) on risers is presented based on computation of the flow by a CFD program, structural dynamics by a nonlinear structural (CSD) code, and a coupling between them.
Abstract: A methodology for computing vortex-induced vibrations (VIV) on risers is presented. It is based on computation of the flow by a CFD program, structural dynamics by a nonlinear structural (CSD) code, and a coupling between them. The CFD computations are performed in 2-D at a number of sections along the riser. The load is imposed on the riser in a strip theory manner. The coupling between the CFD planes takes place through the response of the riser. The local deformation of the riser is taken into account by the CFD program, thus completing a fluid-structure interaction loop each time step. The methodology is validated by comparing results from simulations with results from model tests.

28 citations



Journal ArticleDOI
TL;DR: In this paper, an analytical expression for the prediction of fully plastic moment capacity of pipes subjected to axial loading and internal pressure is derived based on the von Mises yield criterion, which is in good agreement with full-scale experimental results.
Abstract: An analytical expression is derived for the prediction of fully plastic moment capacity of pipes subjected to axial loading and internal pressure. The expression is based on the von Mises yield criterion. The expression predicts pipe moment capacities that are in good agreement with full-scale experimental results. A universal nondimensional moment versus effective axial force-pressure interaction diagram is developed for the design of elevated pipe lines.

24 citations



Journal ArticleDOI
TL;DR: In this article, the design of a catenary riser can be formulated as an optimization problem by using riser costs as the criteria function, design requirements in terms of maximum allowable stress and buckling capacity as constraints, and riser dimensions as free variables.
Abstract: This paper describes how the design of a catenary riser can be formulated as an optimization problem by using riser costs as the criteria function, design requirements in terms of maximum allowable stress and buckling capacity as constraints, and riser dimensions as free variables. The theory has been implemented in a computer program that can generate an optimized riser design for given design parameters such as water depth, diameter, pressure, and platform excursions. The developed software consists of a conventional program for two-dimensional riser analysis and a set of standard routines to minimize a nonlinear function subjected to general constraints. A case study where design parameters and requirements have been varied is also presented. The importance of buckling versus allowable equivalent stress as the most critical constraint has been investigated for varying water depth. The Conclusion of this work is that optimization is a useful tool for riser design, and that the proposed strategy for selection of design variables and constraints will enable an engineer to identify designs with minimum costs in an efficient way.

15 citations


Journal ArticleDOI
TL;DR: In this article, a method to determine the effective friction factor and overall heat transfer coefficient for a high-pressure, natural gas pipeline during fully transient flow conditions is presented. But, the method is limited to a single pipeline.
Abstract: This paper presents a method to determine the effective friction factor and overall heat transfer coefficient for a high-pressure, natural gas pipeline during fully transient flow conditions. Time-varying SCADA (supervisory control and data acquisition) measurements at the pipeline boundaries (i.e., inlet and outlet) provide boundary conditions for a transient flow model, as well as additional information which is utilized to determine these parameters. The resulting friction factor and overall heat transfer coefficient minimize the least-squared difference between the additional SCADA measurements at the pipeline outlet and the corresponding values predicted from the transient flow model. This concept is referred to as parameter estimation. The transient flow model is based on a numerical solution of the one-dimensional conservation equations (i.e., continuity, momentum, and energy) which are discretized using a highly accurate compact finite-difference scheme. The transient flow model and parameter estimation is incorporated into a computer program that is initially tested on a simple pipeline with steady flow conditions. The predicted outlet pressure and temperature using the estimated friction factor and overall heat transfer coefficient exactly matches the corresponding prescribed values. Subsequently, a portion of the Foothills Pipe Line Ltd. transmission system in Alberta is considered using time-varying SCADA flow measurements. The resulting outlet pressure and temperature from the transient flow model are in good agreement with SCADA measurements for this pipeline section.



Journal ArticleDOI
TL;DR: In this article, a simple procedure is proposed to estimate the global failure probability of a submarine oil pipeline subjected to underwater explosion shock wave by solving a simplified fluid-structure interaction problem.
Abstract: A simple procedure is proposed in this paper to estimate the global failure probabilities of a submarine oil pipeline subjected to underwater explosion shock wave. The deterministic response of a pipeline subjected to an underwater shock loading is first given by solving a simplified fluid-structure interaction problem. Compared with an FEM/BEM coupling model, the present method gives good results at much lower computational efforts. Then, the Monte Carlo method is used to find the global failure probabilities of the pipeline. Finally, a practical example is given.




Journal ArticleDOI
TL;DR: In this paper, a reliability-based sensitivity analysis of ship hull structures, which are modeled by combined spatial plate and frame elements, based on the ultimate strength analysis, is presented to offer the expansion of the applicability of the structural reliability based sensitivity analysis combined with collapse mode analysis.
Abstract: This paper is concerned with reliability-based sensitivity analysis of ship`s hull structures, which are modeled by combined spatial plate and frame elements, based on the ultimate strength analysis. In this paper, at first, a newly proposed system using combined plate and beam elements is presented to offer the expansion of the applicability of the structural reliability-based sensitivity analysis combined with collapse mode analysis. Next, the applicability of the proposed system is demonstrated through a numerical example applied to a simple box girder with longitudinal stiffeners. Finally, features of the structural reliability and sensitivity factors of a high speed ship based on collapse mode analysis are also investigated by successively applying the newly developed system.




Journal ArticleDOI
TL;DR: In this article, the problem of estimating the response statistics of moored large-volume offshore structures subjected to short-crested random waves is discussed, and a general second-order theory is described to carry out the entire analysis in the frequency domain, which is computationally more efficient than time domain analysis, which generally requires considerably more CPU time to reach the same level of accuracy.
Abstract: The paper discusses the problem of estimating the response statistics of moored large-volume offshore structures subjected to short-crested random waves. A general second-order theory is described that makes it possible to carry out the entire analysis in the frequency domain, which is computationally more efficient than time domain analysis, which generally requires considerably more CPU time to reach the same level of accuracy.

Journal ArticleDOI
TL;DR: In this paper, a two-dimensional finite-element model is employed to investigate the wave-induced seabed response around a pipe laid on a poro-elastic seabing.
Abstract: The stability of a pipeline has been extensively studied by coastal and marine geotechnical engineers in recent years. However, most previous investigations have been only concerned with the fluid field around the pipelines, not the wave-induced soil response below the pipelines. In this paper, a two-dimensional finite-element model is employed to investigate the wave-induced seabed response around a pipe laid on a poro-elastic seabed. Unlike conventional investigations, both soil permeability and Young’s modulus are considered to vary with soil depth in the present model. Based on the proposed model, the mechanism of the wave-induced pore pressure and effective stresses are explored.

Journal ArticleDOI
TL;DR: In this paper, the authors present results for finite element analysis of full-sized girth-welded specimens of line pipe and compare these results with the behavior exhibited by test specimens subjected to constant axial force, internal pressure and monotonically increasing curvatures.
Abstract: The paper presents results for finite element analysis of full-sized girth-welded specimens of line pipe and compares these results with the behavior exhibited by test specimens subjected to constant axial force, internal pressure and monotonically increasing curvatures. Recommendations for the ‘best’ type of analytical finite element model are given. Comparisons between the behavior predicted analytically and the observed behavior of the experimental test specimens are made. The mechanism of wrinkling is explained and the evolution of the deformed configurations for different wrinkling modes is examined. It is concluded that the analytical tools now available are sufficiently reliable to predict the behavior of pipe in a manner that was not previously possible and that this should create a new era for the design and assessment of pipelines if the technology is properly exploited by industry.Copyright © 1996 by ASME



Journal ArticleDOI
TL;DR: In this article, a differentiated compliance anchoring system (DICAS) is assessed based on stability of its slow-motion nonlinear dynamics using bifurcation theory, which allows the designer to select appropriate values for the mooring parameters without resorting to trial and error, or extensive nonlinear time simulations.
Abstract: The preliminary design of a differentiated compliance anchoring system (DICAS) is assessed based on stability of its slow-motion nonlinear dynamics using bifurcation theory. The system is to be installed in the Campos Basin, Brazil, for a fixed water depth under predominant current directions. Catastrophe sets are constructed in a two-dimensional parametric design space, separating regions of qualitatively different dynamics. Stability analyses define the morphogeneses occurring across bifurcation boundaries to find stable and limit cycle dynamical behavior. These tools allow the designer to select appropriate values for the mooring parameters without resorting to trial and error, or extensive nonlinear time simulations. The vessel equilibrium and orientation, which are functions of the environmental excitation and their motion stability, define the location of the top of the production riser. This enables the designer to verify that the allowable limits of riser offset are satisfied. The mathematical model consists of the nonlinear, horizontal plane fifth-order large-drift, low-speed maneuvering equations. Mooring lines are modeled by open-water catenary chains with touchdown effects and include nonlinear drag. External excitation consists of time-independent current, wind, and mean wave drift.



Journal ArticleDOI
TL;DR: In this article, the third-harmonic component of the third order hydrodynamic loads on a vertical circular cylinder oscillating in water is calculated by a conventional perturbation method within the framework of a potential theory.
Abstract: The third-harmonic component of the third-order hydrodynamic loads on a vertical circular cylinder oscillating in water is calculated by a conventional perturbation method within the framework of a potential theory. Although the third-order forces are expressed in terms of the first, second, and third-order components of the velocity potential, the latter is not directly required for the calculation. It is replaced by a properly defined linearized radiation potential via Haskind-like theorem. The results of the study are applicable to the analysis of high-frequency resonances of deepwater offshore structures under earthquake excitation or under steep waves (ringing problem).