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Showing papers in "Journal of Offshore Mechanics and Arctic Engineering-transactions of The Asme in 1996"



Journal ArticleDOI
TL;DR: In this article, the Fourier-averaged drag and inertia coefficients were calculated as a function of a suitable Keulegan-Carpenter through the use of the instantaneous force acting on each plate.
Abstract: The paper describes an experimental investigation of the damping provided by bilge keels in an oscillating flow (in a large U- shaped water tunnel). Rectangular and square flat plates were placed adjacent to a solid boundary (with no gap) in order to simulate flow about bilge keels. The single free edge of the rectangular plate and the three free edges of the square plates were bevelled to 60-degree angles so as to form sharp edges with included angles of about 60 degrees. For comparison, another sharp-edged rectangular plate was tested without the wall proximity effects. All plates were held normal to the direction of the ambient flow. The Fourier-averaged drag and inertia coefficients were then calculated as a function of a suitable Keulegan-Carpenter through the use of the instantaneous force acting on each plate.

47 citations


Journal ArticleDOI
TL;DR: In this paper, a three-parameter Weibull distribution is fitted to the data and the parameters of the fitted distribution are estimated by the methods of maximum likelihood, of regression, and of the moments.
Abstract: This work examines some aspects involved in the estimation of the parameters of the probability distribution of significant wave height, in particular the homogeneity of the data sets and the statistical methods of fitting a distribution to data. More homogeneous data sets are organized by collecting the data on a monthly basis and by separating the simple sea states from the combined ones. A three-parameter Weibull distribution is fitted to the data. The parameters of the fitted distribution are estimated by the methods of maximum likelihood, of regression, and of the moments. The uncertainty involved in estimating the probability distribution with the three methods is compared with the one that results from using more homogeneous data sets, and it is concluded that the uncertainty involved in the fitting procedure can be more significant unless the method of moments is not considered.

43 citations


Journal ArticleDOI
TL;DR: An Atlas of the European offshore wave energy resource that is being developed within the scope of an European project is presented in this article.It is mainly based on wave estimates produced by the numerical wind-wave model WAM that is in routine operation at the European Centre for Medium-Range Weather Forecasts, Reading, UK.
Abstract: This paper presents an Atlas of the European offshore wave energy resource that is being developed within the scope of an European project. It will be mainly based on wave estimates produced by the numerical wind-wave model WAM that is in routine operation at the European Centre for Medium-Range Weather Forecasts, Reading, UK. This model was chosen after a preliminary verification of two models again buoy data for a one-year period. Wave measurements will be used for the Norwegian Sea and the North Sea. The Atlas will be produced as a user-friendly software package for MS-DOS microcomputers permitting fast retrieval of information as well as saving and printing of statistics and maps. The Atlas will include annual and seasonal statistics of significant wave height, mean and peak period, mean direction and wave power levels (global values as well as directional distributions). These data will be both presented as tables, graphs and as geographic maps.

39 citations


Journal ArticleDOI
TL;DR: In this article, the use of optimal control techniques for improving the energy absorption by a wave-energy converter (WEC) is investigated, and an optimal control strategy based on variations of a Lagrange functional is determined, which gives a set of adjoint equations in addition to the state equations as a necessary condition for optimum.
Abstract: In this paper, the use of optimal control techniques for improving the energy absorption by a wave-energy converter (WEC) is investigated. A mathematical model is developed for a floating body, which is exposed to an irregular incident wave, and is moving relative to a fixed reference. This model includes a control force from the power take-off and control machinery, and a friction force which restricts the oscillation amplitude. This force models end-stop devices, which are necessary to protect the machinery. An optimal control strategy is determined, based on variations of a Lagrange functional. This gives a set of adjoint equations in addition to the state equations, as a necessary condition for optimum. An algorithm is given for solving the problem numerically by iteration, based on a gradient method. It is shown that the optimal motion in a sinusoidal wave is not sinusoidal when the excursion is constrained. Instead, the motion should be stopped in certain intervals. In irregular waves the constrained solution is close to the unconstrained solution when the excursion is small. Moreover, the timings of the extrema and of the zero crossings agree fairly well. When the excursion is constrained, the mean output power is reduced compared tomore » the unconstrained case, but the ratio between the output energy and the total energy passing through the machinery is increased. This means that the conversion efficiency of the machinery is less critical.« less

29 citations



Journal ArticleDOI
TL;DR: In this article, a design methodology is formulated to reveal the dependence of nonlinear slow motion dynamics of spread Mooring systems (SMS) on mooring line arrangement, and catastrophe sets are developed in the parametric design space showing dependence of stability boundaries and singularities of bifurcations on design variables.
Abstract: A design methodology is formulated to reveal the dependence of nonlinear slow motion dynamics of Spread Mooring Systems (SMS) on mooring line arrangement. For a given SMS configuration, catastrophe sets are developed in the parametric design space showing the dependence of stability boundaries and singularities of bifurcations on design variables. This approach eliminates the need for nonlinear simulations. For general SMS design, however, the designer relies on experience rather than scientific understanding of SMS nonlinear dynamics, due to the high number of design variables. Several numerical applications are used to demonstrate counterintuitive ways of improving SMS dynamics. The SMS design methodology formulated in this paper aims at providing fundamental understanding of the effects of mooring line arrangement and pretension on SMS horizontal plane dynamics. Thus, the first guidelines are developed to reduce trial and error in SMS design. The methodology is illustrated by comparing catastrophe sets for various SMS configurations with up to three mooring lines. Numerous examples for a barge and a tanker SMS which exhibit qualitatively different nonlinear dynamic behaviour are provided.

27 citations


Journal ArticleDOI
TL;DR: In this article, the performance of the biplane turbine was investigated in unidirectional steady air flow by varying the model configurations using solidity, gap to chord ratio and stagger angle.
Abstract: The paper describes experimental investigation of the biplane Wells turbine for a wave power plant. Performance of the biplane turbine was investigated in unidirectional steady air flow by varying the model configurations using solidity, gap to chord ratio and stagger angle. It was concluded that the performance of the biplane turbine is considerably influenced by inviscid and viscous flow effects. Adjacent upstream and downstream pressure distributions will either suppress or enhance one-another depending on stagger given sufficiently small G/c ratio. The viscous effect from the upstream wake seems to defer stall by advancing boundary layer transition.

26 citations



Journal ArticleDOI
TL;DR: In this article, the strength of overlapping K joints under axial loading was investigated using a parametric finite element model and various factors such as boundary restraints, hidden welds, loading hierarchy, and failure modes.
Abstract: Overlapped joints are generally regarded as having higher strengths than otherwise identical, simple nonoverlapping joints because of the more efficient load transfer between braces. However, not only that relatively little research has been carried out on such joints, the few test data from which current design guidance was derived has also been recently rejected. This paper reports the first phase of a parametric finite element study into the strength of overlapping K joints under axial loading. The numerical models were validated and calibrated against existing gap and overlapped K joint test results, and various factors which affect the relationship between the strength and the overlap amount, such as boundary restraints, hidden welds, loading hierarchy, and failure modes, were investigated. The results of the work presented lay the foundation for a future parametric study.

19 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of a collinear current on the diffraction of regular waves around three-dimensional surface-piercing bodies are examined, and the boundary value problem is separated into a steady current problem with a rigid wall condition applied at still water level and a linear wave propagation problem in the resulting current field.
Abstract: The effects of a collinear current on the diffraction of regular waves around three-dimensional surface-piercing bodies are examined. With the current speed assumed to be small, the boundary-value problem is separated into a steady current problem with a rigid wall condition applied at the still water level and a linear wave propagation problem in the resulting current field. The boundary conditions of the wave propagation problem are satisfied by a time-stepping procedure and the field solution is obtained by an integral equation method. Free surface profiles, runup, and wave forces are described for a vertical circular cylinder in combined waves and a current. The current is shown to affect significantly the steady drift force and runup predictions. Comparisons of the computed wave forces are made with a previous numerical solution involving a semi-immersed sphere in deep water, and indicate good agreement.

Journal ArticleDOI
TL;DR: In this paper, the authors take a new look at the high frequency range of the wave spectrum and analyze data from two recent field campaigns offshore Portugal and Crete carried out in the MAST II WAVEMOD project, data from WADIC experiment in the North Sea, and deep sea data from Haltenbanken and Voeringplataaet offshore Norway.
Abstract: This paper takes a new look at the high frequency range of the wave spectrum. The analysis is based on data sets from two recent field campaigns offshore Portugal and Crete carried out in the MAST II WAVEMOD project, data from the WADIC experiment in the North Sea, and deep sea data from Haltenbanken and Voeringplataaet offshore Norway. In addition, the authors also show spectra obtained by spectral inversion of ERS-1 SAR imagery. The influence and calibration of wave measuring instrumentation and the use of wavenumber spectra when comparing spectra from shallow water is emphasized.

Journal ArticleDOI
TL;DR: In this paper, the applicability of a parameter identification algorithm based on the Hilbert transform to nonlinear ocean mooring systems is introduced and demonstrated. But the method is not suitable for the case of large-scale experiments.
Abstract: We introduce and demonstrate the applicability of a parameter identification algorithm based on the Hilbert transform to nonlinear ocean mooring systems. The mooring dynamical system consists of a submerged small body and includes a geometrically nonlinear restoring force and a nonlinear dissipation function incorporating both viscous and structural damping. By combining a recently developed methodology with a generalized averaging procedure, parameter estimation from the slowly varying envelope dynamics is enabled. System backbone curves obtained from data generated by numerical simulation are compared to those obtained analytically and are found to be accurate. An example large-scale experiment is also considered.




Journal ArticleDOI
TL;DR: In this paper, the nonlinear dynamic response of a jack-up structure under random wave loads is considered, and the average behavior and variability in extreme forces and responses are found from simulation over many 6-h seastates.
Abstract: The nonlinear dynamic response of a jack-up structure under random wave loads is considered. For a simplified jack-up model, average behavior and variability in extreme forces and responses are found from simulation over many 6-h seastates. Weibull and Hermite analytical models of response extremes are also presented and evaluated. These models use shorter, less expensive simulations to estimate a limited number of response statistics, such as moments or parameters of the response peak distribution, and fit analytical models to estimate global extremes. Necessary simulation lengths are established both for direct simulation of extremes, and for analytical extreme models.

Journal ArticleDOI
O. B. Ness1, R. L. P. Verley1
TL;DR: In this paper, a semi-analytical model was used to establish nonlinear moment-curvature curves at a number of cross sections on the concrete-coated pipe and in the field joint.
Abstract: This paper concerns the strain distribution, and in particular strain concentration in field joints, for concrete-covered pipelines during laying. A semi-analytical model, full-scale tests to verify the model, and results of a parameter study are described. The model is used to establish nonlinear moment-curvature curves at a number of cross sections on the concrete-coated pipe and in the field joint (FJ). These are used to establish a strain concentration factor (SCF) for the FJ, or characteristics for a varying stiffness model of a pipe for direct use in lay analyses. Constant moment, four-point bending tests have been conducted on 16-in and 20-in dia, concrete-coated pipes as well as material tests on the pipe steel, corrosion coating and concrete. The behavior of the pipe, and in particular the SCF at the field joints, is investigated and compared to predictions using the semi-analytical model. The model is found to give a good prediction of the SCF and strain distribution along the pipe joint, for both the steel and the concrete, and is suitable for use in lay analyses for the overbend of S-mode lay vessels.

Journal ArticleDOI
TL;DR: Mortazavi and Bea as discussed by the authors presented the second phase of development and verification of simplified procedures to evaluate environmental loadings and ultimate limit state lateral loading capacities of template-type offshore platforms.
Abstract: During the past three decades, an immense amount of effort has been devoted to development of sophisticated computer programs to enable the assessment of storm wind, wave, and current loadings and the ultimate limit state capacity characteristics of conventional, pile-supported, template-type offshore platforms. These programs require high degrees of expertise to operate properly, are expensive to purchase and maintain, and require large amounts of manpower and time to complete the analyses. Due to the sophistication of these programs, experience has shown that it is easy to make mistakes that are difficult to detect and that can have significant influences on the results. This paper summarizes the second phase of development and verification of simplified procedures to evaluate environmental loadings and ultimate limit state lateral loading capacities of template-type platforms. Reasonable simplifications and high degrees of user friendliness have been employed in development of the software to reduce the engineering effort, expertise, and costs associated with the analyses. The computer program that has been developed to perform the simplified analyses has been identified as ULSLEA (ultimate limit state limit equilibrium analyses) (Mortazavi and Bea, 1994).

Journal ArticleDOI
TL;DR: In this paper, an unsteady flow theory for flow-induced vibration of tubes in crossflow is presented, which includes a general description of motion-dependent fluid forces, characteristics of fluid-force coefficients, and mathematical models.
Abstract: This paper presents an unsteady flow theory for flow-induced vibration of tubes in crossflow. It includes a general description of motion-dependent fluid forces, characteristics of fluid-force coefficients, and mathematical models. The detailed results are presented for the constrained mode in the lift direction of various tube arrangements.


Journal ArticleDOI
TL;DR: In this paper, a new steady-state icing model is presented which explicitly takes into account the dynamics and thermodynamics of a liquid film on the ice accretion surface under high liquid fluxes.
Abstract: A new steady-state icing model is presented which explicitly takes into account the dynamics and thermodynamics of a liquid film on the ice accretion surface under high liquid fluxes. The film is generated by excess unfrozen impinging liquid, is set in motion by the aerodynamic shear stress, and is eventually shed. In order to keep the model simple, it is formulated for a rotating cylinder subjected to a continuous supercooled freshwater spray. The model is used to explore the physics of the liquid film, and confirms that the film is thin and laminar except possibly under extreme liquid fluxes. It predicts supercooling of several degrees at the film surface, in agreement with recent observations. Further, the model is used to investigate the dependence of the icing rate on the following parameters: liquid water content, air temperature, wind speed, spray temperature, cylinder diameter, and heat transfer coefficient.

Journal ArticleDOI
TL;DR: In this paper, a fast and simple method for the determination of the residual deformation for a class of welding problems, namely ring-stiffened pipes, has been proposed, which can predict radial as well as angular distortion of the thin-walled pipe - ringstiffener/flange assembly.
Abstract: A fast and simple method for the determination of the residual deformation for a class of welding problems: ring-stiffened pipes, is proposed. The method can predict radial as well as angular distortion of the thin-walled pipe - ring-stiffener/flange assembly. The pipe and stiffener material is elasto-plastic. In particular, the accumulation of deformation in multi-pass welding is incorporated in the model. Each weld pass is treated separately. This facilitates the assessment of the influence of the sequence in which the weld passes are deposited on the residual deformation state. The method will be included in a conversational knowledge bases "expert" system for the production of a welded ring-stiffened pipe.



Journal ArticleDOI
TL;DR: In this paper, the CTOD transition temperature curves of conventional ship plate grades, including modern high-strength steels, were determined at loading rates representing quasi-static, intermediate, and impact conditions.
Abstract: Full thickness fracture toughness of conventional ship plate grades, including modern high-strength steels, was determined at loading rates representing quasi-static, intermediate, and impact conditions. For testing at impact strain rates ({dot {epsilon}} = 5 s{sup {minus}1}), customized equipment was designed and developed for a drop tower. It was shown from the CTOD transition temperature curves that the transition temperature increase from quasi-static to intermediate rate (a three-order rate increase) is much greater than the increase from intermediate to impact (a two-order rate increase). Comparison of the dynamic CTOD, CVN, and NDTT indicated that the correlation between the CVN and 0.1-mm dynamic CTOD transition temperature does not hold for the TMCP steels, for which the CVN transition temperature is much lower. By contrast, the results display a very good correlation between the NDTT and 0.1-mm dynamic CTOD transition temperature for all of the steels tested in this program.


Journal ArticleDOI
TL;DR: In this article, the reliability of a cracked fillet welded T-joint typically found in offshore structures was investigated, and a formulation for the aspect ratio of a propagating semi-elliptical fatigue crack located at the toe of the weld was developed.
Abstract: This study investigates the reliability of a cracked fillet welded T-joint typically found in offshore structures. A formulation for the aspect ratio (a/c) of a propagating semi-elliptical fatigue crack located at the toe of the weld is developed using Newman and Raju`s stress intensity factor for a cracked flat plate in conjunction with a weld magnification factor. The reliability in terms of fatigue lifetime is then calculated using the aspect ratio and Paris`s law of crack propagation with both fracture toughness and elastic-plastic failure criteria. The variation in crack aspect ratio in the T-joint is compared to that in a cracked flat plate, and examples are provided of reliability calculations for tension and bending loads.

Journal ArticleDOI
TL;DR: In this article, the double-frequency heave response amplitude operator RAO(2) for a freely floating truncated cylinder is presented as a function of cylinder aspect ratio and incident wave frequency.
Abstract: A number of parametric studies were performed on a series of single, truncated cylinders in deep water using second-order potential theory. The cylinders were subjected to a monochromatic incident wave field. Second-order vertical diffraction force components were computed and plotted to illustrate their variation with cylinder aspect ratio and incident wave frequency. These plots make it possible to obtain rough estimates of vertical second-order diffraction force quantities on cylindrical components without having to perform second-order diffraction theory computations. The double-frequency heave response amplitude operator RAO(2) for a freely floating truncated cylinder is presented as a function of cylinder aspect ratio and incident wave frequency. Calculations of the RAO(2) were made using the total double-frequency diffraction force and the first-order radiation coefficients evaluated at the double-frequency.

Journal ArticleDOI
TL;DR: In this paper, a computer model of glaze accretion on wires was developed based on experimental results in the area of ice accumulation on wires, as well as on the related field of the glaze ice accretion in airfoils.
Abstract: The design of power transmission lines requires a knowledge of combined wind and ice loading and of the dynamic behavior of wires loaded with ice accretion. The calculation of the wind forces, in turn, imposes a need for a more detailed computer model for determining glaze accretion shape. For this purpose, a computer model of glaze accretion on wires was developed. It is based on experimental results in the area of ice accretion on wires, as well as on results in the related field of the glaze ice accretion on airfoils. The model incorporates the time dependent on feedback between the growing accretion and the air stream, the variation of the heat transfer coefficient around the cylinder, and the surface runback of water. The main components of the model are the computation of the air flow field, the computation of the impingement water at the control volume level, the solving of the heat balance equation, and the computation of the accretion shape on the wire. The surface air velocity is obtained through the solution of the potential flow around the iced wire and wake, followed by the integration on the surface of the laminar boundary layer. The water flux ismore » computed in each control volume down to the separation point. The heat balance equation derived from the energy equation is solved to determine the freezing fraction and the resulting modified ice surface geometry.« less