Showing papers in "Journal of Offshore Mechanics and Arctic Engineering-transactions of The Asme in 2013"

Analysis of a Two-Body Floating Wave Energy Converter With Particular Focus on the Effects of Power Take-Off and Mooring Systems on Energy Capture

Abstract: The present paper summarizes analyses of a two-body floating wave energy converter (WEC) to determine the mooring tension and the effect of the mooring system on energy capture. Also, the effect of the power take-off (PTO) is assessed. An axisymmetric Wavebob-type WEC is chosen as the object of investigation. However, the PTO system is modeled in a simplified manner as ideal linear damping and spring terms that couple the motions of the two bodies. The analysis is performed using SIMO, which is a time domain simulation tool that accommodates the simulation of multibody systems with hydrodynamic interactions. In SIMO, docking cone features between the two bodies allow movement as per actual operation, and fenders are applied to represent end stops. Six alternative mooring configurations are applied to investigate the effect of mooring on power capture. Mooring analysis is performed to determine the necessary capacity of mooring lines for each configuration to carry the tension due to the WEC motion in extreme conditions. Hydrodynamic loads are determined using WAMIT. We assumed that the WEC will be operated to capture wave power at the Yeu site in France. The analysis is performed for several regular and irregular wave conditions according to wave data available for that site. Simulations are performed to study the effect of the PTO system, end stops settings and several mooring configurations on power capture.

Application of the Contour Line Method for Estimating Extreme Responses in the Mooring Lines of a Two-Body Floating Wave Energy Converter

Abstract: The ultimate limit state (ULS) is one of the design criteria used in the oil and gas industry in mooring system design for floating platforms. The 100 year level response in the mooring line should be applied for the ULS design check, which is ideally estimated by taking into account the dynamic mooring line tension in all sea states available at the operational site. This approach is known as a full long-term response analysis using the all-sea-state approach. However, this approach is time consuming, and therefore, the contour line method is proposed for estimation of the 100 year response by primarily studying the short-term response for the most unfavorable sea states along the 100 year environmental contour line. Experience in the oil and gas industry confirmed that this method could yield good predictions if the responses at higher percentiles than the median are used. In this paper, the mooring system of a two-body wave energy converter (WEC) is considered. Because this system involves the interaction between two bodies, the estimation of the ULS level response using the all-sea-state approach may be even more time consuming. Therefore, application of the contour line method for this case will certainly be beneficial. However, its feasibility for application to a WEC case must be documented first. In the present paper, the ULS level response in the mooring tension predicted by the contour line method is compared to that estimated by taking into account all sea states. This prediction is achieved by performing coupled time domain mooring analyses using Simo/Riflex for six cases with different mooring configurations and connections between two bodies. An axisymmetric Wavebob-type WEC is chosen for investigation, and the Yeu site in France is assumed as the operational site. Hydrodynamic loads including second-order forces are determined using Wamit. Finally, the applicability of the contour line method for prediction of the ULS level mooring tension for a two-body WEC is assessed and shown to yield accurate results with the proper choice of percentile level for the extreme response.

Experimental and Numerical Plastic Response and Failure of Laterally Impacted Rectangular Plates

Abstract: Experimental and numerical results of drop weight impact test are presented, on the plastic behaviour and fracture of rectangular plates stuck laterally by a mass with a hemispherical indenter. Six specimens were tested in order to study the influence of the impact velocity and the diameter of the indenter. The impact scenarios could represent abnormal actions on marine structures, such as ship collision and grounding or dropped objects on deck structures. The tests are conducted on a fully instrumented impact tester machine. The obtained force-displacement response is compared with numerical simulations, performed by the LS-DYNA finite element solver. The simulations aim at proposing techniques for defining the material and restraints on finite element models which analyze the crashworthiness of marine structures. The mesh size and the critical failure strain are predicted by numerical simulations of the tensile tests used to obtain the mechanical properties of the material. The experimental boundary conditions are modelled in order to represent the reacting forces developed during the impact. The results show that the critical impact energy until failure is strongly sensitive to the diameter of the striker. The shape of the failure modes is well predicted by the finite element models when a relatively fine mesh is used. Comments on the process of initiation and propagation of fracture are presented.Copyright © 2012 by ASME

Experimental Investigation of Tsunami Bore Forces on Vertical Walls

Abstract: A series of experiments have been carried out in the large wave flume (LWF) at Oregon State University to quantify tsunami bore forces on structures. These tests included “offshore” solitary waves, with heights up to 1.3 m, that traveled over a flat bottom, up a sloping beach, and breaking onto a flat reef. Standing water depths on the reef varied from 0.05 m to 0.3 m. Resulting bores on the reef measured up to approximately 0.8 m. After propagating along the reef, the bores struck a vertical wall. The resulting forces and pressures on the wall were measured. The test setup in the LWF is described, and the experimental results are reported. The results include forces and pressure distributions. Results show that the bores propagated with a Froude number of approximately 2 and that the forces follow Froude scaling. Finally, a design formula for the maximum impact force is given. The formula is shown to be an improvement over existing formulas found in the literature.

Study of a Floating Foundation for Wind Turbines

Abstract: Offshore wind farms are currently located predominantly in shallow water as it is possible to cost effectively install bottom fixed offshore turbines. Where shallow water sites are not available, floating offshore turbines could be a better solution than bottom fixed turbines. Currently three main concepts are promoted for the design of a floating wind turbine: a ballast stabilized floater (i.e. spar), a buoyancy stabilized floater (i.e. barge or semi-submersible) or a mooring stabilized floater (tension leg platform). In April 2011 the DeepCWind consortium visited MARIN to carry out model tests in the offshore wave basin with these three of floating wind types turbine platform. This paper reports a numerical study of a wind turbine supported by a semi-submersible floater. The response of the floating system to wind and wave conditions is compared to physical measurements at 1:50 model scale. The outcome of these comparisons is discussed in the conclusions of this paper.

Average Properties of the Largest Waves in Hurricane Camille

Abstract: Ocean waves are known to be both random in time and nonlinear. Surface elevation time histories measured in the Gulf of Mexico during Hurricane Camille in 1969 are re-analyzed. The average shapes of large crests and deep troughs in time are shown to be close to symmetric around the instant when the maximum (or minimum) occurs, with only slight evidence of asymmetry from wave breaking in the time histories. There is considerable vertical asymmetry with higher and sharper crests and smaller and more rounded troughs. Overall, the analysis supports the use of a focused wave group based on the scaled autocorrelation function (NewWave) as proposed by Lindgren and Boccotti, with sum harmonic corrections. There is a very small second order difference setup for both large crests and troughs, consistent with considerable directional spreading in the hurricane sea-state. This spreading is likely to be larger than that usually assumed for non-tropical winter storms. The spectral tail is shown to have a decay rate proportional to ―4.5 power law midway between the classical JONSWAP (Phillips) ―5 form and the ―4 slope proposed by Battjes et al. (1987, "A Reanalysis of the Spectra Observed in JONSWAP," J. Phys. Oceanogr., 17(8), pp. 1288―1295) as a correction to JONSWAP.

Phase Averaged Flow Analysis in an Oscillating Water Column Wave Energy Converter

Abstract: The paper presents the application of phase averaging to experimental data obtained during scale model testing of a forward facing bent duct oscillating water column (OWC). Phase averaging is applied to both wave probe data and a two-dimensional velocity field at the centerline plane of the OWC model obtained using particle imaging velocimetry (PIV). Results are presented for one monochromatic wave condition. The influence of varied wave frequency is briefly discussed.

Evaluation of Weld Root Failure Using Battelle Structural Stress Method

Abstract: Weld related fatigue failure is one of the most common concerns in welded structures. From the fatigue design point of view, weld toe failure is preferable to weld root failure. Base plate thickness is a controlling parameter for weld toe failure, while weld metal size is a controlling parameter for weld root failure. However, controlling the weld metal size is not easy because the actual weld penetration and weld leg size vary along a weld and from weld to weld. Therefore, analyzing fatigue test data for weld root failure tends to enlarge scatter band due to variability in weld penetration and weld leg size when the nominal weld size is considered. The structural stress based weld fatigue master S-N curve adopted by 2007 ASME section VIII Div. 2 and new API 579/ASME FFS-1 was constructed by incorporating only clearly defined weld toe fatigue data. In this article, a simplified structural stress procedure was developed and a design master S-N curve for weld root failure was established based on the published fatigue test data. Consequently, the mean design master S-N curve for weld root failure is downshifted relative to the mean master S-N curve for weld toe failure, and has a wider scatter band. To be conservative, a crack path along weld throat is recommended for structural stress calculation. Also, the transverse shear stress effects on structural stress calculation can be ignored.

Experimental Study on the Collapse Strength of Narrow Stiffened Panels

Abstract: The results of four tests on narrow stiffened panels under axial compression until collapse and beyond are presented to investigate the ultimate strength of stiffened panels. Tension tests are used to evaluate the material properties of the stiffened panels. The tests are made on panels with two half bays plus one full bay in the longitudinal direction. Initial loading cycles were used to relief the residual stresses of the stiffener panels. The strain gauges are set on the plates and stiffeners to record the distribution of strain. The displacement load relationship is established. The ultimate strength behavior, modes of failure and load-carrying capacity of the stiffened panels are investigated with the experiment.Copyright © 2011 by ASME

Second-Order Diffraction and Radiation of a Floating Body With Small Forward Speed

Abstract: The formulation of the second-order wave-current-body problem in the inertial coordinate system involves higher-order derivatives in the body boundary condition. A new method taking advantage of the body-fixed coordinate system in the near field is presented to avoid the calculation of higher-order derivatives in the body boundary condition. The new method has advantage over the traditional method when the body surface is with sharp corner or high curvature. The nonlinear wave diffraction and forced oscillation of floating bodies are studied up to second order in wave slope. A small forward speed is taken into account. The results of the new method are compared with that of the traditional method based on a formulation in the inertial coordinate system. When the traditional method applies, good agreement has been obtained.Copyright © 2010 by ASME

The Effect of Third-Order Nonlinearities on the Statistical Distributions of Wave Heights, Crests and Troughs in Bimodal Crossing Seas

Abstract: This paper investigates the effect of third-order nonlinearities on the statistical distributions of wave heights, crests, and troughs of waves mechanically generated in a deep-water basin and simulating two crossing systems characterized by bimodal spectra. The observed statistics exhibits various effects of third-order nonlinearities, in a manner dependent on both the distance from the wave-maker and the angle between the mean directions of the component wave systems. In order to isolate and demonstrate the effects of third-order nonlinearities by themselves, the vertically asymmetric distortions induced by second-order bound waves are removed from the observed time series. It appears then that the distributions of wave crests, troughs and heights extracted from the nonskewed records clearly deviate from the Rayleigh distribution, suggesting that the waves are characterized by nonlinear corrections of higher-order than the typical of second-order waves. Nonetheless, some models developed for weakly nonlinear second-order waves can still be used in describing wave heights, crests and troughs in mixed seas, provided that the relevant distribution parameters are modified, so as to reflect the effects of third-order corrections and some basic characteristics of the mixed seas.

Influence of Model Geometry and Boundary Conditions on the Ultimate Strength of Stiffened Panels Under Uniaxial Compressive Loading

Abstract: The aim of this paper is to find out an appropriate configuration of boundary conditions and geometric model to calculate the ultimate strength of a continuous stiffened panel under uniaxial compressive loading in FE analysis. The 1+1 bays model with periodical symmetric boundary conditions is proposed to be used in FE analysis, whose results are compared with 1/2+1+1/2 bays model with periodical symmetric and symmetric boundary conditions, and 1/2+1+1+1/2 bays model with symmetric boundary conditions. The effects of the continuity of the stiffened panel with different geometric models and boundary conditions on its collapse mode are investigated. A beam tension test has been used to define the true stress-strain relationship.Copyright © 2012 by ASME

Wave Run-Up and Air Gap Prediction for a Large-Volume Semi-Submersible Platform

Abstract: This paper addresses the problem of estimating the air gap for a large semi-submersible production platform. Although it has a great impact on the design of the floating unit, many times the minimum deck height is still defined from simplified methods that incorporate relatively large safety margins. The reason for this is the intrinsic complexity of the associated hydrodynamic problem. Nonlinear effects on the incoming and scattered waves are usually relevant and sometimes non-linear effects on the motions of the floating hull may also play an important role. This discussion is illustrated by means of wave basin tests performed with the model of a large semi-submersible designed to operate in Campos Basin. Significant run-up effects on its squared-section columns were observed for the steepest waves in several design conditions. Also, the unit presented relatively large low-frequency motions in heave, roll and pitch, which also affected the dynamic air gap measurements. In order to evaluate the difficulties involved in modeling such phenomena, simplified tests were also performed with the model fixed and moored in regular waves of varying steepness. Wave elevation in different points was measured in these tests and compared to the predictions obtained from two different numerical methods: a BEM code that incorporates 2nd order diffraction effects (WAMIT) and a VOF CFD code (ComFLOW), the latter employed for fixed model tests only. Results show that a standard linear analysis may lead to significant errors concerning the air gap evaluation. Extending the BEM model to 2nd order clearly improve the results as the wave-steepness increases. Although the VOF analysis is considerably time-consuming, simulations presented very good agreement to the experimental results, even for the steepest waves tested.Copyright © 2010 by ASME

Scour Around Vertical Pile Foundations for Offshore Wind Turbines Due to Long-Crested and Short-Crested Nonlinear Random Waves

Abstract: This paper provides a practical stochastic method by which the maximum scour depth around vertical piles exposed to long-crested (2D) and short-crested (3D) nonlinear random waves can be derived. The approach is based on assuming the waves to be a stationary narrow-band random process, adopting the Forristall (2000) wave crest height distribution representing both 2D and 3D nonlinear random waves, and using the regular wave formulas for scour depth by Sumer et al. (1992b). An example of calculation is provided. Tentative approaches to related random wave-induced scour cases are also suggested.Copyright © 2011 by ASME

Effects of Diameter to Thickness Ratio and External Pressure on the Velocity of Dynamic Buckle Propagation in Offshore Pipelines

Abstract: In this paper, a numerical study of the dynamic buckle propagation, initiated in long pipes under external pressure, is presented. For a long pipe, due to the high exerted pressure, local instability is likely to occur; therefore, the prevention of its occurrence and propagation are very important subjects in the design of pipelines. The 3D finite element modeling of the buckle propagation is presented by considering the inertia of the pipeline and the nonlinearity introduced by the contact between its collapsing walls. The buckling and collapse are assumed to take place in the vacuum. The numerical results of the nonlinear finite element analysis are compared with the experimental results obtained by Kyriakides and Netto (2000, “On the Dynamics of Propagating Buckle in Pipelines,” Int. J. Solids Struct., 37, pp. 6843–6878) from a study on the small-scale models. Comparison shows that the finite element results have very close agreement with those of the experimental study. Therefore, it is concluded that the finite element model is reliable enough to be used for nonlinear collapse analysis of the dynamic buckle propagation in the pipelines. In this study, the effects of external pressure on the velocity of dynamic buckle propagation for different diameter to thickness ratios are investigated. In addition, the mathematical relations, based on the initiation pressure, are derived for the velocity of buckle propagation considering the diameter to thickness ratio of the pipeline. Finally, a relation for the buckle velocity as a function of the pressure and diameter to thickness ratio is presented.

Experimental Research on the Responses of Neoprene Coated Cylinder Subjected to Underwater Explosions

Abstract: The dynamic responses of a neoprene coated cylinder subjected to underwater explosion were experimentally investigated. Two metallic cylinders of the same geometry were simultaneously fire tested in an artificial lake. One of the cylinders was coated with a layer of neoprene and the other had none. The strain and acceleration recorded at some typical locations on the cylinders and inner structures were qualitatively compared. It was shown that the shock environment associated with the inner structure can be improved to some extent after the rubber layer is coated, but the strain at the outer hull, especially in the hoop direction, is notably amplified. It verifies the fact that soft coating can cause concentration of stresses, mainly on the shell that is directly in contact with the coating layer.

Application of Statistical Analysis Techniques to Pipeline On-Bottom Stability Analysis

Abstract: Offshore pipelines are increasingly being employed to transport offshore hydrocarbons to onshore processing facilities. Pipelines laid directly on the seabed are subject to a considerable hydrodynamic loading from waves and currents and must be accurately designed for on-bottom stability. Confidence in the stability of pipelines requires appropriate models for their assessment and, in this paper, particular emphasis is placed on achieving an integrated and balanced approach in considering the nonlinearities and uncertainties in the pipe structure, the reaction of the restraining soil, and the hydrodynamic loading applied. A statistical approach is followed by developing a response surface model for the pipeline maximum horizontal displacement within a storm, while including variability in parameters. The Monte Carlo simulation method is used in combination with the developed response surface model to calculate the extreme response statistics. The benefit of this approach is demonstrated and also used to investigate the sensitivity of the on-bottom pipeline simulation for a variety of model input parameters. These results provide guidance to engineers as to what uncertainties are worth reducing, if possible, before a pipe is designed.

Evaluation of Slug Flow-Induced Flexural Loading in Pipelines Using a Surrogate Model

Abstract: Slug flow induces vibration in pipelines, which may, in some cases, result in fatigue failure. This can result from dynamic stresses, induced by the deflection and bending moment in the pipe span, growing to levels above the endurance limits of the pipeline material. As such, it is of paramount importance to understand and quantify the size of the pipeline response to slug flow under given speed and damping conditions. This paper utilizes the results of an optimization procedure to devise a surrogate closed-form model, which can be employed to calculate the maximum values of the pipeline loadings at given values of speed and damping parameters. The surrogate model is intended to replace the computationally costly numerical procedure needed for the analysis. The maximum values of the lateral deflection and bending moment, along with their locations, have been calculated using the optimization method of stochastic perturbation and successive approximations (SPSA). The accuracy of the proposed surrogate model will be validated numerically, and the model will be subsequently used in a numerical example to demonstrate its applicability in industrial situations. An accompanying spreadsheet with this worked example is also given.

An Efficient Three-Dimensional FNPF Numerical Wave Tank for Large-Scale Wave Basin Experiment Simulation

Abstract: This paper presents a parallel implementation and validation of an accurate and efficient three-dimensional computational model (3D numerical wave tank), based on fully nonlinear potential flow (FNPF) theory, and its extension to incorporate the motion of a laboratory snake piston wavemaker, as well as an absorbing beach, to simulate experiments in a large-scale 3D wave basin. This work is part of a long-term effort to develop a “virtual” computational wave basin to facilitate and complement large-scale physical wave-basin experiments. The code is based on a higher-order boundary-element method combined with a fast multipole algorithm (FMA). Particular efforts were devoted to making the code efficient for large-scale simulations using high-performance computing platforms. The numerical simulation capability can be tailored to serve as an optimization tool at the planning and detailed design stages of large-scale experiments at a specific basin by duplicating its exact physical and algorithmic features. To date, waves that can be generated in the numerical wave tank (NWT) include solitary, cnoidal, and airy waves. In this paper we detail the wave-basin model, mathematical formulation, wave generation, and analyze the performance of the parallelized FNPF-BEM-FMA code as a function of numerical parameters. Experimental or analytical comparisons with NWT results are provided for several cases to assess the accuracy and applicability of the numerical model to practical engineering problems. [DOI: 10.1115/1.4007597]