bluuebunny/crossref_metadata_2025 huggingface dataset:
Large-Wave Simulation of Spilling Breakers Over Constant-Slope Bed
Authors: Dimakopoulos Aggelos S. , Dimas Athanassios A.
Published: July 2009

A subgrid scale model is presented for the large-wave simulation (LWS) of spilling breaking waves over bottom of arbitrary shape and finite depth. According to LWS formulation, large velocity and free-surface scales are fully resolved, while subgrid scales are accounted for by an eddy viscosity model, similar to large-eddy simulation (LES). The LWS-based model is applied on the two-dimensional wave propagation over a constant-slope bed. Fluid motion is described by the Euler equations for inviscid but rotational flow, subject to the fully non-linear free-surface boundary conditions. The application of LWS is facilitated by a boundary-fitted transformation, which introduces free-surface elevation terms in the Euler equations and simplifies the numerical implementation. Subgrid velocity scales are modeled similarly to LES, while the effect of free-surface subgrid scales are modeled by wave SGS stresses model. The resulting equations are solved numerically by a two-stage fractional time-step scheme, while an absorption zone is placed in the outflow region to minimize reflection by the outgoing waves. The simulation is carried out for the propagation and breaking of waves over a flat bed with constant slope 1/35 and results are compared to available experimental data. The numerical predictions for the breaking height, the breaking depth and the free-surface elevation dissipation in the surf zone agree very well with the corresponding measurements. The model predicts the vorticity generation in the breaking face of the wave and the appearance of the undertow current in the surf zone. The predicted shear of the undertow current is higher than the measured one.
A B-Spline Based Higher-Order Panel Method Applied to Marine Hydrodynamic Problems
Authors: Lee Chang-Sup , Ahn Byoung-Kwon , Kim Gun-Do , Lee Hyun Yup , Hong Do-Chun
Published: July 2009

A B-spline based higher order panel method (hereinafter, HiPan) is developed for the motion of bodies in ideal fluid, either of infinite extent or with free boundary surface. In this method, both the geometry and the potential are represented by B-splines, and it guarantees more accurate results than most potential based panel methods. In the present work, we apply the HiPan, which differs with the works at MIT in evaluating the induction integrals, to two major marine hydrodynamic problems: analysis of propulsive performance of the marine propellers and the motion of the floating bodies on the free surface. The present HiPan is shown superior to the constant panel method (hereinafter, CoPan) in predicting flow quantities in the area of the thin trailing edge and blade tip of the propeller. Numerical results are validated by comparison with experimental measurements.
CFD Analysis of Unsteady Flows Around a New Cell-Truss Spar and the Corresponding Vortex-Induced Motions
Authors: Wang Ying , Yang Jianmin , Li Xin
Published: July 2009

A new configuration of Spar platform called the Cell-Truss Spar has recently been put forward by State Key Laboratory of Ocean Engineering (SKLOE) of Shanghai Jiao Tong University. Since the Cell-Truss Spar is a new design concept that has several physical characteristics which are different from those of the present Classic, Truss and Cell Spars, many aspects of its performances in various sea conditions should be carefully studied. For any type of Spars, Vortex-Induced Motions (VIM) under current flow is an important consideration since it not only affects the motion performance of the Spar, but also reacts on the fatigue analysis and Spar mooring design. This paper mainly discusses the unsteady flows around the new Cell-Truss Spar and the corresponding vortex-induced motion performances of the Spar in uniform currents. A CFD model of the Cell-Truss Spar upper hull (without strake designs for VIM mitigation since it’s still at the concept design stage at the present time) with a scale ratio of 1:100 is created and numerical simulations at different current conditions are performed. The software FLUENT is chosen as the computational fluid dynamic tool to simulate the flow fields around the Cell-Truss Spar and the resulting vortex-induced motions. Both 2D and 3D simulations are carried out. Dynamic meshes and user defined functions are used in the fluid-structure interaction for solving the equations of motion. The SST k-ω method is used as the turbulence model in the 2D simulation and the Detached Eddy Simulation (DES) is used in the 3D simulation. In the 2D simulation, the unsteady flows around the Cell-Truss Spar hard tank at different Reynolds numbers are calculated and the corresponding vortex shedding features and other fluid parameters are obtained and analyzed. Moreover, in order to set up a baseline for the comparison and analysis of the Cell-Truss Spar, a typical Truss Spar hard tank with the same diameter and draft is also modeled and calculated. Comparisons with several classical experimental and numerical results are conducted to validate the numerical method. The Cell-Truss Spar hull VIM responses are then simulated. For the 3D simulation, the fluid domain is made of a hybrid mesh comprising of millions of wedgy and hexahedral elements. Since 3D simulation is very time-consuming, only the results of the nonlinear flow field features for the currents passing the stationary Spar are presented in this paper, and more advanced studies related to this subject including both numerical and experimental investigations would be carried out successively.
A Study on the Effects of Blade Profile and Non-Uniform Tip Clearance of the Wells Turbine
Authors: Takao Manabu , Setoguchi Toshiaki , Nagata Shuichi , Toyota Kazutaka
Published: July 2009

Several of wave energy devices being studied under many wave energy programs in the United Kingdom, Japan, Portugal, India and other countries make use of the principle of an oscillating water column (OWC). In such wave energy devices, a water column which oscillates due to wave motion is used to drive an oscillating air column which is converted into mechanical energy. The energy conversion from the oscillating air column can be achieved by using a self-rectifying air turbine. Wells turbine is a self-rectifying air turbine which is expected to be widely used in wave energy devices with OWC. There are many reports which describe the performance of Wells turbine both at starting and running characteristics. According to these results, Wells turbine has inherent disadvantages: lower efficiency, poorer starting and higher noise level in comparison with conventional turbines. In order to enhance the performance of Wells turbine, some rotor blade profiles have been recommended by various researchers. The aim of this study is to investigate the effect of rotor blade profile on the performance of Wells turbine. In the study, four kinds of blade profile were selected and tested by model testing under steady flow condition. The types of blade profile are as follows: NACA0020; NACA0015; modified NACA0015; and modified Eppler472. The experimental investigations have been performed by use of test section with a casing diameter of 300 mm. Further, the effect of non-uniform tip clearance on the turbine performance was tested and the result was compared with that of the case of Wells turbine with uniform tip clearance. As an additional experiment, the effects of blade profile and non-uniform tip clearance on the performance under unsteady flow condition have been investigated numerically by using a quasi-steady analysis.
Frequency, Stochastic and Time Domain Models for an Articulated Wave Power Device
Authors: Caˆndido Jose´ J. , Justino Paulo A. P.
Published: July 2009

A frequency domain model to describe the behaviour of an articulated system in regular waves is developed. A stochastic model is found for the articulated wave power device. Probability density functions for relevant parameters that characterize the wave power system behaviour are found for different sea state conditions. Average values for useful power and capture length are also obtained for these sea state conditions. A time domain model is also developed for the articulated wave power device. Results for a linear power take-off configuration and regular waves are obtained and compared to the results obtained from the frequency model. A non-linear power take-off configuration is found. Results in regular wave conditions are obtained for this configuration, set in terms of different characteristic parameters. Results for this non-linear configuration for the power take-off and irregular waves are also computed and presented. Finally, these results are compared to the results obtained from the stochastic model.
Incremental Dynamic Analysis Considering Pile-Soil-Structure Interaction for the Jacket Type Offshore Platforms
Authors: Asgarian Behrouz , Fiouz Alireza , Shakeri Talarposhti Ali
Published: July 2009

Nonlinear response of piles is the most important source of potentially nonlinear behavior of offshore platforms due to earthquake excitations. It is often necessary to perform dynamic analysis of offshore platforms that accounts for soil nonlinearity, discontinuity condition at pile soil interfaces, energy dissipation through soil radiation damping and structural nonlinear behaviors of the piles. Incremental dynamic analysis is an analysis method that has recently emerged as a promising tool for thoroughly evaluating the seismic performance of structures. It involves subjecting a structural model to a suite of ground motion records, each scaled to several intensities and recording the responses at each level to form IDA curves of response versus intensity. In this paper, jacket and soil-pile system is modeled and the effects of Soil-Pile-Structure Interaction (SPSI) are considered, and the Incremental Dynamic Analysis (IDA) is used to investigate nonlinear behavior of offshore platforms. An attempt is made to introduce a practical BNWF (Beam on Nonlinear Winkler Foundation) model for estimating the lateral response of flexible piles embedded in layered soil deposits subjected to seismic loading. This model was incorporated into a Finite Element program (OpenSees). All the analyses are performed in two directions and the results are compared with each others. A computer program for Nonlinear Earthquake site Response Analyses of layered soil deposits (NERA) is used for analysis nonlinear response of soil layers. Limit state of the jacket is calculated from incremental dynamic analysis of the jacket using fiber elements for the nonlinear modeling of the system.
A Study on a Disaster Relief System for Large-Scale Earthquake Using Pleasure Boats in Tokyo Bay
Authors: Egami Kazuya , Kondo Takeo , Yamamoto Kazukiyo , Oshima Takatomo , Kakoi Tatsuya , Kigoshi Naoyuki , Yamaguchi Hidekazu , Fukuhara Tetsuo , Namba Yoshimasa , Hirao Naoyuki
Published: July 2009

The Central Disaster Management Council of the Cabinet Office predicts that an earthquake of magnitude 7 will occur in Metropolitan Tokyo and the Tokai area within 30 years. In preparation for such large-scale earthquakes, the government is considering various measures. In this study, we focus on the ‘Umi no eki (Sea Station)’ system developed by the Maritime Bureau of the Ministry of Land, Infrastructure and Transport, and propose a manual for ‘relief goods transportation’ and ‘stranded-commuter transportation’ using pleasure boats. Since it is important to determine whether pleasure boat owners are willing to voluntarily participate in relief activities at the Umi no eki, we also conducted a survey among pleasure boat owners concerning their awareness of disaster management. As a result, we found that pleasure boat owners are highly willing to participate. This study clarified the restrictions on volunteer activities carried out by pleasure boat owners, and a disaster management manual for pleasure boats was prepared with the objective of constructing a rescue system using pleasure boats in Tokyo Bay.
Scaled Boundary FEM Solution of 2D Steady Incompressible Viscous Flows
Authors: Tao Longbin , Song Hao
Published: July 2009

In this paper, the scaled boundary finite-element method (SBFEM) proposed for wave-structure interaction [Tao et al, 2007] is extended to solve two-dimensional (2D) steady incompressible viscous flows governed by the Navier-Stokes (N-S) equations. The present SBFEM scheme is validated against existing analytical solutions of the 2D viscous flow with a constant vorticity. Comparisons clearly demonstrate the excellent accuracy and computational efficiency associated with the present SBFEM. Such superiority in significantly outperforming its counterparts in currently available CFD software ensures a great potential of direct application of the present method to many engineering problems. As a crucial step in expanding the application of the SBFEM, further extension of the SBFEM to to solve viscous flow of variable vorticity or even more complex viscous fluid-structure interactions will be a welcome development.
Hydroelastic Response of Floating Fuel Storage Modules Placed Side-by-Side
Authors: Tay Z. Y. , Wang C. M.
Published: July 2009

Presented herein are the hydroelastic responses of two large box-like floating modules that are placed adjacent to each other. These two floating modules form the floating fuel storage facility (FFSF). Owing to the small draft when compared to the length dimensions, the zero-draft assumption is commonly adopted in the modeling of very large floating structures (VLFS) as plates for hydroelastic analysis. However, such an assumption is not applicable to the considered floating modules since the effect of draft on the hydroelastic response is significant when the modules are loaded with fuel. A numerical model taking into account the draft effect is hence developed in order to predict correctly the hydroelastic response and hydrodynamic interactions of floating storage modules placed side-by-side. The floating storage modules are modeled as plates where an improved Mindlin plate element, developed by coupling the reduced integration method and the additional non-conforming modes, is used. Such a plate element does not exhibit spurious modes and shear locking phenomena, thereby making it applicable to both thin and thick plate models. Furthermore, the Mindlin plate theory predicts better stress resultants as compared with its Kirchhoff plate counterpart. The linear wave theory is used to model the water waves. The wave-induced deflections obtained from the numerical model are validated by experimental tests.
FE Analysis of Ship Structures Under Blast Loads
Authors: Sinha Satyaranjan , Sarangdhar D. G.
Published: July 2009

Naval vessels and Submarines structures in their fighting role are susceptible to Underwater shock generated due to explosion of torpedoes, mines, depth charges etc. The damage inflicted by Non-Contact Underwater explosion consists of direct shock wave damage of hull, whipping damage of keel and mechanical damage to onboard equipment and associated systems. Hence in order to design a shock resistant structure or to know the shock withstandibility of the same, it is important to simulate these structures and loads and then subsequently analyze the same to predict the response (as performing experiments would be expensive). The Underwater explosion analysis of large structures like ships could be considered as one of the most complicated numerical analysis. The most important steps of these analyses are, the accurate load calculation and then the solution process. Loads can be calculated using published empirical formulas, which are complicated if calculated for a large structure. Also the application of load time history for large structure is a tedious job. To solve the complications related to load calculation and application, an in-house software named IRUNDEX, was developed, which, not only calculates the complex loading at all panels comprising the ship structure, but could also apply the loads (using ANSYS Macro) within minutes, thus saving considerable percentage of time taken for the analysis. It could be recognized that underwater shock simulation and analysis should form an important criterion to verify the design of any Naval Vessel or structure susceptible to explosions. This present work illustrates the use of the FE Software ANSYS, backed up with the in-house developed software, for Underwater Explosion analysis of structures.
Study of the Measures Manual for Natural Disasters in a Marina
Authors: Yuta Nagae , Kondo Takeo , Yamamoto Kazukiyo , Takasi Hasikawa
Published: July 2009

Currently, natural disaster management at marinas depends on the experience of the marina managers. To improve the safety of marinas, it is necessary to prepare a manual for natural disaster management according to the local conditions at each marina. In this study, we focused on the basic information required for preparing effective manuals to deal with typhoons and tidal waves. We investigated the damage caused by typhoons and tidal waves, and the measures taken by marinas against natural disasters. The damage caused by natural disasters strongly depends on the storage system and shape of the marina.
The Study of Container Yard - Roadway System Harmony in the Container Port Based on Thermodynamics Entropy
Authors: Liu Yun , Shi Hongda , Ma Xichao , Fan Yong
Published: July 2009

Phenomena of port container yard - roadway system is similar to thermodynamics system, the paper analyzes the sameness between such two movements by means of thermodynamics, it also defines the concepts and hypothesis of container yard - roadway system simulation. The results of this article will do help to make port container yard - roadway system easily be modeled in the field of thermodynamics entropy.
Prediction of Increased Jacket Substructure Loads Due to Wave in Deck
Authors: Brodtkorb Benedicte , Nestega˚rd Arne , Johansen Atle
Published: July 2009

When an extreme wave crest hits the deck of a jacket-type structure, the kinematics in the wave beneath the deck is strongly influenced by the deck itself. The increased fluid particle velocities should be accounted for when assessing the load on obstructions located in a zone below the deck, i.e. on the upper part of the substructure and on under-deck attachments. Usually the wave-in-deck loads and substructure loads are calculated separately by two different analysis programs. The undisturbed velocity field is then applied to the substructure. In this study, we compare two methods, both based on CFD (Computational Fluid Dynamics), for calculating the increased substructure loads due to disturbed fluid particle velocities during extreme wave-in-deck events.
Simulation of the Winter Meso-Scale Wind and Tidal Circulation in the Gulf of Patras (Greece)
Authors: Fourniotis Nikolaos Th. , Horsch Georgios M.
Published: July 2009

The Gulf of Patras is a shallow embayment (of 80 m mean depth), in western Greece, leading to the Ionian Sea on the west, and, through the straits of Rio-Antirio, to the Gulf of Corinth, on the east. In the present study, the three-dimensional modelling system [1] was applied to investigate the tide- and wind-driven circulation in the complex bathymetry of the natural basin of the Gulf of Patras. Numerical simulations have been conducted for different scenarios for wind speed and direction, and the results of the circulation forced by a uniform wind stress corresponding to a wind speed of 4 m/s and two different directions are reported. In addition, tidal records measured at both ends of the Gulf were used as forcing in order to simulate the tidal circulation. The numerical study corroborated that the tidal currents at the Rio-Antirio straits are stronger than those induced by the mean wind and are among the strongest tidal currents to be found in Greek waters. Comparisons between the three-dimensional model predictions and available field measurements have shown reasonable agreement both qualitatively and quantitatively.
Experimental Results From an Offshore Wave Energy Converter
Authors: Bostro¨m Cecilia , Lejerskog Erik , Tyberg Simon , Svensson Olle , Waters Rafael , Savin Andrej , Bolund Bjo¨rn , Eriksson Mikael , Leijon Mats
Published: July 2009

An offshore wave energy converter (WEC) was successfully launched at the Swedish west coast in the middle of March 2006. The WEC is based on a permanent magnet linear generator located on the ocean floor driven by a point absorber. A measuring station has been installed on a nearby island where all measurements and experiments on the WEC have been carried out. The output voltage from the generator fluctuates both in amplitude and frequency and must therefore be converted to enable grid connection. In order to study the voltage conversion, the measure station was fitted with a six pulse diode rectifier and a capacitive filter during the autumn of 2006. The object of this paper is to present a detailed description of the existing wave energy system of the Islandsberg project. Special attention will be given to the power absorption by the generator when it is connected to a non linear load.
2nd Order Hydrodynamic Effects on Resonant Heave, Pitch and Roll Motions of a Large-Volume Semi-Submersible Platform
Authors: Simos Alexandre N. , Sparano Joa˜o V. , Aranha Jose´ A. P. , Matos Vini´cius L. F.
Published: July 2009

During the last decades, as oil production offshore Brazil moved to deeper waters, technical and economical constraints led to a new generation of floating platforms. Nowadays, in the Brazilian offshore scenario, design trends concerning hull form, size and mooring configurations bring novel characteristics of wave-induced dynamics, including non-linear resonant effects. As part of an extensive study on new semi-submersible configurations for Campos basin, recent model tests have shown that their hulls may be subjected to second-order slow motions in heave, pitch and roll. These resonant motions are directly related to the large dimensions and relatively low natural frequencies of the floating systems. The unexpected effects caused great concern, since, in some cases, the low-frequency motions presented amplitudes comparable to those of the first-order response. This paper discusses the evaluation of the 2nd order wave-induced motions of a large-volume semi-submersible platform using WAMIT® second-order module. It is shown that the hydrodynamic forces induced by the 2nd-order potential represent the prevailing effect in the resonant response. Important aspects concerning the numerical model are addressed, such as the parameters involved in the hull and free-surface panelization. Numerical predictions are directly compared with experimental results obtained with a 1:40 model of the platform. A very good agreement is obtained both for heave and angular (pitch or roll) motions, attesting that the numerical code is able to predict the 2nd order forces accurately. Finally, a simplified procedure for dealing with the slow vertical motions is evaluated, aiming to reduce the substantial computational effort required by the 2nd order calculations. Such procedure takes advantage from the fact that the resonant response spectra of the vertical motions are usually narrow-banded (due to the low damping levels) to propose a “white-noise” approach. According to this approach, 2nd order forces need to be calculated only for one frequency difference, corresponding to the natural frequency of the particular motion. Computational time is, therefore, greatly reduced. It is shown that resonant motions calculated through the simplified approach match those predicted through the “full” analysis perfectly, making it an interesting choice for the evaluation of 2nd order effects, especially in the early stages of the design.
Aluminum Drill Pipes: Material and Design Developments
Authors: Lourenc¸o Marcelo Igor , Netto Theodoro A. , Pla´cido Joa˜o Carlos Ribeiro
Published: July 2009

An extensive research program on the structural strength of aluminum drill-pipes is being conducted at COPPE/UFRJ. The main objective is to improve the fatigue performance of these components by selecting the appropriate aluminum alloy and by enhancing the mechanical design of the threaded steel connector. This paper presents the experimental test program and numerical analyses conducted on a drill-pipe of different materials (Al-Zn-Mg alloy) and geometries from those previously presented [1,2]. Small-scale specimens were tested to determine its uni-axial stress-strain and fatigue behavior. Full-scale fatigue test results are also presented. A finite element model of the drill pipe, including the tool-joint region, was developed. The model simulates, through different load steps, the tool-joint hot assembly, and then reproduces the physical experiments numerically in order to obtain the actual stress distribution. The correlation between full and small-scale fatigue tests is performed via multi-axial fatigue models. The weak points of the current practice design are highlighted for further development.
Fatigue Calculation of Risers Using a Van Der Pol Wake Oscillator Model With Random Parameters
Authors: Modarres-Sadeghi Yahya , Hover Franz S. , Triantafyllou Michael S.
Published: July 2009

Vortex induced vibrations of long distributed structures (risers and mooring cables) is an inherently complicated phenomenon in which due to the riser multi-mode excitations, various combinations of traveling and standing wave patterns along the length is observed. These observations are made based on a series of model scale experiments conducted on a riser for both uniform and linearly sheared flow cases. In these model scale experiments, strain and acceleration measurements are conducted at selected points along the riser. The contour plots of amplitudes of oscillations in these experiments show a mainly traveling wave behavior for linearly sheared flow cases and a mainly standing wave one for the uniform flow cases. In order to model the vortex induced vibrations of the riser used in these experiments, a wake oscillator model is used. In this model, the riser is assumed to be a tensioned string and the wake dynamics is represented by a Van der Pol oscillator whose driving force is in parallel with the riser acceleration. Randomness in the current, added mass and lift coefficients is taken into account by considering random parameters for the wake oscillator model. By using the proper parameters in this wake oscillator model, its results can be compared with the experimental ones. The comparison is made in terms of dynamical behavior (traveling waves versus standing waves, amplitudes and frequencies of oscillations) as well as the fatigue life calculations. The statistics of fatigue life calculations based on the experimental reconstruction compares well with those of the model results showing that the theoretical model can predict fatigue damage of the riser fairly well.
Empirical Assay of the Use of the Hilbert-Huang Transform for the Spectral Analysis of Storm Waves
Authors: Ortega Joaqui´n , Smith George H.
Published: July 2009

The Hilbert-Huang Transform (HHT) was proposed by Huang et al. [2] as a method for the analysis of non-linear, non-stationary time series. This procedure requires the decomposition of the signal into intrinsic mode functions using a method called empirical mode decomposition. These functions represent the essential oscillatory modes contained in the original signal. Their characteristics ensure that a meaningful instantaneous frequency is obtained through the application of the Hilbert Transform. The Hilbert Transform is applied to each intrinsic mode function and the amplitude and instantaneous frequency for every time-step is computed. The resulting representation of the energy in terms of time and frequency is defined as the Hilbert Spectrum. In previous work [7] using the HHT for the analysis of storm waves it has been observed that the number of IMFs needed for the decomposition and the amount of energy associated to different IMFs differ from what has been observed for the analysis of waves under ‘normal’ sea conditions by other authors. In this work we explore in detail the effect that the sampling rate has in the empirical mode decomposition and in the Hilbert Spectrum for storm waves. The results show that the amount of energy associated to different IMFs varies with the sampling rate and also that the number of IMFs needed for the empirical mode decomposition changes with record length.
Bending Stiffeners for Extreme and Fatigue Loading of Unbonded Flexible Risers
Authors: Smith Russell
Published: July 2009

Bending stiffeners constrain the dynamic radius of flexible risers at offshore platform and subsea interfaces. The extreme bending and fatigue hotspots of a flexible riser occur near these interfaces. Conventional design of bending stiffeners first account for the extreme metocoean environment to ensure the riser does not bend below the minimum allowable bending radius (MBR). The stiffener design for extreme loading normally proves acceptable for less onerous fatigue loading provided the steel armour in riser annulus stays dry. Flexible riser operations have shown that there are several mechanisms for the annulus to loose its dry-state. A flexible riser that is not fatigue tolerant to a non-dry annulus often needs to be replaced at short notice or shut-down due to the rapid onset of corrosion fatigue of the steel armour wires. This paper demonstrates recent advances in designing bending stiffeners so that a flexible riser can sustain life of field operation with a non-dry annulus. These advances are accomplished by optimising the tapered profile of the bending stiffener and accounting for additional stiffening due to bending hysteresis in a flexible riser. A case study is presented to demonstrate the improvement in fatigue performance of a flexible riser.
Analysis of the Statistical Features of Two-Dimensional Wave Fields by Using Stochastic Simulations of Linear and Nonlinear Sea States
Authors: Nieto-Borge Jose C. , Rodri´guez-Rodri´guez German , A´lvarez-Pe´rez Jose L. , Lo´pez-Ferreras Francisco
Published: July 2009

This work uses stochastic simulations of linear and nonlinear two-dimensional wave fields to analyze some the statistical properties of the extreme wave crests for a given sea state. The studies carried out consider different spatial resolutions and extensions of the simulated sea state.
Elastic Wave Tomography for Wide Area Structural Damage Detection of Very Large Floating Structure
Authors: Suzuki Hideyuki , Nakada Satoshi
Published: July 2009

An efficient inspection and maintenance is a key technology to assure and maintain the integrity of the large scale structure like Very Large Floating Structure (VLFS). A rapid detection of relatively large damage caused by an accident or natural disaster in the wide area of a structure is proposed. This is an application of elastic wave tomography method to geometrically complicated structure like VLFS. Position of damaged area is detected and information about size of the damage can be obtained.
Observations of Surface Waves and Wave Breaking: A Tribute to Nick Newman
Authors: Melville W. Kendall , Romero Lenonel , Kleiss Jessica
Published: July 2009

The Gulf of Tehuantepec is well known for having strong offshore winds, which occur predominantly during the winter months when significant atmospheric pressure differences develop between the Gulf of Mexico and the Pacific Ocean, forcing winds through a mountain gap at the head of the gulf. During the Gulf of Tehuantepec Experiment (GOTEX), conducted in February 2004, we collected surface-wave and wave-breaking measurements using a conical scanning lidar (NASA Airborne Terrain Mapper, ATM) and a downward-looking video camera on the NSF/NCAR C-130 aircraft. We present ATM observations of surface waves as well as statistical and spectral descriptions of the wave field. We also present measurements of the occurrence and strength of breaking using digital video data to identify actively breaking waves. In order to infer the dynamics of breaking from simple physical models, the kinematics of the breaking must be separated from the kinematics of the underlying waves. This is done using linear and nonlinear estimates of the underlying orbital velocities from the spatio-temporal surface displacement data measured by the ATM. Nonlinear estimation of the orbital velocities is described in a companion paper in this symposium (Grue et al., OMAE 2008). Frequent breaking is observed under the strong wind forcing. Examples of the spatial structure of waves are compared to simultaneous video imagery, giving an indication of the scale at which wave breaking occurs. The GOTEX observations, to almost full wave development, are tested against the latest wind-wave numerical spectral models that include the effects of wave generation, nonlinear wave-wave interactions and wave dissipation due to breaking. This paper is dedicated to Nick Newman by the lead author in appreciation of Nicks encouragement and assistance when he (WKM) moved to MIT in the early 1980s.
Wavelet Analysis of an Extreme Wave in a Model Basin
Authors: Ewans Kevin , Buchner Bas
Published: July 2009

Analyses of laboratory wave records including extreme crests, based on the continuous wavelet transform, are reported. The analyses have provided further insight into the spectral characteristics of these extreme events. During the period of the extreme wave crest, spectral levels over all frequencies are substantially elevated by comparison with average spectrum for the complete wave record. This was also observed in a similar analysis of the famous New Year wave event that occurred at the Draupner platform. The analyses also indicate that nonlinear effects are active during the crest event, second-order effects being particular strong and indicating phase-locking of high frequency waves to freely-propagating waves with frequencies close to the spectral peak. These nonlinear effects appear to be strong only in the vicinity of the extreme.
Nondestructive Corrosion Inspection Modeling of Tanker Structures
Authors: Zayed Ahmed , Garbatov Yordan , Guedes Soares C.
Published: July 2009

A mathematical model is proposed to evaluate the effect of different factors on the inspection performance of a ship hull subjected to corrosion. Various formulations are proposed to estimate the probability of detection and non-detection of corrosion degradation based on a mathematical model describing the corrosion degradation of the ship hull though her lifetime. The model proposed is adapted to two sets of corrosion data of deck plates of ballast and cargo tanks of oil tankers.
Coupled Seakeeping With Liquid Sloshing in Ship Tanks
Authors: Kim Booki , Shin Yung S.
Published: July 2009

A three-dimensional forward-speed seakeeping theory is proposed by the inclusion of a tank sloshing flow model. The methodology is based on a coupling model of sloshing and seakeeping in the frequency domain. The fluid forces and moments due to the liquid motion inside tanks are incorporated into the equations of the vessel motion for the coupled seakeeping computations. The classical expression for restoring forces is accordingly modified for the coupled analysis. The global motion responses of a modern LNG carrier at forward speed in beam seas are then calculated to investigate the coupling effects between sloshing and seakeeping. No prominent coupling effect was observed in heave and pitch motions while a significant effect was observed in sway and roll motions. The numerical results show that the proposed theoretical model works well in comparison with experimental results.
New Algorithm for Full Hexahedral Finite Element Mesh Generation of Pipe Models With Multiple Corrosion Defects
Authors: Andueza Alejandro , Estefen Segen F.
Published: July 2009

Analysis of corroded pipelines using simulation techniques has become an essential step for the evaluation of the residual ultimate strength of damaged pipes. Problems with multiple corrosion defects present highly complex geometries mainly when the defects are close enough to produce interacting stress fields. In such cases it is easier the mesh generation with all-tetrahedral elements using mature algorithms implemented in commercial programs like Ansys or Patran. The use of all-tetrahedral meshes in many applications yields to less accurate analysis results. Unfortunately, the algorithm for mesh generation of all-hexahedral elements is much more complex than the generation of all-tetrahedral element mesh. Currently, the problem associated with general all-hexahedral element mesh algorithm is a research subject in progress. This paper presents a new algorithm for the mesh generation of all-hexahedral elements to be used in the analysis of damaged pipelines. The algorithm is currently under development and was designed to deal with any number of corrosion defects of arbitrary shape. The application of the new methodology is demonstrated performing the mesh generation of models with one, two and three corrosion defects in order to demonstrate both efficiency and robustness of the new methodology. Finally, computer simulations for the generated models are performed in order to determine the failure pressure of the damaged pipes. The obtained results are compared to the values predicted by the standard DNV RP-F101.
Comparison of Measured and Back Estimated Cone Penetration Resistance of Sand
Authors: Gui Meen-Wah , Jeng Dong-Sheng
Published: July 2009

The application of cavity expansion theory in the back estimation of cone penetration tests conducted in calibration chambers has been carried out by many researchers. However, the theory is seldom employed by centrifuge modelers. Based on the work of spherical cavity expansion of previous researchers, this study proposed an analytical solution that incorporates the effects of cone geometry and surface roughness and the effect of compressibility to estimate the cone tip resistance. The calculated results are compared with the measured cone penetration resistance of four cone penetration tests performed in the centrifuge. The cone penetration tests were conducted in granular soil specimens having relative densities ranging between 54% and 89%. The comparison demonstrates the capacity of the cavity expansion theory in the prediction of the centrifuge cone penetration resistance.
Research of a New Controllable Friction Damper and Its Application on the Gas Compressor’s Vibration Control
Authors: Zhao Dong , Cai Dongmei , Ma Rujian , Dong Meihua
Published: July 2009

The new controllable friction dampers were composed of a housing, a piston, rollers and elastic bodies. The housing was set on the platform under the machineries or sub structures with which the piston was connected. The housing and the piston could move relatively when the machinery or the sub-structure vibrating under the inside or outside loads. The friction pairs, which were composed of the housing and rollers’ contact surfaces, could dissipate the vibration energy by the Coulomb force. The dampers could increase the output forces automatically with the augmentation of the relative vibration displacements by the usage of their interior elastic bodies which can enhance the positive pressure on the friction pairs. The new dampers overcame the shortcomings of the traditional frictional dampers that could only export the invariable force after their assembling. So they had great capabilities to consume more vibration energy. At the same time, the dampers could use the vibration source signal to control their output forces. This merit could increase the sensitiveness of the dampers to control minor vibration and expand the frequencies’ bound of the vibration sources that to be controlled. The simulation using the finite element program proved that the average displacement decrement of the gas compressor that used the new dampers to control their vibration reduced upwards of 67.2% than the machinery that didn’t use dampers. The new dampers have the better vibration control performance.
Frequency Domain Model for Prediction of Stochastic Vortex Induced Vibrations for Deep Water Risers
Authors: Lie Halvor , Larsen Carl M. , Kaasen Karl Erik
Published: July 2009

This paper describes a new model for prediction of fatigue damage from VIV in risers. The method will overcome some of the shortcomings of previous methods. A fully 3D model is proposed, “cross-flow” and “in-line” response are predicted, response at higher order harmonic components will be added, and the stochastic nature of the response is accounted for by introducing a time varying envelope function combined with “time sharing” between dominating response frequencies. A model that reflects this behaviour is considered to be more realistic and is more likely to predict lower fatigue damage than the traditional discrete-frequency models. The model will predict a response that will appear as a combination of standing and travelling waves depending on boundary conditions, damping and load distribution. Fatigue damage will therefore become more evenly distributed along the riser, and less concentrated at anti-nodes for (dominating modes) than seen from traditional discrete frequency models. The proposed model needs empirical coefficients for simultaneous IL and CF response. In principle this requires a data base of added mass, excitation and damping coefficients for varying flow conditions and response frequencies, combinations of response amplitudes in both directions, varying phase between the two response components and even the presence of higher order motion components. Such data do not exist. We have therefore proposed to use the limited information we have on this matter at present. Future improvement of the model might therefore be possible if more data becomes available. The new model will be implemented in the VIVANA program and the enhancement of the code is in progress. The paper will present the background of the model, the basic assumption of the new model and a comparison between preliminary results obtained from a preliminary code and model test results. The cases include both 2D uniform current conditions and 3D (non-uniform) current conditions.
Parallel Dynamic Optimization of Steel Risers
Authors: Tanaka Rafael Loureiro , de Arruda Martins Clo´vis
Published: July 2009

The design of an ultradeepwater steel riser is known to be a challenging task, considering the several environmental load conditions to be simulated and the long time took by each simulation. To aid in this task, optimization algorithms can be used to automate the search for the best design. The problem, however, is very expensive computationally, motivating the implementation and use of a parallel optimization software coupled with a frequency domain dynamic model. This paper deals with the use of two algorithms to optimize a steel riser of given inner diameter and material for both pipe and floater: the genetic algorithm and the simulated annealing. The optimizer is left free to vary the length of the segments and the floater diameter, inside user-defined ranges. Each obtained configuration is tested in a set of load cases with different currents, offsets and waves. Optimization times and performance gain with parallelization is addressed for both algorithms.
A Direct Assessment Approach for Structural Strength Evaluation of Cargo Containment System Under Sloshing Inside LNGC Tanks Based on Fluid Structure Interaction
Authors: Ito Hisashi , Suh Yongsuk , Chun Sangeon , Kumar Y. V. Satish , Ha Munkeun , Park Jongjin , Yu Han C. , Wang Bo
Published: July 2009

The sloshing phenomenon is one of the most important and challenging issues for the design of cargo containment systems of LNG carriers and is being studied by various research groups in many countries. In this paper, a direct assessment approach based on fluid structure interaction is proposed to assess the structural safety of cargo containment systems against extreme sloshing loads. In the course of developing the methodology, two technical issues are mainly dealt with; (1) how to efficiently apply a numerical method for sloshing phenomena and (2) how to verify the structural safety criteria of cargo containment system against sloshing. The developed methodology is applied to a membrane-type cargo containment system of a large LNG carrier built in Samsung Shipyard.
VIV Suppression and Drag Reduction With Pivoted Control Plates on a Circular Cylinder
Authors: Assi Gustavo R. S. , Bearman Peter W.
Published: July 2009

Experiments have been carried out on two-dimensional devices fitted to a rigid length of circular cylinder to investigate the efficiency of pivoting control plates as VIV suppressors. Measurements are presented for a circular cylinder with low mass and damping which is free to respond in the cross-flow direction. It is shown how vortex-induced vibration can be practically eliminated by using free to rotate, two-dimensional control plates. Unlike helical strakes, the devices achieve VIV suppression with drag reduction. The device producing the largest drag reduction was found to have a drag coefficient equal to about 70% of that for a plain cylinder at the same Reynolds number.
Feasibility of Suction Caisson Solutions for the Foundations of Offshore Jacket Platforms in the Persian Gulf
Authors: Zeinoddini M. , Saeidi B. , Parke G. A. R.
Published: July 2009

This paper gives the results of an investigation into the performance of the suction caisson foundations for jacket platforms in the Persian Gulf under quasi static environmental wave loading. A numerical approach has been chosen to study the subject. The installation phase has not been analysed, so the suction caisson foundations have been considered in an in situ condition. At first, three dimensional finite element models of the suction caissons, embedded in the sea bed but in isolation of the jacket, have been examined under static vertical, horizontal and inclined loadings. The results have been verified against available test data from other researchers and, in general, reasonable agreements have been obtained between the numerical and the experimental results. The models of some of the existing jacket structure in the Persian Gulf have then been incorporated with the verified caisson models. The resulting integrated model of the jacket/caissons/soil body has been examined under push-over environmental loads. The results have then been assessed against those obtained from the corresponding jackets with driven pile foundations. From the current study, the suction caisson foundations have been found to be able to contest the deep pile foundation solution due to savings in the material utilization costs and their load carrying capacity.
Resonance Hold Control of Flapping Foil System
Authors: Isshiki Hiroshi
Published: July 2009

In case of MAV (Micro Air Vehicle), the generation of lift and thrust by flapping foils is very attractive, since flying insects such as dragonflies show the very high flight performance. However, in case of the flapping foil drive, the increase of load due to variation of the inertia forces takes place in contrast to the case of the rotary foil drive. So, it is extremely important to suppress the variation of the inertia forces. The possibility of canceling the inertia forces by using resonance is shown below. Furthermore, a control method to maintain the resonance is developed and verified by a numerical simulation.