Showing papers in "International Journal of Offshore and Polar Engineering in 2006"

Dissociation Behavior of Hydrate Core Sample Using Thermodynamic Inhibitor

Abstract: The dissociation behavior of an artificial hydrate core sample in an inhibitor aqueous solution (methanol aqueous solution) was investigated experimentally. A newly constructed experimental setup equipped with a core holder was used to investigate the dissociation behavior of the hydrate core sample under conditions similar to those below the seafloor. It was observed that an inhibitor that changes the hydrate stability condition is effective in accelerating the hydrate dissociation rate. In addition, it was observed that the temperature of the solution from the outlet of the core changes continuously, while the outlet temperature does not change in the case of pure water. This result suggests that the inhibitor concentration around the hydrate surface changes continuously.

Control of vortex shedding around a pipe section using a porous sheat.

Abstract: Passive control of the flow around a fixed circular cylinder is achieved using a porous layer between the obstacle and the fluid The various media are easily handled by means of the penalization method The computational domain is reduced to a close neighbourhood of the body thanks to efficient nonreflective boundary conditions The porous layer changes the vortex shedding and induces a strong reduction of the vorticity magnitude and of the root mean-square lift coefficient

Implementation And Validation of Nonlinear Wave Maker Models In a HOS Numerical Wave Tank

Abstract: This study deals with the development of a novel, fully nonlinear, potential flow model for simulating the generation and propagation of 2-dimensional or 3-dimensional gravity waves in finite depth. This Numerical Wave Tank (NWT) relies on an original nonperiodic High-Order Spectral (HOS) model (Bonnefoy et al., 2004 ). In this new fully spectral formulation, propagation of waves is solved in a fully nonlinear manner, while generation can be modeled up to the 2nd order in wavemaker excursion. The resolution is made by means of only Fast Fourier Transforms (FFT), leading to a fast and accurate method. The efficiency and fast convergence of this spectral method enable modelization of the shortest wavelengths in a wave tank while keeping reasonable computational efforts. Several validation results are presented where numerical simulations are successfully compared to experiments on different 2-D and 3-D complex sea states.

Riser Interference And VIV Amplification In Tandem Configuration

Abstract: Wake Induced Oscillations (WIO) and amplification of Vortex Induced Vibration (VIV) are studied for 2 risers in tandem configuration. The criticality of riser interference is first investigated in term of nondimensional numbers for different types of risers. Theoretical, numerical and experimental approaches are then used simultaneously to define similarity rules and to investigate scale effects with respect to the Reynolds number. Finally, experimental results demonstrating amplification of VIV are given.

Prediction of Deepwater Oil Offloading Buoy Response And Experimental Validation

Abstract: Fully coupled time-domain approaches were applied to predict the vertical plane motions, i.e. surge, heave and pitch, of the deepwater buoy. It is found that the pitch motion in particular is sensitive to the drag effect of the skirt, and is coupled with both surge and heave motions, and that a time-domain fully coupled analysis can capture the viscous drag effect. Results from 2 experiments, one with a freely floating buoy and the other with a moored buoy, are presented to show that the proposed time-domain coupled analysis predicts the buoy motion behavior very well for both cases compared to frequency-domain analyses with a linearized stiffness for the mooring system. Comparison of experimental data and coupled analysis results from the proposed buoy skirt modeling with multiple disks shows that viscous modeling of the buoy skirt by applying a Morison drag force formulation based on relative velocity can be used to better predict the pitch motion.

Wave Generation And Absorption In Wave Basins: Theory And Application

Abstract: Experimental research remains important for evaluating the performance of offshore structures and ships in waves. To conduct useful experiments, it would be advantageous to first develop a high-performance absorbing wave-maker that can generate both regular and irregular waves. The construction of such a wave-maker is investigated in this study via wavemaker theory and wave-absorbing theory. Concerns have been raised recently regarding the configuration of wave basins, because researchers are doubtful as to whether the commonly used configuration of a rectangular wave tank is a suitable geometry within which to conduct experiments. This review paper presents a summary of the theory of wave generation and wave absorption in wave basins, as well as a new experimental concept and results of investigations based on the above theories. Due to space constraints, a pneumatic-type wave-maker is not reviewed here. For this type of wave-maker, a completely different approach is required. And nonlinear problem-related wave generation and absorption are also not reviewed. Those important subjects will be reviewed at some other time.

Biology-Inspired Precision Maneuvering of Underwater Vehicles (Part 4)

Abstract: This paper describes the results obtained in the fourth year of a multi-year program to demonstrate the utility of replacing cumbersome thrusters with a suite of oscillating fins designed to optimize the hovering and low-speed maneuverability of an underwater vehicle. This paper discusses the guidance and control of an underwater vehicle equipped with 2 pairs of oscillating fins in water currents. The rear pair of oscillating fins mounted in the horizontal plane of the original underwater vehicle were reconfigured in the vertical plane. The maneuverability performance in the horizontal plane from the point of turning in a hovering condition and the ability to guide and control around a cylinder in water currents are compared between the original vehicle and the remodeled one.

Wavelet And Local Directional Analysis of Ocean Waves

Abstract: Whereas local and wavelet-based methods have mostly been applied to single time records of wave data, Donelan et al. (1996) derived and applied the Wavelet Directional Method (WDM) to small arrays of wave elevation recorders. After a short review of the Morlet wavelet transform applied to random signals, the WDM is revisited and compared to a Local Directional Analysis (LDA), where the wavelet transform replaces the discrete Fourier transform in the directional analysis. The sampling statistics of the WDM and LDA are summarized, assuming that the wave records are weakly stationary stochastic processes. All methods show consistent results when applied to real data. In particular, time-averaged local estimates of common wave parameters closely fit the estimates from the standard analysis. Frequency-averaged local wave directions show considerable variation and, in addition, the local directional spread tends to make a significant drop during the passage of large wave groups. The methods are illustrated using offshore directional wave data from the Ekofisk field in the North Sea.

Model-based Sliding Mode Control of Underwater Robot Manipulators

Abstract: In this paper, a model-based sliding mode control method is proposed for trajectory tracking of underwater robot manipulators. The proposed sliding mode controller is designed based on the dynamics of underwater manipulators. The controller in this paper has the advantages of precision and robustness, and it is easy to implement. In order to demonstrate the effectiveness of the controller, several experiments using a 3 degrees-of-freedom underwater manipulator are conducted in a test tank. The results show that the proposed controller provides high performance of trajectory tracking in the presence of uncertainties about the dynamics.

FEM For Time Domain Analysis of Hydroelastic Response of VLFS With Fully Nonlinear Free-surface Conditions

Abstract: A finite element method (FEM) is developed for the time domain analysis of the hydroelastic deformation of a pontoontype very large floating structure (VLFS) with fully nonlinear free-surface conditions. A 3-dimensional free-surface flow is formulated in the scope of potential flow theory with the nonlinear free-surface conditions. To describe the motion of VLFS, the Mindlin plate modeling is adopted. The equation of plate motion is discretized by finite elements using the virtual work principle and integrated by the Newmark method. To consider the fluid-structure interaction, the nonlinear free-surface motion and the plate motion are numerically solved through an iterative method at each time step.

Towing And Winch Control Strategy For Underwater Vehicles In Sheared Currents

Abstract: An efficient approach for planning towing maneuvers for an underwater vehicle via ship and winch control is presented. It is demonstrated that it is possible to generate a towing strategy for a submersible vehicle such that the towed body follows a desired trajectory in the presence of known currents. The approach relies on the fact that the system is differentially flat under certain modeling assumptions; hence, it is possible to determine the required ship motion and winch rate so that the specified trajectory of the submersible is followed. The cable is modeled using a lumped mass approximation, including hydrodynamic drag, buoyancy and added mass effects. In the proposed approach, extensive use is made of spectral collocation methods to compute derivatives of motion. The approach is capable of producing open-loop trajectories in very short computation times, which makes it suitable for real-time computation.

Measurement of Residual Stress Distributions by Energy Dispersive X-ray Diffraction Synchrotron Radiation

Abstract: The application of high-resolution strain mapping in large engineering samples with both high-spatial and strain resolution is reviewed in this paper using high-energy photons between 100 and 300 KeV on beam line X17B1 of Brookhaven National Laboratory. This was achieved by using Energy Dispersive X-ray Diffraction (EDXRD) methods and synchrotron radiation for the nondestructive measurement of residual stresses in engineering components. Examples of residual stresses profiles will be presented which include: laser and shot peening and fatigue crack stress fields. The presented results have been validated with other methods such as FE and other model predictions. This technique represents a significant development in the in life prediction

Nonlinear Time-domain Simulation of Pneumatic Floating Breakwater

Abstract: The performance of pneumatic-type floating breakwaters is studied using a numerical wave tank (NWT) simulation in time domain. The 2-dimensional fully nonlinear NWT is developed based on the potential theory, Boundary Element Method (BEM)/Constant Panel Method (CPM), Mixed Eulerian-Lagrangian (MEL)-nonlinear free-surface treatment, and RungeKutta 4th-order (RK4) time-marching scheme. The inner chamber of the pneumatic breakwater is modeled such that the time-dependent air pressure is linearly or quadratically proportional to the change of air volume at each time step, that is, the volume change results in airflow through an opening causing pneumatic damping. The air-chamber effect on wave-blocking performance is then assessed for various wave conditions and damping coefficients. Both fixed and floating breakwaters are considered. It is found that a significant enhancement of performance can be achieved by the damping effect if breakwaters are stationary. When the breakwater is floating against incident waves, the pneumatic damping effect becomes less significant in most wave frequencies considered, since the floating body tends to follow the vertical motion of incident waves and the resulting volume-change effect is small. However, near the resonance frequency, the air damping can play an important role by suppressing large motions. The fully nonlinear simulations with different wave heights are compared with linear ones. The energy conservation formula for pneumatic breakwaters is derived and confirmed by numerical simulations.

Diffraction theory as a tool for predicting airgap beneath a multicolumn gravity-based structure

Abstract: This work investigates the feasibility of using diffraction solutions to predict extreme green-water levels beneath multicolumn gravity based structures. The ultimate aim is to provide improved design tools for predicting the height the deck structure must be raised above mean sea level (airgap) for the lower deck to avoid green-water impact. Such tools, when fully validated, will replace the need to carry out model tests during preliminary design. Results for a real platform configuration are examined in this paper to highlight the key issues complicating the validation of diffraction-based design tools for real structures. Incident regular waves are considered.

Reliability Calculation of RC Offshore Structures Under Extreme Wave Loading

Abstract: In this paper, a general formulation of the section capacities of a circular RC tubular cross-section is first presented assuming that the strain-stress relation of the concrete in the compression zone is simply modelled as a bilinear function with ultimate values given in Eurocode 2. It is also assumed that the concrete works only in the compression zone. In the cross-section, tension stresses are carried by the reinforcement. Having presented a general formulation of the extreme bending moment and normal force of an RC monopod offshore tower subjected to wave loading, uncertainties in both section capacities and loading terms are presented. Then, a reliability calculation of the cross-section is performed to find out the reliability index. In this calculation, the balance of the normal force is used to determine the concrete compression zone during the reliability iteration, and the failure function is defined on the basis of the section capacity and applied bending moments on the cross-section. Variation of the reliability index with various parameters is investigated, and most sensitive uncertainty variables are determined.

Weld Metal Mechanical Properties In Hyperbaric GTAW of X70 Pipeline

Abstract: In the present investigation, 2 different wires for hyperbaric (underwater) GTA (gas tungsten arc) welding of X70 pipelines have been tested with respect to their weld metal mechanical properties. Welding of full coupons at different pressures (seawater depths of 16, 75 and 200 msw) was done with subsequent weld metal chemical analyses, hardness measurements, tensile testing and Charpy V notch testing as well as microstructure characterization. It is shown that both wires satisfied strength requirements set to X70 grade, representing a weld metal overmatch situation. Both wires gave sufficient impact toughness, but the toughness of the Ni-Mo containing weld was reduced with increasing seawater depth. This observation was strongly linked to the positioning of the Charpy V notch, and crack growth in a brittle, partially transformed region as a consequence of reheating by subsequent stringer beads. The embrittling microstructure consisted of high carbon MA (martensite-austenite constituents islands) decorating prior austenite grain boundaries. This microstructure was less pronounced when welding with the high Ni wire, which may explain why no similar toughness drop was found.

Fast multipole method for wave diffraction/radiation problems and its applications to VLFS

Abstract: This paper presents an accelerated higher-order boundary element method for wave diffraction/radiation problems and its applications, especially for wave response analysis of VLFS (Very Large Floating Structures). The Fast Multipole Method (FMM) has been implemented on the higher-order boundary element code using an 8-node quadrilateral element. The method utilizes an iterative solver, multipole expansion of Green’s function, and a hierarchical algorithm using a quadranttree. For solving a hydroelastic problem efficiently using an iterative solver, a new algorithm has been introduced, where the equations of motions representing plate vibration are solved at each iterative step. The numerical benchmark calculations have shown the efficiency of the method both in the storage requirement of O N and computation time of O N logN , where N is the number of unknowns for the velocity potential.

Flow of Ice Through Converging Channels

Abstract: This paper presents a study of the flow of ice in wedge-shaped converging channels. Such flows are encountered in the relatively constricted waters of the Canadian Arctic Archipelago. Ridging, lead opening patterns, development of a highpressure area, and arch formation are some of the processes which take place during ice flow through converging channels. An idealized geometry and steady wind forcing were used in the testing. The results give ice cover velocity, distribution of stresses, ice thickness, area coverage and ridging. Some of the conditions leading to arch formation at the constricted exit of the channel are explored.

Application of Flat DMT And ANN to Korean Soft Clay Deposits For Reliable Estimation of Undrained Shear Strength

Abstract: The flat dilatometer test (DMT) is a geotechnical tool used to estimate in-situ properties of various types of ground materials Undrained shear strength is known to be one of the most reliable and useful parameters that can be obtained by the flat DMT However, a successful application of the existing relationships that have been established for other local deposits depends on regional geotechnical characteristics In addition, although flat DMT data are interpreted using 3 intermediate indices—the material index (ID), the horizontal stress index (KD , and the dilatometer modulus (ED)—undrained shear strength is estimated using only the horizontal stress index (KD) In this paper, the applicability of the flat DMT to Korean soft clay deposits is investigated An artificial neural network (ANN) model is developed to predict undrained shear strength, based on p0, p1, p2 and ′ v, without using the KD The ANN model adopts the back-propagation algorithm and is trained by using DMT data obtained from Korean soft clays To investigate the feasibility of the ANN model, the prediction results were independently evaluated by data that had not been used to train the ANN model They were also compared with data obtained using conventional relationships

Cyclic threshold shear strains of sands based on pore water pressure buildup and variation of deformation characteristics

Abstract: The cyclic threshold shear strain c th is defined as the cyclic strain amplitude above which the pore water pressure in saturated sandy soil increases and/or the modulus and damping values vary with the number of loading cycles during cyclic loadings. In this paper, the cyclic threshold shear strains are thoroughly investigated on sands using torsional shear tests. Toyoura sand is tested at various densities, drainage conditions (dry, saturated drained, and saturated undrained), and effective confining pressures. Based on the test results, the cyclic threshold strains above which the excess pore water pressure increases and/or shear modulus and damping ratio vary with the number of cycles are determined. Cyclic hardening in dry and saturated drained conditions, and cyclic degradation in an undrained condition, are observed for the same sample, using identical testing equipment. c th for cyclic degradation is compared with c th for cyclic hardening. The variation of damping ratio with the number of cycles is observed at different drainage conditions, and c th for damping ratio is also investigated.

Effect of Electric Field Intensity On Electro-cementation of Caissons In Calcareous Sand

Abstract: The effect of the electric field intensity on electro-cementation of calcareous sand was studied via the large model tests and the electric field analyses. In the experiments, the model caissons were embedded in calcareous sand and a dc intermittent electric field was applied via a central electrode. After 7 days of treatment the pullout resistance of the caisson model increased up to 190 % and generated significant soil cementation. The electro-cementation was found closely related to the electric field intensity, which is determined by the applied voltage, electrode layout and soil electrical conductivity. Furthermore the results indicate that the effectiveness of the electrokinetic treatment is directly associated with the electric field intensity surrounding the caisson. This study demonstrates the relationship between the electric field intensity and electro-cementation between calcareous soil and foundation elements, which serves as the fundamental principle in engineering design.

Sensitivity Study On Computational Parameters For Prediction of Slosh-induced Impact Pressure

Abstract: In this study, based on careful observation of experimental data, physics-based numerical models are developed for sloshing flows in ship cargo. The particular scheme of interest is a finite difference method based on the SOLA-SURF method. The technical issues of conventional methods are outlined, and the corresponding remedies are introduced. The present numerical method is validated by comparing computational results with the experimental data measured in model tests. In particular, sensitivity to critical computation parameters, e.g. mesh size and time segment, is observed. The comparison shows a fair agreement of overall fluid motions and hydrodynamic pressures.

Numerical Analysis On 2-D Optimal Profile of Floating Device With OWC-type Wave Energy Converter

Abstract: A floating-type wave power generating system has an Oscillating Water Column (OWC). The device captures the wave energy using the heaving, pitching and surging motion of the device and the heaving motion of the OWC. The investigations are motivated by the experiments and the numerical analyses undertaken to find a possibility different from an OWC-type floating device, e.g. the Mighty Whale of the Japan Marine Science and Technology Center. The 2-dimensional numerical method for analyzing a floating body with an OWC-type wave energy conversion device is introduced where the eigenfunction expansion method is described under the condition that the linear water wave theory is applicable. It is confirmed that these solutions give good agreement with several experimental results in our previous paper. The 2-D optimal profiles are eliminated according to conditions different from high efficiency, i.e. the minimum size per one power unit and so on. The investigations are established by many calculations, and the optimal load damping coefficient is solved. In this way we can obtain the optimal profiles considered for the construction cost.

A Constitutive Equation For Fluid-particle Flow Simulation

Abstract: A constitutive equation, developed in the framework of the averaged continuum for fluid-particle systems and already proposed in previous papers (Lalli and Di Mascio, 1997; Lalli et al., 2005), has been implemented, and some relevant test cases have been carried out. In the proposed model, the effects of fluid-particle interaction are taken into account in terms of an effective viscosity, defined by means of both Newtonian and non-Newtonian schemes, suitably matched. Actually, this rheological model can represent pure fluid, more or less concentrated suspension and fully packed sediment as well; for concentration below a proper threshold value, the mixture behaves like a Newtonian fluid, and Bingham plastic features appear for concentrations approximating maximum packing value. The numerical solution is achieved by an explicit, fractional step finite difference method. Simple 2-dimensional examples, meaningful in the framework of sediment transport problems, are shown: Bingham flow in a lid-driven square cavity; dam break; and scour below pipelines. The obtained results have been discussed in comparison with numerical and experimental data available in the literature.

Fire-Risk Modelling of Machinery Space: An Application of Approximate Reasoning Approach (Fuzzy Averaging Method) In Passenger Ship Engine Room

Abstract: This preliminary fire-safety study intends to assess the potential hazards that would affect the operation of a ship engine room. The risks associated with such hazards are quantified and ranked in order of priority and assessed for decisionmaking purposes. This paper focuses on the fire-risk evaluation of the major hazards threatening the engine room overall rather than on specific areas of the design. The main objective is to propose a framework for modelling system fire safety using an approximate reasoning approach. A case study of the risk to a passenger-ship engine room due to fire during operations is used to illustrate the application of the proposed risk assessment model.

Ultrasonic Effects On Water Flow Through Porous Media

Abstract: This paper presents the results of laboratory tests conducted to investigate ultrasonically enhanced flow rates using specially designed and fabricated equations and equipment. The influencing factors ( i) which indicated the effectiveness of ultrasound were verified for their effect on a soil matrix with flowing liquid. The test conditions involved various soil types, head losses and ultrasonic energies. Results revealed that ultrasound significantly enhanced flow rates; however, the degree of enhancement and values of the influencing factors varied over the test conditions.

Coupled Mini-TLP Barge Response In Random Seas

Abstract: The response behavior of a coupled mini-TLP / barge system in head and beam West Africa sea conditions is studied. Both of the vessels are independently moored and coupling is introduced through the connection system, consisting of a fender and breast line configuration. The dynamic response of the mini-TLP / barge system in uncoupled and coupled configurations is of particular interest. The experimental data obtained during the model test program is characterized using statistical parameters, distribution functions, correlation functions, spectrum and coherence functions. It is confirmed that the connection system reduces the horizontal vessel motions; and that the forces exerted on the fender system are quite sensitive to sea heading conditions.

Mathematical Modeling of Shock Loading of a Solid Ice Cover

Abstract: This paper deals with the linear 2-dimensional task concerning the effect of an impulsive load on a viscoelastic ice plate on the elastic basis, i.e., water. Analysis is made of the following factors: the effect of ice-plate thickness, relaxation time, variable depth of a basin, remoteness and steepness of the coastal bottom-surface slope, and choice of a function type of an instantaneous impulsive load on the amplitude of the plate deflection.

Root Bead Profiles In Hyperbaric GTAW of X70 Pipeline

Abstract: This investigation began with the goal of studying the root bead penetration profiles in hyperbaric GTAW welding of X70 pipelines. Such profiles may vary substantially depending on the wire and bas ...

Dynamic Behavior Characteristics of Clay In Wide Strain Range Based On Viscoelastic-viscoplastic Constitutive Model

Abstract: In this study, a cyclic viscoelastic-viscoplastic constitutive model based on the kinematic hardening rule was proposed. The proposed model, based on the generalized nonassociated flow rule, and the concept of the overconsolidated boundary surface as well as the nonlinear kinematic hardening rule within the context of infinitesimal strain, was calibrated and verified using cyclic triaxial tests. In order to examine the properties of the proposed model, the element simulations considering viscoelastic effects and also viscoplastic effects were studied by both an elastic-viscoplastic model and the viscoelasticviscoplastic model based on the kinematic hardening rule. From the simulation of cyclic triaxial tests, it was found that viscoelastic behavior of clay in the small strain range is an important characteristic during motion. Both the shear modulus and the hysteretic damping ratio are strain level dependent, and these can only be explained within the framework of the viscoelastic-viscoplastic constitutive model. This study reveals that the viscoelastic-viscoplastic model can describe the damping characteristics of clay accurately at small strain levels, namely cyclic softening, while the elastic-viscoplastic model cannot do so.