bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Spray-Strake on Patrol Vessel Manoeuvrability
Authors: Muhammad Andi Haris , Maimun Adi , Yaakob Omar , Priyanto Agoes
Published: July 2009

This paper describes a design method on the spray-strake parameters of a planing hull (patrol vessel) based on manoeuvring performance in calm water condition. The method set a spray-strake mathematical model which has been developed using experimental data from towing tank. The model selects such a spray-strake parameter which has the effects on manoeuvrability characteristics. In showing these effects, some IMO manoeuvring (for turning circle and zigzag manoeuvre) have been simulated for a planing hull with the designated spray-strake parameter. The results indicate that the spray-strakes attached to the planing vessel have effects on its manoeuvring performance.
Wave Pressure Distributions Along the Midship Transverse Section of a VLCC
Authors: Shu Zhi , Moan Torgeir
Published: July 2009

The external wave pressure distributions along the transverse section in the midship region of a VLCC are evaluated in this paper. The commercial hydrodynamic code WASIM issued by DnV has been adopted to perform the hydrodynamic computation. The ship hulls have been discretized with coarser and finer mesh to investigate the effect of panel size on the hydrodynamic pressures. It is found that the difference between these two mesh finenesses is small. It is also found that the roll damping has a significant influence on the wave pressure of vessel especially in beam sea. A sensitivity analysis is carried out in the sense of assessing the influence of the roll damping on the wave pressure. Finally, the long term prediction of the wave pressure has been compared for different roll damping values.
Experimental and Numerical Analysis of the Behavior of a Monocolumn With a Moonpool
Authors: Torres Fernando G. S. , Alho Alexandre T. P. , Sales Joel S. , Sphaier Sergio H. , Nishimoto Kazuo
Published: July 2009

Although in most vessels the usual function of a moonpool is a passage for production lines, equipments and divers, the moonpool may be used as a device to reduce the vertical motion of floating structures as it can be observed in published studies. From these published works one may say that changes of the moonpool’s internal geometry modify the behavior of vertical oscillation of the water inside the moonpool. Looking for a “hydrodynamic design of the monocolumn-moonpool”, Petrobras has carried out a set of numerical and experimental tests with monocolumn hulls provided with moonpool. Some of these results have been published by the authors in the literature. To increase the comprehension of the dynamics of the joint motion, fluid and structure, numerical results were obtained using a commercial code based on the potential theory and a commercial code based on the solution of Navier-Stokes equations. The paper presents results of these experiments and analyzes the behavior of the coupled motions of mono-columns and the vertical motion of the water inside the moonpool, comparing the experimental and the numerical results.
The Effect of Boundary Proximity Upon the Wake Structure of Horizontal Axis Marine Current Turbines
Authors: Myers L. E. , Bahaj A. S. , Rawlinson-Smith R. I. , Thomson M.
Published: July 2009

An experimental and theoretical investigation of the flow field around small-scale mesh disk rotor simulators is presented. The downstream wake flow field of the rotor simulators has been observed and measured in the 21m tilting flume at the Chilworth hydraulics laboratory, University of Southampton. The focus of this work is the proximity of flow boundaries (seabed and surface) to the rotor disks and the constrained nature of the flow. A three-dimensional Eddy-viscosity numerical model based on an established wind turbine wake model has been modified to account for the change in fluid and the presence of a bounding free surface. This work has shown that previous axi-symmetric modelling approaches may not hold for marine current energy technology and a novel approach is required for simulation of the downstream flow field. Such modelling solutions are discussed and resultant simulation results are given. This work has been conducted as part of a BERR-funded project to develop a numerical modelling tool which can predict the flow onto a marine current turbine within an array. The work presented in this paper feeds into this project and will eventually assist the layout design of arrays which are optimally spaced and arranged to achieve the maximum possible energy yield at a given tidal energy site.
A Study of General Buckling Strength of Ring-Stiffened Cylindrical Shells Under External Pressure and Effective Breadth of Ring-Stiffeners
Authors: Yoshikawa Takao , Yoshimura Kenshi
Published: July 2009

Recently, we usually use the numerical method to calculate the buckling strength of plate and shell structures. But at early design stage, we even now use the theoretical or some simplified formula to check the buckling strength. For most of typical plate and shell structures, the precise formulas of buckling strength have been developed. For the general buckling strength of ring-stiffened cylindrical shell under external pressure in elastic range, the several formulas have been also proposed, but the accuracy of those are hardly adequate. In those formulas, the effective breadth of ring-stiffeners is introduced, but the exact value has not been presented. In this paper, the calculation procedure of effective breadth is developed based on mechanical consideration of pre-buckling deformation, and the modified simplified formula of general buckling strength of ring-stiffened cylindrical shells under external pressure in elastic range is proposed. Finely, the accuracy of proposed formula is confirmed by comparison with numerical results.
Analysis of Submarine Pipeline Scour Using Large-Eddy Simulation of Dense Particle-Liquid Flows
Authors: Zamankhan Piroz , Doolatshahi Amir Ali
Published: July 2009

Using large-eddy simulation technique for dense particle-fluid flows, the current-induced scour is predicted for both the mono- and bi-dispersed systems below a horizontal submarine pipeline exposed to unidirectional flow. The simulations are four-way coupled, which implies that both solid-liquid and solid-solid interactions are taken into account. Particles are assumed to behave as visco-elastic solids during interactions with their neighboring particles, and their motion are predicted by a Lagrangian method. The inter-particle normal and tangential contact forces between particles are calculated using a generalized Hertzian model. The other forces on a particle that are taken into account include gravitational, pressure gradient force accounting for the acceleration of the displaced liquid, the drag force resulting from velocity difference with the surrounding liquid, and the Magnus and Saffman lift forces. The predicted scour profiles for monodispersed system are found to compare favorably with the laboratory observations. For the bi-dispersed system, a seepage flow underneath the pipe (which is a major factor to cause the onset of scour below the pipeline) is found to be weakened using an appropriate size for the sand bed. This finding highlights the importance of the bed particle size distribution on the onset of scour below the pipelines.
Response of a Novel Beam-to-Column/Brace-to-Frame Connection to Monotonic and Cyclic Shear Loading
Authors: Khonsari S. V. , England G. L. , Abazarsa F.
Published: July 2009

A new universal structural joint was developed. While in bending it has a high rotational capacity, which can be accompanied by large bending stiffness and strength, in shear, it also has a very high shear deformation capacity, which can again be accompanied with large shear stiffness and strength. While the former characteristic makes it a good candidate for being used as a beam-to-column joint, the latter makes it highly applicable in connecting braces of a braced frame to the frame members. The experimental study carried out previously on this joint, concentrated on the performance of its steel specimens under ‘monotonic’ shear loading as well as that of its aluminium specimens under both ‘monotonic’ and ‘cyclic’ shear loading. The current study, however, comprises the experimental investigation into the behaviour of the mild steel specimens of this joint under ‘monotonic’ and ‘cyclic’ shear loading. As expected, the monotonic shear loading of the specimens of this new joint resulted in great amount of shear deformation, in contrary to basically all currently-used structural connections which lack any appreciable shear deformation capacity. Moreover, the specimens tested under cyclic shear loading also performed very well. The hysteresis loops of these specimens were ‘stable’ and ‘well-rounded’, implying large amount of energy dissipation in each cycle. Such very ductile response of the connections in shear is expected to be exploited in various circumstances in offshore as well as onshore structures to result in a ductile overall behavior of the structure.
Fatigue Strength Assessment of Aged Floating Production System
Authors: Ren Huilong , Li Chenfeng , Li Hui , Feng Guoqing
Published: July 2009

Fatigue is one of the most frequent causes of failure for offshore structures subjected to environmental loads, especially aged structures. An effective method for calculating the fatigue damage of aged platform under regular wave is presented. Three-dimensional source distribution theory in the frequency domain is used to calculate fatigue load of FPS in regular wave. The hot spot stress in the welded knuckles is calculated on the basis of a detailed finite element model. Based on the S-N curve and the Miner linear accumulative damage theory, the calculation process of fatigue damage of components was simplified. The calculated results show that this computing method is convenient and accurate. It is suitable for engineering application.
Generation and Propagation of Long-Crested Finite-Depth Surface Waves: Comparison Between the Corresponding Results From Numerical Models and a Wave Tank
Authors: Zaman M. Hasanat , Parsons Wade , Nwogu Okey , Choi Wooyoung , Baddour R. Emile , Mak Lawrence
Published: July 2009

The evolution of long-crested surface waves subject to side-band perturbations is investigated with two different numerical models: a direct solver for the Euler equations using a non-orthogonal boundary-fitted curvilinear coordinate system and an FFT-accelerated boundary integral method. The numerical solutions are then validated with laboratory experiments performed in the NRC-IOT Ocean Engineering Basin with a segmented wave-maker operating in piston mode. The numerical models are forced by a point measurement of the free surface elevation at a wave probe close to the wave-maker and the numerical solutions are compared with the measured time-series of the surface elevation at a few wave probe locations downstream.
On the Dynamics of Salvador Canyon, Bahia – Brazil (13°S)
Authors: Amorim Fabiola N. , Cirano Mauro , Soares Ivan D. , Campos Edmo J. D.
Published: July 2009

The dynamics of a large (∼ 22 km wide) and deep (∼ 600 m deep) submarine canyon was investigated using data from three ADCP transects perpendicular to the coast: i) at the canyon axis and ii) at the northern and southern adjacent shelves, as well as high resolution CTD data collected at various hydrographic stations located in each transect. In order to represent the seasonal cycle, the region was seasonally covered during a whole year. Preliminary results show that the wind forcing exerted a great influence on the coastal circulation at the northern and southern canyon’s adjacent shelves. At the canyon’s head, a topographically trapped cyclonic circulation was observed, being intensified during downwelling favorable winds. Its cyclonic circulation affected the coastal circulation at northern adjacent shelf, which showed a surface reversal in comparison to the preferential shelf flow. At depths around 200 m, and below the canyon rim, the flow field was also disturbed by the canyon topography. Vertical hydrographic profiles confirm the strong wind influence to the local dynamics. During the winter season a deeper isothermal layer (∼200 m deep) was observed, caused by the intense mixing promoted by stronger winds, due to the influence of cold front passages.
Statistics of Extreme Events in Airgap Measurements
Authors: Naess A. , Stansberg C. T. , Gaidai O. , Baarholm R. J.
Published: July 2009

The paper presents a study of the extreme value statistics related to airgap measurements on a scale model of a semisubmersible platform subjected to random waves in a wave basin. Relative wave elevation records corresponding to totally 24 hours storm duration are considered, made up by 8 × 3-hours realizations. The focus is on a comparison of two alternative methods for the prediction of extreme values from finite recordings at two different locations at the platform. One is a standard method used in the wave basin, making use of a Weibull-tail fitting procedure. The other is a novel method based upon the level up-crossing function combined with an optimization procedure that allows prediction at extreme response levels. Similar results are obtained in the mean values by the two methods, while the latter shows less variability in the predictions from single 3-hours records.
Finite-Memory Nonlinear System Modelling of Offshore Structures
Authors: Najafian G. , Zaki N. I. Mohd
Published: July 2009

Offshore structures are exposed to random wave loading in the ocean environment, and hence the probability distribution of the extreme values of their response to wave loading is of great value in the design of these structures. Due to nonlinearity of the drag component of Morison wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian; therefore, simple techniques for derivation of their extreme response probability distributions are not available. However, according to a recent paper, in the absence of current, the response of an offshore structure exposed to Morison wave loading, can be approximated by the response of an equivalent finite-memory nonlinear system (FMNS). These models can then be used, with great efficiency, to determine the probability distribution of response extreme values. In this paper, the progress made so far in extending these FMNS models to account for the effect of current on response is discussed.
Wave Heights in a 4D Ocean Wave Field
Authors: Liu Paul C. , Schwab David J. , Wu Chin H. , MacHutchon Keith R.
Published: July 2009

This paper presents a preliminary examination and analysis of a small suite of 4-D wave data to explore what new insight or inference we can garner — particularly toward the realm where conventional approaches have not been traversed. While we caught a few glimpses that might indicate a need for new conceptualizations, it by no means to negates the vast positive contributions the conventional approaches have been made in the past century. We feel it is timely to encourage further 4-D ocean wave measurement and thereby facilitate fresh new states of study and understanding of ocean waves.
Determination of Riser Tensioner Properties From Full-Scale Data
Authors: Gupta Himanshu , Nava Vincenzo , Banon Hugh , Gkaras Vasileios , Spanos Pol
Published: July 2009

Tensioners are used for supporting various kinds of production and drilling risers on offshore platforms. All permanent production facilities (such as TLP’s and Spar’s) have employed tensioners to support their dry tree production risers. The key design parameters for the tensioners design are stiffness, stroke and friction properties. Full scale onshore testing of production riser tensioners to determine tensioner properties is difficult due to the magnitude of the loads and pressures required for typical deepwater risers, thus designers frequently rely on theoretical values or small scale tests. This paper presents a determination of riser tensioner properties from full scale offshore observations made on the Holstein Spar production facility during 2005 hurricance season. The paper presents a model for the tensioner system that relates the properties needed by platform and riser designers to the physical tensioner systems. The methodology and results from extraction of parameters from the full scale data is presented. The results presented in this paper will provide a better insight to the designers of such systems.
Spatial Variation of Drag on Long Cylinders in Sheared Flow
Authors: Jhingran Vikas , Jaiswal Vivek , Vandiver J. Kim
Published: July 2009

A method is described for measuring the local drag coefficient on a long cylinder which exhibits vortex-induced vibration (VIV). Results are shown from a field experiment in which a long flexible pipe was instrumented with two-hundred and eighty fiber optic strain gauges. The measured local drag coefficients are compared to a commonly used drag coefficient prediction formula. The formula is shown to be useful as a tool for predicting the average drag coefficients for the whole cylinder but is not able to accurately capture local variation in CD. The local CD measurements also reveal the location of VIV source regions.
Wave Energy Assessments in the Coastal Environment of Portugal Continental
Authors: Rusu Eugen , Guedes Soares C.
Published: July 2009

The potential for wave energy extraction can be obtained from the analysis of the wave climate which can be determined with numerical models. The wave energy devices can be deployed in offshore, nearshore and shoreline. From this reason, it is important to be able to assess properly the spatial distribution of the wave energy in various locations from the offshore to the coastline in a specific area. The methodology proposed here considers a SWAN based wave model system focusing in the Portuguese continental coastal environment from deep water towards the nearshore. An analysis of the average and high energetic conditions was first performed for a ten-year period, between 1994 and 2003, considering the most relevant in situ measurements available in the Portuguese nearshore. In this way both the average and high energetic conditions corresponding to the Portuguese continental costal environment have been properly defined. For the most relevant average wave conditions, SWAN simulations were performed in some medium resolution areas covering the northern and central parts of Portugal continental, which are traditionally considered richer in wave power resources. The present work allows the identification of some locations in the continental coastal environment of Portugal with greater potential from the point of view of wave power resources. An important observation is related to the fact that the wave power depends on the product between the energy density spectrum and the group velocity of waves. This means that, although the significant wave height is a relevant parameter when assessing the wave power in a specific site, a location having in general higher wave heights is not necessarily also the richest in wave power.
Worst Sea–Best Sea Wave Group Spectra From Random Sea States
Authors: Fernandes Antonio Carlos , Hennig Janou , Maia Marcio Domingues , Cozijn Hans , Sales Joel Sena
Published: July 2009

The paper presents a proposition called the worst sea-best sea (WS-BS) method that intends to identify wave group spectra that yield time series that lead, respectively, to the highest wave group excitation (worst sea) and the smallest wave group excitation (best sea) to the second order slowly varying response for a floating structure model submitted to random waves in a wave basin. The methodology is based on the estimation theory. Based on the fact that even with the same wave spectrum it is possible to devise different wave group spectrum, the paper proposes a method to identify the WS-BS time series. Both simulations and concrete model testing are shown indicating the feasibility of the approach.
Residual Strength Assessment of Semi-Submersible Platform Column Due to Supply Vessel Collision
Authors: do Amaral Amante Diogo , Trovoado Leandro , Estefen Segen F.
Published: July 2009

The new structural concept of semi-submersible oil and gas production platforms is based on column square cross sectional area. A successful design is strongly dependent on optimized column structural arrangement. The columns are constituted of longitudinal stiffened panels having their ultimate strength characterized by buckling under compressive loads. A structural damage which may occur in these columns is the effect caused by supply vessel collision. Normally, such damage is in the form of local dents. The aim of the present paper is to evaluate the ultimate buckling residual strength of typical column of semi-submersible platforms damaged by a supply vessel collision. The buckling analysis was validated using a finite element model considering geometric and material nonlinearities considered in previous correlation studies with small scale experiments. Residual strength results from the damaged column are compared with the ultimate strength of equivalent intact column to estimate the safety margin associated with the column structural capability after supply vessel collision.
The Effect of Pitting Corrosion on the Strength Capacity of Steel Offshore Structures
Authors: Abdel-Ghany R. , Saad-Eldeen S. , Leheta H.
Published: July 2009

In steel offshore platforms used for drilling and production of oil and gas, failures of structural steel members can occur due to pitting corrosion and corrosion fatigue at welded nodes where the brace members are joined. These nodes are areas of high stress concentration due to the complex geometry of these locations. The purpose of this paper is the application of FEM structural analysis to a model of K-T joints of a jack-up rig leg to explore the effect of random pitting corrosion on its strength capacity. The results are presented along with those previously published for the same model for the case of uniform corrosion.
Analysis of Bottom Damage Caused by Ship Grounding
Authors: Alsos Hagbart S. , Amdahl Jo̸rgen
Published: July 2009

A last resort for a ship in disabled condition is to set it aground in a controlled manner for later salvage. A prerequisite for proper handling of this action are tools for reliable, structural analysis with respect to local damage and global hull integrity. The paper addresses advances made in the course of an EU funded project. An important issue is the size and shape of the sea floor in grounding analysis. As little information is generally available, the paper discusses sea floor shape in view of the resistance to indentation of the ship bottom and characteristic scenarios are defined. The resistance to indentation is analyzed for a typical oil tanker for various positions of contact. The interaction between global hull bending and local indentation is also addressed. In dynamic grounding a large part of the ship bottom or the entire bottom may be subjected to damage and a complete nonlinear finite element simulation is very demanding, A new, simplified approach is presented, where the horizontal and vertical components of the contact forces are derived from numerical simulation of lateral indentation, only. Very good agreement has been obtained between the simplified procedure and complete numerical simulation. Force indentation relationships are used as input to rigid body simulation of ship motion. The method is illustrated by a numerical simulation of damages to double bottom of a shuttle tanker. As reliable prediction of rupture initiation and crack growth in cargo tanks is essential with respect to oil spill estimation, advanced rupture criteria have been developed and implemented in the numerical tools. For verification purposes lateral indentation tests of unstiffened and stiffened plates have been carried out in laboratory. Initial results are being presented in the paper.
Compressive Tests on Long Continuous Stiffened Panels
Authors: Gordo Jose´ Manuel , Guedes Soares C.
Published: July 2009

Results are presented of 8 tests on long stiffened panels under axial compression until collapse. The specimens are three bay panels with associated plate made of very high tensile steel S690. Four different configurations are considered for the stiffeners, which are made of mild or high tensile steel for bar stiffeners and mild steel for ‘L’ and ‘U’ stiffeners. The influence of the stiffener’s geometry on the ultimate strength of the stiffened panels under compression is analyzed. This series of experiments belong to an extended series of tests that include short and intermediate panels which allows analyzing the effect of space framing on the strength of stiffened panels.
Long Term Failure Probability Calculation Method Applied to Riser Design
Authors: Perdrizet Timothe´e , Averbuch Daniel
Published: July 2009

A time efficient methodology is described to evaluate the non linear extreme response of a riser connected to a FPSO subjected to wave induced loads in a stationary sea state. It is extended to cover all sea states and thus to assess the long term failure probability of the riser. The short term procedure is based on a classic time variant reliability method. It uses an approximation of the mean outcrossing rate, which is computed with the time invariant reliability method FORM (First Order Reliability Method).
Centerwell Water Motions and Hydrodynamic Loading Using Viscous Flow Calculations
Authors: Holmes Samuel , Gebara Joseph , Magee Allan
Published: July 2009

Most Spar platforms have a wet centerwell which provides a termination point for umbilicals and risers. The column of water in the centerwell is a dynamic system which can be excited by the wave action around the Spar as well as the platform’s own motions. When the exciting frequencies are close to the natural frequency of the water column, the vertical motion of the water in the centerwell can become large in large seastates. This might damage structures within the centerwell. A natural response to this problem is to restrict the fluid flow at the bottom of the centerwell by adding a plated structure to partially close the opening. The remaining open area in the centerwell determines the amount of damping as well as the loads on the plating which can be quite large in heavy seas. The problem addressed in this paper is the determination of the appropriate open area in the centerwell plate that will control the fluid vertical motion without requiring expensive reinforcements to the plating beyond the riser guide structure already present. Traditional design tools based on potential flow models appear to perform poorly for this problem because they do not model the viscous damping in the flow correctly. In this paper we use a Navier-Stokes solver to study the centerwell motions and centerwell plate loads for three centerwell plate geometries. It is found that the Spar motions and the free surface waves need to be included in these simulations. The centerwell water motions and centerwell plate loads are compared with those measured in a scale model experiment. Full-scale calculations are also carried out to determine the corresponding centerwell plate loads and centerwell water motions to assess scale effects.
The Influence of Foundation Modeling Assumptions on Long-Term Load Prediction for Offshore Wind Turbines
Authors: Bush Erica , Agarwal Puneet , Manuel Lance
Published: July 2009

In evaluating ultimate limit states for design, time-domain aeroelastic response simulations are typically carried out to establish extreme loads on offshore wind turbines. Accurate load prediction depends on proper modeling of the wind turbulence and the wave stochastic processes as well as of the turbine, the support structure, and the foundation. One method for modeling the support structure is to rigidly connect it to the seabed; such a foundation model is appropriate only when the sea floor is firm (as is the case for rock). To obtain realistic turbine response dynamics for softer soils, it is important that a flexible foundation is modeled. While a single discrete spring for coupled lateral/rotational motion or several distributed springs along the length of the monopile may be employed, a tractable alternative is to employ a fictitious fixed-based pile modeled as an “equivalent” cantilever beam, where the length of this fictitious pile is determined using conventional pile lateral load analysis in combination with knowledge of the soil profile. The objective of this study is to investigate the influence of modeling flexible pile foundations on offshore wind turbine loads such as the fore-aft tower bending moment at the mudline. We employ a utility-scale 5MW offshore wind turbine model with a 90-meter hub height in simulations; the turbine is assumed to be sited in 20 meters of water. For a critical wind-wave combination known to control long-term design loads, we study time histories, power spectra, response statistics, and probability distributions of extreme loads for fixed-base and flexible foundation models with the intention of assessing the importance of foundation model selection. Load distributions are found to be sensitive to foundation modeling assumptions. Extrapolation to rare return periods may be expected to lead to differences in derived nominal loads needed in ultimate limit state design; this justifies the use of flexible foundation models in simulation studies.
Benchmark Studies Using Virtual Model Basin for Resistance and Diffraction: A RANS CFD Approach
Authors: Sasanapuri Balasubramanyam
Published: July 2009

Virtual Model Basin (VMB) developed based on RANS CFD Approach along with VOF model to simulate free-surface has been used to perform benchmark studies and the results are presented in this paper. The VMB based on general purpose CFD solver ANSYS FLUENT has been used to simulate resistance and diffraction problems for a Navy surface combatant hull and the results are validated against experimental data. The resistance simulations are done to assess two turbulence models and best among the two is used to solve the diffraction problem. The validation results suggest that the VMB approach reproduces the flow features, forces and moments accurately.
Computer Simulation Technique to Reduce Resulting Residual Stresses and Distortion in High Strength Thin Butt Welded Ship Panels
Authors: El-Afandy Moustafa M. , El-Gammal Moustafa M. , Rashwan Ahmed M.
Published: July 2009

Welding of thin-plate ship structures often results in warping of finished fabricated panels. Some manufacturers use preheating and/or pre-stressing the plates during assembly or post heating after assembly as a mitigation method to reduce final product distortion with variably satisfactory results. The current paper aims to study the effect of tensile force application (on the plates undergoing welding) on the residual stresses and distortion behaviours of butt welded thin ship panels using the finite element method. The effects on the butt welded thin panel applying several components of different magnitudes of tensile forces before and during welding are shown. Those external forces are released after the welded joint has reached the room temperature. The stretching force leads to a reduction in the longitudinal residual stresses, and converts part of the tensile components to increase the compressive ones. All those diagnostics will demolish the welded plate distortion making it close to zero.
Nonlinear Crest Height Distribution in Three-Dimensional Ocean Waves
Authors: Arena Felice , Ascanelli Alfredo
Published: July 2009

The interest and the studies on nonlinear waves are increased recently for their importance in the interaction with floating and fixed bodies. It is also well known that nonlinearities influence wave crest and wave trough distributions, both deviating from Rayleigh law. In this paper a theoretical crest distribution is obtained taking into account the extension of Boccotti’s Quasi Determinism theory, up to the second order for the case of three-dimensional waves, in finite water depth. To this purpose the Fedele & Arena [2005] distribution is generalized to three-dimensional waves on an arbitrary water depth. The comparison with Forristall second order model shows the theoretical confirmation of his conclusion: the crest distribution in deep water for long-crested and short crested waves are very close to each other; in shallow water the crest heights in three dimensional waves are greater than values given by long-crested model.
Radial Impulse Turbine for Wave Energy Conversion: A New Geometry
Authors: Pereiras B. , Castro F. , El Marjani A. , Rodriguez M. A.
Published: July 2009

The Oscillating Water Column system (OWC) is an interesting concept for ocean wave energy extraction. Several kinds of air turbines have been used for pneumatic energy conversion to mechanical energy. The Wells turbine has been used widely in OWC plants. However, as an alternative the self-rectifying turbine called Impulse turbine has been studied during the last years. We are interested in the radial version of the Impulse turbine, which was initially proposed by McCormick. A former research work aimed to improve the knowledge of the local flow behaviour and the prediction of the performances for this kind of turbine has been carried out using CFD (FLUENT®). The objectives of that work were connected mainly to the elaboration of a suitable 3D model for air flow simulation in a radial Impulse turbine. Model validation was conducted through a comparison with available experimental results. In the present, the objective is, using the numerical model, to develop a new radial impulse turbine geometry that gets better performances than the original one. This new turbine geometry will be exploited next in a project for an OWC of 250 kW. In this paper we describe the flow behaviour and the performances of this new turbine. For that, we study the Torque and Input coefficients, the losses and flow direction in the turbine elements.
Non-Linear Time Domain Simulation of Dynamic Instabilities in Longitudinal Waves
Authors: Ribeiro e Silva S. , Guedes Soares C.
Published: July 2009

A time domain numerical simulation method is developed to determine ship motions in six degrees-of-freedom and to detect dynamic instabilities in both regular and irregular longitudinal waves. The basic approach of the simulation program involves computation of hydrodynamic coefficients of added mass and damping, restoring coefficients and diffraction and Froude-Krylov excitation forces at each step in time according to the instantaneous waterline and vessel position, using a strip theory method and a pressure integration technique along the segments. This paper briefly describes the computational technique utilized and makes comparisons between numerical and experimental roll damping data to infer about its influence on roll amplitude under parametric rolling conditions. An investigation into the dynamic stability in waves of a container vessel example.
Plastic Buckling and Collapse of Tubes Under Bending and Internal Pressure
Authors: Limam A. , Lee L.-H. , Corona E. , Kyriakides S.
Published: July 2009

This paper presents results from a recent combined experimental-analytical study of the inelastic response and the sequence of events that lead to collapse of pipes bent under internal pressure. Experimental results from stainless steel 321 seamless tubes with D/t of 52 are reported. The tubes were loaded by pure bending at fixed values of pressure ranging from zero to a value that corresponds to 0.75 times the yield pressure. The moment-curvature response is governed by the inelastic characteristics of the material. Bending induces some ovalization to the tube cross section while, simultaneously, the internal pressure causes the circumference to grow. Following some inelastic deformation, small amplitude axial wrinkles appear on the compressed side of the tube, and their amplitude grows stably as bending progresses. Eventually, wrinkling localizes, causing catastrophic failure in the form of an outward bulge. Pressure increases the wavelength of the wrinkles as well as the curvature at collapse. The onset of wrinkling is established by a custom bifurcation buckling formulation. The evolution of wrinkling and its eventual localization are simulated using a FE shell model. The material is represented as an anisotropic elastic-plastic solid using the flow theory, while the models are assigned initial geometric imperfections that correspond to the wrinkling bifurcation mode. It will be shown that all aspects of the observed behavior including the failure by localized bulging can be successful reproduced by the models developed.
Development of High Steel Grade Seamless X100 Weldable
Authors: Izquierdo Alfonso , Quintanilla Hector , Richard Gilles , Anelli Ettore , Mannucci Gianluca , Fonzo Andrea
Published: July 2009

The technological evolution in the offshore sector points out a trend towards an increasing use of high strength steels (grade 80ksi and higher), for both pipelines and risers. Pipeline specifications for deepwater offshore fields demand developments in design criteria (i.e. limit state design), welding, installation, and laying technologies. As long as the market goes deeper in offshore exploration and production, the market trend is to use heavier pipes in steel grade X65/X70 and some technological limits from several fronts are faced and more attractive becomes for the market to have a lighter high strength 100ksi seamless steel grade. The joint industrial program (JIP), termed “Seamless 100 ksi weldable” launched by Tenaris in order to address the complex design issues of high strength Q&T seamless pipes for ultra deep water applications has been finalized. The 100ksi steel grade has been achieved in two wall thickness 16 mm and 25 mm. The main results from both phase I devoted to the development and production of seamless pipes with minimum 100ksi and phase II devoted to evaluate the high strength seamless pipe weldability will be addressed in this paper. Main microstructural features promoting the best strength-toughness results obtained from phase I and the results from phase II, where the heat affected zone (HAZ) characterization made using stringent qualifying configuration such as API RP2Z and the promising results after qualifying the girth welds simulating a typical offshore operation and the Engineering Critical Assessment for installation will be addressed. The results from this development are of interest of all oil & gas companies who are facing as an output from the design project analysis the need to have high strength seamless pipes.
An Integrated Wave Monitoring Network for the Hellenic Seas
Authors: Soukissian Takvor , Morfis Ioannis , Stylianos Alexakis
Published: July 2009

In this paper, an integrated wave monitoring network for the Hellenic Seas is presented and its components are described. The network is comprised from wave buoys and nautical X-band radars covering most of the important areas of the Hellenic seas as regards complicated wave propagation patterns. The buoy wave monitoring network is part of the POSEIDON system for the monitoring, information and forecasting of the state of the Hellenic Seas. The X-band wave radar network is under development within the context of the OKEANOS project in co-operation with the Hellenic Navy Hydrographic Service. The measuring techniques and principles of both types of devices (buoys and X-band radars) are also described. The wave measurements from the network are mainly used for improving the operational wave forecasts provided by the Hellenic Centre for Marine Research (HCMR), for individual planning purposes and in the design and construction of coastal and offshore structures. Two experimental coastal sites have been selected for comparing the radar wave data with data from directional wave buoys and the WAM wave model.
Comparison of Safety Factors Used for Determination of Remaining Life of Girth Welded Pipelines Using Fracture Mechanics and Conventional Endurance Assessment Techniques
Authors: Smith Alan , Osman Mohamad
Published: July 2009

This paper compares the effect of using commonly applied safety factors for the estimation of the remaining life of welded components. Two different design philosophies are compared, conventional design (based on the S-N approach) and a fracture mechanics (Paris Law) approach. Both methods use a safety factor based on two standard deviations of the mean experimental data. However, there is no theoretical reason as to why the two methods should give similar degrees of conservatism when used to assess the remaining life of structures or pipelines. In this study a fracture mechanics approach is used to predict a hypothetical initial flaw size such that the mean life of the welded component (in this case a girth welded pipe) is compatible with that predicted from mean S-N data and the effect of growing the fatigue flaw at various rates (based on selected numbers of standard deviations of the Paris Law curve) is used to illustrate the effect of the safety margins used in both methods. From a comparison of the predicted life with the S-N design curve it is possible to compare the conservatism of the two techniques and highlight inconsistencies between the two approaches. The paper then proceeds to describe how the use of the S-N approach and the fracture mechanics approach is incorporated into the procedure for assessing the criticality of welded joints under operational conditions as given in DNV OS F101 Appendix A.
Vessel Steam Pipe Rupture Force on Internals
Authors: Moody F. J.
Published: July 2009

A postulated steam pipe rupture in a water-moderated nuclear power plant would cause a sonic discharge from the rupture and the formation of a large-amplitude step decompression, which would travel through the pipe to the reactor vessel. When the decompression arrived at the reactor, it would propagate into the vessel where it would exert decompression forces on the internal steam dryer and other surfaces. The magnitude of the decompression can be 40 bars or more when first beginning in the steam line. If the initial magnitude entered the vessel as a step, even though attenuated by expanding, damage could occur to the dryer components. However, solution of the propagation equations show that a step decompression in a compressible fluid spread as it travels so that its arrival at the vessel is a ramp disturbance in time rather than a step. This feature causes a decompression force of smaller impulse to be exerted on vessel internal structures. This study presents an analytical model for quickly estimating conservative decompression forces on a flat wall structure (like a dryer surface) inside a reactor caused by a steam line rupture. It is shown that longer steam line lengths and greater distances between the vessel wall and the dryer surface result in smaller transient load magnitudes. Also, smaller dryer surface dimensions cause the dryer force duration to be decreased.
An Improved Structure-Dependent Explicit Method for Structural Dynamics
Authors: Chang Shuenn-Yih , Huang Chiu-Li
Published: July 2009

An explicit method is presented herein whose coefficients are determined from the initial structural properties of the analyzed system. Thus, it is structure-dependent. This method has a great stability property when compared to the previously published method [6], which is unconditionally stable for linear elastic and any instantaneous stiffness softening systems while it only has conditional stability for an instantaneous stiffness hardening system. The most important improvement of this method is that it has unconditional stability for general instantaneous stiffness hardening systems in addition to linear elastic and instantaneous stiffness softening systems. This implies that a time step may be selected base on accuracy consideration only and the stability problem might be neglected. Hence, many computational efforts can be saved in the step-by-step solution of a general structural dynamic problem, where the response is dominated by the low frequency modes and the high frequency responses are of no great interest, when compared to an explicit method, such as the Newmark explicit method, and an implicit method, such as the constant average acceleration method.
The Evaluation of the 3-D Residual Stress Field Due to Hydraulic Autofrettage in a Finite Length Cylinder Incorporating the Bauschinger Effect Factor Based on the “Zero Offset Yield Stress”
Authors: Perry J. , Perl M.
Published: July 2009

The autofrettage process increases the ability of a pressurized cylinder to withstand higher pressure values prior to the onset of yielding. The yield-pressure of an autofrettaged cylinder is strongly affected by the Bauschinger Effect (BE) that results in a reduction of the yield stress in compression. This reduction, which is measured by the Bauschinger Effect Factor (BEF), highly depends on the exact determination of the yield point. The present analysis suggests a new 3-D axisymmetric model for solving the residual stress field in a hydraulically autofrettaged finite-length cylinder including the BEF curve evaluated by using the newly proposed concept of the “zero offset” yield point definition. The numerical model is validated experimentally using axial and tangential strain gauges attached to a thick walled cylinder undergoing hydraulic pressurization. The experimental set-up enables continuous strain measurements vs. the increasing pressure. The calculated strains and displacements as well as the initial yield-pressure were found to be in very good agreement with the measured values.
An Analytical Solution for Calculating Stresses on Lap Patches Used on Pipes and Pressure Vessels
Authors: Nadarajah Chithranjan , Spinelli Guido L.
Published: July 2009

Lap patch repairs are used regularly in petrochemical industries as temporary repairs for short term operations. In this paper a simple methodology is proposed on how to calculate the stresses on a lap patch using simple analytical approximation. The simple analytical results are compared with finite element results and they are found to be in reasonable agreement.
Response Spectrum Method for Submerged Structures
Authors: Ly B. L. , An Y.
Published: July 2009

A submerged structure has virtual mass matrix in the inertial term and wet mass matrix in the driving term in the equation of motion, whereas a dry structure has the same mass matrix in these two terms. If a finite element code for dry structures is used to analyze a submerged structure, the results should be adjusted. Result for a Single Degree of Freedom (SDOF) system is scaled by the ratio of wet mass to virtual mass. Results for a Multiple Degree of Freedom (MDOF) system are adjusted by scaling the Floor Response Spectra (FRS) at each modal frequency by a ratio of modal work done in that mode by the wet mass matrix to that by the virtual mass matrix. The adjustment factor can be estimated as a product of a ratio of participation factors and a ratio of modal masses. Both the participation factors and the modal masses are computed by using the wet mass matrix, and then by using the virtual mass matrix, respectively. This method provides an alternative way to assess the margin in the design based on the current FRS method that does not distinguish the difference between the two mass matrices. More studies should be carried out to implement this method in specific practical applications.