bluuebunny/crossref_metadata_2025_split huggingface dataset:
Likelihood-Weighted Method of General Pareto Distribution for Extreme Wave Height Estimation
Authors: Wada Ryota , Waseda Takuji 
Published: November 2013

In designing ocean structures, estimating the largest wave height it may encounter over its lifetime is a critical issue, but wave observation data is often sparse in space and time. Because of the limited data available, estimation errors are inevitably large. For an economical and robust structure design, the probability density function of the extreme wave height and its confidence interval must be theoretically quantified from limited information available.
Extreme values estimations have been made by finding the best fitting distribution from limited observations, and extrapolating it for the desired long period. Estimations based on frequentist method lack of generality in confidence interval estimations, especially when the data size is small. Another technique recently developed is based on Bayesian Statistics, which provides the inference of uncertainty. Previous studies use informative and non-informative priors and Markov Chain Monte Carlo (MCMC) simulation for estimation.
We have developed a “Likelihood-Weighted Method (LWM)” to objectively evaluate probability density function of the extreme value. The method is based on Extreme Theory and Bayesian Statistics. Our attempt is to use the ignorant prior to relate each parameter set’s likelihood to its probability. This method is pragmatic, because the numerical implementation does not require the use of MCMC.
The theoretical background and practical advantages of LWM are described. Examples from randomly produced data show the performance of this method, and application to real wave data reveals the poor estimations of previous methods that do not use the Bayesian theorem. The quantification of probability for each extreme value distribution enables the probability-weighted evaluation for inference such as maximum wave height probability density function. The new inference derived from this method is useful to change structure design methodologies of ocean structures.
Fatigue Qualification of Heavy Wall Line Pipe and Girth Weld for High Pressure Applications
Authors: Darcis Philippe P. , Mota Noe , Garcia Enrique , Marines-Garcia Israel , Quintanilla Hector M. , Kan Wan C. , Visco Tyler , Ghosh Rupak 
Published: November 2013

As the offshore oil & gas industry moves into deeper waters, more and more offshore projects, specifically the subsea, riser and flowline designs, rely on heavy wall line pipe materials. These pipe materials must be capable of operating in stringent working conditions such as high pressures, high temperatures, large deformations, fatigue loading, sour environments, etc.
Within this context, ExxonMobil Development Company and Tenaris have jointly conducted a detailed technical assessment of the mechanical and fatigue performance of the newly developed heavy wall X65 line pipe (LP) developed by Tenaris.
The main goal of such project is to evaluate and demonstrate via full-scale testing the fatigue performance of this new generation of heavy wall line pipe materials and the associated girth weld also recently developed by Tenaris. Although conclusive fatigue performance results at ID are not included in the present paper, the comprehensive test/qualification plan also includes a fatigue performance comparison at ID and OD.
The present work clearly demonstrates weldability of this Heavy Wall X65 (273 mm OD × 46 mm WT) line pipe. A narrow-groove bevel welding procedure utilizing the STT® process for the root pass and single torch GMAW process for hot-pass, fill, and cap has been successfully developed. Four full scale fatigue tests were also successfully conducted using a resonant fatigue test machine. The presented fatigue results help demonstrate a realistic level of fatigue performance achievable with this thick wall LP/girth weld technology which will help facilitate assessment of riser/flowline design feasibility in ultra-deep water applications and/or high pressures fields.
CFD Calculations for Free-Surface-Piercing Low Aspect Ratio Circular Cylinder With Solution Verification and Comparison With Experiments
Authors: Rosetti Guilherme F. , Vaz Guilherme , Hoekstra Martin , Gonçalves Rodolfo T. , Fujarra André L. C. 
Published: November 2013

The flow around free-surface piercing, low aspect-ratio circular cylinder is investigated by means of unsteady Reynolds averaged Navier-Stokes (URANS) calculations together with verification procedures and comparison with small-scale experimental and Particle Image Velocimetry results. A two-phase interface capturing model is used to handle the free-surface flow, together with k-ω SST turbulence model. We investigate physical and modeling aspects of this problem in order to gain more knowledge about the interaction of free-surface and free-end effects so that this mechanism is better understood and taken into account when modeling the problem in engineering-applied situations, such as the vortex induced motion of spars, tension-leg platforms and semi-submersibles.
The case herein presented is a captive, low aspect-ratio cylinder (L/D = 2.0) with flow velocity corresponding to Reynolds and Froude numbers (both based on diameter) of Re = 4.3 × 104 and FnD = 0.31, respectively. We will show that appreciable free-surface effects are perceived on the flow, but with dominance of free-end effects, at least in terms of forces. Furthermore, we investigate different boundary conditions that would represent this free-surface problem to show that the separation of viscous and free-surface effects is not valid in this instance. Therefore, the interaction between viscous and free-surface effects is also tangentially investigated. In order to support our conclusions, we will show forces with uncertainty estimation and field variables obtained with different modeling strategies, unveiling physical and numerical aspects of this problem.
Ultimate Strength Analysis of a River-Sea Ship Under Combined Action of Torsion and Bending
Authors: Wu Haochen , Wu Weiguo , Gan Jin , Sun Huanxiang 
Published: November 2013

For a River-Sea ship hull with a relatively low torsional rigidity due to the large deck openings, the analysis of the effect of torsion induced warping stress[1] on the structural strength as well as the ultimate hull strength interaction relationships between the torsion and vertical bending is of significant importance. The present study deals with the ultimate strength of a River-Sea ship under combined action of bending and torsion by the means of numerical and experimental research.
Based on the results, a proposed equation defining the ultimate strength interaction relationship for this River-Sea ship is given.
Thruster-Interaction Effects on a DP Semi-Submersible and a Drill Ship: Measurement and Analysis of the Thruster Wake Flow
Authors: Cozijn J. L. , Hallmann R. 
Published: November 2013

Thruster-interaction model tests were carried out in MARIN’s Deepwater Towing Tank. Detailed PIV measurements were performed of the wake flow behind the azimuthing thrusters on two different DP vessels, a Semi-submersible and a Drill Ship. The flow velocities were measured in a large number of cross sections at different distances from the thrusters. The PIV measurements provide a detailed image of the flow velocities in the thruster wake, showing the axial velocities, as well as the transverse and vertical velocity components.
First, measurements were carried out on a DP Semi-submersible (scale 1:40), which was equipped with 8 azimuthing thrusters. The results of the PIV measurements show the wake flow, interacting with nearby thrusters and the opposite pontoon of the semi-submersible. An example is shown in Figure 1 below. Deflection of the thruster wake, caused by the Coanda effect, was observed. The results for thrusters with a 7 deg downward tilt were compared with the results for thrusters with a horizontal propeller axis. Furthermore, the effect of ambient current was investigated.
Second, measurements were carried out on a DP Drill Ship (scale 1:40), which had 6 azimuthing thrusters. The results of these PIV measurements also gave insight in the wake flow behind the azimuthing thrusters and the interactions between neighbouring thrusters. An example is shown in Figure 2 below. In this case, special attention was paid to the development of the thruster wake along the vessel length, up to a distance of more than 40D downstream.
The results of the present research are used to further improve the understanding of the physics of thruster interaction effects. Furthermore, the results will serve as validation material for CFD calculations that are foreseen in the near future.
Effects of the Mooring Line Configuration on the Dynamics of a Point Absorber
Authors: Nava Vincenzo , Rajic Marin , Guedes Soares Carlos 
Published: November 2013

The aim of this paper is to study the dynamics of a floating body with characteristics comparable to a point absorber wave energy converter with different mooring systems, in geometrical configuration or in the materials. To this purpose, the dynamics of a moored buoy is investigated. The point absorber is modeled as a spherical buoy in plane two-dimensional motion, and it is studied under the action of irregular unidirectional wind-generated waves, moored to the seabed by means of one, two or three mooring lines. Two different sets of moorings are considered, and typical wires and chains used in offshore technology are considered, leading to a total of 6 case studies.
A quasi-static approach is used for modeling the restoring forces needed to keep buoy into station, using an innovative iterative procedure able to predict for each time instant and for each cable the lay down length of the cable, being each mooring line allowed to be taut or slack. Approaches in the time and frequency domains are used to obtain the system responses in intermediate waters, where these facilities are usually installed. Results for all case studies are compared both in terms of statistics of response and tensions on the top of the cable.
Swell Genesis, Modelling and Measurements in West Africa
Authors: Prevosto Marc , Ewans Kevin , Forristall George Z. , Olagnon Michel 
Published: November 2013

Swell events show a large variety of configurations when they arrive at sites off West Africa after generation and propagation of waves across the Atlantic Ocean. Within the West Africa Swell Project (WASP JIP), these different configurations have been described and discussed and the ability of numerical models to reproduce faithfully their properties has been assessed from comparisons with in-situ measurements.
During the austral winter months, swells approach West African coast from the south to south-westerly direction. These swells are generated by storms in the South Atlantic mainly between 40°S and 60°S. But during austral summer, north-westerly swells are also observed coming from North Atlantic. Typical situations of superposition of these different swells are illustrated in the paper.
In spite of a poor overlapping between numerical and in-situ measurements databases at the time of the WASP project, and of reduced durations of measurement campaigns, comparisons between in situ measurements and hindcast models permitted identification of the limitations of the different numerical models available. Three sites have been used for this study, one in the Gulf of Guinea with directional Waverider and Wavescan buoys, a second one off Namibia with a directional Waverider and one last instrumented with two wavestaffs off Cabinda (Angola).
In addition, the existence of infra-gravity waves in shallow water measurements has been investigated.
Residual Stresses Measurement in Various Joint Weldments
Authors: Ficquet Xavier , Hossain Sayeed , Kingston Ed J. 
Published: November 2013

Published experimental data from residual stress measurements are generally limited or difficult to find. Experimental data in the offshore industry are even more scarce and difficult to access to. The oil and gas industry can benefit from research done in other industries. For example, the nuclear industry has published a multitude of residual stress measurements that could be beneficial to the offshore industry, gaining more understanding and confidence in the structural integrity of critical components.
For the past year VEQTER Ltd has been developing a web access database for storing and comparing residual stress measurement and modelling results. This paper presents a comparison of through thickness residual stress measurement results that are published. The first case will show numerous measurements using different techniques on girth welded joints. Then, measurements on the temper bead welding processes which have been developed to simulate the tempering effect of post-weld heat treatment will be presented and compared with cladding. Finally, comparison of residual stress measurements on T-section plate fillet weld will be made.
Semi-Coupled Scheme for the Analysis of Floating Production Systems
Authors: Corrêa Fabrício Nogueira , Jacob Breno Pinheiro 
Published: November 2013

Traditionally, the design practice of floating production systems (FPS) employed uncoupled numerical tools where firstly the hydrodynamic analysis of the hull is performed with the lines represented by scalar models (leading to the hull motions); subsequently, these motions are prescribed at Finite Element (FE) models of the lines. Nowadays, it is widely acknowledged that coupled analysis tools should be employed for deep-water applications, considering that the overall behavior is dictated by the interaction between the hydrodynamic behavior of the hull and the structural behavior of the lines.
In this context, considering that in some situations the use of coupled formulations can lead to excessive computing times, this work presents a formulation for the analysis of FPS, referred here as the semi-coupled (S-C) strategy. Its goal is to attain faster simulations than a coupled formulation, with better accuracy than usually provided by the classical uncoupled scheme. In this strategy, for each load case a coupled static simulation is performed. From this simulation a global 6-DOF stiffness matrix that represents the array of lines is automatically calculated and added to the global matrix for the subsequent dynamic analysis to solve the equations of motion of the hull. Therefore, this dynamic analysis will adequately consider the nonlinear stiffness contribution of the lines, as well as the effect of the current profile acting on them, all evaluated at the static mean position for each load case. Case studies are presented to compare the computational costs and accuracy of this S-C strategy with coupled formulations.
VIV Excitation Competition Between Bare and Buoyant Segments of Flexible Cylinders
Authors: Rao Zhibiao , Vandiver J. Kim , Jhingran Vikas 
Published: November 2013

This paper addresses a practical problem: “Under which coverage of buoyancy modules, would the Vortex Induced Vibration (VIV) excitation on buoyant segments dominate the response?” This paper explores the excitation competition between bare and buoyant segments of a 38 meter long model riser. The source of data is a recent model test, conducted by SHELL Exploration and Production at the MARINTEK Ocean Basin in Trondheim Norway. A pipe model with five buoyancy configurations was tested.
The results of these tests show that (1) the excitation on the bare and buoyant regions could be identified by frequency, because the bare and buoyant regions are associated with two different frequencies due to the different diameters; (2) a new phenomenon was observed; A third frequency in the spectrum is found not to be a multiple of the frequency associated with either bare or buoyancy regions, but the sum of the frequency associated with bare region and twice of the frequency associated with buoyancy region; (3) the contribution of the response at this third frequency to the total amplitude is small; (4) the power dissipated by damping at each excitation frequency is the metric used to determine the winner of excitation competition. For most buoyancy configurations, the excitation on buoyancy regions dominates the VIV response; (5) a formula is proposed to predict the winner of the excitation competition between bare and buoyant segments for a given buoyancy coverage.
A Classification of Shallow Water Resonant Sloshing in a Rectangular Tank
Authors: Bouscasse Benjamin , Colagrossi Andrea , Antuono Matteo , Lugni Claudio 
Published: November 2013

A numerical and experimental analysis of sloshing phenomena (i.e. violent fluid motions inside a tank) has been conducted in shallow water regimes. A large range of experimental data from moderate to large amplitude sway motions has been considered for different filling heights. The numerical simulations, performed through a δ-SPH model, aim to cover the configurations where no experiments were available and provide an exhaustive description of the shallow-water sloshing motion.
A convergence analysis for non breaking and breaking cases has also been presented.
Boundaries Influence on the Flow Field Around an Oscillating Sphere
Authors: Mirauda Domenica , Plantamura Antonio Volpe , Malavasi Stefano 
Published: November 2013

This work analyzes the effects of the interaction between an oscillating sphere and free surface flows through the reconstruction of the flow field around the body and the analysis of the displacements.
The experiments were performed in an open water channel, where the sphere had three different boundary conditions in respect to the flow, defined as h* (the ratio between the distance of the sphere upper surface from the free surface and the sphere diameter). A quasi-symmetric condition at h* = 2, with the sphere equally distant from the free surface and the channel bottom, and two conditions of asymmetric bounded flow, one with the sphere located at a distance of 0.003m from the bottom at h* = 3.97 and the other with the sphere close to the free surface at h* = 0, were considered. The sphere was free to move in two directions, streamwise (x) and transverse to the flow (y), and was characterized by values of mass ratio, m* = 1.34 (ratio between the system mass and the displaced fluid mass), and damping ratio, ζ = 0.004. The comparison between the results of the analyzed boundary conditions has shown the strong influence of the free surface on the evolution of the vortex structures downstream the obstacle.
Application of Markov Chain to Determine the Electric Energy Supply System Reliability for the Cargo Control System of LNG Carriers
Authors: Hidalgo Erick Miguel Portugal , Silva Dennis Wilfredo Roldán , de Souza Gilberto Francisco Martha 
Published: November 2013

The natural gas industry, as well as other industrial activities, is not free from accidents, which can cause serious consequences to the integrity of people and properties. For this reason, it is necessary to develop studies to determine what are the possible causes of LNG leakage during the loading or unloading operation. This paper aims to determine the reliability of the Cargo control system electric power supply system applying Markov Chain technique. This reliability modeling tool is used because the system consists of several components in passive parallel. Based on the reliability analysis maintenance recommendations are proposed aiming at reducing system failure probability.
The method used in this paper is divided in three steps. The first step consists in carrying out a study to identify what are the components that are part of electric power supply system. The second step involves the reliability analysis of the electric energy supply system. Finally some maintenance recommendations are presented aiming at reducing system failure rate. This analysis is used for the analysis of a LNG carrier operating in a Brazilian harbor indicating the most probability failure modes of this system and the most suitable maintenance actions to avoid them.
Simulation-Length Requirements in the Loads Analysis of Offshore Floating Wind Turbines
Authors: Haid Lorenz , Stewart Gordon , Jonkman Jason , Robertson Amy , Lackner Matthew , Matha Denis 
Published: November 2013

The design standard typically used for offshore wind system development, the International Electrotechnical Commission (IEC) 61400-3 fixed-bottom offshore design standard, explicitly states that “the design requirements specified in this standard are not necessarily sufficient to ensure the engineering integrity of floating offshore wind turbines” [1]. One major concern is the prescribed simulation length time of 10 minutes for a loads-analysis procedure, which is also typically used for land-based turbines. Because floating platforms have lower natural frequencies, which lead to fewer load cycles over a given period of time, and ocean waves have lower characteristic frequencies than wind turbulence, the 10-min simulation length recommended by the current standards for land-based and offshore turbines may be too short for combined wind and wave loading of floating offshore wind turbines (FOWTs). Therefore, the goal of this paper is to examine the appropriate length of a FOWT simulation — a fundamental question that needs to be answered to develop design requirements.
To examine this issue, we performed a loads analysis of an example FOWT with varying simulation lengths, using FAST, the National Renewable Energy Laboratory’s (NREL’s) nonlinear aero-hydro-servo-elastic simulation tool. The offshore wind system used was the OC3-Hywind spar buoy, which was developed for use in the International Energy Agency (IEA) Offshore Code Comparison Collaborative (OC3) project, and supports NREL’s offshore 5-MW baseline turbine. Realistic metocean data from the National Oceanic and Atmospheric Administration (NOAA) and repeated periodic wind files were used to excite the structure. The results of the analysis clearly show that loads do not increase for longer simulations. In regard to fatigue, a sensitivity analysis shows that the procedure used for counting half cycles is more important than the simulation length itself. Based on these results, neither the simulation length nor the periodic wind files affect response statistics and loads for FOWTs (at least for the spar studied here); a result in contrast to the offshore oil and gas (O&G) industry, where running simulations of at least 3 hours in length is common practice.