bluuebunny/crossref_metadata_2025_split huggingface dataset:
CFD Simulation for Vortex Induced Motions of a Multi-Column Floating Platform
Authors: Tan Jaime Hui Choo , Magee Allan , Kim Jang Whan , Teng Yih Jeng , Ahmad Zukni NorBahrain 
Published: November 2013

The potential of vortex induced motion (VIM) in multi-column floating platforms such as semi-submersibles and tension leg platforms (TLPs) is well-acknowledged although the industry guidelines for design for VIM are not comprehensive and more research effort is required. Significant VIM in multi-column floating platforms will affect the fatigue life of the steel catenary risers and must be quantified and sometimes reduced. Industry-standard design tools used for drag estimation based on model tests of fixed structures may not accurately reflect the effects of drag augmentation due to VIM. Model tests and Computational Fluid Dynamics (CFD) analysis are feasible methods to investigate VIM, with the latter being more resource-efficient, provided sufficient benchmarking has been carried out to ensure reliable results.
Subsequent to the model tests and preliminary Computational Fluid Dynamics (CFD) simulations done for a multi-column floating platform [1, 2], further CFD analyses for the VIM of the floating platform have been carried out using improved simulation techniques with a commercial software. Good agreement between model test results and CFD calculations for VIM of a multi-column floating platform is observed. Sensitivity of CFD results to the modeling assumptions such as mesh size and density, time-step size and different turbulence models is presented.
Evaluation of Residual Stresses in Steel-to-Nickel Dissimilar Joints
Authors: Sotoudeh K. , Zhang S. Y. , Eren S. E. , Kabra S. , Milititsky M. , Gittos M. F. 
Published: November 2013

Welds between ferritic and austenitic materials, commonly described as dissimilar metal welds (DMW’s), are widely employed in various industrial applications. Failure of such joint can occur by a variety of failure mechanisms including creep, thermal fatigue and hydrogen embrittlement/corrosion While in some cases, the presence of long range stresses, due to design and fit-up, have been linked to failures, triaxial stress distribution across these types of joints have not adequately been investigated.
In the present work, forged low alloy connectors were buttered with a nickel alloy and post-weld heat treated, before making closing welds to a pipeline steel. The DMW received no further post-weld heat treatment (PWHT). The residual strain profiles across dissimilar joints were measured in the axial, radial and hoop directions of a series of these joints containing multi-run girth welds, using the neutron diffraction technique. Residual stresses were calculated from these measurements. Three dissimilar metal interface combinations were investigated: (1) ASTM A182 F22 steel to ERNiCrMo-3 (alloy 625) weld metal (PWHT’d joint), (2) AISI 8630M steel to ERNiCrMo-3 (alloy 625) weld metal (PWHT’d joint) and (3) ASTM A964-F65 low alloy steel to ERNiCrMo-3 (alloy 625) weld metal without PWHT. The current study attempts to create a better insight into the unique stress distributions across these joints.
Numerical Simulations for Installation of Offshore Wind Turbine Monopiles Using Floating Vessels
Authors: Li Lin , Gao Zhen , Moan Torgeir 
Published: November 2013

Monopiles are the most commonly used support structures for offshore wind turbines with up to 40m water depth due to the simplicity of the structure. The installation of turbine support structures can be carried out by a jack-up vessel which provides a stable working platform. However, the operational weather window using jack-up vessels is very limited due to the low sea states required for jacking up and down.
Compared to jack-up installation vessels, floating vessels have more flexibility due to fast transportations between foundations. However, the vessel motions will affect the motion responses of the lifting objects, which might bring installation difficulties. Therefore, it is necessary to examine the dynamic responses of the coupled system to ensure safe offshore operations.
In this paper, the installation operation of a monopile using a floating installation vessel is studied by a numerical model. Time domain simulations were carried out to study the installation process of a monopile, including lowering phase, landing phase and steady states after landing. Sensitivity studies were performed focusing on the effects by the gripper device stiffness and landing device stiffness. Comparisons of critical responses by using floating vessel and a jack-up vessel were also studied in the paper.
Study of Use of Supercritical CO2 to Enhance Gas Recovery and its Interaction With Carbonate Reservoirs
Authors: El Hajj Hicham , Odi Uchenna , Gupta Anuj 
Published: November 2013

It is well known that with continued production from wet gas reservoirs, the reservoir pressure eventually falls below the dew point pressure leading to condensation and loss of gas productivity in the reservoir. The concept of simultaneously injecting CO2 in a gas reservoir for long term storage while at the same time accelerating production of the natural gas is intriguing and promising. CO2 may also interact with carbonate matrix by changing porosity and permeability of the host rock; this is true for reservoirs that are found in the Gulf Region.
To maintain field gas production targets, operators routinely set the bottom hole pressure below the dew point pressure which results in condensate blockage. Injecting CO2 can delay the onset of condensate blockage by reducing the dew point pressure of the condensate blockage zone. The approach illustrated, utilizes CO2 to delay the onset of condensate blockage. Factors such as improved effusion were analyzed to justify the use of CO2 for wellbore condensate removal and enhanced gas recovery (EGR). Experimental verification of a new method of determining dew point pressures for wet gas fluids is presented in this work and compared to simulation results.
Core floods experiments with carbon dioxide were conducted in a core sample analogue to carbonate at reservoir conditions in order to study the interaction between CO2 and carbonate reservoir. CO2 sequestration in carbonate formation was evaluated by XRF and AFM.
Experimental and simulation results show increases in productivity index after CO2 injection. Increases in productivity index were caused by CO2 evaporating the condensate blockage. Condensate vaporization was caused by CO2 reducing the dew point pressure of the condensate. Carbonate aging in presence of CO2 shows two mechanism of CO2 trapping which are dissolution and mineralization.
Revisiting the Mechanics of Onset of Scour Below Subsea Pipelines in Steady Currents
Authors: Zhang Qin , Draper Scott , Cheng Liang , An Hongwei , Shi Hongda 
Published: November 2013

This paper summarizes the results of a series of experiments performed in a recirculating flume (namely the mini O-tube) to investigate the onset of scour below pipelines in steady currents. The experiments were performed on model seabed with two different lengths, to assess the effects of sediment supply on the potential for onset of scour.
In each experiment the flow velocity and pipeline embedment were recorded continuously. These measurements have been compared with an empirical formula proposed by Sumer et al. (2001) which predicts the onset velocity of a pipeline partially buried in a flat seabed. In general, present experimental results show that when the upstream sediment supply is limited, onset of scour may happen due to piping at a velocity well below that predicted by the empirical formula, but following significant local scouring around the pipe. This result, although only preliminary, suggests the potential for tunnel erosion beneath deeply embedded pipelines in the field where the sediment supply upstream of the pipe may be interrupted by rock outcrops or seabed slopes.
A series of numerical simulations have also been conducted to investigate the dynamic pressure difference between upstream and downstream of the pipeline prior to onset of scour. The numerical results show that despite local scour significantly alters the surrounding seabed for a deeply embedded pipe, the pressure gradient across the pipe is similar in magnitude to that measured for a flat seabed and is of sufficient size to cause piping.
A Methodology for Calculating Wave Crest Enhancement in Extreme Seas
Authors: Martin Bruce , Rijken Oriol 
Published: November 2013

The deck height of a tension leg platform or semi-submersible depends in large part on the expected crest height. This expected crest height is the result of the sea state, i.e. the incoming wave train, and local enhancement due to the vessels diffraction of the wave train. These local enhancements are usually determined by a combination of numerical computations and model tests. Quite often a crest enhancement factor is defined which takes into consideration these local amplification effects. Extrapolating the enhancement factor from extreme conditions to survival conditions may lead to significantly large crests and result in a very high deck elevation.
Many studies, including the CresT JIP address the characteristics of the crests within a given sea state and in the absence of a vessel. This paper addresses the effect of the presence of a vessel on the crest heights, and in particular the high crests which will ultimately determine deck height. The paper is based on experimental measurements of wave elevations underneath and around various tension leg platforms and semi submersibles. The investigated sea states comprise of a series of long crested irregular waves, generated in a model basin, which describe extreme and survival conditions in the Gulf of Mexico. The crest heights underneath the vessel are measured and compared with crests which occur without the presence of the vessel. Numerical predictions of the local amplification are also made, based on 1st order diffraction analysis and the as-measured incident wave train. A narrative is provided on the differences in crest height and observed phenomena.
Issues With the Density and Rheology of Drilling Fluids Exposed to Extreme Temperatures and Pressures
Authors: Zamora Mario , Roy Sanjit , Slater Kenneth 
Published: November 2013

The 1980’s introduction of costly non-aqueous drilling fluids finally sent the industry a wake-up call to seriously acknowledge the significant impacts of temperature and pressure on the downhole density and rheological properties of drilling fluids. Despite notable progress since then, key issues remain, mostly related to inherent complexities and uncertainties associated with quantifying these effects.
Some issues are technical; others are procedural. Concerns are particularly critical on wells drilled under extreme HTHP (high-temperature/high pressure) conditions, in deepwater, and in Arctic and other ultra-cold locations.
The primary objective of this paper is to discuss existing issues from well-integrity and wellbore-integrity perspectives, with focus on determining hydrostatic pressures and rheological properties under downhole conditions. Also included are new representative volumetric-behavior and viscometric data measured in the laboratory under extreme HTHP conditions.
Sparse Spectrum Seas and Waves Statistics
Authors: Charnotskii Mikhail 
Published: November 2013

A novel Sparse Spectrum (SS) model of the sea surface (M. Charnotskii, Proc. OMAE, 2011) is based on the conjecture that each surface realization contains a finite, possibly random, number of sinusoidal components with random frequency, phase and amplitude. Unlike the conventional FFT-based model, the number of spectral components forming the surface is determined by the sea state, but not the desired spatial resolution and domain size. A single constrain on the probability distribution of the wavenumbers, and the amplitude variances of the components and the number of spectral components allows the SS surface to conform to any prescribed spectral density.
Spectra and single-point probability distribution of the surface elevations are not sensitive to the number of sparse spectral components. In order to gain information about the spectral sparsity of the surface elevation we use our SS-based Monte-Carlo model for the surface elevations to examine the sensitivity of the statistics of the individual waves to the number of the sparse spectral components. We analyze the probability distributions of the wave heights; mean zero crossing period, and exceedance probability of the crest height at a fixed point and over a given region of area for the wave records generated by the SS model and compare it to the statistics based on the traditional FFT-based Monte-Carlo models corresponding to the dense spectrum model.
We use the Elfouhaily et.al. spectral model [3] that is common for the ocean remote sensing applications and discuss the sensitivity of the wave statistics to the spectral model choice.
Peakedness of Wave Systems Observed on the French Wave Energy Test Site (SEM-REV) Using a Spectral Partitioning Algorithm
Authors: Saulnier Jean-Baptiste , Le Crom Izan 
Published: November 2013

Located off the Guérande peninsula, SEM-REV is the French maritime facility dedicated to the testing of wave energy converters and related components. Lead by Ecole Centrale de Nantes through the LHEEA laboratory, its aim is to promote research alongside the development of new offshore technologies. To this end, the 1km2, grid-connected zone is equipped with a comprehensive instruments network sensing met-ocean processes and especially waves, with two identical directional Waverider buoys deployed on the site since 2009.
For the design of moored floating structures and, a fortiori, floating marine energy converters, the knowledge of the main wave resource — for regular operation — but also extreme conditions — for moorings and device survivability — has to be as precise as possible. Also, the consideration of the multiple wave systems (swell, wind sea) making up the sea state is a key asset for the support of developers before and during the testing phase.
To this end, a spectral partitioning algorithm has been implemented which enables the individual characterisation of wave systems, in particular that of their spectral peakedness which is especially addressed in this work. Peakedness has been shown to be strongly related to the groupiness of large waves and is defined here as the standard JONSWAP’s peak enhancement factor γ. Statistics related to this quantity are derived from the measurement network, with a particular focus on the extreme conditions reported on SEM-REV (Joachim storm).
Model Test Correlation Study for a Floating Wind Turbine on a Tension Leg Platform
Authors: Koo Bonjun , Goupee Andrew J. , Lambrakos Kostas , Lim Ho-Joon 
Published: November 2013

The challenges related to floating wind turbine analysis simulations relate to the modeling of the flexible turbine tower dynamics, the rotor dynamics, and the floating platform dynamics. In order to simulate the interactions between the wind turbine and the floating platform, two existing numerical codes, FAST [1], developed by the National Renewable Energy Laboratory (NREL), and MLTSIM, a Technip proprietary software, were integrated into one code, MLTSIM-FAST. In this integrated program, the turbine tower and rotor dynamics are simulated by the subroutines of FAST, and the hydrodynamic loads and mooring system dynamics are simulated by the subroutines of MLTSIM.
This paper presents validation of the MLTSIM-FAST code on the basis of data from the DeepCwind floating wind turbine model tests [2] and [3]. For the present MLTSIM-FAST validation study, the TLP floating wind turbine, which showed the strongest interactions between the wind turbine and the floating platform among the three platforms TLP, SEMI, and SPAR tested, is selected. The validation results are given on the basis of full scale measured and simulated motions and loads.
Development of a Computer Program for Three Dimensional Analysis of Zero Speed First Order Wave Body Interaction in Frequency Domain
Authors: Guha Amitava , Falzarano Jeffrey 
Published: November 2013

Evaluation of motion characteristics of ships and offshore structures at the early stage of design as well as during operation at the site is very important. Strip theory based programs and 3D panel method based programs are the most popular tools used in industry for vessel motion analysis. These programs use different variations of the Green’s function or Rankine sources to formulate the boundary element problem which solves the water wave radiation and diffraction problem in the frequency domain or the time domain.
This study presents the development of a 3D frequency domain Green’s function method in infinite water depth for predicting hydrodynamic coefficients, wave induced forces and motions. The complete theory and its numerical implementation are discussed in detail. An in house application has been developed to verify the numerical implementation and facilitate further development of the program towards higher order methods, inclusion of forward speed effects, finite depth Green function, hydro elasticity, etc. The results were successfully compared and validated with analytical results where available and the industry standard computer program for simple structures such as floating hemisphere, cylinder and box barge as well as complex structures such as ship, spar and a tension leg platform.
High spatial resolution brain functional MRI using submillimeter balanced steady‐state free precession acquisitiona)
Authors: Wu Pei‐Hsin , Tsai Ping‐Huei , Wu Ming‐Long , Chuang Tzu‐Chao , Shih Yi‐Yu , Chung Hsiao‐Wen , Huang Teng‐Yi 
Published: November 2013

Purpose:One of the technical advantages of functional magnetic resonance imaging (fMRI) is its precise localization of changes from neuronal activities. While current practice of fMRI acquisition at voxel size around 3 × 3 × 3 mm3 achieves satisfactory results in studies of basic brain functions, higher spatial resolution is required in order to resolve finer cortical structures. This study investigated spatial resolution effects on brain fMRI experiments using balanced steady‐state free precession (bSSFP) imaging with 0.37 mm3 voxel volume at 3.0 T.Methods:In fMRI experiments, full and unilateral visual field 5 Hz flashing checkerboard stimulations were given to healthy subjects. The bSSFP imaging experiments were performed at three different frequency offsets to widen the coverage, with functional activations in the primary visual cortex analyzed using the general linear model. Variations of the spatial resolution were achieved by removing outerk‐space data components.Results:Results show that a reduction in voxel volume from 3.44 × 3.44 × 2 mm3 to 0.43 × 0.43 × 2 mm3 has resulted in an increase of the functional activation signals from (7.7 ± 1.7)% to (20.9 ± 2.0)% at 3.0 T, despite of the threefold SNR decreases in the original images, leading to nearly invariant functional contrast‐to‐noise ratios (fCNR) even at high spatial resolution. Activation signals aligning nicely with gray matter sulci at high spatial resolution would, on the other hand, have possibly been mistaken as noise at low spatial resolution.Conclusions:It is concluded that the bSSFP sequence is a plausible technique for fMRI investigations at submillimeter voxel widths without compromising fCNR. The reduction of partial volume averaging with nonactivated brain tissues to retain fCNR is uniquely suitable for high spatial resolution applications such as the resolving of columnar organization in the brain.
A comprehensive investigation of the accuracy and reproducibility of a multitarget single isocenter VMAT radiosurgery technique
Authors: Thomas Andrew , Niebanck Michael , Juang Titania , Wang Zhiheng , Oldham Mark 
Published: November 2013

Purpose:Recent trends in stereotactic radiosurgery use multifocal volumetric modulated arc therapy (VMAT) plans to simultaneously treat several distinct targets. Conventional verification often involves low resolution measurements in a single plane, a cylinder, or intersecting planes of diodes or ion chambers. This work presents an investigation into the consistency and reproducibility of this new treatment technique using a comprehensive commissioned high‐resolution 3D dosimetry system (PRESAGE®/Optical‐CT).Methods:A complex VMAT plan consisting of a single isocenter but five separate targets was created in Eclipse for a head phantom containing a cylindrical PRESAGE® dosimetry insert of 11 cm diameter and height. The plan contained five VMAT arcs delivering target doses from 12 to 20 Gy. The treatment was delivered to four dosimeters positioned in the head phantom and repeated four times, yielding four separate 3D dosimetry verifications. Each delivery was completely independent and was given after image guided radiation therapy (IGRT) positioning using Brainlab ExacTrac and cone beam computed tomography. A final delivery was given to a modified insert containing a pin‐point ion chamber enabling calibration of PRESAGE® 3D data to dose. Dosimetric data were read out in an optical‐CT scanner. Consistency and reproducibility of the treatment technique (including IGRT setup) was investigated by comparing the dose distributions in the four inserts, and with the predicted treatment planning system distribution.Results:Dose distributions from the four dosimeters were registered and analyzed to determine the mean and standard deviation at all points throughout the dosimeters. A dose standard deviation of <3% was found from dosimeter to dosimeter. Global 3D gamma maps show that the predicted and measured dose matched well [3D gamma passing rate was 98.0% (3%, 2 mm)].Conclusions:The deliveries of the irradiation were found to be consistent and matched the treatment plan, demonstrating high accuracy and reproducibility of both the treatment machine and the IGRT procedure. The complexity of the treatment (multiple arcs) and dosimetry (multiple strong gradients) pose a substantial challenge for comprehensive verification. 3D dosimetry can be uniquely effective in this scenario.