bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison Study of Various Buckling and Ultimate Strength Assessment Methodologies for Stiffened Panels in Offshore Structures
Authors: Wang Xiaozhi , Sun Haihong , Wang Xiaohong , Wang Zhinong , Thayamballi Anil
Published: January 2011

Strength of offshore structures including FPSOs consists broadly of three aspects which are global intact and damaged strength, and local strength. Any of these strength aspects can be assessed by either prescriptive rule or finite element analysis (FEA). While many considerations relate to behavior in the linear elastic regime, the buckling and ultimate strength of both structural components (plate and stiffened panels) and structural systems can involve material and geometric nonlinearity behavior beyond the elastic region. With the development of computers and robust methods for nonlinear FEA, there has been a tremendous increase in the number of studies of structures under plastic or elasto-plastic behavior. However, even with today’s computers and software, nonlinear FEA of offshore structures remains complex and is not routinely applied in design analysis. Considerable effort therefore continues to be devoted to the development of simplified methods for rapid structural assessment and design analysis, instead of lengthy and complex nonlinear FEA. In this paper, various bucking and ultimate strength methodologies for plate and stiffener panels are first introduced. Each method is then compared with collected test data for buckling and ultimate strength of plate panels and stiffeners. Finally, conclusions are summarized based on the comparison study.
Numerical Computation for Flow-Induced Oscillations of a Circular Cylinder
Authors: Kondo Norio
Published: January 2011

This paper presents numerical results for flow-induced oscillations of an elastically supported circular cylinder, which is immersed in a high Reynolds number flow. The flow-induced oscillations of the circular cylinder at subcritical Reynolds numbers have been investigated by many researchers, and the interested phenomena with respect to the oscillations have been found in a wide range of the Scruton number. For the flow-induced oscillation of the circular cylinder with high mass ratio, it is well-known that there is the peak value of amplitudes at near the critical reduced velocity. Therefore, we computer flow-induced oscillations of a circular cylinder with a mass ratio of 8, which is placed in a high Reynolds number flow, by three-dimensional simulation, and the numerical results are compared with the results of flow-induced oscillations of the circular cylinder immersed in a subcritical Reynolds number flow.
Hull-Mooring Coupled Dynamic Analysis of Two Side-by-Side Operating Vessels
Authors: Mirzaei Mahdi , Shafieefar Mehdi , Moharrami Mohammad Reza
Published: January 2011

This paper contains the results of mooring analyses of two side-by-side operating crane barges in the installation procedure of a jacket type platform in an oil-gas field in the Persian Gulf. Since the vessels are proposed to operate next to a fixed structure, making sure about their displacements and a convenient clearance between them and the jacket is a highly important aspect of the design process. In essence, the analyses consist of setting an appropriate configuration and positioning of the vessels relative to each other, considering a reasonable gap between vessels and the jacket, connecting the vessels together, and studding the behavior of the moored system when it is subjected to a set of environments. In particular, an equilibrium configuration is found when the system is subjected to the mean force of the environment which consists of waves, wind gusts and currents. Then the dynamics of the system about the mean position is investigated. In order to study the influence of a dynamic mooring analysis, two cases are compared; one considering weight and buoyancy as the only forces acting on the lines and carrying out a quasi-static mooring analysis, and the other taking into account the hydrodynamic forces from the mooring lines. An accurate and efficient finite element program for the coupled analysis of the hull-mooring system is used and two types of modeling the connected vessels are compared; one using joints with restraints for all relative motions but in relative roll, and the other using slings and fenders. In all cases, the results of time domain analyses consisting of mooring line loads and motion responses of the vessels are presented in time histories and the statistics are studied.
Test and Analysis of Vibration Characteristic on New Type Dynamic Hydrocyclone
Authors: Lv Fengxia , Wang Zunce , Zhang Jinglong , Li Sen , Xu Yan
Published: January 2011

Dynamic hydrocyclone is currently used in separating oil and water from the crude oil, in which the fluid rolling motion is drived by the external power. Compared with the static type, the dynamic hydrocylone has higher separating property, while its structure is more complex and its separating property is influenced seriously by all the rotary components. Based on the original model, dynamic hydrocyclone of the new type is designed and manufactured, while applying the vibration signal collection and analysis system of IOtech640 type in the vibration characteristic analysis of the model body. The result shows that, when the rotating speed rises from 600r/min to 2000r/min and the flux from 1 to 3 m3/h, the level time-domain vibration peak of the monitoring site both the near the electromotor and the faraway is under 3.4×10−4, while the vertical is under 3.2×10−4, with steady frequency components in the vibration signal. It is illustrated that vibration intensity of the dynamic hydrocyclone of the new optimized type is lightened, which may confirm the improvement of the separation property and the operational life.
Evaluation of Critical Conditions in Offshore Vessel Operation by Response Based Optimization Procedures
Authors: Clauss Gu¨nther F. , Klein Marco , Sprenger Florian , Testa Daniel
Published: January 2011

During the design process of floating structures, different boundary conditions have to be taken into account. Besides the basic determination of the type of vessel, the range of application and the main dimensions at the initial stage, the reliability and the warranty of economical efficiency are an inevitable integral part of the design process. Model tests to evaluate the characteristics and the performance of the floating structure are an important milestone within this process. Therefore it is necessary to determine an adequate test procedure which covers all essential areas of interest. The focus lies on the limiting criteria of the design such as maximum global loads, maximum relative motions between two or more vessels or maximum accelerations, at which the floating structure has to operate or to survive. These criteria are typically combined with a limiting characteristic sea state (Hs, Tp) or a rogue wave. However, the important question remains: What is the worst case scenario for each design parameter — the highest rogue wave or a wave group of certain frequency? And which sea states have to be taken into account for the experimental evaluation of the limiting criteria? As an approach to these challenges, a response based wave generation tool for critical wave sequence detection is introduced. By means of this procedure, model tests can be conducted more efficiently. Besides the theoretical background of the response based wave generation tool, an exemplary practical application for a multi-body system is shown with maximum relative motions as the limiting criterion.
Axial Velocity Distribution at the Efflux of a Stationary Unconfined Ship’s Propeller Jet
Authors: Lam W. , Robinson D. J. , Hamil G. A. , Raghunathan S.
Published: January 2011

This paper is aimed at presenting an up-to-date investigation of the hydrodynamics of the jet (wake) of a stationary, unconfined ship’s propeller. The velocity field of a ship’s propeller jet is of particular interest for the researchers investigating the jet induced damage on a seabed as documented in previous studies. This paper discusses the time-averaged velocity field at the efflux, which is the immediate exit of the downstream propeller jet. The propeller jet is a rotating flow, which has axial, tangential and radial components of velocity. The axial component of velocity is the main contributor to the total velocity magnitude. Researchers are more interested in the axial velocity field within the ship’s propeller jet, due to the large contribution made by the axial velocity to the jet. The axial velocities at the efflux plane were obtained using joint experimental and numerical approaches. The results confirmed the two-peaked ridges axial velocity profile and disagreed with the 0.707Dp contraction suggested by Blaauw & van de Kaa (1978), Verhey (1983) and Robakiewicz (1987) at efflux of a ship’s propeller jet.
Application of Composites for Ship Structural Parts
Authors: Sankunny Surendran , Chandrabose Kannan
Published: January 2011

In the past, there were very serious casualties under the actions of extreme waves including loss of precious lives. There are cases like loss of M V Derbyshire (Faulkner, 2001) due to hatch cover failure in extreme weather conditions. Use of composite materials in marine fields as major or minor components off floating platforms is discussed in this paper. Application of composites on board ships reduces the self weight and lowers the position of vertical centre of gravity of the floating vessel. There are advantages in using composite structures in marine environment. A link-span fitted with a composite deck and the feasibilities of using composite for hatch covers of bulk carrier ship is described in this paper. In the case of bulk carriers, failure of hatch cover especially in the forward part of the vessel leads to flooding of the forward cargo compartment and occasionally results in fatal casualty. The foremost hatch cover and the next one within 25% length of the vessel.
Reliability-Based Design of Torpedo Anchors
Authors: Sagrilo Lui´s Volnei Sudati , de Sousa Jose´ Renato Mendes , Prates de Lima Edison Castro , Porto Elisabeth Campos , Fernandes Jane Vieira Volota˜o , Foppa Diego
Published: January 2011

The use of powerful numerical tools based on the finite element method has been improving the prediction of the ultimate bearing capacity of fixed anchors applied in the offshore oil industry. One of the main achievements of these numerical tools is the reduction of the uncertainty related to the bearing capacity prediction of these anchors. Therefore, it is possible to reduce the design safety factors values that have been calibrated based on prediction models with higher uncertainty, without impairing the original level of the structural safety. This paper presents a reliability-based safety factors calibration study for the design of torpedo anchors considering the statistical model uncertainty evaluated using the results from some experimental tests performed by PETROBRAS and their correspondent finite-element based numerical estimates.
Temperature and Strain-Rate Dependent Characteristics of AISI 300 Series of Austenitic Stainless Steel
Authors: Park Woong-Sup , Lee Chi-Seung , Kim Myung-Hyun , Lee Jae-Myung
Published: January 2011

AISI 300 series austenite stainless steel tensile tests under cryogenic conditions were performed to determine the effect of strain-rate and cryogenic temperature. Four types of commercial austenitic stainless steels widely used in LNG applications, AISI 304L, 316L, 321 and 347, were tested. To analyse strain-rate and temperature dependency, material properties were compared quantitatively. Temperatures ranging from 110K to 293K and strain-rates ranging from 1.6E−4/sec to 1.0E−2/sec have been studied as test conditions. From the test results, both strain-rate and temperature have substantial effect on material properties such as strength and elongation. The experimental results could be used to validate numerical techniques for tested materials as well as structures in cryogenic environment.
Effects of Free Spans on Lateral Stability of Offshore Pipelines: A Local Approach
Authors: Vedeld Knut , Sollund Ha˚var , Aamlid Olav
Published: January 2011

Current industry practice on determination of necessary weight for lateral stability of offshore pipelines makes many important assumptions on the boundary conditions and initial configurations of a subsea pipeline. The pipe is assumed to be on a flat seabed, the distance between local constraints of the pipeline is assumed to be so large that the boundary conditions at the ends have no influence on the maximum lateral displacement, and the contact between seabed and pipeline is assumed to be uniform. Pipelines are in reality placed on rough seabed, and the contact between pipeline and seabed may be very non-uniform. Therefore it is necessary to investigate the effects of free spans and their influence on lateral stability. Pipelines in free spans experience much lower hydrodynamic loads, including lift, drag and inertia, but at the same time a reduced overall resistance. The local influence of the free span is governed by span gap, span length, soil properties, pipe geometry, effective axial force and pipe submerged weight. Local studies of lateral stability on span shoulders have been performed using a simplified approach according to the absolute stability criterion and penetration models according to DNV-RP-F109, and the influence with respect to design according to the absolute static stability criterion is shown. It is demonstrated in this paper that local displacements on free span shoulders may occur even if the pipe satisfies the absolute stability criterion. However, the overall effects of free spans [2] are fairly minor on the global pipeline behaviour, even for pipes with a high ratio of spans to contact points. Therefore, the displacements demonstrated in this paper are likely to be local to smaller areas on span shoulders.
Investigations on Scuba Diving Regulators
Authors: Wang Jung-Chang , Tsai Yu-Pin
Published: January 2011

The paper utilizes numerical method with performance experiment to investigate the key factors affecting the performance of the Self-Contained Underwater Breathing Apparatus (SCUBA) system. A SCUBA system consists of a high pressure compressed air tank and a pressure regulator. The development of scuba diving is based on the invention of the regulator. The high pressure air carried by diver must be reduced to the pressure in the ambient environment by the regulator before the diver can breathe it. The life of the diver thus depends on the performance and stability of the regulator. Results show that the index performance of regulator can be predicted and used to improve the performance of the regulator. The numerical method can thus effectively reduce the develop time for new regulators and related products, as described in this article.
Three Dimensional Simulation of Bore Flow Using SPH
Authors: Liu Huaxing , Tan Soon Keat , Li Jing , Wang Xikun
Published: January 2011

Tidal bore is a fascinating and powerful hydraulic phenomenon. In this paper, the tidal bore’s process is studied using 3D Smooth Particle Hydrodynamic (SPH) model. The Lagrangian nature of SPH suits well to the modeling of the complex fluid flow phenomenon. In the SPH method, the Navier-Stokes equations are discretized with fluid particles in the Lagrangine sense. Boundary conditions, including both no slip wall and bottom wall, are implemented using dynamic boundary particles. Using SPH, the bore’s generation together with its traverse along the channel are presented, including the description of flow field and bore’s configuration. Different types of bores’ behavior are investigated. It is observed that there is a splash of water surge up the wall and the front of the bore becomes a breaker wave when the initial water column travels at high speed. The velocity field and bore heights at different locations are visualized and discussed as well.
Observations and Modeling of Linear and Nonlinear Spatio-Temporal Wave Statistics
Authors: Romero Leonel , Melville W. Kendall
Published: January 2011

We present an analysis of airborne wave observations collected in the Gulf of Tehuantepec. The data includes LIDAR measurements of the surface displacement as a function of two horizontal dimensions and time in fetch-limited conditions, with fetches between 50 and 300 km and winds between 10 and 20 m/s. The spatio-temporal data have an advantage over the commonly used single point time-series measurements allowing direct estimates of the wavelength and wave slope, including spatial information such as the lengths of crests exceeding threshold wave heights and slopes. The statistics of these wave parameters are particularly important for risk assessment of off-shore structures and in other ocean engineering applications. We present an analysis of several statistical wind-wave parameters, including the joint probability distribution function (pdf) of wave amplitudes and wavelengths, the pdf of wave heights, wavenumber vectors, and wave slopes, including the statistics of crests lengths exceeding threshold wave heights or slopes. The empirical findings from the LIDAR data are related to the analytical work by Longuet-Higgins (1957) [1] for a linear spectrum, including the average length of contours surrounding large wave heights. The effect of second-order nonlinearities on the distribution of crest lengths is investigated with numerical stochastic simulations from computed directional wavenumber spectra. The results show that second-order nonlinearities can increase the crest length density of large waves by about a factor of two or more. The results are discussed in the context of predicting wave statistics for ocean engineering applications.
A Proposed Strategy for Risk Analysis of Flexible Pipe Systems
Authors: Valle Longo Carlos Eduardo Costa , Bessa Guilherme de Salles , Brack Marcelo , de Arau´jo Ota´vio Campos , e Silva Wallace Bartholomeu
Published: January 2011

Nowadays, due to the increasing importance of the HSE and also the operational availability requirements, one of the most important challenges for producing oil and gas at subsea environments is to have a mature methodology for quantifying risks. The main objective of this paper is to present a strategy for running risk analysis of flexible pipe systems. These systems are extensively used by Petrobras to flow different kinds of fluids in deep water offshore scenarios. The strategy presented herein includes the classification of threats and the potential failure mechanisms. Probabilistic figures are established depending on the historic data of the components of the flexible pipe system and also on the theoretical results obtained from available models for determination of the component remaining life. The strategy is based on the previous definition of inference rules and technical criteria for probability, consequence and risk assessments. The final goal is to have an adequate tool to help Operators to take decisions, to establish strategies and to improve flexible pipe integrity management.
Investigation of Liquid Loading in Gas Well: Simultaneous Measurements of Drop-Size, Film Thickness and Pressure Drop
Authors: Alamu Mhunir B. , Azzopardi Barry J. , van der Meulen Gerrit P. , Hernandez-Perez Valente
Published: January 2011

The mechanism of atomization of part of the liquid film to form drops in annular two-phase flow is not entirely understood. It has been observed that drop creation only occurs when there are large disturbance waves present on the film interface. Woodmansee and Hanratty [1] observed that ripples on these waves were a precursor to drops. Though it has been reported that drops occur in bursts by Azzopardi [2], all previous drop size or concentration measurements have always been time integrated to simplify data analysis. Dynamic time averaged drop-size measurements are reported for the first time for annular two-phase flow. Experiments were carried out on a 19mm internal diameter vertical pipe with air and water as fluids. Spraytec, a laser diffraction-based, drop size measurement instrument, was used in the data acquisition. Simultaneous time-resolved measurements were made of: film thickness using conductance probes employing a pair of flush mounted rings as electrodes; and pressure gradient. The gas superficial velocity was 13–43 m/s at liquid superficial velocities of 0.05 and 0.15 m/s. Additional tests were carried out with the gas velocity at 14 m/s for liquid superficial velocities of 0.03–0.18 m/s. Though structures are not clearly visible in the signals acquired, they have been analyzed in amplitude and frequency space to yield Probability Density Function (PDF) and to identify the dominant frequency. Cross-correlation between two film thickness probes provides the wave velocities. Based on the signal analysis, links between film thickness, drop concentration and pressure drop have been identified.
Special Characteristics of Tsunami Propagation in the Western Mediterranean Basin
Authors: Heidarzadeh Mohammad , Tsimplis Michael
Published: January 2011

Unusual water-level behavior was observed in the western Mediteranean basin due to the most recent tsunami in this region on 21 May 2003. An analysis of seventeen tide-gauge records of this tsunami showed that the generated variability persisted for more than 1 day in most of the stations consisted of several wave trains. These observations suggest reflections at the boundaries and possibly resonant oscillations in the western Mediterranean basin triggered by the tsunamis. In this study we determine the resonant modes of the western Mediterranean basin on the basis of a numerical model. We then assess whether these modes have contributed to the unusual behavior of the 21 May 2003 tsunami.
Cryogenic Structural Performance of Corrugated Pipe
Authors: Buitrago Jaime , Slocum Scott T. , Hudak Stephen J. , Long Randy
Published: January 2011

One alternative to developing offshore gas reserves is to use a floating LNG plant (FLNG) on site and export the LNG using tankers. This alternative requires the use of a reliable LNG transfer system between the FLNG and the tanker under offshore conditions. One such system involves a cryogenic hose, whose main body is a vacuum insulated, pipe-in-pipe hose made of corrugated stainless steel pipe (c-pipe) and flanged terminations. Given the novelty of the transfer system, ExxonMobil conducted an experimental program to understand the structural performance of the basic c-pipe under static and cyclic loading at room and cryogenic temperatures. This paper discusses overall qualification issues and presents the experimental methodology and results of structural performance tests of the full-scale c-pipe at both ambient and cryogenic temperatures. Fourteen full-scale, c-pipe static tests are reported, including tension, compression, bending, torsion, and internal pressure. In addition, 11 axial and three pressure fatigue tests are presented. One key result is that, overall, cryogenic temperature improves structural performance for the limit states tested, indicating that future qualification at room temperature would be sufficient. Moreover, the fatigue performance at both ambient and cryogenic temperatures surpassed the design curve reported in the literature for c-pipe.
Simplified Fatigue Design Assessment (FDA) Procedure for Slots/Cut-Outs Based on a Stress Concentration Factor (SCF) Approach
Authors: Sia T. , Jago R. , Tong J. , Zhang S. M.
Published: January 2011

The Fatigue Design Assessment (FDA) procedure is a part of ShipRight design, construction and lifetime ship care procedures developed by Lloyds Register. The work presented in this paper is on the application of a Stress Concentration Factor (SCF) approach to the FDA procedure to develop an effective and user friendly tool for the assessment of fatigue performance of cut-outs in transverse web frames for longitudinal stiffeners. Investigation of hull damage statistics has identified that fatigue cracking in way of cut-outs and end brackets of longitudinal stiffeners affects a reasonable proportion of ships in service. This can become an ongoing ‘through life’ maintenance issue as defects can re-occur if the design of the structural detail has insufficient fatigue performance. A spectral fatigue assessment of this structural detail has traditionally been based on localised mesh refinement applied to a global finite element (FE) model and hydrodynamic calculated using first principle methods. These traditional methods are time, resource and labour intensive processes and therefore limited in its application. The SCF approach developed offers a faster and simpler method for the determination of hotspot stresses for standardised cut-out details.
Effect of Seabed Slope on Offshore Pile Lateral Behaviour Under Wave Force
Authors: Dhandapani Sathyanarayanan , Kasinathan Muthukkumaran
Published: January 2011

Fixed offshore platforms supported by pile foundations are required to resist dynamic lateral loading due to wave forces. The response of a jacket offshore tower is affected by the flexibility and nonlinear behavior of the supporting piles. In this study, a typical fixed offshore platform is chosen, and dynamic wave analysis is performed on it. Analysis has been performed for normal environmental conditions and extreme conditions. For the foundation, the deflections and reactions at regular intervals along the vertical direction from the seabed have been found out from the dynamic analysis, and the results have been compared for normal and extreme conditions. The aim of this study is to investigate the effects of the combined lateral and vertical loads on pile group foundation of a fixed offshore structure and the effects of seabed slope on the pile responses. To provide a more accurate and effective design for offshore pile foundation systems under axial structural loads and lateral wave loads, a finite element model which is modelled in FLAC3D is employed herein to determine the soil structure interaction under similar loading conditions. Three dimensional modelling and the analyses are done using FLAC3D — a finite element package.
Multifunctional Nanocomposites for Offshore Wind Turbine Blades: Materials, Processing and Performance
Authors: Gou Jihua , Liang Fei , Tang Yong , Lin Kuo-Chi , Ham Chan , Joo Young Hoon
Published: January 2011

Energy is an essential ingredient of socio-economic development and economic growth. Renewable energy sources like wind energy is indigenous and can help in reducing the dependency on fossil fuels. It has been estimated that roughly 10 million MW of energy are continuously available in the earth’s wind. Wind energy provides a variable and environmental friendly option and national energy security at a time when decreasing global reserves of fossil fuels threatens the long-term sustainability of global economy. Wind energy is the only power generation technology that can deliver the necessary cuts in CO2 in the critical period up to 2020, when greenhouse cases must peak and begin to decline to avoid dangerous climate change. Over the past ten years, global wind power capacity has continued to grow at an average cumulative rate of over 30%, and 2008 was another record year with more than 27 GW of new installations, bringing the total up to over 120 GW.
Research on a Mathematical Model of Storm Surge and Inundation in Bohai Bay
Authors: Li Da-ming , Fu Qing-jun
Published: January 2011

A two-dimensional extratropical storm surge and inundation numerical model was built based on the unsteady flow N-S equations. This model was divided into two sections, one is the simulation of storm surge tidal level and the other is the simulation of storm surge inundation on the coastal area. While simulating the storm surge tidal level, the FDM (finite difference method) was applied for dispersing and solving the 2D storm surge equations using the ADI (Alternating Direction Implicit) difference scheme. In the simulation of storm surge inundation, the 2D unsteady flow equation was dispersed and solved using the structureless mesh method of FVM (finite volume method). The calculation mode that connecting the process of storm surge floodplain with the process of storm surge tidal level variation was also introduced in this paper, therefore the storm surge floodplain process and area can be calculated while simulating and forecasting the process of storm surge tidal level setup. Furthermore, an extratropical storm surge which happened in Bohai Bay was simulated and validated using this numerical model, and the process of storm surge floodplain on the coastal area was also simulated. Data comparisons reveal that this model’s calculation results are in good agreement with the measured results, which showed that this numerical model provides a new method of simulating and forecasting for storm surge and inundation in Bohai Bay.
Probabilistic Assessment of Stiffened Panel Strength: An Interactive Tool Using PROBAN and PULS
Authors: Amlashi Hadi , Ho̸rte Torfinn , Steen Eivind , Kippenes Jon
Published: January 2011

To achieve an economic and reliable ship design, the ship structure has to be designed with adequate safety margins. This can be accomplished by a reliability-based limit states design approach, in which probabilistic methods are used to guide the development of the design criteria. A tool is developed to probabilistically assess the capacity distribution of the stiffened panels. A Monte Carlo simulation scheme, which samples a number of probability distributions, has been applied using PROBAN (DNV) which interactively utilizes PULS (DNV) as an efficient ultimate strength prediction tool for plated panels. It is demonstrated that this tool can successfully link the two internationally recognized programs, i.e. PROBAN and PULS for probabilistic assessment of stiffened panel’s ultimate strength. The results demonstrate that the suitability of the assumed distribution for the strength can be assessed with relatively little computational time, where the yield stress and imperfection sizes are treated as random variables. Such results are very informative and useful for further development of existing safety format ensuring a safe, economic and reliable ship design.
Efficient Reliability Analysis of Structural Systems With a High Number of Limit States
Authors: Naess Arvid , Leira Bernt J. , Batsevych Olexandr
Published: January 2011

In principle, the reliability of complicated structural systems can be accurately predicted by standard Monte Carlo simulation methods, but the computational burden may be prohibitive. A new Monte Carlo based method for estimating system reliability that aims at reducing the computational cost is therefore proposed. It exploits the regularity of tail probabilities to set up an approximation procedure for the prediction of the far tail failure probabilities based on the estimates of the failure probabilities obtained by Monte Carlo simulation at more moderate levels. In the paper the usefulness and accuracy of the estimation method is illustrated by application to some particular examples of structures with several thousand potentially critical limit state functions. The effect of varying the correlation of the load components is also investigated.
An Analytical Solution for Wave-Induced Soil Response and Seabed Liquefaction
Authors: Zhou Xiang-Lian , Wang Jian-Hua , Xu Yun-Feng
Published: January 2011

In this study, an analytical method to solve the wave-induced pore pressure and effective stress in a saturated porous seabed is proposed. The seabed is considered as a saturated porous medium and characterized by Biot’s theory. The displacements of the solid skeleton and pore pressure are expressed in terms of two scalar potentials and one vector potential. Then, the Biot’s dynamic equations can be solved by using the Fourier transformation and reducing to Helmholtz equations that the potentials satisfy. The general solutions for the potentials are derived through the Fourier transformation with respect to the horizontal coordinate. Numerical results show that the permeability and shear modulus of the porous seabed has obvious influence on the response of the seabed. The vertical effective stress and attenuation velocity of pore pressure along seabed depth increase as permeability k increases. The liquefaction may be occur at the surface of seabed when shear modulus decreasing.
Risk Assessment of Riser Operations on DP Drilling Units
Authors: Liu Xiao , Abrahamsen Bjo̸rn
Published: January 2011

Risk of riser operations on dynamic positioning mobile offshore drilling units is addressed in this paper. Primary focus is placed on the blowout risk caused by failure of Emergency Quick Disconnection in the situation of DP failure, i.e. position loss. The risk model is established by an event tree linked with fault trees using RiskSpectrum® software where various equipment failures and operational errors that may lead to failure of DP system and well barriers are identified and modeled. Both overbalanced operation with standard marine drilling risers and underbalanced operations with a set of high-pressure completion/workover risers are analyzed in this paper. And the blowout frequency is presented for a given yearly riser operational activity level. Sensitivity studies are performed for several key variables and recommendations are proposed in order to further reduce the blowout risk of riser operation on DP drilling units.
Sakhalin Single Anchor Leg Mooring (SALM) Repair Project
Authors: Masterton Stephen , Si Michael , Swart Pieter , Efthymiou Mike , Loques Jose
Published: January 2011

In July 2008, Sakhalin Energy Investment Company (SEIC) had successfully retrofitted its SALM system offshore Sakhalin Island in the Russian Sea of Okhotsk following its Universal Joint (Uni-joint) connection failure in November 2007. This SALM system is part of the “Vityaz” Production Complex in which the produced oil is exported from the Piltun-Astokhskoye-A platform via a subsea pipeline and the SALM to the Okha floating storage and offloading (FSO) unit for subsequent loading onto tankers for delivery to customers. Yearly oil production using the SALM system starts normally in June and lasts till December, during the ice free period. Full year round production were planned using a pipeline replacement for the SALM system (on-stream early 2008). However, in the interim period, there was a significant production opportunity if the SALM could be reinstalled in time for the summer 2008 ice-free season. Another significant incentive for SEIC was an early gas flow through the offshore pipelines to the Onshore Processing Facility for commissioning purposes, which expedites first gas delivery for the whole project by an estimated 4 months. In order to make this production season possible, a unique and fast track project to reinstate the SALM was necessary. SEIC, Shell International E&P in Rijswijk and Single Buoy Moorings Inc. in Monaco, had jointly undertaken this project, incorporating proven engineering standards with an immense amount of creativity to meet these time constraints. The SALM project, that would normally take 2 years, has been hailed as a great success with design, fabrication and installation completed in just 7 months time. This paper describes the key challenges overcome to successfully deliver a fully working SALM system to an extreme time schedule. The emphasis is on a description of the technical challenges and the engineering repair solutions developed with reference to the constructibility, transportation, installation and operation. The challenges include: Re-engineering of the SALM Unijoint cast components to meet the limitations of the available foundries, implementation of a novel pull down installation method using a subsea linear winch, design and construction of independent contingency option, installation trials, worldwide logistics and project coordination. All these aspects form an integral part of the project execution strategy with prompt decision making to meet the required schedule.
Evaluation of a simplified simulation approach for thin film type gated field emitters
Authors: Luo Jie , Chen Jun , Deng S. Z. , Xu N. S.
Published: January 2011

A simplified simulation approach aiming at reducing computational complexity has been proposed for gated field emitters. In this approach, a thin-film cold cathode consisting of quasi-one-dimensional nanoemitters is modeled as a flat metallic thin-film. The surface of the thin-film is divided into equal-size grids. Fowler–Nordheim parameters fitted with experimental current-voltage data are used to calculate emission current from these individual grids. The statistics of the total emission current and its distribution between the gates and the anode are analyzed by summing the currents from all of the grids. The validity of the proposed approach is evaluated by comparing the simulation results with experimental data from gated carbon nanotube (CNT) thin-film field emitters. CNTs at the edge of the CNT film stand out as superior emitters, and proper consideration of such emitters plays a critical role in determining the validity of the proposed approach.
Structural characteristics of ZnO films grown on (0001) or (11−20) sapphire substrates by atomic layer deposition
Authors: Liu Kuang-Pi , Yen Kuo-Yi , Lin Ping-Yuan , Gong Jyh-Rong , Wu Kun-Da , Chen Wei-Li
Published: January 2011

The structural characteristics of zinc oxide (ZnO) films deposited on the (0001)- or (11−20)-oriented sapphire substrates were investigated. ZnO films having low temperature ZnO buffer layers were prepared by atomic layer deposition using diethylzinc and nitrous oxide. The ZnO films were analyzed using cross-sectional transmission electron microscopy, high-resolution transmission electron microscopy and x-ray diffractometry. Cross-sectional high-resolution transmission electron microscopic observations showed the presence of extrinsic stacking faults along with Frankel partial dislocations in ZnO near the ZnO/sapphire heterointerface. Based on the results of cross-sectional transmission electron microscopic observations and double-crystal x-ray rocking curve measurements, it was found that buffer-layer annealing treatment tended to reduce the density of threading dislocations in a ZnO film efficiently so that the crystalline quality of the film was greatly improved.
Comprehensive study of the p-type conductivity formation in radio frequency magnetron sputtered arsenic-doped ZnO film
Authors: Fan J. C. , Zhu C. Y. , Yang B. , Fung S. , Beling C. D. , Brauer G. , Anwand W. , Grambole D. , Skorupa W. , Wong K. S. , Zhong Y. C. , Xie Z. , Ling C. C.
Published: January 2011

Arsenic doped ZnO and ZnMgO films were deposited on SiO2 using radio frequency magnetron sputtering and ZnO–Zn3As2 and ZnO–Zn3As2–MgO targets, respectively. It was found that thermal activation is required to activate the formation of p-type conductivity. Hall measurements showed that p-type films with a hole concentration of ∼1017 cm−3 and mobility of ∼8 cm2 V−1 s−1 were obtained at substrate temperatures of 400–500 °C. The shallow acceptor formation mechanism was investigated using x-ray photoelectron spectroscopy, positron annihilation, low temperature photoluminescence, and nuclear reaction analysis. The authors suggest that the thermal annealing activates the formation of the AsZn-2VZn shallow acceptor complex and removes the compensating hydrogen center.
Effect of interfacial formation on the properties of very long wavelength infrared InAs/GaSb superlattices
Authors: Haugan H. J. , Brown G. J. , Grazulis L.
Published: January 2011

In InAs/GaSb superlattices (SLs) designed for infrared detection, the interfacial layers comprise approximately 10%–15% of the heterostructure. As interdiffusion into the InAs and GaSb layers is considered, this percentage is expected to be even higher. Although the primary goal for engineering these transient layers is to balance the SL strain to the GaSb substrate, the interfacial quality can impact the performance of the SL in other ways as well. Many believe that the majority of nonradiative defects that shorten carrier lifetime can be generated from the SL interfaces or regions near them due to the poor interface engineering. Because the degree of lattice mismatch tends to be higher in very long wavelength infrared InAs/GaSb designs, the approach tuning growth parameters to optimize the strain balancing process is different from that for midinfrared SLs. To investigate this optimization, a systematic approach was applied to achieve strain compensated 16 monolayers (MLs) InAs/7 MLs GaSb SLs aimed for a target onset wavelength of 15 μm. The authors systematically explored the effect of growth parameters, such as group V fluxes, growth rates, and shutter sequences, on the SL strain and interfacial quality. For this study, high-resolution x-ray diffraction and atomic force microscopy analysis were extensively used to monitor the effect of interfaces on material properties.
Electronic and field emission properties of two-dimensional nanotori
Authors: Filip Marina Rucsandra , Filip Lucian Dragoş
Published: January 2011

The electronic properties of two-dimensional toroidal surfaces of nanometer size have been investigated by approximately solving the time-independent Schrödinger equation. The effects of the quantum confinement on the electron population of these structures were shown by studying the average electron density on the surface of the torus. The unique electron spread that resulted from such computations encouraged further study on the field emission properties of this two-dimensional manifold. The Wentzel–Kramers–Brillouin approximate approach was used for a preliminary study of the field emission properties of the nanotori for various geometrical parameters.
Pronounced field emission from vertically aligned carbon nanotube blocks and bundles
Authors: Yilmazoglu Oktay , Joshi Ravi , Popp Alexander , Pavlidis Dimitris , Schneider Jörg J.
Published: January 2011

Field emission from aligned carbon nanotube (CNT) blocks and bundles grown by two different chemical vapor deposition (CVD) methods were investigated. A single CNT block consists of about 1600 double-walled nanotubes per μm2 of area with an intertube distance in the range 15–20 nm, while a bundle consists of a few multiwalled CNTs, which contact each other in the tip region. Hydrogen/water assisted catalytic CVD on a silicon wafer resulted in ultrahigh aspect ratios for the CNT blocks with extremely high field enhancement coefficients >11 000, a turn-on field of 0.36 V/μm, and good field emission stability. The water and hydrogen concentration ratio were found to be a critical parameter for establishing high quality, ultralong (up to 2 mm) CNT growth. The second CVD growth method used was a noncatalytic template assisted technique for bundle generation. Special self-organized anodic alumina films controlled the synthesis of highly aligned carbon nanotube arrays with the desired tube geometry and adjustable intertube distances. The CNT bundle generation was adjusted by the CNT tube diameter, wall thickness, and length-controlled growth. Subsequent etching of the Al2O3 template resulted in substrateless flexible CNT bundles permitting simple gate electrode integration process. Field emission tests showed that the field enhancement factor, turn-on field, and field emission current density of the CNT cathode arrays could be optimized by adjusting the block or bundle distance. The authors obtained, for the block emitter, a smaller turn-on field and better stability. The block emitter with the better performance was additionally coated with CVD grown ZnO nanoparticles to obtain further improvement. The ZnO nanoparticle diameter was in the range of 15–20 nm. First results showed partial clustering of the ZnO particles suggesting the need for further optimization of the deposition.
Beamforming with a circular microphone array for localization of environmental noise sources
Authors: Tiana-Roig Elisabet , Jacobsen Finn , Grande Efrén Fernández
Published: January 2011

It is often enough to localize environmental sources of noise from different directions in a plane. This can be accomplished with a circular microphone array, which can be designed to have practically the same resolution over 360°. The microphones can be suspended in free space or they can be mounted on a solid cylinder. This investigation examines and compares two techniques based on such arrays, the classical delay-and-sum beamforming and an alternative method called circular harmonics beamforming. The latter is based on decomposing the sound field into a series of circular harmonics. The performance of the two signal processing techniques is examined using computer simulations, and the results are validated experimentally.
Comparison between direct-sequence and multicarrier spread-spectrum acoustic communications in time-varying channels
Authors: van Walree P. A.
Published: January 2011

Underwater communication experiments have been conducted in the Norwegian Oslofjord. Two modulation schemes are compared in a 7-kHz frequency band on a 14-kHz center frequency. The first scheme is direct-sequence spread spectrum (DSSS), using a 7-chip spreading code to achieve a raw data rate of 1000 bps on a single carrier. The second scheme is multicarrier spread spectrum (MCSS) and accomplishes spreading by using seven subbands. The DSSS receiver equalizes on the chips prior to explicit symbol despreading, whereas MCSS features joint multiband equalization and despreading. Four channels are examined, from nearly static to overspread. In slowly varying channels, MCSS offers the best performance. DSSS has the best tracking potential for rapidly varying channels, where the challenge is to obtain reliable chip decisions before symbol despreading. The tracking potential can be realized to some extent by hypothesis-feedback equalization. It is further shown that adaptive equalizers are capable of code conversion, i.e., the DSSS receiver can demodulate the MCSS waveform, and vice versa. Neither receiver requires knowledge of the spreading code in order to despread the data.
Trans-dimensional geoacoustic inversion
Authors: Dettmer Jan , Dosso Stan E. , Holland Charles W.
Published: January 2011

This paper develops a general trans-dimensional Bayesian methodology for geoacoustic inversion. Trans-dimensional inverse problems are a generalization of fixed-dimensional inversion that includes the number and type of model parameters as unknowns in the problem. By extending the inversion state space to multiple subspaces of different dimensions, the posterior probability density quantifies the state of knowledge regarding inversion parameters, including effects due to limited knowledge about appropriate parametrization of the environment and error processes. The inversion is implemented here using a reversible-jump Markov chain Monte Carlo algorithm and the seabed is parametrized using a partition model. Unknown data errors are addressed by including a data-error model. Jumps between dimensions are implemented with a birth–death methodology that allows transitions between dimensions by adding or removing interfaces while maintaining detailed balance in the Markov chain. Trans-dimensional inversion results in an inherently parsimonious solution while partition modeling provides a naturally self-regularizing algorithm based on data information content, not on subjective regularization functions. Together, this results in environmental estimates that quantify appropriate seabed structure as supported by the data, allowing sharp discontinuities while approximating smooth transitions where needed. This approach applies generally to geoacoustic inversion and is illustrated here with seabed reflection-coefficient data.