bluuebunny/crossref_metadata_2025 huggingface dataset:
A Generic, Transient Model of a Turbocharged, Multi-Cylinder, Common-Rail Diesel Engine
Authors: Ceausu Radu , Taraza Dinu , Henein Naeim A. , Bryzik Walter
Published: November 2008

The paper presents a generic model of a turbocharged, common rail diesel engine developed on a SIMULINK platform. The model is developed for the main task of predicting transient behavior of the turbocharged engine, especially acceleration under load. In order to reduce computer time, the model is zero-dimensional as far as combustion is concerned, but split injection is considered to simulate the main characteristic of modern common rail diesel engines. A simplified generic model of the turbocharger is developed such as to determine the effects of different design parameters on the acceleration of the engine. A detailed dynamic model of the engine is considered to allow estimating the torsional behavior of the crankshaft and evaluate the impact of torsional vibrations. The model is validated on a four-cylinder, 2.5 liter common-rail diesel engine and is further used to identify the fueling strategy and the design parameters that could be improved in order to obtain the best possible engine acceleration under different load conditions.
Railplug Design Optimization to Improve Large-Bore Natural Gas Engine Performance
Authors: Gao Hongxun , Hall Matt J. , Ezekoye Ofodike A. , Matthews Ron D.
Published: November 2008

It is a very challenging problem to reliably ignite extremely lean mixtures, especially for the low speed, high load conditions of stationary large-bore natural gas engines. If these engines are to be used for the distributed power generation market, it will require operation with higher boost pressures and even leaner mixtures. Both place greater demands on the ignition system. The railplug is a very promising ignition system for lean burn natural gas engines with its high-energy deposition and high velocity plasma jet. High-speed photography was used to study the discharge process. A heat transfer model is proposed to aid the railplug design. A parameter study was performed both in a constant volume bomb and in an operating natural gas engine to improve and optimize the railplug designs. The engine test results show that the newly designed railplugs can ensure the ignition of very lean natural gas mixtures and extend the lean stability limit significantly. The new railplug designs also improve durability.
Transient Temperature Estimation for Active-Flow Aftertreatment Control
Authors: Zheng Ming , Wu Yue , Zhao Guochang , Reader Graham T.
Published: November 2008

Diesel exhaust temperatures vary with engine load and speed thereby affecting the thermal behavior and thus performance of exhaust after-treatment systems. The determination of the transient temperature is needed to enable active-flow control after-treatment schemes that include parallel alternating flow, partial restricting flow, periodic flow reversal, and extended flow stagnation. The active schemes are found to be especially effective to treat engine exhausts that are difficult to cope with conventional passive-flow converters, by shifting the exhaust gas temperature, flow rate, and oxygen concentration to more favorable windows for the filtration, conversion, and regeneration processes. This paper reports a thermal-response model that uses the temperature data obtained with two high-inertia thermocouples of different sizes to estimate the diesel engine transient exhaust gas temperature. The thermal inertial difference of the two thermocouples is critical in predicting the transient temperature through a mathematical procedure. To validate the model, the exhaust gas temperature was simultaneously measured with a third thermocouple of high sensitivity that acquired temperature data approximating the real-time value.
Conversion Technologies: A New Option for MSW Management
Authors: Jenkins Stephen D. , Legrand Robert
Published: November 2008

Conversion technologies (CTs) utilize thermal or biological processes to convert municipal solid waste (MSW) or Material Recovery Facility (MRF) residuals into useful products, such as electricity, fertilizers, and chemicals. These technologies, such as pyrolysis, gasification, and anaerobic digestion are very different from conventional waste-to-energy (WTE) technologies for MSW. CTs can provide greater efficiency and environmental benefits compared to WTE, and environmental and societal benefits compared to landfilling.
Gasification/Cogeneration Using MSW Residuals and Biomass
Authors: Schubert Jim , Fichtner Konrad
Published: November 2008

The City of Edmonton presently collects and processes about 230,000 tonnes of municipal solid waste (MSW) and recyclables per year at the composting and materials recovery facilities located at the Edmonton Waste Management Centre. Over 60% of the waste materials that are brought to the facilities are recycled and composted. Remaining residuals from both the composting and materials recovery facilities have little value in terms of being further recycled and are currently being landfilled. The residuals do have a significant calorific value and have the potential to produce enough electricity to provide 100% of the power and heating for facilities at the Edmonton Waste Management Centre (EWMC), with remaining energy for adjacent developments. The City is considering advanced thermal treatment (not conventional incineration) of the residual waste (after recycling and composting) as a way to close the loop in waste management in terms of minimizing waste materials that are landfilled and reducing the net energy requirement for waste processing and disposal to nil. Other renewable biomass waste streams (e.g.: wood or agricultural waste) could complement operation of the facility and make it more economically viable (economies of scale). There are also other environmental benefits such as reductions in the overall greenhouse gas (GHG) and other harmful emissions by displacement of fossil fuel as an energy source.
McKay Bay Refuse-to-Energy Facility Split-Range Control System
Authors: Schwartz Mark P.
Published: November 2008

The McKay Bay Refuse-to-Energy Facility underwent a three-year retrofit program completed in 2001. The major portion of this work involved the replacement of all four combustion trains. The existing turbine generator set, rated at 22.5 MW, was retained. Each of the four boilers had a maximum continuous rating (MCR) of 62,186 lb/hr of steam; i.e., 248,744 lb/hr with all boilers in operation. The turbine generator operated at about 220,000 lb/hr (about 93% capacity), thus allowing for load swings due to fuel inconsistencies. As a result of the difference between the boiler MCR and the rate into the turbine, excess steam (over 26,000 lb/hr) was sent to an existing bypass (dump) condenser. Upon installing a new automatic governor control system for the turbine during the retrofit, the potential for providing additional control capabilities was realized. Utilizing an available second control feature on the governor control system, a major portion of the bypassed steam could be sent to the turbine via an innovative split-range control system configuration. The formerly bypassed steam was now added to the energy recovered, and had a positive effect on the net kwh/ton of waste. This paper discusses the research, design and installation of the split-range control system, as well as the economics of the project. The capital cost of this system enhancement was recovered in the first three months of operation, and the process continues to operate successfully.
Pulse-Jet Baghouse Optimization in WTE: Meeting the Challenges of the Future
Authors: Ladwig Jeff , Linton Robin
Published: November 2008

Like many coal-fired power plants today, the waste-to-energy (WTE) industry is faced with a number of challenges including the need to maximize plant output, lower outlet emissions and increase plant efficiencies. Within WTE, there’s also been a move from reverse-air baghouses to pulse-jet collectors due to lower initial capital costs and the ability to operate pulse-jet collectors at higher air-to-cloth ratios (3–4:1), allowing for a smaller housing footprint. However, the majority of today’s pulse-jet collectors utilize an off-line cleaning mode where modules are taken out of service and pulsed to lower the differential pressure. There are inherent advantages in switching from an off-line cleaning mode to an on-line cleaning mode. This paper discusses the idea of using the fabric filter as a damper and stabilizing draft through the baghouse and boiler. It also outlines the use of pleated filter element (PFE) technology to address increased production concerns, and the need for lower outlet emissions.
Evaluating Waste Incineration as Treatment and Energy Recovery Method From an Environmental Point of View
Authors: Olofsson Mattias , Sundberg Johan , Sahlin Jenny
Published: November 2008

During the last two decades, several research groups as well as consultants have been analysing the environmental impacts of incineration in comparison to other waste treatment options. Methods and models for describing these systems have been developed. Systems studies on local, regional and national level have been performed using a wide range of different modelling approaches. The aim of this paper is to describe the environmental performance of incineration with energy recovery in Europe in comparison with other options for waste treatment/recovery. This includes identifying key factors that largely affect the outcome from environmental systems studies where such comparisons are made. The paper focuses on mixed solid waste and on waste fractions where there has been a lot of controversy whether the material should be recycled, incinerated or treated biologically (e.g. paper, plastics, compostable material). The paper is based on a meta-study, where the above research field is mapped out in order to gather relevant systems studies made on local, regional and national levels in Europe. By thoroughly examining these studies, conclusions are drawn regarding the environmental performance of incineration with energy recovery and regarding key factors affecting the environmental results.
To MACT or Not to MACT: Mercury Emissions From Waste-to-Energy and Coal-Fired Power Plants
Authors: Themelis Nickolas J. , Assaf-Anid Nada
Published: November 2008

During the combustion of fuel in Waste-to-Energy (WTE) and coal-fired power plants, all of the mercury input in the feed is volatilized. The primary forms of mercury in stack gas are elemental mercury (Hg0) and mercuric ions (Hg2+) that are predominantly found as mercuric chloride. The most efficient way to remove mercury from the combustion gases is by means of dry scrubbing, followed by activated carbon injection and a fabric filter baghouse. Back in 1988, the U.S. WTE power plants emitted about 90 tons of mercury (Hg). By 2003, implementation of the EPA Maximum Achievable Control Technology (MACT) standards, at a cost of one billion dollars, reduced WTE mercury emissions to less than one ton of mercury. EPA now considers coal-fired power plants to be the largest remaining anthropogenic source of mercury emissions. Approximately 800 million short tons of coal, containing nearly 80 short tons of Hg are combusted annually in the U.S. for electricity production. About 40% of this amount is presently captured in the gas control systems of coal-fired utilities. Since the concentration of mercury in U.S. coal is ten times lower than in the MSW feed and the volume of gas to be cleaned 55 times higher, the cost of implementing MACT by the U.S. coal-fired utilities is estimated to be about $25 billion. However, when this retrofit cost is compared to the total capital investment and revenues of the two industries, it is concluded that MACT should be affordable. Per kilogram of mercury to be captured, the cost of MACT implementation by the utilities will be twenty times higher than was for the WTE industry. However, implementation of MACT by the utilities will also reduce the emissions of other gaseous contaminants and of particulate matter.
Coating Degradation Models: Cost Optimisation of Inspection, Maintenance and Repair of FPSO’s
Authors: Landet Einar , Lo̸nvik Kari , Sigurdsson Gudfinnur , Fischer Karl P.
Published: November 2008

Corrosion control design and management for a new build Floating Production Storage and Offloading installation (FPSO) operating in certain benign regions, such as West Africa, China and Brazil, can provide significantly increased challenges compared to their North Sea counter parts. Even though there are over 100 FPSO’s operating worldwide, designing and implementing a cost optimal Inspection, Maintenance and Repair (IMR) system for a 20 year service still remains a major challenge. Primarily this is due to the fact that there is limited information available to facilitate the corrosion control design for a 20 year continuous service. Therefore, it is difficult to select a cost effective corrosion control design that addresses both the fabrication and operational aspects. This paper describes a probabilistic model for coating degradation and its application for implementing IMR for long year service of FPSO’s. The model can be utilized as a tool for planning of inspection and maintenance of the corrosion protective coatings and also estimate the associate costs. This enables the operator to model various scenarios for future inspection and maintenance work and thereby select cost optimal solutions for the given FPSO requirements. The paper will also demonstrate the proposed model through a realistic case study.
Modeling of Drillstrings
Authors: Elsayed M. A. , Phung Chin Chin
Published: November 2008

In this paper we show how a drillstring can be modeled in terms of limited numbers of masses and springs for the purpose of building a test rig. The model should represent the drillstring dynamics to a reasonable degree of accuracy. We will use the real component of the Frequency Response Function and the stability diagram as measures of dynamic similarity between the model and drillstring. We will also show how the chosen modes can be decoupled and used in obtaining the bit displacement. The decoupled modes will be used in a proposed test rig configuration that would increase flexibility in adding or removing modes from the system. Obtaining a representative model of the test rig is critical to our ability to extrapolate laboratory data into field applications.
Riser Response Based Optimal Positioning of Deep-Water Vessels
Authors: Hoen Christopher
Published: November 2008

The present paper discusses the mathematical modeling of risers and riser-like structures applied in a positioning context for deep-water floating vessels. The main purpose of the paper is to show that an estimate for the optimal vessel position, sufficient for most practical applications, is obtained from measurements of the riser inclinations or related parameters at lower end, and optionally upper end, through a solution based on the variably tensioned beam differential equation. Due to the ease of implementation this solution is well suited for direct application in on-line riser monitoring systems. The method is an attractive alternative to on-line FE-analyses, application of pre-computed regression curves based on idealized loading or black-box neural networks, which has been proposed by others to be applied as basis for interpretation of the measured riser responses. The basic idea behind the method is based on the observation that the riser inclinations or stress-joint moments at upper and lower end have mainly two causes. Firstly an effect caused by the position of the riser top end relative to the wellhead due to permanent vessel offset and slow drift vessel motions, and secondly the effects of transverse current down the riser. The general theory behind the method will be outlined. It will then be shown how the method adapts to drilling-risers with flex-joints, risers with stress-joints and also to the special case of well intervention with coiled tubing in open sea without applying a work-over or marine riser. The performance of the method is illustrated using simulated vessel and riser dynamic response data. The simulations are performed for selected vessel types both for deep-water and shallower waters applying state-of-the-art software for simulation of the riser and vessel dynamic response in random sea states.
Numerical vs. Experimental Predictions of Yaw Motion of Single Point Moored Vessel
Authors: Brotons Mathieu , Jean Philippe
Published: November 2008

The accurate prediction of SPM vessel yaw motion is important to its mooring system design. Inconsistencies have been observed between the numerical and model test predictions of offloading responses. In some cases, the numerical simulation predicted unstable yaw behavior of the vessel (fishtailing) while the model tests did not show such instability. This discrepancy between experiment and theory casts doubt as to whether the numerical simulation predicts correctly the vessel yaw motion. The work presented in this paper investigates the following two hypotheses to possibly explain the non-expected fishtailing in the numerical simulations: The mooring software may not accurately integrate non-linear differential equations that describe the yaw motion of the SPM vessel. Some damping terms may be under-estimated in the software (user input issue). To validate the integration scheme of the system of non-linear differential equations as implemented in the mooring software, a stability analysis has been conducted on a shuttle tanker moored to a West Africa deep water buoy. Variations of parameters like the hawser length, its axial stiffness and the vessel’s drag coefficients have been studied to explore their impacts on the vessel yaw stability. The approach is to identify without performing any time domain simulations, the domains of stability by linearizing the differential equations of SPM vessel’s yaw motion around its equilibrium point. The validity of the developed approach is then confirmed by performing time domain simulations of the same case. The second conjecture which may explain the non-expected fishtailing in numerical simulations was that some damping terms may be under-estimated. A semi empirical formula for the drag moment can be derived from rotation tests and comparisons were performed with the engineering model implemented in the mooring analysis software. The results show that by calibrating this damping term with the one derived from the experiments, the numerical simulations would match the stable yaw motion behavior as predicted during model tests. Following the above findings, a tool has been developed to fit the yaw drag moment engineering model based on experimental measurements, for any case of mooring analysis.
Pitting Corrosion and Its Influence on Local Strength of Hull Structural Members
Authors: Nakai Tatsuro , Matsushita Hisao , Yamamoto Norio
Published: November 2008

The objective of this study is to obtain basic data for discussing the structural integrity of aged ships, especially hold frames of aged bulk carriers. Firstly, shapes of corrosion pits observed on hold frames of bulk carriers have been investigated. It was shown that the shape of the corrosion pits is a circular cone and the ratio of the diameter to the depth is in the range between 8 to 1 and 10 to 1. Secondly, a series of tensile tests has been conducted to investigate the effect of pitting corrosion on tensile strength. It was pointed out that the tensile strength decreases gradually and the total elongation decreases drastically with the increase of thickness loss due to pitting corrosion. Thirdly, a series of 4-point bend tests with structural models which consist of shell, web and face plates simulating hold frames of bulk carriers has been carried out to investigate the effect of pitting corrosion on collapse behavior and lateral-distortional buckling behavior. Following the experiment, a series of non-linear FE-analyses has been also made. In the case where tensile load acted on the face plate, cracks were initiated at the bottom of the pits when pitting concentrated on the web near the face plate. On the other hand in the case where compression load acted on the face plate, lateral-distortional buckling has been observed and the ultimate load of the structural models where pitting developed regularly on the web was found to be almost the same as that of the structural models where the web has uniform corrosion corresponding to the average thickness loss.
A New Approach to the Design of Mono-Hull FPSOs
Authors: Khaw Tek , Rawstron Phil , Lagstrom Kaj
Published: November 2008

Problems and issues arising in the design and operation of mono-hull FPSOs have been apparent for many years. This has resulted in shortcomings of current design philosophy being highlighted. Environmental downtime of 10 to 14 days per year is not uncommon in both new-built and converted FPSOs. The financial loss resulting from this downtime during the life of a field, is a significant factor in the overall field economics. The importance of selecting the principal dimensions of the hull to optimise the natural periods of the FPSO is discussed in this paper. Converted FPSOs are therefore at a disadvantage as the natural periods are already set by the principal dimensions and mass properties, which may not suit the environmental conditions at site, potentially resulting in high downtime. However, it is also noted that preliminary design of new built-FPSOs has also tended to neglect this aspect of design and opportunities to optimise motion performance have often been lost by following standard ship building practices in selecting initial principal hull dimensions. This paper explores a new rational approach to the design of the FPSOs, relative to the field environmental conditions, particularly for deep water locations. The main objective is to achieve minimum downtime to avoid financial penalties caused by the associated loss of production. Preliminary hull sizing software for optimising the hull principal dimensions, relative to the mooring and riser requirements and the field environmental conditions for a given storage capacity and topside load is presented. Benefits for the riser and mooring systems and for hull material and fabrication costs are also outlined.
Hydrodynamic Analysis of Floating Bodies in General Bathymetry
Authors: Belibassakis K. A.
Published: November 2008

A hybrid technique, based on the coupled-mode theory developed by Athanassoulis & Belibassakis (1999) and extended to 3D by Belibassakis et al (2001) and Belibassakis & Athanassoulis (2004), which is free of any mild-slope assumption, is used, in conjunction with a boundary integral representation of the near field in the vicinity of the body, to treat the problem of hydrodynamic analysis of floating bodies in the presence of variable bathymetry. Numerical results are presented concerning floating bodies of simple geometry lying over sloping seabeds. With the aid of systematic comparisons, the effects of bottom slope on the hydrodynamic characteristics (hydrodynamic coefficients and responses) are illustrated and discussed.
F(P)SO Global Responses in the West of Africa Squall Environment
Authors: Zhong Zhibin , Luo Yong , Curic Dusan
Published: November 2008

Mooring design for F(P)SOs in West of Africa offshore environment is in many cases governed by the squall driven condition. In the past, the squall condition was typically analyzed by using the peak wind speed with associated wind direction. However, due to its inherent transient nature, the squall event formulated in the time history with varying wind speed and direction is more appropriate and could be potentially more critical for the mooring system design. This approach has been adopted in the design and analysis of recent F(P)SO mooring systems. The F(P)SOs are turret-moored in various water depths in offshore West of Africa. A series of squall time histories have been applied to predict the global responses of the F(P)SO in the time domain. Each squall time history, which provides a unique combination of wind speed and direction variations, is analyzed in five nominal directions covering a sector of 90 degrees from East to West. Squall time histories are also applied to analyze the tandem offloading operation. The results are compared with those of the conventional constant wind speed approach and a few interesting observations are made. The paper also provides some insights into the F(P)SO yaw motions, as well as their relations to the changing wind direction. Analysis results demonstrate that using the squall time series with changing wind speed and direction is more critical than the conventional constant wind speed approach in the tandem offloading scenario. It is therefore recommended that mooring analysis using squall time series should at least be used for the tandem offloading simulations.
Thermo-Mechanical Analysis of Buried Heated Pipelines in the Shore Approach Area
Authors: Solano Rafael Familiar , Vaz Murilo Augusto , Rego Vale´ria Souza
Published: November 2008

Often in heated pipeline projects in the shore approach area the pipeline is buried with the disturbed soil but a partial natural recovering inevitably takes place once the trench is open. This condition may not be enough to ensure the required pipeline soil support for its design life. This peculiar condition of shallow and insufficient soil support may lead to an upheaval buckling. Hence, this paper intends to develop a numerical model to simulate the thermo-mechanical buckling phenomenon of buried heated pipelines in shore approach areas. Furthermore, a sensibility analysis adopting geometrical, physical and operational data is carried out for a particular Petrobras project. The in situ geophysical survey, geotechnical and oceanographic information are also taken into account. The influence of the sediment transport after opening the trench and required backfilling with a heavier soil is also addressed. This paper focuses on the challenges for the design and installation group, due to the burial requirements, in order to find a safe solution for the burying process by trenching and backfilling known methods.
Load-Out, Towing and Float-Off Analysis for 105K COT of the First Ground Construction of Ship Type Structure
Authors: Kim Jong-Wook , Yang Young-Tae , Shin Hyun-Soo , Yoon Ki-Young
Published: November 2008

The 105K COT (105,000 DWT Crude Oil Tanker) is being continuously constructed at the ground yard of HHI (Hyundai Heavy Industries), Ulsan, Korea as the first ground construction for ship type structures. She is loaded out transversely from the quayside by skidding onto the DBU (Double Barge Unit). The DBU will then be towed to a pre-determined site and ballasted down to float-off the 105K COT from the DBU. This paper describes the motions, mooring forces, collision possibilities and constraint forces of DBU / 105K COT during load-out, towing and float-off of the 105K COT.
Higher Order Effects on Bending of Helical Armor Wire Inside an Unbonded Flexible Pipe
Authors: Tan Zhimin , Case Michael , Sheldrake Terry
Published: November 2008

This paper investigates the higher order geometrical effects on the deflections of a helical armour wire when the pipe is subjected to uniform bending and is away from any end effects from its remaining fittings (end fittings), and the effect on the subsequent bending stresses. Due to its complexity in both geometrical shape and loading conditions, the approaches found in the literature are often to assume either a bent helix or geodesic as its deflected configuration with linearized mathematical expressions for simplification. The bending stresses are then calculated based on the geometrical difference between the assumed deflection and the original helical shape. The effect of the wire cross-section characteristics, for example the width and thickness ratio, over its deflection are often ignored. This paper presents an analytical strain energy model to quantify the influence of the wire width and thickness ratio over its final deflection. The higher order geometrical effects are fully considered in determining the wire deflection by using the exact mathematical expressions, and in the subsequent wire deformation and stress calculations. The paper also discusses briefly the structural coupling behavior between the pipe axial and bending deformations raised by using the exact expressions. The analytical results are validated by finite element simulation of an identical structure. The results are shown in good agreement in both deflection and the bi-normal bending stress. The results also show desirable conservatism in the normal and the total bending stresses. The presented analytical approach is demonstrated as an efficient and conservative way for investigating the behaviour of such a helical wire.
The Effect of Reduced Water Depth on the Description of Extreme Waves and the Prediction of the Water Particle Kinematics
Authors: Katsardi Vasiliki , Swan Chris
Published: November 2008

This paper concerns the description of large waves in intermediate and shallow water depths. In deep water it is well known that the evolution of the largest waves is governed by linear dispersion. In contrast, as the water depth reduces the effects of wave dispersion are weakened and the relative significance of wave modulation shown to be increasingly important. This leads to very different extreme wave groups, the properties of which are critically dependent upon the directionality of the wave field. The paper also concerns the water particle kinematics arising beneath these nonlinear wave groups and contrasts fully non-linear predictions based on a state-of-the-art wave model with the results of the commonly applied design wave solutions. To explore these effects, and to provide a physical explanation for their occurrence, two wave models are employed. The first, proposed by Bateman, Swan & Taylor [1, 2], allows fully-nonlinear descriptions of the evolution of large waves in realistic seas, involving a significant spread of wave energy in both frequency and direction. The second is a wave evolution equation based upon the early work of Zakharov [3] and written in Hamiltonian form by Kasitskii [4]. This model is only valid to a fourth-order of wave steepness, but has the over-riding advantage that it gives physical insight into the evolution process.
Efficient Dynamic Analysis of a Combined Spar System via a Frequency Domain Approach
Authors: Spanos Pol D. , Ghosh Rupak , Finn Lyle D. , Botros Fikry , Halkyard John
Published: November 2008

The response of a combined Spar/ risers/mooring lines system is conventionally determined by conducting nonlinear time domain analysis. The system nonlinearity is introduced by the mooring nonlinear force, the friction between the buoyancy-can and the preloaded compliant guide, and the quadratic model of the fluid related damping. Obviously, during the design process, it is important to understand the sensitivity of the Spar responses to various parameters. To a great extent, these objectives cannot be readily achieved by using time domain analysis since, in this context, elements with frequency dependent representation such as the added masses and supplementary damping must be incorporated in the analysis; this may require the use of elaborate convolution techniques. This attribute of the time domain solution combined with the necessity of running a significant number of simulations makes it desirable to develop alternative methods of analysis. In the present paper, a frequency domain approach based on the method of the statistical linearization is used for conducting readily a parametric study of the combined Spar system. This method allows one to account by an equivalent linear damping and an equivalent linear stiffness for the mooring nonlinearity, friction nonlinearity, and the damping nonlinearity of the system. Further, frequency dependent inertia and radiation damping terms in the equations of motion are accommodated. This formulation leads to a mathematical model for the combined system, which involves five-by-five mass, damping and stiffness matrices. In the solution procedure, the equivalent parameters of the linear system are refined in an iterative manner, and by relying on an optimization criterion. This procedure is used to assess the sensitivity of representative Spar system responses to various design parameters. Further, the effect of various design parameters on the combined system response is examined. The environmental loadings considered are of the JONSWAP format of a 100-yr hurricane in the Gulf of Mexico.
Deepwater Riser Interaction: Computational Method and Validation
Authors: Herfjord Kjell , Holma˚s Tore
Published: November 2008

A method for predicting interaction between risers is presented. The method is efficient and handles different riser systems exposed to complex environmental data. To achieve this, the method utilizes pre-established data for forces on risers in close proximity. Interaction effects concerning mean values as well as the dynamic forces at vortex shedding frequencies are stored. The interaction effects cause large wake induced oscillations (WIO), and the vortex induced vibrations (VIV) are influenced as well. The method uses the database of forces in a strip theory manner to obtain excitation forces on risers and other slender bodies exposed to current. The method is implemented in a non-linear dynamic finite element tool, HYBER. During the simulation, the clearance between the risers is followed up. If collisions occur, relevant data are recorded, and the simulation continues. Fatigue and possible single event damage are assessed. In the present paper, results from the method are compared with comprehensive measurements in model scale, and the computed results show good agreement with the measurements. Databases exist for bare cylinders with equal and unequal diameters as well as for two geometries of strakes.
Crack Growth in High Strength Chain Steel Subjected to Fatigue Loading in a Corrosive Environment
Authors: Lassen Tom , Arana Jose L. , Canada Luis , Henriksen Jan , Holthe Nina K.
Published: November 2008

The present article presents the fatigue crack growth behavior of new high strength steels designated R4S grade. Eight Compact Tension (CT) specimens with 25 mm thickness were subjected to constant amplitude loading while exposed to seawater without and with cathodic protection. The Cathodic Potential (CP) was set to −890 mV and −1100 mV relative to an Ag/AgCl reference cell. Rates in air are included as a reference. The crack growth parameters were determined from a linear relation between da/dN and δK for a log-log scale. The derived figures are given in the table below. The figures for dry air and cathodic protection are valid for δK between 15 and 30 MPam0.5. Below this range the slope m of the linear relation will change and further investigations have to be carried out for this region. The figures for free corrosion are valid for δK values from 10 to 30 MPam0.5. The threshold value for δK is close to 5 MPam0.5 in this case. The measured growth rates were compared with the rates for medium strength carbon manganese steels found in rules and regulation, i.e. BS7910. The present growth rates are well within the scatter band given for these steels in air and free corrosion. The present growth rates found in seawater with cathodic protection are however substantially lower than the rates given in BS7910. When a cathodic potential of −1100 mV was applied, crack closure was observed at medium levels of δK. The explanation is the formation of calcareous deposit in the wake of the crack front that gives significantly reduced growth rates and finally leads to crack closure. This finding is a surprise for a high strength steel. The results are promising and should be investigated further. Finally, a linear elastic fracture mechanics model was established to study the fatigue behavior in a stud-less link. The model was used to construct S-N curves that are consistent with experimental fatigue lives and the design curve given in the DNV rules. The present growth parameters were used in conjuction with a crack-like initial flaw with depth in the range from 0.12 to 0.20 mm. The difference found between the growth rates in dry air and in free corrosion are in accordance with tested fatigue lives for these two environments.
An Analytical/Numerical Procedure for Structural Analysis of Hybrid Riser Systems
Authors: Rodrigues G. J. O. , Cardoso Daniel C. T. , de Lima Beatriz S. L. P. , Jacob Breno P. , Fernandes Antonio C.
Published: November 2008

In deep and ultra-deep water petroleum exploitation activities, floating production systems such as semi submersible platforms and FPSO (Floating Production, Storage and Offloading) units have been commonly employed. However, the utilization of flexible risers in ultra-deep waters has been hindered by technical and economical reasons. On the other hand, first order motions from the floating unit due to environmental loads are not favorable to the use of Steel Catenary Risers (SCR) in a free-hanging configuration. This fact has motivated several studies on hybrid riser systems, including the system studied in this work, which is based on a sub-surface buoy with large dimensions, moored to the seabed by tethers. This system employs flexible lines connecting the floating unit to the buoy, in the region where dynamic effects are more relevant due to the floating unit motions, and also SCRs that extend from the buoy to the seabed, in the region where dynamic motions are not so significant. The objective of this work is to describe a solution procedure for the analysis of such a hybrid riser system. This procedure is based on an analytical formulation that is solved numerically. One of the main features of this procedure is the fact that it takes into account the effects of current loads acting on the lines. Current profiles can be considered, with direction and velocities varying with depth, therefore configuring a full three-dimensional solution. This procedure can be employed either as a preliminary static analysis tool, to be used in parametric studies in order to assess the feasibility of candidate configurations of hybrid riser systems, or else for the generation of finite-element meshes for a full time-domain nonlinear dynamic simulation. It is important to start the dynamic simulation from a statically balanced configuration, since the transient effects can be dramatically shortened and the total simulation time can be reduced. The results obtained from this procedure are compared with a discrete solution obtained using a nonlinear finite-element based solver. The strategy considered here is intended to be an approach that will speed up the tasks involved in the design of hybrid risers systems based on the subsurface buoy concept.
CFD Analysis of Vortex-Induced Motions of Bare and Straked Cylinders in Currents
Authors: Thiagarajan K. P. , Constantinides Y. , Finn L.
Published: November 2008

It is widely acknowledged that the use of helical strakes for mitigation of vortex-induced motions (VIM) of surface piercing cylinders, such as spar platforms, is only partially effective. Using computational fluid dynamics tools, we compare the oscillation characteristics of a bare cylinder and a straked cylinder in uniform currents. Our model comprised of a straked cylinder with diameter of 0.741 m, aspect ratio of 1:1.9 and three helical strakes of height 13% of cylinder diameter. This geometry corresponds to the hard tank geometry of a scaled truss spar model known to exhibit VIM in tow tank testing. In the CFD simulations the cylinder is moored with linear springs to provide a range of reduced velocities. The fluid domain is made of an unstructured grid comprising of hexahedral elements. Fluid structure interaction utilizes grid stretching and a user defined function for solving the equations of motion. Turbulence modeling uses Detached Eddy Simulation (DES) and the boundary layer is modeled using a wall function with a surface roughness of 0.0003 m. Reynolds numbers are in the range of 50,000 to 100,000. Results for straked cylinder compares reasonably with published results, but under-predicts the peak response. In comparing with corresponding results for a bare cylinder without strakes, the spectral features of the transverse displacement show variations, which are found to be due to the spoiling effect of the strakes.
Ice Loading of Jack-Up Platforms
Authors: Weihrauch Alexandra , Berger Joachim , Bartels Marc
Published: November 2008

The use of drilling platforms for exploration drillings in ice-covered shallow water is expected to increase in the future. Jack-up platforms (JU) are regarded to be most suitable for exploration drillings due to their high mobility, their facile transportation and installation. Although having proved their capability for harsh environments, e.g. in the North Sea, jack-ups have neither been especially designed for nor have been operated frequently in icy waters. The problems associated with the operation of jack-ups in ice waters are related to structural integrity as well as to operational limitations. One objective of the research project MATRA-OSE was to check the design of JU for exploration and oil production in ice-covered, shallow-water areas. The global stability of JU as well as local and global loads acting on a three-legged jack-up drilling unit were investigated. The design loads were determined by analytical approaches. Different ice load cases were defined as the basis for the detail design. These load cases include local loads on single members of the platform legs like chords or braces as well as global loads, which may act on one individual leg or on several legs of the platform. Vertical forces, which result from a change of the water level (up-surge and down-surge) in case ice sheets are frozen to the platform legs were considered. Also the impact of ice being jammed between the platform legs was investigated. It has been found out that the most critical load case results from the ice being jammed between the platform legs. It should therefore be tried in practice to avoid the ice jamming by proper ice management, as the large ice loads calculated for this scenario may result in problems for the stability of a JU. Ice breakers could be used to pre break the ice and should try to clear the site from broken ice pieces before it starts to accumulate.
Improved Model for Ice Force on Conical Structure
Authors: Feng W. , Shi Z. M. , Liu L. M.
Published: November 2008

Ice force is an important factor to be taken into account for offshore structures in cold region, and the calculation method of the ice force is meaningful for the offshore structure design. Cone is now used as optimal ice-resistant structure because it can cause bending failure of the ice sheet. The interaction between ice sheet and conical structure is studied in this paper and Croasdale’s model is modified based on the field observations. The newly built model separates the ice sheet into emersed part and floating part, and the equilibrium analyses are carried out respectively. The bending moment distribution of the ice sheet is analyzed to determine the position of bending failure, which serves as a supplementary restriction. Analytic solution of ice force on conical structure is got and it is verified by the experimental data of previous researches.
Viscoelastic Analysis of Bend Stiffeners
Authors: Caire Marcelo , Vaz Murilo Augusto , de Lemos Carlos Alberto Duarte
Published: November 2008

Bend stiffeners are polymeric structures employed to ensure a smooth and safe transition in the upper connection of risers and umbilical cables, protecting them against accumulation of fatigue damage and excessive curvatures. Recent failures have stimulated a better understanding of the mechanical response in order to increase the reliability in design and analysis of bend stiffeners. This work presents a mathematical formulation that represents the system riser/bend stiffener considering geometric non linearity and polyurethane with viscoelastic behaviour, an inherent characteristic to polymers. The following assumptions are considered: cross-sections remain plane after deformation, large deflections are accepted but it is a small strain bending problem, the self-weight and external forces are disregarded and the material is assumed with linear viscoelastic behaviour. The curves that represent the viscoelastic response of the material have been raised by means of creep tests, whose specimens were cut from actual bend stiffeners. The time dependent data obtained in the experimental tests were well approximated by a third order Prony series which describes the creep function. The set of four first order non linear ordinary differential equations results from geometrical compatibility, equilibrium of forces and moments and linear viscoelastic constitutive relations. The numerical solution of the problem is obtained using a one-parameter shooting method. The results are then compared with the consolidated numerical solution for linear elastic material. It is concluded that the viscoelastic phenomena can lead to excessive curvatures on the upper terminations of risers and umbilical cables if the polymeric structure were designed considering elastic behaviour. The correct characterization of the viscoelastic properties of polyurethane used on bend stiffeners must be taken into account when accurate analysis is desired.
The Hydrodynamic Efficiency of an Oscillating Water Column
Authors: Morris-Thomas M. T. , Irvin R. , Thiagarajan K. P.
Published: November 2008

An oscillating water column device enables the conversion of wave energy into electrical energy via wave interaction with a semi-submerged chamber coupled with a turbine for power take off. This present work concentrates on the wave interaction with the semi-submerged chamber, whereby a shore based oscillating water column (OWC) is studied experimentally to examine energy efficiencies for power take-off. The wave environment considered consists of plane progressive waves of steepnesses ranging from kA = 0.01 to 0.22 and water depth ratios varying from kh = 0.30 to 3.72, where k, A and h denote the wave number, wave amplitude and water depth respectively. The key feature of this experimental campaign is a study on the influence of geometrical parameters of the front wall on the OWC’s performance. More specifically, these parameters include: front wall draught; thickness; and aperture shape. We make use of a two-dimensional inviscid theory for an OWC for comparative purposes and to explain trends noted in the experimental measurements. The work undertaken here has revealed a broad banded efficiency centred about the natural frequency of the OWC. The magnitude and shape of the efficiency curves are influenced by the geometry of the front wall. Typical resonant efficiencies of the OWC are in the order of 70%.
Practical Implementation of the Polynomial Representation of Potential Damping in Time Domain Simulations
Authors: van Zutphen Hermione J. , den Haan Joost
Published: November 2008

Time domain simulations are required when analyzing nonlinear vessel behaviour. The usual approach conducting time domain simulations is to transform a complex valued function of frequency dependent damping and added mass to a convolution integral in the time domain. Evaluating the integrals during time domain simulations is computational expensive and the accuracy of the calculation of the limit value of added mass in diffraction calculations is dependent on the panel size of the model. In this paper, an alternative approach based on a polynomial model for damping proposed by K.E. Kaasen et al is extended from a single degree of freedom to a 6 degrees of freedom model of a heavy lift barge. Polynomials for contributions of velocity to the damping force are constructed generically using a least square curve fitting method. The polynomials then are transformed to the time domain counterpart using a state space representation. The quality of the fits of the damping function has a large influence on the resulting damping force in time domain. Furthermore, the higher the order of the differential equation, the larger the number of variables to integrate during a time domain simulation. Consequently, the presented method is not necessarily more efficient in simulations than the traditional retardation functions.