bluuebunny/crossref_metadata_2025 huggingface dataset:
The Dynamics of Drop Impact
Authors: Josserand Christophe
Published: March 2009

Inkjet printing, rainfall, droplet collision in combustion chambers are different forms of drop impacts. The whole dynamics of these impacts is complex and remains far to be fully understood. In particular the role of the viscosity of the drop liquid is still hard to exhibit. In one hand, the early time of the impact should be considered inviscid, and viscous free calculation give a fair approximation of the short time dynamics. On the other hand, experimental evidences show that the transition between splashing dynamics and deposition is controlled by a so-called splashing parameter K = We · sqrt(Re), where the viscosity enter through the Reynolds number Re (We being the Weber number). Therefore the role of the viscosity for the early time of the impact needs to be elucidated. We will present numerical simulations of the impact of a drop on a liquid layer thanks to a volume of fluid technique (VOF), where the Navier-Stokes equations are solved for both liquid and gas phases. For a given Weber number, we will vary only the viscosity so that viscous effects can be emphasized. The calculation will also determine the relative spreading of the drop inside the liquid layer. For splashing behaviors, a jet is emitted soon after the initiation of the impact; contrarily, no jets are present when deposition happens. The pressure field and the velocity field are studied near the neck of the impact and show no specific dependance on the viscosity. However, viscous effects are observed through the diffusion of the vorticity from the interface into the liquid bulk. Therefore, the viscous length lv = sqrt(vt) controls the gradient fields at the impact and we observe that the width of the emitted jet is determined by this length. Therefore, applying mass conservation to a dynamical solution where a jet of width lv is created, we can estimate the balance between mass ejected by the falling drop with mass coming from a retracting jet. The growth of a jet is thus controlled by this mass balance and the splashing parameter law is retrieved. In particular, the viscous effects appear in the theory as a singular perturbation of the inviscid impact dynamics. Self-similar solutions of the impact are therefore considered in specific gometries. More information at http://www.lmm.jussieu.fr/MEMBRES/JOSSERAND/josserand.html.
Development and Validation of an X-Ray Tomograph for Two-Phase Flows
Authors: Hervieu Eric , Jouet Emmanuel , Desbat Laurent
Published: March 2009

This paper describes the development and validation of a high spatial resolution X-ray tomograph devoted to investigate air-water two-phase flows. The device hardware is mainly composed of a 60 keV X-ray source, a detector and an accurate mechanical bench. This paper concentrates on accuracy quantification and emphasis is given on the reconstruction procedure. It is well known that absorption gradients induce reconstruction artifacts when using standard algorithms based on uniform regularization. In the particular case of two-phase flows in pipes, this leads to a poor measurement accuracy in the vicinity of the walls. To overcome such effects, improved algorithms have been developed in this study, involving different spatially adaptative regularization methods. A first calibration performed on static phantoms clearly exhibits the benefit brought by such advanced reconstruction algorithms. A validation procedure has been carried out on an air-water bubble column, equipped with an optical probe which can be translated in order to explore the 80 mm × 80 mm square cross section. Comparisons of local void fraction measurements have been performed pixel by pixel, and demonstrate the accuracy improvement induced by the advanced reconstruction algorithms.
An Assessment of Margin in Design Steam Hammer Forces for Combined Cycle Power Plants
Authors: Zheng D. , Vieira A. T. , Jarvis J. M.
Published: March 2009

All combined cycle steam plants have rapid-closing stop valves in steam lines to protect the turbine. The rapid valve closure produces a steam hammer in the piping resulting in large forces for which the piping system and supporting structures need to be designed. These forces are typically calculated using the classical Method Of Characteristics (MOC) solution. An evaluation has been conducted which compares the forces computed using the classical methods with a best-estimate approach. This comparison has been done to define margin, and to benchmark and identify potential refinements in the techniques used for evaluating steam hammer loads. The best-estimate approach involves the use of the RELAP5 computer program. RELAP5 is used extensively in the Nuclear Industry to evaluate fast thermal hydraulic transients. It has the capability to analyze subcooled liquid, two-phase and saturated or superheated steam piping system. The models used in RELAP5 are best estimate results in comparison to the MOC solution which are mathematically derived from theory. The compressible flow program GAFT is used to obtain the MOC solution. The main steam line of a single Heat Recovery Steam Generator combined cycle plant is modeled with both the GAFT program and with a PC version of RELAP5. Identical piping lengths, mass flow rates, pressures are used in each model. Also, a stop valve closure time of 100 milliseconds is modeled. As RELAP5 output results are pressure, flow rate, velocity, and density, the resultant forces are generated using the R5FORCE program, a post-processor to compute associated transient forces on straight piping links. The GAFT program, which is specifically designed to compute steam hammer forces, computes the force history internally on straight piping lengths. A comparison of the peak force from GAFT and from RELAP for every piping link has been generated. Through the comparison, both RELAP5 and GAFT have been verified for the evaluation of rapid valve closure reaction loads. The comparison also shows that the classical method typically over-predicts the best-estimate solution by 15% to 20% for straight piping links. Although not confirmed, a better agreement between the two methods would be expected if a more accurate steam sonic velocity correlation and valve closure model are incorporated into the classical solution. Theis study helps to quantify the degree of conservatism inherent in the classical approach.
Overall Performance of H2/O2 Cycle Power Plants Based on Steam-Methane Reforming
Authors: Gambini M. , Vellini M.
Published: March 2009

In this paper the overall performance of a H2/O2 cycle has been evaluated. This typology of power plants requires a material and energetic integration with hydrogen and oxygen production systems. The steam-methane reforming, for the hydrogen-rich fuel production, and the cryogenic air separation, for the oxidiser production, have been investigated, quantifying all their thermal and mechanical requirements. At first, the thermodynamic performance of this cycle has been evaluated, considering the presence of incondensable gases, owing to the processes that provide H2 from fossil fuels and O2 from air. Then, the power plant has been integrated with the oxidiser and fuel production plants as well as with the fuel compression section. Including all the material and energetic flows, the overall performance has been evaluated. The final result is not very encouraging: in fact, even if the H2/O2 cycle has relevant thermodynamic performance if the energetic requirements for oxygen and hydrogen productions are neglected — the efficiency is over 62% —, the overall performance of H2/O2 cycle power plants based on steammethane reforming is very low — the net efficiency attains only the 26%.
Exhaust Gas Analysis in a Catalytic Combustion System With a Methane Mixture
Authors: Hayashida Kazuhiro , Amagai Kenji , Arai Masataka
Published: March 2009

Exhaust gas from the catalytic combustion of methane mixture was analyzed experimentally. A palladium catalyst supported on a cordierite honeycomb was used. The methane/air mixture, which was pre-heated to 530K, was introduced into the catalyst. Combustion states were investigated under various conditions of the equivalence ratio. In the case of lean mixture, homogeneous surface reaction in the catalyst was observed, and combustion reaction was completed within the catalyst. However, in the case of rich mixture, a thermal combustion flame appeared after the catalyst with the homogeneous surface reaction. In order to investigate exhaust gas composition, exhaust gas was sampled by a quartz probe, and was analyzed by a FTIR. Furthermore, low level NO in the exhaust gas was detected by a LIF method. In order to obtain the NO concentration from the NO fluorescence, temperature dependency of the fluorescence was obtained by using a NO/N2 mixture. NO concentration in a exhaust gas just after the catalyst was highest when the equivalence ratio was 0.3. When the thermal combustion flame appeared after the catalyst, CO and CH4 were detected just after the catalyst. These concentrations decreased around the thermal combustion flame. This result suggests that unburnt CH4 and CO which was produced by a partial combustion in the catalyst were converted in a gas phase reaction. In this case, although NO was hardly detected just after the catalyst, it was detected around the thermal flame.
Alstom Power’s Flash Dryer Absorber for Flue Gas Desulfurization
Authors: Ahman Stefan , Barranger Charles B. , Maurin Peter G.
Published: March 2009

ALSTOM’s Flash Dryer Absorber (FDA) system is a dry FGD technology, which aims for simplicity and compactness. This first installation was in Poland at the 2×120 Mwe Lasiska site, and was commissioned in 1996. Since then there have been a number of important developments of technology, notably combination with an ESP, use of integrated lime hydration and use of ash from CFB as the reagent; supported by an ongoing test program. The technology also looks promising for the retrofit area. The combined full-scale plant for the removal of flyash and SO2 can often be fitted into the space occupied by the existing electrostatic precipitators. There are seventeen commercial installations of FDA in various stages of operation, construction, and design.
Numerical Optimization of Stator Blade Sweep and Lean in an LP Turbine Stage
Authors: Lampart Piotr
Published: March 2009

The paper describes results of direct constrained optimization of stator blade axial sweep and circumferential lean for the exit stage of a large power steam turbine, using Nelder-Mead’s method of deformed polyhedron. Values of the minimised objective function, that is stage losses with the exit energy are found from 3D viscous compressible computations. Turbulence effects are taken into account using Menter’s SST model. Among the optimized parameters are stator and rotor stagger angles, stator straight sweep and straight lean, stator compound sweep at the tip and compound lean at the root, giving the total number of 8 optimized parameters. The blade sections (profiles) are assumed not to change during the optimization. There are constraints imposed on the mass flow rate, exit swirl angle and reactions. A number of optimization runs is reported. First, optimization of the exit stage with stator blade circumferential lean, second, optimization of the stage with stator blade axial sweep, then, the stage is optimized with both sweep and lean of stator blades. In the above tasks, the process of optimization is carried out for a nominal load, however, due to the fact that exit stages of steam turbines operate over a wide range of flow rates away from the nominal conditions, the original and final geometries are also checked for low and high loads. Optimization gives designs with new 3D stacking lines of stator blades, and with significantly increased efficiencies, over large part of the assumed range of load, compared to the original design.
New Pipelines, New Opportunities
Authors: Cleveland Tony , Milinusic Mike
Published: March 2009

The development of new pipeline systems through lands where the primary fuel source has been fuel oil or wood offers unique opportunities for the utilization of natural gas from the pipeline. The use of gas in place of oil can improve local economies; reduce heating and energy cost and lower the emissions produced from energy sources. A computer simulation has been built that can examine all the options for the use of gas from a pipeline system, considering direct replacement of fuel oil with a local gas pipeline system, the generation of electric power — locally or centralized at the pipeline, and the option of using cogeneration with the pipeline compressor stations. The simulation provides both graphic and tabular presentation of the results to show payback and the influence of changes in key parameters. The paper discusses this program and the possible applications to areas hitherto remote from the supply of natural gas for fuel.
Development and Evaluation of Improved Model for Engineering Critical Assessment of Pipelines
Authors: Jaske C. E. , Beavers J. A.
Published: March 2009

In past work, the authors developed a very useful model for engineering critical assessment (ECA) of pipelines with stress corrosion cracking (SCC). That model uses the effective area method to characterize the crack-depth profile along with flow strength and fracture toughness failure criteria. In the current work, the model was improved in four areas. Tearing instability was added to the fracture toughness failure criteria, formulations for computing values of the J integral for surface cracks were improved, interaction criteria were developed for co-planar flaws, and relationships for estimating values of the strain-hardening exponent were developed. To help validate the improved model, J fracture toughness tests were conducted using compact-tension (CT) specimens, and burst tests were conducted on pipe samples with large flaw length-to-depth ratios.
Development, Field Testing and Implementation of Automated, Hydraulically Controlled, Variable Volume Loading Systems for Reciprocating Compressors
Authors: Hickman Dwayne A. , Slupsky John , Chrisman Bruce M. , Hurley Tom J.
Published: March 2009

Automated, variable volume unloaders provide the ability to smoothly load/unload reciprocating compressors to maintain ideal operations in ever-changing environments. Potential advantages provided by this load control system include: maximizing unit capacity, optimizing power economy, maintaining low exhaust emissions, and maintaining process suction and discharge pressures. Obstacles foreseen include: reliability, stability, serviceability and automation integration. Results desired include: increased productivity for the compressor and its operators, increased uptime, and more stable process control. This presentation covers: (1) system design features with descriptions of how different types of the devices were developed, initial test data, and how they can be effectively operated; (2) three actual-case studies detailing the reasons why automated, hydraulically controlled, variable volume, head-end unloaders were chosen over other types of unloading devices; (3) sophisticated software used in determining the device sizings and predicted performance; (4) mechanical and field considerations; (5) installation, serviceability and operating considerations; (6) device control issues, including PC and PLC considerations; (7) monitoring of actual performance and comparison of such with predicted performance; (8) analysis of mechanical reliability and stability; and (9) preliminary costs versus return on investment analysis.
Pipeline Risk Mitigation Study
Authors: Pillay Anand
Published: March 2009

This paper presents a case study, which is currently being carried out on a high-pressure sub-sea gas export pipeline. The main objective of the case study is to determine if the risk mitigation measures that are already in place are sufficient considering the level of risk for different sections of the pipeline. The methodology utilizes a Geographical Information System and the COmputer Assisted Shipping Traffic (COAST) database to determine the level of risk caused by vessels traveling over the pipeline. The factors considered are damaged due to anchor drop and drag, vessel foundering and grounding over the pipeline. Live vessel tracks obtained from nearby radar stations are used to determine the annual traffic volume, size, type and speed of vessels as well as vessel headings. This information is then fed into the COAST database and presented graphically. The pipeline is then divided into equal area cells and the probability of anchor dropping and dragging and vessel foundering and grounding are calculated based on the type, size and speed of the vessels identified to have passed over the pipeline. The results of the frequency of occurrences and fatalities are presented and then evaluated against the set As Low As Reasonable Practicable (ALARP) level. The “hot spots” of the pipeline are identified and a base case study is carried out for the risk reduction measures that are in place (if any) for each of the “hot spots” identified. Alternative risk reduction measures are considered and a cost benefit analysis is carried out to determine the most feasible option. The risk levels are then recalculated with the risk reduction measures in place to see if this has reduced the risk to ALARP. The COAST database is thought to be very useful as it can be updated to reflect the current navigational practices of vessels and hence, it would be easy to update analyses as and when required.
Development of Acceptance Criteria for Mild Ripples in Pipeline Field Bends
Authors: Rosenfeld M. J. , Hart James D. , Zulfiqar Nasir , Gailing Richard W.
Published: March 2009

Field bends in large diameter pipe are routinely used in the construction of oil and gas pipelines. Mild ripples along the intrados are often unavoidable where such bends have a high D/t or high yield strength. Present regulations and industry standards differ in their treatment of mild ripples, consequently, the acceptance of such features has been heretofore inconsistent. A review of prior work on the subject was undertaken. Shell finite element analysis was then used to estimate the effect of ripple magnitude and spacing on stresses due to pressure and bending. Stress concentration effects were used with a suitable fatigue damage rule to estimate the effect of ripple parameters on service life. Results were benchmarked against the available test data.
Effect of Changes in Tensile Properties Due to Cold Bending on Large Deformation Behavior of High-Grade Cold Bend Pipe
Authors: Fukuda Naoki , Yatabe Hiroshi , Masuda Tomoki , Toyoda Masao
Published: March 2009

The large deformation behavior of cold bend was experimentally and analytically investigated. Full-scale large deformation experiments were conducted on two API X80 grade cold bends with a bending angle of approximately 9 degrees for both closing and opening modes. Finite element (FE) analyses were also conducted to simulate the large deformation behavior by considering the distribution of tensile properties after the cold bending process. The results of the simulation were in good agreement with the large deformation experiments. The deformability of cold bend in the opening mode was greater than that in the closing mode. Changes in the tensile properties due to the cold bending process had a large influence on the deformability of cold bend. In particular, distribution of the part with work hardening after the cold bending process had large effect on the deformability in the closing mode.
Risk Management to Support Maintenance Planning and Investment in Gas Transmission Pipelines
Authors: Martinez Gonzalez Jose L. , Urencio Castro Claudio F. , Betancourt Enrique Rodriguez
Published: March 2009

Since 1999 PEMEX Gas (PGPB) has adopted a risk management scheme as common practice to support maintenance programming in a pipeline network of more than 11,000 kilometres. The risk management approach has been extensively used in the 14 working sites that comprise the pipeline division. Taking into account that over the past four years the pipeline incident rate has been significantly reduced and has thus lessened the consequence of failure cost in terms of reduced service interruption, property damage and safety, PGPB decided to re-evaluate and upgrade its risk management programme by enhancing the risk assessment tool to better reflect increased awareness and experience with the evaluation of the pipeline network risks. Decisions to implement risk–reduction activities derived from the results of the risk assessment phase are addressed in terms of “Dollars of Risk” by a quantitative risk model. Development of Tolerable Risk Criteria (Ref. 2) indicated the amount of risk that PGPB is willing to maintain in its pipeline facilities. This paper shows a case study of a risk management application, describing the benefits of having a methodology available to support maintenance planning and investment in pipeline systems.
Development of Large Diameter X70 High Toughness HSAW Linepipe for Gas Transmission
Authors: Huo Chunyong , Ma Qiurong , Li Weiwei , Feng Yaorong , Li Helin , Ma Yingli , Liu Wencheng
Published: March 2009

X70 large diameter linepipe with helical seam SAW were developed, with 1016mm OD and 14.6mm WT. Acicular ferrite type linepipe steel is adopted for the base material, which was found having high toughness and low yield strength loss after pipe forming. The very stringent requirements for toughness, i.e. 190J/140J for average/minimum for pipe body and 120J/90J for average/minimum for weld and HAZ were meet successfully. The yield strength loss due to Bauschinger effect was found lower than 20 MPa, which benefited.
Effect of Loading History on Hydrogen Content in Pipeline Steels
Authors: Vadhwana N. M. , Chen W.
Published: March 2009

The application of high strength pipeline steels for oil and gas transmission is believed to provide greater gas flow capacity due to increased design pressure, and reduced line pipe cost due to material tonnage savings. However, the use of high strength pipeline steels is concerned with high risk of brittle failures such as hydrogen induced cracking, fractures due to low ductility. In this study, three grades of modern pipeline steel (X65, X80, X100) were examined to determine their susceptibility to hydrogen permeation and hydrogen trapping under the influence of various mechanical loading conditions. The steel samples were placed in a solution of sulfuric acid poisoned with arsenic trioxide to create an environment where hydrogen can enter the steel. Initially, round bar samples were charged for various times at a low current density to establish that 24 hours was a sufficient charging time for the three steels. Tensile samples were loaded and held at stress levels corresponding to the respective yield strength and the amount of hydrogen entering the steel was then measured. The stress, normalized to the yield strength, and hydrogen contents, normalized to as received contents, were used to rank the three steel grades and to find the steel that was the most susceptible to hydrogen entry. For the samples charged prior to loading, two times as much diffusible hydrogen was found in the X100 as compared to the other steels, but the trapped hydrogen content was equivalent. Four loading conditions were used for each grade of steel: 1) 2% strain; 2) 2% strain and hold at load for 24 hours; 3) 2% strain then 100 cycles at R = 0.1; and 4) 2% strain, 100 cycles at R = 0.1 then hold at load for 24 hours. For the loaded samples, the amount of hydrogen, both diffusible and trapped increased with load severity, with the highest amounts found in the highest grades of steel. The most pronounced increase was not found in the X100, but in the X-80 steel. Micro structural features, such as banded structure, seemed to have a more prominent role on the hydrogen content of the X100 than in the other steels as it seemed less affected by the loading condition than by charging time.
Satellite-Based Monitoring of Subsidence Ground Movement Impacting Pipeline Integrity
Authors: Youden James , Power Desmond , Han Ping , English Jerry , Gailing Rick , Rizkalla Moness
Published: March 2009

Ground movements due to a range of governing mechanisms are recognized to pose hazards to the operating integrity of pipelines in California. As part of an extensive technology management program, Southern California Gas Company (SoCalGas) is involved in the development and implementation of satellite-based monitoring of subsidence ground movements impacting pipeline integrity. By both hosting a Gas Technology Institute (GTI) and Pipeline Research Council International (PRCI) study and undertaking an internal study, SoCalGas is supporting the investigation of two aspects of this promising pipeline integrity management technology. The current project of monitoring ground subsidence due to oil production in the San Joaquin Valley utilizes synthetic aperture radar (SAR) to derive sub-centimeter ground movement measurements from February to September, 2001. The estimates of the subsidence derived from the SAR data are compared with GPS survey results taken at 65 monuments. In addition, archived SAR data from 1992 to 2000 are used to better estimate the movement that has occurred there over the past decade.
Theoretical Study of Surge and Rotating Stall in a Compression System With Wet Compression
Authors: Wang Yunhui , Li Ping , Sun Yufeng , Wang Guoxue
Published: March 2009

A Moore-Greitzer-typed model is established for a compression system with wet compression in this paper, which can reveal effects of wet compression on transients of this compression system when surge or rotating stall occurs. Simulation results show that wet compression affects compressor stable characteristic greatly, as a result post-stall transients of this system will be changed when wet compression is used and what is more interesting is that under some situations wet compression can be used to eliminate surge and rotating stall.
Evaluation of Pipeline Control Center Operator Skills Utilizing High Fidelity Pipeline Simulation Technology
Authors: Fradette Rick
Published: March 2009

The U.S. Department of Transportation Operator Qualification Rule requires pipeline Control Center Operators to be evaluated on their ability to perform “covered tasks”, and recognize / react to abnormal operating conditions. ChevronTexaco has chosen to utilize pipeline simulation technology as a skill evaluation tool for its Control Center Operators. Metso Automation was selected to provide the pipeline simulation-based evaluation system. This paper provides an overview of the pipeline simulation application system that was selected and implemented by ChevronTexaco.
The Evolution of Data Management to Empower Integrity Management Decisions: A Case Study of an Enterprise Implementation
Authors: Dupuis Bruce , Humber Jason
Published: March 2009

BP’s Natural Gas Liquids business unit (NGLBU) has conducted integrity investigation and mitigation activities on its pipelines and has been following this best practice for numerous years. In recent times, NGLBU’s data management initiatives focused on establishing an enterprise Geographic Information System (GIS) coupled tightly with a derivative of the Pipeline Open Data Standard (PODS) data model. During successful implementation of the GIS, an analysis identified gaps in existing data management processes for pipeline integrity information. Consequently, the business unit adopted Baseline Technology’s Pipeline Information Control System (PICS) and its modules to support the pipeline integrity decision-making process on its 9000km of pipeline. The PICS implementation leverages the existing GIS implementation while addressing a number of unresolved data management and integration issues, including: • Integration of inline inspection with excavation results; • Migration of above ground surveys to a common repository; • Integration of multiple inline inspections; • Facilitation of corrosion growth modeling; • Structured process for prioritization of remediation; • Structured process for integration of inline inspections with risk parameters; • Defined data collection, storage, and integration standards. Data management solutions based solely on a GIS require pipeline surveys without explicit positional information to be converted into a common linear reference system (typically chainage or stationing) such that disparate data sets may be overlaid and compared. This conversion, or spatial normalization, process is where much of the data management effort is spent and is often prone to error introduction. Even when small errors are introduced, the normalization process is often performed such that it is not auditable. If the underlying spatial errors are not reported, addressed, and understood, the value of the data integration and any subsequent analysis of the combined data set is questionable.
The Difference Between Probabilistic and Interval Reasoning Under Uncertainty
Authors: Chamanski Alexandre
Published: March 2009

The purpose of this study is to provide a basic comparison of the probabilistic and interval methodologies for processing uncertain information in decision analysis. The awareness of fundamental differences between these methodologies is considered to be useful not only for better understanding their principles, but also for their appropriate application.
Third Generation Wind Wave Model Frequency Dependence of the White-Capping Dissipation Source Function
Authors: Meza Eustorgio , Zhang Jun , Olivares Alejandro , Brambila Jorge
Published: March 2009

This paper proposes new frequency dependence for the empirical formulas presently used to determine wave energy dissipation in ocean wave models. Using an energy focusing technique, several unidirectional transient wave trains were generated. Each of the transient wave train contained an isolated plunging or spilling breaker. By comparing the energy spectra of free-wave components before and after breaker it was found that: 1)the energy loss as function of frequency is almost exclusively from wave components at frequencies higher than the spectral peak frequency; 2)although the energy density of the wave components near the peak frequency are the largest, they do not significantly gain or lose energy after breaking; and 3)wave components of frequencies significantly below or near the peak frequency gain a small portion (about 12%) of energy lost by the high-frequency waves. The empirical formulas presently used to determine white-capping dissipation (Komen et al. 1994; Tolman and Chalikov 1996; Booij 1999) do not agree with the above spectral distribution of energy dissipation. Analysis of the dissipation distribution obtained by Meza et al. (2000), suggest that the dependence of the dissipation rate on the frequency should be described by, (ωωp)(1−(ωωp)E(ω), where ω is the wave frequency, ωp is the spectral peak frequency and E(ω) is the energy density spectrum. An energy dissipation source function with such a frequency dependence is being implemented and tested in a third generation wind wave model.
The Diffraction of Multidirectional Random Waves by Rectangular Submarine Pits
Authors: Lee Hong Sik , Williams A. Neil
Published: March 2009

The diffraction of multidirectional random surface waves with one or more rectangular submarine pits is investigated theoretically. The incident wave conditions are specified using a discrete form of the Mitsuyasu directional spectrum. The numerical model involves the superposition of regular-wave diffraction solutions based on linearized shallow water theory obtained by a two-dimensional boundary integral approach for water of uniform depth. Numerical results are presented for multi-directional random waves that illustrate the effect of the various wave and pit parameters on the diffraction characteristics of typical single and multiple pit systems.
A Method for the Validation of Predictive Computations Using a Stochastic Approach
Authors: Ghanem Roger , Pellissetti Manuel
Published: March 2009

The task of model validation deals with quantifying the extent to which predictions from a particular model can be relied upon as representatives of the true behavior of the system being modeled. This issue is of great importance in assessing the reliability and safety of structures since in most cases their quantification relies on predictions from sophisticated probabilistic models. The paper describes a formalism that will extend the realm of the model to include all aspects of data collection and parameter calibration. Error estimators are developed that permit the quantification of the value of computational efforts (mesh refinement) versus analytical efforts (model refinement) and experimental effort (data acquisition and analysis). Starting with the hypothesis that the material properties of a given medium can be modeled within the framework of probability theory, a very rich mathematical setting is available to completely characterize the probabilistic behavior and evolution of the associated random medium under an external disturbance. The probability measure on the material properties is uniquely transformed into a probability measure on the state of the medium. A computational implementation of related concepts has been developed in the framework of stochastic finite elements and applied to a number of problems. Clearly, great value can be attributed to the ability of performing the forward analysis whereby the probability measure on the state of the system is completely characterized by the measure on the material properties. The assumed probability measure on the material properties, however, is greatly dependent on the amount and quality of data used to synthesize this measure. As this measure is updated, estimates of the performance of the underlying natural or physical system change. Significant interest exists therefore in developing the capability of controlling the error in the probabilistic estimates through designed data collection. The mathematical setting adopted in this paper for describing random variables is ideally suited for treating this problem as one of data refinement. A close parallel will be delineated between this concept and that of adaptive mesh refinement, well established in deterministic finite elements. Underlying this latter problem, are issues related to error estimation that are relied upon to guide the adaptation of the refinement. The paper develops a similar “data refinement” concept and highlights the basic underlying principles.
Determining Appropriate Stiffness Levels for Spudcan Foundations Using Jack-Up Case Records
Authors: Cassidy M. J. , Houlsby G. T. , Hoyle M. , Marcom M. R.
Published: March 2009

The level of soil stiffness under spudcan footings is an area of intense interest and debate, with many practitioners believing that current jack-up assessment guidelines are overly conservative. In order to evaluate appropriate stiffness parameters, back-analysis of case records of jack-up platforms in the North Sea has been performed. The records relate to three different rigs at a total of eight locations, which include a variety of soil conditions, water depths and sea-state severity. For each site the horizontal deck displacements and the sea-state conditions under storm loading are available. Numerical simulation of the platforms under storm loading was undertaken with varying levels of foundation stiffness. For each set of stiffness one-hour of numerical simulation was performed, with the most severe recorded environmental loading conditions for that site used. The horizontal deck displacements of the measured data and the numerical simulation results have been compared in both the frequency domain and by the magnitude of response. On the basis of the analyses, recommendations can be made for higher stiffness factors then are currently suggested in the SNAME, 1997 Technical & Research Bulletin 5-5A, Site Specific Assessment of Mobile Jack-up Units (SNAME, 1997).
Effects of Slip-Stick on Top Tensioned Riser Fatigue
Authors: Murray John J. , Sidarta Djoni E. , Lu Xiaohua , Gupta Himanshu , Beynet Pierre
Published: March 2009

In a Truss Spar system, top tensioned risers are tensioned, and centralized, inside a buoyancy can stem that extends all the way to the keel. This stem and buoyancy can contact the hull by guides at the buoyancy can, at the deck stopper, at the heave plates and at the keel where the riser exits the hull and descends to the seafloor wellhead. The buoyancy cans are in constant contact with the hull because of a preloaded contact guides located on each can. The buoyancy cans and their guides are designed to let the riser stroke with respect to the platform, however the friction forces at the guides and keel joint cause the riser to slip-stick. Thus the riser tension fluctuates. When this tension is large enough to overcome the friction force, and the relative velocity between the riser and the hull is non-zero, the riser slides. Otherwise it sticks. These tension cycles caused by slip-stick makes a significant contribution to the overall fatigue damage. While the riser is stuck it acts as a tendon on the Spar affecting its resonant periods and motions. While the riser is sliding it introduces a Coulomb type damping to the motion. The tension variations are the same at each point along the riser below the keel and the fatigue damage must be added to that caused by hull motions and riser VIV. Coupled time domain analyses that include the inertial, damping and stiffness properties of both the hull and the riser are very complex and time consuming. The amount of computation based on such models is not always practical in a design optimization phase. More approximate uncoupled methods, which produce adequately conservative results, can be applied for design purposes. This paper compares and discusses the results of coupled and uncoupled models to compute the damage in the risers due to slip-stick. The two models are based on identical finite-element models of the riser system. The main difference is how the motion input to each model is computed. In the coupled model, the effects of the riser on the motions are integrated in to the time-domain solution of the motion. In the uncoupled model, motions that are computed assuming only horizontal restoring forces from the risers are imposed on the contacts between the riser and the hull. In general, the coupling effects showed that the risers dampen motion in surge, sway, roll, pitch and yaw but have an insignificant effect on the natural periods. The heave comparison showed a reduction in heave due to damping from the risers sliding as well as from the constraints of the risers while they are stuck. Results of the coupled analysis showed lower fatigue damage than those predicted by the uncoupled model and these results are used to show the level of conservatism introduced by the uncoupled model.
Performance of Formulations for Whitecapping in Wave Prediction Models
Authors: van Vledder Gerbrant Ph. , Hurdle David P.
Published: March 2009

This paper describes work currently being carried out to examine possible methods to improve the computation of the dissipation by whitecapping in third generation wave prediction models. Such alternatives are needed to avoid unphysical dissipation behavior in the case of double-peaked wave spectra. First, an overview is given of the problems associated with the formulation for whitecapping that is now widely used in wave prediction models. Second, a summary is given of existing suggestions to improve the whitecapping formulation. Third, a number of examples are given with the new formulations to illustrate the potential improvements.
Performance Related Parameters of Flexible Connections Used in Offshore Pipelines
Authors: Kumar Alok , Hogan Michael E.
Published: March 2009

Despite continued advances in rubber technology, the design of elastomeric flexible connections, used in offshore pipelines transporting high-pressure/high-temperature hydrocarbon fluid/gas mixture, remains more of an art than a science, primarily due to the intricate behavior of rubber. The mechanical response of rubber is dependent on time, temperature and mode of loading. It is susceptible to explosive decompression damage. Rubber’s non-linear stress-strain curves, creep, hysteresis and other properties are influenced not only by the method of fabrication but also by age. If these characteristics are not accounted for properly, the result can be less precision in design compared with metals. At present, there are no codes or standards that directly address the design, analysis or evaluation of the elastomeric flexible connections used in offshore oilfield applications. Based on the results of the recent research, the significance of key parameters that affect the short term and long term structural performance of elastomeric flexible connections is presented in this paper.
Numerical Simulation of Sloshing of Water in Ship Tanks During Sequential Ballast Water Exchange in Seaways
Authors: Arai Makoto , Makiyama Humberto S. , Cheng Liang-Yee
Published: March 2009

In recent years, ballast water has been blamed for a variety of marine pollution problems, particularly for transporting harmful aquatic organisms from one part of the world to another and damaging the ecosystem of the new areas. A relatively simple mechanism to control this problem is to exchange ballast water on the high seas between ports in order to remove invasive species before the ship reaches its destination. However, some issues regarding ballast exchange on the open sea need to be addressed before this operation is introduced. One of them is the sloshing of the sea water in the ballast tank. In this paper, ballast water exchange on the open sea by means of the Sequential exchange method is simulated. Irregular seaways are generated from the ISSC spectrum, and the sloshing response of the water in the ballast tanks of a large merchant ship is numerically computed by using a finite difference code developed by the authors. The results showed that there is little possibility that severe sloshing presents a serious problem in regard to the ballast tank’s strength, especially in the case of a bulk carrier whose tanks are generally short in length, with sloshing anticipated only at the low water level.
A Novel Concept for a Deformable Joint in Offshore Structures
Authors: Khonsari S. V. , England G. L. , Mahboub-Farimani M. R.
Published: March 2009

A new semi-rigid beam-to-column, as well as bracing-to-frame, connection has been developed. In addition to its application in building structures, due to its special features and characteristics, it can best lend itself to offshore jackets, where fracture caused by fatigue has always been a major concern. The adverse effects of welding, material embrittlement, residual stresses, etc., pronounced by cyclic loading on such structures, can be overcome by using this new connection. Moreover, it can be easily used in the structure of various modules used in the topside of platforms and FPSOs. The particular feature of this connection is its geometry, which is so devised that it allows much higher rotations to be delivered than those normally used. Furthermore, it can work in a ‘sacrificial’ capacity with the ability to ‘contain’ the damage, preventing it from propagating through the jacket/frame members. This, together with its ‘replaceability,’ minimizes the cost of repair, if the structure is subjected to severe loading conditions, such as earthquakes, blast, or ship/ice impact. Being a separate entity, it is of a limited size, and can be easily housed in a heat treatment oven to be annealed and thereby to remove the adverse effects of welding. These features eliminate the need for taking additional measures to prevent the development of cracks in such joints, and can better guarantee the safety of the entire structure. Due to the use of high energy absorbing elements within this connection, it can dissipate high amounts of energy prior to its failure. The normal dilemma of not being able to provide high strength (moment capacity) together with large rotational capacity, presented by almost all conventional joints, is overcome in this new design. These two important features can be adjusted to the demand of the structure quite independently. A comprehensive experimental study of the comprising energy-dissipating elements showed the crucial role of annealing in eliminating the embrittlement effects of welding on the connections, in particular the developed cracks. Finally, the behaviour of two single-bay, single-storey, building frames, fitted with this connection, subjected to the ground acceleration records of four major earthquakes, was studied numerically. Various dynamic response parameters of these frames were worked out and compared with those of their rigidly-connected, as well as hinged-connected, counterparts.
High Resolution Wind Fields Retrieved From Spaceborne Synthetic Aperture Radar Images in Comparison to Numerical Models
Authors: Horstmann Jochen , Koch Wolfgang , Lehner Susanne
Published: March 2009

This paper introduces a recently developed algorithm to retrieve high-resolution wind fields over the ocean surface from spaceborne synthetic aperture radar (SAR) data. The algorithm consists of two parts, the first for determining wind direction and the second for wind speed retrieval. Wind directions are extracted from wind induced streaks e.g. from boundary layer rolls, Langmuir cells, or wind shadowing, which are approximately in line with the mean wind direction. Wind speed is derived from the normalized radar cross section (NRCS) and image geometry of the SAR image, together with the local retrieved wind direction. The application of SAR-wind retrieval in coastal regions is demonstrated using data acquired aboard the European satellites ERS-1 and ERS-2 and the Canadian satellite RADARSAT-1. These data allow to measure wind fields of an area of up to 500 km × 500 km with a resolution of up to 200 m. To improve and validate the set-up of numerical high-resolution models in coastal regions SAR-retrieved wind fields offer an unique opportunity. This is shown by comparisons of wind fields measured by SAR to results of the numerical model REMO, HIRLAM and GESIMA.
Real-Time Reliability Assessment and Management of Marine Pipelines
Authors: Bea Robert , Smith Charles , Smith Bob , Rosenmoeller Johannes , Beuker Thomas , Brown Bryce
Published: March 2009

In-line instrumentation information processing procedures have been developed and implemented to permit ‘real-time’ assessment of the reliability characteristics of marine pipelines. The objective of this work is to provide pipeline engineers, owners and operators with additional useful information that can help determine what should be done to help maintain pipelines. This paper describes the real-time RAM (reliability assessment and management) procedures that have been developed and verified with results from laboratory and field tests to determine the burst pressures of pipelines. These procedures address the detection and accuracy characteristics of results from in-line or ‘smart pig’ instrumentation, evaluation of the implications of non-detection, and the accuracy of alternative methods that can be used to evaluate the burst pressures of corroded and dented – gouged pipelines. In addition, processes are described have been developed to permit use of the information accumulated from in-line instrumentation (pipeline integrity information databases) to make evaluations of the burst pressure characteristics of pipelines that have not or can not be instrumented. Both of these processes are illustrated with applications to two example pipelines; one for which in-line instrumentation results are available and one for which such information is not available.