bluuebunny/crossref_metadata_2025 huggingface dataset:
Strain Aging of C-Mn Line Pipe Steels: An Analytical Approach to Compare Strain Aging Heat Treatments
Authors: Noecker Rick , Nissley Nathan , Ma Ning , Thirumalai Neeraj , Wojtulewicz Pamela , Hukle Martin
Published: November 2011

Static strain aging has been demonstrated to have a significant effect on the yield strength and yield point behavior of modern line pipe steels. Static strain aging can occur at many points during pipeline fabrication: during coating application, girth welding, and field joint coating. Each of these discrete thermal events is described by a unique thermal cycle (time-temperature profile). This makes it difficult to define a simulation heat treatment which would reflect the cumulative strain aging effect of all thermal events. The first section of this work describes the development of an analytical expression based upon classical strain aging theory that allows strain aging heat treatments to be normalized to an equivalent time at an isothermal temperature. The second section of this work compares thermal profiles from various strain aging heat treatments. Lastly, the mechanical response of modern line pipe steel to a variety of carefully controlled time-temperature combinations designed to simulate the causes of strain aging was examined. The analytical approach to compare strain aging heat treatments was found to be overly conservative when comparing longer times at lower temperature to shorter times at higher temperature which had been calculated to have equivalent heat inputs. Additionally, the yield strength was found to concomitantly increase with strain aging temperature, even when the effects of time were normalized. An alternate approach to the development of normalized strain aging heat treatments using a phase field model are briefly introduced.
Influence of Transverse Cracks on Ultimate Strength of a Steel Plate Under Compression
Authors: Bayatfar Abbas , Matagne Jerome , Rigo Philippe
Published: November 2011

This study has been carried out on ultimate compressive strength of a cracked steel plate component, considering the effects of initial imperfections (transverse and longitudinal residual stresses and initial deflection, as well). The main objective of this paper is to numerically investigate the influence of crack location and crack length on ultimate strength of a steel plate under monotonic longitudinal compression. This investigation is performed through non-linear finite element (FE) analysis using ANSYS commercial finite element code in which is employed Newton-Raphson method. The FE results indicate that the length of transverse crack and especially its location can significantly affect the magnitude of ultimate strength where the steel plate is subjected to longitudinal compressive action.
Flow Energy Harvesting by Fluttering Slender Bodies in Axial Currents
Authors: Michelin Se´bastien , Singh Kiran , de Langre Emmanuel
Published: November 2011

Coupled fluid-solid instabilities offer promising perspectives for the development of new technologies to harvest energy from water currents. A fundamental study of these instabilities and of the impact of the damping induced by the energy harvesting on the fluid-solid system is essential to identify promising configurations, quantify energy harvesting potential and characterize possible design optimization. In this work, we focus on the dynamics of long flexible cylinders placed in axial currents. It is well established that above a critical velocity threshold the flexible cylinder’s rest position becomes unstable to flutter and self-sustained limit-cycle oscillations can develop. A fraction of the solid kinetic energy can then be converted into electrical form, acting as a dissipative mechanism on the fluid-solid system. The non-linear dynamics of the dissipative system are studied in this paper using a reduced-order model of the deformable cylinder in the form of a bi-articulated system of two rigid cylinders with energy harvesting at each articulation. The impact of energy extraction on the system’s properties and the optimal placement of energy harvesters are then analyzed and discussed. It is shown that optimal energy harvesting only involves a single harvester at the upstream end, away from the region of useful curvature that drives the instability mechanism at the origin of the self-sustained oscillations.
Basic Parameters of Working Model of Floating Offshore Power Farm
Authors: Timofeev Oleg Ya. , Kartashev Anatoly B. , Trub Mikhail S. , Valdman Nikolay A.
Published: November 2011

As a result of developing the basic designs of up to 25 MW floating offshore power farms (FOPF) using renewable energy sources (wind, wave, current and solar) the efficiency of their operation was shown. Furthermore the investment indices of FOPF are better than those of fixed wind farm or floating diesel power plant.
Grid Integration of Wave and Tidal Energy
Authors: Blavette Anne , O’Sullivan Dara L. , Lewis Antony W. , Egan Michael G.
Published: November 2011

Wave and tidal energy provide a renewable source of electricity. However, their inherent fluctuations may have a negative impact on the power quality of a local electrical network. Grid operators assess this impact through the use of dynamic models of the generation units, which are inserted into the overall power system model. Providing these models is a compulsory step for any power generator to procure a grid connection above a specified power capacity. Significant issues were encountered in the wind energy industry regarding the dynamic modelling of devices, among which were model numerical instability, poor dynamic model quality and model incompatibility. Considering the large diversity of device types in the emerging ocean energy industry, these problems are considered as a major barrier to the larger scale grid-integration of marine energy converters. Dynamic models must clearly demonstrate the compliance of the actual power generation device and array of devices to the grid code requirements for grid-connection to be allowed. A further barrier to grid connection of ocean energy devices is that existing grid codes — mainly written in the context of wind energy — may be irrelevant or inadequate for ocean energy devices. This paper presents an overview of these issues, and details a radically different approach to the dynamic modelling of ocean energy devices that will assist in overcoming the issues previously encountered in the development of wind turbine models. It also highlights the gaps and inadequacy regarding grid code requirements for ocean energy devices, and provides some recommendations for a new ocean energy grid code.
Response Characteristics of the Floating Water Purifier “Water Beetle” in Waves
Authors: Ikegami Kunihiro , Baba Martin , Yamasaki Tomohiro
Published: November 2011

In closed water areas such as reservoirs and ponds, the water quality is prone to deteriorate when water flowing into the area is polluted and over-rich in nutrients. This causes algae to increase. The Water Beetle is a floating water purification unit on new principle to have the high performance with small energy. When the Water Beetle is used in a water area of complicated shape, plural miniaturized units are arranged to adjust the shape of area and connected to each other by use of mechanical connecting devices. To design the mechanical connecting device, it is necessary to grasp the exact response characteristics of the Water Beetle connected to each other in waves. A model basin test was conducted for the Water Beetle connected to each other in waves. In the model basin test, three floating unit models of the Water Beetle were connected by mechanical connecting devices that were simplified. The test was carried out to vary the connecting mechanism at connecting points. The measured results were compared with the calculated results obtained by use of the calculation method for motion analysis of multiple bodies connected to each other in waves, and they were compared between various connecting conditions to evaluate the most suitable connecting condition. As the result, the characteristics of response of the Water Beetle connected to each other in waves were obtained experimentally and theoretically, and the most suitable connection mechanism of connecting device was able to be decided. Therefore, the realization of the multi-connected Water Beetle system was confirmed experimentally.
Model Tests of Pipe-Laying Vessel: Results and Analysis
Authors: Kulikova Alevtina N. , Platonov Victor G. , Foce Sandro
Published: November 2011

The paper deals with the physical modeling of kinematic/dynamic characteristics of multi-component systems in a seakeeping basin. The paper presents the results of model seakeeping tests conducted to study the behavior of a combination comprising the barge + stinger + roller + pipe + mooring system in irregular waves under combined wind and current effects in the process of pipe-laying operations. The tests were conducted with variation of wave headings and flexural stiffness of pipeline. The data contained in the paper can be useful for modeling multi-component systems operating at sea. The following abbreviations are used: mooring system – MS, WDF – wave drift forces, CL plane – centerline plane of barge, CoG – center of gravity.
ISO 19902 Modeling and Analysis Aspects Including Dynamics
Authors: Wægter John
Published: November 2011

ISO 19902 provides a thorough collection and detailed presentation of state of the art procedures for many analysis and design aspects hitherto only sporadically included in design codes or only covered in more specialized literature. Important examples of such topics are specific requirements and guidance in relation to structural modeling and analysis aspects including dynamics. Also, specific requirements for the use of component resistances determined from FE analysis are given and linked to a potential validation by physical testing.
Statistical Analysis of a Set of Basin Waves
Authors: Scharnke Jule , Hennig Janou
Published: November 2011

In a recent paper the effect of variations in calibrated wave parameters on wave crest and height distributions was analyzed (OMAE2010-20304, [1]). Theoretical distribution functions were compared to wave measurements with a variation in water depth, wave seed (group spectrum) and location of measurement for the same initial power spectrum. The wave crest distribution of the shallow water waves exceeded both second-order and Rayleigh distribution. Whereas, in intermediate water depth the measured crests followed the second order distribution. The distributions of the measured waves showed that different wave seeds result in the same wave height and crest distributions. Measured wave heights were lower closer to the wave maker. In this paper the results of the continued statistical analysis of basin waves are presented with focus on wave steepness and their influence on wave height and wave crest distributions. Furthermore, the sampling variability of the presented cases is assessed.
Set-Up due to Random Waves: Influence of the Directional Spectrum
Authors: Barbaro Giuseppe , Foti Giandomenico , Malara Giovanni
Published: November 2011

This paper deals with the estimation of set-up due to irregular waves. Following the logic of Barbaro and Martino [1], it is derived the analytical expression of the set-up. The solution is based on the hypotheses of straight, parallel depth contours and constant average flow parameters in the longshore direction. In this context, the corresponding value of the set-up is calculated from a specified off-shore directional spectrum. The effect of the assumed directional spectrum is investigated. In particular, set-up is estimated by considering the following frequency spectra: Pierson-Moskowitz [2], JONSWAP [3] and Ochi-Hubble [4]. Further, influence of the spreading function is investigated by assuming a cosine-power [5] and a hyperbolic spreading function [6]. It is shown that the assumed off-shore spectrum significantly modifies the estimated set-up. It is proposed a practical application. The estimation has been carried out by considering various Italian and American locations. The model is applied from buoy data, that are provided by ISPRA (Istituto Superiore per la Protezione e la Ricerca Ambientale) and by NDBC (National Data Buoy Center).
How Quartet Resonance Might or Might Not Amplify Large Waves Into Freak Waves
Authors: Tromans Peter
Published: November 2011

We have developed an approximate solution to Zakharov’s equations for the evolution of ocean surface waves. We have applied it with a spectral response surface method to estimate the probability of exceedance of crest elevation in random seas. The method avoids time-marching and allows probabilities to be estimated on practical time scales. The results indicate that quartet resonance can produce very large amplification of extreme crests in uni-directional seas. However, the effect is very much smaller in spread seas. Although the solution is approximate it provides a great deal of physical insight into the mechanisms by which a freak wave might develop and explains the difference in results between uni-directional and spread seas.
Construction of Various Fatigue Design Master S-N Curves for Offshore/Marine Structures Using Battelle Structural Stress Method
Authors: Hong Jeong K.
Published: November 2011

Reliable structural integrity evaluation is a critical part of the design process. Reliable structural integrity evaluation is especially important for large and complex structures such as buildings and offshore/marine installations that have significant implications for human and environmental safety. The design and careful evaluation of welded structures are essential in structural design since the stress concentrations at the welds have significant impact on the overall fatigue lives of the structures. A mesh-insensitive structural stress method has been developed and proven to be highly effective in correlating fatigue behaviors of welded joints by Battelle researchers. The Battelle structural stress method and related weld fatigue master S-N curve approach has been adopted by ASME and API in 2007 [1–2]. The design fatigue master S-N curve has been constructed by incorporating the results of more than 800 fatigue tests, which are clearly categorized as weld toe failure. In addition, a design master S-N curve for weld root failure has been constructed recently [3]. For offshore/marine structures, it is essential to consider weld fatigue damage in corrosive environments as well as that in air, and to understand the effects of techniques commonly applied to improve weld fatigue strength of the structures, e.g., hammer peening, toe grinding, and TIG dressing. In order to meet the industry’s increasing demand for reliably, fatigue resistant structures, design master S-N curves incorporating the effects of corrosive environments and weld improvement techniques have been constructed. These new curves are based on existing weld fatigue data from the literature and class bodies’ fatigue design documents.
Modelling Techniques for Underwater Noise Generated by Tidal Turbines in Shallow Waters
Authors: Lloyd Thomas P. , Turnock Stephen R. , Humphrey Victor F.
Published: November 2011

The modelling of underwater noise sources and their potential impact on the marine environment is considered, focusing on tidal turbines in shallow water. The requirement for device noise prediction as part of environmental impact assessment is outlined and the limited amount of measurement data and modelling research identified. Following the identification of potential noise sources, the dominant flow-generated sources are modelled using empirical techniques. The predicted sound pressure level due to inflow turbulence for a typical turbine is estimated to give third-octave-bandwidth pressure levels of 119 dB re 1 μPa at 20 metres from the turbine at individual frequencies. This preliminary estimate reveals that this noise source alone is not expected to cause permanent or temporary threshold shift in the marine animals studied.
Innovative Supports of Risers and Corresponding Installation Procedures
Authors: Saito Daniel , Cordeiro Leticia R. S. , Castro Felipe A. , Medeiros Cipriano J. , Ribeiro Glauco D.
Published: November 2011

Usage of FPSO in shallow water always presents challenges to keep flexible risers’ stresses at the TDP (TouchDown Point) below acceptable values. Depending on the water depth, the catenary configuration can impose excessive high stresses at the TDP and for this reason, configurations like Lazy S, Steep S, Lazy Wave, Steep Wave, Pliant Wave and other complex configurations should be used. Choosing the type of configuration is always dependent on, among others, local environmental conditions, FPSO’s behavior and supports, the water depth, the number of risers, bottom arrangement of them, possibility of line clashes and mainly the cost of the solution. Depending on the number of risers to be supported, construction, installation and keeping good reliability of structural solutions generate considerable increases in the overall investment and maintenance costs. The paper presents two innovative fixed submerged supports of flexible risers, in lazy S configuration, and their installation procedures, that manage to lower construction and installation costs and also decrease inspection tolls due to increased reliability of the structure in comparison to floating buoy solutions.
Analysis of Foreship and Iceberg Impact Loads Based on Bayesian Networks
Authors: Liu Zhenhui , Garre` Luca , Amdahl Jo̸rgen
Published: November 2011

This paper presents an application of Bayesian Networks (BN) to the estimation of foreship and iceberg impact loads and corresponding damages. The realized BN models the energy dissipated in the collision between a foreship structure and an iceberg. The energy is estimated based on a simplified formulation of the external dynamics of the ship and iceberg collision. At present, the analysis focuses solely on collisions occurring in the front area of the ship, as this scenario is believed to be the most probable. BN requires probability distribution functions (pdfs) of the variables entering the energy formulation. The pdfs utilized are based on the literature available and the resulting pdf of the energy level is illustrated. The internal mechanics of the collision has been modeled by means of the non-linear finite element analysis (NLFEA) program LS-DYNA, with a user-defined material model of the iceberg. From the simulation, the energy levels modeled in the BN are thereafter associated with extents of the damage in the ship. Finally, conclusions and discussions for further improvements are presented.
Experimental Investigation Into Pile Diameter Effects of Laterally Loaded Mono-Piles
Authors: Alderlieste Etienne A. , Dijkstra Jelke , van Tol A. Frits
Published: November 2011

This paper presents the results of model tests on laterally loaded mono-pile foundations in sand. The tests have been performed in a geotechnical centrifuge. The objective of the research is to quantify large diameter effects of these mono-piles on the lateral capacity and the stiffness response for cyclic lateral loading. These large diameters are out of the validity range of the commonly used design methods. For this reason prototype pile diameters up to 4.4 m with a length over diameter ratio of 5 have been investigated. The results show an increase in pile diameter from Ds = 2.2 m to Dl = 4.4 m leads to a significant increase in static lateral capacity and stiffness from cyclic load tests. All tests have been performed with constant L/D = 5, Id = 60% and a load eccentricity up to e = 4.8 m. However, the current test series needs to be extended to higher initial densities and the load control should be more strictly regulated before a clear diameter dependence, for pile diameters > 2.2 m, is proven.
Numerical Investigation of Vortex-Induced Vibration (VIV) of a Circular Cylinder in Oscillatory Flow
Authors: Zhao Ming , Cheng Liang , Zhou Tongming
Published: November 2011

Vortex-induced vibration (VIV) of a circular cylinder in oscillatory flow is investigated numerically in this study. The incompressible Reynolds-Averaged Navier-Stokes equations governing fluid flow around a circular cylinder are solved using Arbitrary Langrangian-Eulerian (ALE) scheme and Petrov-Galerkin finite element method. The equation of motion is solved for the displacements of the cylinder both in the inline and cross-flow directions. The numerical model is firstly validated against the experimental results of one-degree-of-freedom VIV in cross-flow direction. It is found that both VIV frequency and amplitude vary with reduced velocity for a fixed KC number. In most of the simulated cases the vibration comprises of multiple frequencies of different amplitudes. Each frequency component is multiple times of the frequency of the oscillatory flow. Two-degree-of-freedom VIV is investigated with the same parameters used in the one-degree-of-freedom case. By examining the XY-trajectory of the vibration, it if found that the vibration follows different trajectory for different KC numbers or reduced velocities.
Short-Term Fatigue Analysis of Semi-Submersible Wind Turbine Tower
Authors: Kvittem Marit I. , Moan Torgeir , Gao Zhen , Luan Chenyu
Published: November 2011

Coupled time domain analyses of a semi-submersible wind turbine are performed with the intention to study motions affecting fatigue damage at the base of the tower. The software applied is SIMO/RIFLEX with the extension TDHmill, which gives the wind thrust force and gyro moment on the wind turbine as point loads in the tower top. Short term environmental conditions are chosen from a joint wind and wave distribution for a site in the Northern North Sea. Variance spectra, mean value, standard deviation, kurtosis, skewness and Vanmarcke’s bandwidth parameter are calculated for stresses at the base of the tower. Damage is calculated for each short term condition by two methods; rainflow counting and narrow band approximation. The accuracy of narrow band approximation estimates for fatigue are examined for the structure in question. Time domain simulations are carried out for different sea states and fatigue damage is calculated for each case. Simulations show that turbulent wind dominates the response at low wind speeds and the response spectral density function tends to be very wide-banded. For wave dominated response, spectra have lower bandwidth, and narrow banded approximation for fatigue damage gives estimates 20–50% above rainflow counted damage.
Analysis Tool for Environmental Impact of Seafloor Resources Development
Authors: Yamamoto Joji , Oka Hideyuki , Nakajima Yasuharu , Uto Shotaro , Inoue Shunji , Ishida Shigesuke
Published: November 2011

The exclusive economic zone (EEZ) of Japan has a very wide area due to a lot of islands in Japanese Archipelago. As the worldwide supply and demand of natural resources and foods are being tight, the technology for the effective use of the EEZ will become the key for the sustainable development of Japan. From this background, the Ministry of Land, Infrastructure, Transport and Tourism of Japan (MLIT) started an R&D project of floating offshore platform technologies in 2007[1]. In this project an integrated design support tool is developed for evaluating the economical and safety aspects of the offshore platforms. This tool contains the function to estimate an environmental impact of seafloor resource development. We postulated the process of seafloor resource development as follows. The ores mined on seabed are sent up to the platforms as a mixture of the ore particles and water, then the waste water containing inorganic suspended particles is discharged into sea. The particles exhausted in the sea are transported by the advection and diffusion, and sink gradually on seafloor. We developed an analytical code simulating the distribution of discharged particle in both sea and sediments by a simple technique. Mass Consistent Flow Model is used to save the time in the calculation of ocean flow. In addition, the ecosystem model in sediment is developed to estimate the ecological impact of seafloor resource development. It includes benthic organism, zooplankton, bacteria, and particulate organic matter as the components. The ecological impact was assessed based on the recovery period of the biomass. In this paper, the outline of this environmental impact analysis tool and results of the trial calculation for seafloor resource mining are shown.
Fatigue Strength Characteristics of Cast Steel Weld Joints
Authors: Kim Ho Jung , Kim Seong Min , Lee Jae Myung , Kim Myung Hyun
Published: November 2011

The aim of the present paper is to investigate and compare the fatigue characteristics of weld joints in terms of S-N curve between cast steel and conventional mild steel. Two different joints such as thick butt weld joints and fillet weld joints are considered in this study. Fatigue strength of cast steel weld joints with various geometric designs are tested and compared. The fatigue life and the fatigue crack propagation characteristics are investigated by experimental and numerical methods in a systematic manner. The conventional hot spot stress, structural stress and notch stress approaches are employed for the precise interpretation of the fatigue test result of cast steel weld joints. Finally, the estimation of the fatigue limit of cast steel weld joints are attempted based on material properties and geometrical parameters. The results can be used for the efficient fatigue design of various cast steel welded joints.
Reliability Assessment for TLP Tether Overload in Nonlinear Waves
Authors: Hagen O̸istein
Published: November 2011

Non-linear effects caused by extreme crest loading might be governing for important structural limit states. For TLP’s, non-linear effects caused by extreme crest might lead to tether overload and tether slack. Simple scaling of ULS results is not necessarily adequate if wave in deck events (or other NL phenomena) might occur with annual probability >10−4. In this case ALS analysis should be performed, and detailed knowledge of the physics and statistics of extreme crest events, and of the loading caused by these waves, is required. Ignoring the possibility for wave in deck events and establishing the characteristic tension from data where there are no wave in deck events, might lead to unrealistic low estimates for the extreme tether tension. The present study demonstrates effects of the CresT JIP (Cooperative Research on Extreme Seas and their impacT) findings regarding extreme and rogue waves through a simplified reliability analysis of a generic TLP defined by the project. In this paper, SRA for tether overload is discussed, with emphasize on the effect of tension Twid caused by wave in deck events. Data from TLP basin test undertaken as part of the CresT JIP are applied, and parametric models for tether tension from measured crests and measured tether tension data are established. The model for Twid is used in the SRA for analysis of tether overloads for design sea states and for long term analysis for different geographic locations. Special attention is given to assessment of the effect of higher order crest non-linearities beyond 2nd order. The crest models are based on results from other work packages of the Crest JIP. Recommendations for future research needs are given.
On the Dynamic Interaction Between Drifting Level Ice and Moored Downward Conical Structures: A Critical Assessment of the Applicability of a Beam Model for the Ice
Authors: Wille Sjoerd F. , Kuiper Guido L. , Metrikine Andrei V.
Published: November 2011

Downward conical structures are believed to be an interesting concept of a floating host for oil and gas developments in deeper Arctic waters. The conical structure forces the ice to fail in bending, thereby limiting the ice loads on the structure. During the last two years, several conical structures were investigated at the Hamburg Ship Model Basin (HSVA) as part of a Joint Industry Project. This paper presents a numerical model for drifting level ice interacting with a moored downward conical structure. The goal of this development was to get insight in the key processes that are important for the interaction process between moving ice and a floating structure. The level ice is modelled as a moving Euler-Bernoulli beam, whereas the moored offshore structure is modelled as a damped mass-spring system. The ice-structure interaction process is divided into two phases. During the first phase, the ice sheet bends down due to interaction with the structure until a critical bending moment is reached at a cross-section of the beam. At this moment, the beam is assumed to fail at the critical cross-section in a perfectly brittle manner. During the second phase, a broken off block of ice is pushed further down the slope of the structure. These phases were built into one, piece-wise in time continuous model. A key result found by means of the numerical analysis of the model is that the motions of the moored floating structure do not significantly influence the bending failure process of level ice. Also the influence of the in-plane deformation and the heterogeneity of ice on the bending failure process is negligible. As a consequence, the dynamic response of the structure is for the biggest part determined by the ice failure process. Although the response of the structure can be dynamically amplified due to resonance for some particular ice velocities, no frequency locking of the ice failure onto one of the natural frequencies of the structure was observed. The model output showed qualitative agreement with the HSVA test results. It was however concluded that one-dimensional beamlike models of level ice sheets cannot accurately predict loading frequencies on downward conical moored floating structures because the ice blocks that break off are too long. Modelling level ice in two dimensions using plate theory is expected to give better results, since it takes into account the curvature of a structure and both radial and circumferential ice failure.
Estimating Long-Term Distributions of Extreme Response of a Catenary Riser
Authors: Passano Elizabeth , Mainc¸on Philippe
Published: November 2011

The purpose of this paper is to present a method for efficient and unbiased estimation of the long-term extreme response distribution of a catenary riser. In this approach a computationally inexpensive, nonlinear response predictor is used to estimate the response in all sea states, thus allowing selection of relevant sea states and intervals within sea states for detailed, nonlinear finite element simulations. This method requires significantly less simulation time than the conventional approach with extensive nonlinear simulations of many sea states. The method is applied to a catenary riser case. In an earlier study (Passano and Larsen, OMAE 2006), a strong relation was found between the prescribed vertical motions at the top and the axial force and bending moment near the touch down area. This relation was then used to limit simulations to intervals where the largest maxima values were expected. This gave an estimate of the upper part or tail of the extreme response in the selected sea states. In a later study (Passano and Larsen, OMAE 2007), nonlinear response predictors based on the prescribed vertical motions were established. These were used to estimate the short-term extreme response distribution directly. In the case study of this paper three long-term response distributions are compared: 1) A distribution established directly from the predicted short-term response distributions. 2) A distribution based on response simulated in the relevant time intervals of the relevant sea states. 3) A distribution obtained from extensive nonlinear simulations of the sea states in the scatter diagram. It is found that distribution 2) provides a fast and reliable estimate of 3). This allows a significant reduction in analysis work.
Examining a Comprehensive Dataset Containing Thousands of Freak Wave Events: Part 1—Description of the Data and Quality Control Procedure
Authors: Christou Marios , Ewans Kevin
Published: November 2011

This paper concerns the description of the formation of a very large, quality-controlled dataset of raw wave measurements. As part of the CresT (Cooperative Research on Extreme Seas and their impacT) Joint Industry Project (JIP), the participants provided raw field measurements of water surface elevation from various installations across the globe. This paper will describe the data collection from the different installations, the strict quality control procedure employed to ensure a reliable dataset, and an overview of the occurrences of records containing freak wave events. Part 2 of this paper will then go on to describe the analysis performed on this dataset as well as the findings from the study.
Further Investigations on Vortex Self Induced Vibration (VSIV)
Authors: Fernandes Antonio Carlos , Sefat Sina Mirzaei , Casca˜o Luis Victor , Boas Pedro Vilas , Francis Ricardo
Published: November 2011

An experimental facility has been designed to study the transversal and response associated with vortex-induced vibration of a totally submerged horizontal cylinder hang on horizontally elastic support under the influence of vertical harmonic oscillating motion. The study aims to analyze the vortex self induced vibration (VSIV) phenomena, which occurs for instance with Steel Catenary Risers (SRCs) the deep offshore connected to floating platforms. These vibrations occur even in the absence of a current. The experiments were conducted in a tank at the LOC/COPPE/UFRJ (Laboratory of Waves and Current of COPPE, Federal University of Rio de Janeiro). The tests were carried out with Keulegan-Carpenter numbers equal to 10 and 20 and for several exciting frequencies. The vertical trajectories of the VSIV motion were then classified based on its different configurations such as oval-shape, eight-shape and etc. Different ranges of reduced velocities were also considered. The amplitude and frequency of transversal motion and also the forces calculated.
Flow Fields Inside Stocked Fish Cages and the Near Environment
Authors: Gansel Lars C. , Rackebrandt Siri , Oppedal Frode , McClimans Thomas A.
Published: November 2011

This study explores the average flow field inside and around stocked Atlantic salmon (Salmo salar L.) fish cages. Laboratory tests and field measurements were conducted to study the effects of biofouling and especially fish behaviour on the flow patterns around and through fish cages. Currents were measured around an empty and a stocked fish cage in a fjord to verify the results obtained from laboratory tests without fish and to study the effects of fish swimming in the cage. Fluorescein, a non-toxic, fluorescent dye, was released inside a stocked fish cage for visualization of 3-dimensional flow patterns inside the cage. Atlantic salmon tend to form a torus shaped school and swim in a circular path, following the net during the daytime. Current measurements around an empty and a stocked fish cage show a strong influence of fish swimming in this circular pattern: while most of the oncoming water mass passes through the empty cage, significantly more water is pushed around the stocked fish cage. Dye experiments show that surface water inside stocked fish cages converges towards the center, where it sinks and spreads out of the cage at the depth of maximum biomass. Furthermore, the converging surface water swirled in the direction of the swimming fish. In order to achieve a circular motion, fish must accelerate towards the center of the cage. This inward-directed force must be balanced by an outward force that pushes the water out of the cage, resulting in a low pressure area in the center of the rotational motion of the fish. Thus, water is pulled from above and below the fish swimming depth. The laboratory tests with empty cages agree well with field measurements around empty fish cages, and give a good starting point for further laboratory tests including the effect of fish-induced currents inside the cage to document the details of the flow patterns inside and adjacent to stocked fish cages. The results of such experiments can be used as benchmarks for numerical models to simulate the water flow in and around net pens, and model the oxygen supply and the spreading of wastes in the near wake of stocked fish farms.
Multiple Axial Fatigue of Pressure Armors in Flexible Risers
Authors: Ye Naiquan , Sævik Svein
Published: November 2011

The design of flexible risers has been challenged by the exploration of oil and gas goes into ever-deep regions as in Mexico Gulf and West Africa. Comparing to the fatigue analysis of the tensile armors which have been extensively investigated in recent years, much less effort has been devoted in the fatigue of pressure armor. The fatigue of the pressure armor is much more complicated than the tensile armors. For the tensile armor, the longitudinal stress along the helix path dominates the fatigue behavior, while for the pressure armor, more stress components will play together to affect its fatigue. If the fatigue of the tensile armor can be characterized as a uni-axial fatigue phenomenon, the fatigue of the pressure armor will be a typical multi-axial problem. The stress components in the pressure armor consist of contribution from the following sources: stress in the hoop direction due to internal/external pressure, stress in the radial direction due to the pressure and contact pressure from the tensile armor layer, stresses caused by the ovalization when the riser is bent, and local stresses due to local bending (nub/valley contact). The friction between the nub and valley interface is reflected in the local stress components as well. A Finite Element (FE) based computer program BFLEX developed by MARINTEK for the stress analysis of flexible risers are capable of calculating the complicate stress components of the pressure armors. In order to perform fatigue damage calculation for the pressure armor, mean stress and stress range must be computed based on these stress components. Mean stress correction becomes very important due to large mean stress experienced by the pressure armor. There are several ways to make use of these stress components to derive the mean stress and stress range. Equivalent stress models and critical plane models are the main models to address the general feature of the multi-axial fatigue. The application of these models on the fatigue of the pressure armor of the flexible risers will be discussed in this paper. The best suited model will be suggested based on the specific stress components in the pressure armor.
Characteristics of Steep Second-Order Random Waves in Finite and Shallow Water
Authors: Stansberg Carl Trygve
Published: November 2011

The theoretical formulation of second-order random waves in deep and finite water is reviewed. In particular, the increased nonlinear interactions with decreasing depth are addressed, including both the sum-frequency as well as the slowly varying difference-frequency components. Depth-defined limitations in the valid range for random waves are suggested based on the Ursell number. Numerical time series realizations at various depths and for two sea states are obtained by an efficient bifrequency summation procedure. Resulting time series show moderate average second-order energy contents, except for the steep sea state Hs = 15m, Tp = 14s in depths of 30m and 20m which are outside the suggested valid second-order range. The two largest wave events from the simulations are studied in particular for the different depths. Nonlinear interactions increase significantly with decreasing depth. Still, within the valid range, extreme second-order crests and peak particle velocities are only moderately increased with decreasing depth, while the negative peaks increase significantly. This is because the difference-frequency component almost compensates for the sum-frequency part at crests, while it is opposite at troughs. Maximum slopes, however, are clearly increased in shallow water, eventually leading to increased breaking (which is beyond second order of course). Velocity profiles under the crests are also shown, confirming the findings from the elevation.
Flow Induced Forces on Multi-Planar Rigid Jumper Systems
Authors: Nair Aravind , Chauvet Christian , Whooley Alan , Eltaher Ayman , Jukes Paul
Published: November 2011

Rigid subsea jumper systems are typically used as interface between subsea structures and are required to accommodate significant static and dynamic loads. Due to constraints imposed by in-line planar jumpers (e.g. U shaped and M shaped jumpers), the industry is shifting towards the use of multi-planar jumper systems (e.g., Z-shaped jumpers). These multi-planar jumper systems have increased tolerance to end displacements and can be tailored to accommodate cyclic end motions of subsea structures. Multi-planar systems, however, come with unique challenges of their own including the coupling of flexural and torsional responses under vortex induced vibrations (VIV), fluid induced vibration (FIV) and slugging. In particular, the development of hydrodynamic slug flow is a common occurrence in oil and gas pipelines. It is understood to be initiated by instabilities of wave on the gas-liquid interface. It is also understood that slug flows are the source of vibration within pipework when a change of direction occurs e.g. 90° bend at a subsea riser base or top side piping. In standard slug flow vibration analysis, averaged slug frequency and length are used to calculate the force generated. In the case of a multi-planar rigid jumper, several changes of direction occur within a short length of pipe. After each bend the characteristics of the slug flow are modified. It is necessary to accurately capture these changes in order to reproduce the forces generated at critical points along the jumper length. This paper presents a methodology for analyzing slugging induced fatigue that has been developed in an on-going study undertaken by MCS Kenny for design of multi-planar rigid jumper systems. In this methodology, Computational Fluid Dynamics (CFD) is used to accurately simulate the flow within the jumper and provide pressure fluctuations on the internal pipe wall for the vibration analysis. The pressure fluctuations are then incorporated in a Finite Element (FE) model of the jumper system and further used to determine the slugging fatigue damage. CFD (Star-ccm+) and FE (Flexcom, ABAQUS) software programs are used to accurately capture the response of the jumper system. Key conclusions and challenges overcome during the course of this study are presented herein.
Structural Analysis of Cryogenic Flexible Hose
Authors: Bardi Francois C. , Tang Huang , Kulkarni Mohan , Yin Xiaolei
Published: November 2011

Significant gas resources remain to be developed in remote offshore locations. These resources can be produced through the use of a floating liquefied natural gas plant (FLNG) and exported through liquefied natural gas (LNG) carriers. Cryogenic flexible hoses provide a structurally compliant solution for the transfer of LNG between the FLNG and LNG carrier in harsh offshore environments. One of several cryogenic flexible hoses currently being developed in the industry is a structure composed of several layers; a corrugated stainless steel pipe in pipe structure reinforced using two layers of armoring wires. Thermal insulation of the inner hose is obtained by creating a vacuum in the annulus of the pipe in pipe. To ensure sound structural performance and integrity of the flexible hose during offloading operations, a Failure Mode and Effects Criticality Analysis (FMECA) was performed. 3D finite element models of the fully assembled hose and selective individual components were generated to assess their structural response to different loading scenarios, resulting stress concentrations and layer interactions. A sensitivity study of the corrugation profile of the corrugated pipe was performed to minimize stress concentrations and allowable bending radius.
Numerical Wave Tank Analysis of Wave Run-Up on a Truncated Vertical Cylinder
Authors: Kim Jang , Jaiman Rajeev , Cosgrove Steve , O’Sullivan Jim
Published: November 2011

A new far-field closure condition for a CFD-based numerical wave tank that uses a potential wave solution to overlay the outer computational domain of a CFD solution is described. A prescribed potential wave solution covers the region beyond a diameter more than 10 times of floater footprints. The diffracted waves from the body are absorbed by the ‘potential-attractor’ terms applied in the intermediate CFD domain where the CFD solution for Navier-Stokes equation is gradually blended into far-field potential solution. The proposed model provides an efficient numerical wave tank for the case when incoming wave length is much longer than floater. In this case, the required mesh and domain size for numerical accuracy is mainly affected by the floater geometry and local wave kinematics near the floater and less dependent on the length scale of the incoming waves. The new numerical wave tank is first tested for a diffraction of a truncated cylinder exposed to long regular waves. Comparison with theoretical and experimental results demonstrates accuracy and efficiency of the new method.
Hydrokinetic Energy Harnessing Using the VIVACE Converter With Passive Turbulence Control
Authors: Chang Che-Chun , Bernitsas Michael M.
Published: November 2011

Passive turbulence control (PTC) in the form of selectively applied surface roughness is used on a rigid circular cylinder supported by two end-springs in transverse steady flow. The flow-induced motions are enhanced dramatically reaching the limits of the experimental facility and motion mechanism at amplitude to diameter ratio A/D ≅ 3. In comparison to a smooth cylinder, in the fully turbulent shear layer flow regime at Reynolds number on the order of 100,000, PTC initiates VIV earlier at reduce velocity U* ≅ 4, reduces VIV amplitude depending on damping, and initiates galloping at U* ≅ 10 rather than 20. Thus, back-to-back VIV and galloping are achieved expanding the synchronization range of Flow Induced Motion (FIM) beyond U* ≅ 15 and the capabilities of the experimental set-up. The harnessed horizontal hydrokinetic power increased by a factor of four due to increased velocities in the synchronization range without any adjustment of the motion mechanism particulars.
Experimental Study on the Collapse Strength of Narrow Stiffened Panels
Authors: Xu Mingcai , Guedes Soares C.
Published: November 2011

The results of four tests on narrow stiffened panels under axial compression until collapse and beyond are presented to investigate the ultimate strength of stiffened panels. Tension tests are used to evaluate the material properties of the stiffened panels. The tests are made on panels with two half bays plus one full bay in the longitudinal direction. Initial loading cycles were used to relief the residual stresses of the stiffener panels. The strain gauges are set on the plates and stiffeners to record the distribution of strain. The displacement load relationship is established. The ultimate strength behavior, modes of failure and load-carrying capacity of the stiffened panels are investigated with the experiment.