bluuebunny/crossref_metadata_2025 huggingface dataset:
Design Issues for Connecting Two Floating Structures
Authors: Morandini Cedric , Orozco Jean-Marc , L’Hostis Didier
Published: November 2008

The proposed paper shall list and present the issues from a mooring and hydrodynamic point of view relative to the design of a floating structure connected to another floating structure in surface and to the seabed with traditional anchoring legs, such as a Flotel with an F(P)SO. The paper will propose tentative ways to properly design the connections between the two floating structures which will limit their relative excursions in order to allow the setting of a gangway between them. In addition, the design should forbid at any time any potential contact between the two structures in intact and damage conditions, and should trigger at any time acceptable tensions in the various anchoring and mooring systems. The paper will address the appropriate mooring and hydrodynamic analysis to be used for such exercise. Safety issues such as quick disconnection in case of fire are investigated. Operational issues are to be discussed as well.
Time-Variant Reliability Assessment of FPSO Hull Girder With Long Cracks
Authors: Ayala-Uraga Efre´n , Moan Torgeir
Published: November 2008

An efficient time-variant reliability formulation for the safety assessment of an aging Floating Production Storage and Offloading (FPSO) vessels with the presence of through-thickness cracks (i.e. long cracks), is presented in this paper. Often in ship structures, cracks are detected by means of close visual inspection when they have already propagated through the thickness. The propagation of long cracks in stiffened panels is therefore considered, as they may be present in critical details of the deck and/or bottom plating of the vessel. The mean stress effect due to the continuously varying still-water loading as well as residual stresses is explicitly accounted for in the crack growth calculation. An analytical model is established for determining the equivalent long-term stress range. Although it has been found that stiffened panels are tolerant to fatigue cracking, the probability of brittle fracture of an aging hull structure becomes more relevant with the presence of long cracks. The continuously varying still-water load effects due to the operational nature of FPSOs introduce additional uncertainties in the estimation of fatigue damage as well as in the likelihood of fracture failure mode. These uncertainties are accordingly considered in the analyses.
A Parametric Study on Relative Wave Height of Ships in Short Crested Irregular Sea With Forward Speed
Authors: Inoue Yoshiyuki , Zakaria N. M. Golam , Nakai Ryoji
Published: November 2008

This paper deals with the parametric study of relative wave heights of ships in short crested irregular seas. Here linear potential theory has been used to describe the fluid motion and 3-D sink-source technique with forward speed has been used to determine hydrodynamic forces for surface ship advancing in waves at constant speed. The time domain simulations of relative wave heights of typical container ship, bulk carrier, pure car carrier (PCC) and general cargo ships in short crested irregular waves have been carried out for three different sea states and for the parametric study of these vessels, different sizes of ships have been considered. Empirical roll damping has been taken into account in time domain analyses of motion responses of sea going ships in rough seas and for ensuring longer time simulation of the random sea waves, unequal frequency spacing has been used. Finally, the numerical results of the maximum and the significant values of irregular relative wave heights are discussed by comparing with some requirements by a classification society of shipping for the different sizes of ships which might provide some important information for the designer of ocean going vessel to assess possible deck load or bottom impact force on future regulation of designing ship.
Flow Curvature Effect on Dynamic Behaviour of a Novel Vertical Axis Tidal Current Turbine: Numerical and Experimental Analysis
Authors: Coiro Domenico , Nicolosi Fabrizio , De Marco Agostino , Melone Stefano , Montella Francesco
Published: November 2008

This paper presents a summary of the work done by the authors regarding the design, construction and test of vertical axis hydro turbines to exploit tidal currents. Double Multiple Streamtube (DMS) model and Vortex model have been used to predict turbines performances either with fixed blades or with self-acting variable pitch blades. Within the DMS model, VAWT and VAWT_dyn codes have been developed to analyze steady and dynamic performances; within Vortex model, VAT-VOR3D code has been developed. Theoretical analysis and numerical predicted performances have been compared and validated with experimental test results on both model and real scale turbines. A comparison between DMS and Vortex model results has been presented. Moreover, the recent activities in terms of numerical investigations on the flow curvature effects are presented.
Fatigue of Jacket Platforms: Effect of Marine Growth Modelling
Authors: Schoefs Franck , Boukinda Morgan , Guillo Christophe , Rouhan Antoine
Published: November 2008

After decades of developments of methods and software, reassessment of jacket structures is still a challenge. This paper focus on marine growth effects in case of fatigue behavior for Jacket offshore structures. Two main items are pointed out: selection of marine growth profile and modeling of hydrodynamic coefficients in presence of marine growth. A complete approach is provided and completed with both sensitivity and uncertainty studies.
Temporal Variability in Corrosion Modeling and Reliability Updating
Authors: Straub Daniel , Faber Michael Havbro
Published: November 2008

Many parameters influencing corrosion degradation are time-variant and the corrosion process is thus subject to temporal variability, the real characteristics of which are commonly neglected in reliability assessment. After a short overview on the quantitative modeling of corrosion loss, a comparison is made between different temporal models of corrosion degradation and the consequences of applying an inappropriate model are investigated. The effect of temporal variability is then investigated in detail and illustrated on an example considering CO2 corrosion in pipelines. It is demonstrated how the time-variant corrosion process can be consistently represented by constant parameters, using equivalent values of the corrosion rate. Finally the influence of temporal variability on the reliability updating following inspections is investigated and it is shown how this effect can be accounted for in inspection planning.
Three-Dimensional CFD Analysis of Two Circular Cylinders Arranged Perpendicular to Each Other
Authors: Schulz Karl W. , Minyard Tommy , Barth William
Published: November 2008

A three-dimensional numerical method combining solution of the incompressible Reynolds Averaged Navier-Stokes (RANS) equations with a rigid body structural dynamics response has been developed previously to aid in the prediction of the loads and motions of offshore structures. In this paper, we use the tool to compute the hydrodynamic flow around two tandem cylinders oriented perpendicularly to each other. The flow conditions and gap distances between the cylinders are chosen to match a set of water tunnel experiments carried out at the University of Queensland. Comparisons of Strouhal frequencies and example flowfield visualizations are presented between the experimental measurements and associated CFD results.
Subsurface Buoy Configuration for Rigid Risers in Ultra Deepwater
Authors: Franciss Ricardo
Published: November 2008

The Exploration and Production Department of Petrobras asked the R&D Center the development of a production system for 1800 m water depth, in Campos Basin, which would allow the installation of Steel Caterany Risers (SCR) in the starboard side of a production platform. The subsurface buoy concept was chosen as one of the alternatives. This concept has being developed since the preliminary studies conducted in the first phase of JIP Deepstar. This concept has an advantage of uncoupling the movements of the platform from the risers, reducing the loads due to the risers in the platform and allowing the installation of this system before the installation of the production vessel, anticipating the production of the field. This article shows the main characteristics of the buoy, its sizes, results of structural analyses and installation procedures for a buoy which sustains 14 SCR and 5 umbilicals in one side and 14 flexible jumpers and the same 5 umblicals in the opposite side. This concept was tested in two model test tanks, where it was verified that this concept is feasible. Also, information related to a field trial with a prototype installed in Brazilian waters will be presented.
Fatigue Performance of Friction Welded Studs
Authors: Hsu T. M. , Buitrago Jaime , Herman Arthur , McKeighan Peter C.
Published: November 2008

Early in the life of the Genesis spar, cracking developed at the welded connections between the riser guide supports and the hull wall plate. The cracking was caused by the movements of the top-tensioned risers within the steel guide frames in the moon pool of the structure. The remedial action taken to minimize the riser movements and its effects on the hull involved the use of novel rubber bumpers, which were installed in lieu of the steel guides. The bumpers around the periphery of the moon pool were fastened to the hull wall via threaded studs that were friction welded to the hull wall plate underwater. This paper describes a testing program specifically designed to qualify the fatigue performance of the stud-plate friction welds. Results verify the use of the F2 S-N curve from British Standard 7608 with a single slope for the design of the friction-welded connections subjected to axial load. It was also found that the fatigue performance of friction welds is sensitive to the stud preload. One unique feature of the fatigue failure mode of the connection, when the load is transferred through the stud into the plate, is that cracking takes place along the semi-circular heat-affected zone (HAZ) of the bond-line between the stud and the plate, and not through the hull plate thickness. As a result, failure of a stud connection does not compromise the structural integrity of the spar hull.
Nested Grid Models for Ocean Processes
Authors: Katopodes Nikolaos D. , Kao Kuo-Cheng
Published: November 2008

This paper presents a novel two-way nested grid scheme that allows information to travel freely through the computational boundaries of the grid interface. The method tentatively assumes the pressure boundary conditions between subdomains, but uses a receding boundary approach to minimize the pressure errors. Initially, the domains overlap, but within a few time steps the overlap is eliminated by the receding boundaries, so repeated use of approximate boundary conditions is avoided. This prevents the accumulation of errors on the subdomain boundaries and, furthermore, decreases the transfer of errors to the rest of the domain by continuously discarding boundary data. To avoid continuous shrinking of the subdomains, the boundaries are reset to their original positions every few time steps. Successful applications of the method are presented for surface wave propagation, gravity currents and a combination of both surface and internal waves.
ATLAS AUVs for Offshore Applications: Status and Perspectives
Authors: Hornfeld Willi
Published: November 2008

The scientific and economic significance of underwater technology is considerable, particularly as the oceans are becoming more and more important not only as a source of energy, raw materials and food but also due to their influence on climatic changes. Underwater technology, especially at great depths, represents a very particular challenge which is being tackled by ATLAS ELEKTRONIK Autonomous Underwater Vehicles (AUVs) DeepC©, SeaOtter Mk1 and SeaOtter Mk2. Many actions that would normally require high cost ROV systems or which are not possible (e.g. under ice investigations) can now accomplished using AUVs, at less cost and manpower. DeepC© is a high efficient AUV system for ultra deep missions (4000m), with long endurance (40–60h) and for a lot of applications. The development objectives are: Low vehicle weight, modular design, high efficient energy generation, autonomous mission execution, obstacle avoidance, in mission re-planning capability, precise navigation, advanced diagnosis and fault recovery system, high manoeuvrability, hover capability and containerized payload. The system is final integrated and ready for water tests and demonstrations till end of 2004. The paper deals with the system performance and shows the development status. The AUV SeaOtter Mk1, based on the ATLAS Maridan M600 vehicle and adapted to modern requirements in navigation software and mission management, is the ideal payload sensor carrier for military and commercial purposes. It ensures precise positioning, stability and manoeuvrability for accurate data collection. The on-board system provides all necessary position, attitude, status and time information to ensure effective and efficient performance throughout the mission. High positional accuracy and accurate stabilization of the vehicle’s attitude throughout the collection of data allows the mapping of raw data to be implemented directly, without post-processing, to obtain the necessary correlation of the survey mosaic. The advanced version of the SeaOtter AUV, the Mk2 is designed for various purposes, strictly adhering to a modular approach with regard to superstructure, propulsion, energy, communication, navigation and payload. The basic vehicle can easily be extended by the means of one or two extension modules to carry heavier payloads. The paper shows the SeaOtter Mk1 performance and gives an overview over the Mk2 Philosophy and the development status.
Reeling Effect on the Ultimate Strength of Sandwich Pipes
Authors: Castello Xavier , Estefen Segen F.
Published: November 2008

Sandwich pipes composed of two steel layers separated by a polypropylene annulus can be used for the transport of oil&gas in deepwaters, combining high structural resistance with thermal insulation in order to prevent blockage by paraffin and hydrates. In this work, sandwich pipes with typical inner diameters of those employed in the offshore production are analyzed numerically to evaluate the ultimate strength under external pressure and longitudinal bending as well as the effect of the reeling installation method on the collapse pressure. Numerical models were developed using the commercial finite element software ABAQUS. The validation was based on experimental results. The analyses for combined loading were performed using symmetry conditions and the pipe was reduced to a ring with unitary length. The analysis of bending under a rigid surface was simulated numerically according to the experiments performed using a bending apparatus especially built for full scale tests. Symmetry conditions were employed in order to reduce the analysis to a quarter of a pipe. Mesh sensitivity studies were performed to obtain an adequate mesh refinement in both analyses. The collapse pressure was simulated numerically either for the pre or post reeling process. Bauschinger effect was included by using kinematic hardening plasticity models. The influences of plasticity and out-of-roundness on the collapse pressure have been confirmed.
The Behaviour of Tugs in Waves Assisting LNG Carriers During Berthing Along Offshore LNG Terminals
Authors: Buchner Bas , Dierx Pieter , Waals Olaf
Published: November 2008

For future offshore LNG terminals tugs are planned to assist LNG carriers during berthing and offloading operations. A model test study was carried out to better understand the tug behaviour in waves and to make a first step in the quantification of the related weather limits. Scale 1:35 model tests were performed in the two important ‘modes’ of a tug during this type of operation: the ‘push’ mode and the ‘pull’ mode. Realistic weather conditions were used and the tugs were working at the unshielded and shielded sides of the LNG carrier. Based on the results presented in this paper, it can be concluded that the motions of tugs in waves are significant, even in wave conditions that are considered to be mild for the berthing and offloading LNG carriers. The resulting push or pull loads may hamper these tug operations significantly. Special measures are necessary to take this behaviour into account in tug design, LNG carrier design and development of operational procedures and equipment. The paper gives insight in the typical tug behaviour in different weather conditions. One should be careful, however, to generalize the present results: with an optimised tug design and operation the tugs can be used in more severe conditions.
Designing Efficient Trajectories for Underwater Vehicles Using Geometric Control Theory
Authors: Chyba M. , Haberkorn T.
Published: November 2008

In this paper, we consider the minimum time problem for underwater vehicles. Using Lagrangian mechanics, we write the equations of motion for marine vehicles with 6 degrees of freedom as a controlled mechanical system. We then apply the necessary conditions from the maximum principle for a trajectory to be time optimal. Using techniques from differential geometry we analyze the resuls. Finally we supplement the theoretical study with numerical simulations.
Intercomparison of Wave Data Between Triaxys Directional Wave Buoy, ADCP, and Other Reference Wave Instruments
Authors: Shih H. H. , Long C. , Bushnell M. , Hathaway K.
Published: November 2008

The use of Triaxys directional wave buoy and acoustic Doppler current profiler (ADCP) for wave measurements are relatively recent. The US National Oceanic and Atmospheric Administration’s (NOAA) National Ocean Service (NOS) acquired these instruments in 2001 and systematic laboratory and field tests were conducted during 2001–2002. This paper describes the field tests conducted near the US Army Corps of Engineers’ Field Research Facility (FRF) ocean pier and near the Barren Islands in the Chesapeake Bay. At the FRF site, Triaxys buoy wave measurements were compared with FRF’s field standards of pressure sensor arrays and Datawell Waverider buoy. For the Bay test, ADCP was compared with the Triaxys buoy. There are significant numbers of outlier in the Triaxys peak periods at both test sites. In the Chesapeake Bay, which is dominated by high frequency and low energy waves, there is much scatter in the Triaxys data for significant wave heights below 0.2 m. Detailed analyses were performed after these outliers and noisy data were removed. Statistics of differences in significant wave heights, peak periods and directions between each comparative pair were computed and characteristics of frequency and frequency-direction spectra were examined. Overall, correlations between each instrument pair are very good in significant wave heights, fair in wave peak periods (except the ADCP/Triaxys pair), and marginal in wave directions. Triaxys buoy compared better with Waverider buoy than with others. Both ADCP and FRF pressure sensor array exhibit higher resolution in detecting multi-modal and multi-frequency waves. In most cases, energy distribution of spectral peaks in Triaxys buoy data differs significantly from those obtained from FRF pressure sensor array and ADCP.
Analytical and Experimental Modal Characteristics of Cylindrical Shells
Authors: Alzahabi Basem
Published: November 2008

Cylindrical Shells are widely used in many structural designs, such as offshore structures, liquid storage tanks, submarine hulls, and airplane hulls. Most of these structures are required to operate in a dynamic environment. The acoustic signature of submarines is very critical in such high performance structure. Submarines are not only required to sustain very high dynamic loadings at all time, but also being able maneuver and perform their functions under sea without being detected by sonar systems. Reduction of sound radiation is most efficiently achieved at the design stage, and the acoustic signatures may be determined by considering operational scenarios, and modal characteristics. The acoustic signature of submarines is generally of two categories; broadband which has a continuous spectrum; and a tonal noise which has discrete frequencies. Therefore, investigating the dynamic characteristics of cylindrical shells is very critical first step in developing a strategy for modal vibration control for specific operating conditions. Unlike those of beam structure, the lowest natural frequency does not necessarily correspond to the lowest wave index. In fact, the natural frequencies do not fall in ascending order of the wave index in cylindrical shells. Mode shapes associated with each natural frequency are combination of Radial, Longitudinal, and Circumferential modes. In this paper, a scaled model of submarine hull segment under shear diaphragm boundary conditions is analyzed analytically and numerically. Then experimental modal analysis of the scaled model utilizing a fixed response approach was performed to obtain the modal characteristics of the cylindrical shell between 0 and 800 Hz. The cylinder was excited at predetermined points with an impact hammer, while the response was measured using an accelerometer at specified fixed point. Designing a boundary condition that simulate a shear diaphragm is very challenging task by itself. A total of ten natural frequencies were found within that range with their corresponding mode shapes. The experimental data were correlated with those results obtained analytically and numerically using the finite element methods using MSC.NASTRAN software. The results were found to be in excellent agreement.
On the Effects of Aero Boundary Layer Control on Pressure Drag Reduction in Supercavitating Bodies
Authors: Khakpour Yasmin , Yazdani Miad
Published: November 2008

Supercavitation is known as the way of viscous drag reduction for the projectiles, moving in the liquid phase. In recent works, there is distinct investigation between cavitation flow and momentum transfer far away from the cavity surface. However, it seems that there is strong connection between overall flow and what takes place in the sheet cavity where a constant pressure distribution is assumed. Furthermore as we’ll see, pressure distribution on cavity surface caused due to overall conditions, induct nonaxisymetric forces and they may need to be investigated. Primarily we describe how pressure distribution into the cavity can cause separation of the aero boundary layer. Then we present some approaches by which this probable separation can be controlled. Comparisons of several conditions exhibits that at very low cavitation numbers, constant pressure assumption fails particularly for gradient shaped profiles and separation is probable if the flow is sufficiently turbulent. Air injection into the NATURALLY FORMED supercavity is found as an effective way to delay probable separation and so significant pressure drag reduction is achieved. In addition, the position of injection plays a major role to control the aero boundary layer and it has to be considered. Moreover, electromagnetic forces cause to delay or even prevent separation in high pressure gradient flows and interesting results obtained in this regard shows significant drag reduction in supercavitating vehicles.
Feedforward-Feedback Control of Offshore Platforms Excited by Random Wave
Authors: Ji Chunyan , Meng Qingmin
Published: November 2008

Conventional optimal control theories usually deal with optimal control force that involves only structural information. In order to control the excessive vibrations of offshore platforms more effectively, this paper presents a control strategy, which employs both information of the controlled system and external loading, namely feedforward-feedback (FF-FB) control strategy. FF-FB control strategy requires that the input signal be a process of white noise for obtaining the optimal control result. So, an analog filter is designed in this paper to approximately represent the random wave force by a process of white noise. In order to investigate the feasibility and effectiveness of the proposed method, a numerical example applied to an offshore platform is presented in this paper. The numerical results demonstrate that the proposed method offers advantage in performance over the control method employing only state feedback.
Mechanism of Local Scour Around Submarine Pipelines Based on Numerical Simulation of Turbulence Model
Authors: Lu Lin , Li Yucheng , Chen Bing
Published: November 2008

In this paper, the mechanism of local scour around submarine pipeline is studied numerically based on a Renormalized Group (RNG) turbulence model. To validate the numerical model adopted in this paper, the equilibrium profiles of local scour for two cases are simulated and compared with the experimental data. It shows that the RNG turbulence model can give an appropriate prediction for the configuration of equilibrium scour hole, and it is applicable to this situation. The local scour mechanism around submarine pipeline including the flow structure, shear stress distribution and pressure field is then analyzed and compared with experiments. For the further comparison and validation, especially for the flow structure, a numerical model of Large Eddy Simulation (LES) is also developed in this paper. The numerical results of RNG demonstrate that the critical factor governing the equilibrium profile is the seabed shear stress distribution in the case of bed load sediment transport, and the two-equation RNG turbulence model coupled with the law of wall is capable of giving a satisfying estimation for the bed shear stress. Moreover, the piping phenomena due to the great difference of pressure between the upstream and downstream parts of pipeline and the vortex structure around submarine pipeline are also simulated successfully, which are believed to lead to the onset of local scour.
Benchmarking of Truss Spar Vortex Induced Motions Derived From CFD With Experiments
Authors: Halkyard John , Sirnivas Senu , Holmes Samuel , Constantinides Yiannis , Oakley Owen H. , Thiagarajan Krish
Published: November 2008

Floating spar platforms are widely used in the Gulf of Mexico for oil production. The spar is a bluff, vertical cylinder which is subject to Vortex Induced Motions (VIM) when current velocities exceed a few knots. All spars to date have been constructed with helical strakes to mitigate VIM in order to reduce the loads on the risers and moorings. Model tests have indicated that the effectiveness of these strakes is influenced greatly by details of their design, by appurtenances placed on the outside of the hull and by current direction. At this time there is limited full scale data to validate the model test results and little understanding of the mechanisms at work in strake performance. The authors have been investigating the use of CFD as a means for predicting full scale VIM performance and for facilitating the design of spars for reduced VIM. This paper reports on the results of a study to benchmark the CFD results for a truss spar with a set of model experiments carried out in a towing tank. The focus is on the effect of current direction, reduced velocity and strake pitch on the VIM response. The tests were carried out on a 1:40 scale model of an actual truss spar design, and all computations were carried out at model scale. Future study will consider the effect of external appurtenances on the hull and scale-up to full scale Reynolds’ numbers on the results.
Blue Stream Collapse Test Program
Authors: DeGeer D. , Timms C. , Lobanov V.
Published: November 2008

The Blue Stream pipeline is a gas transmission line delivering natural gas from the Russian grid. across the Black Sea, to Turkey. The submarine portion of this pipeline consists of a pair of 24-inch diameter, 31.8 mm wall thickness, API grade X65 pipelines running almost 400 km along the floor of the Black Sea. Over one half of the submarine pipeline lies at a water depth of more than 2000 metres, with the deepest portion of the line reaching a depth of 2150 metres. First gas was supplied through the lines in February of 2003. There were numerous engineering obstacles facing this technically challenging pipeline undertaking. including a lack of heavy-wall, large diameter pipe experimental data to support the pipeline design. Recognizing this need, PeterGaz commissioned a collapse test program during the preliminary engineering phase of the project to generate these data and to gain a better understanding of pipe behavior under combined loading conditions. Numerous full-scale tests were performed on prototype pipe samples, including external pressure testing, combined external pressure and bend testing, and bend testing. Hundreds of material coupon tests were also performed to characterize material stress strain behavior around the circumference of the pipe, through the wall thickness of the pipe, and before and after UOE manufacturing. Tests were also performed to quantify the strength recovery of thermally treated pipe material resulting from the pipe coating process. This paper presents the results of this experimental work and provides some comparisons to collapse predictions.
Hydrofoil Attack Angle Effect on Hydrodynamic Resistance of Semi-Planing Catamaran
Authors: Rad M. , Nakisa M. , Golnezhad G. , Askari M.
Published: November 2008

The present paper contains results of experimental study on a semi-planing catamaran. The aim of this study is to find the effect of hydrofoil usage on resistance of semi-planing catamaran and achieving optimal attack angle, arrangement of hydrofoils and displacement center of gravity. Tests have been done without and with hydrofoils in certain arrangement with different angles of attack then the results are compared. Results show that considerable resistance reduction obtained in semi-planing vessels which equipped with hydrofoil in optimized angle of attack in service speed range. Meanwhile the hydrofoils arrangement and the angle of attack are important so unsuitable design causes the instability in the motion and may be increases hydrodynamic resistance.
Probabilistic Analysis of Live Loads on Offshore Platform Piles
Authors: Zaghloul Hassan , Ronalds Beverley , Cole Geoff
Published: November 2008

Relatively accurate techniques are available to assess structural behavior under given loads, yet the loads themselves remain an estimate based in part on field measurements, in part on professional logic and experience, and in part on trial and error. The design of piled foundations for fixed offshore platforms must consider operating and extreme weather conditions. In the operating condition, the magnitude of live loads on open areas of topside structure is an important consideration. Unfortunately, the design live load intensity that applies to open areas on offshore platforms is not identified in international codes and standards. There does not appear to be any consensus on the value to be adopted in the industry. Some operators suggest the open area live loads need not be considered for pile foundation design, while others stipulate values such as 10 kPa. This is partly due to the variability associated with the different live loads sources. The objective of this study is to obtain a better understanding of open area live loads on offshore platforms and develop a methodology to obtain the long-term and extreme open area live load. A load survey was conducted for the purpose of this study, and a probabilistic analysis was carried out to derive the maximum axial load on piles that is expected during platform lifetime. The results of this study indicate that the use of a single value for the open area live load (OALL) may not be appropriate and suggest appropriate values for Load Resistance Factor Design (LRFD) or Working Stress Design (WSD) methods.
Long-Term Performance of Syntactic Foam Materials in Unusual Environments
Authors: Wang Wen-Tsuen , Watkins Lou
Published: November 2008

Syntactic foam, a composite material made by combining spherical fillers in a polymeric binder, has been used for over thirty years in the offshore oil industry. To date, the applications of this material have fallen into two categories: (1) buoyancy modules or floats to support drilling risers, or (2) thermal insulation for subsea equipment and flowlines. In the first category, the syntactic foam is exposed only to cold water (4° C). In the second category, the insulation may be subjected to temperatures as high as 150° C. The contrast of these two separate applications has led to two distinct classes of materials, each with its own properties and accepted standards and criteria. Now a new category of usage has arisen: Vertical production risers that require buoyant lift, and sometimes some degree of thermal insulation, for long-term service (20–25 years) in “warm” water that may be in the range of 40° C to 65° C. By combining the buoyancy requirement of lowest possible density with the insulation requirement of prolonged hydrothermal stability, this application poses new challenges for syntactic foam development and demands new directions in testing and analysis. Because of the increasingly large size of emerging offshore projects, the potential requirement here is for very large volumes. This paper describes the materials that have been identified as candidates for the new service, and outlines the testing philosophy that is being evolved to test and qualify them with confidence for very long periods of service. Preliminary test data is presented, along with predictions of long-term performance. Lessons learned during the project will have implications for all syntactic materials, and will be useful to any managers and technologists involved in marine engineering.
Effects of Local Metal Loss on Deformability of Line Pipes Subjected to Compressive Load
Authors: Hashimoto Yoshikazu , Yatabe Hiroshi , Hagiwara Naoto , Oguchi Noritake
Published: November 2008

In this paper, the deformability of line pipe with local metal loss was examined. A full-scale experiment and a finite element (FE) analysis were carried out for line pipe with local metal loss subjected to an axial compressive load. As a result, a good agreement was obtained between the analytical and experimental results. This indicated that the present analytical method was applicable to evaluate the deformability of line pipes with local metal loss subjected to a large ground movement. Parametric studies were then conducted to clarify the relationship between the geometry of the local metal loss and the deformability using the FE analytical method.
On Stability Improvement of Gravity Type Onshore and Offshore Structures
Authors: Hazarika Hemanta
Published: November 2008

A protective cushion layer sandwiched between a gravity type caisson and the surrounding soil can improve the seismic stability of the caisson. In this paper, an interaction model was developed for analyzing such structure under earthquake loading. Interfaces on either side of the cushion were modeled as elements of finite thickness having different stiffness and constitutive properties. Participation from each element of the interaction system was taken into the account by incorporating appropriate factors at the respective interfaces. Seismic analyses were performed on a gravity-type caisson subjected to an actual earthquake motion. Comparison of the analysis results with a similar caisson without any protective cushion showed that the use of cushion yields a significant reduction of seismic load on the structure.
Optimization and Time-Domain Simulation of the SEAREV Wave Energy Converter
Authors: Babarit Aure´lien , Cle´ment Alain H. , Gilloteaux Jean-Christophe
Published: November 2008

This paper introduces a new second generation wave energy converter concept named SEAREV [Systeme Electrique Autonome de Recuperation d’Energie des Vagues]. The working principle and linearized equations of the device are described. It is shown how energy absorption depends on the shape of the external floating body and on the mechanical characteristics of the moving mass. This allows to numerically optimize the geometry of the device. Latching control is used to further improve the capture width of the system, with success in regular waves.
Series Expression and Applications of Random Drag Forces
Authors: Ma Rujian , Li Guixi , Zhao Dong
Published: November 2008

The series expression of random wave drag forces is studied in this paper by means of nonlinear spectral analysis. A new method is proposed in this study since the derivative of the wave drag force is discontinuous. The characteristics of random process are used to obtain the coefficients of the series in this method. Numerical verification indicates that the series expression given in the paper approaches the original expression satisfactory. The maximum error is less than 4% when measured in the peak values of the time series and less 3% when measured in the root mean square values. That is to say, the series expression given in the study has reasonable accuracy and can be used in engineering computations. The application example indicates that the spectral densities of wave forces, as well as the root mean squares of wave forces, can be easily obtained without lose much accuracy using the series expression of wave drag forces.
Revealing Nonlinear Dynamics Phenomena in Mooring Due to Slowly-Varying Drift
Authors: Bernitsas Michael M. , Matsuura Joa˜o Paulo J.
Published: November 2008

The effects of slowly-varying wave drift forces on the nonlinear dynamics of mooring systems have been studied extensively in the past 30 years. It has been concluded that slowly-varying wave drift may resonate with mooring system natural frequencies. In recent work, we have shown that this resonance phenomenon is only one of several possible nonlinear dynamic interactions between slowly-varying wave drift and mooring systems. We were able to reveal new phenomena based on the design methodology developed at the University of Michigan for autonomous mooring systems and treating slowly-varying wave drift as an external time-varying force in systematic simulations. This methodology involves exhaustive search regarding the nonautonomous excitation, however, and approximations in defining response bifurcations. In this paper, a new approach is developed based on the harmonic balance method, where the response to the slowly-varying wave drift spectrum is modeled by limit cycles of frequency estimated from a limited number of simulations. Thus, it becomes possible to rewrite the nonautonomous system as autonomous and reveal stability properties of the nonautonomous response. Catastrophe sets of the symmetric principal equilibrium, serving as design charts, define regions in the design space where the trajectories of the mooring system are asymptotically stable, limit cycles, or non-periodic. This methodology reveals and proves that mooring systems subjected to slowly-varying wave drift exhibit many nonlinear phenomena, which lead to motions with amplitudes 2–3 orders of magnitude larger than those resulting from linear resonance. A turret mooring system (TMS) is used to demonstrate the harmonic balance methodology developed. The produced catastrophe sets are then compared with numerical results obtained from systematic simulations of the TMS dynamics.
Reliability of Mooring Systems for a Spar
Authors: Ding Y. , Gilbert R. B. , Theckum Purath Basil , Dangyach S. , Zhang J. , Choi Y. J.
Published: November 2008

Mooring systems for floating production systems, including the mooring lines and anchors, are currently designed on the basis of individual components. The most heavily loaded line and anchor are checked under extreme loading conditions with the system of lines intact and with one line removed. However, the performance of the floating production system depends more directly on the performance of the system of lines and anchors rather than on the performance of a single line or anchor. The on-going study conducts the reliability analyses using realistic probabilistic descriptions of the extreme met-ocean conditions (hurricanes and loop currents) for the Gulf of Mexico. Presented in this paper is the methodology of calculating the probabilities of failure of a classical Spar during a 20-year design life for individual components.
Development of Structural Steel With High Resistance to Fatigue Crack Initiation and Growth: Part 3
Authors: Katsumoto Hiroshi , Konda Noboru , Arimochi Kazushige , Hirota Kazuhiro , Isoda Atsushi , Kitada Hiroshige , Sakano Masahiro , Yajima Hiroshi
Published: November 2008

In recent years, higher safety and reliability of steel welded structures have been required as it shows growing concern about environmental problems. To prevent fatigue fracture is one of the most important challenges to improve the safety and reliability. A lot of studies how reduce stress concentration at critical areas have been carried out from the viewpoint of structural design as prevention measures while nothing has been studied from the viewpoint of material because fatigue strength of welded joints converges in limited range regardless of material strength. On the other hand, it was found that an appropriate dual phase microstructure could reduce the fatigue crack growth rate remarkably. The newly developed steel plate with high resistance to fatigue crack growth could extend the fatigue life of structures. The developed steels have already been applied to some ships and vessels, and a new bulk carrier applied the developed steels acquired the notation and descriptive note as the valuable ship with resistance to fatigue fracture by Nippon Kaiji Kyokai for the first time in the world. From further studies, it was found the developed steels had also high resistance to fatigue crack initiation as well as the growth even in welded structure. In this study, it was clarified that the fatigue strength of HAZ, where fatigue crack generally initiates, in the developed steel was higher than that in conventional steel and the stress concentration at toe of weld in the developed steel was smaller than in the conventional steel. It was considered the mechanism of suppression of fatigue crack initiation with FEM analysis and fatigue test. The newly developed steel can effectively extend fatigue fracture life of welded structure from the viewpoint of material.
Non-Linear Analysis for Ultimate Loading Capacity of Cast Tubular T-Joints
Authors: Sun Peng , Wang Yuanqing , Shi Yongjiu
Published: November 2008

In this paper, we performed a lot of parametric analysis of cast T-joints. The geometry model was established using Solidworks. Non-linear analysis was carried out using the commercial finite element programme Ansys. Parametric equations of ultimate loading capacity derived from the results of finite element analysis are presented for the usual range of basic shapes of T-joints under axial loading, in-plane and out-of-plane bending. The sensitivity of the ultimate loading capacity in cast tubular joints to variation in the geometric parameters has been assessed. Besides the parameters which governing the stresses in welded joints, an additional parameter ρ that is defined by C. D. Edwards has been introduced to describe the size of fillet. In this paper, the sensitivity of ultimate loading capacity of cast tubular joints to the parameter ρ is presented.
Water Entry and Exit of a Horizontal Circular Cylinder
Authors: Zhu Xinying , Faltinsen Odd M. , Hu Changhong
Published: November 2008

This paper describes the fully nonlinear free-surface deformations of initially calm water caused by water-entry and water-exit of a horizontal circular cylinder with both forced and free vertical motions. This has relevance for marine operations as well as for the ability to predict large amplitude motions of floating sea structures. A new numerical method called the CIP (Constrained Interpolation Profile) method is used to solve the problem. In this paper, the circular cylinder and free surface interaction is treated as a multiphase problem, which has liquid (water), gas (air) and solid (circular cylinder) phases. The flow is represented by one set of governing equations, which are solved numerically on a non-uniform, staggered Cartesian grid by a finite difference method. The free surface as well as the body boundary is immersed in the computational domain. The numerical results of the water entry and exit force, the free surface deformation and the vertical motion of the cylinder are compared with experimental results, and favorable agreement is obtained.
Computation of Hydrodynamic Interaction of Multiple Bodies in Waves With a Panel-Free Method
Authors: Qiu Wei , Peng Hongxuan , Calisal Sander M. , Wang Jianhong
Published: November 2008

The hydrodynamic interaction of multiple floating bodies in waves has been computed in the frequency domain based on the panel-free method developed earlier for the time-domain analysis. The integral equations are first desingularized by removing the singularity in the Green function and then discretized by Gaussian quadrature over the exact geometry. Non-uniform rational B-splines (NURBS) surfaces are employed to represent the exact body surface. Robustness and accuracy of the method has been demonstrated by its application to vertically floating cylinders. Computed motions, hydrodynamic coefficients and wave exciting forces due to interactions are presented and compared with other published results. The numerical method has also been applied to the performance study of a wave energy converter. The computed results were compared with the experimental ones.
In-Plane Deflection of Nonprismatic Curved Beam Structures Considering the Effect of Shear Deformation Solved by DQEM
Authors: Chen Chang-New
Published: November 2008

The development of differential quadrature element method in-plane deflection analysis model of curved nonprismatic beam structures considering the effect of shear deformation was carried out. The DQEM uses the differential quadrature to discretize the governing differential equation defined on each element, the transition conditions defined on the inter-element boundary of two adjacent elements and the boundary conditions of the beam. Numerical results solved by the developed numerical algorithm are presented. The convergence of the developed DQEM analysis model is efficient.
Vibration Control of Jacket Platforms by DTMD System
Authors: Zhao Dong , Ma Rujian , Wang Weiqiang , Cai Dongmei
Published: November 2008

The turned mass damper (TMD) system has found extensive applications in vibration control nowadays. The main disadvantage of the system is to add additional mass to the structures. In order to make use of its merits and overcome its disadvantage, the developed turned mass damper (DTMD) vibration control system is studied in this paper. It uses the equipment set on the structure instead of additional mass to control the vibration of structures. The theory analysis shows that the frequency range is enlarged and the resonant band is reduced with the mass ratio increasing. The FEM simulation shows that the DTMD vibration control system could absorb more vibration energy when the frequency of the DTMD is near the excitation frequency. The platform has better vibration control effect when the mass ratio between 3.75 and 5 and when the frequency ratio is in the range of (0.3,0.35) and (0.35,0.5). The whole system, including the platform and the DTMD, has the smaller vibration response when the frequency ratio is between 0.5 and 2. The results of the simulation also showed that the DTMD is more effective in the vibration control for the jacket platforms.
Nonlinear Internal Waves in Stratified Fluid With Homogeneous Layer
Authors: Makarenko Nikolai I. , Maltseva Janna L.
Published: November 2008

The problem of steady internal waves in a weakly stratified two-layered fluid is studied analytically. We discuss the model with a constant density in lower layer and exponential stratification in the other one. The long-wave approximation using a scaling procedure with small Boussinesq parameter is constructed. The nonlinear ordinary differential equation describing large amplitude solitary waves and internal bores is obtained.