bluuebunny/crossref_metadata_2025 huggingface dataset:
Separated Flow About a Slender Body in Forward and Lateral Motion
Authors: Yeung Ronald W. , Seah Robert K. M. , Imamura John T.
Published: July 2009

This paper presents a solution method for obtaining the hydrodynamic forces and moments on a submerged body translating at a yaw angle. The method is based on the infinite-fluid formulation of the Free Surface Random Vortex Method (FSRVM), which is re-formulated to include the use of slender-body theory. The resulting methodology is given the name: Slender-Body FSRVM (SB-FSRVM). It utilizes the viscous-flow capabilities of FSRVM with a slender-body theory assumption. The three-dimensional viscous-flow equations are first shown to be reducible to a sequence of two-dimensional viscous-fluid problems in the cross-flow planes with the lowest-order effects from the forward velocity included. The theory enables one to analyze effectively the lateral forces and yaw moments on a body undergoing prescribed forward motion with the possible occurrence of flow separation. Applications are made to several cases of body geometry that are in steady forward motion, but at a yawed orientation. These include the case of a long “cone-tail” body. Comparisons are made with existing data where possible.
Deformation of Steel Straight Pipes With Internal Pressure Under Axial Compression and Bending Load by Seismic Action
Authors: Sa´nchez Sa´nchez He´ctor A. , Corte´s Salas Carlos
Published: July 2009

Due to the high risk seismic in many zones of Mexico it has been observed that the ground motions often show large horizontal displacement. This displacement causes large deformations of buried pipelines. Then, the knowledge of study and design recommendations related to deformability of the pipes has not been sufficiently provided. A grand number of studies have been reported concerned about the plastic deformations or buckling of the straight pipe. Most of them are performance of column pipe without internal pressure. Therefore, in this work are analyzed the steel straight pipes, for the purpose of clarifying the deformations of the pipes with internal pressure under large displacement and bending. Effect of internal pressure on deformability of pipe is investigated both under load bending. Stress analysis using FEM is performed in order to simulate the large deformations of the pipes.
Collision Prevention of Two Different Top Tensioned Risers by Automatic Control
Authors: Rustad Anne M. , Larsen Carl M.
Published: July 2009

It has previously been demonstrated that collision between two identical risers in a tandem arrangement can be prevented by controlling the payout of the heave compensators. A control strategy based on “equal effective length” will in an effective way cause the two risers to respond dynamically in parallel. This effect might, however, be more difficult to obtain if the two risers are different with respect to diameter, weight and tension. In this paper a system consisting of a tension leg platform (TLP) and two different risers is presented. The control strategy previously applied to two identical risers has been used in this case as well. Slowly varying and wave frequency surge motions are introduced in order to create dynamic response that normally would lead to collision between the risers. From this study it can be concluded that the control strategy previously applied for identical risers in tandem will also be effective for risers with significantly different design parameters such as diameter, top tension and effective weight.
Application of Methane Supply Process Unit in Mass Balance Ecosystem Model Around Cold Seepage
Authors: Yamazaki Tetsuo , Monoe Daisuke , Oomi Tomoaki , Nakata Kisaburo , Fukushima Tomohiko
Published: July 2009

Natural cold seepages are characterized as rapid upward transports of methane from deeper parts of geological structures to the seafloor. The original methane supply source is expected to locate below BSR (Bottom Simulating Reflector). The methane moved up to seafloor is mainly consumed by microorganisms living in anoxic marine sediments. When the methane supply is very large or rapid, remaining unconsumed methane escapes into the water column and is consumed by oxidizing bacteria. The supply mechanism of methane from the supply source to the cold seepages has not yet being clarified. In order to integrate the methane consumption processes in sediments and water column, a simple methane supply mechanism is developed.
Review and Assessment of Fatigue Data for Offshore Structural Components Containing Through-Thickness Cracks
Authors: Zhang Y. H. , Stacey A.
Published: July 2009

In recent years, structural integrity management schemes for offshore installations have placed increased reliance on the use of flooded member detection (FMD) as the principal inspection method. This method can be routinely employed in a remotely operated vehicle, which enables a large number of members to be inspected fairly quickly at a much reduced cost compared to using diver operated techniques. However, reliance on FMD for safety assurance requires that welded joints retain sufficient fatigue life and static strength after through-thickness cracking. A comprehensive examination of published work containing data on fatigue lives beyond through-thickness cracking in offshore structures was carried out, resulting in the development of a database of 281 relevant tests. The database was used to perform a statistical assessment of the effects of different testing conditions and geometrical parameters on the remaining fatigue life beyond the occurrence of through-thickness cracking, N3, which was represented by a parameter Re. Whilst the data showed a large amount of scatter, it was found that Re depends strongly on chord thickness, loading mode, type of joint and testing environment. In some cases, a significant amount of remaining life existed. This was often associated with T-type tubular joints with thin chord thickness under out-of-plane loading and a seawater (with CP) environment. The influence of the relevant parameters on Re is discussed and attributed to their effect on crack shape, stress distribution, cracking location and crack propagation path.
Core Material Performance on the Propagation Pressure of Sandwich Pipes
Authors: Lourenc¸o Marcelo Igor , Pasqualino Ilson Paranhos , da Silva Palermo Tiago
Published: July 2009

Due to their insulative nature and ability to withstand high hydrostatic pressures, sandwich pipes have been considered feasible for use in ultra deepwater pipelines. As defined here, sandwich pipes consist of an inner and outer metallic pipe with a suitable core material in the annulus. In this paper, a non linear three-dimensional finite element model, previously presented and compared with experiments, is used in an extensive parametric study of the quasi-static propagation of buckles in sandwich pipes. The primary purpose of this study is to investigate the actual contribution of the core material to the propagation pressure. Parameters addresses include bonding condition (no adherence, perfect adherence), metallic pipe thickness, and core material thickness.
Investigating the Relevance of Strip-Theory for Pipelines Subjected to Vortex Induced Vibration
Authors: Soni Prashant K. , Larsen Carl M.
Published: July 2009

Empirical models for prediction of vortex induced vibrations (VIV) apply hydrodynamic coefficients to represent the fluid forces on the structure. The coefficients are found by measuring forces on a rigid cylinder under harmonic pure inline (IL) and pure cross flow (CF) forced motions, and presented as functions of non-dimensional motion amplitude and frequency. In the response analysis the forces at a specific cross-section are assumed to be defined by the motion of this cross-section, which implies that possible three-dimensional (3D) hydrodynamic effects are neglected. The approach is often referred to as ‘strip-theory’, which is a term originally used in ship hydrodynamics. Here, a classical method for calculation of motions and beam forces is based on the same type of simplification. The strip-theory has been used for VIV analysis both in combination with empirical coefficients but also combined with 2D numerical solutions of the Navier-Stokes equation for force calculation. The approach as such has never been verified and the loss of accuracy from neglecting 3D hydrodynamic effects has never been quantified. The purpose of the present work is to contribute to such verification. The investigation reported herein consists of three steps. – Experiments with a flexible beam subjected to VIV. Response amplitudes on CF and IL directions were measured so that the trajectories for several cross-sections along the beam could be found; – Measurement for hydrodynamic forces on a rigid cylinder that was forced to follow the same trajectories as found from the beam experiments; – Use of a finite element program to calculate the dynamic response of a flexible beam with the same properties as for the first test and subjected to forces from the second test. If the calculated response is found to be identified to the measured the verification exercise could be accepted as successful. Discrepancies between measured and calculated response might be caused by 3D hydrodynamic effects or poor quality of the experiments. The present study is a first attempt and the reported results do not lead to a firm conclusion.
Deterministic Reconstruction and Prediction of a Non-Linear Wave Field Using Probe Data
Authors: Blondel Elise , Ducrozet Guillaume , Bonnefoy Fe´licien , Ferrant Pierre
Published: July 2009

The accurate simulation of non-linear sea states evolution over long time periods represents a great challenge, with number of applications in oceanography, marine engineering, security of people or marine transportation, etc... The aim of this study is to develop an efficient deterministic prediction model for irregular wave fields based on the exploitation of wave elevation time series given by one or more probes. We use the High-Order-Spectral model (HOS) to simulate numerically the wave field evolution, in order to take the non-linear effects up to a desired order into account. In this paper, we report on the development of an effective reconstruction scheme, for two dimensional wave fields and using one wave record, that allows us to get proper initial conditions for numerical simulations and nonlinear wave fields forecast.
Heave Added Mass and Damping of a Suction Can in Proximity to the Sea Floor
Authors: Roe T. F. , Macfarlane G. , Drobyshevski Y.
Published: July 2009

Suction cans are usually deployed by the crane of a construction vessel, which must have adequate capacity to withstand the dynamic hook loads generated by motions of the vessel and heave response of the suction can. Before the structure is placed on the sea floor, it must be positioned above the target location; in this phase the suction can is manoeuvred into position being suspended in proximity to the sea floor. Hydrodynamic properties of the structure in the positioning phase are different from those experienced during the decent, due to the effect of the bottom proximity. As a result, the dynamic hook loads experienced in this phase may be also different from the deep water condition. The objective of this study is to quantify these effects; in particular the impact of the bottom proximity on the heave added mass and hydrodynamic damping. The added mass and damping of a 6-metre diameter suction can, of dimensions typical for Australian North West Shelf conditions, have been determined by testing a 1:10 model in the 4.2 m deep water tank of the Australian Maritime College. Free decay tests were conducted at several heave frequencies, and the heave added mass and damping determined. Four clearances of the model from the sea floor were investigated ranging from 1.20 to 0.20 of the can height. For each clearance, several sizes of open hatches were examined, by testing the model with 3 pairs of hatches of various diameters, with up to 4.8% of the relative area open. Model tests demonstrate that the heave added mass and damping increase as the suction can approaches the sea floor. Increase in added mass is found to be within 20% of its deep water value, and is made less pronounced by opening hatches of larger area. Linear (proportional to velocity) hydrodynamic damping also increases moderately as the under-bottom clearance reduces. Quadratic (proportional to velocity squared) damping is strongly affected, especially at very small clearances, with drag coefficient reaching unusually high values; this is attributed to substantial constraining effect of the bottom, which causes increasing flow velocities past the lower edge of the can. Results of the tests are presented, and their application for the installation lift analysis is discussed.
Computational Techniques for Stabilized Edge-Based Finite Element Simulation of Free-Surface Flows
Authors: Elias Renato N. , Gonc¸alves Milton A. , Coutinho Alvaro L. G. A. , Esperanc¸a Paulo T. T. , Martins Marcos A. D. , Ferreira Marcos D. A. S.
Published: July 2009

Free-surface flows occur in several problems in hydrodynamics, such as fuel or water sloshing in tanks, waves breaking in ships, offshore platforms, harbors and coastal areas. The computation of such highly nonlinear flows is challenging since free-surfaces commonly present merging, fragmentation and breaking parts, leading to the use of interface capturing Eulerian approaches. In such methods the surface between two fluids is captured by the use of a marking function which is transported in a flow field. In this work we discuss computational techniques for efficient implementation of 3D incompressible SUPG/PSPG finite element methods to cope with free-surface problems with the Volume-of-Fluid (VOF) method [1]. The pure advection equation for the scalar marking function was solved by a fully implicit parallel edge-based SUPG finite element formulation. Global mass conservation is enforced adding or removing mass proportionally to the absolute value of the normal velocity of the interface. We introduce parallel edge-based data structures, a parallel dynamic deactivation algorithm to solve the marking function equation only in a small region around the interface. The implementation is targeted to distributed memory systems with cache-based processors. The performance and accuracy of the proposed solution method is tested in the simulation of the water impact on a square cylinder and in the propagation of a solitary wave.
VSIV (Vortex Self-Induced Vibration) Kinematics
Authors: Fernandes Antonio C. , Silva Erika M. C. , Franciss Ricardo , Coelho Fabio M. , Neto Severino F. S.
Published: July 2009

The Vortex Induced Vibration (VIV) of cylindrical lines that may occur when the lines are submitted to currents has been extensively discussed in the past few years and its behavior has become well known. However, it is not so well known that the vibrations may occur in a current-less situation, induced by the lateral motion of the structure itself. The present work refers to the last as the Vortex Self-Induced Vibration, the VSIV. This occurrence has been made clear in the LOC/COPPE/UFRJ (Laboratory of Waves and Currents of COPPE, the Graduate School of Federal University of Rio de Janeiro) by specifically designed tests. In these tests, a totally submerged horizontal cylinder was submitted to harmonic forced oscillations, being free to move in the transverse direction of the forced excitation. The VSIV then showed up, with the cylinder segment, describing vertical trajectories in two (vertical 8-shape), three, four, etc., almost circular trajectories (called the rings in the work). Subsequently, the work shows that the measurements in full scale with the VIV bottle on a Steel Catenary Riser in the PETROBRAS 18 platform also indicate the existence of the VSIV. The tests were carried out with Keulegan-Carpenter equal to 10, 20 and 30 and for several amplitudes. The response of the cylinder was represented in non-dimensional parameters corresponding to the amplitude, the excitation and the response frequencies.
Numerical Investigation Into Parameters Influencing the Installation of Suction Caissons in Sand
Authors: Zeinoddini Mostafa , Shariati Woorya H. , Nabipour Mahmood
Published: July 2009

This paper reports results from a numerical investigation into the suction caissons penetration in sand. Two dimensional axisymmetric models have first been calibrated and verified against several laboratory and field test data from other researchers. Soil nonlinearities and soil/caisson interactions have been taken into account. The verified models have then been used to evaluate the effects from various soil/structure characteristics on the performance of the suction caissons during the installation phase in sand. The results of the current study show that the total installation force required for the full penetration of the caisson has a second order relationship with the soil/caisson interface strength reduction factor. The soil cohesion has also been found to have a second order effect on the total installation force. The soil internal friction angle, and the soil modulus of elasticity have each been noticed to present an increasing linear effect on the total installation force. It has also been observed that while the caisson diameter remains constant, with an increase in the caisson length the total installation force almost linearly increases. This is the same when the caisson length is kept constant but its diameter increases. Dilatancy angle and Poisson’s ratio have been realized to have a second order monotically increasing effect on the total installation force.
The Wave Loads Acting on the Underwater Pipeline
Authors: Egorov Gennadiy V. , Vorobyov Yuriy L. , Foedorova Katrin Y.
Published: July 2009

The nonlinear boundary value problem is studied for potential function of progressive surface waves with finite amplitude in shallow water. For the first time the method of space deformation is used to get an approximate solution for evaluation of wave load on a horizontal pipe lying at the bottom. The theoretical results are compared with experimental data and showed practically good conformity.
Modelling and Characterization of Artificial Marine Growth
Authors: Baarholm Rolf , Skaugset Kjetil
Published: July 2009

The Riser and Mooring project of the Norwegian Deepwater Programme (NDP) has undertaken a parametric study of marine growth on circular cylinders fitted with helical strakes. The study was performed at MARINTEK. The aim is to provide guidance on the effect that marine growth has on the dynamic response of the riser. The tests were conducted in a controlled laboratory condition where organic material is not allowed. Hence artificial marine growth had to be modelled and manufactured. Both hard and soft marine growth have been modelled. Further it is vital to document the properties of the marine growth. The present paper will discuss issues in modelling artificial hard and soft marine growth, as well as characterizing the growth for reference after testing. Results from the NDP test campaign will be presented in the paper.
Estimation of Ice Load on a Ship Hull Based on Strain Measurements
Authors: Leira Bernt J. , Bo̸rsheim Lars
Published: July 2009

The increased activity related to the oil and gas industry in polar waters implies that proper operation of ships such areas also will be in focus. The loading on a particular ship hull depends strongly on the route selection and vessel speed. Lack of information about the actual ice condition and the corresponding loads acting on the hull is identified to be among the most critical factors when operating in Arctic waters. This implies that there is a challenging interaction between strength-related design rules and schemes for operation of ships in arctic regions. In particular, the possibility of monitoring ice-induced stresses in order to provide assistance in relation to ship manoevering becomes highly relevant. The present paper is concerned with estimation of ice loads acting on the hull of the coast guard vessel KV Svalbard based on strains that were measured during the winter of 2007 as part of a project headed by DNV. Application of a finite element model of the bow structure is also applied in order to correlate the loading with the measured strains. The influence of ice thickness and vessel speed on the measured strain levels is also investigated.
Current Modeling Experience in an Offshore Basin
Authors: Buchner Bas , de Wilde Jaap
Published: July 2009

A reliable current simulation is vital to achieve the objectives of model testing. In the present paper the development and testing of the current generation in an Offshore Basin is presented. First the objectives of such a development are discussed. Then the actual system as developed is described in detail. Finally the results of current measurements in the basin are presented. It is concluded that with a dedicated current modeling system in a model basin, it is possible to achieve a constant current over the measurement area with low variations in time and space.
Numerical Methods for Hull Structure Strength Analysis and Ships Service Life Evaluation, for a LPG Carrier
Authors: Domnisoru Leonard , Dragomir Dumitru , Ioan Alexandru
Published: July 2009

In this paper the authors focus on the ship hull structure strengths and fatigue analyses, in order to estimate the ship service life period at the initial design stage. The topic of the paper is divided in three-interlinked parts. The first part includes the method for the hull strength analysis, based on 3D/1D-FEM models, under equivalent quasi-static head wave loads. The second part presents the method for the ship hull dynamic response analysis, based on non-linear hydroelasticity theory with second order wave spectrum. The last part includes the fatigue analysis method for the initial ship hull structure, based on the long-term prediction ship dynamic response, the cumulative damage ratio and the design S-N material curves. The numerical analyses are carried out for a LPG carrier with independent cargo-tanks type A. Two significant load cases are considered: full and ballast. The numerical results outline the extreme wave loads and the ships initial service life evaluation.
Modeling the UOE Pipe Manufacturing Process
Authors: Toscano Rita G. , Raffo Javier , Fritz Marcelo , Silva Ronaldo C. , Hines Joshua , Timms Chris
Published: July 2009

It has been demonstrated in previous work that, for deepwater applications, the cold forming processes involved in UOE pipe manufacturing significantly reduces pipe collapse strength. To improve the understanding of these manufacturing effects, Tenaris has embarked on a program to model the phases of the UOE manufacturing process using finite element analysis simulations. Phase 1 of this work, presented previously in the literature [1], formulated the basis for the model development and described the 2D approach taken to model the various steps of manufacture. This paper presents the results of the Phase 2 work, and includes a description of the enhancements made to the modeling approach, a summary of the full-scale collapse testing performed at C-FER, and a comparison of the model predictions to the test results. Variations are made to the simulated manufacturing process in order to evaluate the sensitivity of collapse strength to key parameters. Based on the modeling approach taken, the findings of the Phase 2 work have shown that the deterioration of the collapse pressure diminishes with increasing O-press compression. The residual stress value is the most sensitive parameter when the strain hardening varies. It increases with the compression ratio and with the strain hardening value. In addition, given the assumed compression ratio of the test pipes, predictive behavior of the test results was found to be acceptable.
Response Characteristics of a Trailer Type Multi-Connected Barge System in Waves
Authors: Ikegami Kunihiro , Ohta Makoto , Matsuura Masami , Toshimitsu Kazuki , Nagaosa Satoshi , Zhang Guo
Published: July 2009

A trailer type multi-connected barge system is a new type of ocean transportation system composed of several barges connected to each other with a mechanical connecting device and a tug ship. To design the trailer type multi-connected barge system, it is necessary to grasp the exact response characteristics of the trailer type multi-connected barge system in waves. A model basin test was conducted for the trailer type multi-connected barge system in waves. In the model basin test, the tug ship and three barges were connected to each other by the mechanical connecting device, and the tug ship towed three barges in waves. As a result, the response characteristics of the trailer type multi-connected barge system in waves were obtained. A non-linear phenomenon of a longitudinal connecting force caused by relative pitch motions of barges was observed. A way to avoid the non-linear phenomenon and reduce the longitudinal connecting force could be found, and the realization of the trailer type multi-connected barge system was experimentally confirmed.
Probabilistic Model of Pitting Corrosion and the Simulation of Pitted Corroded Condition
Authors: Yamamoto Norio
Published: July 2009

Pitting corrosion is a great concern with respect to the integrity of ship hull structures such as the hold frames in way of the cargo holds of those bulk carriers which exclusively carry coal and/or iron ore. The actual strength of pitting corroded member depends on the pitting corroded surface condition; a condition which varies according to the generation and the progress of the corrosion pits. In order to investigate a practical strength evaluation method of the pitting corroded member, a serial strength evaluation for various pitting corroded conditions is necessary. However, because it was difficult to obtain actual pitting corroded members which satisfied the purpose of our analysis, we needed to use an alternative method of conducting this strength analysis; a method that used simulated pitting corroded members. In order to do this, the simulated pitting corrosion surface we used had to have been a good representation of the actual pitting corrosion condition. In this paper, we investigated the pitting corroded surface conditions of hold frames in way of cargo holds of bulk carriers in order to comprehend the statistical nature of such pitting corroded surface. We developed a simulation procedure based upon a probabilistic model and used it to generate an optional pitting corroded surface. Through our investigation, we were able to confirm that the simulated pitting corrosion condition is visually similar to the actual condition and has the same statistical nature.
Plastic Behavior of Web Girders in Ship Collision and Grounding
Authors: Hong Lin , Amdahl Jo̸rgen
Published: July 2009

Some recent studies on the plastic behavior of web girders are described in the present paper. During the process of ship collision and grounding, various web girders will be subjected to large impact loads from various directions. Therefore, different deformation patterns can be identified. The recent developed plastic mechanism for local denting of web girders is depicted briefly and compared to various existing theoretical models. On the other hand, as a newly identified problem in ship grounding and collision process, the problem of lengthwise sliding deformation of web girders is described. Emphasis is placed on the description of the plastic mechanism and various energy dissipation patterns. The developed simplified methods on local denting and sliding deformation of web girders are compared with the results of experimental tests and finite element analysis respectively, both show satisfactory agreement. The proposed models will contribute substantially to the establishment of efficient methods for fast and reliable assessment of the outcome of accidental collision and grounding events. Such methods may in turn be incorporated into decision support tools for crisis handling in emergency situations, e.g. for tankers in disabled conditions.
Computer Simulation and Potential Measurement of the Cathodic Protection System in Ballast Tank
Authors: Nakayama Atsushi , Amaya Kenji , Yamamoto Norio
Published: July 2009

Numerical analyses and experimental measurements on potential distributions inside a ballast tank were studied. The relationship between the paint conditions and potential distributions was quantitatively examined. The actual potential distribution in a ballast tank were measured. A boundary element method for a cathodic protection system was employed to the numerical analyses. A polarization resistance of a metal was obtained by a laboratory experiment for the numerical analyses. A differential analysis and a differential measurement were proposed. The differential numerical results were compared with the differential experimental results in the ballast tank. It was seen that the differential potential distributions were well modeled in the numerical analyses.
Performance of Perforated Wall for the Conditioning of the Flow in an Ocean Basin
Authors: Alho Alexandre T. P. , Sphaier Sergio H. , Levi Conceic¸a˜o Carlos A. , Esperanc¸a Paulo de Tarso T.
Published: July 2009

This paper discusses the investigations conducted to develop the hydraulic design of the flow conditioner of the current generation system of LabOceano. The performance of different flow conditioner configurations were numerically investigated by means of computational fluid dynamics techniques. The numerical results were validated against experimental data, showing a good performance of the numerical model in predicting the main flow characteristics. It was observed that undesirable high angles of incidence of the flow occur upstream of the flow conditioner, indicating that the culvert geometry is not of much help to contribute to the conditioning of the flow. However, the results demonstrate that, in spite of such an adverse inflow conditions, the classical solution based on the perforated plate concept works very well.
Numerical Modelling of Automatic Mooring Winches Applied to Moored Ships in Harbours
Authors: Santos Joa˜o Alfredo
Published: July 2009

In order to control excessive motions of moored ships, automatic mooring winches, namely constant tension winches and automatic tension winches, have been employed. It has been common practice the recovering force in automatic tension winches never becoming lower than one third of the rendering force. However, the existence of such a non-zero recovering force implies that the cable where that force is applied to may be counteracting the rendering force produced on an opposing cable by another winch. Ideally, winches should only produce forces to reduce ship motions, hence the recovering force should be zero. This paper describes the use of a numerical model for moored ship behaviour in the study of the effects of automatic tension winches in the motion amplitudes of a 108 416 m3 displacement ship moored in two different conditions: a) alone at an exposed location; b) 30 m apart from a vertical breakwater. Additional cables connected to automatic mooring winches are considered and the change from idealized conditions where the cables apply forces on the ship with no vertical component to realistic conditions where such component exists is investigated as well as the influence of the recovering force on the ship motions for the ship subject to regular head waves.
Structural Reliability Assessment of Aging Offshore Platforms With Corrosion and Cracks
Authors: Ji Chunyan , Li Shanshan
Published: July 2009

Corrosion and fatigue cracks are major threats to the structural integrity of aging offshore platforms. To rationally estimate the safety levels of the structures, a reliability assessment approach for aging offshore platform with corrosion and fatigue cracks is presented in the paper. The advanced second-order reliability method is adopted to calculate the probability index based on the limit state equation of global failure mode. The base shear capacity is taken as the ultimate strength of the platform. It is modeled as a random process that decreases with time in the presence of corrosion and fatigue crack propagating. And corrosion and fatigue crack growth rates in the main members and key nodes are modeled as random variables. A simulation method of the extreme wave loads that are applied on the structures is proposed too. Finally the reliability index of an example jacket platform is estimated in the case study.
Numerical Study on the Effects of Water Surface Shield and Oxygen Injection on Water Quality in Lake Biwa
Authors: Kitazawa Daisuke , Kumagai Michio
Published: July 2009

Numerical simulation was carried out by means of a three-dimensional hydrostatic and ecosystem coupled model in order to predict the effects of water surface shield and oxygen injection on the water quality in Lake Biwa. Recent observations show the degradation of water quality particularly around the water bottom in Lake Biwa. One of the possible reasons of the degradation is the depletion in the vertical mixing of waters due to climate change. The meteorological observations at Hikone show that the atmospheric temperature has increased by 1°C in the recent 20 years. If the vertical mixing of waters is weakened, the waters with rich oxygen are not supplied enough around the water bottom during winter seasons, and the concentration of dissolved oxygen is hardly recovered to the saturated level. The depletion in the vertical mixing of waters has been increasingly worried since Intergovernmental Panel on Climate Change (2007) reported that the globally mean atmospheric temperature will increase by a few °C in a century. In order to cope with such problems, the present study proposes a feasible system to inject the oxygen produced by the electrolysis of water. In the proposed system, water surface is shielded by a floating platform, which blocks off the penetration of solar radiation through the water surface and generates the density-driven current due to the horizontal difference in water temperature. In addition, solar cell modules are installed on the floating platform to produce oxygen by means of the electrolysis of water. The produced oxygen is then released from the electrodes installed around the water bottom. As a result of numerical simulation using the three-dimensional hydrostatic and ecosystem coupled model, the effect of water surface shield was negligible since the surface area of the floating platform was quite smaller than that of Lake Biwa. The effect of oxygen injection was remarkable and localized in summer than in winter. The concentration of dissolved oxygen was then expected to increase by more than 0.2mg/l within 15m from the bottom surface in summer, in the cases when 730,000m3 of oxygen was injected in an year. The effect of seasonal change in the production of oxygen was negligible since little difference was found in the concentration of dissolved oxygen. As future studies, the nested grid system should be utilized to predict precisely the diffusion of oxygen both in the vicinity of the electrode and in the whole Lake Biwa.
Influence of Location and Instrumentation on Wave Group Characteristics
Authors: Mercelis Peter , Vanmarcke Nele , Smith George , Dumon Guido , Toffoli Alessandro , Monbaliu Jaak
Published: July 2009

Wave group characteristics from water of intermediate depths ranging from relatively deep to relatively shallow are discussed. The analysis is based on measurements of the surface elevation, which were collected with both floating and fixed sensors. Results indicate that, for similar spectral conditions, the general distributions of the run lengths are remarkably similar for the waves in the relatively deep and relatively shallow water depths. However, for less steep sea states, it has been observed that the group tendency is more pronounced for the deeper water location due to more relevant swell activities. The analysis of the fixed sensor data gives measured run lengths that are shorter than the ones obtained from wave buoys. For both locations, the empirical findings agree relatively well with probabilistic predictions using the approach given in [1] where a correlation parameter κ is estimated using spectral shape parameters.
Optimizing Hot Tapping Using the Fuzzy Logic Modeling Approach
Authors: Almostaneer Hamad , Liu Stephen
Published: July 2009

Strong demands for energy compels the oil and gas industries to increase productivity. Since transportation of oil and gas products through pipelines is the most efficient way to deliver these products, there is an ever increasing network of pipelines from the production fields to the processing plants. Construction and operating costs, however, limit the proliferation of pipelines. Hot tapping is an economical technology that the oil and gas industries use to minimize oil and gas pipeline segments by connecting a new facility to the nearest operating unit processing the same product. Technologically speaking, hot tapping is challenging especially during field installation. Welding a branch pipe/pipeline to an in-service pipeline has two major concerns: burn through and heat-affected zone (HAZ) cracking. Thin pipe wall thickness and the use of welding electrodes with high hydrogen contents can promote burn through and HAZ cracking. Improper hot-tapping procedures applied on a live pipeline may lead to disasters that can be life threatening to the welding operators. Parameter optimization is essential to ensure successful hot tapping. In this paper, fuzzy logic modeling was used to optimize the hot-tapping procedure, i.e. to minimize burn through and HAZ cracking, for processes that use shielded metal arc (SMA) welding. A total of 324 rules were built to consider multiple welding parameters, pipeline and operating characteristics such as heat input, electrode type, pipe inside surface temperature, wall thickness, hardness, and fluid flow to study the effects of these parameters on the weldment. These parameters were the fuzzy logic model inputs for successful hot-tapping conditions. For each pair of parameters, the output of the model is a processing surface on which successful hot tapping can be conducted. A graphical user inference (GUI) easily retrieves the different sets of parameters and their respective safe hot-tapping surfaces on which burn through and HAZ cracking can be avoided. As conclusion, the present study demonstrates that fuzzy logic modeling can be used to provide guidelines for in-service pipeline hot tapping. Careful modeling can result in safe welding space for hot tapping and allow for the specification of standard welding procedures within the safe welding zone. The guidelines and range of safe welding parameters from fuzzy logic modeling can reduce the size of the experimental matrix required during field weld testing for the determination of the final parameters.
Response of a Novel Beam-to-Column Connection to Monotonic and Cyclic Flexural Loading
Authors: Khonsari S. V. , England G. L. , Shahsavar-Gargari M. , Parvinnia S. M. H.
Published: July 2009

A new beam-to-column joint with high rotational as well as shear deformation capacity was devised. This high rotational ‘capacity’ is required to fulfill the great ‘demand’ for rotation arising during earthquakes, severe waves and current loads, etc. Due to its ability to contain damage during an overload, it leaves the connected elements intact. This, together with its replaceability can reduce the cost of post-event repair substantially. Its bending as well as shear performance under “monotonic” loading had already been assessed experimentally (OMAE’02-28864, OMAE’03-37292, OMAE’04-51494 & OMAE’05-67361) and proved well superior to that of conventional joints. In order to study its performance under “cyclic” flexural loading experimentally, new bending tests were conducted on mild steel specimens of the connection. These tests clearly showed the ability of the devised joint to withstand adequate number of cycles in bending and dissipate energy through well-shaped hysteresis loops. This would result in large amount of energy being dissipated in each cycle. Such very ductile response of this connection in bending is expected to be exploited in various circumstances in offshore as well as onshore structures to give rise to a ductile overall behavior of the structure. In particular, it can be utilized for the repair and retrofitting of the aging offshore platforms which need to be treated in a non-destructive manner.
Reciprocal Form of the Wave Drift Force and Moment Acting on Floating Body
Authors: Tsubogo Takashi
Published: July 2009

This paper provides the reciprocal form on wave drift force and moment from the momentum theory. The author in Japan has transformed from the pressure integration on the wetted body surface oscillating in regular waves into the reciprocal form at the near field, then transformed into the form at the far field owing to Green’s second identity, and transformed into Maruo’s and Newman’s formulas. But in this paper the start point is the momentum theory and the goal is the reciprocal form. The obtained reciprocal form at the far field can be transformed into the integration over the wetted floating body surface owing to Green’s second identity.
Rogue Wave Impact on a Semi-Submersible Offshore Platform
Authors: Rudman Murray , Cleary Paul , Leontini Justin , Sinnott Matthew , Prakash Mahesh
Published: July 2009

Full three-dimensional simulation of the impact of a rogue wave on a semi-submersible platform is undertaken using the Smoothed Particle Hydrodynamics (SPH) technique. Two different mooring configurations are considered: A Tension Leg Platform (TLP) system and a Taut Spread Mooring (TSM) system. It is seen that for a wave impact normal to the platform side, the heave and surge responses of the platform are significantly different for the two mooring systems. The TLP system undergoes large surge but comparatively smaller heave motions than the TSM system. The degree of pitch is very similar. The total tension in the mooring cables is approximately four times higher in the TSM system and exceeds the strength of the cables used in the simulation. SPH is seen to be an attractive alternative to standard methods for simulating the coupled interaction of highly non-linear breaking waves and structural motion.
Response Comparisons of a Large Floating Structure by Grillage and Shell FE Models
Authors: Iijima Kazuhiro , Kim Junghyun , Yao Tetsuya
Published: July 2009

At the early design stage of a large floating structure, it is firstly important to know the hydroelastic response characteristics in waves. For this purpose, the structure is modeled by three-dimensional grillage, and hydroelastic analysis is performed in order to estimate the overall behaviour. At this stage, main design parameters are: floater shapes, their arrangement and rigidity distributions. They are optimized by referencing to the hydroelastic responses estimated by the analysis. As the design work develops, more detailed modelling is possible. At the final design stage, the design must be confirmed by checking the response against criteria. The structure is re-modeled by shell FE elements for skin structures and beam elements for stiffeners. It is considered that the more correct estimations are performed by employing the refined model. However, there might be significant differences in the modelling and also in the resultant estimations between the first and final stages even when the subject structure is identical. Then, it is necessary to evaluate the differences between the results estimated by using these two models in order to assure the actual response level estimated by using the beam model at the early design stage. In this paper, three-dimensional grillage and shell FE structural models of a large floating structure are prepared. Hydroelastic analyses are performed on the two models. The results are compared in terms of motion, member force and stress responses.
Numerical Analysis of Bearing Capacity of Suction Caisson
Authors: Ghazavi Mahmoud , Barkhordari Kazem , Dabirmanesh Hooman
Published: July 2009

This paper presents a finite element modeling for determination of bearing capacity of a suction caisson subjected to vertical uplift loading. The approach looks at the uplift capacity of the caisson considering a non-uniform cross sectional area to account for a geometry optimization. For this purpose, the numerical simulation is first verified using available data from other research work especially centrifuge data. Parametric studies are then performed to investigate the role of influencing factors including taper angel. The results show that the bearing capacity of caisson increases with increasing the taper angle.
Numerical Simulation of Tidal Current or Wind Over the Sea and Applications to Navigational Simulation of a Sailing Ship
Authors: Shiotani Shigeaki
Published: July 2009

This paper deals with numerical estimations of the ocean surface wind and tidal currents in a bay. Such estimates are very important because the wind and tidal current react to environmental factors, such as the effects of a marine structure or a sailing ship. The final object of this research was to develop a weather routing system for small vessels in coastal water. First, a computer simulation of tidal current was carried out in the same bay in Japan. The results showed that tidal simulation was very effective for the weather routing system of a small vessel. Next, an estimation of the ocean surface wind was carried out in Osaka Bay using the MM5 mesoscale meteorological model. A comparison of the calculated numerical results with the observed ones was confirmed that the ocean surface wind in Osaka Bay was accurately estimated. Third, as an application of the estimation of wind or tidal current in the bay, navigational simulations of a sailing ship under the effects of the wind or tidal current were conducted using the numerical data of the ocean surface wind or tidal current separately. The importance of this simulation method for ship positioning was confirmed, as was the accurate estimation of the effects of wind or tidal current in coastal water on a sailing ship in a navigational simulation for optimum weather routing.