bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural Behavior of Umbilicals: Part I—Mathematical Modeling
Authors: Pesce Celso Pupo , Ramos Roberto , da Silveira Lauro Massao Yamada , Tanaka Rafael Loureiro , Martins Clo´vis de Arruda , Takafuji Fernanda Cristina Moraes , Novaes Joa˜o Paulo Zi´lio , Godinho Carlos Alberto Ferreira
Published: January 2011

Umbilicals for offshore application are very complex, since they combine elements of different mechanical behavior, such as steel tubes, thermoplastic hoses and power cores in a single structure, not to mention helically laid-up armouring layers and polymeric sheathes. This motivates continuous research on their mechanical modeling. This paper presents research undertaken in the structural behavior of umbilicals and focus on the mathematical modeling of the elements, which are gathered into concentric layers. Interaction between layers is included as well as helical lay-up of elements. The model here presented will be compared to a set of experimental results in a separate paper (Part II). An analytical model was developed combining equilibrium equations, geometric compatibility and constitutive relations to obtain a set of equations that describe the umbilical behavior under external loads. This set of equations is numerically solved to obtain contact pressures (or gaps) among layers, radial variations and strains in the elements (used to calculate the stresses). The model was built to be general in order to be able to cope with complex cross-sections designs often encountered. It was then implemented in a tailor made local analysis software, called UTILFLEX®. Both modeling and software resulted from a development program partnership between Prysmian Cables & Systems and University of Sa˜o Paulo. The paper will present modeling hypotheses and structural models that were used for steel tubes, hoses (thermoplastic and reinforced) and power cables and how the interaction among them has been treated.
Structural Behavior of Umbilicals: Part II—Model-Experimental Comparisons
Authors: Pesce Celso Pupo , Fujarra Andre´ Lui´s Condino , Rabelo Marcos , Tanaka Rafael Loureiro , Martins Clo´vis de Arruda , da Silveira Lauro Massao Yamada , de Barros Eduardo Esterqui , Godinho Carlos Alberto Ferreira
Published: January 2011

A set of tests was performed in a non-armored Steel Tube Umbilical (STU), including pure pressure loading, constant and variable tension loads and combinations of constant and cyclic bending moment and tension. Tests were made for pressurized and non pressurized conditions. Strains were measured with strain gages attached to the external surface of selected tubes. Instrumentation was performed in four windows that were opened on the umbilical outer sheath to provide access to the tubes. Besides the strains, tension, internal pressure and imposed angle were measured. Comparisons with results obtained using the model presented in Part I, [1], are presented for different load conditions.
2nd Order Pitch and Roll Slow Motions of a Semi-Submersible Platform: Full Scale Measurements and Theoretical Predictions Comparative Study
Authors: Matos Vini´cius L. F. , Ribeiro Eric O. , Simos Alexandre N. , Sphaier Sergio H.
Published: January 2011

In October 2007, the semi-submersible platform PETROBRAS 52 (P-52) was installed in Campos Basin (Roncador Field) offshore Brazil. This Unit is moored through 16 lines in taut-leg configuration in a water depth around 1.800m. Its displacement at the operational draft (T = 27.5m) is 80.986t. The maximum production capacity is 180.000bpd. During the design phase of this floating system, a model test campaign was performed in a wave basin and slow drift motions in the vertical plane (heave, roll and pitch) were identified. It is known that resonant responses vary considerably with the damping loads. As these loads are affected by scale effects, by that time, it was a doubt if this phenomenon would happen during the platform operation. Since June 2008, PETROBRAS has been monitoring P-52 motions with the use of accelerometers and rate-gyros. Through spectral analysis of the measured signals, it was possible to verify the presence of slow motions with frequencies around the natural frequencies of roll and pitch during almost the whole monitoring period. Sometimes, the 2nd order amplitudes were even grater than the 1st order ones. Furthermore, the environmental conditions have also been monitored through wave radars, ADCPS (current) and meteorological stations (wind) in the vicinity of P-52 location, making the excitation loads identification possible. A comparative study confronting full-scale measurements and theoretical predictions was performed. First and second-order forces and responses were calculated using WAMIT® second order module. This study permitted to estimate the full scale damping values of this offshore system (hull plus mooring and riser lines) for one of the environmental conditions measured. This work demonstrates the importance of considering the resonant roll and pitch motions in the seakeeping analysis of large-volume semi-submersible platforms, contributing with an important feedback to future designs.
Effect of Constraint on Fatigue Crack Propagation in Notched Plates
Authors: Wang Xin
Published: January 2011

It has been observed that the loss of crack tip constraint leads to enhanced resistance to both cleavage and ductile tearing. Recent developments on constraint-based fracture mechanics have enabled the practical assessment of defective components including the constraint effect. In fatigue analyses, it is common practice to use the Paris crack propagation law, which is based on the stress intensity factor alone. Although in recent years several research attempts have been made to investigate the effect of constraint on fatigue, how to quantify the effect of crack tip constraint on the fatigue crack propagation rate is still an open issue. In this paper, the effect of crack tip constraint on the fatigue crack propagation of small cracks at notched plates is investigated. A fatigue crack propagation rate model developed earlier which incorporates both low and high constraint conditions is used. It is shown that the effect of constraint on crack propagation rate is significant when the crack is small (within the stress concentration of the notches). This effect can be accounted for by using the fatigue crack propagation rate model.
Investigation of Fluid-Structure Interactions on Controlled Oscillations of a Cylinder at Moderate Reynolds Number
Authors: Kamarudin Muhamad H. , Thiagarajan Krish P.
Published: January 2011

This paper focuses on the flow dynamics associated with uniform flow past an oscillating cylinder using a 2D Computational Fluid Dynamics (CFD) approach. The simulations were carried out using the Reynolds-Averaged Navier-Stokes (RANS) k-w turbulence model. The numerical model has been validated with experimental data to ensure accuracy of the results. The results were examined for Re = 10,000 and a fixed motion amplitude ratio of A/D = 0.3 and the frequencies of the oscillations were varied in the vicinity of the Strouhal frequency of a fixed cylinder in a free stream. It is noted that a phase switch of the vortex formation will occur as the frequency of the oscillation passes through the Strouhal cylinder frequency. This transition is characterized by a jump in the in-phase and out-of-phase with the velocity components of the forces. The results were examined by analyzing the transfer of energy either from the fluid to the structure or vice versa to quantify the jump in the force components. In the lock-in region, a limit cycle of the cylinder motion and the lift force will form. The energy transfer is then related to the traverse direction of the limit cycle.
Stress Assessment in Armour Layers of Flexible Risers
Authors: Leroy Jean-Marc , Perdrizet Timothe´e , Le Corre Vincent , Estrier Pascal
Published: January 2011

The accurate modeling of offshore flexible risers behaviour remains a great challenge because of (i) their complex internal structure, (ii) the variable nature of the loads along the pipe (tension, curvature, internal and external pressures), (iii) and the interactions with structures used to limit the pipe curvature. Technip and IFP have been codeveloping for many years models dedicated to stresses calculation in the armour wires, to assess the flexible lifetime. These models must account for a large number of potential inner contacts (contacts between upper and lower layers, lateral contacts between adjacent armour wires in the same layer) as well as external contacts (bend stiffener, arch, bellmouth or other curvature limitation setup). The paper presents a comparison between 3 models with different level of complexity and realism. The first one is a in house model, whereas the two others were developed on the basis of the commercial FE code Abaqus. The first model, Life6, is based on some simplified assumptions in particular the fact that periodic solutions are assumed (given constant curvature in the pipe) and uses analytical solutions of equilibrium of wires on a torus (the bend pipe). The effect of non uniform curvature (in particular end-fitting proximity) is not considered in this model. The second model, namely 3D/Periodic model, is still based on a periodic assumption, but can cope with severe loadings (such as large curvature of the pipe or compression) leading to specific wires contact interactions. Abaqus Standard (Implicit scheme) is used. The third model, called 3D/Explicit model, is a full length model, as it includes end fittings effects, outer structure (like stiffener) interactions and any curvatures variations along the pipe. All contacts interactions are considered. The number of DOFs involved in the analysis requires the use of an explicit integration scheme (Abaqus Explicit) running on a parallel platform. These models are cross validated on a dedicated case study that consisted of a pressurized pipe cyclically bent with constant curvature. The validation of the model results is very satisfying even when lateral contacts between wires occur. Finally, a comparison of the 3D/Explicit model results with experimental data is presented. This model provides a very good estimation of the flexible behavior and of the end fitting effects.
Dynamic Non-Linear Analysis of Offshore Jacket Type Platforms Under Extreme Wave Loads
Authors: Enderami S. M. , Zeinoddini M. , Shabakhty N. , Ameryoun H.
Published: January 2011

Regular wave theories such as Airy and 5th order Stokes are widely employed to compute wave loads on offshore platforms. These deterministic approaches are based on certain wave heights and frequencies, so, it does not suitably account for the randomness of the sea state. The most accurate methods for estimating extreme responses are based on time domain but they need long duration’s storm inputs and are computationally very time consuming. NewWave theory offers a different time-domain approach for the extreme response evaluation. By constraining a NewWave with a pre-determined height within a completely random background Constrained NewWave is generated. Constrained NewWave allows for the easy and efficient evaluation of extreme response statistics; achievable without the need to simulate many hours of real time random seas. In the present study the extreme response of a Jacket offshore platform has been evaluated using a 5th order Stokes wave; long duration random reproduction of the sea surface; and a Constrained NewWave method. In general, it has been found that the Constrained NewWave method provides reasonably accurate results, as compared to those from long duration random reproduction of the sea surface, with significantly lower computational time.
The CFD Simulation of a Lifting Hydrofoil in Close Proximity to a Free Surface
Authors: Gretton Gareth I. , Bryden Ian G. , Couch Scott J. , Ingram David M.
Published: January 2011

This paper presents results from the CFD simulation of a lifting hydrofoil in close proximity to a free surface. The case considered is that of a NACA 0012 hydrofoil at a Froude number of 0.567. The depth of submergence is approximately equal to the chord length. All results were produced using the commercial CFD code ANSYS-CFX. The primary emphasis of this paper is on the detailed and quantitative verification of the CFD simulations, focussing especially on the spatial discretization and the choice of boundary conditions. Quantitative assessment is by examining the free surface deformation and the lift, drag and moment coefficients. It is found that a very fine mesh (having approximately 1×106 nodes for this 2D case) is required for an accurate prediction of the free surface deformation, whereas a considerably coarser mesh (having approximately 62×103 nodes) gives comparatively accurate prediction of the lift, drag and moment coefficients.
Crack Propagation Versus Crack Initiation Lives of FPSO Weld Details
Authors: Lotsberg Inge , Salama Mamdouh M.
Published: January 2011

Documentation of a long crack propagation phase is important for planning a sound inspection program for fatigue cracks in FPSOs. Test results of full scale FPSO weld details have shown that fatigue lives of FPSO details are governed by crack propagation and that crack propagation lives are several times that of the crack initiation life. However, some analysis packages predict a short crack propagation life until failure compared to the crack initiation life. These predictions are not consistent with full scale test results and thus cannot be relied on in developing inspection strategy. The reason for this inconsistency in analysis as compared with test results may be due to limitations in the analysis program packages. The paper presents analysis of fatigue testing data on several full scale FPSO weld details. The paper also discusses the effect of “shake-down’ that is not simulated in the full scale constant amplitude testing and would even lead to longer crack propagation lives under the actual long term loading on FPSOs.
An Alternative to Model the Non-Linear Roll Damping of a FPSO Hull
Authors: de Oliveira Allan C. , Fernandes Antonio Carlos
Published: January 2011

The ship rolling problem is a subject that has been studied from a long time. From Froude (in the XIX century) to nowadays, this problem was revisited several times in order to adjust the theory to the changes in ship hulls, dimensions, materials, appendages and others. On the other hand, the ship analysis resources technologies including both the experimental techniques and the computational capacity (that not exist in Froude’s time) also have amazingly increased. Although all this technological apparatus, that allows testing a model for several conditions a large number of times, the standard way to estimate the non linear roll damping have not been significantly modified (since Froude’s time!). As a result, some ship applications (as a FPSO, for instance) that require more effective roll suppressor appendages have evidenced behaviors that are not in accordance with the ship industry methodology to predict them. The roll standard theory for ship rolling is, however, a well established theory, based on a solid knowledge about viscous effects in fluids. Hence the question is would it be possible to use all the technological development to get a better approach and estimative of roll damping? This paper will discuss how this may be possible, showing an alternative method to estimate the roll damping and testing for a FPSO with strong non-linear behavior observed in model tests suggesting among other thing alternative ways to perform model testing.
Towards the Understanding of Manifold Fluttering During Pendulous Installation: Flow Induced Rotation of Flat Plates in Uniform Flow
Authors: Fernandes Antonio Carlos , Sefat Sina Mirzaei , Coelho Fabio Moreira , Ribeiro Mario
Published: January 2011

The pendulous installation method of a manifold has a first phase that can be considered as a free fall in water. Of course, this is not free due to the fluid action. The consequence is that the manifold may oscillate rotationally which characterizes a fluttering behavior. However, the manifold is a complex body with non-uniform shape, several modules, porosity etc. Hence, in order to improve the understanding of the fluttering, this work presents advances in the observation of flow induced rotation on a flat plate in uniform flow. This has been started experimentally and subsequently numerical models yielded a confirmation of quasi-steady observations. The experimental results were obtained at the Laborato´rio de Ondas e Correntes (LOC) [Laboratory of Waves and Current] in COPPE/Federal University of Rio de Janeiro. The drag and lift forces coefficients and the center of pressure have been obtained for angles of attack θ = 0°–90° and for different Reynolds numbers.
Comprehensive Study on Energy Dissipation Performances of Two-Layer Porous Energy Dissipation Plate
Authors: Wang Hong , Chen Daming , Shen Liyu , Wang Yong
Published: January 2011

In this paper, we introduce a new type of two-layer porous energy dissipation plate breakwater, consisting of one upper porous plate and a lower plate. The system is used to control energy dissipation in breakwaters. The structure performance of dissipating waves has been investigated in detail in the regular wave tests. The factors identified with the characteristics of the energy dissipation plate are discussed, such as the relative width (B/L), the relative hole-size (A/A0) and the wave steepness (H/L). The comparison and analysis of the energy dissipation coefficients with respect to different factors are presented. Model test results indicate that the two-layer porous energy dissipation system is effective in dissipating a significant amount of wave energy.
Experimental and Numerical Studies on Super-Cavitating Flow of Axisymmetric Cavitators
Authors: Ahn Byoung-Kwon , Kim Hyoung-Tae , Lee Chang-Sup
Published: January 2011

Recently underwater systems moving at high speed such as a super-cavitating torpedo have been studied for their practical advantage of the dramatic drag reduction. In this study we are focusing our attention on super-cavitating flows around axisymmetric cavitators. A numerical method based on inviscid flow is developed and the results for several shapes of the cavitator are presented. First using a potential based boundary element method, we find the shape of the cavitator yielding a sufficiently large enough cavity to surround the body. Second, numerical predictions of supercavity are validated by comparing with experimental observations carried out in a high speed cavitation tunnel at Chungnam National University (CNU CT).
An Improved Methodology for the Design of Marine Drilling Riser Couplings
Authors: Persent Emmanuel , Averbuch Daniel , Guesnon Jean
Published: January 2011

The current trend in offshore drilling is a significant increase in water depth in the specification of drilling risers, associated with high density muds. This situation presents a real challenge for the design of the drilling riser which depends to a large extent on these parameters, as well as other related to operational and environmental conditions. In particular, improved design methodologies are required to better assess the margins of riser couplings regarding their static performance and fatigue life. As recommended by API Spec 16R, the stress linearization and classification in one of the key steps to design a riser connector. The designers are encountering some difficulties in the application of this methodology to 3D finite element results. IFP has then proposed a simple approach that applies to non-axisymmetric geometries of connectors. It consists in calculating the membrane and bending stresses in a given plane by averaging over a suitable portion of a cross-section the results of the linearization in the stress classification lines (SCLs) located in the selected plane. A short presentation of a breech-block type riser connector, on which the methodology has been applied, is given at the beginning of this paper. The API specification 16R requirements regarding the design criteria are then discussed. A simple approach to extend the stress linearization and classification methods to three-dimensional FEA is proposed. The proposed methodology is applied to the design of the Clip connector. At last, the R&D work aiming at improving the fatigue analysis of riser connectors is introduced at the end of the paper.
Simple Wake Models for Tidal Turbines in Farm Arrangement
Authors: Palm Moritz , Huijsmans Rene , Pourquie Mathieu , Sijtstra Anne
Published: January 2011

From wind turbines it is known that the wake, induced by a turbine, has a negative impact on the energy production of downstream devices. Basically, the wake is a zone with reduced velocity behind a turbine. Further downstream, the velocity recovers gradually by turbulent mixing with the ambient flow. In order to optimize the design of a tidal farm, the aim of this paper is to find simple relations that can be used to predict the energy output of a given farm configuration. The energy output of a turbine depends on its inflow velocity. Therefore, the strategy is to find a model that is able to predict the velocity field in the tidal farm. Such ‘wake models’ exist already for wind turbines and thruster-thruster interaction. In this research, the applicability of these wake models to tidal turbines is investigated by comparing their results to reference data of tidal turbines. Only limited measurement data for tidal turbines are available; therefore a CFD model of a tidal turbine is used to generate the reference data. The velocity in the wake is simulated for different conditions with the CFD model. The CFD model is validated with the available data in the literature. The velocity in the wake for a single turbine is predicted accurately for different initial conditions. Modeling of the turbulence showed some discrepancies in the far wake, consequently the wake of turbines in farm configurations is predicted less accurate. Three wake models, selected from the literature, are compared to the CFD simulations of the wake behind a single turbine. The wind turbine wake model of Jensen performed best; the velocity in the wake is calculated accurate for different situations. Mutual interaction of wakes will occur inside tidal farms. Several methods from wind turbines theory are used to estimate the velocity in interaction situations. Three basic situations of wake interaction are distinguished: tandem operation, wake interference and overlapping inflow. The interaction methods are tested with CFD reference data for each situation separately. Most methods compared reasonably well; the most suitable interaction methods are selected. A small tidal farm case study is performed to test the combination of wake model and interaction methods. The flow in the cluster of 5 turbines is predicted satisfactorily by the wake model for different inflow velocities. All results indicate that the principle of applying wind turbine wake models to tidal turbine has good potential. However the number of test cases conducted in the thesis is limited and the incorrect turbulence modeling of the CFD model caused some uncertainties for multiple turbine situation.
Experimental Study of Impact Pressures and Reaction Forces on a VLGC Tank Due to Sloshing
Authors: Cai Zhonghua , Zhong Yinghua , Li Zhe , Wang Deyu
Published: January 2011

This study presents results of an experimental study of slosh-induced impact pressures and reaction forces on a 80,000 m3 VLGC (Very Large Gas Carrier) tank. The scale of the model tank was 1/45.5. Experiments were performed with 30% and 70% filling levels. Single and coupled motion tests were carried out. Excitation amplitudes and frequencies were based on the guideline of Lloyd’s Register. For measuring impact pressures and reaction forces, 12 pressure transducers and 48 force transducers were installed on the model. The experimental results showed that coupled pitch and roll motion causes the maximum impact pressures. Heave motion has little effect on impact pressures. The impact pressures of side transducers on the longitudinal tank wall are larger than those of middle transducers which are of the same height. Coupled motion causes larger reaction forces.
Evaluation of Seabed Stability and Scour Control Around Subsea Gravity Protection Structures
Authors: Whitehouse Richard J. S. , Lam Carlos , Richardson Stephen , Keel Peter
Published: January 2011

Results from an advanced 3-dimensional Computational Fluid Dynamics (CFD) model have proven to form an effective basis on which to design stable and scour resistant subsea structures in areas of seabed which are prone to scouring. A case study application from the UK sector of the southern North Sea is presented to demonstrate the benefits of the CFD analysis.
Vibration Absorber Technology and Conceptual Design of Vibration Absorber for TLP in Ultradeep Water
Authors: Rijken Oriol , Spillane Michael , Leverette Steve J.
Published: January 2011

Vibration absorber technology can be used to change the TLP’s resonant behavior and make the TLP performance basically independent of the water depth, allowing a TLP to be a technically acceptable solution for field developments in water depths exceeding 6000 ft and making the TLP an ideal structure to support multiple risers in ultra deep waters, particularly under harsh environmental conditions. Vibration absorber technology for the TLP consists of several vertical caissons partially filled with water and a sealed air chamber near the top capped at the top. A throttle plate is present between the air chambers to dissipate energy. The implementation of a vibration absorber into the TLP hull is discussed. The performance of a single vibration absorber is investigated using CFD, and the results is compared with experimental findings. A discussion is provided regarding the comparative cost of a deep water TLP solution with vibration absorbers and a spar and semi submersible.
Sour Environmentally Assisted Fatigue of Welded SCR Materials: Post-Weld Finishing Treatment Evaluation
Authors: Darcis Philippe P. , Marines-Garcia Israel , Hudak Stephen J. , Armengol Mariano , Quintanilla Hector M.
Published: January 2011

The current work aims to point out the influence of sour brine environment on the fatigue resistance of welded SMLS (seamless) steel pipe used for design and fabrication of risers for oil and gas development. A C-Mn steel X65 pipe 10.75″ (273.1 mm) outside diameter (OD) and 25.4 mm wall thickness (WT) was chosen for this program. The Welding Procedure designed for girth welds manufacturing involved the use of Lincoln STT-GMAW™ process for the root pass and SAW process with twin wire configuration for the fill and cap passes. This welding procedure presents a special post-weld finishing treatment, which consist in flapping the inner and outer weld overfills to produce a flush profile between weld metal and outer/inner pipe surfaces. The experimental approach focused on quantifying the effect of H2S using a sour brine environment. For this purpose, intermediate-scale fatigue data in the sour brine environment, using full thickness’ strip specimens extracted from the welded SMLS (seamless) pipe, have been generated. Intermediate-scale fatigue tests in air have also been obtained to provide a baseline for comparison with the sour brine data. Those results have been compared with full-scale fatigue tests in air environment. Finally, results were statistically analyzed to determine which standard fatigue design curves best represent the measured S-N fatigue endurance in air and sour brine environments. Results were also compared with available literature and results on other seamless’ welded pipe of the same API 5L, Grade X65 steel in comparable environments.
Computing Methods and Application Development of Longitudinal Strength of Damaged Ships
Authors: Yan Renjun , Qi Yao , Luo Bailu , Lei Jiajing , Li Peng
Published: January 2011

In this paper, it starts by providing a description of the scenarios of damages that can be found in the various accidental events. Based on a traditional standardized approach for calculating longitudinal strength of ships, an improved method for longitudinal strength of damaged ships, which can be applied to any traditional type of ships, is presented in next section. The variation of loads in damaged ships is related to the effects of the ingress of water with the consequences of changing the ship displacement, a non-symmetric floating body, and the load case of compartments before damage, etc. The computing methods used in this paper allow for research on corresponding applications. Using this computer program, a few case studies are carried out and the results are discussed.
Study on Hydrodynamic Performance of a Dish-Shaped Underwater Vehicle
Authors: Su Yumin , Wang Zhaoli
Published: January 2011

A new kind of dish-shaped underwater vehicle was designed. The maneuverability of the dish-shaped underwater vehicle (UV) is predicted in this paper. Hydrodynamic coefficients of the vehicle were calculated in numerically. The numerical method applied is one of the tools available in the commercial computational fluid dynamics software FLUENT. The dynamic mesh system and post-processing system are adopted in the numerical method. By simulating numerically straight motion, inclined motion and planar motion mechanism (PMM) experiment, the hydrodynamic performance in different states were obtained. Based on the least square method, the hydrodynamic coefficients of maneuverability were obtained. The calculated results indicate that the numerical method is suitable.
Numerical Simulation of Compressible Flow in Gas Liquid Separator
Authors: Li Sen , Wang Zunce , Xu Yan , Lv Fengxia , Yan Yuejuan , Song Yujie
Published: January 2011

The Gas Liquid Separator (GLS) has been widely used by petroleum industry, chemical engineering, the area of environmental protection, etc. A large quantity of works on the GLS available in literature includes experimental data, numerical simulations and field applications. However, previous studies on the GLS were based on gas incompressible circumstances. In fact, the gas flows from high to low pressure area that it lead to the density fluctuations of gas in separator, the changes of density cause volume expansion of gas, so that separation performance of the GLS is reduced. Numerical simulation for the GLS was developed based on compressible and incompressible flow. Research results show that flow field of two ways and separation performance were different.
Ultimate Compressive Capacity of Rectangular Plates With Partial Depth Pits
Authors: Jiang Xiaoli , Guedes Soares C.
Published: January 2011

Pitting corrosion has been one of main corrosion type of immersed ship hulls, which can bring heavy damage and even accidents to in-service ships, particularly to aged ships. To investigate the effects of pits on the ultimate compressive strength of mild steel plates, a series of non-linear FEM analysis on plates with partial depth corrosion pits are undertaken in the paper, changing the size, intensity and location of pits and the slenderness of plates. It is shown that the eccentricity induced by single side distributed pits has considerable degrading effects on the ultimate compressive capacity of plates. Although the degree of pit corrosion (DOP) does reflect the effect of pits to a large extent, it is not enough to rely on DOP exclusively to represent the extent of damage caused by pits, as “volume effect” should be considered.
A Numerical Study of Wave Energy Converter in the Form of an Oscillating Water Column Based on a Mixed Eulerian-Lagrangian Formulation
Authors: Liu Chunrong , Huang Zhenhua , Law Wing Keung Adrian , Geng Nan
Published: January 2011

A desingularized boundary integral equation method (DBIEM) is employed to study the wave energy extraction by an oscillating water column (OWC) device. The method is based on a mixed-Eulerian-Lagrangian formulation. We examine the effects of the relative draught on the efficiency of 2D OWC energy converters. The oscillating air pressure inside the OWC chamber is modeled by assuming that the air is incompressible and the air-turbine mass-flow rate is proportional to the pressure difference (a linear turbine). For shallow draughts the numerical results agree well with available analytical results. The wave-excited seiching inside the extraction chamber is discussed and the variation of extraction efficiency with dimensionless air-chamber width for different immersion depths is reported.
Numerical Simulation of the Interaction Between Two Bubbles Rising in the Water
Authors: Zhu Ren-qing , Li Yan-cheng , Ni Yong-yan , Zhang Pei-tao
Published: January 2011

Based on the VOF model, the rising movement and deformation of two bubbles placed horizontally or vertically with separation distance under the action of the buoyancy are simulated by using FLUENT software. The effects of distance between two bubbles on deformation, coalescence, rising velocity and path are considered. The bubble deformation is monitored and the change of velocity with time is analyzed. The influence of bubble diameter on rising and merging are discussed and the velocity history of bubble is obtained in rising and merging process. The results show that: The distance has great effect on the interaction between two bubbles in rising and merging process, the smaller the bubble spacing is, the stronger the interaction between bubbles is, and the larger the bubble diameter is, the larger the affected area is. The two bubbles placed vertically attract each other in rising process. The upper bubble has accelerative effect on the speed of lower bubble. The two bubbles placed horizontally in water attract and exclude each other periodically.
Application of a New Semi-Active Control Strategy to Dynamic Response Control of an Offshore Platform Using MR Dampers
Authors: Yu Xiaochuan , Ma Tianwei , Falzarano Jeffrey M.
Published: January 2011

A new semi-active control strategy is developed to adjust the voltage/current of magnetorheological (MR) dampers to track the optimal/desired damping force by the Linear Quadratic Regulator (LQR) method. Integrated with the new strategy, a modified inverse dynamic model is proposed to evaluate the value of evolutionary variable based on the Bouc-Wen model of MR dampers. This new strategy is firstly applied to a one-story building under El-Centro earthquake, and it is further applied to the dynamic response control of a fixed jacket offshore platform. The results show that this new semi-active control strategy can achieve efficient vibration control.
Nonlinear Finite Element Analysis of a Toroidal Shell With Ring-Stiffened Ribs
Authors: Du Qing-Hai , Cui Wei-Cheng , Wan Zheng-Quan
Published: January 2011

The toroidal shell is a special type of shells of revolution, which is hardly solved by analytical method. To show the nonlinear structural characteristics of a circular toroidal shell with ring-stiffened ribs due to external pressure, both material nonlinear and geometric nonlinear Finite Element Analyses (FEA) have been presented in this paper, especially for the stability to the type of pressure hull. In the presented Finite Element Method (FEM), the elastic-plastic stress-strain relations have been adopted, and the initial deflection of toroidal shell created by manufacture was also taken into account. The analytic results eventually indicate that by nonlinear FEA such a new type of ring-stiffened circular toroidal shell could be used to a main pressure hull as the traditional ring-stiffened circular cylindrical shell, which could obtain kinds of performance in underwater engineering, such as better stability and more reserve buoyancy to the classical ring-stiffened cylindrical shell.
Effects of Cross-Section Ovalization of Subsea Thin-Walled Bends Under In-Plane Bending
Authors: Banneyake Ranil , Eltaher Ayman , Jukes Paul
Published: January 2011

Ovalization of the cross-section of bends under in-plane bending (a.k.a. Brazier effect) is a known phenomenon caused by the longitudinal stress acting on the cross-section as the pipe bends. Besides its tendency to induce stresses in the bend above what is predicted using simple beam theory, excessive cross-section ovalization is particularly critical to subsea pipes, as it can lead to collapse of the pipe under external pressure. Also, being in a plastic regime may cause the bend material to ratchet and undergo excessive strains under cyclic operational loads, especially under high-pressure high-temperature (HPHT) conditions. Ovalization normally results in local increase of stresses and could lead to failure of the bend before the bend globally reaches its limiting capacity. The offshore industry standards and design codes address the impact of initial ovality in straight pipes, but their applicability to bends is not clear. Therefore, this paper presents an investigation into the increased tendency of thin-walled bends to ovalize, and the effect of bend cross-section ovalization on their stiffness and yielding and collapse limit states, with emphasis on offshore applications. Due to the lack of analytical solutions for the bend response taking into account cross-section ovalization, finite element analysis (FEA) is used in this study. Predictions of the bend models are compared with those of straight pipe models and predictions of models of the bend made of beam elements (with pipe section) are compared with those of models made of brick /shell elements. The increased tendency of thin-walled bends to ovalize compared to straight pipes is investigated (e.g. 100 times in the linear range), and the impact and significance of ovalization in bends are assessed (e.g., stress increase of the order of 35% has been observed in some example situations). Also discussed in the paper is the selection of proper element specifications in order to accurately capture the ovalization response while keeping the computational cost manageable. Recommendations as to how to account for ovalization effects are presented. This paper helps to gain a better understanding of the response of subsea thin-walled bends under in-plane bending and their comparatively high tendency to ovalize compared to straight pipe, and emphasizes the significance of local effects such as cross-section ovalization, the overlooking of which may result in a significant underestimation of involved stresses and strains.
Numerical Simulation of Ship Maneuvering in Pack Ice
Authors: Zhan Dexin , Agar Daniel , He Moqin , Spencer Don , Molyneux David
Published: January 2011

This paper presents preliminary results from a computer program for simulating ship maneuvering in ice covered waters. The program is derived from two programs, Ship Maneuvering Laboratory (SML) and a discrete element numerical modeling program (DECICE). SML is an in-house code developed by Oceanic Consulting Corporation for simulating ship maneuvering in open water. It is based on a ship maneuvering model originally developed by the Japanese mathematical maneuvering group (MMG). DECICE is a discrete element method which was developed by INTERA Technologies and is used to calculate the ice loads on the ship and the interactions between ice pieces. The paper presents a summary of the mathematical methods used together with the results of some case studies for ships EXM004, PSM004 and Esso Osaka Tanker. These computer predictions include turning circle and Zig-Zag maneuvers. Comparisons and discussion of the simulated results between cases with and without ice are also provided.
Motion Characteristics of TLP Type Offshore Wind Turbine in Waves and Wind
Authors: Nihei Yasunori , Fujioka Hiroyuki
Published: January 2011

In this research, we propose a new offshore wind energy generation system that uses a Tension Leg Platform (TLP) and performed the experimental test of the TLP type wind turbine both in waves and in wind. The authors used a 1/100 scale wind turbine. Not only the motion characteristics, but also the loads of the tension legs and the bending moments of the tower were revealed in this paper. From the research, the following conclusions were mainly obtained. 1) In the case of waves-wind coexisting condition, the wind effect stabilizes the pitch motion compared with in only waves. 2) The wind effect is decreasing the vibration of the mooring lines in waves and wind coexisting field. Especially, the springing (2nd order or 3rd order force) is also decreasing in this field. 3) It can be estimated that the amount of reduction rate of electricity generation power is up to about 6% from the results of the heel angle.
Numerical Simulation of 2D Sloshing in Liquid Tanks Based on SPH Method
Authors: Zhu Renqing , Chen Zhengyun , Wang Quan
Published: January 2011

Impact pressures induced by liquid sloshing may damage wall structures of containment system. And when coupling with ship motions liquid sloshing can cause violent motions and even capsizing of liquid cargos under extreme conditions. At the same time, sloshing of liquid tank is a strong nonlinear mechanical problem with free surface, which is very difficult to predict exactly. In this paper, the implementation of the Smoothed Particle Hydrodynamics (SPH) method is presented to tackle two-dimensional large amplitude liquid sloshing in tanks. The classical SPH method is modified to calculate the pressure on the wall. The wave height tracing module is improved for more accurate output of the computed wave height. Two examples of computations of liquid sloshing in a two-dimensional rectangular liquid tank are performed by above improved method, and mechanism of sloshing phenomenon was analyzed. The results including wave surface profile and pressure acting on the bulkhead are compared with experimental results, showing a good agreement. The capability of this method to be used to simulate large amplitude liquid sloshing is validated.
Motion Monitoring and Decision Support During Heavy Transport
Authors: Peters Onno A. J. , Adegeest Leon J. M.
Published: January 2011

During transports of large heavy cargo like jack-up rigs or semi-submersibles, the Motion Monitoring and Captain Decision Support system is a valuable tool to ensure a safe and economical voyage. Using the dynamic characteristics of the vessel in combination with 5-day weather forecasts and design limits like maximum accelerations at the cargo location, roll motion and/or leg bending moment, more and better information is available to the Master to choose a safe heading, speed and route. This way the best knowledge what to expect is contributing to the safety of cargo, transport vessel and crew. Besides use in heavy transport, this system is widely used on container ships, LNG carriers, all kinds of offshore vessels and many other types of floating structures. With daily communication, all important information is made available on internet to the operator’s main office, from which clients are informed with a comprehensive and concise overview of what is happening with their property. After the voyage, clients can be provided with the recorded Motion Monitoring data, which is valuable information for the lifetime assessment. The paper is presenting background information of the Motion Monitoring and Captain Decision Support system, a brief overview of methods used by the system and is describing the relations between transport vessel, main office and client and between the Transport Manual and the system. Results of two independent measurement systems are giving proof of high accuracy of the measurements. Comparison between measurements and predicted vessel response are shown and explained.
Condition Assessment and Preventive Maintenance of Concrete Structures in Severe Marine Environment
Authors: Cao Weiqun , Fan Hong , Zhao Tiejun , Wan Xiaomei
Published: January 2011

Concrete structures are generally designed on the assumption that they will require no maintenance during their service life. The experience with reinforced concrete structures in the severe marine environment shows that this assumption is not valid. Almost everywhere in the world it is possible to find coastal structures, even some relatively new ones, which contain cracked or spalled concrete usually involving corrosion of the reinforcement. For most concrete structures, the typical situation during operation of the structures is that maintenance and repairs are mostly reactive, and the need for taking appropriate measure is usually realized at a very advanced stage of deterioration. For chloride-induced corrosion, repairs at such a stage are both technically difficult and disproportionately expensive compared to carrying out regular condition assessments and preventive maintenance. Therefore, for all concrete structures where high safety, performance and service life are of special importance, regular condition assessment and preventive maintenance should be carried out including chloride profiles, potential mapping, local inspection, etc. For all concrete structures in chloride-containing environments, however, special procedures for monitoring and control of chloride penetration during operation of the structures are needed. The establishment of such procedures should always be an impartment and integral part of the durability design. In this paper, some procedures for the preventive maintenance are described and discussed in more detail.
Experimental Studies on Dynamic Response Behavior of Multi-Legged Articulated Tower
Authors: Chandrasekaran Srinivasan , Bhaskar K. , Hashim Mohammed
Published: January 2011

Articulated towers consist of surface piercing columns pinned to sea floor and have increased applications in deep water oil exploration. Vital component is the buoyant shaft connected to sea bed through a universal joint. Design methodologies of these towers ensure reduced motion characteristics with less deck acceleration while loads at the articulated joint are kept to minimum; this is required to establish sufficient stability under working conditions. A Scaled model of a multi-legged articulated tower is experimentally investigated under regular and random waves. Influence of different parameters on the tower response, namely, deck load and wave approach angle are examined in detail. Apart from having increased deck area, multi-legged articulated towers showed controlled dynamic response behavior under environmental loads. Conclusions drawn from the study bring a detailed insight to the design of such platforms. Though few observations inferred from the study are not new, important dynamic response characteristics like bending stress variations are quantified through experimental investigations.