bluuebunny/crossref_metadata_2025 huggingface dataset:
Buckling Strength of Damaged Stiffened Panels
Authors: Amante Diogo do Amaral , Estefen Segen F.
Published: November 2011

Numerical and experimental simulations for small scale damaged stiffened panels are performed. Six small scale stiffened panels were fabricated following special techniques to keep them in accordance with usual tolerances related to initial geometric imperfections due to fabrication. Ultimate buckling strength analyses were carried out in order to perform a correlation study to adjust the numerical model for further use in parametric studies. The damage was imposed with a local indentation on the panels. Measurements of the geometric imperfection distributions and damage shapes were accomplished using the equipment laser tracker. It is a portable contact measurement equipment that uses laser technology with sub-millimeter accuracy. The numerical models are represented by shell elements assuming finite membrane strains and large rotations, considering both geometric and material nonlinearities. The aim of the work is to study the failure behavior up to and beyond buckling to evaluate the strength loss due to the damage imposed to the panel. Additionally, some numerical simulations of damaged stiffened panels were performed. In these analyses the damage was done with explicit nonlinear finite element code from ABAQUS program. Therefore the distortions and the residual stresses due to the damage are both considered in subsequent compression analysis.
Surface Piercing Drag in CFD
Authors: Bowen Will , Alexander Keith
Published: November 2011

Drag force on surface piercing flat plates oriented normal to a flow has been modeled using Ansys CFX v12.1 and compared to experimental results. Surface piercing refers to a body which is subjected to a two phase external flow, with the surface being the boundary between the two phases, in this case water and air. It is known that at high velocities flow behind a water piercing flat plate will separate due to a low pressure region created in the wake of the plate. This is known as the mechanism of ventilation. Once ventilation occurs, profile drag is decreased on the plates, due to the back side of the plate being exposed to air and thus atmospheric pressure. However, for surface piercing flow energy is dissipated in the form of waves and spray which increases the drag. The resulting change in drag can be expressed as a surface piercing drag coefficient of a plate, which changes as a function of speed or Froude Number and has been empirically shown to fluctuate from approximately 1.1 to 1.7 for a square plate. (Hoerner) The aim of the report is to verify and present the CFD results for surface piercing drag at a range of speeds from 0–10 m/s (0–36 kph). The results for a fully simulated domain are compared to experimental results at speeds of 0–3 m/s. One rectangular plate of aspect ratio 1 was tested in a towing tank and speed was limited. For speeds of 1–3 m/s the comparison was within 10%.
Suppressing Vortex Induced Vibration of an Elastic Mounted Circular Cylinder by Wavy Wall
Authors: Ni Yongyan , Zhang You-lin , Zhu Renqing
Published: November 2011

Technique of wavy wall with the circumferential direction is used to suppressing vortex induced vibration of an elastically mounted circular cylinder which is free to move along the stream-wise (X-Direction) and transverse (Y-Direction) direction. The simulation is performed at low Reynolds number (Re) and based on finite volume method (FVD). The results show that wavy wall is very effective for suppressing VIV by detuning vortex shedding. Transverse response is reduced greatly while applying appropriate frequency and the amplitude is located between 0 and 0.05D (D is the diameter of cylinder) with the increase of reduced speeds and fixed Reynolds number 500. Besides, the difference-value between wavy wall cylinder and standard cylinder increases.
Development of a Vertical Axis Type Marine Turbine Model With Variable-Pitch Blades and Its Basic Performance
Authors: Ikoma Tomoki , Masuda Koichi , Rheem Chang-Kyu , Yoshimura Yusuke , Maeda Hisaaki
Published: November 2011

This paper describes performance of a vertical axis type marine turbine with variable-pitch blades for marine current power utilization. A cyclic mechanism is applied to pitch control of turbine blades. In this paper, the experimental model is developed, which type is variable pitch control one. At the first, conditions of marine currents around Japanese islands is explained, the necessarily of development of marine turbines in low to middle current velocity range is shown. Effectiveness of pitch control of a vertical axis turbine is described from results of CFD calculations. In the calculations, as the basic model, which pitch angles are fixed, that of a past experimental study is applied. From these investigations, the vertical axis type water turbine with variable-pitch blades for model experiment in a channel is developed. Detail of the model developed is introduced. The model experiment is explained. In the experiment, performance of the self-starting and the number of rotations of the turbine is measured. From the results, effectiveness of the pitch control is shown. However, the characteristics are different between past quasi-statistical investigation with the single stream theory and the experimental results. From the experiments, it is confirmed that the turbine of the variable-pitch types can start by itself in very low velocity range in which conventional vertical axis turbines cannot start and also rotate even if the inertia moment is acted.
The Application of Supercapacitors Based Energy Storage Systems on Heave Compensation Systems for Offshore Lifting Equipment
Authors: Delrue Andre A. G. , van der Veen Peter , Ekelaar Jens
Published: November 2011

An Electrical Heave Compensation System will cause large power demand fluctuations with a period time e.g. of around 8 to 12 seconds. These large power fluctuations are not desirable for the stability of the ship’s electrical power grid. This paper proposes the application and control of a supercapacitor based ESS in a power electronic system with a common DC Bus to limit the effect of the power fluctuations on the ship’s electrical power grid. The regenerated power from the heave compensation movements is temporarily stored in supercapacitors and used again when power is demanded. This causes a more constant power flow from the ship’s electrical power grid, which will cope with the losses in the system. Two topologies are discussed, namely the direct connection of the supercapacitors to the common DC bus and the indirect connection of the supercapacitors to the common DC bus through a DCDC converter. The result of the modelling and experiments of both ESS topologies shows that the high power fluctuations caused by an electrical heave compensation system are significantly decreased.
On Vortex Induced Vibration in Two-Degree-of-Freedoms of Flexible Cylinders
Authors: Huang Weiping , Yu Weihong
Published: November 2011

In this paper, an experimental study on the in-line and cross-flow vortex-induced vibration (VIV) of flexible cylinders is conducted. The relationship of two-degree-of-freedoms of vortex-induced vibration of flexible cylinders is also investigated. The influence of natural frequency of flexible cylinders on vortex shedding and VIV are studied through the experiment in this paper. Finally, A nonlinear model, with fluid-structure interaction, of two-degree-of-freedom VIV of flexible cylinders is proposed. It is shown that the ratio of the frequencies and amplitudes of in-line and cross flow VIV of the flexible cylinders changes with current velocity and Reynolds number. The natural frequency of flexible cylinder has great influence on the vortex-induced virbation due to the strong fluid-structure coupling effect. Under given current velocity, the natural frequency of flexible cylinder determines its forms of vibration (in circular or ‘8’ form). The ratio of the VIV frequencies is 1.0 beyond the lock in district and 2.0 within the lock in district respectively. And the ratio of the VIV amplitudes is 1.0 beyond the lock in district and 1/3 to 2/3 within the lock in district. The results from this paper indicates that in-line vibration should be considerated when calculating the vibration response and fatigue damage.
Progressive Drift of Moored Floating Wind Turbines in a Wind Farm: Improved Mooring Capacity Model
Authors: Suzuki Hideyuki , Kitahara Yu , Fukumoto Yukinari
Published: November 2011

A wide range of platform concepts have been investigated for a floating wind turbine. So far analysis and design of motion characteristics of the platform is main research concern. One key research area less focused is floating platform related risk. If the wind energy would be one of the major sources of electric power supply, wind farms which are comprised of large number of floating wind turbines must be deployed in the ocean. Wind turbines are relatively closely arranged in a wind farm. In such an arrangement, a wind turbine accidentally started drifting will have some possibility to collide with floater and moorings of neighboring moored floating wind turbines, and might initiate another drift which might cause progressive drifting of wind turbines. In the previous report, a scenario of progressive drifting of wind turbines was investigated and associated risk was formulated. Quantitative risk of several arrangements of wind farm was estimated. Effects of arrangement of wind turbines in a wind farm and safety factor used in the design of moorings is discussed. Probability of initial drift was evaluated analyzing past records of accidents and design of mooring. In this research, strength of mooring system was modeled more precisely and probabilistic model was developed considering aged deterioration. Risk of progressive drifting was evaluated and safety factor required to realize a acceptable risk of a wind farm was discussed.
Hydrodynamic Loading on Mid Water Arch Structures
Authors: Russell Colin , Vignaud Bertrand
Published: November 2011

In the design and analysis of subsea structures, the hydrodynamic coefficients play a significant role in determining the behaviour of the system, particularly when they are under the combined action of steady and oscillatory flow. For small regularly shaped structures and slender members there is a significant amount of literature that provides assistance in the selection of suitable values, however this is not the case for larger and irregular shaped structures where there is a scarcity of information. This paper details the results of a study to develop techniques to derive hydrodynamic coefficients for representative mid water arch configurations commonly used in shallow water flexible riser system applications. The results of a literature review are presented outlining where these structures fit into the different theoretical models. The use of the potential flow approach is verified by means of computational fluid dynamic simulations for a representative structure and both methods are utilised to assist in the generation of appropriate coefficients. A case study on a typical shallow water North Sea project is presented that demonstrates the importance of selecting appropriate hydrodynamic values to represent the structure. The influence of each of the coefficients on the arch motion, which has an impact on the riser response, is studied. It is shown that it is not always possible to be conservative and instead it is important to select appropriate coefficients for the structure in question, which may differ from those typically used. The method developed has wide ranging applicability and can in principle be applied to the determination of hydrodynamic coefficients for other large structures subject to hydrodynamic loading, including disconnectable turret buoys and hybrid riser tower systems.
Experimental Studies and Numerical Modeling of Copper Nets in Marine Environment
Authors: Tsukrov Igor , Drach Andrew , DeCew Judson , Swift M. Robinson , Celikkol Barbaros , Baldwin Kenneth C.
Published: November 2011

Copper alloy netting is increasingly used for offshore aquaculture, harbor protection and other marine applications. Its advantageous characteristics include high resistance to biofouling and increased strength compared to polymer nets. However, the hydrodynamic properties of copper nets are not well studied. In this paper, the results of experimental studies of drag forces on copper alloy net panels are reported. Based on these studies, empirical values for drag coefficients are proposed for various types of copper nets, and compared to the corresponding data for polymer netting. It is shown that copper nets exhibit significantly lower resistance to the current flow which corresponds to lower values of drag coefficient. Coefficients obtained from the experiments are incorporated into the finite element program Aqua-FE, developed at the University of New Hampshire for analysis of flexible structures subjected to waves and currents in marine environment. The results of the numerical simulations for a small volume fish cage, subjected to two different sets of environmental conditions, are analyzed to compare how introduction of copper netting instead of traditional nylon nets affects the dynamic response of the system.
Alternative Method of Buoy Supporting Riser (BSR) Installation
Authors: Franciss Ricardo , Casaprima Gonzales Enrique , de Almeida Jose´ Carlos Lima , de Arau´jo Jairo Bastos , Fernandes Antonio Carlos
Published: November 2011

Due to the 2200m water depth and harsher environmental conditions, one option that Petrobras is considering for the production of the Pre-Salt fields is the use of a subsurface buoy known as a Buoy Supporting Riser (BSR). It is composed of a subsurface tethered buoy, flexible jumpers connecting the Floating Production Unit (FPU) to the BSR and Steel Catenary Risers (SCRs) connecting the BSR to the flowlines on the sea bottom. The main advantages of this system are that it decouples the FPU motions from the SCRs, reducing fatigue damage in the touch down zone. It may also be installed independently of the FPU, except for the flexible jumpers, which would reduce the risers load on the FPU. Petrobras has been studying this concept since 1997 and has established, as a final stage of the study, a field test with the actual installation of the BSR. This was performed through an alternative method using only Petrobras AHTS boats, in order to avoid critical and expensive resources such as lift barges. With the purpose of validating this new installation procedure, Petrobras performed the referred installation of a 27.2m × 27.2m square ring shaped buoy in Congro Field in the Campos Basin over a water depth of 500m. The buoy was positioned at 80m depth, where the incidence of loads caused by waves is negligible, thus increasing the fatigue life of risers. After the BSR installation, the riser pull-in procedure was also conduced. This paper describes why this technology is necessary for these fields and the model tests made to validate the installation procedures. It also discusses how Petrobras tested the pull-in operations for two flexible risers after the actual buoy was installed. Monitoring systems were designed to check all forces and displacements during the referred installation. These actions will consolidate the BSR technology for Petrobras leading to another riser system option for production in ultra deep waters.
Safety Format and Acceptance Criteria for Analysis of Marine Structures Using Non-Linear Methods
Authors: Solland Gunnar , Johansen Atle , Sigurdsson Gudfinnur
Published: November 2011

This paper deals with how structural integrity can be documented by use of non-linear analysis methods. The focus is on structures that are exposed to extreme environmental loads. In order to prove the structural integrity for aging offshore installation, non-linear analysis methods are finding increased applications. The codes ISO 19902, API RP 2A and Norsok N-006 give recommendations for use of non-linear methods for capacity checks of existing structures. This paper discusses the different safety formats given in these codes. Also presented is how practical acceptance criteria that have to be applied in conjunction with such analyses can be formulated. The topics that are dealt with are issues that the analyst performing such analyses needs to address like: definition of failure, representation of joints, issues on cyclic loading, instability of cross sections and member buckling. In cases of novel design solutions or special structural details, non-linear analysis methods may also find application when planning new structures.
Burst Capacity Estimation of Pipeline With Colonies of Interacting Corrosion Defects
Authors: Li Xin , Chen Yanfei , Su Chenliang
Published: November 2011

Based on the elastic plastic finite element method, the burst capacity of steel pipeline with corrosion defects i.e. longitudinal and circumferential aligned double corrosion defects are analyzed, respectively. The corresponding interaction rules for longitudinal and circumferential aligned corrosion defects are proposed, respectively. A new method is suggested for pipeline with more general interaction type named as compound aligned corrosion defects. Through the projections of the corrosion profile on the longitudinal and circumferential directions, the effective length, effective width and effective depth of adjacent corrosion defects are defined. Using the burst capacity expression of pipeline with single corrosion defect, the burst capacity of the pipeline with double interacting corrosion defects is calculated. The predictions of burst capacity of the pipe with compound aligned corrosion defects are in good agreement with experimental results. Furthermore, the method is extended for pipeline with colonies of corrosion defects and the assessment procedure is presented for the corroded pipe under internal pressure only. Due to the incorporation of the effective length, width and depth in those solutions, the resistance effect of uncorroded region between corrosion defects can be reasonably taken into account. Therefore, the procedure is validated to yield more accurate predictions than those in current assessment codes after compare with the experimental results.
Investigation on the Effects of Current in the Dynamics of Floating Systems
Authors: Rateiro Felipe , Malta Edgard Borges , Fujarra Andre´ L. C.
Published: November 2011

This paper presents the experimental results obtained by means of model tests, focused on the analysis of the influence of current effects in the dynamic behavior of a semi-submersible. The tests were accomplished with and without current, in order to establish the comparison basis. As an additional investigation procedure, the experimental results were compared to a numerical code based on the potential theory. As will be seen, considerable changes in the natural periods and in the damping levels was observed.
Response of a Spar Platform Under the Occurrence of Extreme Waves
Authors: Nava Vincenzo , Arena Felice
Published: November 2011

In this note, the response of a Spar Platform is studied when subject to very high waves. The novelty of this paper consists in adopting the Quasi Determinism (QD) theory in its first formulation for the determination of the response of the structure. In fact, one of the peculiarities of QD theory is that it can be adopted under any boundary condition. The Spar Platform has been modeled as a two-dimensional 3-degree-of-freedom (3dof) rigid cylindrical floating body, moored to the seabed, floating in deep water. Further, it is supposed to be a slender body under the action of extreme waves neglecting any diffraction effect. Moreover, such a system is considered to be nonlinear, because of the nonlinear damping derived from the drag force and because of the stiffness of the moorings, which in the model have been studied as nonlinear springs. The non linear coupled set of differential equations of motion have been integrated in order to obtain the response of the structure by means of numerical methods commonly adopted in Literature. Thus, the response under the occurrence of very high wave crests is studied by using Quasi Determinism theory and then the results compared and validated with those obtained through Monte-Carlo simulations.
Design Challenges of Top Tensioned Risers in Ultra Deepwater Applications
Authors: Cheng Yongming , Xu Lixin , Stanton Paul
Published: November 2011

A Top Tensioned Riser (TTR) system provides direct access to subsea wells from a floating platform for drilling, workover, and completion operations. TTRs have been widely used with floating production systems such as Spars and TLPs in deepwater field developments. This paper investigates design challenges of TTRs in ultra deepwater applications. As application moves to ultra deepwater, challenges of TTRs increase in terms of riser design, analysis, installation and global performance. This paper first introduces a typical TTR configuration in ultra deepwater applications. The TTR design issues cover riser wall thickness which is especially driven by extreme high pressure and high temperature, tension limits, high payload, and long tensioner stroke. The installation concerns come from hook capacity limit, riser Vortex-Induced Vibration (VIV) when the riser is disconnected, and riser bottom drift caused by vessel motion during installation. This paper further investigates the coupling between the risers and floating platform, the interference between riser pairs, riser VIV, and tensioner stick-slip phenomenon. Examples are given to illustrate various aspects of TTRs in ultra deepwater applications. In addition, this paper also explores likely solutions to TTR design challenges.
Hydrodynamic Considerations for FLNG Concepts
Authors: Clauss Gu¨nther F. , Mavrakos Spyros A. , Sprenger Florian , Testa Daniel , Dudek Matthias
Published: November 2011

The current demand in liquefied natural gas (LNG) encouraged the design of various concepts for floating LNG (FLNG) liquefaction or regasification facilities. With increasing transport distance, e.g. from remote marine locations to the onshore gas supply net, gas pipelines become uneconomic compared to shuttle carriers for LNG (LNGC). Due to its high energy density, offshore transfer from processing terminals to carriers and from carriers back to receiving terminals has to be analyzed in detail. During the transfer period, free fluid surfaces occurring in the cargo tanks of the LNGC are leading to a significant decrease of the initial intact stability and altered motion behavior. This paper focusses on the influence of resonant tank sloshing on the LNGC’s roll and surge motions. Analyses of transverse and longitudinal sloshing yield a surprising phenomenon: the frequency shift Δω between the theoretical natural frequency of the tank alone and the respective motion peak for a vessel with four tanks mounted to the hull. Force measurements between tank and hull reveal a peak at the tank’s natural frequency that causes strong liquid motions with related forces and moments on the hull but no increased vessel motions. Additional investigations comprise the offloading situation with a multi-body arrangement of LNGC and a FLNG (the MPLS20 system) in tandem and briefly also in side-by-side configuration. The slow drift motions on the turret-moored FLNG are exemplarily investigated in head seas.
The WAG Platform Structural Effects Study
Authors: Jeans Gus , Johnson Robert
Published: November 2011

This paper describes a detailed investigation into structural effects on wind measured on an offshore platform. Squalls dominate the extreme wind regime in the West Africa region. The West Africa Gust (WAG) Joint Industry Project was initiated to provide critical new data to support engineering design. The WAG squall measurement system was carefully designed to characterise the horizontal and vertical structure of squall winds. A total of 8 anemometers were installed at various locations on a platform offshore Congo. This application required a high level of confidence in wind speed differences measured at separate locations. An intensive investigation into platform structural effects was undertaken using the in-situ data, supported by independent wind tunnel testing. This paper will explain how the wind tunnel results were effectively used to quantify structural contamination at each key anemometer location, for the full range of incident wind directions. The wind tunnel results were verified using empirical structural correction factors derived from the in-situ data. The study successfully established the required high level of confidence in the measured data, so that squall structure could be characterised for engineering design.
Dynamic Analysis of Mooring Lines for Deep Water Floating Systems
Authors: Gurumurthy K. , Ahmad Suhail , Chitrapu A. S.
Published: November 2011

Efficient dynamic analysis of mooring lines and risers is necessary for deepwater floating systems that typically consist of a number of mooring lines and risers. In deepwater, the interactions between the floater motions and the large number of risers and mooring lines become significant and must be considered for accurate prediction of floater motions as well as line dynamics. Time-domain coupled dynamic analysis procedures have been proposed which can account for the coupling effects and consider most of the nonlinearities present in the problem. These methods have been shown to give more accurate results compared to traditional de-coupled analysis methods although they tend to be computationally more expensive. If the system has a large number of mooring lines and risers, it becomes very difficult and impractical to perform time domain coupled analysis. A number of efficient methodologies have therefore been proposed in the past to balance the accuracy of results with computational efficiency. Such methods include the frequency domain approach, combination of frequency and time domain methods, and combination of coupled and uncoupled analysis methodologies. Enhanced de-coupled dynamic analysis is an efficient method and is similar to the traditional de-coupled dynamic analysis method except that the floater motions are computed by coupled analysis considering a coarse finite element model of the mooring lines. In this paper, dynamic analysis of mooring lines for a deep water classical spar floater under random waves is performed by using the enhanced de-coupled dynamic analysis method and the response statistics are compared with results obtained from coupled dynamic analysis. The spar is modeled as a rigid body with six degrees-of-freedom and the mooring lines are modeled as finite element assemblage of elastic rods. All major non-linearities and the dynamic interaction between spar and its mooring lines are considered while determining the tension time histories. Hinge connection is assumed at the fairleads. At every time step of the integration of equations of motion of the spar, a series of nonlinear dynamic analyses of the mooring lines is performed using a subcycling technique. From the analyses, it is found that the enhanced de-coupled dynamic analysis provides results comparable in accuracy with the results obtained from coupled dynamic analysis in terms of predicting the response statistics, but requires only one third of the computational time. Therefore, enhanced de-coupled dynamic analysis can be used for accurate prediction of mooring line dynamics for deep water floating systems.
Potential Impact of Climate Change on Tanker Design
Authors: Bitner-Gregersen Elzbieta M. , Ho̸rte Torfinn , Skjong Rolf
Published: November 2011

Global warming and extreme weather events reported in the last years have attracted a lot of attention in academia, industry and media. The ongoing debate around the observed climate change has focused on three important questions: will occurrence of extreme weather events increase in the future, which geographical locations will be most affected, and to what degree will climate change have impact on future ship traffic and design of ships and offshore structures? The present study shortly reviews the findings of the Intergovernmental Panel on Climate Change Fourth Assessment Report, AR4, [1] and other relevant publications regarding projections of meteorological and oceanographic conditions in the 21st century and beyond with design needs in focus. Emphasis is on wave climate and its potential implications on safe design and operations of ship structures. A risk based approach for marine structure design accounting for climate change is proposed. The impact of expected wave climate change on ship design is demonstrated for five oil tankers, ranging from Product tanker to VLCC. Consequences of climate change for the hull girder failure probability and hence the steel weight of the deck in the midship region is shown. Recommendations for future research activities allowing adaptation to climate change are given.
Passive Energy Extraction Through Oscillation of a Yawing Flat Plate Operating in a Uniform Current
Authors: Fernandes Antonio Carlos , Armandei Mohammadmehdi
Published: November 2011

The work studies the behavior of a plate positioned vertically to incoming current allowed only to yaw. The intention is to use this or similar device to extract energy from the current. Soon, it became clear that positioning a forward disturbance in front of the flat plate creates a wake that improves dramatically the energy extraction. This disturbance was made by a pile with a semi circle transversal section (D-section pile). Several tests have been run, all with the same dimensions of the flat plate and the D-section pile, but each with a unique combination of the incoming velocity (U), the yawing axis position (a/c), the external spring coefficient (k), the distance between the D-section pile and the leading edge of the flat plate (d). The experimental tests are done in LOC (Laborato´rio de Ondas e Correntes — Waves and Currents Laboratory) in COPPE/Federal University of Rio de Janeiro. The results demonstrate that one such system, if controlled appropriately, has the ability to extract energy efficiently.
Modeling VIV Supression Using Negative Lift Coefficients
Authors: Jhingran Vikas , Vogiatzis Johnny
Published: November 2011

Vortex-Induced Vibration (VIV) is a complex, non-linear fluid-structure interaction problem with important consequences for offshore risers, tendons and other tubulars. The prevalent approach in the industry is to use semi-empirical formulations to estimate VIV amplitudes, frequencies and the resulting fatigue damage. These semi-empirical techniques estimate VIV response amplitude by considering the balance of power input into the pipe due to vortex-shedding and the loss of power from the pipe due to damping. At the heart of this method are lift coefficient curves, which are used to estimate power input into the pipe. A key difficulty of this method is modeling the response of pipes with mitigation devices (strakes and fairings). This paper discusses the use of negative lift coefficient curves for estimating VIV response from mitigation devices. The author’s show how damping is handled in Shear7 using this approach. Results of comparisons between predicted and measured data show that negative lift curves are an effective means of modeling damping from VIV mitigation devices.
Experimental Investigation of Flow Induced Rotation of Hinged Plates With Shapes to Avoid Fluttering
Authors: Fernandes Antonio Carlos , Sefat Sina Mirzaei , Coelho Fabio Moreira , Albuquerque Amanda Silva
Published: November 2011

This paper addresses the flow induced rotation phenomena of plates hinged to allow flow induced rotating about their vertical axis. Different transversal shape configurations are studied. The aim of this study is to simplify the fluttering problem that may occur with falling objects in water during installation of offshore devices. The investigation intent is to propose an optimized configuration for stabilizing the fluttering motion of pendulous installation method of manifolds. The experiments and dimensional analysis confirmed that natural frequency is linearly proportional to the incoming flow velocity and inversely proportional to the flat plate width, and also the equivalent harmonic angle of rotation for small oscillation angles is approximately constant in different velocities. Experiments show that the bluffer plates (plate with two stabilizers and plate with stabilizers and nose), by increasing of period of rotation and also decreasing of equivalent harmonic angle of rotation have stabilizing effect in the fluttering motion of falling objects.
Assessment of Lateral Buckles in a HP/HT Pipeline Using Sidescan Sonar Data
Authors: Hansen Alexandre Santos , Antunes Bruno Reis , Solano Rafael Familiar , Roberts Graeme , Bedrossian Arek
Published: November 2011

During design stage of high pressure/high temperature pipelines, some conservative parameters are adopted along with sensitivity analyses to assure safe operation in the presence of uncertainties that influence buckle formation, e.g. pipe-soil interaction, as-laid out-of-straightness and initial heat-up. After operation starts and lateral buckles appeared along the line, a survey may provide valuable information regarding confirmation of the design assumptions, evaluation of actual behaviour and the possibility of increase the operating conditions. This work presents the methodology applied to analyse the configuration of the P-53/PRA-1 12″ oil export pipeline in operation using data from a sidescan sonar survey. The aim of such analyses was to gather information for an FE model calibration as well as to obtain preliminary estimates for the bending strains at lateral buckling locations. Special attention was dedicated to smoothing and interpolation of the pipeline coordinates extracted from sonar imagery in order to avoid unrealistic strains estimates.
An Improved Anti-Diffusive VOF Method to Predict Two-Fluid Free Surface Flows
Authors: Chen Y. G. , Price W. G. , Temarel P.
Published: November 2011

This investigation continues the development of an anti-diffusive volume of fluid method [1] by improving accuracy through the addition of an artificial diffusion term, with a negative diffusion coefficient, to the original advection equation describing the evolution of the fluid volume fraction. The advection and diffusion processes are split into a set of two partial differential equations (PDEs). The improved anti-diffusive Volume of Fluid (VOF) method is coupled with a two-fluid flow solver to predict free surface flows and illustrated by examples given in two-dimensional flows. The first numerical example is a solitary wave travelling in a tank. The second example is a plunging wave generated by flow over a submerged obstacle of prescribed shape on a horizontal floor. The computational results are validated against available experimental data.
Hydro-Dynamic Simulation of a Cylinder Buoy for Wave Energy Conversion
Authors: Velez Carlos , Papesh Brent , Ilie Marcel , Qu Zhihua
Published: November 2011

Development of technology to harness the vast amount of renewable energy available in nature has been ever-increasing in popularity. A worldwide desire to limit dependency on fossil fuels as a means to produce power has motivated research in solar, wind, and wave energies, as well as other clean, naturally-abundant energy sources. With a density approximately 1000 times greater than air, the energy potential of ocean water is tremendous, and it is capable of providing power to locations in which other forms of renewable energy are not applicable—namely coastal regions with minimal wind or sunshine, or offshore structures. This research details the hydro-dynamic modeling of an innovative buoy design for a wave energy harvester that converts the heaving motion of waves into electrical power. Power is generated through the use of a bi-directional turbine which is driven by the relative water velocity created by the heaving buoy. In order to predict the changing velocity profile in which the bi-directional turbine will experience, a hydro-dynamic model has been created with a smoother particle hydro-dynamics code, SPHysics. The model can accurately simulate the motion of the buoy as it is excited by various ocean waves for different ocean depths. In order to maximize the flow velocity through the turbine, various geometric parameters will be altered to attempt to have the buoy and ocean wave perfectly out of phase. Additionally, the buoys stability is studied to determine the optimal geometry to promote a vertical motion as any yaw or pitching motion can not be harnessed by the bi-directional turbine.
A Data-Driven Mode Identification Algorithm for Fatigue Damage Assessment in Instrumented Marine Risers
Authors: Shi C. , Park J. , Manuel L. , Tognarelli M. A.
Published: November 2011

A well-established empirical procedure, which we refer to as Weighted Waveform Analysis (WWA), is employed to reconstruct a model riser’s response over its entire length using a limited number of strain measurements. The quality of the response reconstruction is controlled largely by identification of the participating riser response modes (waveforms); hence, mode selection is vital in WWA application. Instead of selecting a set of consecutive riser vibratory modes, we propose a procedure that automatically identifies a set of non-consecutive riser modes that can thus account for higher harmonics in the riser response (at multiplies of the Strouhal frequency). Using temporal data analysis of the discrete time-stamped samples, significant response frequencies are identified on the basis of power spectrum peaks; similarly using spatial data analysis of the sparse non-uniformly sampled data, significant wavenumbers are identified using Lomb-Scargle periodograms. Knowing the riser length, the most important wavenumber is related to a specific mode number; this dominant mode is in turn related to the dominant peak in power spectra based on the temporal data analysis. The riser’s fundamental frequency is estimated as the ratio of the empirically estimated dominant spectral frequency to the dominant mode number. Additional mode numbers are also identified as spectral peak frequencies divided by the fundamental frequency. This mode selection technique is an improvement over similar WWA procedures that rely on a priori knowledge of the risers fundamental frequency or on knowledge of physical properties and assumptions on added mass contributions. At selected target locations, we compare fatigue damage rates, estimated based on the riser response reconstructed using the WWA method with the proposed automated mode selection technique (we refer to this as “improved” WWA) and those based on the “original” WWA method (that relies on a theoretically computed fundamental natural frequency of the riser). In both cases, predicted fatigue damage rates based on the empirical methods and data at various locations (other than the target) are cross-validated against damage rates based directly on measurements at the target location. Results show that the improved WWA method, which empirically estimates the riser’s fundamental natural frequency and automatically selects significant modes of vibration, may be employed to estimate fatigue damage rates quite well from limited strain measurements.
Short and Long Term Extreme Reliability Analysis Applied to Floating Wind Turbine Design
Authors: Perdrizet Timothe´e , Averbuch Daniel
Published: November 2011

This paper describes and exemplifies an efficient methodology to assess, jointly and in a single calculation, the short and long terms failure probabilities associated to the extreme response of a floating wind turbine, subjected to wind and wave induced loads. This method is applied to the realistic case study OC3-Hywind used in phase IV of the IEA (International Energy Agency) Annex XXIII Offshore Code Comparison Collaboration. The key point of the procedure, derived from the outcrossing approach, consists in computing the mean of the outcrossing rate of the floating wind turbine response in the failure domain over both the short term variables and the ergodic variables defining long term parameters.
Direct Numerical Simulation of Effects of Small Angle of Incidence on Honji Instability
Authors: Yang Kun , Cheng Liang , An Hongwei , Zhao Ming
Published: November 2011

This paper concerns Honji instability generated around a circular cylinder in an oscillatory flow with a small oblique angle. In this study, direct numerical simulation has been conducted for an oscillatory flow past a stationary cylinder with small incidence angles (α) of 5° and 10° at KC number of 2 and β number of 200. The three-dimensional Navier-Stokes equations are solved using the Petrov-Galerkin finite element method. Flow structures around the cylinder are visualized through using streamlines, velocity vectors and vorticity contours. Honji instability has been captured at both chosen inclination angles. However Honji vortex pairs are asymmetric at α = 5° and 10° due to the inclination of the oscillation direction and can only be observed during the flow reversal. It is also found that the flow inclination appears to suppress the three-dimensional instability.
Investigation of the Hydrodynamic Characteristics of Asymmetric Cross-Sections Advancing in Regular Waves
Authors: Ribeiro e Silva Sergio , Uzunoglu Emre , Guedes Soares Carlos , Maro´n Adolfo , Gutierrez Cesar
Published: November 2011

A series of experiments were carried out to assess the instantaneous hydrodynamic coefficients and the parametric rolling characteristics of a C11 class container vessel model. The experiments consisted of captive model tests at various heel angles, forced oscillation tests, free roll decay tests and parametric rolling tests. The results obtained from the forced oscillation tests in calm water on symmetric (upright) and asymmetric (heeled) cross-sections are discussed in here. The main objective of the forced oscillation tests was to obtain the heave, roll and pitch added masses and damping coefficients for two different speeds and also three different amplitudes of forced motions. The most important aspect was to assess the influence of taking instantaneous cross-section into account on calculation of these hydrodynamic coefficients. In addition, these tests allowed the identification of the level of non-linear effects with the amplitude of the forced motions for both symmetric and asymmetric hull forms. The experimental data obtained is compared with the results from two strip theory codes for symmetric and asymmetric cross-sections.
Mathematical Modeling and Numerical Simulation of Fluid Flow Effects on Marine Current Turbines
Authors: Singh Amit J. , Arockiasamy Madasamy
Published: November 2011

This paper presents a numerical model to study the fluid flow effects in an idealized, full scale marine current turbine. The effect of changing fluid flow conditions on the operation and structural integrity of a marine current turbine is of utmost importance in designing the shape, size and composition of the turbine blade. The model uses the measured current velocity offshore the coast of Ft. Lauderdale, Florida. A probability distribution function is used to describe the probability of exceedance of the current velocity. The effects of free surface, kinematic viscosity and pitch angle are considered in this study. The turbine rotor is modeled using a 3D computer aided design (CAD) tool, SolidWorks. The rotor and the computational domain are meshed using geometric mesh generation software, ANSYS ICEM CFD. ANSYS FLUENT software is used to model the fluid flow interactions by solving the conservation equations for mass and momentum, considering non-uniform inflows and turbulence. The model will provide a methodology for predicting the lift and drag coefficients, bending moments and center of pressure in the turbine rotor. The results from this study can be used to predict the fatigue life of a turbine blade based on the statistical data of the current profile at a particular location. Furthermore, the estimation of power generation and efficiency of the turbine can be calculated to provide the information needed to evaluate the feasibility and economics of the energy converter.
Detailed Analyses of the Tow Line Behaviour in Single, Double and Triple Towages in Case of Emergency Stop and Catenary
Authors: Bos A. J. , Heemskerk R.
Published: November 2011

Two phenomena have been studied in order to enhance the contingency plans and improve the safety of towages. 1. For multi tug towages it is important to prepare proper contingency plans for the case that a tug fails and overrun by the tow is a probability, especially the towages of FPSO’s and huge rigs performed by more tugs. A model has been developed to assess the time between the failure and the moment the tow will collide. After thorough research not the stopping distance proved important, but the time it takes for both objects to collide. In case the tug is allowed to be pulled towards the FPSO, the time from engine failure to collision is 380 [s] or 6 minutes 20 seconds. Both objects will collide with a speed difference of 2.28 [m/s]. In case the towline is cut, the time until collision is 1479 [s], or 24 minutes and 40 seconds. In this case it is very important to cut the towline instantly after the engine failure. Otherwise the tug will gain a negative speed of 1 m/s within 2 minutes, and the distance between the FPSO and the tug will be reduced to 683 [m] already. 2. Grounding of tow lines must be avoided the standard of the catenary approach to assume a hyperbolic shape is investigated and a detailed finite element model approach shows that the standard assumptions are not accurate enough. A numerical approach has been used to calculate the effect of current and loss of tension in the wire. The influence of current along the towing-wire depends on the speed, diameter, length and the angle of the towing-wire in the water. The maximum depth increases when the speed increases or the tension in the wire decreases. In the example the depths are on the safe side for depths below 35[m], but above 35[m] the values are too optimistic when current is involved.
MODELING OF CONTACT ANGLES AND WETTING EFFECTS WITH PARTICLE METHODS
Authors: LIU M. B. , CHANG J. Z. , LIU H. T. , SU T. X.
Published: November 2011

The physics of fluid–fluid–solid contact line dynamics and wetting behaviors are closely related to the inter-particle and intra-molecular hydrodynamic interactions of the concerned multiple phase system. Investigation of surface tension, contact angle, and wetting behavior using molecular dynamics (MD) is practical only on extremely small time scales (nanoseconds) and length scales (nanometers) even if the most advanced high-performance computers are used. In this article we introduce two particle methods, which are smoothed particle hydrodynamics (SPH) and dissipative particle dynamics (DPD), for multiphase fluid motion on continuum scale and meso-scale (between the molecular and continuum scales). In both methods, the interaction of fluid particles and solid particles can be used to study fluid–fluid–solid contact line dynamics with different wetting behaviors. The interaction strengths between fluid particles and between fluid and wall particles are closely related to the wetting behavior and the contact angles. The effectiveness of SPH and DPD in modeling contact line dynamics and wetting behavior has been demonstrated by a number of numerical examples that show the complexity of different multiphase flow behaviors.