bluuebunny/crossref_metadata_2025 huggingface dataset:
Spar VIM Model Tests at Supercritical Reynolds Numbers
Authors: Finnigan Tim , Roddier Dominique
Published: May 2009

There have been a number of publications on spar Vortex-Induced-Motions (VIM) model testing procedures and results over the past few years. All tests allowing full 6 DOF response to date have been done under sub-critical Reynolds Number conditions. Tests under super-Critical Reynolds Number conditions have only been done with a fully submerged 1 DOF rig. Early in 2006, Chevron Energy Technology Company (CETC) completed a series of model tests to investigate the effect of Reynolds Number and hull appurtenances on spar vortex induced motions (VIM) for a vertically moored 6DOF truss spar hull model with strakes. Tests were done at both sub- and super-critical Reynolds Numbers, with matching Froude Numbers. In order to assess the importance of appurtenances (chains, pipes and anodes) and current heading on strake effectiveness, tests were done with several sets of appurtenances, and at various headings and reduced velocities. This paper addresses the challenges of performing spar VIM model tests at Super Critical Reynolds Numbers, and how they were resolved without the restrictions noted in earlier publications. Certain aspects of the effect of appurtenances and current heading on strake effectiveness and VIM response are discussed.
A Study of the Forces on an Oscillating Cylinder
Authors: Marzouk Osama A. , Nayfeh Ali H.
Published: May 2009

Flow over a cylindrical structure is a problem of interest in different engineering fields, especially offshore structures like risers and spars, which are constantly subject to sea waves. In this study, numerical simulation of the flow over a fixed cylinder at a low Reynolds number was carried out using the Reynolds-averaged Navier-Stokes model. It is well known theoretically and experimentally that the lift is self excited and has a frequency equal to the vortex shedding frequency. Then the cylinder was forced to oscillate in the cross-stream direction with different frequencies and amplitudes. The flow structure, including the lift and drag, was examined within the so-called lock-in frequency range, where the lift frequency is entrained by the cylinder frequency. The frequency-response curves for the lift and drag and the phase between the hydrodynamic force and the cylinder were produced for different motion amplitudes. These curves change significantly as either the motion amplitude or frequency is varied. Two modes of response were identified associated with high lift and low lift; each one has different characteristics.
Numerical Study on Sea Keeping Performances of Ship by Exact Linear 3-D Potential Method
Authors: Golam Zakaria N. M. , Baree M. S.
Published: May 2009

This paper deals with the numerical calculations of sea-keeping performances of ship in irregular sea condition. Here linear potential theory has been applied for describing the fluid motion and 3-D sink-source technique has been used to determine hydrodynamic forces for surface ship advancing in waves at constant forward speed. Numerical coding based on 3-D potential method has been tested in an extensive manner keeping an eye with the criteria recommended by various ITTC committees [1]. The numerical accuracy of the coding has been examined using some experiment results as well as some other contemporary numerical calculations given by some authors for the case of frequency domain analysis. Taking a typical Panamax Container Vessel and in order to simulate its sea-keeping performances in real sea condition, the frequency domain analysis has been performed. The result is then used for time domain simulation in short crested irregular waves. Unequal frequency spacing has been taken into account to get longer simulation time and also empirical nonlinear roll damping has been taken in the way of time domain simulation. From this time domain simulation, relative wave height has been calculated which could sometimes damage deck equipment as well as posing a risks to personnel in severe sea condition. The effect of speed & wave direction on relative wave height has been considered and finally the numerical results of the maximum and significant values of irregular relative wave heights for these conditions are discussed.
Development of Design Technologies for Optimum Moonpool Shapes of Drill Ship
Authors: Park Jong Jin , Kim Mun Sung , Lee Hee Sung , Ahn Young Kyu , Kim Young Bok , Ha Mun Keun
Published: May 2009

The present study is concerned with the numerical and experimental analysis of a moonpool in rough sea. From hydrodynamic viewpoint, a moonpool of drill ships can cause various problems. Among them, two major problems arise such as an increased resistance and overflow on the deck due to pumping up phenomena. To overcome these inherit problems, we have carried out various numerical analysis to find optimum moonpool shapes. The three-dimensional numerical model adopting SOLA-VOF scheme is used to predict violent free surface movement inside of a moonpool due to irregular ship motions with six degree of freedom. For accurate input data of ship motions, a three dimensional panel method program is applied for seakeeping analysis. The resistance and seakeeping model test have been carried out at MARINE to validate the proposed moonpool shapes, which have been designed based on numerical simulation results.
Metallurgical Design of Newly Developed Material for Seamless Pipes of X80–X100 Grades
Authors: Arai Yuji , Kondo Kunio , Hirata Hiroyuki , Hamada Masahiko , Hisamune Nobuyuki , Hitoshio Keisuke , Murase Tsuneo
Published: May 2009

With the increasing development of oil and gas fields in deepwater or ultra-deepwater with deep well depth, the development of high strength seamless pipe has become necessary. This paper describes a metallurgical design of seamless pipe with high strength reaching X80–X100 grade (minimum yield strength, 552 MPa–689 MPa) manufactured by steel containing very low carbon and with a microstructure of uniform bainite. The effect of microstructure of quenched and tempered (QT) steel on strength and toughness is investigated in laboratory. Uniform bainitic structure without coarse martensite-austenite constituent (M-A) is obtained by lowering bainite transformation temperature during quenching process by controlling the alloying elements. Moreover the structure is very effective in obtaining good toughness for tempered steel even with the high strength X100 grade. Sufficiently low hardness and good toughness in heat affected zone (HAZ) are confirmed by welding tests. The trial production of developed steel is conducted by applying inline QT process in medium-size seamless mill according to an alloying design obtained in laboratory tests. The seamless pipes of the trial production achieve grades X80 to X100 by changing tempering temperature. Some data of mechanical properties of the produced pipes is introduced.
Stress Measurements at Hot Spots Using StressProbe
Authors: Lotsberg I. , Salama M. M. , Dover W. D. , Kare´ R. F. , Stone N.
Published: May 2009

The estimation stress concentration factor (SCF) of weld details of floating structures is critical parameter for fatigue life predictions. The common practice for predicting the SCF value is the use of code-specified empirical equations and through detailed finite element analysis (FEA) for critical joints. Under certain conditions, it becomes necessary to reassess the SCF value while the structure is in service. The most accurate approach is to measure the SCF value on the real joint since variations in FEA results always exist due to variation on modeling techniques by different analysts. While the use of strain gauges is the standard approach, applying strain gauges in service can be extremely complicated. Therefore, an alternative approach for direct measurement that does not require removal of coating and bonding to the surface is attractive. The StressProbe that takes advantage of the change in the magnetic permeability of steel due to the presence of a mechanical stress offers the required alternative as a non-contacting strain measurement method. The paper presents the results of a study to assess that capability of the StressProbe in measuring SCF value by measuring variations in strain in a high strain gradient region of a hopper corner detail that is typical in floating structures and compare the results with those obtained using both strain gauges and finite element analysis. The results show that the StressProbe can be used for the determination of the SCF at a weld under various scenarios and it can also be used to monitor cyclic stresses during periods when there is wave loading. While there is some variability in the StressProbe results but given the variability found with strain gauges it is considered that the StressProbe could be used to replace strain gauges.
Galloping 2D Tests of a Two Cylinder Bundle
Authors: Braaten Henning , Lie Halvor , So̸reide Martin , Svardal Stian
Published: May 2009

Elastic mounted noncircular cross section geometries can produce large displacements and/or rotations often denoted galloping or fluttering, see e.g. Blevins (1990) and Faltinsen (1990). For such a structure the fluid forces that act on the structure change with orientation to the flow. If the oscillating fluid force tends to increase vibration, the structure is dynamically unstable and very large-amplitude vibration, galloping, can result. Statoil ASA has commissioned a model test campaign at MARINTEK to study the behaviour of flow induced vibrations for a 2D model of two pipes tied together with a special emphasize on the possibility of galloping response. The results were used in a Statoil study on the behaviour of free-span pipelines near sea bottom. However, the test results are generic and can also be relevant for deep water risers and umbilicals. The short rigid pipe section model was elastically mounted and was free to move as a single degree of freedom (SDOF) in the cross-flow direction. In some tests it was also allowed to rotate around the cylinders axial axis (2-DOF). By adjusting the torsion stiffness the same natural frequencies in the two DOFs were obtained. The cylinder was constrained in in-line direction. The tests were done for several different headings and towing velocities. Altogether 189 towing tests were done. As key result relative large displacement-to-diameter ratios were observed in the reduced velocities range 1–20. By evaluation of the oscillating frequency the response was characterized. For the lower reduced velocity range (say up to 10) the response could easily be characterized as VIV and for the highest reduced velocities as galloping. In an intermediate reduced velocity range a mixed kind of behavior is observed. These results coincide with results reported of flow induced vibrations of bridge decks with rectangular cross-sections, confer Blevins (1990). Most galloping analysis utilizes quasi-steady fluid assumption. A fundamental assumption for the quasi-steady assumption is that the fluid forces are quasi-steady and the structure oscillates around its natural frequency. Oscillating vortex shedding forces are at much higher frequencies and do not matter in the galloping response. Since the galloping response frequencies overlap with the vortex-shedding frequency it was concluded that the quasi-steady fluid assumption is not applicable for the present system.
Vortex Induced Vibration of a Rigid Cylinder Oscillating With a Given Trajectory Profile
Authors: Soni Prashant K. , Larsen Carl M. , Muthanna Chittiappa
Published: May 2009

Pipeline laid on irregular seabed terrain may have free spans. Due to current, such spans may experiences vortex induced vibrations (VIV), which may lead to fatigue failure. The dynamic properties of free spanning pipelines cause a very complex response pattern and adjacent spans may also have some kind of dynamic interaction. A first set of experiments with flexible pipe model, see Soni & Larsen (2006), showed that the maximum response amplitudes for two interacting spans are higher than for equivalent single span cases. The interaction between IL and CF response will probably have some influence on the response level, in addition to the interaction between adjacent spans. A second set of experiments has been conducted with a motion controlled rigid cylinder in order to find the hydrodynamic coefficients for different flow conditions and also to observe how combination of IL and CF motions will influence the hydrodynamic forces. The cylinder was forced to follow an oscillatory pattern found from the first set of experiments with flexible pipe model. The Reynolds’s number and the dimensionless frequency were kept the same for both types of tests in order to ensure that the flow conditions are identical. The vortex shedding process for the motion controlled rigid cylinder has been mapped using Particle Imaging Velocimetry (PIV) under varying oscillation conditions. Improved understanding of correlation along a flexible beam and the interaction between cylinder motions and vortex shedding is hence obtained. The variation of lift coefficient along the pipe length supports the theory given by the authors; see Soni & Larsen (2005), for energy transfer between the spans. Thus, the spans interact dynamically.
Effect of Constraint on Fatigue Crack Propagation in T-Plate Welded Joints
Authors: Wang Xin , Gao Qing
Published: May 2009

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. However, how to quantify the effect of crack tip constraint on the fatigue crack propagation rate is still an open issue. It is common practice to use the Paris crack propagation law, which is based on the stress intensity factor alone. In this paper, the effect of crack tip constraint on the fatigue crack propagation of small cracks at T-plate welded joints is investigated. A fatigue crack propagation rate model is developed incorporating both low and high constraint conditions. 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 weld joints). This effect can be accounted for by using the proposed fatigue crack propagation rate model.
Getting Your Feet Wet: New Risks and Rewards in Offshore Wind Energy
Authors: Tadich Josef Kryger , Feld Tove
Published: May 2009

The move of wind energy offshore involves unique technical considerations during the project’s development. With multiple key stakeholders such as: designers, manufacturers, developers, investors and regulatory bodies, each inevitably focusing on their own areas of interest during project development. This paper will focus on some of the differences between onshore and offshore wind energy technology, the new risk environment in which they operate, and how project certification can provide a useful tool for boosting stakeholder confidence during development while insuring cross-disciplinary integrity for the project as a whole. The European offshore wind energy market will be outlined, highlighting the major project developments in Northern Europe, and the associated market forces that have driven this development. Focusing on the technology itself, some of the driving technical issues for offshore wind energy today and for the foreseeable future will be discussed. An overview of recent developments in the large wind turbines used in offshore projects themselves, such as increases in rotor diameters and rated powers is presented, leading to a discussion on some of the key component developments facilitating this. An introduction to energy production will be given that focuses on offshore wind farms, and this will be tied in with a discussion on various foundation concepts for offshore wind turbine support structures. Encompassing it all, the role of project certification is discussed in its key phases being: design basis, design, and manufacturing, installation, commissioning, in-service, and decommissioning. This overview will be provided with reference to practical industry experience and the various existing DNV rules currently in publication.
Finite Element Analysis of Large Diameter Grouted Connections
Authors: Nielsen Lars P.
Published: May 2009

When considering offshore monopile foundations designed for wind turbine support structures, a grouted connection between the monopile and an overlapping transition piece has become the de facto standard. These connections rely on axial loads being carried primarily by the bond between the steel and grout as shear. Given the critical nature of the grouted connection in a system with zero redundancy, the current design verification requirement is that a finite element analysis is performed to ascertain the viability of the connection with respect to combined axial and bending capacity whilst pure axial capacity is handled as a decoupled phenomenon using simple analytical formulas. The present paper addresses the practical modeling aspects of such a finite element model, covering subjects such as constitutive formulations for the grout, mesh density, and steel/grout interaction. The aim of the paper is to discuss different modeling approaches and, to the extent possible, provide basic guidelines for the minimum requirements valid for this type of analysis. This discussion is based on the accumulated experience gained though the independent verification of more than 10 currently operational offshore wind farms that have been certified by DNV, as well as the significant joint research and development with industry captured in the DNV Offshore Standard for Design of Offshore Wind Turbine Structures DNV-OS-J101. Moreover, general observations relating to the basic subjects such as overall geometric extent of the model, inclusion of secondary structures, detail simplification, boundary conditions, load application etc. are presented based on the authors more than 3 year involvement on the subject at DNV.
Vortex-Induced Vibration of a Variable Tension Riser
Authors: Josefsson Per M. , Dalton Charles
Published: May 2009

The transverse vibratory response of a long, slender vertical top-tension riser, subject to an ocean current, is studied. The problem is treated as a coupled fluid-flow/vibration problem which is solved numerically. The fluid flow part is represented by the 2-D Navier-Stokes equations, with LES and strip theory, which are solved numerically to obtain the flow field and determine the vortex-shedding behavior in the flow. The approach flow is a shear flow ranging in Reynolds number from 8000 to 10,000. Given the flow field and vortex-shedding behavior, the transverse fluid forcing function can be determined at a given instant, which becomes the input to the Euler-Bernoulli beam equation to calculate the displacement of the riser, using a technique that involves the WKB method and modal decomposition. The boundary conditions for the fluid-flow equations are updated each time step as the cylinder moves. The natural frequency of the riser is tension-dominated, not bending stiffness-dominated. With the decrease in tension with increasing depth, the natural frequency is affected. Therefore, the solution will be influenced by the depth-dependent tension. This study has indicated some interesting features regarding the VIV of a variable-tension riser. The vibrational response is greater for a variable-tension riser than for a constant-tension riser, when the variable-tension riser is assumed to have the same top tension as the constant-tension riser. Therefore, it is important to take into account the variable tension when estimating fatigue failures of marine risers.
C-Manipulator: An Autonomous Dual Manipulator Project for Underwater Inspection and Maintenance
Authors: Spenneberg Dirk , Albiez Jan , Kirchner Frank , Kerdels Jochen , Fechner Sascha
Published: May 2009

We present the new project C-Manipulator (funded by the German Ministry of Economics (BMWI), Grant No. 03SX231). The goal of C-Manipulator is the development of an autonomous, modular, dual manipulator system for underwater applications. This paper provides an overview over the project. It explains shortly the relevance of autonomous underwater manipulation. Then it describes briefly the state-of-the-art, explains the new vision-based control approach featuring visual servoing techniques and the planned manipulator system design featuring the Sub-C Network. Furthermore, a new developed indoor test-bed using a gantry crane for UUV-simulation is introduced, which will be used to test the manipulator system under realistic conditions and to prepare the system for a final test in the Baltic sea, which is planned for 2009.
Mechanical Behavior of the Risers With Helicoidally Weld Under Combined Actions
Authors: Sa´nchez Sa´nchez He´ctor A. , Corte´s Salas Carlos
Published: May 2009

The growing need to satisfy the national oil industry demand has required the evaluation and retrofit of the existing pipelines, in addition to create new risers for transportation and distribution of oil products. The current development of offshore structures requires better designs of the risers capable to assuring an optimal level of security. It is common to get situations, in the performance of fixed offshore platforms, in which risers might be found with lowly internal pressure or empty due to maintenance or repairing. This condition could represent an unfavorable situation that might become highly critical given the conditions they are found in. Therefore, this research is focused to study the mechanical behavior and their structural integrity of the pipelines such as, risers with helicoidally welds under combined load (external pressure, bending moment and axial compression). In particular, for safety prediction, the failure modes and uncertainties involved in each loading condition need to be incorporated in the analysis in order to specify the pipelines use thresholds that keep them over acceptable safety levels within their operating lifetimes.
Scaled Boundary FEM Solution of Wave Diffraction by a Circular Cylinder
Authors: Song Hao , Tao Longbin
Published: May 2009

The scaled boundary finite-element method (SBFEM) is a novel semi-analytical approach, with the combined advantages of both finite-element and boundary-element methods. The basic idea behind SBFEM is to discretize the surface boundary by FEM and transform the governing partial differential equations to ordinary differential equations of the radial parameter. The radial differential equation is then solved analytically. It has the inherent advantage for solving problems in unbounded medium with discretization to the interface only. In this paper, SBFEM is applied to solve the wave diffraction by a circular cylinder. The final radial matrix differential equation is solved fully analytically without adoption of any numerical scheme. Comparisons to the previous analytical solutions demonstrate the excellent computation accuracy and efficiency of the present SBFEM approach. It also revealed the great potential of the SBFEM to solve more complex wave-structure interaction problems.
Stability Analysis of Unburied Offshore Pipeline on Carobonate Soil Under Severe Storm Condition
Authors: Takatani Tomiya , Kaya Takanori
Published: May 2009

The purpose of this paper is to investigate the stability of an unburied offshore pipeline resting on carbonate sand under severe storm condition. Pore pressure accumulation and pipeline movement during cyclic loading caused by waves and currents are numerically investigated. Both drag and lift forces are numerically obtained for 100 years return period storm condition using the Fourier decomposition method. Non-linear spring element is used to simulate a slip phenomenon between pipeline and seabed. The effects of both bi-linear and tri-linear spring element models on pipeline movement and pore pressure response are numerically investigated. Pipeline movement during cyclic loading greatly depends on the mechanical properties of non-linear spring element. In addition, pore water pressure response as well as shear strain are more sensitive to the initial unit tangential stiffness of spring element.
Development of Ultrasonic Phased Array Probe for SAW Seam Inspection of UOE Pipe
Authors: Yamano Masaki , Okubo Hiroyuki , Horikiri Takumi , Nagase Makoto
Published: May 2009

The requirements of Ultrasonic Testing, such as DNV, ISO specification etc., for longitudinal submerged arc weld (SAW) seam of UOE pipe become severe to the offshore steel line-pipe with high grade and heavy wall thickness. In order to satisfy the requirements, testing method with multi-probe arrangement have been widely applied in the pipe manufacture. Recently, the phased array technique has been applied for inspection of girth welds of pipeline and longitudinal ERW (electric resistance welded) seam instead of multi-probe method. But the inspection system with conventional phased array probe has many difficulty to apply for inspection longitudinal SAW seam of UOE pipe, because of defect detection capabilities, array elements arrangement, and so on. The authors have developed an applicable ultrasonic phased array probe to satisfy the severe requirements for longitudinal SAW seam of UOE pipe. This paper presents the results of our original designed array probe, which has been composed of 32-transducer elements mounted on cylindrical-shaped plastics, and also compares the inspection results to those obtained on the conventional multi-probe and our developed array probe in the UOE pipe mill.
Dynamic Response and Fatigue Reliability Analysis of Marine Riser Under Random Loads
Authors: Khan Rizwan A. , Ahmad Suhail
Published: May 2009

Marine riser is a major component of offshore drilling and productions systems that are either fixed or floating. Since, a marine riser is intended to remain in station for the productive life of an oil field; it will be exposed to wide variety of hazards, with the potential for environmental damage, structural failure or damage to the material. As part of the design process, there are requirements of structural strength based on criteria referring to failure modes, such as rupture by over loading, fatigue failures, buckling or unstable fracture. 3D Nonlinear dynamic analysis of riser is carried out in the time domain using finite element solver ABAQUS/Aqua. The response histories so obtained are employed for the study of fatigue and fracture reliability analysis of riser under random waves and random waves together-with vessel motion. In the present study application of structural concepts for the evaluation of the fatigue resistance of marine risers, including reliability techniques has been presented. The limit state function has been established for cumulative fatigue damage using S-N curve approach and fracture mechanics approaches considering number of parameters, random in nature. Reliability methods deal with the uncertain nature of loads, resistance etc. and lead to prediction of the failure and a rational measure of the safety coefficient. Response surface method (RSM) in conjunction with First Order Reliability Method (FORM) has been used for reliability estimation. The results are compared with Monte Carlo simulation method. The design point important for the probabilistic design is located on the failure surface. The effects of the uncertainties in various random variables on riser fatigue reliability are highlighted.
Design Synthesis Studies for the Development of Deep Ocean Manganese Nodule Collecting Systems
Authors: Raphael Deepak C. , Kumaraswamy Sivasailam , Rao Medisetti M. , Ravindran Muthukamatchi
Published: May 2009

Deep ocean mining of manganese nodules presents one of the major challenges in technology in areas pertaining to complex design of machinery for long duration operations at about five thousand metres depth on soft ocean floor. Design synthesis for such systems has to be done keeping in view device interactions, cascading effects of probable failures in hyperbaric subsea conditions apart from main functional requirements. In the present study, conventional design synthesis approaches have been enhanced with design fusion methodologies developed like structured mutual fusion, function sharing approaches for reinforcement, buoyancy and protection, mutual balancing of dynamic forces and reactions of devices. Bionic design methods with analogies to nature can be used effectively in design fusion for manganese nodule collection systems. Case studies of failure and its cascading effects experienced in deep seas have been studied and factors to be taken care in design synthesis mentioned. The paper describes in detail the methodologies developed and design synthesis approaches that can be used for designing deep ocean manganese nodule collector systems.
Techniques for the Analysis of the Stochastic Dynamics of Offshore Systems
Authors: Ellermann Katrin
Published: May 2009

This paper addresses the modeling of the dynamics of moored structures and different techniques for the evaluation of the resulting differential equations. Benefits and limits of simplifying assumptions such as linearizations or deterministic descriptions of random processes are discussed.
An Investigation on the Effect of Tension Variation on VIV of Risers
Authors: da Silveira Lauro M. Y. , Martins Clo´vis de A. , Cunha Leandro D. , Pesce Celso P.
Published: May 2009

This paper aims at investigating the effect of vertical motion (or equivalently the effect of variable tensioning) of the floating unit on the vortex-induced vibrations of vertical risers. This is done using a numerical procedure, based on modeling assumptions, which, though simple, succeeded in describing some expected dynamic behaviors. The model simulates the riser dynamics using a finite element model coupled to a wake-oscillator model, of the van der Pol type, used to emulate the fluid dynamics. Vertical motion (or dynamic tension) is directly imposed to the top. The transverse amplitudes at each section feed the wake-oscillator, which responds with a transverse force that is applied to the riser. The rigidity matrix is updated at each time integration step. The analysis is also carried out with a commercial simulation code dedicated to riser analysis, with a similar wake-oscillator VIV module. Amplitude envelopes are extracted from the time series, showing response mode jumps. The application of the Hilbert-Huang spectral analysis technique helps distinguishing mode jumps by tracking frequency responses in time. The results of the two different dynamic models are compared with very good agreement.
Influence of an Improved Sea-State Description on a Wave Energy Converter Production
Authors: Kerbiriou Marie-Aure´lie , Prevosto Marc , Maisondieu Christophe , Babarit Aure´lien , Cle´ment Alain
Published: May 2009

Sea-states are usually described by a single set of 5 parameters, no matter the actual number of wave systems they contain. We present an original numerical method to extract from directional spectra the significant systems constituting of a complex sea-state. An accurate description of the energy distribution is then given by multiple sets of parameters. We use these results to assess the wave climatology in the Bay of Biscay and to estimate the power harnessable in this area by a particular Wave Energy Converter, the SEAREV. Results show that the fine description of sea-states yields a better assessment of the instantaneous device response. The discrepancy between the classical and multi-sets descriptions show that the new one is preferable for the assessment of harnessable power and for device design.
Gulf of Mexico Sub-Surface Jet Screening
Authors: Jeans Gus , Fan Shejun
Published: May 2009

There have been reports of strong submerged jet currents in the Gulf of Mexico in recent years which have implications for the design and operation of some offshore facilities. This paper describes the methodology and results of a screening study which searched for such events within the extensive body of recent data collected by the offshore oil and gas industry, made available on the NOAA National Data Buoy Center web site. This study was conducted on behalf of a consortium of industry clients with participation by relevant US government bodies. After a first phase investigation, initially well defined screening criteria were revised to avoid the numerous events triggered by clearly invalid data and the potential for missing some critical submerged events. The automatic screening criteria were replaced by manual event selection based on plots showing all available data. The identified events fall into the following broad categories: • Submerged speed peaks due to inertial period currents, most notable after the 2005 hurricanes. • A few submerged jet like events isolated in time with no clear periodicity. • Submerged speed peaks with high vertical and error velocities and often incoherent structures. • Persistent high speed currents near the limit of the ADCP range.
Three-Dimensional Large Amplitude Body Motions in Waves
Authors: Zhang Xinshu , Beck Robert F.
Published: May 2009

Three-dimensional, time-domain, wave-body interactions are studied in this paper for cases with and without forward speed. In the present approach, an exact body boundary condition and linearized free surface boundary conditions are used. By distributing desingularized sources above the calm water surface and using constant-strength panels on the exact submerged body surface, the boundary integral equations are solved numerically at each time step. Once the fluid velocities on the free surface are computed, the free surface elevation and potential are updated by integrating the free surface boundary conditions. After each time step, the body surface and free surface are regrided due to the instantaneous changing submerged body geometry. The desingularized method applied on the free surface produces non-singular kernels in the integral equations by moving the fundamental singularities a small distance outside of the fluid domain. Constant strength panels are used for bodies with any arbitrary shape. Extensive results are presented to validate the efficiency of the present method. These results include the added mass and damping computations for a hemisphere. The calm water wave resistance for a submerged spheroid and a Wigley hull are also presented. All the computations with forward speed are started from rest and proceed until a steady state is reached. Finally, the time-domain forced motion results for a modified Wigley hull with forward speed are shown and compared with the experiments for both linear computations and body-exact computations.
Numerical Simulations of the Flow Past Cross-Flow Oscillating Circular Cylinders and Marine Riser Pipes
Authors: McSherry Richard J. , Willden Richard H. J. , Graham J. Michael R.
Published: May 2009

Presented in this paper is a comparison between numerical and experimental investigations of the Vortex-Induced Vibrations of a model scale marine riser pipe, length-to-diameter ratio of approximately 1400, that was subjected to sub-critical Reynolds number flows. The experiments, which were commissioned by the Norweigan Deepwater Programme, were carried out at Marintek’s Ocean Basin in Trondheim in 2003. The numerical simulations are performed using a coupled Computational Fluid-Structural dynamics code. The fluid flow is computed using Large Eddy Simulation, on multiple two-dimensional strips that are positioned at intervals along the length of the riser. The numerical riser response predictions are found to agree well with the experimental measurements. Of particular note was the occurrence in the simulations of harmonic responses at multiples of the principal oscillation frequency; in the cross-flow direction at three and five times the cross-flow oscillation frequency, and in the in-line direction at two and three times the in-line oscillation frequency. Also presented in the paper are the results of a rigorous benchmarking of the code’s two-dimensional flow solver against high-quality data from forced cross-flow oscillation experiments performed at sub-critical Reynolds numbers; the code predicted the lift force and phase angle variations well, although agreement in the mean drag force variation was less good.
Time Domain Hydroelastic Analysis of a Flexible Marine Structure Using State-Space Models
Authors: Taghipour Reza , Perez Tristan , Moan Torgeir
Published: May 2009

This article deals with time-domain hydroelastic analysis of a marine structure. The convolution terms in the mathematical model are replaced by their alternative state-space representations whose parameters are obtained by using the realization theory. The mathematical model is validated by comparison to experimental results of a very flexible barge. Two types of time-domain simulations are performed: dynamic response of the initially inert structure to incident regular waves and transient response of the structure after it is released from a displaced condition in still water. The accuracy and the efficiency of the simulations based on the state-space model representations are compared to those that integrate the convolutions.
Importance of Control Strategies on Fatigue Life of Floating Wind Turbines
Authors: Skaare Bjo̸rn , Hanson Tor David , Nielsen Finn Gunnar
Published: May 2009

Exploitation of wind energy at deep-waters locations requires floating wind turbine foundations. Several floating wind turbine foundation concepts are reported in the literature, and a common challenge is to make a low cost foundation with acceptable motion characteristics. In order to analyze the fatigue life of floating offshore wind turbines, the coupled action of wind, waves, current and blade pitch control strategy must be considered. State-of-the-art computer programs for motion analysis of moored offshore bodies, Simo-Riflex from Sintef Marintek, are coupled to a state-of-the-art aero-elastic computer program for wind turbines, Hawc2 from Riso̸ National Laboratory. The wave loads on the body may include wave diffraction and radiation loads as well as viscous forces. The mooring lines are modelled using cable finite elements with inertia and drag forces. The wind load on the rotor is based on common rotor aerodynamics including corrections for skew inflow and relative motion caused by large displacement and large tilt and yaw rotations of the rotor. Conventional wind turbine control strategies lead to wind-induced loads that may amplify or damp the motions of the floating wind turbine. The first case is a result of the blade pitch control strategy above rated wind speed for the wind turbine, and can result in large resonant motions that will reduce the fatigue life of the floating wind turbine significantly. The latter case implies energy extraction from the waves. This paper addresses the importance of control strategies on fatigue life for a given floating offshore wind turbine. A fatigue life time comparison between a conventional blade pitch control strategy and an estimator based blade pitch control strategy show that the fatigue life of floating offshore wind turbines can be significantly increased by use of alternative blade pitch control strategies.
A Study of the Interaction of a Gravity Current With a Square Cylinder Using Two-Dimensional Numerical Simulations
Authors: Gonzalez-Juez Esteban , Constantinescu George , Meiburg Eckart
Published: May 2009

We studied the interaction of a compositional lock-exchange gravity current with a square cylinder at moderate Reynolds numbers using two-dimensional high-resolution simulations. Recent experimental work obtained measurements of the drag and lift for circular and rectangular cylinders. However, a detailed quantitative description of the flowfield was not given. Our objectives were to provide such description and, from it, explain the physical mechanisms behind the values and the time variation of the drag and lift coefficients. Such information is valuable for the design of submarine structures under the potential impact of gravity currents. Our numerical results agreed well with available experimental measurements. The interaction can be divided into impact, transient, and quasisteady-state stages. The shedding of vortices during the impact stage had an important effect on the drag and lift variation with time. For example, for a specific set of parameters, the suction created by these vortices produced about 40–50% of the drag. The maximum drag and the maximum lift amplitude occurred during the initial impact stage, and their minimum values were found when the square cylinder was closer to the bottom wall.
Stress Concentration Factors for a Drilling Riser Containing Corrosion Pits
Authors: Benjamin Adilson C. , Cunha Divino J. S. , Silva Rita C. C. , Guerreiro Joa˜o N. C. , Campello George C. , Roveri Francisco E.
Published: May 2009

The residual fatigue life of a corroded riser joint can be evaluated by means of a fatigue analysis based on S-N data. In this case nominal stresses are determined through a global riser analysis in which the drilling riser is modeled as a tensioned beam subjected to loads throughout its length and with boundary conditions at each end. The effect of the corrosion defects is taken into account multiplying the nominal stresses by stress concentration factors (SCFs) derived by local Finite Element (FE) analyses of the riser joints containing corrosion defects. In this paper stress concentration factors for a drilling riser containing corrosion pits are calculated using solid FE models. These pits are situated on the external surface of the riser joints. Three shapes of corrosion pits are considered: semi spherical, cylindrical wide and cylindrical narrow. Five depths of corrosion pits are considered: 12.6%, 20.1%, 30.2%, 40.3% and 50.3% of the riser wall thickness. The riser outside diameter and the riser wall thickness are 533.4 mm (21 in) and 15.9 mm (0.625 in), respectively.
Hybrid Riser Tower for Deepwater Drill Centres
Authors: Saint-Marcoux Jean-Francois , Blevins Robert D.
Published: May 2009

Hybrid Riser Towers, whether single bore or bundles minimise the riser load on Floating Processing Units and are field proven. Recent work has emphasised the effect of wake instability of parallel risers, and provided guidance for minimum distance between the risers. The paper describes the general architecture of a minimum Hybrid Riser Tower riser production system to be used for a drill centre, and the results of wake interference analysis for single bore HRTs and for a drill centre HRT’s in deepwater West Africa and GOM for turret-moored FPSO’s.
Virtual Model Basin for Resistance, Maneuvering, and Seakeeping: A RANS CFD Based Approach
Authors: Sasanapuri Balasubramanyam , Shirodkar Viraj Suresh , Wilson Wesley , Kadam Samir , Rhee Shin Hyung
Published: May 2009

A Virtual Model Basin (VMB) is developed based on a Computational Fluid Dynamics (CFD) approach to solving the Reynolds Averaged Navier-Stokes (RANS) equations along with the Volume of Fluid (VOF) method for predicting the free surface. The primary objective of this work is to develop methodologies for the VMB and to demonstrate the capabilities for a generic multi-hull ship geometry. The VMB is used to simulate various model basin tests for steady resistance, maneuvering and seakeeping. For a generic catamaran hull configuration, the methodologies are used for solving these problems and the results are discussed in this paper. VMB results are compared with the results of a benchmarked potential flow theory method for calm water resistance.
Experimental Investigation of Hydrodynamic Coefficients of a Wave-Piercing Tumblehome Hull Form
Authors: Bassler Christopher C. , Carneal Jason B. , Atsavapranee Paisan
Published: May 2009

A systematic series of calm-water forced roll model tests were performed over a range of forward speeds using an advanced tumblehome hull form (DTMB model #5613-1) to examine the mechanisms of roll damping. This experimental investigation is part of an ongoing effort to advance the capability to assess seakeeping, maneuvering, and dynamic stability characteristics of an advanced surface combatant. The experiment was performed to provide data for development and validation of a semi-empirical roll damping model for use in validation of ship motion and viscous flow simulation codes, as well as to provide a basis for future work with additional experiments, contributing to the development of an improved analytical roll damping model. Two hull configurations were tested: barehull with skeg, and bare hull with skeg and bilge keels. Measurements of forces and moments were obtained over a range of forward speeds, roll frequencies, and roll amplitudes. Stereo particle-image velocimetry (SPIV) measurments were also taken for both zero and forward speeds. Test data was used to calculate added mass/inertia and damping coefficients. Two different system modeling techniques were used. The first method modeled the system as an equivalent linearly-damped second-order harmonic oscillator with the time-varying total stiffness coefficient considered linear. The second technique used equivalent linear damping, including higher-order Fourier components, and a non-linear stiffness formulation. Results are shown, including plots of added inertia and damping coefficients as functions of roll frequency, roll amplitude, and forward speed and SPIV measurements. Trends from the experimental data are compared to results from traditional component roll damping formulations for conventional hull from geometries and differences are discussed.
Limitations of “Marching Algorithms” in the Analysis of Multiphase Flow in Pipelines
Authors: Ayala Luis F. , Alp Doruk
Published: May 2009

Marching algorithms are the rule rather than the exception in the determination of pressure distribution in long multiphase-flow pipes, both for the case of pipelines and wellbores. This type of computation protocol is the basis for most two-phase-flow software and it is presented by textbooks as the standard technique used in steady state two-phase analysis. Marching algorithms acknowledge the fact that the rate of change of common fluid flow parameters (such as pressure, temperature, and phase velocities) are not constant but vary along the pipe axis while performing the integration of the governing equations by dividing the entire length into small pipe segments. In this algorithm, governing equations are solved for small single sections of pipe at a time, and the calculated outlet conditions for the particular segment and then propagated to the next segment as its prescribed inlet condition. Calculation continues in a “marching” fashion until the entire length of the pipe has been integrated. In this work, several examples are shown where this procedure cannot longer accurately represent the physics of the flow. The implications related to the use of this common technique are studied, highlighting its lack of compliance with the actual physics of the flow for selected examples. This paper concludes by suggesting remedies to these problems, supported by results, showing considerable improvement in fulfilling the actual constrains imposed by the set of simultaneous fluid dynamic continuum equations governing the flow.