bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Investigation of Flow-Induced Vibrations Interference Between Two Circular Cylinders in Tandem Arrangements
Authors: A´ssi Gustavo R. S. , Meneghini Julio R. , Aranha Jose´ A. P. , Bearman Peter W. , Carmo Bruno S. , Casaprima Enrique
Published: November 2008

This paper presents experimental results concerning flow-induced oscillations of rigid-circular cylinders in tandem. Preliminary results are presented: new measurements on the dynamic response oscillations of an isolated cylinder and flow interference of two cylinders in tandem are shown. The oscillations are due to vortex-induced vibrations (VIV). Models are mounted on an elastic base fitted with flexor blades and instrumented with strain gages. The base is fixed on the test section of a water channel facility. The flexor blades possess a low damping characteristic [ζ ≈ 0.008 and less] and they are free to oscillate only in the cross-flow direction. The Reynolds number of the experiments is from 3,000 to 13,000 and reduced velocities, based on natural frequency in still water, range up to 12. The interference phenomenon on flow-induced vibrations can be investigated by conducting experiments in two ways: first, the upstream cylinder is maintained fixed and the downstream one is mounted on the elastic base; subsequently, an investigation will be carried out letting both cylinders oscillate transversally. The results for an isolated cylinder are in accordance with other measurements in the literature for m* ≈ 2 and m* ≈ 8. For the tandem arrangement (m* ≈ 2), the trailing cylinder oscillation presents what previous researchers have termed interference galloping behaviour for a centre-to-centre gap spacing ranging from 3·0D to 5·6D. These initial results validate the experimental set up and lead the way for future work; including tandem, staggered and side-by-side arrangements with the two cylinders free to move.
Validation of Numerical Motion Simulations by Direct Comparison With Time Series From Ship Model Tests in Deterministic Wave Sequences
Authors: Clauss Gu¨nther F. , Hennig Janou , Cramer Heike , Brink Kay-Enno
Published: November 2008

Ship safety under normal and severe weather conditions is determined by ship design, approval, and operation. Numerical simulation of ship motions has proven to be a valuable tool for ship design evaluation — not only for accident investigations and studies addressing fundamental stability related phenomena, but also during the design process. A sufficient number of simulations provide a data basis for polar plots to judge the ship’s situation in a sea state — defined by significant wave height and characteristic period — with respect to load case, encounter angle and ship speed. Application of data provided by numerical tools has to take into account the validity range of the model and has to be validated sufficiently by model test data. For providing useful validation data, the exact correlation of wave excitation and ship motion in model testing is indispensable. In the framework of the German research project SINSEE, this is achieved by a fully automated test procedure with a free running ship model in combination with deterministic generation of tailored realistic wave sequences and their transformation to the moving reference frame of the cruising ship. The resultant wave train can be directly correlated with time series of motions — registered by an optical system — and forces. In this paper, these methods are applied to investigate pre-simulated seakeeping scenarios in the model basin and compare the results directly to the simulation results.
Structural Mechanics Problems With Structures on a Two-Parameter Foundation Solved by DQEM
Authors: Chen Chang-New
Published: November 2008

The development of DQEM solution of structural problems with structures resting on a two-parameter foundation was carried out. The DQEM uses DQ or EDQ to discretize the governing differential equation defined on each element, the transition conditions defined on the inter-element boundary of two adjacent elements and the boundary conditions of the beam. Some EDQ models can be generated by DQ. They are DQ generated EDQ. Numerical results solved by the developed numerical algorithms are presented. The convergence of the developed DQEM analysis models is efficient.
Parametric Analysis of a Lazy-Wave Steel Riser
Authors: Roveri Francisco Edward , de Arruda Martins Clo´vis , Balena Rosianita
Published: November 2008

Lazy-wave steel risers appear as a possible solution for ultra deepwater oil fields in Campos Basin. The design of such a solution, however, is a very time consuming task as several configurations must be studied, including static, dynamic and fatigue analysis. In the first cycle of design, simplified models can be used to speed up the selection process of feasible configurations. This paper presents a parametric analysis that was implemented on a computer tool, aiming to select the feasible geometric configurations for a lazy-wave steel riser in the first cycle of design.
Experimental Results on the Combined 1st and 2nd Order Dynamics of a Small-Scale Taut-Leg Mooring Line
Authors: Simos Alexandre N. , Fujarra Andre´ L. C. , Palazzo Fa´bio G.
Published: November 2008

A set of experimental results on the dynamics of taut-leg mooring-lines was generated by means of towing-tank tests. These results will be employed by Petrobras as an experimental paradigm for the calibration and validation of numerical codes based on finite-elements method (FEM). The setup allowed combining first and second order motions on the top of the line with different amplitudes and frequencies. The first order motions were emulated by means of circular harmonic motions while alternate horizontal translations represented the drift motions. It was also possible to emulate a uniform in-plane current profile along the suspended length of the model. The model was composed by three different segments. An intermediate rubber hawser connected the top and anchor chain segments and allowed considerable elongation of the line during the tests. Tension at the top of the model was measured by means of a load cell and the second-order motion was registered optically. This paper presents the procedure adopted for the tests and also some preliminary comparisons between experimental results and numerical simulations.
Critical Analysis of Control and Filtering Algorithms Used in Real Dynamic Positioning Systems
Authors: Tannuri Eduardo A. , Morishita Helio M. , Veras Vinicius L. M. , Lago Glenan A.
Published: November 2008

Dynamic positioning systems (DPS) comprise the utilization of active propulsion to maintain the position and heading of a vessel. Sensors are used to measure the actual position of the floating body, and a control algorithm is responsible for the calculation of forces to be applied to each propeller, in order to counteract all environmental forces, including wind, waves and current loads. The controller cannot directly compensate motions in the sea waves frequency range, since they would require an enormous power to be attenuated, possibly causing damage to the propeller system. A filtering algorithm is then used to separate high frequency components from the low frequency ones, which are indeed controlled. Usual commercial systems apply Kalman filtering technique to perform such task, which includes a full model of the system. Furthermore, an adaptive on-line estimation algorithm is also used to evaluate the wave peak frequency, since the model in Kalman Filter depends on such parameter. The controller itself is based on a simple proportional-derivative (PD) actions. This paper presents all the mathematical formulation of the Kalman Filter, adaptive algorithm and the controller used in commercial DPS and performs a critical analysis of those models. Some illustrative results of a dynamic positioned shuttle vessel are presented, considering the incidence of waves, current and winds.
Component Approach for Confident Predictions of Deepwater CALM Buoy Coupled Motions: Part 1 — Philosophy
Authors: Huang Z. J. , Santala M. J. , Wang H. , Yung T. W. , Kan W. , Sandstrom R. E.
Published: November 2008

This paper describes a component approach of coupled motions for design of deepwater CALM offloading system in West Africa environment. Confident offloading buoy motions coupled with mooring and offloading line dynamics is identified as one of the key design challenges. In deepwater systems, components from the wave forces (exciting forces and radiated wave forces), viscous damping forces and mooring forces follow different scaling laws. We can not properly scale up the measured global responses of the coupled system to full-scale to verify the design. Component approaches overcome many of the test engineering and scale up weaknesses associated with truncated physical modeling. An effective application of a component approach develops a model test strategy for the purpose of validating the design analysis tools. In this paper we present a strategy for model testing, design tool validation and full-scale analyses. Differences between the component approach and the current industry practice are highlighted.
A Method for Fatigue Analysis of Pipelines on Topsides of FPSO Systems
Authors: Spanos Pol , Sofi Alba , Wang Juan , Peng Berry
Published: November 2008

Pipelines located on the decks of FPSO systems are exposed to damage due to sea waves induced random loading. In this context, a methodology for estimating the fatigue life of conveying-fluid pipelines is presented. The pipeline is subjected to a random support motion which simulates the effect of the FPSO heaving. The equation of motion of the fluid-carrying pipeline is derived by assuming small amplitude displacements, modeling the empty pipeline as a Bernoulli-Euler beam, and adopting the so-called “plug-flow” approximation for the fluid (Pai¨doussis, 1998). Random vibration analysis is carried out by the Galerkin method selecting as basis functions the natural modes of a beam with the same boundary conditions as the pipeline. The discretized equations of motion are used in conjunction with linear random vibration theory to compute the stress spectrum for a generic section of the pipeline. For this purpose, the power spectrum of the acceleration at the deck level is determined by using the Response Amplitude Operator of the FPSO hull. Finally, the computed stress spectrum is used to estimate the pipeline fatigue life employing an appropriate S-N fatigue curve of the material. An illustrative example concerning a pipeline simply-supported at both ends is included in the paper.
Modeling of Flow Around FPSO Hull Sections Subject to Roll Motions: Effects of Nonlinear Boundary Conditions
Authors: Vinayan Vimal , Kinnas Spyros A. , Yu Yi-Hsiang
Published: November 2008

The paper presents the development of BEM (Boundary Element Method) and FVM (Finite Volume Method) based models for the analysis of the flow around 2-D FPSO hull-sections fitted with bilge keels and subject to forced roll motions. Through these models an attempt is made to gain an insight into the two important aspects of the flow: separation around bilge keels and the effect of the free-surface. The effect of the free-surface and the resulting wave-body interaction is studied using a 2-D BEM model coupled with a Mixed-Eulerian-Lagrangian (MEL) time marching scheme for the free-surface boundary conditions. The separation around the bilge keels and viscous aspects of the flow are studied using a FVM based 2-D Navier Stokes (2DNS) solver with linear free-surface boundary conditions. The primary aim of the BEM model is to investigate the effects of the linear and nonlinear boundary conditions on the predicted flow within the scope of the parameters of the FVM model.
Dynamic and Frictional Interaction Between Buoyancy Can Riser Tensioner System and SPAR Hull
Authors: Li Yousun , Yun Heedo , Zhang Jane Q. , Aramanadka Shankar Bhat , Zhang Heping
Published: November 2008

Buoyancy Can Riser Tensioner (BCRT) systems provide tension to the Top Tensioned Riser (TTR) systems. The main benefit of employing BCRT is that it can minimize the interaction between the floating platform and riser system. The possible lateral impact and dynamic loads which may occur between the SPAR hull and BCRT are minimized by placing the so-called Compliant Guides (CGs) at the sliding interfaces. The CGs are designed to allow relatively free sliding of the BCRT in the riser axial direction, while preventing/minimizing lateral impact loads by providing lateral compliance. CGs are made of high stiffness and abrasion resistant material, such as elastomer materials. Objective of this study is to develop a numerical capability and analysis procedure to evaluate the spar-riser interaction mechanism and their effects on spar motion and riser response. The newly developed frictional interface element is an essential component of an FE model for the time simulation of the coupled motions of SPAR and riser systems. The algorithm can be used to investigate the dynamic and frictional interaction between the SPAR and BCRTs, such as the friction’s impact on the SPAR motion and riser stresses, and the dynamic load/wear requirements on the CGs.
A Deep-Ocean Mass Spectrometer to Monitor Hydrocarbon Seeps and Pipelines
Authors: McMurtry Gary M. , Wiltshire John C. , Bossuyt Arnaud
Published: November 2008

New developments in instrumentation for ocean environmental engineering are allowing unprecedented levels of trace contaminant measurement in the deep ocean. With funding from the U.S. National Science Foundation (NSF), our engineering design team constructed a new mass spectrometer-based in situ analysis system for work in the deep ocean environment over prolonged deployment periods. Our design goals were a depth capability of up to 4,000 m water depth (400 bars hydrostatic pressure) and autonomous operation for periods of up to six months to a year, depending upon the type of external battery system used or other deployment circumstances, e.g., availability of a power cable or fuel cell power source. We chose a membrane introduction mass spectrometry (MIMS) sampling approach, which allows for dissolved gases and volatile organics introduction into the mass spectrometer vacuum system. The MIMS approach and the hydrophobic, silicon-coated membrane chosen both draw upon our previous experience with this technology in the deep ocean. The membrane has been tested to 400 bars in a series of long-term hydrostatic pressure tests, which extend the 200-bar working depth rating of this membrane by a factor of 2. Long-term deployment capability of the moderately powered, approximately 100 W system, was accomplished by power management of the embedded computer system and custom electronics with Windows-based and custom software now fully-developed and bench tested. The entire system fits within a 6.5-inch outside diameter pressure housing that is approximately five feet long. It consists of a 1 to 200 amu range quadrupole mass spectrometer equipped with Faraday and electron multiplier detectors, compact turbo-molecular and backing diaphragm vacuum pumps, internal rechargeable batteries, and internal waste vacuum chamber. Sample routing past the MIMS is accomplished by computer-controlled solenoid valves. We designed the pressure housings of both 6AL4V and type 2 titanium alloys that are rated to working depths of >4,000 m and are essentially corrosion proof over long-term deployments. We designed and integrated a fail-safe valving system for both rapid response to high-pressure MIMS failure and a pressure-switch circuit and high-pressure solenoid valve to detect and protect against slow leaks of the MIMS. To route sample waters to the MIMS-based instrument, we also designed and built a rugged plastic plenum that couples to the face of the sampler head, the latter of which consists of the MIMS inlet and a full-ocean rated thermister temperature probe with an operational range from −5 to 50°C. These instrumentation innovations will be described in the paper.
On Hydrodynamic Interaction Between Several Freely Floating Vertical Cylinders in Waves
Authors: Ali Mir Tareque , Khalil Gazi Md.
Published: November 2008

This paper investigates the hydrodynamic interaction between several vertical cylinders freely floating in each other’s close vicinity. The physical aspect of hydrodynamic interaction is rather complicated and numerically accurate scheme is highly recommended to study this complex phenomenon. In the present study, 3D source-sink method has been adopted to determine the hydrodynamic coefficients and wave exciting forces by taking into account accurately the effect of hydrodynamic interaction among the different floating bodies. In order to check the convergence of numerical results, mean wetted surface of the floating cylinders are divided into different numbers of rectangular panels. A computer code is developed using 3D source-sink formulations and the validation of the computer code has been justified by comparing the present results with that of the published ones. In order to study the interaction effect, numerical results for a single cylinder have been compared with those of the multi-cylinder ones. The separation distance between the cylinders has been varied to examine the interaction effect for head sea condition. Finally some conclusions have been drawn on the basis of the present analysis.
Simulation of Flow Around Circular Cylinder Using a Collocation Spectral Method
Authors: Rizales Johnny J. M. , Esperanc¸a Paulo T. T. , Bueno Andre´ Belfort
Published: November 2008

The purpose of this paper is to develop a Fourier-Chebyshev collocation spectral method for computing unsteady two-dimensional viscous incompressible flow past a circular cylinder for low Reynolds numbers. The incompressible Navier-Stokes equations (INSE) are formulated in terms of the primitive variables, velocity and pressure. The incompressible Navier-Stokes equations in curvilinear coordinates are spectrally discretized and time integrated by a second-order mixed explicit/implicit time integration scheme. This scheme is a combination of the Crank-Nicolson scheme operating on the diffusive term and Adams-Bashforth scheme acting on the convective term. The projection method is used to split the solution of the INSE to the solution of two decoupled problems: the diffusion-convection equation (Burgers equation) to predict an intermediate velocity field and the Poisson equation for the pressure, it is used to correct the velocity field and satisfy the continuity equation. Finally, the numerical results obtained for the drag and lift coefficients around the circular cylinder are compared with results previously published.
Cost-Benefit Evaluation of Inspection and Repair Planning for Ship Structures Considering Corrosion Effects
Authors: Li Dianqing , Tang Wenyong , Zhang Shengkun
Published: November 2008

To ensure the safety and reliability of ship structures during their service lifetime, inspections are essential to evaluate corrosion or fatigue damage and schedule maintenance or repair. This paper aims to propose a cost-benefit model of risk based inspection and repair planning for ship structures subjected to corrosion deterioration. The models of expected costs and expected benefit are formulated. The benefit-cost ratio is adopted to select the optimal inspection and repair planning. The planning problem is formulated as an optimization problem where the total benefit-cost ratio in the expected lifetime is maximized with a constraint on the minimum acceptable reliability index. A component subjected to corrosion deterioration is presented to illustrate the proposed approach. In addition, sensitivity studies are presented to account for the effects of uncertainties in basic parameters on the benefit-cost ratio and the optimal inspection and repair planning. The results indicate that the proposed approach can effectively integrate the economy with reliability of the inspection and repair planning. Furthermore, a balance can be achieved between the risk cost and total expected inspection and repair costs, which is very effective in selecting the optimal inspection and repair planning. Parametric studies show that the benefit-cost ratio is sensitive to the time of inspection and repair, the reference period, the repair criterion and the failure criterion.
The Solution-Focused Design Process Organization Approach Applied to Offshore Platforms Design
Authors: Schachter Richard David , Fernandes Antonio Carlos , Bogosian Neto Sdepan , Jordani Carlos Gomes , de Castro Gustavo Adolfo V.
Published: November 2008

The objective of this work is to describe and discuss a design process organization approach named ‘Solution-Focused Design’. This method has been created and used successfully for AMV, Ship and Offshore Platform Design over the last ten years with reasonable advantages. It was first created in a context were the decision of what concept to be adopted supersedes the use of the classical design spiral, suggesting a combination of the spiral with morphological charts, since the design processes of the concurring concepts are completely different from each other. The advantages are in terms of allowing for the introduction of creative ideas into the Conceptual Design process, eventually leading to an innovative product or design solution, establishing a sound design sequence and rationalizing the search for design tools and knowledge, parameter and task organization and classification (free variables, restrictions, performance specifications) and the design process itself. In this work the Solution-Focused Design process is described and compared to some usual ones, which normally rely on ‘problem-focused’ strategies for problem solving. It is shown how the process evolves from a pre-established scenario and a design briefing or specification to a Design Methodology. This is done in brainstorming sessions, using sketches and interactive design flowcharts (similar to design spirals), adapted for this approach. The objective is to provide the designer with means to define quickly and efficiently the optimal configuration of the design, while incorporating novelties into it. In order to show some features of the approach, three different examples of designs previously developed are presented: one for a SPAR Buoy Platform, other for an FPSO, and another for the feasibility and concept of a ‘fully submersible semi-displacement fast boat’, with morphological comparisons of different concepts of means of support in the later.
FPSO: Tanks Optimization Design Approach
Authors: Vasconcellos Jose M. , Fernandes Antonio C. , Santos Alberto , Ferreira Marcos D. A. S.
Published: November 2008

Brazil Campos Basin is under large offshore oil production activity. Many new giant oil fields are under development. Brazil has been using tankers ship converted as FPSO platform vessel. At this moment three FPSO are in final construction phase to be installed in Campos Basin. Many oil cargo tankers are transformed to work as FPSO. Although they are showing good characteristics, some problems are having been emerging. The turret design approach was used in the primary FPSO’s but recently the spread mooring system with rigid risers is also under installation in Campos Basin. New ideas about Floating Production Storage and Offloading (FPSO) system have been in focus since some projects started working in an appropriately FPSO design direction. This paper spotlights one of these ideas: the FPSO tankers arrangement as a possible optimization procedure. MARPOL rule is followed but an alternative procedure is suggested to consider vessel control under wave’s action and at the same time looking for stress minimization. The aim in this procedure is increase the operator capability. The idea is allow the FPSO operator change the load condition using ballast in segregated tanks assuring a more suitable vessel condition in waves. Stability, wave response, stresses and operation ability should be also merit functions in any optimization process. A box shape FPSO-BR is used to highlight the procedure application. Results are presented and conclusion pointed out.
Proposal for Planning of Strategic Signs Inducing Actions for Vitalization of Coastal Cities
Authors: Yamamoto Kazukiyo , Tsukada Kenji , Kondo Takeo
Published: November 2008

At present, coastal cities in advanced countries are losing their vitality and their sprawl has been depriving these cities of the unique attraction provided by their coastal position with seaside spaces. For the vitalization of coastal cities, it is important to increase the visiting population to seaside spaces from other areas. While the rapid development of advanced countries in the modern era has been founded on the economic order originally established by the Industrial Revolution, the phenomenon of an aging society with fewer children which is currently in progress makes such economic foundations vulnerable, constituting a social problem which must be urgently dealt with. This study features the planning of signs as a technique for the development of a universal design in consideration of the imminent increase of the number of the elderly and disabled and as a measure to increase the visiting population to vitalize coastal cities while focusing on the planning of signs as a system to assist ocean recreational activities. The underlying concept is that these activities have the potential to secure or even increase the number of visitors to seaside spaces which have a variety of resources and characteristics. As the introduction of a new strategic concept for the planning of signs as a measure to increase the number of visitors is essential, the focus is placed on pictogrammes which are visual signs with a superior information conveyance function to other types of signs. The outcomes of this study presented here are (i) techniques to configure pictogrammes which are easy to understand by all and which are capable of inducing people’s desire for actions and (ii) concrete pictogramme designs.
Kinematics in a Diffracted Wave Field: Particle Image Velocimetry (PIV) and Numerical Models
Authors: Kristiansen Trygve , Baarholm Rolf , Ro̸rtveit Geir J. , Hansen Ernst W. , Stansberg Carl Trygve
Published: November 2008

As the use of CFD in industrial applications increases, so does the need for verification and validation of the theoretical/numerical results. This paper focuses on tools for validation and in particular, on the use of Particle Imaging Velocimetry (PIV) as such a tool. Diffraction of regular waves due to a single, fixed vertical cylinder is investigated. Theoretical results of wave run-up and wave kinematics are compared to measurements from model tests. Theoretical results are obtained by second order potential theory and by fully non-linear CFD computations. The second order potential theory frequency-domain results are computed by the industry standard code WAMIT, while the fully nonlinear time-domain simulations are performed by the commercial CFD code Flow-3D. Measurements are obtained by means of wave probes, PIV and snapshots taken with a high-speed camera. The experiments are made with the model in place as well as without the model, for validation of the incident flow field. For the identification of non-linear effects, the steepness of the waves is varied. The surface elevation is measured by means of the wave probes, while the PIV equipment measures the kinematics. High quality photos taken by the high-speed camera give a detailed overview of the surface elevation for inspection. In addition to focusing on validation tools, the paper also addresses some critical aspects associated with the CFD computations, such as the modeling of boundary conditions. The work is based partly upon results from the WaveLand JIP, Phase 2.
Free Standing Hybrid Riser for 1800 m Water Depth
Authors: Roveri Francisco E. , Pessoa Paulo Ricardo F.
Published: November 2008

Petrobras is considering the single-line FSHR (Free Standing Hybrid Riser) design as an option for 18-in export risers in 1800 m water depth. This paper provides the background on the studies carried out to achieve confidence that the concept is feasible for such application, and explains the main features of the design that take best advantage of local practices and Petrobras capabilities. The installation studies consider a Mobile Offshore Drilling Unit (MODU) for deployment of the FSHR system. The analyses assess the allowable sea states for such operation. Another design driver is the fatigue damage due to VIV (Vortex Induced Vibrations), which will also be addressed in the paper. Model tests were performed in order to support the conclusions of the numerical analyses that established the operation window for the FSHR system deployment.
Pipe Response Under Concentrated Lateral Loads and External Pressure
Authors: Karamanos Spyros A. , Eleftheriadis Charis
Published: November 2008

The present paper examines the denting deformation of offshore pipelines and tubular members (D/t≤50) subjected to lateral (transverse) quasi-static loading in the presence of uniform external pressure. Particular emphasis is given on pressure effects on the ultimate lateral load of tubes and on their energy absorption capacity. Pipe segments are modeled with shell finite elements, accounting for geometric and material nonlinearities, and give very good predictions compared with test data from non-pressurized pipes. Lateral loading between two rigid plates, a two-dimensional case, is examined first. Three-dimensional case, are also analyzed, where the load is applied either through a pair of opposite wedge-shaped denting tools or a single spherical denting tool. Load-deflection curves for different levels of external pressure are presented, which indicate that pressure has significant influence on pipe response and strength. Finally, simplified analytical models are proposed for the two-dimensional and three-dimensional load configurations, which yield closed-form expressions, compare fairly well with the finite element results and illustrate some important features of pipeline response in a clear and elegant manner.
Numerical Simulation on Mooring Performance of LNG-FPSO System in Realistic Seas
Authors: Inoue Yoshiyuki , Kamruzzaman Md.
Published: November 2008

The LNG-FPSO concept is receiving much attention in recent years, due to its active usage to exploit oil and gas resources. The FPSO offloads LNG to an LNG carrier that is located close to the FPSO, and during this transfer process two large vessels are in close proximity to each other for daylong periods of time. Due to the presence of neighboring vessel, the motion response of both the vessels will be affected significantly. Hydrodynamic interactions related to wave effects may result in unfavorable responses or the risk of collisions in a multi-body floating system. Not only the motion behavior but also the second order drift forces are influenced by the neighboring structures due to interactions of the waves among the structures. A study is made on the time domain analysis to assess the behavior and the operational capability of the FPSO system moored in the sea having an LNG carrier alongside under environmental conditions such as waves, wind and currents. This paper presents an analysis tool to predict the dynamic motion response and non-linear connecting and mooring forces on a parallel-connected LNG-FPSO system due to non-linear exciting forces of wave, wind and current. Simulation for the mooring performance is also investigated. The three-dimensional source-sink technique has been applied to obtain the radiation forces and the transfer function of wave exciting forces on floating multi-bodies. The hydrodynamic interaction effect between the FPSO and the LNG carrier is included to calculate the hydrodynamic forces. For the simulation of a random sea and also for the generation of time depended wind velocity, a fully probabilistic simulation technique has been applied. Wind and current loads are estimated according to OCIMF. The effects of variations in wave, wind and current loads and direction on the slowly varying oscillations of the LNG and FPSO are also investigated in this paper. Finally, some conclusions are drawn based on the numerical results obtained from the present time domain simulations.
Influence From Helical Strakes on Vortex Induced Vibrations and Static Deflection of Drilling Risers
Authors: Larsen Carl M. , Baarholm Gro Sagli , Lie Halvor
Published: November 2008

Helical strakes are known to reduce and even eliminate the oscillation amplitude of vortex induced vibrations (VIV). This reduction will increase fatigue life, and also reduce drag magnification from cross-flow vibrations. But sections with strakes will also have a larger drag coefficient than the bare riser. Hence, the extension of a section with strakes along a riser should be large enough to reduce oscillations, but not too long in order to limit drag forces from current and waves. The optimum length and position for a given riser will therefore vary with current profile. Dynamic response from waves should also be taken into account. The purpose of the present paper is to illustrate the influence from strakes on VIV, as well as on static and dynamic response for a drilling riser. Hydrodynamic coefficients for a cylinder with helical strakes are found from experiments and applied in an empirical model for the analysis of VIV. The result from the VIV analysis is used for a second calculation of drag forces that are applied in an updated static analysis. Dynamic stresses from regular waves are also presented, but VIV are not considered for these cases. A simple study of length and position of the section with strakes is carried out for some standard current profiles. Results are presented in terms of oscillation amplitudes, fatigue damage, bending stresses and riser angles at ends. The study is based on test data for one particular strake geometry, but the analysis method as such is general, and the computer programs used in the study can easily apply other test data.
Marine Growth Colonisation Process and Profile in Guinea Gulf: From Inspection Data to Load Computing
Authors: Boukinda Morgan , Quiniou-Ramus Valerie , Schoefs Franck , Birades Michel , Lahaille Alain , Garretta Raymond
Published: November 2008

Reassessment of existing offshore structures needs rational aid tools to update new information (metocean data, new regulations, ...). This paper focuses on marine growth management on Jacket offshore structures. After years of exploitation, marine growth thickness reaches up to the initial beam diameter. Inspection and cleaning are actions to be optimized, keeping in mind that safety must be preserved according to cost reduction. The paper first presents the building of an exhaustive data base in view to use the whole available information. An illustration is given for structures placed in Gulf of Guinea: main species, colonization process as well as modeling of thickness increasing are discussed. A modeling of hydrodynamic coefficient is then suggested according to kinematics field and physical response surface.
CFD Simulations of the Vortex-Induced Vibrations of Model Riser Pipes
Authors: Willden Richard H. J. , Graham J. Michael R.
Published: November 2008

The paper reports results from two strip theory CFD investigations of the Vortex-Induced Vibrations of model riser pipes. The first investigation is concerned with the vibrations of a vertical riser pipe that was subjected to a stepped current profile. An axial spatial resolution study was conducted to determine the number of simulation planes required to achieve tolerably converged numerical solutions. It was found that six to seven simulation planes are required per half-wavelength of pipe vibration in order to obtain convergence. The second investigation is concerned with the simultaneous in-plane and out-of-plane vibrations of a model Steel Catenary Riser that was subjected to a uniform current profile. The pipe’s simulated vibrations were found to agree very well with those determined experimentally. This result was achieved despite the questionable usage of simulation planes at high angles to the flow direction.
Hydrodynamic Aspects of the Torpedo Anchor Installation
Authors: Fernandes Antonio Carlos , dos Santos Melquisedec Francisco , de Araujo Jairo Bastos , Lima de Almeida Jose´ Carlos , Diniz Roge´rio , Matos Vinicius
Published: November 2008

The torpedo anchor is a novel kind of device to moor floating offshore structures. It has been proved in practice that this kind of anchoring may be used for both drilling and production offshore activities. For drilling, it is indeed easily recoverable and for large production, it has enough holding power even for large production platforms. There are a lot of soil-interaction aspects to be considered and the installation is one of them. The installation procedure is to release the torpedo from a high enough position from the sea bottom to allow the device to reach the terminal velocity: A correct amount of kinetic energy at the bottom is essential for the penetration. Besides this, the anchor has to reach the bottom in a vertically up right in order to maximize the final holding power in all directions. Therefore, the work addresses two hydrodynamic aspects for the installation design and analysis. The first is the drag minimization and the second is the directional stability. If the drag is be kept to a minimum (without compromising, later on, the soil interaction) then the terminal velocity is higher. The work shows that parameters like the mass and the shape are essential for this. On the other hand, the shape and mass distribution have a strong influence on the directional stability. One important parameter is the rear line length connected to the anchor, which is necessary for further connection with the final mooring line: this parameter influences both the terminal velocity and the directional stability. The presence of the rear line and its role is a novel problem and it seems to have no parallel in other filed applications. The work addresses all this aspects under the light of a novel model testing performed in a model basin that is 15 m deep. It is important to say that this model testing procedure has been conceived to attend specifically the torpedo anchor evaluation. For that matter, the work presents an extrapolating mathematical model. Besides that, an analytical model is shown for the directional stability, together with time domain numerical evaluation. Different model have been used in the tests performed with and without the rear line. Finally, the work presents the model testing design including the use of imaging processing to get the anchor tracking during the launching.
VIV of a Composite Riser at Moderate Reynolds Number Using CFD
Authors: Rakshit T. , Atluri S. , Dalton C.
Published: November 2008

The vibratory response of a long slender riser, made of composite materials and subject to an ocean current, is studied for a range of conditions. The influence of the number of modes of vibration is studied as is the influence of the mass ratio and the value of the damping coefficient. The flow past the riser is represented by a shear flow, ranging from Re = 8000 at the lower end of the riser to Re = 10,000 at the upper end of the riser. The riser vibration is treated as a coupled fluid-flow/vibration problem. The fluid-flow equations are represented by a Large Eddy Simulation model for the wake turbulence present in the flow. Strip theory is used to represent different forcing locations along the length of the riser. Since the composite riser has a variable damping coefficient, which decreases with increasing frequency, its response is different from, say, a steel riser with a constant damping coefficient. The composite riser, with variable damping, has a larger RMS displacement than a riser with constant damping. The vibration amplitude is found to increase with an increase in number of modes.
Validation of Wave Propagation in Numerical Wave Tanks
Authors: Bunnik Tim , Huijsmans Rene´
Published: November 2008

During the last few years there has been a strong growth in the availability and capabilities of numerical wave tanks. In order to assess the accuracy of such methods, a validation study was carried out. The study focuses on two types of numerical wave tanks: 1. A numerical wave tank based a non-linear potential flow algorithm. 2. A numerical wave tank based on a Volume of Fluid algorithm. The first algorithm uses a structured grid with triangular elements and a surface tracking technique. The second algorithm uses a structured, Cartesian grid and a surface capturing technique. Validation material is available by means of waves measured at multiple locations in two different model test basins. The first method is capable of generating waves up to the break limit. Wave absorption is therefore modeled by means of a numerical beach and not by mean of the parabolic beach that is used in the model basin. The second method is capable of modeling wave breaking. Therefore, the parabolic beach in the model test basin can be modeled and has also been included. Energy dissipation therefore takes place according to physics which are more related to the situation in the model test basin. Three types of waves are generated in the model test basin and in the numerical wave tanks. All these waves are generated on basin scale. The following waves are considered: 1. A scaled 100-year North-Sea wave (Hs = 0.24 meters, Tp = 2.0 seconds) in deep water (5 meters). 2. A scaled operational wave (Hs = 0.086 meters, Tp = 1.69 seconds) at intermediate water depth (0.86 meters) generated by a flap-type wave generator. 3. A scaled operational wave (Hs = 0.046 meters, Tp = 1.2 seconds) in shallow water (0.35 meters) generated by a piston-type wave generator. The waves are generated by means of a flap or piston-type wave generator. The motions of the wave generator in the simulations (either rotational or translational) are identical to the motions in the model test basin. Furthermore, in the simulations with intermediate water depth, the non-flat contour of the basin bottom (ramp) is accurately modeled. A comparison is made between the measured and computed wave elevation at several locations in the basin. The comparison focuses on: 1. Reflection characteristics of the model test basin and the numerical wave tanks. 2. The accuracy in the prediction of steep waves. 3. Second order effects like set-down in intermediate and shallow water depth. Furthermore, a convergence study is presented to check the grid independence of the wave tank predictions.
Numerical Study of Oscillatory Flow Past a Pair of Cylinders in a Side-by-Side Arrangement
Authors: Anagnostopoulos P. , Koutras A. , Seitanis S. A.
Published: November 2008

The results of a numerical study of the viscous oscillating flow around a pair of circular cylinders are presented herein, for a constant frequency parameter, β, equal to 50, and Keulegan-Carpenter numbers, KC, ranging between 0.2 and 10. The cylinders were placed side-by-side to the oncoming flow, for a pitch to diameter ratio, P/D, equal to 2. The finite-element method was employed for the solution of the Navier-Stokes equations, in the formulation where the stream function and the vorticity are the field variables. The vorticity contours generated from the solution were used mainly for the flow visualization, whereas the stream-lines and isobars are shown in some cases. At low values of the Keulegan-Carpenter number the flow remains symmetrical with respect to the horizontal axis of symmetry of the solution domain. As the Keulegan-Carpenter number is increased asymmetries appear in the flow, which are eventually amplified and lead finally to more complicated vortex-shedding patterns. These asymmetries generate an aperiodic flow configuration at consecutive cycles, which becomes almost chaotic as KC grows larger. For the various Keulegan-Carpenter numbers examined the time-histories of the hydrodynamic forces are presented, and the r.m.s. values of the hydrodynamic forces and the coefficients of the in-line force were evaluated.
Global Loads on an FPSO Induced by a Set of Freak Waves
Authors: Fonseca Nuno , Guedes Soares Carlos , Pascoal Ricardo
Published: November 2008

The paper presents a systematic study of the structural loads induced by abnormal waves on a FPSO. This work is a follow-up of a previous investigation that explored the possibility of using freak, abnormal or episodic waves as additional wave load conditions to be considered in the design of ships and offshore platforms. In the previous work, a procedure was developed and implemented to adopt deterministic time series of wave elevation, which may include abnormal waves, as reference design conditions to calculate the wave induced structural loads on ships. An application example was presented for a FPSO subjected to the well known New Year Wave trace that was measured during a severe storm in Central North Sea. In the present paper, the same procedure is applied to obtain the wave induced structural loads on a FPSO, but a systematic investigation is carried out by using a large set of wave traces. These wave traces have been measured at different occasions in the North Sea, additionally at one location in the Central Gulf of Mexico, and they all include episodic freak waves. In this way it is possible to assess the influence of realistic rogue wave characteristics on the wave induced structural loads. Finally, and based on the platform responses to all wave traces, some statistics are produced, regarding the platform responses and structural loads induced by rogue waves.
Fatigue Analysis and Risk Based Inspection Planning for Life Extension of Fixed Offshore Platforms
Authors: Faber Michael Havbro , Straub Daniel , Chakrabarti Partha , Abu-Odeh Ibrahim , de la O. Rami´rez Juan de Dios
Published: November 2008

The present paper reports on the application of Risk Based Inspection (RBI) planning as conducted on a recent project for Pemex Exploracio´n y Produccio´n. The project considers the engineering assessments required for a possible service life extension of 23 fixed steel jacket structures located in the Bay of Campeche. In the project RBI plays an important role as this quantifies the relation between the overall performances of the platforms in their present condition with their future deterioration and facilitates the identification of strategies for cost efficient condition control by means of targeted inspection and maintenance activities and if necessary repair and strengthening. For the purpose of facilitating efficient RBI for larger number of structures of more or less similar characteristics, a generic scheme for RBI is pursued. The specific aspects of the generic RBI for the considered group of structures in regard to risk acceptance criteria, modeling of inspection qualities and the probabilistic fatigue and fracture mechanical modeling are outlined. Finally also a study is reported whereby the benefit of the RBI approach is assessed as a means of risk control in comparison to standard code based requirement for inspection planning.