bluuebunny/crossref_metadata_2025 huggingface dataset:
Unsteady Force Measurements on a Responding Circular Cylinder in the Wake of an Upstream Cylinder
Authors: Assi Gustavo R. S. , Bearman Peter W. , Meneghini Julio R.
Published: May 2009

This paper presents force measurements during flow-induced vibration of a pair of circular cylinders with low mass ratio (m* = 2.0) and low damping (ζ = 0.7%) aligned in a tandem arrangement. A particular case with a gap of 3 diameters centre to centre is used to examine flow-interference mechanisms occurring on a downstream cylinder, free to oscillate only in the transverse direction. The Reynolds number varies within the range 1500 < Re < 20000. A cylinder immersed in the wake of another can develop flow-induced oscillations persisting for a large range of reduced velocities. Oscillations are observed for reduced velocities, based on cylinder natural frequency measured in air, as high as 35. Apparently, the amplitude of oscillation is reaching a level of saturation of about 1.5 diameters, while the frequency of vibration is increasing at an approximate constant rate. As reduced velocity is increased two regimes of flow-induced vibration are observed: first vortex-induced vibration and then a wake-induced vibration regime. In addition, the presence of the second cylinder affects the dynamics of the upstream wake, but it is found not to synchronize the vortex shedding frequency of the upstream cylinder for the second regime of oscillations.
Estimation of Ultimate Wave Height Due to Strength Criteria for Vessels During Onetime Trip
Authors: Egorov Gennadiy V. , Kozlyakov Vitaly V.
Published: May 2009

Vessels that works on shelf in constrained conditions and in shallow water very often are of restricted navigation area vessels type. Operation area and place of building for such vessels may be situated on significant distance from each other so onetime trip outside of allowable by Classification Society sailing regions. In this paper method of definition of minimum allowable value of 3% probability wave height due to overall and local strength criteria for hulls of restricted navigation area vessels (RNAV) is substantiate. This method allows to carry out onetime voyage projects for sailing outside of allowable sailing regions.
Low Frequency Wave Forces and Wave Induced Motions of a FPSO in Shallow Water
Authors: Xiao Longfei , Yang Jianmin , Hu Zhiqiang
Published: May 2009

The low frequency (LF) response of a soft yoke moored 160kDWT FPSO in shallow water is investigated by conducting frequency domain computations and wave basin model tests. An incident wave with Hs = 4.1m and Tp = 8.9s is applied. An obvious LF part appears in the measured wave spectrum at water depth of 16.7m. As a result, the 1st order LF wave force exists and is much larger than the 2nd one. The difference of the spectrums is about one hundred times. The LF wave drift force increases enormously. Consequently, much larger resonant surge response is induced. The LF surge amplitude at h = 16.7m is about 7 times the one at h = 29.0m and 9 times the one in deep water, although the 2nd order response changes a little. Therefore, in very shallow water, LF part of incident waves should be taken into account carefully and LF wave forces and wave induced motions will be very serious.
Development and Design Verification of a Floating Tidal Power Unit
Authors: Berstad Are Johan , Tronstad Harald
Published: May 2009

This paper presents how the design verification process for a tidal current power plant was carried out, and points out the challenges needed to be addressed in the technical assessment phase of the concept. The rules and regulations found suitable for the design process are presented. The paper focus mainly on what analysis is required to validate the capacity of the structure. The structure is exposed to a harsh offshore environment. The relative importance of the different loads and load effects is shown and discussed. The need for hydroelastic coupled analysis is demonstrated. The work shows that interactions between the main components are of large importance. There are interaction effects between moorings and structure as well as between turbine blades and structure. In fact the fully integrated system needed to be analysed as an integrated system, hence the need for coupled analysis. Although the structure was designed to derive energy from tidal currents, wave induced forces showed as important as currents for the structural integrity of the unit. The analysis demonstrated the versatility of the used FE analysis program AquaSim. This program is developed specifically to carry coupled analysis with hydroelastic interactions between structure and fluid. The FE program is in wide use within the Aquaculture industry. It is also in wide use in the offshore industry including moored systems and towed seismic equipment.
Damping Properties of Sandwich Metal-Polymer-Ceramic Coating
Authors: Kireitseu M. V. , Bochkareva L.
Published: May 2009

The focus in this paper is directed toward to the investigation into hybrid metal-polymer-ceramic damping coatings for pipelines and fuel tanks. Recently sandwiched coatings have been manufactured by advanced technology based on thermal flame spraying of foamy aluminum or its alloy and viscoelastic polymeric layers (ultra high molecular weight polyethylene). Technological aspects of manufacturing are discussed for tailoring safety and reliability of pipeline structure affected by external shock impact and vibrations. The particle-reinforced coatings is further formed as a constrained-layer damping coating system (CLD) showing the greatest range over which damping coatings have excellent energy absorbing/damping properties over bulk materials. Modal loss factors of clamped specimen obtained at 60–80% coverage were higher than maximum damping values obtained in the case of 100% specimen coverage. FEM-based computer modeling was used to predict the damping of multilayer materials by a strain energy method.
Conceptual Design for Utilizing Buoyancy Can to Support Spar Drilling Riser
Authors: Xu Lixin , Stanton Paul
Published: May 2009

Platform drilling has been effectively employed for several Spar applications in the Gulf of Mexico (GOM). As the drilling day rates surge in the recent market, this option becomes more attractive and has been increasingly considered by operators. The Spar drilling riser has been traditionally supported by a hydraulic tensioner. On the other hand, buoyancy can riser tensioner (BCRT) systems have been successfully used for top tensioned production risers on Spars in deepwater oil and gas developments in the GOM. The authors herein propose a conceptual design for using a BCRT system to support the Spar drilling riser in possible practical applications. The paper presents system configuration and general arrangement of the BCRT system supported drilling riser for application in 5,000 ft of water in the GOM. The maximum pressure rating of the drilling riser is assumed 10,000 psi. The BCRT system will provide required top tension, as well as redundancy capacity, required for various drilling operations. Design requirements are carefully considered for this previously-unused concept. Furthermore, as needed, the drilling riser BCRT system can be converted for supporting a production riser after all drilling operations are completed.
Expected Shape of Extreme Waves in Storm Seas
Authors: Tayfun M. Aziz , Fedele Francesco
Published: May 2009

The theoretical expected structure of large nonlinear waves can be described, using the Gram-Charlier approximations of Jensen et al. (1995) and Jensen (1996, 2005), and the quasi-deterministic model of Fedele & Arena (2005). The second-order narrow-band approximation offers a simpler alternative to these models, as recently suggested in Tayfun & Fedele (2006). Herein, this latter alternative is elaborated further, deriving theoretical expressions for predicting the expected shape of large waves, conditional on somewhat more general constraints than those previously considered. The theoretical results are verified favorably with oceanic measurements gathered at deep and transitional water depths in the North Sea. Some comparisons of the present model with those of Jensen et al. (1995) and Fedele & Arena (2005) are also given, showing that all three models do in fact reasonably well in representing the expected profile of large waves in storm seas.
FPSO Transient Responses During Cyclonic Storms
Authors: Curic Dusan , Luo Yong
Published: May 2009

This paper presents the key results and conclusions of the study of FPSO transient responses in the cyclonic storm. The measured wind, wave and current data of recent cyclonic events are utilized to simulate the FPSO responses in terms of mooring loads, vessel yaw motion and relative FPSO heading to waves as it weathervanes in the wind, waves and current, input as time series. The primary objectives are to check the FPSO responses as the cyclone (eye or fringe) passes over it, causing rapid changes in the intensity and the direction of environmental loads, and to confirm the adequacy of the present mooring system design criteria. The results of the study serve as a good benchmark of the current industry standard for mooring design and address industry’s concern of the safety of FPSO platforms in the event of cyclonic storm. This study has used the hindcast data to inspect the event of a strong cyclonic storm passing over an FPSO. Despite the fact that the wind direction changes for about 140° in only one hour in the path of the cyclone eye, higher mooring line tension has not been observed due to reduced wind speed in the eye of the storm. The extreme mooring line tension is still governed by the responses in the path of cyclone fringe due to its maximum wind speed. Note that the transient analysis has shown that, during the rapid change of wind direction, the vessel can potentially be exposed to beam sea waves. Although this does not correspond with the highest tension in mooring legs, it can lead to critical green water impact.
Theoretical Analysis and Experimental Study of Dynamic Hydrocyclones
Authors: Zhao Lixin , Li Feng , Ma Zhanzhao , Hu Yanqing
Published: May 2009

Characteristics of dynamic hydrocyclonic separator are simply introduced. The advantages of dynamic hydrocyclones used as separators, such as wider applicable flowrate range and smaller cut size, etc., are analyzed compared with normal-used static hydrocyclones. By analyzing tangential and axial velocity field distributions inside hydrocyclones, the reason why dynamic hydrocyclone has higher efficiency than static one is further described. The laboratory experiments and field tests of dynamic hydrocyclones were carried out. Relationships of flowrate, outer shell rotation speed, and split ratio with pressure were studied. Pressure and pressure drop inside hydrocyclones were measured and analyzed. The effect of main operating parameters, such as split ratio and rotation speed, on hydrocyclonic separation performance was also studied. It is shown that the rise of split ratio is beneficial to enhancing separation efficiency, but the split ratio must be controlled in an appropriate range, so as to obtain satisfied separation result. The increase of rotation speed is helpful for the forming of oil core inside the hydrocyclone, but the resonating phenomenon should be avoided when using dynamic hydrocyclones. Field tests indicated satisfied results as anticipated.
Characteristic Study of Bi-Cone Solid-Liquid Separation Hydrocyclones Based on Computational Fluid Dynamics Analysis
Authors: Zhao Lixin , Zhu Baojun , Hu Yanqing , Ma Zhanzhao
Published: May 2009

Numerical simulation of the bi-cone solid-liquid separation hydrocyclones based on Reynolds Stress Model (RSM) is carried out by Computational Fluid Dynamics (CFD) analysis. Re-circulation flow and short-circuit flow phenomena are indicated by simulating results, and distribution rule of axial velocity, radial velocity and tangential velocity of the flow field inside hydrocyclone are obtained. Some of the simulation results are qualitatively in accordance with the Laser Doppler Anemometer (LDA) measuring data, which proves the correctness of turbulence model and computational method. Moreover, the wall erosion problem of the hydrocyclones is also studied. The positions of erosion, which include tangential inlets, boundary of cylinder and large cone section, and the boundary of large cone section and fine cone section, are analyzed and recognized, based on Discrete Phase Model (DPM). Erosion of the revised hydrocyclone is obviously improved and the separation efficiency is enhanced by CFD analysis. At the same time, characteristic of pressure was analyzed.
Simulation Based Estimation of Extreme Response of Floating Offshore Structures
Authors: Naess A. , Gaidai O. , Teigen P. S.
Published: May 2009

The paper presents a study of the extreme response statistics of a tension leg platform (TLP) subjected to random seas. Two different approaches are compared: A numerical integration method based on saddle point integration and the Monte Carlo method. While the saddle point method is a mathematically attractive technique, which gives numerically very accurate results at low computational costs at any response level, the advantage of the Monte Carlo method is its simplicity and versatility. It is demonstrated in this paper that the commonly assumed obstacle against using the Monte Carlo method for estimating extreme responses, i.e. excessive CPU time, can be circumvented, bringing the computation time down to affordable levels. The agreement between the two approaches is shown to be remarkably good.
Coupled Algebraic Multigrid for Free Surface Flow Simulations
Authors: Zwart Philip J. , Burns Alan D. , Galpin Paul F.
Published: May 2009

An accurate, efficient algorithm for solving free surface flows with ANSYS CFX is described. Accuracy is achieved using a compressive advection discretization which maintains a sharp free surface interface representation without relying on a small timestep. Efficiency is obtained using a solution algorithm which implicitly couples velocity, pressure, and volume fractions in the same matrix, and solves these equations using algebraic multigrid. This coupled strategy overcomes difficulties encountered with segregated volume fraction algorithms, where heavy underrelaxation and long solution times are required. The resulting solution algorithm is scalable, leading to solution times which increase linearly with mesh size.
Pull-Down Analysis of Jack-Up Rigs
Authors: Chakrabarti Partha , Joshi Sanjay P. , Maiti Manoj K.
Published: May 2009

During demobilization of jack-up rigs, when the rig’s legs are stuck in the seabed due to resistance from the surrounding soil, the hull is lowered more than the neutrally buoyant condition to draw the legs up. This operation of pulling down the hull to provide net upward buoyancy force to extricate the legs is called ‘pull-down’ operation. The hull, being partly underwater, attracts considerable wave forces even during relatively calm weather. Due to the changing support condition at bottom of the leg, while it is being extricated, the natural period of the system changes continuously. The dynamic amplification may be high when the wave periods are close to the natural periods of the structure. This makes it imperative to consider dynamic analyses of the rig. A unique but simplified ‘pull-down’ analysis procedure is developed in this study considering the harmonic wave forces, added mass of the hull in water, boundary condition of the legs in soil, and distributed buoyancy springs under the hull. Wave excitation loads and added mass for the hull are computed using diffraction-radiation analysis of the hull in water. A number of steady state dynamic analyses of the complete jack-up rig structural model have been performed for a range of wave periods, water depths, and drafts of the hull. Three different bottom boundary condition scenarios have been considered — three legs supported, one leg free but two legs still stuck in soil, and two legs free but one leg still stuck. The static and the dynamic load cases are combined to get the maximum effects on the stresses of the leg members. Using this procedure the allowable safe wave heights are predicted for a range of wave periods for a particular water depth, and draft of the hull. Results are presented for one class of jack up rig. The results show that the leg stresses are strongly dependent on the wave periods, indicating the importance of including dynamic effects in the pull-down analysis.
Numerical Study on the Hydrodynamic Behavior of a New Cell-Truss Spar Platform
Authors: Zhang Fan , Yang Jian-Min , Li Run-Pei , Chen Gang
Published: May 2009

The spar platform concept develops quickly in the oil and gas exploitations in offshore areas, especially in deep and ultra-deep water. From the first generation of classic spar, the spar concept has developed into the second generation of truss spar and the third generation of cell spar, and many new spar concepts are being put forward by the ocean structure design companies and research institutes. The purpose of those designs mainly focuses on the reduction of fabrication difficulties and cost, while satisfying the requirement of exploitation at meantime. This paper presents a numerical study on the hydrodynamic behavior of a cell-truss spar platform. Features of truss spar and cell spar were both taken into account in the new spar concept, aiming to take advantage of the heave plate damping feature of the truss spar to obtain satisfactory heave motion performance, while reduce manufacture and installation difficulties with cell concept. The whole spar concept with risers and mooring system were modeled by means of numerical simulation, and fully time-domain coupled analysis was conducted to investigate hydrodynamics of the spar platform both in operating and survival conditions.
Hydraulic Performances of Current Generation System in the New Deepwater Offshore Basin
Authors: Lu Haining , Yang Jianmin , Peng Tao
Published: May 2009

The state-of-the-art deepwater offshore basin is presently under construction at Shanghai Jiao Tong University. Its current generation system is considered as one of the most challenging and crucial equipments in the basin. Hydraulic performances of the current generation system in the new deepwater offshore basin are investigated using both CFD and experimental methods. 3D numerical models are applied and calculated. Besides a 1:10 scaled model test is also carried out to validate the numerical simulation. Uniformities of the flow field in the basin, including horizontal and vertical current profiles with various mean values are studied in detail. In addition turbulence levels are also analyzed. Satisfactory agreements can be achieved between numerical calculation and model test. It is concluded that hydraulic performances of current generation system in the new basin can satisfy the requirements of model tests in deepwater. Uniform and constant current field can be obtained in measuring area. Besides various typical vertical current profiles in deep water, including uniform profiles and sheared profiles etc. can be modeled. And suggestions are also presented which will be helpful instructions for the design and operation of the new deepwater offshore basin.
Experimental Investigation on Dynamic Response of Submarine Pipeline Over Flat Beds in Waves
Authors: Sha Yong , Wang Yongxue , Pearson Lee M.
Published: May 2009

Model tests have been conducted on flexible submarine pipelines over flat beds in both regular and irregular waves and the second order effect induced by waves are concerned within the certain range of Keulegan-Carpenter numbers and Reynolds numbers. The tests were conducted in the wave flume with 55m in length, 4m in width and 2.5m in depth. The pipelines were made by flexible pipe with 60mm in diameter and placed at various distances from a flat bed. Gap to diameter ratio varies from 0.4 to 0.8 when the pipelines are not sagging. The wave period in the model tests is in the range from 0.8 to 2.0 and water depth is 0.4m. The Keulegan-Carpenter numbers are less than 8.0 and the Reynolds numbers are in the subcritical regime. Bending strain was measured by strain gauges bonded on the inner surface of the pipeline. The strain amplitudes and second order effect are analyzed and discussed against various Keulegan-Carpenter numbers and gaps between the pipeline and the flat bed.
Longitudinal Forces and Bending Moments of a FPSO
Authors: Clauss Gu¨nther F. , Kauffeldt Andre´ , Jacobsen Katja
Published: May 2009

For the design of FPSOs the vertical bending moment is a key parameter to ensure safe operation. If analyzed at water line level, however, the unknown influence of longitudinal forces may distort the results. Hence, a segmented FPSO model with midship force transducers at two levels is investigated in various deterministic wave sequences to identify the vertical bending moment and its associated neutral axis as well as the superimposing longitudinal forces. It is shown that the neutral axis is far below the water line level, with the consequence, that extreme cyclic loads at deck level would be expected. However, as the associated longitudinal forces — even if significant — generate a counteracting moment, this effect is largely compensated. Both, frequency- and time-domain results are presented. With frequency-domain analysis the profound data for the standard assessment of structures, concerning seakeeping behaviour, operational limitations and fatigue are obtained. In addition, time-domain analysis in real rogue waves gives indispensable data on extremes, i.e. motions and structural forces.
Hydrodynamic Optimization of an Ocean-Going Oil Recovery System in Harsh Seas
Authors: Clauss Gu¨nther , Abu Amro Mazen
Published: May 2009

Environmental consequences due to oil spills are disastrous — like the Prestige catastrophe in 2002. Therefore oil combating systems must be developed, which are capable of operating in rough seas. This paper presents the design and optimization of a Seaway independent Oil Skimming system — SOS. The principle of the system is to separate the wave damping from the skimming process without using moving parts. The system consists of a solid pontoon with a bow segment (inverse ramp) and an integrated collection tank (moon pool). The skimmer glides over the oil polluted water surface, smoothing the waves. Subsequently, the oil flows beneath the bow segment to an inlet slit, where a separation blade guides the oil film into the moon pool. Inside the moon pool, a vortex develops behind the bow segment, separating the oil from the main flow and transporting it to the free surface of the moon pool, where it is pumped off. Computational Fluid Dynamics (CFD) calculations and model tests at different scales (1:9, 1:15 and 1:22) with and without oil layer have been conducted. Based on results at different speeds and random seas, the geometry of the separation blade and the moon pool are optimized. Oil tests yield an efficiency of the skimming process up to 90% in random seas and up to 95% in calm water. The assessment of the system includes tests with the skimmer integrated into the carrier vessel MPOSS (Multi Purpose Oil Skimming-System).
Application of CFD Methods to the Analysis of the Flow in Air-Lift Pump
Authors: Pougatch Konstantin , Salcudean Martha
Published: May 2009

In order to improve the understanding of the air-lift process a Computational Fluid Dynamics (CFD) model has been developed. A multi-fluid Eulerian approach is used in the modeling. Three independent interpenetrating phases are considered: a continuous liquid phase (water) and two discrete phases (air bubbles and solid particles). The simulations are conducted two-dimensionally. After the transient computational results have been obtained for a sufficiently long time period, averaging is done to obtain mean values of the flow rates for all the phases. The model has been verified by comparison with published experimental results for the air-lift pipes in the range of 300–450 m in height. Good agreement between experiment and computations has been obtained. The solution process proves to be sufficiently robust for a fairly wide range of particle sizes, densities, flow rates, pipe and injection depths, and inlet volume fractions. Flow analysis reveals the distribution of velocities and volume fractions for all phases inside the airlift pipe that improves our understanding of the process. The computational results highlight some interesting transient behavior of the multiphase flow in the pipe. The numerical model can be utilized during various stages of the design of the air-lift pumps to help answer fundamental questions on the process, and during their operation to select optimal process parameters and to address possible problems.
Reeling of Tight Fit Pipe (TFP)
Authors: Focke E. S. , Karjadi E. , Gresnigt A. M. , Meek J. , Nakasugi H.
Published: May 2009

A 12.75 inch outer diameter single walled pipe bending test was executed and theoretical and FE analysis of this test was performed as preparation for 12.75 inch outer diameter TFP bending tests. The main objective of the TFP bending tests was to determine the initiation and degree of liner wrinkling occurring during the TFP spooling-on phase when simulating the reeling pipelay installation method. Due to lack of a definition of liner wrinkling initiation, the crossing of a certain threshold of the liner wrinkle height was defined as liner wrinkling initiation. The bending tests results indicated that (1) the extent of liner wrinkling decreased if TFP with a high mechanical bonding strength was used. (2) The presence of a circumferential weld in the highly bonded TFPs initiated higher liner wrinkles at lower curvatures than in case no circumferential weld was present. (3) The ERW outer pipe longitudinal weld did not result in higher liner wrinkles. API residual compressive stress tests showed that the initial mechanical bonding strength in the 12.75 inch TFP used in this research was significantly reduced, irrespective of whether a high or a low initial mechanical bonding strength had been used prior to spooling-on. These findings justify further research into this phenomenon as the eventual mechanical bonding strength after reeling installation may be vital for its anticipated application during operation.
FPSO Conceptual Design System Tools Considering Hurricane Data Base and Production Requirements
Authors: Vazquez-Hernandez Alberto Omar
Published: May 2009

The conceptual design is an important stage of the engineering process of any floating production system. This design stage takes into account the specific site conditions and production requirements to find a feasible technical alternative of the system characteristics, from which in the next engineering stage the structure is improved. Among the basic characteristics of a floating production system are the hull main dimensions, which have to meet the production and stability requirements, as well as the mooring system characteristic. The mooring system definition mainly depends on the water depth, hull dimensions and the environmental site conditions. The environmental conditions are different from site to site, where in some sites around the world harsh conditions like hurricanes or typhoons may occur. Therefore, in the dimensioning of the mooring system such environmental conditions have to be considered. In the case of Gulf of Mexico, hurricane conditiones have presented a risk to the oil and gas existing facilities as it is evident on the severe damages and failures they have provoked in some cases. The present study aims to consider a computational tool that takes into account in the conceptual design stage a data base of hurricane conditions in Gulf of Mexico, as well as the production and site requirements so that to carry out the basic sizing of hull and mooring systems. The tool also considers a data base considering different hull sizes (wave force coefficients) and mooring materials to estimate the environmental loads and the mooring system design.
Methodology for Calculating Irregular Wave Stress Time Histories of Tensile Wires in Flexible Risers
Authors: Doynov Krassimir , Nilsen-Aas Christoffer , Haakonsen Rune , Kan Wan , Bjærum Robert
Published: May 2009

Flexible risers are being deployed in more and more demanding applications in terms of water depth, remote locations, temperature, pressure and corrosive fluids. Focus has been put on long term riser integrity in general, and on fatigue performance in particular, as knowledge of pipe behavior and properties has been advanced over the last decade. In this context, accurate and consistent estimation of riser global and local response to external loading is essential. A methodology has been developed to efficiently calculate irregular wave stress time histories of tensile armour wires for flexible risers. The stress time histories are calculated directly from the global loads which are usually generated by using commercially available well proven global analysis tools. The methodology elevates the dynamic analysis of flexible risers from the conventional regular-wave approach to irregular-wave time-domain approach. This in turn allows a better assessment of the fatigue performance and provides a better fit-for-service assessment or an opportunity to reduce design conservatism. This methodology also allows for consistent stochastic fatigue evaluations to be performed in time domain simulations using the well established stochastic analysis approach. All flexible riser non-linear hysteretic effects are included and phase shift between tension and curvature is also fully accounted for. The key ingredient lies in the generation of transfer functions of all stress components using a validated local analysis (LA) tool based on finite element method. This is done because direct use of the LA tool for long time domain simulations is very computationally intensive and impractical. The stress transfer functions allow direct mapping of the tension and curvature readings to individual stress components, which are combined in a phase consistent manner to obtain the total stress-time histories. This methodology should also work well for other systems having complicated cross sections such as dynamic umbilicals and integrated production bundle, etc. Accuracy of the proposed methodology should be equivalent to that of using the LA tool directly provided that the stress transfer functions are constructed appropriately. In comparison with the traditional regular-wave methodology, this irregular wave approach has been shown to provide a significant fatigue-life improvement for the flexible riser tensile-wire in a deep water West Africa application.
Interference Between Risers
Authors: Franciss Ricardo , Fujarra Andre´
Published: May 2009

This article shows the results of the tests of interference between rigid risers, in relation of Vortex Induced Vibration (VIV), made in the Institute de Pesquisas Tecnolo´gicas do Estado de Sa˜o Paulo (IPT), Brazil. It was tested several conditions with different arrangements with two cylinders in tandem and side by side positions, with different distances between them. The models were installed in an elastic base with two degrees of freedom for each cylinder. The stiffness and the natural frequencies were calibrated to have the maximum amplitude of VIV within the possible range of velocities in the IPT towing tank. The final lift and drag coefficients were measured, for one cylinder with and without strakes and for two cylinders. All these data are used in Riser Analyses giving more real results in relation of VIV analysis, clashing and interference between risers.
Development of a Battered Four-Column TLP Concept
Authors: Heidari Homayoun , Williams Neil , Large Sean , Rangaraju Nagaraju , Byrne Jim
Published: May 2009

This paper discusses the development of the Atlantia’s new battered four-column TLP, called the FourStar™. As a result of battering the columns, the footprint of the new TLP is larger than a conventional vertical-column TLP, for a particular deck size. This results in several advantages, namely: • Increased stability, enabling quayside hull–topsides integration, wet tow of the TLP system, and self-supported tendon lock-off. • Better hydrodynamic behavior and smaller size tendons. • Improved pontoon access for SCR installation. The FourStar TLP consists of four columns of (rounded) rectangular cross-section, aligned along the diagonals of a square and battered towards the geometric center of the platform. The columns are connected by conventional rectangular pontoons, which aspect-ratio’s are optimized to improve the hydrodynamic performance. The basenode (column-pontoon connection) is structurally optimized for a ‘hot-spot-free’ stress distribution. Furthermore, the hull is designed for effective load transfer through the outer shell, which minimizes the steel weight. The tendon porches are located on the outer face of the base node, which maximizes their effective arm in the critical design case. The application of this concept to various payloads, including wet- and dry-tree production platforms, and various environmental conditions, including GOM, West Africa, and offshore Brazil is studied. The emphasize of this paper is on the hydrodynamic and structural aspects of the concept. Moreover, the results from an extensive series of recent wave basin model tests are discussed. Finally, the advantages of this new concept over a vertical column design are quantified for a typical GOM application, and its scalability is discussed.
Numerical Modelling of Floating Breakwaters and Shape Optimization Using a Nonlinear Programming Method
Authors: Elchahal Ghassan , Younes Rafic , Lafon Pascal
Published: May 2009

In this paper, shape optimization is addressed through sequential quadratic programming (SQP). The recent increase in information technologies dedicated to optimal design, associated with the progress of the numerical tools, allows significant improvement in the design optimization of floating breakwaters. First of all, the physical and mechanical constraints, related to the environment of floating breakwaters, are expressed in terms of the geometrical dimensions of the latter in form of mathematical expressions. Then, the optimization procedure is developed based on SQP method and satisfactory results are obtained demonstrating the capability of this work.
Model Test Verification of a Cell Truss Spar Using Hybrid Model Testing Technique
Authors: Su Yihua , Yang Jianmin , Xiao Longfei , Chen Gang
Published: May 2009

Hybrid model testing technique is a prospective method for experiments on deepwater platforms. Full-depth mooring/riser system should be truncated according to model scale and available work depth of the basin, and model test results need to be calibrated and extrapolated by numerical software. A model test of Cell Truss SPAR based on the method is presented in this paper. Vertical and horizontal truncation factors are defined respectively to illustrate the difficulties in designing the equivalent truncated mooring system. A special mechanism which can eliminate the effect of horizontal truncation is used in the truncated mooring line. The model test results are reconstructed and extrapolated to full depth. It is found that the numerical and experimental results agree well.
Physical Simulation of the Hull-Tether Coupled Dynamics of a Deepwater TLP
Authors: Jayalekshmi R. , Idichandy V. G. , Sundaravadivelu R.
Published: May 2009

The behavior of TLP merits rigorous numerical and experimental studies. Understanding the coupled dynamics is crucial towards establishing the safety of the structure on survival conditions in one hand and optimization of structure geometry for superior performance on the other. The hull-tether coupled dynamics that occur in very deep water cannot be accurately studied through model tests due to depth limitations of the existing model testing facilities and possible scale effects (in the case of ultra-small scale models). In this paper an experimental methodology is presented for physically simulating the coupled behavior of a single column TLP in a wave basin having dimensions 30m × 30m × 3m. Combination of model tests and numerical analysis were carried out. The experimental results are presented for hull motions, tether displacements and dynamic tether tension. The tether profiles indicate hull-tether coupling vividly. The results establish the success of the experimental methodology and importance of coupled behavior.
Materials Selection for Sandwich Pipes Under the Combined Effect of Pressure, Bending and Temperature
Authors: de Souza Allan R. , Netto Theodoro A. , Pasqualino Ilson P.
Published: May 2009

Recent researches point to the great potential of the sandwich pipe conception for ultra deepwater exploitation and production of oil and natural gas. Its configuration is very simple and comprises two concentric metallic pipes with a core material, polymeric or ceramic, in the annulus. The main functions of the annular layer are: to provide satisfactory thermal insulation so as to avoid the formation of wax and hydrates along the pipeline during production shutdown; to improve the overall structural strength of the system. Polypropylene and cement have been recently proposed for these applications. The reason for the choice of these materials was the low cost and the extensive availability in industry. Here a systematic material selection approach is employed in order to assess the applicability of other polymeric materials. The attributes of materials needed to meet the design specification are thoroughly studied. The list of possible materials was enlarged and the modified digital logic approach is used with the purpose to define a top group of materials for further numerical comparative study. Finite element analyses are carried out to assess the structural strength of the sandwich pipe under pure external pressure or longitudinal bending and combined external pressure and bending. Additionally, the effect of thermal gradient is included to the numerical analyses to evaluate each pre-selected material of the top group. Results indicate that other potential materials such as PEEK and polycarbonate can improve the structural performance of the sandwich pipe conception and yet meet other several design criteria.
Construction and Optimization of the Anti-Vibration Mega-Frame Platforms
Authors: Zhao Dong , Cai Dongmei , Ma Rujian , Chen Zhonghe , Qu Zhaofu , Wang Jungang
Published: May 2009

A new 16-leg mega-frame platform (MFP) was constructed based on an actual jacket platform (W12–1) using the mega-frame theory. Two control platforms, traditional jacket platform and jacket platform with upright column, were also built based on principle that all the platforms had the equal or similar weight, the same upper deck structure and the same function. Every column of the MFP had the most equal load and the smallest stress under the same horizontal and vertical unequal loads. So the MFP had the best loads bearing ability of all the three types’ platforms above. In order to find out the effect of the structure parameters’ change to the platforms’ static performance, many types of MFPs were constructed by changing the number, the height and the stiffness of the major beams and/or by changing the stiffness and the angle of the major columns on the basic model MFP-313707. The analyses of all types of MFPs with different parameters under the same horizontal and vertical unequal loads show that: 1) The increase of the major beams’ height makes the platforms’ columns bear more equally than the increase of the major beams’ number; 2) The increase of major columns’ stiffness has better effect on the platform’s maximal deformation decrease and the columns’ loads equality. And the effect of the increase of the major beams’ and columns’ stiffness on the MFP’s static capability has the optimal values. Beyond the optimal values, the increase of the platforms’ weight caused by the increase of the stiffness is much bigger than the decrease of the platform’s deformation and the increase of the columns’ loads’ equality. The analyses also show that the slope angle of the platforms’ columns must be set around 10°.
How to Increase the Capacity of an Existing Sea Water Intake
Authors: Mazaheri Said , Sabouri Shargh Zeynab
Published: May 2009

Sea Water Intakes supplying water for desalination, cooling systems, or other parts of many important industrial components, such as refineries, and power plants are usually built in the shore lines and close to the target systems. In some conditions, such as expansion of the refinery, or other plants and in order to avoid building a new sea water intake because of lack of the suitable land or expending a lot of money, it is required to enhance the amount of withdrawing water and increase the capacity of the existing sea water intake. In such conditions, several important factors such as, minimum required submergence depth for the pumps, maximum allowed current velocity at the entry of the suction chambers, maximum allowed current velocity inside the intake and near the filters, and the flow pattern should be checked. In this paper, it is tried to describe these factors and restrictions. In addition, a case study sea water intake located in South Pars Gas Field at the northern shore line of the Persian Gulf in the province of Boushehr, Iran, is considered and the mentioned factors and restrictions for increasing the capacity of sea water intake from 25,000 to 35,000 is discussed. Besides, a hydraulic mathematical model has been used to check the flow line of the developed system. At the end the results are discussed.
Numerical Study on the Wind Drag Stress in Storm Surge
Authors: Kong Jun , Song Zhiyao
Published: May 2009

In estuary and coastal areas the storm surge will usually occur in typhoon season. When simulating the storm surge by numerical model, the wind speed at the height of 10m above the mean sea level will be usually used. Determining the wind drag stress on free surface reasonably plays an important role to simulate the storm surge accurately. In the past numerical models, the wind drag stress on free surface was calculated only considering the relative wind speed. There are many formulas about wind stress can be used, whereas these formulas are usually got in laboratory, where the water surface fluctuates in a small range, and the water elevation itself has not been considered in formula. Actually in some place, the astronomical tidal range is large, such as Yangtze estuary and Hangzhou bay in China, and during typhoon season, the water fluctuation range is much larger than ever there. In conventional method the wind stress will be underestimate in flood tide and be over-valuated in ebb tide without considering the fluctuation of water, so it is obviously unsuitable to take no account of the influence of tidal level on wind stress. Therefore in the intensive tide coastal area, the relationship of the relative wind speed, tidal level should be considered together. A new kind of wind stress formula has been established in this paper and been adopted in simulating the storm surge of typhoon Winnie in Yangtze estuary, and the results are better and satisfying.
CFD Truss Spar Hull Benchmarking Study
Authors: Oakley Owen H. , Constantinides Yiannis
Published: May 2009

Blind comparisons of computational fluid dynamics (CFD) predictions are made against large scale experiments of a truss spar hull. The overall objective of the work is to try and mature CFD modeling capability related to motion and loads for offshore platforms. Specific goals include the testing of newly developed meshing techniques and seeking an understanding of how spar appurtenances interact. The key comparisons are illustrated in comparisons of the simple case of a spar hull with stakes alone against a second, far more complex case, involving multiple appurtenances. The latter exhibits very different vortex induced motion (VIM) behavior. The CFD predictions were made with knowledge of the as-built model details, but without access to the experimental results. The estimates were found to be in close agreement with the experiments or slightly conservative. We continue to observe that the better the model fidelity, the better will be the comparison with the benchmark. The computations show that high quality predictions are now feasible, given access to sufficient computational hardware and accurate meshing of the body in question.
Mooring Line Damping Estimation by a Simplified Dynamic Model
Authors: Lie Halvor , Gao Zhen , Moan Torgeir
Published: May 2009

When predicting slowly varying resonant vessel motions, a realistic estimate of the motion damping is crucial. Mooring line damping, which is mainly induced by the drag force on line, can dominate the total damping of catenary moored systems and methods for predicting mooring line damping are therefore required. Based on a simplified dynamic model of mooring line tension, an approach to estimate the corresponding damping is presented in this paper. Short-term time domain simulations of dynamic line tension are carried out to verify the accuracy of the simplified frequency domain approach. Compared with the simulation results, the practical simplified method proposed herein gives a maximum 30% lower prediction of the damping coefficient of each mooring line and an about 20% smaller estimate of the total line damping and therefore yields conservative estimates of the low frequency vessel motions.
Identifying the Power-In Region for Vortex-Induced Vibrations of Long Flexible Cylinders
Authors: Swithenbank Susan B. , Vandiver J. Kim
Published: May 2009

The primary objective of this research is to locate the source of the vortex-induced vibrations (VIV) for long flexible cylinders at high-mode number and to help determine the source region for future predictions. The two Gulf Stream tests were conducted to collect data on a scale-model pipe that was excited at high-mode numbers. The high density of the sensors on the pipe allowed for analysis that had not previously been done. Two methodologies are presented to locate the area of the region that is the source of the vibration. In VIV, the current which causes the vibrations is important, because the speed of the current will determine the frequency of the vibration. Therefore, one important question is which section of the pipe will be the source of the vibrations for a known current profile. This source region is known as the power-in region. Regions on the pipe that are not a source of power instead damp the structural vibrations. Once the region where the vibration originated has been found, the different phenomena that effect the location of the power-in region that were discovered are shown. Four different factors are presented that effect the locations of the power-in region: the angle of the pipe with respect to the vertical, the gradient of the current direction, the current profile, and the end effects at high mode number. A dimensionless parameter is presented which help in the prediction of VIV given a current profile. The power-in factor predicts the region where the source of the vibration occurs using a combination of the current velocity and the source region length.