bluuebunny/crossref_metadata_2025 huggingface dataset:
Methodology for Time Domain Fatigue Life Assessment of Risers and Umbilicals
Authors: Steinkjer Oddrun , So̸dahl Nils , Gryto̸yr Guttorm
Published: January 2011

Risers and umbilicals are exposed to dynamic loading from waves and floater motions. These structures are known to have a pronounced non-linear response characteristic. Non-linear time-domain finite element analyses is in general required to give an adequate description of the non-linearities involved. Analyses of a large number of short-term environmental conditions considering stochastic wave loading are required to give a representative description of the long-term fatigue loading on the structure. Short term fatigue damage is established by means of rain-flow cycle (RFC) counting in each stationary short-term condition. It is has been experienced that significant statistical uncertainties can be present in the short-term fatigue damage estimates. This is because the accumulated fatigue damage in a stationary condition normally has significant contribution from the largest stress cycles in the realisation. Selection of proper simulation length is hence essential to obtain reliable fatigue life estimates. Applicable codes and standards for risers and umbilicals provide Design Fatigue Factors (DFF) to secure adequate safety against failure due to wave induced fatigue. The total uncertainty in the calculated fatigue damage comes from various sources and the DFFs in e.g. DNV-OS-F201 “Dynamic Risers” and API-RP-2RD corresponds to a certain uncertainty level in the fatigue damage estimate. A recommended target value for the statistical uncertainty of the fatigue damage estimates is given with basis in these design codes. The objective of this paper is to give a description of a methodology recommended for time domain fatigue assessment. Special focus will be on the importance of adequate simulation time for predicting the short-term fatigue damage and selection of the short-term seastates in the scatter diagram. Statistical uncertainty is one source that the analyst actually can influence by selecting proper analysis methodology. A statistical uncertainty meassure can be used to evaluate the robustness in the estimated fatigue life. Assessment of statistical uncertainty in fatigue damage estimate is demonstrated by case studies. The fatigue assessment methodology discussed in this paper, will be described in an update of DNV-RP-F204 “Riser Fatigue” 2010.
Research and Design of Ballast System for a Science Icebreaker
Authors: Yong Yang , Jie Ma
Published: January 2011

With the process of industry productions, the polar region which is the last maiden land for human being on the earth is confronted to the environment pollution increasingly. In accordance with the development of green shipbuilding technologies in the world, the ballast system of a science icebreaker should be researched and designed particularly because the International Maritime Organization (IMO) has developed international legislation, the International Convention for the Control and Management of ships’ ballast water and sediments, to regulate discharges of ballast water and reduce the risk of introducing non-native species from ships’ ballast water. Not only the scientific research icebreaker is ensured to work normally in the polar region, but also the weak ocean environment is protected.
Control of Unstable Ship Motions Using Anti-Rolling Tanks: Erosion of Safe Basins
Authors: Neves Marcelo A. S. , Rodri´guez Claudio A. , Merino Jorge A. , Vivanco Jerver E. M. , Rosales Jose´ C. Villago´mez , Agarwal Radhdesh
Published: January 2011

The objective of the paper is to apply modern numerical techniques of nonlinear dynamics to the problem of control of the roll motion employing U-shaped anti rolling tanks (ART). Parametric rolling in head seas is the focus of the paper. A transom stern small vessel, well known for her tendency to develop strong parametric excitation is investigated. Nonlinear equations are employed to describe the liquid motion inside the tank, the forces and moments generated by the tank on the ship and the coupled ship motions (heave, roll and pitch). These are numerically solved for different initial conditions. An analysis of the dynamical behavior of the vessel with stabilization is presented in the form of numerical limits of stability, safe basins, integrity curves and integrity surfaces. Finally, curves of critical amplitude for different wave tunings are computed. A design procedure for quantitative assessment of the level of parametric rolling mitigation by means of ART’s is discussed.
Numerical Prediction and Suppression of VIV of Deepwater Riser
Authors: Nozawa Toshiki , Suzuki Hideyuki , Ohta Makoto
Published: January 2011

Deepwater risers and long pipe structures are used for offshore oil development and CO2 dilution into ocean concerning mitigation of global warming. Risers are installed in increasingly deeper water and sometimes catenary shape is used utilizing the relative flexibility. For example, riser designs such as Steel Catenary Riser (SCR), Compliant Vertical Access Riser (CVAR), Lazy Wave Steel Catenary Riser, etc. are investigated and put into practice. Vortex Induced Vibration (VIV) is one of the key technical issues which needs to be examined to maintain the integrity of the riser system. In this paper, development of a time domain VIV simulation scheme LINE3D_VIV is reported, and using the scheme fatigue damage of the structure is evaluated and the energy balance of riser is visualized to understand the behavior of VIV from the viewpoint of energy-balance. The analysis method is basically a nonlinear Finite Element Method (FEM) of underwater line structure. The VIV hydrodynamic force is calculated at each time step consulting database of VIV hydrodynamic force. VIV hydrodynamic force on a real scale pipe section subjected to harmonic oscillation was measure at high Reynolds number and arranged into the database. A numerical filter which evaluates amplitude, frequency and phase of local response of a riser from the time history of pipe vibration was developed. The database is consulted with the parameters and instantaneous VIV force is evaluated. Moreover, the validity of the simulation method was tested by comparison with the experiments carried out in Holland in 2005. In this paper, hydrodynamic force database for riser section with fairing was also constructed and numerical analysis to evaluate the effect of fairing was carried out.
A Holistic Design Approach for Considering Rogue Buckle Formation Due to Pipelay-Induced Out-of-Straightness
Authors: Rathbone Andrew , Cumming Gary
Published: January 2011

As oil and gas reserves become more hostile and more remote, the associated flowline mechanical design faces increasing challenges. The trend is that longer, larger diameter, hotter, and lighter (with increased insulation requirements, and minimal stability issues in deep water) flowlines are required. Global buckling is therefore an increasingly prevalent part of the design process of modern field developments. In order to ensure a robust global buckling design, all potential buckle triggers must be considered. A challenge for this is that imperfections introduced by pipelay can not be known until installation is complete. A common approach is therefore to consider high quality survey data from previous projects to characterise the severity of pipelay features for future projects. This paper considers the effect of pipelay catenary length, pipe-soil interaction, pipeline bending stiffness and vessel motion on the as-laid horizontal imperfections, and consequently details the interrelationship between the causes and effects of the imperfection. Further, there is an interrelationship between the severity of the feature, its resistance to buckling and the level of strain in the post-buckled state. An integrated approach is therefore advocated, to obtain a robust design range of imperfection sizes in a deterministic fashion.
Numerical Analyses of Scaling Effects and the Flow Field Inside an Innovative Sea State Independent Oil Skimming System: SOS
Authors: Clauss Gu¨nther F. , Sprenger Florian , Kosleck Sascha
Published: January 2011

Due to improved regulations and high safety standards, the quantity of oil spilled in tanker accidents is notably decreasing over the last years. Despite this progress, the factor of human failure and hence the eventuality of an oil spill with catastrophic ecological and economical consequences can never be excluded. Current oil recovery systems are able to operate in wave heights up to 1.5 m. In severe weather they have to wait until conditions are improving. To prevent emulsification and weathering processes, it is necessary to skim the oil film off the sea surface shortly after the accident. This can only be achieved by an oil recovery system with high transit velocities on the one side, and the capability of operating in rough seas on the other side. A Sea state-independent Oil Skimming System (SOS) that satisfies these requirements has been developed and gradually optimized in various numerical and experimental analyses. The skimming process was already successfully validated in compliance with Froude’s law, but scaling effects were observed. In order to understand the complex flow phenomena around and through the SOS, numerical analyses based on a RANSE/VOF (Reynold-Averaged Navier-Stokes Equations / Volume Of Fluid) approach are conducted. Here, in addition to Froude’s and Morton’s law, viscous effects and the physical characteristics of oil and water are also scaled correctly according to Reynolds’ law. The results of these investigations present essential information for the transfer of the entire oil skimming process to full scale. This paper presents a further step of development towards a marketable system (see Fig. 1). The latest innovations — which are already applied for a patent — comprise a hermetically closed moon pool with pressure regulation and a hatch system with separate inlet and outlet flaps. In addition to the improved hull strength, this new design enables better individual control of the oil skimming process — hence the efficiency in dependency of environmental conditions — by pressure control of the water fluid level inside the moon pool.
Overview of Deepwater and Ultra-Deepwater Spar Risers
Authors: Yiu F. , Stanton P. , Burke R.
Published: January 2011

For more than a decade, Spar Top Tension Risers (TTRs) and Steel Catenary Risers (SCRs) have established a good track record through technology innovations. This paper describes how Spar TTR and SCR riser configurations have evolved to meet increasing industry demands and discusses the future of these riser systems. TTRs on the first Spar, Neptune, installed in the GOM in 1996, were supported by buoyancy cans. The next several Spar risers also used buoyancy cans with various improvements and modifications to the buoyancy can system design and installation method. In 2003, BP’s Holstein Spar was the first to use hydro-pneumatic tensioners to support its TTRs. The Kikeh and Perdido Spars also used tensioners. Optimization of TTRs is continuing with new construction methods such as the use of threaded and coupled (T&C) connectors instead of weld-on threaded connectors. Spar SCRs have also received widespread acceptance for deep and ultra-deepwater oil and gas production in recent years. The SCR has the advantages of relative low cost, conceptual simplicity, ease of fabrication and offshore installation. SCR hang-off options on a Spar include porch and pull tube. The pull tube option provides the most efficient technical solution for installation, brings the SCR interface above the water, which facilitates the piping hookup, and is better suited for the Spar’s architecture and transportation method. SCRs supported from a Spar have good fatigue performance in the touchdown region due to the relatively low motions of the Spar. As exploration and production activities move into deep and ultra-deepwater, and the metocean data increases in severity, Spar risers face additional design and analysis challenges. Adoption of high strength materials and strain-based design to meet these challenges is discussed.
Numerical Simulation of a Farm of Vertical Axis Marine Current Turbines
Authors: Goude Anders , A˚gren Olov
Published: January 2011

For commercial applications of marine current turbines, it can be useful to build several turbines close to each other in a farm, similar to wind turbine parks. To create a good farm configuration, the turbines’ mutual interaction needs to be studied. Here, to obtain detailed information, several turbines were simulated together using a 2D vortex method. To limit the computational cost, the vortex method was combined with known profile section data for the blades. First, a single turbine was compared against two turbines in close proximity. The two turbines were tested both with equal and opposite rotational direction, and the two blade pitch angles 0 and 3 degrees were tested. For both a single turbine and the two turbine case, a 3 degree pitch angle gave higher power coefficients than 0 degrees. The differences between 3 and 0 degrees were more significant for the single turbine. In all cases, the two turbine system had higher power coefficient per turbine than the single turbine. A five turbine park was simulated with three different combinations, one with all turbines on a row, and two with a zigzag pattern, where the difference was that the last simulation had larger turbines than the other two. For 0 degrees incident flow angle, the turbines on the row obtained the highest power coefficient, while the larger turbines in zigzag pattern obtained higher total power. The case with the turbines on the row was most insensitive to changes in flow direction, and for a 30 degree change, the row produced the highest total power as well. By locating the turbines inside a channel, all turbines obtained higher power coefficients, and the increase was largest for the large turbines, which blocked the channel to a larger extent.
Wave Run-Up and Air Gap Prediction for a Large-Volume Semi-Submersible Platform
Authors: Matsumoto Fabio Tadao , de Andrade Watai Rafael , Simos Alexandre Nicolaos , Ferreira Marcos Donato A. S.
Published: January 2011

This paper addresses the problem of estimating the air gap for a large semi-submersible production platform. Although it has a great impact on the design of the floating unit, many times the minimum deck height is still defined from simplified methods that incorporate relatively large safety margins. The reason for this is the intrinsic complexity of the associated hydrodynamic problem. Nonlinear effects on the incoming and scattered waves are usually relevant and sometimes non-linear effects on the motions of the floating hull may also play an important role. This discussion is illustrated by means of wave basin tests performed with the model of a large semi-submersible designed to operate in Campos Basin. Significant run-up effects on its squared-section columns were observed for the steepest waves in several design conditions. Also, the unit presented relatively large low-frequency motions in heave, roll and pitch, which also affected the dynamic air gap measurements. In order to evaluate the difficulties involved in modeling such phenomena, simplified tests were also performed with the model fixed and moored in regular waves of varying steepness. Wave elevation in different points was measured in these tests and compared to the predictions obtained from two different numerical methods: a BEM code that incorporates 2nd order diffraction effects (WAMIT) and a VOF CFD code (ComFLOW), the latter employed for fixed model tests only. Results show that a standard linear analysis may lead to significant errors concerning the air gap evaluation. Extending the BEM model to 2nd order clearly improve the results as the wave-steepness increases. Although the VOF analysis is considerably time-consuming, simulations presented very good agreement to the experimental results, even for the steepest waves tested.
USP Active Absorption Wave Basin: From Conception to Commissioning
Authors: de Mello Pedro Cardozo , Carneiro Mario Luis , Tannuri Eduardo Aoun , Nishimoto Kazuo
Published: January 2011

This paper presents the new active absorption wave basin constructed at the University of Sa˜o Paulo (USP), in the Numerical Offshore Tank (TPN) Laboratory. The square (14m × 14m) tank is able to generate and absorb waves from 0.5Hz to 2.0Hz, by means of 148 active flap-type wavemakers. An independent mechanical system drives each flap by means of a 1HP servo-motor and a ball-screw based transmission system. A customized ultrasonic wave probe is installed in each flap, and is responsible for the measurement of wave elevation in the flap. These sensors do not require constant calibration, differently from the capacitive or resistive sensors normally used in similar tanks. A complex automation architecture was implemented, with 3 Programmable Logic Computers (PLC), and a low-level software is responsible for all the interlocks and maintenance functions of the tank. Furthermore, all the control algorithms for the generation and absorption are implemented using higher level software (MATLAB®/Simulink block diagrams). These algorithms calculate the motions of the wavemakers both to generate and absorb the required wave field by taking into account the layout of the flaps and the limits of wave generation. The experimental transfer function that relates the flap motion to the generated wave is used for the calculation of the motion of each flap. Absorption tests were conducted with a prototype wave generator in a 2D wave flume with regular waves. Two different algorithms were tested. The first one is the frequency domain method based on Maeda et al. (2004), in which the commanded variable is the motor velocity. Furthermore, the time domain algorithm proposed by Schaffer (1996) was also tested. It is based on a digital filter and uses the position of the motor as the commanded variable. Both algorithms have hydrodynamic feedback based on the measurement of surface elevation at each flap. The first algorithm needs an extensive test procedure to calibrate its control parameters while the second one, after optimizing the digital filter, is ready to use. Both algorithms presented similar results with reflection coefficient smaller than 10.7% for regular waves in the frequency range of 0.5 to 2.0 Hz. The paper also presents the first results obtained in the tank.
Sleeved Stress Joint for Steel Catenary Riser Application
Authors: Chang S.-H. Mark , Stanton Paul , Kuriakose Sunil , Thompson Hugh
Published: January 2011

Steel catenary risers (SCRs) have been widely used for oil/gas/water transport on floating platforms for the last fifteen years. Flex joints and tapered stress joints are often used for interface between the SCR and platform. Flex joints and tapered stress joints need to be designed to meet both the stiffness and flexibility requirements. A stress joint requires high stiffness to withstand the bending moment induced by the SCR and at the same time needs to be sufficiently flexible so as not to overstress the SCR. To achieve these complex requirements, a sleeved stress joint (SSJ) provides a sound technical and economical alternative for the interface between the SCR and platform. A sleeved stress joint utilizes multiple pipes to provide variable stiffness and to meet the strength and flexibility requirements. In the design of a SSJ, the number of sleeves, and the outer diameters and wall thicknesses of the sleeved pipes can be adjusted to achieve the design requirements. In addition, the locations of welds in the sleeved pipes can be placed to achieve the high fatigue performance that is important in stress joint design. Feasibility of the SSJ design is verified through state-of-the-art computer modeling. Generic cases of SSJ design applied to the porch and pull tube of a floating platform are presented. The design concept is compared with traditional flex joint and tapered stress joint designs. The technical and economic advantages of such a design are discussed.
Dynamic Response Analysis on Different Types of TLPS
Authors: Yan Gongwei , Ou Jinping
Published: January 2011

Different structural forms of TLP have different capacities and different scopes of application. Each type of TLP is globally designed. The hydrodynamic parameters and dynamic responses of different types of TLPs are studied considering the effect of different directions of wave and tendon stiffness. The unsteady potential flow theory and boundary element method are used to analyze flow field and hydrodynamic calculation. The software AQWA suite which is a set of advanced hydrodynamic analysis programs is used to perform the numerical simulation of TLP motion response. The results show that the dynamic responses of different types of TLPs are observably different. Changes of wave directions and added stiffness from tendons have certain effect on the motion response of a TLP.
Engineering Critical Assessment and Burst Strength Calculations
Authors: Wang Wei , Sun Liping , Arif Abu Bakar Mohd , Badaruddin Mohd Fauzi
Published: January 2011

Transmission pipelines, which are usually subjected to a variety of loads ranging from internal pressure to external pressure ground loading, transport gases and liquids at high pressures. Furthermore, the pipelines exhibit various forms of damage from environmentally induced cracks to manufacture and operation induced cracks. Therefore, it is of great engineering significance to establish acceptable levels for the flaws revealed. The acceptance levels for flaws are based on the concept of fitness for purpose. By this principle, a particular fabrication is considered to be adequate, provided that the conditions to cause failure are not reached. Less severe flaws are accepted without further consideration. Moreover, if flaws are more severe, rejection is not necessarily automatic. Based on fitness for purpose, decisions on whether rejection or repair might be justified, either in the light of previously documented experience with similar material, stress and environmental combinations or on the basis of an engineering critical assessment (ECA). The present paper focuses on two objectives. Firstly, the remaining strength and fracture toughness of the pipeline with defects along the longitudinal direction of the pipeline have been obtained. Secondly, the utilizing reliability of the pipeline, which has been tested to have inner flaws and to what extent this kind of defects may exist in the pipeline purchased from the same manufacturer, has been evaluated. Based upon the description of the defects found in the pipeline, the remaining strength assessment is performed with the application of different codes such as B31G and DNV. Also, an ECA sample for the embedded flaw in plates will be performed according to the existing BS 7910 methodology. The initial crack depth is 2a and the width is 2c. According to the BS7910 standard, the fracture and fatigue assessment is applied to obtain the fracture ratio Kr and the load ratio Lr. A 4th Runge-Kutta method is used to calculate crack growth rate da/dN and the final size of the crack af and cf. Also the Failure Assessment Diagram (FAD) can be changed according to the material stress-strain curve. This work might provide useful reference to safety design and application of pipelines.
Numerical Simulation of Vortex-Induced Vibrations on a Flexible Riser in Uniform Currents
Authors: He Changjiang , Duan Zhongdong , Ou Jinping
Published: January 2011

A numerical model in a quasi-three-dimensional fashion is developed in this paper to simulate vortex-induced vibration of a flexible riser (the aspect ratio = 250, mass ratio, m* = 2.9, damping ratio, ζ = 0.01) in uniform current U∞ = [0.06, 0.80] m/s, (Reynolds number, Re = [0.01, 1.28]×104). Finite element method are and Finite volume method are applied in the structural and fluid domains respectively. Effects of fluid-structure interaction (FSI) are reckoned in by making use of kinematic equivalence of the relative flow between fluid and the body in inertial and non-inertial frames of reference. It is found transeverse motion and streamwise motion are strongly coupled, they have same changing trend at the same reduced velocity range, the upper branch appears in the range Vrn = U∞/fnD ≈ 5–7 for the generated nth mode, whilst the lock-in remains in the range Vrn ≈ 3–10, the phase angles decrease from about 90° in the initial branch to less than 45° in the lower branch. The RMS and envelop values of cross-flow displacements are 4∼6 times those of in-line, maximum amplitudes of about 1.2 diameters at cross-flow and 0.25 diameters at in-line have been observed. Standing wave response was observed as Vr1 = 6, the in-line response even contains the first and the second modes at the same time. The strouhal number, St is about 0.17 in the present cases.
Experimental Studies of Vortex Structures in the Wake of a Cylinder With Helical Strakes
Authors: Hao Z. , Zhou T. , Wang X. , Tan S. K.
Published: January 2011

This work is experimental studies of the turbulent vortex structures in the wake of a fixed cylinder with helical strakes. The effect and mechanism of a triple helical strake system on suppression of vortex induced vibrations (VIV) are investigated experimentally in a water tunnel. Strakes with a length of 25D in pitch and 0.1D & 0.2D in height are fitted onto a rigid cylinder of diameter D = 12mm, and subjected to a transverse water flow. PIV techniques are used to measure the vector field to identify the vortex structures. The hot-film techniques are also employed to identify the shedding frequency. Measurements for the bare cylinders are also conducted for comparison. The mechanism of the VIV reduction by helical strakes is discussed in detail.
Application of the Random Melnikov Method for Single-Degree-of-Freedom Vessel Rolls Motion
Authors: Hu Kaiye , Ding Yong , Wang Hongwei , Li Jide
Published: January 2011

Basing on the nonlinear dynamics theory, the global stability of ship in stochastic beam sea is researched by the global bifurcation method. In this paper, bounded noise is first briefly introduced. Bounded noise is a harmonic function with constant random frequency and phase. It has finite power and its spectral shape can be made to fit a target spectrum, such as Pierson-Moskowitz spectrum, by adjusting its parameters. This paper considered the stochastic excitation term as bounded noise and the influence of nonlinear damping and nonlinear righting moment, setup the random single degree of freedom nonlinear rolling equation. Then the random Melnikov process for the nonlinear system with homoclinic orbits under both dissipative and bounded noise perturbations is derived. The random Melnikov mean-square criterion is used to analysis the global stability of this system. The research indicates that the bounded noise can approximately simulate the wave excitation and if the noise exceeds the threshold value, the ship will undergo stochastic chaotic motion. That will lead ships to instability and even to capsizing.
Physical Modeling of Untrenched Pipeline Breakout From Sand-Bed in Ocean Currents
Authors: Yan Shu-Ming , Gao Fu-Ping , Cao Jing , Zhang En-Yong , Li Guo-Hui , Wu Ying-Xiang
Published: January 2011

The ultimate lateral soil resistance for pipe losing lateral stability on a sandy seabed under the action of ocean currents is investigated with a newly developed test facility by employing mechanical actuators to simulate hydrodynamic loads on the pipe. Two kinds of constraint conditions, i.e. anti-rolling pipe and freely-laid pipe, are taken into account, respectively. The experimental observations indicate that, the horizontal lateral soil resistance increases gradually to its maximum (ultimate) value when the additional settlement is fully developed. The buildup of the ultimate lateral soil resistance to the anti-rolling pipe benefits from not only the additional settlements but also the sand-particle collections in front of the moving pipe, especially for the anti-rolling pipes. The lateral-soil-resistance coefficient for the anti-rolling pipe is much larger than that for the freely-laid pipe. The pipe surface roughness also affects the lateral stability of anti-rolling pipes. A comparison is made between present mechanical-actuator tests and the previous water-flume tests, indicating the results of two types of tests are comparable and the local scour may reduce the pipe lateral stability in ocean currents.
Measuring Ocean Waves From Space: Objectives and Characteristics of the China-France Oceanography SATellite (CFOSAT)
Authors: Hauser Danie`le , Tison Ce´line , Lefe`vre Jean-Michel , Lambin Juliette , Thierry Amiot , Aouf Lotfi , Collard Fabrice , Castillan Patrick
Published: January 2011

The Chinese and French Space Agencies are jointly preparing a satellite mission devoted to the monitoring of the ocean surface and related science and applications. This is the so-called “China France Oceanography SATellite” (CFOSAT), to be launched around 2013. This mission will provide simultaneous and collocated observations of wind at the ocean surface and spectral properties of surface ocean waves using two scatterometers, both in Ku-Band: SWIM for measurements of directional wave spectra and SCAT for wind vector measurements. The SWIM instrument will use a real aperture observation technique so as to avoid limitations encountered with SAR systems. This paper describes the main objectives and characteristics of the mission with a focus on the SWIM instrument designed and developed under French responsibility to measure directional spectra of ocean waves.
Nonlinear Wave Statistics
Authors: Fedele Francesco , Cherneva Zhivelina , Tayfun M. Aziz , Guedes Soares Carlos
Published: January 2011

Third-order quasi-resonant interactions among free waves and associated modulational instabilities can significantly affect the statistics of various surface features in narrowband waves. In particular, modulational instabilities tend to induce intermittent amplifications on the surface displacements, causing their statistics to deviate from the linear Gaussian and second order models. Herein, we investigate the nature of such instabilities on the statistical and spectral characteristics of deep-water waves generated in a large wave basin. We analyze the spectral changes that occur as waves propagate along the basin, develop bounds on the spectrum bandwidth, and interpret various statistics based on third-order Gram-Charlier distributions.
Numerical and Experimental Comparisons of Vortex-Induced Vibrations of Marine Risers in Uniform/Sheared Currents
Authors: Srinil Narakorn , O’Brien Patrick , Wiercigroch Marian
Published: January 2011

This paper presents a general theoretical reduced-order model capable of evaluating the multi-mode nonlinear dynamics of marine risers subject to uniform and sheared currents. The main objectives are to predict the vortex-induced vibration responses and parametrically compare between numerical and experimental results. The emphasis is placed on the analysis of cross-flow vibrations due to unsteady lift forces. The nonlinear equations governing riser axial/transversal motions are derived based on a top-tensioned beam model with typical pinned-pinned boundary conditions. The riser geometric nonlinearities owing to possible large dynamic displacements and multi-mode interactions are accounted for. To approximate the space-time varying lift force, the empirical hydrodynamic model, based on a nonlinear van der Pol wake oscillator with a distributed diffusive term, is used. A low-dimensional dynamic model and computationally-robust time-domain tool are then developed to evaluate the multi-mode fluid-riser interactions. These are very useful in dealing with large parametric studies involving varying system parameters. Comparisons of numerical and experimental results are performed by estimating riser response amplitudes and fatigue damage indices. Both linear and nonlinear risers are considered in the present numerical model whereas only linear riser has been considered by a referenced literature in the reconstruction of experimental displacements through measured strains. It is found that riser geometric nonlinearities play a significant role in both numerical simulations and comparisons with experiment post-processed results. In some cases, quantitative/qualitative discrepancies in riser response predictions are remarkable with linear vs. nonlinear models. These may be recognized as one of the factors why recent numerical and experimental comparisons in literature have been unsuccessful.
Numerical Simulation of Flow Control on Marine Riser With Attached Splitter Plate
Authors: Wang Jiasong , Liu Hua , Gu Fei , Zhao Pengliang
Published: January 2011

Attaching a splitter plate (SP) on the base of a riser wall is used to control the flow of risers and evaluated by using the CFD technique in this paper. A finite-volume total variation diminishing (TVD) approach for solving incompressible turbulent flow with renormalization group (RNG) turbulence model was used to simulate the hydrodynamic characteristics of the riser system with additional SP for the different aspect ratio of length to diameter L/D. It was shown that the present numerical method has high order of accuracy by comparing with the available experimental and numerical simulation data for typical circular cylinder flow. A riser system attached with SPs of L/D = 0.5∼2.0 for Reynolds number 1000, and 30000 respectively can obviously reduce the lift and drag coefficient and alter the vortex shedding frequency. The mean drag coefficient can be reduced up to 20% and 35% and the maximum lift coefficient can be reduced up to 94% and 97%, for Re = 1000 and 30000, respectively. The lift can be effectively suppressed after a relative long time. L/D = 0.5∼1.0 may be considered as more practical geometries considering the real conditions, which also have good flow control effect.
Streamwise Evolution of a Yawed-Cylinder Wake
Authors: Wang Hanfeng , Zhou Tongming
Published: January 2011

The wake of a yawed circular cylinder was investigated by measuring all the velocity and vorticity components using an eight-wire vorticity probe. All experiments were performed at a Reynolds number of 7200. The tested yaw angle (α) is 0°, 15°, 30° and 45°. The measurements were conducted at different downstream positions to investigate the streamwise evolution of the wake. It is found that the effects of yaw angle on the coherent vorticity are negligibly small for α ≤ 15°. When α increases to 45°, the maximum of coherent spanwise vorticity reduces by about 50%, while that of streamwise vorticity increases by about 70%, indicating the suppression of spanwise vortices and the enhancement of three dimensionality of the wake with the increase of α. The streamwise decaying rate of the coherent spanwise vorticity is smaller for a larger α. It is because the streamwise spacing between the spanwise vortices is bigger for a larger α, thus this results in a weak interaction between the vortices and hence decays slowly. The coherent contribution to u2 and v2 decay slower for a larger yaw angle as moving downstream, consistent with the slowly streamwise decaying rate of ω˜z* for a larger yaw angle.
Flow Behavior Behind Two Side-by-Side Circular Cylinders With Unequal Diameters
Authors: Gao Yangyang , Yu Dingyong , Tan Soon Keat , Wang Xikun , Hao Zhiyong
Published: January 2011

In this study, flow behavior behind two side-by-side circular cylinders with unequal diameters, was investigated by using the technique of particle-image-velocimetry (PIV). The experiments were conducted in a re-circulating open water channel within the subcritical Reynolds number regime and a center-to-center pitch ratio T/D range of 1.2 to 3.6. Instantaneous and average velocity patterns, vorticity and Reynolds stress contours acquired with the PIV were used to analyze the flow behavior behind the two cylinders. The effects of the gap ratio and Re on vortex shedding were studied. The results showed that three flow patterns including the asymmetric biased flow, symmetric biased flow and two wake modes with different scales for different gap ratios respectively were observed.
Methods for Improvement of Drift Forecast Models
Authors: Maisondieu Christophe , Breivik O̸yvind , Roth Jens-Christian , Allen Arthur A. , Forest Bertrand , Pavec Marc
Published: January 2011

Over the past decades, various operational drift forecast models were developed for trajectory prediction of objects lost at sea for search and rescue operations. Most of these models are now based on a stochastic, Monte Carlo definition of the object’s initial position and its time-evolving search area through computation of an ensemble of equally probable trajectories (Breivik [1]). Uncertainties in environmental forcing, mainly surface currents and wind, as well as the uncertainties inherent in the simplified computation of leeway speed and direction relative to the wind are also accounted for through this ensemble-based approach. Accuracy of the drift forecast obviously depends to a large extent on the quality of the environmental forecast data provided by numerical weather prediction models and ocean models, but it also depends on the level of uncertainty associated with the estimation of the drift properties (leeway) of the objects themselves. The present work mostly focuses on this second aspect of the problem. Drift properties of objects can be described by means of their downwind and crosswind leeway coefficients, according to the definition of leeway as stated by Allen [2, 3]. Assessment of the leeway coefficients is based on a direct method, which requires measurements acquired during field tests. Such field experiments basically entail deploying one or more objects at sea and simultaneously recording the environmental parameters (namely wind speed and motion of the object relative to the ambient water masses, i.e., its leeway) as well as the object’s position while adrift for periods ranging from several hours to several days. Using this method, a large database providing leeway coefficients for more than sixty object classes ranging from medical waste to a person-in-water to small fishing vessels was compiled over the years by the United States Coast Guard (Allen [2]). More recently additional trials were conducted, which allowed evaluation of new objects, including 20-ft shipping containers. We present in this paper the methods and analysis procedures for field determination of leeway coefficients of typical search-and-rescue objects. As an example we present the case study of a 20-ft container and discuss results obtained from a drift forecast model assessing sensitivity of such a model to the quality of environmental data as well as uncertainty levels of some reference parameters.
Numerical Investigation for Air Cavity Formation During the High Speed Water Entry of Wedges
Authors: Wang Jingbo , Faltinsen Odd M.
Published: January 2011

In this paper, a nonlinear boundary element method is developed for investigating the air cavity formation during the high speed water entry of wedges. A novel technique is introduced to remove the saw-tooth instability of the free surface profile, which is often encountered in solving violent free surface flow problems by boundary element methods. This technique applies to both the equally spaced grids and the non-equally spaced grids, and is demonstrated to be quite efficient and practical by numerical simulations. When the flow reaches the knuckle of the wedge, separation will occur. The authors develop a purely numerical method to simulate the non-viscous flow separation. This nonlinear BEM has been verified by comparisons with similarity solutions. We also compare the numerical results with experimental results.
An Integral Design Approach for Deep Sea Mining Systems Using Dredging Technology
Authors: Vercruijsse P. M. , Lotman R.
Published: January 2011

Experience in the area of wet mining and the dredging industry learns that the excavation system cannot be seen separate from the slurry transportation system. These two key systems in a deep sea mining operation interrelate to such extend that they must be developed towards an integral solution. The nominal production, peak production and variability of these figures must match for all sub-systems in the overall mining system to optimize for mining efficiency; we call this the ‘game of capacities’. Also the configuration of the excavation and transport system has great consequences. For instance an important question is whether to place the first pump and its drive of the slurry transport system on the seafloor mining tool or in the riser system. The choices made impact amongst others on; the mining tool’s reach (and thus efficiency), the seafloor mining tool’s propulsion system and its geo-mechanical interaction, the slurry transport flow and pumping power requirements. This paper will discuss the several dependencies of the (producing) subsystems and important choices for configuration and their consequences regarding technology, capex, opex, reliability and maintainability.
Collision Character Research for Semi-Submersible Through Model Test, Simplified Analytical and Numerical Simulation Method
Authors: Hu Zhiqiang , Liu Zhenhui , Amdahl Jo̸rgen
Published: January 2011

The characters of the collision scenario when a semi-submersible is struck by a containership are studied in this paper, through the model test, simplified analytical method and numerical simulation. The model test is conducted in the Deepwater Offshore Basin in Shanghai Jiao Tong University. Two special devices are designed to fulfill the model test. One is Ship Launching Device, simplified as SLD, who can launch the striking ship with controllable velocity and in any horizontal direction. The other is Energy Absorbing Device, simplified as EAD, who can simulate the buffer effect of the column structure and collect the collision force as well. A numerical simulation is completed to get the approximate stiffness of the column structure, which is used to adjust the property of EAD. The motions of semi-submersible are obtained, and the collision force and the tension forces of mooring lines are also got. Collision scenario characters for semi-motion and tension force are summarized by the analysis of the model test results. The second collision phenomenon is observed. The collision force dominates the collision moment and the tension force of the mooring lines lags behind. A NTNU in-house program developed by analytical simplified method is also verified by the model test result. The comparison proves the feasibility of the program.
Euler Buckling of Idealised Horizontal Pipeline Imperfections
Authors: Cumming Gary , Rathbone Andrew
Published: January 2011

Imperfections introduced by pipelay can not be known until installation is complete; therefore a common approach is to perform finite element analysis of idealised horizontal imperfections to determine critical buckling forces. Rundsag et al 2008 [1], showed that the critical buckling force for a snake lay geometry is directly proportional to the pipeline bend radius. Rathbone et al 2008 [2] showed that, with decreasing arch lengths, the pipeline critical buckling force is proportional to the change in the offset angle. This paper looks at the relationship between the minimum critical buckling force and the horizontal offset angle of the pipeline, considering an Euler buckling approach. The resulting relationship that estimates the critical buckle load based on pipeline stiffness and weight, offset angle and friction factor is then compared against idealised finite element models.
A Case Study on Landslide-Generated Tsunami and Its Impact on the U.S. Atlantic Coast
Authors: Zhou Hongqiang , Moore Christopher W. , Weiss Robert , Titov Vasily V. , Wei Yong
Published: January 2011

Recent studies suggested that the potential eruption of Cumbre Vieja Valcano on La Palma Island may cause catastrophic failure of the west flank and generate a tsunami that threats the American coasts (e.g. Ward and Day, 2001; Lo̸vholt et al., 2008). In the present study, we revisit this problem with a credible worst case scenario and advanced numerical approaches.
Feasibility and Design of the ClubStead: A Cable-Stayed Floating Structure for Offshore Dwellings
Authors: Aubault Alexia , Sitler-Roddier Wendy , Roddier Dominique , Friedman Patri , Gramlich Wayne
Published: January 2011

The ClubStead is a novel type of offshore floating platform, which provides comfortable and safe ocean-going dwellings for communities of a few hundred people. The prospect of large, unclaimed ocean spaces has encouraged people to consider developing sea-going settlements. A number of attempts have been made on former oil platforms or cruise ships. But these structures are not designed for permanent living at sea and fall short of meeting dwellers’ expectations. Efforts to build large, spacious floating living facilities have struggled to balance cost-effectiveness and structural integrity. This paper describes an innovative, cost-efficient solution to maximize space on offshore structures. To control the cost, the submerged volume of the floater is minimized. To maximize comfort, the available living surface area is also maximized, while the motions of the platform are limited. The proposed solution is based on the principles of tensegrity, which are commonly used on bridges. Cable stays are tensioned at the top of towers to support the weight of both light and cantilevered top-sides. The floater is column-stabilized with four submerged columns. A feasibility study was performed for the design of a Clubstead based off the coast of California. The platform is dynamically positioned and can house up to 270 people. Due to its primary function, as a floating living facility, the architectural design and the engineering studies are intertwined. Iterations are necessary to determine the global characteristics of the ClubStead. The buildings and living spaces are arranged by an architect, within specific offshore-related constraints. The resulting payload is calculated and thus used in the design basis to perform the engineering analysis. The feasibility study focuses on survivability and passenger comfort to assess the novel design. The survivability analysis is based on structural strength and motion predictions in a 100-year storm. Passenger comfort is evaluated in operational conditions.