bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of a Quadratic Boundary Element Method to Ship Hydrodynamic Problems
Authors: Zhang Sheguang , Weems Kenneth M. , Lin Woei-Min , Yan Hongmei , Liu Yuming
Published: July 2009

This paper describes the implementation and application of a Quadratic Boundary Element Method (QBEM) to the 3-D, time domain potential flow solution of ship-wave hydrodynamic interaction problems. In QBEM, the geometry, singularity, and solution distributions on each panel are represented by bi-quadratic parametric functions defined by nine nodal values. QBEM has been shown to provide many theoretical advantages in accuracy and convergence versus the more conventional Constant Panel Method (CPM), but its application to real ship flow problems presents many challenges. This paper describes the implementation of QBEM into an existing time-domain ship motion and wave load prediction code, the Large Amplitude Motions Program (LAMP), and identifies and addresses some important numerical issues related to QBEM, such as mismatched grid points between neighboring panels, surface normal calculation on degenerate QBEM panels, the stability of the free surface and the solution and treatment of QBEM panels for body-nonlinear calculations. A number of practical cases are provided to compare QBEM with CPM and to validate QBEM against analytical solutions and experiments. The advantages of using QBEM for ship hydrodynamic problems and associated/unresolved numerical issues are also discussed.
Determination of Required Reserved Strength Ratio in Ultimate Strength Limit for Assessment of Existing Offshore Platforms in the Persian Gulf
Authors: Amiri M. M. , Asgarian B.
Published: July 2009

The offshore infrastructures are used by the oil and gas industry to meet over twenty percent of the world’s demand for the hydrocarbon productions. Offshore platforms are subjected to various environmental, operational and accidental loads. Environmental loads include wave, current, wind earthquake and ice loads. Considering the reuse of old platforms, design criteria for offshore structures have changed significantly over the post decades. Most of existing structures have been designed based on the previous codes. Many of these structures are now called upon for extended service beyond their original design life. This makes the need to assess the platforms under different environmental conditions such as wave, current and wind loading. Since the proposed amounts of Reserve Strength Ratio (RSR) in the guidelines are for the special geographical zones (i.e. Gulf of Mexico and other parts of US), and also since the methodologies used for estimation of RSR have not been determined explicitly, this study aims to explain more the effective parameters for the assessment of existing offshore platforms. Because the calculation of these parameters is based on statistical data for specific regions, to achieve the RSR ratio related to the Persian Gulf, we utilize the environmental statistical data for this region. And consequently a relevant RSR is proposed. Ultimately, the reserved strength of three sample platforms located in this zone will be assessed according to proposed criterion.
Numerical Simulation of Ship Motion in Offshore and Harbour Areas
Authors: Christensen Erik Damgaard , Jensen Bjarne , Mortensen Simon Brandi , Hansen Hans Fabricius , Kirkegaard Jens
Published: July 2009

A method for simulating the motions and mooring forces of a moored ship subject to wave forcing has been further developed and validated for both the open water case and inside harbour areas. The method was originally developed and reported in Bingham (2000). The simulation tool is named WAMSIM and it solves the equations of motions in the time domain. The package applies the WAMIT® model to provide the frequency domain hydrodynamic characteristics (the frequency response functions or FRFs) of the body. Examples from both open waters and enclosed waters in harbours are presented.
Integrated Subsea Design: An Informed and Practical Approach
Authors: Underwood Nigel , Odina Lanre , Hansen Kevin , Lassila Harry
Published: July 2009

In a sector that is seeing unprecedented levels of activity, the opportunity to assess skill sets to take an integrated approach to subsea system design can be challenging. Contracting strategies that compartmentalise engineering scopes at any stage of the project lifecycle also require a high level of interface and project management. An informed methodology ensures a holistic approach to subsea design and interfaces is undertaken. Many challenges to subsea design are influenced by other disciplines such as reservoir, production technology, process, flow assurance and facilities engineering. Traditionally these considerations are outside of subsea engineering scopes and are controlled at the next level in the supply chain, or within other designers or contractors. This paper considers not only the regional challenges to pipeline and riser design in the Australian region, but also how the inclusion of an integrated approach encompassing multi-disciplined teams allows a more informed outcome. Using these highly skilled teams, this approach can: • Reduce interface management requirements; • Reduce design cycles; • Create optimised solutions within a shorter timescale. This has the potential to reduce overall CAPEX and OPEX costs by identifying value and reducing uncertainties in key areas. This adds significant value in the current climate of extended procurement lead times and resource constraints. Examples of current projects demonstrating how this approach can be applied in concept selection through to detailed design are described. Practical examples of how the possible design outcomes have been assessed using this approach are detailed. Considerations of how key factors such as subsurface, field layout, architecture, materials selection, operability, construction and HSE influence pipeline and riser design are also discussed.
Robust Neural-Network Hybrid Tracking Control of Underwater Vehicles
Authors: Luo Wei-Lin , Zou Zao-Jian , Huang Lan-Ping
Published: July 2009

A cascade system of an autonomous underwater vehicle is considered. It consists of the nonlinear equation of motion and the equations of actuator dynamics. In the motion equation, unmatched uncertainties are taken into account, including the modeling errors and the bounded disturbances. The modeling errors result from the parameter errors, the ignored high-order modes and unmodelled dynamics. The bounded disturbances refer to environment forces or unknown random disturbances. To obtain accurate manoeuvring of the underactuated system, a hybrid robust controller is proposed by using backstepping Lyapunov functions. A two-layer feedforward neural-network is applied to compensate the modeling errors and the derivatives of desired control inputs, while the H∞ control strategy is used to achieve the L2-gain performance with respect to the bounded disturbances. The on-line tuning algorithms of the neural-network weights are given. The uniformly ultimately bounded stabilities of the tracking errors and the neural-network weights errors are analyzed. Moreover, selection of the gains in controller is recommended by analysis of the upper boundedness of errors. Simulation results have demonstrated the validity of the controller proposed.
Assessment of Empirical VIV Analysis Tools and Benchmark With Experiments
Authors: Constantinides Yiannis , Oakley Owen H.
Published: July 2009

In the current engineering practice, riser VIV design is typically based on empirically derived models. These tools have been under development for some time, but are based primarily on small scale rigid cylinder tests and flexible cylinders with relatively low aspect ratio. As high aspect ratio and full scale measurements become available for a wide variety of configurations, an improved understanding of the physics is beginning to emerge. This study employs a design based methodology for benchmarking VIV prediction tools against a comprehensive set of experiments. Configurations with bare, partially and fully straked risers operating at low and high current speeds are used to cover a broader range of validation conditions. Additional work to include the effect of high harmonics over a range of cases is necessary to draw more definitive conclusions. Given the difficulties in predicting VIV response for these small scale experiments the effectiveness in handling full scale risers is in question.
The Reliability Analysis for Coastal Structure Based on Monte Carlo Method
Authors: Huang Yam , Shi Honda , Wang Wei , Fan Yong
Published: July 2009

The Monte Carlo method is often a favorite selection to estimate the reliability of structure since it involves no complex mathematical analysis, especially when systems are so complex or large that deterministic methods are difficult. In this paper, Monte Carlo method is used to analyse the failure probability of a gravity wharf in Yellow Sea. In addition, in view of the uncertainty exited in calculation model or other aspects, the sensitivity of failure probability is calculated.
Update on the Design of a 1:33 Scale Model of a Modified Edinburgh Duck WEC
Authors: Lucas Jorge , Cruz Joa˜o , Salter Stephen , Taylor Jamie , Bryden Ian
Published: July 2009

A modified version of the Edinburgh Duck wave energy converter has been studied recently at the University of Edinburgh. From the design point of view the key innovation was a modification of the wetted profile. Wave energy is converted into useful work by the same pitching motion as in the original Duck, but by means of a circular cylinder with an off centred axis of rotation. This recent study was focused on a Duck version designed for vapour compression desalination rather than electricity production. An hydrodynamic numerical model (WAMIT) was used to predict first-order hydrodynamics quantities and to select and optimize configurations. The results obtained showed that it was possible, following the appropriate control strategies, to obtain similar energy absorption capabilities as the in the cam shaped original Duck. A 1:33 scale model was built to validate the numerical predictions. This paper extends the already published numerical predictions and experimental results obtained with this model. Experimental tests in random waves and measurements of the mooring forces for different submerged volumes will be reported for the first time.
A Straight-Bladed Vertical Axis Wind Turbine With a Directed Guide Vane: Mechanism of Performance Improvement
Authors: Kuma Hideki , Takao Manabu , Beppu Toshiyuki , Maeda Takao , Kamada Yasunari , Kamemoto Kyoji
Published: July 2009

A straight-bladed vertical axis wind turbine with a directed guide vane has been proposed in order to enhance its torque. The experimental study of the proposed wind turbine was carried out by a wind tunnel with an outlet diameter of 1.8m. The tested rotor has 3 straight rotor blades with a profile of NACA0018, a radius of 0.35 m and a height of 0.7 m. The guide vane which consists of an arc plate rotates around the rotor and is directed to the wind by aerodynamic force generated by tail vanes, so as to put the guide vane in upstream of the rotor. As a result, the performance of the straight-bladed vertical axis turbine was improved by means of the directed guide vane and the power coefficient of the proposed wind turbine was approximately 1.2 times higher than that of the original wind turbine which has no guide vane. Further, flows around the proposed wind turbine have been investigated by use of the vortex method which provides a Lagrangian simulation of unsteady and vortical flows.
A Semi-Linear Method for Particle Kinematics Under Steep Random-Wave Crests
Authors: Ojieh Nwaka Chuks , Barltrop Nigel
Published: July 2009

The determination of random wave kinematics in high sea-states especially in the region above mean water surface has been a major challenge and most proposed methods are not robust. In this work, a method which is similar to stretching and extrapolation in its simplicity is developed based on a simplified form of wave-wave interaction which is robust to 2nd order. To check the usefulness of the new method, horizontal particle velocity results are compared with Zhang’s HWM, Wheeler stretching, linear extrapolation above MWL, experimental measurements and theoretical computations of Longridge et al (1996), Skjelbriea et al (1991) and Stansberg et al (2006). Results of the new method show good agreement with the results of Skjelbriea et al (1991) and are closer to experiment than 2nd order results of Stansberg et al (2006) while only showing a qualitative agreement with Longridge et al (1996). The poorer comparison with Longridge et al (1996) is probably because it was not possible to reproduce the exact time series used by Longridge et al as the exact amplitudes and phases were not available. The improved capability of the new method over existing methods has thus been demonstrated and its advantage over the HWM lies in being simpler to implement.
Application of a VOF Method to Model Compressible Two-Phase Flow in Sloshing Tanks
Authors: Wemmenhove Rik , Luppes Roel , Veldman Arthur E. P. , Bunnik Tim
Published: July 2009

The growing transport of LNG in partially filled tanks raises the demand to have accurate methods to predict the fluid behaviour in these sloshing tanks and the effect of the sloshing fluid on the tanker motion. To examine the motion of the sloshing fluid, model experiments have been carried out on a scale of 1:10. Different tank filling ratios and types of motion have been tested to study the sloshing fluid behaviour for various sea states. The model experiments have been carried out to provide extensive validation material for the numerical method ComFLOW. The details of this improved Volume Of Fluid (iVOF) method are presented in the paper. The method resolves the governing equations in both fluids, one of them being compressible. The compressibility of the second phase is especially important for more violent flow conditions, when two-phase phenomena such as air entrapment and air entrainment occur frequently. Particular attention in the numerical method has been paid to the treatment of the flow variables around the interface, especially the density. The fluid is convected by means of a first-or second-order upwind scheme. The behaviour of the sloshing fluid strongly depends upon the regularity of the tank motion and the filling ratio of the tank. Video frames, wave probes and pressure transducers have been used to compare the fluid flow of simulation and experiment. Two-phase effects such as air entrapment are more common for increasing tank filling ratios and for more irregular tank motion. A realistic simulation of these effects is possible by modeling two-phase flow, especially when using a relatively fine grid and applying the less-dissipative second-order upwind scheme. Compared to the earlier paper on the numerical simulation of sloshing in LNG tanks [8], where the numerical method was validated for regular sway motion, more extensive attention is paid to the accuracy of the applied discretisation schemes in space and time. The results of different schemes are now evaluated for both regular and irregular sway and roll motion of LNG tanks.
Determination of Vortex Shedding Frequency Around Offshore Pipeline Using Unsteady Drag Force Model
Authors: Valipour Reza , Yeganeh Bakhtiary Abbas , Ghaheri Abbas , Kazeminezhad Mohammad H.
Published: July 2009

Laying offshore pipelines on the uneven sea floor makes the flow turbulence and instability behind the pipe, which is termed as the vortex shedding. Vortex shedding causes a periodic change in the net hydrodynamic pressure around the pipe which in turn increases significantly the potential of fatigue damage of a pipeline at the free span section. This potential especially will rise when the vortex frequency is synchronized with one of the natural frequency of the pipeline span and/or resonance occurs. Thus, during the design process for avoiding resonance, it is crucially important to estimate the frequency of vortex shedding behind the pipeline to limit the length of pipeline on the free span section to its allowable value. Due to the essential role of obtaining vibration drag forces and as a consequence vortex shedding frequency as well as its induced time dependent pressure and velocity distribution, using numerical simulation model for simulation of hydrodynamic forces acting on the pipe become necessary. In the present study, the vortex shedding frequency around an offshore pipeline is calculated by employing the pattern of unsteady dynamic drag force. The unsteady pattern of dynamic drag force is obtained by the model, which is constructed based on the Reynolds Averaged Navier-Stokes (RANS) equations in conjunction with a standard k-ε turbulence model for turbulence closure. As a results of employing this numerical model, vortex shedding frequency, caused by current, around the offshore pipelines is calculated along with its induced turbulent pressure and velocity field. Finally, the results of estimating vortex shedding frequency behind the pipeline is compared with those of experimental data, for different situations of laying offshore pipeline on the seabed and various environmental conditions. The results reveal that the employed numerical model is capable of evaluating vortex shedding frequency around an offshore pipeline.
The Theory of Molecular Dispersion in Flowing Fluids: An Analytic Solution of the Navier-Stokes Equations
Authors: Todd William
Published: July 2009

This development is a description of the transport of mass, energy and momentum in flowing viscous fluids at the molecular level; and results in: • A thermostatistical link between Reynolds’ number and momentum and free energy, • A wave characterization of the behavior of flowing fluids using the forces of attraction between molecules as a basis, • Calculation of the velocity components in flowing fluids for all Reynolds’ numbers greater than 535; thus defining a mathematical theory of turbulence, • An analytic solution of the Navier-Stokes equations for incompressible fluids in 3-dimensions. The following steps lead to the solution: • Definition of the fluid Model, • A re-characterization of Reynolds’ number in terms of momentum and free energy, • Calculation of the shear and circulatory components of velocity, • Transformation of the Navier-Stokes equations into the curvilinear coordinates of the intermolecular force waves, • Using the transformed equations to calculate the velocity components and Pressure-wave front resulting from the current, • Corroboration of the theoretical results with: a) wave fronts as manifest in the behavior of sails in uniform flow, b) boundary layer definition/behavior compared to theoretical and empirical developments of Schlichting and others, and c) empirical results for forces measured in the OCEANIC/DeepStar high Re beam-tow tests.
Ultraviolet Protection by Beach Shade
Authors: Kawanishi Toshimasa , Sueda Yuko
Published: July 2009

Ultraviolet radiation is divided into UV-A (320–400nm), UV-B (290–320nm), UV-C (under 290nm). Of these, UV-B may be cause serious health problem, such as cataracts, loss of skin immunity, and increase of skin cancer. For this reason, it is important to know how to protect human body against the UV-B radiation. Beach shade is very effective to prevent the radiation. The radiation enters to the shade from sun and sky. The first step is to measure the sky radiance distribution of sun and sky radiation spectrum. We calculate erythema action radiation from the radiation spectrum. Sunburn chart is made using distribution of the erythema action ultraviolet radiation. Using the Sunburn chart, the shade is designed for protect against the UV radiation.
The Floating Performance Stage @ Marina Bay, Singapore: New Possibilities for Space Creation
Authors: Koh H. S. , Lim Y. B. , Seow T. H. , Stocks D. , Thapar A.
Published: July 2009

This paper presents the systems engineering solutions implemented in developing a large floating performance stage that was constructed at the Marina Bay of Singapore. The Marina Floating Platform is designed to be a multi-purpose facility on the bay for mass spectator events, sporting activities and cultural performances, as well as be a re-configurable “piers” for water sports and boat shows. This floating platform was completed in 2007 and is believed to be the world’s largest floating performance stage on water. It hosted the nation’s National Day Parade 2007, the first parade to be held on water. Since then, the floating platform and its seating gallery have been used as the venue for lifestyle events, extreme sports, the Singapore Fireworks Festival and Water Carnival. The floating platform opens up new possibilities in space creation in land-scarce countries such as Singapore.
Centrifuge Modelling of Riser-Soil Stiffness Degradation in the Touchdown Zone of a Steel Catenary Riser
Authors: Hodder M. S. , White D. J. , Cassidy M. J.
Published: July 2009

Steel catenary risers (SCRs) are economical to assemble and install compared to conventional vertical risers. However, accurate evaluation of the fatigue life of an SCR remains a major challenge due to uncertainty surrounding the interaction forces at the seabed within the touchdown zone (TDZ). Fatigue life predictions are heavily dependant on the assumed stiffness between the riser and the seabed and therefore an accurate assessment of seabed stiffness — or more specifically the nonlinear pipe-soil resistance — is required. During the lifespan of an SCR, vessel motions due to environmental loading cause repeated penetration of the riser into the seabed within the TDZ. This behaviour makes assessment of seabed stiffness difficult due to the gross deformations of the seabed and the resulting soil remoulding and water entrainment. This paper describes a model test in which the movement of a length of riser pipe was simulated within the geotechnical beam centrifuge at the University of Western Australia. The model soil was soft, lightly over-consolidated kaolin clay with a linearly increasing shear strength profile with depth, typical of deepwater conditions. The pipe was cycled over a fixed vertical distance from an invert embedment of 0.5 diameters to above the soil surface. This range represents a typical vertical oscillation range of a section of riser within the TDZ during storm loading. The results indicate a significant degradation in the vertical pipe-soil resistance during cyclic vertical movements. Due to the cyclic degradation in soil strength, the component of the vertical resistance created by buoyancy was significant, particularly due to the influence of heave. A new approach to the interpretation of heave-enhanced buoyancy was used to extract the separate influences of soil strength and buoyancy, allowing the cyclic degradation in strength to be quantified. During cycling, the soil strength reduced by a factor of 7.5 relative to the initial penetration stage. This degradation was more significant than the reduction in soil strength during a cyclic T-bar penetration test. This contrast can be attributed to the breakaway of the pipe from the soil surface which allowed water entrainment. This dramatic loss of strength and therefore secant stiffness, and the significance of the buoyancy term in the total vertical pipe-soil resistance, has implications for the fatigue assessment of SCRs.
Study on Conduct Guidance Signs Used at a Beach
Authors: Katayama Yoshiyuki , Kondo Takeo , Yamamoto Kazukiyo , Nagae Yuta
Published: July 2009

Currently, signs for promoting recreational marine activities have not been proposed. In this study, we proposed an action-prompting sign using a pictogram of beach volleyball, and we conducted a survey with the objective of determining the effectiveness of the sign in prompting action, as well as its problems. According to the results of a questionnaire concerning the action-prompting sign used in the study, we found that a sign representing beach volleyball has a prompting effect and that the sign is suitable for use near water.
Production of Metallurgically Cladded Pipes for High End Applications in the Oil and Gas Industry
Authors: Reichel Thilo , Beissel Jochem , Pavlyk Vitaliy , Heigl Gernot
Published: July 2009

The paper describes the different industrially used options to produce a clad pipe and explains in detail the manufacture of metallurgically cladded pipes starting with the production of roll bonded plates. In plate manufacturing the advantages as well as the limitations of thermo-mechanical (TM) rolling are discussed. The TM-technology is shown to improve weldability, HIC-resistance, strength and toughness properties of the carbon steel section of the pipe. Moreover, it also improves corrosion resistance of the CRA layer. The pipe manufacturing procedure, which involves two welding technologies for longitudinal welds is described. The carbon steel parts of the pipe are joined using double-sided multi-pass Submerged-Arc-Welding (SAW). The single-pass Electroslag-Welding (ESW) is subsequently used for recladding of the CRA layer. The multi-pass SAW results in excellent mechanical properties of the weld joint, whereas the ESW technique ensures low dilution of CRA with the carbon steel, a smooth weld bead shape and a high corrosion resistance of the deposited layer. With the aid of thermodynamic modeling and numerical simulations it is shown, that the high corrosion resistance is promoted by an intensive mixing within the ESW weld pool and relatively low segregation level of Cr and Mo during solidification. Furthermore, FEM analysis is applied to examine the plastic deformation and residual stresses distribution in the pipe during forming, welding and final calibration. The obtained information assists in optimization of manufacturing procedure, and can also be included in prediction of resulting pipe fatigue during operation.
Numerical Prediction of Sloshing Loads in Ship Tanks
Authors: Singh Sanjay P. , Lal Anant , Dhavalikar Sharad S.
Published: July 2009

The present work is about the estimation of sloshing loads in partially filled tanks of a ship for design purpose. Two oil tankers of different dimensions were taken for this study. Ship motions for several wave-heading angles were computed using potential flow solver. Relevant period for sloshing was determined based on the seakeeping analysis. Critical fill levels of the tanks (with respect to sloshing) were identified from all possible set of motions. The numerical simulation of tank fluid motions for critical fill level was performed, using general fluid flow solver, ANSYS CFX. Prior to applying the method to ship tanks a validation study was carried out. The method was validated against the experimental results obtained by Hinatsu et al. (2001). Pressures at various locations of the tank were computed and were compared with the Common Structural Rules for Oil Tankers (CSR). Pressure time history obtained from computational fluid dynamics (CFD) simulations was applied on the tank bulkhead to get the structural response, using ANSYS Mechanical.
Emergency Towing: Field Exercises and Simulator-Based Training
Authors: Berg Tor E. , Gudmundset Gunnar , Klevstad Ulf
Published: July 2009

Recently there have been a number of incidents involving drifting ships and offshore structures in the North Sea. Increasing traffic density combined with harsher weather and less experienced crews has led the Norwegian Coastal Administration to focus on improving the skills of personnel on board emergency response vessels operating off the coast of Northern Norway. This is order to reduce the consequences of incidents with disabled vessels sailing in the vulnerable coastal environment of the Norwegian and Barents Seas. In addition to field exercises involving dedicated Norwegian emergency response vessels and different types of vessels made available by shipping companies, training takes place on full mission bridge simulators. As part of an ongoing R&D project, MARINTEK and SMS invite stakeholders to participate in short workshops on topics related to the specification of functional requirements for emergency towing vessels and the sharing of experience from recent emergency towing operations.
Influence of Surge on Extreme Roll Amplitudes
Authors: Vidic-Perunovic Jelena , Rognebakke Olav , Jensen Jo̸rgen Juncher , Pedersen Preben Terndrup
Published: July 2009

Interference of the wave-induced ship surge motion with roll dynamics has been studied. The surge motion has been included in a previously derived hydrodynamic roll prediction model in order to account for the ship speed variation due to the longitudinal incident wave pressure force. Depending on the position of the wave crest relatively to the ship, the ship will be slowed down when she meets the wave. In this paper attempts are made to accurately account for added resistance and additional thrust, applied in order to maintain the ship speed. The surge equilibrium equation has been longitudinally balanced in order to determine the added thrust term that would represent actions to maintain speed The resulting forward speed variation affects the frequency of encounter and the parametric roll resonant condition is directly influenced by this speed variation. The analysis procedure is demonstrated for an example containership sailing mainly in head sea condition and higher sea states. Sensitivity of the results to the added thrust model and vertical motion calculation is discussed.
Influence of Viscous Effects on the Hydrodynamics of Ship-Like Sections Undergoing Symmetric and Anti-Symmetric Motions, Using RANS
Authors: Que´rard A. , Temarel P. , Turnock S. R.
Published: July 2009

The aim of this investigation is to assess the influence of viscous effects on the predicted hydrodynamic coefficients for a range of ship-like sections, such as rectangular, triangular, chine and bulbous. Hydrodynamic coefficients, of added mass or inertia and fluid damping, for two-dimensional sections harmonically heaving, swaying and rolling at the undisturbed free surface are obtained using the ANSYS-CFX11.0 RANS solver, for a range of frequencies of oscillation. All predictions are compared with available experimental measurements and other numerical predictions (potential flow and RANS). It is concluded from these comparisons that the proposed RANS approach can offer a better prediction for the hydrodynamic coefficients when viscous effects become significant, in particular for sway and roll motions. It is important that a reliable and systematic approach is adopted for the application of the unsteady free surface RANS methodology.
Numerical Investigation of the Effects of the Cross-Shore and Groyne-Wall Slopes on Flow Pattern Around an Impermeable Groyne
Authors: Ghaldarbandi R. , Hakimzadeh H.
Published: July 2009

The groynes which are typical of shore protective structures can provide several aims. Although these structures may partly help to shoreline protection, they would also create some major problems in adjacent regions. Therefore the real performance of these structures needs to be considered carefully before going to construction. In this research, the effects of the cross shore and groyne wall slopes on flow pattern around an impermeable groyne were considered using a three-dimensional numerical CFD model (i.e., FLUENT). The widely accepted eddy viscosity concept and k-ε turbulence model were used to evaluate the Reynolds stresses and eddy viscosity coefficients, respectively. The finite volume method used in the software makes attractive flexibility to use any shape of grids to cover the cross and structural slopes in the computational domain. The model was first applied to a vertical groyne on a flat bed and the numerical model results were compared with experimental data. The model results of this numerical test showed a very good agreement with the corresponding experimental measurements, in terms of water elevation and velocity magnitudes. The model was then applied to a series of structures with different lateral slopes on various cross sectional bed slope. It was found that the flow pattern around the groyne was not changed significantly when the slopes of the structure and bed were slightly changed. The numerical model results, however, showed that by increasing the cross shore slope in any case of the lateral slope of the structure, the magnitude of the maximum velocity was decreased. The bed shear stresses were also decreased when the cross shore slope was increased. Moreover, these values were further decreased when the groyne-wall slope was reduced.
Directional Wave Partitioning and Its Applications to the Structural Analysis of an FPSO
Authors: Lawford Ruth , Bradon Jill , Barberon Thomas , Camps Claude , Jameson Richard
Published: July 2009

A full characterisation of the individual components of a sea-state is key to enabling the response of an offshore structure to be accurately calculated. This paper discusses the partitioning of a time series of directional wave spectra into wind-sea and swell components with distinct frequency and direction characteristics. Once the wave data have been partitioned, JONSWAP or Pierson-Moskowitz parameters can be fitted to each spectrum using ‘best-fit’ techniques. The result of the partitioning and fitting analyses is a time series of wave parameters defining the wave spectrum for each component of the sea state. A 10-year site specific time series of directional wave spectra has been partitioned in this way and used in the analysis of the Triton FPSO, a turret moored FPSO in the central North Sea. The representation of the directionality and magnitude of each environmental force acting simultaneously on the vessel, allows the relative heading of the vessel to be determined and the mooring and hydrodynamic analyses to be performed. These analyses provided input to a structural analysis of the FPSO, which resulted in an inspection plan for monitoring the effects of the metocean conditions on the unit.
A Numerical Investigation of Vortex Induced Vibration on an Elastically Mounted Rigid Cylinder
Authors: Wanderley Juan B. V. , Sphaier Sergio H. , Levi Carlos
Published: July 2009

The Vortex-Induced Vibration on an elastically mounted circular cylinder is investigated by the numerical solution of the two-dimensional Reynolds Averaged Navier-Stokes equations and results are compared with experimental data. The upwind TVD scheme of Roe – Sweby is used to solve the governing equations and the k-ε turbulence model is used to simulate the turbulent flow in the wake of the cylinder. The cylinder is laterally supported by a spring and a damper and is free to oscillate in the transverse direction. Results for the lift coefficient amplitude, displacement amplitude, frequency, phase angle, and power absorbed by the system are presented and compared to experimental data. The code was tested for the fixed cylinder case, and for the moving cylinder. The comparison with experimental data obtained from the literature showed the good quality of the numerical results and validated the code for simulations of vortex-induced vibration.
Ultimate Capacity of Pipe Bends Under Bending and Pressure
Authors: Pappa Patricia , Tsouvalas Dimitris , Karamanos Spyros A. , Houliara Sotiria
Published: July 2009

The structural capacity of pressurized pipe elbows under in-plane bending is examined. The investigation is conducted through advanced finite element analysis tools. The numerical results are successfully compared with available experimental measurements. In addition, a parametric study is conducted, which is aimed at investigating the effects of the diameter-to-thickness ratio on the ultimate capacity of 90 deg elbows, focusing on the failure mode (buckling or flattening), the maximum bending strength, and the deformation capacity. The effects of moderate internal and external pressure levels on the ultimate moment and the failure mode are examined extensively.
Fatigue Reliability of Catenary Mooring Lines Under Corrosion Effect
Authors: Lardier Julien , Moan Torgeir , Gao Zhen
Published: July 2009

Experiences show that mooring line failures have occurred due to crack propagation in chain links. Moreover, the crack propagation is accelerated by corrosion. To maintain the safety of floating structures in a long-term period of, say 20–30 years, it is therefore crucial that these chain links are properly designed as well as inspected and repaired during operation. The aim of this paper is to establish a reliability-based formulation for assessing mooring chains under the deterioration due to combined fatigue cracking and corrosion wastage. The crack growth in chain links has been modelled using the 2-dimensional Paris-Erdogan equation, which is validated by comparison with the SN data on a reliability basis. The corrosion is treated by considering the diameter reduction and the fatigue crack growth rate, using three zones of corrosion, namely the splash, catenary and bottom zones. A fatigue reliability case study of studless chain links is carried out considering various system models and proper correlation between links. The material correlation is modelled by splitting variables for the global and local effect. Accumulated failure probabilities of one year up to twenty years have been calculated to illustrate the effect of corrosion on fatigue reliability. A sensitivity study has been carried out on the initial crack size, the crack aspect ratio and the material correlation between links, when applying the fracture mechanics approach. Based on the case study, it has been found that the splash zone has an important effect on the failure probability of the entire mooring line, especially under corrosion of long time.
Steel Catenary Riser Design Based on Coupled Analysis Methodology
Authors: Rodrigues Marcos V. , Bahiense Rodrigo A. , Steinkjer Oddrun , Raposo Celso V.
Published: July 2009

In a riser design, the traditionally assessment adopted by industry for attainment of platform motions applied to the top of riser consists in the use of de-coupled methodologies. These formulations consider the static environmental loads over the platform (current and wind) through a static offset and the dynamic environmental loads due to wave through imposition of top riser displacement calculated from cross spectral response of sea spectrum and vessel’s RAOs (Response Amplitude Operators). Nowadays due to shift of oil and gas exploitation to deeper waters more accurate methodologies, based on coupled analysis, have been introduced. The coupled analysis considers the interaction between the hydrodynamic behavior of the hull and the structural behavior of mooring lines and risers submitted to environmental loads. For deep waters the coupling effects of lines over platform motions are especially significant and a reduction of the amplification of platform motions if compared to the platform motions obtained from de-coupled analysis is observed. This paper presents a typical Steel Catenary Riser design, connected to a semi-submersible platform, where the motions applied to the top of riser are obtained from the “traditional” way (de-coupled) and from the coupled analysis. Numerical application is presented in order to assess the comparison of the two presented methods in terms of SCR analysis results and to confirm the reduction of platform motions due to coupling effects. The coupled model here studied is composed of approximately 80 lines connected to the platform which requires an excessive computational effort. In order to reduce the computational time some additional studies are performed considering the variation of line mesh discretization and time step size. The objective of this study is the adoption of an optimized model where the required accuracy is achieved and the computational time consuming is the minimum possible. The computational time consuming and platform motions are also presented. The conclusion of this work is that coupled analysis should be adopted by industry as usual practice of SCR design in deep waters, where more realistic and optimum results are presented, without an excessive computational effort.
A Unified Coupled-Mode Approach to Nonlinear Waves in Finite Depth: Potential Flow
Authors: Athanassoulis G. A. , Belibassakis K. A.
Published: July 2009

A non-linear coupled-mode system of horizontal equations is presented, as derived from Luke’s (1967) variational principle, which models the evolution of nonlinear water waves in intermediate depth over a general bottom topography. The vertical structure of the wave field is represented by means of a complete local-mode series expansion of the wave potential. This series contains the usual propagating and evanescent modes, plus two additional terms, the free-surface mode and the sloping-bottom mode, enabling to consistently treat the non-vertical end-conditions at the free-surface and the bottom boundaries. The present coupled-mode system fully accounts for the effects of non-linearity and dispersion, and has the following main features: (i) various standard models of water-wave propagation are recovered by appropriate simplifications, and (ii) it exhibits fast convergenge, and thus, a small number of modes (up to 5) are usually enough for the precise numerical solution, provided that the two new modes (the free-surface and the sloping-bottom ones) are included in the local-mode series. In the present work, the coupled-mode system is applied to the numerical investigation of families of steady traveling wave solutions in constant depth, corresponding to a wide range of water depths, ranging from intermediate to shallow-water wave conditions and its results are compared vs. Stokes and cnoidal wave theories, respectively. Also, numerical results are presented for waves propagating over variable bathymetry regions and compared with second-order Stokes theory and experimental data.
Effects of Surface Tension in the Near Field Wave Breaking of Ships
Authors: Pistani Fabrizio , Olivieri Angelo , Campana Emilio
Published: July 2009

When model experiments are performed the viscous and surface tension forces are not scaled accordingly. Thus not all of the features of the flow can be satisfactorily reproduced at model scale. A comparative set of experiments for measuring the model resistance, the free surface elevation and the flow velocity in the near field, have been carried out for models of different scales for evaluating the influence of the dimensions in reproducing the complete wave breaking dynamics. The resistance curves of the models show that the scale effect is present both for low and high speeds. Comparison of the averaged surface elevation reveals that the largest model possess already some of the full scale features. The comparison of the flow velocity fields highlights substantial differences among the models in the formation of the vortical structures. The influence of these vortices on the free surface is discussed and a correlation with surface scars is proposed.
Efficient Analysis of Offshore Pipeline Buckling on Uneven Seabeds
Authors: Klæbo Frank , Giertsen Egil , Sævik Svein
Published: July 2009

Many offshore pipelines are required to operate at high temperatures and pressures. This results in increased axial stress in the pipe-wall and potentially unexpected buckling, which may have serious consequences for the integrity of the pipeline if not taken into account during the design phase. In a buckling and stability analysis, a detailed representation of the interaction between the seabed and the pipeline is very important. To capture necessary geometric effects and to ensure accurate results, a full 3D description of the seabed is desirable. The present paper deals with recent developments related to efficient algorithms for accurate prediction and simulation of buckling effects in offshore pipelines on uneven seabeds. The algorithms include a full 3D representation of the seabed, special contact elements and automatic procedures to place the pipeline along a pre-defined route. The buckling and stability analysis capabilities of the new algorithms were verified by analyzing a section of one of the Ormen Lange import lines. Ormen Lange is a very challenging field when it comes to offshore pipe design because of extreme seabed topography, strong currents, large water depth and low temperatures. The paper presents the developed methods, analysis procedures and results from several analyses demonstrating both lateral buckling effects as well as comparison with results from a commercial FEA-solver.
Metocean Data: Maximising Potential Cost Benefit
Authors: Calverley Mark J. , Fleet Richard C.
Published: July 2009

Metocean data are critical through the oil and gas development life cycle. However, the approach taken to acquisition of metocean data (whether modelled or measured), and its subsequent storage and distribution, is often not properly addressed to consider requirements beyond individual components of the life cycle. Additionally a disconnected approach to data-driven projects lacks the benefits afforded through an integrated system that can lead to both operational and financial inefficiencies. The proper consideration of metocean data is briefly discussed in this paper. However the focus of the paper is the description of a web based system that provides access to metocean data in a range of formats that best meet the needs of non-expert end users. The system can deliver: • hindcast data, • forecast data, • near real time in-situ measurement data, • and earth observing data. All readily queried through a simple a ‘GIS’ style interface. The system can be considered as a number of elements: • Data import and retrieval; • Data storage; • Data interface; • Client management system; • Client interface. The data platform underpinning the architecture is designed to readily interface to all types of data delivery, both into and out of the system. The Client Management System ensures that access to data is restricted to bona fide users of the companies owning the data. The system aims to utilise the best available technology and where appropriate to utilise available ‘off the shelf’ solutions. The focus of the work for these elements has been determining data structures and the interface between the elements. The client interface was considered to be the highly critical element as it is the only element seen by the client. It has been developed with a range of end users in mind. The information delivered will be tailored by the system to meet the requirements of various end user applications. For example meteorological and wave data may be delivered to meet the requirements of legislation to support helicopter operations. The same data may be delivered in a different way to support construction activity such that a traffic light system is applied to operational limits. The system permits delivery via standard internet browsers on a PC or via PDAs, thus enabling decision makers onshore to be aware of in-field conditions. Alerts on conditions can also be applied to ensure that personnel are aware of potential impact of metocean conditions on offshore operations.