bluuebunny/crossref_metadata_2025 huggingface dataset:
URANS Calculations for Smooth Circular Cylinder Flow in a Wide Range of Reynolds Numbers: Solution Verification and Validation
Authors: Rosetti Guilherme F. , Vaz Guilherme , Fujarra André L. C.
Published: August 2013

The flow around circular smooth fixed cylinder in a large range of Reynolds numbers is considered in this paper. In order to investigate this canonical case, we perform CFD calculations and apply Verification & Validation (V&V) procedures to draw some conclusions regarding numerical error, and afterwards, assess the modelling errors and capabilities of URANS method to solve this problem. Eight Reynolds numbers between Re = 10 and Re = 5×105 will be presented with five geometrically similar grids and five time steps for each case, together with strict control of iterative and round-off errors, allowing a consistent verification analysis with uncertainty estimation. In these calculations, two-dimensional Unsteady RANS calculations were performed making use of the k–ω SST turbulence model. The Validation procedure is performed by comparing the numerical results with an extensive set of experimental results compiled from the literature and also made available in the VIV Data Repository website (http://oe.mit.edu/VIV/).
Preliminary Modeling and Analysis of a Horizontal Pressure Differential Wave Energy Converter
Authors: McNatt J. Cameron , Özkan-Haller H. Tuba , Morrow Michael , Delos-Reyes Michael
Published: August 2013

Wave energy converters (WECs) have been proposed that take advantage of spatially varying pressure differentials (PDs) in a wave field to drive a fluid flow. In order to accurately assess the pressure forcing on PD devices, physics-based relationships between major device parameters and device performance need to be determined. Herein, a transfer function is developed that relates horizontally oriented PD device configurations and wave conditions to the amount of pressure forcing available to the device. Investigation of the transfer function confirms intuitive expectation, but also yields surprising results. The transfer function can be applied to a wave spectrum to create a pressure resource spectrum. By manipulating the device length and orientation, an optimal configuration can be found that maximizes the total harnessable pressure resource for a given wave condition or a wave climate. Optimal device lengths for directional seas are longer than those for non-directional seas, and a wide range of suboptimal configurations yields a reasonable pressure resource. The pressure resource transfer function is a fundamental tool for understanding how horizontal PD WECs work and designing an optimal device.
Analysis of Ship Hull Plate Vibrations Induced by Wave and Slamming Loads
Authors: Lakitosh F. , Ananthakrishnan P.
Published: August 2013

The hydroelasticity problem related to multi-hull ship plate vibrations excited by periodic and transient surface wave forces is analyzed. Theoretical method based on the Wagner theory and empirical methods based on classification society ABS rules are considered for determining the transient slamming force on the ship hull. A boundary integral method based on the simple source distribution (Yeung [1]) and mixed Eulerian and Lagrangian (MEL) formulation (Longuet-Higgins and Cokelet [2]) for the determination of the slamming force is in progress. A suite of plate theories, ranging from small-amplitude linear undamped isotropic plate theory to damped nonlinear, stiffened-plate theories, are considered to determine the vibrations of ship-hull plates subject to the wave forces. In the present work, finite-difference algorithms are developed to solve the nonlinear plate equations. Results for range of sea states and hull scantlings are obtained to determine key structural parameters affecting the structural integrity. The algorithms and the methodology developed can be used for efficient design of multi-hull ships, and perhaps also to update classification society rules on structural design.
EdgeCFD-ALE: A Stabilized Finite Element System for Fluid-Structure Interaction in Offshore Engineering
Authors: Alves José L. D. , Silva Carlos E. , Guevara Nestor O. , Coutinho Alvaro L. G. A. , Elias Renato N. , Rochinha Fernando F. A. , Martins Marcos A. D. , Ferreira Marcos D. A. S. , Silva Daniel F. C.
Published: August 2013

This work presents the development of EdgeCFD-ALE, a finite element system for complex fluid-structure interactions designed for offshore hydrodynamics. Sloshing of liquids in tanks, wave breaking in ships, offshore platforms motions and green water on decks are important examples of these problems. The software uses edge-based parallel stabilized finite elements for the Navier-Stokes equations and the Volume-Of-Fluid method for the free-surface, both described by an Arbitrary Lagrangian Eulerian (ALE) formulation. Turbulence in is treated by a Smagorinsky model. Mesh updating is accomplished by a parallel edge-based solution of a non-homogeneous scalar diffusion problem in each spatial coordinate. Boundary conditions involve the motion of the immersed body’s surface, i.e., the fluid-structure interface, taken as the Lagrangian portion of the domain in the overall problem. The simulation capabilities of the present software are demonstrated in the solution of two problems, the interaction of two cylinders in tandem and the free fall of a sphere.
Dynamic Fracture Toughness of Coated Structural Components at Different Temperatures
Authors: Manjunath G. L. , Surendran S.
Published: August 2013

Fracture toughness is a material property in the same sense that yield strength is a material property. The determination of fracture toughness for dynamic loading conditions is not very straightforward, as dynamic crack growth speed in supersonic speed and the speed range is 1 to 2 km/sec. The improvement of fracture toughness of metals plays a vital role in the design and manufacturing of structural components. To achieve this purpose, industries rely up on coatings which are an integral part of manufacturing. These coated samples are tested by Charpy V-notch impact testing for estimating dynamic fracture toughness. These coatings improve the wear and corrosion resistance of the materials and they tend to reduce the strength of the materials, because of the increased residual stresses due to the coating process. These defects cannot be precluded from these coated and treated components. The strength of those components in the presence of such defects can be analyzed by fracture mechanics approach. An attempt has been made to analyze the effect of coating methods like electroplating and PVD (Physical Vapour Deposition), coating thickness, heat treatment and the service temperature on the fracture behavior of metals. The experiments have been carried out on EN8 steel and aluminium for different temperatures. The specimen preparation and experimentations were carried out according to the ASTM standard E-23. The FRANC 2D (Fracture Analysis Code) has been relied upon for estimating the stress intensity factor at different crack length and temperature.
Simulated Field Measurements Based Response Amplitude Operators Estimation via the TVAR Model
Authors: Yazid Edwar , Liew Mohd. Shahir , Parman Setyamartana
Published: August 2013

The purpose of this paper is to propose and investigate a new approach for estimating field measurements based response amplitude operators (RAOs) for offshore structures. The approach is based on applying time-varying autoregressive (TVAR) model. This work is virtually unexplored in offshore engineering field. In the literatures, a number of works have shown that RAO are discrete Fourier transform (DFT). Here, we outline a practical algorithm for TVAR model which uses expectation-maximization (EM) algorithm based Kalman smoother to generate RAOs of surge, heave and pitch motions. The method is then applied to sampled discrete wave as excitation input and motion responses of offshore structures as outputs, generated from simulated field measurements. The proposed approach outlined here has shown tremendous potential in the application of RAO estimation. The results indicate that TVAR model bring enhanced accuracy, less noisy in obtaining RAOs estimates over other Fourier based methods. TVAR model also allows for the creation of time-varying RAOs.
Gravity Anchors Astride Subsea Pipelines Subject to Oscillatory and Combined Steady and Oscillatory Flows
Authors: Zhao Xu , Cheng Liang , Zhao Ming , An Hongwei , He Wei
Published: August 2013

This writing presents results of simulating oscillatory and combined steady and oscillatory flows past gravity anchors astride subsea pipelines. It can be considered a companion to a previous numerical study on steady currents past gravity anchors. The gravity anchor system comprises large arch-shaped concrete blocks positioned at intervals astride offshore pipelines, and it is engineered to provide innovative and cost-effective secondary stabilisation for high-capacity gas-transporting pipelines serving in severe metocean conditions, e.g. cyclone-prone offshore areas. A free-settling marine object bottom-seated on the seabed, however, the gravity anchor may subside into scour pits around its base due to locally disturbed flow regimes, imposing integrity risks on the pipe. Also, the effect of gravity anchors on hydrodynamic loading on nearby pipeline lengths is of interest. The present study employed a Petrov-Galerkin finite element method to solve the three-dimensional Navier-Stokes equations in direct numerical simulation. Firstly sinusoidal flow oscillating perpendicularly to the pipe beneath gravity anchors on an immobile bed was simulated at a Keulegan-Carpenter number of 10 and a pipe Reynolds number of 1000. Then, a steady current co-directionally superimposed on the aforementioned oscillatory flow was modelled at a ratio of steady current velocity to oscillatory flow velocity amplitude of 1. With sediment transport capacity related to bed shear stresses, the time-averaged bed shear stress amplification around gravity anchors in oscillatory flow was revealed first, and found to be consistent with laboratory observations of scour patterns. The effect of superimposing steady flow onto oscillatory flow on bed shear stress amplification was then explored. Lastly, hydrodynamic forces on pipelines in the vicinity of gravity anchors were gauged. The present work intends to shed light on the initial seabed responses with regard to the scour process around gravity anchors immersed in the oceanic wave boundary layer, as well as the effect of gravity anchors on hydrodynamic loadings on pipelines.
Fatigue Design of Roller Bearing for Large FPSO Turrets
Authors: Lassen Tom , Hansen Svein Erik , Askestad Sigmund
Published: August 2013

The present report presents the fatigue design and a fatigue life prediction method for large roller bearings applied in the turret turn table for large Floating Production Storage and Offloading (FPSO) units. The contact point between wheel and rail in these bearings is subjected to a multi-axial stress situation and both surface wear and fatigue cracking may occur. Stress analyses with contact elements are carried out and a methodology based on the Dang Van fatigue criterion is adopted. The criterion is based on an equivalent stress defined as a combination of the fluctuation of the shear stress from its mean value and the associated hydrostatic stress at a critical plane at any time. The criterion has been used in the railway industry and is regarded as an appropriate approach for the crack initiation phase in the high cycle fatigue regime. A realistic case study is included where material combinations with high strength stainless steel S165M and high strength carbon steel 42CrMo4 are applied. It is demonstrated that a favorable wheel-rail design is characterized by an extreme load condition with a stress situation within the linear elastic behavior of the steel. Furthermore, the Dang Van equivalent stress function shall be below the shear stress fatigue limit. These two design criteria will ensure robust design and long durability of the turret bearings.
Dynamic Response of a Porous Seabed Combined Wave and Current Loadings: Effects of Inertial Terms
Authors: Liu Bo , Jeng Dong-Sheng
Published: August 2013

In this paper, an analytical solution for the response of a porous seabed to combined wave and current loadings is presented. Unlike most previous investigations, currents are considered in this study. Based on the newly analytical solution, a detailed parametric study will be carried out to explore the effects of combined wave and current loading and inertial terms on the pore pressure in marine sediments. Numerical examples demonstrate the significant effects of currents and inertial terms on the pore pressures in shallow water. The relative difference of the maximum pore pressures between the present model and the previous model (without currents) can reach 7% of the static water pressure.
Numerical Modeling of Seabed Response to Combined Wave and Current Loading
Authors: Zhang Ji-Sheng , Zhang Yu , Jeng Dong-Sheng , Zhang C.
Published: August 2013

An integrated model is developed to study the response of a porous seabed to combined wave-current loading. While the Reynolds-Averaged Navier-Stokes (RANS) equations with k-ε turbulence closure scheme and internal wave-maker function are solved for the wave-current interactions, Biot’s poro-elastic “u-p” model is adopted for the seabed response. After validated by the laboratory measurement, this model is applied to investigate the effects of wave, current and soil parameters on the wave-current induced seabed response. Numerical examples conclude that interacting with the following currents, waves with a shorter period or greater height lead to smaller values of maximum pore pressure.
Physical Modelling of Offshore Structures Founded on Sea Bed
Authors: de Freitas Fagundes Diego , Almeida Maria Cascão F. , Almeida Marcio S. S. , Rammah Khader I.
Published: August 2013

This work presents an experimental study using centrifuge modelling on mudmat shallow foundations which are mainly used to support offshore subsea equipment. An optimized design of these foundations requires the maximizing of the ratio between the installation and the pull-out resistance of the foundation. International standards, based on classical theories, are often limited and do not meet the complexity of the projects. The bibliography is not yet conclusive about the load capacity factors for this particular problem. In this work, physical modelling has been performed on both perforated and solid mudmats subjected to vertical load and simulating both installation and extraction scenarios.

The soil used in this study is a marine clay taken from the Roncador field at Campos basin located offshore of the state of Rio de Janeiro, Brazil. The physical modelling experiments have been carried out on the mini-drum geotechnical centrifuge at COPPE/UFRJ. The technique of lumps was adopted for the preparation of the soil models and a temporary surcharge using a sand layer has been applied to develop a soil strength profile similar to the one found in the field. The results obtained in this study provided a better understanding of the soil-structure interaction problem with regard to the installation and pull-out resistance of the studied type of foundation focusing on their bearing capacity factors.
The Formation of Surfable Waves in a Circular Wave Pool: Comparison of Numerical and Experimental Approaches
Authors: Javanmardi Mohammadreza , Binns Jonathan , Renilson Martin R. , Thomas Giles , Schmied Steven , Huijsmans Rene
Published: August 2013

This paper investigates the capability of a numerical approach to address the problem of designing a wave pool. The numerical approach developed has the potential to reduce the number of design alternatives which require testing by eliminating poor performing designs early in the design cycle. For the three dimensional computations in the present study, the CFD software FLUENT (which solves the RANS equations with finite-volume approach and uses the volume of fluid technique to simulate the free-surface motion) was utilized. Pressure source models in straight and round tracks were simulated. Predicted results agreed closely with experiment data.
Wavemaker Limitations and Difficulties in Model Testing of Small High Speed Craft
Authors: Fu Thomas C. , Fullerton Anne M. , Schleicher Christine C.
Published: August 2013

Model testing of marine platforms has always focused on preserving geometric similitude and Froude number. While Froude scaling does provide a good approximation to true dynamic similitude, there are viscous effects which are not scaled. Additionally, the dynamic testing of small high speed marine craft presents difficulties due to their small size, so while one would prefer to minimize these effects by testing large models, model basin characteristics limit the physical size of the models that can be tested. These issues as they relate to model testing, including sea state scaling and scale effects, are discussed.
The Effect of Exposure Length on Vortex Induced Vibration of Flexible Cylinders
Authors: Rao Zhibiao , Vandiver J. Kim , Jhingran Vikas , Sequeiros Octavio
Published: August 2013

This paper addresses a practical problem: “What portion of fairing or strake coverage may be lost or damaged, before the operator must take corrective measures?” This paper explores the effect of lost fairings (the exposure length) on Vortex-Induced Vibration (VIV) of flexible cylinders. The source of data is a recent model test, conducted by SHELL Exploration and Production. A 38m long pipe model with varying amounts of fairings was tested. Response as a function of percent exposure length is reported. Unexpected results are also reported: (i) the flexible ribbon fairings used in the experiment did not suppress VIV at speeds above 1 m/s; (ii) Above 1 m/s, a competition was observed between VIV excited in the faired and bare regions of the cylinder, (iii) Unusual traveling wave behavior was documented—waves generated in the bare region periodically changed direction, and exhibited variation in VIV response frequency.

The results of these tests showed that (1) the excitation on the bare and faired regions could be identified by frequency, because the faired region exhibited a much lower Strouhal number; (2) as expected, the response to VIV on the bare region increased with exposure length; (3) the response to VIV on the faired region decreased with exposure length.
Experimental Study on the Dynamics of Four Flexible Cylinders in Square Arrangement Subjected to Uniform Cross-Flow
Authors: Sanaati Bijan , Kato Naomi
Published: August 2013

Groups of cylinders can be found in many engineering fields such as marine and civil applications. The behaviors of the group cylinders can be very complex because it undergoes the mutual effects of adjacent cylinders arranged in different positions. In this paper, we present the results of a study on the dynamics of a group of flexible cylinders in square arrangements along with a single (isolated) cylinder subjected to uniform cross-flow (CF). Four cylinders of the same size, properties, and pretensions were tested in two configurations with different centre-to-centre separations. Horizontal and vertical separations were 2.75D & 2.75D and 5.50D & 2.75D for the first and second configurations, respectively. The tandem (horizontal) separations between the downstream and upstream cylinders, i.e., 2.75D and 5.5D, correspond to the reattachment and co-shedding regimes, respectively. Vertical separation, i.e., 2.75 was chosen in a range where the side-by-side cylinders can have proximity interference. Reynolds number ranged from 1400 to 20000 (subcritical regime). The parameter of reduced velocity reached up to 19. The aspect ratio of all the cylinders was 162 (length/diameter). Mass ratio (cylinders mass/displaced water) is 1.17, a low mass ratio. The amplitude ratio of the CF vibration of the downstream cylinders, hydrodynamic force coefficients including mean and fluctuating components of the drag and lift forces, and frequency responses for both CF and inline (IL) directions were analyzed. All the cylinders excited up to the second and fourth mode of vibrations for CF and IL directions, respectively. Mean drag coefficient of the upstream cylinders are almost twice those of the downstream cylinders at high reduced velocities. The mean lift coefficient is much higher for the upstream cylinders than the downstream cylinders with a negative value. Obvious IL and CF lock-in regions exist for all four cylinders at low reduced velocities. Among the four cylinders, the upper downstream cylinder shows the least and the most fluctuating lift and drag forces, respectively. The IL and CF frequencies of the downstream cylinders are much lower than those of the upstream ones and the single cylinder.
Supercritical Group Velocity for Dissipative Waves in Shallow Water
Authors: Jung Tae-Hwa , Lee Changhoon
Published: August 2013

The group velocity for waves with energy dissipation in shallow water was investigated. In the Eulerian viewpoint, the geometric optics approach was used to get, at the first order, complex-valued wave numbers from given real-valued angular frequency, water depth, and damping coefficient. The phase velocity was obtained as the ratio of angular frequency to realvalued wave number. Then, at the second order, we obtained the energy transport equation which gives the group velocity. We also used the Lagrangian geometric optics approach which gives complex-valued angular frequencies from real-valued wave number, water depth, and damping coefficient.

A noticeable thing was found that the group velocity is always greater than the phase velocity (i.e., supercritical group velocity) in the presence of energy dissipation which is opposite to the conventional theory for non-dissipative waves. The theory was proved through numerical experiments for dissipative bichromatic waves which propagate on a horizontal bed.

Both the wave length and wave energy decrease for waves with energy dissipation. As a result, wave transformation such as shoaling, refraction, and diffraction are all affected by the energy dissipation. This implies that the shoaling, refraction, and diffraction coefficients for dissipative waves are different from the corresponding coefficients for non-dissipative waves. The theory was proved through numerical experiments for dissipative monochromatic waves which propagate normally or obliquely on a planar slope.
Numerical Investigation of Flow Around a ROV With Crawler Based Driving System
Authors: Katsui Tokihiro , Kajikawa Satoshi , Inoue Tomoya
Published: August 2013

The Remotely Operated Vehicle, so called “ROV” which has crawler based moving system is considered as one of the appropriate underwater vehicles for seafloor exploration or seabed resources development [1][2][3][4][5][6][7]. The advantages of crawler driven ROV are to be able to stay on a fixed sea bottom location and to be capable to do heavy works such as digging the seafloor. However, the ROV moving on the sea bottom with crawler based driving system easily turn over due to the buoyancy and hydrodynamic forces [8][9][10][11][12]. Therefore, it is important to know the moving capability of the ROV on the sea bottom for the design point of view. The authors have shown the condition for the normal running of the ROV which moves on horizontal and inclined flat sea bottom by means of a simple dynamic model [11]. Normal running means that the ROV runs without bow-up or stern-up situations and the crawlers touch the ground normally. The normal running condition of ROV indicates the constrained condition of the relation between gravity and buoyancy center locations for any given design parameters such as geometry, weight, displacement and running speed of the ROV. Though this method estimates the ROVs’ moving capability with acceptable accuracy, the hydrodynamic forces on the ROV and its application point are required for accurate estimation. In the previous research, those quantities are roughly estimated from the past experimental investigations. The present study investigated the flow around the crawler driven ROV which runs on seafloor with CFD (Computational Fluid Dynamics) analysis to evaluate the characteristics of hydrodynamic forces acting on the ROV. The open source CFD code, OpenFOAM [13] was applied for flow calculation and the results were validated with model experiments. By using the calculated hydrodynamic forces on ROV, the moving capability of ROV was evaluated with a method the authors had shown. The estimates of the running capability of the ROV by using the CFD calculations are quite different from past estimations in some running conditions.
Planning for Construction Work in Cold Climate Regions
Authors: Gudmestad Ove T. , Karunakaran Daniel
Published: August 2013

With increased interests in oil and gas exploration in cold climate regions, it is not realistic that all construction activities can take place during the short summer and work will continue into the early fall and possibly later. The offshore contractors must, therefore, be ready to participate in construction work in these regions during an extended season, i.e. outside the summer season with milder weather conditions. It is also important that some key work-intensive activities (e.g. pipe laying) can start as early as possible in the season.

This paper will discuss the challenges associated with construction work in cold climate regions with emphasis on the physical conditions, in particular with reference to Polar Low Pressures and the potential for icing, as well as the logistics of working long distances from established supply bases.

Large uncertainties in weather forecasts call for proper management decisions accounting for the specifics of the area. Long periods of “waiting on weather” might result and management must have the patience to wait until safe operations can commence.

Emphasis will be on the Barents Sea where recent hydrocarbon findings have proven very encouraging and where a huge area soon will be opened for exploration, following the agreement on the border between Norway and Russia, potentially calling for joint Norwegian–Russian construction projects (Bulakh et al., 2011).
Seed Variations of Extreme Sea States and Repeatability of Extreme Crest Events in a Model Test Basin
Authors: Scharnke Jule , van den Berg Joris , de Wilde Jaap , Vestbostad Tone , Haver Sverre
Published: August 2013

New methods for estimation of extreme wave crest height have resulted in an increase of the estimated 10,000 year crest height. In the Central North Sea this increase is typically 2 to 4 m. As a result several fixed platforms designed prior to 2000 may experience negative air gap if being hit by the 10,000 year wave crest height. Numerical methods have been used for assessing wave-in-deck impact loads. The model tests discussed in this paper were conducted to be used as verification of the numerical codes. For the model tests two sea states along the 10,000 year contour line were considered. Several 3-hour (full scale time) realizations were calibrated in order to capture the natural variability of the most extreme crest heights. Realizations were generated both by using random phase (and deterministic amplitude) and by using random coefficients (i.e. random phase and random amplitude), for the purpose of discussing possible difference between the two approaches.

For wave deck impact problems, one is merely interested in the few very large wave crests out of a 3-hour simulation. A more efficient test scope would, therefore, be to generate only the largest wave groups of the realizations. For this reason the most extreme crest(s) per sea state were identified and most wave-in-deck tests were conducted by generating only the part of the time series containing the large crest(s). The adequacy of the crest characteristics of the short realization as compared to the crest characteristics of the full series will be discussed.

Impact tests will by nature involve rather steep waves that may be in the vicinity of breaking. The repeatability of such waves can be challenging. For the purpose of simplifying the interpretation of wave-in-deck impact loads, the repeatability (the uncertainty in the crest that actually approaches the platform) of these wave groups will be discussed.
Blind Estimation of a Ship’s Relative Wave Heading
Authors: Nielsen Ulrik D. , Iseki Toshio
Published: August 2013

This article proposes a method to estimate a ship’s relative heading against the waves. The procedure relies purely on ship-board measurements of global responses such as motion components, accelerations and the bending moment amidships. There is no particular (mathematical) model connected to the estimate, and therefore it is called a ‘blind estimate’. The approach is in this introductory study tested by analysing simulated data. The analysis reveals that it is possible to estimate a ship’s relative-heading on the basis of shipboard measurements only.
Slamming Simulations in a Conditional Wave
Authors: Seng Sopheak , Jensen Jørgen Juncher
Published: August 2013

A study of slamming events in conditional waves is presented in this paper. The ship is sailing in head sea and the motion is solved for under the assumption of rigid body motion constrained to two degree-of-freedom i.e. heave and pitch. Based on a time domain non-linear strip theory most probable conditional waves are generated to induce short term extreme responses of 4500 MNm sagging and hogging vertical bending moment (VBM) amidships on a modern 9,400-TEU post-Panamax container ship and 3000 MNm (sag) on a Panamax container ship. The results of the strip theory are compared to the results of free surface NS/VOF CFD simulations under the same wave conditions. In moderate seas and no occurrence of slamming the structural responses predicted by the methods agree well. When slamming occurs the strip theory overpredicts VBM but the peak values of VBM occurs at approximately the same time as predicted by the CFD method implying the possibility to use the more accurate CFD results to improve the estimation of slamming loads in the strip theory through a rational correction coefficient.
Towards Efficient Fully-Nonlinear Potential-Flow Solvers in Marine Hydrodynamics
Authors: Shao Yan-Lin , Faltinsen Odd M.
Published: August 2013

Solving potential-flow problems using the Boundary Element Method (BEM) is a strong tradition in marine hydrodynamics. An early example of the application of BEM is by Bai & Yeung [1]. The bottleneck of the conventional BEM in terms of CPU time and computer memory arises as the number of unknowns increases. Wu & Eatock Taylor [2] suggested that the Finite Element Method (FEM) field solver is much faster than the BEM based on their comparisons in a wave making problem. In this paper, we aim to find a highly efficient method to solve fully-nonlinear wave-body interaction problems based on potential-flow theory. We compare the efficiency and the accuracy of five different methods for the potential flows in two dimensions (2D), two of which are BEM-based while the other three are field solvers. The comparisons indicate that it is beneficial to use either an accelerated matrix-free BEM, e.g. Fast Multipole Method accelerated BEM (FMM-BEM), or any field solvers whose resulting matrix are sparse.

Another highlight of this paper is that an efficient numerical potential-flow method named the harmonic polynomial cell (HPC) method is developed. The flow in each cell is described by a set of harmonic polynomials. The presented procedure has approximately 4th order accuracy, while its resulting matrix is sparse similarly as the other field solvers, e.g. Finite Element Method (FEM), Finite Difference Method (FDM) and Finite Volume Method (FVM). The method is verified by a linear wave making problem for which the steady-state analytical solution is available, and the forced oscillation of a semi-submerged circular cylinder for which the frequency-domain added mass and damping coefficients are compared. The fully-nonlinear wave making problem and nonlinear propagating waves over a submerged bar are also studied for validation purposes. Only 2D cases are studied in this paper.
Using CFD to Assess Low Frequency Damping
Authors: Pistidda Alessio , Ottens Harald , Zoontjes Richard
Published: August 2013

During offshore installation operations, floating bodies are often moored using soft mooring which are designed to withstand the environmental forces. Large amplitude motions often occur due to excitation by slowly varying wind and wave drift forces. To analyze these motions the dynamic system has to be accurately described, which includes an estimation of the added mass and damping coefficients. In general, the added mass can be accurately calculated with traditional potential theory. However for the damping this method is not adequate because viscous effects play an important role. Generally these data are obtained using model tests. This paper validates the CFD methodology as an alternative to model tests to evaluate the viscous damping. The aim is to define a standard procedure to derive viscous damping coefficients for surge, sway and yaw motion of floating bodies.

To estimate viscous damping in CFD, a 3D model of the launch and float-over barge H-851 was used. For this barge, model test data is available which could be compared with the results of the CFD analysis. For the simulations, the commercial package STAR-CCM+ with the implicit unsteady solver for Reynolds-Averaged Navier-Stokes (RANS) equations was used. The turbulence model implemented was the k-Omega-SST. Numerical errors have been assessed performing sensitivity analysis on time step and grid size. Damping has been investigated by performing decay simulations as in the model tests, taking the effect of coupling among all motions into account. The P-Q fitting method has been used to determine the linear and quadratic component of the damping. Numerical results are validated with those obtained from the towing tank.

Results show that CFD is an adequate tool to estimate the low frequency damping in terms of equivalent damping. More investigations are required to determine the linear and quadratic component.
Wet/Dry Areas Modelling of Interaction Between Wind and Water Waves Under Zero Wind Speed
Authors: Wen Xianyun
Published: August 2013

Accurate prediction of the distribution of shear stress is essential for the numerical investigation of the interaction between the wind and water waves on ocean surface since the shear stress plays a key role in this type of interfacial flows. The numerical velocity distribution provided by the computational fluid dynamics should have high accuracy as the shear stress is computed by the derivative of the numerically predicted velocity. The recently developed wet/dry areas method based on the conservative integral form of the Navier-Stokes equations mathematically reveals that the convection terms in the Navier-Stokes equations should be calculated on the surface of control volume and the mass flux on the surface of the control volume should be calculated by the areas exposed to the water and air. In this paper the new numerical method, the wet/dry areas method, is briefly explained and applied to a two-dimensional viscose air-water flow when the wind speed is zero.
Utilizing TLCD (Tuned Liquid Column Damper) in Floating Wind Turbines
Authors: Shadman Milad , Akbarpour Abbas
Published: August 2013

Among the floating wind turbine support concepts for carrying large-scale wind turbines, the barge type is more simple and inexpensive to install. The ability to install barge type platforms over a broad range of sea depths increases the number of site options suitable for its installation. Although there are several advantages related to barge type platforms, its significant angular motions which induce dynamic loads in the rotor, tower and drivetrain, hinder its commercial development. In this study, a single degree-of-freedom TLCD (Tuned Liquid Column Damper), which is placed on the turbine’s tower, is incorporated into a modified version of the aero-elastic code FAST. The response of a floating wind turbine with a barge type support controlled by a TLCD subjected to couple hydrodynamic and aerodynamic loads is investigated. The solution of multi degree-of-freedom floating wind turbine coupled with a TLCD dynamic system is done by a sequential method. In this method, two coupled systems of nonlinear differential equations are solved separately by a modified version of FAST in which an added module solves the nonlinear differential equation of motion of the TLCD. The results are compared to the baseline system. The results indicate that this passive type control approach can be used to improve the structural response of floating wind turbines.
Prediction of Sheet Cavitation on Marine Current Turbines With a Boundary Element Method
Authors: Baltazar J. , Falcão de Campos J. A. C.
Published: August 2013

For marine current turbines under certain operation conditions cavitation on the blades may occur. Therefore, it is important from the design stage of such systems to be able to predict the presence and extent of cavitation on the blades. In this paper a boundary element method for the prediction of sheet cavitation of a horizontal axis marine current turbine is presented. The boundary element method is based on a low-order potential formulation. Dipoles and sources are placed on the rigid body surfaces either on the wetted part and beneath the cavities. Kinematic boundary conditions are applied on the wetted surfaces and kinematic and dynamic boundary conditions are applied on the surface of the cavities. The blade wakes are modeled with an empirical formulation. The method is applied to analyze a marine current turbine in steady flow conditions and results are compared with the cavitation observations available in the literature.
A Two-Dimensional Numerical Simulation of Roll Damping Decay of a FPSO Using the Upwind TVD Scheme of Roe-Sweby
Authors: Guarniz Avalos Gustavo O. , Wanderley Juan B. V.
Published: August 2013

The study of roll damping is investigated for a Floating Production Storage and Offloanding (FPSO). For this purpose, a roll decay test of FPSO is simulated by means of the numerical solution of the slightly compressible Navier-Stokes equations in 2D. The governing equations are solved using the finite volume method and the upwind TVD scheme of Roe-Sweby. The roll damping for rectangular hulls is dominated by viscous effects. Strong vortices are formed around the bilge keel. Hence, in this zone, there is a discontinuity of pressure that the TVD scheme will resolve and capture the physics of the phenomenon without spurious oscillations. The numerical results are compared with experimental data for validating the numerical scheme implemented.
Time Domain Simulation of a Ship Drifting in a Seaway
Authors: Subramanian Rahul , Beck Robert F.
Published: August 2013

The effects of waves on floating bodies are of great importance to the naval architect. In addition to the first order responses to the waves, second order slowly varying forces play a vital role in various scenarios. In this research, a fast and robust method for simulating the drifting of a vessel in a seaway has been developed based on a body-exact strip theory method (cf. Bandyk (2009)). The problem is solved directly in the time domain by setting up a boundary value problem for the perturbation potentials. A boundary integral method is used for solving the Laplace equation. The forces are computed by direct pressure integration. Body accelerations and velocities are computed by using a nonlinear 6-DOF Euler equation of motion solver. Free motion drift simulations have been carried out for long times, on the order of hundreds of wave periods for the Wigley-I hull and the containership S-175. The long time simulations establish the stability and robustness of the numerical scheme. The results also show the effectiveness of the methodology to capture slowly varying 2nd order forces in addition to the wave frequency responses.

The authors take great pleasure in presenting this paper in this symposium honoring Prof. Ronald W. Yeung. Prof. Yeung has worked extensively in the areas of ship hydrodynamics and offshore mechanics. He was one of the first authors to have used a source distribution on the free surface instead of Green’s function to solve time domain free surface problems. The present research work also makes use of a source distribution on the free surface.
Fully Coupled Three-Dimensional Dynamic Response of a TLP Floating Wind Turbine in Waves and Wind
Authors: Ramachandran G. K. V. , Bredmose H. , Sørensen J. N. , Jensen J. J.
Published: August 2013

A dynamic model for a tension-leg platform (TLP) floating offshore wind turbine is proposed. The model includes three-dimensional wind and wave loads and the associated structural response. The total system is formulated using 17 degrees of freedom (DOF), 6 for the platform motions and 11 for the wind turbine. Three-dimensional hydrodynamic loads have been formulated using a frequency- and direction-dependent spectrum. While wave loads are computed from the wave kinematics using Morison’s equation, aerodynamic loads are modelled by means of unsteady Blade-Element-Momentum (BEM) theory, including Glauert correction for high values of axial induction factor, dynamic stall, dynamic wake and dynamic yaw. The aerodynamic model takes into account the wind shear and turbulence effects. For a representative geographic location, platform responses are obtained for a set of wind and wave climatic conditions. The platform responses show an influence from the aerodynamic loads, most clearly through a quasi-steady mean surge and pitch response associated with the mean wind. Further, the aerodynamic loads show an influence from the platform motion through more fluctuating rotor loads, which is a consequence of the wave-induced rotor dynamics. In the absence of a controller scheme for the wind turbine, the rotor torque fluctuates considerably, which induces a growing roll response especially when the wind turbine is operated nearly at the rated wind speed. This can be eliminated either by appropriately adjusting the controller so as to regulate the torque or by optimizing the floater or tendon dimensions, thereby limiting the roll motion. Loads and coupled responses are predicted for a set of load cases with different wave headings. Based on the results, critical load cases are identified and discussed. As a next step (which is not presented here), the dynamic model for the substructure is therefore being coupled to an advanced aero-elastic code Flex5, Øye (1996), which has a higher number of DOFs and a controller module.
Comparison of Calculated In-Line Vortex Induced Vibrations to Model Tests
Authors: Passano Elizabeth , Larsen Carl M. , Lie Halvor
Published: August 2013

The purpose of the present paper is to compare vortex-induced vibrations (VIV) in both in-line and cross-flow directions calculated by a semi-empirical computer program to experimental data. The experiments used are the Bearman and Chaplin experiments in which a model of a tensioned riser is partly exposed to current and partly in still water.

The VIVANA program is a semi-empirical frequency domain program based on the finite element method. The program was developed by MARINTEK and the Norwegian University of Science and Technology (NTNU) to predict cross-flow response due to VIV. The fluid-structure interaction in VIVANA is described using added mass, excitation and damping coefficients. Later, curves for excitation, added mass and damping for pure in-line VIV response were added. These curves are valid for low current levels, before the onset of cross-flow VIV response.

Recently, calculation of response from simultaneous cross-flow and in-line excitation has been included in VIVANA. The in-line response frequency is fixed at twice the cross-flow response frequency and the in-line added mass is adjusted so that this frequency becomes an eigenfrequency. A set of curves based on forces measured during combined cross-flow and inline motions are used. At present, the in-line excitation curves are not dependent on the cross-flow response amplitude.

In the paper, in-line and cross-flow response predicted by VIVANA will be compared to the Bearman and Chaplin model tests. The choice of added mass and excitation coefficients will be discussed.
Friction Forces Between Seabed and Fishing Gear Components
Authors: Enerhaug Birger , Ivanović Ana , O’Neill Finbarr , Summerbell Keith
Published: August 2013

Bottom trawls are still the most common, and most energy consuming type of fishing gear. For this type of fishing gear, as well as other types with bottom contact, the contact forces between gear elements and seabed have a significant influence on both resistance and shape.

This paper describes the work that has been done in research programs at SINTEF in Norway and Marine Scotland Science in Scotland. In both studies sections of rock hoppers were examined, full scale sections and 1:5 scaled model sections were tested at sea and in laboratory, respectively. Scaled models at SINTEF were tested with the submerged models in water along with the additional tests in order to establish associated hydrodynamic forces. The tests were undertaken at angle of attack 0–90 degrees. Similar procedure was followed for full scale models giving a scope for potential comparison. The coefficients of friction for each model are presented as a function of penetration depth, towing speed and angle of attack, and the relevance as friction load models for fishing and off-shore gear are discussed.
Deepwater Current Profile Data Sources for Riser Engineering Offshore Brazil
Authors: Jeans Gus , Prevosto Marc , Harrington-Missin Liam , Maisondieu Christophe , Herry Christelle , Lima José Antonio M.
Published: August 2013

A variety of current profile data sources are compared for a deepwater site offshore Brazil. These data were gathered for consideration as part of the Worldwide Approximations of Current Profiles (WACUP) Joint Industry Project, described separately in OMAE2012-83348. The primary source of data for current profile characterisation is site specific full water column measurement. Sufficiently high vertical and temporal resolutions are required to capture the dominant oceanographic processes. Such in-situ data are generally expensive and time consuming to collect, so there is an increasing tendency for numerical model current data to be considered for engineering applications. In addition to being relatively inexpensive and quick to obtain, model data are also typically of much longer duration. This potentially allows inter-annual variability and rare extreme events to be captured. However, the accuracy and reliability of numerical model data remains questionable, or unproven, in many deepwater development regions. This paper explores the suitability of such models to represent a deepwater site offshore Brazil, in relation to the key oceanographic processes revealed within the in-situ data.