bluuebunny/crossref_metadata_2025_split huggingface dataset:
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.
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.
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.
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.
Stress Development Inside Large Diameter Pipe Piles Using a Soil Plug Forcing System
Authors: Fischer Jan , Henke Sascha , Höhmann Sebastian 
Published: August 2013

It is well known that soil plugging inside tubular steel piles will only appear in rather small diameter piles during impact driving. Therefore, large open ended steel piles, which are often used for the fixation of offshore buildings, such as wind farms, are highly unlikely to develop an internal soil plug. To take advantage of a soil plug, where a significant rise in the piles’ bearing capacity generally appears, a large diameter pipe-pile with an inner steel ring was designed by the third author. The location of the steel ring was determined by the soil formation in situ. To avoid increasing pile driving energy, the internal ring should dip into dense soil conditions only for the last few decimeters of driving. In October 2010, a full scale test was performed in the harbor of Hamburg, using two tubular piles with an outer diameter of 1220 mm. One pile was equipped with an inner steel ring as described above. The second pile was a typical tubular pile without any attached systems. To better compare the results, both piles were driven next to each other. Both piles were equipped with internal total stress and pore water pressure sensors at the pile tip to investigate the radial stress development during and after installation. Acceleration and strain at the pile head were measured to predict the bearing capacity. Using the numerical analysis program CAPWAP (Case Pile Wave Analysis Program) [15], the distribution of shaft and toe friction can be determined additionally. Furthermore, the internal soil movement was surveyed during driving. The results of the measurements showed, that when using an inner steel ring, a significant rise in internal radial stresses and the piles’ bearing capacity occurs.
To better understand the stress development inside and outside the two investigated piles during driving, a numerical back-calculation of the recorded measurements was performed. The results of the full scale and numerical simulations, with a particular focus on the use of an internal steel ring to force the soil to plug behavior in large diameter pipe piles, is presented in the following paper.
Slender Buoy FSHR Vortex Induced Rotations
Authors: Minguez Matthieu , Luppi Ange , Berger Anthony 
Published: August 2013

The paper addresses the Flow Induced Response (FIR) of a sub-surface Buoyancy Can (BC) and more particularly the yaw motions possibly due to the current vortices.
2D Computational Fluid Dynamic (CFD) simulations have been performed to analyse the turbulent flow regime over a BC. Thereafter 1 DOF and 2 DOF BC responses i.e. yaw and cross flow responses are analysed.
Focusing on the yaw behaviour, a lock-in has been observed from which yaw rotations develop. The Yaw and Cross Flow (CF) appear to be coupled. As already mentioned in [8], the presence of yaw tends to weaken the CF vortex induced motions (VIM). When the 2 DOF natural periods are somehow closer, the CF response tends to be recovered.
Occurrence of High Amplitude VIV With Time Sharing
Authors: Swithenbank Susan B. , Larsen Carl Martin 
Published: August 2013

When using the theory of Time Sharing of frequencies for predicting damage and fatigue from Vortex-Induced Vibrations (VIV), the percent of time that a riser is vibrating with high amplitude VIV will significantly influence the fatigue rate of a riser. To accurately predict the fatigue of a riser using time sharing or another stochastic prediction method an accurate model is needed for the percent of time spent at these high amplitude vibrations.
Using multiple laboratory tests with both sheared and uniform flow, the percent of time that each section of a pipe vibrated at high amplitude was determined. The larger issue was determining the factors that control the percentage of time at high amplitude vibration. Energy in the system was determined to be the largest influence on the percentage of time. At both low levels and high levels of energy the system had a low percentage of high amplitude VIV. A middle set of energy is the condition that produced high amplitude VIV for the largest percentage of time.
Rankine Source Method for Seakeeping Predictions
Authors: Söding Heinrich , von Graefe Alexander , el Moctar Ould , Shigunov Vladimir 
Published: August 2013

Model tests are usually used for the traditional seakeeping predictions (transfer functions of ship motions and loads in regular waves). Experience shows that numerical solution of Reynolds-averaged Navier-Stokes equations (RANSE) can provide accurate results for this task, however, such computations require too much computational time for the required large number of the loading conditions, ship speeds and wave directions and periods. Traditionally, potential flow methods are used for such computations at early design stages. Although potential flow methods can produce results very quickly for large number of conditions, viscosity effects (most important for the roll motion) have to be taken into account using measurements or RANSE computations.
Rankine source method, applied to seakeeping problems perhaps for the first time by Yeung [1] to oscillating ship sections, is increasingly used in practical seakeeping analysis. This paper presents a three-dimensional Rankine source code GL Rankine. Patch method is used instead of the usual collocation method to satisfy boundary conditions on the solid body surface. Periodic flow due to waves is linearized with respect to wave and motion amplitude, taking into account interactions between the nonlinear steady flow and periodic flow due to waves and ship motions. The steady flow solution accounts for the nonlinear free-surface conditions, ship wave and dynamic squat. The paper shows results of the method for ship motions in waves in comparison with model measurements and RANSE simulations.
Effects From Fully Nonlinear Irregular Wave Forcing on the Fatigue Life of an Offshore Wind Turbine and its Monopile Foundation
Authors: Schløer Signe , Bredmose Henrik , Bingham Harry B. , Larsen Torben J. 
Published: August 2013

The effect from fully nonlinear irregular wave forcing on the fatigue life of the foundation and tower of an offshore wind turbine is investigated through aeroelastic calculations. Five representative sea states with increasing significant wave height are considered in a water depth of 40 m. The waves are both linear and fully nonlinear irregular 2D waves. The wind turbine is the NREL 5-MW reference wind turbine. Fatigue analysis is performed in relation to analysis of the sectional forces in the tower and monopile.
Impulsive excitation of the sectional force at the bottom of the tower is seen when the waves are large and nonlinear and most notably for small wind speeds. In case of strong velocities and turbulent wind, the excitation is damped out. In the monopile no excitation of the force is seen, but even for turbulent strong wind the wave affects the forces in the pile significantly. The analysis indicates that the nonlinearity of the waves can change the fatigue damage level significantly in particular when the wave and wind direction is misaligned.
The Stabilizing Effects of U-Tanks as Passive and Controlled Anti-Rolling Devices
Authors: Ribeiro e Silva Sergio , Vásquez Guillermo , Guedes Soares C. , Marón Adolfo 
Published: August 2013

In this study a U-type tank, consisting of two lateral reservoirs connected by means of an horizontal channel, is considered either as a passive or controlled anti-rolling device for ships.
Initially, forced oscillation tests have been conducted at CEHIPAR for the U-type tank on a moving platform for different frequencies and also for different amplitudes of the pure roll forced motions adopting two distinct configuration: passive and controlled by means of two air valves installed on top of two lateral reservoirs. The most important aspect was to obtain a large set of roll restoring moments, for both passive and controlled configurations, which could give a discrete distribution of the roll restoring moments along the entire range of wave frequencies of interest. On a first study, a detailed account of model tests was presented, the method of analyzing the experimental results was discussed, and the experimental data of the U-type tank was compared with results from Lloyd’s linear frequency domain model, where non-linear effects with the amplitude of the forced motions was thoroughly addressed.
In this paper, the stabilizing effects of the same U-tank installed onboard a typical containership has been first analyzed with two distinct tools: WaMIT wave interaction 3D analysis software, and the two degrees-of-freedom numerical model referred above. In second place, the experimental data of the controlled U-type tank has been compared and contrasted with results from a nonlinear time domain model, where the controller of the air valves has been also included. Therefore, some preliminary conclusions could finally be drawn about the stabilizing effects of U-type tanks as passive and controlled anti-rolling devices.
Wave Loads on a Monopile in 3D Waves
Authors: Nielsen Anders Wedel , Schlütter Flemming , Sørensen Jacob V. Tornfeldt , Bredmose Henrik 
Published: August 2013

During the last decades more and more wind farms have been erected offshore. Most of these wind farms are located at relatively shallow water. The majority of the offshore wind turbines are founded on monopiles. Many of these offshore wind farms are exposed to large and steep waves, in some cases even breaking waves. The loads on the piles caused by waves can be significant and a better knowledge of the forcing caused by real sea states, including irregular waves and directional spreading is required to optimize the design.
The present physical model tests have been conducted in order to determine the effects of wave directionality and breaking of irregular waves. Piles with and without secondary structures have been tested. The waves were shoaled over a sloping bed and the pile was placed at two different positions with varying bed slopes. The three forcing components (Fx, Fy, Fz) were measured at the bottom of the pile during the experiments.
Breaking waves occurred around the pile in most of the tests and significant slamming forces were observed in the cases where breaking waves hit the pile which is well known from the literature. The experimental results indicated that the slamming force may be reduced when the wave spreading is increased, similar to the case of non-breaking waves.
Numerical and Experimental Investigation of a Novel Wave Energy Converter
Authors: Rogne Øyvind Ygre , Ersdal Svein , Moan Torgeir 
Published: August 2013

This paper deals with the numerical and experimental work carried out as part of the development of a new wave energy converter device suggested by Aker Solutions. The device consists of one catenary moored platform hinged to several smaller buoys. Energy is extracted through the relative rotation in the hinges. Model tests have been carried out in a towing tank with the aim to verify the use of linear hydrodynamic theory as a mean to predict the annual mean power production. The mathematical model of the device relies on linear hydrodynamic theory, and the commercial BEM code WAMIT is used to obtain the hydrodynamic coefficients. The equation of motion is formulated using generalized coordinates and Lagrange equation. The non-linear characteristics of the pneumatic cylinders, used to represent the power take-off forces in the model tests, necessitate the use of a time domain model. The convolution integral representing the radiation forces is replaced by an approximate state-space model, which is obtained using an frequency domain identification method.
The results show that the simulation model overpredicts the mean absorbed power by approximately 20% in sea states that result in moderate hinge response, whereas in one particular sea state that produce large hinge response, and consequently large power absorption, the overprediction is as large as 49%. This trend is expected, since large hinge response imply that the linear assumption is violated.