URANS Predictions of Low-Frequency Damping of a LNGC

2019 ◽  
Author(s):  
Frédérick Jaouën ◽  
Arjen Koop ◽  
Lucas Vatinel
Keyword(s):  
Low Frequency ◽  
Offshore Structures ◽  
Viscous Damping ◽  
Wave Loads ◽  
Viscous Effects ◽  
Offshore Structure ◽  
Time Step ◽  
Cfd Simulations ◽  
Damping Coefficients ◽  
Hydrodynamic Damping

Abstract The horizontal motions of a moored offshore structure in waves are dominated by the resonance phenomena that occur at the natural frequencies of the system. Therefore, the maximum excursions of the structure depend on both the wave loads and the damping in the system. At present, potential flow calculations are employed for predicting the wave loads on offshore structures. However, such methods cannot predict hydrodynamic damping which is dominated by viscous effects. Therefore, the current practice in the industry is to obtain the low-frequency damping based on model testing. Nowadays, CFD simulations also have the potential to predict the low-frequency viscous damping of offshore structures in calm water. To obtain confidence in the accuracy of CFD simulations, a proper validation of the results of such CFD calculations is essential. In this paper, the flow around a forced surging or swaying LNGC is calculated using the CFD code ReFRESCO. The objective is to assess the accuracy and applicability of CFD for predicting the low-frequency viscous damping. After a description of the code and the used numerical methods, the results are presented and compared with results from model tests. Both inertia and damping coefficients are analyzed from the calculated hydrodynamics loads. Extensive numerical studies have been carried out to determine the influence of grid resolution, time step and iterative convergence on the flow solution and on the calculated damping. The numerical uncertainty of the results are assessed for one combination of amplitude and period for the surge motion. The CFD results are compared to experimental results indicating that the calculated damping coefficients agree within 5% for both surge and sway motion.

Using CFD to Assess Low Frequency Damping

2012 ◽  
Author(s):  
Alessio Pistidda ◽  
Harald Ottens ◽  
Richard Zoontjes
Keyword(s):  
Standard Procedure ◽  
Low Frequency ◽  
Added Mass ◽  
Model Tests ◽  
Viscous Damping ◽  
Viscous Effects ◽  
Floating Bodies ◽  
Time Step ◽  
Damping Coefficients ◽  
Quadratic Component

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.

Downtime Analysis Techniques for Complex Offshore and Dredging Operations

2004 ◽  
Author(s):  
Remmelt J. van der Wal ◽  
Gerrit de Boer
Keyword(s):  
Wind Waves ◽  
Weather Forecast ◽  
Offshore Structures ◽  
Operational Mode ◽  
Offshore Structure ◽  
Hydrodynamic Models ◽  
Time Step ◽  
Made In

Offshore operations in open seas may be seriously affected by the weather. This can lead to a downtime during these operations. The question whether an offshore structure or dredger is able to operate in wind, waves and current is defined as “workability”. In recent decades improvements have been made in the hydrodynamic modelling of offshore structures and dredgers. However, the coupling of these hydrodynamic models with methods to analyse the actual workability for a given offshore operation is less developed. The present paper focuses on techniques to determine the workability (or downtime) in an accurate manner. Two different methods of determining the downtime are described in the paper. The first method is widely used in the industry: prediction of downtime on basis of wave scatter diagrams. The second method is less common but results in a much more reliable downtime estimate: determination of the ‘job duration’ on basis of scenario simulations. The analysis using wave scatter diagrams is simple: the downtime is expressed as a percentage of the time (occurrences) that a certain operation can not be carried out. This method can also be used for a combination of operations however using this approach does not take into account critical events. This can lead to a significant underprediction of the downtime. For the determination of the downtime on basis of scenario simulations long term seastate time records are used. By checking for each subsequent time step which operational mode is applicable and if this mode can be carried out the workability is determined. Past events and weather forecast are taken into account. The two different methods are compared and discussed for a simplified offloading operation from a Catenary Anchor Leg Mooring (CALM) buoy. The differences between the methods will be presented and recommendations for further applications are given.

Wave Load Prediction on a Stationary Bergy Bit Near a Fixed Offshore Platform

2017 ◽  
Author(s):  
Dong Cheol Seo ◽  
Tanvir Sayeed ◽  
M. Hasanat Zaman ◽  
Ayhan Akinturk
Keyword(s):  
Water Level ◽  
Offshore Structures ◽  
Air Gap ◽  
Offshore Platform ◽  
Load Prediction ◽  
Wave Load ◽  
Offshore Structure ◽  
Cfd Simulations ◽  
Simulation Performance ◽  
Fixed Offshore Platform

Offshore oil and gas operations conducted in harsh environments such offshore Newfoundland may pose additional risks due to collision of smaller ice pieces and bergy bits with the offshore structures, including their topsides in the case of gravity based structures particularly in extreme waves. In this paper, CFD (Computational Fluid Dynamics) prediction for wave loads acting on a bergy bit around a fixed offshore platform is presented. Often the vertical column of a gravity based structure is designed against ice collisions, if operating in such an environment. In practices, topsides are usually protected by being placed sufficiently high from the still water level, away from the reach of the bergy bits. This vertical clearance between the still water level and the topside deck is known an air gap. Hence, the amount of the air gap planned for such an offshore structure is an important factor for the safety of the topsides at a given location. In this study a CFD method is applied to estimate the dynamic response of the bergy bit and provide a reliable air gap to reduce the potential risk of the bergy bit collision. In advance of more complex collision simulations using a free-floating ice for the airgap design, CFD analysis of wave load prediction on a stationary bergy bit is carried out and reported in this paper. In the experiments and CFD simulations, the location of the bergy bit is changed to quantify the change of wave load due to the hydrodynamic interaction between the bergy bit and the platform. Finally, the results of the CFD simulations are compared with the relevant experiment results to confirm the simulation performance prior to the free floating bergy bit simulations.

Wave–current effects on large offshore structures

1989 ◽  
Vol 16 (4) ◽  
pp. 543-551 ◽  
Author(s):  
Michael Isaacson ◽  
John Baldwin
Keyword(s):  
Potential Flow ◽  
Offshore Structures ◽  
Wave Forces ◽  
Ocean Engineering ◽  
Viscous Effects ◽  
Offshore Structure ◽  
Time Stepping ◽  
Waves And Currents ◽  
Flow Effects ◽  
Interaction Problem

The various effects that influence loads acting on a large offshore structure due to the combination of waves and currents are reviewed. These may be broadly associated with potential flow effects and viscous effects. The potential flow effects are nonlinear and may generally be investigated by perturbation or time-stepping methods. Viscous effects include the onset of flow separation, which affects the validity of the assumed potential flow, as well as steady and oscillatory forces. The fluid mechanics of the complete wave–current–structure interaction problem are not yet well understood and areas in need of additional research are identified. Key words: currents, drag, drift forces, hydrodynamics, ocean engineering, offshore structures, waves, wave forces.

Computing Large Amplitude Motions of a Floating Offshore Structure in the Time Domain

2008 ◽  
Author(s):  
Wei Qiu ◽  
Hongxuan Peng
Keyword(s):  
Time Domain ◽  
Large Amplitude ◽  
Offshore Structures ◽  
The Body ◽  
Surface Boundary ◽  
Offshore Structure ◽  
Floating Structure ◽  
Time Step ◽  
Large Amplitude Motions ◽  
The Time Domain

Based on the panel-free method, large-amplitude motions of floating offshore structures have been computed by solving the body-exact problem in the time domain using the exact geometry. The body boundary condition is imposed on the instantaneous wetted surface exactly at each time step. The free surface boundary is assumed linear so that the time-domain Green function can be applied. The instantaneous wetted surface is obtained by trimming the entire NURBS surfaces of a floating structure. At each time step, Gaussian points are automatically distributed on the instantaneous wetted surface. The velocity potentials and velocities are computed accurately on the body surface by solving the desingularized integral equations. Nonlinear Froude-Krylov forces are computed on the instantaneous wetted surface under the incident wave profile. Validation studies have been carried out for a Floating Production Storage and Offloading (FPSO) vessel. Computed results were compared with experimental results and solutions by the panel method.

The Effect of Wave Directionality on Low Frequency Motions and Mooring Forces

2009 ◽  
Author(s):  
Olaf J. Waals
Keyword(s):  
Water Depth ◽  
Transfer Functions ◽  
Low Frequency ◽  
Sensitivity Study ◽  
Wave Loads ◽  
Offshore Structure ◽  
Frequency Wave ◽  
Drift Theory ◽  
Mooring Forces ◽  
Drift Forces

Operability of offshore moored ships can be affected by low frequency wave loads. The low frequency motions of a moored ship may limit the uptime of an offshore structure such as an LNG offloading terminal. The wave loads that cause the main excitation of these low frequency motions are usually computed using second order wave drift theory for long crested waves, which assumes that the low frequency components are only related to waves coming from the same direction. In this method short crested seas are dealt with as a summation of long crested seas, but no interaction between the wave components traveling in different directions is usually taken into account. This paper describes the results of a study to the effect of 2nd order low frequency wave loads in directional seas. For this study the drift forces related to the interaction between waves coming from different directions is also included. This is done by computing the quadratic transfer functions (QTF) for all possible combinations of wave components (frequencies and directions). Time traces of drift forces are generated and compared to the results without wave directional interaction after which the motions of an LNG carrier are simulated. A sensitivity study is carried out towards the number of direction steps and the water depth. Finally the motions of an LNG carrier in shallow water (15m water depth) are simulated and mooring forces are compared for various amounts of wave spreading.

On Wind-Wave Misalignment, Directional Spreading and Wave Loads

2013 ◽  
Author(s):  
Gerbrant Ph. Van Vledder
Keyword(s):  
Numerical Modeling ◽  
Wind Wave ◽  
Offshore Structures ◽  
Integration Time ◽  
Wave Loads ◽  
Wave Direction ◽  
Time Step ◽  
The Difference ◽  
Wind Sea ◽  
Spectral Partitioning

Causes of wind-wave misalignment, the difference between wind and mean wave direction, are investigated for stationary and non-stationary situations using numerical modeling. This includes the effects of upwind fetch restrictions, refraction, choice of source terms and integration time step on wind-wave misalignment are illustrated. A statistical analysis is performed to quantify wind-wave misalignment as a function of wind speed and significant wave height. In addition, the effect of spectral partitioning in separate wind sea and swell systems on the statistics of wind-wave misalignment is illustrated. Apart from the differences in mean direction, attention is given to the associated directional spreading. Implications for the design of offshore structures and the movements of moored ships are discussed.

scholarly journals OC6 Phase Ia: CFD Simulations of the Free-Decay Motion of the DeepCwind Semisubmersible

Energies ◽  
2022 ◽  
Vol 15 (1) ◽  
pp. 389
Author(s):  
Lu Wang ◽  
Amy Robertson ◽  
Jason Jonkman ◽  
Jang Kim ◽  
Zhi-Rong Shen ◽  
...  
Keyword(s):  
Damping Ratio ◽  
International Energy Agency ◽  
Offshore Wind ◽  
Cfd Simulations ◽  
Energy Agency ◽  
Equivalent Damping ◽  
Damping Coefficients ◽  
Hydrodynamic Damping ◽  
Free Decay ◽  
Quadratic Damping

Currently, the design of floating offshore wind systems is primarily based on mid-fidelity models with empirical drag forces. The tuning of the model coefficients requires data from either experiments or high-fidelity simulations. As part of the OC6 (Offshore Code Comparison Collaboration, Continued, with Correlation, and unCertainty (OC6) is a project under the International Energy Agency Wind Task 30 framework) project, the present investigation explores the latter option. A verification and validation study of computational fluid dynamics (CFD) models of the DeepCwind semisubmersible undergoing free-decay motion is performed. Several institutions provided CFD results for validation against the OC6 experimental campaign. The objective is to evaluate whether the CFD setups of the participants can provide valid estimates of the hydrodynamic damping coefficients needed by mid-fidelity models. The linear and quadratic damping coefficients and the equivalent damping ratio are chosen as metrics for validation. Large numerical uncertainties are estimated for the linear and quadratic damping coefficients; however, the equivalent damping ratios are more consistently predicted with lower uncertainty. Some difference is observed between the experimental and CFD surge-decay motion, which is caused by mechanical damping not considered in the simulations that likely originated from the mooring setup, including a Coulomb-friction-type force. Overall, the simulations and the experiment show reasonable agreement, thus demonstrating the feasibility of using CFD simulations to tune mid-fidelity models.

A Practical Method to Determine the Effect of Current on Wave Loads and Motions of Offshore Structures

2011 ◽  
Author(s):  
Tim Bunnik
Keyword(s):  
Free Surface ◽  
Surface Condition ◽  
Diffraction Method ◽  
Stationary Flow ◽  
Offshore Structures ◽  
Boundary Integral ◽  
Wave Loads ◽  
Viscous Effects ◽  
First Order ◽  
Free Surface Condition

In this paper a linear diffraction method is presented which is able to determine accurately and fast the effect of current on motions of, and first- and second-order wave loads on offshore structures. The current flow is modeled by double-body potential flow, thereby neglecting viscous effects and stationary wave effects. The interaction between the double-body flow and the first-order non-stationary flow is modeled up to first order in the current speed, thereby restricting the method to small current speeds. A direct pressure integration method is used to evaluate the wave drift forces. Therefore, the gradient of the double-body velocities is required, which is evaluated using an additional boundary integral. The interaction between the double-body flow and the non-stationary flow leads to a modified free-surface condition which is satisfied using panels on the free surface. On these panels pulsating sources are placed satisfying the zero-speed free surface condition in the frequency domain. The method is validated against results published by Zhao and Faltinsen for a fixed vertical truncated cylinder. Furthermore, experimental data for a tanker, published by Wichers, [11] is used to validate the method for realistic ship shapes. In both cases a good agreement is obtained.

Reliable Estimation of Low-Frequency Viscous Damping for a Turret-Moored FLNG in Current

2018 ◽  
Author(s):  
Z. Huang ◽  
S. Ryu ◽  
D. Lee ◽  
C. S. Hughes
Keyword(s):  
Low Frequency ◽  
Test Method ◽  
Inertia Force ◽  
Total Force ◽  
Viscous Damping ◽  
Damping Force ◽  
Damping Coefficients ◽  
Calm Water ◽  
Oscillation Test ◽  
Measured Force

For a turret-moored Floating Liquefied Natural Gas Plant (FLNG), it is important to use confidently derived low frequency viscous damping coefficients in the prediction of its motions and mooring loads in wind, wave and current conditions. In this paper we present our recent experimental work on the low frequency sway and yaw viscous damping in calm water and in current. In general, damping force is a relatively small portion of the total hydrodynamic force on an oscillatory model. In a previous ExxonMobil damping test in calm water (Huang et al., 2010), i.e. without current and wave, a deeply submerged double-body model was forced to oscillate to avoid surface wave contamination. An inertia compensation system was also designed to cancel the inertia force and the restoring force during oscillations, then the measured force was mainly damping force. Because of the schedule constraints of the present study, it was not possible to perform the submerged oscillation test. Instead, a forced oscillation test in water surface was performed based on KC-number and β-number. In order to obtain reliable damping coefficients, we had to carefully design the test conditions, i.e. current speeds, oscillation amplitudes and frequencies so that an adequate portion of damping force within the total force could be achieved with no significant surface waves that could contaminate the damping results being generated by the oscillating model. Good damping results were obtained. To check the acceptance of the test method based on Froude scaling, a limited number of tests were performed in which the oscillation amplitudes and frequencies were scaled down based on the Froude scaling. Magnitudes of the measured force and moment are significantly low. The time series of the measurements have drifting and significant noise. We could not confidently determine viscous damping results from the measurements.

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