Slowly Varying Wave Drift Forces Analyzed From Model Test Data on a Moored Ship in Shallow Water

2009 ◽  
Author(s):  
Carl Trygve Stansberg ◽  
Csaba Paˆkozdi
Keyword(s):  
Noise Reduction ◽  
Shallow Water ◽  
Model Test ◽  
Transfer Functions ◽  
Numerical Study ◽  
Wave Frequency ◽  
Irregular Waves ◽  
Spectral Peak ◽  
Frequency Range ◽  
Wave Drift

Model test estimation of quadratic transfer functions (QTFs) is investigated for slowly varying wave drift excitation on a large moored ship in shallow water. Cross-bi-spectral analysis in irregular waves is used. A numerical study is run first, with a known, synthetical QTF model characterized by a strong off-diagonal variation, combined with a very lightly damped linear slow-drift dynamical system. The purpose is to check the accuracy of the analysis. For this simple model, a good accuracy is obtained in the estimated QTF. This is because of a refined noise reduction method which works well in this case. The wave frequency range of valid estimates is where the wave spectrum S(f) is higher than 7% of the spectral peak. Without the refinement, the useful range is reduced to where S(f) is higher than 15% of the spectral peak, based on a 3-hour sea state simulation. The method is then applied on experimental surge motion records from 1:50 scaled model tests carried out in an offshore basin, simulating 15m water depth. It is found that the QTF estimation procedure works reasonably well, but the accuracy is lower than that in the numerical study because the refined noise reduction could not be used due to the particular characteristics of the QTF. Therefore a basic version without the refinement had to be used. Still, results appear to be fairly reliable in the reduced wave frequency range with S(f) > 15% of the spectral peak, i.e. from 0.07Hz to 0.10Hz in this case.

Numerical study of self-interaction of Bernstein waves by nonlinear Landau damping

1992 ◽  
Vol 48 (3) ◽  
pp. 465-476
Author(s):  
Masao Sugawa
Keyword(s):  
Numerical Study ◽  
Magnetized Plasma ◽  
Wave Frequency ◽  
Landau Damping ◽  
Frequency Range ◽  
Electron Heating ◽  
Dominant Mechanism ◽  
Bernstein Waves ◽  
Nonlinear Landau Damping ◽  
Bulk Plasma

When Bernstein waves (B waves) are excited in a magnetized plasma, their self-interaction by nonlinear Landau damping (NLD) becomes the dominant mechanism for the electron heating of the bulk plasma. We examine this behaviour numerically. This occurs only for B waves with relatively small k∥ because the damping of the B waves becomes very small. This occurs in the relatively broad B-wave frequency range betweenω/ωc = 1.45 and 1.78. For B waves with large k∥ (k∥R > 0.15), virtual waves are not generated via self-interaction due to NLD because the quasi-linear cyclotron damping of the B waves becomes the dominant mechanism. The numerical results agree well with experimental ones.

Model Tests With an FPSO in Design Environmental Conditions

2004 ◽  
Author(s):  
Jesper Skourup ◽  
Martin J. Sterndorff ◽  
Susan F. Smith ◽  
Xiaoming Cheng ◽  
R. V. Ahilan ◽  
...  
Keyword(s):  
Model Test ◽  
Environmental Conditions ◽  
Transfer Functions ◽  
Model Tests ◽  
Irregular Waves ◽  
Wind Condition ◽  
Test Programme ◽  
Wave Basin ◽  
Wide Range ◽  
Wave Conditions

An extensive model test programme has been carried out with a turret moored FPSO model in design environmental conditions. The model tests were carried out in the 3D offshore wave basin at DHI Water & Environment at a scale of 1:80. The objectives of the model tests were two-fold: 1. To determine quadratic transfer functions for the slow-drift forces. 2. To determine the turret moored FPSO response in design environmental conditions with wave spreading. The model tests were made with a wide range of monochromatic and bi-chromatic wave conditions and also with long- and short-crested irregular wave conditions. For the tests in design conditions the irregular waves were combined with the corresponding wind condition. The model (which was segmented into two parts) was equipped with instruments to measure forces in mooring lines and turret, 6 dof motions of the FPSO, bending moments on the FPSO hull and wave run-up on the FPSO model. The present paper describes the details of the experimental work and the measurements made in the tests. Comparisons between model test results themselves to demonstrate the effects of wave spreading on the responses and comparisons to numerical results are given. The model test programme is part of the REBASDO project, funded by the European Union, and involving companies and institutions from several European Countries. The overall objective with the REBASDO project is to develop met-ocean and hydrodynamic models, which will capture the significant features of directional wave effects on FPSO design so that relevant enhancements in the design process can be incorporated in the future.

Diffraction/Radiation of 135,000M3 Storage Capacity LNG Carrier in Shallow Water: A Benchmark Study

2009 ◽  
Author(s):  
Mamoun Naciri ◽  
Emmanuel Sergent
Keyword(s):  
Shallow Water ◽  
Load Transfer ◽  
Storage Capacity ◽  
Transfer Functions ◽  
Second Order ◽  
Diffraction Radiation ◽  
Hull Form ◽  
Benchmark Study ◽  
Wave Drift ◽  
Key Issues

The HAWAI (sHAllow WAter Initiative) JIP was launched in 2005. The objective was to improve the reliability of Offshore (LNG) Terminals by combining the expertise of offshore hydrodynamics and coastal engineering to better address key issues regarding motion and mooring prediction methods in shallow water. One of the key issues identified was the diffraction/radiation calculation as this is the main foundation of all motion and mooring analyses. Comparisons of second order wave drift load transfer functions predicted by leading diffraction/radiation software for a typical 135,000m3 storage capacity LNG Carrier (LNGC) had shown notable differences (see Ref [1]). A benchmark study was launched for a standard LNGC in 15m water depth. Seven leading commercial diffraction/radiation software were used for this comparison (AQWA, DELFRAC, DIFFRAC, DIODORE, HYDROSTAR, WADAM and WAMIT). Comparison was first done by specifying the hull form in CAD format and then by specifying the mesh. First and second order results are presented and conclusions are drawn regarding the robustness of these codes.

A New Interpolation Method for FPSO Motion and Wave Drift Load Transfer Functions

2011 ◽  
Author(s):  
Ste´phanie Stafrach ◽  
Mamoun Naciri
Keyword(s):  
Spectral Density ◽  
Shallow Water ◽  
Load Transfer ◽  
Storage Capacity ◽  
Transfer Functions ◽  
Interpolation Method ◽  
New Method ◽  
Wave Drift ◽  
Floating System ◽  
Response Amplitude Operators

A new method of direction-wise interpolation is proposed. Its merits are first presented by considering the interpolation of 1st order motion Response Amplitude Operators (RAOs) and 3D (ω1, ω2, θ) wave drift load Quadratic Transfer Functions (QTFs). In a recent publication (see Ref. [1]), the importance of wave spreading on the spectral density of wave drift loads for a standard storage capacity LNG Carrier (135,000m3) in shallow water has been demonstrated. The computation of above-mentioned spectral densities requires the precalculation of a large number of 4D (ω1, ω2, θ1, θ2) wave drift load QTF and interpolations between the calculated directions. Application of the new method is investigated in this more challenging context. Examples are selected in the buoyant LNG floating system area with an LNG Carrier in shallow water and an FLNG in deep water.

scholarly journals A Numerical Study: Transitional Hydrodynamic Behaviour of a Moored Barge in Different Ultra-shallow Water Depths of Malaysia

2019 ◽  
Vol 13 (1) ◽  
pp. 238-259
Author(s):  
M.S. Patel ◽  
Nurliyana Azizan ◽  
M.S. Liew ◽  
Zahiraniza Mustaffa ◽  
Montasir Osman Ali ◽  
...  
Keyword(s):  
Shallow Water ◽  
Numerical Study ◽  
Simulation Software ◽  
Operating Conditions ◽  
Dynamic Responses ◽  
Irregular Waves ◽  
Peak Time ◽  
Hydrodynamic Behavior ◽  
Hydrodynamic Behaviour ◽  
Water Depths

Background: Malaysia has most of its oil reservoirs in the South China sea. The water depth ranges from 50 m to 200 m. The effects of ultra-shallow water are of prime importance in the exploration of marginal oil fields. Hence, there is an increasing demand for understanding the hydrodynamic behavior of FPSO in ultra-shallow water depths. Objective: A simulation study in both frequency-domain and time-domain analyses has been performed to understand the dynamic responses of a moored barge in varying shallow water depths. The objective of this study was to observe the transitional hydrodynamic behavior of the moored barge under varying shallow water depths. Methods: The moored barge was administered under regular and irregular waves. Operating conditions for irregular waves in terms of significant wave height and peak time period were incorporated from PETRONAS Technical Standards (PTS). The wave-body interactions and mooring effects have been numerically modelled using a commercial Computational Fluid Dynamics (CFD) and simulation software (ANSYS AQWA) successfully. In order to gain confidence in the simulation software, additional experimental validation was performed for a FPSO model. Results: Though the barge was primarily free to rotate in all Degrees Of Freedom (DOF), however, only three DOFs were considered for our study; viz, heave, roll and yaw respectively. The force spectral density, cable RAO’s in addition to the time series of cable forces, along with the effect of significant motions on the mooring cables behavior have been discussed. Conclusion: In irregular beam sea state, the significant motions in ultra-shallow water were greater than that for deep waters, this was primarily the main reason for higher cable responses in ultra-shallow water.

Low Frequency Wave Loads and Damping of Four MODUs in Severe Seastates With Current

2020 ◽  
Vol 143 (1) ◽  
Author(s):  
Nuno Fonseca ◽  
Carl Trygve Stansberg ◽  
Kjell Larsen ◽  
Rune Bjørkli ◽  
Tjerand Vigesdal ◽  
...  
Keyword(s):  
Empirical Formula ◽  
Potential Flow ◽  
Transfer Functions ◽  
Low Frequency ◽  
Irregular Waves ◽  
Experimental Program ◽  
Frequency Wave ◽  
Numerical Predictions ◽  
Wave Drift ◽  
Semi Empirical

Abstract Model tests have been performed with four mobile offshore drilling units (MODUs) with the aim of identifying wave drift forces and low-frequency damping. The MODUs configuration is different, namely on the number and diameter of columns; therefore, the sample is representative of many of the existing concepts. The model scale is the same as well as the wave and current conditions. The experimental program includes irregular waves with systematic variations of the significant wave height, wave peak period, current velocity, and vessel heading. A nonlinear data analysis technique (cross bi-spectral analysis) is applied to identify the surge and sway quadratic transfer functions (QTFs) of the slowly varying excitation, together with the linearized low-frequency damping. The paper also presents a semi-empirical formula developed in the scope of the EXWAVE JIP to correct potential flow mean wave drift force coefficients of Semis in high seastates with current. The empirical QTFs are then compared with numerical predictions. Comparisons with potential flow coefficients lead to conclusions on the role of viscous drift. The semi-empirical formula is assessed based on comparisons with test results and concluded that it provides a significant improvement compared to potential flow predictions.

Second-Order Low-Frequency Wave Forces on a SPM Offloading Tanker in Shallow Water

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
S. Ma ◽  
M. H. Kim ◽  
S. Shi
Keyword(s):  
Shallow Water ◽  
Transfer Functions ◽  
Low Frequency ◽  
Wave Force ◽  
Second Order ◽  
Irregular Waves ◽  
Mooring Line ◽  
Frequency Wave ◽  
Water Region ◽  
Shallow Water Region

This paper studies the influence of three different calculation methods of the second-order low-frequency (LF) wave-force quadratic transfer functions (QTFs) for a single point mooring (SPM) tanker system in relatively shallow water region. The multivessel-mooring hawser coupled dynamic analysis is used to simulate the floater relative motions and mooring and hawser tensions. Because the SPM tanker is deployed in shallow water region and the slowly varying drift motions are to be dominant in typical operational conditions, the accurate calculation of LF wave-force QTFs become important especially for mooring and hawser-tension prediction. The practically popular Newman’s approximation and another approximation excluding complicated free-surface integrals are used to calculate the LF QTFs on the offloading tanker and they are compared with the complete QTF results. Further comparison is carried out by calculating the resulting LF wave-force spectra and motion time histories and analyzing their impacts on hawser and mooring line tensions. Through the example studies, the limitation of the Newman’s approximation in the case of shallow water and longer period irregular waves is underscored.

Numerical study on the airfoil self-noise of three owl-based wings with the trailing-edge serrations

2021 ◽  
pp. 095441002098822
Author(s):  
Dian Li ◽  
Xiaomin Liu ◽  
Lei Wang ◽  
Fujia Hu ◽  
Guang Xi
Keyword(s):  
Flow Field ◽  
Noise Reduction ◽  
Numerical Study ◽  
Barn Owl ◽  
Aerodynamic Noise ◽  
Frequency Noise ◽  
Trailing Edge ◽  
Reduction Mechanisms ◽  
Trailing Edge Serrations ◽  
Bionic Airfoil

Previous publications have summarized that three special morphological structures of owl wing could reduce aerodynamic noise under low Reynolds number flows effectively. However, the coupling noise-reduction mechanism of bionic airfoil with trailing-edge serrations is poorly understood. Furthermore, while the bionic airfoil extracted from natural owl wing shows remarkable noise-reduction characteristics, the shape of the owl-based airfoils reconstructed by different researchers has some differences, which leads to diversity in the potential noise-reduction mechanisms. In this article, three kinds of owl-based airfoils with trailing-edge serrations are investigated to reveal the potential noise-reduction mechanisms, and a clean airfoil based on barn owl is utilized as a reference to make a comparison. The instantaneous flow field and sound field around the three-dimensional serrated airfoils are simulated by using incompressible large eddy simulation coupled with the FW-H equation. The results of unsteady flow field show that the flow field of Owl B exhibits stronger and wider-scale turbulent velocity fluctuation than that of other airfoils, which may be the potential reason for the greater noise generation of Owl B. The scale and magnitude of alternating mean convective velocity distribution dominates the noise-reduction effect of trailing-edge serrations. The noise-reduction characteristic of Owl C outperforms that of Barn owl, which suggests that the trailing-edge serrations can suppress vortex shedding noise of flow field effectively. The trailing-edge serrations mainly suppress the low-frequency noise of the airfoil. The trailing-edge serration can suppress turbulent noise by weakening pressure fluctuation.

Semi-Empirical Crest Distributions of Long-Crest Nonlinear Waves of Three-Hour Duration

2017 ◽  
Author(s):  
Zhenjia (Jerry) Huang ◽  
Qiuchen Guo
Keyword(s):  
Nonlinear Wave ◽  
Wave Height ◽  
Model Test ◽  
Significant Wave Height ◽  
Spectral Peak ◽  
Wave Crest ◽  
Empirical Formulas ◽  
Peak Period ◽  
Significant Wave ◽  
Semi Empirical

In wave basin model test of an offshore structure, waves that represent the given sea states have to be generated, qualified and accepted for the model test. For seakeeping and stationkeeping model tests, we normally accept waves in wave calibration tests if the significant wave height, spectral peak period and spectrum match the specified target values. However, for model tests where the responses depend highly on the local wave motions (wave elevation and kinematics) such as wave impact, green water impact on deck and air gap tests, additional qualification checks may be required. For instance, we may need to check wave crest probability distributions to avoid unrealistic wave crest in the test. To date, acceptance criteria of wave crest distribution calibration tests of large and steep waves of three-hour duration (full scale) have not been established. The purpose of the work presented in the paper is to provide a semi-empirical nonlinear wave crest distribution of three-hour duration for practical use, i.e. as an acceptance criterion for wave calibration tests. The semi-empirical formulas proposed in this paper were developed through regression analysis of a large number of fully nonlinear wave crest distributions. Wave time series from potential flow simulations, computational fluid dynamics (CFD) simulations and model test results were used to establish the probability distribution. The wave simulations were performed for three-hour duration assuming that they were long-crested. The sea states are assumed to be represented by JONSWAP spectrum, where a wide range of significant wave height, peak period, spectral peak parameter, and water depth were considered. Coefficients of the proposed semi-empirical formulas, comparisons among crest distributions from wave calibration tests, numerical simulations and the semi-empirical formulas are presented in this paper.

A Numerical Study on the Reflection of a Solitary Wave in Shallow Water

1982 ◽  
Vol 51 (3) ◽  
pp. 1018-1023 ◽  
Author(s):  
Mitsuaki Funakoshi ◽  
Masayuki Oikawa
Keyword(s):  
Solitary Wave ◽  
Shallow Water ◽  
Numerical Study
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