Experimental Study of Slow-Drift Ship Motions in Shallow Water Random Waves

2011 ◽  
Author(s):  
Carl Trygve Stansberg ◽  
Trygve Kristiansen
Keyword(s):  
Spectral Analysis ◽  
Shallow Water ◽  
Transfer Functions ◽  
Second Order ◽  
Ship Motions ◽  
Random Wave ◽  
Random Waves ◽  
Low Frequencies ◽  
Wave Basin ◽  
Drift Forces

Slowly varying motions and drift forces of a large moored ship in random waves at 35m water depth are investigated by an experimental wave basin study in scale 1:50. A simple horizontal mooring set-up is used. A second-order wave correction is applied to minimize “parasitic” long waves. The effect on the ship motion from the correction is clearly seen, although less in random wave spectra than in pure bi-chromatic waves. Empirical quadratic transfer functions (QTFs) of the surge drift force are found by use of cross-bi-spectral analysis, in two different spectra have been obtained. The QTF levels increase significantly with lower wave frequencies (except at the diagonal), which is special for finite and shallow water. Furthermore, the QTF levels frequencies at low frequencies increase significantly out from the QTF diagonal. Thus Newman’s approximation should preferrably not be used in these cases. Using the LF waves as a direct excitation in a “linear” ship force analysis gives random records that compare reasonably well with those from the cross-bi-spectral analysis. This confirms the idea that the drift forces in shallow water are closely correlated to the second-order potential, and thereby by the second-order LF waves.

scholarly journals Random wave-driven drag forces on near-bed vegetation in shallow water based on deepwater wind conditions

2019 ◽  
Vol 233 (4) ◽  
pp. 1287-1290
Author(s):  
Dag Myrhaug
Keyword(s):  
Shallow Water ◽  
Drag Force ◽  
Wind Waves ◽  
Wave Spectrum ◽  
Random Wave ◽  
Random Waves ◽  
Coastal Zones ◽  
Drag Forces ◽  
Mean Wind Speed ◽  
Wave Induced

This article provides a simple analytical method for giving estimates of random wave-driven drag forces on near-bed vegetation in shallow water from deepwater wind conditions. Results are exemplified using a Pierson–Moskowitz model wave spectrum for wind waves with the mean wind speed at the 10 m elevation above the sea surface as the parameter. The significant value of the drag force within a sea state of random waves is given, and an example typical for field conditions is presented. This method should serve as a useful tool for assessing random wave-induced drag force on vegetation in coastal zones and estuaries based on input from deepwater wind conditions.

Laboratory Wave Modelling for Floating Structures in Shallow Water

2006 ◽  
Author(s):  
Carl Trygve Stansberg
Keyword(s):  
Shallow Water ◽  
Low Frequency ◽  
Second Order ◽  
Wave Groups ◽  
Large Wave ◽  
Frequency Wave ◽  
Wave Modelling ◽  
Floating Structures ◽  
Wave Basin ◽  
Wave Drift Forces

The analysis of moored floating vessels in shallow water requires special attention, when compared to similar problems in deep water. In particular, low-frequency wave drift forces need to be studied. Model testing is essential in validation of numerical prediction tools for these problems. Wave-group induced low-frequency wave components is an important part of the problem. Their reproduction in laboratories needs special attention. In general, two types of low-frequency waves are present: “bound” waves following the wave groups, and “free” waves propagating with their own speed. The former is included in second-order numerical codes for floater is included in second-order numerical codes for floaters, while the latter is normally not. Therefore, identification and possible reduction of the free components is of interest. A practical way to do this in a large wave basin is described in this paper. Results from generation of bi-chromatic waves without and with correction are presented. Corrected results show a clear reduction of the free wave component.

Numerical Research on the Interaction of Multidirectional Random Waves With a Large-Scale Offshore Wind Turbine Foundation

2021 ◽  
Author(s):  
Xinran Ji ◽  
Daoru Wang
Keyword(s):  
Wind Turbine ◽  
Potential Theory ◽  
Large Scale ◽  
Coupling Model ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Random Wave ◽  
Random Waves ◽  
Outer Domain ◽  
Wave Basin

Abstract Real sea waves are multidirectional, but most of researches are focused on the unidirectional wave. Special to the numerical wave basin based on OpenFOAM to simulate the propagation of multidirectional random wave and its interaction with structure has the insufficient of large amount of calculation, to overcome this problem, a one-way coupling model is established based on the potential theory and OpenFOAM wave basin, and the amount of calculation is reduced and the computational efficiency is improved. Base on the coupling model, the multidirectional random waves and its interaction with a large-scale offshore wind turbine foundation are simulated. In the outer domain, the multidirectional random wave is generated by the potential theory quickly. The interaction of multidirectional waves with the offshore wind turbine foundation is simulated in the inner domain by solving the Navier-Stokes equation. The result shows that the wave directionality has a significant effect on the interaction of multidirectional irregular waves with cylinder.

Calculation of Second Order Drift Forces on a FLNG Accounting for Difference Frequency Components

2008 ◽  
Author(s):  
Nuno Fonseca ◽  
Joa˜o Pessoa ◽  
Carlos Guedes Soares
Keyword(s):  
Transfer Functions ◽  
Computational Effort ◽  
Second Order ◽  
Difference Frequency ◽  
Order Problem ◽  
Usual Procedure ◽  
Slow Drift ◽  
Frequency Components ◽  
The Difference ◽  
Drift Forces

The usual procedure for calculation of slow drift forces is to simplify the quadratic transfer function by representing the difference frequency components in terms of the zero difference results. In this way the second order problem is much simplified as well as the computational effort. However this approximation has some limitations and in particular for the slow drift oscillations problem it may be important to consider correctly the difference frequency components. The paper presents an analysis of the slow drift exciting forces on a FLNG (Floating Production Storage and Offloading system for production of LNG). The complete second order forces are computed by WAMIT V6.1s which is based on a boundary element method. Results of the horizontal second order drift forces are presented for several difference frequency values. Deep water and two shallow water depths are considered, since water depth seems to have an important effect on the quadratic transfer functions.

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.

Second-Order Random Wave Kinematics and Resulting Loads on a Bottom-Fixed Slender Monopile

2013 ◽  
Author(s):  
Carl Trygve Stansberg ◽  
Andreas Amundsen ◽  
Sebastien Fouques ◽  
Ole David Økland
Keyword(s):  
Second Order ◽  
Offshore Wind ◽  
Random Wave ◽  
Wave Loads ◽  
Random Waves ◽  
Shear Forces ◽  
Traditional Use ◽  
Wave Kinematics ◽  
Drag Forces ◽  
Particle Velocities

The importance of including second-order nonlinear random wave kinematics in the numerical prediction of drag-induced shear forces and moments, at various levels on a bottom-fixed slender monopile in 40m water depth, is investigated. A vertical circular cylinder of diameter 0.5m is considered, representing typical dimensions of members in jacket type foundations of offshore wind turbines. The focus is here on the wave loads only, and wind and a propeller are therefore not included in this study. In particular, the main focus is on the effects from second-order random wave kinematics on the structural quasi-static time-varying loads due to drag forces in heavy storm wave conditions. Comparisons are made to the traditional use of Airy waves with various ways of stretching. An in-house numerical FEM code developed for structural analysis, NIRWANA, is used for this study. Thus one purpose of the present work is also to verify the implementation of the second-order random waves in the code. The results show significant effects, especially in the wave zone. Extreme crests are around 15%–20% increased, free-surface extreme particle velocities increase by around 30%–40%, while the velocities at levels below MWL are, on the other hand, somewhat reduced. The resulting peak shear forces, and in particular the moments, are thereby increased by typically 50%–100% in the upper parts of the column. At the base the peak shear forces are comparable to the traditional methods, while moments are still somewhat higher. Another effect is the generation of more high-frequency load contributions, which may be important to address further with respect to natural frequencies of such towers.

Spectral Analysis of Nonlinear Random Wave Loadings

2005 ◽  
Author(s):  
Rujian Ma ◽  
Guixi Li ◽  
Dong Zhao
Keyword(s):  
Spectral Analysis ◽  
Finite Amplitude ◽  
Wave Profile ◽  
Circular Cylinders ◽  
Random Wave ◽  
Wave Spectra ◽  
Random Waves ◽  
First Order ◽  
Nonlinear Random Wave ◽  
Nonlinear Spectral Analysis

The spectral analysis of nonlinear random wave loadings on circular cylinders is performed in this paper by means of nonlinear spectral analysis. The study is carried out by expressing the wave profile and velocities of water particles as a nonlinear composition of the first order wave profile. Under the assumption of the first order wave profile being a zero-mean Gaussian process, the random wave spectra of finite amplitude waves are given. In order to solve the loading spectra of the finite amplitude random waves, the drag force is extended into power series of velocity. The loadings of the finite amplitude random waves are then expressed as nonlinear compositions of the first order wave profile and its derivatives. These techniques made it easier to compute the spectral densities of the finite amplitude random wave loadings.

scholarly journals On the Shoaling of Solitary Waves in the Presence of Short Random Waves

2015 ◽  
Vol 45 (3) ◽  
pp. 792-806 ◽  
Author(s):  
Miao Tian ◽  
Alex Sheremet ◽  
James M. Kaihatu ◽  
Gangfeng Ma
Keyword(s):  
Solitary Wave ◽  
Solitary Waves ◽  
Kdv Equation ◽  
Statistical Significance ◽  
Laboratory Data ◽  
Systematic Research ◽  
Random Wave ◽  
Random Waves ◽  
Tsunami Forecasting ◽  
Wave Basin

AbstractOverhead video from a small number of laboratory tests conducted by Kaihatu et al. at the Tsunami Wave Basin at Oregon State University shows that the breaking point of a shoaling solitary wave shifts to deeper water if random waves are present. The analysis of the laboratory data collected confirms that solitary waves indeed tend to break earlier in the presence of random wave field, and suggests that the effect is the result of the radiation stresses gradient induced by the random wave fields. A theoretical approach based on the forced KdV equation is shown to successfully predict the shoaling process of the solitary wave. An ensemble of tests simulated using a state-of-the-art nonhydrostatic model is used to test the statistical significance of the process. The results of this study point to a potentially significant oceanographic process that has so far been ignored and suggest that systematic research into the interaction between tsunami waves and the swell background could increase the accuracy of tsunami forecasting.

Characteristics of Steep Second-Order Random Waves in Finite and Shallow Water

2011 ◽  
Author(s):  
Carl Trygve Stansberg
Keyword(s):  
Time Series ◽  
Shallow Water ◽  
Frequency Component ◽  
Second Order ◽  
Difference Frequency ◽  
Nonlinear Interactions ◽  
Random Waves ◽  
Theoretical Formulation ◽  
Sum Frequency ◽  
The Difference

The theoretical formulation of second-order random waves in deep and finite water is reviewed. In particular, the increased nonlinear interactions with decreasing depth are addressed, including both the sum-frequency as well as the slowly varying difference-frequency components. Depth-defined limitations in the valid range for random waves are suggested based on the Ursell number. Numerical time series realizations at various depths and for two sea states are obtained by an efficient bifrequency summation procedure. Resulting time series show moderate average second-order energy contents, except for the steep sea state Hs = 15m, Tp = 14s in depths of 30m and 20m which are outside the suggested valid second-order range. The two largest wave events from the simulations are studied in particular for the different depths. Nonlinear interactions increase significantly with decreasing depth. Still, within the valid range, extreme second-order crests and peak particle velocities are only moderately increased with decreasing depth, while the negative peaks increase significantly. This is because the difference-frequency component almost compensates for the sum-frequency part at crests, while it is opposite at troughs. Maximum slopes, however, are clearly increased in shallow water, eventually leading to increased breaking (which is beyond second order of course). Velocity profiles under the crests are also shown, confirming the findings from the elevation.

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