Evaluation of Response Amplitude Operator of Ship Roll Motions Based on the Experiments in White Noise Waves

2017 ◽  
Author(s):  
Sebastian Bielicki ◽  
Antoni Bednarek ◽  
Marek Kraskowski
Keyword(s):  
White Noise ◽  
Response Amplitude ◽  
Irregular Waves ◽  
Scale Model ◽  
Roll Motion ◽  
Viscous Damping ◽  
Regular Waves ◽  
Irregular Wave ◽  
Decay Test ◽  
Ship Roll

Evaluation of the response amplitude operator (RAO) function for ship wave frequency motions by means of scale model tests in regular waves is a standard procedure conducted by hydrodynamic model testing institutions. The resulting RAO function allows for evaluating sufficiently reliable seakeeping predictions for low to moderate sea states. However, for standard hull forms, correct prediction of roll motion in irregular wave (and also in regular waves different than these used in the experiment) on the basis of RAO function presents a substantial challenge due to considerable contribution of viscous damping to roll response. In other words, the RAO values depend strongly on the amplitude of the waves used in the experiment, so the final prediction requires careful application of relevant correction of RAO, dependent on the actual significant wave height, for which the prediction is computed. Thus, in order to collect complete data for ship roll prediction, the roll decay test is usually also required. Additional drawback of evaluating the seakeeping prediction on the basis of RAO is the fact that the experiment in regular waves is quite time-consuming, which refers to the experiment itself as well as to the processing. The following paper presents a proposal of the alternative method for experimental evaluation of response amplitude operator of roll motion in beam waves, consisting in exposing the ship model to irregular wave characterized by white noise spectrum, i.e. the spectrum of uniform energy density. In theory, RAO function is equivalent to the square root of the spectrum of the response to white noise wave. The results of experiments in white noise waves were verified on the basis of the results of comprehensive experiments conducted in usual way. Additionally, the effect of non-linearity of viscous damping was widely studied by comparing the calibrated RAO-based predictions with actual response to irregular waves of different heights. As a result, a method for including the non-linear effects in prediction based on white noise was proposed. It was proved that the proposed method is capable of providing equally valuable information in significantly shorter time.

Damping Coefficient Analyses for Floating Offshore Structures

2010 ◽  
Author(s):  
Edgard B. Malta ◽  
Rodolfo T. Gonc¸alves ◽  
Fabio T. Matsumoto ◽  
Felipe R. Pereira ◽  
Andre´ L. C. Fujarra ◽  
...  
Keyword(s):  
Numerical Models ◽  
Linear Method ◽  
Offshore Structures ◽  
Damping Coefficient ◽  
Irregular Waves ◽  
Scale Model ◽  
Viscous Damping ◽  
Viscous Effects ◽  
Random Decrement Technique ◽  
Decay Test

The damping evaluation of floating offshore systems is based on the viscous effects that are not considered in numerical models using the potential theory. Usually, different techniques for different systems are used to evaluate these hydrodynamic coefficients. The total damping is separated by potential and viscous damping, the first one is evaluated numerically and the second through experiments at reduced scale model. Common techniques considering linear motion equations cannot be applied to all degrees of freedom. Some methods were compared for results of decay test, such as: exponential and quadratic fit. Fourier transform (FT) spectral analysis and Hilbert Huang transform (HHT) can be used to evaluate the signal natural frequency and with HHT this can be done during the time domain. Also, analysis through the Random Decrement Technique (RDT) is presented to demonstrate the damping evaluation for irregular waves. The method to obtain external damping was presented for the different techniques in an ITTC semi-submersible model. The linear method is not sufficient to predict the damping coefficient for all the cases, because in most of them, the viscous damping was better represented by a quadratic fit. The HHT showed to be a good alternative to evaluate damping in non-linear systems.

The Effect of Pitch Gyradius on Added Resistance of Yacht Hulls

1991 ◽  
Author(s):  
James F. Moran
Keyword(s):  
Water Resistance ◽  
Wave Spectrum ◽  
Response Amplitude ◽  
Added Resistance ◽  
Regular Waves ◽  
Low Frequencies ◽  
Irregular Wave ◽  
Wave Data ◽  
The Difference ◽  
Response Amplitude Operators

The purpose of this investigation was to determine the effect of pitch gyradius on added resistance of yacht hulls. Tank testing of a model yacht in head seas was performed in the Webb Robinson Model Basin. The model was tested in regular waves at two speeds and five variations of gyradius. The model was also evaluated in irregular seas of the Pierson-Moskowitz spectrum at various speeds with two gyradii. Response Amplitude Operators were developed from the regular wave data and comparisons made. The irregular wave data were analyzed for the effect of speed on the difference in added resistance between the maximum and minimum gyradius settings. Several conclusions were arrived at after analyzing the data. The Response Amplitude Operaters shift as the gyradius changes. In regular waves, at low frequencies of encounter, a lower, gyradius resulted in less added frequencies of encounter in regular waves, this trend reverses itself and the higher gyradii result in reduced added resistance. However, at higher frequencies of encounter in regular waves, this trend reverses, reverses itself in reduced added resistance. The peaks of the RAO curves shift to higher frequencies at higher gyradii. It was also concluded that at the higher speed, Froude Number of 0.3, the added resistance was lower relative to the still-water resistance for each gyradius tested. The irregular wave testing revealed the effect of the lower frequencies dominating the irregular wave spectrum. The minimum gyradius, in irregular seas showed less added resistance than the maximum gyradius. In addition, the irregular wave testing verified, the reduction of added resistance, relative to still-water resistance, at increasing speeds for both the minimum and maximum gyradii.

Hydrodynamic Performance of a Novel Free Surface Pitching Breakwater

2014 ◽  
Author(s):  
Vengatesan Venugopal ◽  
Stefan Zlatev
Keyword(s):  
Body Shape ◽  
Hydrodynamic Performance ◽  
Irregular Waves ◽  
Scale Model ◽  
Reflected Waves ◽  
Irregular Wave ◽  
Wave Flume ◽  
Floating Breakwater ◽  
Wave Heights ◽  
Transmission Ct

A new concept floating breakwater was developed and tested to evaluate its hydrodynamic performance in this paper. This innovative floating breakwater has a rocking body shape which could also be used as a wave power device. A scale model was tested in a wave flume under regular and irregular wave conditions for various combinations of wave frequencies and wave heights. The breakwater has been tested for three immersion depths of 0.05 m, 0.09 m and 0.13 m from still water level. The measured transmitted and reflected waves were used to evaluate the coefficients of transmission (CT), reflection (CR) and dissipation (CL). The results illustrated that the breakwater model performed at its best when submerged at 0.13m, as this immersion depth produced lower coefficients of transmission (CT), lower reflection coefficients (CR) and higher energy dissipation (CL) coefficients. The comparison between regular and irregular waves produced similar ranges of transmission, reflection and energy coefficients.

Maneuvering Behavior of Ships in Irregular Waves

2013 ◽  
Author(s):  
Renato Skejic ◽  
Odd M. Faltinsen
Keyword(s):  
Deep Water ◽  
Cross Flow ◽  
Irregular Waves ◽  
Scale Model ◽  
Wave Loads ◽  
Ship Maneuvering ◽  
Slender Body Theory ◽  
Statistical Point ◽  
Irregular Wave ◽  
Mmg Model

Ship maneuvering in waves is analyzed by using a unified seakeeping and maneuvering two-time scale model in irregular sea that has been applied by Skejic and Faltinsen [1] for regular waves. The irregular wave effects are accounted for by Newman’s [2] approximation of the slow-drift 2nd order wave loads valid for deep water (Faltinsen [3], Pinkster [29]). The modular type maneuvering model (MMG model) based on Söding’s [4] nonlinear slender-body theory is used for the maneuvering analysis. Forces and moments due to rudder, propeller, and viscous cross-flow are accounted for as presented by Skejic and Faltinsen [1] and Yasukawa [5, 6]. In particular, the behavior of the propulsive coefficients (the thrust deduction and wake fraction) in waves (Faltinsen et al. [7], Faltinsen and Minsaas [8]) are discussed from the perspective of ship maneuvering characteristics in both regular and irregular wave environments. The unified model of seakeeping and maneuvering for deep-water irregular waves is validated for the ‘S7-175’ (‘SR 108’) container ship in calm water and regular deep-water wave scenarios by comparison with experimental results by Yasukawa [5, 6]. The maneuvering model is applied to a ‘MARINER’ ship performing turning maneuver in irregular waves. The obtained results of the ships main maneuvering parameters are discussed from a statistical point of view.

Behaviour of a Suspended Wellbay Module and Flare Tower in Waves During Transit to Shore

2019 ◽  
Author(s):  
Hoi-Sang Chan ◽  
Evren Armaoğlu ◽  
Matthew Thomson ◽  
Alistair Garner
Keyword(s):  
Frequency Domain ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Irregular Waves ◽  
Scale Model ◽  
Response Analysis ◽  
Regular Waves ◽  
The North ◽  
Response Amplitude Operators

Abstract The extended lift operation to deliver the Wellbay module (M5) combined with the Flare Tower (M8) from the Miller Platform in the North Sea to the shore using the Semi-Submersible Crane Vessel S7000 was restricted by the clearances between M5/M8 and the vessel crane booms. A method to calculate the clearances of the M5/M8 normal to the vessel crane booms has been developed and used in a frequency-domain response analysis to define operability limits. Investigations based on a series of scale model tests in regular waves and irregular short-crested waves including motion decay tests in calm water, conducted by the Maritime Research Institute (MARIN) in the Netherlands, were also made to further evaluate the behaviour of the suspended M5/M8 on S7000’s main hooks during transit. The time series of decay motions of the suspended M5/M8 obtained from the decay motion tests and a time domain analysis are compared and used to derive rigging damping. The numerical results of the frequency-domain analysis are validated with the experimental data for response amplitude operators (RAOs) found in regular waves and pink noise waves, significant and 3 hour most probable maximum/minimum (MPM) responses of interest in irregular waves.

scholarly journals REPRODUCTION MODELS OF BEACH CHANGE BY STORM WAVES

1988 ◽  
Vol 1 (21) ◽  
pp. 115
Author(s):  
Masahiro Ito ◽  
Yoshito Tsuchiya
Keyword(s):  
Grain Size ◽  
Model Test ◽  
Irregular Waves ◽  
Scale Model ◽  
Regular Waves ◽  
Wave Duration ◽  
Storm Waves ◽  
The Mean ◽  
Wave Properties ◽  
Sand Grain Size

This paper presents a technique to reproduce, by a twodimensional moveable-bed model, beach change due to the timedependent storm waves which are generated by the passage of an atmospheric depression. In the model test, scaling conditions for sand grain-size, vertical and horizontal lengths, and wave height and period characteristics were established by applying the authors' scale-model relationship which was reported; and wave duration time also was decided. A method of employing regular waves in the model to represent irregular waves in the field is proposed. From the results, it was shown that the model can reproduce well the beach change in the field using the regular waves having the mean wave properties in the irregular waves.

scholarly journals A Heuristic Approach for Inter-Facility Comparison of Results from Round Robin Testing of a Floating Wind Turbine in Irregular Waves

2021 ◽  
Vol 9 (9) ◽  
pp. 1030
Author(s):  
Sebastien Gueydon ◽  
Frances Judge ◽  
Eoin Lyden ◽  
Michael O’Shea ◽  
Florent Thiebaut ◽  
...  
Keyword(s):  
Wind Turbine ◽  
Round Robin ◽  
Irregular Waves ◽  
Scale Model ◽  
Test Results ◽  
Floating Platform ◽  
Global Trends ◽  
Irregular Wave ◽  
Floating Wind Turbine ◽  
Decay Tests

This paper introduces metrics developed for analysing irregular wave test results from the round robin testing campaign carried out on a floating wind turbine as part of the EU H2020 MaRINET2 project. A 1/60th scale model of a 10 MW floating platform was tested in wave basins in four different locations around Europe. The tests carried out in each facility included decay tests, tests in regular and irregular waves with and without wind thrust, and tests to characterise the mooring system as well as the model itself. While response amplitude operations (RAOs) are a useful tool for assessing device performance in irregular waves, they are not easy to interpret when performing an inter-facility comparison where there are many variables. Metrics that use a single value per test condition rather than an RAO curve are a means of efficiently comparing tests from different basins in a more heuristic manner. In this research, the focus is on using metrics to assess how the platform responds with varying wave height and thrust across different facilities. It is found that the metrics implemented are very useful for extracting global trends across different basins and test conditions.

scholarly journals IRREGULAR WAVE TESTS FOR COMPOSITE BREAKWATER FOUNDATIONS

1982 ◽  
Vol 1 (18) ◽  
pp. 128 ◽  
Author(s):  
Katsutoshi Tanimoto ◽  
Tadahiko Yagyu ◽  
Yoshimi Goda
Keyword(s):  
Wave Height ◽  
Significant Wave Height ◽  
Irregular Waves ◽  
Stability Number ◽  
Regular Waves ◽  
Significant Wave ◽  
Irregular Wave ◽  
Concrete Blocks ◽  
Rubble Mound ◽  
The Stability

The stability of armor units for the rubble mound foundations of composite breakwaters has been investigated under the action of irregular waves. The tests establish that irregular waves are more destructive than regular waves, when the height of regular waves is set equal to the significant wave height. The stability number, defined by Hudson, for quarry stones and concrete blocks with simple shapes is formulated on the basis of irregular wave tests. The stability number is expressed by two parameters of h'7/7]/3 and K, where h' is the crest depth of the rubble mound foundation, #1/3 is the design significant wave height, and K is a parameter for the combined effects of the relative water depth and the relative berm width of the rubble mound foundation to the wavelength. The design mass of armor units can be calculated by the stability equation with the stability number. The application of the proposed method to the results of the irregular wave tests demonstrates that the damage percent for the quarry stones is at most 3.5% at the design condition and the damage progresses rather gradually for the action of higher waves. On the other hand, the damage of the concrete blocks almost jumps beyond the design wave height. In particular, the drastic damage is often caused in the case of high rubble mound foundations. The proposed method is confirmed, however, to be applicable for the ordinary low mound foundations with a sufficient safety.

scholarly journals Study on the Resonant Behaviors of a Bottom-Hinged Oscillating Wave Surge Converter

2021 ◽  
Vol 10 (1) ◽  
pp. 2
Author(s):  
Yao Liu ◽  
Yong-Hwan Cho ◽  
Norimi Mizutani ◽  
Tomoaki Nakamura
Keyword(s):  
Maximum Velocity ◽  
Irregular Waves ◽  
Width Ratio ◽  
Wave Surface ◽  
Regular Waves ◽  
Peak Period ◽  
Natural Period ◽  
Irregular Wave ◽  
Cfd Method ◽  
The Time Domain

This paper studied the resonant behaviors of a bottom-hinged oscillating wave surge converter (OWSC) as well as the relationship of resonance with the response and capture width ratio (CWR). The time-domain dynamic equation of an OWSC in shallow water based on the boundary element method (BEM) was solved by a Python code, considering the corrected wave surface and the nonlinearities of restoring moment, drag, and friction. The unknown factors, such as wave surface corrected factor and drag coefficient, were effectively calibrated with computational fluid dynamics (CFD) method. An intermediate initial angle in free decay is appropriate for use to determine the natural period. Under regular waves, the resonance occurs near the natural period for the uniform wave amplitude, rather than the uniform wave torque amplitude, and can disappear due to the amplification of Power Take-Off (PTO) friction. Under unit-amplitude regular waves, the period of maximum CWR is relatively close to the period of maximum velocity, but far from the resonant period. Under irregular waves, no stable resonance is observed because the maximum equivalent pitch angle appears at different peak periods of wave spectra with the variation in PTO damping. When the period of a regular wave or the peak period of an irregular wave is close to the natural period, a phase hysteresis of velocity relative to wave torque always occurs.

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