Impact Forces on a Jacket Deck in Regular Waves and Irregular Wave Groups .

1997 ◽  
Author(s):  
J.J. Murray ◽  
F.N. Winso ◽  
P. Kaplan
Keyword(s):  
Regular Waves ◽  
Wave Groups ◽  
Irregular Wave ◽  
Impact Forces

The Slow Drift Oscillations of a Moored Object in Random Seas

1972 ◽  
Vol 12 (03) ◽  
pp. 191-198 ◽  
Author(s):  
G.F.M. Remery ◽  
A.J. Hermans
Keyword(s):  
Wave Train ◽  
Model Tests ◽  
Regular Wave ◽  
Regular Waves ◽  
Frequency Oscillation ◽  
Wave Groups ◽  
Irregular Wave ◽  
Random Seas ◽  
Head Waves ◽  
Constant Part

Abstract The phenomenon of the slowly varying drifting force on a mowed object in a random sea is explained and illustrated from the results of several model tests with a rectangular barge. These tests, conducted at the Netherlands Ship Model Basin, were an extension of an object executed program. Using the results of measured or calculated drifting forces on an object moored in regular waves, a prediction can be Made of the drifting forces induced by wave trains consisting of regular wave groups. Also, for an irregular wave train the drifting force on the barge can be computed as a function of time, which makes it possible to calculate the surge motion of the barge. The results of tests and calculations show a reasonable agreement. Introduction In the last few years the problems concerning the mooring of objects in random seas have gained much attention as a result of the necessity to load and discharge big tankers in open sea, or because the sea bottom has to be explored and exploited by vessels operating from the water surface. Generally a floating object moored in waves will be subjected to forces causing horizontal and vertical motions and to moments causing angular motions about the horizontal and vertical axes. Here we will deal with the horizontal surge motion of a rectangular barge moored by means of linear springs in head waves. The surge motion can be split up into a mean excursion, a slowly varying motion, and a higher frequency oscillation around the slowly varying position. The period of the higher frequency oscillation is equal to that of the wave motion; and since a considerable amount of literature is available concerning this part of the motion, it will not be treated in this paper. From the results of model tests in regular waves the mean drifting force on the barge could be determined as a function of the wave frequency. Using these data, the long-periodical surge motion of the barge was calculated for different stiffnesses of the mooring system for the condition in which the barge was moored in a wave train consisting of regular wave groups. The results of these calculations are compared with model test results. From these and earlier executed tests it is clear that resonance phenomena may occur when the period with which the wave groups encounter the barge equals the natural period of the surge motion of the moored barge. period of the surge motion of the moored barge. It also appears to be possible to calculate the drifting force induced by regular wave groups when such a wave train is taken to consist of two regular waves with a small difference in frequency. The regular wave groups, used for a clear demonstration of certain long-periodical phenomena, have mainly educational value. Regular wave groups will seldom occur. Generally the wave height changes irregularly. To estimate the drifting forces exerted on an object in a particular irregular wave train as a function of time, a method exists which produces reasonable results. This method, based on the principle of known drifting force in regular waves, principle of known drifting force in regular waves, will be dealt with. Starting from the obtained drifting force, the surge motion of the object moored in this particular wave train can be calculated. This is illustrated by comparison of some calculated surge motion records with those of measured ones. THE DRIFTING FORCE IN REGULAR WAVES The hydrodynamic forces on an object floating in regular waves may be resolved in an oscillatory part and in a constant part, of which the latter is part and in a constant part, of which the latter is known as the steady drifting force. Maruo shows that, for the two-dimensional case of an infinitely long cylinder floating in regular waves with its axis perpendicular to the wave direction, the steady drifting force Fd per unit length satisfies: Fd = 1/2 pga . SPEJ P. 191

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.

CFD Analysis of Deck Impact in Irregular Waves: Wave Representation by Transient Wave Groups

2011 ◽  
Author(s):  
O̸ystein Lande ◽  
Thomas B. Johannessen
Keyword(s):  
Wave Field ◽  
Irregular Waves ◽  
Computational Domain ◽  
Cfd Analysis ◽  
Wave Group ◽  
Regular Waves ◽  
Transient Wave ◽  
Wave Groups ◽  
Wave Induced ◽  
Random Background

Analysis of wave structure interaction problems are increasingly handled by employing CFD methods such as the well known Volume-of-Fluid (VoF) method. In particular for the problem of deck impact on fixed structures with slender substructures, CFD methods have been used extensively in the last few years. For this case, the initial conditions have usually been treated as regular waves in an undisturbed wave field which may be given accurately as input. As CFD analyses become more widely available and are used for more complex problems it is also necessary to consider the problem of irregular waves in a CFD context. Irregular waves provide a closer description of the sea surface than regular waves and are also the chief source of statistical variability in the wave induced loading level. In general, it is not feasible to run a long simulation of an irregular seastate in a CFD analysis today since this would require very long simulation times and also a very large computational domain and sophisticated absorbing boundary conditions to avoid build-up of reflections in the domain. The present paper is concerned with the use of a single transient wave group to represent a large event in an irregular wave group. It is well known that the autocovariance function of the wave spectrum is proportional to the mean shape of a large wave in a Gaussian wave field. The transient nature of such a wave ensures that a relatively small wave is generated at the upwave boundary and dissipated at the downwave boundary compared with the wave in the centre of the domain. Furthermore, a transient wave may be embedded in a random background if it is believed that the random background is important for the load level. The present paper describes the method of generating transient wave groups in a CFD analysis of wave in deck impact. The evolution of transient wave groups is first studied and compared with experimental measurements in order to verify that nonlinear transient waves can be calculated accurately using the present CFD code. Vertical wave induced loads on a large deck is then investigated for different undisturbed wave velocities and deck inundations.

Numerical Simulation of Breaking Waves in a Wave Group by SPH

2015 ◽  
Vol 776 ◽  
pp. 151-156
Author(s):  
Ni Nyoman Pujianiki
Keyword(s):  
Surf Zone ◽  
Breaking Waves ◽  
Return Flow ◽  
Wave Group ◽  
Regular Waves ◽  
Wave Groups ◽  
Smoothed Particle ◽  
Wave Heights ◽  
Smoothed Particle Hydrodynamic ◽  
Wave Break

Smoothed Particle Hydrodynamic (SPH) numerical model is used to investigate wave group effects at breaking and after breaking by comparing individual waves in a group with equivalent regular waves. Regular wave break almost at the same position and with the same wave height. Meanwhile in a wave group, the wave breaks in the variant positions and with variant wave heights. These phenomena cause the breaking point to be more scattered in a wave group rather than in regular waves. Return flow due to the breaking of wave groups appears more significant and is extended to the full depth in the surf zone rather than in regular waves. Swash oscillations of the wave group in the surf zone appear irregular. Meanwhile in regular waves, swash oscillations are almost constant.

Interaction of Waves With a Moored Semisubmersible

1984 ◽  
Vol 106 (4) ◽  
pp. 419-425 ◽  
Author(s):  
S. K. Chakrabarti ◽  
D. C. Cotter
Keyword(s):  
Diffraction Theory ◽  
Nonlinear Damping ◽  
Irregular Waves ◽  
Test Results ◽  
Regular Waves ◽  
Mooring Lines ◽  
Wave Groups ◽  
First Order ◽  
Oscillating Motion ◽  
Theoretical Results

A semisubmersible moored in waves experiences a steady offset and two types of motion—a first-order motion at frequencies corresponding to the incident wave frequencies and a slowly oscillating motion near the natural frequency of the semisubmersible/mooring system. An extensive wave tank testing of a semisubmersible model was undertaken in which the motions of the semisubmersible and the loads in the mooring lines were measured. The semisubmersible was tested in the tank in a head sea as well as a beam sea heading in a series of regular waves, regular wave groups and irregular waves. The test results of the steady offset and first-order and slowly oscillating motions are presented for each heading and for each of these wave series as functions of the wave period. The experimental results are correlated with theoretical results based on a 3-D diffraction theory which takes into account the appropriate first and second-order terms. It is found that the nonlinear damping terms are quite important in explaining the behavior of the moored semisubmersible in waves and that the steady drift loads in wave groups can be determined from results based on regular waves.

Propagation of Steep and Breaking Short-Crested Waves: A Comparison of CFD Codes

2018 ◽  
Author(s):  
Øystein Lande ◽  
Thomas Berge Johannessen
Keyword(s):  
Wave Propagation ◽  
Wave Spectrum ◽  
Ocean Waves ◽  
Offshore Structures ◽  
Practical Reasons ◽  
Breaking Wave ◽  
Linear Wave ◽  
Wave Groups ◽  
Irregular Wave ◽  
Highly Nonlinear

Using the computational fluid domain for propagation of ocean waves have become an important tool for the calculation of highly nonlinear wave loading on offshore structures such as run-up, wave slamming and non-linear breaking wave kinematics. At present, there are many computational fluid dynamics (CFD) codes available which can be employed to calculate water wave propagation and wave induced loading on structures. For practical reasons, however, the use of these codes is often limited to the propagation of regular uni-directional waves initiated very close to the structure, or to investigating the properties and loading due to measured waves by fitting a numerical wave to a measured wave profile. The present paper focuses on the propagation of steep irregular and short crested wave groups up to the point of breaking. Indeed, this is challenging because of the highly nonlinear behavior of irregular wave groups as steepness increases and they approach the point of breaking. The complexity further increases with the introduction of short-crestedness requiring computation in a large 3-dimentional domain. Two CFD codes are used in this comparison study which are believed to be well conditioned for wave propagation, the commercial code ComFLOW (available through the ComFLOW JIP project) and the open-source code BASILISK. The primary objective of this paper to show the two CFD codes capability of recreating measured irregular wave groups, using the known linear wave components from the model test as input to fluid domain. Wave elevation is measured at several locations in the close vicinity of the focus point. The comparison is carried out for a selection of events with variation in steepness, wave spreading and wave spectrum.

scholarly journals LOAD ANALYSIS FROM WAVE GROUPS

1978 ◽  
Vol 1 (16) ◽  
pp. 140
Author(s):  
A.I. Kuznetsov ◽  
G.D. Khaskhatchikh
Keyword(s):  
Random Process ◽  
Group Structure ◽  
Theoretical Models ◽  
Point Of View ◽  
Irregular Waves ◽  
Regular Waves ◽  
Shore Zone ◽  
Wave Groups ◽  
Load Analysis ◽  
Sea Wave

At the present time sea wave is described by means of two theoretical models, the first is based on regular waves, components of which do not change in time and space, and the second model is based on irregular waves, components of which are randomly changed. The latter coincides to the greater extent with the rolling sea, but even this model does not characterize it to the full» Taking sea wave for a random process, the model of irregular waves does not take into account the sequence of their alternation. Prom the point of view of probability, on which the model of irregular waves is based, the maximum wave may be followed by the minimum wave, and the greater period may be followed by the smallest one. The real sea wave, especially in shore zone, where the main engineering constructions are placed, is characterized by clearly expressed group structure, which includes alternation of a number of great and small waves and the maximum wave is always followed by the wave having almost the same parameters.

Assessment of Seakeepability

1966 ◽  
Vol 3 (04) ◽  
pp. 454-468
Author(s):  
Norman Hamlin ◽  
Roger Compton
Keyword(s):  
Systematic Research ◽  
Regular Waves ◽  
Ship Model ◽  
Irregular Wave ◽  
Single Screw ◽  
Wave Patterns ◽  
Ship Performance ◽  
Ship Speed ◽  
Naval Architect ◽  
Selection Of

This paper attempts to apply presently available knowledge of ship model behavior in regular waves to the prediction of trends of ship performance in realistic irregular wave patterns. Performance is studied in the terms of tendency to ship water forward and to slam, and of magnitude of heaving accelerations. The variables of particular interest considered are severity of sea, ship speed, ship heading, and hull proportions. It is believed that the trends obtained in this exploratory project should provide the naval architect with some guidance in the selection of basic hull characteristics for one particular type of ship (in this case, the single-screw dry-cargo ship). The results should also be of assistance in planning further systematic research.

Kinematics of Experimentally Generated Severe Wave Conditions and Implications for Numerical Models

2011 ◽  
Author(s):  
Lisa Minnick ◽  
Christopher Kent ◽  
Christopher Bassler ◽  
Scott Percival ◽  
Lauren Hanyok
Keyword(s):  
Free Surface ◽  
Ship Motions ◽  
Wave Group ◽  
Regular Waves ◽  
Validation Data ◽  
Data Set ◽  
Wave Groups ◽  
Wave Kinematics ◽  
Seakeeping Performance ◽  
Numerical Wave

An experiment was performed to measure and characterize wave kinematics in a large-scale experimental basin. The primary objective was to measure and characterize the wave kinematics of both regular waves of varying steepness and scaled irregular seas with embedded large-amplitude wave groups. The secondary objective was to provide a validation data set for wave theory and numerical simulation tool development. Measurements included free surface elevations and velocity field measurements under the free surface using particle image velocimetry (PIV). A discussion of free surface elevation data, the effect of wave steepness on the velocity profile of regular waves and an introduction to the analysis of the wave kinematics of the embedded wave groups was presented in a previous paper by the authors (OMAE2010-20240). The current paper expands on the analysis presented in the previous paper, providing a discussion of the effect of wave group composition, wave group location within irregular seas, and seaway scaling on the kinematics. This experiment is part of an ongoing effort at the Naval Surface Warfare Center Carderock Division (NSWCCD) to improve predictions and measurement of ship motions in waves and assess the dynamic stability and seakeeping performance of naval ships. NSWCCD has developed a deterministic wave generation method to be used in its seakeeping basin facility. The exploration of the above factors provides a better understanding of the employed method and its effectiveness for creating wave groups. Specifically, to model realistic severe conditions that a ship may encounter. These findings, in turn, lead to the possibility of improvements to current model testing methods at NSWCCD. In addition to assessing the experimental methods employed, the experimental data set can also be used to validate and improve current numerical wave models for ship motions prediction. A comparison of the measured wave elevations and kinematics with a pseudo-spectral numerical wave model is also presented and discussed.

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