scholarly journals Comparison of Extreme Offshore Structural Response from Two Alterna-tive Stretching Techniques

2013 ◽  
Vol 7 (1) ◽  
pp. 273-281 ◽  
Author(s):  
N.I. Mohd Zaki ◽  
M.K. Abu Husain ◽  
G. Najafian
Keyword(s):  
Wave Theory ◽  
Surface Elevation ◽  
Random Wave ◽  
Surface Zone ◽  
Offshore Structure ◽  
Near Surface ◽  
Water Particle ◽  
Wave Kinematics ◽  
Water Particle Kinematics ◽  
Extreme Responses

Linear random wave theory (LRWT) has successfully explained most properties of real sea waves with the ex-ception of some nonlinear effects for surface elevation and water particle kinematics. Due to its simplicity, it is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record; however, predicted water particle kinematics from LRWT suffer from unrealistically large high-frequency compo-nents in the vicinity of mean water level (MWL). To overcome this deficiency, a common industry practice for evaluation of wave kinematics in the free surface zone consists of using linear random wave theory in conjunction with empirical techniques (such as Wheeler and vertical stretching methods) to provide a more realistic representation of near-surface wave kinematics. It is well known that the predicted kinematics from these methods are different; however, no systematic study has been conducted to investigate the effect of this on the magnitude of extreme responses of an offshore structure. In this paper, probability distributions of extreme responses of an offshore structure from Wheeler and vertical stretching methods are compared. It is shown that the difference is significant; consequently, further research is required to deter-mine which method is more reliable.

Comparison of Extreme Responses From Wheeler and Vertical Stretching Methods

2013 ◽  
Author(s):  
N. I. Mohd Zaki ◽  
M. K. Abu Husain ◽  
G. Najafian
Keyword(s):  
Wave Theory ◽  
Random Wave ◽  
Surface Zone ◽  
Offshore Structure ◽  
Near Surface ◽  
Water Particle ◽  
Realistic Representation ◽  
The Difference ◽  
Water Particle Kinematics ◽  
Extreme Responses

Linear random wave theory (LRWT) is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record. However, it is well known that LRWT leads to water particle kinematics with exaggerated high-frequency components in the vicinity of mean water level (MWL). To avoid this problem, empirical techniques such as Wheeler and vertical stretching methods are frequently used to provide a more realistic representation of the wave kinematics in the near surface zone. In this paper, the Monte Carlo time simulation technique is used to investigate the effect of these two different methods of simulating water particle kinematics on the probability distribution of extreme responses. It is shown that the difference could be significant leading to uncertainty as to which method should be used.

The Effect of Different Methods of Simulating Water Particle Kinematics on the 100-Year Responses

2016 ◽  
Author(s):  
N. I. Mohd Zaki ◽  
M. K. Abu Husain ◽  
N. Abdullah Shuhaimy ◽  
G. Najafian
Keyword(s):  
Wave Theory ◽  
Random Wave ◽  
Offshore Structure ◽  
Near Surface ◽  
Water Particle ◽  
Wave Kinematics ◽  
Realistic Representation ◽  
Frequency Components ◽  
Offshore Industry ◽  
Water Particle Kinematics

Linear random wave theory (LRWT) is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record. However, it is well known that LRWT leads to water particle kinematics with exaggerated high-frequency components in the vicinity of mean water level (MWL). A number of empirical techniques have been suggested to provide a more realistic representation of near surface wave kinematics. The empirical techniques popular in the offshore industry include Wheeler stretching, linear extrapolation, delta stretching, and vertical stretching. Each of these methods is intended to calculate sensible kinematics above the MWL, yet they have been found to differ from one another in the results yielded. In this paper, two new methods of simulating water particle kinematics are introduced. In this study, the values of 100-year responses derived from different methods of simulating wave kinematics are compared.

Simulation of Water Particle Kinematics in the Near Surface Zone

2009 ◽  
Author(s):  
G. Najafian ◽  
N. I. Mohd Zaki ◽  
G. Aqel
Keyword(s):  
Water Level ◽  
High Frequency ◽  
Wave Theory ◽  
Surface Zone ◽  
Offshore Structure ◽  
Near Surface ◽  
Water Particle ◽  
Frequency Components ◽  
Mean Water Level ◽  
Water Particle Kinematics

Linear Random Wave Theory (LRWT) is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record. It is, however, well known that wave kinematics calculated from LRWT suffer from unrealistically large high-frequency components in the vicinity of mean water level. To overcome this deficiency, a common industry practice consists of using linear wave theory in conjunction with empirical techniques, such as the Wheeler or the vertical stretching methods, to provide a more realistic representation of the near-surface water particle kinematics. In this paper, a modified version of LRWT is introduced, which, unlike the standard LRWT, does not lead to unrealistically large high-frequency components in the vicinity of mean water level. The proposed method leads to predicted kinematics in the near surface zone which lie between corresponding values from the Wheeler and the vertical stretching methods, respectively.

Derivation of Water Particle Kinematics in the Near Surface Zone by the Effective Water Depth Procedure

2010 ◽  
Author(s):  
N. I. Mohd Zaki ◽  
G. Najafian
Keyword(s):  
Water Depth ◽  
High Frequency ◽  
Random Wave ◽  
Surface Zone ◽  
Offshore Structure ◽  
Near Surface ◽  
Water Particle ◽  
Frequency Components ◽  
Water Particle Kinematics ◽  
Effective Water

Linear Random Wave Theory (LRWT) is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record. However, it is well known that LRWT leads to water particle kinematics with exaggerated high-frequency components in the vicinity of mean water level (MWL). To avoid this problem, empirical techniques (such as Wheeler & vertical stretching methods) are frequently used to provide a more realistic representation of the wave kinematics in the near surface zone. In this paper, a modified version of LRWT, based on the derivation of an effective water depth, is introduced. The proposed technique leads to predicted kinematics (in the near surface zone) which lie between corresponding values from the Wheeler and the vertical stretching methods. Furthermore, it does not suffer from exaggerated high-frequency components in the near surface zone.

scholarly journals PERIODIC THEORY VELOCITY PREDICTION IN RANDOM WAVE

1978 ◽  
Vol 1 (16) ◽  
pp. 18
Author(s):  
John H. Nath ◽  
Koji Kobune
Keyword(s):  
Wave Theory ◽  
Ocean Waves ◽  
Periodic Wave ◽  
Random Wave ◽  
Random Waves ◽  
Wave Kinematics ◽  
Velocity Prediction ◽  
Water Particle Kinematics ◽  
Predictive Theory ◽  
Random Ocean Waves

Large waves in a series of random ocean waves are considered in the design of ocean structures. When random structural vibrations can be ignored, periodic wave theories are used to predict the water particle kinematics for a design wave even though the real wave is irregular. This paper presents the authors' first attempt to quantify the validity of using periodic wave theory for random waves. Measurements of maximum horizontal and vertical velocities were made in laboratory generated periodic and random waves. They compared favorably with predictions from periodic wave theories (even with Airy theory) particularly for the large waves in a series. Since the design wave concept is applied to the largest waves, the conclusion is that periodic wave theory may be adequate, providing an appropriate factor of safety is used to account for the differences between the actual maximum wave kinematics in nature and those in the predictive theory.

DISPERSION IN SEISMIC SURFACE WAVES

Geophysics ◽  
1951 ◽  
Vol 16 (1) ◽  
pp. 63-80 ◽  
Author(s):  
Milton B. Dobrin
Keyword(s):  
Surface Waves ◽  
Water Waves ◽  
Seismic Refraction ◽  
Wave Theory ◽  
Wave Dispersion ◽  
Surface Zone ◽  
Surface Wave Dispersion ◽  
Near Surface ◽  
Arrival Times ◽  
Ground Roll

A non‐mathematical summary is presented of the published theories and observations on dispersion, i.e., variation of velocity with frequency, in surface waves from earthquakes and in waterborne waves from shallow‐water explosions. Two further instances are cited in which dispersion theory has been used in analyzing seismic data. In the seismic refraction survey of Bikini Atoll, information on the first 400 feet of sediments below the lagoon bottom could not be obtained from ground wave first arrival times because shot‐detector distances were too great. Dispersion in the water waves, however, gave data on speed variations in the bottom sediments which made possible inferences on the recent geological history of the atoll. Recent systematic observations on ground roll from explosions in shot holes have shown dispersion in the surface waves which is similar in many ways to that observed in Rayleigh waves from distant earthquakes. Classical wave theory attributes Rayleigh wave dispersion to the modification of the waves by a surface layer. In the case of earthquakes, this layer is the earth’s crust. In the case of waves from shot‐holes, it is the low‐speed weathered zone. A comparison of observed ground roll dispersion with theory shows qualitative agreement, but it brings out discrepancies attributable to the fact that neither the theory for liquids nor for conventional solids applies exactly to unconsolidated near‐surface rocks. Additional experimental and theoretical study of this type of surface wave dispersion may provide useful information on the properties of the surface zone and add to our knowledge of the mechanism by which ground roll is generated in seismic shooting.

Numerical Simulation of Non-Gaussian Wave Elevation and Kinematics Based on Two-Dimensional Fourier Transform

2006 ◽  
Author(s):  
Xiang Yuan Zheng ◽  
Torgeir Moan ◽  
Ser Tong Quek
Keyword(s):  
Fourier Transform ◽  
Wave Interaction ◽  
Wave Theory ◽  
Two Dimensions ◽  
Second Order ◽  
Interaction Matrix ◽  
Random Wave ◽  
Two Dimensional ◽  
Near Surface ◽  
Wave Elevation

The one-dimensional Fast Fourier Transform (FFT) has been extensively applied to efficiently simulate Gaussian wave elevation and water particle kinematics. The actual sea elevation/kinematics exhibit non-Gaussianities that mathematically can be represented by the second-order random wave theory. The elevation/kinematics formulation contains double-summation frequency sum and difference terms which in computation make the dynamic analysis of offshore structural response prohibitive. This study aims at a direct and efficient two-dimensional FFT algorithm for simulating the frequency sum terms. For the frequency difference terms, inverse FFT and FFT are respectively implemented across the two dimensions of the wave interaction matrix. Given specified wave conditions, not only the wave elevation but kinematics and associated Morison force are simulated. Favorable agreements are achieved when the statistics of elevation/kinematics are compared with not only the empirical fits but the analytical solutions developed based on modified eigenvalue/eigenvector approach, while the computation effort is very limited. In addition, the stochastic analyses in both time-and frequency domains show that the near-surface Morison force and induced linear oscillator response exhibits stronger non-Gaussianities by involving the second-order wave effects.

scholarly journals NEAR-SURFACE ORBITAL VELOCITIES IN IRREGULAR WAVES

1986 ◽  
Vol 1 (20) ◽  
pp. 8
Author(s):  
K.-F. Daemrich ◽  
A. Gotschenberg
Keyword(s):  
Wave Theory ◽  
Surface Elevation ◽  
Irregular Waves ◽  
Linear Wave ◽  
Simulation Methods ◽  
Servo Motor ◽  
Near Surface ◽  
Linear Wave Theory ◽  
Wave Flume

The paper deals with measurements of horizontal orbital velocities near the surface of mechanically generated waves in a wave flume. Due to the characteristics of most velocity probes, it is difficult or impossible to measure with a fixed probe in the area above the lowest trough. As the probe is not submerged continuously, failures or uncertainties in the measurements may occur. To overcome these limitations, a movable frame for the velocity probe was designed, which can be moved vertically up and down with the surface elevation by a disc rotor servo motor, controlled by a wave gauge. By that continuously velocities up to 3 cm below the surface could be measured. Theoretical velocities have been calculated for comparison with different simulation methods for irregular waves, based on linear wave theory.

scholarly journals FULL SCALE NEAR SURFACE WATER PARTICLE VELOCITIES AND PRESSURES ACTING ON AN INCLINED TUBULAR MEMBER

1980 ◽  
Vol 1 (17) ◽  
pp. 113
Author(s):  
Fritz Busching ◽  
Eckehard Martini
Keyword(s):  
Reynolds Numbers ◽  
Wave Force ◽  
Field Investigation ◽  
Velocity Measurements ◽  
Near Surface ◽  
Water Particle ◽  
Wave Kinematics ◽  
Pressure Distributions ◽  
Particle Velocities ◽  
Offshore Wave

A field investigation programme on simultaneous wave force and water particle velocity measurements is decribed with reference to an inclined tubular member subjected to offshore wave kinematics. First measurements at supercritical Reynolds numbers indicate strong irregularities in successively taken pressure distributions on the circumference of the test section as well as in the velocity vectors. The influence of superimposed tidal currents is obvious.

Numerical Simulation of Random Wave Forces Near the Free Surface

1991 ◽  
Vol 113 (1) ◽  
pp. 14-22 ◽  
Author(s):  
M. Isaacson ◽  
K. Subbiah
Keyword(s):  
Numerical Simulation ◽  
Free Surface ◽  
Standard Deviation ◽  
Probability Density ◽  
Random Wave ◽  
Wave Forces ◽  
Water Particle ◽  
Time Histories ◽  
The Mean ◽  
Water Particle Kinematics

The present paper describes the numerical simulation of random wave forces acting on a section of fixed slender vertical cylinder near the free surface, taking account of the intermittency of submergence. Time histories of water particle kinematics corresponding to a specified wave spectrum are generated using linear numerical transforms and corresponding force time histories at different sections are computed using the Morison equation. Analytical predictions of various statistical properties of water particle kinematics and forces for the intermittent flow are compared with results of the numerical simulations. These include the probability density of particle kinematics, the spectral density of the force, the probability density of force maxima, and the mean and standard deviation of the force maxima. In general, the agreement is found to be quite satisfactory. The effects of simulation time and random phases on the mean and standard deviation of intermittent force maxima are also investigated.

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