Amplification of Waves by a Concrete Gravity Sub-Structure: Linear Diffraction Analysis and Estimating the Extreme Crest Height

2004 ◽  
Author(s):  
E. J. van Iperen ◽  
G. Z. Forristall ◽  
J. A. Battjes ◽  
J. A. Pinkster
Keyword(s):  
Wave Model ◽  
Diffraction Theory ◽  
Peak Frequency ◽  
Model Tests ◽  
Second Order ◽  
Irregular Waves ◽  
Regular Wave ◽  
Experimental Surface ◽  
Elevation Data ◽  
Using Data

Diffraction of both regular and irregular waves by a Concrete Gravity Sub-structure (CGS) was investigated using experimental surface elevation data and computational results of the linear diffraction code DELFRAC. The influence of the box-shaped base that supports the four vertical columns was studied independently from the columns, using data from regular wave model tests of the Malampaya CGS. DELFRAC was shown to give accurate results for the focusing of waves over the submerged structure. Results from regular wave data analysis of model tests of the complete Sakhalin II project Lunskoye CGS were compared to the predictions by the linear diffraction code. For the wave cases tested, the first-order amplitudes were accurately predicted. Diffraction of irregular waves at the Lunskoye CGS was studied in a similar way and linear diffraction theory for random seas gave an excellent prediction of incident wave spectral diffraction, including the peaks in the diffracted spectrum near twice the peak frequency in the input spectrum. The results obtained for the Lunksoye CGS in the present study were consistent with results found in similar studies on less complex structures. An attempt to predict the extreme crest heights from the diffracted spectrum was made using a Weibull distribution, and a second-order expansion of the sea surface that captures the effects of wave steepness, water depth and directional spreading with no other approximation than the truncation of the expansion at second order. Depth induced breaking appeared to be an important phenomenon limiting the crest heights. The crest heights in a 100-year sea state at the Lunskoye CGS were accurately predicted.

Experimental Investigation on Dynamic Response of Submarine Pipeline Over Flat Beds in Waves

2007 ◽  
Author(s):  
Yong Sha ◽  
Yongxue Wang ◽  
Lee M. Pearson
Keyword(s):  
Reynolds Numbers ◽  
Order Effect ◽  
Model Tests ◽  
Second Order ◽  
Irregular Waves ◽  
Strain Gauges ◽  
Submarine Pipeline ◽  
Flexible Pipe ◽  
Wave Flume ◽  
Bending Strain

Model tests have been conducted on flexible submarine pipelines over flat beds in both regular and irregular waves and the second order effect induced by waves are concerned within the certain range of Keulegan-Carpenter numbers and Reynolds numbers. The tests were conducted in the wave flume with 55m in length, 4m in width and 2.5m in depth. The pipelines were made by flexible pipe with 60mm in diameter and placed at various distances from a flat bed. Gap to diameter ratio varies from 0.4 to 0.8 when the pipelines are not sagging. The wave period in the model tests is in the range from 0.8 to 2.0 and water depth is 0.4m. The Keulegan-Carpenter numbers are less than 8.0 and the Reynolds numbers are in the subcritical regime. Bending strain was measured by strain gauges bonded on the inner surface of the pipeline. The strain amplitudes and second order effect are analyzed and discussed against various Keulegan-Carpenter numbers and gaps between the pipeline and the flat bed.

Time Domain Simulation of Jack-Up Platform in Second-Order Irregular Seas

2017 ◽  
Author(s):  
Michael Binsar Lubis ◽  
Sverre Haver ◽  
Jørgen Amdahl
Keyword(s):  
Wave Model ◽  
Second Order ◽  
Irregular Waves ◽  
Stokes Wave ◽  
Time Domain Simulation ◽  
Wave Load ◽  
Extreme Wave ◽  
Irregular Wave ◽  
Load Component ◽  
Jack Up

This paper reviews the significance of applying second-order irregular waves for assessing hydrodynamic loads on a jack-up platform. The study is based on a realistic jack-up model. The wave load is determined utilizing Morison equation. The study focuses on extreme wave elevations when the drag load component dominates and the magnitude of wave particle horizontal velocity is crucial. Both extreme surface elevation and wave particle kinematics are observed. For wave particle kinematic, two different stretching methods are compared. The static response of a pile structure using the second-order model and the 5th Stokes wave are compared. In the end, a dynamic analysis of the jack-up utilizing a second-order irregular wave model is performed.

An Experimental Investigation of Nonlinear Wave Generation by Flap Wavemakers

2021 ◽  
Author(s):  
Sébastien Fouques ◽  
Sébastien Laflèche ◽  
Andreas Akselsen ◽  
Thomas Sauder
Keyword(s):  
Experimental Investigation ◽  
Large Amplitude ◽  
Second Order ◽  
Surface Elevation ◽  
Irregular Waves ◽  
Regular Wave ◽  
Third Order ◽  
Times Series ◽  
Long Period ◽  
Sinusoidal Motion

Abstract It is well known that flap wavemakers behave in a nonlinear way when either the flap angle or the flap velocity becomes large. Moreover, the hinge depth should be adapted to the period of the generated waves in order to minimize linear evanescent modes, which may contribute to the formation of nonlinear spurious waves. For example, imposing a sinusoidal motion with a relatively long period and a large amplitude to a short flap will result in a surface elevation composed of a regular wave with the same period as the flap motion, but also of a variety of harmonics with higher frequencies. Second-order harmonics can be predicted theoretically for regular and irregular waves, and they can be corrected by modifying the control signal of the wavemaker. However, there is no theory that can describe nor mitigate effects of orders higher than two. The design of the wavemaker is then essential to generate extreme sea states with good quality and predictability in a laboratory. In this paper, the nonlinearities of flap wavemakers are investigated experimentally for regular and irregular waves generated in SINTEF Ocean’s laboratories. Nonlinearities of order two and three are estimated from times series of the surface elevation measured at different locations by an array of wave probes. Particular focus is put on identifying the effects of the classical second-order correction on the second- and third-order harmonics.

Kinematics Under Extreme Waves

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
Carl Trygve Stansberg ◽  
Ove T. Gudmestad ◽  
Sverre K. Haver
Keyword(s):  
Free Surface ◽  
Water Level ◽  
Deep Water ◽  
Wave Model ◽  
Second Order ◽  
Irregular Waves ◽  
Random Wave ◽  
Extreme Waves ◽  
Near Surface ◽  
Particle Velocities

Nonlinear contributions in near-surface particle velocities under extreme crests in random seas can be important in the prediction of wave loads. Four different prediction methods are compared in this paper. The purpose is to observe and evaluate differences in predicted particle velocities under high and extreme crests, and how well they agree with measurements. The study includes linear prediction, a second-order random wave model, Wheeler’s method [1970, “Method for Calculating Forces Produced by Irregular Waves,” JPT, J. Pet. Technol., pp. 359–367] and a new method proposed by Grue et al. [2003, “Kinematics of Extreme Waves in Deep Water,” Appl. Ocean Res., 25, pp. 355–366]. Comparison to laboratory data is also made. The whole wave-zone range from below still water level up to the free surface is considered. Large nonlinear contributions are identified in the near-surface velocities. The results are interpreted to be correlated with the local steepness kA. Some scatter between the different methods is observed in the results. The comparison to experiments shows that among the methods included, the second-order random wave model works best in the whole range under a steep crest in deep or almost deep water, and is therefore recommended. The method of Grue et al. works reasonably well for z>0, i.e., above the calm water level, while it overpredicts the velocities for z<0. Wheeler’s method, when used with a measured or a second-order input elevation record, predicts velocities fairly well at the free surface z=ηmax, but it underpredicts around z=0 and further below. The relative magnitude of this latter error is slightly smaller than the local steepness kA0 and can be quite significant in extreme waves. If Wheeler’s method is used with a linear input, the same error occurs in the whole range, i.e., also at the free surface.

Analysis on the Performance of Deckwetness of Sandglass-type and Cylindrical Floating Bodies

2016 ◽  
Vol 60 (03) ◽  
pp. 145-155
Author(s):  
Ya-zhen Du ◽  
Wen-hua Wang ◽  
Lin-lin Wang ◽  
Yu-xin Yao ◽  
Hao Gao ◽  
...  
Keyword(s):  
Transfer Functions ◽  
Linear Response Theory ◽  
Second Order ◽  
Irregular Waves ◽  
Wave Loads ◽  
Floating Bodies ◽  
Drift Motion ◽  
First Order ◽  
Series Of Experiments ◽  
Deck Wetness

In this paper, the influence of the second-order slowly varying loads on the estimation of deck wetness is studied. A series of experiments related to classic cylindrical and new sandglass-type Floating Production, Storage, and Offloading Unit (FPSO) models are conducted. Due to the distinctive configuration design, the sand glass type FPSO model exhibits more excellent deck wetness performance than the cylindrical one in irregular waves. Based on wave potential theory, the first-order wave loads and the full quadratic transfer functions of second-order slowly varying loads are obtained by the frequency-domain numerical boundary element method. On this basis, the traditional spectral analysis only accounting for the first-order wave loads and time-domain numerical simulation considering both the first-order wave loads and nonlinear second-order slowly varying wave loads are employed to predict the numbers of occurrence of deck wetness per hour of the two floating models, respectively. By comparing the results of the two methods with experimental data, the shortcomings of traditional method based on linear response theory emerge and it is of great significance to consider the second-order slowly drift motion response in the analysis of deck wetness of the new sandglass-type FPSO.

Extreme Response of Very Large Floating Structure Considering Second-Order Hydroelastic Effects in Multidirectional Irregular Waves

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
Xujun Chen ◽  
Torgeir Moan ◽  
Shixiao Fu
Keyword(s):  
Second Order ◽  
Irregular Waves ◽  
Bending Moments ◽  
Floating Structure ◽  
First Order ◽  
Fluid Forces ◽  
Principal Coordinates ◽  
Vertical Displacements ◽  
Exciting Forces ◽  
Nonlinear Fluid

Hydroelasticity theory, considering the second-order fluid forces induced by the coupling of first-order wave potentials, is introduced briefly in this paper. Based on the numerical results of second-order principal coordinates induced by the difference-frequency and sum-frequency fluid forces in multidirectional irregular waves, the bending moments, as well as the vertical displacements of a floating plate used as a numerical example are obtained in an efficient manner. As the phase angle components of the multidirectional waves are random variables, the principal coordinates, the vertical displacements, and the bending moments are all random variables. Extreme values of bending moments are predicted on the basis of the theory of stationary stochastic processes. The predicted linear and nonlinear results of bending moments show that the influences of nonlinear fluid forces are different not only for the different wave phase angles, but also for the different incident wave angles. In the example very large floating structure (VLFS) considered in this paper, the influence of nonlinear fluid force on the predicted extreme bending moment may be as large as 22% of the linear wave exciting forces. For an elastic body with large rigidity, the influence of nonlinear fluid force on the responses may be larger than the first-order exciting forces and should be considered in the hydroelastic analysis.

Sum-Frequency Diffraction Loads on Tension-Leg Platforms in Bidirectional Waves

1997 ◽  
Vol 119 (1) ◽  
pp. 14-19
Author(s):  
J. H. Vazquez ◽  
A. N. Williams
Keyword(s):  
Numerical Technique ◽  
Diffraction Theory ◽  
Potential Method ◽  
Second Order ◽  
Surface Integral ◽  
Hydrodynamic Loads ◽  
Infinite Depth ◽  
Sum Frequency ◽  
Wave Directionality ◽  
Green Function Approach

Second-order diffraction theory is utilized to compute the sum-frequency diffraction loads on a deepwater tension-leg platform (TLP) in bidirectional waves. The linear diffraction solution is obtained utilizing a Green function approach using higher-order boundary elements. The second-order hydrodynamic loads explicitly due to the second-order potential are computed using the indirect, assisting radiation potential method. An efficient numerical technique is presented to treat the free-surface integral which appears in the second-order load formulation. Numerical results are presented for a stationary ISSC TLP in water of infinite depth. It is found that wave directionality may have a significant influence on the second-order hydrodynamic loads on a TLP and that the assumption of unidirectional waves does not always lead to conservative estimates of the sum-frequency loading.

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