Variations of Water Particle Kinematics of Offshore TLPS with Perforated Members: Numerical Investigations

2014 ◽  
pp. 629-645
Author(s):  
Srinivasan Chandrasekaran ◽  
N. Madhavi
Keyword(s):  
Numerical Investigations ◽  
Water Particle ◽  
Water Particle Kinematics

Estimation of Force Reduction on Ocean Structures With Perforated Members

2015 ◽  
Author(s):  
Srinivasan Chandrasekaran ◽  
Madhavi Natarajan ◽  
Lognath Radhakrishnan Sreeramulu
Keyword(s):  
Outer Cylinder ◽  
Wave Structure ◽  
Cylindrical Structure ◽  
Water Particle ◽  
Design Charts ◽  
Outer Cover ◽  
Force Reduction ◽  
Water Particle Kinematics ◽  
Existing Structure ◽  
Wave Structure Interaction

While retrofitting and rehabilitation are usually related to strengthening of members, the presented concept is a novel attempt as it addresses decrease in the encountered forces on the members. Presence of perforated members in ocean structures reduces the wave-structure interaction significantly; breakwaters with perforated members are classical examples of such kind. This concept of encompassing the perforated outer cylinder with inner existing structure is found to be most feasible rehabilitation concept as it does not demand replacement of any damaged members. The presence of outer perforated cover alters the water particle kinematics significantly and eventually this remains the reason for the force reduction mechanism. In this study, the variation of water particle kinematics along the depth of the cylinder is estimated on the cylindrical structure with and without perforated outer cover. Forces on the inner cylinder are quantified numerically and experimentally; experimental results show a close agreement with that of the numerical ones. Velocity variations along the water depth are quantified in the form of design charts, which shall be helpful for the practicing professionals while attempting for retrofitting or re-design. Force variations derived through numerical analyses, which are functions of the water particle kinematics along the depth shall be useful in the design offices for cylindrical members encompassed with perforated outer cover. Introduction of perforated member over the existing cylindrical structure showed a significant force reduction around 60% on an average for all the wave steepness indexes considered for the study, when compared to the force on the member without perforated cover.

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.

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.

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.

The Effect of Reduced Water Depth on the Description of Extreme Waves and the Prediction of the Water Particle Kinematics

2005 ◽  
Author(s):  
Vasiliki Katsardi ◽  
Chris Swan
Keyword(s):  
Water Depth ◽  
Wave Model ◽  
Wave Dispersion ◽  
Linear Dispersion ◽  
Wave Groups ◽  
Relative Significance ◽  
Water Particle ◽  
Applied Design ◽  
Wave Models ◽  
Water Particle Kinematics

This paper concerns the description of large waves in intermediate and shallow water depths. In deep water it is well known that the evolution of the largest waves is governed by linear dispersion. In contrast, as the water depth reduces the effects of wave dispersion are weakened and the relative significance of wave modulation shown to be increasingly important. This leads to very different extreme wave groups, the properties of which are critically dependent upon the directionality of the wave field. The paper also concerns the water particle kinematics arising beneath these nonlinear wave groups and contrasts fully non-linear predictions based on a state-of-the-art wave model with the results of the commonly applied design wave solutions. To explore these effects, and to provide a physical explanation for their occurrence, two wave models are employed. The first, proposed by Bateman, Swan & Taylor [1, 2], allows fully-nonlinear descriptions of the evolution of large waves in realistic seas, involving a significant spread of wave energy in both frequency and direction. The second is a wave evolution equation based upon the early work of Zakharov [3] and written in Hamiltonian form by Kasitskii [4]. This model is only valid to a fourth-order of wave steepness, but has the over-riding advantage that it gives physical insight into the evolution process.

Variations of Hydrodynamic Characteristics With the Perforated Cylinder

2014 ◽  
Author(s):  
Srinivasan Chandrasekaran ◽  
Madhavi Natarajan ◽  
Natarajan Chithambara Thanu
Keyword(s):  
Water Depth ◽  
Hydrodynamic Characteristics ◽  
Offshore Platforms ◽  
Wave Impact ◽  
Direct Wave ◽  
Water Particle ◽  
Design Charts ◽  
Perforated Cylinder ◽  
Outer Cover ◽  
Water Particle Kinematics

Presence of the perforated outer cover on the existing column members of offshore platforms reduces the direct wave impact on these members. Such applications are common in the coastal structures where perforated covers are provided on the seaside to dissipate the wave energy and to reduce the pressure on the members. Detailed studies on the variations of the hydrodynamic characteristics on the inner cylinder, encompassed by a perforated outer cover are scarce in the literature. Present study is focused the development of numerical model to investigate the variations in the water particle kinematics on the inner cylinder encompassed by perforated outer cover. Hydrodynamic characteristics are examined along the water depth through computation fluid dynamics (CFD) for perforation ratios (p%) varying in the range of 10 to 15%. Velocity profiles for different wave steepness are developed along with the design charts for the chosen perforation ratios. These design charts can be readily used for estimating the water particle kinematics for perforated members along the water depth.

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.

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