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.

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.

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.

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.

Forced oscillation experiments

1991 ◽  
Vol 334 (1634) ◽  
pp. 199-211
Keyword(s):  
Numerical Methods ◽  
Water Depth ◽  
Forced Oscillation ◽  
Vertical Plane ◽  
Forced Oscillations ◽  
Hydrodynamic Characteristics ◽  
Scale Models ◽  
Ship Speed

Forced oscillation experiments with scale models are carried out to determine hydrodynamic characteristics of ships, with respect to motions in waves or steering and manoeuvring qualities. Depending on the considered motion components, in a horizontal or vertical plane, various methods are used to induce forced oscillations which are discussed briefly. Some results of forced oscillation experiments are presented as examples of this technique and compared with calculations based on numerical methods. The comparisons include, among others, the effects of ship speed and restricted water depth.

Hydrodynamic Properties of an Asymmetric Deepwater Riser Bundle

2006 ◽  
Author(s):  
Charles Zimmermann ◽  
Richard James ◽  
Blaise Seguin ◽  
Mattias Lynch
Keyword(s):  
Cross Section ◽  
Water Depth ◽  
Hydrodynamic Characteristics ◽  
Cfd Analysis ◽  
Effective Solution ◽  
Hydrodynamic Properties ◽  
Global System ◽  
Field Development ◽  
Hydrodynamic Behaviour ◽  
Deepwater Riser

The BP operated Greater Plutonio field development offshore Angola comprises a spread-moored FPSO in 1,300 m water depth, serving as a hub processing the fluids produced from or injected into the subsea wells. The selected riser system is a riser tower tensioned by a steel buoyancy tank at its top end and distributed foam buoyancy along a central structural tubular. The riser bundle is asymmetric in cross-section and this paper presents the work performed to determine the specific hydrodynamic characteristics of the design. Both basin tests and CFD analysis results are presented with discussion on some specific hydrodynamic issues: vortex-induced vibration (VIV) of the global riser tower system, VIV of individual risers, and the dynamic stability of the global system (i.e. galloping). Finally, guidelines for the assessment of the hydrodynamic behaviour of such system geometries are proposed. The results of this paper demonstrate that the Greater Plutonio riser bundle represents an effective solution in term of hydrodynamic behaviour and is not sensitive to VIV fatigue or galloping.

Wave Impact Load and Corresponding Nonlinear Response of a Semi-Submersible

2019 ◽  
Author(s):  
Yinghao Guo ◽  
Longfei Xiao ◽  
Handi Wei ◽  
Lei Li ◽  
Yanfei Deng
Keyword(s):  
Spatial Distribution ◽  
Structural Integrity ◽  
Impact Load ◽  
Wave Crest ◽  
Impulsive Effect ◽  
Offshore Platforms ◽  
Wave Impact ◽  
Global Response ◽  
Local Wave ◽  
Incident Waves

Abstract Offshore platforms operating in harsh ocean environments often suffer from severe wave impacts which threaten the structural integrity and staffs safety. An experimental study was carried out to investigate the wave impact load and its effect on the global response of a semi-submersible. First, two typical wave impact events occurring successively in the wave test run are analyzed, including the characteristics of incident waves, relative wave elevations and the spatial distribution of the wave impact load. Subsequently, the corresponding global response of the semi-submersible under these two wave impacts are investigated in time domain. It reveals that compared with the incident wave, the relative wave elevation has a more straightforward relationship with the wave impact load. The relative wave crest height is associated with the spatial distribution of the wave impact load, while the local wave steepness matters more in the magnitude of the wave impact load. The impulsive effect of the wave impact load on the motion behaviors is not obvious. But severe wave impacts can introduce excessive horizontal accelerations and nonlinear behaviors like ringing in the acceleration response.

Estimates of Wave Elevations Around Structures

2010 ◽  
Author(s):  
Anne Katrine Bratland ◽  
Ragnvald Bo̸rresen ◽  
Per Ivar Barth Berntsen
Keyword(s):  
Water Depth ◽  
Wave Theory ◽  
Higher Order ◽  
Offshore Platforms ◽  
Test Results ◽  
Regular Wave ◽  
First Order ◽  
Wave Elevation ◽  
Higher Order Terms ◽  
Wave Elevations

When designing offshore platforms the still water air gap has to be large enough to avoid major wave-in-deck impact. Since wave elevation in harsh weather is highly non-linear, corrections to the calculated first order solution are necessary. The present method is a pragmatic approach to estimate the higher order contributions, utilizing the first order response amplitude operator and higher order wave elevations. For infinite water depth it is shown that regular wave theory is a good approximation for calculating second order wave elevation in irregular seas. So the higher order waves are calculated with regular wave theory, and the QTF and higher order terms are approximated by the first order RAO. Comparison with model test results have been performed for a GBS in moderate water depth and a semi-submersible is relatively deep water. The agreements with model tests are satisfactory.

Physical and Numerical Investigations on Wave Run-Up and Dissipation under Breakwater with Fence Revetment

2021 ◽  
Vol 9 (12) ◽  
pp. 1355
Author(s):  
Enjin Zhao ◽  
Lin Mu ◽  
Zhaoyang Hu ◽  
Xinqiang Wang ◽  
Junkai Sun ◽  
...  
Keyword(s):  
Wave Height ◽  
Water Depth ◽  
Significant Wave Height ◽  
Irregular Waves ◽  
Structure Stability ◽  
Wave Impact ◽  
Significant Wave ◽  
Tide Level ◽  
Run Up ◽  
The Impact

Revetment elements and protective facilities on a breakwater can effectively weaken the impact of waves. In order to resist storm surges, there is a plan to build a breakwater on the northern shore of Meizhou Bay in Putian City, China. To better design it, considering different environmental conditions, physical and numerical experiments were carried out to accurately study the effects of the breakwater and its auxiliary structures on wave propagation. In the experiments, the influence of the wave type, initial water depth, and the structure of the fence plate are considered. The wave run-up and dissipation, the wave overtopping volume, and the structure stability are analyzed. The results indicate that the breakwater can effectively resist the wave impact, reduce the wave run-up and overtopping, and protect the rear buildings. In addition, under the same still water depth and significant wave height, the amount of overtopped water under regular waves is larger than that under irregular waves. With the increase of the still water depth and significant wave height, the overtopped water increases, which means that when the storm surge occurs, damage on the breakwater under the high tide level is greater than that under the low tide level. Besides, the fence plate can effectively dissipate energy and reduce the overtopping volume by generating eddy current in the cavity. Considering the stability and the energy dissipation capacity of the fence plate, it is suggested that a gap ratio of 50% is reasonable.

An Experimental Study of the Run-Up Process of Breaking Bores Generated by Dam-Break Under Dry- and Wet-Bed Conditions

2018 ◽  
Vol 12 (02) ◽  
pp. 1840005 ◽  
Author(s):  
Senxun Lu ◽  
Haijiang Liu ◽  
Xiaohu Deng
Keyword(s):  
Water Depth ◽  
Laboratory Experiments ◽  
Bottom Friction ◽  
Dam Break ◽  
Hydrodynamic Characteristics ◽  
Initial Water ◽  
Water Head ◽  
Run Up ◽  
Front Profile ◽  
Two Stages

In this study, a series of dam-break laboratory experiments were carried out to investigate the run-up process of breaking bores under dry- and wet-bed conditions. Detailed measurements were conducted to reveal differences in the run-up hydrodynamic characteristics under these two conditions, e.g. the bore front profile, the maximum run-up height and duration, and the instantaneous bore front velocity. Two successive bores were observed under the wet-bed run-up process, while multiple bores (three bores in general) were generated during the dry-bed run-up process due to the significant bottom friction effect. A linear relationship with the uniform gradient is found between the maximum run-up height and the initial water head for both dry- and wet-bed conditions, indicating that difference in the maximum run-up height between the dry- and specified wet-bed cases or among various wet-bed cases is not sensitive to the initial water head. Under the same initial water head, although the dry-bed run-up process takes a longer duration than that of wet-bed cases, the maximum run-up height is smallest for the dry-bed case and gradually increases with the increase of the initial downstream water depth for wet-bed cases. Under the wet-bed conditions, temporal variation of the bore front run-up velocity can be classified into two stages, i.e. the acceleration stage induced by the relatively large incident bore front water depth (large onshore hydrostatic pressure gradient) and the deceleration stage governed by the offshore-directed gravity force and bottom friction. Nevertheless, due to the small incident bore front water depth, run-up process under the dry-bed conditions does not show the acceleration stage.

Response Behaviour of Perforated Cylinders in Regular Waves

2011 ◽  
Author(s):  
Srinivasan Chandrasekaran ◽  
S. Parameswara Pandian
Keyword(s):  
Outer Cylinder ◽  
Wave Structure ◽  
Offshore Structures ◽  
Experimental Investigations ◽  
Coastal Protection ◽  
Wave Loads ◽  
Wave Impact ◽  
Protection Measures ◽  
Cylindrical Structures ◽  
Perforated Cylinder

In order to reduce the direct wave impact, coastal and offshore structures are often constructed with one or more perforated layers. Several permeable breakwaters and docks have been deployed in coastal protection measures to reduce direct impact due to encountered wave loads and to reduce wave reflection in front of these structures as well; compared with the traditional ones, such structural forms are found to be more economical. Such structures result in lesser surface fluctuation in harbours due to the low reflection, which is vital for loading and unloading of ships. Deploying permeable breakwaters posses other advantages namely: i) increasing water circulation; ii) retaining water quality and iii) enhances coastal protection. Emerged and submerged perforated cylindrical structures reduce wave-structure interactions and scouring problems considerably, but their use on floating structures is scarce in the literature. This study is focused on detailed experimental investigations carried out on impermeable inner cylinder encompassed by a larger outer cylinder with perforatios along its length. By varying the porosity and perforatio diameter, their influence on the hydrodynamic response of the cylindrical member is highlighted through the current study. The conclusions on the comparison of forces in the cylinder with and without perforated cylinder coverage are presented.

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