2008 Vol.(4)
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2008, (4): 533-546.
Abstract:
This paper presents the Hill instability analysis of Tension Leg Platform (TLP) tether in deep sea. The 2-Dnonlinear beam model, which is undergoing coupled axial and transverse vibrations, is applied. The governing equations are reduced to nonlinear Hill equation by use of the Galerkin's method and the modes superposition principle. The Hill instability charted up to large parameters is obtained. An important parameter M is defined and can be expressed as the functions of tether length, the platform surge and heave motion amplitudes. Some example studies are performed for various environmental conditions. The results demonstrate that the nonlinear coupling between the axial and transverse vibrations has a significant effect on the response of structure. It needs to be considered for the accurate dynamic analysis of long TLP tether subjected to the combined platform surge and heave motions.
This paper presents the Hill instability analysis of Tension Leg Platform (TLP) tether in deep sea. The 2-Dnonlinear beam model, which is undergoing coupled axial and transverse vibrations, is applied. The governing equations are reduced to nonlinear Hill equation by use of the Galerkin's method and the modes superposition principle. The Hill instability charted up to large parameters is obtained. An important parameter M is defined and can be expressed as the functions of tether length, the platform surge and heave motion amplitudes. Some example studies are performed for various environmental conditions. The results demonstrate that the nonlinear coupling between the axial and transverse vibrations has a significant effect on the response of structure. It needs to be considered for the accurate dynamic analysis of long TLP tether subjected to the combined platform surge and heave motions.
2008, (4): 547-559.
Abstract:
In this paper a new nondestructive damage identification method is introduced. The method based on flexibility matrix can be used to detect and locate structural damage and evaluate the severity of damage in legs of jacketplatforms by modal parameters of structure. With the modal data for only the few lower modes in both the intact and damaged states, the one dimensional and two dimensional distributed curvatures can be used to analyze damage location and the severity. Instead of directly comparing the curvatures before and after damage, the methodhere uses modal parameters only in the damaged structure to detect the damage and it consists of three parts. First, flexibility matrix is obtained by use of the absolute maximum in each column. Second, because the legs of jacket platforms are the pipe-like structure, the circumferential flexibility curvature matrix is obtained by use of the circular curvature. At last, equivalent curvature ratio is defined and the curve meaning equivalent curvature ratio andthe severity of damage relationship for one element is given through the data of damage severity from ten percent toninety percent by numerical simulation. Many existing damage detection methods need two steps, locate the damage firstly and evaluate the severity of the damage. However, the method presented! in this paper can locate andthen evaluate the severity of damage at the same time. The numerical analysis results indicate that the presentmethod is effective, useful and only need the first and the second mode data of the structure.
In this paper a new nondestructive damage identification method is introduced. The method based on flexibility matrix can be used to detect and locate structural damage and evaluate the severity of damage in legs of jacketplatforms by modal parameters of structure. With the modal data for only the few lower modes in both the intact and damaged states, the one dimensional and two dimensional distributed curvatures can be used to analyze damage location and the severity. Instead of directly comparing the curvatures before and after damage, the methodhere uses modal parameters only in the damaged structure to detect the damage and it consists of three parts. First, flexibility matrix is obtained by use of the absolute maximum in each column. Second, because the legs of jacket platforms are the pipe-like structure, the circumferential flexibility curvature matrix is obtained by use of the circular curvature. At last, equivalent curvature ratio is defined and the curve meaning equivalent curvature ratio andthe severity of damage relationship for one element is given through the data of damage severity from ten percent toninety percent by numerical simulation. Many existing damage detection methods need two steps, locate the damage firstly and evaluate the severity of the damage. However, the method presented! in this paper can locate andthen evaluate the severity of damage at the same time. The numerical analysis results indicate that the presentmethod is effective, useful and only need the first and the second mode data of the structure.
2008, (4): 561-574.
Abstract:
Free spanning pipelines are suspended between two points on an uneven seafloor. The variations of structural conditions, such as the changes in soil property, flow velocity, axial force and span length etc. directly affectworking performance of the whole submarine pipeline system. But until now few researches have focused on condition identification for free span (CIFS). A method to identify the operational conditions of free spanning submarinepipelines based on vibration measurements is proposed in this paper. Firstly, the ill-posedness of CIFS is analyzedin detail. Secondly, the framework for CIFS based on the nonlinear kernel discriminant analysis (KDA) is established. Thirdly, the internal structural characteristics of natural frequencies, normalized frequencies and frequencychange ratios are studied. And then the condition feature vector for CIFS is extracted by use of the vibration measurements. Finally, the validity of the proposed approach is evaluated by a case study. The results demonstrate that the proposed approach can effectively identify each condition of free span when condition variation occurs even ifunder measurement noise. It is concluded that the proposed method is promising tool for CIFS in real applications.
Free spanning pipelines are suspended between two points on an uneven seafloor. The variations of structural conditions, such as the changes in soil property, flow velocity, axial force and span length etc. directly affectworking performance of the whole submarine pipeline system. But until now few researches have focused on condition identification for free span (CIFS). A method to identify the operational conditions of free spanning submarinepipelines based on vibration measurements is proposed in this paper. Firstly, the ill-posedness of CIFS is analyzedin detail. Secondly, the framework for CIFS based on the nonlinear kernel discriminant analysis (KDA) is established. Thirdly, the internal structural characteristics of natural frequencies, normalized frequencies and frequencychange ratios are studied. And then the condition feature vector for CIFS is extracted by use of the vibration measurements. Finally, the validity of the proposed approach is evaluated by a case study. The results demonstrate that the proposed approach can effectively identify each condition of free span when condition variation occurs even ifunder measurement noise. It is concluded that the proposed method is promising tool for CIFS in real applications.
2008, (4): 575-584.
Abstract:
This paper discusses some previous, and presents some new experimental results on wave transmission oversubmerged breakwaters. The objective of this study is to evaluate wave transmission coefficient and develop two-dimensional (2D) model as an improvement to the existing wave transmission coefficient models. Factors which affect wave transmission over submerged breakwaters are discussed through series of laboratory experiments. Basic recommendations for evaluation and design of submerged rubble-monud breakwaters are presented.From the test results, calculation formula of wave transmission coefficient is proposed.
This paper discusses some previous, and presents some new experimental results on wave transmission oversubmerged breakwaters. The objective of this study is to evaluate wave transmission coefficient and develop two-dimensional (2D) model as an improvement to the existing wave transmission coefficient models. Factors which affect wave transmission over submerged breakwaters are discussed through series of laboratory experiments. Basic recommendations for evaluation and design of submerged rubble-monud breakwaters are presented.From the test results, calculation formula of wave transmission coefficient is proposed.
2008, (4): 585-594.
Abstract:
Quarter circular breakwater ( QCB) is new-type breakwater developed from semi-circular breakwater(SCB). The superstructure of QCB is composed of quarter circular front wall, horizontal base slab and vertical rear wall. The width of QCB's base slab is about half that of SCB, which makes QCB suitable to be used onrelatively firm soil foundation. The numerical wave flume based on the Reynolds averaged Navier-Stokes equationsfor impressible viscosity fluid is adopted in this paper to simulate the hydraulic performances of QCB. Since the ge鄄ometry of both breakwaters is similar and SCB has been studied in depth, the hydraulic performances of QCB aregiven in comparison with those of SCB.
Quarter circular breakwater ( QCB) is new-type breakwater developed from semi-circular breakwater(SCB). The superstructure of QCB is composed of quarter circular front wall, horizontal base slab and vertical rear wall. The width of QCB's base slab is about half that of SCB, which makes QCB suitable to be used onrelatively firm soil foundation. The numerical wave flume based on the Reynolds averaged Navier-Stokes equationsfor impressible viscosity fluid is adopted in this paper to simulate the hydraulic performances of QCB. Since the ge鄄ometry of both breakwaters is similar and SCB has been studied in depth, the hydraulic performances of QCB aregiven in comparison with those of SCB.
2008, (4): 595-610.
Abstract:
New hyperbolic mild slope equations for random waves are developed with the inclusion of amplitude dispersion. The frequency perturbation around the peak frequency of random waves is adopted to extend the equations forregular waves to random waves. The nonlinear effect of amplitude dispersion is incorporated approximately into themodel by only considering the nonlinear effect on the carrier waves of random waves, which is done by introducinga representative wave amplitude for the carrier waves. The computation time is greatly saved by the introduction of the representative wave amplitude. The extension of the present model to breaking waves is also considered in orderto apply the new equations to surf zone. The model is validated for random waves propagate over shoal and in surf zone against measurements.
New hyperbolic mild slope equations for random waves are developed with the inclusion of amplitude dispersion. The frequency perturbation around the peak frequency of random waves is adopted to extend the equations forregular waves to random waves. The nonlinear effect of amplitude dispersion is incorporated approximately into themodel by only considering the nonlinear effect on the carrier waves of random waves, which is done by introducinga representative wave amplitude for the carrier waves. The computation time is greatly saved by the introduction of the representative wave amplitude. The extension of the present model to breaking waves is also considered in orderto apply the new equations to surf zone. The model is validated for random waves propagate over shoal and in surf zone against measurements.
2008, (4): 611-621.
Abstract:
The study in this paper is focusing on trajectories of particles in the irrotational progressive water waves coexisting with uniform current. The parametric equations of particle trajectories over range of levels in Lagrangian type of description are developed analytically via the Euler-Lagrange transformation.The Lagrangian wave period ofparticle motion differing from the Eulerian wave period and the mass transport can also be obtained directly. The third-order solution of particle trajectory exhibits that they do not move in closed orbital motion but represent netmovement that decreases exponentially with the water depth. Uniform current is found to have significant effect on the trajectories and drift velocity of gravity waves. Overall, the influence of increased uniform current is to increase the relative horizontal distance traveled by particle, as well as the magnitude of the time-averaged drift velocityon the free surface. For adverse current cases, reverse behavior is found. The obtained third-order solutions satisfy the irrotational condition contrasted to the Gerstner waves and are verified by reducing to those of two-dimensional gravity waves in Lagrangian coordinates.
The study in this paper is focusing on trajectories of particles in the irrotational progressive water waves coexisting with uniform current. The parametric equations of particle trajectories over range of levels in Lagrangian type of description are developed analytically via the Euler-Lagrange transformation.The Lagrangian wave period ofparticle motion differing from the Eulerian wave period and the mass transport can also be obtained directly. The third-order solution of particle trajectory exhibits that they do not move in closed orbital motion but represent netmovement that decreases exponentially with the water depth. Uniform current is found to have significant effect on the trajectories and drift velocity of gravity waves. Overall, the influence of increased uniform current is to increase the relative horizontal distance traveled by particle, as well as the magnitude of the time-averaged drift velocityon the free surface. For adverse current cases, reverse behavior is found. The obtained third-order solutions satisfy the irrotational condition contrasted to the Gerstner waves and are verified by reducing to those of two-dimensional gravity waves in Lagrangian coordinates.
2008, (4): 623-634.
Abstract:
A numerical irregular wave flume with active absorption of re-reflected waves is simulated by use of volume offluid (VOF) method. An active absorbing wave-maker based on linear wave theory is set on the left boundary ofthe wave flume. The progressive waves and the absorbing waves are generated simultaneously at the active wave generating-absorbing boundary.The absorbing waves are generated to eliminate the waves coming back to the generating boundary due to reflection from the outflow boundary and the structures. SIRW method proposed by Frigaard and Brorsen (1995) is used to separate the incident waves and reflected waves. The digital filters are designed based on the surface elevation signals of the two wave gauges. The corrected velocity of the wave-maker paddle is the output from the digital filter in real time. The numerical results of regular and irregular waves by the active absorbing-generating boundary are compared with the numerical results by the ordinary generating boundaryto verify the performance of the active absorbing-generator boundary.The differences between the initial incident waves and the estimated incident waves are analyzed.
A numerical irregular wave flume with active absorption of re-reflected waves is simulated by use of volume offluid (VOF) method. An active absorbing wave-maker based on linear wave theory is set on the left boundary ofthe wave flume. The progressive waves and the absorbing waves are generated simultaneously at the active wave generating-absorbing boundary.The absorbing waves are generated to eliminate the waves coming back to the generating boundary due to reflection from the outflow boundary and the structures. SIRW method proposed by Frigaard and Brorsen (1995) is used to separate the incident waves and reflected waves. The digital filters are designed based on the surface elevation signals of the two wave gauges. The corrected velocity of the wave-maker paddle is the output from the digital filter in real time. The numerical results of regular and irregular waves by the active absorbing-generating boundary are compared with the numerical results by the ordinary generating boundaryto verify the performance of the active absorbing-generator boundary.The differences between the initial incident waves and the estimated incident waves are analyzed.
2008, (4): 635-647.
Abstract:
In order to investigate the effect of wind input and whitecapping dissipation on the simulation of typhoon-waves, three experiments are conducted with the latest version of SWAN (Simulating WAves Nearshore) model.The three experiments adopt the Komen, Janssens, and Westhuysen expressions for wind input and whitecappingdissipation, respectively. Besides the above-mentioned source terms, other parameterization schemes in these experiments are the same. It shows that the experiment with the Westhuysen expression result in the least simulationerrors while that with the Janssens expression has the most. The results from the experiments with Komen and Westhuysen expressions show that the differences in significant wave height (SWH) have good correlation with thedifferences in dissipation energy caused by whitecapping. This indicates that the whitecapping dissipation sourceterm plays an important role in the resultant differences of the simulated SWH between the two experiments.
In order to investigate the effect of wind input and whitecapping dissipation on the simulation of typhoon-waves, three experiments are conducted with the latest version of SWAN (Simulating WAves Nearshore) model.The three experiments adopt the Komen, Janssens, and Westhuysen expressions for wind input and whitecappingdissipation, respectively. Besides the above-mentioned source terms, other parameterization schemes in these experiments are the same. It shows that the experiment with the Westhuysen expression result in the least simulationerrors while that with the Janssens expression has the most. The results from the experiments with Komen and Westhuysen expressions show that the differences in significant wave height (SWH) have good correlation with thedifferences in dissipation energy caused by whitecapping. This indicates that the whitecapping dissipation sourceterm plays an important role in the resultant differences of the simulated SWH between the two experiments.
2008, (4): 649-662.
Abstract:
Spatial, environmental, and material resources are important for tidal flats and wetlands, thus, harmonious balance between utilization and protection should be maintained. In the present study, the concept of tidal flat is defined, and its spatial limitation is also clarified. Located at the Yangtze Estuary, tidal flats in Shanghai area are selected for study. The relationship between water discharge and sediment transport of the Yangtze River, allowedexploration intensity and needs for wetland protection are discussed in the paper. The dynamic process and changing trend for different types of tidal flats are illustrated in detail. Regulations are proposed for reasonable exploitation, dynamic protection, and ecosystem restoration of each individual tidal flat and wetland. Some valuable suggestions are put forward for further study about the sustainable exploitation of offshore resources in Shanghai area.
Spatial, environmental, and material resources are important for tidal flats and wetlands, thus, harmonious balance between utilization and protection should be maintained. In the present study, the concept of tidal flat is defined, and its spatial limitation is also clarified. Located at the Yangtze Estuary, tidal flats in Shanghai area are selected for study. The relationship between water discharge and sediment transport of the Yangtze River, allowedexploration intensity and needs for wetland protection are discussed in the paper. The dynamic process and changing trend for different types of tidal flats are illustrated in detail. Regulations are proposed for reasonable exploitation, dynamic protection, and ecosystem restoration of each individual tidal flat and wetland. Some valuable suggestions are put forward for further study about the sustainable exploitation of offshore resources in Shanghai area.
2008, (4): 663-674.
Abstract:
The Computational Fluid Dynamics (CFD) approach is adopted to study the steady and unsteady performances of the podded propulsor by the Fluent software package. While the interactions of the propeller blades with the pod and strut are time-dependent by nature, the mixing plane model is employed firstly to predict the steadyperformance, where the interactions are time-averaged.Numerical experiments are carried out with systematicallyvaried mesh sizes to investigate the dependence of the predicted force values on the mesh sizes. Furthermore, thesliding mesh model is employed to simulate the unsteady interactions between the blades, pod and strut. Based on the numerical results, the characteristics of unsteady hydrodynamic forces are discussed, and the applicability of the mixing plane model is investigated for puller-type podded propulsor.
The Computational Fluid Dynamics (CFD) approach is adopted to study the steady and unsteady performances of the podded propulsor by the Fluent software package. While the interactions of the propeller blades with the pod and strut are time-dependent by nature, the mixing plane model is employed firstly to predict the steadyperformance, where the interactions are time-averaged.Numerical experiments are carried out with systematicallyvaried mesh sizes to investigate the dependence of the predicted force values on the mesh sizes. Furthermore, thesliding mesh model is employed to simulate the unsteady interactions between the blades, pod and strut. Based on the numerical results, the characteristics of unsteady hydrodynamic forces are discussed, and the applicability of the mixing plane model is investigated for puller-type podded propulsor.
2008, (4): 675-682.
Abstract:
In consideration of the effect of the internal flowing fluid and the external marine environmental condition on the vortex-induced vibration (VIV) of top tensioned riser (TTR), the differential equation is derived based on work-energy principles and the riser near wake dynamics is modeled by Facchinetti's wake oscillator model.Then Galerkin's finite element approximation is implemented to derive the nonlinear matrix equation of the coupled equations and the corresponding numerical programs are compiled which solve the coupled equations directly in the timedomain. The comparison of the predicted results with the recent experimental results and the prediction of SHEAR7is performed. The results show the validity of the proposed method on the prediction of VIV of deep water risers.The effect of internal flow on the dynamic characteristics and dynamic response of the riser is analyzed and severalvaluable conclusions are drawn.
In consideration of the effect of the internal flowing fluid and the external marine environmental condition on the vortex-induced vibration (VIV) of top tensioned riser (TTR), the differential equation is derived based on work-energy principles and the riser near wake dynamics is modeled by Facchinetti's wake oscillator model.Then Galerkin's finite element approximation is implemented to derive the nonlinear matrix equation of the coupled equations and the corresponding numerical programs are compiled which solve the coupled equations directly in the timedomain. The comparison of the predicted results with the recent experimental results and the prediction of SHEAR7is performed. The results show the validity of the proposed method on the prediction of VIV of deep water risers.The effect of internal flow on the dynamic characteristics and dynamic response of the riser is analyzed and severalvaluable conclusions are drawn.
2008, (4): 693-704.
Abstract:
Waves generated by vertical seafloor movements are simulated by use of fully nonlinear two-dimensional numerical wave tank. In the source region, the seafloor lifts to designated height by generation function. The numerical tests show that the linear theory is only valid for estimating the wave behaviors induced by the seafloormovements with small amplitude, and the fully nonlinear numerical model should be adopted in the simulation ofthe wave generation by the large amplitude seafloor movements. Without the background surface waves, many numerical tests on the stable maximum elevations η0max are carried out by both the linear theory and the fully nonlinear model. The results of two models are compared and analyzed. For the fully nonlinear model, the influences of the amplitudes and the horizontal lengths on ηmax are stronger than that of the characteristic duration times. Furthermore, results reveal that there are significant differences between the linear theory and the fully nonlinear model.When the influences of the background surface waves are considered, the corresponding numerical analyses revealthat with the fully nonlinear model the ηmax nearlinearly varies with the wave amplitudes of the surface waves, andthe ηmax has significant dependences on the wave lengths and the wave phases of the surface waves. In addition,the differences between the linear theory and the fully nonlinear model are still obvious, and these differences aresignificantly affected by the wave parameters of the background surface waves, such as the wave amplitude, thewave length and the wave phase.
Waves generated by vertical seafloor movements are simulated by use of fully nonlinear two-dimensional numerical wave tank. In the source region, the seafloor lifts to designated height by generation function. The numerical tests show that the linear theory is only valid for estimating the wave behaviors induced by the seafloormovements with small amplitude, and the fully nonlinear numerical model should be adopted in the simulation ofthe wave generation by the large amplitude seafloor movements. Without the background surface waves, many numerical tests on the stable maximum elevations η0max are carried out by both the linear theory and the fully nonlinear model. The results of two models are compared and analyzed. For the fully nonlinear model, the influences of the amplitudes and the horizontal lengths on ηmax are stronger than that of the characteristic duration times. Furthermore, results reveal that there are significant differences between the linear theory and the fully nonlinear model.When the influences of the background surface waves are considered, the corresponding numerical analyses revealthat with the fully nonlinear model the ηmax nearlinearly varies with the wave amplitudes of the surface waves, andthe ηmax has significant dependences on the wave lengths and the wave phases of the surface waves. In addition,the differences between the linear theory and the fully nonlinear model are still obvious, and these differences aresignificantly affected by the wave parameters of the background surface waves, such as the wave amplitude, thewave length and the wave phase.
2008, (4): 705-711.
Abstract:
High-pile and beam-slab quays have been widely used after several years development.They are mature enough to be one of the most important structural types of wharves in China coastal areas. In order to accommodate large tonnage vessels, wharves should be constructed in deep water gradually. However, conventional high-pileand beam-slab structures are hard to meet the requirements of large deep-water wharf.According to arch's stresscharacteristics, a new type of wharf with catenary arched longitudinal beams is presented in this paper. The new wharf structure can make full use of arch's overhead crossing and reinforced concrete compression resistance, improve the interval between transverse bents greatly, and decrease underwater construction quantity. Thus, the construction cost cab be reduced. Take the third phase project of the Yangshan Deep-water Port for example, comparative analysis on catenary arched longitudinal beams and conventional longitudinal beams has been made. The result shows that with the same wharf length and width, the same loads and same longitudinal beam moment, catenary arch structure can improve the interval between bents up to 28 m, decrease the number of piles and underwater construction quantity.
High-pile and beam-slab quays have been widely used after several years development.They are mature enough to be one of the most important structural types of wharves in China coastal areas. In order to accommodate large tonnage vessels, wharves should be constructed in deep water gradually. However, conventional high-pileand beam-slab structures are hard to meet the requirements of large deep-water wharf.According to arch's stresscharacteristics, a new type of wharf with catenary arched longitudinal beams is presented in this paper. The new wharf structure can make full use of arch's overhead crossing and reinforced concrete compression resistance, improve the interval between transverse bents greatly, and decrease underwater construction quantity. Thus, the construction cost cab be reduced. Take the third phase project of the Yangshan Deep-water Port for example, comparative analysis on catenary arched longitudinal beams and conventional longitudinal beams has been made. The result shows that with the same wharf length and width, the same loads and same longitudinal beam moment, catenary arch structure can improve the interval between bents up to 28 m, decrease the number of piles and underwater construction quantity.
2008, (4): 983-691.
Abstract:
Excited by ocean currents, random wave and vessel motion, deepwater drilling risers exhibit significant dynamic response. In time domain, method is proposed to calculate the nonlinear dynamic response of deepwaterdrilling risers subjected to random wave and dynamic large displacement vessel motion boundary condition. Structural and functional loads, external and internal pressure, free surface effect of irregular wave, hydrodynamicforces induced by current and wave, as well as wave and low frequency (drift) motion of the drilling vessel are allaccounted for. An example is presented which illustrates the application of the proposed method. The study shows that long term drift motion of the vessel has profound effect on the envelopes of bending stress and lateral displacement, as well as the range of lower flex joint angle of the deepwater riser. It can also be concluded that vessel motion is the principal dynamic loading of nonlinear dynamic response for the deepwater risers rather than wave force.
Excited by ocean currents, random wave and vessel motion, deepwater drilling risers exhibit significant dynamic response. In time domain, method is proposed to calculate the nonlinear dynamic response of deepwaterdrilling risers subjected to random wave and dynamic large displacement vessel motion boundary condition. Structural and functional loads, external and internal pressure, free surface effect of irregular wave, hydrodynamicforces induced by current and wave, as well as wave and low frequency (drift) motion of the drilling vessel are allaccounted for. An example is presented which illustrates the application of the proposed method. The study shows that long term drift motion of the vessel has profound effect on the envelopes of bending stress and lateral displacement, as well as the range of lower flex joint angle of the deepwater riser. It can also be concluded that vessel motion is the principal dynamic loading of nonlinear dynamic response for the deepwater risers rather than wave force.
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