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The offshore jacket platform is a complex and time-varying nonlinear system, which can be excited of harmful vibration by external loads. It is difficult to obtain an ideal control performance for passive control methods or traditional active control methods based on accurate mathematic model. In this paper, an adaptive inverse control method is proposed on the basis of novel rough neural networks (RNN) to control the harmful vibration of the offshore jacket platform, and the offshore jacket platform model is established by dynamic stiffness matrix (DSM) method. Benefited from the nonlinear processing ability of the neural networks and data interpretation ability of the rough set theory, RNN is utilized to identify the predictive inverse model of the offshore jacket platform system. Then the identified model is used as the adaptive predictive inverse controller to control the harmful vibration caused by wave and wind loads, and to deal with the delay problem caused by signal transmission in the control process. The numerical results show that the constructed novel RNN has advantages such as clear structure, fast training speed and strong error-tolerance ability, and the proposed method based on RNN can effectively control the harmful vibration of the offshore jacket platform.
Corrosion and fatigue cracks are major threats to the structural integrity of aging offshore platforms. For the rational estimation of the safety levels of aging platforms, a global reliability assessment approach for aging offshore platforms with corrosion and fatigue cracks is presented in this paper. The base shear capacity is taken as the global ultimate strength of the offshore platforms. It is modeled as a random process that decreases with time in the presence of corrosion and fatigue crack propagation. And the corrosion and fatigue crack growth rates in the main members and key joints are modeled as random variables. A simulation method of the extreme wave loads which are applied to the structures of offshore platforms is proposed too. Furthermore, the statistics of global base shear capacity and extreme wave loads are obtained by Monte Carlo simulation method. On the basis of the limit state equation of global failure mode, the instantaneous reliability and time dependent reliability assessment methods are both presented in this paper. Finally the instantaneous reliability index and time dependent failure probability of a jacket platform are estimated with different ages in the demonstration example.
A technique for the evaluation of the hydrodynamic coefficients of ships is outlined for ship oscillating in a numerical wave tank, which is established on Computational Fluid Dynamics (CFD) theories. The numerical simulation of ship sections and bodies forced oscillating in the tank are carried out. The added mass and damping coefficients are obtained by the decomposition of the computational results, which agree well with the corresponding ones of potential theories.
This paper discusses the numerical prediction of the induced pressure and lift of the planing surfaces in a steady motion based on the potential flow solver as well as the spray drag by use of the practical method. The numerical method for computation of the induced pressure and lift is potential-based boundary element method. Special technique is identified to present upwash geometry and to determine the spray drag. Numerical results of a planing flat plate and planing craft model 4666 are presented. It is shown that the method is robust and efficient and the results agree well with the experimental measurements with various Froude numbers.
In this paper, the numerical model of the net cage with the grid mooring system in waves is set up by the lumped mass method and rigid kinematics theory, and then the motion equations of floating system, net system, mooring system, and floaters are solved by the Runge-Kutta fifth-order method. For the verification of the numerical model, a series of physical model tests have been carried out. According to the comparisons between the simulated and experimental results, it can be found that the simulated and experimental results agree well in each condition. Then, the effects of submerged depth of grid and direction of incident wave propagation on hydrodynamic behaviors of the net cage are analyzed. According to the simulated results, it can be found that with the increase of submerged depth of grid, the forces acting on mooring lines and bridle lines increase, while the forces on grid lines decrease; the horizontal motion amplitudes of floating collar decrease obviously, while the vertical motion amplitudes of floating collar change little. When the direction of incident wave propagation changes, forces on mooring lines and motion of net cage also change accordingly. When the propagation direction of incident wave changes from 0○ to 45○, forces on the main ropes and bridle ropes increase, while the forces on the grid ropes decrease. With the increasing propagation direction of incident wave, the horizontal amplitude of the forces collar decreases, while the vertical amplitude of the floating collar has little variation.
In the present study, a generalized active contour model of gradient vector flow is combined with the video techniques of Argus system to delineate and track sequential nearshore wave crest profiles in the shoaling process, up to their breaking on the shoreline. Previous applications of active contour models to water wave problems are limited to controllable wave tank experiments. By contrast, our application in this study is in a nearshore field environment where oblique images obtained under natural and varying condition of ambient light are employed. Existing Argus techniques produce plane image data or time series data from a selected small subset of discrete pixels. By contrast, the active contour model produces line image data along continuous visible curves such as wave crest profiles. The combination of these two existing techniques, the active contour model and Argus methodologies, facilitates the estimates of the direction wave field and phase speeds within the whole area covered by camera. These estimates are useful for the purpose of inverse calculation of the water depth. Applications of the present techniques to His-tzu bay where a beach restoration program is currently undertaken are illustrated. This extension of Argus video techniques provides new application of optical remote sensing to study the hydrodynamics and morphology of a nearshore environment.
Based on the results of the tidal flow Reynolds stresses of the field observations, indoor experiments, and numerical models, the parabolic distribution of the tidal flow Reynolds stress is proposed and its coefficients are determined theoretically in this paper. Having been well verified with the field data and experimental data, the proposed distribution of Reynolds stress is also compared with numerical model results, and a good agreement is obtained, showing that this distribution can well reflect the basic features of Reynolds stress deviating from the linear distribution that is downward when the tidal flow is of acceleration, upward when the tidal flow is of deceleration. Its dynamics cause is also discussed preliminarily and the influence of the water depth is pointed out from the definition of Reynolds stress, turbulent generation, transmission, and so on. The established expression for the vertical distribution of the tidal flow Reynolds stress is not only simple and explicit, but can also well reflect the features of the tidal flow acceleration and deceleration for further study on the velocity profile of tidal flow.
Based on theory of three-dimensional hydrodynamics, an Euler-Lagrangian particle model is established to study the transport and water exchange capability in the Jiaozhou Bay. The three-dimensional hydrodynamic model, driven by tide and wind, is used to study the effects of wetting and drying of estuarine intertidal flats by the dry-wet grid technology based on the Estuarine, Coastal and Ocean Model (ECOM). The particle model includes the advection and the diffusion processes, of which the advection process is simulated with a certain method, and the diffusion process is simulated with the random walk method. The effect of the intertidal zone, the turbulent diffusion and the timescales of the water exchange are also discussed. The results show that a moving boundary modelcan simulate the transport process of the particle in the intertidal zone, where the particles are transported for a longer distance than that of the stationary result. Simulations with and without the turbulent random walk show that the effect of turbulent diffusion is very effective at spreading particles throughout the estuary and speeding up the particle movement. The spatial distribution of residence time is given to quantify the water exchange capability that has very important ramifications to water quality. The effect of wind on the water exchange is also examined and the southeasterly wind in summer tends to block the water exchange near the northeast coast, while the northerly wind in winter speeds up the transport process. These results indicate that the Lagrangian particle model is applicable and has a large potential to help understanding the water exchange capability in estuaries, which can also be useful to simulate the transport process of contaminant.
For the simulation of the three-dimensional (3D) nearshore circulation, a 3D hydrodynamic model is developed by taking into account the depth-dependent radiation stresses. Expressions for depth-dependent radiation stresses in the Cartesian coordinates are introduced on the basis of the linear wave theory, and then vertical variations of depth-dependent radiation stresses are discussed. The 3D hydrodynamic model of ELCIRC (Eulerian-Lagrangian CIRCulation) is extended by adding the terms of the depth-dependent or depth-averaged radiation stressesin the momentum equations. The wave set-up, set-down and undertow are simulated by the extended ELCIRCmodel based on the wave fields provided by the experiment or the REF/ DIF wave model. The simulated results with the depth-dependent and depth-averaged radiation stresses both show good agreement with the experimental-data for wave set-up and set-down. The undertow profiles predicted by the model with the depth-dependent radiation stresses are also consistent with the experimental results, while the model with the depth-averaged radiation stresses can not reflect the vertical distribution of undertow.
Pipes inevitably encounter high ambient pressure and bending moment during the deepwater pipe-laying process, which can lead to elliptical buckling and even deterioration failure. For the safety of pipe-laying operation, available formulas for the pipe stability are established on the basis of the assumption of uniform deformation along the tube length and symmetrical buckling. This method can predict the nonlinear response of elliptical collapse of steel circular tubes for different ratios of diameter to thickness (D / t) under pure bending or combined bending and external pressure. In these formulas, the strain-displacement relationship is deduced from the nonlinear ring theory, and the Ramberg-Osgood constitutive model is applied to simulate the inelastic material behavior. Meanwhile, the principle of virtual work is adopted to derive the equilibrium equations. A set of equations is solved by the Newton-Raphson method, and the iterative scheme contains nested iteration for the constitutive relation. In order to check the effectiveness of this theoretical method, illustrative examples are presented in this paper. Besides, the numerical simulation is carried out by use of ANSYS. A comparison of the results shows that the theoretical method can provide reasonable prediction for engineering practice.
For the study on the ice-induced vibration of a compliant mono-cone structure, a series of model tests were performed from 2004 to 2006. In these tests, the ice sheet before the compliant conical structure was found to be failed in two-time breaking. Based on this important finding, model tests study of the ice force on a compliant multi-cone structure were performed from 2006 to 2007. In these tests, the ice sheet broke before each single cone non-simultaneously. The exciting energy of the total ice force was found to be in a wide range of frequencies, and the structure can be easily excited with nonlinear resonance.
In this paper, macro- and micro- properties of natural marine clay in two different and representative regions of China are investigated in detail. In addition to in-situ tests, soil samples are collected by use of Shelby tubes for laboratory examination in Shanghai and Zhuhai respectively, two coastal cities in China. In the laboratory tests, macro-properties such as consolidation characteristics and undrained shear strength are measured. Moreover, X-ray diffraction test, scanning electron microscope test, and mercury intrusion test are carried out for the investigation of their micro-properties including clay minerals and microstructure. The study shows that: (1) both clays are Holocene series formations, classified as either normal or underconsolidated soils. The initial gradient of the stress strain curves shows their increase with increasing consolidation pressure; however, the Shanghai and the Zhuhai clays are both structural soils with the latter shown to be more structured than the former. As a result, the Zhuhai clay shows strain softening behavior at low confining pressures, but strain hardening at high pressures. In contrast, the Shanghai clay mainly manifests strain-hardening. (2) An activity ranges from 0. 75 to 1. 30 for the Shanghai marine clay and from 0. 5 to 0. 85 for the Zhuhai marine clay. The main clay mineral is illite in the Shanghai clay and kaolinite in the Zhuhai clay. The Zhuhai clay is mainly characterized by a flocculated structure, while the typical Shanghai clay shows a dispersed structure. The porous structure of the Shanghai clay is characterized mainly by large and medium-sized pores, while the Zhuhai clay porous structure is mainly featured by small and medium-sized pores. The differences in their macro- and micro- properties can be attributed to different sedimentation environments.
The stability of the motion control system is one of the decisive factors of the control quality for Autonomous Underwater Vehicle (AUV). The divergence of control, which the unstable system may be brought about, is fatal to the operation of AUV. The stability analysis of the PD and S-surface speed controllers based on the Lyapunov's direct method is proposed in this paper. After decoupling the six degree-of-freedom (DOF) motions of the AUV, the axial dynamic behavior is discussed and the condition is deduced, in which the parameters selection within stability domain can guarantee the system asymptotically stable. The experimental results in a tank and on the sea have successfully verified the algorithm reliability, which can be served as a good reference for analyzing other AUV nonlinear control systems.
A two-degree freedom model for an ALT-tanker system is established corresponding to the pitch of the ALT and the surge of the tanker. Tension in the mooring cable between the ALT and the tanker is represented by an un symmetrical, piecewise nonlinear function. Wave load on the tower is evaluated by use of the Morison equation. The first order wave load acting on the tanker is calculated by the linear diffraction theory based on the 2-D Helm-holtz equation, and the near field approach of Pinkster is used to evaluate the second order drift force. The dynamo ic equation of motion is established based on the principle of D'lembert. Dynamic response and cable tension of a mooring system composed of an 88. 4 m ALT and a 100000 t grade tanker are calculated. The influence of wave frequency, wave excitation amplitude, wind and current force on ALT-tanker motion and cable tension is discussed.
It is necessary to pay more attention to the durability of concrete undergoing freeze-thaw cycles and seawater attack simultaneously. Investigated are the effects of water-binder ratio, fly ash (FA) contents and air-entraining agent on resistance to frost and chloride diffusion of marine concrete blended with FA in natural seawater. The results show that fly ash does not improve the frost resistance of concrete but can improve its resistance to chloride diffusion by addition of less than 30%. The resistance to frost and chloride diffusion of FA concrete can be improved with the decrease of water-binder ratio, and FA may improve both of them simultaneously only being mixed with air-entraining agent. A ratio (named as R) of the frost-resisting durability factor to chloride diffusion coefficient can be used to evaluate the durability of marine concrete. Scanning electron microscope (SEM) analyses are consistent with the evaluations by the value of R.
The assembly coastal building technique initiated at home and abroad, is for a novel vertical standing harbor structure. Its main concept is the assembling components which can be combined and locked together to form a large caisson. Its application and future are discussed for a building. After many years of application and tests, the technique has a high stability, a wide range of application, low workload and fast construction speed. It can be widely applied in future for harbor engineering projects.
ScholarOne Manuscripts Log In
- Volume 33
- Issue 6
- December 2019
- Superintended by:
CHINA ASSOCIATION FOR SCIENCE AND TECHNOLOGY
- Sponsored by:
Chinese Ocean Engineering Society （COES）
- Edited by:
Nanjing Hydraulic Research Institute
Adaptive Predictive Inverse Control of Offshore Jacket Platform Based on Rough Neural Network
Numerical Simulation of Water Exchange Characteristics of the Jiaozhou Bay Based on A Three-Dimensional Lagrangian Model
A Global Reliability Assessment Method on Aging Offshore Platforms with Corrosion and Cracks