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This paper proposes an optimum design model for the offshore jacket platform considering multi-design criteria, multi-design constraints and the structure-pile-soil interaction, and gives an optimum design procedure in which the proposed optimum design model is used together with structural analysis software SAP91 and optimum algorithm software OPB1. The Chengbei #11 offshore platform, which lies in the Shengli oil field, is designed by use of the above optimum design model. The results show that the optimum design model is stable, and it depends on neither the optimization algorithm nor initial values of design variables. All values of the objective function converge to the same minimum value, and the speed of convergence is high, showing that the proposed optimum design model is reasonable.
For the computation of wave forces on structures, a B-spline expansion is applied to discretize the body surface, and represent the velocity potentials on the body surface. The expansion coefficients for the body geometry are determined by the Least Square Method, and the coefficients for velocity potentials by the Galerkin method. The method can give continuous description of velocity potentials and their derivatives on the whole smooth body surface. The method has been implemented, and numerical results show that the method gives very accurate results and its convergence is fast.
Considering the requirement of direct design and fatigue test for ships and floating structures by use of FEM technique, a computational procedure of spectral analysis for wave load on the hull surface is developed in this paper. The response of hydrodynamic pressure on the body surface to a designated sea state for ships and floating structures is calculated by use of the revised strip method with the hull bound perturbation flow concept. The spectral function of wave load for the defined point on the body surface can be determined from the Wiener-Khinhine theorem and the characteristic load value can be also obtained from spectral moment analysis. A container ship is taken as a computational example and the sample of wave load with a certain probability and corresponding encountered frequency is provided.
On the basis of a comparison of the field hydrological, sediment and topographic data observed before and after the completion of the West Breakwater in Lianyungang Harbor, which has led to the change of topography from a strait to a bay, a synthetical analysis is conducted on the change of back siltation. The results show that it is the change of coast line and the consequent change of hydrological condition and sediment movement that has led to the change of back siltation in the harbor. This phenomenon is caused by the lag effects of water current, sediment movement and local adjustment of topography after the construction, and it tends to be stable. The results of analysis provide not only a basis for extension of Lianyungang Harbor into a deep water harbor, but also propose a case for the study of the change of back siltation in a harbor on muddy coast.
Stiffened Panels are important strength members in ship and offshore structures, A new method based on counterpropagation neural networks (CPN) is proposed in this paper to predict the ultimate compres-sive strength of stiffened panels. Compared with two-parametric polynomial, this method can take more parameters into account and make more use of experimental data. Numerical study is carried out to verify the validation of this method. The new method may find wide application in practical design.
Based on the 2D horizontal plane numerical model, a quasi-3D numerical model is established for coastal regions of shallow water. The characteristics of this model are that the velocity profiles can be obtained at the same time when the equations of the value of difference between the horizontal current velocity and its depth-averaged velocity in the vertical direction are solved and the results obtained are consistent with the results of the 2D model. The circulating flow in the rectangular area induced by wind is simulated and applied to the tidal flow field of the radial sandbanks in the South Yellow Sea. The computational results from this quasi-3D model are in good agreement with analytical results and observed data. The solution of the finite difference equations has been found to be stable, and the model is simple, effective and practical.
The Explicit Upstream FEM (EU-FEM) proposed in this paper not only possesses the advantages of saving memory and CPU time for FDM, but also fits boundary easily, arranges nodal points flexibly and makes local grids fine down conveniently. The software package, which consists of EU-FEM models and pre/post-processing skills, has been widely used to estuary, near-shore, bay, lake and complex waters with many islands and channels. In addition to flow field, this model can be used to calculate the distribution fields of pollutants, temperature, salinity, sediment, and oil spill, and can be used to case study of estuary regulation projects. Some practical applications are presented and some problems discussed.
Probability distributions of wave phases in association with distributions of surface elevations are studied with wave records. Wave records of different nature are used for comparison. These are surface fluctuations acquired during wind wave flume experiments, representing wave generation under strong wind; and wave records measured in the northern part of Taiwan for waves in natural environments. Three probability models, the unifrom distribution, the beta distribution, and a model from Tayfun and Lo (1989) are adopted to study the possible distributions of wave phases. It is found that when surface elevations become skewed, wave phases deviate from the usually assumed uniform distribution and a better model would be the beta distribution .
Ocean wave propagation is slow, visible and measurable, so a wave theory can be used to approximately predict the imminnent wave force on an offshore structure based on measured, real-time wave elevation near the structure. This predictability suggests the development of a more efficient algorithm, than those that have been developed for structures under wind and seismic loads, for the active vibration control of offshore structures. The present study delveops a mutiple-step predictive optimal control (MPOC) algorithm that accounts for multiple -step external loading in the determination of optimal control forces. The control efficiency of the newly developed MPOC algorithm has been investigated under both regular (single-frequency) and irregular (multiple-frequency) wave loads, and compared with that of two other well-known optimal control algorithms: classical linear optimal control (CLOC) and instantaneous optimal control (IOC).
Based on observed wind waves, the relationships between wave spectrum products and wave characteristics are established successfully, and the apparent energy distribution of sea waves is expressed as a function of wave characteristics.
Based on the non-equilibrium suspended load transport equation, bed load transport equation and sediment transport capacity formulas derived by Don et al. , a 2-D numerical model of total sediment transport in the Yangtze Estuary is presented. In the model, the actions of tidal currents and wind waves and the effect of salinity on sediment transport are considered. An automatically generated boundary-fitted grid is used to fit the boundaries of the estuary and the boundaries of engineering projects. The verification of calculations shows that the sediment concentration, the deformation of riverbed and siltation in the channels caused by typhoons can be successfully simulated.
Nonlinear effect is of importance to waves propagating from deep water to shallow water. The non-linearity of waves is widely discussed due to its high precision in application. But there are still some problems in dealing with the nonlinear waves in practice. In this paper, a modified form of mild-slope equation with weakly nonlinear effect is derived by use of the nonlinear dispersion relation and the steady mild-slope equation containing energy dissipation. The modified form of mild-slope equation is convenient to solve nonlinear effect of waves. The model is tested against the laboratory measurement for the case of a submerged elliptical shoal on a slope beach given by Berkhoff et al. The present numerical results are also compared with those obtained through linear wave theory. Better agreement is obtained as the modified mild-slope e-quation is employed. And the modified mild-slope equation can reasonably simulate the weakly nonlinear effect of wave propagation from deep water to coast.
The accumulative shear deformation of soft clays under cyclic loads is considered as pseudo-static creep. A pseudo-static elasto-plastic cyclic creep model is developed based on the visco-elasto-plastic theory. The parameters in the model are determined by cyclic triaxial soil tests. A method for analyzing the stability of offshore soft foundation under wave loads is given by combining the model with pseudo-static creep analysis. An example is analyzed by the method. The results show that the horizontal and vertical stability of foundations under wave loads can be analyzed by it and the analytical results are qualitatively consistent with the observed failure modes of shallow foundations.
Silt deposition occurs in the downstream approach-channel of the tidal lock as in a closed channel or excavated dock basin. It is often difficult to calculate or predict siltation because of complex flow and sediment conditions and many other affecting factors. In this paper, the characteristics of flow movement in the approach channel (including its mouth) of the tidal lock are analyzed, the basic laws of sediment movement and siltation mechanism are investigated, the conditions for three types of siltation (circumfluence siltation, density flow siltation and slow flow siltation) are discussed, and corresponding calculating formulas are proposed. A practical example shows that the difference between measured and calculated results is small, indicating that the present calculating methods could be used in design and management of practical engineering projects.
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- 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