Seafloor Seating Platform Stability Analysis And Optimization Design

Advances in science and technology will lead to innovations in our observation techniques.For example,some inventions like microscopes,telescopes,and satellites,could make us to observe the micro and macro world,and even to establish observation views outside the surface of the earth.Today,however,we still almost know noting about what is going on the seafloor.Thus,it is urgent to establish a seafloor observation view that will help us to understand something more about the oceans,the earth's crust,diversity of species,the exploration and extraction of energy,and so on.This observation view will be built based on a secure and stable seabed platform.The platform will spend more than 98%of its life,sitting on the seafloor of the sea,which is a dark and high pressure environment.We cannot monitor the status of the platform in real time,so the excellent stability of the platform is the guarantee of its safety.The thesis will study seafloor seating stability analysis and its influencing factors based on the hydrodynamic characteristics of the platform after seated on the seafloor.After the platform and environment models are parameterized,we can change the hyper-parameter of the models to obtain different structure of the platform.From two aspects of its structure and environmental factors,the thesis use the method of CFD numerical simulation to calculate the 14 series working conditions,which are nearly 150 calculations,to obtain the lifts,resistances and lift-to-resistance ratios data of each working condition.CFD calculations are carried out after the size independence experimentation of the grid.And what's more,the thesis illustrates the causes and influencing factors of platform hydrodynamic,according to the speed and relative pressure cloud map.And then,we could get the corresponding conclusions.According to the calculations data and conclusions,one optimization method based on regression analysis and another optimization method based on fuzzy theory are respectively applied to carry out the optimization design research.The optimization objective is its optimal seafloor seating stability of the seabed platform,and the optimized platform parameters are obtained.(1)The size,shape and relative position of the platform itself are changed,to study the influence of four factors,including the folds height of the upper and lower decks,the number,size and relative position of buoys,the length of the deck-buoy and the existence or absence of weight loss holes.The study shows that the existence of the lower deck causes the upward lift turn to downward,which is conducive to the seafloor seating stability of the platform.Regardless of the number of buoys,a change in the height of the upper deck folds of the platform would result in a linear change of lift force and resistance force,and on the other hand,a change in the height of the lower deck folds of the platform would result in a nonlinear change of lift force and a linear change of resistance force.Therefore,there is an optimal lower deck folds height for the seafloor seating stability of the platform.The lift force is mainly generated by the lower deck of the inflow section,and the resistance is proportional to the inflow area.The seafloor seating stability is more sensitive to the folds height of the lower deck than that of the upper deck.At the same time,larger buoy's diameter and distance of the buoys are more conducive to the stability.In order to avoid the emergence of wake vortex,the buoy should be close to the lower surface of the upper deck,which can also improve the stability of the platform.The lift force and resistance force of the platform are linearly related to the length of the deck-buoys,but the lift-resistance ratios tends to a stable value as the increase of the deck-buoys length.The diaphragm plate will cause the pressure zone that generates lift force to be discontinuous,and the lift decreases.With the same effective deck-buoy length,two sub-platforms connected produce a downward lift force and lift-to-resistance ratio,which are about 3/4 produced by one sub-platform,and their resistance forces are similar.The platform with weight loss hole has a large negative lift-resistance ratio,which is conducive to the stability.Therefore,during the progress of design,the deck-buoy length could be appropriately increased,the number of diaphragm plate could be reduced,and the proper weight loss holes could be designed to improve the stability,on the premise of satisfying the stability of structure and material.(2)The terrain and velocity of the environment after sitting on the seafloor are changed to study the influence of the environment on the hydrodynamic characteristics of the platform.The terrain includes peak and valley environment,and the velocity includes steady velocity,sinusoidal velocity,and step velocity.The research shows that the seafloor seating stability of the valley is better than that of the horizontal seafloor,and the environment of the peak with small peak height and small peak slope angle is more conducive to the seafloor seating stability of the platform.The change of flow velocity results in the nonlinear change of lift force and resistance force,but the lift-resistance ratio changes stably.Therefore,if the platform satisfies the seafloor seating stability at one flow velocity,it can be considered that the platform also satisfies the seafloor seating stability at other flow velocities.At the moment when the velocity changes,the unstable factors of the platform increase,especially when the velocity drops,the positive lift-to-resistance ratio appears.Thus,extreme velocities can threaten the stability of the platform.(3)Based on the data and conclusions obtained from CFD numerical simulation mentioned earlier in the thesis,we study the optimal design of the platform about some hyper-parameters such as_uh?w?d?D?_dh,including two optimization methods based on regression analysis and fuzzy theory.The optimization objective is to obtain a optimal hyper-parameters of the seabed platform which own a better seafloor seating stability.Regression analysis is used to carry out regression fitting for lift force,drag force and lift-drag ratio about the five parameters above respectively.The relation formulas are solved according to the optimization objective,and the optimal solution of the five parameters are obtained respectively.The global optimal solution can be obtained by this method,but the parameters are independent of each other.In order to comprehensively evaluate the seafloor seating stability of the platform,a set of comprehensive evaluation system composed of the five parameters mentioned above was established.Analytic hierarchy process,fuzzy theory,mathematical optimization and other methods are applied to optimize the platform.The method can obtain the local optimal solution,but it can take five parameters into consideration.The two optimization methods have their own advantages and disadvantages,so they should be used according to specific problems.The research results and calculation methods in this thesis can provide ideas for the optimal design and hydrodynamic performance analysis of seabed platform,and also can provide experience reference.This is of positive significance for the development of undersea scientific and technological equipment,the establish of an undersea observation network and an undersea space station.