Reliability-based Multidisciplinary Design Optimization Of Deep Ocean Profiling Float

With the rapid economic development in our country,China has put forward the strategic concept of "One Belt and One Road".The construction of the "Maritime Silk Road" has become an important part of the "the Belt and Road initiative".The rational development and utilization of marine resources has become especially important in the 21 st century.Understanding the ocean is a prerequisite for rational development and utilization of ocean resources.With the global warming,the greenhouse effect intensifying,the marine climate and environment deteriorating,extreme weather events have been occurring with startling frequency.There are many uncertainties and challenges for the development and utilization of the ocean resources.The monitoring of the marine environment can be used not only for marine-climate models,marine disaster prediction,weather forecasting,but also for fishery production.The monitoring of marine environment can help fishery researchers to analyze and forecast the formation,migration and movement of marine fisheries,so as to guide human beings in their fishery production activities.As a kind of ocean observation platform,ocean profiling float has been applied to ocean observation.It has the advantages of small size,convenient deployment,independent observation and low price.The research,development and utilization of deep ocean profiling float can help us to construct oceanrelated statistical models more accurately and effectively,and to understand the ocean circulation,sea water energy exchange better,to understand the laws deeper,then do good service for human society.At present,the profiling floats are mainly represented by COPEX of National Oceanographic Technology Center of State Oceanic Administration(SOA)and HM2000 of No.710 research institute of China Shipbuilding Industry Corporation(CSIC)at home.COPEX and HM2000 are both conventional profiling floats with a measured depth about 2000 m.Foreign deep ocean profiling floats have been put on the market,while the domestic scientific research institutions have a high demand for deep ocean profiling floats.Under this background,this thesis selects “Reliability-Based Multidisciplinary design optimization(RBMDO)of deep ocean profiling float” as the research topic.In order to solve various issues related to the design of a deep ocean profiling float,the following problems are studied in this thesis:Firstly,this dissertation reviews the history of profiling floats and compares them with the conventional profiling floats and deep-ocean profiling floats at home and abroad.The overall design for the profiling float largely depends on the design of the buoyancy regulation system.The existing conventional profiling floats are designed by using cylindrical shell,while most of the deep-ocean profiling floats are designed by using spherical pressure structure,which provides a reference for the design of deepocean profiling floats pressure structure.To select the appropriate multidisciplinary design optimization(MDO)method,several comonly used multidisciplinary design optimization methods are compared and analyzed by an example of gear reducer model.Aiming at the problem of nonconvergence of the CO algorithm of the gear reducer model,a variety of constructions and improvements of gear reducer model have been made.The results show that both the construction form and the relaxation factor have an influence on the optimization result,and the improved CO configuration can be used to obtain a reliable optimization result.Then the scope of application of various multidisciplinary design optimization methods is given to facilitate the researchers choosing the appropriate method in engineering optimization calculations.The buoyancy regulation system of deep ocean profiling float is the premise for realizing its descent and ascent.In order to meet the needs of South China Sea,the target design depth of the deep ocean profiling float is set to be 4500 m.In-depth analysis and study of the existing conventional profiling floats and deep-ocean profiling floats are carried out.Its buoyancy regulation system is discussed based on the design goal,then a design scheme is formulated.A hydraulic system design plan is proposed,and the relevant components are selected.In order to obtain the performance of hydraulic components,a hydraulic test bench is designed to simulate seawater pressure,then the pressure flow-rate characteristic curve and energy consumption of key hydraulic components have been obtained,leakage have been tested,and these data provide a good foundation for the establishment of subsequent multidisciplinary models.Test results of hydraulic system show that the energy consumption of the motor has become a major contradiction of the design of deep-ocean profiling float.In order to save energy,a control mode of intermittent starter motor is proposed to drive the hydraulic pump to discharge hydraulic oil from the internal oil bladder to the external oil bladder.According to the pressure flow-rate characteristic curve,energy consumption of the key components and seawater density,the simulation study on whether the float compresses under the deep sea environment,the result shows that the intermittent control strategy saves 50.69% of the energy and at the same time it can obtain the energy consumption for the single-period motion of profiling float.The design of the deep-ocean profiling float needs to meet the structure and balance requirements,however,a large number of design points which meet the structural requirements and balance requirements are selected for motion simulation study,the results show that not all the design points which meet the structural requirements and balance requirements are reasonable design points,reasonable design points should not only meet the structural and balance requirements but also need to dive to the maximum design depth.The study found the existence of sensitive areas which may invalidate the designed deep ocean profiling float or fail to reach the specified maximum working depth.Therefore,it is recommended that this area should be avoided during the design process of deep ocean profiling float.To some extent,the discovery of this sensitive area may help to explain the phenomenon that the current conventional profiling floats cannot reach the maximum design depth.The multidisciplinary design optimization model of deep ocean profiling float based on reliability is established.Deep-ocean profiling floats are divided into five modules: hydrodynamics,structure,weight and balance,volume,energy.During the optimization process of the hydrodynamics module,the concept of equivalent average velocity is proposed for model optimization.Deep-ocean profiling floats depend on glass sphere.The glass sphere not only provide a safe and reliable environment for nonpressure structures,but also provide buoyancy for deep-ocean profiling floats.The structural performance of glass sphere is a key point during the design process of deep ocean profiling float.Therefore,structure module mainly considers the pressure resistance and deformation of glass sphere.It is necessary to have static analysis on glass sphere of different sizes by ANSYS,then the stress formulas of different sizes of glass sphere under the designed working pressure were obtained,and the reliability based design optimization of glass sphere is ensured.As a system-level optimization goal,the discipline of weight balances the weight and buoyancy of deep ocean profiling float.Due to the gravitation of the earth,the gravitational acceleration produces slight deviations between different latitudes and longitudes,and there is an uncertain deviation in seawater density between different sea areas.Therefore,free weights are considered at the same time in this module.Due to the changes of sea-water density and environmental factors in different sea areas,the role of free weights is to adjust the weight of deep ocean profiling float slightly to meet the measurement requirements.Through the energy consumption,the volume of fixed equipment in the cabin,the total required volume can be optimized.In this dissertation,the reliability methods are compared and an example analysis is carried out through the formula of ultimate strength of pressure spherical hulls.The mainstream reliability optimization algorithms are compared and reviewed,and a single-loop algorithm is used to analyze the gear reducer model.It is found that the single-loop algorithm can be better applied to reliability based design optimization.In order to maximize the measurement period,a double-loop algorithm based on the CO method is proposed.After RBMDO of the deep ocean profiling float,the total weight is 47.808 kg,and the total weight is increased by 42 g compared to MDO,but the reliability of the glass sphere is 99.87%.This largely ensures the safety of floats under deep sea and complex conditions,it can meet the design requirements,and RBMDO is an important way to improve survivability of the float.Within the reasonable design range,the number of measurement profiles of the float has reached 124 profiles,an increase of 24 profiles over the 100 profiles of initial design schedule.