Research Status and Development Trend of Pressure Resistant Structure of Deep Submersibles

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(1.College of Naval Architecture and Ocean Engineering,Naval University of Engineering,Wuhan 430033,China;2.Wuhan Institute of Shipbuilding Technology,Wuhan 430050,China)

Abstract:This paper introduces the current situation and trend of the development of deep-sea submersibles in several powerful countries.The advantages and disadvantages of different materials for pressure resistant structure of deep-sea submersibles are analyzed,and the research status of compressive structure of composite materials is reviewed.According to the requirement of hydrodynamic characteristics of the third-generation submersibles,the research status of pressure-resistant structure configuration is summarized,and an ellipsoidal configuration is proposed.Finally,the development trend of pressure resistant structure of deep-sea submersibles is discussed.

Key words:deep-sea submersibles;pressure resistant structure;configuration;ellipsoid

0 Introduction

With the overexploitation and decrease of land resources,ocean resources will become an important source of energy for human beings.Deep-sea submersibles can effectively help humans to explore the ocean world,which mainly include manned submersibles(HOV)and unmanned submersibles(UUV).Deep-sea submersibles have developed from the first generation that needed to carry buoyancy chambers,through the second generation that could sail freely without carrying buoyancy chambers,to the third generation that emphasizes the speed of submersion at 10,000 meters.In recent years,many scholars have expressed their views on the development status and trend of deep submersibles.However,most of them focused on the development history,scale parameters,loading capacity,working time,submerged speed,and other performance indicators.Most of the development trend judgments made were based on these points,and the next step is proposed to be deeper,more reliable and collaborative.

In the view of structural design,more concerns are the form of compressive structure,material selection,bearing capacity and other indicators.In recent years,the research on deep-sea submersibles of various countries has been increasing continuously,among which the United States,France,Russia,Japan,and China are leading the development in this field.This paper summarizes and analyzes the development of deep-sea submersibles in the above-mentioned countries,and introduces the development direction of deep submersibles.According to the development trend of deep submersibles,the advantages and disadvantages of different materials are compared.The research status of compressive structure of composite materials at home and abroad is reviewed.The advantages and disadvantages of the pressure resistant structure and configuration of typical deep-sea submersibles are introduced according to the requirements of the third-generation submersibles for the submerged velocity.And the research status of the pressure-resistant structure and configuration of deep submersible vehicles is summarized,and the possibility of developing an ellipsoidal composite pressure shell is presented.Finally,the development trend of pressure resistant structure of deep submersibles is presented.

1 Development status of deep-sea submersibles

The development of the deep-sea submersibles began in 1934 when the United States opened the door to the exploration of marine life.Trister was the first deep-sea submersible with a depth of more than 10 000 m[1].However,due to its large size,lack of navigation and operational capacity,it had few practical applications in scientific research.The submersible that had explored the ocean the most so far was Alvin.Since 1964 when it was built,Alvin successively participated in diving of more than 5 000 times[2],like the Mediterranean hydrogen bomb salvage mission,seafloor hydrothermal and biota detection task to a depth of 2 500 m,Pacific high temperature black smokers’mission,study of the Titanic,and many other search and rescue mission.In 2004,the United States proposed modifications to the Alvin to improve its volume,visibility,maneuverability,and speed of ascent and descent.From 2007 to 2016,the United States released four versions of unmanned systems(integration)road map,Autonomous Undersea Vehicle Requirement for 2025,Autonomy and Next-Generation Unmanned Undersea System,these documents and reports from the point of view of different levels and reaction to the U.S.for unmanned submersible deep system’s overall development[3-9].At present,the deep-sea submersible of the United States is located at the depth of the whole sea,and is being developed such as‘Deepflight Challenger’,‘Triton 36 000/3’,‘Deep Search’,etc..The maximum working depth of these deep-sea submersibles is 11 000 m[10].Among them,‘Deepflight Challenger’manned capsule will use carbon fiber/epoxy resin composite material.The highlight of the Triton 36 000/3 is the high-strength glass for the crewed capsule.‘Deep Search’improvement is more comprehensive in the floating mechanism,body material,buoyancy material and other aspects of innovation.

Fig.1 Deep Search

Fig.2 Nautile

France also developed early in the field of deep-sea submersibles,with the construction of a 10 000-meter manned submersible‘Archimède’in 1961,which dived to 10 515.6 m in the Kuril Trench near the Sea of Japan[11].In 1972,the Archimède worked with the submersible Alvin to help French and American scientists confirm the theory that the seafloor was expanding along the ridge of oceanic waters.It was decommissioned in 1981[12].Since then,France has built a 6 000-meterdeep Nautile manned submersible using titanium alloy.The submersible projects include polymetallic nodules,ocean trenches and deep seabed ecosystems,with a payload of 200 kilograms[13].The submersible is characterized by light weight,fast ascent/descent speed,lateral mobility,good visibility,and the ability to carry a small ROV,and has gone down more than 1 000 times.

Russia has more deep-sea submersibles than any other countries in the world.The most famous are the two 6 000-meter manned submersibles,Mir1 and Mir2,jointly designed by the former Soviet Union and Finland in 1987.The latent energy is sufficient,can stay underwater for 17-20 hours a day,once successively participated in scientific investigation in the Pacific ocean,the Indian ocean,Atlantic ocean,the Arctic ocean thousands of times,such as mid-ocean ridge water temperature field measurement,nuclear submarine‘communist youth league’nuclear radiation detection,underwater photography of the sinking of the Titanic,arctic-2007 marine science expedition,and so on.In 2011,Russia launched two more manned submersibles,CONSUL and CONSUL at the current level of 6 000 m to the Russian navy.

Fig.3 Mir1

Fig.4 Shinkai 12000

Japan has developed rapidly in the field of deep-sea submersibles and is at the international advanced level.The Shinkai 6500,built in 1989,reached a depth of 6 527 m,setting a record for manned submersibles[14].Japanese KAIKO deep-sea submersible,equipped with a lot of advanced equipment and sensors at that time,went to the deepest part of the Mariana Trench in 1995,setting the world diving depth record at that time-10 911.4 m[15].The deep-sea submersible‘Shinkai 12000’,which is currently being developed in Japan,was approved in 2013,and is expected to be completed around 2025.The submersible will have good endurance,guaranteeing 6 people to sail in the deep sea for 2 days.The submersible may be made of titanium alloy or reinforced glass or carbon fiber reinforced plastic.

Since the late 1970s,China began to conduct research on deep-sea submersibles,with a rapid development momentum.The most famous submersible is Jiaolong,China’s first manned submersible designed and independently developed.In June 2012,Jiaolong’s maximum diving depth reached 7 062 m,marking that China’s deep-sea manned submersible research and development has reached the world’s advanced level.At present,the team is independently developing another deep-sea manned submersible,the main body of which is made of titanium alloy,and plans to conduct sea trials after 2020.Haidou,developed by the Shenyang Institute of Automation of the Chinese Academy of Sciences,had a maximum depth of 10 767 m in 2016.It is the first deep-sea submersible in China that has exceeded 10 000 m and carried out scientific research,making China the third country after the United States having developed a 10 000-meter deep-sea submersible.Rainbow fish,a 10 000-meter deep-sea submersible developed by Shanghai Ocean University Abyss Science and Technology Research Center(HAST),uses martensitic high-strength nickel steel to build a manned capsule and is expected to complete sea trials by 2020.‘Haima’deep-sea submersible is an unmanned remotely operated submersible system with the largest depth and the highest localization rate independently developed in China so far,which has realized the localization of key core technologies.‘Hailong’,developed by the Institute of Underwater Engineering of Shanghai Jiao Tong University,is a deep-sea submersible with the largest depth and the strongest function in China.It has participated in many ocean scientific research activities,and successfully discovered the deep-sea hydrothermal‘black chimney’,acquired polymetallic sulphide samples,and took a lot of precious photos.‘Hailong’is China’s only high-precision technical equipment capable of conducting marine surveys and operations under the special conditions of high seabed temperature and complex terrain.The‘Haiyan 11 000’underwater glider developed by the Qingdao National Laboratory of Marine Science and Technology has reached a depth of 8 213 m and is also advancing to 10 000 m[16-18].

Fig.5 Jiaolong

Fig.6 Rainbow fish

2 Research status of composite material pressure resistant structure of deep submersibles

Upon review of the development of deep-sea submersibles at home and abroad,it can been seen that reducing the weight of pressure resistant structure is one of its important development directions,and weight reduction has a great correlation with pressure resistant structure design.

For the research and development of deep-sea submersibles,the focus of materials used gradually turns to the following aspects:(1)selection of non-magnetic or low-magnetic materials;(2)choosing materials with good sound permeability and function of vibration and noise reduction;(3)choosing lightweight materials;(4)choosing corrosion-resistant materials;(5)choosing materials with good mechanical properties[19].Available materials are high-strength steel,titanium alloy,high-strength aluminum alloy,composite materials,ceramics,etc..High strength steel has large density,and will cause the overall structure heavy,and has impact on the weight of deep-sea submersible structure.High strength aluminum alloy has poor solderability and is sensitive to stress corrosion.The inherent brittleness of ceramics limits its application[20].At present,the pressure resistant shell structure of underwater vehicles is mainly made of aluminum alloy,titanium alloy,high-strength steel,and other materials in order to obtain a higher specific strength and stiffness,to reduce the weight of the pressure resistant shell structure and achieve non-magnetic,corrosion resistance and other properties.But overall,the metal pressure shell structure weight is still heavy,and there are many shortcomings,such as low elastic modulus of aluminum alloy,high cost of titanium alloy,and difficult manufacturing process.With the increase of the payload requirement of deep-sea submersibles,the composite material with a relatively small density gradually entered the designer’s field of vision in order to reduce the weight of pressure resistant structure.

The use of composite materials in the pressure resistant structure of deep submersibles can achieve the design objectives of light weight,high strength and corrosion resistance,so as to effectively reduce the weight of the carrier structure of deep submersibles,increase the space utilization rate,improve the endurance and stealth performance,and create favorable conditions for the greater development of deep submersibles[21].In recent years,the non-pressure resistant structure and even pressure resistant structure of several underwater unmanned vehicles developed abroad have been extensively applied with carbon fiber materials,which reduces the weight and is conducive to more configuration of task loads.The outer shell of the Talisman aircraft in the UK,for example,is made of carbon fiber composite materials and contains carbon fiber composite pressure vessels containing electronic systems and mission loads[22].The research status of the pressure resistant structure of the composite submersibles is summarized below.

Fathallah et al studied the influence of parameters such as ellipse radius,ring rib spacing,composite fiber thickness and laying angle on the structural strength as well as the sensitivity coefficient for the composite compression-resistant column-shell structure with elliptical section,and gave the failure strength and buckling load of the composite compression-resistant structure with elliptical section.The characteristics of an elliptic submarine pressure hull under non-contact underwater explosion were simulated,and the time history of plastic strain,wet surface displacement,velocity and Mises stress were given.The research results provide a certain reference value for the design of compressive column shell with elliptic section[23-26].Tan carried out theoretical and numerical simulation analysis on the stress of the composite compression chamber structure of a 7 000-meter deep-sea glider based on the cylindrical shell,and the water pressure test and compression deformation test of carbon fiber pressure-proof cylinder were carried out.The research results can be used to guide the design of cylindrical deep-sea submersibles.Unfortunately,this paper did not analyze and test the ultimate bearing capacity of the composite pressure shell[27].

Jiang and Zhou et al of Huazhong University of Science and Technology carried out research on multi-plane reinforced cylindrical shell structure of composite materials.The buckling and strength properties of multi-plane reinforced cylindrical shells were analyzed considering the angle of fiber laying and thickness of composite materials,and pointed out that the adjustment of laying angle and thickness influenced the elastic stability and tensile strength of the structure.The advantages of multi-plane cylindrical shell compared with cylindrical shell and annular rib cylindrical shell were analyzed,and the axial compression test of composite multi-plane cylindrical shell was carried out.The research results provide a reference for the design of new deep-sea pressure resistant shells[28-29].

Cheng and Li,from Harbin Engineering University,studied the design and calculation methods and optimization methods of composite materials for cylindrical pressure resistant shell structures by using finite element method,damage mechanics theory and approximate model method.The collaborative optimization design of composite pressure shell based on the parameters of the layer was carried out,which solves the problem of low optimization efficiency of taking the layer angle as the design variable directly.The optimization method combining 6σdesign and reliability evaluation can accurately,efficiently,and reliably optimize the pressure resistant shell of underwater vehicle annular rib composite material,and reasonably solve the problem that the structure reliability is not high due to the change of random factors.The study is systematic and can be used for reference in the design of cylindrical composite pressure shells[30-32].

Wang et al of Northwestern Polytechnical University carried out an analysis on the compressive stress of the ellipsoidal structure with unequal pole holes of fiber winding,and found that the stress state of the ellipsoidal container of fiber winding was the first place where the failure occurred at the equator,and local failure occurred.This research result provides guidance for the arrangement of composite materials’ellipsoidal compressive structure fibers[33].Pan,Shen and Wang et al carried out numerical calculation of nonlinear buckling for the pressure resistant structure of composite cylindrical shell,compared the nonlinear buckling behavior of the pressure resistant structure of cylindrical shell of different materials,and pointed out that carbon/epoxy resin is an ideal material for underwater cylindrical shell.The effects of orientation angle and length of shell laminate on buckling shape and buckling pressure were studied numerically.The laying mode,fiber angle and layer number of carbon fiber/epoxy resin,borane epoxy resin and glass/epoxy resin composites were optimized by using the combination of genetic algorithm and numerical analysis method.New methods such as variable thickness and composite reinforcement were proposed to solve the contradiction between buckling pressure and material failure,and increased the design pressure by adding smaller bars.The research results provide a reference for the design of cylindrical compressive structure of composite materials[34-36].

Zhang,Wang and Zhou et al from the team of Jiangsu University of Science and Technology studied the eggshell-shaped pressure resistant structure of composite materials,and derived the quantitative relationship between ultimate strength load,critical buckling load and thickness.They also got the solving equations of thickness and buoyancy coefficient.The mechanical properties of spherical shell under the same conditions are studied.It is pointed out that the mechanical properties of an egg-shaped shell are better than that of a spherical shell.Compared with composite cylindrical shell,egg-shaped shell has a better axial bearing capacity and a lower defect sensitivity,but egg-shaped pressure resistant shell structure has great difficulty in processing.This study provides a reference for the selection of a new type of pressure resistant structure for the 4 000-meter deepsea submersibles[37-39].

Zhu,Chen and Li et al from the Naval Engineering University team were committed to selecting the composite sandwich structure in the pressure resistant shell of deep-sea submersibles,and carried out the research on the mechanical properties of the wound composite sandwich cylindrical shell under the action of axial pressure.By comparing the experimental data with the numerical results,a new method for the calculation of wound composite sandwich cylindrical shells was presented.In order to improve the critical buckling load,the winding optimization of composite sandwich cylindrical shell was carried out.The optimization method could not only meet the requirements of lightweight,but also meet the requirements of structural strength and stability.In order to better fit the project,the axial pressure bearing characteristics of the composite sandwich cylindrical shell with precrack were analyzed.The effects of crack direction and fiber winding angle on the ultimate bearing capacity of the structure were investigated by means of experiment and numerical calculation,and the effects of wall thickness and winding angle on the ultimate bearing capacity were proposed.The research results provide a reference for the structural design of composite sandwich cylindrical shells[40-42].

3 Research status of deep submersible’s pressure resistant configuration

There are three main types of pressure resistant shells:spherical,ellipsoidal,and cylindrical.For the above structures of metal materials,their advantages and disadvantages are shown in Tab.1[43].

Tab.1 Advantages and disadvantages of typical pressure resistant structures

With the increasing demand for operating time of deep-sea submersibles,their process time needs to be further compressed,and the hydrodynamic performance of pressure resistant structure will also affect the submerged velocity and process time of deep-sea submersibles.It is an important direction for the development of deep-sea submersibles to improve their floating speed,so the hydrodynamic performance of the pressure resistant structure of deep-sea submersibles should also be considered.

Li et al from China Ship Scientific Research Center pointed out that the submersible speed of Jiaolong needs to be improved,and took‘Deepsea Challenger’,‘Deepflight Challenger’and‘Deepsearch’as research objects to analyze their resistance characteristics.It is pointed out that the shape of a torpedo can reduce bow section height,shorten inflow section length,increase total heightH,increase length-diameter ratio,etc..(what has been described resembles an ellipsoid),which could reduce running resistance.Xu compared the deep-sea submersibles with different configurations,and pointed out that the submersibles with a single vertical flat body configuration and a double vertical flat body configuration had the fastest submerged speed,i.e.,the droplet shape submersibles,and had the best hydrodynamic performance.These results provide a theoretical basis for the structural design of the third generation of all-sea submersibles in China[44-48].

Mohammad et al analyzed six different end-to-end cylindrical submersible models.It was pointed out that the elliptical bow and the circular or parabolic fan tail were the best to reduce the resistance of the submersibles.The research results are based on the main configuration of submarines[49].Lüet al studied the lift-drag ratio of the round saucer,flying saucer and ellipsoid glider.It was pointed out that the lift-to-drag ratio of the circular plate glider and the flying saucer glider at a low angle of attack was better than that of the elliptical glider,while the elliptical glider at a high angle of attack had a better lift-to-drag ratio than the other two gliders.The research results provide some ideas for the selection of the pressure resistant structure of deep-sea submersibles[50].

In comparison with spherical and cylindrical pressure resistant structures,ellipsoidal pressure resistant structures of deep-sea submersibles may be more suitable.On the one hand,ellipsoid is like water droplet,which is the optimization of cylindrical structure.It can reduce seawater resistance,increase the speed and maneuverability of submersibles and lower energy consumption[51].On the other hand,ellipsoidal pressure shell is like spherical pressure shell.The transverse section is round,and the stress of the film is the main stress.The longitudinal arc is smooth,and the bearing capacity is higher than that of the longitudinal straight cylindrical shell.

4 Development trend of pressure resistant structure of deep submersibles

Deep-sea submersibles,especially the whole deep-sea submersibles,have become an important development direction of this type of equipment,and are a hot spot in the research and development of various countries.This paper summarizes the research status and development focus of pressure resistant structure of deep-sea submersibles at home and abroad,mainly including the following aspects[52-55]:

(1)Reducing pressure structure weight.With the increase of the depth of submergence,the thickness of the pressure resistant shell will increase gradually,which will lead to an increase of the pressure resistant structure’s weight.The payload that a deep-sea submersible can carry is directly related to its mission capability.Under certain buoyancy conditions,reducing the weight of the pressure resistant structure can effectively increase the payload carried by the submersible.

(2)Drag reduction configuration design.An important development direction of deep submersibles is to increase the submersible speed and decrease the process time.Reducing the resistance of the pressure resistant structure in the process of subsurface flotation is of great significance for improving the overall hydrodynamic performance and the subsurface velocity of the submersible.Especially for a single-hull submersible,the design of drag reduction configuration of pressure resistant structure is more important.

(3)Development and application of new materials.In order to effectively improve the bearing capacity of the pressure resistant structure,the thickness of the pressure resistant shell can be increased on one hand,and on the other hand,new materials with a higher strength and a lower density can be developed and applied in the pressure resistant structure,to reduce the weight of the pressure resistant structure while achieving a high bearing capacity.