Journal of Marine Science and Engineering
Article Design and Application of Buoy Single Point Mooring System with Electro-Optical-Mechanical (EOM) Cable
Junwei Yu 1,2,*, Shaowei Zhang 1,*, Wencai Yang 1, Yongzhi Xin 1 and Huaining Gao 1
1 Institute of Deep-Sea Science and Engineering Chinese Academy of Sciences, Hainan 572000, China; [email protected] (W.Y.); [email protected] (Y.X.); [email protected] (H.G.) 2 College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence: [email protected] (J.Y.); [email protected] or [email protected] (S.Z.)
Received: 17 July 2020; Accepted: 27 August 2020; Published: 1 September 2020
Abstract: This paper presents the design of a hybrid system named the Mooring Buoys Observation System for Benthic with Electro Optical Mechanical Cable (MBOSBC) for long-term mooring buoy observation in deep oceans. MBOSBC is comprised of three main modules: a surface buoy, a benthic node, and an Electro-Optical-Mechanical (EOM) cable. The Surface buoy provides energy for the entire system, the Benthic node is used to observe scientific phenomena on the seabed, and the Electro-Optical-Mechanical (EOM) cable connects the sea surface buoy and the benthic observation node, serving as the information and power transmission link. This paper provides design criteria for a single point mooring buoy using an EOM cable. The environmental load of the buoy under varying wind, wave, and current conditions is analyzed and the mooring hydrodynamic force of EOM is calculated. After calculating the load of the EOM cable under extreme marine conditions, the hydrodynamic force needed for the system is analyzed. After physically testing the strength of the designed EOM cable, a near-shore test of MBOSBC was carried out, in order to verify that the system has the expected function.
Keywords: ocean buoy mooring; Electro-Optical-Mechanical (EOM) cable; mooring force analysis; single point mooring
1. Introduction It is indisputable that the ocean is rich in oil, natural gas, minerals, and marine living resources. It was reported in the 2018 Blue mining Public Report that the global demand for materials, such as cobalt, nickel, lithium, and copper, will increase ten- to hundred-fold in the foreseeable future. However, the prerequisite for the development of these resources is the ability to grasp the marine data of relevant sea areas. At the same time, observing various ocean phenomena is also a prerequisite for us to be able to forecast the ocean climate. Observations of seafloor crustal movement, seafloor earthquakes, warmness and salinity of ocean water, dissolved oxygen, carbon cycle, and the resulting ocean circulation can help us to take corresponding countermeasures early in the face of natural disasters, and may help us to avoid or reduce economic losses. In the interior of the ocean, due to the inconsistency of the salinity and temperature in various places, the density of seawater in different water areas or depths is also inconsistent. It is very important to understand the physical information of the ocean for the navigation of warships, especially submarines. If a submarine inadvertently drives into less dense waters from denser waters without taking corresponding measures, the submarine will dive violently, resulting in a sudden increase of the pressure on it, which is likely to cause the submarine to collapse,
J. Mar. Sci. Eng. 2020, 8, 672; doi:10.3390/jmse8090672 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2020, 8, 672 2 of 25 taking the lives of its crew members. Therefore, observing and mastering marine information is of great significance to the marine economy, marine climate forecast, and marine military. In addition to having abundant resources, the ocean also contains tremendous energy. Human beings have been working hard to develop and utilize this energy and have done a lot of work on it. Simeon Doyle et al. discussed the potential of M-OWC (multi-oscillating water columns) and the improvements they can bring to the OWC (oscillating water column) principle, in order to increase capture efficiency. Thus, M-OWC has the potential to provide encouraging results and support the further development of wave energy development [1]. Buoy-based observation systems can continuously observe a specific sea area over a long time. With a set of corresponding deployment and recovery equipment, the observation platform can migrate flexibly to obtain long-term, wide-range, and high-precision marine scientific data. Therefore, at present, in the field of marine meteorological forecast and marine military, buoys and submarine observation networks are mainly used to obtain marine data. Buoy-based ocean observation platforms are mainly composed of buoys, seafloor nodes, mooring cables, and deployment recovery devices. They are equipped with different observation instruments, from sea surface to seafloor. Therefore, the power supply and communication systems should be redesigned, according to the observation needs of the system. The mooring technology of the buoy is directly related to the survival of the entire observation platform. There are three typical mooring methods for buoys: chain-catenary mooring, semi-taut mooring, and inverse-catenary mooring [2]. The factors that determine the type of mooring include the depth of deployed waters, cost factors, the deployment strategy, and so on. Catenary mooring is mainly used in shallow water, and its configuration is also the simplest. As shown in Figure1a, a mooring chain is usually used as the mooring string when the water depth is lower than 50 m; when the water is deeper, the upper part of the mooring chain is usually replaced by a cable to reduce its weight, as shown in Figure1b. The cost of this kind of mooring configuration is low, but it can only be placed in shallow water and it is difficult for this mooring configuration to resist strong wind and waves; it is also harmful to the ecosystem of the seabed, as the mooring chain is in J.contact Mar. Sci. with Eng. 20 the20, seabed.8, x FOR PEER REVIEW 3 of 25
Buoy Buoy
Wire-rope
Depth 50m Depth 200m
Anchor Chain
Anchor Chain
Anchor Anchor
(a) Catenary mooring using Anchor chain (b) Catenary mooring using wire-rope
Figure 1. CatenaryCatenary mooring. mooring.
A semi-taut mooringBuoy system moors the buoy on the sea surfaceBuoy using an elastic mooring string, as shown in Figure2. The mooring string is usually made of nylon, polyester, and polypropylene.
Elastic material (Nylon, Polyester, etc.) Stretch hoses Depth 500m
Depth 100m Recovery floatation
Acoustic releaser Chain Wire-rope Anchor Anchor
(a) Semi-taut mooring using Stretch hoses (b) Semi-taut mooring using Elastic material
Figure 2. Semi-taut mooring.
J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 3 of 25
Buoy Buoy
Wire-rope
Depth 50m Depth 200m J. Mar. Sci. Eng. 2020, 8, 672 3 of 25 Anchor Chain
The bottom of the semi-taut mooring system is not contact with the seabed, thus having little impact on the marine ecological environment. However, it is necessary to know the water depth before laying a semi-taut mooring system, as the length of the mooring chain needsAnchor to be determinedChain according to the water depth, such that the mooring string always maintains a certain tension. According to Anchor Anchor the requirements of the whole system, some sensors (e.g., CTD (conductivity temperature depth), ADCP (acoustic doppler current profilers), and so on) may be installed in the middle of the mooring string.(a) ACatenary float and mooring acoustic using releaser Anchor are chain usually installed(b) Catenary at the mooring lower part using of wire the mooring-rope string, to facilitate the recovery of mooring stringFigure and 1. Catenary the observation mooring. instruments.
Buoy Buoy
Elastic material (Nylon, Polyester, etc.) Stretch hoses Depth 500m
Depth 100m Recovery floatation
Acoustic releaser Chain Wire-rope Anchor Anchor
(a) Semi-taut mooring using Stretch hoses (b) Semi-taut mooring using Elastic material
FigureFigure 2.2. Semi-tautSemi-taut mooring.mooring.
The most significant feature of inverse-catenary moorings is that the middle section of the mooring string has the shape of an ‘S’, as shown in Figure3. There is not much di fference of the mooring design for differing sea water densities. Common materials are polyolefin, polyester, and nylon. This type of mooring system is usually deployed in waters with a depth greater than 500 m. It can resist the impact of high-velocity currents and has high survivability in extreme marine environments. It is suitable for deployment in deep seas, but requires a longer mooring chain, its assembly is more difficult, and this type of mooring system is more costly. Many scholars have studied the mooring technology of floating structures. Imanol Touzon et al. analyzed the numerical results of applying three different well-known mooring design methods to floating wave energy converters moored by four catenaries. The simulation was carried out under extreme environmental conditions. After analyzing the simulation results, it was found that the wire tension is greatly affected by resistance and inertial force. The author believed that this effect comes from the sudden load caused by the fluctuation of the float [3]. Sergej Antonello Sirigu et al. experimentally studied the mooring system of a wave energy converter, where the influence of the mooring layout on loads in extreme wave conditions was the focus of the research. The peak load of the mooring system was reduced by changing the configuration of the mooring system [4]. Magnus Thorsen Bach-Gansmo et al. presented the dynamic behavior and line loads of a FOWT (Floating offshore wind turbines). They used physical experiments and numerical codes to investigate the mooring and floater response when moored with elastic mooring lines. The influence of the mooring line angles and the pretension in the mooring line of the mooring system was studied experimentally [5]. J. Mar. Sci. Eng. 2020, 8, 672 4 of 25 J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 4 of 25
Buoy Sea Surface
Float EOM Cable Depth 500m
Additional Weight
SeaBed Anchor
FigureFigure 3. 3. InverseInverse-catenary-catenary mooring. mooring.
ManyMany scholars scholars have have studied studied the the mooring mooring technology technology of of floating floating structures. structures. Imanol Imanol Touzon Touzon et et al. al. analyzedanalyzed the the numerical numerical results results of of applying applying three three different different well well-known-known mooring mooring design design methods methods to to floatingfloating wave wave energy energy converters converters moored moored by by four four catenaries. The The simulation simulation was was carried carried out out under under extremeextreme environmental environmental conditions. conditions. After After analyzing analyzing the thesimulation simulation results, results, it was it found was found that the that wire the tensionwire tension is greatly is greatly affected aff ectedby resistance by resistance and inertial and inertial force. The force. author The authorbelieved believed that this that effect this comes effect fromcomes the from sudden the sudden load caused load caused by the by fluctuation the fluctuation of the of the float float [3]. [3 Sergej]. Sergej Antonello Antonello Sirigu Sirigu et et al. al. experimentallyexperimentally studied studied the the moo mooringring system system of ofa wave a wave energy energy converter, converter, where where the theinfluence influence of the of mooringthe mooring layout layout on loads on loadsin extreme in extreme wave waveconditions conditions was the was focus the of focus the research. of the research. The peak The load peak of theload mooring of the mooring system was system reduced was reducedby changing by changing the configuration the configuration of the mooring of the system mooring [4]. system Magnus [4]. ThorsenMagnus BachThorsen-Gansmo Bach-Gansmo et al. presented et al. presented the dynamic the dynamic behavior behavior and line and loads line of loadsa FOWT(Floating of a FOWT offshore(Floating wind offshore turbines) wind turbines) They used They physical used physical experiments experiments and numerical and numerical codes codes to investigate to investigate the mooringthe mooring and floater and floater response response when whenmoored moored with elastic with mooring elastic mooring lines. The lines. influence The influence of the mooring of the linemooring angles line and angles the pretension and the in pretension the mooring in theline mooringof the mooring line of system the mooring was studied system experimentally was studied [5].experimentally [5]. CUMASCUMAS (Cabled (Cabled Underwater Underwater Module Module for for Acquisition Acquisition of of Seismological Seismological data) data) was was designed designed to to extendextend the the land land surveillance surveillance ne networktwork towards towards the the wide wide marine marine sector sector of of calderas, calderas, as as shown shown in in [6 [6,7,7].]. TheThe system system was was deployed deployed within within 100 100 m m of of depth. depth. The The system system is is located located at at the the Campi Campi Flegrei Flegrei caldera caldera (southern(southern Italy), Italy), one one of of the the most most hazardous hazardous and and populated populated volcanic volcanic areas areas in in the the world world.. Th Thee scientific scientific sensorssensors include include a a seismometer, seismometer, hydrophone, hydrophone, ocean ocean current current meter, meter, and water and waterthermometer. thermometer. The MBARI The MBARIOcean ObservatoryOcean Observatory System System (MOOS) (MOOS) prototype, prototype, which which is shown is shown in [8– 10in], [8 utilizes–10], utilizes an EOM an EOM cable cableto deliver to deliver power power from from the the buoy buoy to to the the seafloor seafloor instruments, instruments, realize realize communicationcommunication between the the instrumentsinstruments and and the the buoy, buoy, and and which which can can be bedeployed deployed in the in thedeep deep ocean ocean (up (upto 4000 to 4000m depth). m depth). The surfaceThe surface buoy buoy collects collects solar solar and and wind wind energy, energy, where where horizontal horizontal axis axis wind wind turbines turbines and and solar solar panels panels areare utilized utilized for power collection. TheThe windwind turbineturbine hashas aa rotor rotor diameter diameter of of 1.2 1.2 m m with with 300 300 W W output output in in12.5 12.5 m /m/ss wind. wind. The The actual actual power power is is 60 60 W, W, which which depends depends on onthe the windwind speed.speed. The wind turbine and and solarsolar provide provide 48 V DC power busbus forfor thethe battery.battery. Through Through a a DC–DC DC–DC Converter, Converter, the the system system provides provides 12 12V DCV DC for for the the buoy buoy equipment equipment and and 375 375 V DCV DC for for the the ocean ocean benthic benthic instruments. instruments. SatelliteSatellite communication communication o ffers offers a low-bandwidth a low-bandwidth path for path buoys, for while buoys, optical while fiber communication optical fiber communicationprovides a high-bandwidth provides a path high from-bandwidth the seafloor path to from sea surface; the seafloor for example, to sea through surface; anfor EOM example, cable. throughThe cable an utilized EOM cable. in the The MOOS cable systemutilized is in di thefferent MOOS from system that inis different the seafloor from observation that in the network.seafloor observation network. The traditional EOM cables utilized in seafloor networks are armored EOM cable, which lie on the sea bottom statically. An armored EOM cable is not suitable for deep sea buoy
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The traditional EOM cables utilized in seafloor networks are armored EOM cable, which lie on the sea bottom statically. An armored EOM cable is not suitable for deep sea buoy mooring, however. The EOM cable has to be designed to have net buoyancy, in order for it to hang in the water for a long time. This kind of EOM cable requires high strength, excellent bending property, watertight performance, and wear resistance. Vectran fibers, nylon, and polyester are generally selected as the strength material. The properties of EOM cables with different strength materials have been compared in [11–14], especially the properties of the static and dynamic responses of mooring EOM cables. The tether technology is generally implemented in shallow ocean areas, as shown in [6,7], where the anchoring rope and coaxial copper conductors are roped together as the mooring component [13,15], such that the coaxial cable does not have to be pulled. The analog signals are transmitted by the coaxial cable and transferred to digital signals by the A/D converter at the buoy and seafloor. This kind of mooring method is difficult to utilize in deep sea, however, as it is hard to launch and recover the system in such conditions, while the anchor chain and cable are entangled together. DEOS (Dynamics of Earth and Ocean Systems) [16] provides the design concept of the moored buoy observatory, which is comprised of a cable link and a high-bandwidth observatory employing a discus buoy, where the buoy uses an electro-optic cable to connect the seafloor and moored instruments to the surface. The C-Band satellite telemetry system is utilized in DEOS with continuous high-bandwidth communication (64–128 kbps or higher). The buoy is equipped with a diesel generator to supply enough power (up to 1000 W) for the benthic node. Acoustic communication was selected to construct the link between the surface and seafloor. A solar-powered, low-power satellite telemetry system delivers ~5 Mbyte data to shore. The prototype system of Cyprus-TWERC (Tsunami Warning and Early Response system of Cyprus) and CSnet’s Offshore Communications Backbone (OCB), for up to 3000 m depth, were designed to support a modular architecture which may be linked together with several buoys, as shown in [17–19]. The OCB consists of a buoy, a buoy riser cable (mooring/power and communications), an anchor, a seafloor cable, and four seafloor nodes. Cyprus-TWERC serves as the prototype Tsunami Warning and Early Response system. The surface buoy is equipped with a C-Band satellite antenna and can offer data bandwidth in excess of 2 Mbps. The buoy is powered by a diesel electrical generator which offers 1000 W power to the seafloor. The seafloor junction box is equipped with wet-mateable “plug-in” ports. Cyprus-TWERC can quickly transmit tsunami information to coastal residents and local government departments, such that they can take corresponding preventive measures. GEOSTAR (GEOSTAR is a single-frame autonomous seafloor observatory) was developed as a single-frame seafloor observatory for geophysical and environmental monitoring, which is shown in [20]. This system is able to operate from shallow waters to deep-sea (down to 4000 m). GEOSTAR includes a Bottom Monitoring System, a communication buoy, and a MODUS vehicle (MODUS, a simplified ROV, is the special vehicle for the deployment/recovery procedures.) The MODUS vehicle and hydraulic winch are utilized for the launch and recovery of the Bottom Monitoring System. The communication buoy is equipped with an acoustic hydrophone and a satellite antenna, in order to establish the data transition link from the Bottom Monitoring System to a shore station. A ship controls the MODUS remotely, through the EOM cable. The hydraulic winch launches and recovers the Bottom Monitoring System. GEOSTAR does not utilize wet-mateable “plug-in” for seafloor installation. The MODUS separates from the Bottom Monitoring System after deploying it on the seafloor. Dimitris Konovessis et al. studied the development of relevant laws and regulations for offshore floating structures in recent years, summarizing some representative cases of sexual maritime safety accidents. After analyzing the destabilization evaluation rules of related organizations (e.g., classification societies), the key issues of stability requirements were pointed out and, finally, some reasonable safety methods were proposed for the future development of stability requirements of floating offshore structures [21]. K.P. Thiagarajan and H.J. Dagher reviewed some key studies involving the floating infrastructure of offshore wind turbine systems. These floating foundation structures can be divided into three categories, according to their stability sources; namely, (1) ballast stabilized J. Mar. Sci. Eng. 2020, 8, 672 6 of 25
(low center of gravity; e.g., spar), (2) mooring stabilized (e.g., tension leg platform), and (3) buoyancy or water-plane stabilized (e.g., semi-submersible). Through the demonstration of key research on three types of floating infrastructure, the authors finally showed that floating offshore wind turbine systems require an intelligent combination of land-based systems and offshore engineering expertise, in order to develop sustainable solutions for the future [22]. Agbomerie Charles Odijie et al. (2017) gave a detailed review of the origin and background of semi-submersible hulls, summarized the major advances in the application of semi-submersible hulls in marine engineering, and compared the technical differences of different semisubmersible hulls and their respective advantages and disadvantages. The authors found that the deep-water semisubmersible hull has better motion characteristics and can offer favorable payloads [23]. Haicheng Zhang et al. established a generalized non-linear network dynamics model of floating airports of arbitrary topological form. Then, the dynamic response was analyzed and the numerical results showed that the non-linearity of connectors plays an important role in the dynamic behavior of the system. In addition, a new concept of stability design for floating airports based on the amplitude death mechanism was proposed for the weak oscillation state of the floating airfield module, which provided theoretical guidance for the safety design of floating airports [24]. Chiemela Victor Amaechia et al. studied the load generated by waves and currents on the hose of the CALM (Catenary Anchor Leg Mooring) buoy, and discussed the influence of flow angle and hose hydrodynamic load on the hose structure response. The bending moment and tensile force distributed along the arc length of the hose were analyzed, and the influence of the increase in bending stiffness at the top connection and the bottom contact point were discussed [25]. Chiemela Victor Amaechia et al. also performed numerical stress analysis for offshore composite risers at a depth of 2000 m. The kind of composite pipe was made of carbon fiber, PEEK (Poly ether ether ketone), and so on. The authors used six different lining materials and designed different composite riser configurations—each configuration having 18 floors—and used the ANSYS ACP 19.0 software to develop a finite element model of the stand pipe, according to the strength of the composites used for composite stress distribution in the riser. The Factor of Safety for The Composite Risers for different Load cases was presented, in order to guide Offshore designers of composite Risers [26]. In this paper, the MBOSBC is designed for the seafloor and surface observation of the South China Sea, including (but not limited to) observations of ocean meteorology, ocean physical processes, and water quality of sea surface and seafloor. As the mooring of the buoy is directly related to the survival of MBOSBC, it is extremely important to study the mechanical characteristics of the buoy and the hydrodynamic characteristics of the EOM cable. This is a prerequisite for designing a reasonable mooring scheme. MBOSBC includes a Mooring Buoy and a Benthic Node, these two components being connected by an EOM cable. The Mooring Buoy plays the roles of observation, power generation, and data transmission. The Benthic Node is designed for the requirements of ocean physical processes and seafloor video imaging. On the other hand, the Benthic Node plays the role of anchoring the Mooring Buoy. A cable- and acoustic-linked observatory for mooring buoy-based ocean observation was established. The winch system was designed to launch and recover the MBOSBC. MBOSBC is designed for the seafloor observatories off islands, as well as Ocean regions far from the coast. MBOSBC can be relocated or deployed rapidly for transient or long-term observations, with different combinations of cable link and acoustic link. In particular, the sea surface buoy, as the power generator and data transmitter, has several capabilities for long-term observation and real-time data transmission for deep-sea observations. This report is organized as follows: Firstly, the MBOSBC is briefly introduced and, then, the parameters of buoy and benthic nodes are given. Secondly, the preliminary mooring method of the system is designed. Thirdly, the force of the buoy under wind and waves is calculated. The resultant force of the buoy under wind and wave action is used as the boundary condition of the mooring EOM cable at the upper node. The fourth step is to divide the mooring rope into several micro-sections J. Mar. Sci. Eng. 2020, 8, 672 7 of 25 and analyze the stress of the micro-sections. The hydrodynamic relationship between two adjacent segments is derived. Fifth, an iterative calculation program based on the mechanical relationship between micro-segments is developed, in order to calculate the tension value of each micro-segment.
Sixth,J. Mar. according Sci. Eng. 2020 to, 8, the x FOR calculation PEER REVIEW results in the fifth step, the technical indices of the EOM7cable of 25 are determined and the strength of the EOM cable required in the system is derived. Next, an experiment withwith the the mooring mooring system system is is introduced, introduced, in in order order to to verify verify the the reliability reliability of of the the system. system. Finally, Finally, the the work ofwork this paper of this is paper summarized is summari andzed the and emphasis the emphasis of future of future work work is discussed. is discussed.
2.2. Description Description of of Mooring Mooring BuoysBuoys Observation System System
2.1.2.1. System System Introduction Introduction MBOSBCMBOSBC is isdesigned designed for for the the regional regional observation observation ofthe of coast the coast of the of South the South China China Sea. It Sea. integrates It seafloorintegrates and seafloor sea surface and instruments sea surface forinstru aments broad for spectrum a broad of spectrumenvironmental, of environmental, meteorological, andmeteorological, hydrological and observations. hydrological observation The typess of. The instruments types of instruments include waterinclude temperature, water temperature, pressure, salinity,pressure, current, salinity, wind current, speed wind/direction, speed/direction, air temperature, air temperature, relative humidity, relative barometric humidity, barometric pressure, and chemistrypressure, sensors. and chemistry The seafloor sensors. module The seafloor includes module light, includescamera, CTD, light, andcamera, ADCP CTD sensors,, and ADCP to verify thesensors, power to capacity verify the of MBOSBC.power capacity The overallof MBOSBC. system The architecture overall system is shown architecture in Figure is shown4, including in Figure three modules:4, including the mooringthree modules: buoy, the benthicmooring node, buoy,and the thebenthic EOM node, cable. and The the Mooring EOM cab buoyle. The plays Mooring the roles buoy plays the roles of sea surface observation, data communication, and energy supply. The benthic of sea surface observation, data communication, and energy supply. The benthic node contains the node contains the junction box, the oceanographic instruments, the seafloor video module, and the junction box, the oceanographic instruments, the seafloor video module, and the buoy anchor block. buoy anchor block. The benthic node contains power management, control and data acquisition, and The benthic node contains power management, control and data acquisition, and seafloor junction box seafloor junction box status monitor modules. The EOM cable moors the sea surface buoy by the statusbenthic monitor node. modules. The EOM cable moors the sea surface buoy by the benthic node.
Satellite
Real Time Solar Power Real Time Transmission Transmission
Wind Power
Wireless Transmission Sea Surface Ocean Buoy Acoustic Transmission
Shore Station Floating Ball Data Control and Management EOM Cable
EOM Cable Power Transmission Data Transmission Additional Weight
Benthic Node Gravity anchor
FigureFigure 4.4. ConceptConcept design design of Mooring of MooringBuoys Observation Buoys ObservationSystem for Benthic System with forElectro Benthic-Optical- with Electro-Optical-MechanicalMechanical Cable (MBOSBC). Cable (MBOSBC).
2.2. Design of Mooring Buoys and Benthic Observation Node
2.2.1. Mooring Buoy
J. Mar. Sci. Eng. 2020, 8, 672 8 of 25
2.2. Design of Mooring Buoys and Benthic Observation Node
2.2.1. Mooring Buoy J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 8 of 25 The Mooring Buoy is a rigid structure made of 304 steel, as shown in Figure5. The main diameter of the buoy is 3 m, while the bottom diameter of the buoy is 1.5 m. The weight of the buoy is 4.2 tons. The Mooring Buoy is a rigid structure made of 304 steel, as shown in Figure 5. The main diameter The buoy body is divided into two parts: a watertight tank in the center, with four buoyancy cabins of the buoy is 3 m, while the bottom diameter of the buoy is 1.5 m. The weight of the buoy is 4.2 tons. arranged around it. The electronic instruments are equipped within the watertight tank. The buoy is The buoy body is divided into two parts: a watertight tank in the center, with four buoyancy cabins equipped with 24 rechargeable batteries (2 V 500 Ah each), which are connected to eight solar panels arranged around it. The electronic instruments are equipped within the watertight tank. The buoy is and the wind turbine. Each solar panel is nominal 100 W with dimensions of 1200 mm 540 mm. equipped with 24 rechargeable batteries (2 V 500 Ah each), which are connected to eight solar× panels The buoy is designed to endure such extreme ocean conditions as: and the wind turbine. Each solar panel is nominal 100 W with dimensions of 1200 mm × 540 mm. The buoy workis designed depth: to 1000endure m; such extreme ocean conditions as: ≤ ➢ workwind depth: speed: ≤<100070 m m;/s; ➢ windwave speed: height: <70<15 m/s; m; and ➢ waveocean height: current <15 velocity: m; and< 6 knot. ➢ ocean current velocity: <6 knot.
Figure 5. Mechanical Design of the Mooring Buoy.
2.2.2. Benthic Observation Node The design working depth of the benthic observation nodenode isis 10001000 m.m. The junctionjunction box includes a titanium cylinder for power supply and a titanium cylinder for data communication and and control. control. The weight of the junction box is less than 300 kg,kg, andand itit isis surroundedsurrounded byby aa cage.cage. The junction box hosts several instruments, including a seafloorseafloor video imaging module, CTD, and ADCP ADCP,, as shown in Figure6 6.. The cage has dimensions of 2.25 m 2.25 m, with a height of 2.53 m. The seafloor junction box × and the video module are fixed on the grid of the cage. The diameter of the titanium cylinder of the junction box is 320 mm, with a length of 1060 mm. The DC–DC converter and data sample and transmission module are fixed in the two titanium cylinders separately. The Benthic Observation Node plays the role of the seafloor anchor. The cage is 1 ton, which holds the scientific instruments inside it. The anchor block at the bottom center of the cage is 3 tons, in order to offer enough weight to moor the buoy. The anchor block can be released by the acoustic releaser while recovering MBOSBC.
J. Mar. Sci. Eng. 2020, 8, 672 9 of 25 J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 9 of 25
Figure 6. Mechanical design of the Benthic Observation Node.
2.3. AnalysisThe cage of has Buoy dimensions Load in Wind, of 2.25 Waves, m × and 2.25 Current m, with a height of 2.53 m. The seafloor junction box and theThe video buoy module is subjected are fixe to thed on external the grid force of the load cage. of the The wind, diameter waves, of and the currenttitanium at cylinder sea surface, of the as shownjunction in box Figure is 3207. where mm, with the resultant a length force of 1060 acts mm. on the The mooring DC–DC point converter of the and optical data cable. sample In this and section,transmission the wind module and are wave fixed load in the of two the buoytitanium are cylinders calculated. separately. The buoy is affected by gravity and buoyancy,The Benthic where Observatio the buoy bodyn Node provides plays reservethe role buoyancy.of the seafloor Environmental anchor. The hydrodynamic cage is 1 ton, which forces holds the scientific instruments inside it. The anchor block at the bottom center of the cage is 3 tons, include the wave force Fwave, wind force Fwind, and current force Fcurrent. These forces, under certain in order to offer enough weight to moor the buoy. The anchor block can be released by the acoustic J.boundary Mar. Sci. Eng. conditions, 2020, 8, x FOR were PEER analyzed REVIEW and it was found that the buoy can endurance the max10wind of 25 releaser while recovering MBOSBC. speed Fwind = 70 m/s, max wave height h = 20 m, and max current velocity Fcurrent = 6 kont.
2.3. Analysis of Buoy Load in Wind, Waves, and Current The buoy is subjected to the external forceF loadV of the wind, waves, and current at sea surface, as shown in Figure 7. where the resultant force acwavets on the mooring point of the optical cable. In this section, the wind and wave load of the buoy are calculated. The buoy is affected by gravity and buoyancy, where the buoy Fbodywind provides reserve buoyancy. Environmental hydrodynamic forces include the wave force 퐹푤푎푣푒, windH force 퐹푤𝑖푛푑, and current force 퐹푐푢푟푟푒푛푡. These forces, under certain boundary conditions, wereF analyzedwave and it was found that the buoy can endurance the max wind speed 퐹푤𝑖푛푑 = 70 m/s, max wave height ℎ = 20 m, and max current velocity 퐹푐푢푟푟푒푛푡 = 6 kont. Fcurrent
Fmooring
EOM Cable
FigureFigure 7. 7. ExternalExternal loads loads on on buoys buoys other other than than buoyancy buoyancy and and gravity. gravity.
The net force FBuoy generated by the weight and buoyancy of the buoy is
H FBuoy 0 (1) FBuoy ==V FBuoy W-B 퐻 푉 where 퐹퐵푢표푦 is the horizontal force of buoy and 퐹퐵푢표푦 is the vertical force of the buoy. As the weight 푉 is equal to the buoyancy, 퐹퐵푢표푦 = 0. The wind force 퐹푤𝑖푛푑 of the buoy platform is: 1 F H H CAV 2 wind Fwind 1 wind FWind ===2 (2) FV 0 wind 0 퐻 푉 2 where 퐹푤𝑖푛푑 is the horizontal force of the wind, 퐹푤𝑖푛푑 is the vertical force of the wind, 퐴1 = 4.55 m is the frontal area acreage of the buoy above the buoy waterline, 퐶 = 0.5– 0.8 is the air resistance 2 4 퐻 coefficient; and 휌 = 0.1228 kg ∙ s /m is the air mass density. So 퐹푤𝑖푛푑 = 821.3 kg. The current force 퐹푐푢푟푟푒푛푡 of the buoy platform is: 1 F H H CA V 2 currtent Fcurrtent 2 current FCurrent ===2 (3) FV 0 current 0 2 where 퐶 = 0.2– 0.4 is the water resistance coefficient, 퐴2 = 2.2 m is the frontal area acreage of the 2 4 퐻 buoy below the buoy waterline; and 휌 = 104.49 kg ∙ s /m is the water mass density. So, 퐹푐푢푟푟푒푛푡 = 328.5 kg.
The wave force 퐹푤푎푣푒 of the buoy platform is:
J. Mar. Sci. Eng. 2020, 8, 672 10 of 25
The net force FBuoy generated by the weight and buoyancy of the buoy is FH " # Buoy 0 F = = (1) Buoy FV W B Buoy −
H V where FBuoy is the horizontal force of buoy and FBuoy is the vertical force of the buoy. As the weight is V = equal to the buoyancy, FBuoy 0. The wind force Fwind of the buoy platform is: " # " # " # FH H 1 2 wind Fwind 2 CρA1Vwind FWind = V = = (2) Fwind 0 0
H V = 2 where Fwind is the horizontal force of the wind, Fwind is the vertical force of the wind, A1 4.55 m is the frontal area acreage of the buoy above the buoy waterline, C = 0.5–0.8 is the air resistance coefficient; and ρ = 0.1228 kg s2/m4 is the air mass density. So FH = 821.3 kg. · wind The current force Fcurrent of the buoy platform is:
" H # " H # " 1 2 # Fcurrtent Fcurrtent 2 ρCA2Vcurrent FCurrent = V = = (3) Fcurrent 0 0
2 where C = 0.2–0.4 is the water resistance coefficient, A2 = 2.2 m is the frontal area acreage of the buoy below the buoy waterline; and ρ = 104.49 kg s2/m4 is the water mass density. So, FH = 328.5 kg. · current The wave force Fwave of the buoy platform is:
2 " # 4π2 4π Dth H ρCiHAV01 2 exp 2 Fwave T − T g FWave = V = 2 (4) F 2 4π2 8π Dth wave 0.5ρC A SV exp DV T2 − T2 g where ρ = 104.49 kg s2/m4 is the water mass density, g is the acceleration of gravity, C = 2 is the · iH inertial force coefficient with respect to the horizontal plane, A = 10 m is the max wave amplitude, 3 V01 = 4.6 m is the buoy wet volume, T is the wave period, Dth = 0.81 m is the draught depth, 2 CDV = 0.3 is the drag coefficient with respect to the vertical plane, and SV = 4.55 m is the acreage of the head wave with respect to the vertical plane. When the wave height is 15 m and the wavelength is 300 m, the wave period is: s r 2πλ 2π 300 T = = × = 13.86 s (5) g 9.81 where λ is the length of the wave (λ = 300 m). So, " H # " # Fwave 1772.2 kg FWave = V = (6) Fwave 1463.4 kg
The total force on the mooring EOM cable is the sum of FBuoy, FWave, FWind, and FCurrent, as:
Tmoor = FBuoy + FWind + FWave + FCurrent (7)
Therefore, the total mooring force in scalar form is expressed as: r " H # " # Tmoor 2922.1 kg V 2 H 2 Tmoor = Tmoor = V = = Tmoor + Tmoor = 3268.0 kg (8) k k k Tmoor k k 1463.4 kg k J. Mar. Sci. Eng. 2020, 8, 672 11 of 25
2.4. Design of Mooring System
2.4.1. Mooring Design of System for 1000 m Water Depth The buoy mooring system is mainly composed of the buoy, the mooring cable, and the anchor blocks. The anchor block is installed under the benthic node, supported through the acoustic release device. In the buoy-based Seabed Observation System, oceanographic instruments are used to obtain scientific data from the depths of the ocean. Hydrologic, meteorological, and water quality information are obtained from the seabed and surface nodes. If the system is mooring by a traditional anchor chain, using nylon or polyethylene material for the mooring, energy supply from the buoy to the benthic node cannot be achieved, nor can data be transmitted from the benthic node to the buoy. If an electro-optic-mechanical (EOM) cable is used for mooring, the collected seabed information can be transmitted to the sea surface by using the optical fiber, as well as realizing energy transmission from the buoy to seabed node. Our goal was to moor the system at 1000 m depth with the EOM cable. Firstly, tests and experiments were carried out near the coast, in order to verify the stability of the whole system.
Moreover,J. Mar. Sci. aEng. 1000 2020 m, 8,depth x FOR PEER mooring REVIEW scheme was designed, as shown in Figure8. 12 of 25
Buoy Sea Surface Weight- Bearing Joints Photoelectric L1=550m Converter Box
Float EOM Cable
L2=200m
Additional Weight
L3=250m
Benthic Observation Node SeaBed Anchor Block
FigureFigure 8. 8.S-type S-type mooring mooringin in 10001000 m depth with with 1400 1400 m m Electro Electro-Optical-Mechanical-Optical-Mechanical (EOM (EOM)) cable cable..
TheThe top top of of the the “S “S tether” tether” doesdoes notnot interfere with with ship ship activities, activities, while while the the bottom bottom of the of “S the tether” “S tether” cannotcannot connect connect with with the the seabed. seabed. TheThe systemsystem was was launched launched at at 1000 1000 mm depth depth,, so sowe we have have
l1 l 2 l 3 ldepth (9) l1 + l2 + l3 = ldepth (9)
where 푙푑푒푝푡ℎ = 1000 m is the water depth, 푙1 is the distance from sea surface to the top of the “S where ldepth = 1000 m is the water depth, l1 is the distance from sea surface to the top of the “S tether” tether” shape, 푙2 is the height of the “S tether” shape, and 푙3 is the distance from the bottom of “S shape,tether”l2 is shape the height to the ofseabed. the “S tether” shape, and l3 is the distance from the bottom of “S tether” shape to the seabed. The total length of the EOM cable 푙푡표푡푎푙 = 1400 m, with: The total length of the EOM cable ltotal = 1400 m, with: ldepth+2 l2 = l total (10)
ldepth + 2l2 = ltotal (10) By Equations (9) and (10), the length of l2 can be calculated: l = 200m 2
We selected 푙1 = 500 m and 푙2 = 250 m, to make the top of “S-tether” far from the sea surface. Considering the net weight of the EOM cable, the floats were used to balance the weight generated from the seabed to the top of the “S-tether”.
2.4.2. Mooring Scheme for Nearshore In order to test the data collection and communication ability of the entire system, as well as the power supply of MBOSBC, the system first needed to be tested inshore. Therefore, a mooring scheme with a depth of 20 m near the shore was also designed, as shown in Figure 9.
J. Mar. Sci. Eng. 2020, 8, 672 12 of 25
By Equations (9) and (10), the length of l2 can be calculated:
l2 = 200 m
We selected l1 = 500 m and l2 = 250 m, to make the top of “S-tether” far from the sea surface. Considering the net weight of the EOM cable, the floats were used to balance the weight generated from the seabed to the top of the “S-tether”.
2.4.2. Mooring Scheme for Nearshore In order to test the data collection and communication ability of the entire system, as well as the power supply of MBOSBC, the system first needed to be tested inshore. Therefore, a mooring scheme withJ. Mar. a depthSci. Eng. of2020 20, 8 m, x nearFOR PEER the shoreREVIEW was also designed, as shown in Figure9. 13 of 25 J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 13 of 25 Bouy Sea Surface Bouy Sea Surface Weight- Bearing Joints Weight- Photoelectric Bearing Joints ConverterPhotoelectric Box Converter Box
Depth=20m Depth=20m EOM Cable EOM Cable
Benthic Observation Node ABenthicnchor B Observationlock Node SeaBed Anchor Block SeaBed Figure 9. Mooring design of system in 20 m depth with 60 m EOM cable. Figure 9. Mooring design of system in 20 m depth with 60 m EOM cable. Figure 9. Mooring design of system in 20 m depth with 60 m EOM cable. ForFor thethe convenienceconvenience of of sending sending divers divers to tosalvage salvage the theequipment equipment in case in of case system of system failure, failure, the For the convenience of sending divers to salvage the equipment in case of system failure, the thenear near-shore-shore laying laying depth depth was was set set at at20 20m. m. The The EOM EOM cable cable was was directly directly used used to toconnect connect the the Surface Surface Buoynear- andshore the laying Benthic depth Observation was set at Node, 20 m. in The the EOMform ofcable a catenary. was directly The length used ofto theconnect EOM the cable Surface was Buoy and the Benthic Observation Node, in the form of a catenary. The length of the EOM cable was aboutBuoy 3and times the of Benthic the water Observation depth. Figure Node, 10 in shows the form the ofcable a catenary. and cable The reel length used offor the the EOM inshore cable test. was about 3 times of the water depth. Figure 10 shows the cable and cable reel used for the inshore test. about 3 times of the water depth. Figure 10 shows the cable and cable reel used for the inshore test.
Figure 10. Cable and cable tray for inshore test. FigureFigure 10. 10.Cable Cable andand cablecable tray for insho inshorere test. test. 3. Calculation Method of Mooring Force of the Cable 3. Calculation Method of Mooring Force of the Cable The buoy was moored by the EOM cable, while the EOM cable was moored the buoy by the benthicThe node. buoy At was the moorsameed time, by the EOMcable wascable, affected while theby theEOM ocean cable current was moored force in the the buoywater. by The the buoybenthic and node. the benthic At the samenode time,were theconnected cable was together affected by bythe the cable, ocean such current that theforce force in the added water. to theThe buoybuoy and and the the benthic benthic node node by were the connectedocean environment together bywas the used cable, as thesuch boundary that the conditionsforce added for to the the cablebuoy force and theanalysis. benthic node by the ocean environment was used as the boundary conditions for the cableIn force this section,analysis. the hydrodynamic force of the cable caused by the ocean current is analyzed. For analysisIn this of thesection, hydrodynamic the hydrodynamic force, the force mooring of the cabl cablee caused was divided by the ocean into current several is micro analyzed.-element For segments.analysis ofFirst, the the hydrodynamic force formulae force, of two the adjacent mooring micro cable-segments was divided were obtained. into several Then, micro considering-element thesegments. current First, hydr theodynamic force formulae force received of two adjacent by each micro micro-segments-segment, were the forceobtained. relationship Then, considering between adjacentthe current micro hydr-segmentsodynamic was force derived received and the by iterative each micro calculation-segment, algorithm the force of relationship the hydrodynamic between forceadjacent on the micro EOM-segments cable was was deduc deriveded. Therefore,and the iterative the tension calculation of the EOMalgorithm cable ofcan the be hydrodynamic solved using theforce iterative on the calculation EOM cable algorithm, was deduc selectinged. Therefore, the two the boundary tension ofconditions the EOM as cable the initialcan be conditions solved using at seathe surface iterative and calculation seafloor. algorithm, selecting the two boundary conditions as the initial conditions at sea surface and seafloor.
J. Mar. Sci. Eng. 2020, 8, 672 13 of 25
3. Calculation Method of Mooring Force of the Cable The buoy was moored by the EOM cable, while the EOM cable was moored the buoy by the benthic node. At the same time, the cable was affected by the ocean current force in the water. The buoy and the benthic node were connected together by the cable, such that the force added to the buoy and the benthic node by the ocean environment was used as the boundary conditions for the cable force analysis. In this section, the hydrodynamic force of the cable caused by the ocean current is analyzed. For analysis of the hydrodynamic force, the mooring cable was divided into several micro-element segments. First, the force formulae of two adjacent micro-segments were obtained. Then, considering the current hydrodynamic force received by each micro-segment, the force relationship between adjacent micro-segments was derived and the iterative calculation algorithm of the hydrodynamic force on the EOM cable was deduced. Therefore, the tension of the EOM cable can be solved using the iterative calculation algorithm, selecting the two boundary conditions as the initial conditions at sea J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 14 of 25 surface and seafloor. To facilitate calculation, the following assumptions were made before the EOM cable hydrodynamic To facilitate calculation, the following assumptions were made before the EOM cable force analysis: hydrodynamic force analysis: I. The effect of the ocean current on the cable in the vertical direction is negligible; this means
I. The effectthat the of currentthe ocean was current assumed on tothe be cable a planar in the flow vertical field, direction and the system is negligible; was simplified this means to a that the currenttwo-dimensional was assumed problem; to be a planar flow field, and the system was simplified to a two- II.dimensionalThe current problem; velocity gradually decreased from 6 knots at the surface to 0 at a depth of 600 m, II. The currentas shown velocity in Figure gradually 11. We assumed decreased the velocity from 6 wasknots 0 from at the the surface water depth to 0 at of a 600 depth m to of 1000 600 m. m, as III.shownThe in mooring Figure cable11. We does assumed not transmit the velocity bending was moment 0 from and the torque. water depth of 600 m to 1000 m. III. The mooring cable does not transmit bending moment and torque.
Change of velocity with water depth current current speed 4 Current Speed 3
2
（m/s) 1
0 0 100 200 300 400 500 600 700 800 900 1000 depth(m)
FigureFigure 11. 11. TheThe relationship relationship between between velocityvelocity and and water water depth. depth.
UnderUnder extreme extreme sea sea conditions, conditions, the the mooring mooring system system adoptsadopts the the shape shape of of an an “S” “S mooring” mooring system, system, whichwhich is shown is shown in Figure in Figure 12. 12 We. We selected selected a amicro micro-segment-segment onon the the mooring mooring cable cable to to analyze analyze its tension.its tension. The force analysis of the micro-segment subjected to the ocean current is shown in Figure 13. The force analysis of the micro-segment subjected to the ocean current is shown in Figure 13.
Figure 12. The approximate shape of the mooring line under extreme sea conditions.
J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 14 of 25
To facilitate calculation, the following assumptions were made before the EOM cable hydrodynamic force analysis: I. The effect of the ocean current on the cable in the vertical direction is negligible; this means that the current was assumed to be a planar flow field, and the system was simplified to a two- dimensional problem; II. The current velocity gradually decreased from 6 knots at the surface to 0 at a depth of 600 m, as shown in Figure 11. We assumed the velocity was 0 from the water depth of 600 m to 1000 m. III. The mooring cable does not transmit bending moment and torque.
Change of velocity with water depth current current speed 4 Current Speed 3
2
（m/s) 1
0 0 100 200 300 400 500 600 700 800 900 1000 depth(m)
Figure 11. The relationship between velocity and water depth.
Under extreme sea conditions, the mooring system adopts the shape of an “S” mooring system,
J.which Mar. Sci. is Eng.shown2020 in, 8 ,Figure 672 12. We selected a micro-segment on the mooring cable to analyze its tension.14 of 25 The force analysis of the micro-segment subjected to the ocean current is shown in Figure 13.
J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 15 of 25 FigureFigure 12.12. The approximate shapeshape ofof thethe mooringmooring lineline underunder extremeextreme seasea conditions.conditions.
(a) The speed of the current on the EOM cable (b)Force diagram of EOM cable micro-segment
FigureFigure 13. 13. TheThe hydrodynam hydrodynamicic force force on on an an EOM EOM cable cable micro micro-segment.-segment.
DecomposingDecomposing the the force force of ofthe the current current on the on micro-segment the micro-segment into tangential into tangential and normal and directions, normal directions,according toaccording the hydrodynamic to the hydrodynamic formula, we formula, have: we have:
112 22 1 2 = 1 2 2 FtFt = CDdsVρC t t (πD ds) (Vcurrent cosφ ) = ρ C CDVtπ t D V current current cos cosφ ds , ds (11 (11)) 222 · · · · 2 · · · ,
111 22 1 2 2 Fn = ρCn (D ds) (Vcurrent sin φ) = ρ Cn D Vcurrent22sin φ ds. (12) Fn2 CDdsV n· · · current· sin 2 · CDV n· · current · sin · ds (12) 22. The meanings of the variables in the Equations (12) and (13) and Figure 13 are as follows: The meanings of the variables in the Equations (12) and (13) and Figure 13 are as follows: Ft—Tangential flow force, 퐹푡—Tangential flow force, Fn—Normal flow force, 퐹 —Normal flow force, ρ푛—Density of seawater, 휌—Density of seawater, Ct—Tangential flow resistance coefficient Ct = (0.01–0.03) Cn, 퐶푡—Tangential flow resistance coefficient 퐶푡 = (0.01–0.03) 퐶푛, Cn—Normal flow resistance coefficient, 퐶푛—Normal flow resistance coefficient, D—Diameter of EOM cable, 퐷—Diameter of EOM cable, 푑푠ds—micro-segment—micro-segment length,length, V —Current speed, 푉푐푢푟푟푒푛푡current —Current speed, ∅∅——Micro-sectionMicro-section inclination, inclination, 푃P——NetNet weight of micro micro-section-section in water ( 푃P = 푊WM− 퐵BM)),, 푀 − 푀 푊푀—Weight of Micro-section, and 퐵푀—Buoyancy of Micro-section.
Suppose the tensions at both ends of the micro-segment ds are 푇 and 푇 + 푑푇, and the respective inclination angles are and -d . Then, the static balance equations at both ends of the micro- segment can be expressed as: (T d )cos d P sin T F 0 Tt , (13) d( F P cos ) / T n . (14)
As d is a quantity approaching 0, we have sin dd and cosd 1 , where we have ignored the second-order infinitesimal quantity and, so Equations (14) and (15) can be simplified to: T dT T ( F P sin ) , (15)
J. Mar. Sci. Eng. 2020, 8, 672 15 of 25
WM—Weight of Micro-section, and BM—Buoyancy of Micro-section.
Suppose the tensions at both ends of the micro-segment ds are T and T + dT, and the respective inclination angles are φ and φ d . Then, the static balance equations at both ends of the micro-segment − φ can be expressed as: (T + dT) cos d + P sin φ T Ft = 0, (13) φ − − J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 16 of 25 dφ = (Fn + P cos φ)/T. (14) As d is a quantity approaching 0,d we have( Fsin Pd cosd ) /and T cos d = 1, where we have ignored φ n φ ≈ φ φ (16) the second-order infinitesimal quantity and, so Equations (14) and. (15) can be simplified to: From this, the iterative relationship of the force of each micro-section on the mooring cable can T + dT = T + (F P sin φ), (15) be obtained: −
TTFPdφ = (Fn + P cos φ)sin/T. (16) i1i i ti , (17) From this, the iterative relationship of the force of each micro-section on the mooring cable can be FPT cos / obtained: i 1 ini i i , (18) T + = T + (F P sin φ ), (17) i 1 PWBi ti − i Mi Mi Mi , (19) φ + = φ (F + P cos φ )/T , (18) i 1 i − ni i i where 𝑖 is the number of each microsegment, from top to bottom. PMi = WMi BMi, (19) The inclination of the first micro-section of the− cable can be derived from the force of the cable whereat thei buoyis the end: number of each microsegment, from top to bottom. The inclination of the first micro-section of the cable can be derived from the force of the cable at F the buoy end: cos1 H 1 F (20) 1 HT0 φ1 = cos− .. (20) T0 ByBy subsequent subsequent iterative iterative algorithm, algorithm, the the tension tension of of each each section section of of the the mooring mooring cable cable can can be be computed,computed, in in order order to to obtain obtain the the requirements requirements of of the the EOM EOM cable cable in in the the system. system. TheThe cable cable has has buoyant buoyant floats floats and and counterweight counterweight ballasts ballasts attached attached to some to some micro-sections. micro-sections. The force The analysisforce anal of theseysis ofmicro-segments these micro-segments should take should into account take into the influence account the ofthe influence float and of counterweight. the float and Thecounterweight. hydrodynamic The force hydrodynamic analysis of force the EOM analysis cable of of the the EOM micro-segment cable of the withmicro respect-segment to thewith ballasts respect andto the floats ballasts are shown and floats in Figures are shown 14 and in 15Figures, respectively: 14 and 15, respectively:
FigureFigure 14. 14.Hydrodynamic Hydrodynamic force force analysis analysis of of the the EOM EOM cable cable with with attached attached ballast. ballast.
Figure 15. Hydrodynamic force analysis of the EOM cable with attached float.
J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 16 of 25
d( F P cos ) / T n . (16) From this, the iterative relationship of the force of each micro-section on the mooring cable can be obtained: TTFP sin i1i i ti , (17) FPT cos / i 1 ini i i , (18) PWB Mi Mi Mi , (19) where 𝑖 is the number of each microsegment, from top to bottom. The inclination of the first micro-section of the cable can be derived from the force of the cable at the buoy end: F cos1 H 1 T (20) 0 . By subsequent iterative algorithm, the tension of each section of the mooring cable can be computed, in order to obtain the requirements of the EOM cable in the system. The cable has buoyant floats and counterweight ballasts attached to some micro-sections. The force analysis of these micro-segments should take into account the influence of the float and counterweight. The hydrodynamic force analysis of the EOM cable of the micro-segment with respect to the ballasts and floats are shown in Figures 14 and 15, respectively:
Figure 14. Hydrodynamic force analysis of the EOM cable with attached ballast. J. Mar. Sci. Eng. 2020, 8, 672 16 of 25
Figure 15. Hydrodynamic force analysis of the EOM cable with attached float. Figure 15. Hydrodynamic force analysis of the EOM cable with attached float. The force analysis idea of the micro-segments with floats and counterweights was the same as other micro-segments. Additionally, considering the influence of the buoyancy of the float, the micro-segment force iteration formula with counterweight becomes:
( ) Ti+1 = Ti + Fti (P + G) sin φi − φi+1 = φi (Fni + (P + G) cos φi)/Ti , (21) − P = W B Mi Mi − Mi where G is the net weight in water of the counter weight. The iteration formula of micro-segment force with respect to a buoyant float is changed into:
( ) Ti+1 = Ti + Fti (P BF) sin φi − − φi+1 = φi (Fni + (P BF) cos φi)/Ti (22) − − P = W B Mi Mi − Mi where BF is the net buoyancy of the float in water.
4. Numerical Simulation of Single-Point Mooring Cable According to the above calculation method, a Matlab program was written to iteratively analyze the mooring force in 1000 m water depth. The calculation process is shown in Figure 16. First, we set the initial environmental conditions. Then, we defined the required variables, according to the calculation process. Next, we calculated the horizontal load, vertical load, and comprehensive load of the buoy, according to the wind, wave, and current conditions. Finally, we re-calculated the initial angle of the mooring cable, based on the buoy load. With this iterative algorithm, the initial angle of the cable and the load of the buoy are updated as the initial conditions of the cable force calculation program into the iterative formula, and the force of each section of the cable is iteratively calculated in turn, until the calculation achieves the precision we need, with the mooring force returned as the output of the iterative algorithm. J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 17 of 25
The force analysis idea of the micro-segments with floats and counterweights was the same as other micro-segments. Additionally, considering the influence of the buoyancy of the float, the micro-segment force iteration formula with counterweight becomes:
TTFPGi1i i ti ( + )sin i 1 i FPGTni ( )cos i / i (21) PWBMi Mi Mi , where 퐺 is the net weight in water of the counter weight. The iteration formula of micro-segment force with respect to a buoyant float is changed into:
TTFPBi1i i ti ( F )sin i 1 i FPBTni ( F )cos i / i (22) PWBMi Mi Mi where 퐵퐹 is the net buoyancy of the float in water.
4. Numerical Simulation of Single-Point Mooring Cable According to the above calculation method, a Matlab program was written to iteratively analyze the mooring force in 1000 m water depth. The calculation process is shown in Figure 16. First, we set the initial environmental conditions. Then, we defined the required variables, according to the calculation process. Next, we calculated the horizontal load, vertical load, and comprehensive load of the buoy, according to the wind, wave, and current conditions. Finally, we re-calculated the initial angle of the mooring cable, based on the buoy load. With this iterative algorithm, the initial angle of the cable and the load of the buoy are updated as the initial conditions of the cable force calculation program into the iterative formula, and the force J.of Mar. each Sci. section Eng. 2020 of, 8 the, 672 cable is iteratively calculated in turn, until the calculation achieves the precision17 of 25 we need, with the mooring force returned as the output of the iterative algorithm.
Start
Determine input conditions
Define variables
Calculate buoy force
Calculate the initial cable angle
Iterative calculation Calculate Calculate segmented node force forces
yes Whether the cycle condition is met
no
Output result
End
Figure 16. The iterative algorithm of mooring force calculation. The calculated results of the buoy and cable under varying wind and current speeds are shown in Table1:
Table 1. Mooring forces of buoys and cables with respect to different wind, waves, and currents.
Wind speed (m/s) 0.5 10 20 30 40 50 60 70 Current speed (m/s) 0.1 0.6 1.2 1.8 2.1 2.5 2.8 3.0 Wave height (m) 0.1 1.4 2.9 4.3 5.7 7.1 8.6 10 Buoy horizontal load (kg) 18.1 277.3 630.6 1024.6 1430.4 1892.8 2397.9 2903.8 Buoy vertical load (kg) 0.14 28.7 123.1 270.6 475.5 737.7 1082.3 1463.3 Buoy comprehensive load (kg) 18.1 278.8 642.5 1059.7 1507.4 2031.5 2630.8 3251.7 Cable initial angle (◦) 0.5 5.9 11.0 14.8 18.4 21.3 24.3 26.8 Maximum cable load (kg) 49.6 279.0 646.0 1070.9 1522.6 2054.5 2659.7 3284.3
The mooring force property of the buoy under the wind speed of 10 m/s and the current velocity of 0.6 m/s was selected, as shown in Figure 16. Further, the mooring force property of the buoy under the wind speed of 70 m/s and the current velocity of 3.0 m/s was also selected, as shown in Figure 17. Figure 17 shows the variation of tension value of the EOM cable, from top to bottom, at low velocity. The tension on the EOM cable gradually decreases from top to bottom. The tension value changes greatly in the ‘S’ type segment: In the micro-segment with counterweight, the tension value decreases faster. In the middle of the ‘S’ type segment, the tension value retains the trend of decreasing, but its change range is small. In the micro-segment with float, the tension value in the EOM cable increases, where the change range depends on the buoyancy provided by the float for the EOM cable. The tension at the end of the EOM cable retains a small trend of decreasing. Figure 18 shows the variation of tension value of the EOM cable, from top to bottom, at high current velocity. The tension value on EOM cable increases gradually, at first. Below the position of about 400 m, the tension value gradually decreases. In the ‘S’ section of the EOM cable, the change trend of tension value is the same as that under the conditions of low velocity. The reasons for this change are as follows: J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 18 of 25
Figure 16. The iterative algorithm of mooring force calculation.
The calculated results of the buoy and cable under varying wind and current speeds are shown in Table 1:
Table 1. Mooring forces of buoys and cables with respect to different wind, waves, and currents.
Wind speed (m/s) 0.5 10 20 30 40 50 60 70 Current speed (m/s) 0.1 0.6 1.2 1.8 2.1 2.5 2.8 3.0 Wave height (m) 0.1 1.4 2.9 4.3 5.7 7.1 8.6 10 Buoy horizontal load (kg) 18.1 277.3 630.6 1024.6 1430.4 1892.8 2397.9 2903.8 Buoy vertical load (kg) 0.14 28.7 123.1 270.6 475.5 737.7 1082.3 1463.3 Buoy comprehensive load (kg) 18.1 278.8 642.5 1059.7 1507.4 2031.5 2630.8 3251.7 Cable initial angle ( ) 0.5 5.9 11.0 14.8 18.4 21.3 24.3 26.8 Maximum cable load (kg) 49.6 279.0 646.0 1070.9 1522.6 2054.5 2659.7 3284.3
The mooring force property of the buoy under the wind speed of 10 m/s and the current velocity J.of Mar. 0.6 Sci.m/s Eng. was2020 selected,, 8, 672 as shown in Figure 16. Further, the mooring force property of the buoy18 under of 25 the wind speed of 70 m/s and the current velocity of 3.0 m/s was also selected, as shown in Figure 17.
FigureFigure 17.17. MooringMooring forceforce property property of of the the buoy buoy under under the the wind wind speed speed of of 10 10 m /m/ss and and current current velocity velocity of 0.6of 0.6 m/ s.m/s. J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 19 of 25 Figure 17 shows the variation of tension value of the EOM cable, from top to bottom, at low velocity. The tension on the EOM cable gradually decreases from top to bottom. The tension value changes greatly in the ‘S’ type segment: In the micro-segment with counterweight, the tension value decreases faster. In the middle of the ‘S’ type segment, the tension value retains the trend of decreasing, but its change range is small. In the micro-segment with float, the tension value in the EOM cable increases, where the change range depends on the buoyancy provided by the float for the EOM cable. The tension at the end of the EOM cable retains a small trend of decreasing. Figure 18 shows the variation of tension value of the EOM cable, from top to bottom, at high current velocity. The tension value on EOM cable increases gradually, at first. Below the position of about 400 m, the tension value gradually decreases. In the ‘S’ section of the EOM cable, the change trend of tension value is the same as that under the conditions of low velocity. The reasons for this change are as follows:
FigureFigure 18. 18.Mooring Mooring force force property property of of the the buoy buoy under under the the wind wind speed speed of of 70 70 m/ sm/s and and current current velocity velocity of 3.0of m3.0/s. m/s.
DueDue to to thethe actionaction ofof thethe sea-current,sea-current, thethe buoybuoy isis inin thethe downstreamdownstream directiondirection ofof thethe benthicbenthic node,node, suchsuch thatthat thethe cablecable formsforms anan angleangle ofof inclination.inclination. AsAs shownshown inin thethe rightright ofof FigureFigure 1212,, thethe ooffsetffset buoybuoy producesproduces aa diagonaldiagonal upwardupward forceforce onon thethe cable,cable, whichwhich isis calledcalled T푇,, andand thethe flowingflowing seawaterseawater alsoalso producesproduces aadiagonal diagonal upwardupward forceforce onon thethe cable,cable, whichwhich isis calledcalled F퐹t.푡. However,However, thethe net net weight weight ofof thethe cablecable in in the the water water causes causes the the cable cable to to bear bear a downward a downward force, force, which which is called is calledP. As 푃.T Asis far푇 is greater far greater than thethan other the forces,other forces, the cable the mainlycable mainly bears anbears upward an upward pulling pulling force. force.
InIn the the case case of of low low sea-current sea-current velocity, velocity, the etheffect effect of Ft isof not 퐹푡 asis obviousnot as ob asvious that of asP ,that so the of tension푃, so the in thetension cable in gradually the cable decreases. gradually In decreases. the case of In high the flow case velocity, of high theflow eff velocity,ect of Ft is the greater effect than of 퐹 that푡 is ofgreaterP, so thethan tension that of in 푃 the, so cablethe tension gradually in the increases. cable gradually However, increases the velocity. However, of the the sea-current velocity of becomes the sea- smallercurrent asbecomes the depth sma ofller seawater as the increases, depth of andseawater the eff ectincreases, of Ft becomes and the smaller effect thanof 퐹P푡 .becomes Therefore, smaller the tension than in푃. theTherefore, cable gradually the tension decreases. in the cable In the gradually ‘S’ section decreases. of the cable, In the the ‘ variationS’ section of ofP theis greatercable, the than variation the effect of of푃 Fist, greater caused than by the the influence effect of of퐹푡 the, caused counterweight. by the influence So, the of tension the counterweight.T in the cable So, decreases the tension quickly, 푇 in wherethe cable the degree decreases of the quickly, decrease where depends the on degree the weight of the of decrease the counterweight. depends on the weight of the counterweight.In the micro-segment with floats, the buoyancy of the float causes P to act in the opposite direction, such thatIn the the micro tension-segmentT quickly with increases, floats, the where buoyancy the increase of the depends float causes on the 푃 buoyancy to act in provided the opposite by thedire float.ction, such that the tension 푇 quickly increases, where the increase depends on the buoyancy provided by the float. If the EOM cable is heavier, the pulling force of the cable from the top to the bottom can be reduced greatly. However, considering that the buoy must have some reserve buoyancy, the EOM cable utilized in the system should not be too heavy. For example, if the cable is 1 kg per meter, then the cable with 1000 m length needs the surface buoy to provide 1 ton of buoyancy and, so, the buoy needs a larger reserve buoyancy. Considering the aim of convenience of buoy deployment and recovery, the weight of the cable should limited within a suitable range.
5. EOM Cable Design The difference between an anchor chain and an EOM cable is that when the optical fiber fails, the EOM cable will not work even if the load-bearing aramid fiber is not broken. The difference between an EOM cable and a mooring nylon anchor chain is that the EOM cable has the capacity of transmitting power and data. Therefore, the minimum breaking strength of the EOM cable should be selected to be as large as the maximum breaking load of the mooring nylon chain. It can be found, from the above mooring force calculation, that the maximum tension on the mooring line does not exceed 3500 kg, even under the extreme conditions of wind speed of 70 m/s and water current speed of 3.0 m/s. Therefore, the safety factor was selected as 5 and the breaking strength required for the mooring line was:
J. Mar. Sci. Eng. 2020, 8, 672 19 of 25
If the EOM cable is heavier, the pulling force of the cable from the top to the bottom can be reduced greatly. However, considering that the buoy must have some reserve buoyancy, the EOM cable utilized in the system should not be too heavy. For example, if the cable is 1 kg per meter, then the cable with 1000 m length needs the surface buoy to provide 1 ton of buoyancy and, so, the buoy needs a larger reserve buoyancy. Considering the aim of convenience of buoy deployment and recovery, the weight of the cable should limited within a suitable range.
5. EOM Cable Design The difference between an anchor chain and an EOM cable is that when the optical fiber fails, the EOM cable will not work even if the load-bearing aramid fiber is not broken. The difference between an EOM cable and a mooring nylon anchor chain is that the EOM cable has the capacity of transmitting power and data. Therefore, the minimum breaking strength of the EOM cable should be selected to be as large as the maximum breaking load of the mooring nylon chain. It can be found, from the above mooring force calculation, that the maximum tension on the mooring line does not exceed 3500 kg, even under the extreme conditions of wind speed of 70 m/s and water current speed of 3.0 m/s. Therefore, the safety factor was selected as 5 and the breaking strength requiredJ. Mar. Sci. Eng. for the2020 mooring, 8, x FOR PEER line RE was:VIEW 20 of 25
T = n T 17500 kg (23) Tbreak = n T· cable ≈ 17500 kg (23) break cable Thus, the EOM cable required byby thethe mooringmooring systemsystem mustmust bebe ableable toto withstandwithstand thethe pullpull ofof kg.kg. # The EOM cable is composed of four optical fibers fibers and eight 18 # AWGAWG copper copper wires wires (18 (18 AWG). AWG). The two optical fibersfibers are utilized for data transmission and the eight copper wires are utilized for power transmission. The diameter of the EOM cable is 30 mm, while the breaking strength is 200 kN and its weight in the water is 0 2.0 N/m. and its weight in the water is 0 ± 2.0 N/m. The EOM cable was designed with vectran fibersfibers as the strength member.member. The conductors and optical fibersfibers were in the core andand thethe vectranvectran fibersfibers werewere wrappedwrapped outsideoutside them.them. There were four layers of vectran fiber fiber,, which were reverse spiral braided doubly around the core to reduce the torque of the cable. Polyester Polyester was was selec selectedted as as the the outer outer sheath sheath layer, layer, to to protect protect the the vectran vectran fibers fibers from from water. water. The cable is shown in Figure 1919 andand detaileddetailed informationinformation isis givengiven inin TableTable2 .2.
Figure 19. Appearance of the EOM cable.
Table 2. Properties of the EOM cable.
Cable Properties Unit Nominal Value Diameter mm 30 Air weight kg/m 0.9 ± 0.1 Weight in sea water N/m 0 ± 2.0 Minimum breaking strength kN 200 Maximum working load kN 100 Safe working load kN 50 Working voltage V DC, 1000 Current of each copper conductor A ≤5
The EOM cable had sufficient strength and was verified by tensile test, in order to make sure that it could moor the buoy well. Figure 20 shows the tensile test experiments. The EOM cable was first tested with a rated safe working load of 7 tons and kept for about an hour, after which the EOM cable was tested with a breaking load of 20 tons and kept for about 10 min. The test results showed that the EOM cable could withstand the working load and Minimum breaking strength.
J. Mar. Sci. Eng. 2020, 8, 672 20 of 25
Table 2. Properties of the EOM cable.
Cable Properties Unit Nominal Value Diameter mm 30 Air weight kg/m 0.9 0.1 ± Weight in sea water N/m 0 2.0 ± Minimum breaking strength kN 200 Maximum working load kN 100 Safe working load kN 50 Working voltage V DC, 1000 Current of each copper conductor A 5 ≤
The EOM cable had sufficient strength and was verified by tensile test, in order to make sure that it could moor the buoy well. Figure 20 shows the tensile test experiments. The EOM cable was first tested with a rated safe working load of 7 tons and kept for about an hour, after which the EOM cable was tested with a breaking load of 20 tons and kept for about 10 min. The test results showed that the
J.EOM Mar. Sci. cable Eng. could 2020, 8 withstand, x FOR PEER the REVIEW working load and Minimum breaking strength. 21 of 25
Figure 20. EOM cable tension experiments.
6. Experiments in the Shallow Coast
6.1. Launch of MBOSBC In this this section, section, the the deployment deployment process process of of the the system system is discussed. is discussed. The The process process of the of inshore the inshore test istest as is follows: as follows: First, First, we weinstalled installed the the system system at atthe the wharf wharf and and tested tested the the system system to to e ensurensure it works properly, asas shownshown inin FigureFigure 2121.. Second, we put the buoy into the water with a crane from the wharf and towed the buoy to the launch position, as shown in Figure 2222.. Finally, we utilized the winch to lift the benthic node to the seafloor. seafloor. Af Afterter deploying the benthic benthic node node to to the the seafloor, seafloor, we released the lifting point to separate the lift wire rope from the system, as shown in Figure 23,23, and put the EOM cable into the water.
Figure 21. System testing and installation at the wharf.
Figure 22. Towing the buoy to the area of deployment.
J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 21 of 25
Figure 20. EOM cable tension experiments.
6. Experiments in the Shallow Coast
6.1. Launch of MBOSBC In this section, the deployment process of the system is discussed. The process of the inshore test is as follows: First, we installed the system at the wharf and tested the system to ensure it works properly, as shown in Figure 21. Second, we put the buoy into the water with a crane from the wharf and towed the buoy to the launch position, as shown in Figure 22. Finally, we utilized the winch to lift the benthic node to the seafloor. After deploying the benthic node to the seafloor, we released the liftingJ. Mar. Sci. point Eng. to2020 separate, 8, 672 the lift wire rope from the system, as shown in Figure 23, and put the21 EOM of 25 cable into the water.
Figure 21. System testing and installation at the wharf.
Figure 22. Towing the buoy to the area of deployment. J. Mar. Sci. Eng. 2020, 8, x FOR PEERFigure RE VIEW22. TowingTowing the buoy to the area of deployment. 22 of 25
Figure 23. Launching the benthic node on the seabed.
6.2. Recovery of MBOSBC The systemsystem waswas recoveredrecovered in in the the opposite opposite order. order. First, First, a diver a diver dove dove down down to the to seafloor, the seafloor, in order in orderto hook to thehook lift the point lift point of the of benthic the benthic node node and haul and thehaul wire the ropewire torope the to ship the winchship winch of the of mother the mother ship. Then,ship. Then, the benthic the benthic node was node lifted was with lifted the with ship the winch. ship Lastly,winch. the Lastly, buoy the and buoy EOM and cable EOM were cable recovered were withrecovered an “A” with frame an “A” and frame winch. and winch. After recovery, recovery, it it was was found found that that there there was was no nodamag damagee or fracture or fracture of the of EOM the EOM cable, cable, and there and werethere few were crops few attached crops attached to the seabed to the seabedsupport. support. The appearance The appearance of the EOM of the cable EOM was cable intact, was without intact, scratches, abrasion, folding, or other problems. After recovery, there was less silt on the frame of the benthic node, indicating that the seabed was dominated by sandy bottom. Figure 24 shows the appearance of the Benthic Node and EOM cables, after they were recovered, on the ship.
(a) (b)
Figure 24. Recovery of the Benthic Node and EOM cable: (a) Benthic Node recovered on the ship deck; and (b) appearance of recovered EOM cable.
7. Conclusions and Future Works In this paper, an ocean observation system, MBOSBC, which uses an EOM cable to tether a buoy to a benthic node, was designed. First, the load of the buoy under wind, wave, and current forces was analyzed. The calculation results showed that, under the extreme conditions of wind speed at 70 m/s and current velocity of 3 m/s, about 3.2 T were needed to moor the buoy. Then, the mooring method
J. Mar. Sci. Eng. 2020, 8, x FOR PEER REVIEW 22 of 25
Figure 23. Launching the benthic node on the seabed.
6.2. Recovery of MBOSBC The system was recovered in the opposite order. First, a diver dove down to the seafloor, in order to hook the lift point of the benthic node and haul the wire rope to the ship winch of the mother ship. Then, the benthic node was lifted with the ship winch. Lastly, the buoy and EOM cable were J.recovered Mar. Sci. Eng. with2020 an, 8 ,“A” 672 frame and winch. 22 of 25 After recovery, it was found that there was no damage or fracture of the EOM cable, and there were few crops attached to the seabed support. The appearance of the EOM cable was intact, without without scratches, abrasion, folding, or other problems. After recovery, there was less silt on the frame scratches, abrasion, folding, or other problems. After recovery, there was less silt on the frame of the of the benthic node, indicating that the seabed was dominated by sandy bottom. Figure 24 shows the benthic node, indicating that the seabed was dominated by sandy bottom. Figure 24 shows the appearance of the Benthic Node and EOM cables, after they were recovered, on the ship. appearance of the Benthic Node and EOM cables, after they were recovered, on the ship.
(a) (b)
Figure 24. RecoveryRecovery of the the Benthic Benthic N Nodeode and EOM cable cable:: ( (aa)) Benthic Benthic N Nodeode rec recoveredovered on the ship ship deck; deck; and (b) appearance ofof recoveredrecovered EOMEOM cable.cable.
7. Conclusion Conclusionss and Future Works In this paper, anan ocean observation system,system, MBOSBC,MBOSBC, whichwhich usesuses an EOM cable to tether a buoy to a benthic benthic node, node, was was designed. designed. First, First, the the load load of ofthe the buoy buoy under under wind, wind, wave, wave, and andcurrent current forces forces was wasanalyzed. analyzed. The calculation The calculation results results showed showed that, under that, underthe extreme the extreme conditions conditions of wind of speed wind at speed 70 m/s at 70and m current/s and currentvelocity velocity of 3 m/s, of about 3 m/s, 3.2 about T were 3.2 needed T were neededto moor tothe moor buoy. the Then, buoy. the Then, mooring the mooring method method for 1000 m water depth based on the EOM cable was designed and the tension of the EOM cable was calculated. The calculation results showed that EOM cables need to withstand a maximum force of 3284.3 kg under the extreme conditions of 70 m/s wind speed and 3 m/s current velocity. Under the condition that the safety factor was selected as 5, the strength of EOM cable was higher than kg. After carrying out a 20 ton tensile test on the EOM cable, it was confirmed that the EOM cable had enough strength to tether the buoy. The performance of the EOM cable was verified in a nearshore experiment at a water depth of 20 m. In future work, its performance will be verified in deep water and deployed in waters with a depth of more than 1000 m.
Author Contributions: Conceptualization, S.Z. and J.Y.; methodology, J.Y. and S.Z.; software, J.Y. and Y.X.; validation, W.Y., H.G. and Y.X. and S.Z.; formal analysis, W.Y. And J.Y.; investigation, W.Y., H.G. and Y.X. and S.Z.; resources, S.Z.; data curation, J.Y.; writing—original draft preparation, J.Y.; writing—review and editing, J.Y. and S.Z.; visualization, J.Y., Y.X.; supervision, S.Z. and W.Y.; project administration, S.Z. and H.G.; funding acquisition, S.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by “Natural Science Foundation of China (NSFC), grant number NO.51809255”; and “National Key Research and Development Plan of China NO.2018YFC0307906”; and “China Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDA13030301”; Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences (CAS). And the authors would like to thank reviewers, for their careful work. Conflicts of Interest: The authors declare no conflict of interest. J. Mar. Sci. Eng. 2020, 8, 672 23 of 25
Abbreviations
EOM Electro-Optical-Mechanical MBOSBC Mooring Buoys Observation System for Benthic with Electro-Optical-Mechanical Cable CUMAS Cabled Underwater Module for Acquisition of Seismological data MOOS MBARI Ocean Observatory System DEOS Dynamics of Earth and Ocean Systems OCB Offshore Communications Backbone FBuoy Net force of buoy weight and buoyancy Fwind The force of wind on the buoy Fwave The force of waves on the buoy FCurrent The force of current on the buoy ρ Density of seawater g Acceleration of gravity CiH Inertial force coefficient with respected to the horizontal plane CDV Drag coefficient with respected to the vertical plane A Max wave amplitude V01 Buoy wet volume SV Acreage of head wave with respect to the vertical plane T Wave period Dth Draught depth Tmoor Total mooring force ldepth Water depth l1 Distance from sea surface to the top of “S tether” shape l2 Height of “S tether” shape l3 Distance from the bottom of “S tether” shape to the seabed ltotal The total length of net buoyancy EOM cable Ft Tangential flow force Fn Normal flow force Ct Tangential flow resistance coefficient Cn Normal flow resistance coefficient D Diameter of EOM cable ds Micro-segment length Vcurrent Current speed ∅ Micro-section inclination PM Net weight of micro-section in water WM Weight of Micro-section BM Buoyancy of Micro-section dT Tension change of a micro-segment d∅ The angle change of a micro-segment T The tensions at end of the micro-segment G Net weight in water of the counterweight BF Net buoyancy of the float in water
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