The Research on the Mobile Drilling Rig for Deep Seabed Shallow Strata

： At present, the drilling rig for deep seabed shallow strata is widely used in the exploration of ocean cobalt-rich crust resources and other fields, which not only can obtain solid core samples at a specific station, but the operation process is relatively safe. This paper mainly presents the structure and mechanism of mobile drilling rig in acquiring the seafloor cores (up to 1.5 m long). Based on the function of the AMESim software, the hydraulic propulsion system model of mobile drilling rig is established, which is the basis and core part of the mobile drilling rig. Moreover, the control methods of closed-loop and PID are respectively used to control the hydraulic propulsion system for simulation analysis. Through the comparison of simulation results, it is found that the PID control method is more convincing in verifying the design rationality of hydraulic propulsion system. In the simulation of the PID-controlled hydraulic propulsion system, the co-simulation technology of AMESim and Matlab/Simulink not only establishes the hydraulic model and control model, but also determines the relevant simulation parameters, which is helpful to improve the system simulation efficiency. In its verification deployment in the South China Sea, the mobile drilling rig has been operated for many times at different depths, and some cores have been successfully obtained. Furthermore, the mobile drilling rig has been used during the 55th Voyage of China Oceanic Scientific Expedition supported by China Ocean Mineral Resources R&D Association. Several sites were explored and a large number of cobalt-rich crust cores were obtained. The powerful theory and sea trails are provided to support for the further research on survey of the abyssal resource.

engineering Consortium, which can core up to 1.5m deep and was hanged in the form of steel cable [8].
In 2003, Changsha Institute of Mining Research developed the first deep-sea shallow drilling prototype in China, which was designed for 4000m depths and can obtain 0.7 m of 56-mm diameter core [9]. Under the sponsorship of the China National High-tech R&D Program, the 20 m seafloor core sampling drill was developed by Hunan University of Science and Technology at the end of 2010, which can drill up to 20-m penetration depth by the way of wire-line coring [10]. Cobalt-rich crusts are mostly distributed on the slopes of seamounts, and the slope is relatively large. These devices, however, cannot choose exploration sites independently, thus severely restricting the comprehensive study of cobalt-rich crusts and nodules. In order to improve survey efficiency, a new method of mobile drilling rig to acquire the core samples of deep sea is proposed. This paper is organized as follows. First, the structure and working principle of the mobile drilling rig are briefly described. Thereafter, the propulsion system is presented, which is at the heart of the mobile drilling rig. Based on the function of the AMESim software, the hydraulic propulsion system model of mobile drilling rig is established. Moreover, the control methods of closed-loop and PID are respectively used to control the hydraulic propulsion system for simulation analysis. Finally, the simulation results based on data analysis are elaborated. The mobile drilling rig was used successfully and a large number of cobalt-rich crust cores were obtained during the 55th Voyage of China Oceanic Scientific Expedition supported by China Ocean Mineral Resources R&D Association. In addition, certain suggestions are proposed for future studies.

The whole frame
The mobile drilling rig for deep seabed shallow strata is necessary equipment for marine geologic and environmental investigation, which is designed for 4500 m depths and can obtain 1.5 m of 56-mm diameter cores. It mainly consists of drilling unit, hydraulic system unit, propulsion unit and so on. As shown in Figure.1, the mobile drilling rig mainly includes overall framework, the lifting device, buoyancy device, power rotator, drill pipe, solenoid valve box, underwater motor, compensator, and propeller. The drilling unit, which is used to acquire the cores, is composed of a drilling frame, a drilling driving device, a double length mechanism of wire rope driven by hydraulic cylinder, and a washing mechanism. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 14 December 2020 doi:10.20944/preprints202012.0345.v1 The drilling frame is installed in the center of the drilling rig, the lower part is fixed with the chassis by bolts, and the upper part is connected with the hanger for load bearing. The power rotator is installed in front of the drill frame, and the propulsion driving cylinder and pulley block are located inside the drill frame. The power rotator consists of the sliding frame, a hydraulic motor, bearing box, water inlet, etc.
Under the control of the hydraulic system, the hydraulic motor has the functions of high speed and slow speed, which can cooperate with different drilling pressures to achieve the best drilling effect. The stroke of drilling driving device can reach 1.5 m through the double length mechanism of wire rope driven by hydraulic cylinder with a maximum stroke of 0.9 m. Through the water pump driven by hydraulic motor, sea water is discharged from the water inlet of the drilling driving device, and then flows through bottom of the drilling hole to realize the functions of drilling slag and cooling the drill bit.
(2) Hydraulic system Each reversing valve and proportional control valve are electromagnetically driven and controlled by an underwater computer control system. When used underwater, the valve pack cover, electronic control interface box and other actuators must be oil filled and compensated using suitable positive pressure compensators. (

3) Propulsion unit
The propulsion unit is mainly composed of four vertical propellers and two parallel propellers and the hydraulic system which drives them to work. In the process of deployment, the compass mounted on the system is used to monitor its attitude in real time. If its attitude changes significantly, the propellers are used to correct the deviation to ensure its stability and reduce the impact of ocean currents. When it is near the sea floor, the mobile drilling rig can move smoothly, and the drilling sites can be selected intuitively and quickly, utilizing the underwater propulsion system and visualization system.

Control system
According to the actual working process, the overall structure of the control system is presented, as shown in Figure.   The mobile drilling rig is equipped with various types of external equipment with 12V and 24V power supply. Each equipment requires independent power supply to ensure that it can be isolated separately without affecting the normal use of other equipment when an external device fails. As shown in Figure   4, the MCU based on STM32F103C8T6 is adopted to network with the communication bus through the RS485. The 4-channels 12V and 8-channel 24V power supply circuits with overload protection function, can supply power to equipment with a rated power of below 30W and 100W. Two leakage detection circuits provide real-time leakage detection for the light compartment. if the flag of watchdog is obtained within the reload time, the watchdog will be updated, otherwise the auxiliary control chip system will be reset. Finally, the serial port of the main control chip interrupts the received data and checks the data frame. The data sent by the auxiliary control chip is stored in the buffer.
When the inquiry instruction is sent by the upper computer, the collected data transmitted by the auxiliary control chip or the inquiry command sent by the upper computer are stored if the instruction is correct, and then, the latest data status is written into the data frame and sent to the host computer through the serial port.
(2) Design of peripheral power control unit software The functions of this software are presented as follows. Firstly, the system is predefined and initialized based on the software. Thereafter, it can receive the data and judge the correctness of the data. If the correct data is received, it will judge whether the data frame ID is the ID of the peripheral power control board, and then the instruction byte of the data frame will be judged, after that, the corresponding IO port is operated according to the status of command byte. If the data frame received is the inquiry instruction, the leakage detection data collected by the system will be sent to the upper computer. In addition, the system will automatically reload if it does not receive the operation instruction from the upper computer again during the time when the independent watchdog reloads the value. Due to the reasonable spatial arrangement of the propeller, the mobile drilling rig can move freely in the near seabed. No matter how the drilling rig moves, there is a corresponding thrust or torque output by the propeller, and the combined force of all thrusters is greater than the resistance of its navigation in the sea.

Working principle of the propulsion system
In order to realize the independent movement with three degrees of freedom, 6 thrusters are selected to be distributed on the mobile drilling rig, among which, two horizontal thrusters mainly control the moving direction, while and four vertical propellers mainly control the vertical lifting and horizontal movement of the mobile drill rig. According to Newton's third law, as the propeller spins, it also applies a reaction force to the mobile drilling rig, causing it to rotate in the opposite direction. In order to avoid the rotation of the rig, two adjacent propellers rotate in opposite directions, as shown in Figure   5.According to the actual process, the mechanical model of the system is presented, as shown in Figure   6.  generation, and precise control. The disadvantage, however, is that this method would make the hydraulic system become complex, thereby increasing costs [11][12][13].
Since the pressure and flow of hydraulic propulsion system vary greatly, the hydraulic propulsion system in this paper adopts the mixed control hydraulic system of pump and valve, combining the advantages of the two method, which can ensure that the hydraulic propulsion system can have a good dynamic response performance. Furthermore, the flow adaptive hydraulic pump is used to make the output flow of system always match the flow required by the hydraulic components, so as to reduce the excess flow and the overflow loss of the hydraulic system, thereby improving the system effectiveness [14].
Constant pressure variable pump is applied to hydraulic system, which can continuously provide hydraulic oil with constant pressure for hydraulic propulsion system, and adjust the output flow of the system according to the flow required by the workload. Since the mobile drilling rig transmits instructions and data through the cable and the console in both directions, electro-hydraulic proportional technology is adopted in the hydraulic propulsion system, in order to accurately control the speed of propellers. The electro-hydraulic proportional pressure reducing valve is used to limit the hydraulic pressure of propeller, while the electro-hydraulic proportional flowrate regulating valve is applied for adjusting oil flow of the branch to satisfy the requirements of the propeller [15][16][17]. According to the actual working process, the hydraulic schematic of a single propeller is presented, as shown in Figure 7.
In order to enable propeller output accurate speed, the real-time speed measured by the speed sensor is uploaded to the console through the cable and compared with the target speed, and then the deviation processed by the control system is fed back to proportional valve for real-time adjustment. According to the actual working principle [18], the simulation model of electro-hydraulic proportional pressure valve is presented, as shown in Figure 8. This proportional pressure valve is the three-position four-way directional control valve, and its specific parameters are presented as follows. The diameter of this proportional valve l is 10mm, the maximum working hydraulic pressure of the oil inlet (P) is 315bar, the maximum allowable flow rate of the oil circuit is 120L/min, and the power voltage is 24V. The  proportional flowrate regulating valve is presented, as shown in Figure 10. Its specific parameters are presented as follows. The diameter of this proportional valve l is 10mm, the maximum allowable flow rate of the oil circuit is 45L/min. Based on the data analysis through AMESim software, the relationship between output flow rate and input voltage is obtained during working process, as shown Fig 11. The simulation analysis results show that the output pressure increases with the increase of the input voltage.
The flow rate is 0L/min when the voltage is less than 3v, and the maximum output flow rate is 45L/min when the voltage is 10v, which is basically consistent with the steady-state flow rate curve. pressure and flow rate of the oil flowing into the propeller is regulated to reach the target speed of rotation.
During the regulating process, the real-time rotational speed detected by the rotary speed sensor is compared with the target rotational speed, and the deviation is then applied to the proportional pressure valve and flowrate regulating valve, so as to adjust the oil pressure and flow rate again. The cycle process is infinite, and finally the rotational speed is infinitely close to the target rotational speed. Based on the above working principle of hydraulic propulsion system, the simulation model is established with the method of closed-loop control, as shown in Figure 13. rotational speed is in fluctuation from the beginning to the end of the simulation, and there is a large amount of over-adjustment. Therefore, we can know that it is difficult to justify the performance of the hydraulic propulsion system in terms of design with the method of closed-loop control. The method of PID control is adopted to simulate the sampling process to deal with the above problem.
The AMESim software is characterized by its ability to use various model libraries in AMESim to design the required system, thus achieving the purpose of simulation [23]. Matlab\Simulink is widely used in many fields due to its powerful numerical processing capability [24]. With the joint simulation technology, AMESim's excellent simulation functions of fluid mechanics can not only be fully utilized, but also the powerful numerical processing function of Matlab\Simulink can be used to achieve the best simulation results [25][26]. The PID controller is to control the proportional (P), integral (I) and differential (D) deviation to meet the control requirements [27][28].
The PID controller is a negative feedback regulation mechanism, composed of operational amplifiers, capacitors and resistors, which is widely used in industrial control systems. As a classic traditional controller, its advantages are mainly in its simple structure, relatively strong robust control, and easy implement in actual operation. The basic theory of PID control is presented that the deviation between real-time data with the target value is transformed into proportional, integral and differential respectively, and then superimposed on an input control object to control its output, which is an infinite loop control process [29].
The mathematical equation for PID control equation can be expressed as: where ( ) is control quantity of controlled object; is proportional factor; is integral time constant; is differential time constant; ( ) is control deviation.
The deviation between the real-time rotational speed and the target rotational speed should be processed by the PID controller and then fed back to the proportional pressure valve and proportional flowrate regulating valve, so that the real-time rotational speed can be closer to the target rotational speed.
According to above working principle, the simulation model is established with the method of PID control, as shown in Figure 15. And the model of control system is shown in the Figure 16. provides basic data for future marine geological science research and resource estimation. The Figure 19 shows underwater photograph of plate curst and cores of plate crust.

Conclusion
In this paper, the mobile drilling rig is proposed to acquire the seafloor core (up to 1.5 m long). Based on the function of the AMESim and Matlab/Simulink software, the hydraulic propulsion system model of mobile drilling rig is established and simulated. Moreover, the control methods of closed-loop and PID are respectively used to control the hydraulic propulsion system for simulation analysis. Through the comparison of simulation results, it is found that the PID control method is more convincing in verifying the design rationality of hydraulic propulsion system, which provides a suitable control method for motion simulation of other freedom degrees for mobile drilling rig. On the basis of theoretical analysis and simulation analysis results, the mobile drilling rig was designed and tested in 2019. In its verification deployment in the South China Sea, the mobile drilling rig was operated for many times at different depths, and some cores had been successfully obtained, which verified the feasibility of this equipment.
Furthermore, the mobile drilling rig was used during the 55th Voyage of China Oceanic Scientific Expedition supported by China Ocean Mineral Resources R&D Association. Several sites were explored and a large number of cobalt-rich crust cores were obtained, which provided powerful support for the further research on survey of the abyssal resource.