A Deep-Sea Environment Simulated Test System for Subsea Control Modules, Part A: Prototype and Test

Abstract

This paper proposes a version of the deep-sea environment simulated test system for subsea control modules to solve the problem of incomplete testing systems for electro-hydraulic subsea control modules. Based on the subsea control module test requirements specified in APISTD17F, the test system in this paper is a highly integrated system, including a test hydraulic power unit, a control module test bench, a signal simulator, an electronic test unit, an umbilical simulator, a high-pressure chamber, and an incubator. Firstly, the design indicators of the test system were determined by analyzing the various functions of the subsea control module and its working environment. Secondly, the design scheme for the test system was proposed, and a detailed design was carried out. Finally, a hydro-electrical subsea control module for the Bohai Sea was fully tested with this system, with tests including the qualification test and the factory acceptance test. The test results show that all parts of the test system coordinated well and have achieved the design indicators, and the test system can simulate the working environment and complete a land test. The effectiveness and feasibility of the test system have been verified through the test. By adopting this system, the risk of subsea control module failure can be minimized, laying the foundations for future research and improvement of subsea control module testing equipment.

1. Introduction

As the economy developed, traditional onshore oil energy could no longer meet the growing energy demands. More countries and oil companies thus turned their attention to the deep sea. The production and proven reserves of global submarine oil have steadily increased [1]. After investigation, it was found that a quarter of global oil and 45% of exploitable resources are buried on the seabed [2]. Since 2000, submarine oil fields have accounted for 30% of global oil production. The future of the oil industry thus relies on submarine oil [3].

There are many challenges in extracting submarine oil [4], such as deep-water exploration problems and the reliability of subsea facilities. Subsea production systems are very suitable for developing submarine oil due to their advantages [5]. Among them, the electro-hydraulic control system is the most widely used and is the mainstream control system [6,7]. The subsea control module (SCM) is the brain of the electro-hydraulic control system, and its safety and reliability are critical [8]. It is a highly integrated piece of electromechanical and hydraulic equipment, and research shows that it is the type of equipment that causes the greatest loss in subsea production systems [9]. Once an SCM breaks down, the repair work is hard and expensive. It has been shown that, since the 2010 Gulf of Mexico oil spill, the maintenance cost of a deep-water wellhead has been USD 200,000 per day, with an average maintenance cost of USD 10 million for each piece of maintenance work [10]. The stability of SCMs affects the safe and proper operation of subsea production systems. Therefore, comprehensive land testing of an SCM is of great significance for ensuring the reliability of subsea oil and gas extraction.

The land testing of an SCM requires a reasonable testing process and corresponding testing equipment, which is one of the key technologies for deep-sea oil and gas development [11]. Before the installation of an SCM, a comprehensive land test is required to ensure its safe and reliable operation. However, due to the lack of test equipment, it is difficult to test an SCM before installation. According to the APISTD17F [12] standard, SCM test equipment should include a test hydraulic power unit, a control module test bench, an umbilical simulator, an electronic test unit, and a sensor simulator. The test hydraulic power unit developed by FMC can provide redundant high- and low-pressure oil [13]. It is controlled via a dedicated PLC and has both local and distance control modes. The test hydraulic power unit developed by AKER has a compact structure and can be operated at 20 MPa [14]. It is designed with an external pump, which is easy to maintain. The test hydraulic power unit developed by FMC is equipped with a portable unit and a fixed sliding plate [15]. This improves mobility and durability on land and in the sea. The company also developed a control module test bench; however, this can only test the SCM developed by Weatherford and is not universal. Cameron developed an umbilical power simulation device to simulate the electrical transmission characteristics [16]. At present, there is a lack of research and development on electronic test units and sensor simulators. The research on the above equipment is limited to the individual devices in the testing systems rather than the entire test system.

In addition to the aforementioned suppliers, researchers have also conducted studies relevant to this issue. W. P. Rickey et al. [17] introduce the land integration test of the subsea control system for the Zinc project. This test uses a combination of devices, such as a control system, an umbilical, a manifold, and a Christmas tree, to measure, adjust, and record the operational characteristics of the system. However, no solution to the problems encountered during the test process has been proposed. Wang et al. [9] propose a reliability and safety model for an SCM based on Markov and multi-beta factor models to improve its reliability. The model considers the influence of multiple factors, including the fault detection rate, common cause faults, and failure rate of each module, to evaluate the reliability and safety of the system. Yang et al. [18,19] propose a composite digital-driven fault diagnosis method that combines virtual data and real data and establish a fault diagnosis model based on Bayesian networks to address the fault problem of subsea production systems. The effectiveness of this method is verified using measured data from a certain offshore platform in the South Sea, China. Ge et al. [20] proposed a fault diagnosis method for subsea control systems based on digital twins and an improved digital twin fault diagnosis framework and established a fault diagnosis model based on Bayesian networks. The effectiveness of this method was verified through redundant control systems. Mahmoudi et al. [21] propose an integrity-level analysis based on the typical OREDA database to address the reliability of subsea control systems. Their analysis shows that signal failure is a failure mode that occurs more frequently than other failure modes. The above scholars have all conducted research on fault diagnosis of subsea production systems and how to improve the reliability and safety of these systems. There has been no research on the testing process performed before a subsea production system is launched.

Qiu et al. [22] emphasize the important role of integrated test technology for subsea oil production systems for ensuring the production and safety of offshore oil fields. Chen et al. [23] analyze the shallow water test function and test site classification requirements and introduce the main test content and shallow water test process, using a subsea production system as an example. Liang et al. [24] introduce the factory acceptance test, the system integration test, the spot reception test, etc., as well as the test purpose, test content, test methods, and requirements. Han et al. [25] describe the test requirements for subsea production systems, including the Christmas tree and subsea control systems based on the design standards. Zhang [26] et al. designed a specialized installation tool for subsea control modules to ensure that subsea control modules can be installed smoothly and reliably on subsea equipment. Gong et al. [27] analyzed the working principle of an SCM hydraulic system and the performance of the electro-hydraulic control valve. They proposed a testing method for an SCM hydraulic system, designed a hydraulic testing system, and conducted a simulation analysis. Jia et al. [28] developed testing equipment through research on a subsea production system and an SCM, which included a control module test bench, a test hydraulic power unit, a sensor simulator, and an electronic test unit. This test equipment cannot simulate the length of the umbilical, which is not suitable for the deep-sea environment. Wang [29] et al. used AMESim to establish a venue model and equivalent model for umbilical cable hydraulic pipes and conducted a simulation based on the environmental parameters and some working conditions of a subsea oil and gas field in the South Sea. Li et al. [30,31] apply the similarity principle to create a mathematical model for the hydraulic transmission characteristics of the umbilicals. By analogizing transmission characteristics of the hydraulic with the electric power, an equivalent method for the hydraulic transmission characteristics of the umbilical is proposed. Based on this method, a set of umbilical hydraulic simulation equipment is designed. But the simulation equipment has low accuracy and can only simulate partial pipe diameters. Zhao et al. [32] propose a compact and low-cost piece of simulation equipment that is comparable to real umbilicals in power transmission characteristics and can adjust parameters within a given range. The research on subsea control module testing equipment and testing techniques conducted by the above-mentioned scholars is relatively one-sided, and a complete testing system has not been established, making it impossible to conduct full functional testing of subsea control modules.

This paper proposes a concept of a deep-sea environment simulated test system for SCMs that can be used for the full testing of a deep-sea SCM. The test system consists of a test hydraulic power unit, a control module test bench, an umbilical simulator, an electronic test unit, a sensor simulator, a high-pressure chamber, and an incubator. A prototype of this system was produced, and an SCM for the Bohai Sea was successfully tested with this system.

2. Deep-Sea Environment Simulated Test System for Subsea Control Modules

An SCM is the core of a subsea production system that ensures reliable, safe, and stable oil extraction. Electro-hydraulic subsea production systems are currently the mainstream. A multi-channel electro-hydraulic subsea production system consists of an above-water part and an underwater part, as shown in Figure 1.

The main components of the above-water part are the hydraulic power unit, uninterruptible power supply, electric power unit, master control station and chemical injection unit. The electric power unit provides power for the master control station and the underwater devices, the hydraulic power unit provides hydraulic power for the subsea production system, and the chemical injection unit injects chemicals into the subsea production system. The main components of the underwater part are the SCM, Xmas tree [33], and subsea distribution unit. The SCM is located in the Xmas tree [34].

The SCM communicates with the master control station through the power line carriers, optical fibers, or digital subscriber cables in the umbilical [35]. The SCM sends its internal status and sensor information, as well as the monitored information of other devices, to the master control station. In the meantime, it also receives instructions from the master control station to open or close the internal direction control valve (DCV), to control the valve status of the subsea devices [36]. The design life cycle of SCM is generally 20 years. If the SCM malfunctions, it usually causes the subsea production system to shut down. In severe cases, oil leaks can pose a huge threat to the marine environment. Moreover, its maintenance cost is high, and the replacement is difficult. Therefore, a series of tests need to be carried out on an SCM before it is used in production.

2.1. Subsea Control Module Deep-Sea Environment Simulation Test System

Based on the SCM test requirements specified in APISTD17F, the test system is a highly integrated system, including a test hydraulic power unit, a control module test bench, an umbilical simulator, a sensor simulator, an electronic test unit, a high-pressure chamber, and an incubator, as shown in Figure 2.

The test hydraulic power unit supplies the hydraulic fluid to the control module test bench under the operation pressure and provides hydraulic power to the entire test system. Simultaneously, the test hydraulic power unit can also perform hydraulic flushing for the pipeline. A redundant oil supply is adopted for the control module test bench. The control module test bench mainly simulates the valve actuator on the Xmas tree and can perform a series of tests on the DCV and SCM. The docking plate of the control module test bench is used to simulate the SCM installation base, which can verify the performance of the mechanical and functional interfaces between SCM and the SCM installation base, and also complete a series of mechanical tests. The umbilical simulator can simulate the hydraulic transmission characteristics, with a simulation distance of up to 30 km. The sensor simulator can simulate the sensor signals of Xmas trees, manifolds, and downhole temperature and pressure sensors and send them to SCM. The signal types include 4–20 mA signals, Modbus signals, and Canopen signals, and the signal waveforms include sine waves, cosine waves, sawtooth waves, square waves, triangular waves, etc. The electronic test unit can simulate the master control station, communicate with the SCM, and monitor and control the SCM. The communication methods supported by the electronic test unit include digital subscriber line communication, fiber optic communication, and power line carrier communication. The high-pressure chamber mainly simulates the deep-sea high-pressure environment, and the SCM is placed into the high-pressure chamber for the pressure test, which can verify the sealing performance of the SCM. The incubator simulates the temperature of the deep-sea environment, and the ability of subsea electronic modules in extreme environments can be verified.

The testing system is shown in Figure 3. The SCM is installed in a high-pressure chamber and coupled to electrical and hydraulic connectors when being tested. The hydraulic oil circuit connects the SCM and the test bench; it includes high-pressure oil supply circuits, low-pressure oil supply circuits, and a return oil circuit. The oil supply circuit and return oil circuit of the test hydraulic power unit are connected to the test bench through the umbilical simulator. The sensor simulator and electronic test unit are connected to the SCM through electrical connectors.

2.2. Tests

The test system is mainly used to conduct the qualification test and factory acceptance test for SCMs. The purpose of the qualification test is to verify the performance of an SCM under specified working conditions and its compatibility with the environment. The qualification tests are listed in

Table 1. Qualification tests.

Test Type	Test Description
Qualification test of hydromechanical components	Internal Hydrostatic Pressure Test	External Hydrostatic Pressure Test
	LP System Internal Leakage Test	HP System Internal Leakage Test
	Shuttle Valve Function Test	Solenoid Valve Function Test
	Choke Control Valve Function Test	Valve Latch-In and Drop-Out Test
	Contaminated Fluid Cycle Test	Cycle Test
	Minimum and Maximum Temperature Test
Qualification test of the electrical module	Shock Test	Temperature Test
	Electromagnetic Compatibility Test	Vibration Test
	Electrical Power System and Communication System Sensitivity Test

.

Before an SCM is released from the factory, it must pass full functional testing to demonstrate that its functionality satisfies design specifications and is suitable for production use. The factory acceptance tests are the final tests an SCM is subjected to before it leaves the factory, and they are listed in

Table 2. Factory acceptance tests.

Test Type	Test Content
Hydraulic Test	Solenoid Coil Polarity Test	LP System Standard Pressure Test
	Return System Standard Pressure Test	HP System Standard Pressure Test
	Return Line Compensator Installation	Pressured Interface Checks
	Valve Latch-In and Drop-Out Test	Valve Function Leakage Test
	Choke Control Valve Function Test	Shuttle Valve Function Test
	Internal Pressure Sensor Function Test	Leak Pressure Test
	Solenoid Valve Function Test	Cleanliness Verification
Electrical System and Communication Test	External Analog Sensor Channel Test	Downhole Sensor Interface Test
	External Serial Channel Test (Modbus and Canopen)

.

3. Design of the Deep-Sea Environment Simulated Test System

According to the APISTD17F standard, all test equipment should be compatible with the environment in which it is used. The test system should be able to control and detect subsea production devices and simulate and test all necessary operations.

3.1. Design Requirements

The design requirements of the test system are as follows:

(1)

The test system can simulate the hydraulic power unit, the master control station, the subsea valves, the umbilical, the sensors, and other devices to accomplish the SCM qualification tests and factory acceptance tests.

(2)

It can test different models of SCMs.

(3)

The output pressure can be adjusted, with a high pressure of 0–70 MPa and low pressure of 0–35 MPa.

(4)

It is equipped with 24 low-pressure oil circuits and 6 high-pressure oil circuits.

(5)

It is equipped with two high-pressure oil supply circuits, two low-pressure oil supply circuits, and one backflow circuit.

(6)

The methods of communication with an SCM should include power line carrier communication, fiber optic communication, and digital subscriber line communication.

(7)

It can simulate 48 channels of 4–20 mA signals, 16 channels of Modbus signals, and 4 channels of Canopen signals. The signal type and range can be flexibly configured.

(8)

It can simulate the hydraulic transmission characteristics of 1–30 km umbilicals.

3.2. Functions

The following functions are designed to meet the requirements.

3.2.1. Simulation of the Hydraulic Power Unit

The hydraulic power unit provides hydraulic power for the subsea production system. A test hydraulic source is designed to simulate the hydraulic power unit and provide hydraulic power for the test system. According to the design requirements and testing standards, the main parameters of the test hydraulic power unit are listed in

Table 3. The parameters of the test hydraulic power unit.

Parameter	Value
High-pressure, low-pressure output pressure	0–75 MPa, 0–50 MPa
High-pressure, low-pressure output oil circuits	2, 2
Return oil circuits	2
Hydraulic oil grade	Class 6

.

The schematic diagram of the test hydraulic power unit is shown in Figure 4.

The test hydraulic power unit consists of a filtering unit, a power unit, an oil supply unit, a pressure regulation unit, and an emergency stop pressure relief unit. The principles of low-pressure and high-pressure systems are similar, but the parameters are different. The low-pressure system is taken as an example to introduce the system design.

(1)

The filtering unit

The function of the filtering unit is to ensure the hydraulic oil is clean inside the oil tank. The filtering unit mainly consists of an oil tank, an air filter, a liquid level gauge, a circulating motor, a circulating pump, and a filter. The air filter is installed on the top of the tank to filter out dust and sand particles in the air. The circulation motor, the circulation pump, and the filter form a circuit. The circulation motor and pump extract the hydraulic oil from the tank, filter it, and finally flow back to the tank. The cleanliness of the hydraulic oil inside the tank can be ensured by regularly performing filtration.

(2)

The power unit

The power unit can provide oil to the oil supply unit. To ensure cleanliness, two filters are connected at the outlet of the pump, with a grid of 5 μm and 3 μm in diameter. After the filter, there is a two-port electromagnetic valve, a direct relief valve, and a ball valve. The direct relief valve regulates the outlet pressure of the power unit and plays the role of the constant pressure overflow. The two-port electromagnetic valve is in the pressure relief position when the motor starts, to prevent a sudden pressure jump in the system. After the motor is fully started, the two-port electromagnetic valve is in the cut-off position.

(3)

The oil supply unit

The oil supply unit is mainly composed of a one-way valve, an accumulator, a pressure sensor, and a pressure gauge. The pressure sensor and pressure gauge monitor the pressure of the output oil from the accumulator and then provide feedback to the control system. The internal pressure of the low-pressure accumulator fluctuates within the range of 37–45 MPa. When the pressure sensor detects that the output pressure is higher than 45 MPa, the two-port electromagnetic valve acts, and the power unit begins to relieve pressure. When the pressure is below 37 MPa, the valve closes, and the power unit continues to supply oil to the accumulator. The one-way valve ensures that the oil will not be discharged when the two-port electromagnetic valve releases pressure.

(4)

The pressure regulation unit

The pressure regulation unit controls the output pressure of the system. It consists of a direct relief valve and a pressure-reducing valve, which is connected to a pressure sensor and a pressure gauge. The pressure-reducing valve can adjust the output pressure, and the pressure gauge and the pressure sensor can monitor the adjusted pressure.

(5)

The emergency stop pressure relief unit

The emergency stop pressure relief unit can relieve the pressure in the test hydraulic power unit in the event of a hydraulic system malfunction. It consists of a two-position three-way manual directional valve, a two-position three-way hydraulic control directional valve, and a one-way valve. The operator can switch off the manual directional valve to cut off the external pressure output of the test hydraulic power unit.

3.2.2. Simulation of the Subsea Valves

The subsea valves are generally installed on Xmas trees to control various production pipelines. The opening and closing of valves are controlled by a DCV inside the SCM. To simulate the subsea valves and conduct a series of tests on the DCV, a control module test bench is developed. According to the design requirements, the test bench includes a hydraulic system and a mechanical structure. The parameters of the test bench are listed in

Table 4. The parameters of the control module test bench.

Parameter	Value
High-pressure, low-pressure design pressure	100 MPa, 50 MPa
High-pressure, low-pressure oil working circuits	6, 24
High-pressure, low-pressure oil supply circuits	2, 2
Oil supply method	Redundant oil supply
Return oil circuits	2
Docking plate	Replaceable design

.

(1)

The design of the hydraulic system

To enhance the testing capability and improve test efficiency, the test bench is equipped with 24 low-pressure oil circuits and 6 high-pressure oil circuits. Two SCMs can be tested simultaneously. The schematic diagram is shown in Figure 5.

Each circuit is equipped with a needle valve and a pressure gauge, which are responsible for opening and closing the oil circuit and monitoring the pressure. The L1–L24 circuits are low-pressure circuits, and the H1–H6 circuits are high-pressure oil circuits, which can test the opening and closing functions of the DCV valve. The LP1 and LP2 circuits are low-pressure redundant oil supply circuits, while the HP1 and HP2 circuits are high-pressure redundant oil supply circuits. The redundant oil supply makes the oil supply more reliable and allows for conducting a series of tests on the SCM shuttle valves. The input part is connected to the test hydraulic power unit through a quick connector. The output part of the hydraulic system is connected to the SCM hydraulic system through a threaded joint.

(2)

The design of the mechanical structure

The mechanical structure includes a docking plate, an operation panel, a connector panel, an input panel, a rack, etc., as shown in Figure 6.

All the needle valves and the pressure gauges in the hydraulic system mentioned are integrated into the operation panel. The output panel is integrated with a threaded joint for the hydraulic system output part. The input panel is integrated with a quick conversion connector for the hydraulic system input part. The docking plate is exchangeable and can be used for different models of SCMs. In order to facilitate the transport of the control module test bench, a forklift hole is set at the bottom, and four lifting eyes are set at the top.

3.2.3. Simulation of the Subsea Umbilical

The function of the subsea umbilical is to provide electricity and hydraulic power for the subsea production system, inject chemicals into subsea oil fields, transmit control signals from the master control station to subsea devices, and feed the sensor information from subsea production devices back to the master control station. A simulation device for the hydraulic transmission of umbilical cables is proposed; it is called an umbilical simulator. The parameters of the umbilical simulator are listed in

Table 5. Design parameters of umbilical simulator.

Parameter	Value
High-pressure, low-pressure, return design pressure	70 MPa, 35 MPa, 35 MPa
Simulated pipe diameter	1/2 inch
Simulated distance	1–30 km

.

To simulate the different lengths of the hydraulic lines of the umbilical, the umbilical simulator is modularly designed. Multiple modules for different lengths can be connected in series according to the requirements. The schematic diagram of a single module is shown in Figure 7.

S1 and S2 are needle valves, the upper oil line is the line for the simulation, and the lower oil line is the short path. When it is necessary to use this module, S2 is closed and S1 is opened. The hydraulic fluid passes through the simulation circuit. When the module is not needed, S1 is closed and S2 is opened. The hydraulic fluid passes through the short path.

As shown in Figure 8, the umbilical simulator simulates the hydraulic transmission characteristics of 1–30 km umbilical cables through the free combination of six units: 1 km, 2 km, 2 km, 5 km, 10 km, and 10 km.

3.2.4. Simulation of the Master Control Station

The master control station is the monitoring and control platform for the subsea production system. As a result of instructions sent to the SCM, the subsea valve can be opened or closed. The real-time monitoring of the subsea production environment is achieved as a result of communication with the SCM. According to the design requirements and APTSTD17F standard, an electronic test unit is developed for simulating the master control station. The electronic test unit can simulate the master control station to conduct a series of tests on the SCM. It can also simulate the SCM to test the master control station. The control system of the electronic test unit is shown in Figure 9.

Figure 9a shows the schematic diagram of the electronic test unit, and Figure 9b is a photo of the control system. The electronic test unit is composed of three parts: a display processing unit, a PLC control unit, and a data exchange unit. The display processing unit consists of a monitor and an industrial computer and is used to control and monitor the subsea production system. The data exchange unit is composed of an Ethernet switch, a power line carrier machine, and a digital subscriber line communication module, which is for the data exchange between the electronic test unit and external devices. The PLC control unit is composed of a power module, a PLC controller, and a serial communication module. The power module provides power to the PLC control unit. The PLC controller is the core unit of the control system, which is for the data acquisition, information processing, control output, and other tasks in the system. The serial communication module is a serial heterogeneous device intervention module. It utilizes standard protocols to connect data from third-party devices to the control system, and it can also send data to third-party devices through standard protocols. The PLC control unit is shown in Figure 10.

The Zhejiang Central Control G5 control system is adopted for the PLC control unit, and the UCP unified protocol is adopted for the control system. The PLC control unit can receive management information from the upper layer and communicate with the SCM and the master control station. The signal input module is used to periodically collect real-time information about the master control station and the SCM, and it outputs signals after executing the corresponding algorithm. It can control the master control station and the SCM in real time. There are two sets of serial communication modules in the PLC control unit, both of which use module redundancy. One group supports RS235/485 communication, and the other supports TCP communication. In order to facilitate manual operation and monitoring, the upper computer of the electronic test unit is configured using SCADA, and controlling and monitoring can be carried out through the interface.

3.2.5. Simulation of the Subsea Device Sensors

The sensors of the subsea devices are located in SCMs (internal sensors) and on XTs, manifolds, and flowlines and in downhole conditions (external sensors). The sensors send the collected information to the subsea electronic module inside the SCM. According to the design requirements and the APISTD17F standard, a sensor simulator is proposed to simulate sensor signals of the subsea devices. It is capable of simulating the sensor information inside the SCM. The parameters of the signal simulator are listed in

Table 6. The parameters of the signal simulator.

Parameter	Value
Signal type and quantity	48 channels 4–20 mA, 16 channels Modbus, 4 channels Canopen, 8 channels DO, 8 channels DI
Signal waveform	Triangle, zero start triangle, rectangular, sine, cosine, constant value

.

The sensor simulator principle and its control system are shown in Figure 11.

Figure 11a shows the schematic diagram of the sensor simulator, and Figure 11b shows the corresponding control system. The sensor simulator is composed of three parts: a display processing unit, four PLC control units, and a signal output unit. The display processing unit is composed of a display and an industrial computer, which can output, display, and control the output signal. There are four PLC control units, and each comprises a PLC controller, four RS232/485 modules, six analog output modules, a digital output module, and a terminal module. Figure 12 shows one of the PLC control units. The PLC CPU is a compact programmable controller with network and fieldbus interfaces. It can output Modbus signals. The analog output module is used to output 4–20 mA signals, and the digital output module is used to output digital signals.

The signal output unit consists of different types of terminals that can output different types of signals generated by the signal simulator to the SCM. To facilitate manual operation and monitoring, the upper computer of the signal simulator is configured using SCADA so that the operator can simulate the signals through the interface. The simulation of different signal types can be carried out through the interface.

3.2.6. Simulation of the Pressure Environment

To simulate the pressure environment and verify the sealing performance of the SCM, a high-pressure chamber is used to simulate the deep-sea environment. According to the APISTD17F standard, the parameters of the high-pressure chamber are listed in

Table 7. The design parameters of the high-pressure chamber.

Parameter	Value
Simulated pressure	0–35 MPa
Test medium	Water, seawater, oil
Effective inner diameter	2400 mm
Effective depth	2500 mm

.

The high-pressure chamber is composed of a circulation system, a pressurization system, a cabin, and a subsea monitoring system. The cabin is the core device and is used to place the SCM. A connector is installed on the cabin to facilitate electrical and hydraulic connections between the cabin inside and outside. The circulation system is used to inject the test medium into the cabin. The pressurization system pressurizes the cabin to the test pressure. The subsea monitoring system is used to monitor the test devices.

3.3. Parameters

Figure 13 shows the entire test system. It consists of seven parts: the test hydraulic power unit, the control module test bench, the signal simulator, the electronic test unit, the umbilical simulator, the high-pressure chamber, and the incubator. Each part consists of several subsystems.

The specifications of SCMs that can be tested with the test system are listed in

Table 8. The specifications of SCMs that can be tested.

Parameter	Value
Structural style	Circular or square
Size	Height ≤ 2000 mm, width ≤ 1200 mm
Weight	≤5000 kg
Pressure	High pressure 0–70 MPa, low pressure 0–50 MPa
Circuits	≤28 channels of low-pressure oil working circuits
	≤4 channels of high-pressure oil working circuits
	≤4 channels of low-pressure oil supply circuits
	≤4 channels of high-pressure oil supply circuits
	≤2 channels of oil return circuits
Communication	Power carrier, fiber optic, digital subscriber line
Voltage	220 V~600 VAC
Electrical circuits	≤4 channels
Sensor signals	≤48 channels 4–20 mA, ≤16 channels Modbus, ≤6 Canopen

.

4. Tests

Conducting full functional testing on an SCM allows the prompt identification of potential safety hazards, such as communication faults between the SCM and the master control station, missing sensor signals, DCV valve opening or closing faults, and internal hydraulic pipeline leaks. Eliminating these faults in advance can effectively reduce the rate of failure after the application of an SCM, so its service life can be improved.

An SCM for the Bohai Sea underwent the qualification tests and the factory acceptance tests using this test system. Some of the tests are listed below. The test hydraulic power unit, the control module test bench, the umbilical simulator, the electronic test unit, the sensor simulator, and the high-pressure chamber used in the tests were all developed by us.

4.1. Temperature Test

The temperature test is performed to verify the ability of a subsea electronic module in extreme temperatures. It includes a high-temperature test, a low-temperature test, and a high–low-temperature cycle test. The test processes are listed as follows:

(1)

The subsea electronic module is placed in the incubator, and its external circuits are connected to the sensor simulator and the electronic test unit.

(2)

The temperature of the incubator is adjusted to −18 °C. Then, the subsea electronic module opens, and the temperature cycle test starts.

(3)

The initial temperature is −18 °C for 30 min, and then the temperature is increased to 70 °C at a rate of ≥5 °C/min. After being maintained at 70 °C for 30 min, the temperature is decreased to −18 °C at a rate of ≥5 °C/min. The temperature cycle is performed 10 times, and then full functional testing is conducted.

(4)

The subsea electronic module is powered off, and the temperature is adjusted to 70 °C. The high temperature is maintained for 48 h after the power is turned on. Then, full functional testing is conducted.

(5)

The subsea electronic module is powered off, and the temperature is adjusted to −18 °C. The low temperature is maintained for 48 h at −18 °C after the power is turned on. Then, full functional testing is conducted.

Continuous full functional testing should be conducted during the temperature tests, and no defects are acceptable. The temperature tests are shown in Figure 14.

4.2. External Hydrostatic Pressure Test

The external hydrostatic pressure test is also called the high-pressure chamber test. The purpose of the high-pressure chamber test is to verify the sealing performance of the SCM. The test processes are listed as follows:

(1)

The SCM is installed and fixed in the high-pressure chamber and connected to the external test equipment.

(2)

The high-pressure chamber is closed and filled with water. The pressure inside increases from the ambient pressure to 1.1 times the specified pressure at a rate of at least 24 bar/min, and it is held for 2 h.

(3)

Then, the pressure reduces to the ambient pressure at the rate of at least 36 bar/min.

During the high-pressure chamber test, full functional testing on the SCM is conducted to verify its functionality. The high-pressure chamber test is shown in Figure 15.

4.3. DCV Self-Locking and Unlocking Function Test

The purpose of the DCV self-locking and unlocking function test is to verify that the DCV can self-lock under the operation pressure and maintain the self-locking state and that it can be unlocked below the pilot pressure. The low-pressure system is taken as an example, and the test processes are listed as follows:

(1)

The SCM low-pressure system is pressurized to 35 MPa by the test hydraulic power unit, and the backflow circuit is opened.

(2)

The duration of electromagnetic pulses for all DCVs is set to 1 s, and all DCVs are sequentially opened. The pressure gauge value is observed at the control module test bench to see if it is around 35 MPa.

(3)

After 30 s, the pressure gauge value is observed again to see if it remains unchanged.

(4)

The output pressure of the test hydraulic source system is slowly reduced, and each DCV is closed within the specified range. The pressure when the DCV is closed is recorded.

4.4. SCM Docking Test

During the installation process, the SCM needs to be docked, positioned, and locked on the installation base to ensure the smooth docking and connection of the electro-hydraulic connector. When recycling, the SCM needs to be unlocked from the installation base. So, it is necessary to complete docking, locking, and unlocking tests on land. The SCM test bench docking plate is used to simulate the installation base of the SCM. The test steps are as follows:

(1)

The crane is used to simulate the crane ship in subsea engineering. It lifts the SCM and moves it to the upper part of the guiding mechanism, preparing for docking and positioning.

(2)

The SCM is lowered and aligned with the guiding mechanism to reach the designated position.

(3)

The SCM is lowered down further to the docking position and begins docking on the docking plate. Before docking, the alignment of the electro-hydraulic connector and the position of the positioning pin at the bottom of the SCM should be checked.

(4)

After docking, the SCM is locked onto the control module test bench and powered on for full functional testing.

(5)

The above steps should be repeated five times.

5. Results and Discussion

The test results are as follows:

(1)

The results of the temperature test

The sensor data sent by the signal simulator to the subsea electronic module are read through the electronic test unit, which includes 16 channels of 4–20 mA signals, 6 Canopen signals, and 4 Modbus signals. The Modbus signals and Canopen signals receive data normally, as they are transmitted through communication protocols with an error rate of zero. The 4–20 mA signal is the current signal, and the test results in the extreme temperature conditions are shown in Figure 16. Figure 16a shows the test results in the low-temperature environment. The signal simulator uniformly sends a 12 mA signal to the subsea electronic module. The maximum 4–20 mA signal received by the subsea electronic module is 12.03 mA, the minimum is 11.98 mA, and the maximum error is 0.03 mA. Figure 16b shows the test results in the high-temperature environment. The signal simulator uniformly sends 16 mA to the subsea electronic module. The maximum 4–20 mA signal received by the subsea electronic module is 16.04 mA, the minimum is 15.96 mA, and the maximum error is 0.04 mA. During the tests, the communication between the subsea electronic module and the electronic test unit works well, and the signal from the sensor simulator is received correctly. The errors are all within the allowable acceptance range.

(2)

The results of the external hydrostatic pressure test

Figure 17a shows the stamping curve of the high-pressure chamber. At 1.2 min, the internal pressure is 2.31 MPa. Figure 17b shows the pressure holding curve. During the six-hour pressure holding test, the pressure decreases from 2.31 MPa to 2.25 MPa, indicating that the internal pressure remains basically unchanged. Figure 17c shows the pressure relief curve. The pressure drops from 2.25 MPa to 0.06 MPa within 0.7 min, and the pressure relief is complete. From Figure 17, it can be seen that the pressurization rate, the depressurization rate, and the pressure holding process all meet the test requirements. The appearance of the SCM does not deform, and there is no leakage at the sealing point.

(2)

The results of the DCV self-locking and unlocking function test

The production main valve is tested as an example, and the results are shown in Figure 18. After the production main valve is opened, the system pressure remains at 35 MPa. After 70 s, the pressure remains unchanged. Then, the output pressure of the test hydraulic power unit reduces. When the pressure drops to 7 MPa at 87 s, the production main valve closes, and the pressure behind the valve drops to 0 MPa. It can be seen that the self-locking and unlocking functions of the electromagnetic directional valve in the low-pressure system meet the system requirements.

(4)

The results of the SCM docking test

The docking processes are shown in Figure 19.

The results of the five docking tests are listed in

Table 9. The results of the five docking tests.

Number	Docking Time (min)	Jamming (Yes/No)	Hydraulic Connectors All Docked (Yes/No)	Electrical Connectors All Docked (Yes/No)	Docking in Place (Yes/No)	Hydraulic/Electrical Functions (Yes/No)
1	6.6	No	Yes	Yes	Yes	Yes
2	7.2	No	Yes	Yes	Yes	Yes
3	7.6	No	Yes	Yes	Yes	Yes
4	7.5	No	Yes	Yes	Yes	Yes
5	7.9	No	Yes	Yes	Yes	Yes

. The entire docking process takes an average of 7 min. The hydraulic and electrical connectors are all correctly docked. There is no oil leakage on the joint surface of the hydraulic connectors. After the power is turned on, the hydraulic and motor functions of SCM are all tested. The SCM locking and unlocking functions perform properly. The SCM locking and unlocking functions are normal and have passed full functional testing.

Through the qualification tests and the factory acceptance tests, it is found that the deep-sea environment simulated test system can simulate the working environment of the SCM. The test hydraulic power unit has stable output pressure, convenient pressure regulation, and low energy consumption and can meet the test requirements. The docking plate of the control module test bench can be flexibly replaced and can be adapted to different types of SCMs. Its hydraulic supply is redundant, with 24 low-pressure oil circuits and 6 high-pressure oil circuits. The pressure of each oil circuit can be detected, and two SCMs can be tested at the same time. The signal simulator works well. The electronic test unit can simulate SCM and the master control station well, and it has the functions of optical fiber communication, power line carrier communication, and digital subscriber line communication. The umbilical simulator has a modular design and can simulate the hydraulic transmission characteristics of a 1–30 km umbilical cord. The high-pressure chamber can simulate the deep-sea pressure environment, and the sealing performance of the SCM can be tested.

6. Conclusions

(1)

This paper studied an electro-hydraulic composite subsea control module and its working environment. In combination with the APISTD17F standard, a deep-sea environment simulated test system for the subsea control module has been developed. The test system is highly integrated and includes a test hydraulic power unit, a control module test bench, a signal simulator, an electronic test unit, an umbilical simulator, a high-pressure chamber, and an incubator.

(2)

This test system can simulate the hydraulic power unit, the subsea valves, the master control station, the subsea umbilical, and the subsea device sensors. It can also simulate the deep-sea high-pressure environment and temperature, and it can perform full functional testing on the subsea control module before it is launched for operation.

(3)

With the prototype of the deep-sea environment simulated test system, the qualification tests and the factory acceptance tests were conducted successfully, and all indicators were normal. The feasibility and capability of the test plan and the test system have been verified.

Further work for this deep-sea environment simulated test system includes the reliability and safety analysis of the hydraulic power unit (Part B), an analysis of the hydraulic transmission characteristics of the umbilical simulator (Part C), and the optimization of the design of the hydraulic pipeline for control module test bench (Part D).