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.
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.
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 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.