Abstract: With the rapid development of my country's marine industry and deep-sea technology, the requirements for positioning accuracy, stability, and reliability in marine operations such as deep-sea drilling platforms, subsea pipeline and cable laying, crane vessels, and marine salvage and rescue are becoming increasingly stringent. As a result, dynamic positioning systems (DP systems) are being increasingly widely used in marine engineering. A dynamic positioning system uses its control system to drive the ship's propellers to counteract external environmental forces such as wind, waves, and currents, thereby maintaining the ship in a fixed position or along a pre-set track. This paper introduces the definition and classification of dynamic positioning systems, briefly describes their composition, working principle, and key control technologies, and analyzes the technical characteristics of electric propulsion systems. Based on the above, a ship dynamic positioning system with completely independent intellectual property rights, based on a B&R dual-master station and dual-network redundant X20PLC (PCC, programmable computer controller), was developed. The paper focuses on describing the composition and hardware configuration, main human-machine interface, and advantages of this DP system.
introduction
Dynamic Positioning System (DPS) is a marine closed-loop control system that drives the ship's propellers through its control system to counteract the environmental forces such as wind, waves, and currents acting on the ship, thereby keeping the ship at a certain position at the required sea level. DP continuously detects the deviation between the ship's actual position and the target position through the measurement system, and then calculates the amount of thrust required to restore the ship to the target position based on the influence of environmental forces. It then distributes the thrust to each propeller of the ship so that each propeller generates corresponding thrust to overcome the influence of environmental forces such as wind, waves, and currents, so that the ship can stay at a certain position or sail along a preset track [1]. DP is widely used for fixed-point mooring of offshore operating vessels and offshore platforms. It has many advantages such as high positioning accuracy, good flexibility, strong maneuverability, and applicability to various sea conditions, and has received widespread attention [2].
Figure 1 External environmental factors to be considered for ship dynamic positioning
Figure 2 Internal control factors to be considered for ship dynamic positioning
1. Definition and Classification of Dynamic Positioning Systems
Definition of DP system
Initially, in the International Marine Contractors Association (IMCA)'s "Guideline for the Design and Use of Dynamically Positioned Ships," a dynamic positioning system was defined as comprising three parts: power, control, and references. Power can be further divided into power generation, power distribution, and power consumption (propeller system); control refers to the control and management of power, which can be automatic or manual, and also includes a position control system; references refer to sensors related to position, environment, and ship bearing.
Later, the International Maritime Organization (IMO) and the International Association of Marine Contractors (IMCA) defined dynamic positioning (DP) as all the equipment required for a dynamically positioned vessel, including the power system, propulsion system, and dynamic positioning control system.
1.2 Hierarchy of DP Systems
The classification standards for dynamic positioning (DP) mainly consider the reliability and redundancy of the equipment. The purpose is to specify the design standards, required equipment, operating requirements and test procedures of the dynamic positioning system, so as to ensure the safe and reliable operation of DP and avoid damage and hazards to personnel, ships and other equipment during DP operations.
As the reliability requirements for dynamic positioning systems on vessels operating at sea become increasingly stringent, the IMO and classification societies in various countries have imposed very strict requirements on DP systems. In addition to the basic requirement of being able to use both manual and automatic control under various conditions, the following three levels of standards have been established:
(1) Equipment Class 1 (DP1): Location malfunction may occur under single fault conditions.
(2) Equipment Level 2 (DP2): When a single fault occurs in an active component or system such as a generator, thruster, switchboard, or remote control valve, positioning abnormality will not occur. However, when a fault occurs in a static component such as a cable, pipe, or manual valve, positioning abnormality may occur.
(3) Equipment Level 3 (DP3): No single fault will cause positioning failure. The ship dynamic positioning system designed and developed based on B&R dual master station and dual network redundancy PCC belongs to this level.
1.3DP systems are high-barrier applications.
DP systems are not simply for monitoring offshore platforms. Unlike typical SCADA systems, they are high-reliability and high-availability systems with global requirements. Furthermore, they must pass rigorous certifications from internationally authoritative organizations such as IMO, ABS, DNV, IMCA, and Lloyd's Register to be recognized as DP1-3 level. In addition, all DP systems have strict regulations regarding operator qualifications, system documentation, redundant system design, and testing and verification.
Currently, products from major international DP manufacturers all meet the requirements of the IMO and major classification societies. However, fewer than ten companies worldwide can provide DP3-level systems. Their products offer manual, semi-automatic, and automatic operation modes, allowing for independent or simultaneous control of the ship's position and heading. Due to differences in design philosophies and product applications, the configurations of these products vary, but generally include a control console, control and signal processing units, measurement systems, propulsion systems, and networks.
As the largest marine equipment and system research and development institution in China, ZPMC Marine Equipment Research Institute has been actively promoting the independent research and development and application of this technology in order to meet the most stringent challenges from international ship certification bodies and gain global industrial competitiveness.
Based on the successful collaboration with B&R in the AGV and quay crane anti-sway system projects at the digital terminal, the superior performance of the B&R system has earned the trust of ZPMC Marine Equipment Research Institute, which is why B&R systems were directly selected when developing the DP system. The redundant control system (dual CPU, dual network) provided by B&R can directly meet the requirements of DP2. By adding a manual operation mode to the existing system, the highest DP3 level requirements can be met.
2. Composition and Working Principle of Dynamic Positioning System
2.1 Composition of DP System
The ship's DP system mainly consists of three parts: position measurement system, control system, and thrust system, as shown in Figure 3.
Figure 3 Schematic diagram of the composition of a ship dynamic positioning system
(1) Position measurement system
Measurement systems refer to sensor systems that can obtain relevant motion parameters and environmental parameters of ships. They mainly include: acoustic positioning systems, gyrocompasses that provide the ship's heading, DGPS that provides the ship's accurate position, vertical reference sensors that provide the ship's attitude, wind vanes and anemometers that provide wind speed and direction changes, and tensioning cables, etc.
(2) Control system
The control system mainly processes the ship motion information and current environmental information obtained by the measurement system, calculates the control signal of the propeller to control the propeller, so that the dynamically positioned ship can maintain the desired position and heading or sail towards the preset position under the combined action of external forces such as wind, current, and waves and the thrust of the propeller.
(3) Thrust system
The thrust system is the actuator of the dynamic positioning system, comprising the power system and the thruster. The thrust system controls the magnitude and direction of the thrust emitted by the thruster according to commands from the control system, in order to resist interference forces and torques from the external environment.
2.2 Composition and Characteristics of Electric Propulsion Systems
2.2.1 Composition of the electric propulsion system [5]
Electric propulsion systems are a type of thrust system within DP systems. Common marine electric propulsion systems typically consist of the following components: prime mover and generator, power distribution unit, converter, electric motor, propeller, and related control equipment.
(1) Prime mover and generator
A power station consisting of a prime mover, generator, and power distribution equipment provides the necessary electrical energy for the electric propulsion system. The prime mover is typically a diesel engine, but may also be a steam turbine, gas turbine, nuclear power, or fuel cell. Depending on the ship type, the electric propulsion system can be powered by an independent power station or shared with other loads on board via a common power station, commonly referred to as an integrated electric propulsion system.
(2) Power distribution equipment
Power distribution equipment distributes power to the electric propulsion system and provides protection for related equipment and the power grid. Shipboard power distribution systems typically consist of a main switchboard, sub-switches, a motor control center, and an emergency switchboard. Depending on the voltage level and electrical system requirements of the power grid, the main switchboard can use high- or medium-voltage distribution, while the low-voltage switchboard can use different forms such as DC and AC distribution.
(3) Converter
The converter is mainly used to control the speed of the propulsion motor and is one of the core devices of marine electric propulsion.
(4) Electric motor
Electric motors are used to drive propellers. Depending on the requirements, DC motors, AC asynchronous motors, or synchronous motors can be selected. The power of propulsion motors ranges from hundreds of kilowatts to several megawatts. Synchronous motors are usually used in high-power propulsion devices.
(5) Propeller
Propellers are used to convert the driving torque of an electric motor into the power to move a ship. Because electric propulsion systems can easily change the speed and direction of the motor, electric propulsion propellers are usually stationary propellers.
2.2.2 Characteristics of electric propulsion systems [5]
(1) New types of high-efficiency thrusters can be used.
Since electric propulsion uses variable speed drive to change the ship's speed, there is no need to change the structure and shape of the propeller, so a fixed propeller can be used.
(2) Propulsion performance has been significantly improved.
Electric propulsion has a wide range of speed adjustment, fast rotational reversal, and large starting torque, which can not only meet the performance requirements of various ships under different propulsion working conditions, but also greatly improve the survivability of the propulsion system.
(3) Save effective space
The biggest advantage of electric propulsion is that the prime mover and propeller can be arranged separately, which is the biggest difference from direct diesel propulsion. The power unit for electric propulsion is a diesel generator set, which can be flexibly arranged throughout the ship.
(4) Energy saving and environmental protection
Ships using electric propulsion have lower fuel consumption and less waste emissions because their diesel generator sets are always able to operate at full load, with almost no light load conditions. This allows them to maintain optimal efficiency and protect the environment.
2.3 Working principle of the DP system
The Dynamic Positioning (DP) system analyzes and compares the ship's position and heading with desired values based on the ship's motion information and environmental parameters measured by its measurement system. The control system then calculates the thrust based on this deviation and allocates the thrust appropriately. Under the control system's regulation, the ship drives its propellers through its own power, forming a main force capable of withstanding external time-varying environmental loads to counteract the negative effects of wind, waves, and currents, ensuring the ship remains a system with a certain position and orientation. The working principle of the DP system is shown in Figure 4. The dynamic characteristics of a ship at sea are difficult to describe with precise mathematical models, and wind, waves, and currents vary significantly with time and different sea states. When formulating control strategies based on the working mechanism of the Dynamic Positioning System (DPS), both the control accuracy of the DP system itself and the system's response speed and energy consumption must be considered.
Figure 4. Simplified block diagram of the DP system's working principle.
2.4 Key Control Technologies of DP System
The control system is the core of the entire DP system. Its positioning accuracy and speed directly affect the performance of the DP system; in fact, the performance of the control system determines the technical level of the DP system. Currently, the main control technologies used in DP systems include PID control, LQG control, and intelligent control technologies represented by fuzzy control and neural network technology.
(1) PID control
Based on classic PID control technology, this system controls the ship's sway, pitch, and bow roll. The thrust value is calculated from the deviation between position and heading, the thrust distribution logic is determined, and thrust is generated to position the ship. PID technology is mature, widely used, low-cost, and easy to operate; therefore, early ship dynamic positioning systems mostly adopted this technology. However, with the increasing demands for control accuracy and speed in dynamically positioned ships, this technology is no longer sufficient and has been gradually phased out in newly developed systems.
(2) LQG control
Linear Quadratic Gaussian (LQG) control technology, primarily composed of Kalman filtering and optimization control, is widely used in second-generation dynamic positioning (DP) systems. This technology solves the lag problem caused by filtering in positioning control and offers significant improvements in safety, energy efficiency, and robustness compared to PID control. The control accuracy and response speed of LQG control technology generally meet system requirements, making it one of the most commonly used control technologies currently available.
(3) Intelligent control
For some complex systems, it is difficult to establish effective mathematical models and use conventional control theory for quantitative calculation and analysis. Instead, a reasoning and judgment method that combines quantitative and qualitative approaches must be adopted. The purpose is to enable the control device to use wisdom and experience similar to those of humans to guide the solution process.
Here, intelligent control is represented by fuzzy control technology. The quantitative region condition judgment and reasoning form of fuzzy control technology mimics human intelligence and is one of the effective means to handle nonlinear problems and problems with uncertainties in mathematical expressions. Considering the characteristics of the ship dynamic positioning process itself, fuzzy control technology is particularly suitable. Intelligent control technology has advantages such as strong anti-interference ability and fast response speed. According to relevant information, fuzzy control algorithms and DRNN neural networks are already being used in DP3 level DP systems.
3 Technical Solution for Ship Dynamic Positioning System Based on B&R PLC (PCC)
With the rapid development of my country's marine industry, the requirements for the reliability of dynamic positioning of various ships are getting higher and higher. The country's demand for the "ship stabilizing needle" with completely independent intellectual property rights - the highest level of ship dynamic positioning system DP3 is also becoming increasingly urgent. The DP system jointly developed by ZPMC Marine Engineering Group and B&R is launched against this background.
3.1 System Development Principles
(1) It can adapt to harsh marine environments
Products used in offshore engineering projects must be able to withstand the erosion and impact of salt spray, humidity, wind, and waves. B&R Control Systems' entire product line is certified by the demanding GL-DNV classification society and features anti-corrosion coatings specifically designed for long-term offshore applications.
(2) High reliability
The DP system based on B&R X20 PLC (PCC) provides a redundant control scheme, adopts dual CPUs and dual communication networks, has a complete hardware system configuration, and is designed according to the DP3 level, which has the highest reliability requirements.
(3) High system confidentiality
Globally, there are fewer than 10 companies with the capability to design and develop DP3-level systems, and in China, the number is even smaller. Therefore, for users, the choice of control system, its security performance, and its ability to effectively protect their core know-how will be key factors in their selection.
(4) Powerful software functions
The dynamic positioning algorithm of the DP system is highly complex and may also involve intelligent control algorithms, which places high demands on the system's software platform.
(5) Good openness
As a crucial control system for engineering vessels, offshore platforms, and oil drilling platforms, the DP system needs to communicate with other subsystems, including the propulsion system, potentially involving a large amount of data exchange. Therefore, its openness is also highly demanding.
3.2 System Composition and Hardware Configuration
The structure of the B&R DP system is shown in Figure 5. Figure 5 illustrates the main control components and data flow of the DP system.
Figure 5. Schematic diagram of B&R DP system configuration
Figure 6 shows the hardware configuration of the DP system based on B&R X20 PLC (PCC), which is designed according to the requirements of DP3 level. As can be seen from the figure, its main controller and communication network adopt a highly reliable fully redundant architecture.
Figure 6. Hardware configuration of the DP3 system based on B&R X20 PLC (PCC)
3.3 System Human-Machine Interface
ZPMC Offshore Engineering Group's DP system, based on the B&R X20 PLC (PCC), is equipped with a rich set of visual graphical interfaces for human-machine interaction. It boasts powerful and convenient human-machine interaction and operation functions, facilitating the implementation of ship dynamic positioning operations. Figure 7 shows the main interface.
Figure 7 Main Interface
3.4 System Advantages
(1) High reliability resulting from high availability of the system
For a ship's DP system, from a high availability perspective, the system must support redundancy capabilities. This is because in the rough seas at sea, sudden system failures (potential risks such as power outages and communication interruptions) must be automatically switched over by a reliable system to maintain continuous control. Such instantaneous changes pose huge risks to the platform at sea.
B&R's X20 controller supports multiple redundancy modes, including POWERLINK high-speed real-time Ethernet for automatic high-speed switching in case of failure. In addition to the dual-network redundancy system design, it also supports multi-master redundancy mode.
It can be said that B&R's X20 system is a perfect control system that meets the requirements of classification societies, possessing high availability and reliability, open interconnectivity, complex algorithm design, and program security capabilities. This is why ZPMC decided to choose this system. As R&D Manager XXX said, "This is indeed a good choice. Of course, the support from B&R's experienced engineers is also an important factor in the successful development of our system."
(2) Good open interconnection capabilities
Dynamic positioning (DP) systems, as crucial control systems for engineering vessels, offshore platforms, and oil platforms, require communication and extensive data exchange with numerous substations, including thrusters. B&R platforms offer significant advantages in terms of openness.
Communication between the ship's central control center and its various substations is achieved using OPCUA/database. B&R's X20 series PLCs support direct access to each client provided by OPCUA Server.
Marine subsystems typically have industry-specific characteristics, therefore, they are generally developed using dedicated chips. Communication with various subsystems, including the thrusters, is achieved through various fieldbuses. Here, however, the open-source nature of high-speed real-time Ethernet POWERLINK is utilized. ZPMC independently developed POWERLINK slave stations. These slave stations can achieve high-bandwidth data transmission with the master station via POWERLINK, including various operating status information and parameters of the field thrusters. For networks up to 100Mbps, highly dynamic data transmission and response can be easily achieved, and seamless connection with the DP system can be realized.
Because the X20 CPU supports multiple fieldbuses, third-party devices can be connected via a bus controller.
B&R provides the system with multi-level data support capabilities, including real-time communication, third-party fieldbus, and OPCUA, which is also a symbol of the strong openness of B&R systems.
(3) Strong algorithm design capabilities
B&R's AutomationStudio software development platform supports high-level language (C/C++) programming, which is beneficial for customers' software teams to implement the development of complex algorithms.
Considering the complex algorithm design requirements of DP systems, the selected controller must have high-level language development capabilities. B&R's X20 series PLCs (PCCs) are based on an RTOS real-time operating system, have compilation capabilities, and support C/C++ high-level language development. These features reduce the difficulty of algorithm design while also improving the modular encapsulation capabilities of the software.
(4) High program security
Another crucial factor to consider in software design is algorithm encryption, which is especially important for ZPMC, which invests heavily in R&D. Choosing the X20 controller facilitates enhanced algorithm security measures. Due to the use of binary code compilation and additional hardware TG protection modes, programs running on the X20 system are as secure and reliable as if they were secured with two strong locks.
B&R provides users with the following two lines of defense to ensure the security of users' core know-how:
(i) Binary code, cannot be downloaded;
(ii) Technology Guarding.
(5) Powerful simulation modeling capabilities
B&R provides MATLAB/Simulink software, which offers an integrated environment for modeling, simulating, and synthesizing dynamic systems. Within this environment, complex systems can be constructed through simple and intuitive visualization operations, eliminating the need for extensive programming.
Traditional systems often lack simulation modeling capabilities, requiring initial PC modeling followed by porting to a PLC platform, or direct PC control. B&R's X20 series PLCs and AutomationStudio software development platform offer maximum convenience for system development through their simulation modeling capabilities. Furthermore, regardless of whether complex fuzzy control or Kalman filtering algorithms are used in the design, the seamless integration of B&R's AutomationStudio with MATLAB/Simulink allows for easy integration of these algorithms into the PLC (PCC).
Simulation modeling is performed on the entire system's operation process. It determines how each propulsion system should respond to changes in the external environment, which are key factors in ensuring the system's stable operation.
4. Conclusion
In recent years, with the rapid development of my country's shipbuilding and marine engineering, dynamic positioning systems (DPS), as a crucial foundational technology for ship and offshore platform operation, have received considerable attention and national emphasis. Currently, my country has strengthened its research and discussion on ship DPS, with research topics mainly focusing on ship motion models, control models, and control algorithms.
The design of a DP system involves modeling power systems, safety systems, and parameter systems. Its design is similar to that of a functional safety system, and it incorporates the concept of full life cycle management from beginning to end. This includes the entire process of system risk assessment, model development, engineering changes, system operation, and maintenance. Complete evaluation, testing, and verification procedures must be established to ensure the safe operation of the system.
