Displacement Energy Storage Control Method for Power Positioning Offshore Platform Based on Electric Propulsion

Abstract

A displacement energy storage control method for a dynamic positioning ocean platform based on electric propulsion. To address the problems that it is difficult to characterize the quantitative relationships between the displacement deviation of dynamic positioning and the displacement energy storage power, a displacement energy storage model for the dynamic positioning ocean platform is constructed according to the electro-mechanical energy conversion relationship of the ocean platform's dynamic positioning. To address the problem that the response delay of the DP controller makes it difficult for the displacement energy storage to quickly follow the load power fluctuations, a displacement energy storage compensation torque model is constructed. Then, based on the variable frequency control of the propulsion motor and the constructed models, system frequency-displacement energy storage-electric propulsion torque control is carried out to ensure that the displacement energy storage responds to the changes in the operating load power on the electro-mechanical time scale, improving the stability of the power system. Without adding additional equipment, the displacement energy storage directly realizes rapid power regulation through DP control while taking into account the dynamic response of the system frequency, and has a smoothing effect on the power fluctuations generated under different sea conditions and working conditions of the ocean platform's operating load.

Description

Technical Field

[0001] The present invention relates to a technology in the field of ship power control, specifically a displacement energy storage control method for a dynamic positioning ocean platform based on electric propulsion. Background Art

[0002] In order to alleviate the power fluctuation of an ocean platform in high sea states, a large number of energy storage devices need to be added in the existing hybrid energy storage technology, which increases the weight and economic cost of the platform and cannot take into account the frequency regulation of the AC bus side. The existing idea of using the inertia of the ocean platform itself as power station energy storage cannot actively regulate the magnitude of energy storage response according to real-time power fluctuations and ignores the control delay problem of the dynamic positioning system, etc. Summary of the Invention

[0003] Aiming at the problems that the existing technology cannot improve the stability of the ship power system, does not consider the instantaneous charge and discharge under transitional conditions, maintain long-term continuous power supply, and the power fluctuation problems generated when the operating load conditions and sea states of the ocean platform change, the present invention proposes a displacement energy storage control method for a dynamic positioning ocean platform based on electric propulsion. Combining the dynamic characteristics of the ocean platform, by establishing a dynamic positioning system and a three-degree-of-freedom kinematic model of the ocean platform, the quantitative relationship between the platform displacement deviation and the displacement energy storage power is obtained, and a control method for using displacement energy storage to respond to the power fluctuation of the power system is proposed. Aiming at the problem of DP control response delay, the present invention establishes a system frequency - dynamic positioning ocean platform displacement energy storage - electric propulsion compensation torque response model to ensure that the displacement energy storage responds to the change of the operating load power on the electromechanical time scale and improves the stability of the power system. Without adding additional equipment, the displacement energy storage directly realizes rapid power regulation through DP control while taking into account the dynamic response of the system frequency, and has a smoothing effect on the power fluctuation generated when the operating load sea state and working conditions of the ocean platform change.

[0004] The present invention is realized through the following technical solutions:

[0005] The present invention relates to a displacement energy storage control method for a dynamic positioning ocean platform based on electric propulsion. Aiming at the problem that it is difficult to describe the quantitative relationship between the dynamic positioning displacement deviation and the displacement energy storage power, according to the electromechanical energy conversion relationship of the dynamic positioning of the ocean platform, a displacement energy storage model of the dynamic positioning ocean platform is constructed; aiming at the problem that the displacement energy storage is difficult to quickly follow the load power fluctuation due to the response delay of the DP controller, a displacement energy storage compensation torque model is constructed; based on the variable frequency control of the propulsion motor and the constructed models, system frequency - displacement energy storage - electric propulsion torque control is carried out.

[0006] Technical Effects

[0007] The present invention adjusts the displacement energy storage by frequency droop control, and can orderly suppress the complex power impact of the operation load of an offshore platform. This method does not require additional equipment. The displacement energy storage directly realizes fast power regulation through DP control while taking into account the dynamic response of the system frequency, and has a suppressing effect on the power fluctuations generated under the sea conditions and working conditions of the operation load of the offshore platform. Compared with the prior art, the present invention provides displacement energy storage for the DP offshore platform power system to quickly suppress power fluctuations. The present invention realizes the instantaneous power of the displacement energy storage provided by a 12MW dynamic positioning system, which has the same response effect as the hybrid energy storage array composed of a lithium iron phosphate battery with a rated capacity / rated power of 0.37MWh / 1.85MW and a supercapacitor with a rated capacitance / rated power of 0.29MWh / 0.43MW. On the premise of meeting the floating stability of the offshore platform and without introducing additional energy storage equipment, the offshore platform can release displacement energy storage through dynamic positioning control, while taking into account frequency dynamic regulation, reducing system frequency fluctuations, and effectively improving the stability of the offshore platform power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is the schematic diagram of the principle of the present invention;

[0009] Figure 2 is the schematic diagram of the power system of the dynamic positioning offshore platform in the embodiment;

[0010] Figure 3 is the flow chart of the embodiment;

[0011] Figure 4 is the schematic diagram of the electric propulsion control system considering displacement energy storage;

[0012] Figure 5 is the schematic diagram of the power curve of the displacement energy storage;

[0013] Figures 6 - 8 is the schematic diagram of the power suppression effect of the displacement energy storage in Scenarios 1-3 of the embodiment;

[0014] Figure 9 is the schematic diagram of the power suppression effect of the displacement energy storage and the hybrid energy storage with different capacity configurations. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0015] As Figure 1As shown in the figure, this embodiment relates to a schematic principle of a displacement energy storage control method for a dynamic positioning offshore platform based on electric propulsion, where: The dynamic positioning offshore platform calculates the required thrust of the thruster when reaching the expected position by using a DP controller according to the displacement deviation relationship between the target position and the actual position feedback by the measurement system, and feeds the thrust command back to the propulsion system. By controlling the magnitude of the thrust received by the dynamic positioning offshore platform, the rotational speed of the electric propulsion motor is changed. The propulsion system converts electrical energy into mechanical energy and then into kinetic energy, and finally forms a displacement. The electrical energy during the process is stored in the form of the platform's potential energy.

[0016] As Figure 2 shown, the power system of the dynamic positioning offshore platform described above includes: a generator set, an inverter, a propulsion motor, a dynamic positioning control system, a dynamic positioning measurement system, and a dynamic positioning propulsion system, where: The generator set converts the chemical energy of fuel into electrical energy and provides power to the entire offshore platform in the form of electrical energy; The inverter converts the alternating current of the offshore platform power station into alternating current with adjustable voltage frequency and amplitude; The propulsion motor is directly connected to the inverter side and drives the thruster to rotate through the shafting, enabling the power to be transferred from the network side to the thruster, completing the electromechanical conversion process of the dynamic positioning propulsion system; The dynamic positioning measurement system feeds back the displacement deviation relationship between the target position and the actual position of the offshore platform; The dynamic positioning control system calculates the required thrust of the thruster when the offshore platform reaches the expected position by using a DP controller, and feeds the thrust command back to the propulsion system. By controlling the magnitude of the thrust received by the offshore platform, the rotational speed of the electric propulsion motor is changed; The dynamic positioning propulsion system converts electrical energy into mechanical energy and then into the kinetic energy of the offshore platform.

[0017] As Figure 3 shown, this embodiment relates to a displacement energy storage control method for a dynamic positioning offshore platform based on electric propulsion, which specifically includes:

[0018] Step 1: Monitor the bus frequency information through the dynamic positioning displacement energy storage control model monitoring system of the offshore platform;

[0019] Step 2: Determine whether the system frequency or the frequency change rate exceeds the limit. When the frequency or the frequency change rate exceeds the limit (i.e., f0 > 60.15 Hz or f0 < 59.85 Hz or |df / dt| > 0.2 Hz / s), the displacement energy storage of the offshore platform enters the pre-participation power smoothing state;

[0020] Step 3: Determine whether the displacement deviation and the displacement deviation limit duration are within the constraints, that is, when the dynamic positioning displacement deviation of the offshore platform is less than 0.5 m and the displacement deviation limit duration is less than 0.5 s, the displacement energy storage of the offshore platform participates in power smoothing;

[0021] Step 4: Filter the power fluctuations of different frequency bands according to the selected low-pass filter time constant, and the low-frequency load power fluctuations are suppressed by the displacement energy storage of the offshore platform;

[0022] Step 5: Determine the displacement energy storage compensation torque under the conditions that satisfy the constraints of Step 2 and Step 3

[0023] Step 6: Judge whether the propulsion motor is within the limit range, that is, T min ≤T≤T max where: T min =-560 kN,

[0024] T max =560 kN. Implement the displacement energy storage control of the dynamic positioning offshore platform based on electric propulsion, specifically including:

[0025] Step a: When the power of other operating loads changes, monitor the real-time deviation Δω = ω g，ref -ω g，mes ;

[0026] Step b: Introduce a first-order low-pass filter before the dynamic positioning controller to achieve bandwidth separation between the inner and outer control loops and avoid mutual interference between the inner and outer control loops of the propulsion motor. Since the response time of the DC voltage controller is greater than the response times of the VOC control and FOC control in the frequency converter, the control bandwidth B afe of the DC voltage controller is usually used as the inner-loop control bandwidth of the propulsion motor frequency converter. Determine the control bandwidth B afe through the DC voltage controller model, and select the reciprocal of the bandwidth as the time constant of the first-order low-pass filter. When the power of other operating loads on the offshore platform power station fluctuates, the introduction of the first-order low-pass filter can also effectively improve the anti-interference ability of the droop control.

[0027] Step c: With the help of droop control, adjust the power magnitude of the displacement energy storage of the dynamic positioning offshore platform according to the frequency deviation e ω , specifically: P sup，ref =K sup,ref (ω ω -ω g,ref -ω g,mes ). Steps b and c together form the droop control link;

[0028] Step d: Determine the real-time compensation torque τ of the propulsion motor according to the displacement dynamic positioning offshore platform displacement energy storage model established by combining the offshore platform dynamics, kinematics equations and dynamic positioning control system, specifically sup as Step d is the displacement energy storage control link.

[0029] Step e: Combine the real-time reference torque τ of the propulsion motor ref and the real-time compensation torque τ of the propulsion motor obtained in step d sup , and use it as the FOC input reference torque to achieve dynamic regulation of the system power fluctuation and frequency.

[0030] Therefore, the outer-loop control transfer function of the dynamic positioning electric propulsion of the offshore platform considering displacement energy storage shown in steps a to e is as follows:

[0031] Step 7: Determine whether the frequency has recovered to the limit value. If it has recovered to the limit value, the displacement energy storage control exits. Otherwise, repeat the above steps.

[0032] As Figure 4 shown, the displacement energy storage control system of the dynamic positioning offshore platform for implementing the above method according to this embodiment includes: an electric propulsion variable frequency control unit and an outer-loop module of the displacement energy storage control for the dynamic positioning of the offshore platform composed of a droop control unit and a displacement energy storage control unit, where: ω g,ref 、ω g,mes 、e ω are the system reference frequency, real-time frequency, and frequency deviation respectively, τ ref is the rated torque of the propulsion motor, τ sup is the displacement energy storage compensation torque, τ ref is the real-time reference torque of the propulsion motor, C dc is the DC-side capacitor, Q g,ref is the reference reactive power, V dc,ref is the reference DC voltage, t L is the load torque, l m,ref is the reference flux linkage, v g,abc 、i g,abc are the system grid-side AC voltage and current respectively, i m,abc is the inverter-side AC current, v abc,ref 、v m,abc,ref are the system grid-side and propulsion motor-side reference AC voltages respectively, ω r is the rotational angular frequency of the propulsion motor.

[0033] The electric propulsion variable frequency control unit adopts a vector control strategy and includes a frequency converter composed of two back-to-back three-phase inverters. Among them: the first frequency converter is used as an active front-end rectifier and uses a voltage vector orientation control strategy (VOC) to control the DC voltage v dc ; the second frequency converter is used as a torque control frequency drive and adopts a magnetic field vector orientation control strategy (FOC) to control the output torque t m; The VOC control adopts current and voltage double closed-loop control, which has good steady-state performance and is easy to design parameters. The FOC control realizes the decoupling of the excitation component and torque component of the stator current, ensuring that the propulsion motor has good speed regulation performance.

[0034] Simulations and tests are carried out on the RT-LAB hardware-in-the-loop platform, including a computer, an RL-LAB OP5700 hardware simulation platform, a Tektronix MSO44 oscilloscope, Speedgoat baseline control devices, and a controller sampling IO397 module. A dynamic positioning ocean platform system model is built in the real-time simulator, and the parameters are shown in Table 1. The power system architecture of the dynamic positioning ocean platform is as Figure 2 shown, consisting of a diesel generator, an inverter, a propulsion motor, and other operating loads. A displacement energy storage control model for the dynamic positioning ocean platform based on electric propulsion is established, and the control strategy hardware simulation is realized using the Speedgoat baseline controller. The frequency sampling is achieved by connecting the RT-LAB AO board through the IO397 module. The electric propulsion control of the dynamic positioning ocean platform considering displacement energy storage is completed in the Speedgoat baseline controller, and the propulsion system control signal is generated. The control signal input is realized by connecting the RTLAB DI board to the IO397 module to form a simulation closed-loop. The sampling step of the hardware-in-the-loop simulation is set to 0.001 s.

[0035] Table 1 System parameters of the dynamic positioning ocean platform

[0036]

[0037]

[0038] The power fluctuation types of operating loads such as drilling equipment and heave compensators are divided into two categories: working condition switching and sea condition change. By simulating the dynamic positioning ocean platform under different simulation scenarios, the stability of the power system is studied, and the power smoothing effect of the present invention is verified. Denote Scenario 1 as the sudden increase in power generated by a single working condition switching of the operating load, Scenario 2 as the sudden change in power caused by the periodic working condition switching of the operating load, and Scenario 3 as the continuous power fluctuation caused by the influence of sea conditions on the operating load.

[0039] Scenario 1: Single working condition switching of the operating load

[0040] Scenario 1 simulates that there is a working condition switching in the power station operating load of the dynamic positioning ocean platform, and a sudden increase in load power of 1.25 MW occurs at 16 s and lasts for a long time. The power systems of the dynamic positioning ocean platform with conventional variable frequency without energy storage control strategy and displacement energy storage control strategy for the propulsion motor are respectively simulated, and the results are as Figure 6 shown.

[0041] At the beginning, both the system frequency and power are near the rated values, and the offshore platform maintains normal dynamic positioning. At 16 s, an instantaneous load mutation causes the frequency of the power station bus adopting the existing variable-frequency control strategy to drop to 59.60 Hz, and it takes 4.02 s to recover to the rated frequency. While for the power station with the displacement energy storage control of the dynamic positioning offshore platform proposed by the present invention, the frequency peak is 59.93 Hz, and it takes 1.85 s to recover to the rated frequency. Due to the displacement energy storage effect of the dynamic positioning offshore platform, the propulsion system takes 0.047 s to respond to the power demand of the power station, reducing the system frequency overshoot from 0.67% to 0.12%, saving 2.17 s of adjustment time, and there is a maximum displacement deviation of 0.21 m for the offshore platform to provide temporary electrical energy storage. For the instantaneous power mutation generated by the switching of the operating load conditions, the displacement energy storage control scheme proposed by the present invention can provide a maximum power demand response of 1.15 MW, reducing the power fluctuation by 92.74%. The above results show that the displacement energy storage control strategy of the dynamic positioning offshore platform can suppress the power impact and reduce the frequency fluctuation to a certain extent. When the condition switching is completed and the system is stable, the dynamic positioning offshore platform returns to the set position, and the diesel generator undertakes the output of the operating load to ensure the safe operation of the system.

[0042] Scenario 2: Periodic condition switching of operating load

[0043] Scenario 2 simulates the sudden increase in load power generated by the periodic operation of offshore platform drilling equipment, etc. Limited by the simulation running time, the condition switching period in the prior art (A. Veksler, T. A. Johansen, R. Skjetne, et al. Thrust Allocation With Dynamic Power Consumption Modulation for Diesel-Electric Ships [J]. IEEE Transactions on Control Systems Technology, 2016: 24, (2): 578 - 593.) is compressed to 4 s, and the single operation of the load shows a sudden increase in power of 1.3 MW. The displacement energy storage effect of the operating load under high-frequency condition switching is as Figure 7 shown. After 16 s, the operating load such as offshore platform drilling equipment switches conditions every 4 s, and the system shows periodic sudden increases in power. Within each cycle, without energy storage

[0044] The frequency amplitude of the controllable offshore platform system reaches 59.85 Hz, and the frequency regulation time is 3.85 s. For the offshore platform system with displacement energy storage control, the frequency is 59.96 Hz and the frequency regulation time is 1.89 s, reducing the frequency overshoot from 0.25% to 0.06%. For the sudden power increase of 1.29 MW caused by the change of the operating load condition, the offshore platform has a maximum displacement deviation of 0.13 m to provide a 0.95 MW energy storage response, and the energy storage response time is 0.041 s. The displacement energy storage control strategy proposed in the present invention can reduce the overall power overshoot of the power station caused by the periodic change of the working condition from 11% to 3%, while realizing the dynamic response to the frequency, reducing the frequency regulation time by 1.96 s, and improving the stability of the system.

[0045] Scenario 3: Power fluctuation of the operating load under the influence of sea conditions

[0046] Affected by the change of sea conditions, the power fluctuation of the operating load of the dynamic positioning offshore platform is mostly random and continuous. Scenario 3 simulates the access of an operating load with a maximum power fluctuation of 1.8 MW from 16 s to 22 s, and compares the bus frequency of the offshore platform system and the overall power fluctuation of the power station under different control strategies.

[0047] From Figure 8It can be seen that when the power of the operation load fluctuates due to sea conditions, the power and frequency stability of the power propulsion system adopting the displacement energy storage control strategy considering the dynamic positioning of the ocean platform is better. At 16 s to 22 s, the frequency amplitude of the ocean platform system without the energy storage control strategy reaches 60.5 Hz, and the power fluctuation range is 6.65 MW to 10.34 MW. While for the system adopting the control strategy proposed in the present invention, the frequency amplitude is 60.07 Hz, and the power fluctuation range is 8.52 MW to 9.15 MW, reducing the frequency overshoot from 8.3% to 0.12% and the power overshoot from 26.1% to 5.3%. At 22 s, the fluctuating operation load is cut out. The power station without energy storage control has not yet recovered to the rated frequency at 25 s, while the power station adopting displacement energy storage recovers to the rated frequency at 22.9 s, reducing the frequency regulation time by 4.9 s. Adding displacement energy storage control to the power propulsion can provide a maximum power reserve of 1.58 MW, reducing the system frequency fluctuation by 0.43 Hz and the overall power fluctuation range by 3.06 MW. The power of other loads of the ocean platform power station reaches an amplitude of 1.75 MW at 21.1 s. At this time, the displacement energy storage of the dynamic positioning of the ocean platform provides a power response of 1.58 MW, effectively reducing the load power fluctuation by 90.29%. The corresponding maximum displacement deviation of the dynamic positioning of the ocean platform is -0.28 m, within the allowable maximum displacement deviation of 0.5 m. The simulation results show that the displacement deviation of the dynamic positioning of the ocean platform and the response power trend of the displacement energy storage are consistent. The proposed control strategy for the displacement energy storage of the dynamic positioning of the ocean platform can effectively fill the power shortage without introducing additional energy storage devices. Adding a maximum displacement deviation constraint at the dynamic positioning controller can utilize the platform potential energy to provide power reserve for the system on the premise of ensuring the safe operation of the dynamic positioning ocean platform. The system simulation results under different scenarios are summarized in Table 2 as follows.

[0048] Table 2 Summary of displacement energy storage control effects under different scenarios

[0049]

[0050]

[0051] A marine platform power system adopting a hybrid energy storage control of lithium battery - supercapacitor is built for comparison. The lithium battery adopts a constant voltage charge (discharge) control strategy, and the supercapacitor adopts a single current loop control strategy. In order to quantify the configuration relationship between displacement energy storage and hybrid energy storage under the same control effect, the power smoothing and frequency response effects of Scenario 2 in the present invention and hybrid energy storage with different capacity configurations are simulated and analyzed. By adjusting the rated voltage, rated capacity of the battery array, and the number of batteries and supercapacitor banks, etc., the maximum instantaneous response power change of the hybrid energy storage is realized.

[0052] Table 3 Simulation results of displacement energy storage and hybrid energy storage with different capacity configurations

[0053]

[0054] When there is a sudden power increase of 1.29 MW in the operating load of the offshore platform power system, the system simulation results without energy storage control strategy are as Figure 7 shown, and the simulation results of using displacement energy storage and hybrid energy storage with different capacity configurations are as Figure 9 shown in Table 3. The lithium iron phosphate battery of CATL and the supercapacitor of Maxwell are selected as the monomers of the hybrid energy storage. According to the peak power demand of the operating load, the capacity of the battery pack, the number of battery monomers, and the number of supercapacitors are selected. Without introducing additional energy storage devices, the maximum instantaneous energy storage response of the present invention is equivalent to that of Hybrid Energy Storage 1. Since the displacement energy storage control of the present invention takes into account the requirements of frequency dynamic response while suppressing the power fluctuation of the offshore platform, the frequency response of the displacement energy storage control is better than that of Hybrid Energy Storage 1, where: Hybrid Energy Storage 1 consists of a lithium iron phosphate battery with a rated capacity and rated power of 0.37 MWh and 1.85 MW respectively, and a supercapacitor with a rated capacity and rated power of 0.29 MWh and 0.43 MW respectively. Therefore, the control strategy proposed by the present invention meets the lightweight requirements of the dynamically positioned offshore platform and saves space on the offshore platform. The present invention responds to power changes through the dynamic positioning electric propulsion control. Since the electric propulsion is connected to the operating load on the same bus through an inverter, there is a response delay of about 0.04 s.

[0055] For the actual load power curve of offshore engineering equipment with a maximum power fluctuation of 1.8 MW, based on considering the battery life cycle, with the double-objective function of the total cost and total mass of the energy storage system being optimal, under the constraint conditions of meeting the system peak power and maximum energy, the optimal cost of the hybrid energy storage is determined to be 514,042 yuan, and the total mass is 1,296 kg. The lithium battery needs to be replaced twice during the entire life cycle of the platform. Compared with the existing hybrid energy storage technology, the present invention does not need to introduce additional equipment under the same power fluctuation suppression effect, reduces the cost by 514,042 yuan, reduces the energy storage device with a total mass of 1,296 kg, and improves the utilization rate of the limited space on the offshore platform. Compared with the existing electric propulsion frequency conversion control technology, due to the addition of displacement energy storage control on the offshore platform in the present invention, the size of the displacement energy storage is adjusted through frequency droop control, the complex power impacts of the operating load on the offshore platform are suppressed in an orderly manner, and the frequency dynamic regulation is participated in, reducing the frequency and power overshoot and the adjustment time. The specific indicators are shown in Table 2.

[0056] The above specific implementation can be locally adjusted in different ways by those skilled in the art without departing from the principles and purposes of the present invention. The protection scope of the present invention is subject to the claims and is not limited by the above specific implementation. All implementation solutions within its scope are subject to the constraints of the present invention.

Claims

1. A displacement energy storage control method for a dynamic positioning marine platform based on electric propulsion, characterized in that: According to the electromechanical energy conversion relationship of the dynamic positioning of the marine platform, a displacement energy storage model of the dynamic positioning marine platform is constructed; a displacement energy storage compensation torque model is constructed; and then based on the propulsion motor variable frequency control and the constructed model, the system frequency-displacement energy storage-electric propulsion torque control is carried out; The dynamic positioning offshore platform displacement energy storage model is specifically: Determine the real-time compensation torque τ of the propulsion motor sup , specifically 2. The displacement energy storage control method of a dynamic positioning marine platform based on electric propulsion according to claim 1 is characterized in that: The power system of the dynamic positioning offshore platform comprises: a generator set, a frequency converter, a propulsion motor, a dynamic positioning control system, a dynamic positioning measurement system and a dynamic positioning propulsion system, wherein: the generator set converts the chemical energy of the fuel into electrical energy, and provides power to the entire offshore platform in the form of electrical energy; the frequency converter converts the alternating current of the offshore platform power station into alternating current with adjustable voltage frequency and amplitude; the propulsion motor is directly connected to the inverter side of the frequency converter, and drives the propeller to rotate through the shaft system, so that power is transmitted from the net side to the propeller, completing the electromechanical conversion process of the dynamic positioning propulsion system; the dynamic positioning measurement system feeds back the displacement deviation relationship between the target position and the actual position of the offshore platform; the dynamic positioning control system uses a DP controller to calculate the thrust required by the propeller when the offshore platform reaches the expected position, and feeds back the thrust command to the propulsion system, and changes the speed of the electric propulsion motor by controlling the thrust size received by the offshore platform; the dynamic positioning propulsion system converts electrical energy into mechanical energy and then into kinetic energy of the offshore platform.

3. The displacement energy storage control method of a dynamic positioning marine platform based on electric propulsion according to claim 1 or 2, characterized in that: include: Step 1: Monitor the system bus frequency information through the offshore platform dynamic positioning displacement energy storage control model; Step 2: Determine whether the system frequency or frequency change rate exceeds the limit. When the frequency or frequency change rate exceeds the limit, that is, f0>60.15Hz or f0<59.85Hz or |df / dt|>0.2Hz / s, the offshore platform displacement energy storage enters the pre-participation power leveling state; Step 3: determine whether the displacement deviation and the displacement deviation limit duration are within the constraint range, that is, when the displacement deviation of the offshore platform dynamic positioning is less than 0.5m and the displacement deviation limit duration is less than 0.5s, the offshore platform displacement energy storage participates in power leveling; Step 4: According to the selected low-pass filter time constant, the power fluctuations in different frequency bands are filtered, and the low-frequency load power fluctuations are smoothed by the offshore platform displacement energy storage; Step 5: Under the condition of satisfying the constraints of step 2 and step 3, determine the real-time compensation torque of the propulsion motor Step 6: Determine whether the propulsion motor is within the limit, that is, T min ≤T≤T max , where: T min =-560kN, T max =560kN, implementing displacement energy storage control of dynamic positioning offshore platform based on electric propulsion; Step 7: Determine whether the frequency has recovered to the limit value. If it has recovered to the limit value, the displacement energy storage control is exited. Otherwise, repeat the above steps.

4. The displacement energy storage control method of a dynamic positioning marine platform based on electric propulsion according to claim 3 is characterized in that: The step 6 specifically includes: Step a: When the power of other operating loads changes, the real-time deviation of the bus frequency of the offshore platform power system is monitored by Δω=ω g，re f-ω g，mes ; Step b: introduce a first-order low-pass filter before the dynamic positioning controller to achieve bandwidth separation between the inner and outer loops and avoid mutual interference between the inner and outer loops of the propulsion motor control. Since the response time of the DC voltage controller is greater than the response time of the VOC control and FOC control in the inverter, the control bandwidth B of the DC voltage controller is usually afe As the inner loop control bandwidth of the propulsion motor inverter, the control bandwidth B is determined by the DC voltage controller model. afe , and the inverse of the bandwidth is selected as the time constant of the first-order low-pass filter. When the power of other operating loads of the offshore platform power station fluctuates, the introduction of the first-order low-pass filter can also effectively improve the anti-interference ability of the droop control; Step c: With the help of droop control, according to the frequency deviation e ω Adjustment of offshore platform dynamic positioning Offshore platform displacement energy storage P sup，ref Power size, specifically: P sup,ref =K ω (ω g,ref -ω g,mes ), step b and step c together constitute the droop control link; Step d: Based on the displacement energy storage model of the displacement dynamic positioning marine platform derived and established by combining the marine platform dynamics, kinematic equations and dynamic positioning control system, specifically: Determine the real-time compensation torque τ of the propulsion motor sup , specifically Step d is the displacement energy storage control link; Step e: Combine the real-time reference torque τ of the propulsion motor ref And the real-time compensation torque τ of the motor obtained in step d sup , as the FOC input reference torque, to achieve dynamic regulation of system power fluctuations and frequency; The outer loop control transfer function of the marine platform dynamic positioning electric propulsion taking into account the displacement energy storage in steps a to e is:

5. A dynamic positioning offshore platform displacement energy storage control system for implementing the method described in any one of claims 1 to 4, characterized in that: include: An electric propulsion frequency conversion control unit and an offshore platform dynamic positioning displacement energy storage control outer loop module consisting of a droop control unit and a displacement energy storage control unit, wherein: the electric propulsion frequency conversion control unit adopts a vector control strategy, including a frequency converter consisting of two back-to-back three-phase inverters, wherein: the first frequency converter serves as an active front-end rectifier, and uses a voltage vector directional control strategy to control the DC voltage v dc The second inverter is used as a torque control variable frequency drive, which uses a magnetic field vector oriented control strategy to control the output torque t of the induction motor. m ; VOC control adopts current and voltage dual closed-loop control, and FOC control realizes the decoupling of the stator current excitation component and the torque component.

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