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The automatic positioning control of mooring system for deepwater semi-submersible platform has become a key issue in the research and development field of deep-sea resources. The Dual-Stage Actuator (DSA) proposed in this paper can replace the single actuator to achieve the high speed and high precision positioning by cooperative control. The relative model and control algorithm of motion trajectory (CAMT) are designed and validated, which proves that the method proposed in this paper is effective.

With the development of deep-sea resources has become the focus of research [

The simplified automatic positioning control systems for the deepwater semi-submersible platform are de- signed by the research institutes including Marin in Netherland and Marintek in Norway, in which, the PID con- trol method is adopted and the control accuracy obtained is lower [

Jason I. Gobat and Mark A. GroSenbaug put forward an experiential model to explain the coupling relation between the surging of platform and the tension of mooring chain, and then developed a control strategy to achieve automatic positioning for the deepwater semi-submersible platform. However, the maximum error reached up to 11 percent [

Based on the given constraints of surging and swaying, Aubault (2007) presented a model and strategy to op- timize the distribution and operation of anchors using the genetic algorithm, whereas, the control method for automatic positioning of mooring system is not involved from the perspective of process control for the platform [

The control strategy and development of automatic positioning control mooring system are researched in this paper from the aspect of predictive control, and a novel Smith-Fuzzy-PID predictive control strategy is proposed with comprehensive utilization of more accurate mathematical model analysis and rich operation experience knowledge, and the high accurate and stable control can be proposed and implemented for the mooring auto- matic positioning control system.

By coordinating windlass and analog windlass simultaneously, automatic positioning control mooring system can simulate and control the movement of deepwater semi-submersible platform and balance the distribution of the force field.

The research object in this paper is the platform with four symmetric windlasses (12 anchor chains) to remain the platform stable in the horizontal direction as

In the deepwater mooring automatic positioning experiment and simulation systems, the real one anchor and the other three simulation anchors would be controlled by the center control desk. At the same time, the sea conditions and operating parameters of each anchor are shown in the human machine interface of the control desk. The simulation system to establish the dynamic model of anchor chain, platform, windlass, sea conditions to simulate the three sets of virtual windlass.

The system could simulate the platform control and movement, and balance the distribution of the force field, as shown in

The composition of functional unit is shown as

The automatic positioning control strategy for the plat form is shown in

In the control strategy, how to realize the automatic positioning is the key point, which should be further ana- lyzed on the basis of the follows model analysis.

Dynamic characteristics of anchor chain greatly influence the movement of the deepwater platform. The anchor chain has the dynamic properties of the flexible components, and the dynamic model possesses features of un- certainty and nonlinear.

Considering the dynamical stiffness of anchor chain, 3-dimensional elastic rod theory is adopted to deduce the dynamic equation of the anchor chain [

where

As shown in the

The relationship between horizontal distance and vertical distance can be expressed with Equations (5) and (6).

The relationship between tension

The longitudinal component of chain tension is

The horizontal component of chain tension is

Basic constraint 1: The chain tension

Taking the windlass as the research object, by reasonable simplification of the torque balance equation [

where

High frequency movement of deepwater semi-submersible platform only shows the periodic oscillation and will not lead to the change of the average position. The nonlinear coupling for low frequency motion equation in the direction of surging [

where

is the velocity vector in three direction under the moving coordinate system,

trix with the same dimension,

Basic constraint 2: In the process of mooring automatic positioning control, the changes of related parameters need to ensure the safety of the platform.

Control principle for mooring automatic positioning system for deepwater semi-submersible platform is aimed to securely satisfy the location accuracy requirement under the above 2 basic dynamic constraints. A closed-loop feedback mooring automatic positioning system is designed in this paper and the control block diagram as shown in

Set the controller output is

In the process of automatic positioning, mooring position deviation calibration has certain hysteresis, due to the unsteady speed of hauling in and paying out the anchor chain, the error, the change of the coefficient of anchor chain tension and the speed of tension adjustment. Equation (11) can be obtained.

where

Uncertainty

The system contains many nonlinear factors, thus the exact theoretical mode cannot be determined.

Hysteresis

According to Equation (11), system lags. The length of the lag time is relevant with deviation adjustment speed, the tension coefficient and the size of error.

Big overshoot

With the existence of the lag factors, actual position deviation cannot be immediately eliminated after adjusting error for the first time and the system error should be adjusted once again, which leads to big overshoot. Cor- respondingly, system will be adjusted back in the same way, thus big overshoot and oscillation phenomenon ap- pear.

There are many control algorithms for the lag system at present. The common control algorithm for mooring automatic positioning system is traditional digital PID, in which, control accuracy is limited. Based on the above model, a new comprehensive control strategy is proposed in this paper by combining PID, Fuzzy control and Smith predictive control.

1) Fuzzy control

Fuzzy control has simple algorithm, good performance and strong robustness [

2) Smith predictive control strategy

There must be delay component

The system output is relevant with the input in the future time. A certain predictive mechanism is needed to obtain the future input to realize the efficient control.

3) A new comprehensive predictive control strategy is proposed in this paper.

From Equation (13), an equivalent system model can be obtained as

where

where

which are both constants relevant with time delay

From Equation (14), compared with Smith predictive control, the new comprehensive predictive control stra- tegy can inherit the advantages of Smith predictive control and overcome the dependence on the precise ma- thematical model of the system.

The Smith-PID control model and Smith-Fuzzy control model are analyzed at first, the new integrated compre- hensive control model is then proposed in this part.

1) Smith-PID control model

Combining the Smith predictive control and PID control, the Smith-PID control can compensate the pure lag system like Smith predictive control and is easy to realize as PID control, which is shown as

The disadvantage of Smith-PID control model is that the controller includes the precise mathematic model, which is difficult to realize for the research object in this paper. Thus, the Fuzzy control strategy is introduced to Smith predictive control model.

2) Smith-Fuzzy control model

The Smith-Fuzzy controller deals with the time-varying system and compensates the pure lag characteristic simultaneously. The Smith-Fuzzy control model and controller are shown as

In

As shown in

The domain of discourse of input displacement deviation is

for the platform in this paper, which is swaying displacement deviation and surging displacement deviation in

turn. And the fuzzy subset is

dium), NS (negative small), ZE (zero), PS (positive small), PM (positive medium), PL (positive large) are the membership functions value. The domain of discourse of output movement is

Mamdani method is adopted in Fuzzy rules, the fuzzy sets would be translated into the clear value to represent

Surge Sway | NL | NM | NS | ZE | PS | PM | PL |
---|---|---|---|---|---|---|---|

NL | NL/NL | NM/NL | NS/NL | ZE/NL | PS/NL | PM/NL | PL/NL |

NM | NL/NM | NM/NM | NS/NM | ZE/NM | PS/NM | PM/NM | PL/NM |

NS | NL/NS | NM/NS | NS/NS | ZE/NS | PS/NS | PM/NS | PL/NS |

ZE | NL/ZE | NM/ZE | NS/ZE | ZE/ZE | PS/ZE | PM/ZE | PL/ZE |

PS | NL/PS | NM/PS | NS/PS | ZE/PS | PS/PS | PM/PS | PL/PS |

PM | NL/PM | NM/PM | NS/PM | ZE/PM | PS/PM | PM/PM | PL/PM |

PL | NL/PL | NM/PL | NS/PL | ZE/PL | PS/PL | PM/PL | PL/PL |

after fuzzy reasoning, namely “anti-fuzzy”. Anti-fuzzy uses the center of gravity method, that is, to find the center of area surrounded by the subjection function curve and the abscissa. The abscissa value of the center is selected as the representative of the value of the fuzzy sets.

3) A new Smith-Fuzzy-PID predictive control strategy implementation

According to the above analysis, combining the Smith-PID control and Smith-Fuzzy control, with some approximate equivalent processing for Smith predictive control, that is, take the d steps sliding average of the historical data before the sampling instant of system output to predict the feedback value of the current sampling instant, Equation (15) can be obtained.

where d value is larger, the more accurate prediction.

The new comprehensive predictive control algorithm inherits the advantages of Smith predictive control and overcomes the dependence on the precise mathematical model of the system. The comprehensive predictive control algorithm structure is shown as

The predictive weight of feedback channel

According to the environmental condition every year, considering the most unfavorable situation with the wind wave and current in the same direction, and taking 600 s as computation time, 14 m is taken as the desired value of platform displacement in the 1500 m water depth and wave direction Angle (135). The simulation result with and without mooring damp with the traditional mooring positioning PID control strategy is shown as

Four mooring equipments are centralized monitored on the central control desk [

During the course of the multi-point mooring position operation, the deepwater mooring position system to be used to simulate the environment and equipment parameters. After analysis and treatment, the central control system sends out the correction command to pay out or haul in anchor chain. In order to ensure the safety of the platform, the platform movement would be limited in less than 5% the working depth of water. In the control process, the environmental load, the platform data and the anchor parameters would be showed on the HMI in real-time.

When the tension of the anchor chain reaches its maximum, the control system should take protective meas- ures, and send out alarm signals to prevent the accidents.

As shown in

Adopting the touch screen operation of panel, each anchor can be monitored and operated by the center control desk, which can realize the anchor chain choose, the anchor chain mode selection, the anchor chain speed selection, the anchor chain pay out and haul in, the anchor chain emergency release, the sea conditions detection and the equipment status monitoring [

During the operation of anchor chain, selecting the corresponding anchor chain, the work mode, the run speed, the pay out and haul in of anchors chain are operated by the joystick which mounted on the control desk. The related parameters about the anchors and platform can be displayed on the HMI.

According to the above analysis and simulation, adopting the new comprehensive Smith-Fuzzy-PID predictive control algorithm proposed in this paper, the overshoot decreases and the adjustment process is accelerated, and the performance of the mooring automatic positioning system is greatly improved for deepwater semi-submersible platform.

The utility system is developed to realize accurately automatic positioning under satisfying the dynamic con- straints and has alarm protection functions by the central control, on-site control and monitoring.

Thus, the control strategy and the monitoring system integrated software and hardware are validated to have the ability to play an important guiding role on the development of marine engineering.

This research was financially supported by the marine engineering equipment scientific research project of National Ministry of Industry and Information Technology of China (Department of Industry and Information Technology Equipment [