SeaDP. Dynamic Positioning System for Split Hopper Vessels

Author(s):
Miguel Taboada - Seaplace S.L

Industry:
Industrial Controls/ Devices/ Systems

Products:

The Challenge:
Controlling and keeping the position of a 1000 m3 Split Hopper Vessel (2500 t displacement) during the dumping operation, within a 10m tolerance and withstanding a force 6 wind (25 knots), long crested sea up to 3 meter height and sea current up to half of a knot.

The Solution:
A combined PXI and Compact RIO platform performs in real time data acquisition, signal processing, control algorithms and interfacing with the control panel and the electronic thrusters. A client / server TCP architecture carries out the signals monitoring and the user input through a touch panel monitor. NI LabVIEW allows the fulfillment of the system in a record time.

A Split Hooper Vessel is a specialized ship, with its main structure is composed of two articulated semi-hulls forming a hopper which uses a hydraulic system to open and close it. This ship is used in maritime works for transporting and dumping materials which serves as a foundation for dam construction.   

 During the dumping operation, the ship undergoes a two thirds decrease in displacement and a double increase in wind exposed area. This causes a rapid variation of wind, currents and wave forces leading to an undesired offset. This fact is unallowable during precision dumping operations, and could lead to a very expensive material replacement.

 SeaDP is a Dynamic Positioning System specially adjusted to operation of Split Hopper Vessels, which control the position and heading of the ship during all the steps of the dumping process; approaching the target, controlling and keeping the position, and orientation while dumping.  

 System Description

 Sensors, Thrusters, Signal Acquisition and Processing

 Data Acquisition from GPSs, Gyro, Anemometer, Speed Log and Draught Sensor is carried out in the PXI platform through serial interfaces RS 232/RS 422 (NMEA protocol), the sampling rate ranges from 10 s of the Speed Log to 10 Hz of the DGPS (RTK). PXI is also sampling the IMU (Inertial Motion Unit) at 75 Hz through RS 232 serial interface. The Thruster System of the ship comprises three (3) thrust devices, each capable of giving variable force in any planar direction, with an overall thrust of 200 kN at 2100 kW power.

 The System has two CompactRIOs working synchronously (interrupt method) with PXI RT Controller at 10 Hz. One of them performs Digital Logic Interface with Schottel Thrusters, acquisition of revolutions and azimuth thrust direction signals, and sending command signals; throttles to Diesel Engine and thrust azimuth direction.

 The Control Panel contains all indicators and buttons that allow the system operation even in case of a communication breakdown between the Control Computer and the Monitoring & Supervision Computer. In addition there is a Joystick to manually control the ship position and heading. Three dimensional position of the antenna location, from both GPS/DGPS, is transferred to the hopper center, having into account RPY (Roll, Pitch, Yaw) angles. The ship position is given in UTM coordinates.  The Anemometer measures relative speed and wind direction which will feed, after wind gust removal, an aerodynamic drag force model to estimate the mean wind forces. 

 The IMU gives rotation rates and accelerations in body axes in the center of gravity of the ship. Through the use of Kalman Filters roll and pitch angles are calculated whereas yaw angle (True Heading) is obtained from the Gyro. Frequency domain processing of acceleration time series is used for extracting HF surge, sway and yaw movements. Roll angle is also used in an RLS estimation of the period of wave induced ship movements.

Sensors (azimuth angle and propeller revolutions) in thrust devices are used for estimating the total force and moment acting upon the ship.

 The starting point for recursive estimation of the ship model contained in SeaDP is the taking draughts procedure.  A detailed ship model is used in a nonlinear State Observer and in the LQR Controller.

State Observer and Controller

Ship movements in waves, wind and current can be considered as the superposition of wave frequency movements (0.05-0.2 Hz) and low frequency movements caused by wave grouping forces. In practice, it is impossible to counteract the wave movements as they are produced by forces of ship weight magnitude. Despite of this, and due to the fact that these frequencies falls into the bandwidth of thrust devices, adequate filtering of the wave movements has to be performed to avoid excessive wear and tear.

 The wave filtering is achieved through a Non-Linear State Observer that consists of a model of LF ship motion and thruster system response, and of stochastic models of wave motions and environmental disturbances working together to give smooth estimation of LF motion and velocity of the ship.

 The Controller uses the ship LF motion, velocity and target position deviation to calculate the commanded forces and moments through a Linear Quadratic Regulator (LQR). It minimizes a weighted integral of the deviation and power/thrust effort. Furthermore wind forces estimated in real time are feed-forwarded in order to increase the Controller performance. The Controller allows either manual/automatic control in surge, sway and yaw movements. The Controller commanded forces in longitudinal and transversal directions, and the moment about the gyration point has to be allocated among the different thrust devices, this is done in SeaDP using on line quadratic programming techniques.

 Future Developments of SeaDP

SeaDP belongs to DP class I, this means that a single failure (thrust device, controller, etc) could lead to lose the position. Higher requirements of class II and III involve physical and logical redundancy and special application features (i.e. on line single-failure consequence analysis). Seaplace will rely on NI hardware and software for these future challenges.

 

Author Information:
For more information on this Case Study, contact:
Miguel Taboada
Seaplace S.L

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