Dynamic Positioning for Dummies - Thrusters (3)

Summary

This video lecture provides an in-depth explanation of dynamic positioning (DP) systems, focusing on the role of thrusters, power management, and trust allocation. It covers the various trust modes used for positioning control, such as bias, fixed, and push-pull, and addresses common issues like thruster interaction and friction. The lecture also discusses power overload control, thruster estimation, and feedback mechanisms, emphasizing the importance of accurate control for stability. Techniques like Kalman filtering are introduced to improve predictions and system performance in dynamic conditions, ensuring optimal thrust and efficient vessel movement.

Takeaways

• 😀 Thrusters play a critical role in Dynamic Positioning (DP) systems by calculating and distributing thrust for position and heading control.
• 😀 The actuator is responsible for allocating the thrust based on the system’s knowledge of which thrusters are active and their respective modes.
• 😀 The DP system aims to optimize thrust usage to minimize power consumption and wear and tear on propulsion equipment while meeting directional force and rotational movement demands.
• 😀 When thruster availability is limited, the system prioritizes heading control over position control, unless manually adjusted by the operator.
• 😀 Thrust loss due to friction along the hull or thruster interactions can result in errors up to 40% in directional control and 25 degrees in limited thrust scenarios.
• 😀 Forbidden zones are used to avoid thruster wake interaction, though these zones cannot fully eliminate disturbances caused by thruster crossing.
• 😀 Different trust modes like bias, fixed, and push-pull are employed to optimize thrust distribution for specific operational needs.
• 😀 Momentum calculation relies on the 90-degree angle between the thruster force and the rotational point, but if this angle varies, adjustments are made using sine calculations.
• 😀 The power overload control system reduces the pitch and RPM demand to avoid overload, but it cannot prevent blackouts or initiate standby generators. This is handled by the vessel's Power Management System (PMS).
• 😀 Accurate thruster feedback is essential for preventing oscillations, and thruster models must monitor differences between set points and feedback to ensure stability.
• 😀 Kalman filtering is employed to improve thruster feedback accuracy and manage external disturbances such as wind, waves, and currents for better thrust allocation.

Q & A

What is the role of the thruster in a DP (Dynamic Positioning) system?

The thruster's role in a DP system is to generate thrust and provide directional force and rotational movement required for position and heading control. It ensures optimal thrust usage with minimum power consumption and minimal wear on propulsion equipment.

How does the DP system handle the loss of thrust?

If thrust is lost, the system automatically redistributes the remaining thrust among the operational thrusters. The system can prioritize heading control over position control if needed, especially when thrusters are out of service or deselected.

What is thruster hull interaction, and how does it affect the DP system?

Thruster hull interaction occurs when the thrust of one thruster affects the operation of another, often due to friction or propeller wash. This can lead to errors in thrust calculations, causing directional alteration up to 40% and reduced thrust capabilities by up to 25 degrees.

What is the purpose of specifying forbidden zones in a DP system?

Forbidden zones are used to prevent thruster interactions by restricting where the thrust wake can go. While this helps reduce the risk of interference between thrusters, it is not a complete solution, as thrusters may still occasionally cross into these zones.

What are the different trust modes used in the DP system?

The DP system can use various trust modes, including bias mode (thrusters are placed opposite to each other), fixed mode (thrusters set at fixed angles), and push-pull mode (used for thrusters with two propellers and rudders). Each mode helps manage the direction and force of the thrusters for better control.

How does the DP system calculate the momentum from thrusters?

Momentum is calculated by multiplying the force exerted by the thruster by the distance from the rotation point. If the thrust angle is not 90 degrees, the formula is adjusted by multiplying the distance by the force and the sine of the angle.

How does the power overload control system interact with the DP system?

The power overload control system ensures that the power available to the thrusters is sufficient. If power is insufficient, it reduces the pitch and RPM demand of the thrusters to minimize impact on position and heading control. However, this system cannot prevent blackouts or manage other power-consuming systems like the PMS (Power Management System).

What is the purpose of a vessel's power management system (PMS) in relation to DP?

The PMS works alongside the DP system to manage power consumption, including load reduction and ensuring the vessel's power demand is met. It helps prevent blackouts and ensures that power distribution between different systems is balanced.

What types of thrusters are mentioned in the script?

The types of thrusters mentioned include tunnel thrusters, main propellers, rudders, ultimate thrusters, water-jet thrusters, and specific thrusters like a white Schneider propeller or a chill chat thruster.

What is the role of the thruster estimator in the DP system?

The thruster estimator's role is to calculate the estimated thrust based on available data and thruster characteristics provided by the manufacturer. It helps predict the thruster's response to control commands, ensuring the system adjusts effectively for accurate control.