Propulsion Control Part 1 of 2

1. Propulsion ControlPart 1 of 2 Øyvind Smogeli and Asgeir J. Sørensen, Department of Marine Technology, Norwegian University of Science and Technology, Otto Nielsens Vei 10, NO-7491 Trondheim, Norway E-mail: Oyvind.Smogeli@ntnu.no Asgeir.Sorensen@ntnu.no

2. Outline Part 1: Modelling • Motivation and problem formulation • Mathematical modelling • Propeller characteristics • Propeller losses and dynamics • Experimental results Part 2: Local control • Conventional local thruster control • Combined power and torque control • Sensitivity to thrust losses • Propeller load torque observer • Torque loss calculation • Anti-spin control • Simulation results • Experimental results

3. Propulsion and Thruster Control • Thrust and power allocation (over/under actuated systems) • Pitch/rpm/torque/ power control • Combined torque and power control • Anti-spin thruster control • Combined rudder and propeller control PITCH or RPM CONTROL POWER TORQUE CONTROL POWER VARIABLE TORQUE CONSTANT TORQUE

4. Dynamic Positioning and Transit • Demand for vessels to conduct all-year operationin harsh environment and extreme conditions • High positioning accuracy required • DP system and propulsion system must be robust to any single failure • It’s a trend towards physical and functional integration between the power and automation systems

5. Motivation

6. Thrusters affected by waves, current and vessel motion: Rapidly changing operating conditions Load fluctuations Motivation

7. Thrusters affected by waves, current and vessel motion: Rapidly changing operating conditions Load fluctuations Motivation Effects of bad low-level thruster control: • Danger of blackout • Wear and tear of the propulsion system • Increased fuel consumption • Reduced thrust capability

8. Propeller Types: FPP and CPP Two controllable parameters: Shaft speed and pitch • Shaft speed: Fixed pitch propellers (FPP) • No hydraulics needed to control the pitch • Preferable for electric motors with variable speed • Optimized for one advance speed • Pitch: Controllable pitch propellers (CPP) • Used for direct-driven shafts when varying thrust is needed • Fast response, produces thrust in two directions • Better hydrodynamics for varying advance speed • Consolidated control: Combination also possible (CPP) • Typically two or three speed setpoints • Varying pitch dynamically Pitch P measured at 0.7R, commonly given as pitch/diameter ratio P/D Shaft speed given as RPM, n = RPS = RPM/60 or ω = 2n

9. Conventional Propulsion Control FPP: Fixed Pitch Propeller with controllable speed (RPM) CPP: Controllable Pitch Propeller with fixed speed RPM INPUT PITCH INPUT POWER POWER RPM PITCH

10. Propulsion Courtesy to Rolls-Royce Marine: http://www.rolls-royce.com/marine/default.jsp

11. Propulsion Courtesy to ABB Marine: http://www.abb.com/

12. Functionality: Control Modes • Manoeuvring models • Linearized about some Uo • Sea keeping • Motion damping • Station keeping models • Marine operation models • Slender structures • Multibody operations High speed tracking/Transit Low speed tracking Marked position Station keeping Speed [knots] 5 6 7 ….. 0 4 1 2 3

13. Control Modes: - Speed range and actuation • Control actuation: • Main propellers/pods • Tunnel thrusters • Azimuthing thrusters • Control actuation: • Main propellers/pods • Rudders High speed tracking/Transit Low speed tracking Marked position Station keeping Speed [knots] 5 6 7 ….. 0 4 1 2 3

14. Propeller Blade Model Integrate over propeller blades to get total thrust and torque

15. Propeller Characteristics Typical characteristics of actual propeller thrust and torque : is the density of water _ w is the propeller diameter D is the propeller speed (RPS) n The non-dimensional thrust and torque coefficients are given as: is the advance speed V a is the pitch ratio P / D is the number of blades A / A e O is the propeller expanded blade area ratio is the Reynolds number Z R n is the max. blade thickness t is the propeller chord length c

16. Desired Thrust and Moment for Speed Controlled Propellers The desired/reference thrust and torque coefficients for zero advance speed, KT0 and KQ0, are used for control since Va is unknown to the control system: is the desired propeller speed (RPS)

17. Open Water Tests as Function of Advance Ratio 0.8 R o 10K 0.6 Q K T 0.4 0.2 P / D 0 . 7 , 0 . 8 9 and 1 . 1 = 0 0 0.2 0.4 0.6 0.8 1 1.2 Advance ratio Open water propeller efficiency in undisturbed water: Work done by propeller in producing a thrust/ work required to overcome shaft torque

18. Linear Thruster Characteristics Common simplification: Advance ratio: The thrust and torque are then expressed as: The nominal thrust and torque coefficients, used for control since Va is unknown to the control system (simplest possible representation):

19. 4 Quadrant Thrust Model Fixed pitch propeller in the Wageningen series:

20. Controllable Pitch Propeller For a given pitch, the propeller behaves like a fixed pitch propeller: A “cut” along one P/D value gives the conventional KT curve as function of advance ratio.

21. Propulsion Efficiency (1) Axial water inflow velocity to the propeller Va due to vessel velocity U and wake fraction number w is: The hull reduces the inflow to the propeller 0 < w < 0.4 is the wake fraction caused by the wave motion of the water particles is the wake fraction caused by so-called potential effects for a hull advancing forward in an ideal fluid, is the wake fraction caused by viscous effects due to the effect of boundary layers

22. Propulsion Efficiency (2) In steady state the effective thrust is equal to the total resistance R: Propeller suction on the aft ship given by the thrust-deduction coefficient 0 < td < 0.2 increase resistance (drag) Overall propulsion efficiency is given by the ratio of useful work done by the product of drag and ship speed divided by the required work to overcome the shaft torque: where Relative rotational efficiency Hull efficiency in the range of 1-1.2 Mechanical efficiency in the range of 0.8-0.9 Open water propeller efficiency in undisturbed water

23. Propeller and Thruster Losses The actual thrust Taand torque Qa are affected by: • The vessel hull • Coanda effect • Tunnel thruster suction losses • Thrust deduction • Velocity fluctuations • In-line change of advance velocity • Cross-coupling drag • Thruster-thruster interaction • Ventilation and in-and-out-of water effects

24. Thrust Losses: General formulation The actual thrust Taand torque Qa may be expressed as: where: represents dynamic states (vessel motion, propeller submergence, environmental conditions). represents propeller dependent parameters. hT and hQ are termed the thrust and torque reduction functions Thrust and torque loss factors:

25. Thrust Tth [kN] KT = 0.4 KT = 0.36 800 KT = 0.30 400 200 0 -3 -2 -1 1 2 3 4 5 -200 Speed n [RPS] Example: Thrust curves for varying KT

26. Thrust Losses: Velocity fluctuations Axial in-line fluctuations: Variation in Va and hence thrust coefficient Transverse fluctuations: Cross-coupling drag

27. Thrust Losses: - Thruster-thruster interaction Loss of thrust because of: • Change of advance velocity due to inline jet velocity component Vj,a, which leads to change in the thrust coefficient • Cross-coupling drag due to transverse jet velocity component Vj,t • Other interaction effects, harder to model

28. Thrust Losses: Coanda effect • Propeller slipstream is drawn towards the hull and deflected • Severe loss of thrust for unfortunate thrust angles

29. Cavitation Tunnel Experiments:- Ventilated ducted propeller • Ducted propeller • Varying shaft speed / loading and submergence • Measuring thrust and torque • Steady state • Propeller: • D = 250 mm • Z = 4 • P/D = 1 • EAR = 0.55 • Duct: • L/D = 0.5 • L = 118.8 mm • Di = 252.1 mm

30. Experiments: - Cavitation tunnel results Thrust for Ja= 0.2 Ventilation

31. Experiments: - Cavitation tunnel results Thrust for Ja= 0.2 Ventilation

32. Experiments: MCLab • Same ducted propeller as in the cavitation tunnel • Operating in waves with ventilation • Varying submergence and propeller speed

33. Experimental Results MCLab

34. Experimental Results MCLab: Waves

35. Experiments: MCLab time series Thrust for increasing shaft speed in h/R = 1.5 Loss of thrust during ventilation

36. Experimental results from cavitation tunnel at NTNU • Ventilation loss model for simulation Modelling of Ventilation Loss Effects

37. Propeller Shaft Model Power delivered by the motor: Actual propeller shaft power accounting for the effect of thrust losses: torque generated by the motor Q m actual torque experienced by the propeller Q a moment of inertia of the propeller shaft I s angular shaft speed 2 n g = ^ Km friction coefficient

38. Torque Loop in Electrical Motor Drive Q Torque c controller - Q ~ PWM calc PWM ~ Converter Y Flux c controller I - sa Y Motor calc Model I sb Induction Motor n Flux weakening T Tacho m The closed loop of thrust motor and torque controller is assumed to be equivalent with a 1st order model: where 20 < Tm < 200 milliseconds

39. Thruster Modelling for Control: A summary Thruster dynamics: first order motor model rotational dynamics with friction propeller load torque propeller thrust propeller power Desired thrust and torque: Thrust and torque loss factors:

40. Propeller Block Diagram q x Thruster p p unit . . ( ) f q x Q p p Q a T T Q Q n Shaft Motor ref . m a c ( ) f Control Dynamics Dynamics T n