1. Control Aspects Related to Positioning and Motion Damping of Large-Scale Interconnected Marine Structures
Asgeir J. Sørensen
Centre for Ships and Ocean Structures, NTNU
Workshop on Very Large Floating Structures,
October 28-29, 2004, Trondheim, Norway

2. Outline Background Terms and Definitions Control strategies Examples
Locally Multiobjective H2 and H∞ Control of Large-scale Interconnected Marine Structures
Modelling and Control of Single Point Moored Interconnected Structures
Floating LNG Barge
Conclusions
References

3. Background (1) Control of structures: active field since 1980s
Several disciplines involved
Material technology
Fluid mechanics
Signal processing and control engineering
Computer science
Engineering fields
Aerospace (e.g. low weight, high reliability, large variations in temperature)
Civil engineering (e.g. earth quake resistance)
Automotive
Marine technology?

4. Background (2)
Definitions and terminology varies based on “traditions” within each engineering field
Enabling technologies are:
Materials
Sensors and actuators
Micro-electro-mechanical systems (MEMS) + nano technology
Communication technology
Computer science
Process insight (e.g. hydrodynamics, structural mechanics)

5. Definitions (1) Many terms: Classification dependent on: Size:
Active, sensing, adaptive, smart, intelligent, controlled, modern, biological materials
Systems, structures and materials
Classification dependent on:
Sensors
Actuators
Control distribution
Control strategy
Process
Learning ability
Size:
Large-scale structures
Hinged structures
Hydroelastic structures
Crawley, 1994:
Structural control

6. Definitions (2)
Boller, 1996:
Crawley, 1994:

7. Definitions (3) Controllable Fluids: Tensegrity Structures
Preumont, 2002:
Smart structures with various sensors and actuators
Controllable Fluids:
Electro-rheological (ER)
Magneto-rheological (MR)
Tensegrity Structures
SMA- Shape Memory Alloys
Piezoelectric materials …
Skelton, 2001:
Snelson’s tensegrity structures

8. Our Definitions (1)
Conventional structure is defined as structure without any control and monitoring
Monitored structure is defined as structure equipped with one or more sensors. Hence, state or condition of the structure is monitored, but no control actions will be taken.
Passive controlled structure is defined as structure equipped with both sensor(s) and actuator(s).

9. Our Definitions (2)
Active controlled structure includes automatic control. Hence, the resulting structure consists of sensor(s), actuator(s) and controller. The more joints with these properties in the structure, the higher distribution or resolution of control.
Smart or intelligent structure is defined as structure consisting of sensors, actuators and control with a high degree of distribution or resolution.

10. Our Definitions (3)
Rustad, 2004: Proposed definitions

11. Marine Applications Floating platforms Sea farming Airfields Cities
Oil installations

12. Two Model Classes Process Plant Model (PPM):
Comprehensive description of the actual process.
The main purpose of this model is to simulate the real plant dynamics.
The process plant model may be used in numerical performance and robustness analysis of the control systems.
Control Plant Model (CPM):
Simplified mathematical description containing only the main physical properties of the process.
This model may constitute a part of the observer and controller, e.g. LQG, H2/H∞, nonlinear feedback linearization controllers, back-stepping controllers, etc.
The control plant model is also used in analytical stability analysis, e.g. Lyapunov stability.

13. Control Structure Office Systems Business enterprise/ Fleet management
Office Network
Ship 3:
Ship 2:
Ship 1:
Operational management
Real-Time Control
Local optimization (min-hour)
Control layers
Fault-Tolerant Control
High level
(0.1-5 s)
Plant control
Real-Time Network
Low level
( s)
Actuator control

14. Control Strategies and Challenges
Marine environment
Large forces in harsh environment
Safety versus performance
Local or central control
Flexibility in operation
Dependencies
Optimized control
Control objectives
Sustainability wrt. fatigue and maximum loadings
Motion damping
Configuration control and positioning
Control strategies:
Model-based control
Passivity-based control
Multi-objective control
Controller #1
Controller #2
Controller #3
Controller #4
Localized
versus
Centralized
control
Controller

15. Example 1: Dynamic Positioning and Motion Damping
Hydrodynamic coupling between:
Surge, heave and pitch
Sway, roll and yaw
Geometrical coupling by thruster configuration and suspension joints
Control objectives:
Dynamic positioning (DP) in surge, sway and yaw
Motion damping in heave, roll and yaw
Locally multiobjective H2 and H with pole constraint using LMI optimization, Ref. Scherer, Gahinet and Chilali

16. Process Plant Model
Nonlinear 6N DOF low-frequency model - surge, sway, heave, roll, pitch and yaw
where

17. Suspension Joints (1)
Earth-fixed position and velocity of suspension joint ij
Restoring and damping forces between unit i joint j and unit k=i+1 joint l
Resulting suspension joint force and moment vector for unit i

18. Suspension Joints (2) Assuming small angles such that
Linear suspension joint model
where

19. Control Plant Model Linear vessel dynamics where State space model

20. Controller Design
Plant
State-feedback controller
Closed-loop system

21. Controller Design Control objective: minimize the sum
for some weights c1, c2 > 0 subject to Q=QT>0 and feasibility of the LMIs
which ensures an H gain from w to z below , and
which ensures an H2 gain from w to z2 below 

22. Simulation Study Main dimensions for each unit:
Three interconnected units coupled by suspension joints at each corner on the deck
Each unit equipped with 4 azimuthing thrusters each able to produce 200 kN
Main dimensions
for each unit:
Mass = 4388 tons
Length = 50 m
Breadth = 45 m
Draft = 15 m

23. Earth-Fixed Positions and Angels, Unit 1
Controller:
3 DOF (green)
6DOF (red)
Environmental load:
Current = 1 m/s

24. Power Spectrum of Pitch Angle
Controller:
3 DOF (blue)
6DOF (red)

25. Example 2: Control of Single- Point Moored Structures
First module connected to the seabed through a spread mooring system

26. Simulation Study
Five modules connected together and to the sea bed via a spread mooring system
Exposed to slowly varying tidal with high eccentricity
Maximum allowed deviation of module 1 from the origin is set to 28 meters

27. Simulation Results (1)
Comparison between open loop and closed loop deviation from the origin
Open loop
Closed loop
Extreme travel reduced with approximately 20 meters

28. Simulation Results (2) Stress on the mooring lines Peaks removed.
Increasing operational safety.
Solid: controlled
Dotted: open loop

29. Statoil: Floating Liquid Natural Gas

30. Conclusions Definitions related to control of structures are reviewed
Based on own experience from marine control systems another definition related to control of structures is proposed
Control challenges are briefly mentioned
Three examples on control of large-scale interconnected structures are shown

31. Some References
Berntsen, P. I. B, O. M. Aamo and A. J. Sørensen (2003). Modelling and Control of Single Point Moored Interconnected Structures. In Proceedings of 6th Conference on Manoeuvring and Control of Marine Crafts (MCMC2003), September 16-19, Girona, Spain.
Boller, C. (1996). Intelligent Materials and Systems as a Basis for Innovative Technologies in Transportation Vehicles. In: the third ICIM/ECSSM’96. Lyon, France
Crawley, E. .F. (1994). Intelligent Structures for Aerospace: A technology Overview and Assessment. AIAA Journal 32 (8).
Joshi, S. M (1989). Control of Large Flexible Space Structures. Springer, Berlin, Germany.
Rustad, A. M. (2004). Motion Damping Control of a Heat Exchanger on a Floating Barge. Master Thesis, Department of Engineering Cybernetics, NTNU, Norway.
Skelton, R. E., R. Adhikari, J.-P. Pinaud, W. Chan and J. W. Helton (2001). An Introduction to the Mechanics of Tensegrity Structures. In: Proc. Of the 40th IEEE CDC, Florida, USA.
Sørensen, A. J., K.-P. W. Lindegaard and E. D. D. Hansen (2002). Locally Multiobjective H2 and Hinf Control of Large-scale Interconnected Marine Structures. In Proceedings of CDC'02, 41st IEEE Conference on Decision and Control, Las Vegas, US.