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Control Aspects Related to Positioning and Motion
Damping of Large-Scale Interconnected Marine
Structures Asgeir J. Sørensen Centre for
Ships and Ocean Structures, NTNU E-mail
Asgeir.Sorensen_at_ntnu.no Workshop on Very Large
Floating Structures, October 28-29, 2004,
Trondheim, Norway
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Outline

• Background
• Terms and Definitions
• Control strategies
• Examples
• Locally Multiobjective H2 and H8 Control of
  Large-scale Interconnected Marine Structures
• Modelling and Control of Single Point Moored
  Interconnected Structures
• Floating LNG Barge
• Conclusions
• References

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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?

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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)

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Definitions (1)

• Many terms
• 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
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Definitions (2)
Boller, 1996
Crawley, 1994
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Definitions (3)
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 Snelsons tensegrity structures
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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).

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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.

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Our Definitions (3)
Rustad, 2004 Proposed definitions
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Marine Applications

• Floating platforms
• Sea farming
• Airfields
• Cities
• Oil installations

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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/H8, nonlinear
  feedback linearization controllers, back-stepping
  controllers, etc.
• The control plant model is also used in
  analytical stability analysis, e.g. Lyapunov
  stability.

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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 (0.001-1 s)
Actuator control
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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

Localized versus Centralized control
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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
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Process Plant Model
Nonlinear 6N DOF low-frequency model - surge,
sway, heave, roll, pitch and yaw
where
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Suspension Joints (1)

• Earth-fixed position and velocity of suspension
  joint ij

• Restoring and damping forces between unit i joint
  j and unit ki1 joint l

• Resulting suspension joint force and moment
  vector for unit i

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Suspension Joints (2)

• Assuming small angles such that

• Linear suspension joint model

where
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Control Plant Model
Linear vessel dynamics
where
State space model
where
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Controller Design

• Plant
• State-feedback controller
• Closed-loop system

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Controller Design

• Control objective minimize the sum
• for some weights c1, c2 gt 0 subject to QQTgt0 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 ?

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Simulation Study

• 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

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Earth-Fixed Positions and Angels, Unit 1
Controller 3 DOF (green) 6DOF (red)
Environmental load Current 1 m/s
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Power Spectrum of Pitch Angle
Controller 3 DOF (blue) 6DOF (red)
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Example 2 Control of Single-Point Moored
Structures
First module connected to the seabed through a
spread mooring system
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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

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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
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Simulation Results (2)
Stress on the mooring lines
Peaks removed. Increasing operational safety.

• Solid controlled
• Dotted open loop

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Statoil Floating Liquid Natural Gas
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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

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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/ECSSM96. 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.