1. TMR4225 Marine Operations,
Dynamic stability of underwater vehicles
Comments to Exercise 4
ROVs
Definitions
Types
Typical tasks
Mathematical models
Estimation of force and moment coefficients
Simulators
Future developments

2. Dynamic stability Linear equations can only be used when
The vehicle is dynamically stable for motions in horisontal and vertical planes
The motion is described as small perturbations around a constant motion, either horisontally or vertically
Small deflections of control planes (rudders)

3. ROV overview
ROV:
Remotely Operated Vehicle with umbilical connection to mother vessel
Umbilical is used for power transfer to the vehicle and for communication between it and its pilot

4. Dynamic stability (cont.)
For horisontal motion the equation (2.15) can be used if roll motion is neglected
The result is a set of two linear differential equations with constant coefficients
Transform these equations to a second order equation for yaw speed
Check if the roots of the characteristic equation have negative real parts
If so, the vehicle is dynamically stable for horisontal motion

5. Dynamic stability (cont.)
Characteristic equation for linear coupled sway-yaw motion:
( A*D**2 + B*D + C) r’ = 0
Dynamic stability criteria is:
A > 0, B > 0 and C>0
The last inequality is the important one and it describes the relations between sway and yaw damping forces and moments

6. ROV types/classes A common classification is:
Low cost observation ROV (electric)
Small observation ROV (electric)
Large observation/light work ROV (electric)
Ultra-deep observation/sampling ROV (electric)
Medium light/medium work ROV (electro/hydraulic)
Large heavy work/large payload ROV (electro/hydraulic)
Ultra-deep heavy work/large payload ROV (electro/hydraulic)

7. Examples of ROVs Check organisational web sites
Check suppliers and operators websites:

8. NTNU ROV Minerva NTNU has bought an ROV for biological research
2 Dr. ing studies have been allocated to develop tools and procedures for scientific use of the ROV
For more info see the web site:

9. Minerva ROV

10. ROV operational goals Visual inspection
Inspection of underwater structures
Observation of ongoing work tasks on subsea structures
Biological observation
Different types of mechanical inspection
Non destructive testing
Mechanical work
Biological sampling, water column and bottom

11. Animation of scientific ROV operation
Take a look at
for an animation of a planned scientific ROV operation for biological sampling at sea bottom

12. Equation of motion for ROVs
6 degree of freedom (6DOF) model
No defined steady state motion as a baseline for development of motion equations
ROVs are usually asymmetrical up-down and fore-aft
As far as possible the ROVs are designed for port-starboard symmetry
See section 4.6 of lecture note for ROV motion equation

13. Hydrodynamic added mass/moment of inertia
6 x 6 matrix
Non-diagonal terms exists
Terms may have different values for positive and negative accelerations, especially for heave and pitch motion
Ideal fluid sink-source methods can be used
Motion decay tests can be used to find some terms
Generalized Planar Motion Mechanism tests can be used to find all terms
Simplified 2D crossections can be used to estimate some of the terms

14. Velocity dependent forces (drag and lift)
Non linear terms are important
Streamlining of bouyancy elements influence both drag and lift forces and moments
Motion decay tests can be used to find some drag terms
Generalized Planar Motion Mechanism tests can be used to find all terms

15. Minerva 1:5 scale model test

16. Minerva 1:5 scale model test

17. Other forces Gravity and buoyancy forces and moments
Thruster forces and moments
Control forces from any additional control units
Umbilical forces
Environmental forces
Interaction forces from bottom and/or sea bed structures