TMR4225 Marine Operations, 2004.03.04 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
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)
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
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
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
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)
Examples of ROVs Check organisational web sites http://www.rov.org http://www.rov.net http://www.rovworld.com http://www.diveweb.com/rovs/features/uv-wi99.01.htm Check suppliers and operators websites: http://www.kystdesign.no http://www.sperre-as.com http://www.oceaneering.com http://www.stoltoffshore.com
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: http://www.ivt.ntnu.no/imt/minerva
Minerva ROV
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
Animation of scientific ROV operation Take a look at http://www.rpos.com/desc/anime.htm for an animation of a planned scientific ROV operation for biological sampling at sea bottom
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
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
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
Minerva 1:5 scale model test
Minerva 1:5 scale model test
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