Marine systems simulation
+ Collaboration diagram for Trawl cable:
Author
Jørgen Haavind Jensen
Date
Q4 2011 JHJ: Initial version.

Cable model with bending/torsional stiffness and winch-functionality. The cable is discretized as "6 degrees of freedom" rigid cylinders which are connected with axial and angular constraints. For high bending stiffness the cable will work as a truss/beam.

The constraints are regularized with a compliant variation of the Baumgarte method. In effect the cylinders can be thought of as connected with axial and angular springs like a traditional spring-mass-damper model, but high frequency dynamics are filtered out which reduces some of the numerical instability problems associated with high stiffness cables.

The cable solves full collision detection and reaction forces, both self-collision and other geometry that can be added through other SimObjects. Additionally the cable itself includes the collision geometry of a full winch drum.

Note
The hydrodynamic load model does not currently include the effect of added mass.

Configuration parameters

ParameterDefaultComment
CableLength-Total cable length [m]
NumElements-Number of discrete rigid mass elements in cable [#]
CableDiameter-Nominal cross sectional diameter [m]
TensileCrossSectionAreaAssumes the nominal area encircles 7 circular strands in a hexagonal patterncross sectional area of load bearing material [m^2]
TensileCrossSectionSecondAreaMomentAssumes the nominal area encircles 7 circular strands in a hexagonal patternsecond area moment of load bearing material [m^4]
TensileCrossSectionTorsionalAreaMomentAssumes the nominal area encircles 7 circular strands in a hexagonal patterntorsional area moment of load bearing material [m^4]
YoungsModulus2*10^11 (200GPa)Young's modulus of load bearing material [Pa],[N/m^2]
ShearModulusYoungsModulus/2.6Shear modulus of load bearing material [Pa],[N/m^2]
DisplacementCrossSectionArea90% of nominal areafluid displacement cross section area [m^2]
CableWeightTensileCrossSectionArea*7900kg/m^3Cable weight per meter [kg/m]
CableFrictionCoefficient1.2

Skin friction coefficient for the cable

RegulatorFrequencyCoefficient-Time response coefficient of the structure. A typical value would be less than 1/timestep, but it depends on the integrator method used.
StructuralDampingCoefficient1.0

Structural damping factor. A value of 1 gives critical damping.

TensionStabilityFactor1.0

Numerical stabilizer for when the cable is under heavy load. Increases effective bending stiffness, and decreases bending frequency response. Values from 1.0 and up.

WinchLength-Inner length of the winch drum
WinchInnerRadius-Inner radius of the winch drum
WinchOuterRadius-Outer radius the winch drum
WinchFrictionCoefficient0.8

Skin friction coefficient for the winch drum

InitialCableLengthOnWinch0For setting initial conditions. Determines how much cable should be already on the winch. [m]
InitialCableWindingOnWinchRightWindingShould the first layer of the initial winding be "RightWinding" or a "LeftWinding"?
WayPointN-Intermediate waypoints for initializing cable. An arbitrary number of waypoints (starting from #1) can be included. Cable elements are distributed in a hanging catenary shape between waypoints (the winch position and free end position are implied start/end-points). Takes 4 numbers, where the first three are coordinates and the last one is the length of cable to distribute between that waypoint and the last. [m]

Input ports

TagSizeComment
WinchPosition3Position of winch. [m]
WinchVelocity3Velocity of winch. [m]
WinchOrientation4Orientation of winch. [Quaternion(w,x,y,z)]
WinchAngularVelocity3

Angular velocity of winch. [rad/s]

EndPosition3Position of endpoint. [m]
EndVelocity3Velocity of endpoint. [m/s]
EndConnectionStrength1Relative strength of endpoint connection. Typically between [0,1]
InternalConnections2Interface for cutting/splicing cable. The port can be used to adjust the relative strength of one internal connection at a time. The first value is the index number of the constraint to adjust, and the second is the relative strength to apply, typically between [0,1].

Output ports

TagSizeComment
WinchForce3Reaction force on winch. [N]
WinchTorque3Reaction torque on winch. [Nm]
EndForce3Reaction force on endpoint. [N]

States

TagSizeComment
posN3The position of element N, from 0 -> NumElements-1. [m]
velN3The velocity of element N, from 0 -> NumElements-1. [m/s]
omegaN3The angular velocity of element N, from 0 -> NumElements-1. [rad/s]
thetaN4The quaternion(w,x,y,z) orientation of element N, from 0 -> NumElements-1. [quaternion(w,x,y,z)]

This SimObject is referred to as TrawlCable