Tutorial 6 — Synthetic ropes
Goal: choose between the three polyester/nylon models — linear EA, viscoelastic (a
series-Kelvin model with slow and dynamic stiffness), and Syrope (working curve with load
history) — and see where each one matters.
Decks: polyester_catenary_mooring.dat, ve_polyester_catenary_mooring.dat,
ve_nylon_loaddependent_mooring.dat, ve_polyester_dynamic_waves.dat,
syrope_polyester_mooring.dat (+ data/syrope/) · Route: EI = 0 cable path ·
Run time: < 1 s each
Why a rope needs more than one EA
A polyester or nylon rope is stiffer under wave-frequency cycling than under a slowly applied
mean load, and its stiffness depends on the largest load it has ever carried. A single EA
must therefore be chosen for one purpose: the static (slow) stiffness sets the mean offset,
the dynamic stiffness sets wave-frequency tension ranges. CableDyn offers three levels,
selected entirely by the EA and BA tokens of the LINE TYPES row — no extra columns:
TypeName Diam Mass EA BA ...
polyester 0.1438 22.42 1.42e8 -1.0 ...
poly_ve 0.1438 22.42 1.424e8|2.50e8 4.0e9|1.1e7 ...
nylon_mean_load 0.1500 24.00 6.0e7|1.00e8|0.4 4.0e9|1.1e7 ...
rope 0.1438 22.42 "SYROPE:data/syrope/syrope_settings.dat|1.53e8|23.12" 5.0e10|1.0e5 ...
Model |
|
Use it when |
|---|---|---|
linear |
|
first estimates, or when a single secant stiffness is justified for the load case |
viscoelastic (series-Kelvin), constant dynamic stiffness (MoorDyn |
|
the vendor gives a static and a dynamic stiffness; mean offset and wave ranges both matter |
viscoelastic, load-dependent dynamic stiffness ( |
|
nylon or polyester whose dynamic stiffness grows with mean load |
Syrope |
|
polyester with a measured original working curve and a known load history (pretension and storm peaks shift the working curve) |
The equations are in Theory; the token rules in Deck format reference (.dat).
Statics: the slow stiffness decides
New-Item -ItemType Directory -Force results | Out-Null # already there after the quickstart
.\CableDyn_driver.exe .\examples\polyester_catenary_mooring.dat .\results\poly_lin
.\CableDyn_driver.exe .\examples\ve_polyester_catenary_mooring.dat .\results\poly_ve
Both decks describe the same 690 m semi-taut polyester leg in 200 m of water:
poly_lin.out FairTen1 = 4.2291139E+004 AnchTen1 = 3.1765601E+004 FairIncl1 = 4.0917746E+001
poly_ve.out FairTen1 = 4.2291825E+004 AnchTen1 = 3.1766281E+004 FairIncl1 = 4.0917412E+001
They agree to 0.002 %: the static solution of a viscoelastic rope uses the slow stiffness Es
(1.424e8 N, versus 1.42e8 N in the linear deck). The dynamic branch Ed only engages when
the tension cycles. ve_nylon_loaddependent_mooring.dat (a 46.6°-inclined nylon leg,
FairTen1 = 30.01 kN) behaves the same way in statics.
Dynamics: the dynamic stiffness decides
ve_polyester_dynamic_waves.dat marches the viscoelastic leg for 10 s under a 1.5 m, 10 s
Airy wave. Make a linear twin by replacing the line type with poly_lin 0.1438 22.42 1.424e8
-1.0 0.0 1.2 0.2 1.0 0.0 (and the SECTIONS type name), then run both:
linear twin FairTen1 mean 42.29 kN min 42.15 kN max 42.45 kN
viscoelastic FairTen1 mean 42.29 kN min 42.15 kN max 42.42 kN
With the fairlead held, the wave only acts on the rope itself and the tension barely moves, so the models agree. Drive the fairlead with platform motion (Exercise 1) and the stiffer dynamic branch produces visibly larger tension ranges for the same motion — the effect that governs fatigue of synthetic moorings.
Syrope: working curve and load history
The Syrope deck is a 20 m taut polyester test leg. Its EA token names a settings file; the
settings file names the original working-curve (OWC) table:
data/syrope/syrope_settings.dat:
syrope_owc.dat OWC - Original working-curve table file (relative to this file)
EXP WCType - Working-curve formulation {LINEAR; QUADRATIC; EXP}
0.20 k1 - First working-curve shape parameter
1.50 k2 - Second working-curve shape parameter
and a SYROPE IC section gives the rope’s history before the simulation starts — the
largest tension it has seen (Tmax0) and its mean tension (Tmean0):
--------------------- SYROPE IC ----------------------------------------
Line(s) Tmax0 Tmean0
1 2.0e6 1.5e6
.\CableDyn_driver.exe .\examples\syrope_polyester_mooring.dat .\results\syrope
The static initial condition is solved on the Tmax0 working curve, and the line starts in
that equilibrium: FairTen1 is 972.8 kN at t = 0 and stays there over the 2 s run (the fairlead
is held). With both ends fixed, the geometry and Tmax0 set the mean tension, so Tmean0
does not enter the state. The driver prints a note because this deck’s 1.5 MN differs from the
972.8 kN equilibrium. Syrope lines are single-section and run dynamically (dtM/TMax).
Important
Copy the Syrope deck together with data/syrope/syrope_settings.dat and
data/syrope/syrope_owc.dat. Paths are relative to the file that names them; the OWC table
is part of the material model and belongs in your analysis record.
Exercises
Cycled rope. Give
ve_polyester_dynamic_waves.data motion file with a 2 m, 10 s surge of point 2 (see Tutorial 4 — Prescribed fairlead motion for the file format), run the viscoelastic deck and its linear twin, and compare tension standard deviations over the last two cycles.Load history. In the Syrope deck (copied with its
data/syropefolder), raiseTmax0from 2.0e6 to 3.0e6 N and rerun. The higher past peak leaves more permanent elongation and a softer working curve, so at the same 20.5 m span the rope carries 709.2 kN instead of 972.8 kN. Load history is a first-order input for polyester.Vendor data. For your own rope, write down the source of
Es,Ed, the damping pair, the MBL scaling, and the working curve before running anything.