Migrating from MoorDyn or OrcaFlex

CableDyn reads MoorDyn v2 vocabulary and uses the line-and-section arrangement and output conventions that users of OrcaFlex (by Orcina) know, so both kinds of model can be ported. This page maps the concepts, shows what changes, and ends with a checklist.

From MoorDyn 

A stock MoorDyn deck runs almost unchanged 

CableDyn accepts the MoorDyn v2 file layout: the --- MoorDyn Input File --- banner and title, LINE TYPES with the stock Cd Ca CdAx CaAx column order (recognised from the header row), POINTS with Fixed/Vessel/Coupled/Free/Connect attachments, the stock 7-column LINES row, SOLVER OPTIONS, and OUTPUTS closed by END.

A stock LINES row

ID     LineType   AttachA   AttachB   UnstrLen   NumSegs   Outputs
1        chain       1         4        850.0       50         -

is read as a line from point 1 to point 4 made of one section of chain, 850 m long, 50 elements — exactly equivalent to a CableDyn LINES row 1 4 1 - plus one SECTIONS row 1 chain 850.0 50. MoorDyn writes lines anchor-first; when AttachA is a Fixed anchor and AttachB a Vessel/Coupled/Body fairlead, CableDyn swaps the ends so that End A is the fairlead, as everywhere in CableDyn. Endpoint channels therefore keep their physical meaning: FairTen is at the fairlead, AnchTen at the anchor.

examples/openfast/MoorDyn_UMaine.dat is such a stock deck. For a standalone run it needs only what OpenFAST (maintained by NLR, the National Laboratory of the Rockies, formerly NREL) would otherwise provide — a water depth — and either no dtM or both dtM and TMax, and it must not ask for the host SeaState field:

200.0    WtrDpth   - water depth (m)          <- add (OpenFAST supplies it when coupled)
0.0002   dtM       - ...                      <- delete for a static run, or add TMax
SeaState WaterKin  - ...                      <- delete (coupled runs only)

With those three edits CableDyn_driver.exe reproduces the same 2.437 MN fairlead tension as the native deck of Tutorial 2 — A spread mooring and its output channels. In OpenFAST the stock deck works unchanged as a MooringFile for CompMooring = 5; its dtM = 0.0002 is below the glue step, so CableDyn says so on the console and uses dtM = DT (0.025 s) instead. Keywords that only steer MoorDyn’s dynamic-relaxation start (dtIC, TmaxIC, CdScaleIC, threshIC) are accepted and have no effect: CableDyn’s initial condition is a Newton static equilibrium.

When to use SECTIONS instead 

The 7-column row can describe only a single-type line. A composite (chain–polyester–chain, or a power cable with bare, buoyancy, and bend-stiffener zones) is, in MoorDyn, several lines joined at Connect points; in CableDyn it is one line with several ordered sections:

--------------------- LINES --------------------------------------------
ID    NodeA   NodeB   Outputs
1     2       1       -
--------------------- SECTIONS -----------------------------------------
LineID   LineType    Length   NumSegs
1        chain       100.0    20
1        polyester   600.0    60
1        chain       250.0    25

One line object keeps one continuous arc length, one static profile, and exact continuity at the joints, with no artificial connection mass. A line is defined either by one 7-column row or by a 3/4-column LINES row plus its SECTIONS — not both.

Keyword map 

MoorDyn	CableDyn
`LINE TYPES` `Name Diam MassDen EA BA/-zeta EI Cd Ca CdAx CaAx`	same row; the native order is `Cd_n Cd_t Ca_n Ca_t` and the header decides which is meant. `EI > 0` switches the line to the bending element.
`ElasticMod 2/3` viscoelastic `EA`/`BA`	`Es\|Ed` / `Es\|alphaMBL\|vbeta` in `EA` and `BA_s\|BA_d` in `BA`
Syrope line type and `SYROPE IC`	`"SYROPE:<settings>\|alpha\|beta"` and the same `SYROPE IC` section
`POINTS` `Fixed` / `Vessel` / `Coupled`	same; fairleads are `Coupled` (`Vessel` is an alias)
`POINTS` `Free` / `Connect` with `M V CdA CA`	same, on dynamic decks
`BODIES`, `RODS`, `ROD TYPES`	supported subset: `Point3`/`Rigid6` bodies and rigid rods, lines attached through `Body<N>` points and at rod ends (`R<N>A`/`R<N>B` in `LINES`, or `Rod<N>A`/`Rod<N>B` points) (Tutorial 7 — Buoys, bodies, and rods); `ROD TYPES` `Cd Ca CdEnd CaEnd` as in MoorDyn, with optional axial side `CdAx CaAx` columns
`dtM`, `TMax` (standalone), `WtrDpth`, `rhoW`/`WtrDnsty`, `g`	same
`kBot`, `cBot`, seabed friction coefficient	`kBot`, `cBot`, `frictionMu` (alias `mu`)
`WaterKin` file (`CurrentMod 1`, `WaveKinMod 1`)	same file; `WaveKinMod 2` and `SEASTATE` only when coupled to OpenFAST
`FAILURE`, `CONTROL` sections	`FAILURE` on `EI = 0` decks, standalone and in OpenFAST; `CONTROL` on `EI = 0` decks in OpenFAST (the standalone driver rejects it). A deck with a finite-EI cable rejects both
`FairTen`, `AnchTen`, point/node channels	same names; tensions in N (Output files and channels)

What differs on purpose 

Topic	CableDyn
Initial condition	always a Newton static equilibrium from an analytical seed; no dynamic relaxation, no `CdScaleIC` tuning, and no dependence on the time step. Delete any `ICmode` row: it is not a MoorDyn option and CableDyn rejects it
Element	finite elements with an implicit generalised-α integrator, not a lumped-mass explicit scheme; `EI > 0` lines use a geometrically exact bending element
Time step	chosen for accuracy, not stability: an implicit `dtM` of 0.025–0.1 s is usual, while an explicit lumped-mass scheme needs a far smaller step on stiff chain (the MoorDyn twin deck in `examples/openfast/` uses 2e-4 s)
Unsupported input	rejected at parse time with a message naming the feature, never silently ignored
Node identity	CableDyn meshes are its own; MoorDyn node numbers and rod-node histories are not reproduced

Inside OpenFAST 

Set CompMooring = 5 instead of 3 and point MooringFile at the CableDyn (or stock MoorDyn) deck. Both modules are in the same openfast.exe, so an A/B comparison is one integer (Tutorial 9 — A floating wind turbine in OpenFAST). The supported coupled surface and its limits are in Capabilities and route selection.

From OrcaFlex 

CableDyn uses the same line arrangement and conventions as OrcaFlex: a line runs from End A to End B through sections of different line types and mesh densities; tension is effective tension; declination, azimuth, and the global frame follow OrcaFlex (Conventions).

OrcaFlex	CableDyn
line type (outer diameter, mass per length, EA, EI, Cd, Ca)	`LINE TYPES` row; `Diam` is the hydrodynamic and displacement diameter
line with sections of different line types and target segment lengths	one `LINES` row + ordered `SECTIONS` rows with explicit `NumSegs`
End A / End B	identical: End A is the fairlead or hang-off, End B the anchor or termination
end connection stiffness (bend stiffness at a line end)	`END CONNECTIONS` row: numeric rotational stiffness, `Pinned`, or `Rigid`
vessel / fixed / anchored connections	`Coupled` / `Fixed` points (`Coupled` is driven by a `motionFile`, a `vesselMotion` record, a `vesselRAO` table, or OpenFAST)
6D buoy, 3D buoy, clump	`Rigid6` / `Point3` bodies, `Free` points with mass and volume
lazy wave with distributed buoyancy modules	a net-buoyant middle section (equivalent diameter and mass), or an `EQUIVALENT BUOYANCY` table (Deck format reference (.dat))
seabed stiffness and friction	`kBot`, `cBot`, `frictionMu`; flat `WtrDpth` or a `bathymetryFile`
statics	automatic: no catenary pre-shape, no user starting shape
range graph (tension, curvature, bend moment vs arc length)	`<root>.static.out` for the static state; `<root>.Line<L>.range.out` (`LINES` `Outputs` flag `r`) for the minimum, maximum and mean over a dynamic run (Output files and channels)
time-history results	`OUTPUTS` channels in `<root>.out`; `p`/`t` per-line files
wave and current environment	`waves` rows (for example `airy`, `jonswap`, `stream`, `pm`, `torsethaugen`, `ochihubble`), `wavetrain` rows and `current` (`uniform`/`profile`) options, or a WaterKin file (OPTIONS reference and defaults); OpenFAST SeaState when coupled

Practical differences: the model is a plain-text SI deck rather than a .dat/.yml model file; section meshes are given as element counts; and each option is a single keyword row. The lazy-wave reference cables of Tutorial 3 — A lazy-wave power cable were compared with OrcaFlex 11.6d; the results are in CableDyn verification and validation.

Porting checklist 

Decide per line: a single-type line may keep the stock 7-column row; a composite becomes one LINES row plus ordered SECTIONS from End A (fairlead) to End B (anchor).
Give cables and bending-dominated lines EI > 0; keep chain and rope at EI = 0.
Standalone: add WtrDpth (and g/rhoW if not default); set either no dtM (static) or both dtM and TMax. Coupled: OpenFAST supplies depth, density, gravity, and time.
Translate the environment: waves, current, kBot/cBot/frictionMu, bathymetryFile.
List one channel per OUTPUTS row; set p/t on lines whose full profile you need.
Run the static case first and compare fairlead tension, touchdown, and — for cables — the curvature profile with the source model before running dynamics.
If the parser rejects something, the message names the feature and the line; Troubleshooting lists every message and its fix.