MoorDyn: Feature/syrope for polyester by zhilongwei · Pull Request #3324 · OpenFAST/openfast

zhilongwei · 2026-05-01T14:14:48Z

Feature or improvement description

This pull request implements the Syrope model for polyester mooring ropes to MoorDyn-F.

The same implementation has already been completed and approved in MoorDyn-C. See PR #369.

This implementation captures:

nonlinear, path-dependent, and loading-rate-dependent tension-strain behavior of polyester ropes,
the dependence of dynamic stiffness on mean tension in both wave-frequency and low-frequency regimes,
quasi-static response based on working curves.

Impacted areas of the software

MoorDyn-F

Additional supporting information

Documentation has been updated to describe how to use the model. See PR #369.
A detailed reference for the numerical implementation is provided in:
ESOMOOR D5.1: Extended MoorDyn solver and validation report. The theory and validation remain valid but the input file format has changed since our discussion during the MoorDyn-C PR.
I tested the new functionality with the IEA-15-250-RWT semi-sub floater. Examples are available at https://github.com/zhilongwei/IEA-15-240-RWT/tree/ESOMOOR-Mooring/OpenFAST/IEA-15-240-RWT-UMaineSemi-ESOMOOR, where a README is also provided.

Generative AI usage

Assisted by: GitHub Copilot support@github.com

Test results

Three tests failed (see LastTest.log):
- 27 - 5MW_OC4Jckt_DLL_WTurb_WavesIrr_MGrowth (Failed) aerodyn elastodyn hydrodyn offshore openfast servodyn subdyn
- 71 - Damped_Beam_Rotating (Failed) beamdyn linear openfast
- 83 - MHK_RM1_Floating_MR_Linear (Failed) aerodyn elastodyn hydrodyn linear mhk moordyn multirotor offshore openfast servodyn
An r-test branch merge is required. A new test, md_syrope, has been added to r-test/modules/moordyn following the existing md_viscoelastic test. Like md_viscoelastic, this test is skipped by default. See https://github.com/zhilongwei/r-test/tree/feature/syrope-regtest/modules/moordyn/md_syrope.
OpenFAST version

**************************************************************************************************
 OpenFAST

 Copyright (C) 2026 National Renewable Energy Laboratory
 Copyright (C) 2026 Envision Energy USA LTD

 This program is licensed under Apache License Version 2.0 and comes with ABSOLUTELY NO WARRANTY.
 See the "LICENSE" file distributed with this software for details.
 **************************************************************************************************

 OpenFAST-v5.0.0-69-gf0549eae
 Compile Info:
  - Compiler: GCC version 14.2.0
  - Architecture: 64 bit
  - Precision: double
  - OpenMP: No
  - Date: May  1 2026
  - Time: 11:08:49
 Execution Info:
  - Date: 05/01/2026
  - Time: 11:23:27+0000


  OpenFAST terminated normally.

Notes for reviewers

I am not sure about the workflow for creating a PR to r-test. Should I complete this PR to OpenFAST first? Clarification on this would be appreciated.

…re/syrope-for-polyester

Copilot

Pull request overview

Implements the Syrope polyester rope model in MoorDyn-F by extending line-type parsing, line state/data structures, and line tension/working-curve evaluation to support nonlinear, path-dependent rope behavior with optional initial conditions.

Changes:

Add input parsing for SYROPE:<working-curve-file> in line-type EA entries and a SYROPE IC section for initializing Syrope history variables.
Extend MoorDyn line/line-type data structures + registry serialization to store Syrope working-curve parameters and history state (e.g., Tmax, Tmean).
Add a new 1D interpolation helper and integrate Syrope working-curve setup/evaluation into line initialization and state-derivative calculations.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
modules/moordyn/src/MoorDyn.f90	Parses Syrope configuration/ICs and adds a working-curve file reader.
modules/moordyn/src/MoorDyn_Line.f90	Allocates/uses Syrope state and implements working-curve setup + Syrope tension/damping behavior.
modules/moordyn/src/MoorDyn_Misc.f90	Adds `calculate1DinterpolationXY` for table lookups used by Syrope.
modules/moordyn/src/MoorDyn_Types.f90	Adds Syrope fields to types and updates copy/pack/unpack/destroy logic.
modules/moordyn/src/MoorDyn_Registry.txt	Registers new Syrope fields for serialization.

Comments suppressed due to low confidence (1)

modules/moordyn/src/MoorDyn.f90:2809

Lines using ElasticMod==3 also store dl_1 in the state vector (Line_Initialize sets dl_1 for ElasticMod>1 and <4), but this initialization only runs for ElasticMod==2 or 4. Include ElasticMod==3 here so the initial state vector is consistent with the line object's dl_1 values.

         ! if using viscoelastic model or Syrope model, initialize the internal states
         if (m%LineList(l)%ElasticMod == 2 .or. m%LineList(l)%ElasticMod == 4) then
            do I = 1,N
               x%states(m%LineStateIs1(l) + 6*N-6 + I-1) = m%LineList(l)%dl_1(I)   ! should be zero
            end do
         end if

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zhilongwei · 2026-05-06T07:45:32Z

+      CHARACTER(40)                :: owcPath              !
+      CHARACTER(40)                :: wcFormula            !


I followed the same convention as for TempString1–TempString6, where TempString1 stores the path to the nonlinear stiffness lookup file. I assumed this is intentional, likely to limit the path length.

zhilongwei · 2026-05-06T07:15:34Z

+
+                  ! check for correct number of columns in current line
+                  IF ( CountWords( Line ) /= N+2 ) THEN 
+                     CALL SetErrStat( ErrID_Fatal, ' Unable to parse line failure '//trim(Num2LStr(l))//' on row '//trim(Num2LStr(i))//' in input file. Row has wrong number of columns. Must be 5 columns.', ErrStat, ErrMsg, RoutineName )


I simply copyed the similar syntex from the line failure section and forgot to update the error message.

+                     if (TempIDnums(il) <= p%nLines) then      ! ensure line ID is in range
+                        DO j = 1, m%LineList(TempIDnums(il))%N
+                           m%LineList(TempIDnums(il))%Tmax(j) = Tmax0
+                           m%LineList(TempIDnums(il))%Tmean(j) = Tmean0
+                        END DO
+                     else
+                        CALL SetErrStat( ErrID_Fatal, ' Unable to parse Syrope line initial conditions '//trim(Num2LStr(l))//'. Line number '//TRIM(Int2LStr(TempIDnums(il)))//' out of bounds.', ErrStat, ErrMsg, RoutineName )
+                        CALL CleanUp()


+                  IF (ErrStat2 == 0) THEN
+                     READ(Line,*,IOSTAT=ErrStat2) TempIDnums(1:N), Tmax0, Tmean0


zhilongwei · 2026-05-06T07:55:21Z

+            ErrMsg  = ' Error allocating Syrope arrays, Tmax and Tmean.'
+            !CALL CleanUp()
+            RETURN
+         END IF


Line%Tmax and Line%Tmean are already initialized when reading the SYROPE IC section in MoorDyn.f90.

DO il = 1, N if (TempIDnums(il) >= 1 .and. TempIDnums(il) <= p%nLines) then ! ensure line ID is in range DO j = 1, m%LineList(TempIDnums(il))%N m%LineList(TempIDnums(il))%Tmax(j) = Tmax0 m%LineList(TempIDnums(il))%Tmean(j) = Tmean0 END DO else CALL SetErrStat( ErrID_Fatal, ' Unable to parse Syrope line initial conditions '//trim(Num2LStr(l))//'. Line number '//TRIM(Int2LStr(TempIDnums(il)))//' out of bounds.', ErrStat, ErrMsg, RoutineName ) CALL CleanUp() return endif END DO

+      ! find static strain of the slow spring on the original working curve if Syrope model is used
+      if (Line%ElasticMod == 4) then
+         DO I = 1,Line%nEApoints
+            Line%stiffZs(I) = Line%stiffXs(I) - 1.0 / Line%vbeta * log(1.0 + Line%vbeta / Line%alphaMBL * Line%stiffYs(I))
+         END DO
+      end if


+      REAL(DbKi)                             :: WCK1          ! Syrope working curve parameter 1
+      REAL(DbKi)                             :: WCK2          ! Syrope working curve parameter 2
+



+      Real(DbKi)                       :: WCK1           ! coefficient for the Syrope working curve formula
+      Real(DbKi)                       :: WCK2           ! coefficient for the Syrope working curve formula


+                      CALL MD_ReadSyropeWorkingCurves(tempString6, owcPath, wcFormula, wcK1, wcK2, ErrStat2, ErrMsg2)
+
+                      if (ErrStat2 /= ErrID_None ) then
+                          CALL SetErrStat( ErrID_Fatal, 'Failed to read SYROPE working curves for line type '//trim(Num2LStr(l))//'. Check error message for details.', ErrStat, ErrMsg, RoutineName )
+                          CALL CleanUp()
+                          RETURN
+                      end if


andrew-platt · 2026-05-05T15:20:43Z

Dear @zhilongwei,

Thanks for adding this feature! We'll review this shortly.

For regression tests, the best method we have found is the following:

create a fork of the r-test repository
using the dev base branch, add your new test on an r-test branch (use the same name as this feature)
for a new test case, edit the <openfast>/reg_tests/CTestList.cmake on your OpenFAST branch.
commit the r-test sub-module change to your OpenFAST branch

If everything tests out ok, we will merge the r-test branch during the merge of this PR. This is admittedly not the cleanest workflow, but we haven't found a better solution yet.

Regards,

zhilongwei · 2026-05-06T15:33:19Z

Dear @zhilongwei,

Thanks for adding this feature! We'll review this shortly.

For regression tests, the best method we have found is the following:

create a fork of the r-test repository

using the dev base branch, add your new test on an r-test branch (use the same name as this feature)

for a new test case, edit the <openfast>/reg_tests/CTestList.cmake on your OpenFAST branch.

commit the r-test sub-module change to your OpenFAST branch

If everything tests out ok, we will merge the r-test branch during the merge of this PR. This is admittedly not the cleanest workflow, but we haven't found a better solution yet.

Regards,

Dear @andrew-platt,

I found that r-test/dev is not currently synchronized with the OpenFAST branch used here: some cases are missing the new AutoRelax and RelaxFactor inputs. So I based my r-test branch on ~~818aa5a855a9adfbf6d9913775453b48dd4a6b2c~~ff9b577119f5d056168d711b291d0156ee033955, the r-test commit this OpenFAST branch was already using, and added the Syrope test on top. See https://github.com/zhilongwei/r-test/commits/feature/syrope-for-polyester/.

I have added the md_syrope as well as the existing md_viscoelastic to the CTestList.cmake. All tests passed except for the three I reported in the PR:

27 - 5MW_OC4Jckt_DLL_WTurb_WavesIrr_MGrowth (Failed) aerodyn elastodyn hydrodyn offshore openfast servodyn subdyn
71 - Damped_Beam_Rotating (Failed) beamdyn linear openfast
83 - MHK_RM1_Floating_MR_Linear (Failed) aerodyn elastodyn hydrodyn linear mhk moordyn multirotor offshore openfast servodyn

The new test md_syrope passed. See LastTest.log

Best regards

zhilongwei added 7 commits April 10, 2026 14:59

Defined new variables of Syrope model

a976072

1D interpolation from x and y arrays

4244f33

Read Syrope settings

fc7b738

Implementation for the Syrope model

7fe99b9

Allocate memory with stat

38b3dc6

Merge branch 'dev' of https://github.com/OpenFAST/openfast into featu…

3acaa3c

…re/syrope-for-polyester

Merge branch 'dev' of https://github.com/OpenFAST/openfast into featu…

f0549ea

…re/syrope-for-polyester

jjonkman added Type: Enhancement Module: MoorDyn labels May 1, 2026

andrew-platt requested review from RyanDavies19, andrew-platt and Copilot May 5, 2026 15:14

andrew-platt added this to the v5.1.0 milestone May 5, 2026

Copilot started reviewing on behalf of andrew-platt May 5, 2026 15:15 View session

Copilot AI reviewed May 5, 2026

View reviewed changes

zhilongwei added 5 commits May 6, 2026 07:23

Updated the error message in the SYROPE IC section

25cca6d

Line ID must be >=1 and used a robust way to read the ICs

3c146e4

Removed unused varaibles WCK1 and WCK2

94e001c

Handle Syrope working-curve warnings without aborting MD_Init

4dda4d2

New tests passed

d78fa63

dl_1 denotes stretch instead of strain

aca2b2f

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MoorDyn: Feature/syrope for polyester#3324

MoorDyn: Feature/syrope for polyester#3324
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		IF (ErrStat2 == 0) THEN
		READ(Line,*,IOSTAT=ErrStat2) TempIDnums(1:N), Tmax0, Tmean0

		REAL(DbKi) :: WCK1 ! Syrope working curve parameter 1
		REAL(DbKi) :: WCK2 ! Syrope working curve parameter 2

		Real(DbKi) :: WCK1 ! coefficient for the Syrope working curve formula
		Real(DbKi) :: WCK2 ! coefficient for the Syrope working curve formula

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zhilongwei commented May 6, 2026 •

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