MoorDyn: Feature/syrope for polyester

[zhilongwei]

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.

[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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[andrew-platt]

Dear @zhilongwei,

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

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

1. create a fork of the r-test repository
2. using the dev base branch, add your new test on an r-test branch (use the same name as this feature)
3. for a new test case, edit the <openfast>/reg_tests/CTestList.cmake on your OpenFAST branch.
4. 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]

Dear @zhilongwei,

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

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

1. create a fork of the r-test repository
2. using the dev base branch, add your new test on an r-test branch (use the same name as this feature)
3. for a new test case, edit the <openfast>/reg_tests/CTestList.cmake on your OpenFAST branch.
4. 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 818aa5a855a9adfbf6d9913775453b48dd4a6b2cff9b577119f5d056168d711b291d0156ee033955, 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

[andrew-platt]

I won't claim I understand the code, but the form looks good.

Thanks @zhilongwei for posting this, and thank you @RyanDavies19 for reviewing it!

[andrew-platt]

Added a merge commit to resolve the r-test conflict. I'll merge after tests complete.