Taut leg mooring synthetic ropes

Topics of general, but wind-related interest.

Moderator: Bonnie.Jonkman

Quentin.Agoulon
Posts: 1
Joined: Fri Jul 14, 2017 2:34 am
Organization: UPC - ICIT
Location: Barcelona

Taut leg mooring synthetic ropes

Postby Quentin.Agoulon » Fri Jul 14, 2017 8:18 am

Hello,

For my master thesis, I'm currently working on the OC3 spar buoy concept.
I experiment with the mooring systems and want to change the catenary system made of chains to taut leg system with synthetic ropes.
All the other system parameters are unchanged.

However, during the simulation I get the following error message "Maximum outer_loop iterations reached" after 115 seconds.

Even increasing the defaut value of the tolerance from 1.0E-6 to 1.0E-3 doesn't solve the problem.

Please find enclosed my MAP++ input file.

Thank you for your help

Kind regards,

Quentin AGOULON
Attachments
MAP++ input file_mooring synthetic ropes.JPG
Taut leg mooring synthetic ropes
MAP++ input file_mooring synthetic ropes.JPG (63.01 KiB) Viewed 2519 times

Vincent.Arnal
Posts: 1
Joined: Mon Oct 02, 2017 7:50 am
Organization: LHEEA Laboratory
Location: France

Re: Taut leg mooring synthetic ropes

Postby Vincent.Arnal » Thu Oct 05, 2017 1:08 am

Dear Quentin,

What you are experiencing is the difficulty to solve catenary equations.
You have 2 connections points, which make the equations more difficult to solve.

Did you carefully select the initial values for the connection points ?

I had the same kind of troubles. Solving catenary equations is not really robust.

I recommend you some few things :
- Initial values should be reasonable (if a segment is too streched it doesn't work)
- try first without connection points ( with an equivalent mass for the full line)
- from my experience, changing the solver options usually don't solve the problem

Kind regards,
Vincent

Kristopher.Reed
Posts: 4
Joined: Tue Jan 08, 2019 7:15 pm
Organization: University of Maine
Location: Maine

Re: Taut leg mooring synthetic ropes

Postby Kristopher.Reed » Fri Jan 11, 2019 10:40 am

Good Afternoon,

I'm running into similar trouble. I am modeling the DeepCwind spar mooring line connection. The system consists of a similar arrangement as the example input file described in the MAP++ user manual. The system has a delta connection, which a 424.4 m long line connects to the sea bed and two 30 m lines that are considered 'infinitely' rigid and connected to the spar. The last two mooring lines in the model are 'rigid' sections that connect the 30m lines to the spar.

My current error is ' Zero pivot detected in LU factorization.', which I know is an error in the numerical calculations. With the help of a MAP++ support representative and reading previous posts, I have tried a number of different solutions. First, I have confirmed my node's coordinates and line lengths in SolidWorks (first attached file). Second, in the 'Solver Options' section of the MAP input file, I have adjusted the maximum outer loop iterations or 'outer_max_its', the outer and inner tolerances, and tried the different Jacobian solvers. As described by a MAP++ support representative, I've made sure the inner tolerances are less than the outer tolerances (see file 'map_kristopher3'). Maybe the system cannot be solved with the number of connect-nodes?

I have also tried modeling a system without the 'connect' - node types. I tried to model a simple system which has three lines connecting to the spar and the seabed and the error 'Node 18 does not project onto any line2 element.' appears (see file 'map_kristopher2'). I don't understand why this error occurs. The geometry is correct, so the lines and nodes should be connected.

Any help would be appreciated.

Take care,
Kristopher Reed, EIT
Graduate Mechanical Engineering Student
University of Maine


MAP++ input file:
https://map-plus-plus.readthedocs.io/en ... html#flags
Attachments
Untitled document.docx
Delta connection mooring line system - SolidWorks file.
(309.6 KiB) Downloaded 18 times
map_kristopher3.txt
Delta connection mooring line system - MAP++ input file.
(1.71 KiB) Downloaded 21 times
map_kristopher2.txt
Simplified mooring system - MAP++ input file.
(1.11 KiB) Downloaded 20 times

Marco.Masciola
Posts: 20
Joined: Fri Apr 12, 2013 3:53 pm
Organization: Siemens Gamesa
Location: In Colorado

Re: Taut leg mooring synthetic ropes

Postby Marco.Masciola » Sat Jan 12, 2019 3:28 am

I noticed input file map_kristopher3.txt has three materials with EA varying between 96E6 and 96E9, and individual line lengths varying between 424 m and 0.5 m. Given this variance in stiffness, it's not surprising the inverse Jacobian is failing. Zero pivot tells us the outer loop Jacobian is not full rank and the matrix is insufficient to find a unique solution. I quickly estimated the Jacobian condition number to be order(1000000), which is indicative of an ill conditioned problem. The Krylov boostrap option may work, but the source of the trouble is the wide stiffness range (and especially the stiff 0.5 meter line). You'll find easier convergence to a solution omitting the half meter line.

The solver tolerances look a bit loose for the degree of stiffness in your system. The outer loop solver tolerance is 0.1 with a 0.5 m length element defined. If you remove material matThree and apply these options:

outer_tol 1e-3
inner_ftol 1e-6
inner_gtol 1e-6
inner_xtol 1e-6
outer_max_its 1000
outer_fd
outer_epsilon 1e-5
repeat 120 240
krylov_accelerator 4

Then MAP will converge.
Marco Masciola

Kristopher.Reed
Posts: 4
Joined: Tue Jan 08, 2019 7:15 pm
Organization: University of Maine
Location: Maine

Re: Taut leg mooring synthetic ropes

Postby Kristopher.Reed » Sat Jan 12, 2019 10:36 am

Dear Dr. Masciola,

Thank you for your response and help. I will try your suggestions.

Take care,

-Kris

Kristopher.Reed
Posts: 4
Joined: Tue Jan 08, 2019 7:15 pm
Organization: University of Maine
Location: Maine

Re: Taut leg mooring synthetic ropes

Postby Kristopher.Reed » Mon Jan 14, 2019 11:11 am

Hey,
Unfortunately the program failed to converge because the LAPACK library is not compiled in the MAP++ version that I am using. I am using the MAP++ v.1.20.10 for Windows from the following link: https://nwtc.nrel.gov/MAP.

Are you using the Linux version or a modified Windows version?

Thanks for your help,

-Kris

Marco.Masciola
Posts: 20
Joined: Fri Apr 12, 2013 3:53 pm
Organization: Siemens Gamesa
Location: In Colorado

Re: Taut leg mooring synthetic ropes

Postby Marco.Masciola » Mon Jan 14, 2019 11:01 pm

Hi Kristopher,

The version of map I used is indeed built on linux.

I wasn't aware of this, but your error message confirms the windows binary distributed on the NWTC site is compiled without lapack. It likely went unnoticed for a while because the krylov_accelerator option is rarely used and relies on a single call to a lapack (LAPACK_dgels) matrix factorization routine. This option can be activated by recompiling MAP with the MKL library (if using Intel icc) and the "WITH_LAPACK" compiler flap activated. Unfortunately I don't have access to a windows compiler (otherwise I could compile it).

As an aside, the Jacobian condition number created by your geometry requires extreme measures to solve. Although the krylov_accelerator flag will help, needing it is a sign the problem is ill-posed. If you remove the half meter lines in your model it will converge easier. MAP wasn't envisioned to solve lines discretized into half meter resolution, so I think your problem pushes the bounds of what MAP was designed for.
Marco Masciola


Return to “General”

Who is online

Users browsing this forum: No registered users and 1 guest