## Moordyn

Topics of general, but wind-related interest.

Moderator: Bonnie.Jonkman

Krishnaveni.Balakrishnan
Posts: 22
Joined: Mon Feb 11, 2019 10:31 am
Organization: University of Massachusetts
Location: United States

### Moordyn

Dear Dr.Jonkman,

Am trying to understand how the Moordyn line properties are arrived in default OC4 Semi submersible and OC3 Hywind. OC4 default semisub has a Moordyn file of its own but OC3 does not have.Still in Moordyn manual there is an example file posted for OC3 Hywind Moordyn. So am comparing these two. Based on the paper 'validation of lumped mass mooring line model with DeepC semisubmerisble model test data' and other references I understand that we
step 1:choose Diameter& massdensity (MassDen)of the line first(may be from manufactures)
step 2:then find (equivalent)volume diameter i.e Diam_vol=sqrt((4 x MassDen)/(7850 x pi)).
step 3:Then followed by finding the using the below formula from DNV-GL
step 4:Modulus=[6 −0.0033 x Diam_vol] x 〖10〗^10 (Diam in mm)
step 4.a: If modulus if less than 5.6e10 then modulus is 5.6e10 based on DNV OS E301
step 5:And then Area=MasDen/7850 and finally E*A which is the stiffness of the line
If I follow these steps I could able to achieve the default OC4 line properties but am not able to get the default OC3 line properties(from Moordyn manual)

Default OC4 Line
1 NTypes - number of LineTypes
Name Diam MassDen EA BA/-zeta Can Cat Cdn Cdt
(-) (m) (kg/m) (N) (N-s/-) (-) (-) (-) (-)
main 0.0766 113.35 7.536E8 -1.0 0.8 0.25 2.0 0.4

OC3 Line(From Manual)

Name Diam MassDen EA BA/-zeta Can Cat Cdn Cdt
(-) ( m) (kg/m) (N) (N-s/-) (-) (-) (-) (-)
main 0.09 77.7066 384.243E6 -0.8 1.0 0.0 1.6 0.1

Kindly correct if my understanding on this concept is wrong and if is not kindly guide me on why am not able to get the values in case of OC3 Hywind.

Thanks
Krishnaveni

Jason.Jonkman
Posts: 4777
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Moordyn

Dear Krishnaveni,

I'm not familiar with the formulas you obtained from DNV GL, but your equations relating MassDen and Diam assume that the mooring line cross section is circular and made of steel. This may not always be a great assumption.

Regardless, the values of Diam, MassDen, and EA that you've stated for the OC4-DeepCwind semisubmersible and OC3-Hywind spar match those given in their specifications reports: https://www.nrel.gov/docs/fy14osti/60601.pdf (for OC4) and https://www.nrel.gov/docs/fy10osti/47535.pdf (OC3). I'm assuming these reports were the sources used to create these MoorDyn models.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Krishnaveni.Balakrishnan
Posts: 22
Joined: Mon Feb 11, 2019 10:31 am
Organization: University of Massachusetts
Location: United States

### Re: Moordyn

Dear Dr.Jonkman,

Thank you for your response to my earlier queries. I have one another question related to Moordyn Module
Am trying to do an analysis with anchors at varying depth both in Moordyn and in MAP++ modules
But am not able do it successfully.Below are my corresponding input files and the outputs am getting.
Platform -OC3 Hywind Spar
Water depth- 200m
Mooring line properties modified accordingly.

Moordyn Input file:

--------------------- MoorDyn Input File ------------------------------------
Mooring system for OC3-Hywind
FALSE Echo - echo the input file data (flag)
----------------------- LINE TYPES ------------------------------------------
1 NTypes - number of LineTypes
Name Diam MassDen EA BA/-zeta Can Cat Cdn Cdt
(-) (m) (kg/m) (N) (N-s/-) (-) (-) (-) (-)
main 0.09 168 1.15E9 -0.8 0.55 0.27 1.0 0.19
---------------------- CONNECTION PROPERTIES --------------------------------
6 NConnects - number of connections including anchors and fairleads
Node Type X Y Z M V FX FY FZ CdA CA
(-) (-) (m) (m) (m) (kg) (m^3) (kN) (kN) (kN) (m^2) (-)
1 Fixed 400.00 692.8 -202.0 0 0 0 0 0 0 0
2 Fixed -800.00 0.00 -200.0 0 0 0 0 0 0 0
3 Fixed 400.00 -692.8 -201.0 0 0 0 0 0 0 0
4 Vessel 2.6 4.50 -70.0 0 0 0 0 0 0 0
5 Vessel -5.2 0 -70.0 0 0 0 0 0 0 0
6 Vessel 2.60 -4.50 -70.0 0 0 0 0 0 0 0
---------------------- LINE PROPERTIES --------------------------------------
3 NLines - number of line objects
Line LineType UnstrLen NumSegs NodeAnch NodeFair Flags/Outputs
(-) (-) (m) (-) (-) (-) (-)
1 main 830 20 1 4 p
2 main 830 20 2 5 p
3 main 830 20 3 6 p
---------------------- SOLVER OPTIONS ---------------------------------------
0.001 dtM - time step to use in mooring integration (s)
3.0e6 kbot - bottom stiffness (Pa/m)
3.0e5 cbot - bottom damping (Pa-s/m)
2.0 dtIC - time interval for analyzing convergence during IC gen (s)
200 WtrDpth - water depth (m)
60.0 TmaxIC - max time for ic gen (s)
4.0 CdScaleIC - factor by which to scale drag coefficients during dynamic relaxation (-)
0.01 threshIC - threshold for IC convergence (-)
------------------------ OUTPUTS --------------------------------------------
FairTen1
FairTen2
FairTen3
AnchTen1
AnchTen2
AnchTen3
Con1fX
Con1fY
Con1fZ
Con2fX
Con2fY
Con2fZ
Con3fX
Con3fY
Con3fZ
END
------------------------- need this line --------------------------------------

Output

Am getting really large anchors forces (10000x) than when the anchors are at constant depth.

Map++ input file

---------------------- LINE DICTIONARY ---------------------------------------
LineType Diam MassDenInAir EA CB CIntDamp Ca Cdn Cdt
(-) (m) (kg/m) (N) (-) (Pa-s) (-) (-) (-)
Material 0.09 168 1.15E9 1 00 0.61 -1.0 0.19
---------------------- NODE PROPERTIES ---------------------------------------
Node Type X Y Z M B FX FY FZ
(-) (-) (m) (m) (m) (kg) (mË†3) (N) (N) (N)
1 fix 400.0 692.00 - 202 0 0 # # #
2 Vessel 2.6 4.5 -70.0 0 0 # # #
3 fix -800.0 0.0 -200 0 0 # # #
4 Vessel -5.2 0.0 -70.0 0 0 # # #
5 fix 400.0 -692.00 -201 0 0 # # #
6 Vessel 2.6 -4.5 -70.0 0 0 # # #
---------------------- LINE PROPERTIES ---------------------------------------
Line LineType UnstrLen NodeAnch NodeFair Flags
(-) (-) (m) (-) (-) (-)
1 Material 830.00 1 2 tension_fair tension_anch
2 Material 830.00 3 4 tension_fair tension_anch
3 Material 830.00 5 6 tension_fair tension_anch

Output

In case of using MAP++ am not able to get anchor forces from line 2 and line 3 ,they are always zero. Only line 1 has forces in it.

Kindly help me in fixing it /guide me where I could have gone wrong.

Thanks
Krishnaveni

Matt.Hall
Posts: 21
Joined: Tue Apr 05, 2011 2:41 pm
Organization: NREL
Location: Boulder, CO

### Re: Moordyn

Dear Krishnaveni,

MoorDyn is not set up to handle non-uniform seabed depths at this time. You can specify different anchor depths, but only one depth (specified by WtrDpth) will be used to model lines resting on the seabed. Positioning a line end below the seabed depth, as you do in your MoorDyn input file, could create very large seabed contact forces. I'd suggest ensuring WtrDpth is greater than or equal to the depth of all your anchors, then trying the simulation again.

I'm not as familiar with MAP++. My understanding is it will permit different lines to have different anchor depths and assume that seabed contact occurs at that same depth. If that's correct, I'm not sure why some of the lines would have zero anchor tension.

Best,
Matt