Topics of general, but wind-related interest.

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Joined: Mon Feb 11, 2019 10:31 am
Organization: University of Massachusetts
Location: United States


Postby Krishnaveni.Balakrishnan » Tue Nov 26, 2019 2:10 pm

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.


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Location: Boulder, CO

Re: Moordyn

Postby Jason.Jonkman » Wed Nov 27, 2019 10:27 am

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: (for OC4) and (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

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