Properties of OC4 Semi Mooring

Wind & Water General
1

Dear All,

I am trying to understand the mooring dynamics of OC4 semi-submersible using OpenFAST. I am using MoorDyn for getting the mooring line tension. In the Definition of OC4 semi-submersible report (https://www.nrel.gov/docs/fy14osti/60601.pdf ), the diameter of the mooring line is given as 0.0766 m as shown below:

Mooring

The MoorDyn input file also has a diameter of 0.0766 m as shown below :

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

I understand that the diameter to be used in Moordyn input should be the volume-equivalent diameter of the line – the diameter of a cylinder having the same displacement per unit length (m) as defined in MoorDyn user manual. However, it is not clear from the report https://www.nrel.gov/docs/fy14osti/60601.pdf whether the diameter given in Table 5-1 is the nominal diameter or the volume equivalent diameter. I think if the value in the report is the nominal diameter, then the value used in the MoorDyn input file should have been different (i.e. volume equivalent diameter).

Moreover, the report does not mention the details of the line breaking strength, which is required to check if the mooring line tension is within allowable limits. Can someone kindly provide this information as well.

Regards
Ramya

2

Dear @Ramya.Niranjan,

The OC4-DeepCwind specifications report and the MoorDyn input file you refer to where developed at the same time, and so, their mooring line diameter specifications are consistent and specified correctly.

The line breaking strength of the as-built system tested in the wave tank were not specified; if you need this value, you’ll have to make your own assumptions for your own purposes.

Best regards,