Allowable heave in FOWT

Wind & Water Structural Analysis
1

Dear sir,

Am trying to design mooring line for 200m water depth for OC3 hywind model. Am taking the platform motions for default 320m spar as my target platform motions and changing the mooring line properties accordingly. While doing so I have ended up with a mooring line which is twice heavier than the default one. With this line I could control surge but the heave has become more in the negative direction . I would like to know is there any allowable limit for heave like as such in surge 10% of water depth .

Thanks
Krishnaveni

2

Dear Krishnaveni,

I’m not aware of any heave-specific constraint. One general constraint commonly used is a nacelle acceleration constraint (limiting motion to about 1/3 gravity); I would assume this would apply as a vector magnitude from motions in any direction.

That said, I would not expect a spar to be have much heave motion anyway because of the lack of wave-surface area in the heave direction, deep draft, and low wave excitation in the heave direction.

Best regards,

3

Thank you Dr.Jason that was very informative. Am dealing with surge of about 18m and heave around -4m in a water depth of about 200m.

Are these numbers acceptable for a spar?

4

Dear Krishnaveni,

Are you referring to extreme values (min/max)? I’m a bit surprised about a (minimum?) heave of 4m considering, again, that there is little heave excitation of a spar.

Best regards,

5

Dear Dr.Jonkman

These values which am referring are mean surge and mean heave for a particular one hour simulation after getting rid off the initial ten minutes. And also these mean heave values have shifted from positive to negative as I move from lighter to denser chain.

Krishnaveni

6

Dear Krishnaveni,

I’m assuming the 18-m surge mean displacement is the result of wind excitation at a given mean wind speed?

I would not expect a properly designed mooring system for a spar to result in a mean heave of -4m, unless the lines are taut such that surge motion results in corresponding heave motion. In your mooring design, are you at least ensuring that the mean heave is near zero in the absence of wind and wave excitation (I would expect that in general)?

Best regards,

7

Yes Dr.Jonkman this 18m surge and -4m heave is for DLC 1.6 (wind dominated case) . And I will have to check the mean heave in static condition.