Hi,

I’m doing some research on the control of the OC3 Hywind wind turbine.

In this relation I have a question regarding the weight of the entire structure. If I do calculations I do not get the same location of the still water level (10m down the structure of the buoy).

My results for the updraft force is the same as the 80,708,100 N mentioned in “Definition of the Floating System for Phase IV of OC3”.

So my guess is that there’s something about the weight I do not understand. I have used the following data:

Blades: 53,220 kg

Hub: 56,780 kg

Nacelle: 240,000 kg

Tower: 249,718 kg

Platform: 7,466,330 kg

and then the thing that’s a bit confusing the weight of the mooring lines. In the report it says that the mass of the platform is calculated based on the combined weight of the previous mentioned structures… “plus the weight of the mooring system (not including the small portion resting on the seafloor)”. The sentence in the parenthesis can have two meanings, but I guess it means that the weight of the portion resting on the seafloor is not included? How much is resting in the undisplaced position?

If I calculate the length of the mooring lines needed to balance out the updraft I get a length of about 767m, but that doesn’t seem like a “small portion” resting then. This is calculated with an apparent weight of 71.19 kg/m of the lines.

Best Regards

Jesper Larsen

M.Sc.E.E student

Aalborg University, DK

Dear Jesper,

All of the numbers you mention in your post are correct, so, I’m not sure why you say you don’t end up with the same location of the still water level. With your numbers, the weight of the blades, hub, nacelle, tower, and platform combined balances with the buoyancy and pretension the mooring system in the undisplaced position. Each line is 902.2 m long unstretched, and 767.3 m of this do not lie on the seabed. The remaining 134.9 m is the “small” portion resting on the seabed in the undisplaced position.

I hope that helps.

Best regards,

Tower: 249,718 kg

Is this correct? In table 1-1 of the document it has Tower Mass as 347,460 kg. Could someone explain this discrepancy?

Thanks!

Sean Q

Dear Sean,

The tower used in the OC3-Hywind model is different from the tower used by the land-based NREL 5-MW turbine. In the OC3-Hywind model, the tower mass is 249,718 kg. In the land-based NREL 5-MW model, the tower mass is 347,460 kg. See these models’ respective specifications documents for further details.

Best regards,

Dr. Jonkman,

Thanks for your quick response, although I’m now more confused. Where are the differences between these two turbines (the onshore and offshore) spelled out? Maybe a better way to ask this is which papers are about the offshore and which are about the onshore? I got the mass tower as 347,460 from 3 different papers (NREL/TP-500-41958, Wind Energ. 2009; 12:459–492, and NREL/TP-500-38060) and I was not aware that it is not the right mass. It appears that I have some catching up to do.

Where can I find these differences?

Thanks!

Sean Q

Dear Sean,

The NREL 5-MW specifications report, NREL/TP-500-38060, describes the land-based version of this turbine. For some variations of the NREL 5-MW turbine, the rotor-nacelle assembly — including the aerodynamic and structural properties — remain the same, but the support structure (tower and substructure) and control system properties have been changed. For the OC3-Hywind system, NREL/TP-500-47535 describes the changes to the support structure and control system relative to the land-based version.

I hope that clarifies things.

Best regards,