Question about IEA 15MW power output in RAFT

[HYW-1113]

Hi everyone,

I’m quite new to offshore wind turbine simulations and have recently started using RAFT with the IEA 15 MW turbine (VolturnUS-S).

I noticed something that confused me a bit. In my simulation, at around 11 m/s wind speed (with IIB_NTM turbulence), the rotor power sometimes reaches about 16 MW, which is higher than the rated 15 MW.

In my setup:

Turbine status: Operating
aeroServoMod = 2
I did not set up any controller (just using the default RAFT setup)

I was wondering:

Is it normal for RAFT to give rotor power higher than the rated power?
Does RAFT include any kind of pitch or torque control by default, or is it purely aerodynamic?
Would this difference be the reason why results from OpenFAST (with controller) look different?

Sorry if this is a basic question — I’m still learning how these tools work.

Thanks a lot for any help!

[mattEhall]

Hi @HYW-1113,
When you say the power "sometimes reaches 16 about MW", what do you mean? Is the mean power reported by RAFT 16 MW in some case? Is this the maximum reported power specified by some output of RAFT? If it's a maximum/dynamic value, can you also mention if there are waves?
Matt

[HYW-1113]

Hi @HYW-1113, When you say the power "sometimes reaches 16 about MW", what do you mean? Is the mean power reported by RAFT 16 MW in some case? Is this the maximum reported power specified by some output of RAFT? If it's a maximum/dynamic value, can you also mention if there are waves? Matt

Hi Matt,

Thanks for your follow-up — that helped me clarify my results more carefully.

In my case, the ~16 MW I mentioned corresponds to the mean rotor power, not a transient peak. From the RAFT output, the rotor power is reported as a single value (1.58e+07 W) without max/min statistics, so I interpret it as a mean (steady) value.

Regarding the simulation setup:

• Wind speed = 11 m/s with IIB_NTM turbulence
• No waves (wave height = 0 m)
• aeroServoMod = 2

In addition, I also have a related question regarding the comparison between RAFT and OpenFAST results.

In a broader comparison performed under identical operating conditions, I observed that the two models begin to diverge noticeably at wind speeds above approximately 10 m/s.

Specifically:

In OpenFAST, rotor power increases with wind speed and then gradually stabilizes once the turbine enters the rated region (around 10–13 m/s), which is consistent with expected pitch and torque control behavior.
In contrast, RAFT shows a different trend where rotor power does not clearly reach a stable plateau. Around approximately 11 m/s, the power can exceed the expected rated level, and at higher wind speeds, the power may fluctuate and in some cases decrease even as wind speed continues to increase.

I would like to ask whether this difference could be related to how rotor power is defined in RAFT compared to OpenFAST.

In particular:

Does RAFT output represent aerodynamic (rotor/shaft) power, while OpenFAST typically reports generator (electrical) power after drivetrain and control effects?
If so, to what extent can this difference in power definition explain the higher-than-rated power observed in RAFT around 11 m/s?

Additionally, could the observed non-monotonic trend at higher wind speeds be related to differences in modelling or control implementation?

Thanks again for your help!

For reference, here is my input file:
VolturnUS-S_example.yaml

And the corresponding output is shown below:

A comparison between RAFT and OpenFAST power is shown below: