OpenFAST Seastate Monopile Extreme Wave Loads

Hi Jason and all NREL Forum members,

I am now running a ‘theoretical’ wave load increasing from 2m to 25m height at around 70m waterdepth on a monopile with IEA 15MW Turbine. With that waterdepth, it ‘theoretically’ can accommodate up to ~45m wave height before breaking.

For now, I switch the Aerodyn, Servodyn, and Inflowwind off to ensure that my Hydrodyn simulation runs successfully.

I am using dt = 0.0005 sec (to ensure the timestep is small enough to minimize numerical instability) and simulation time = 800 sec.

I discarded the first 200 sec to ensure the transient startup effect disappeared.

Seastate module with: SeaState Driver-v3.5.1-573-g889501f8.

I have these errors that occur when Wave Height is > 10 m.

Morison_CalcOutput:getElementHstLds_Mod1:Element cannot be parallel to the free surface. 
This has happened for Member ID 1

getElementHstLds_Mod1:Element cannot be parallel to the free surface. 
This has happened for Member ID 1

However, in cases at larger wave heights, the simulations were completed without any errors. Here is the list of the sims and its status:

Wave height (m) Success?
2 Y
3 Y
4 Y
5 Y
6 Y
7 Y
8 Y
9 Y
10 Y
11 N
12 N
13 N
14 N
15 Y
16 N
17 N
18 N
19 Y
20 N
21 N
22 N
23 Y
24 Y
25 Y

Member ID 1 spans from the seabed (-70 m) to +20 m.

I created a series of insignificant JONSWAP (Wavemod = 2) with Hs = 0.1 m. Then I put a constrained wave with the below flag:

2 ConstWaveMod
1.86*wave height CrestHmax
500 CrestTime
0 CrestXi
0 CrestYi

The simulation succeeded with the following water elevation (Hs = 15 m) - fig1:
But the mudline moment is weird - fig2:
Here is one of the unsuccessful simulations (Hs = 11 m), the wave elevation stops at t ~= 500 sec -fig3:
The bending moment also stop at max -fig4:

My question will be:

  1. What caused the error?
  2. Even if the simulation was successful, why I still have weird mudline moment? Is it because of the plastic deformation?

Dear @Danu.Riyanto,

You mention the SeaState Driver, but also seem to be running a coupled OpenFAST simulation. Can you clarify which version of OpenFAST you are compiling and running? Presumably you are running some branch of OpenFAST newer than v3.5.1 with ElastoDyn, SubDyn, SeaState, and HydroDyn enabled; is that correct?

In your plots, you refer to the “mudline moment about Y=axis”. Is this an output from SubDyn?

When you get the error about an “Element cannot be parallel to the free surface”, do you get this when the constrained wave is impacting the monopile, or before that?

Can you share your input files so that I can better understand your simulation setup?

Best regards,