I have one more question please.
If I want to study the effect of water depth, like 16m -25m -41 m for monopile IEA 22 MW, in this case I will change the water depth in HydroDyn
should I change the substructure part in SubDyn to be the same as water depth 16m, 25m, 41 m? or I can keep it 34 m as the manual for all of them? but there will be problem for 41 m, wright?
When I make the substructure length is 30m for cases (10m , 20m, 30m), the difference in response between the three water depths was around 18%
If you want to change the water depth, in addition to WtrDpth, you should change the JointZss (or add joints) in SubDyn such that the lowest joint is at the revised seabed. Likewise, you should change the Jointzi or (add joints) in HydroDyn such that the lowest joint is at the revised seabed. This may require a redesign of the monopile because its change of length.
Hello again,
I have another question please and it would be appreciated if you could help me with any comments.
For turbine IEA 15 MW on monopile, I modeled the soil pile system as coupled springs through SSI.txt file. I compared the effect of foundation flexibility considering different DLCs. There is no optimization here, I want to study the effect of SSI
as shown in the figure, some DLCs, the effect of SSI is minimum like DLCs (1.1,1.3,1.5,1.6,2.1,2.3), for DLC 5.1 around 30%, DLCs 6.1 and DLC7.1 is maximum effect around 90% and 80%, while DLC6.2 and 6.3 the trend is the opposite, it should be noted that DLC 6.2 and DLC 6.3 the results are taken from yaw error (20-60) which has very high post-stall angles-of-attack outside of the verified/validated linear regime and are not to be trusted as conventional numerical models that rely on 2D airfoil polars and 1D beams struggle to reproduce reality, which is characterized by unsteady aerodynamics and 3D effects because a the AeroDyn’s induction or dynamic stall models were not enabled in these DLCs. what was surprising that when I tested DLC 6.2 under yaw error 160 degree and wind wave are codirectional, the trend was the same as 6.1 which has yaw error 8 degrees and DLC 7.1, My questions are
1- Parked idling DLCs (6.1, 7.1, “6.2 with yaw error outside range ±20 to 60”) the SSI effect is higher than operation DLCs as the aerodynamic damping is least in these DLcs?
2- why DLC 5.1 has the maximum effect in operating DLCs? is it due to emergency shutdown so after shut down the aerodynamic damping is minimum? but why this does not happened with DLC 2.1 and 2.3 as they stopped also at specific time?
3- why DLCs 6.2 and 6.3 with these DLCs the trend is the opossite? is it like putting the structure on spring , so it absorb the energy?
4- with foundation flexibility, should the load increased due to (p- delta effect) secondary moment effect? or should it decreased because it is like put the structure on springs and it absord the energy so the moment decrease?
I doubt I can comment on your specific results without knowing a lot more about your set-up and results.
I would just say I would expect the biggest impact of the SSI on the global response is from the change to the support structure natural frequencies (moving closer or further away from excitation frequencies) and damping. So, when you change from fixed foundation to a foundation with SSI, I would suggest comparing the impact of SSI on the support structural natural frequencies and damping.
as shown in this figure that as the soil type became more softer, the natural frequency decreased ( fixed base =0.195 Hz, stiff soil = 0.183 Hz, soft soil = 0.134 Hz).
but I don’t know why this effect changes from DLC to another? I mean if we design for example the turbine for only (DLC 101 and DLC1.3 (which is the case of IEA 22 MW decomentation) the effect of foundation flexibility is minimum, but when the same turbine experience another DLC like for example 6.1, it will have almost douple the load that it designed for, that’s why I want to study the influence on the DLCs.
Well, the excitation frequencies (from wind and waves) change with DLC, so, that is likely part of the answer. But to fully understand would likely require a deep dive into other response metrics associated with each DLC.
In addition to the change to the first-bending mode, I would also expend the second-bending mode to change, although this is a bit difficult to interpret from the PSD you shared.
Thanks Dr. Jason for your quick reply.
1- you said that “the excitation frequencies (from wind and waves) change with DLC”, I think that’s why the effect of foundation flexibility is almost the same for DLC 1.1 and 2.1 which both have (NTM) and (NSS), but for DLC 2.3 it has (EOG), DLC 1.3 it has (ETM), and for DLC 1.6 it has (ESS). for DLC 5.1 and 6.1 and 6.3 maybe due to the aerodynamic damping is minimum?
2- what should I do to make the second bending mode more clear?
I’m not sure how you are computing your PSD, but you can make PSDs more clear by binning within frequency bins and/or averaging across multiple time-series intervals.
Dear Dr. Jason, thanks for your quick reply. I had another basic question please.
I read the paper of “Influence of Foundation Damping on Offshore Wind Turbine Monopile Design Loads” Carswell et al, 2021. they also investigated the effect of foundation flexibility in addition to damping considering different DLcs. as shown from the table, DLC 5.1 was the most DLC that has significant reduction in load after adding foundation damping
in addition, for DLC 6.1, they mentioned “The resulting small error in eigenfrequency using the criteria cited above was considered negligible for our analysis (it should be noted that the natural frequency from the representative mudline stiffness matrices was on average 0.23 Hz and varied by less than 0.01 Hz). The relative change in natural frequency can be seen in one example time history of tower top displacement (stochastic load case DLC 6.1a, refer to Section 4 and Figure 2A), where there is a significant phase and amplitude difference between the fixed base and first iteration flexible foundation case; with a second iteration, the difference in frequency and amplitude is substantially minimized. Methodology for the analysis herein relied on differences in foundation response: the difference in cyclic mudline moment amplitude from the fixed base to flexible base case was 39% but differed by only 2% after the second iteration (Figure 2B).”
How the natural frequency of the turbine changed from DLC to another (like what they have in DLC 6.1)? I thought that the natural frequency and mode shapes are depends only on the stiffness and the mass of the turbine and do not changes from DLC to another. Can you please explain this to me?
I have a similar response to that paper, where the foundation flexibility effects is maximum in DLCs 5.1 and 6.1. Is there any explanation?
3)Should I exclude the results from DLC 6.2 and 6.3 with yaw errors between 20-60 from the results as they have the opposite trend?
I want to add something more in paper “Effect of monopile foundation modeling on the structural response of a 5-MW offshore wind turbine tower” (jung el al. 2015)
he mentioned that"
he studied the turbine under DLC 1.1 rated wind speed. and in my results for DLC 1.1 the stiff soil difference is 0.15% and for stiff soil 4.6% similar to him (I studied turbines NREL 5 MW and IEA 15 MW and they both have the same trend).
I mean that when we studied the effect of foundation flexibility and soil effect in operation condition cases, the effect is not significant , but when we consider other DLCs such as DLC 5.1 and 6.1, the effect is more apparent.
1- Is my conclusion is correct please? and can you please give me more explanation.
2- for the second mode shape, I get the PSD from white noise in OpenFAST
I’m not familiar with the papers you are citing, and as I said, you’d have to dig into the details to fully understand the results you are showing. I can’t comment more on that.
Regarding (1), the natural frequency will depend on the mass and stiffness and boundary conditions. Depending on the nonlinearities of the model, these may change with magnitude of applied loads, which can vary between DLCs.
Regarding (3), I would not expect DLC 6.3 to have yaw errors larger than 20 degrees. This is certainly possible in DLC 6.2 and in the yaw error range of 20-60 degrees, I expect that you are running into the known blade edgewise / tower side-side instability that happens under parked/idling conditions under high winds and sizeable yaw error, which has been discussed in other topics on our forum:
