Report typo in soil modulus of elasticity for monopile?

[jennirinker]

Description

Per Table 4-1 in the original report the soil is listed as having a shear modulus of 140 MPa.

However, this is causing stability issues in the HAWC2 model, and I think it might be a typo. From this reference, the range of the Young's modulus for the soil of dense sand and gravel is from 90 - 200 MPa, not the shear modulus. A shear modulus of 140 MPa corresponds to a Young's modulus of 392 MPa, which is much, much too large.

So is the value in the table meant to be the Young's modulus, and not the shear modulus? If that's the case, then the soil shear modulus should be 50 MPa, not 140 MPa, is that correct?

[jennirinker]

Sorry, quick follow-up.

Related to this query, can you confirm that these are the correct distributed lateral stiffness values for the embedded part of the monopile?

Distance under MWL [m]	Lateral stiffness [kN/m]
30	1263158
35	2374737
40	3486316
45	4597895
50	5709474
55	6821053
60	7932632
65	9044211
70	10155789
75	11267368

[gbarter]

@jennirinker - The reference in the report is an old one and I think the modeling method came from Rick Damiani many years ago. The shear modulus is taken from Table 4-8 (pg 69 in the book / 83 in the pdf) by converting 20k psi to MPa. The equivalent spring-stiffness is in Table 4-1 (pg 58/72) suresh-aryapincus-design-of-structures-and-foundation-for-vibrating-machinepdf.pdf

I honestly don't know enough of the geotech fundamentals to square this reference with the site/paper you linked to.

On the question of the lateral stiffness- are you asking about the soil model? The monopile has constant diameter & thickness below the mudline, so the side-side stiffness shouldn't change.

[jennirinker]

Regarding the shear modulus value:

I also don't feel qualified to speak knowledgeably on realistic soil stiffness values. This reference list the shear modulus range of 34 - 49 MPa (Young's modulus of 96-192 MPa), whereas this one lists the range from 57 - 114 MPa (Young's modulus of 160 - 320 MPa).

Long story short, there is a quite a range of reported values. Does anyone at NREL have an industry contact we could ask, just to get a ballpark figure? I'm okay with sticking with the original reported value, I just want to make sure it's not completely unrealistic.

Regarding the soil stiffness calculations:

Ahhh thanks for sharing the reference. The appendix in the report is quite sparsely explained, just gives equations without explaining what they are.

To my understanding, the distributed stiffness value depends on the embedment depth, which is the h parameter in the eta equations given in Table 4-2 of the Suresh PDF. So the stiffness should increase the further down you go in the grade, unless I'm misinterpreting something.

[RBergua]

@jennirinker and @gbarter: The characterization of the soil is always challenging. It's important to note that the shear modulus increases nonlinearly with depth. Also, not all "sand soils" are the same. You can have loose sand or dense sand. Below you can see some actual shear modulus measurements vs depth that are publicly available:

1. https://www.sciencedirect.com/science/article/pii/S0029801818315142?via%3Dihub: Note that the Bolders Bank soil is clay, the Egmond Ground soil is sand, and the Swarte Bank soil is clay.

2. https://www.sciencedirect.com/science/article/pii/S0267726117308953?via%3Dihub:

Based on this, I would say that a shear modulus of 140 MPa for sand is plausible. But it really depends on the depth.

In case it's useful, during the OC6 Phase II project (https://www.nrel.gov/docs/fy21osti/79938.pdf) we analyzed the monopile-based offshore wind system from the WAS-XL project. For reference, we were using a 10 MW reference wind turbine. In our case, the soil was made of clay. You can find the p-y curves in Appendix B. If you want to have the equivalent force-displacement relationship (i.e., to use lumped springs along the monopile foundation), you can do it according to the explanation provided in section 3.3.4 Distributed Springs: p-y Method.

[gbarter]

You rock, @RBergua !

[jennirinker]

Excellent -- I also asked some of my industry contacts and they agree the value assumed in the report is reasonable.

My colleague pointed out a constraint issue I had with the monopile (model is unstable without a torsional constraint), so the numerical issue is now resolved.

Since we have established the shear modulus is a reasonable value for dense sand, I think it is fine to close this issue. We are using a simplistic (linear) soil model for the IEA 15 MW which we know is unrealistic, but I think it is fine to do so. We can do something more realistic for the 22 MW. :)

I will therefore close this issue, but @gbarter feel free to reopen if you think something about the model should be changed.

[jennirinker]

Just a final note for future researchers, here are the lateral distributed stiffness values for the monopile, from -30 m to -75 m in kN/m. -30 3536842 -35 6649263 -40 9761684 -45 12874105 -50 15986526 -55 19098947 -60 22211368 -65 25323789 -70 28436211 75 0 31548632