# Compute stifness and damping for soil interaction

Hello,

I’m doing my masters thesis in the structural behaviour of the NREL 5 MW offshore wind turbine and I have some questions about the interaction with the soil.

I am using version 7 of FAST so I can use the routines UserTwrLd and UserPtfmLd to simulate the interaction with the soil. I wanted to use both routines to compare the interaction with and without damping but the damping and the added mass matricies are all zero.

My questions are:

1 - Why is that?
2 - How to compute those matricies?
3 - How to compute the springs’ stifness in both routines?

I downloaded “OC3-Soil-Pile-Interaction Models_ReadMe” along with “OC3-LPILE_Results” and “OC3-Soil-Pile_Interaction_Model” to understand how it is done but there’s nothing raleted on how to compute the linear springs.

Dear Miguel,

I’m not an expert in soil-structure interaction (perhaps someone else on this forum can respond in more detail), but here are some general comments:

• In simple engineering models, often the soil is only modeled with springs (without added mass and damping), such as in the coupled springs (CS) model (stiffness matrix applied at the seabed) or distributed springs (DS) model (linear or nonlinear springs (p-y curves) distributed along the pile.
• The linear models are typically computed by linearization of a more specialized nonlinear soil-structure interaction model about a given load e.g. by perturbing the displacement and calculating the change in soil load.
• Damping can be added to these models using e.g. stiffness-proportional damping based on the assumed (or known/measured) damping ratio, or neglected altogether to be conservative.
• More advanced engineering models including nonlinear springs with hysteresis, which intrinsically include hysteretic damping.
• It is rare for engineering models to include soil added mass.

Best regards,

Dear Jason,

When you say:
“The linear models are typically computed by linearization of a more specialized nonlinear soil-structure interaction model about a given load e.g. by perturbing the displacement and calculating the change in soil load.”

I agree completely with you. In fact, those documents that I quoted in the previous post say that a linearization of the p-y model, defined by the API, was preformed to get the springs’ constants of stifness.

My question is: How is that linearization preformed?

In “OC3-LPILE_Results” they give us the given load you were talking about:

“Pile-head boundary conditions are Shear and Moment (BC Type 1)
Shear force at pile head = 3910.000 kN
Bending moment at pile head = 124385.000 m- kN”

They also give us the Axial load at pile head but that won’t disturbe the deflection y.

So what I did was apply these loads at the pile head (In this case, at the mudline) and determine the deflection y for a point z, assuming a cantilivered beam at z = -36m.

From that, you can compute p from the API p-y function. But I can’t get to the stiffnesses values from OC3-Soil-Pile_Interaction_Model.