# NREL 5MW monopile OWT modelling details

Sir,

In Fastv7 for the platform model 2 i.e, for the monopile offshore wind turbine. what were the mesh sizes, type of element, and total no.of nodes used for the turbine, tower, and monopile assembly.

Thank you.

Dear Dhaneesh,

Are you asking about the aerodynamic, hydrodynamic, or structural discretization, or all of these? These properties are all specified in the FAST v7 input files; do you have those?

Best regards,

Sir,

Sorry for my late reply. Yes, I do have FAST input files. Currently for my Mtech thesis, I was working on Platform model 2 i.e, Bottom fixed offshore wind turbine with foundation modeled as coupled springs. I have some doubts and I am asking it here for my clarification. Please correct me if my understanding is wrong.

1. The NREL 5MW wind turbine was modeled with nonlinear beam elements in the FAST code. For the NREL 5MW wind turbine, the blade structural properties are defined at 49 locations along the blade from the blade root to tip. And for aerodynamic and structural forces calculation at nodes, the blade is discretizing into 17 nodes. And the support structure properties for both tower and monopile were defined at 13 locations along the support structure i.e, from the seabed (-20m) to tower top(87.6m). And the tower nodes above mean sea level was discretized into 20 nodes for aerodynamic and elastic forces calculation on the tower. Then for the calculation of hydrodynamic loading, into how many nodes the monopile has been discretized?

2. FAST code uses Kane’s method for deriving the equation of motion. According to Kane’s method the equations of motion for a holonomic system is defined as
Fr + F*r = 0 where Fr = Fr|aero + Fr|gravity + Fr|elastic + Fr|drivetrain + Fr|platform

`````` where  Fr|platform = -(Addedmass*qdd) + Fr|hydro + Fr|seismic ( If monopile OWT is under action of wind, wave and seismic effects)

Fr|seismic is the seismic forces required to achieve the ground motion given by user and is caluclated as

Fr|seismic = k * (PtfmDisp-X) + c * (PftmVel-XD)
``````

where PtfmDisp and PtfmVel are desired displacement and velocity at the base of the monopile at seabed and X, XD are realized displacement and velocity.
Fr|hydro is calculated by using morison equation because D/l ratio is less than 0.2 for monopile substructure in seawater. And I have used JONSWAP spectrum for modeling irregular waves.

``````     And where F*r = F*r|hub + F*r|blades + F*r|tower + F*r|nacelle +F*r|platform

(and what F*r|platform indicates then. Is it equals to mass of the (monopile+Platform mass) multiplied by input ground motion?)
``````

The Addedmass term in the Fr|platform is added to Mass matrix and complete nonlinear equation of motion is written in matrix form as

M(q,u,t)*qdd = f(q,qd,u,ud,t)

where,

M = mass matrix depending on a nonlinear combination of displacements (q), control inputs (u), and time (t)
qdd = accelerations
f = forcing vector depending on a nonlinear combination of displacements (q), velocities (qd), control inputs (u), wind inputs (ud) (input disturbances), and time (t).

These non linear equations of motion is linearized to find the full system modes. So linearization of nonlinear equations of motion is done about several operating points and reducing the nonlinear equation to second order representation of
Mxdd + C xd + Kx = F(t) where xdd,xd, x are vectors of second derivative of DOF’s, first derivative of DOF’s and DOF’s
and it was further defined in first order representation and all the state matrices obtained are given to mbc post processing matlab scripts and averaged eigen values, eigen vectors are obtained. Can we stop the linearization at second order representation and can we get mass and stiffness matrices as out of mbc?

Sorry for posting such a lengthy question. Please correct me if my understanding is wrong.

Thank you.

Dear Dhaneesh,

1. In FAST v7, the entire support structure (monopile + tower) is modeled together via the tower-bending DOFs. The 20 tower nodes you refer to (TwrNodes = 20) are equally spaced elements (with nodes located at the centers of the elements) from the seabed (assuming you’ve set TwrDraft > 0 in the platform file) to the tower top. Whichever of these elements/nodes are below the water level will be used for the hydrodynamic load calculation. Increasing TwrNodes will improve the hydrodynamic discretization. Please note that the discretization approach has changed drastically in FAST v8 and OpenFAST.

2. I agree with your commentary, but I’m not sure I understand your question.

Best regards,

Sir,

My question was whether my understanding regarding the F*r|platform and Fr|platform forces when seismic loading is also acting on a monopile OWT is correct? And Can we get mass and stiffness matrices as a direct output from mbc?

Thank you.

Dear Dhaneesh,

Yes to both.

Best regards,