Design of support structures for offshore wind turbines - Wind Power Generation and Wind Turbine Design - page 584

M M

F F

Deflection [m]

Penetration depth

Deflections

Depth

Figure 32 : Pile defl ection.

Vertical Tangent: Max. Mudline

deflection (120 mm):

Max. Pile toe
deflection (20 mm ):

Deflection [m]

(1) (2) (3)

Depth [m]

Figure 33: The three checks for foundation pile length design: vertical tangent
at the pile toe, maximum defl ection at the mudline, and maximum
toe defl ection.

The foundation pile can be modelled separately in a program incorporating
detail soil models following the p - y method described in the previous chapter.
The defl ection of the pile is shown in Fig. 32. To start the pile penetration depth
design, a pile length of seven times the pile diameter is chose to be sure the pile
tip defl ection is negligible.
With the foundation pile modelled in the fi nite element program, the pile length can
be reduced while the following three checks are monitored after each step (Fig. 33):

1. the pile tip should reach a vertical tangent

2. the defl ection of the pile at mudline is less than 120 mm

3. the defl ection at the toe is less than 20 mm

5.6 Buckling & shear check

Now that the foundation pile has been modelled, also the pile-soil interaction is
known and the defl ection of the structure under different loads. The structural steel
can now be checked for integrity under extreme load cases.

5.7 Fatigue check

The biggest step in optimising the design of the support structure is checking
the fatigue. Fatigue is the phenomenon of slow deterioration of the steel due to