Design of support structures for offshore wind turbines - Wind Power Generation and Wind Turbine Design - page 577

The wind loads on an offshore wind turbine can be split into operational loads
on the turbine and loads on the structure. A description of the operational loads on
the turbine and the load cases that should be considered can be found elsewhere in
this work. The operational loads result in bending moments, normal forces and
shear forces on the tower top.
The wind load on the tower structure itself results from drag forces only. To
determine the total load on the tower structure the instantaneous wind speed should
be evaluated at several elevations to account for wind shear. Subsequently, eqn
(13) can be used to determine the drag force on each segment:

1
2

2

Ft () = r ⋅ CDut ⋅ ⋅ () (13 )

tower air w av wind

4.4 Soil

The soil contributes to the loading of the structure by providing the support reac-
tions. In the case of piled foundations, these reactions are dependent on the lateral
and axial pile-soil interaction. For GBSs the support reactions are generated by
the vertical bearing capacity and the resistance against sliding.
Soil is generally a granular material, either cohesive such as clay, or non-cohesive
such as sand. Other soil types that may be encountered are gravel, silt and peat. Soil
originates either through erosion of rocks or through accumulation of organic mate-
rial. Due to its geological history soil is highly inhomogeneous. The inter-particle
voids are fi lled with water which may prevent or slow deformations [5].
The characterisation of loose to dense sand and soft to hard clay only gives a fi rst
indication of the ability of the soil to carry load. For design, more detailed knowledge
is required. This is usually gathered through in-situ sampling and analysis of drilled
samples in the laboratory. The fi rst property measured for all types is the density r soil
(kg/m 3 ), usually for submerged soil, which is the dry density minus the density of
water. A typical value is between 400 and 1000 kg/m 3 . For clay, the undrained shear
strength s u and the strain at 50% of the maximum stress e 50 are measured. Table 1

gives an overview of typical values when no reliable soil data is available [4].
For sand the friction angle and the relative density of sand D r are derived

directly from in-situ measurements. The initial modulus of horizontal subgrade
reaction, k s , can then be found with the graph in Fig. 25 [6].

φ ′

Table 1: Characteristic parameters for clay.

Clay type s u (kPa) e 50 (%)

Soft 0-25 1.5

Firm 25-50 1.5

Stiff 50-100 1.0

Very stiff 100-200 0.5

Hard >200 0.5