This article covers the design of a monopile to support the turbines of a proposed Offshore Wind Farm. The water depth at the site is 24 and the turbine is the REPower 5M.

Assumptions

Water Levels
Water depth: 24 m
HAT  1.16
MSL  0.00
LAT  -1.05

Using the provided water level data we calculate the tidal range and surge at the site
Tidal range 2,21

tidal level = water level – surge

Storm surge
Positive1.95 m
Negative-1.15 m

The highest (max) still water level and the lowest (mix) still level are: 

HSWL3,11 m

HAT1,16 m

LAT-1,05 m

LSWL-2,20 m

Tidal range2,21 m

 

 

Temperatures and Densities

Ice

It is very unlikely that sea ice occurs. Therefore, it will not be taken into account. However, icing at the structure is possible and will be dimensioned according the following values:

 

Atmospheric ice formation: thickness is 30mm
Ice formation due to sea water spray from MSL to HSWL: thickness is 100mm 
Ice formation due to sea water spray from HSWL up to 60m above MSLThickness: decreasing linearly to 30mm
Density of ice: 900 kg/m³

Marine growth

 

For design purposes, marine growth has to be assumed. The density has to be taken as 1100 kg/m³. shows the thickness as determined according to DNV standard.

 

Density1100 kg/m³
Thickness100 mm from Level -2 to -4 of MSL

Waves

Currents

Based on 3-hour measurements for 30 years the current velocities at surface level are:

 

Wind Speed Distribution

 

Wind speed - Extreme values 

 

Soil Parameters 

From the available soil investigation data, we can see that the soil parameters vary from borehole to borehole. Roughly, the soil is mainly dense sand interrupted by two layers of clay (approx. 5 m thick). When designing the foundation for each turbine we will use the particular properties from the nearest borehole.

 

 

Codes and standards

For the calculation of the loads and for determining the load combinations the relevant DNV standards should be used:

  • DNV-OS-J101-2007:Design of offshore wind turbine structures
  • DNV-RP-C203: Fatigue design of offshore steel structures
  • API RP 2A LRFD: Planning, Designing and Constructing Fixed Offshore Platforms – Load and Resistance Factor Design