1. Design of Monopile for Offshore Wind Turbine – Simple Example

This article covers the design of a monopile to support the turbines of a proposed Offshore Wind Farm.
Project Name: OWF ALPHA
Location: North Sea, EZZ of the Netherlands, Coordinates of Center: 53° 17’ N and 04° 01’ E
Water depth: 20-24 m
Distance from shore: 70 km the Port of Den Helder
Turbine Type: REPower 5 MW

1. Design Basis – Design Parameters and Assumptions

First step is to write down the Design Basis. Design Basis is simply a summary of all the input for the design. It provides all the necessary information, parameters and assuptions for the design of the monopile foundation. The Design Basis is also very important because it serves as a common reference for the design team. To make reference easier, we prefer to have the data in list form and avoid lengthy text.

1.1. Codes and Standards

DNV-OS-J101-2007: Design of offshore wind turbine structures
DNV-RP-C203: Fatigue design of offshore steel structures

1.2. Design Life

Design life: 20 years

1.3. Environmental Data (Metocean Data)

1.3.1 Water depth

Design water depth: 24 m

1.3.2. Tidal Levels

Using the provided water level data we calculate the tidal range and surge at the site. Tidal level = Water level – Surge
Highest Astronomical Tide, HAT1.16 m (max of all tidal values from 1979 to 2010)
Lowest Astronomical Tide, LAT: -1.05 m (min of all tidal values from 1979 to 2010)
Mean Sea Level, MSL: 0.00
Tidal Range: 2.21 m

1.3.3. Storm Surge

Positive Storm Surge: 1.95 m
Negative Storm Surge: -1.15 m

1.3.4. Still Water Levels

The highest (max) still water level and the lowest (mix) still level are: 
Highest Still Water Levels, HSWL: 3,11 m
Lowest Still Water Levels, LSWL: -2,20 m

Design Water Levels, HAT - Highest Astronomical Tide, LAT - Lowest Astronomical Tide, MSL - Mean Sea Level, Tidal Range, Positive Storm Surge, Negative Storm Surge

1.3.5. Temperatures

Water temperatures at the surface
Mean: 10°C
Standard deviation: 5°C
Min/Max: 0°C / 22°C
Yearly amplitude: 7°C

Air temperature
Mean: 15°C
Extremes: -20°C to +50°C

1.3.6. Densities

Water Density: 1025 kg/m³
Water Salinity: 3.5 %
Air density: 1.225 kg/m3 at 20°C

1.3.6. Ice

Sea ice does not occur at the location. However, icing at the structure is taken into account:
(1) Atmospheric ice: 30 mm thickness
(2) Ice due to sea water spray:
From MSL to HSW: 100 mm thick
From MSL to 60 m above MSL: 100 mm decreasing linearly to 30 mm
(3) Ice Density: 900 kg/m³

1.3.7. Marine growth

(1) Density: 1100 kg/m³ (ref. DNV-….)
(1) Thickness: 100 mm from Level -2 m to -4 m MSL

1.3.7. Waves

The wave data came from the Agross database. Summary of wave data as below:
Hs, 1y = 6,4 m (3-hour storm, 1-year return period)
Hs, 5y = 7,1 m (3-hour storm, 5-year return period)
Hs, 50y = 8,1 m (3-hour storm, 50-year return period)
Hs, 100y = 8,3 m (3-hour storm, 100-year return period)
Hmax, 5y = 13,21 m (3-hour storm, 5-year return period)
Hmax, 50y = 15,07 m (3-hour storm, 50-year return period)
Hmax, 100y = 15,44 m (3-hour storm, 100-year return period)
Hsd = 240 deg
Tm = 4 s
Tp = 7 s

Scatter Diagram: Hs – To as below

Wave scatter diagramm - North Sea - Offshore Wind foundation design

1.3.7. Currents

Based on 3-hour measurements for 30 years the current velocities at surface level are:
Va = 0.19 m/s, direction= 181 deg (Average Current Velocity)
Vmax, 30y = 0.82 m/s, direction= 76.2 deg (30-year return period)
Vmax, 5y = 0.6 m/s, direction= 76.2 deg (5-year return period)
Vmax, 50y = 1.1 m/s, direction= 76.2 deg (50-year return period)

1.3.8. Wind

(1) Data source: Agross database
(2) Wind Speed Distribution: as figure below

Agross wind speed distribution

(3) Probabilities of exceedance:

Prob. of exc. (%) Wind speed (m/s)
902.8
804.4
705.4
606.4
507.2
408.4
309.4
2010.6
1012.8
514.4
2.516
117.4

(4) Extreme Wind speed values
Weibull shape parameter is 2.7
Centre of area is at 53° 17’N, 4° 01’E

Return periodwind speedlower limitupper limit
1 month18.618.218.9
1 yr21.62122.3
2 yr 22.421.623.1
5 yr 23.322.424.1
10 yr 23.922.924.9
25 yr 24.723.625.8
50 yr 25.324.126.5
100 yr 25.924.627.1
1000 yr 27.626.129.1

Above data are valid at elevation +10 from MSL. By assuming a logarithmic profile, we find the relevant values at hub height.

Umean = 7.6 m/s (at +10 from MSL)
Umean = 9.14 m/s (at +90 from MSL)
U5-year = 23.3 m/s (at +10 from MSL)
U5-year = 27.9 m/s (at +85 from MSL)
U50-year = 25.3 m/s (at +10 from MSL)
U50-year = 30.3 m/s (at +85 from MSL)
U100-year = 25.9 m/s (at +10 from MSL)
U100-year 31.0 m/s (at +85 from MSL)
Hub height 85 m
Roughness length z 0.0002
Dominant wind direction 225 deg

1.3.9 Scatter diagrams

The following scatter diagrams are available and will be used accordingly:

1. 3D-scatter diagrams of Hs , To and Vw: Hs and To values are given for wind speeds Vw that cover a range of 2 m/s. For example the diagram for Vw = 6 m/s refers to wind speeds from 5 to 7 m/s.
2. 3D-scatter diagrams of Hs , Vw  and wind direction: Hs and Vw values are given for directional sectors of 30 degrees. The sectors start from 0 degrees and cover all 360 degrees.
3. 3D-scatter diagrams of Hs , To  and wave direction: Hs and To values are given for directional sectors of 30 degrees. The sectors start from 0 degrees and cover all 360 degrees.
4. 2D Monthly scatter diagram of Hs  and wave direction: Hs values are given for directional sectors of 30 degrees for each month. The sectors start from 15 degrees and cover all 360 degrees.
5. 2D Monthly scatter diagram of Vw  and wind direction: Vw values are given for directional sectors of 30 degrees for each month. The sectors start from 15 degrees and cover all 360 degrees.
6. 2D Monthly scatter diagram of swell height and swell direction: Swell height values are given for directional sectors of 30 degrees for each month. The sectors start from 15 degrees and cover all 360 degrees.

1.4. Soil Parameters

Data Source: Soil parameters from on-site geotechnical soil investigation data (boreholes)
General: The soil is mainly dense sand interrupted by two layers of clay (approx. 5 m thick). Average soil parameters as below

Depth [m]Soil descriptionγ’
[kN/m³]
Strength parametersCone resistance qc [MPa]
0.0 – 3.5Sand (medium to dense)10φ=30°10-20
3.5 – 10.7Clay10su = 80 kPa, OCR=4.82-25
10.7 – 27.0Sand (very dense)11φ=35°28-80
27.0 – 34.3Sand (dense)11φ=30°12-80
34.3 – 70.0Clay10su = 100 kPa10-80
70.0 – 90.5Sand (medium)11φ=30°28-80

For each tubine location, we derived the soil profile by correlating data from the nearest boreholes.

1.5. Turbine Specifications

Below list of turbine characteristics relevant to the foundations design

Model: REpower 5M (5.0 MW)
Turbulence intensity Class: IEC Ib /GL offshore type class I
Structural design lifetime: 25 years
Hub height: 85 m above MSL
Blade Tip Height: 153 m above MSL
Rotor Diameter: 126 m
Swept Area: 12’469 m²
Mass of nacelle (without rotor): 290 tonnes (approx.)
Rotor: 120 tonnes (approx.)
Cut-in Wind Speed: 3,5 m/s
Rated Wind Speed: 13,0 m/s
Cut-out Wind Speed: 30 m/s
Operational rotor speed: 7.7 – 12.1 rpm
Nominal rotor speed:10.5 rpm
Structure Type: Steel Tubular

Tower Dimensions
BASE: D=6.00 m, t = 35 mm
TOP: D=4.50 m, t = 20 mm
Mass initial estimate =π*5.25*0.0275*70*7,850 = 250 t

1.6. Structural Steel

Steel grade: S355
Yield strength: 355 MPa
Modulus of elasticity E = 210 GPa
Poisson’s ratio ν = 0.3
Coefficient of linear thermal expansion (T ≤ 100 oC) α = 12e-6 K-1
Partial material factors:
– Ultimate limit state strength checks, γs = 1.10
– ULS buckling check, γs = 1.20
– Serviceability limit state, γs = 1.00
– Seismic ultimate limit state, γs = 1.15

1.7. Corrosion

Steel structure components in the splash zone and below are protected by coating and corrosion allowance.
(1) Within the splash zone: 0.3 mm/year, Corrosion allowance: 20 y x 0.3 mm = 6.0 mm
(2) 3m below seabedup to the splash: 3.0 mm (20 years)

The splash zone is the part of a support structure that is intermittently exposed to air and immersed in the sea. The splash zone is calculated according to DNV (DNV, 2011):
Upper limit: HAT+ 0.6 *(1/ 3) Hmax, 100y = 1.16 + 0.6 * 1/3 * 15.44 = 4.25 m MSL
Lower limit: LAT- 0.4 * (1/ 3) Hmax, 100y = – 1.05 – 0.4 * 1/3 * 15.44 = -3.10 m MSL

1.8. Important Elevations

1.8.1 Interface elevation

Interface elevation is equal to:
interface­= LAT + Tidal range + Storm surge + 0,65 * ζ max (100 years) + Air gap
interface­= -1.05 + 2,21 + 1,95 + 0,65 * 15,44 + 1,5 = 14.65 ≈ 15 m
Interface elevation: +15,00 m from MSL

1.8.2 Hub Height

hub ­=  z­interface­ + Clearance + 0,5 Rotor Diameter
hub= 15 + 8 + 0.5 * 126 = 85 m
Hub Height: 85 m from MSL

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