1. Dept. of Mechanical & Industrial Engineering
The International Design Standard for Offshore Wind Turbines: IEC
IGERT Seminar
February 21, 2013
J. F. Manwell, Prof.
Wind Energy Center
Dept. of Mechanical & Industrial Engineering
Univ. of Mass., Amherst, MA 01003

2. Why Are Standards Necessary?
Without proper design standards, failures are much more likely 
Offshore presents particular challenges!

3. Who Cares About Standards?
Regulators
Banks
Insurance companies
Designers
Project developers/owners

4. The Larger Context Design Standards
Component Certification
Type Certification
Project Certification
Analytical Assessment of Components
Component Testing
Analytical Assessment of Entire Turbine
Prototype Testing
Analytical Assessment of Project

5. Design Standards Process: IEC 61400-3 International Electrotechnical Commission (IEC)
Prepare preliminary design (“PD”)
Develop structural dynamic model of PD
Specify external conditions
Specify load cases
Determine structural loads and stresses
Check that stresses are acceptable, given chosen material
Adapt design if necessary and repeat

6. Antecedent: IEC
The offshore wind turbine design standard started with conventional, land-based turbine design standard, IEC
IEC is still directly relevant, especially to the rotor nacelle assembly (RNA), and IEC is compatible with it to the extent possible

7. What is an Offshore Wind Turbine?
A wind turbine shall be considered as an offshore wind turbine if the support structure is subject to hydrodynamic loading.
Note! is not sufficient for floating offshore wind turbines
But an IEC working group is presently developing guidelines for floating OWTs

8. Parts of an Offshore Wind Turbine
RNA
Defined here
Includes:
Rotor/nacelle assembly (RNA)
Support structure
Tower
Substructure
Foundation
Tower
Substructure
Foundation

9. Common Types of Fixed Bottom Support Structures
Monopiles
Gravity base
Jackets
Others
Tripods
Suction bucket

10. Parts of Floating Offshore Wind Turbine
RNA
Tower
Floating substructure (hull)
Mooring system (moorings and mooring lines

11. Scope of 6100-3 Requirements (beyond IEC 61400-1) for:
Assessment of the external conditions at an offshore wind turbine site
Essential requirements to ensure the structural integrity of offshore wind turbines
Subsystems such as control and protection mechanisms, internal electrical systems and mechanical systems

12. Design Methods
Requires the use of a structural dynamics model of PD to predict design load effects
Load effects to be determined for all relevant combinations of external conditions and design situations
Design of support structure to be based on site-specific external conditions
Design of RNA to be based on IEC (to extent possible)

13. Structural Dynamics Model
Example: FAST
From US National Renewable Energy Laboratory
FAST is “an aeroelastic computer-aided engineering tool for horizontal axis wind turbines…[it] models the wind turbine as a combination of rigid and flexible bodies.”
FAST for offshore also includes modules for incorporating effect of waves
Accompanying software: TurbSim (turbulent wind input), BModes (dynamic properties)

14. External Conditions Wind conditions Marine conditions
Waves, sea currents, water level, sea ice, marine growth, seabed movement and scour
Other environmental conditions
Soil properties at the site
Including time variation due to seabed movement, scour and other elements of seabed instability
Meteorological /oceanographic or “Metocean” Conditions

15. Occurrences of External Conditions
Normal
Recurrent structural loading conditions
Extreme
Rare external design conditions of greater than normal magnitude or effect

16. Wind Turbine Classes Follows that of IEC 61400-1
Based on: wind speed and turbulence parameters (I, II, II) and special conditions (S)
Design lifetime: at least 20 years

17. Wind Conditions Normal: Extreme:
More often than once per year
Extreme:
Recurrence of once per year or per 50 years
For RNA, use wind conditions as in , with some differences:
Wind shear, inclination of mean flow

18. Marine Conditions Assumed to primarily affect support structure
Conditions include at least:
Waves, sea currents, water level, sea ice, marine growth, scour and seabed movement
Normal:
More often than once per year
Extreme:
Recurrence of once per year or per 50 years

19. Waves Stochastic wave model assumed Design sea state:
Wave spectrum, S (f) (m2/Hz)
Significant wave height, Hs (m)
Peak spectral period, Tp (s)
Mean wave direction, wm (deg)
Normal, severe, extreme conditions
Breaking waves
Wind/wave correlations

20. Sea Currents
Sub-surface currents generated by tides, storm surge, atmospheric pressure variations, etc.
Wind generated, near surface currents
Near shore, breaking wave induced surf currents running parallel to the shore
Current models:
Normal, extreme
See standard for details

21. Water Level Reasonable range must be considered
Includes tidal range, storms

22. Sea Ice Sea ice may seriously affect design of support structure
Special consideration, such as ice cones may be needed
Detailed information given in standard
Cone breaks ice

23. Marine Growth
Marine growth may influence hydrodynamic loads, dynamic response, accessibility and corrosion rate of the structure
Classified as “hard” (e.g. mussels and barnacles) and “soft” (seaweeds and kelps)
Barnacles on ship

24. Seabed Movement and Scour
Seabed soil may move due to currents
Protection (“rip-rap”) may be needed around structure

25. Situations As in 61400-1 Power production
Power production plus occurrence of fault
Start up
Normal shut down
Emergency shut down
Parked (standing still or idling)
Parked and fault conditions
Transport, assembly, maintenance and repair

26. For Each Situation… Wind conditions Waves Wind and wave directionality
Sea currents
Water level
Other conditions
Type of analysis
Partial safety factor

27. Types of Loads As in 614000-1 Ultimate (U) Fatigue (F)
Normal (N), abnormal (A), or transport and erection (T)
Consider: material strength, blade tip deflection and structural stability (e.g. Buckling)
Fatigue (F)
Fatigue loads/fatigue strength

28. Method of Analysis
Characteristics loads predicted by design tools (e.g. computer codes)
Method of partial safety factors
Expected "load function (effect)," multiplied by a safety factor, must be less than the "resistance function”
Design properties for materials from published data
Safety factors chosen according to established practice

29. Ultimate Strength Analysis
Find characteristic load effect, Sk, from analysis
Find design load effect, Sd, using load safety factor
Find characteristic material resistance, fk, from literature (or other source)
Find design material resistance, Rd, using material safety factor
Acceptable

30. Assessment of Metocean External Conditions
Wind speeds and directions
Significant wave heights, wave periods and directions
Correlation of wind and wave statistics
Current speeds and directions
Water levels
Occurrence and properties of sea ice
Occurrence of icing
Other parameters: air, water temperatures, densities; water salinity; bathymetry, marine growth, etc

31. Assessment of External Electrical Conditions (examples)
Normal voltage and range
Normal frequency, range and rate of change
Voltage imbalance
Method of neutral grounding;
Method of ground fault detection / protection;
Annual number of network outages;
Total lifetime duration of network outages;
Auto-reclosing cycles;
Required reactive compensation schedule;

32. Assessment of Soil Conditions
Geological survey of the site
Bathymetric survey of the sea floor including registration of boulders, sand waves or obstructions on the sea floor
Geophysical investigation
Geotechnical investigations consisting of in-situ testing and laboratory tests

33. Scope of IEC 61400-3: 2nd Edition
Consideration of comments from national committees during pre-publication review
Comments from others and from EU’s Upwind research program
Incorporating recent experience of the design of offshore wind turbines and their support structures

34. General Areas of Interest
Load calculations and simulations
External conditions
Assessment of external conditions
Support structure and foundation design
The various annexes on design approaches
Text referring to issues treated by IEC

35. Changes Likely… General corrections Wave models Hurricanes/cyclones
Wind shear as affected by waves
Floating ice
Boat (service vessel) impact
Soil characterization
Vortex induced vibrations

36. Issues for US Wind/wave conditions (e.g. hurricanes)
100 yr vs. 50 yr events
Role of American Petroleum Institute (API), other US standards
Role of Bureau of Ocean Energy Management (BOEM)
Other standards referenced by
US vs. European or international
English units vs. metric (SI) units

37. Little or No Detail… Foundation design (soil/structure interaction)
Material properties
Offshore data collection
Environmental impact