Knut O. Ronold 16 March 2011
Design standards for floating wind turbine structures
EWEA 2011 - Brussels
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Design standards for floating wind turbine structures
16 March 2011
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Outline of presentation
Background – why is a floater standard needed?
Existing relevant DNV documents
Plans for development of full-fledged standard
Key issues to be covered in development of standard
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Design standards for floating wind turbine structures
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Background for initiating development of floater standard Existing standards are in practice restricted to bottom-fixed structures only:
- IEC61400-3- DNV-OS-J101- GL (IV Part 2)
With regard to use for design of floaters, shortcomings of these standards exist with respect to:- Stability - Station keeping- Site conditions (related to LF floater motions) - Floater-specific structural components
(tendons, mooring lines, anchors)- Accidental loads- ALS design in intact and damaged condition- Other: Simulation periods, higher order responses, safety level...
DNV guideline 2009 (technical report):- Addresses some of the issues not dealt with in existing standards Photo: Trude Refsahl, Statoil
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Design standards for floating wind turbine structures
16 March 2011
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Current DNV documents
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Design standards for floating wind turbine structures
16 March 2011
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Next step
Development of full-fledged design standard for floating support structures
Approach- Joint Industry Project – industry involvement- Quality assurance by
- Technical reference group- Internal and external hearings
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Design standards for floating wind turbine structures
16 March 2011
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Technical issues to be covered by design standard
Safety philosophy and design principles
Site conditions, loads and response
Materials and corrosion protection
Structural design
Foundation design
Stability
Station keeping
Control and protection system
Mechanical system and electrical system
Transport and installation
In-service inspection, maintenance and monitoringK. Ronold
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Design standards for floating wind turbine structures
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Safety philosophy and design principles; safety class approach
Safety philosophy as for fixed wind turbine structures in DNV-OS-J101- Safety class methodology; three classes are considered depending on severity of failure
consequences:- Low- Normal- High
- Target failure probability is set depending on required safety class
Design principles- Partial safety factor method- Requirements for partial safety factor
are set depending on required target failure probability
Safety class- It is important to determine/decide an adequate safety class for the various structural
components of floating wind turbine structures
L.C. Nøttaasen
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Design standards for floating wind turbine structures
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What shall the safety level be for floating support structures?
The target safety level of the existing standards is taken as equal to the safety level for wind turbines on land as given in IEC61400-1, i.e. normal safety class
The scope for this fixed target safety level has been expanded several times:- Extrapolation from smaller turbines to larger turbines- Extrapolation from onshore turbines to offshore turbines- Extrapolation from turbine+tower to support structure- Extrapolation from individual structures to multiple structures in large wind farms
Cost-benefit analyses would likely show a need to go up one safety class, from normal to high, at least for some structural components
The DNV guideline for floating wind turbine structures recommends design of station keeping system to high safety class (with a view to consequences of failure)
Target safety level is likely to depend on the number of turbines in the wind farm
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Design standards for floating wind turbine structures
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Site conditions – particular issues for floaters
Special issues to be considered relative to current requirements in existing codes:
Adequate representation of wind in low frequency range
Adequate representation of dynamics may require more thorough/improved representation of simultaneous wind, waves and current
Gust events based on gust periods in excess of 12 sec must be defined; must cover expected events and reflect frequencies encountered for dynamics of floaters
For floaters which can be excited by swell, the JONSWAP wave spectrum is insufficient and an alternative power spectral density model must be applied
For tension leg platforms, water level and seismicity may be of significant importance
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Design standards for floating wind turbine structures
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Loads – particular issues for floaters
Special issues to be considered relative to current practice for bottom-fixed structures:
Simulation periods to be increased from standard 10 min to 3 to 6 hrs- Purpose: Capture effects of nonlinearities, second-order effects, slowly varying responses- Challenge: Wind is not stationary over 3- to 6-hr time scales
Loads associated with station keeping system include permanent loads- Pretension of tendons (permanent load)- Pretension of mooring lines (permanent load)
Ship impact loads (from maximum expected service vessel) need more thorough docu- mentation than for bottom-fixed structures- Larger consequences of ship collision- Motion of two bodies with different motion
characteristics
Photo: Øyvind Hagen, Statoil
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Design standards for floating wind turbine structures
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Loads – continued
Additional load cases to be defined, accounting for- Changes necessitated by new/additional gust events- The fact that the control system is used to keep turbine in place by preventing excitations
Accidental loads to be considered. Examples:- Dropped objects- Change of intended pressure difference- Unintended change in ballast distribution- Trawling- Collision impact from unintended ship collisions- Explosions and fire
A. Grimsby
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Design standards for floating wind turbine structures
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Structural design
Reliability-based calibration of partial safety factor requirements for design of structural components not covered by DNV-OS-J101- Examples: tendons, mooring lines
Existing design standards from other industries may be capitalized on:- DNV-OS-C101 and DNV-OS-C105 for tendons - DNV-OS-E301 for mooring lines- Difficulties because of different definition of characteristic loads- Shortcomings because of rotor-filtrated wind loads are not covered by existing standards
Need for data to define a representative set of design situations for safety factor calibrations- Load and response data for various
structural components- Full scale data (example Hywind)- Model scale data- Data from analytical models
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Design standards for floating wind turbine structures
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Stability
Sufficient floating stability is an absolute requirement- In operation phase and in temporary phases- In intact as well as in damaged condition
Additional compartmentalization is usually not required for unmanned structures
The need for a collision ring in the splash zone depends on- Manned/unmanned- Substructure material (concrete/steel/composites)- Size of service vessel and resistance against ship impacts
Location and design of manholes and hatches to be carried out with a view to avoid water ingress
Dropped objects and ship collisions may pose threats to stability
C.F. Salicath
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Design standards for floating wind turbine structures
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Station keeping Three types are foreseen:
- Catenary or taut systems of chain, wire or fibre ropes- Tendon systems of metal or composites for restrained systems such as TLPs- Dynamic positioning
Various issues for catenary and taut moorings:- Mooring system is vital for keeping wind turbine in position such that it can produce electricity
and maintain transfer of electricity to receiver- Optimization of mooring systems may lead to non-redundant systems where a mooring
failure may lead to loss of position and conflict with adjacent wind turbines- Sufficient yaw stiffness of the floater must be ensured
Various issues for tendon systems:- Systems with only one tendon will be compliant in roll and pitch- Floaters with restrained modes will typically experience responses in three ranges of
frequencies- High frequency, wave frequency, low frequency- More complex to analyse than other structures
- Terminations are critical components, regardless of whether tendon is metallic or composite
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Design standards for floating wind turbine structures
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Needs for information
Load/response data for various structural components- tendons- mooring lines- structural components in floater
from - analysis models - model tests - full scale measurements
Wind data for definition of new gust events
Wind data in low frequency range
Ship impact load data
Data for accidental loads and frequencies of accidental events causing damage of wind turbine structure
Courtesy: Statoil
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Design standards for floating wind turbine structures
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Acknowledgments
Illustration contributions from
Statoil
C.F. Salicath
L.C. Nøttaasen
A. Grimsby
are gratefully acknowledged
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Design standards for floating wind turbine structures
16 March 2011
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Thank you
Thank you for
your attention
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Design standards for floating wind turbine structures
16 March 2011
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