design standards for floating wind turbine...

Knut O. Ronold16 March 2011

Design standards for floating wind turbine structuresEWEA 2011 - Brussels

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Design standards for floating wind turbine structures

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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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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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Current DNV documents



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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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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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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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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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Site conditions – particular issues for floatersSpecial 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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Loads – particular issues for floatersSpecial 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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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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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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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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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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Needs for information Load/response data for various

structural components- tendons- mooring lines- structural components in floaterfrom - 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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AcknowledgmentsIllustration contributions from

Statoil

C.F. Salicath

L.C. Nøttaasen

A. Grimsby

are gratefully acknowledged



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Thank you

Thank you for

your attention



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Safeguarding life, property and the environment

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