Technical Contents - Ke y Issues - SINTEF · Technical Contents - Ke y Issues Johan Sandberg 20 January 201120 January 2011. Background ... - DNV-OS-C101 for tendonsC101 for tendons

Standard for floating wind turbine structuresTechnical Contents - Key Issues

Johan Sandberg20 January 2011

y

20 January 2011

Backgroundg Existing standards are in practice restricted to bottom-fixed structures only:

- IEC61400-3- DNV-OS-J101- GL (IV Part 2)

Shortcomings of existing standards exist with respect to:Shortcomings of existing standards exist with respect to:- Stability - Station keeping- Site conditions (related to LF floater motions)- Site conditions (related to LF floater motions) - Floater-specific structural components

(tendons, mooring lines, anchors)Accidental loads- 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

© Det Norske Veritas AS. All rights reserved.

Standard for floating wind turbine structures

20 January 2011

2

Current DNV documents



20 January 2011

3

Technical tasks Safety Philosophy and Design Principles

Site conditions loads and response Site conditions, loads and response

Materials and corrosion protection

Structural design Structural design

Foundation design

St bilit Stability

Station keeping

C t l d t ti t Control and protection system

Mechanical system and electrical system

Transport and installation

In-service inspection, maintenance and monitoring



20 January 2011

4

Safety philosophy and design principles; safety levely p p y g p p ; y 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

Hi h- High- Target failure probability; is set depending on required safety class

Design principlesg p p- Partial safety factor method- Requirements for partial safety factor; are set depending on required target failure probability

S f t l l Safety level- It is an important task of the project to determine/decide an adequate safety level for various

structural components of floating wind turbine structures



20 January 2011

5

Safety levely 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 the target safety level has been expanded several times:- Extrapolation from smaller turbines to larger turbines

Extrapolation from onshore turbines to offshore 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 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)(with a view to consequences of failure)



20 January 2011

6

Site conditionsSpecial issues to be considered relative to current requirements in existing codes:

Adequate representation of wind in low frequency range 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 significantand seismicity may be of significant importance



20 January 2011

7

LoadsSpecial 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 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

Load categorization to be supplemented by loads associated with station keeping system- Pretension of tendons (permanent load)(p )- Pretension of mooring lines (permanent load)

Ship impact loads (from maximum expected service vessel) need more thorough docuservice vessel) need more thorough docu-mentation than for bottom-fixed structures- Larger consequences of ship collision

M ti f t b di ith diff t ti- Motion of two bodies with different motion characteristics



20 January 2011

8

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 compensating for motions

Accidental loads to be considered; examples:D d bj t- Dropped objects

- Change of intended pressure difference- Unintended change in ballast distribution

T li- Trawling- Collision impact from unintended ship collisions- Explosions and fire



20 January 2011

9

Structural designg Calibration of partial safety factor requirements for design of structural components

not covered by DNV-OS-J101- Examples: tendons, mooring lines

Existing design standards may be capitalized on to some extent:DNV OS C101 for tendons- DNV-OS-C101 for tendons

- DNV-OS-E301 for mooring lines- Difficulties because of different definition of characteristic loads

Sh t i b f t filt t d i d l d t d b i ti t d d- 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, which can be made available to

the project- Full scale data (example Hywind)- Model scale data- Data from analytical models



20 January 2011

10

Stabilityy 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)( p )- Size of service vessel and resistance against ship impacts

Location and design of manholes and hatches to be carried out ith i t id t iwith a view to avoid water ingress

For some concepts, dropped objects may pose a threat in case of repairs and lifting operationsp g p



20 January 2011

11

Station keepingp g 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: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 mooringOptimization 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: 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

frequenciesfrequencies- 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



20 January 2011

12

- Terminations are critical components, regardless of whether tendon is metallic or composite

Needs for information Load/response data for various structural components

- Tendons- Mooring lines- Structural components in floaterfrom analysis models and/or full scale measurementsy

Wind data for definition of new gust events

Wind data in low frequency range (?)Wind data in low frequency range (?)

Ship impact load data

Data for accidental loads and frequencies of accidentalData for accidental loads and frequencies of accidental events causing damage of wind turbine structure

List to be expanded...



20 January 2011

13

Key floater issuesyIn design:

Mathieu Instability and Vortex Induced Motions must be avoided or be controllableMathieu Instability and Vortex Induced Motions must be avoided or be controllable

Cautious selection of eigenperiods in heave, pitch and roll

State-of-the-art offshore design practice provides guidanceg p p g

In particular for compliant floaters:

Location of fairleads

Use of ”crowfoots” to ensure sufficient restoring stiffness in yaw

In particular for restrained floaters:

Terminations are usually critical

Caution to be exercised with respect to risk of higher order responses ( i i i i ) i i i d d t d i(ringing, springing); springing is very dependent on damping

Eigenperiods to be above the fundamental wave periods to avoid resonance



20 January 2011

14

Key floater issues – continued yIn operational mode:

Effects of rotating turbine on global motions must be accounted for Effects of rotating turbine on global motions must be accounted for

Control software and algorithms to be used to - limit inclinations and thereby limit motions, accelerations, bending moments (roll and pitch y g ( p

wind damping effects may be vital)- positively influence mooring and cable hang-off motions with respect to fatigue- positively influence stability of floater



20 January 2011

15

Safeguarding life, property g g , p p yand the environment

www dnv comwww.dnv.com



20 January 2011

16

Documents

Technical Contents - Ke y Issues - SINTEF · Technical Contents - Ke y Issues Johan Sandberg 20 January 201120 January 2011. Background ... - DNV-OS-C101 for tendonsC101 for tendons