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Optimization of floating support structures for deep water wind

turbines

EWEA 201114-17 March 2011, Brussels, Belgium

byPetter Andreas Berthelsen

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Background and motivation

Growing interest for floating wind turbines (FWT) Limited access to shallow water areas world wide Can be installed further offshore

in areas with stronger and steadier wind with less visual impact

Potential is huge provided cost can be brought down to a competitive level

Additional technical challenges for FWT E.g. exposed to wave induced motions

Conceptual design need to Limit wave and wind induced motions Minimize cost

There is a need for efficient methods for optimal design of floating offshore wind turbines

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Scope of work

Develop a tool for:

Optimization of floater and mooring system for a given wind turbine size and given design requirements.

Optimization in this context is the same as minimizing the material cost while satisfying functional and safety related design requirements

Based on existing software tools

Consider optimization of Spar type floaters only

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Design tool

WINDOPT Efficient design tool for minimum cost design of floating support

structures, including mooring system and cable connection Spar type concepts

Building block for WINDOPT MOOROPT – Optimization tool for mooring- and riser systems MIMOSA 6.3 – Response analysis in frequency domain

WF+LF motion Mooring and power cable forces

WAMOF 3 – Hydrodynamic response analysis of slender structures NLPQL – Nonlinear optimisation with arbitrary constraints

Useful tool for conceptual design and parametric studies

4

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Problem formulation

3 key items for optimization

1. Objective function Cost function to be minimized: Spar buoy cost + mooring line cost

2. Constraints Design requirements: Floater motions, heel angle, nacelle accelerations,

capacity (safety factors), offset limitations, etc…

3. Variables Design parameters influencing the objective function and/or constraint

related responses: Spar diameter and length, mooring line diameter, lengths, pretension, etc…

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Need a simplified representation of the spar buoy

Parameterizations and cost model

Assume that a representative mass and cost figure for an initial structure is available

Mass per unit length governed by depth and diameter

Spar buoy cost

Mooring line cost

Depth dependency

Diameter dependency

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Constraints

Spar buoy: Maximum spar draught Maximum tower inclination Max/min requirements on heave and pitch period Maximum nacelle acceleration

Mooring lines: Maximum mooring line tension Maximum allowable horizontal offset Minimum static horizontal pretension (for minimum yaw stiffness)

…

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Variables

Spar buoy Cylinder variables

Height and diameter Diameter of footing Vertical position of fairleads

Mooring system Line direction Pretension or distance to anchor Segment length Segment diameter

Tower and turbine are fixed

Col 2 Transition section

Col 1 Water plane section

Col 3 Main buoyancy section

Col 4 Heavy ballast section

Footing, Bottom plate

Mooring lines

Power export cable

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Analysis option

Extreme response analysis Repeated NCASE times, each one with NENV different environments. Typical cases

Operational (rotor running) Survival (passive rotor) Damage (line break, etc…)

Check for design constraints

Fatigue response calculation Check for fatigue life constraints (mooring lines only)

(not included in present work)

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Example

Reference case based on:

NREL/IEA OC3 5MW turbine

Water depth is 320 m Extreme conditions:

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Example: Floater definitions

Initial data: Material mass- and

cost assumption

Consider three different design options

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Example: Performance

Constraints

Max allowable draught is 120 m

Results

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Example: Spar shape

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Example: Cost

Mooring lines:

Initial Design A Design B Design C

Spar buoy 78880 60293 58855 52711

Mooring lines 17494 14145 13703 13270

10000

30000

50000

70000

90000

110000

Cost comparisonkN

OK

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Concluding remarks

Useful tool for finding improved solutions for floating support and mooring system

Efficient for parametric studies and valuable for early evaluations of spar concepts (not intended as detailed simulation tool)

Weight and cost models can be further improved Realistic design constraints need to be obtained Further development to be considered are:

Fatigue life constraints (completed, but not included in present work) Optimize power cable (completed, but not included in present work) Include tower design Other types of floaters Improved description of wind loads

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Acknowledgement

This work has been carried out as part of NOWITECH (Norwegian Research Centre for Offshore Wind Technology) which is co-funded by the Research Council of Norway, and participating industrial companies and research organisations (www.nowitech.no)

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References

1. Fylling and Berthelsen (2011), WINDOPT – An optimization tool for floating support structures for deep water wind turbines, OMAE 2011