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Reliability of wave energy devices WP5 - overview
J.D. Sørensen, S. Ambühl, J.P. Kofoed Department of Civil Engineering, Aalborg University, Denmark
C.B. Ferreira Det Norske Veritas BV, London, UK
1 Department of Civil Engineering • Aalborg University
Contents • Introduction • Characteristics of Wave Energy Devices (WED) • Reliability of wind turbines vs wave energy devices • Reliability modeling of WED • Target reliability level for WED • Summary
Department of Civil Engineering • Aalborg University 2
Introduction Objectives: • Development of methodologies for
• Reliability assessment of a WED • Electrical, mechanical and structural components + control system
• Risk analysis of a WED system incl. • Planning of O&M
• Assess ‘optimal’ reliability level for WED • Proposals for codes / standardization / safety factors for WED
Use experience / methods from: • (Offshore) Wind turbines • Oil & gas structures • Coastal structures
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Introduction Risk / reliability analysis concepts for wind turbines wave energy devices:
Structural components: - use Structural Reliability Methods
Electrical / mechanical components: - use System / Classical Reliability Methods
Risk-based methods for life cycle management: • Operation & Maintenance
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WT Failure Rates and Downtimes (examples)
ISET: 2006
Reliability – wind turbines
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Development / qualification phases:
From Carbon Trust (DnV) 2005
Reliability
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From Thies et al. 2009
Reliability modelling of WED
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WED subsystems: • Reaction subsystem (foundation or moorings, the structural
reference elements) • Hydrodynamic subsystem (structural elements responsible
for the primary power capture, typically where the wave power is turned into mechanical/hydraulic/pneumatic power)
• Power take-off subsystem (mechanical and electrical elements responsible for conversion of the mechanical/hydraulic/pneumatic power into electrical power)
• Control subsystem (electronic elements including sensors and actuators needed for optimization and control of the power take-off subsystem)
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Reliability modeling of WED
Reliability assessment – WED vs WT Wave energy devices (WED): ratio between structural loadings in
extreme and production conditions is in most cases very high Wind turbines (WT): ratio is significantly smaller, as wind turbine
blades are pitched out of the wind in extreme conditions, making extreme loadings of the same order of magnitude as production loads.
As extreme loadings and survivability drive the costs of the
devices, and as income is only generated in everyday production conditions, it is of tremendous importance for WED to increase reliability and reduce cost.
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Failu
re R
ate
Time
‘Burn-in’ failures Improve quality control
Wear out Inspections Robustness
Random failure Improve reliability
Constant failure rate = 1 / Mean Time Between Failure
Bath tub curve
Reliability – elec. / mech. components
10 Department of Civil Engineering • Aalborg University
Limit state equation: Probability of failure: Requirements: • Formulation of limit state equation • Stochastic modeling of uncertain parameters
• Physical uncertainties • Statistical uncertainties • Model uncertainties • Measurement uncertainties
( ) 0=xg
( )( ) ( )β−Φ≈≤= 0XgPPF
Probability of failure,
10-2 10-3 10-4 10-5 10-6 10-7
Reliability index,
2,3 3,1 3,7 4,3 4,8 5,2 FP
β
Reliability – structural components
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ULS limit states: • Fatigue failure of welded details • Mooring failure by sliding of anchor • Mooring failure by breaking of mooring line(s) • Failure of structural element, leading to disintegration/change
of geometry/loss of part(s) • Local structural failure due to wave impact (slamming)
(potentially leading to capsizing/sinking) • Wear out of hinged connections • ...
Reliability – structural components
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Department of Civil Engineering • Aalborg University
Building codes: e.g. Eurocode EN1990:2002: • annual PF = 10-6 or β = 4.7
Fixed steel offshore structures: e.g. ISO 19902:2004 • manned: annual PF ~ 3 10-5 or β = 4.0 • unmanned: annual PF ~ 5 10-4 or β = 3.3
IEC 61400-1: land-based wind turbines • annual PF ~ 10-3 or β = 3.1
IEC 61400-3: offshore wind turbines • annual PF ~ 2 10-4 or β = 3.5
Wave energy devices: ???
• annual PF ~ 2 10-4 or β = 3.5
Reliability level
Load combinations - proposal • Power production – control system is to some degree limiting the
load effects due to wave (and wind) actions. Extreme load effects have to be determined by load
extrapolation • Power production and occurrence of fault(s). faults in e.g. the
electrical or hydraulic system may imply extreme load effects • Parked (out-of-operation): for some WED types power
production is stopped for very large wave heights and loads are limited.
• Transportation, installation and maintenance
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Summary Task 5.1: Development of methodology for reliability assessment of WEDs • Based on existing techniques for reliability analysis will be applied for
assessment of the reliability of electrical, mechanical and structural components in a WED
Task 5.2: Collection and analysis of statistical information for structural, electrical and mechanical components
• Available data on failure rates and uncertain parameters - statistical analysis • Estimation of reliability of selected components / WEDs Task 5.3: Risk analysis and operation & maintenance • Life cycle approach - optimal planning of operation and maintenance –
minimize cost of energy • Assessment of the minimum reliability level for different WED components • Recommendations for deterministic design using e.g. the partial safety factor
design methods for structural components
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Thank you for your atttention
This work is part of the project SDWED (Structural Design of Wave Energy Devices) Supported by Danish Council for Strategic Research www.sdwed.civil.aau.dk
Contact: • Jens Peter Kofoed: [email protected] • John Dalsgaard Sørensen: [email protected] • Simon Ambühl [email protected]
Department of Civil Engineering • Aalborg University 16
