reliabilityof waveenergyconverters …department of civil engineering•aalborg university failure...

Department of Civil Engineering•Aalborg University

RELIABILITYOF WAVE ENERGYCONVERTERS–RELIABILITYASSESSMENT

John Dalsgaard Sørensen, Simon Ambühland Jens Peter KofoedDepartment of Civil Engineering, Aalborg University, Denmark

1Department of Civil Engineering•Aalborg University

3rd SDWED SYMPOSIUM


Contents• Introduction

• Reliability modeling of SWED

• Target reliability level for SWED

• Case study: Wavestar prototype

• Conclusions / Comments

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Introduction: WP5 - Reliability

Minimize the Total Expected Life-Cycle Costs

Minimize Levelized Cost Of Energy (LCOE)


Introduction: WP5 - ReliabilityObjectives:

• Develop methodologies for• Risk analysis of a WEC (→ presentation by C Bittencourt,

DNV-GL)• Probabilistic reliability assessment of a WEC

• Assess ‘optimal’reliability level• Proposals for codes / standardization

Use experience / methods from:• (Offshore) Wind turbines• Oil & gas structures• Coastal structures


Introduction: WP5 - ReliabilityRisk / reliability analysis for wind turbinesàwave energy devices:

• Structural components:- use Structural Reliability Methods

• Electrical / mechanical components:- use System / Classical Reliability MethodsWT Failure Rates and Downtimes (examples):

Source: ISET: 2006


Reliability analysis –structuralcomponents

• Use Structural Reliability Methods• ULS: Extreme loads

• Normal operation (without and with faults)• Parked

• FAT: Fatigue (→ presentation by S Ambühl)• ULS & FAT: Transport & Installation• ALS: Accidental situations ?• SLS: Serviceability ?

• Damage tolerant design / robustness• Calibration of ‘Partial safety factors’


Structural Reliability MethodsLimit state equation:

Probability of failure:

• Formulation of limit state equation• Stochastic modeling of uncertain parameters

• Physical uncertainties• Statistical uncertainties• Model uncertainties• Measurement uncertainties

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

Department of Civil Engineering•Aalborg UniversityDepartment 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 ~ 5 10- 4 or β= 3.3

Wave energy devices: ???• annual PF ~ 10- 4 - 10- 3 or β= 3.1–3.7

Reliability level


ULS limit states:•Failure of structural elements, leading to disintegration/change of

geometry/loss of part(s)•Local structural failure due to wave impact (slamming) (potentially

leading to capsizing/sinking)•Mooring failure by sliding of anchor•Mooring failure by breaking of mooring line(s)•...

Fatigue limit states:•Fatigue failure of welded details, …

Reliability –structural components



Methodology for Probabilistic ReliabilityAssessments of Wave Energy Devices



Load cases of Importance for Wave Energy Devices(WEDs)

The following load cases are generally of importance:• Extreme wave and wind loads during normal operation.• Extreme wave and wind loads during operation

simultaneous with a fault of• electrical component.• mechanical component.• control system.

• Extreme wave and wind loads when the WEDis in ‘parked’

position.• Extreme loads during Transport & Installation• Fatigue failure due to wave and wind loads.



Consideration of limit state s with faults of Mechanical/ Electr icalComponents and Control System


Annual prob. of failure of failure mode i.

Annual faultrate of failure mode i

Extreme load effect for failure mode i

Consequences of system due to fault i


Petrochemical industry (Offshore) wind turbines Generic databases

Failure Rates of Components

• Often modeled by time-independent failure rates

• Data for WEDs not available due to lack of knowledge

• Basic reliability data of mechanical/electrical components

available from related industries:

Alternatively for standardization: Introduce partial safety factorsfor fault DLCs dependent on the system fault occurrence rate estimatedusing e.g. ISO 13849-1or IEC 62061on functional safety



Case Study - Wavestar Prototype



Failure cases considered• Failure mode: sliding of gravity-based foundation

• 5 different system failure modes / failure sequences:

• At piles: only wave loads considered

• At platform: only wind loads considered

• At floater: only wave loads when

a fault occurs



Failure and Load cases (I)

• Failure modes considered:


Case Title Descript ion

0 Normaloperation Floater out of water during storm.

1 Total loss of electricity Electricity connection broken and onboard

diese l generator fails.

2 Failure floater lifting system Failure of auxiliary pump or its valve and failure

of motor (generator) or pump (turbine).

3 Failure bearing Failure of one (out of 4) bearing.

4 Wrong wave state measurements Pressure sensor and ultrasonic sensor broken.

5 Failure control system Software failure of control system.


Failure and Load cases (II)


Case i 1 2 3 4 5

Tit leTotal loss of

electricityFailure floater lifting

systemFailure bearing Wrong wave state

measurementsFailure control

system

Effect

Blocking ofhydraulic cycle

(valves in standbyposition).

Floater remains onwater surface ,

brokenturbine /generatorleads to blocking.

Increased frictionmovement of floater.

Floate r remains inoperation, wave

state measurementstoo low.

Floater remains inoperationalmode .

Number ofaffectedfloaters

2 1 1 2 2

Load case(wave loads onfloater)

Extreme wave loadsand high damped

floate r motionduring 490 hrs


floater motionduring 490 hrs



Extreme wave loadsand normal damped


Extreme wave loadand normal damped

floate r motionduring 48 hrs


Load Calculation for the Failure Cases



Limit state –Sliding of Gravity-based Foundation• Limit state equation including failure case i:



Design Equation

• Used to calculate design parameter zi:



Stochastic Model



Results



Conclusions / Comments• Framework developed for reliability assessment and probabilistic design of

wave energy devices:• Electrical / mechanical components: classical system reliability methods

• Structural components: structural reliability methods

o Identification and selection of structural elements to be included in theprobabilistic basis: e.g. anchor block, mooring line(s), structuralelements

o Identification and modeling by limit states of important failure modes

o Stochastic models for the uncertain parameters

o Recommendation of methods for estimation of the reliabilityo Recommendations for target reliability level

o Recommendation for system aspects and damage tolerant design.

• Wave energy devices require consideration of system reliability effects androbustness to unexpected incidents and errors –using a risk-based approach



Conclusions / Comments• Design Load cases (DLCs) could be:

• Extreme loadso Normal operation (without and with faults)

o Parked

• Fatigue• Transport & Installation DLCs

• Accidental situations ?• Serviceability ?

• Standardization / calibration of safety factors:

• Choice of required reliability level

• Introduction of WEC classes?

• Calibration of safety factors assisted by reliability assessments

• Reliability is an important input for optimal planning of Operation &Maintenance and estimation of LCOE


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reliabilityof waveenergyconverters …department of civil engineering•aalborg university failure...

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