Structural Integrity of Offshore Wind Foundations Feargal Brennan September 2013 Photo: DONG Energy
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Structural Integrity of Offshore
Wind Foundations
Feargal Brennan
September 2013
Photo: DONG Energy
Presentation
Overview
• Risk based design optimisation;
• Tools to ensure Structural Integrity;
• Inspection Reliability;
• NDT/NDE or SHM?
• Summary & Conclusions.
Fixed Foundations
Risk & Reliability –
learning from Offshore
Oil & Gas
Risk = Probability x
Consequence
Cost with respect to
PoF
US contractor Fluor
has lost the major
arbitration case
against the owners
of the Greater
Gabbard offshore
wind farm SSE and
RWE and will take
a pre-tax hit of
$400m
Structural integrity
Assessment &
Management
Crack
Crack
tip ¼ node
Fla
w S
ize
(mm
)
No of Cycles
Actual
Behaviour
Most Likely Life Prediction not
taking POS into account
Extreme Life
Prediction
a
critical
Structural Analysis
Inspection & Monitoring
Inspection Reliability Damage Model
Repair/Remaining life
Assessment
Reliability Based Life-
Cycle Design,
Reassessment & life
Extension
8
Relia
bili
ty index
Time
Target Reliability
Inspection Strategies
Def
ect
Siz
e
Life
Deterministic Approach
Reliability (or Probabilistic)
inspection intervals
Reliability Based
Design
• Stochastic Modelling of Variables
• Design for Reliability
• Target Reliability set from standards for
existing configurations (DNV-OS-J101) or
generic standards (DNV, ISO)
• Basis for structural optimization and
reliability based inspection
Reliability Based
Design
DNV Classification Note 30-6
Reliability
Assessment
• To quantitatively assess reliability
based on detailed
characterisation of the asset
through inspections, monitoring
and damage modelling.
Offshore Design
Standards
Offshore Structures
Design Guidance
• DNV-OS-J101 Design of Offshore Wind Turbine Structures, October 2007.
• HSE Offshore Technology Report 2001/015 – Steel.
• Recommended Practice for Planning, Designing and Constructing Fixed
Offshore Platforms – Working Stress Design, API RP 2A-WSD, Dec 2002.
• Structural Welding Code – Steel, AWS D1.1/D1.1M2002, American Welding
Society/ANSI.
• Eurocode 3: Design of Steel Structures, BS EN 1993-1-1:2005.
• DNV-RP-C203 Fatigue Design of Offshore Steel Structures, Apr 2008.
• DNV-OS-C201 Structural Design of Offshore units (WSD Method), Oct
2008.
• HSE Offshore Technology Report 92 390 - Background to New Fatigue
Guidance for Steel Joints and Connections in Offshore Structures, 1999.
xxx
xxx
The fictitious Hot-
Spot Stress
21
DNV and BS 7608 Class C curves with and without thickness correction for 87 mm.
22
Damage reduction by misalignment on the C1 S-N curve in air.
Barriers to optimisation
of offshore structures
• The Hot-Spot Stress;
• 30 year old S-N curves;
• S-N Curves that were never
intended for large diameter
circumferential welds;
• API RP 2A etc., i.e. load or stress
based design.
Structural Integrity:
Session 5a
24
Surface Improvement
and good fabrication
practice
Structural Integrity:
Session 5a
25
Peening
Distance into Material Surface
Str
ess
Surface Residual Stress with Applied Bending Stress
Surface Residual Stress
Cold Working
Structural Integrity:
Session 5a
27
Cold Worked & Laser
Peened crack
Stoppers
0
5
10
15
20
25
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Cycles x 1000
Cra
ck D
ep
th,
(mm
)
Reference -Crack 1
Reference -Crack 2
Test 1
Test 2
Newman and Raju
Offshore Inspection
POD
Flaw Size
% P
OD
Ideal POD
Behaviour
Likely Behaviour
Spurious / False Calls
POD Trials
Blind inspection trials are carried out on a
series of groups of representative
defective specimens
POD = No of successful inspections
No of attempts
P = S
N
POD Confidence
Confidence that the measured POD is
representative of the population is
dependent on sample size
Using a Binomial Distribution:
)!(!
!
,
)1()( )(
SNS
NC
Where
ppCSP
NS
SNS
NS
POD Confidence
)(1 NPC
The confidence that the point
estimate of P is representative
of the population
Probability of obtaining
N successes in N trials
=> for 90/95% POD the minimum
sample size is:
0.95 = 1 - 0.9N
=> N=28.4 i.e. 29 samples
RBI using POD and POS
Actual Behaviour
Fla
w S
ize
(mm
)
No of Cycles
a critical
Initial defect
distribution
POS of
detected
defect
POS of 90/95%
POD defect
Life prediction from initial
defect distribution
Life prediction from POS of
detected defect
Life prediction from
POS of 90/95% defect
POD Trials
NDT, NDE or SHM?
Emerging
Technologies to
support RBI
• The key to a quantitative risk assessment is
good quality information;
• The more information from different sources
leads to greater confidence in RBI input values;
• Structural Integrity Monitoring (SIM) of critical
components coupled with advanced damage
and failure models is becoming a rapidly
developing activity, however, the performance
reliability of SIM needs to be better understood.
How Inspections and
SIM work together
• Crack growth can be monitored and
assessed;
• Repairs and untested design details can be
assessed;
• SIM can point to areas that have been most
highly stressed helping to optimize
inspections and increase POD by simply
looking in the right place;
• SIM can monitor inaccessible areas allowing
relaxation of inaccessibility imposed design
penalties.
Summary &
Conclusions
Summary &
Conclusions
• The offshore and marine renewables industry
can not afford the “over design” luxury that Oil
& Gas has enjoyed;
• Materials, structural analysis techniques and
inspection/monitoring technology and
knowhow is unrecognizable compared to 30
years ago when Oil & Gas platforms were
designed;
Summary &
conclusions (ctd.)
• A combination of risk based design and
maintenance coupled with modern high
strength weldable steels and weld toe
improvement methods promise to deliver cost
effective advanced steel offshore foundations
for wind & marine renewables.
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