Advanced analysis for offshore wind inter-array cables anderson... · Advanced analysis for offshore wind ... – Highly conservative, no VIV allowed – Multiple span mitigations
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woodplc.com
Advanced analysis foroffshore windinter-array cables
Keith AndersonSubsea and export systems manager
Cable claims• Multiple reasons
• 1/3 includes poor workmanship orengineering
• Industry needs to improve
• Advanced engineering techniques canhelp
3 A presentation by Wood.
0
20
40
60
80
100
Cable % contribution tooverall offshore wind
development
%CAPEX %Cable Claims %Cost of Cable Claims
• Cable protection system engineering
• Freespan analysis
• Local cross-section modeling
• Conclusion
Scope of presentation
4 A presentation by Wood.
• Cable hang-off at platform level
• Cable entry location near seabed
• Cable protection options– Internal to monopile
• Internal J-tube• Internal guides• Free hanging• If J-tubes are not used bend protection
required
– External to monopile• J-tube• Bend protection system
Cable entry systems for monopilesOverview
6 A presentation by Wood.
• Cable modelled from exit to seabed
• Cable exit point:– Axially fixed point– Rotational motions permitted up to constraints
• Cable and ancillary protection equipment modelledas equivalent beam– Combined stiffness– Combined weight
• Cables highly dynamic– High lateral deflections– Limited overlength to accommodate motion– Load transferred to burial and cable entry point
Basic system modelling – option 1 (split model)
7 A presentation by Wood.
Case study• Cable - 150mm diameter; dry weight 40kg/m• Current speed 2m/s; 100yr wave
External cable model
• External model loads applied as boundaryloads at cable entry
• With internal J-tube:– Restricts curvature within the monopile– Overtension remains
• Without internal J-tube:– High curvatures and tensions
• Cable entry location• Hang-off / guides• CPS required
Basic system modelling – option 1 (split model)Internal cable model
8 A presentation by Wood.
Case study• Cable - 150mm diameter; dry weight 40kg/m• Current speed 2m/s; 100yr wave
• Ancillary equipment modelledindependently from cable– Pipe in pipe contact modelling– No axial load transfer– Allows relative CPS / cable
movement
• Improved external system loads at thecable entry point
• Reduced input loads to internal modelimproves the curvatures within themonopile– Less onerous CPS requirements
Intermediate modelling – option 2 (split model PiP)Independent CPS modelling
9 A presentation by Wood.
Case study• Cable - 150mm diameter; dry weight 40kg/m• Current speed 2m/s; 100yr wave
• Full pipe-in-pipe model of the system– Explicit definition of CPS, J-tubes and
cable elements
• Continuous load transfer and axialmovement of cable within CPS / cableentry point– Significantly reduces tension response– Improves curvature response
• Step change in results from fail to pass
• Optimise CPS requirements
Recommended system – option 3 combined modelSingle model from hang-off to burial
10 A presentation by Wood.
Case study• Cable - 150mm diameter; dry weight 40kg/m• Current speed 2m/s; 100yr wave
Full combined modelling recommended
• Scour around monopiles– Cables freespans
• Risk– Cable strength overload– Fatigue failure– Vortex induced vibration (VIV)
• Development had 18 turbines with cable spans to bemanaged
Span challenge
12 A presentation by Wood.
• System modelling– Pipe in pipe modelling in J-tube– Local cross-section stress analysis
• Extreme strength– 100yr response proven within criteria
• Wave fatigue– Residual life sufficient for life of field operations
Strength and wave fatigue
13 A presentation by Wood.
• Company initial approach– 100yr peak wave velocity– Highly conservative, no VIV allowed– Multiple span mitigations required
• Wood approach– Accept VIV via engineering– Model system responses– Determine fatigue life
Advanced VIV approach
14 A presentation by Wood.
Scou
r Pit
Dep
th (m
)
Scour Pit Diameter (m)
Scour pit diameter vs depth
Survey Scour Profiles Wood Scour Profile Value
• Environmental considerations– 10% of wave scatter not applicable
• Large motion response decouples VIV• 100yr wave not applicable
– Waves and tidal cycle are oscillatory• Peak velocity not critical• Combined duration above onset velocities
• Flow normal to pipeline– Reduced effective in-plane current
Advanced VIV models
15 A presentation by Wood.
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.0 2.0 4.0 6.0 8.0
Velo
city
(m/s
)
Time (s)
• Response excitation assessment– In-line VIV
• In-plane current + normal mode• Transverse current + transverse mode
– Cross flow VIV• In-plane current + transverse mode• Transverse current + normal mode
– Span length selected to mitigate cross-flow
• Fatigue analysis for all excited modes– >400yrs fatigue life predicted– VIV less onerous than wave fatigue
Advanced VIV models
16 A presentation by Wood.
• VIV limits acceptable span lengths– Extreme performance less onerous
• Allow VIV onset and manage behaviour– Allowable span lengths increased 50%– No mitigations required
• 80:20 rule– Project ended with acceptable performance achieved– Further increases in span lengths possible
• e.g. current scatter tables
VIV summary
17 A presentation by Wood.
• Independent, validated cable modelling– Comprehensive FEA contact models– Benchmarked with test data
• Required for cross-section fatigue calcs– Wave and VIV
• Coupled with advanced global modelling– Accurate, efficient fatigue life prediction
Advanced cross-section modelling
19 A presentation by Wood.
Beams
Shells
Solids
Fatigue life prediction
20 A presentation by Wood.
Hs (m) 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Tz (s)
4 0.3 0.2 0.15 0.8 0.6 0.4 0.26 1 0.8 0.7 0.5 0.4 0.37 1.3 2 2.1 1.8 0.8 0.7 0.5 0.58 0.6 1.5 3 2 1.5 1.2 1.1 0.8 0.7 0.69 1 2 4 3.5 2 1.5 1.3 0.7 0.5 0.7 0.410 1.8 5 4.5 3.1 2.4 1.9 0.6 0.7 0.5 0.411 3 3.2 2.8 2.2 1.6 1 0.8 0.7 0.112 1 2.5 1.8 1.1 0.8 0.7 0.6 0.313 1.6 1.2 0.8 0.6 0.5 0.414 1 0.9 0.5151617
• Global response loads
• Local stresses– Core slippage?
• Material fatigue data
• Environmental occurrences
• Output = fatigue life
• Reduces conservatism in design loadings
• Supports engineering mitigation anomalies
• Enhanced fatigue life prediction
Advanced system modelling
22 A presentation by Wood.
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