welcome at our masterclass...dnvgl-st-0126 –tolerances defined in dnvgl-os-c401 ch2 section 11 δ...
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Welcome at our Masterclass
Centre Line Aligned assembly of cans
Daniel Butterworth
DNV GL
What is Centre Line Alignment?
• OD Aligned
• ID Aligned
• CL Aligned
• Stress at ID circa 2% lower than stress at OD
• Preferential to have a larger SCF internally than externally for double sided welding
• Fatigue life for worse on the side with the thickness transition
• For single sided welding – best ID aligned as root has a worse fatigue classification – F3. SCF internally becomes 1.
DNVGL-ST-0126 – tolerances defined in DNVGL-OS-C401 Ch2 Section 11
δt = Change in thickness
δm = Fabrication misalignment
δ0 = Inherent factor within SN curve – 0.05t double sided, 0 for single sided or ground (table 3-1)
The norm
4
• SCF’s defined in DNVGL-RP-C203 Section 3, 3.3.7.3 SCF butt welds at
thickness transitions at girth welds in tubulars
SCF’s
5
Internal
External (3.3.5)
(3.3.6)
• Where fatigue is critical it is governed by the side with the misalignment
Typical SCF’s
6`
External
SCF
Internal SCF
OD 1.0 to 1.05
(eqn 3.3.6)
1.0 to 1.45
(20mm step)
(eqn 3.3.5)
ID 1.0 to 1.45
(20mm step)
(eqn 3.3.5)
1.0 to 1.04
(eqn 3.3.6)
CL 1.0 to 1.1
(10mm step)
(eqn 3.3.5)
1.0 to 1.1
(10mm step)
(eqn 3.3.5)
Potential Thickness reductions
7
At all fatigue sensitive step changes in
wall thickness
CL alignment allows larger step changes
in can thickness
Influence of array cable hole can be
limited to a single thicker can
Driving damage reduction at CL welds
Where is it Beneficial
8
Lower SCF = Lower Driving induced damage
Thinner walls reduce pile tip energy
For 10mm tinner over 45% of pile - blowcount > by 7%
Damage at unalterred section 90% to 105%
Stress per blow reduces below thinner section
For 10mm thinner section CL alignment damage 60% of OD aligned
At larger step changes driving damage will reduce further
Differences will increase when driving is hard.
Driving induced fatigue
9
SCF from Equations 3.3.12 & 13
A - FE analysis required not covered by
parametrics
B - No other SCF’s required if transition is
outside at the base, and inside at the top
C – Internal transition at base would require eqn
3.3.5 in addition to conical SCF
Cones
10
BA C
Conical SCF Variations
11
A good reason to avoid thickness transitions at conical junctions
Can be improved with longer Tapers, CL alignment & FE
Centreline alignment will mean additional fatigue life relative to OD or ID aligned for the same pile weight – Capacity redistributed better
Lower Driving induced fatigue as SCF lower
Does not change other details – cable entry holes, & attachements
Reliable corrosion control measures needed for extended design life – coatings ICCP....
Does not influence parts of the structure with constant wall thickness – often these are not fatigue driven
Option to remove sensitivity to large step changes – useful at cable entry cans
As WTG’s get larger, % of strucure driven by D/t increases – CL allows sensitive portion to be optimised
Life Extension
12
Colin EmmettAtkins
Three real design case studies to understand the implications of the
‘classical’ vs. Centre Line alignment welding:
• Triton Knoll offshore wind farm
• Another recently installed monopile project offshore wind farm, and
• A recent deepwater jacket offshore wind farm
Risks and Rewards
• Geotechnical implications
Benefit of Centre Line Alignment
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What governs a MP’s design?
• Fatigue Limit State (FLS)
• Attachments
• Circumferential welds
• Ultimate Limit State (ULS)
• Natural Frequency
• Servicibility Limit State (SLS)
• Accidental Limit State (ALS)
• Connection characteristics
Benefit of Centre Line Alignment –Monopile design drivers
15
Weight reduction of 27.2 tonnes per pile16
Benefit of Centre Line AlignmentPotential weight savings
Benefit of Centre Line Alignment –Triton Knoll
17
Triton Knoll – Concept development
Average saving across 90 foundations - 1431 tonnes across the site
Benefit of Centre Line Alignment – Weight savings
18
Classical Centre Line Change
Transition Piece 106.5 T 106.5 T 0 T
Monopile 550.8 T 534.9 T 15.9 T
Total 657.4 T 641.5 T 15.9 T
Triton Knoll – lifetime damage
At the critical location this equates to a reduced probability of failure of about 20%
Benefit of Centre Line Alignment – enhanced integrity
19
Classical Centre Line
TP – CW2 0.633 0.372
TP – CW3 0.962 0.581
MP – CW2 0.998 0.817
MP – CW5 0.865 0.660
MP – CW6 0.883 0.567
MP – CW8 0.776 0.547
MP – CW9 0.998 0.676
MP – CW10 0.891 0.439
MP – CW11 0.165 0.159
What governs a pin-pile’s design?
• Connection characteristics
• Fatigue Limit State (FLS)
• Circumferential welds
• Ultimate Limit State (ULS)
• Servicibility Limit State (SLS)
Benefit of Centre Line AlignmentJacket pin-piles
20
• Limited application for the CL alignment
welding
• Only 2 thickness transitions in the pin- pile
• Achievable in a single transition with a saving
of approx. 1 tonne per pile
• 344 piles - this equates to between 3 and 4
piles worth of steel saved
Benefit of Centre Line Alignment –Jacket Pin Piles
21
• Clays
• Driving shoes are often used to aid driving
• Where used, there is no need for limitations on the use of CL welding
• Sands
• Reduced resistance of sections following a thicker can
• Short term effect – reconsolidation
• Chalk
• Low skin friction used in design – minimal effect
Applying Centre Line Alignment –Geotechnical implications
22
Possible reduced capacity?
Not appreciable for thickness
changes less than 15mm
• Sands – short term
• Chalks - Permanent(?)
Applying Centre Line Alignment –Geotechnical risks
23
William LafleurSif Netherlands
• Stress concentrations at tubular weld connections are due to eccentricities
resulting from different sources e.g. out of roundness or differences in thickness of
joined tubulars.
• Where the differences in plate thickness of butt welds exceeds 4 mm the thicker
plate shall be tapered not steeper than 1:3. Butt joints prone to fatigue loading shall
be tapered not steeper than 1:4 (DNVGL-OS-C401).
• The transition shall be smooth and gradual.
The standard
25
• Some standards or codes indicate that the transition may be formed by any process that
will provide a uniform taper (ASME BPVC VIII.1-2015 UW-9).
The transition may be formed by adding additional weld metal beyond what would
otherwise be the edge of the weld.
• The normal interpretation of DNVGL is that the taper starts on the bevelled edge from the
thicker part.
The standard
26
• From a fabricators point of view (align with DNVGL requirements), thickness jumps
up to 4 mm can be accommodated in the weld joint and do not require tapering.
The standard
27
Transition in butt welds of unequal thickness
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Butt weld No.
Thicker part
T1 [mm]
Thinner part
T2 [mm]
Change in thickness
[mm]
DNVGL-OS-C401: 2017
Non-Symmetrical (offset alignment)
Symmetrical (centre alignment)
1
80
80 0 No taper No taper
2 78 2
No taper. Transition incorporated in the weld
No taper. Transition (on each side) incorporated in the weld
3 76 4
4 74 6
Taper 1: 4 5 72 8
6 70 10
Taper 1:4 on each side
• Taper (inside) is preferably made by machining in flat condition after which the
plate is formed to a shell or shell segment by cold rolling.
• Taper length of 100 mm can be performed by machining in flat condition, but this
length is further restricted by the rolling process to approximately max. 75 mm.
Sif fabrication
29
max l
T
ΔT
plate
Inside
A
• Inside tapers, when made in flat condition, end just before the longitudinal joint
preparation, as to maintain a full cross section and facilitate longitudinal welding.
Sif fabrication
30
Ta
pe
r
Machined taper
Circumferential joint preparation
Longitudinal joint
• Plate bevelling (with or without taper) executed at plate mill or at Sif.
Sif fabrication
31
• Outside tapers and inside tapers with a length 75 < l < 200mm, must be made by
flame cutting & grinding after cold rolling → longitudinal welding → re-rolling.
Sif fabrication
32
Centre line alignment fabrication aspects
33
Advantages Disadvantages
Less machined tapers needed -Thickness
jumps up to 8mm can be equalized in the weld.
More welding passes needed when taper is
accommodated in the weld joint – slightly
increased weld width.
Applied in Oil & Gas industry – experience.
No new feature.Measuring misalignment slightly more difficult.
Enhanced integrity / Potential weight savings.Symmetrical taper → additional fabrication
activity to perform outside taper.
No impact on back milling and welding process.
Rotational speed due to different diameters is
automatically compensated by the roller beds.
Both sides of the taper need to be restored by
grinding (at the longitudinal weld location).
Flush grinding of girth welds (if required), will
take more time and require more caution
during execution.
Thank you
for your attention