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

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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

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Weight reduction of 27.2 tonnes per pile16

Benefit of Centre Line AlignmentPotential weight savings

Benefit of Centre Line Alignment –Triton Knoll

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Triton Knoll – Concept development

Average saving across 90 foundations - 1431 tonnes across the site

Benefit of Centre Line Alignment – Weight savings

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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

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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

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• 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

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• 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

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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

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• 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

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• 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

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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

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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

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• 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