FE based structural design – a stress diffuser for underwater welding

8/7/2019 FE based structural design a stress diffuser for underwater welding

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FE based structural design a

stress diffuser for underwaterwelding

Wangwen Zhao


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Concerns for Integrity of Conductors:

y Conductor guides and guide framing at -2m have been removed,which means the conductor guides and guide framing at -29m are

now non-redundant SCE components failure of a sleeve or guide

brace at -29m will rapidly lead to the failure of one or more

conductors.

y - If a guide at the -29m level were to fail, the conductor would span from +10m to

the next guide level down at -61m - a span of 71m.

- The conductor may fail immediately because of static loads, or there is

significantly increased likelihood of damaging vibration to the conductor and/or

flow lines, and likelihood of failure from increased fatigue damage.

y Conductor max 100-yr storm reaction at -29m has increased from

40kN to 70kN as result of removing -2m guides

y Without strengthening or other mitigation it is difficult to continue to

assure the integrity of the -29m conductor guide framing.


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Conductor frame


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ABAQUS screenshot of stress contours and deformation for guides:

With FourBraces With Three Braces

-29m CONDUCTOR FRAME INTEGRITY


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Ultimate strength is much below extremeload

SCFs are very high up to 54

Fatigue life around 3 years for some jointsbased on design S-N curves

Loss of one component is serious leading

to a number of over-utilised guides, i.elikely to lead to failure of all guides in a

Bay

-29m CONDUCTOR FRAME INTEGRITY


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Evolution of design options


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- Welding of doubler plates on the sleeve

Mitigation Option 1 and 2

Simple to install but not sufficient strength

1: Shell attachment 2. Doubler attachment


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- Wet Welding of ring stiffeners

Mitigation Option 3

Poor fatigue details


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-Wet-Welded Ring Stiffeners with snipe back and rat holes

N

Guide sleeve with new

stiffeners

Wet-welded

Stiffeners

Three-brace Ring Stiffener

Mitigation Option 4

Satisfy ultimate and fatigue strength requirements but requires

full penetration wet welding --- hence not feasible


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-Option 4 without welding to the sleeve

Mitigation Option 5

Easier to weld but very high SCFs are found in near sleeve points

High SCF at termination points


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-Introducing diffusers

Mitigation Option 6

Stiffener ends are welded with diffusers onshore. The combined diffuser andstiffener plate is welded onto the brace faces offshore using fillet welds.

Fatigue strength is increased as SCfs are decreased drastically.

This is a key design feature but it needs refinement for practical considerations.


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-Option 6 with optimised stiffener plates

Mitigation Option 7

Fatigue strength is enhanced buy cutting radius at the ends of the stiffeners.Greens on the edge of the stiffeners are left for site adjustment.


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-Option 7 with modified diffuser shape

Mitigation Option 8

The inner edge of the inboard diffuser does not have mush stress but has

poor underwater welding accessibility.

New diffuser shape reduces the welds in difficult areas.


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-Option 7 with further modified diffuser shape

Mitigation Option 9

Inboard diffuser to have completely open end for even easier accessibility

Weld between diffuser and stiffener plates are simplified.


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Finite element analysis option 9

meshes size and type for different parts of the model :

Part Thickness Element Type Size

Diffuser 15mm4 node linear

tetrahedronApproximately 5mm --. 1/3 thickness

Stiffener 15mm

4 node linear

tetrahedron when

contacting with the

diffuser,8-node linear brick,

incompatible modes in

other part

Approximately 5mm near the diffuser or

at the flange ends, larger size away

from the regions of high stress

concentration.At least 4 layers of elements across the

thickness of 15mm of the main stiffener

plate

2 layers of elements across the

thickness of 10mm of the flange

Fillet

weldL=15mm

4-node linear

tetrahedronApproximately 5mm

Conduct

or and

Braces

C -15mm,

B1-11mm,

B2-10mm

8-node linear brick,

incompatible modes

5mm at areas of high stressconcentration and larger size away fromhot spots

At least 2 layers of elements across

thickness of 10mm or 11mm


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Finite element analysis mesh


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Finite element analysis mesh


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Ultimate strength

Design

Option

Load from

large brace (OD 480mm)

Load from

small brace (OD = 273mm)

Compression

(kN)

Tension

(kN)

In

plane

Out of

plane Compression

(kN)

Tension

(kN)

In

plane

Out of

plane

Bending relative

to sleeve (kNm)

Bending relative to

sleeve (kNm)

4940 940 86 380 918.7 1147.2 128.3 244

71030.66 1240.53 80.0 401.74 1198 1223.8 113 180

91001.0 1134.0 84.4 366.5 1170.2 1217.0 127.7 168.0


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Compressive ultimate strength

Maximum compressive strain contour at around 5% maximum principal strain


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UR for ultimate strength analysis

Minimum UR for option 9 is 0.33 for all joints with stiffened sleeves


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Fatigue analysis hot spots

w8 (w8t)

w11 (+45)

w1 (w1t)

w2 (w2t,we2)

w3

w4

w5 (w5t)

w6 (w6t,we6)

w9 (w9t, we9)

w10

w12 (-45)

w7

w series 12 weld toes --- use D curve

w+t series 6 weld throat for fillet weld, use W curve

we series 3 weld start use F curve


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Fatigue lives for as designed

LocationS-N

curve

SCF Lowest Fatigue Life (years)

Axial IP B OPBWithout

grinding

With grinding

(Sy= 355 MPa)Diffuser side

or in weld

Brace side

weld toe

(relative to sleeve)

w1 D 12.50 10.5 1.20 5.00 75 268

w2 D 8.50 4 1.20 4.50 276 982

w3 T 8.00 6.00 3.50 1187 1187

w4 D 10.00 35.00 1.20 79 281

w5 D 8.00 9.0 1.20 5.00 265 940

w6 D 19.00 10 5.50 4.50 20 70

w7 T 20.00 2.20 3.70 47 47

w8 D 10.00 8 2.00 1.20 227 806

w9 T 20.00 11.00 4.50 5.50 16 59

w10 T 8.00 6.00 3.50 1187 1187

w11 T 3.20 32.00 1.20 323 323

w12 T 1.20 55.00 1.20 91 91

w1t W 8.43 1.27 3.98 347 347

w2t W 6.21 2.83 3.36 1177 1177

w5t W 7.19 1.51 3.52 701 701

w6t W 12.87 3.80 2.95 80 80

w8t W 6.46 1.03 1.01 1451 1451

w9t W 9.74 1.94 2.89 236 236

we2 F 3.60 21.60 2.16 370 370

we6 F 5.40 5.40 2.16 2174 2174

we9 F 9.00 30.60 2.16 84 84


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SCF for as inspected conditionsHot Spot G7, sleeve t 15,

480x11,273x10

G3, 11, 14, 15, 22, 24,

sleeve t 13, 480x10,273x9pup piece G10 G2, 6, 9, 12, 13, 16, 19, 23,

with 4 braces, sleeve

t =13, 480x10, 273x9

G17, sleeve t =13,395x10.5,

273x10.5

Axial IP B OPB Axial IP B OPB Axial IP B OPB Axial IP B OPB Axial IP B OPB

w1 12.50 0.85 3.54 10.50 1.20 5.00 11.00 1.20 4.50 10.00 1.20 3.50

w2 8.50 0.85 3.18 8.50 1.20 5.00 10.00 4.50 5.50 6.50 4.00 3.00

w3 8.00 4.24 2.48 8.00 8.00 2.50 5.50 7.00 1.70 12.00 1.50 25.00 6.00 3.50 1.50

w4 10.00 24.75 0.85 10.00 40.00 1.20 4.50 24.00 1.20 7.50 25.00 1.20

w5 8.00 0.85 3.54 10.00 1.50 4.50 11.00 2.20 5.70 12.00 1.20 3.50

w6 19.00 3.89 3.18 21.00 5.50 5.50 20.00 6.00 3.00 15.00 3.50 3.00

w7 20.00 1.56 2.62 22.50 5.50 4.00 19.00 5.00 3.00 12.00 1.50 25.00 20.00 3.50 2.50 w8 10.00 1.41 0.85 9.50 2.40 1.20 9.00 6.50 3.50 6.00 1.40 1.20

w9 20.00 3.18 3.89 26.00 5.00 6.00 12.00 1.20 3.00 15.00 2.75 3.50

w10 8.00 4.24 2.48 6.50 2.50 3.50 11.00 4.00 4.00 6.00 3.00 14.00 3.00 2.25 2.20

w11 3.20 22.63 0.85 12.00 27.00 2.00 3.60 32.00 1.20 8.50 27.00 1.20

w12 1.20 38.90 0.85 2.50 15.00 2.00 0.00 67.19 1.20 2.50 12.00 1.20

w1t 8.43 0.90 2.81 13.12 0.37 5.93 12.82 0.36 5.33 10.14 0.34 3.71

w2t 6.21 2.00 2.38 7.20 6.25 3.75 7.18 5.81 3.42 4.97 4.58 2.20

w5t 7.19 1.07 2.49 12.52 1.67 5.56 9.62 3.24 5.01 11.32 0.43 3.95

w6t 12.87 2.69 2.09 13.84 3.03 3.60 12.99 4.94 2.68 9.07 2.54 1.37

w8t 6.46 0.73 0.71 4.89 1.08 0.13 12.59 0.19 0.35 3.74 0.72 0.22

w9t 9.74 1.37 2.04 11.97 2.62 3.22 15.13 0.71 2.89 7.62 1.05 1.70

we2 3.60 15.28 1.53 3.60 19.80 2.16 2.70 16.20 0.71 5.40 22.50 0.71

we6 5.40 3.82 1.53 2.70 2.16 2.16 7.20 7.20 0.71 15.30 23.40 0.71

we9 9.00 21.64 1.53 12.60 9.90 2.16 3.24 2.16 0.71 7.20 7.20 0.71


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Fatigue lives - inspected conditionsGuide Node

Fatiguelife

Guide Node

Fatiguelife

1 403376 96.6 13 406380 143928.0

1 403476 297.5 13 406480 10882.0

2 403576 4501.1 14 406180 3253.9

2 403676 25492.0 14 406280 2665.8

3 403776 1412.1 15 405780 1453.9

3 403876 1576.9 15 405880 4560.8

4 404176 2333.8 16 405580 5253.0

4 404276 3916.5 16 405680 42657.0

5 404376 2050.2 17 405380 216.9

5 404476 479.3 17 405480 747.6

6 404576 8106.5 18 404780 166.4

6 404676 1911.2 18 404880 64.3

7 404776 435.8 19 404580 5856.0

7 404876 157.4 19 404680 1898.0

8 405376 111.8 20 404380 2467.9

8 405476 377.0 20 404480 536.4

9 405576 5800.5 21 404180 1244.6

9 405676 46498.0 21 404280 5486.8

10 405776 4911.5 22 403780 2645.710 405876 9064.4 22 403880 2737.7

11 406176 2011.4 23 403580 8225.4

11 406276 1316.2 23 403680 63616.0

12 406376 45068.0 24 403380 138.7

12 406476 5191.5 24 403480 517.1


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Sensitivity study smaller green

Location Axial In plane bending Out of plane bending

w1 10 1.33 2.5

w2 7 1.33 2.1

w3 8 5 1.33

w4 5 12.5 1.33

w5 9 1.33 2.5

w6 15 1.33 2.5

w7 19 5 2

w8 8 1.33 1.33

w9 13 1.33 3

w10 7.5 2.5 2

w11 4 12 1.33

w12 2 8.5 1.33

w1t 7 1 2

w2t 5 2 2

w5t 8 1 2

w6t 10 1.5 1.5

w8t 6 1 1

w9t 8 1 1.3

The table shows the SCF of option 7design with edge of the stiffener

having a distance of 10 mm above

the diffuser instead of20 mm above

the diffuser.

5mm top up

weld

Stiffener edge distance

Diffuser

They are all smaller than those for largerdistance. Hence the worst case with 20

mm edge distance in further calculation.


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Sensitivity study Brace angular

rotation

SCfs for weld toes are

similar to perfect case

hence fatigue lives are

similar with the lowest

being 47 years for w7.

There is an increase of

SCfs for weld throat with

the lowest fatigue life = 62

years for w9t.

1


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Onshore manufactured integral

stiffener

Date post:	09-Apr-2018
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FE based structural design – a stress diffuser for underwater welding

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