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Fatigue and Residual Strength of Fiber Rope Tuglines

Rafael ChouOctober 23, 2003

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Presentation Overview

• Fiber, Ropes and Testing• Project Overview • Testing and Analysis • Results and Conclusions

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Dyneema FiberDyneema Fiber CharacteristicsCharacteristics• Maximum strength/minimal weight – long breaking length

• Low elongation

• Floating

• Excellent tension and bending fatigue

• Good UV and chemical resistance

• High cut and abrasion resistance

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Amsteel-Blue is a coated 12 strand single braided construction, 100% HMPE fiber rope.

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Amsteel-Blue• Physical Properties

– Has excellent UV properties– Floats (0.98 Specific Gravity) – Is only one-tenth the weight of an equal

diameter size and strength wire rope• Mechanical Properties

– Has maximum strength-to-weight ratio and is stronger than wire rope

– It has the best flex-fatigue and wear resistance of all HMPE fiber working ropes

– Has less than 1% elastic elongation at 30% of rope breaking load – and approximately 4% elongation at break

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Samson 1.1 MM lb. Test Machine

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Synthetic Rope Testing HMPE Rope with and without precycling

50% level pre-cycling

0

5000

10000

15000

20000

25000

30000

35000

0 2 4 6 8Elongation, %

Stre

ngth

, lb

20% level Pre-Cycle

0

5000

10000

15000

20000

25000

30000

35000

0 2 4 6 8Elongation, %

Stre

ngth

, lb

No cycle

0

5000

10000

15000

20000

25000

30000

35000

0 2 4 6 8Elongation, %

Stre

ngth

, lb

HMPE Rope Strength, lbNo cycling 31,782

20% cycling 32,73450% cycling 34,652

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9

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Yarn properties; UV-resistance

0

20

40

60

80

100

0 4 8 12 16 20 24

exposure time [months outdoors]

stre

ngth

rete

ntio

n [%

]

Dyneema

aramid

polyester

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Rope properties; bending fatigue (D/d = 25)

1000

10000

100000

1000000

10000000

0 10 20 30 40 50 60peak load [% MBL]

cycl

es to

failu

re

Dyneema

aramid

polyester

steel wire rope

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1. 50% of rope’s breaking strength – 1000 cycles2. 60% of rope’s breaking strength – 1000 cycles3. 70% of rope’s breaking strength – 1000 cycles.4. …..continue the procedure at 10% incremental of

breaking strength till rope breaks.

• The results are then used to calculate a TCLL value which theoretically represents the load (expressed as a % of the breaking strength) at which the rope would fail at exactly 1000 cycles.

TESTING METHOD

TCLL – Level of load that fatigues rope to break at the 1000th cycle

- fatigue resistance test

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Amsteel-Blue Application:Tractor Tug Winch line

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The Application for the Study:Tractor Tug (single drum) Winch Lines

64 mm diameter Amsteel-Blue winch line on Crowley Marine Services Inc. Harbor-Class tractor tug

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Project Overview• Project Goals:

– Investigate and understand forces of vessel and machinery dynamics on HPME rope

– Create a testing method to inspect, test, catalog and analyze physical changes to HMPE rope

– Determine the residual strength from used ropes under laboratory conditions

– Use laboratory model studies to investigate strength reduction mechanisms

– Create a platform for the modification of the HMPE ropes to provide greater service life

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Residual Strength Testing

• Phase 1:– Mainlines:

• Ends– Worked for 1000-1700 jobs then End-for-ended– End-for-ended then Worked for 1000-1700 jobs– Worked no bury (~1100 jobs)

• Mid-sections – Pendants

• Phase 2:– Pendants-worked for 6 specific intervals between 100 and 1250

jobs

END ENDMID-SECTIONS

X-HOURS Y-HOURS

PHASE 1 DIAGRAM

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Residual Strength Testing

Procedure: Pendants were generally used for (1) year and main towlines were used for (2) years – Main lines were used for (1) year – end-for-ended, and used

for another year, thus 2 year life– Visual inspections and break tests were performed– Ropes were re-spliced with new eyes – before each break

test when necessary– Testing was done on the mainline ends and the mainline

midsections– Pendants were cataloged separately– Jobs, hours and notations about service were cataloged for

all samples– All lines were cycled to 10 times to 20% of their published

Minimum Breaking Load prior to failure

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8” circ. Amsteel BlueResidual Strength - Mid-Sections

0

20

40

60

80

100

120

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000No. of jobs

% o

f MB

L (5

95,3

50 lb

f)

Mid-Section

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8” circ. Amsteel BlueResidual Strength - Ends

(Mainline Ends & Pendants)

0

20

40

60

80

100

120

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000No. of Jobs Worked

% o

f MB

L (5

95,3

50 lb

f)

P1-EndsP1-Pendants *

* Was not buried on winch. No significant drum compression

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Combined Graph

0

20

40

60

80

100

120

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000

No. of Jobs Worked

% o

f MB

L (5

95,3

50 lb

f)

All EndsP1-Pendants *P2-Pendants *Upper BoundLower Bound

* Was not buried on winch. No significant drum compression

No Drum Compression

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Literature HMPE-towline retention strength

Source: “Latest Synthetic Fibre Rope Developments in the Towage Industry”, John Hooker, Marlow Ropes Ltd., 16th ITS, 2000

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Literature HMPE-towline retention strength

Source: “High Performance Steelite Ropes For Harbour Towing”, Andrew Street, David Potter, Marlow Ropes Ltd., 12th ITS, 1992

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Possible Mechanisms of strength reduction

• Shock Loading• Twists• Drum Compression• Cyclic Fatigue • Abrasion

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Strength loss in towing ropes

• Shockloading–Literature on shockloading–Analysis of data of Crowley tugsProtector and Nanuq

–Cyclic shockloading test–Strain rate and fibre properties

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Shockloading literature“The large difference between the planned

force and the measured value could not be explained.”

“An energy analysis on shock load situations should give reasonable results provided the load / extension curve used is representative of the rope condition.”

Source: “Latest Synthetic Fibre Rope Developments in the Towage Industry

John Hooker, Marlow Ropes Ltd, 16th ITS, 2000

Source: “Experiments with shock load on steel wire ropes

L.Wiek, TU Delft, OIPEEC Bulletin 70, 1995

LAEHgmFimpact

.....2=

Hgvinit ..2=

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Effects of Shock Loading

Strength loss from shock loading

• The sudden increase in applied load can cause some of the rope’s fibers to uncharacteristically break

• Computerized line-loading strength measuring capability was employed on 3 Crowley VS drive tractor tugs (different classes)

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Equipment:Marine Services Harbor-Class Tractor Tugs

• 4800 HP• Bollard Pull of 51 tons (US)• West Coast (US) deployment

– Seattle– Long Beach

• Propulsion – “Voith Schneider” drive• Line handling – mainline Amsteel-Blue

on single drum hydraulic winch – going through stainless steel-clad “staple”

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Crowley Marine Services Inc. – Harbor-Class tractor tugs in Long Beach, CA harbor

302-5/8” (64MM) diameter Amsteel-Blue on Crowley Harbor Class

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Prince William Sound Class Escort Vessel

Nanuq (’99), Tan’erliq (’99)Valdez, Alaska

• 153 ft. Voith-Schneider Escort Tug

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Crowley Nanuq Line Tension data

5,5% MBS

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Protector Class ship-assist vessel

• Protector (’96), Guard (’97) Los Angeles / Long Beach

• 123 ft Z-Drive Escort Tug

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Crowley Protector Line Tension data

18% MBS

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Crowley Protector Line Tensiondata

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Crowley Protector Line Tensiondata

58123 lbs / 1,1 sec

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Crowley Protector Line Tension data

70321 lbs / 1,4 sec

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Crowley Protector Line Tension data

Extreme Line Tension Buildup:• 60-70,000 lbs in 1-2 sec.

• Towed Vessel height: 15 m.MBS 9-inch rope: 747,495 lbsStrain at break: 3%Tension build-up: 60,000 lbs (8% MBS) in 2 seconds

• Up-angle 75° 30°Line-length 15,5 m. (short) 30 m. (long)

• Impact velocity (Amount of strain / seconds)(8%*3%*15,5 / 2 sec.) (8%*3%*30 / 2 sec.)= 0,02 m/s. = 0,04 m/s

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Practical Shock Loading Scenarios:• 60-70,000 lbs in 1-2 sec.• Towed Vessel height: 15 m.

MBS 9-inch rope: 747,495 lbsElongation at break: 3%Tension build-up: 60,000 lbs (8% MBS) in 2 seconds

30 m.15,5 m.Line-length

0,04 m/s0,02 m/sImpact Velocity

30°75°Line Angle

Long leadShort lead

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Shock Loading Simulation Testing

• Loading Rates:– 20 sec sine + 0 sec dwell time– 3 sec sine + 17 sec. dwell time– 1.1-1.8 sec sine + 18-19 sec dwell time

• Laboratory Test Procedure:– 1000 cycles 5-50% BL– 1000 cycles 5-60% BL– 1000 cycles 5-70% BL– 2000 cycles 5-80% BL– Load to failure

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Effects of Cyclic Fatigue

Residual Strength=134% MBL120 s.sine5-80% BL200020 s.sine5-70% BL100020 s.sine5-60% BL100020 s.sine5-50% BL1000Cycle timeLoad cycle#cycles

5

55

0 .00 20 .00 40 .00 60 .00 80 .00 100 .00

T im e (sec )

Load

(%M

BL)

C yc le 1

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Effects of Cyclic Fatigue

Residual Strength=139% MBL13 s. sine +17 s.5-80% BL20003 s. sine +17 s.5-70% BL10003 s. sine +17 s.5-60% BL10003 s. sine +17 s.5-50% BL1000Cycle timeLoad cycle#cycles

5

55

0 20 40 60 80 100

T im e (sec)

Load

(%M

BL)

C yc le 2

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Effects of Cyclic Fatigue

Residual Strength=135% MBL11,8 s. sawtooth+18,25-80% BL20001,6 s. sawtooth+18,45-70% BL10001,4 s. sawtooth+18,65-60% BL10001,1 s. sawtooth+18,95-50% BL1000

Cycle timeLoad cycle#cycles

5

55

0 20 40 60 80 100

T im e (sec )

Load

(%M

BL)

C yc le 3

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Cyclic shockloading tests13/16 inch AmSteel Blue rope

Break Strength 29,600 kg

OCIMF Thousand Cycle Load Level – test (dry)

#cycles Load cycle Cycle time Cycle Time Cycle time

1000 5-50% BL 20 s.sine 3 s. sine +17 s. 1,1 s. Sawtooth +18,9

1000 5-60% BL 20 s.sine 3 s. sine +17 s. 1,4 s. Sawtooth +18,6

1000 5-70% BL 20 s.sine 3 s. sine +17 s. 1,6 s. Sawtooth +18,4

2000 5-80% BL 20 s.sine 3 s. sine +17 s. 1,8 s. Sawtooth +18,2

Break Strength

39,735 kg 41,096 kg 39,894 kg

134 % 139 % 135 %

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Literature: Strain rate at other fibres

Source: “Dynamic properties of marine fibre ropes”, G.R.Borwick, G.Elliot, National Engineering Laboratory, 2nd ITC, 1982

Dynamic Breaking Load vs. impact speed Absorbed energy vs. impact speed

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Strain rate and fibre properties

At increasing deformation rate:Tenacity increases and Elongation at break

decreases

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Strain rate and fibre properties

At increasing deformation rate:Tenacity increases more at higher

temperatures

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Strain rate and fibre properties

At increasing deformation rate:Elongation at break decreasesover whole temperature range

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Strain rate and fibre properties

At increasing deformation rate:E-modulus (stiffness) increases over whole temperature range

0,155 m/s (l=15,5 m.)(75° up-angle)

0,3 m/s (l=30 m.)(30° up-angle)

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Strength loss & shock loading

• Computerized line-loading strength measuring capability was employed on 3 Crowley VS drive tractor tugs (different classes)– A high-sensitivity, self triggering, frequent-

capture strain gauge attached to winch – Measure loading and velocities on Amsteel-Blue

during ship assists in rough water– Traditional mentality is “shock-loading”, the rapid

loading of synthetic rope, could severely decrease residual strength of the Amsteel-Blue lines

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Effects of Twist

Strength loss due to line rotation and twist

• Traditional “rope theory” states that twist rapidly decreases strength

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Line twist

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Efficiency of a Amsteel Blue RopeTypical Crowley Sample = 3 turns/meter

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

0 2 4 6 8 10 12 14 16

Turns per meter

% R

etai

ned

Stre

ngth

DSM 10mm

SRT 24mm

54Effects of Twist

3-turns per meter5-turns per meter

7-turns per meter

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TwistTypical Crowley Sample = 1 turns/meter

Normalized to 1” dia = 3 turns/meter

40.0%

50.0%

60.0%

70.0%

80.0%

90.0%

100.0%

0 1 2 3 4 5 6 7 8 9 10

Turns per meter

% R

etai

ned

Stre

ngth

Amsteel 1in dia

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Conclusions

Line twist–1-1.5 turns/foot equates to a ~15% strength loss

–No data available on effect of twist on fatigue behavior. Study is in progress to understand this effect.

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Effects of Drum Compression

Deformation of Amsteel-BlueDue to compression

• Compression of Synthetic Fiber ropes cause constructional inefficiencies that could lead to decreased residual breaking strengths

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Amsteel-Blue deformation

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Amsteel-Blue

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Amsteel-Blue

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Deformation of Amsteel-Blue on winch drum

• HMPE ropes such as 12 strand braided Amsteel-Blue are relatively soft and compliant – This is very good for ergonomics and

high strength– Temporary distortion can be removed

by working the rope

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Measured strength (burried splice)

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4

1 -3

At t = 0

Reference 31.500 kg 100 %

At t = 4190 cycles

1. Drum layer 1 24.860 kg 79 %

2. Drum layer 2 24.260 77 %

3. Drum layer 3 26.610 84 %

4. Buried rope 26.750 85 %

5. Horizontal part 31.920 101 %

6. Bending point 30.550 97 %

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Fatigue

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Measured strength (fatigue)

Fatigue StudyAt t = 0 Braid (16x1/1760 dtex ) Yarn

Reference 6.300 N 100 % 465 N 100%

At t = 1000 cycles @ 50% breaking strength

1. Drum layer 1 4.915 N 78 % 460 N 99 %

2. Drum layer 5 6.270 99,5 % 440 94,5 %

3. Free end 6,330 100,5 % 450 97 %

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Cyclic Fatigue

• Testing at NEL Lab Scotland• Control Sample: 5/16” Amsteel Blue w/ 63.4 kN Breaking

Strength• Lifetime Test:

– Load Cycle = 5-75% BL (wet)– Cycle time = 0.5 sec.– Measured lifetime = 426,600 cycles

• Determine Residual Strength at:– 90% Lifetime = ~384,000 cycles– 80% Lifetime = ~341,000 cycles– 30% Lifetime = ~128,000 cycles

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Cyclic Fatigue• Testing at NEL Lab Scotland

Cyclic fatigue not a major issue up to 80% of lifetime

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Conclusions

Compression and fatigue on drum–Rope and small braid samples show tohave a 10-15% strength loss due to compression on the drum.

–Rope and do not lose strength after cyclic fatigue/loading

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Abrasion and Cutting Damage to fiber rope

• Rope contact surfaces aboard tugs were smooth and not promoting cuts –however cutting does occur in client ship’s chocks

• Mechanical damage on test samples was cataloged and analyzed to find a correlation between a percentage loss of rope strength from severity or number of strand and yarn cuts or abraded in the rope

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Effects of Abrasion

• Pendant (8”circ ASB) analysis

• Two pennants tested: 300 and 1250 jobs

strand

Braiding periodrope

strand

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Effects of Abrasion

1250 job, fuss but no apparent damage on surface

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Effects of Abrasion

Cut rope yarns in almost every strand

1250 job

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Effects of Abrasion

12 %65 %Outside (15 yarns)

21 %42 %

71 %70 %

Averaged (26 yarns)Rope Strength

31 %78 %Middle (9 yarns)

47 %87 %Center (2 yarns)

1250300No of jobs

Rope yarnPosition in strand

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Conclusions

• 8” Circ. Amsteel Blue– Current retirement practice for both

mainlines and pendants yields an average safety factor of 2.2:1• After 400-500 jobs the residual strength in

the worked end has decreased from a safety factor of 5:1 to an average of 2.2:1

• After 500 jobs there is no further decrease up to 1700 jobs

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Academic Conclusions• Shock loading

– Not a strong mechanism for the ropes tested.

• Line twist during application– High twist levels adversely effect the residual strength and should be

avoided

• Compression of rope on the winch drum– From Lab experiment data: 10 ~15% strength reduction tested when

compressed.– From Actual sample data: Decrease observed in un-worked (mid-

section) samples, unable to quantify the effects once worked (ends).

• Cyclic Fatigue– Not a strong mechanism for the ropes tested

• Abrasion and cutting damage – Abrasion –Major mechanism in strength reduction

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Real World Conclusions• OK to retire lines in 2 years• Handle ropes with care – train the people

– avoid cutting– no twist– routine Inspection

• Higher Retained Strength– Increased Volume-Surface Area Ratio

• 8” Circ. Residual strength = 45%• Testing data -

– 9” circ. Residual strength = 50%– 10” circ. Residual Strength = 60%

• However - more weight and still dives on drum– Increased Twist in Strands

• Lower initial strength size-for-size, but higher abrasion resistance

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Questions & Comments• Cans of worms

• UV?

• Creep?

• Shark biting?

• Melting/fusion?

• Testing induced disturbance? Splicing & handling

• coating degradation?

• longer term to retire – risk management