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Table of contents TABLE OF CONTENTS..................................................................................................2 APPENDIX A ANCH...

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Table of contents TABLE OF CONTENTS..................................................................................................2 APPENDIX A ANCHORHEADS .....................................................................................3 APPENDIX B WEDGE PROPERTIES ...........................................................................5 APPENDIX C ∅18 MM DYFORM STRANDS .................................................................6 APPENDIX D OPERATION CYCLES STRANDJACK UNIT ..........................................7 D.1 JACK UP CYCLE (LIFTING THE LOAD) .....................................................................................7 D.2 JACK DOWN CYCLE (LOWERING THE LOAD) ..........................................................................10

APPENDIX E CALCULATIONS WEDGE MODEL.......................................................15 E.1 E.2 E.3 E.4 E.5

AREAS AND PROJECTED WEDGE AREAS AIN , AOUT , AINEFF EN AOUTEFF ........................................16 HORIZONTAL WEDGE EQUILIBRIUM ......................................................................................17 VERTICAL WEDGE EQUILIBRIUM ..........................................................................................18 WEDGE OPERATING RANGE ................................................................................................18 VERTICAL CABLE EQUILIBRIUM ...........................................................................................19

APPENDIX F MALFUNCTION OF THE WEDGE .......................................................20 F.1 WEDGE OPERATION WITH : ΜKIN CONSTANT , INCREASING ΜKOUT ...............................................20 F.2 WEDGE OPERATION WITH : ΜKOUT CONSTANT , DECREASING ΜKIN .............................................21 F.3 WEDGE OPERATION WITH: INCREASING ΜKOUT , DECREASING ΜKIN ............................................21

APPENDIX G MICRO SLIP DURING JACK UP ..........................................................23

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Appendices

By H.G.M.R. van Hoof

2

APPENDIX A Anchorheads An anchor head is drilled with a number of tapered holes suitable for accepting jack temporary wedges. An anchor head of the 830 SSL strand jack unit for example contains 54 tapered holes (Figure 3). In Figure 1 a characteristic example of an anchor head is presented. The quantity of tapered holes of the anchor heads is scaled to the required capacity.

SECTION A-A

Figure 1:Anchor head of 830 SSL strand jack unit

The main specifications of the anchor heads are presented in Table 1. Table 1: Specifications Anchor heads. Source: [ Mammoet] Anchor heads Specifications Main dimensions(mm) : Weight (kg) : Material : Wedges / holes (Qty) : Surface treatment :

SSL830

SSL550

SSL300

SSL100

∅520x190

∅450x190

∅354x100

∅200x150

271

207

60

40

34CrNiMo6

34CrNiMo6

34CrNiMo6

34CrNiMo6

54

36

18

7

Nitrocarburizing(dutch: teniferen) Nitrocarburizing(dutch: teniferen) Nitrocarburizing(dutch: teniferen) Nitrocarburizing(dutch: teniferen)

Layer thickness surface treatment (mm) : Solid lubricant : Optimal layer thickness lubricant (µm) :

0.1-1.6

0.1-1.6

0.1-1.6

0.1-1.6

Molycote D321 R

Molycote D321 R

Molycote D321 R

Molycote D321 R

15-25

15-25

15-25

15-25

Note that the surface treatment of the anchor heads is nitro carburizing.

Figure 2: 830 SSL anchor head with 54 wedges Figure 3: Tapered wedge seatings of an anchor head

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Appendices

By H.G.M.R. van Hoof

3

In order to reduce friction, the wedge seats of the anchor head are pre-treated with Molykote D321 R, a dry lubricant (see Figure 4).

Figure 4: Molykote D321 R Pre-treated Anchor head

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Appendices

By H.G.M.R. van Hoof

4

APPENDIX B WEDGE PROPERTIES The wedges used in the strand jack units contain three wedge parts. The wedge is equipped with a friction or grip profile on the inside of each part. The main dimensions of the wedge and wedge parts are presented in Figure 5 en Figure 6.

DETAIL GRIP PROFILE

WEDGE

Figure 5: Assembled wedge and wedge part Figure 6:Detail grip profile

The three wedge parts are assembled with an elastic rubber ring. This ring retains the three wedge segments/parts together during operation (Figure 7). Table 2: Specifications TT 18/44 wedge (sources : Mammoet,KANIGEN )

Wedge Main specifications

Main dimensions(mm) :

TT 18/44 Wedge

∅25x∅44x∅18x80

Total weight (kg) :

Rubber ring

0.450

Material :

DIN 1.6523 (SAE 8620)

Wedge parts (Qty) :

3

Weight wedge part (kg) : Hardening (Carburizing) temperature (°C):

0.150 900-1000

Tempering temperature (°C):

190-200

Hardness of substrate after tempering (indication*) (HV):

700-760

Surface treatment :

Electroless Nickel plated (KANIGEN method)

Proces temperature surface treatment (°C) :

200-280

Hardness of surface layer (indication*) (HV):

500-550

Hardness substrate after surface treatment (indication*) (HV):

560-620

Layer thickness surface treatment (µm) : Solid lubricant :

25-50 Molycote D321 R

Optimal layer thickness lubricant (µm) :

5-20

Total cost (euro):

Figure 7: Assembled wedge with rubber ring

20

Manufacturer :

*

TT Fijnmechanica

Values for reference only, exact values have to be determined by material tests

For main specifications of the currently used TT 18/44 wedge is referred to Table 2(In Table 2 several specifications are noted with “indication”, these values need to be reviewed by hardness tests). In order to reduce friction, the wedges are pre-treated before operation with Molykote D321 R, a dry lubricant.

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Appendices

By H.G.M.R. van Hoof

5

APPENDIX C ∅18 mm DYFORM strands The strand bundle contains several Dyform strands with a maximum operation capacity of 167 kN/strand (specified by Mammoet). The quantity of strands depends on the used unit type.The Dyform strand contains 7 twisted, high capacity, steel wires with a flattened side (see Figure 8). They are specially developed for heavy lifting and its length can be up to 1500 meters to meet the job requirements. DYFORM STRAND

Cross section P-P Flattened side(6x)

P

P

Wire (7x)

Figure 8: Dyform strand

The main specifications of the Dyform strand are presented in Table 3. Table 3: Main specifications Dyform strand. Source :[ Bridon wire Ltd ]

Strand Main specifications

BS5896 (DYFORM)

Nominal values

Tolerances

Nominal diameter (mm) :

∅18

+0.4 -0.2

Mass (kg/m) :

1.75

+0.4% -2%

Tensile strength (Rm) (N/mm2)

1700

Surface hardness of wires (HV)

430-480

Steel area (mm2)

223

Breaking load (Fm) (kN)

380

0.1% proof load (Fp 0.1) (kN)

323

Load at 1% elongation (Ft 1.0) (kN)

334

Wires (Qty) Manufacturer :

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7 Carrington Wire Ltd / Bridon Wire Ltd

Appendices

By H.G.M.R. van Hoof

6

APPENDIX D Operation cycles Strandjack unit In this appendix a review is presented on two main operation cycles of the strand jack unit SSL to familiarize the reader with the general concepts of the Strand jack-unit SSL.: • •

Jack up cycle (lifting the load) Jack down cycle (lowering the load)

D.1 Jack up cycle (lifting the load) In the next paragraph the Jack up cycle of the hydraulic Strand jack units is described. This principle concerns all types.

START POSITION

STEP 1

WEDGE UPPER ANCHOR HEAD RELEASE PIPE UPPER RELEASE CILINDER UPPER RELEASE PLATE

STROKE

WEDGE LOWER ANCHOR HEAD RELEASE PIPE LOWER RELEASE CILINDER LOWER RELEASE PLATE LOAD

LOAD STROKE

Figure 9: Start position and Step 1 of the Jack up cycle

The start position (see Figure 9): The 18 mm dyform compact strands are installed through the unit and the load is connected. Both upper and lower anchor heads are “locked”(red in Figure 9). Step 1 (see Figure 9):The piston of the jack extends and raises the upper anchor head including the locked strands and connected load. During this movement the wedges in the lower anchor head are pulled up slightly by the movement of the strands; the lower anchor head is still closed. In case of failure of the upper anchor head, the wedges in the lower anchor head secure the load.

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7

STEP 2

STEP 3

STROKE -15 mm

LOAD

LOAD

Figure 10: Step 2 and Step 3 of the Jack up cycle

Step 2 (see Figure 10): In top position the load is transferred from the upper anchor head to the lower anchor head by slightly retracting the piston, approx. 15 mm. During this load transfer both anchor heads remain locked. Step 3 (see Figure 10): After the transfer, the upper anchor head is opened hydraulically. The upper release cylinder lifts the release plate with the release tubes and shifts the wedges/grips up and out of their seatings (Note the difference between left and right detail picture) .The upper anchor head is now “unlocked”(blue in Figure 10) allowing free passage of the strands.

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By H.G.M.R. van Hoof

8

STEP 4

STEP 5

-STROKE

LOAD

LOAD

Figure 11: Step 4 and Step 5 of the Jack up cycle

Step 4 (see Figure 11): With the upper anchor head “unlocked”, the piston of the jack retracts and returns to the starting position. The strands slide through the wedges in the upper anchor head. Step 5 (see Figure 11): At the end of step 4, the upper anchor head is “locked” again by the upper release cylinder retracting the release plate and release tubes. The jack up cycle of step 1 to 5 is repeated till the required jack up distance is accomplished.

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Appendices

By H.G.M.R. van Hoof

9

D.2 Jack down cycle (lowering the load) In the next paragraph the Jack down cycle of the hydraulic Strand jack units is described. This principle concerns all types.

START POSITION

STEP 1

WEDGE UPPER ANCHOR HEAD RELEASE PIPE UPPER RELEASE CILINDER UPPER RELEASE PLATE

WEDGE LOWER ANCHOR HEAD RELEASE PIPE LOWER RELEASE CILINDER LOWER RELEASE PLATE

LOAD

Figure 12: Start position and Step 1 of the Jack down cycle

The start position (see Figure 12): The 18 mm dyform compact strands are installed through the unit and the load is connected. Both upper and lower anchor heads are “locked”(red in Figure 12). Step 1 (see Figure 12): The upper anchor head is opened hydraulically. The upper release cylinder lifts the release plate with the release tubes and shifts the wedges/grips up and out of their seatings (Note the difference between detail picture of the upper anchor head in Start position and Step1). The upper anchor head is now “unlocked”(blue in Figure 12) allowing free passage of the strands.

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Appendices

By H.G.M.R. van Hoof

10

S TEP 2

S TEP 3

LO AD

LO AD

Figure 13: Step 2 and Step 3 of the Jack down cycle

Step 2 (see Figure 13): With the upper anchor head “unlocked”, the piston of the jack extends till approx. 15 mm before end of the outward stroke (compare the position of the main hydraulic cylinder in step 1 and step 2). The strands slide through the wedges in the upper anchor head during the stroke. Step 3 (see Figure 13): In the position “15 mm before end of the outward stroke” the upper anchor head is “locked” by the upper release cylinder retracting the release plate and release tubes (red in detail upper anchor head in see Figure 13 step 3).

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Appendices

By H.G.M.R. van Hoof

11

STEP 4

STEP 5

LOAD

LOAD

Figure 14: Step 4 and Step 5 of the Jack down cycle

Step 4 (see Figure 14): The load is transferred from the lower anchor head to the upper anchor head by slightly extending the piston of the main cylinder further (approx. +15 mm) to the end of the stroke. During this load transfer both anchor heads remain “locked” (both red in Figure 14 step 4). During this movement the wedges in the lower anchor head are pulled up slightly by the movement of the strands; the lower anchor head is still closed. In case of failure of the upper anchor head, the wedges in the lower anchor head secure the load. Step 5 (see Figure 14): After the transfer, the lower anchor head is opened hydraulically (“unlocked”; blue in detail of lower anchor head step 5). The lower release cylinder lifts the release plate with the release tubes and shifts the wedges/grips up and out of their seatings (Note the difference between left and right detail picture step 4 and step 5) .The lower anchor head is now “unlocked” (blue in Figure 14) allowing free passage of the strands.

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By H.G.M.R. van Hoof

12

STEP 6

Figure 15: Step 6 and Step 7 of the Jack down cycle

Step 6 (see Figure 15): The piston of the jack retracts and lowers the closed upper anchor head including the strands and load (movement is –stroke) until approx. 15 mm before end of the inward stroke. The lower anchor head is still “unlocked” (blue in Figure 15) during this movement and allows free passage of the strands. Step 7 (see Figure 15): In the position “15 mm before end of inward stroke”, the lower anchor head is “locked” hydraulically by the lower release cylinder retracting the release plate and release tubes (red in detail lower anchor head in Figure 15 step 7).

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Appendices

By H.G.M.R. van Hoof

13

Figure 16: Step 8 of the Jack down cycle

Step 8 (see Figure 16): After “locking” the lower anchor head, the load is transferred from the upper anchor head to the lower anchor head by slightly retracting the piston of the main cylinder further to the end of the inward stroke, approx. -15 mm. During this load transfer both anchor heads remain locked. The jack down cycle of step 1 to 8 is repeated until the required jack down distance is accomplished.

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Appendices

By H.G.M.R. van Hoof

14

APPENDIX E Calculations wedge model In this appendix a mathematical model of the strand locking system is presented. In the following considerations a single wedge part is examined and the spring force of the prestress spring is neglected. STRAND WEDGE PART

ANCHORHEAD

D strand 2 x Ri

kout

CW

kin

out

out

cw

L

CW

S
Nout

cw

=

FL A cable

FL Figure 17 : Schematic presentation of mathematical wedge model

µkout = Kinetic coefficient of friction between outer wedge surface and anchor head surface µkin = Kinetic coefficient of friction on inner wedge surface and cable surface Nout = Normal force on outer wedge surface Nin= Normal force on inner wedge surface σcw = Compressive stress between wedge and cable σNout = Compressive stress perpendicular wedge surface and anchor head σc = Tensile stress in cable as result of FL FL= Load force α = Wedge angle τout = Friction shear stress between outer wedge surface and anchor head τcw = Friction shear stress between wedge and cable S = Position of slip front (for detailed information is referred to Appendix G2.2) Ri = Inner radius of wedge and therefore outer radius of strand/cable Acable = Cross section area of cable From Figure 17 , the following equations can be obtained:

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Appendices

By H.G.M.R. van Hoof

15

τ out = µ kout ⋅ σ Nout

(a)

τ cw = µ kin ⋅ σ cw

and

(b)

E.1 Areas and projected wedge areas Ain , Aout , Aineff en Aouteff The estimation of the theoretical outer and inner surface of a single wedge part (assuming cylindrical), Ain and Aout, is formulated as :

A in =

2 π ⋅ Ri ⋅ S 3

A out =

(c)

2 S π ⋅ R out ⋅ 3 cos(α )

and

(d)

This according to Figure 18.

SIDE VIEW WEDGE PART

Ri

R out

TOP VIEW WEDGE PART

S

S COS ( )

Figure 18: Top view and side view wedge part in relation to Ain and Aout

The estimated projected effective outer and inner surface of a single wedge part, Aineff and Aouteff, can formulated as:

A ineff = R i ⋅ S ⋅ 3

(e)

and

A outeff = R out ⋅

S ⋅ 3 cos(α )

(f)

These equations can be derived from Figure 19.

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Appendices

By H.G.M.R. van Hoof

16

R out

Ro

Ri

Ri

Ri

x

ut

3

x

3

R out

TOP VIEW WEDGE PART

Figure 19: Top view and side view wedge part in relation to Aineff and Aouteff

The total normal force Nout acting perpendicular on the effective surface of the wedge part becomes (see also Figure 17) :

Nout = σ Nout ⋅ A outeff = σ Nout ⋅ R out ⋅

S ⋅ 3 cos(α )

(g)

The total normal force Nin acting perpendicular on the inner surface of the wedge part becomes (see also Figure 17) :

Nin = σ cw ⋅ A ineff = σ cw ⋅ R i ⋅ S ⋅ 3

(h)

E.2 Horizontal wedge equilibrium The total equilibrium of the horizontal forces on the wedgepart results in the following equation (according to Figure 17):

Nout ⋅ cos( α ) = Nin − τ out ⋅ sin(α ) ⋅ A outeff

(i)

After substitution of equations a, g , h and f in i

σ Nout ⋅ R out ⋅

S S ⋅ 3 ⋅ cos( α ) = σ cw ⋅ R i ⋅ S ⋅ 3 − µ kout ⋅ σ Nout ⋅ sin(α ) ⋅ R out ⋅ ⋅ 3 cos(α ) cos(α )

After solving and rearranging, we get

σ Nout ⋅ R out (1 − µ kout ⋅ tan(α )) = σ cw ⋅ R i

or

σ Nout ⋅

R out Ri

(1 − µ kout ⋅ tan(α )) = σ cw (j)

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Appendices

By H.G.M.R. van Hoof

17

E.3 Vertical wedge equilibrium The total equilibrium of the vertical forces on the wedgepart results in the following equation (according to Figure 17):

A in ⋅ τ cw − τ out ⋅ cos(α ) ⋅ A out − σ Nout ⋅ sin(α ) ⋅ A out = 0

+

(k)

After substitution of equations b, c , a, d in k, we get

2 2 S π ⋅ R i ⋅ S ⋅ µ kin ⋅ σ cw − µ kout ⋅ σ Nout ⋅ cos(α) ⋅ π ⋅ R out ⋅ − 3 3 cos(α ) 2 S σ Nout ⋅ sin(α) ⋅ π ⋅ R out ⋅ =0 3 cos(α ) After solving and rearranging, this results in

Ri ⋅ µkin ⋅ σcw = σNout ⋅ Rout ⋅ (µkout + tan(α))

µkin ⋅ σcw = σNout ⋅

or

R out ⋅ (µkout + tan(α)) Ri

(l)

E.4 Wedge operating range Substituting equation j in l results in

µkin ⋅ σNout ⋅

Rout R (1− µkout ⋅ tan(α)) = σNout ⋅ out ⋅ (µkout + tan(α)) Ri Ri

After rearranging

µ kout =

µ kin − tan(α ) 1 + µ kin ⋅ tan(α )

and so

µ kin =

µ kout + tan(α ) 1 − µ kout ⋅ tan(α )

(m)

These two equations represent the “borderline” of the wedge operating range.

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Appendices

By H.G.M.R. van Hoof

18

E.5 Vertical cable equilibrium When the equilibrium of the vertical forces on the cable/strand in Figure 17 is considered, the following equation can be acquired:

FL = 2 ⋅ π ⋅ R i ⋅ τ cw ⋅ S

(n)

With

FL = σ c ⋅ A cable = σ c ⋅ π ⋅ R i

2

(o)

and

τ cw = µ kin ⋅ σ cw

(b)

After substitution of o and b in equation n, the result is: 2

σ c ⋅ π ⋅ R i = 2 ⋅ π ⋅ R i ⋅ µ kin ⋅ σ cw ⋅ S

(p)

After rearranging :

σc = 2 ⋅

S ⋅ µ kin ⋅ σ cw Ri

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(q)

Appendices

By H.G.M.R. van Hoof

19

APPENDIX F Malfunction of the wedge For the benefit of the analysis, three possible cases are described to clarify the malfunction of the wedge, according to the mechanical wedge model: 1. Wedge operation with : µkin Constant , increasing µkout 2. Wedge operation with : µkout Constant , decreasing µkin 3. Wedge operation with : Increasing µkout , decreasing µkin In the following paragraphs these load cases are described in detail.

F.1 Wedge operation with : µkin Constant , increasing µkout When the wedge operation is started, a proper and normal operation of the wedge is assumed.This implies that the friction conditions are below the “good/bad” borderline in Figure 20. For example, fictive estimated start values of µkout and µkin in operation are 0,2 and 0,5 (see start operation point A in Figure 20).

Friction coefficients 0.6

0.4

Malfunction of wedge (slipping of strand, situation 2)

B

`

A

0.2

Wedge operating properly (situation 1)

µkout

0

0.1

0.2

0.3

0.4

0.5

0.6

α =8.19˚

0.2 µkin Figure 20:Wedge operation with increasing µkout

With increased µkout , the operation point B is positioned in the red “Malfunction Area” .The friction force between wedge outer surface and tapered hole surface is too high and consequently gripping action is less; the strand slips through the wedge.

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Appendices

By H.G.M.R. van Hoof

20

F.2 Wedge operation with : µkout Constant , decreasing µkin When the wedge operation is started, a proper and normal operation of the wedge is assumed. For example, fictive estimated start values of µkout and µkin in operation are 0,2 and 0,5 (see start operation point A in Figure 21).

Friction coefficients 0.6

0.4

Malfunction of wedge (slipping of strand, situation 2)

A

0.2

C

µkout

0

0.1

0.2

0.3

Wedge operating properly (situation 1)

0.4

0.5

0.6

α =8.19˚

0.2 µkin Figure 21: Wedge operation with decreasing µkin

When µkin decreases during operation below the value ≤ 0,35 and the value µkout is constant , the operation point of the wedge is moved from A to point C (see Figure 21 ). Operation point C is positioned in the red “Malfunction Area” .The friction force between wedge inner surface and cable surface is too low, consequently the gripping action is less; the strand slips through the wedge.

F.3 Wedge operation with: Increasing µkout , decreasing µkin In most practical cases the values of the friction coefficients will change simultaneously, influenced by environmental circumstances (temperature, relative humidity etc.).

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21

A combination of increasing µkout and decreasing µkin will lead to mal-function of the wedge conform Figure 22, the operation point of the wedge is moved from A to point D. Malfunction of the wedge appears.

Friction coefficients 0.6

0.4

Malfunction of wedge (slipping of strand, situation 2)

D A

0.2

Wedge operating properly (situation 1)

µkout

0

0.1

0.2

0.3

0.4

0.5

0.6

α =8.19˚

0.2 µkin Figure 22: Wedge operation with decreasing µkin and increasing µkout

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22

APPENDIX G Micro slip during jack up In every load cycle (jack up and jack down) micro slip will occur in the wedges of the upper and lower anchor head. This is a result of the difference in axial strain (as a result of the load force FL) between the cable and wedge in consequence of the difference in stiffness. The axial strain in the cable is larger than in the wedge. Hence the strain in the loaded cable can not be “followed” by the wedge what results in a relative displacement between the cable outer surface and wedge inner surface. Micro slip in combination with a load force results in wear of the inner friction surface of the wedge and eventually in a reduction of the inner friction coefficient µkin. Finally this will cause malfunction of the wedge. When the piston of the jack extends, the closed upper anchor head is raised including the strands. The load force FL and related strain in the strand increase during this movement until the anchor head bears the total present load force FL. The situation of the full loaded wedge in the anchor head is described schematic in Figure 23.

Figure 23: Loaded wedge during raising of upper anchor head

FL= Load force V = Spring force (pre-stress)

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23

Nin= Normal force on inner wedge surface µkin = Kinetic coefficient of friction on inner wedge surface Q = Length arc of wedge part α = Wedge angle σcw = Compressive stress between wedge and cable σc = Tensile stress in cable as result of FL εc = Strain in cable S = Position of slip front L = Length of friction profile Acable = Cross section area of cable Ec= Modulus elasticity of cable τcw = Friction shear stress At the end of the trajectory of the increasing load force FL (the upper anchor head bears the total load force FL ) , the slip front has propelled to distance S. In the cross section of the cable on position S, the tensile stress σc= 0. Along the inner friction surface of the wedge part, width Q and length L, exists a compressive stress σcw between wedge and cable as a result of the normal force Nin. The (pre-stress) spring force V is neglected and the stiffness of the wedge is considered infinite in the following considerations. This compressive stress σcw present on the 3 wedge parts is formulated as:

σ cw =

Nin 3⋅Q ⋅L

The compressive stress σcw is able to transmit an average friction shear stress τf on the contact area:

τ cw = µ kin ⋅ σ cw The load force FL is compensated by three inner friction surfaces, with an area of Q ⋅ s and thus is derived for FL :

FL = 3 ⋅ Q ⋅ s ⋅ τ cw = 3 ⋅ Q ⋅ s ⋅ µ kin ⋅ σ cw =

3 ⋅ Q ⋅ s ⋅ µ kin ⋅ Nin s ⋅ µ kin ⋅ Nin = 3 ⋅Q ⋅L L

A increasing force FL results in a increasing value of s, as soon as s > L total slip and FL= Fmax = µ kin ⋅ Nin occurs. • •

If s > L : macroslip occurs, the strand slips through the wedge If 0
For the tensile stress

σc =

σ c and strain ε c in the cable we can write

FL A cable

εc =

FL σc = E c E c ⋅ A cable

The length s can be calculated from the relation

s=

FL ⋅L µ kin ⋅ Nin

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Appendices

(Eq. G.1)

By H.G.M.R. van Hoof

24

The tensile stress

σ c diminishes linear with values on positions x = 0 to x = s (see Figure 23 )

according to

σ c ( x =0 ) = 0

σ c( x= s) =

and

And analogically with the equations for the tensile stress

ε c( x =0) =

σc 0 =0 = E c E c ⋅ A cable

and

A graph of the x- position against strain

ε c( x=s)

FL A cable σ c , equations for strain ε c are FL σ = c = E c E c ⋅ A cable

ε c is provided in Figure 24.

Figure 24 Graph of position x, strain and tensile stress in cable wedge

With usage of Figure 24 the total cable elongation dl in the wedge is derived. The elongation dl of the cable in the wedge corresponds with area P

dl = P =

1 ⋅ ε c(x =s) ⋅ s 2

With

ε c( x=s) =

σc FL = E c E c ⋅ A cable

and

s=

FL ⋅ L µ kin ⋅ Nin

After substitution, dl becomes

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Appendices

By H.G.M.R. van Hoof

25

dl =

Function

1 F2L ⋅ L ⋅ ε c( x=s) ⋅ s = 2 2 ⋅ E c ⋅ A cable ⋅ µ kin ⋅ Nin

ε c (x ) can be described as (see Figure 24 ) ε c (x ) =

FL ⋅ x E c ⋅ A cable

for 0 ≤ x ≤ s

du = ε c (x ) follows dx FL ⋅ x du = dx E c ⋅ A cable

With the definition

To obtain the local relative displacement function u(x)

u(x ) = ∫ [

FL ⋅ x du ] ⋅ dx = ∫ [ ] ⋅ dx dx E c ⋅ A cable

for 0 ≤ x ≤ s

This leads to the relative displacement function u(x)

u(x ) =

With

FL ⋅ x 2 +C 2 ⋅ E c ⋅ A cable

for 0 ≤ x ≤ s

u(0) = 0 consequently C = 0 , relative displacement function u(x) becomes

FL ⋅ x 2 u(x ) = 2 ⋅ E c ⋅ A cable

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for 0 ≤ x ≤ s

Appendices

(Eq. G.2)

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Equation G-2 is presented graphically in Figure 25.

Figure 25: Graph of relative displacement u(x) of the outer cable surface against inner friction profile

The magnitude of relative displacement u(x) (and thus wear) of the outer cable surface against the friction profile depends mainly on the location of the slip front S , assuming FL , Acable , Ec constant. Maximum slip is located at the loaded “outlet” of the cable at position S.

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Appendices

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STRANDJACK WEDGES Friction coefficients, micro slip and handling Report nr. DCT 2005-78 By: H.G.M.R. van Hoof Idnr : 501326 20 November, 2005

Abstract The company Mammoet BV meets difficulties with cable wedges/clamps of their strand jack units. Slipping of strand through the wedges and high maintenance costs of the strand jack units hence, are the main aspects. In this report general operation fundamentals of the current strand jack unit are explained and detailed information of essential components is provided. It also provides a straightforward theoretical/mathematical approach on the probable causes of malfunction and unreliability of the strand jack wedge . To improve the service life, reliability and handling properties of the wedge and strand, an alternative wedge design is presented.

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Summary Mammoet is a leading company specialized in solving extreme heavy lifting and transport challenges. With offices in 30 countries and over more than 1500 employees around the world Mammoet provides its customers with knowledge, skills and equipment at onshore or offshore location. The solutions that Mammoet provide are used in economic sectors like the Petro Chemical industry, Civil works, Power plants and Offshore. The best project example of the latter is of course the salvation of the Russian nuclear submarine the Kursk. In various projects Mammoet applies a compact Strand jack unit placed on a gantry or mast for precise lifting, lowering or pulling of heavy loads. This unit is, due to its compact design, very suitable in areas where conventional equipment as cranes cannot be placed. This Strand jack unit is generally based on a hydraulic cylinder with a wedge principle to lock the strands in each stroke. Two wedges, situated in the lower and upper anchor head in the cylinder, lock each strand in the unit when the load is lifted or lowered. Mammoet meets difficulties especially with these wedge clamps of the strand jack units. Slipping of strand through the wedges and high maintenance costs of the strand jack units hence, are the main aspects. In the specialized and prestigious market of heavy lifting and transport safety and operating reliability are important topics, decisive in maintaining a good reputation. It’s obvious that research on the used wedge became more than desirable. According to the calculations performed in Appendix E the proper operation area of the wedge can be characterized as:

µ kout <

µ kin − tan(α ) 1 + µ kin ⋅ tan(α )

and so:

µ kin >

µ kout + tan(α ) 1 − µ kout ⋅ tan(α )

With: µkout = Kinetic coefficient of friction between outer wedge surface and anchor head surface µkin = Kinetic coefficient of friction on inner wedge surface and cable surface α = Wedge angle FL= Load force The wedge gripping quality depends on inner-and outer friction coefficients µin , µout and the wedge angle α . The characteristic graph of Figure 13 provides the proper operation area and malfunction area of the wedge, as a function of µin , µout (with α=8.19°). Several environmental circumstances and principles can have an influence on the value of µkout : o o

Adhesion: between wedge outer surface and tapered hole surface Plowing /third body effects: between wedge outer surface and tapered hole surface

o

Accumulation of dirt : between strand outer surface and inner surface of the wedge Wear on inner friction profile of wedge during load cycles

For µkin :

o

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During operation relative displacement occurs between the inner friction profile and strand outer surface. The relative displacement u(x) can be characterized as :

u(x ) =

FL ⋅ x 2 2 ⋅ E c ⋅ A cable

FL= Load force Acable = Cross section area of cable Ec = E-modulus of cable material This relative displacement which initiates wear is called micro slip The micro slip reducing design, presented in this report, can increase service life of the wedge. Applying 190 high capacity solid strands with diameter of 9 mm can realize the following objectives: o

No internal relative displacements between wires during operation: less elastic power is lost by friction/wear. Therefore no internal wear is initiated. In general a strand with a diameter of > 9 mm contains multiple wires, and the internal wear and elastic energy loss will increase drastically by an increasing number of strand wires.

o

The 9 mm strand also improves handling due to a weight reduction of 1.25 kg/m compared to the original 18 mm strand. In a proper 900-ton configuration of the strand jack unit SSL, 190 wires of 9 mm are used.

In this 9 mm configuration, 380 small 9 mm “low cost” wedges are used.

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Table of contents SUMMARY .....................................................................................................................2 TABLE OF CONTENTS..................................................................................................4 1

INTRODUCTION ......................................................................................................5

2

MAMMOET...............................................................................................................7 2.1 2.2

3

MERGER .........................................................................................................................7 CLIENTS AND SCOPE OF WORK..........................................................................................7

THE MAMMOET STRAND JACK SYSTEM.............................................................9 3.1 HYDRAULIC STRAND JACK-UNIT SSL ................................................................................9 3.2 COMPONENTS HYDRAULIC STRAND JACK-UNIT SSL .........................................................10 3.3 STRAND LOCKING PRINCIPLE ..........................................................................................11 3.3.1 “Locked” principle .................................................................................................11 3.3.2 “Unlocked” principle..............................................................................................13

4

THEORETICAL PROBLEM ANALYSIS.................................................................14

5

WEAR BY MICRO SLIP .........................................................................................16

6

MICRO SLIP / WEAR REDUCING DESIGN...........................................................18 6.1 6.2

7

WEAR REDUCTION ON THE BASE OF VERTICAL CABLE EQUILIBRIUM ....................................19 “9 MM” WEDGE / ANCHOR HEAD CONFIGURATION ..............................................................21

GENERAL CONCLUSION .....................................................................................22 7.1

FUTURE PERSPECTIVES/ RECOMMENDATIONS ..................................................................23

REFERENCES..............................................................................................................24

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1 Introduction Mammoet is a leading company specialized in solving extreme heavy lifting and transport challenges. With offices in 30 countries and over more than 1500 employees around the world Mammoet provides its customers with knowledge, skills and equipment at onshore or offshore location. The solutions that Mammoet provide are used in economic sectors like the Petro Chemical industry, Civil works, Power plants and Offshore. The best project example of the latter is of course the salvation of the Russian nuclear submarine the Kursk. In various projects Mammoet applies a compact Strand jack unit (see Figure 1and Figure 2) placed on a gantry or mast for precise lifting, lowering or pulling of heavy loads. This unit is, due to its compact design, very suitable in areas where conventional equipment as cranes cannot be placed.

Figure 1: Lifting 6.700 mTonnes deck with 12 strand jack units

Figure 2: Two strand jack units on top of a mast

This Strand jack unit is generally based on a hydraulic cylinder with a wedge principle to lock the strands in each stroke. Two wedges, situated in the lower and upper anchor head in the cylinder, lock each strand in the unit when the load is lifted or lowered. Mammoet meets difficulties especially with these wedge clamps of the strand jack units. Slipping of strand through the wedges and high maintenance costs of the strand jack units hence, are the main aspects. In the specialized and prestigious market of heavy lifting and transport safety and operating reliability are important topics, decisive in maintaining a good reputation. It’s obvious that research on the used wedge became more than desirable. In this research report chapter two provides an overall view on Mammoet, its clients, mission statement and scope of work. In chapter three a profound and extensive examination is performed on the existing strand jack system and its components. General operation fundamentals of the current strand jack unit are explained and detailed information of essential components is provided. Chapter four provides a theoretical approach on the probable causes of mal-function and unreliability of the strand jack wedge. Mathematical equations, which

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characterize the operation of the wedge, are derived and a graphical presentation of the “wedge malfunction area” is presented. In chapter five wear by micro slip is discussed and a micro slip/wear reducing wedge design is presented in chapter six. The general conclusion and future perspectives/recommendations are carried out in chapter seven.

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2 Mammoet Mammoet is a specialist in heavy lifting and transport; it has subsidiaries and agencies throughout the world. Besides contracting on a turnkey basis, Mammoet offers crane rental and related services.

2.1 Merger The new heavy lift combination was formed on 12 July 2000, when the shares of Mammoet Transport BV were taken over by Van Seumeren Holland BV from Royal Ned Lloyd. Van Seumeren Group has acquired Mammoet’s entire business, including its activities in the United States, Asia, the Middle East and Europe. The merger of the two companies provided the possibility to offer a total package at the top of the market by scaling up to customers. Synergy results in: • • • • •

Mobilisation/demobilisation savings Capacity utilisation of equipment International commercial effectiveness A more efficient organisation Purchasing power

Both Dutch, heavy lifting and special transportation companies Mammoet and Van Seumeren Holland BV were nearly equally sized when they joined forces. Mammoet was slightly stronger in horizontal transportation. Van Seumeren was slightly stronger in vertical transportation. The combined expertise, the experience and efforts of the 1500 employees of Mammoet Holding BV form a crucial factor for the success and further growth opportunities of the company in the international project market. The new company is positioned worldwide under the name and logo “Mammoet” , with the mention “Van Seumeren Group”.

2.2 Clients and scope of work Mammoet provides clients with tailor-made solutions and services in all scopes of work for engineered heavy lifting and multimodal transport worldwide. In Table 1 Mammoet’s clients and scope of work are indicated. Table 1:Mammoet’s clients Mammoet’s clients: Power generating industry Chemical industry Petrochemical Industry Offshore industry

Forwarders EPC contractors Ship yards Vessel Fabricators

Construction companies Crane companies Engineering companies Machine buildings companies Steel construction companies

Table 2: Mammoet’s scope of work Mammoet’s scope of work: Lifting Transporting Load outs Load ins

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Site moves Shutdowns Placing Shifting

Jacking Skidding Ballasting Weighing

Report

Rigging Barging From factory to foundation

By H.G.M.R. van Hoof

7

Figure 3 shows a gantry-lifting system, lifting an offshore module. Figure 4 is an example of a project in the chemical industry.

Figure 3: Lifting a offshore module 600 mTonnes with 4 strand jack units on a gantry Figure 4: Lifting a 830 mTonnes vessel in the power chemical industry

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3 The Mammoet strand jack system This chapter gives a broad view on of the Mammoet strand jack system with the Strand Jack Units SSL and description of the working mechanism with occurring defects in the strand-locking principle. Also a detailed description of the used wedges is given. Mammoet has designed the Mammoet strand jack system to be used for installation of heavy equipment, such as offshore-structures, equipment for petrochemical and nuclear power plants and bridges. The system has a modular set-up, designed for safe a precise lifting, lowering or pulling of heavy loads.

3.1 Hydraulic Strand jack-unit SSL

STROKE

The (multi) Strand Jack Unit SSL is designed for usage in the MSG (Mammoet Sliding Gantry) and in the transport of heavy loads, horizontally or vertically. Mammoet has several Strand jack units available with different capacities and dimensions. The capacity is (of course) coupled with the quantity of strands and the diameter of the main cylinder. The main dimensions and technical data of the available Mammoet equipment are presented in Figure 5.

HEIGHT

WIDTH

LENGTH

Strand jack units SSL830 (Double Anchored)

SSL830

SSL550

SSL300

SSL100

Capacity (kN):

9000

9000

6000

3000

1000

Strands (Qty):

54

54

36

18

7

Strand diameter (mm) :

18

18

18

18

18

Specifications

Stroke (mm):

400

400

400

400

480

Weight (kg)

4350

3850

2390

2000

940

Length (mm):

880

880

790

550

800

Width (mm):

880

880

790

550

500

Height (mm): Type of cable :

2590

1880

1845

1560

1765

Dyform Strand

Dyform Strand

Dyform Strand

Dyform Strand

Dyform Strand 180

Working oil pressure (bar)

400

400

370

310

Wedges (Qty) :

108

108

72

36

14

10600

10600

7185

3900

1450

Test capacity (kN) :

Figure 5: Technical data and main dimensions of the strand jack equipment

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3.2 Components Hydraulic strand jack-unit SSL In Figure 6 shows a general cross-section of a strand jack unit. The strand jack unit consists of the following main components: •

Base plate: this plate is fixed on the foundation



Lower anchor head (see Appendix A)



Wedges (see Appendix B)



Lower wedge-release-cylinder: with the lower wedge-release-cylinder the lower anchor head opens and closes.



Hydraulic cylinder: the hydraulic cylinder extends and retracts the piston. This way the strand can be moved over a stroke of 400 or 480 mm.



Upper wedge-release-cylinder: with the upper wedge-release-cylinder the upper anchor head opens and closes.



Upper anchor head (see Appendix A)



Strand guide tube (or pipe): the guide tubes to ensure that strands passing not deflect and apply side loadings onto the top anchor assembly.



Load force

∅18 mm compact strands (DYFORM) (see Appendix C) Figure 6: Cross-section Strand jack unit

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3.3 Strand locking principle The Hydraulic strand jack unit SSL contains a characteristic strand (or cable) locking principle. This locking-principle is a decisive factor in the quality of performance of the strand jack unit SSL. In this paragraph a detailed description is provided. The locking principle comprises two main positions; “locked” and “unlocked” (examine Figure 7). For a detailed description of the operation cycles (jack-up and jack down) of the Hydraulic Strand jack-unit SSL is referred to Appendix D.

"LOCKED"

"UNLOCKED"

Figure 7: Locking principle in “locked” and “unlocked” position

3.3.1 “Locked” principle In the “locked” position, the release plate (or kick out plate) as well as the connected release tubes are fully retracted; allowing the wedges in their tapered seatings, held down by the prestress springs (see Figure 7). A special washer of either spigot or socket type is used between the spring and the wedge, to ensure that the spring pressure is applied equally to all three parts of the wedge (Figure 8). The initial spring force (of magnitude ±400 N /spring) provokes an essential “pre-bite” of the wedge on the strand (Figure 8). The bite depth b of the “pre-bite” is negligible, however not to be overlooked for a proper operation of the gripping action of the wedge. When the load force on the strand (and thus on the wedge) increases, the wedge drops over a distance D downward in the tapered seating and the bite depth b (or distance inwards) enlarges pro rata of the load force (examine Figure 9). The strand is now “locked” by the “bite” of the grip profile of the wedge.

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The bite depth b and distance D have values of roughly 0.15-0.22 mm and 1.5-2 mm respectively (depending on circumstances in operation and differences in hardness of grip profile and strand surface). These values relate to the maximum Mammoet operation load of 167 kN /strand.

SPRING SPIGOT OR SOCKET TYPE WASHER WEDGE

b

D

STRAND

STRAND

Figure 8: Pre-bite of wedge in strand Figure 9: Increase of load force results in bite depth b and distance downward D

The radial movement (distance inward b) of the three wedge parts during an increase of the load force (conform Figure 8 and Figure 9) is presented schematically in Figure 10.

TOPVIEW WEDGE (DISTANCE INWARD b)

b Increasing loadforce

Figure 10: Radial movement of the wedge parts (segments) with increasing load force

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WEDGE

3.3.2 “Unlocked” principle Before the “Unlocking” procedure is actuated, the load force on the wedges is reduced to zero (to exclude any damage on release plate and tubes). Subsequently, by means of the additional hydraulic release cylinders (Figure 7) the release plate (or kick out plate with the connected release tubes) is lifted into the anchor head and shifts the wedges out of their tapered seatings (Figure 7 and Figure 11). The strand is now “Unlocked”; note the difference between Figure 9 and Figure 11.

WEDGE STRAND

WEDGE

RELEASE TUBE (shifted upwards)

Figure 11: "Unlocked” position; the wedge shifted upwards

By unlocking, free passage of the strands through the wedges is allowed; a negligible force is applied on the friction profile and no “bite” is generated (see detail Figure 11).

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4 Theoretical problem analysis In this chapter a mechanical wedge model is presented. With this model the theoretical situations of mal-function of the wedge “strand locking” system are explained. In Figure 12 the mechanical wedge model is presented with relevant parameters.

kout

kin

F L Figure 12: Mechanical wedge model

µkout = Kinetic coefficient of friction between outer wedge surface and anchor head surface µkin = Kinetic coefficient of friction on inner wedge surface and cable surface α = Wedge angle FL= Load force According to the calculations performed in Appendix E the “borderline” of operation of the wedge can be characterized as:

µ kout =

µ kin − tan(α ) 1 + µ kin ⋅ tan(α )

and so

µ kin =

µ kout + tan(α ) 1 − µ kout ⋅ tan(α )

(4a)

In the different load cases of the wedge, the following two situations can occur : Situation 1 The strand jack wedge operates properly. And thus

µ kout <

µ kin − tan(α ) 1 + µ kin ⋅ tan(α )

and so:

µ kin >

µ kout + tan(α ) 1 − µ kout ⋅ tan(α )

(4b)

Situation 2 The strand jack wedge doesn’t operate properly. And thus

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µ kout >

µ kin − tan(α ) 1 + µ kin ⋅ tan(α )

and

µ kin <

µ kout + tan(α ) 1 − µ kout ⋅ tan(α )

(4c)

The equations 4a , 4b and 4c combined in graph below for the wedge angle α = 8.19˚.

Friction coefficients 0.6 “Borderline”

0.4

Malfunction of wedge (slipping of strand, situation 2)

0.2 Wedge operating properly (situation 1)

µkout

0

0.1

0.2

0.3

0.4

0.5

0.6

α =8.19˚

0.2 Figure

µkin Figure 13: areas of proper resp. wrong operation of the wedge in a coefficients of friction diagram

The combinations of friction coefficients µkin and µkout, positioned in red area of Figure 13, will cause malfunction of the “strand locking” action of the wedge (values of µkout in Figure 13 below zero are of no practical use). Note : With a wedge angle of 8.19 ° , the inner friction coefficient has to be µkin > 0.14 for a proper wedge operation ! For the interested reader, Appendix F clarifies several cases of wedge malfunction. Several environmental circumstances and principles can have an influence on the value of µkout : • •

Adhesion: between wedge outer surface and tapered hole surface Plowing /third body effects: between wedge outer surface and tapered hole surface

For µkin : • Accumulation of dirt : between strand outer surface and inner surface of the wedge • Wear on inner friction profile of wedge during load cycles This last wear principle is considered in detail in the following chapter.

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5 Wear by micro slip In every load cycle (jack up and jack down) micro slip will occur between the wedges of the upper and lower anchor head and the strands. This is a result of the difference in axial strain (as a result of the load force FL) between the cable and wedge in consequence of the difference in stiffness. The axial strain in the cable is larger than in the wedge. Hence the strain in the loaded cable can not be “followed” by the wedge what results in a relative displacement between the cable outer surface and wedge inner surface. Micro slip in combination with a load force results in wear of the inner friction surface of the wedge and eventually in a reduction of the inner friction coefficient µkin. Finally this will cause malfunction of the wedge (according to paragraph 4 and Appendix F). The load force FL and related strain in the strand increase during this movement until the anchor head bears the total present load force FL. At the end of the trajectory of the increasing load force FL (the upper anchor head bears the total load force FL ) , the slip front has propelled to distance S. In the cross section of the cable on position S, the tensile stress σc= 0 According to the calculations performed in Appendix G the position of slipfront S can be characterized as:

s=

FL ⋅L µ kin ⋅ Nin

(5a)

Nin= Normal force on inner wedge surface µkin = Kinetic coefficient of friction on inner wedge surface FL= Load force S = Position of slip front L = Total length of friction profile

=0

c

Nin

=FL/Acable

c

Figure 14: Wedge with relevant parameters

FL

concerning microslip

An increasing force FL results in a increasing value of s, as soon as s > L total slip and FL= Fmax = µ kin ⋅ Nin occurs. • •

If s > L : macroslip occurs, the strand slips through the wedge If 0
According to Appendix G the relative displacement u(x) between strand and wedge becomes:

u( x ) =

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(5b)

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This equation is presented graphically in Figure 15.

X

Figure 15: Graph of relative displacement u(x) between the outer cable surface and inner friction profile of the wedge

FL= Load force Acable = Cross section area of cable Ec = E-modulus of cable material The magnitude of relative displacement u(x) (and thus wear) of the outer cable surface against the friction profile depends on the location. Maximum slip (relative displacement) is located at the loaded “outlet” of the cable at x = S (see Figure 15). Throughout the load cycles, wear takes place on the friction profile of the wedge due to the load force and relative motion (micro slip). The friction profile is reduced and flattened during operation (see Figure 16). Consequently the “strand gripping action” of the wedge is less.

Figure 16: Flattened friction profile of a wedge, mal-functioning after 501 strokes at nominal load force of 120 kN

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6 Micro slip / wear reducing design As indicated in Appendix G and chapter 5 the relative displacement u(x) between wedge friction profile and cable outer surface is characterized as (see Figure 17):

u(x ) =

FL ⋅ x 2 2 ⋅ E c ⋅ A cable

(6a)

Figure 17: The parabolic characterisation of the relative displacement u(x) between friction profile and cable outer surface .

FL= Load force Acable = Cross section area of cable Ec = E-modulus of cable material According to Ref [3], the design of a micro slip/hysteresis reducing grip construction of a plate is equipped with “fingers” which can “absorb” the relative displacement u (x) .The lengths of of the “fingers” are arranged a parabolic pattern (conform the equation of u(x)) . The length F of each finger can be designed proportional to the relative displacement u(x). The –3 stiffness C of each finger is proportional to finger length F (see Figure 18). The micro slip (and relative displacement u(x)and thus friction) is reduced by dimensioning the finger length locally in relation to u(x) . Locally the stiffness of wedge is adapted; the wedge friction surface can now “follow” the displacement (or strain) of the outer cable surface and reduce the micro slip and thus wear of the grip profile. This results in the micro slip (and wear) reducing wedge designs of Figure 18.

Figure 18: Micro slip and wear reducing wedge designs A and B

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Figure 18a is theoretically the best alternative, the length of the fingers is conform the parabolic equation of u(x). Practically the alternative B of Figure 18 can also reduce the wear on the inner grip profile and is very straightforward to produce. The exact amount, dimensions and pattern of the “fingers” must be determined by practical results.

6.1 Wear reduction on the base of vertical cable equilibrium From the vertical cable equilibrium of Appendix E , equation 6.1a is obtained:

σc = 2 ⋅

S ⋅ µ kin ⋅ σ cw Ri

(6.1a)

With: µkin = Kinetic coefficient of friction on inner wedge surface and cable surface σcw = Compressive stress between wedge and cable σc = Tensile stress in cable S = Position of slip front (for detailed information is referred to Appendix G2.2) Ri = Inner radius of wedge and therefore outer radius of strand/cable The values of the parameters µkin , σcw , σc of equation 6.1a can be considered as: µkin : Constant, assuming a value between 0,05 and 1. σcw : Constant ,being a maximum design or material stress value σc : Constant , being a maximum design or material stress value As a result of the assumptions of the above, the ratio S/Ri of equation 6.1a must also have a constant value. Nevertheless numerous combinations of S and Ri can satisfy this condition. Ergo , if we design a wedge configuration with the assumed values of µkin = 0.2 , σcw=300 N/mm2 , σc= 745 N/mm2 and α = 8.19˚ , the ratio will be :

σc S = 6.2 = R i 2 ⋅ µ kin ⋅ σ cw

(6.1b)

and with equation 5a the ratio S/L becomes:

FL S = L µ kin ⋅ Nin

(6.1c)

and with Nin (see Figure 14) being roughly :

Nin =

FL tan α

(6.1d)

Equation (6.1c) becomes:

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S tan α = 0.72 = L µ kin

(6.1e)

If we combine equations 6.1b, 6.1c, 6.1d and 6.1e, Table 3 can be comprised: Calculated properties of different strand/wedge combinations Capacity of one strand

Position slip front

Lenght of friction profile

Weight/m

Radius Ri (mm)

Dimensions Strand Steel Area (mm²)

Ratio S/Ri

FL (kN)

S (mm)

L (mm)

kg/m

n strands needed for 900 ton n

9**

223**

6.2

167**

55.8

77.5

1.75**

54

6

113*

6.2

84.3

37.2

51.7

0.89

107

5

78.5

6.2

58.5

31

43

0.62

153

4.5

63.6

6.2

47.4

27.9

38.8

0.5

190

4

50.3

6.2

37.5

24.8

34.4

0.4

240

3

28.3

6.2

21.1

18.6

25.8

0.22

427

* Calculated with p*Ri p* ² , ** Reference value of strand (Appendix C)

Table 3: Calculated wedge/strand configurations

When a strand with a diameter of 9 mm (Ri =4.5) is chosen for example, a solid single high capacity wire can be used. This solid 9 mm wire has better wear/handling properties than the original 18 mm strand; •

No internal relative displacements between wires during operation: less elastic power is lost by friction/wear. Therefore no internal wear is initiated. In general a strand with a diameter of > 9 mm contains multiple wires, and the internal wear and elastic energy loss will increase drastically by an increasing number of strand wires. (See Figure 19)



The 9 mm strand also improves handling due to a reduced weight (Table 3; a weight reduction of 1.25 kg/m!). In a 900-ton configuration of the strand jack unit SSL, 190 wires of 9 mm are used. 31 WIRES 1 WIRE

7 WIRES

During operation :

During operation :

* No internal relative displacement between wires * No internal wear between wires * No elastic power loss due to internal friction/wear

* Internal relative displacement between wires * Internal wear between wires * Moderate elastic power loss due to internal friction/wear

During operation : * Internal relative displacement between wires * Internal wear between wires * High elastic power loss due to internal friction/wear

Figure 19: Examples of strands with 1 wire, 7 wires and 31 wires. Increasing the number of wires will increase the relative elastic power loss by friction/wear between the wires.

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6.2 “9 mm” wedge / anchor head configuration When the 9 mm wire of paragraph 6.1 is applied, the anchor head /wedge configuration is modified. This configuration is shown in Figure 20.

DETAIL B Anchor head with 190 tapered holes

Total of 190 Wedges

A

A

B

SECTION A-A

Figure 20: Wedge / anchor head configuration with 190 "9 mm" wedges

The geometry of the “9 mm” wedge is presented in Figure 21.

Figure 21: The geometries of the "9 mm" wedge and the original 18 mm wedge

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7 General conclusion •

According to the calculations performed in Appendix E the proper operation area of the wedge can be characterized as:

µ kout <

µ kin − tan(α ) 1 + µ kin ⋅ tan(α )

and so:

µ kin >

µ kout + tan(α ) 1 − µ kout ⋅ tan(α )

With: µkout = Kinetic coefficient of friction between outer wedge surface and anchor head surface µkin = Kinetic coefficient of friction on inner wedge surface and cable surface α = Wedge angle FL= Load force The wedge gripping quality depends on inner-and outer friction coefficients µin , µout and the wedge angle α . The characteristic graph of Figure 13 provides the proper operation area and malfunction area of the wedge, as a function of µin , µout (with α=8.19°). Several environmental circumstances and principles can have an influence on the value of µkout : o o

Adhesion: between wedge outer surface and tapered hole surface Plowing /third body effects: between wedge outer surface and tapered hole surface

For µkin : o Accumulation of dirt : between strand outer surface and inner surface of the wedge o Wear on inner friction profile of wedge during load cycles



During operation relative displacement occurs between the inner friction profile and strand outer surface. The relative displacement u(x) can be characterized as :

u(x ) =

FL ⋅ x 2 2 ⋅ E c ⋅ A cable

FL= Load force Acable = Cross section area of cable Ec = E-modulus of cable material

This relative displacement which initiates wear is called micro slip. •

The micro slip reducing design, presented in this report, can increase service life of the wedge.



By applying 190 high capacity solid strands with diameter of 9 mm the following objectives can be realized:

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By H.G.M.R. van Hoof

22



o

No internal relative displacements between wires during operation: less elastic power is lost by friction/wear. Therefore no internal wear is initiated. In general a strand with a diameter of > 9 mm contains multiple wires, and the internal wear and elastic energy loss will increase drastically by an increasing number of strand wires.

o

The 9 mm strand also improves handling due to a reduction of 1.25 kg/m compared to the original 18 mm strand. In a proper 900-ton configuration of the strand jack unit SSL, 190 wires of 9 mm are used.

In this 9 mm configuration, 380 small 9 mm “low cost” wedges are used.

7.1 Future perspectives/ Recommendations •

Cost / service life ratio in case of the 9 mm strand is an important issue to be studied.



The wear and reliability tests on the alternative wedge designs have to be evaluated (conform standard procedures) with the reference properties attained from a reference test.



Less influence of dirt, corrosion and environment can be accomplished with a totally new strand jack design. A new construction can be investigated with a cable “locking” system not dominated by friction (in case of the current wedge principle, operation quality is dominated by friction coefficients and thus by the operation environment), resulting in a very reliable lifting system not influenced by the environment.

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References 1. E. Rabinowicz, Determination of compatibility of Metals Through Static Friction tests, ASLE Trans., Vol 14, 1971, p 198-205 2. D. Landheer, M.J.W. Schouten,J. van Vollenhoven and C.J.M. Meesters, Dictaat Tribotechniek en Aandrijvingen , 1983, p. 5.4.9, Faculteit der Werktuigbouwkunde Technische Universiteit Eindhoven. 3. Van der Hoek,W ., Koster, M.P., Rosielle ,P.C.J.N., Dictaat Constructieprincipes 1 , Faculteit Werktuigbouwkunde Technische Universiteit Eindhoven, Maart 2000.

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APPENDICES STRANDJACK WEDGES Friction coefficients, micro slip and handling Report nr. DCT 2005-78 By: H.G.M.R. van Hoof Idnr : 501326 20 November, 2005

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By H.G.M.R. van Hoof

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Table of contents TABLE OF CONTENTS..................................................................................................2 APPENDIX A ANCHORHEADS .....................................................................................3 APPENDIX B WEDGE PROPERTIES ...........................................................................5 APPENDIX C ∅18 MM DYFORM STRANDS .................................................................6 APPENDIX D OPERATION CYCLES STRANDJACK UNIT ..........................................7 D.1 JACK UP CYCLE (LIFTING THE LOAD) .....................................................................................7 D.2 JACK DOWN CYCLE (LOWERING THE LOAD) ..........................................................................10

APPENDIX E CALCULATIONS WEDGE MODEL.......................................................15 E.1 E.2 E.3 E.4 E.5

AREAS AND PROJECTED WEDGE AREAS AIN , AOUT , AINEFF EN AOUTEFF ........................................16 HORIZONTAL WEDGE EQUILIBRIUM ......................................................................................17 VERTICAL WEDGE EQUILIBRIUM ..........................................................................................18 WEDGE OPERATING RANGE ................................................................................................18 VERTICAL CABLE EQUILIBRIUM ...........................................................................................19

APPENDIX F MALFUNCTION OF THE WEDGE .......................................................20 F.1 WEDGE OPERATION WITH : ΜKIN CONSTANT , INCREASING ΜKOUT ...............................................20 F.2 WEDGE OPERATION WITH : ΜKOUT CONSTANT , DECREASING ΜKIN .............................................21 F.3 WEDGE OPERATION WITH: INCREASING ΜKOUT , DECREASING ΜKIN ............................................21

APPENDIX G MICRO SLIP DURING JACK UP ..........................................................23

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APPENDIX A Anchorheads An anchor head is drilled with a number of tapered holes suitable for accepting jack temporary wedges. An anchor head of the 830 SSL strand jack unit for example contains 54 tapered holes (Figure 3). In Figure 1 a characteristic example of an anchor head is presented. The quantity of tapered holes of the anchor heads is scaled to the required capacity.

SECTION A-A

Figure 1:Anchor head of 830 SSL strand jack unit

The main specifications of the anchor heads are presented in Table 1. Table 1: Specifications Anchor heads. Source: [ Mammoet] Anchor heads Specifications Main dimensions(mm) : Weight (kg) : Material : Wedges / holes (Qty) : Surface treatment :

SSL830

SSL550

SSL300

SSL100

∅520x190

∅450x190

∅354x100

∅200x150

271

207

60

40

34CrNiMo6

34CrNiMo6

34CrNiMo6

34CrNiMo6

54

36

18

7

Nitrocarburizing(dutch: teniferen) Nitrocarburizing(dutch: teniferen) Nitrocarburizing(dutch: teniferen) Nitrocarburizing(dutch: teniferen)

Layer thickness surface treatment (mm) : Solid lubricant : Optimal layer thickness lubricant (µm) :

0.1-1.6

0.1-1.6

0.1-1.6

0.1-1.6

Molycote D321 R

Molycote D321 R

Molycote D321 R

Molycote D321 R

15-25

15-25

15-25

15-25

Note that the surface treatment of the anchor heads is nitro carburizing.

Figure 2: 830 SSL anchor head with 54 wedges Figure 3: Tapered wedge seatings of an anchor head

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In order to reduce friction, the wedge seats of the anchor head are pre-treated with Molykote D321 R, a dry lubricant (see Figure 4).

Figure 4: Molykote D321 R Pre-treated Anchor head

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APPENDIX B WEDGE PROPERTIES The wedges used in the strand jack units contain three wedge parts. The wedge is equipped with a friction or grip profile on the inside of each part. The main dimensions of the wedge and wedge parts are presented in Figure 5 en Figure 6.

DETAIL GRIP PROFILE

WEDGE

Figure 5: Assembled wedge and wedge part Figure 6:Detail grip profile

The three wedge parts are assembled with an elastic rubber ring. This ring retains the three wedge segments/parts together during operation (Figure 7). Table 2: Specifications TT 18/44 wedge (sources : Mammoet,KANIGEN )

Wedge Main specifications

Main dimensions(mm) :

TT 18/44 Wedge

∅25x∅44x∅18x80

Total weight (kg) :

Rubber ring

0.450

Material :

DIN 1.6523 (SAE 8620)

Wedge parts (Qty) :

3

Weight wedge part (kg) : Hardening (Carburizing) temperature (°C):

0.150 900-1000

Tempering temperature (°C):

190-200

Hardness of substrate after tempering (indication*) (HV):

700-760

Surface treatment :

Electroless Nickel plated (KANIGEN method)

Proces temperature surface treatment (°C) :

200-280

Hardness of surface layer (indication*) (HV):

500-550

Hardness substrate after surface treatment (indication*) (HV):

560-620

Layer thickness surface treatment (µm) : Solid lubricant :

25-50 Molycote D321 R

Optimal layer thickness lubricant (µm) :

5-20

Total cost (euro):

Figure 7: Assembled wedge with rubber ring

20

Manufacturer :

*

TT Fijnmechanica

Values for reference only, exact values have to be determined by material tests

For main specifications of the currently used TT 18/44 wedge is referred to Table 2(In Table 2 several specifications are noted with “indication”, these values need to be reviewed by hardness tests). In order to reduce friction, the wedges are pre-treated before operation with Molykote D321 R, a dry lubricant.

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5

APPENDIX C ∅18 mm DYFORM strands The strand bundle contains several Dyform strands with a maximum operation capacity of 167 kN/strand (specified by Mammoet). The quantity of strands depends on the used unit type.The Dyform strand contains 7 twisted, high capacity, steel wires with a flattened side (see Figure 8). They are specially developed for heavy lifting and its length can be up to 1500 meters to meet the job requirements. DYFORM STRAND

Cross section P-P Flattened side(6x)

P

P

Wire (7x)

Figure 8: Dyform strand

The main specifications of the Dyform strand are presented in Table 3. Table 3: Main specifications Dyform strand. Source :[ Bridon wire Ltd ]

Strand Main specifications

BS5896 (DYFORM)

Nominal values

Tolerances

Nominal diameter (mm) :

∅18

+0.4 -0.2

Mass (kg/m) :

1.75

+0.4% -2%

Tensile strength (Rm) (N/mm2)

1700

Surface hardness of wires (HV)

430-480

Steel area (mm2)

223

Breaking load (Fm) (kN)

380

0.1% proof load (Fp 0.1) (kN)

323

Load at 1% elongation (Ft 1.0) (kN)

334

Wires (Qty) Manufacturer :

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7 Carrington Wire Ltd / Bridon Wire Ltd

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By H.G.M.R. van Hoof

6

APPENDIX D Operation cycles Strandjack unit In this appendix a review is presented on two main operation cycles of the strand jack unit SSL to familiarize the reader with the general concepts of the Strand jack-unit SSL.: • •

Jack up cycle (lifting the load) Jack down cycle (lowering the load)

D.1 Jack up cycle (lifting the load) In the next paragraph the Jack up cycle of the hydraulic Strand jack units is described. This principle concerns all types.

START POSITION

STEP 1

WEDGE UPPER ANCHOR HEAD RELEASE PIPE UPPER RELEASE CILINDER UPPER RELEASE PLATE

STROKE

WEDGE LOWER ANCHOR HEAD RELEASE PIPE LOWER RELEASE CILINDER LOWER RELEASE PLATE LOAD

LOAD STROKE

Figure 9: Start position and Step 1 of the Jack up cycle

The start position (see Figure 9): The 18 mm dyform compact strands are installed through the unit and the load is connected. Both upper and lower anchor heads are “locked”(red in Figure 9). Step 1 (see Figure 9):The piston of the jack extends and raises the upper anchor head including the locked strands and connected load. During this movement the wedges in the lower anchor head are pulled up slightly by the movement of the strands; the lower anchor head is still closed. In case of failure of the upper anchor head, the wedges in the lower anchor head secure the load.

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STEP 2

STEP 3

STROKE -15 mm

LOAD

LOAD

Figure 10: Step 2 and Step 3 of the Jack up cycle

Step 2 (see Figure 10): In top position the load is transferred from the upper anchor head to the lower anchor head by slightly retracting the piston, approx. 15 mm. During this load transfer both anchor heads remain locked. Step 3 (see Figure 10): After the transfer, the upper anchor head is opened hydraulically. The upper release cylinder lifts the release plate with the release tubes and shifts the wedges/grips up and out of their seatings (Note the difference between left and right detail picture) .The upper anchor head is now “unlocked”(blue in Figure 10) allowing free passage of the strands.

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STEP 4

STEP 5

-STROKE

LOAD

LOAD

Figure 11: Step 4 and Step 5 of the Jack up cycle

Step 4 (see Figure 11): With the upper anchor head “unlocked”, the piston of the jack retracts and returns to the starting position. The strands slide through the wedges in the upper anchor head. Step 5 (see Figure 11): At the end of step 4, the upper anchor head is “locked” again by the upper release cylinder retracting the release plate and release tubes. The jack up cycle of step 1 to 5 is repeated till the required jack up distance is accomplished.

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9

D.2 Jack down cycle (lowering the load) In the next paragraph the Jack down cycle of the hydraulic Strand jack units is described. This principle concerns all types.

START POSITION

STEP 1

WEDGE UPPER ANCHOR HEAD RELEASE PIPE UPPER RELEASE CILINDER UPPER RELEASE PLATE

WEDGE LOWER ANCHOR HEAD RELEASE PIPE LOWER RELEASE CILINDER LOWER RELEASE PLATE

LOAD

Figure 12: Start position and Step 1 of the Jack down cycle

The start position (see Figure 12): The 18 mm dyform compact strands are installed through the unit and the load is connected. Both upper and lower anchor heads are “locked”(red in Figure 12). Step 1 (see Figure 12): The upper anchor head is opened hydraulically. The upper release cylinder lifts the release plate with the release tubes and shifts the wedges/grips up and out of their seatings (Note the difference between detail picture of the upper anchor head in Start position and Step1). The upper anchor head is now “unlocked”(blue in Figure 12) allowing free passage of the strands.

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S TEP 2

S TEP 3

LO AD

LO AD

Figure 13: Step 2 and Step 3 of the Jack down cycle

Step 2 (see Figure 13): With the upper anchor head “unlocked”, the piston of the jack extends till approx. 15 mm before end of the outward stroke (compare the position of the main hydraulic cylinder in step 1 and step 2). The strands slide through the wedges in the upper anchor head during the stroke. Step 3 (see Figure 13): In the position “15 mm before end of the outward stroke” the upper anchor head is “locked” by the upper release cylinder retracting the release plate and release tubes (red in detail upper anchor head in see Figure 13 step 3).

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STEP 4

STEP 5

LOAD

LOAD

Figure 14: Step 4 and Step 5 of the Jack down cycle

Step 4 (see Figure 14): The load is transferred from the lower anchor head to the upper anchor head by slightly extending the piston of the main cylinder further (approx. +15 mm) to the end of the stroke. During this load transfer both anchor heads remain “locked” (both red in Figure 14 step 4). During this movement the wedges in the lower anchor head are pulled up slightly by the movement of the strands; the lower anchor head is still closed. In case of failure of the upper anchor head, the wedges in the lower anchor head secure the load. Step 5 (see Figure 14): After the transfer, the lower anchor head is opened hydraulically (“unlocked”; blue in detail of lower anchor head step 5). The lower release cylinder lifts the release plate with the release tubes and shifts the wedges/grips up and out of their seatings (Note the difference between left and right detail picture step 4 and step 5) .The lower anchor head is now “unlocked” (blue in Figure 14) allowing free passage of the strands.

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STEP 6

Figure 15: Step 6 and Step 7 of the Jack down cycle

Step 6 (see Figure 15): The piston of the jack retracts and lowers the closed upper anchor head including the strands and load (movement is –stroke) until approx. 15 mm before end of the inward stroke. The lower anchor head is still “unlocked” (blue in Figure 15) during this movement and allows free passage of the strands. Step 7 (see Figure 15): In the position “15 mm before end of inward stroke”, the lower anchor head is “locked” hydraulically by the lower release cylinder retracting the release plate and release tubes (red in detail lower anchor head in Figure 15 step 7).

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Figure 16: Step 8 of the Jack down cycle

Step 8 (see Figure 16): After “locking” the lower anchor head, the load is transferred from the upper anchor head to the lower anchor head by slightly retracting the piston of the main cylinder further to the end of the inward stroke, approx. -15 mm. During this load transfer both anchor heads remain locked. The jack down cycle of step 1 to 8 is repeated until the required jack down distance is accomplished.

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APPENDIX E Calculations wedge model In this appendix a mathematical model of the strand locking system is presented. In the following considerations a single wedge part is examined and the spring force of the prestress spring is neglected. STRAND WEDGE PART

ANCHORHEAD

D strand 2 x Ri

kout

CW

kin

out

out

cw

L

CW

S
Nout

cw

=

FL A cable

FL Figure 17 : Schematic presentation of mathematical wedge model

µkout = Kinetic coefficient of friction between outer wedge surface and anchor head surface µkin = Kinetic coefficient of friction on inner wedge surface and cable surface Nout = Normal force on outer wedge surface Nin= Normal force on inner wedge surface σcw = Compressive stress between wedge and cable σNout = Compressive stress perpendicular wedge surface and anchor head σc = Tensile stress in cable as result of FL FL= Load force α = Wedge angle τout = Friction shear stress between outer wedge surface and anchor head τcw = Friction shear stress between wedge and cable S = Position of slip front (for detailed information is referred to Appendix G2.2) Ri = Inner radius of wedge and therefore outer radius of strand/cable Acable = Cross section area of cable From Figure 17 , the following equations can be obtained:

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τ out = µ kout ⋅ σ Nout

(a)

τ cw = µ kin ⋅ σ cw

and

(b)

E.1 Areas and projected wedge areas Ain , Aout , Aineff en Aouteff The estimation of the theoretical outer and inner surface of a single wedge part (assuming cylindrical), Ain and Aout, is formulated as :

A in =

2 π ⋅ Ri ⋅ S 3

A out =

(c)

2 S π ⋅ R out ⋅ 3 cos(α )

and

(d)

This according to Figure 18.

SIDE VIEW WEDGE PART

Ri

R out

TOP VIEW WEDGE PART

S

S COS ( )

Figure 18: Top view and side view wedge part in relation to Ain and Aout

The estimated projected effective outer and inner surface of a single wedge part, Aineff and Aouteff, can formulated as:

A ineff = R i ⋅ S ⋅ 3

(e)

and

A outeff = R out ⋅

S ⋅ 3 cos(α )

(f)

These equations can be derived from Figure 19.

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R out

Ro

Ri

Ri

Ri

x

ut

3

x

3

R out

TOP VIEW WEDGE PART

Figure 19: Top view and side view wedge part in relation to Aineff and Aouteff

The total normal force Nout acting perpendicular on the effective surface of the wedge part becomes (see also Figure 17) :

Nout = σ Nout ⋅ A outeff = σ Nout ⋅ R out ⋅

S ⋅ 3 cos(α )

(g)

The total normal force Nin acting perpendicular on the inner surface of the wedge part becomes (see also Figure 17) :

Nin = σ cw ⋅ A ineff = σ cw ⋅ R i ⋅ S ⋅ 3

(h)

E.2 Horizontal wedge equilibrium The total equilibrium of the horizontal forces on the wedgepart results in the following equation (according to Figure 17):

Nout ⋅ cos( α ) = Nin − τ out ⋅ sin(α ) ⋅ A outeff

(i)

After substitution of equations a, g , h and f in i

σ Nout ⋅ R out ⋅

S S ⋅ 3 ⋅ cos( α ) = σ cw ⋅ R i ⋅ S ⋅ 3 − µ kout ⋅ σ Nout ⋅ sin(α ) ⋅ R out ⋅ ⋅ 3 cos(α ) cos(α )

After solving and rearranging, we get

σ Nout ⋅ R out (1 − µ kout ⋅ tan(α )) = σ cw ⋅ R i

or

σ Nout ⋅

R out Ri

(1 − µ kout ⋅ tan(α )) = σ cw (j)

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E.3 Vertical wedge equilibrium The total equilibrium of the vertical forces on the wedgepart results in the following equation (according to Figure 17):

A in ⋅ τ cw − τ out ⋅ cos(α ) ⋅ A out − σ Nout ⋅ sin(α ) ⋅ A out = 0

+

(k)

After substitution of equations b, c , a, d in k, we get

2 2 S π ⋅ R i ⋅ S ⋅ µ kin ⋅ σ cw − µ kout ⋅ σ Nout ⋅ cos(α) ⋅ π ⋅ R out ⋅ − 3 3 cos(α ) 2 S σ Nout ⋅ sin(α) ⋅ π ⋅ R out ⋅ =0 3 cos(α ) After solving and rearranging, this results in

Ri ⋅ µkin ⋅ σcw = σNout ⋅ Rout ⋅ (µkout + tan(α))

µkin ⋅ σcw = σNout ⋅

or

R out ⋅ (µkout + tan(α)) Ri

(l)

E.4 Wedge operating range Substituting equation j in l results in

µkin ⋅ σNout ⋅

Rout R (1− µkout ⋅ tan(α)) = σNout ⋅ out ⋅ (µkout + tan(α)) Ri Ri

After rearranging

µ kout =

µ kin − tan(α ) 1 + µ kin ⋅ tan(α )

and so

µ kin =

µ kout + tan(α ) 1 − µ kout ⋅ tan(α )

(m)

These two equations represent the “borderline” of the wedge operating range.

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E.5 Vertical cable equilibrium When the equilibrium of the vertical forces on the cable/strand in Figure 17 is considered, the following equation can be acquired:

FL = 2 ⋅ π ⋅ R i ⋅ τ cw ⋅ S

(n)

With

FL = σ c ⋅ A cable = σ c ⋅ π ⋅ R i

2

(o)

and

τ cw = µ kin ⋅ σ cw

(b)

After substitution of o and b in equation n, the result is: 2

σ c ⋅ π ⋅ R i = 2 ⋅ π ⋅ R i ⋅ µ kin ⋅ σ cw ⋅ S

(p)

After rearranging :

σc = 2 ⋅

S ⋅ µ kin ⋅ σ cw Ri

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(q)

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APPENDIX F Malfunction of the wedge For the benefit of the analysis, three possible cases are described to clarify the malfunction of the wedge, according to the mechanical wedge model: 1. Wedge operation with : µkin Constant , increasing µkout 2. Wedge operation with : µkout Constant , decreasing µkin 3. Wedge operation with : Increasing µkout , decreasing µkin In the following paragraphs these load cases are described in detail.

F.1 Wedge operation with : µkin Constant , increasing µkout When the wedge operation is started, a proper and normal operation of the wedge is assumed.This implies that the friction conditions are below the “good/bad” borderline in Figure 20. For example, fictive estimated start values of µkout and µkin in operation are 0,2 and 0,5 (see start operation point A in Figure 20).

Friction coefficients 0.6

0.4

Malfunction of wedge (slipping of strand, situation 2)

B

`

A

0.2

Wedge operating properly (situation 1)

µkout

0

0.1

0.2

0.3

0.4

0.5

0.6

α =8.19˚

0.2 µkin Figure 20:Wedge operation with increasing µkout

With increased µkout , the operation point B is positioned in the red “Malfunction Area” .The friction force between wedge outer surface and tapered hole surface is too high and consequently gripping action is less; the strand slips through the wedge.

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F.2 Wedge operation with : µkout Constant , decreasing µkin When the wedge operation is started, a proper and normal operation of the wedge is assumed. For example, fictive estimated start values of µkout and µkin in operation are 0,2 and 0,5 (see start operation point A in Figure 21).

Friction coefficients 0.6

0.4

Malfunction of wedge (slipping of strand, situation 2)

A

0.2

C

µkout

0

0.1

0.2

0.3

Wedge operating properly (situation 1)

0.4

0.5

0.6

α =8.19˚

0.2 µkin Figure 21: Wedge operation with decreasing µkin

When µkin decreases during operation below the value ≤ 0,35 and the value µkout is constant , the operation point of the wedge is moved from A to point C (see Figure 21 ). Operation point C is positioned in the red “Malfunction Area” .The friction force between wedge inner surface and cable surface is too low, consequently the gripping action is less; the strand slips through the wedge.

F.3 Wedge operation with: Increasing µkout , decreasing µkin In most practical cases the values of the friction coefficients will change simultaneously, influenced by environmental circumstances (temperature, relative humidity etc.).

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A combination of increasing µkout and decreasing µkin will lead to mal-function of the wedge conform Figure 22, the operation point of the wedge is moved from A to point D. Malfunction of the wedge appears.

Friction coefficients 0.6

0.4

Malfunction of wedge (slipping of strand, situation 2)

D A

0.2

Wedge operating properly (situation 1)

µkout

0

0.1

0.2

0.3

0.4

0.5

0.6

α =8.19˚

0.2 µkin Figure 22: Wedge operation with decreasing µkin and increasing µkout

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APPENDIX G Micro slip during jack up In every load cycle (jack up and jack down) micro slip will occur in the wedges of the upper and lower anchor head. This is a result of the difference in axial strain (as a result of the load force FL) between the cable and wedge in consequence of the difference in stiffness. The axial strain in the cable is larger than in the wedge. Hence the strain in the loaded cable can not be “followed” by the wedge what results in a relative displacement between the cable outer surface and wedge inner surface. Micro slip in combination with a load force results in wear of the inner friction surface of the wedge and eventually in a reduction of the inner friction coefficient µkin. Finally this will cause malfunction of the wedge. When the piston of the jack extends, the closed upper anchor head is raised including the strands. The load force FL and related strain in the strand increase during this movement until the anchor head bears the total present load force FL. The situation of the full loaded wedge in the anchor head is described schematic in Figure 23.

Figure 23: Loaded wedge during raising of upper anchor head

FL= Load force V = Spring force (pre-stress)

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Nin= Normal force on inner wedge surface µkin = Kinetic coefficient of friction on inner wedge surface Q = Length arc of wedge part α = Wedge angle σcw = Compressive stress between wedge and cable σc = Tensile stress in cable as result of FL εc = Strain in cable S = Position of slip front L = Length of friction profile Acable = Cross section area of cable Ec= Modulus elasticity of cable τcw = Friction shear stress At the end of the trajectory of the increasing load force FL (the upper anchor head bears the total load force FL ) , the slip front has propelled to distance S. In the cross section of the cable on position S, the tensile stress σc= 0. Along the inner friction surface of the wedge part, width Q and length L, exists a compressive stress σcw between wedge and cable as a result of the normal force Nin. The (pre-stress) spring force V is neglected and the stiffness of the wedge is considered infinite in the following considerations. This compressive stress σcw present on the 3 wedge parts is formulated as:

σ cw =

Nin 3⋅Q ⋅L

The compressive stress σcw is able to transmit an average friction shear stress τf on the contact area:

τ cw = µ kin ⋅ σ cw The load force FL is compensated by three inner friction surfaces, with an area of Q ⋅ s and thus is derived for FL :

FL = 3 ⋅ Q ⋅ s ⋅ τ cw = 3 ⋅ Q ⋅ s ⋅ µ kin ⋅ σ cw =

3 ⋅ Q ⋅ s ⋅ µ kin ⋅ Nin s ⋅ µ kin ⋅ Nin = 3 ⋅Q ⋅L L

A increasing force FL results in a increasing value of s, as soon as s > L total slip and FL= Fmax = µ kin ⋅ Nin occurs. • •

If s > L : macroslip occurs, the strand slips through the wedge If 0
For the tensile stress

σc =

σ c and strain ε c in the cable we can write

FL A cable

εc =

FL σc = E c E c ⋅ A cable

The length s can be calculated from the relation

s=

FL ⋅L µ kin ⋅ Nin

Research strand jack wedges

Appendices

(Eq. G.1)

By H.G.M.R. van Hoof

24

The tensile stress

σ c diminishes linear with values on positions x = 0 to x = s (see Figure 23 )

according to

σ c ( x =0 ) = 0

σ c( x= s) =

and

And analogically with the equations for the tensile stress

ε c( x =0) =

σc 0 =0 = E c E c ⋅ A cable

and

A graph of the x- position against strain

ε c( x=s)

FL A cable σ c , equations for strain ε c are FL σ = c = E c E c ⋅ A cable

ε c is provided in Figure 24.

Figure 24 Graph of position x, strain and tensile stress in cable wedge

With usage of Figure 24 the total cable elongation dl in the wedge is derived. The elongation dl of the cable in the wedge corresponds with area P

dl = P =

1 ⋅ ε c(x =s) ⋅ s 2

With

ε c( x=s) =

σc FL = E c E c ⋅ A cable

and

s=

FL ⋅ L µ kin ⋅ Nin

After substitution, dl becomes

Research strand jack wedges

Appendices

By H.G.M.R. van Hoof

25

dl =

Function

1 F2L ⋅ L ⋅ ε c( x=s) ⋅ s = 2 2 ⋅ E c ⋅ A cable ⋅ µ kin ⋅ Nin

ε c (x ) can be described as (see Figure 24 ) ε c (x ) =

FL ⋅ x E c ⋅ A cable

for 0 ≤ x ≤ s

du = ε c (x ) follows dx FL ⋅ x du = dx E c ⋅ A cable

With the definition

To obtain the local relative displacement function u(x)

u(x ) = ∫ [

FL ⋅ x du ] ⋅ dx = ∫ [ ] ⋅ dx dx E c ⋅ A cable

for 0 ≤ x ≤ s

This leads to the relative displacement function u(x)

u(x ) =

With

FL ⋅ x 2 +C 2 ⋅ E c ⋅ A cable

for 0 ≤ x ≤ s

u(0) = 0 consequently C = 0 , relative displacement function u(x) becomes

FL ⋅ x 2 u(x ) = 2 ⋅ E c ⋅ A cable

Research strand jack wedges

for 0 ≤ x ≤ s

Appendices

(Eq. G.2)

By H.G.M.R. van Hoof

26

Equation G-2 is presented graphically in Figure 25.

Figure 25: Graph of relative displacement u(x) of the outer cable surface against inner friction profile

The magnitude of relative displacement u(x) (and thus wear) of the outer cable surface against the friction profile depends mainly on the location of the slip front S , assuming FL , Acable , Ec constant. Maximum slip is located at the loaded “outlet” of the cable at position S.

Research strand jack wedges

Appendices

By H.G.M.R. van Hoof

27