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Research Papers: Materials Technology

Improved Friction Joint With Self-Locking Grips

[+] Author and Article Information
Andrei Costache

Department of Mechanical Engineering, Technical University of Denmark,
Nils Koppels Allé, Building 403,
Kongens Lyngby 2800, Denmark
e-mail: ancos@mek.dtu.dk

Kristian Glejbøl

National Oilwell Varco,
Subsea Production Systems—Flexibles, Priorparken 480,
Brøndby 2605, Denmark

Ion Marius Sivebæk

Department of Mechanical Engineering, Technical University of Denmark,
Produktionstorvet, Building 427,
Kongens Lyngby 2800, Denmark

Christian Berggreen

Department of Mechanical Engineering, Technical University of Denmark,
Nils Koppels Allé, Building 403,
Kongens Lyngby 2800, Denmark

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received December 16, 2015; final manuscript received March 29, 2016; published online June 2, 2016. Assoc. Editor: Jonas W. Ringsberg.

J. Offshore Mech. Arct. Eng 138(5), 051401 (Jun 02, 2016) (8 pages) Paper No: OMAE-15-1126; doi: 10.1115/1.4033331 History: Received December 16, 2015; Revised March 29, 2016

Flexible risers are used in the oil industry to transport liquids and gas from the seafloor to extraction and production equipment at the sea surface. Ongoing research aims at using composite materials instead of steel, in order to reduce weight and increase stiffness. Ensuring an optimal load transfer between the composite and metal components is very important. This paper presents an improved method for anchoring a flat fiber-reinforced tendon using a double grip system with self-locking grips. The novelty is the combination of new experimental results and finite element (FE) analysis to develop a superior dry friction grip. Experimental results are carried using a dedicated test setup, through which the test parameters can be accurately controlled. The efficiency of the grip system during pullout is superior to results obtained with flat grips. Numerical results offer an in-depth understanding of the influence between friction, geometrical parameters, and performance, making it possible to optimize the design. Results show that this grip system offers immediate technical applications, in a variety of conditions.

Copyright © 2016 by ASME
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References

Rytter, J. , 2002, “ A Novel Compression Armour Concept for Unbonded Flexible Pipes,” Annual Offshore Technology Conference, Houston, TX, May 6–9, pp. 573–577.
Bryant, M. , 2007, “ Nonmetallic Unbonded Flexible Pipes for Deep Water,” 2nd International Oil Conference and Exhibition, Society of Petroleum Engineers, Veracruz, Mexico, June 27–30, pp. 104–108.
Nielsen, N. J. R. , Glejbøl, K. , and Kulakov, V. , 2001, “ Innovative Un-Bonded Flexible Pipe Design With Hybrid CFRP/Metal Armour,” Deep Offshore Technology, pp. 1–10.
Al-Mayah, A. , Soudki, K. A. , and Plumtree, A. , 2001, “ Experimental and Analytical Investigation of a Stainless Steel Anchorage for CFRP Prestressing Tendons,” PCI J., 46(2), pp. 88–100. [CrossRef]
Al-Mayah, A. , Soudki, K. , and Plumtree, A. , 2007, “ Novel Anchor System for CFRP Rod: Finite-Element and Mathematical Models,” J. Compos. Constr., 11(5), pp. 469–476. [CrossRef]
Schmidt, J. W. , Bennitz, A. , Täljsten, B. , Goltermann, P. , and Pedersen, H. , 2012, “ Mechanical Anchorage of FRP Tendons—A Literature Review,” Constr. Build. Mater., 32, pp. 110–121. [CrossRef]
Rytter, J. , Portnov, G. , and Kulakov, V. , 2005, “ Anchoring and a Load Transfer Technique in Uniaxial Tension of Unidirectional High-Strength Composites,” Mech. Compos. Mater., 41(3), pp. 217–228. [CrossRef]
Burtscher, S. L. , 2008, “ Wedge Anchorage for CFRP Strips,” J. Compos. Constr., 12(4), pp. 446–453. [CrossRef]
Portnov, G. G. , Kulakov, V. L. , and Arnautov, A. K. , 2013, “ Grips for the Transmission of Tensile Loads to a FRP Strip,” Mech. Compos. Mater., 49(5), pp. 457–474. [CrossRef]
Costache, A. , Glejbøl, K. , Sivebæk, I. M. , and Berggreen, C. , 2015, “ Friction Joint Between Basalt-Reinforced Composite and Aluminum,” Tribol. Lett., 59(2), p. 30. [CrossRef]
ISO, 1998, “ Geometriske produktspecifikationer (GPS). Overfladebeskaffenhed. Profilmetode. Termer, definitioner og overfladebeskaffenhedsparametre,” Dansk Standard, Denmark, Standard No. ISO 4287.
Zenkert, D. , 2006, Foundations of Fibre Composites: Notes for the Course: Composite Lightweight Structures, DTU, Lyngby, Denmark.
Foye, R. , 1972, “ The Transverse Poisson’s Ratio of Composites,” J. Compos. Mater., 6(2), pp. 293–295. [CrossRef]
Costache, A. , Glejbøl, K. , Sivebæk, I. M. , and Berggreen, C. , “ Numerical Investigation of Friction Joint Between Basalt Reinforced Composite and Aluminum,” Proc. Inst. Mech. Eng., Part J (submitted).

Figures

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Fig. 1

Typical end-fitting for unbonded flexible pipes. The tensile armor wires give the pipe its tensile strength. They terminate in the end-fitting and have to be anchored in order to transfer loads from the pipe to the flange. The anchoring method is critical for the structural integrity of the pipe/end-fitting assembly.

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Fig. 3

Test setup with stochastic pattern for digital image correlation (DIC). The FRP tendon is squeezed between two triangular grips. The actuator pulls the tendon at constant speed. A load cell is installed opposite to the actuator to record the pullout force. Reference surfaces are used to eliminate rigid body movement.

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Fig. 2

Model geometry. The system consists of two v-shaped grips and a unidirectional basalt fiber-reinforced polymer in between. The grips are held in a fixture to which pressure is applied. The grips’ dimensions are heights h1 and h2, and length lg. The FRP can extend to both sides of the grip. The force Fx is obtained by pulling the FRP in x-direction.

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Fig. 4

(a) Pullout test for a normal force Fn = 16 kN. Fx is the pullout force. Fx/Fn is the grip coefficient. In (b), Fn(1) is the initial normal force value, at the beginning of the test. A displacement of 0.8 mm/min is applied to the FRP. The grip surface is sandblasted.

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Fig. 5

Grip displacement for Fn = 16 kN. Grip 1 in (b) is the displacement of the top grip. Grip 2 in (a) is the displacement of the bottom grip. A displacement of 0.8 mm/min is applied to the FRP in positive x-direction. The grip surface is sandblasted.

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Fig. 8

Grip coefficient Fx/Fn and standard deviation Std(Fx/Fn) for Fn = 16 kN. a1 represents results where a new specimen is used every time. a2 are results obtained by gripping and pulling the same specimen, without cleaning the grips. a3 are creep tests after 67 and 116 hrs. Their standard deviation is given in (b). All results are obtained using sandblasted grips.

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Fig. 9

Force equilibrium for (a) the grips to fixture contact, μ = 0.3. (b) The grips to FRP contact, μ = 0.25. Ff is the friction force, Ftg is the tangential force between the grips and fixture, and FT is the force equilibrium. Ff is the contact friction force and Fx is the total force equilibrium in x-direction.

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Fig. 10

Parametric analysis. (a) The influence between the friction at the contact of the grips and fixture, with respect to the grip angle α. (b) The grip coefficient Fx/Fn. For all cases, between the grips and the FRP, μ = 0.25.

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Fig. 11

Pressure distribution at the contact between the grips and the FRP. Line a1 corresponds to 9530 DOF, and line a2 corresponds to 55,302 DOF. The pressure P and the maximum pressure Pmax are normalized with the nominal applied pressure Pnom = 5.33 MPa. For this aluminum to FRP contact, the static coefficient of friction is μs1 = 0.25. μs2 = 0.3 is used at the contact between the grip and fixture. The contact length is 50 mm, μs/μd = 1.2 and α = 15 deg.

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Fig. 12

Grip coefficient Fx/Fn. The pullout force Fx is obtained by applying displacement to the FRP in positive x-direction. For the results in (a), the normal force is Fn = 16 kN. The static coefficient of friction for the contact between the top grip and the FRP is μs1 = 0.25. The coefficient of friction at the contact between the lower grip and the FRP is μs2, and the reduction in contact area takes the following values: a1—μs2 = 0.2, a = 0%, a2—μs2 = 0.2, a = 33%, a3—μs2 = 0.2, a = 20%, and a4—μs2 = 0.22, a = 20%. (b) Fn effect. The friction properties are μs1 = 0.25, μs2 = 0.22, and a = 20%. b1—Fn = 1 kN, b2—Fn = 4 kN, b3—Fn = 8 kN, and b4—Fn = 16 kN. The aluminum to aluminum friction is μs3 = 0.3. μs/μd = 1.2. The grip angle is α = 15 deg.

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Fig. 13

Experimental versus numeric results. Maximum grip coefficient Fxmax/Fn as a function of the normal force Fn. The tilt angle is α = 15 deg. μs1 = 0.25, μs2 = 0.22, μs3 = 0.3, μs/μd = 1.2, and a = 20%.

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Fig. 15

Friction between grip and the fixture. (a) After normal force application. (b) Before pullout. The coefficient of friction for line 1 is μ = 0.1. For line 2 is μ = 0.3. For the aluminum to FRP contact μs1 = 0.25, μs2 = 0.22, and contact area reduction is a = 20%. The contact length is 50 mm. The grip angle is α = 10 deg.

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Fig. 6

FRP displacement for Fn = 16 kN. P1 is the point closest to displacement application. P3 is the point farthest from displacement application. P2 is in the middle of the contact area. The FRP is pulled with 0.8 mm/min. The grip surface is sandblasted. The linear part of the grip coefficient is between Fx/Fn = 0.2 and 0.45.

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Fig. 16

Boundary conditions influence. (a) After normal force application. (b) Before pullout. The top line of the top fixture is constrained with ux = 0 for line 1 results. For line 2, the entire top fixture is constrained with ux = 0. For line 3, the entire top fixture is constrained with ux, uy = 0. For the aluminum to FRP contact, μs1 = 0.25, μs2 = 0.22, and contact area reduction is a = 20%. At the grip to fixture contact, μs2 = 0.3. The contact length is 50 mm. The grip angle is α = 10 deg.

Grahic Jump Location
Fig. 17

Boundary conditions. Line 1: top fixture constrained with ux = 0 and Fn = 4 kN; line 2: top and bottom fixture constrained in all directions, while displacement is applied to the grips in positive x-direction. (a) After gripping the FRP tendon. (b) Before pullout. For the aluminum to FRP contact, μs1 = 0.25, μs2 = 0.22, and contact area reduction is a = 20%. At the grip to fixture contact, μs2 = 0.3. The contact length is 50 mm. The grip angle is α = 15 deg.

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Fig. 14

Grip angle α effect. Three α values are compared, namely, 3 deg, 8 deg, and 20 deg. The contact pressure P and the pullout force Fx are normalized with the nominal applied pressure Pnom and normal force Fn = 16 kN. For the first load step, pressure is applied to the top fixture, and the contact pressure distribution is given in (a). In the second load step, displacement is applied to the right end of the FRP. The pressure distribution just before pullout is given in (b). The grip coefficient Fx/Fn is given in (c). μs1 = 0.25, μs2 = 0.22, μs3 = 0.3, and the contact area reduction is 20%. The contact length is 50 mm.

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Fig. 7

Pullout force Fx and grip coefficient Fx/Fn for a normal force Fn = 1, 4, 8, and 16 kN. Results using the sandblasted grips are marked with lines a1 and b1. Results using the smooth grips are marked with lines a2 and b2.

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