Local plastic deformation in pressure and tensile armour layers of flexible risers

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by  Caroline Fixter

Local plastic deformation in pressure and tensile armour layers of flexible risers
ALAVANDIMATH, Shivaraj
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SHEFFIELD HALLAM UNIVERSITY
Local Plastic Deformation in
Pressure & Tensile Armour
Layers of Flexible Risers
by
Shivaraj Alavandimath
The thesis is submitted in partial fulfilment of the requirements of
Sheffield Hallam University for the degree of Doctor of Philosophy
January 2009
Abstract
Flexible pipes are commonly used for connecting seabed flow-lines to floating pro­
duction facilities. The general riser design consists of an internal carcass for col­
lapse resistance, a polymer fluid barrier, carbon steel interlocked circumferential
pressure armour layer for resisting internal pressure loads, helically wound carbon
steel tensile armour layers to resist axial loads and a watertight external sheath.
Much of the analytical work that has been carried out on flexible composite pipe
is based on the early stress analyses of wire ropes, and this is primarily because
of the helical geometry of many of the metallic elements such as pressure armour
and tensile armour layers of a flexible riser. The general design philosophy of the
layer is defined in API 17J (American Petroleum Institute) in terms of the stress
“utilisation” factor that specifies the maximum allowable average hoop stress in
the layer, which is conventionally produced by the elastic stress analysis.
The layers are subjected to severe cyclic bending and twisting deformation during
manufacturing of the pipe which makes the material to exceed the yield point.
Consequently, residual stresses are developed in the pipe material and variable
amounts of non zero stresses exist in the metallic layers of the newly manufactured
pipes. An attempt has been made to model the amount of residual stress evolved
during manufacturing stage for two different metallic layers, namely; pressure
armour and tensile armour layers and its effects on overall pipe’s performance
while in service.
The strategy is to apply the finite element (FE) method by creating a 3 dimensional
wire model of the segment of pipe. Solid structural elements with plasticity were
employed for the analyses. Kinematic hardening with Baushinger effect, contact
and friction effects were also taken into account. Precise boundary conditions were
applied to the model for elastic-plastic bending. The resulting residual stresses
have been transferred to 3D axisymmetric model to investigate the internal and
external pressure effects. This new procedure was carried out for both layers
satisfying equilibrium and compatibility conditions for the pressure and tensile
armours to find the common interface pressure and contact loading. FAT (Factory
Acceptance Test) condition is simulated to investigate the level of residual stress
variation. Experimental measurements of residual stress by X-Ray diffraction are
in well agreement with FE results.
Contents
Abstract i
List of Figures iv
Declaration x
Acknowledgements xi
1 The Flexible Risers 1
1.1 Introduction................................................................................................ 1
2 Literature Review 7
2.1 Flexible Risers............................................................................................. 7
2.2 Residual Stresses....................................................................................... 11
2.3 Fatigue and Residual Stresses....................................................................26
2.4 Elastic Plastic Bending.................................................................................59
2.5 Residual stress measurement techniques....................................................75
2.6 Summary.......................................................................................................88
3 The Finite Element Strategy 91
3.1 Introduction....................................................................................................91
3.2 FE Model of Pressure and Tensile Armour wires....................................95
3.3 Contact Modelling......................................................................................102
3.4 2D and 3D model Comparison...................................................................117
4 The Bending & Pressure Analysis 121
4.1 Bending Analysis.........................................................................................121
4.2 The Manufacturing Process of the Pipe...................................................122
4.3 Pressure Analysis.........................................................................................129
5 The Experimental Work 133
5.1 Measurement of Residual Stresses............................................................133
6 Results and Discussion 139
7 Conclusions & Suggestions for Further Work 148
ii
Contents iii
A Pressure armour wire stress plots 151
B Tensile armour wire stress plots 156
C Software Input Listing 160
D Offshore Operations and Riser Installation 193
D.l Oil Platforms...............................................................................................193
D.2 Fixed Platforms............................................................................................194
D.3 Compliant Towers:......................................................................................195
D.4 Semi-submersible Platform: 195
D.5 Jack-up Platforms:......................................................................................195
D.6 Drillships .....................................................................................................196
D.7 Floating production systems......................................................................196
D.8 Tension-leg platform ..................................................................................197
D.9 Spar Platforms............................................................................................197
D.10 Installation of risers in deep w aters.........................................................198
D.ll Flexible Catenary Risers............................................................................199
D.12 Steel Catenary Risers..................................................................................200
D.13 Single Line Offset Risers............................................................................202
Bibliography 203
Notation
At = Tensile force acting on an arbitrary area
Ac = Tensile force acting on an arbitrary area
b = Width of the beam
E = Young’s Modulus
e = y/R
F = Force
h = Height of the beam.
I = Second moment of Inertia
L = Length of the beam
M = Bending Moment
Me = Elastic Moment
Mp = Plastic Moment
NN = Neutral Axis
R = Radius of Curvature
W = Load
Y = Bending Stress
y = Distance from the neutral axis
v = Poisson’s Ratio
a = Stress
e = Strain
5W = Deflection under the load W
A = Deflection
AE = Elastic Springback
AF = Final deflection
Declaration
This thesis is submitted in partial fulfilment of the requirements of Sheffield HelI-
lam University for the degree of Doctor of Philosophy. It contains an account of
research carried out between November 2004 to July 2008 in Materials Engineer­
ing and Research Institute, Sheffield Hallam University under the supervision of
Dr Richard Crampton and Dr Mohammed Islam. Except where acknowledgement
and reference is appropriately made, this work is, to the best of my knowledge,
original and has been carried out independently. No part of this thesis has been,
or is currently being submitted for any degree or diploma at this or any other
University.
Shivaraj Alavandimath
November 2008
x
Acknowledgements
I would like to acknowledge the support and encouragement of my director of
studies Dr Richard Crampton and Dr Mohammed Islam during the course of
this research project. I also acknowledge the Materials Engineering and Research
Institute and the Faculty of Arts Computing Engineering and Sciences at Sheffield
Hallam University for the provision of laboratory and computing facilities.
I thank all the technical staff in the Materials Engineering and Research Institute
for their help and assistance during the course of research work. Special thanks
are extended to Professor Robert Akid, Dr Doug Cleaver, Dr Rosemary Booth
and Rachael Ogden.
I would also like to thank Dr Upul Fernando, Professor John D Atkinson and
Dr Terry Campbell for their Constructive criticisms and discussions. I am also
indebted to Dr Terry Sheldrake and Richard Clements of Wellstream International
Ltd for their technical data and test samples.
My special thanks goes to Ruminda Wimalasiri for his help in using X-Ray diffrac­
tion machine and Tim O’ Hara, Roger Tingle, Brian Didsbury and Tony Earnshaw
for their support in Mechanical testing workshop.
Thanks are also extended to my family and my dear friends Shivaprakash and
Narasimha Raj for their unfailing support and encouragement. Special thanks are
extended to my friends Anna Lasilla, Chandra Sekhar, Dawn Hughes, Gavin Tolan,
Graham Lister, Janardhan Saithala, Jayne Wellings, Mark Jones, Pradeep Hegde,
Russell Bailey, Thomas Scott, Tom Phol and Willie Brink. All have unknowingly
inspired me to complete this thesis.