Development of the End Fitting Tensile Wires Fatigue Analysis Model: Sample Tests and Validation in an Unbonded Flexible Pipe
- Yijun Shen (Wellstream International) | Frank Ma (Wellstream International) | Zhimin Tan (Halliburton Co.) | Terry H. Sheldrake (Wellstream International)
- Document ID
- Offshore Technology Conference
- Offshore Technology Conference, 5-8 May, Houston, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2008. Offshore Technology Conference
- 4.3.4 Scale, 4.2.4 Risers, 4.1.5 Processing Equipment
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Fatigue analysis of tensile wires inside an end fitting structure is very important but has been difficult to develop due to the complexity of stress calculation. The stress distribution of the tensile wire inside the end fitting is quite different from that inside a flexible pipe. Many factors have resulted in this complexity of the stress characteristics: the bonding and debonding behaviour between the wire and the epoxy surrounding it, pipe tension, pipe bending with end fitting effect, localised twist, geometrical shape due to end fitting configuration, wire crimp shape, end fitting tensile wire assembly method, etc. Therefore, investigating the tensile wires inside the end fitting presents highly technical challenges in terms of fatigue modelling.
This paper focuses on the development of the fatigue analysis model, which can be used to assess the tensile wire fatigue damage along its entire length inside an end fitting. Complicated loading conditions and geometrical effects on the tensile wires are considered. Interaction between the tensile wires and the epoxy are analysed.
In this fatigue analysis model, it is flexible to change the bonding condition between the tensile wire and the epoxy by re-defining a debonded region between the wire and the epoxy inside the end fitting. Furthermore, some sample tests of the small scale test rigs have been carried out to validate the finite element analysis model for an unbonded flexible riser.
This model presents an efficient way to simulate the complicated stress characteristics and to predict the fatigue damage of the end fitting tensile wires of an unbonded flexible pipe.
Unbonded flexible pipes as a technical alternative to the traditional steel pipe have been widely used in oil/gas fields. The End fitting is an important mechanical device in the design of flexible pipes, which forms the transition between the flexible pipe body and the connector. Besides connecting pipe units, the end fitting also combines and terminates the tensile wires and other relevant layers of the flexible pipe. Further details on the function and design specification of the end fitting can be obtained in [1, 2]. The tensile armour wires are the fatigue critical component in the end fitting structure. The key fatigue stress component in the tensile amour wire is the axial stress in the wire cross section .
Figure 1 illustrates the end fitting assembly, where four regions are defined according to the wire configuration. In region 1, the tensile wire remains in the helical shape as expected in the pipe section. Once the tensile wire approaches the region 2, it first changes its direction at the outer sleeve and then over the Flexlok collar. In region 3, the wire is laid back flat over the slope of the body in a helical form. Lastly, the designed crimp section of the tensile wire is defined as region 4.
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