Local Buckling Behaviour of a Corrosion Resistant Alloy Liner in Tight Fit Pipe due to Axial Compression

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Abstract

A promising possibility to reduce corrosion resistant pipeline costs is the concept of Tight Fit Pipe, which is a double wall pipe where a Corrosion Resistant Alloy liner is mechanically fitted inside a carbon steel outer pipe. The mechanical bonding of the Tight Fit Pipe is made through a thermohydraulic manufacturing process. Problem definition Buckling of cylinders subjected to flexural loads (applied to Tight Fit Pipe during cost effective reeling) correlates in a number of respects to buckling of axially compressed cylinders: in both cases the critical stresses (or strains) are of the same order of magnitude and the failure modes have the same characteristics. Results from the axial compression tests provided better understanding of the buckling behaviour of Tight Fit Pipe during bending and results from this thesis study have been used as input for a bending rig construction. Research The main objective of this thesis is to investigate, theoretically and experimentally, the local buckling behaviour due to axial compression of the Corrosion Resistant Alloy liner, whilst fitted in the outer pipe of the Tight Fit Pipe configuration. The liner of the Tight Fit Pipe as tested is first analysed when not fitted in the outer pipe. Secondly the liner is analysed when fitted in the outer pipe of the Tight Fit Pipe. The liner has a higher capacity in stress and strain when the liner is placed in the outer pipe, i.e. mechanically fitted, compared with the liner alone. Results, Conclusions and Recommendations [a.] Buckling of the liners (of Tight Fit pipes (TFP) used in the experiments), without the outer pipe, occurred outward in a single axi symmetrical wrinkle. [b.] The liners in the tight fit pipes, confined in an outer 12 ¾ inch pipes used in the experiments, buckle inward in a non-symmetrical wrinkle. [c.] Wrinkles (of liners confined in an outer 12 ¾ inch pipe) do not exceed 90º of the circumference. The wrinkles have an axis which is circumferential. Wrinkles do not tend to connect with each other if more than one is present at about the same height. [d.] The maximum force in the load - strain diagram was considered to be the point at which wrinkling occurred. There was a good agreement between the Finite Element (FEM) results and test results for the maximum force. The FEM results for the strain at the maximum force were much higher than in the tests. [e.] Buckling of a liner in a 12 ¾ inch pipe has a larger critical buckling force than a liner which is not confined in an outer pipe. This is due to the resistance to outward buckling. Bigger wrinkles must fit in a smaller radius which costs more energy than free outward buckling. This could also be the reason why the wrinkles do not tend to form as one big wrinkle in a 360 degrees radius. Outward buckling tend to occur in one axi symmetrical wrinkle. [f.] The buckling behaviour of the liner in the TFP is believed to be influenced by three different TFP properties: [f1.] Low or high fit (residual hoop stress), [f2.] Type of CRA (Material strength) and [f3.] Type of outer pipe used, seamless or UOE-pipe (Liner to outer pipe contact irregularities ) [f1] has been proven in the test results; [f2] and [f3] are still to be proven in future work.