Influence of friction between layers of mechanically lined pipes on collapse resistance

Mechanically lined pipes (MLP) are characterized by their two-layer structure consisting of an external carbon steel pipe responsible for the mechanical strength of the component and an inner pipe with a thinner wall made of a corrosion-resistant alloy (CRA). There is no metallurgical bond between these layers, meaning that their interaction is mainly defined by contact mechanisms influenced by the surface preparation, material properties and hydraulic expansion manufacturing processes used. This leads to difficulties in determining the operating limits of these pipes. This work focuses on the effects of friction between the layers of MLP on its resistance to collapse due to external pressure. Currently, the criteria established by the DNV-ST-F101 standard consider the contribution of the CRA layer in the collapse pressure calculation only for clad pipes (where there is metallurgical bonding between the layers), and there is no specific calculation procedure defined for MLP. In order to contribute to the current design codes for these components, this work aims to measure the friction coefficient between the layers of an MLP, to demonstrate the effect of this parameter on the collapse resistance of MLP. For this purpose, an experimental apparatus was assembled to obtain the static friction coefficient (s) of samples extracted from different production batches of MLP. Finite element modelling results will be used to illustrate the importance of having accurate measurements of this parameter when establishing the collapse pressure of MLP.