Modelling the time-dependent mechanical behaviour of steel Reinforced Thermoplastic Pipes Kruijer, M.P. Advisor: Prof.dr.ir. R. Akkerman Co-advisor: Dr.ir. L.L. Warnet University of Twente, 2006 ISBN: 90-365-2341-9 EM research theme: Computational and Experimental Mechanics Composite pipes are increasingly used in the oil and gas industry, instead of conventional carbon steel pipes. The main reason is the better corrosion resistance of composite pipes compared with steel pipes. The first composite alternatives to steel pipelines consisted of composite pipes based on thermoset matrices. However, the increasing demands of the oil and gas industry, involving high-temperature resistance and the need for damage tolerance and flexibility, often exceed the capabilities of thermosets. As thermoplastics can satisfy these demands, several types of fibre Reinforced Thermoplastic Pipe (RTP) systems are currently under development The pipe studied here, called steel Reinforced Thermoplastic Pipe (sRTP) is manufactured by overwrapping a thermoplastic liner (i.e. plain HDPE pipe) with high-strength steel reinforced tapes. The reinforced tapes consist of high-strength steel cords embedded in an HDPE matrix. During production of sRTP, the liner with the tapes is heat-welded, thereby providing a sturdy construction. Due to the flexible and tough character of the HDPE, the sRTP is coilable, which enables fast installation as no prefabricated bends are needed. Experimental pre-study with prototype sRTP indicated considerable time-dependent mechanical behaviour of the sRTP. The deformation behaviour as well as the pipe failure mechanism appeared to be time-dependent. This could be explained by the viscoelastic behaviour of the HDPE. Due to the creep / relaxation of the HDPE, the cord load increases in time for a pipe subjected to, for example, a constant inner pressure. This can lead to failure of the sRTP, when a critical cord load is reached. This thesis describes the development of an accurate numerical tool to be used by engineers to design and engineer steel Reinforced Thermoplastic Pipes (sRTPs). To this end, firstly the mechanical behaviour of the pipe constituent materials, the HDPE and the steel(cords) was analysed. The nonlinear viscoelastic behaviour of the HDPE was described by a Schapery creep model. The Schapery creep model was implemented in a multilayer pipe model based on generalised plane strain conditions. The model was verified with experimental data obtained from an extensive experimental program carried out on prototype sRTP. Both the time-dependent deformation behaviour and time-to-failure were accurately predicted by the nonlinear viscoelastic multilayer model. Finally, a design strategy was proposed, which efficiently incorporated the theory described in this thesis for design purposes. Based on the desired load-case, the desired pipe geometry and the operating time and temperature, an sRTP system can be accurately designed, with the desired deformation properties and strength. The effectiveness of this design strategy was confirmed in practise. An sRTP system was designed for water injection with an operating pressure of 129 bar, at 65°C for a period of 6 months, followed by a successful field trial in Syria.