The mechanical properties of the fixture of internal pressure sheath for marine composite flexible pipes

The effective fixing of internal pressure sheath at the end fitting of composite flexible pipe can reduce the leakage risk of transported fluid at the end fitting. In this paper, the fixing form of internal pressure sheath for composite flexible pipes was introduced. Then, its elastic modulus, linear coefficient of thermal expansion, shear strength and compressive stress relaxation property were tested. In addition, its fixing effect was checked on the basis of its properties. It is indicated that the compressive stress applied on the internal pressure sheath attenuates gradually as the time goes, and its decline rate is 83.9% during the designed service lifetime of the pipe. When the teeth of the fixture of internal pressure sheath are completely pressed in the internal pressure sheath, the axial force provided by the fixture in the initial stage can offset the thermal contraction force resulted from pipeline shutdown. If the stress relaxation behavior of internal pressure sheath is taken into account, the axial force provided by its fixture will be decreased and the internal pressure sheath may be pulled out. In order to avoid this risk, the axial force provided by the fixture can be enhanced by increasing the design length and tooth amount of the fixture of internal pressure sheath. The research results can guide the structure design of the fixture and evaluate the valid fixing period of internal pressure sheath.