Flexible riser fatigue design and testing

The development and qualification of new large diameter flexible riser structures with higher strength to allow installation in deep waters is considered of paramount importance to increase Petrobras oil & gas production. Hence the flexible pipe manufacturers are constantly motivated to develop new concepts and to extend their design and construction capabilities. The development of new prototype pipe design requires a myriad of studies and tests to certify their strength and fitting for purpose. Among them, the fatigue design and prototype test have increased in importance due to the ever greater depths of the new fields. Once the design methodology depends on the manufacturer and the fatigue tests required by the international standards consider only the design life, the limitations and conservatisms of the design methodologies are unknown by the operators as well as the real safety factors considered. It is felt that there is a lack of test results that guarantee the applicability of the actual fatigue design methodologies. Furthermore, more and better information about the flexible pipe structural design is needed. Hence, a new fatigue design specification has been issued by Petrobras including the directional influence of bi-modal seas and the regular fatigue test was modified to include a second test step where the fatigue life is pursuit. This new step tries to reach the theoretical pipe failure increasing the loads applied during the test until the pipe reaches the accumulated damage equal to 1 (considering the Miner ' s law). This paper presents a short description of the Working Programme carried out in Petrobras considering the flexible riser fatigue life design methodology and testing performed in a sample of the Marlim Sul gas pipeline, which is connected to the P-40 platform. The sample was tested until the theoretical fatigue life was reached.