Performance of pressure relief devices on flow start-up of gelled waxy crude oils in pipelines

Under low temperatures that can take place in Brazilian offshore production lines and Alaska oil pipelines, waxy crude oil can gel reducing flow capacity and demanding high pumping pressures for flow restart. A novel device developed to lessen the required pressure for starting up flows of gelled fluid was investigated in the current work. The device connected to the pipeline wall consists of a cylinder that contains gas separated from the gelled material by a piston. A mathematical model to study the start-up of gelled materials in pipelines equipped with these pressure relief devices was developed. The conservation equations of mass and momentum were used to assess the weakly compressible flow and the gelled oil was approached by using a thixotropic constitutive model. Two kinds of inlet boundary condition were considered: a constant flow rate and a constant pressure. The results for the first type of boundary condition show that the device reduced significantly the maximum restart pressure but also increased the start-up time. The maximum pressure peak was reduced by 24% when a device size of 1% of the pipeline total volume was employed. For the latter type of boundary, on the other hand, the use of the device only increased the start-up time but did not affect the required pressure.