Effect of System Pressure on Restart Conditions of Subsea Pipelines

Abstract

Abstract As the oil industry invests billions of dollars in oil and gas production from deep waters, the concern for flow assurance of reservoir fluids to the surface and their subsequent delivery to relevant markets becomes very critical to justifying project economics and meeting the ever growing demand for petroleum products. For various reasons, production systems could be shut down long enough to form waxy oil gels in subsea pipelines which can completely hinder further production. Hence, the need for a good understanding of the various phases of waxy crude transformation in a cold subsea pipeline and all factors affecting restart conditions cannot be over emphasized. This paper provides a comprehensive review of the various studies done on wax crystallization, deposition and gelation as well as the yielding behavior of waxy crude gels during pipeline restart. The study focuses on detailed experimental investigation of temperature and pressure effects on gel strength. The influence of testing temperature on gelation kinetics was studied on a controlled stress rheometer (CSR) and analyzed using the modified phenomenological Avrami model for isothermal crystallization proposed by Lopes-da-Silva and Coutinho (2007). On the other hand, the effect of system pressure – a parameter widely neglected in pipeline restart studies, on restart conditions was studied on a novel model pipeline system which produced the best reproducibility of yield pressures reported so far in literature. The model pipeline results were further compared to yield point tests on a CSR. The results of the experimental work conducted in this study suggest that at higher shut-in pressures, weaker gels may be formed which facilitates pipeline restart.