A Theoretical and Experimental Investigation of Non-Newtonian Crude Oil Flow Behaviour in Heated Subsea Pipelines Under Deepwater Conditions in Nigeria’s Offshore Basin

The rheological complexity of high-wax-content Nigerian deepwater crude oils, which exhibit pronounced non-Newtonian viscoplastic behaviour at the low temperatures prevalent in Nigeria's deepwater offshore basin at water depths exceeding 1,000 metres, creates fundamental challenges for the theoretical prediction of restart pressure requirements and flow assurance management that existing pipeline flow models developed for conventional Newtonian crude oils cannot adequately address. This research conducts a theoretical and experimental investigation of the non-Newtonian flow behaviour of Nigerian deepwater crude oil in heated subsea pipeline systems, with the objective of developing an experimentally validated constitutive and hydrodynamic model for flow assurance design. Wax appearance temperature, pour point, and rheological characterisation including yield stress, apparent viscosity, and thixotropic recovery kinetics are experimentally determined for crude oil samples from three deepwater fields in the Nigerian offshore basin across a temperature range of 5 to 40 degrees Celsius. A structural kinetics rheological model is developed to represent the time-dependent gelation and thixotropic breakdown behaviour under flow restart conditions, and its parameters are calibrated against experimental stress growth and creep recovery measurements. The constitutive model is implemented in OpenFOAM computational fluid dynamics software to simulate three-dimensional transient flow restart scenarios in representative heated pipeline configurations, accounting for thermal stratification, wall yielding propagation, and flow-induced thixotropic breakdown kinetics. Experimental validation using a high-pressure flow loop confirms model predictions of restart pressure within 12 percent. Keywords: non-Newtonian crude oil, deepwater pipeline, flow assurance, thixotropic model, Nigeria offshore.