Experimental study on yield–slip characteristics during shutdown–restart of deepwater high-wax oil–water pipelines

Deepwater shutdowns in high-wax oil–water pipelines can form wax-gelled segments with very high yield strength, making conventional yield-stress models unreliable for restart-pressure prediction. Thermally regulated loop experiments together with rheometer measurements were conducted to quantify post-shutdown oil–water distribution, internal water-content characteristics of the gelled segment, and restart-pressure dynamics. Owing to coupled oil–water emulsification and gravitational stratification, the gel exhibits a pronounced non-uniform water-content gradient, and its effective length is ∼1.45 × the theoretical pure-oil gel length. Increasing water content and gel length both markedly raise restart pressure. Restart is governed by two concurrent mechanisms: bulk yielding of the crude oil and wall slip. Wall slip dominates well below the pour point, whereas bulk yielding prevails as temperature approaches the pour point, with a distinct critical temperature separating the two regimes. Based on these observations, we propose a restart-pressure model that couples yielding and slip while accounting for internal water-content gradients, enabling improved prediction accuracy. The results provide guidance for the design and safe operation of deepwater high-wax oil–water transportation pipelines. • Bulk yielding and wall slip mechanisms in oil-water pipeline restart were clarified. • The critical temperature for transition from bulk yielding to wall slip was identified. • The length and water-cut gradient of the oil-water mixed section were quantified. • A restart-pressure model coupling yield-slip and water-cut gradient was developed.