Blockage of pipelines due to hydrate formation is a major problem for subsea flow assurance. Induction time for hydrate formation from the multiphase system within a pipeline is a critical parameter to determine whether hydrates may form at a given time. In this work, the induction time for hydrate formation in water-in-oil emulsions was investigated under different conditions. For this purpose, an autoclave with an online viscometer was designed and built. Based on the viscosity variation observed in the experiments during hydrate formation, a new avenue for defining induction time is proposed, which should be more convenient for determining the hydrate formation time in some pipelines. As hydrate formation in emulsions is more complicated than in pure water, the effects of several factors were considered in this study, including water cut of the emulsions, shear rate, driving force, and memory effect. Additionally, wax precipitation is also a common problem in subsea pipelines and can impact flow assurance when hydrate formation and wax precipitation both occur. Consequently, the effect of wax solid particles on hydrate formation was also considered in this work. The presence of wax particles is observed to impede hydrate formation. In this work, it is determined from induction time that the hydrate formation is initiated at the water–oil surface for water-in-oil emulsion. Moreover, the memory effect can shorten induction times of hydrate formation due to the remaining small CO2 bubbles at the surface of water droplets.

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Knowledge of droplet dynamics provides the basis of predicting pressure drops, holdups and corrosion inhibitor distribution in multiphase fl ow. Droplet size and its distribution also determine the separation efficiency between different phases. Experimental observations were conducted for droplet impingements with different fluids, droplet sizes and velocities, and film thicknesses. The observed transition boundaries were compared with the models developed by different authors. For impingement on a deep pool surface, the Marengo and Tropea correlation for splashing does not agree with the experimental results in this study. The Bai and Gosman critical Weber number for bouncing agrees with the water results but not the oil results. Three new correlations for transition boundaries between bouncing, coalescence, jetting and splashing were proposed and compared with the experimental observations. Key words: Droplet, liquid fi lm, transition boundary, high-speed video are different from each other. In the coalescence regime, the droplet submerges in the fi lm accompanied with vortex rings. In the splashing regime, a crown is formed at the early stage of splashing. A number of daughter droplets are formed from the tip of the crown. The crown collapses and recoils toward the crown center at the late stage of splashing. This recoiling will eventually form a liquid jet at the center and generate a secondary droplet from the top of the jet. Normally, bouncing and coalescence only occur under fairly low Weber numbers. Rein (1996) further analyzed the possible sub-regimes during the transition from coalescence to splashing using the Weber number as a criterion.For complicated phenomena, the study of the regime boundaries remains the first priority for a lot of work in the literature. Knowledge of the transition criteria between regimes is critical for the analysis of droplet impact. Several correlations were proposed for prediction of these transitions. Bai and Gosman (1995) collected a number of results from the literature, trying to find a coherent category of the impact phenomena and their thresholds in terms of the dimensionless numbers. For wetted walls with surface temperature lower than boiling temperature, they proposed simple criteria for transitions between bouncing, coalescence and splashing. Cossali et al (1997) investigated the transition from coalescence to splashing by analyzing a large number of pictures. Their correlation was based on the Weber and Ohnesorge numbers, and the liquid fi lm thickness was normalized with the droplet diameter. From detailed experimental studies performed with visualization techniques, Mundo et al (1997;1998) found a correlation that describes the splashing-deposition (coalescence) limit of an individual droplet impacting on a surface with a defined surface

As pipeline transportation in the oil and gas industry is moving to offshore conditions, the prevalent high-pressure and low-temperature conditions in the subsea flow lines may lead to hydrate formation and wax precipitation occurring simultaneously. The presence of wax may alter the interfacial properties and particle interactions, resulting in the change in hydrate cohesion behavior. In this study, cyclopentane hydrate cohesive forces are measured with different wax contents using a micromechanical force (MMF) apparatus. A custom wax sample with the composition from C17 to C39 was mixed with cyclopentane and used as the bulk phase. It was found that the cohesive force decreased with increasing wax content from 0 to 5 wt % then increased with further wax contents from 5 to 8.75 wt %. Dilution MMF measurements demonstrated that two competitive mechanisms, the oleophilic effect and reduced hydrate conversion rate were synergistically responsible for the observed changes in the cohesive force. In an MMF measurement with 10 wt % wax and 6 h annealing period, the wax was found to deposit on the hydrate surface and effectively reduced the cohesive force, indicating that wax crystals have a potentially inhibiting effect on hydrate cohesion. Furthermore, the bulk phase/water interfacial tension decreased with increasing wax contents. Finally, a possible mechanism is presented to illustrate the effect of wax on the hydrate cohesive force, considering the oleophilic effect, hydrate conversion, and wax deposition. This work provides insight into the influencing mechanisms of wax on hydrate cohesion, which can be useful for flow assurance applications where both hydrates and waxes are present.