## 3 Wax Deposition in Multiphase Flow  
In comparison to single-phase, literature data on two-phase wax deposition studies (e.g., gas-oil and oil-water) are limited due to the complexity of the experiment. In fact, there is no published study on three-phase gas-oil-water paraffin deposition due to various variables that need to be controlled and considered. Nevertheless, there is a need to extend the study of wax deposition under multiphase flowing conditions since the actual crude oil transportation via subsea pipelines could encounter multiphase flow conditions which lead to different in flow patterns [48; 49].  
Figure 2i,ii [50] shows two categories of flow pattern which resulted from the inclination or elevation of the crude oil pipeline. For a horizontal pipeline, the flow patterns are bubbly flow, wavy flow, intermittent flow (plug and slug), and annular flow, as shown in Figure 2i. Meanwhile, flow patterns in a vertical pipeline are bubbly flow, intermittent flow (slug and churn), annular flow, and annular mist, as shown in Figure 2 ii. In general, wax deposition distribution depends on the position of the pipeline, the flow pattern, and the multiphase system (such as the gas-oil system or the oil-water system).  
A limited database can be found for wax deposition studies in gas-oil two-phase flow conducted by researchers worldwide [48; 49; 51; 52; 53]. Figure 3i,ii shows the variation of wax deposit distribution for a gas-oil system, based on the pipeline position and flow pattern. It is reported that wax deposited around the circumference of the pipeline (i.e., the internal pipeline wall) with different thickness and hardness when the pipeline position is vertical. It is notable that the flow pattern has no major effect on the shape of the deposited wax. A similar observation has been reported when the pipeline position is horizontal. For horizontal stratified flow, the deposit was distributed in a crescent shape and was found to be softer and thicker. Meanwhile, for horizontal intermittent flow, a harder deposit was produced than that of stratified flow, while the wax hardness for annular and intermittent flow was similar.  
In terms of increasing gas velocity, the wax deposit thickness in gas-oil two-phase flow was reported to be thicker [48]. In contrast, the wax deposit thickness in single-phase flow was reported to be thinner at increasing flow velocity [16]. The equivalent wax deposition in gas-oil two-phase flow depends on of two factors: the deposition-promoting and the deposition-inhibiting factors. The deposition-promoting factor is an increase in heat transfer or thermal gradient due to the increase in the crude oil velocity simultaneously as the gas velocity increases. In contrast, the deposition-hindering factor is attributed to the surface area of the crude oil pipeline which is in-contact with the oil when the liquid holdup decreases as the gas velocity increases. Few studies have found that the deposition-hindering factor is amplified for bubbly, wavy, and intermittent flow patterns [49; 54].  
Figure 2: Flow patterns in pipelines. (**i**) Flow patterns of near-horizontal pipeline [50]. (**ii**) Flow patterns of near-vertical flow [50].