Modelling ice and wax formation in a pipeline in the Arctic environment

Highlights

• In a pipeline, if the oil temperature is below freezing temperature, ice may form in addition to wax formation.

• Ice and wax formation can lead to pipeline blockage, component failure, and release of hazardous liquid.

• Ice and wax deposition rates are required to be estimated to prevent the plugging issue.

• A fundamental model for both ice and wax deposition is proposed using the first principles of heat and mass transfer.

Abstract

In the Arctic environment, the fluid temperature in the pipeline can drop below the freezing point of water, which causes wax and ice to form on the pipeline surface. Solid formation on the pipeline surface can lead to flow assurance and process safety issues, such as blockage of the pipeline, pipeline component failure, and release of hazardous liquid. Remediating the plugging requires a shutdown of pipeline operation, which incurs tremendous cost and delays the entire production system. In order to prevent blockage, the pigging operation can be used to remove the deposits on the pipeline surface on a regular interval. Ice and wax depositions in the pipeline are a slow process. However, if the deposition grows too thick, pipeline blockage can still occur after pigging operation. So, ice and wax deposition rates are required to be estimated accurately. This paper investigates ice and wax deposition rates in a 90,000 m pipeline. A fundamental model for both ice and wax deposition is proposed using the first principles of heat and mass transfer.

Introduction

The Arctic is the region within approximately 66° North parallel, including parts of Alaska (USA), Canada, Greenland (Denmark), Iceland, Sweden, Norway, Finland, and Russia (Johnstone, 2014). The existing fields beyond the Arctic Circle account for 10% of the world's existing conventional resources (Gautier et al., 2009). The hydrocarbon potential of the Arctic region is considered as the final frontier for conventional hydrocarbon development. In order to ensure continuity in oil supply to global energy needs, oil companies led oil and gas exploration and production activities in a harsh environment including extremely low temperatures (Kaiser et al., 2016).

Process safety and flow assurance challenges exist due to the harsh operating conditions in the Arctic (Khan et al., 2015). In addition to common flow assurance challenges, such as wax, hydrates, and corrosion issues, water or ice can also be a problem (Xu et al., 2018). In the low-temperature conditions, even traceable amounts of water in pipeline can cause ice formation, leading to pipeline blockages and associated risks. For example, it was reported that ice plugging delayed the restart of the Poplar pipeline system gathering crude oil from Montana and North Dakota (Sunne, 2015). The potential risk of ice formation also drew the attention of Trans-Alaska Pipeline Systems (TAPS) (Alyeska Low Flow Impact Study Team, 2011). The possible declining throughputs in the pipeline can decrease its oil temperature below the water freezing point. Ice formation can lead to restricting flow and plugging of a pipeline. Fig. 1 show pipeline flowable area were significant reduced by depositions. The reduced flowable area can increase frictional loss and lead to no flow and blockage. If the flow rate is maintained as constant, the pressure required to deliver the fluid increases. When the required pressure is greater than the pressure available or the line MAOP, production halts and the pipeline get blocked.

Blockage of pipeline due to flow assurance solids is one way of pipeline fault, which can lead to process safety issues (Datta and Sarkar, 2016). Between 1991 and 1998, 16 cases of hydrate and 39 paraffin wax blockages in flowlines were reported (Makogon, 2019). Makogon (2019) summarized cases where blockage caused leak, leak leading to blockage, or a projectile movement causing human life losses. In one incident on February 18, 1957, hydrate blockages in a Europe pipeline caused pipeline ruptures. The remediation of the blockage lead to the projectile movement of the plug, which caused multiple injuries and eight fatalities. Ice deposition and blockage also can be a problem. It was reported that an ice blockage caused a 24-in long rupture (Alaska, 2010). It was estimated that about 1091 barrels of oily material was released. Wax build up can also lead to a process safety event. Pigging is usually used to mechanically remove the deposition. In order to safely operate the pigging method to mitigate deposition, it is required to know the deposition rate and thickness. If the deposition amount is not well calculated, the pigging operation may end up with a stuck pig in the pipeline. The remediation of a plug may be costly and can lead to leaking, environmental issues or even field abandonment (Comfort et al., 2008; Huang et al., 2011). In another example, Staffa 8-inch flowline was blocked due to a combination of wax and hydrates and the field wax eventually abandoned as a result of the unremovable blocks (Makogon, 2019). Another record by US Minerals Management Service shows a wax build-up contributed to a process event (Makogon, 2019). Paraffin plugged the ceramic saddles in the still column of the glycol reboiler and caused a fire.

The current study focuses on two solids in the pipeline: wax and ice. Wax deposition is an existing flow assurance issue for pipelines in a cold environment, such as the Arctic or subsea. Ice deposition is a potential issue for a pipeline in the regions, where environmental temperature can drop below the freezing point of water. When hot oil flows along the pipeline in cold regions, such as subsea and Arctic regions, heat is lost through the pipe wall to the cold surroundings. The decreasing oil temperature leads to the decreasing solubility of the heavier components in the bulk oil (Han et al., 2010) resulting in potential wax deposition. Deposited paraffin, which is a porous medium and immobile, may contain gums, resins, asphaltic material, crude oil, sand, silt, and water (Burger et al., 1981; Noll, 1992).

A variety of theories were proposed to describe the wax deposition process, including molecular diffusion, shear dispersion, Brownian diffusion, and gravity settling (Burger et al., 1981). In a pipeline under cold environment, centerline temperature is usually higher than wall temperature. As wax solubility at the center is higher than that near the wall, molecular diffusion causes wax molecules to move from high concentration are (i.e., centerline) to low concentration area (i.e., the wall). In addition to the growth of wax deposition layer, the wax molecules can diffuse within the deposition layer, increasing wax fraction in the deposition layer, which is so-called “wax aging” (Aiyejina et al., 2011). Wax deposition models based on molecular diffusion were proposed to calculate the location and the growth rate of the wax layer, which involves mass transport of wax molecules from the bulk into the deposit (Lee, 2008; Matzain, 2002; Panacharoensawad and Sarica, 2013; Ravichandran, 2018; Singh et al., 2000; Venkatesan, 2004; Zheng et al., 2017). Matzain (2002) proposed a model to predict the growth of deposit, incorporating the equilibrium model and shear reduction mechanisms. Singh et al. (2000) proposed a wax deposition model based on complete super saturation of mass transfer boundary layer. A model by Venkatesan (2004) is based on complete precipitation in boundary layer. Another model proposed by Lee (2008) is based on partial super saturation in the boundary layer. Zheng et al. (2017) investigated the effect of non-Newtonian characteristics on wax deposition. On the contrary, later experimental studies shows that the contribution from shear dispersion is insignificant compared to molecular diffusion and hence, can be neglected (Azevedo and Teixeira, 2003; Singh et al., 2000). For this reason, current study considers molecular diffusion to be the primary mechanism for wax deposition.

The second issue addressed in the current study, modelling of ice formation and its deposition on a surface, has been widely studied in meteorology, aviation, and refrigeration industries (DeMott et al., 1983; Kim et al., 2014; Messinger, 1953; Myers and Charpin, 2004; Niezgoda-Żelasko and Zalewski, 2006a; Walker, 2002). In meteorology, hoar frost, rime, or glaze ice can form on a surface depending on the environmental temperature and the state of water (Walker, 2002). Modelling the growth rate of one ice morphology needs to consider its forming conditions. For example, Hoar frost is due to the sublimation of water vapor on a surface, and thus molecular diffusion is the main forming mechanism. Other morphologies need to take consideration of water films on surface or droplets in the ice formation process. In aviation industry, aircraft icing can pose a serious threat to flight safety (Cooper et al., 1984). The modelling of aircraft icing considers airflow, water droplet trajectories, as well as surface growth modes (Myers and Charpin, 2004). Although few published works studied ice formation in hydrocarbon flow lines, experimental studies and numerical simulation of ice slurry flow in the pipe are widely reported in refrigeration industries (Grozdek et al., 2009). Models for ice slurry flow considers the factors such as compositions of ice slurry, pipe shape, and concentration (Chhabra and Richardson, 2011; Niezgoda-Żelasko and Zalewski, 2006b).

The objective of this study is to model ice and wax depositions in a liquid hydrocarbon and low water cut system. Molecular diffusion is considered as the formation mechanism for wax deposition, ice deposition, and deposition of wax and ice mixture. The interaction between ice and wax in the deposition layer is also studied.