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Deadlegs are defined as pipe sections in intermittent use for production or special services in oil/gas production systems. Due to the absence of flow, deadlegs are commonly much colder than the main flowlines or the process equipment to which they are connected. Therefore, water vapor tends to condense on the pipe wall of deadlegs, resulting in a potential risk for hydrate deposition. Over time the deadleg may be blocked completely by hydrates. Proper management of hydrates in deadlegs is critical, for both economical and safety reasons. To better address the remediation strategies in design and operation to mitigate the formation of hydrate deposits in vertical deadleg systems, we established an experimental protocol with gas-filled vertical deadlegs of various ratios of length over inner diameter (Lpipe/ID). The pipes have inner diameters of 2, 3, or 4 in. Each pipe system has five temperature-controlled sections. Water is supplied from the header at the bottom dominantly by natural convection. Experiments in each of the systems provide quantitative results for the center temperature profile, the hydrate growth in the top section through visual observation, the amount of water recovered, and the hydrate deposit distribution (inspected via borescope). On the basis of the results, we discuss the effects of inner diameter (ID), pipe length (Lpipe), and Lpipe/ID. The results also show a possibility of scaling results (the hydrate deposit distribution and plug position) over the pipe size by using the characteristic length Lpipe/ID.

Deadlegs in oil and gas production systems often encounter hydrate plugs by deposition. Temperature is generally known to be an important variable in hydrate formation, but the effects in deadlegs are not exactly known. This study focuses on the effects of the header temperature on the hydrate deposition in gas-filled vertical deadlegs at constant wall temperature. All experiments are conducted with a methane/ethane gas mixture at constant pressure. The pipe wall temperature is kept constant while considering different header temperatures. The tests show that the header temperature has a significant impact in the hydrate deposit growth rate and distribution in the deadleg. It is also found that the hydrate deposit can, in turn, change the temperature field inside the pipe. The header temperature or the pipe temperature field can be used to estimate the hydrate distribution in the deadleg. Under the right conditions, hydrates can form a restriction in the deadleg and its location is usually close to the boundary of a hydrate-stable region. The location of the restriction can be correlated to the header temperature. At 80 °C, the location is estimated to be 15–18 ID, and at 30 °C, the location is estimated to be 9–12 ID. The results of this study contribute to the understanding of the hydrate deposition mechanism in deadlegs.

Deadlegs are the pipe sections used for specific services in production and transportation of oil and gas, and they often encounter hydrate management challenges. Despite stagnant fluids in deadlegs, warm water vapor readily condenses on the cold pipe wall, resulting in a risk of hydrate blockages by deposition. Proper management of hydrates in deadlegs is therefore required for economic and safety reasons. Here, we discuss the development of an 1-in. vertical pipe system that is designed to study hydrate deposition from water saturated gas. From a series of hydrate formation and dissociation, the hydrate deposits are characterized to obtain gas/water consumption, thickness/volume hydrate deposit distribution, hydrate morphology, and hydrate porosity and wetness. These characteristic properties are correlated with the header temperature and the time duration for hydrate deposition. Qualitative and quantitative information obtained from the present study contribute to our understandings of hydrate deposition and give insight into establishing management strategies to avoid or minimize the risk of hydrate deposition in deadlegs in oil and gas production and transportation systems.