Experimental Study and Prediction Model for Hydrate Plugging Formation in Single-Pass Gas-Dominated Pipes with Diameter Reduction

Summary Gas hydrate plugging in flowlines acts as a major blockage risk in oil, gas, and natural gas hydrate production. Current studies on hydrate plugging is mainly conducted in pipes with a constant diameter, whereas the effects of varying diameters have been less explored. Pipes with diameter reductions are very common in the oil and gas production process. Herein, by performing experiments with pipes of four different sizes, including one with a constant diameter and three with diameter reductions, the hydrate plugging in single-pass gas-dominated pipes with diameter reduction is investigated systematically, the results of which show that the existence of diameter reduction can facilitate the deposition of hydrate particles on pipe walls and the formation of a hydrate deposition layer. Meanwhile, hydrate sloughing occurs during the growth of the hydrate deposition layer under fluid shear force. With the increase in the diameter reduction ratio or subcooling for hydrate formation, the hydrate deposition is observed to increase significantly, thus resulting in the reduction of time for hydrate plugging. According to these results, the hydrate plugging mechanism in pipes with diameter reduction is proposed. Further, in combination with the hydrate deposition difference between the inside and outside of the arc-shaped low-speed area, a new numerical model is established for hydrate plugging prediction in the pipe with diameter reduction, which can predict the growth and evolution of the hydrate deposition layer accurately with the consideration of the diameter’s reduction. The results of this work provide a valuable guidance for the targeted prevention and management of hydrate plugging in flowlines with diameter reductions.

Flow Assurance in Deep-Water Gas Pipelines: Locating Hydrate Plugging Position in a Fast Way

Hydrate-associated issues are of great significance to the oil and gas sector when advancing the development of offshore reservoir. Gas hydrate is easy to form under the condition featuring depressed temperature and elevated pressure within deep-water gas pipeline. Once hydrate deposition is formed within the pipelines, the energy transmission efficiency will be greatly reduced. An accurate prediction of hydrate-obstruction-development behavior will assist flow-assurance engineers to cultivate resource-conserving and environment-friendly strategies for managing hydrate. Based on the long-distance transportation characteristics of deep-water gas pipeline, a quantitative prediction method is expected to explain the hydrate-obstruction-formation behavior in deep-water gas pipeline throughout the production of deep-water gas well. Through a deep analysis of the features of hydrate shaping and precipitation at various locations inside the system, the advised method can quantitatively foresee the dangerous position and intensity of hydrate obstruction. The time from the start of production to the dramatic change of pressure drop brought about by the deposition of hydrate attached to the pipe wall is defined as the Hydrate Plugging Alarm Window (HPAW), which provides guidance for the subsequent hydrate treatment. Case study of deep-water gas pipeline constructed in the South China Sea is performed with the advised method. The simulation outcomes show that hydrates shape and deposit along pipe wall, constructing an endlessly and inconsistently developing hydrate layer, which restricts the pipe, raises the pressure drop, and ultimately leads to obstruction. At the area of 700m-3200m away from the pipeline inlet, the hydrate layer develops all the more swiftly, which points to the region of high risk of obstruction. As the gas-flow rate increases, the period needed for the system to shape hydrate obstruction becomes less. The narrower the internal diameter of the pipeline is, the more severe risk of hydrate obstruction will occur. The HPAW is 100 days under the case conditions. As the concentration of hydrate inhibitor rises, the region inside the system that tallies with the hydrate phase equilibrium conditions progressively reduces and the hydrate deposition rate slows down. The advised method will support operators to define the location of hydrate inhibitor injection within a shorter period in comparison to the conventional method. This work will deliver key instructions for locating the hydrate plugging position in a fast way in addition to solving the problem of hydrate flow assurance in deep-water gas pipelines at a reduced cost.

Thickness Investigation and Prevention Strategy of Hydrate Deposition During Avoidance Typhoon for Deep Water Gas Well

The investigation of hydrate deposition is a significant basis for the hydrate prevention strategy during avoidance typhoons for deepwater gas wells. Shut-in operation is a necessary procedure due to avoidance typhoon, the hydrate intrinsic kinetic potential increases the growth rate under shut-in conditions. We establish the model of hydrate thickness investigation and the simulations illustrate that the hydrate thickness layer is relatively minor during avoidance typhoon. Meanwhile, hydrate is aggravated by the environment of low temperature and strong disturbance near the mud line due to production after avoidance typhoon. We discuss the variation of temperature profile and concentration of ethylene glycol during restart operation. The concentration variation of the ethylene glycol on the induced flow show after restart operation that the hydrate inhibitor satisfies the hydrate prevention requirements. This research provides a significant design for the hydrate management program during avoidance typhoon for deepwater gas wells, which is useful to the development of traditional avoidance strategy and decreased the cost of hydrate inhibitors.

Understanding the Impacts of Backpressure & Risk Analysis of Different Gas Hydrate Blockage Scenarios on the Integrity of Subsea Flowlines

Offshore oil and gas pipelines are subjected to high pressure and high temperature (HP/HT) from the inner hydrocarbon content during operation. Both the rise in temperature and internal pressure may cause longitudinal expansion of the pipeline. This expansion is restrained or semi-restrained by the pipe end devices and the soil which results in build-up of compression stresses in the pipe wall. These pipelines are also exposed to so many familiar and unfamiliar forces related to static, dynamic and environmental forces. This study presents a thorough review of various sources from literature on the integrity challenges of subsea flowlines and pipelines amid challenging operating conditions especially with regards to flow assurance. This paper evaluates the impact of hydrate deposition and agitation on the overall integrity of the subsea flowlines, riser-base and fitting e.g. elbows, valves e.t.c. A bow tie model was developed to determine the threats, causes, consequences, the top event and the impending hydrates that are to be designed and cause blockage and failure. Stress analysis were done with finite element tools which are ANSYS and Autodesk INVENTOR with only the hoop, Von Mises stress and the corresponding back pressure that occurred with the scenario of 0, 10,30,50,70,90 and 100% blockage of flowlines being analyzed and taking the 0% or null blockage as the pilot case with no hydrate formation. The result gotten from both results were validated with hand calculation with excel and the initial design values for the stress values before the initial operation of the wells after the first commissioning. In addition, HAZOP was done to understand the inherent risk involved in all the cases under study and results gotten would serve as a tool of precautions to operators and stakeholders in period of adversity when facing similar scenario.