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• W2040642228 abstract "Summary For subsea tiebacks in ultradeep water, restarting from shut-in conditions can pose significant flow-assurance issues, particularly with respect to hydrate formation. Achievable restart rates are a function of available hydrate inhibition and shut-in conditions. The overall chemical volume required is highly dependent upon the tieback type, length, and insulation level. Current practice is to evaluate alternate procedures, such as preheating the flowline to reduce warm-up times and chemical requirements. Introduction As developments move into deeper waters, transient issues, such as restart, become increasingly important. While the system may operate with few difficulties under steady-state conditions, how the flowlines and other subsea equipment are treated during a shutdown as well as how they are brought back on line once the process upset is cleared may control the overall feasibility of the entire development. In particular, restart philosophy has a significant impact on the maximum tieback length, insulation type chosen, chemical-injection-line sizes in the umbilical, and overall topside chemical storage. During restart from a prolonged shutdown, cold fluids in the wellbore come into contact with ambient seabed temperatures at the mudline. Depending on the hydrate-mitigation strategy during shutdown, the flowline may be at elevated shut-in pressures and filled with treated production fluid, untreated (live) production fluid, or degassed/dewatered crude oil. To prevent hydrate formation during restart, the live production fluids in the wellbore need to be treated with hydrate inhibitor as they enter the flowline. While the wellbore typically warms relatively quickly, chemical injection must be continued until the entire flowline/riser is out of the hydrate-formation region. For long subsea tiebacks or particular insulation scenarios, such as a buried pipeline, the time for the entire flowline to warm up to greater than hydrate-formation temperatures can be significant. This time directly impacts the overall chemical consumption and the required chemical-storage volumes. Field Layout The hypothetical development under consideration consists of a black-oil reservoir located at a water depth of 7,000 ft. The reservoir fluid has a base gas/oil ratio (GOR) of 800 scf/STB. The reservoir conditions vary during field life, but a temperature and pressure of 180°F and 4,000 psia, respectively, are used for the majority of the results presented here. Two wells are drilled into the reservoir, both of which produce into a single 8-in. flowline. The total production rate (two wells) ranges from 5,000 to 30,000 STB/D, and the separator operates at a pressure of 550 psia. Flowline lengths range from 3 to 15 miles, with a gradual 1° upslope from the wellhead to the riser touchdown point. Several different insulation scenarios were considered for the flowline.Conventional insulation.Pipe-in-pipe insulation.Flexible pipe (no external insulation).Buried pipe.Microporous insulation in pipe-in-pipe. The warm-up temperature profile for the system during restart is determined by the production rate and by the heat-transfer properties of the flowline/riser. The overall heat-transfer coefficient (U-value) of the flowline is a measure of the ability to transfer energy and is related to the total thermal resistance. Eqs. 1 and 2 give the total energy transfer for a system as a function of U-value, area, and temperature gradient. First, as the flow rate increases, the amount of energy input into the system increases. During restart, the higher production rates result in the shortest warm-up times because of the increased energy input. Second, the temperature increases fastest for those configurations that maintain the most thermal energy within the production fluid rather than losing it to the surroundings. The lower U-value configurations lose less heat to the surroundings and, thus, maintain thermal energy in the fluid, expediting the warm-up. The overall U-values for the various configurations considered are given here and represent the general technological limits for each type of insulation considered:Conventional insulation: 0.55 Btu/hr-ft2 - ° F.Pipe-in-pipe insulation: 0.25 Btu/hr-ft2 - ° F.Flexible pipe: 1.00 Btu/hr-ft2 - ° F.Buried pipe: 0.70 Btu/hr-ft2 - ° F.Microporous insulation: 0.09 Btu/hr-ft2 - ° F. The overall U-value is a function of the various materials used in the flowline/riser design and their respective properties (namely, thermal conductivity). In addition, the amount of thermal mass for each configuration plays an important role in determining the time required to warm up the system. The thermal mass is a function of the density, thickness, and heat capacity of the various materials and is a measure of a system's ability to retain/store heat. While this is beneficial during a shutdown, the greater thermal mass can prolong warm-up times because there is more physical material to heat up, taking energy away from warming up the production fluid. The thermal mass of each configuration is addressed in a subsequent section. The combination of the U-value and the thermal mass impacts the thermal behavior during restart, and these can actually be competing effects in many cases. Chemical Requirements/Deliverability While higher and faster restart flow rates are preferable with regard to expediting warm-up times by increasing the amount of energy input into the flowlines, the production fluids from the wellbore must be inhibited against hydrate formation. Sufficient chemical delivery must be available to treat the fluids at the wellhead during restart. Chemical availability often constrains restart rates. Fig. 1 illustrates the hydrate inhibitor (methanol) dosage rate required to inhibit the fluid at a seabed temperature of 40 ° F and a range of shut-in pressures. Inhibitor must be injected in sufficient quantities to treat the water as well as to take into account the inhibitor partitioning into the vapor and hydrocarbon liquid phases. The figure shows that the methanol dosages range from 0.40 to 0.60 bbl MeOH/bbl H2O. For a higher-gas-content fluid, the methanol dosage may be significantly higher, on the order of 1 to 2 bbl MeOH/bbl H2O. A low-dosage hydrate inhibitor may be considered at high water cuts to reduce the overall chemical volumes, but this technology is not as proven as methanol, and there are other flow-assurance and chemical-degradation aspects to consider as well." @default.
• W2040642228 created "2016-06-24" @default.
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• W2040642228 date "2004-05-01" @default.
• W2040642228 modified "2023-09-26" @default.
• W2040642228 title "Thermal Behavior During Restart of Ultradeepwater Flowlines" @default.
• W2040642228 doi "https://doi.org/10.2118/88443-pa" @default.
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