For subsea tiebacks in ultradeepwater, restarting from shut-in conditions can pose significant flow assurance issues. For both planned and unplanned shutdowns, live fluids from the wellbore will come into contact with a cold flowline and can form hydrates during restart since the conditions at the mudline are often well within the hydrate formation region. As the water depth increases, the potential for even higher shut-in pressures exists, exacerbating the probability of hydrate formation.
The achievable restart flowrates are directly related to the available inhibitor rates. Ultradeepwater systems often require high hydrate inhibitor (i.e. methanol) dosage rates to protect the flowline from hydrate formation. In general, the temperature warm-up trend is exponential and is a function of the amount of energy input into the system. If the restart flowrate is too low, limited by inhibitor availability, the temperature may slowly approach a condition at which the system is free from hydrate formation and ultimately require a significant inhibitor volume. For these low restart flowrates, the time to reach conditions above the hydrate formation region can be very significant or the flowline may never warm up above hydrate conditions. At high restart flowrates, the flowline warms up quickly, but the inhibitor delivery rates are high, often significantly higher than those required for a similar development in shallow water. There is a trade-off between inhibitor deliverability (line size) and consumption (storage) that must be accounted for at the design stage of a project when considering how to restart the system.
Operational procedures such as preheating the flowline with dead (de-gassed/de-watered) crude oil to warm up the pipe walls before restarting the wells have been suggested as a means of expediting the overall flowline warm-up. Preheating the pipe walls maintains the thermal energy in the production fluid and reduces temperature losses associated with the fluid warming up the pipe walls. The effectiveness of this technique, while potentially reducing the warm-up time, may again be limited by the allowable restart flowrates.
As developments move into deeper waters, transient issues such as restart become increasingly more 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, the insulation type chosen, chemical injection line sizes in the umbilical, and the overall chemical storage topsides.
During restart from a prolonged shutdown, cold fluids in the wellbore will 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, un-treated (live) production fluid, or de-gassed/de-watered crude oil. The live production fluids in the wellbore need to be treated with hydrate inhibitor as they enter the flowline in order to prevent hydrate formation during restart. While the wellbore typically warms up 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 above hydrate formation conditions can be significant. This time directly impacts the overall chemical consumption and the required chemical storage volumes.