Blowdown Feasibility for Downhill Flowlines
Authors: Dykhno L.A. , Jayawardena S.S. , Schoppa W. 
Published: October 2008

Flowline depressurization ("blowdown") is an effective flow assurance operation to protect flowlines from hydrate formation for unplanned or extended shutdowns. Both numerical simulations and field experience have shown that blowdown is generally feasible for uphill flowlines over a wide range of gas-liquid ratios (GOR), water cuts and operating conditions. It is often assumed, however, that blowdown is not feasible for downhill-sloping flowlines, where liquid accumulated in the riser cannot be fully discharged to reduce the flowline pressure. OLGA calculations presented here indicate that blowdown is in fact successful for downhill flowlines, although in a more narrow operating envelope than for monotonic uphill flowlines. Requirements for blowdown feasibility are identified over a range of governing parameters, including fluid GOR and water cut, and flowline/riser geometry (i.e. diameter, water depth, offset length, inclination). Based on this generalized analysis, operating strategies to increase blowdown effectiveness for downhill flowlines are evaluated. The impact of terrain slugging on blowdown feasibility, which generally accompanies flowlines with downhill inclination, is also evaluated. Reasonably good agreement of computational results with available deepwater field data for blowdown is observed.

Introduction

System design and op erating procedures for subsea oil systems are usually developed to protect flowlines from hydrate plugging, because remediation of hydrate plugs is very costly and may require weeks to even months to melt a plug. Hence, the fluid temperature in the flowline is maintained above the hydrate dissociation temperature at any operating pressure. Steady state and transient thermal-hydraulics analysis is employed to determine insulation requirements to ensure operation outside the hydrate region during normal operation, as well as start-up and shut-down. For start-up operations in which the flowline or riser enter the hydrate region, temporary injection of a hydrate inhibitor (e.g. methanol or LDHI) is required. Alternatively, for dual flowline systems, circulation of a heated dry fluid (e.g. dead oil or diesel) may be used to preheat the flowlines. To protect the flowline from hydrate formation during shut-down, the elapsed time must be carefully monitored to not exceed cooldown time of the flowline and riser to hydrate formation conditions.

For some shutdown scenarios, additional operations are needed to secure the flowline against hydrate formation. For example, interrupted start-up may occur before system warmup has been achieved, or the shut-down time may exceed the flowline cooldown time, typically 8-12 hours. In these extended shutdown situations, flowline depressurization, or blowdown, can be the "last resort" to prevent hydrate formation. Alternatively, circulation of dry oil may be used to remove wet fluids in dual flowline systems. However, dryoiling is an inherently "active" operation requiring topsides power availability and sufficient oil storage. Further, flowlines are temporarily pressurized by dry-oiling (due to the riser hydrostatic head), which may increase the hydrate risk. In contrast, the blowdown operation is a "passive" operation to reduce the flowline pressure to below the hydrate dissociation pressure at ambient seabed conditions. As a result, even if the blowdown is only partially effective, operations can "buy time" for troubleshooting of the shutdown cause. Additionally, flowline depressurization - if effective - can be used as a means of hydrate remediation.