Page 74 - Handbook Of Multiphase Flow Assurance

Hydraulic restrictions boundaries and management 69

of the production chemical. Normally the chemical backpressure is controlled by a chemical
injection valve, but a valve set for operation in early life at initially higher wellhead pressure
may need to be recalibrated for late life lower pressure, and if that maintenance is deferred
the chemical may experience vacuum conditions at the top of the umbilical riser, causing
carrier solvent in the chemical to vaporize and leaving the viscous or solid active ingredient
residue in the chemical tube gradually blocking it.
These static hydraulic restrictions such as a holdup may be easier to plan for than dynamic
flow resistances such as hydrodynamic slug flow because static restrictions are caused by
localized conditions such as pressure, temperature or geometry and thus can be designed
out of the planned system. Dynamic restrictions emerge during the course of production,
and planning of the optimum balance of the production system capacity versus its cost is the
main focus of hydraulic analysis in flow assurance which relies on the anticipated production
flow rates derived from reservoir modeling that carries inherent uncertainty. The same may
be said about hydraulic design of production chemical injection systems.

Scope
Hydraulic assessment and management has the principal priority in flow assurance work
scope. The flow assurance analysis relies on fluid properties from reservoir model or laboratory
data and on a production profile from a recent reservoir model, and latest surface geometry or
subsea bathymetry. Liquid holdup accumulations forming a hydraulic restriction to flow and
erosional velocity limits for liquid and gas production need to be evaluated. For a tieback to
an existing facility, matching of hydraulic and insulation performance should be included to
calibrate the effective roughness and thermal insulation U-value based on existing operations
data from operations and third party engineering vendor experience. Note that the theoretical
roughness and insulation values of a system provided by vendors may be more optimistic than
field performance under subsea or onshore operation. Actual insulation performance may be
affected by flooded sections in pipe-in-pipe insulation or water ingress into the wet polymeric
insulation. Actual roughness is affected by corrosion and erosion. Both insulation performance
and roughness become worse with time.

Hydraulic analysis deliverables
The engineering analysis needs to identify when the system selected in early design oper-
ates with a hydraulic restriction or excessive velocity threats, quantify the duration of system
operating with flow instability, and provide a design for the selected one or more technically
mature technologies to mitigate the identified threats and to remediate the hydraulic restric-
tion, flow instabilities or surge.
Deliverables of hydraulic analysis include the recommended in-field flowline network and
export pipeline sizes with updated corrosion and erosion allowances to permit maximum
uninterrupted production during the life of field, maximum flow velocities for produced
fluids, mitigation method and intervention frequency for liquid inventory management in
flowlines based on transient modeling of scraping and estimated based on historic analogs,
liquid surge capacity for onshore or topsides facilities sizing, chemical injection requirements
(location, dosage, storage, compatibility with materials and with other chemicals) if a chemi-
cal is used to manage liquid inventory (such as a foamer).