Page 75 - Handbook Of Multiphase Flow Assurance

70 4. Hydraulic and thermal analysis

Analysis provides gas and liquid flow velocity upper boundaries for erosion, flow induced
vibration, and thermal material limits for JT expansion cooling (e.g. gas in Deepwater riser).
No credit should be taken for ability to monitor erosion in a dry tree vs subsea tree sys-
tem and the erosional analysis should provide the same gas and liquid flow velocity upper
boundaries. Guideline chosen for the project needs to be used for erosion engineering anal-
ysis. There are several erosion guidelines available such as DNV-0501, API-14E or NORSOK
P-001. Additional models such as SPPS are also used for analysis. A summary overview of the
models is presented by Arabnejad et al. (2015).
Hydraulic multiphase flow analysis needs to be aligned with surface or topsides process
design procedures to make sure the flow rates at the outlet of the production system are
compatible with the flow rates at the intake of the process system. A slug catcher may act as
a surge suppressor if the production system output may have sudden high flow rates such
as during scraping or severe slugging. An overview of occurrence and solutions for severe
slugging is presented by Montgomery (2002).
A turndown analysis indicates what lowest flow rate is sustainable in the production sys-
tem. Deliverables of a turndown sensitivity study relative to the design production profile
indicate when produced fluid flow remains above slug flow onset, and when start-up or
ramp-up surge volume exceeds the facilities process capacity.
Analysis also provides updated steady state temperature and pressure profiles through
time, including turndown sensitivity relative to the design production profile.

Overall design
Overall design for hydraulic management needs to rely on using proven technology to
avoid holdup accumulation irreversible by normal operating procedure. Sensitivity to pro-
duced water cut should include an assumption for planned and deferred water injection for
pressure support, based on reservoir simulation production profiles.
Operations with flow instability may include producer or injector well transient opera-
tion such as cold or warm start-up, planned or unplanned shut-in, and steady operation.
During FEED phase, at least steady operation and start-up surge analyses need to be
performed. Depressurization analysis should be performed if hydrate management relies
on it.
Mitigation of severe slugging and liquid surges may include topsides choking, riser
base gas lift, sufficiently sized slug catcher, subsea pumping, pressure management, etc.
Use of novel methods such as actively controlled topsides choke valve for riser severe
slugging is growing in acceptance in Deepwater and may be considered as technically
mature.
Remediation methods used to control hydraulic losses may include scraping to sweep
accumulated liquids, drag reducing agent chemical injection, or foamer to control liquid
inventory. Mechanical removal of liquids with swabbing has not been widely used in
Deepwater, but has been used successfully in onshore wells, and may be applicable to dry
tree risers in late life. Mechanical methods to reduce flow line cross section such as veloc-
ity strings or artificial lift such as ESP or multiphase pumps to propel liquids may be used
both in onshore and offshore production. Combinations of the methods may also be used
such as an ESP on a coiled tubing velocity string in onshore wells or in offshore risers.