Page 92 - Handbook Of Multiphase Flow Assurance

Flow modeling 87

Specialists working on hydraulic modeling for both projects and operations offshore prefer to
use transient multiphase simulators because the production system flow path geometry includes
multiple significant changes in elevation such as wellbore and riser. Transient multiphase soft-
ware allows to build the system model once and perform many different studies for both steady
state and transient or time-dependent operations. Onshore systems specialists have a prefer-
ence for steady state tools as onshore geometry is more uniformly flat, wells can be modeled as
sources, and large networks of onshore gathering pipelines with hundreds or thousands of wells
can be more reliably modeled and more accurately converged using steady state tools.
With the advent of en-masse long-reach horizontal wells the flow geometry in onshore
production systems becomes more reminiscent of subsea production, with a wellbore vertical
section similar to an offshore riser. Transient multiphase simulator tools can accurately cap-
ture and predict flow instability such as slugging or liquid loading in horizontal wells.
Both classes of software have been extensively verified against laboratory and field data
and updated over the past decades. These tools provide sufficient accuracy in most cases.
Accuracy of commercial tools makes flow assurance engineering more routine because the
used preferred correlations limit the need for the specialty knowledge of how to set up a
model for a given system.

Erosion modeling

Erosion modeling has to be performed to ensure that the production system remains intact.
Typical flow velocities (as an approximation) should be limited to 70 m/s for gas sys-
tems and 70 ft/s for multiphase systems where liquids are present. At higher velocities the
liquid droplets may impinge on the pipe wall and affect the layer of corrosion inhibitor or
corrosion product usually present inside a carbon steel pipe, which may accelerate the rate
of corrosion.
Several methods exist which allow to predict the erosional velocity limit for fluid motion.
Some of these include:
° DNV-0501 guideline
° API 17E guideline
° NORSOK P-001
° SPPS
An overview and comparison of six models for single phase and multiphase flow is avail-
able in the literature (Parsi et al., 2016). However, each company should perform own selec-
tion and validation of an erosion modeling tool for their use.

Multiphase production problems
The more common problems associated with multiphase flow include

° Slug movement and impacts on structures
° Increased pressure drop (depends on holdup and on flow regime) and lower production
° Liquids holdup (cause for corrosion, equipment weight, need for scraping)
° Solids deposition