Page 219 - Handbook Of Multiphase Flow Assurance

218 9. Flow assurance integrity issues

Types of corrosion
A large amount of NACE literature on corrosion exists. General details on corrosion are
available (Fontana 1975; Dillon 1982).
Two key categories of corrosion are:
physico-chemical corrosion.
microbially influenced corrosion.
Some of the common types of physico-chemical corrosion include:
Uniform—caused by electrochemical reaction leading to dissolution of metal;
Galvanic—may occur due dissimilar weld material and pipe material;
Pitting—localized corrosion enhanced by localized difference in ion concentration;
Crevice—when a gap between pipe wall and another material such as flow assurance
deposit creates a localized difference in ion concentration;
Intergranular—when metallurgy has dissimilar grains of metal present;
Stress corrosion cracking—due presence of chloride ions released from fluid. May occur in
stainless steels such as chemical injection systems or sour service systems (Fischer et al., 2016).
Hydrogen embrittlement—may occur due hydrogen evolution in the system and ingress
into metals due to its small molecule size.
Microbially influenced corrosion (MIC) affects the rate of corrosion processes due to a bio-
film formation on a surface of pipe.
MIC may occur in crude pipelines near the locations of water holdup in low spots, which
relates the MIC to flow assurance hydraulic analysis.
A recent overview of MIC in petroleum systems is provided by Al-Saleh et al. (2011).

Corrosion monitoring methods

There are several methods in the operations to monitor corrosion rate listed:
Weight loss coupons.
Electrical resistance probes.
Linear polarization resistance (LPR).
Field signature method (FSM).
Electrochemical noise.
Flexible UT Mats.
Ultrasonic pipe thickness measurement.
ILI in-line inspection with magnetic flux leakage intelligent scraper tool.
Radioactive methods.
Indirect monitoring may also be done by
Ultrasonic sand detection.
Process stream analysis.
Corrosion monitoring can provide data for tuning of the integrated multiphase flow and
corrosion models. An overview summary of corrosion monitoring methods is available in
Hedges and Bodington (2004).