Page 220 - Handbook Of Multiphase Flow Assurance

Corrosion 219

Currently used corrosion control techniques
Corrosion prevention is accomplished by several methods simultaneously: use of corro-
sion inhibitor chemicals, use of maintenance scraping to remove water holdup from the pipe-
lines, use of corrosion-resistant materials.
Corrosion inhibitor chemicals are among the most widely used methods of corrosion
methods. Chemicals performance is evaluated in laboratory by tests and in field by in-line
inspection.
Lab methods aim to measure:
Inhibitor efficiency;
Inhibitor partitioning behavior between hydrocarbon and water;
Compatibility with other production chemicals;
Film stability and persistence;
Optimum concentration.

Inhibitor efficiency may be measured with:
Bubble test apparatus;
Rotating electrode, including rotating disk (laminar flow) and rotating cylinder
(turbulent flow, high shear);
Jet impingement test;
Recirculating flow loop, which allows to make tests at a controlled shear stress;
In-line inspection (ILI) uses magnetic flux leakage probes installed circumferentially on a
ILI tool which records wall thickness along the whole perimeter and length of the tested pipe.

Integrated models
Multiphase flow models can be combined with corrosion rate models to provide an inte-
grated assessment of the expected corrosion rates for a given flow scenario. There are com-
mercial tools available which allow to estimate corrosion rate.
Alternatively it is possible to find multiphase flow parameters and then use these in the
corrosion rate prediction model. Commercial multiphase flow simulators have modules of
published corrosion models such as NORSOK M-506, deWaard-95 and Top-Of-the-Line-
Corrosion (Wang and Nesic, 2003). The first two are for CO2 based corrosion, and the TOLC
is for condensed fresh water corrosion. The use of these built-in modules for corrosion rate
assessment may be limited if corrosion engineers at operator companies develop and main-
tain in-house corrosion rate prediction models. The limitation for the use of multiphase tools’
corrosion modules for corrosion rate prediction is the limited ability to tune the model input
parameters. It may be useful as an initial check of the corrosion rate. Nonetheless, multi-
phase flow modeling tools are very useful and indispensable in analyzing the two parame-
ters which are required by the corrosion specialists: thermal conditions of flowing fluids and
shear rates exerted by fluids on pipe wall. Both of these are parameters used in the in-house
models. Temperature determines both the corrosive species’ diffusion and corrosion reaction
rate and the condensation rate of fresh water. Shear affects the corrosion inhibitor layer and
the protective corrosion product layer. Some operators, to derive the desired shear and tem-
perature distribution, as well as flow regime, attempt to couple the in-house corrosion rate