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At
level 1, the initial set of applicable materials is obtained
by determining the application for which the CRA is to be selected.
If the application is not known, all classes of materials known
to the system become part of the solution set. These include the
following classes of materials:
Stainless Steels: Conventional austenitic, high alloy austenitic,
martensitic, precipitation hardened, duplex and ferritic.
Precipitation hardened and solid solution nickel-base alloys,
nickel copper alloys.
Cobalt-base, titanium and zirconium alloys.
The requirements
(or data) at this level include yield strength, material condition
(heat treatment/cold work) and hardness limitations. At this level,
the requirements operate independently and hence are applied sequentially.
For example, a material in a required yield strength (SMYS) range
(say, 110-130) can also be cold worked. Here, materials available
in the required yield strength range are selected prior to applying
the material condition (cold worked) requirement.
At
level 2, the environment is characterized in terms of severity
for general corrosion using the following environmental parameters:
 |
H2S
and CO2 partial pressure |
|
Bicarbonates |
|
pH |
|
Minimum
operating temperature |
|
Chlorides
and Sulfur |
|
Water
to gas ratio and gas to oil ratio |
|
Operating
Temperature |
The
parameters used in the material evaluation based on the environment
operate concurrently. An environmental severity index, based on
the environmental parameters, is computed to represent the synergistic
effect of the different parameters operating in concurrence. The
environmental severity index is determined based on the pH, chloride
content and the H2S partial pressure of the environment.
The program determines the pH based on acid gas partial pressures,
bicarbonates and temperature. Typical environmental variables and
corresponding severity indices are shown in the Table below.
|
Low
(pH <= 4) |
Moderate
(4 < pH <=5.5) |
High
(pH >= 5.5) |
|
| H2S
pp <= 1.5 psi |
20 |
15 |
10 |
Low (Cl-
<= 10,000 ppm) |
| " |
25 |
20 |
15 |
Moderate
(10,000 < Cl- <=100,000ppm) |
| " |
30 |
25 |
20 |
High (Cl-
> 100,000 ppm) |
| 1.5 psi
< H2S pp<=15 psi |
30 |
15 |
10 |
Low |
| " |
50 |
20 |
20 |
Moderate |
| " |
60 |
50 |
30 |
High |
Environmental
Severity Chart for Sour Systems with GOR >= 5000
The
system, at this point, also determines applicability of steels with
inhibitor systems. Steels are evaluated from a stand point of maximum
potential corrosion rate. This evaluation also includes consideration
of inhibitor systems based on temperature and flow conditions.
At
level 3, materials are evaluated for their susceptibility
to stress corrosion cracking. Factors in evaluation here include,
|
H2S
partial pressure |
|
pH |
|
Chlorides |
|
Maximum
operating temperature |
|
Material
composition in terms of nickel, chromium, molybdenum, tungsten
and columbium |
At
level 4, the materials are evaluated for their susceptibility
to pitting corrosion. A Required Minimum Pitting Index (RMPI) is
computed based on maximum temperature, and environmental severity
determined at level 2. Further, a pitting index is computed for
each of the materials in the solution set using the relation,
Pitting
Index = Cr + 3.3Mo + 11N + 1.5(W + Cb)
Where Cr, Mo,
N, W and Cb represent Chromium, Molybdenum, Nitrogen, Tungsten and
Columbium content of the alloy. All alloys that have a pitting index
greater than or equal to the RMPI are selected for further evaluation.
At
level 5, all applicable materials from level 4 are evaluated
for qualification based on sulfide stress cracking requirements
as well as certain rules of thumb based on application requirements.
The Socrates system contains 160 commonly used CRAs and their metallurgies
are stored in a mdb Access database file. Socrates' ODBC capabilities
ensure that the end-user can use any database management system
in working with the Socrates databases. The architecture of the
Socrates system also facilitates easy modification of the database
and its alloys without affecting the reasoning embodied in the system
or the knowledge base. The list of alloys in the Socrates system
is given in Appendix I.
At
level 6, all applicable materials from level 5 are evaluated
for qualification based on NACE MR0175 / ISO 15156 Requirements
(edition 2003). This level 6 characterization is optional and users
can choose to apply or ignore this level based on their requirements.
The Socrates
system also provides advice to the user on usability of steels in
a given production environment. The system evaluates steels for
SSC resistance and recommends feasibility of steel or inhibited
steel systems as alternatives to CRAs. The system also allows the
user to compare costs of using steels, inhibited steels or CRAs
through a cost analysis module and a material cost database.
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