Socrates System Description
The organization of different frames and the flow of data in the Socrates system is shown below. The earlier versions of Socrates system performed material selection by applying constraints at different hierarchical levels. The current Windows implementation is more flexible in that the user can use and apply constraints at any specific level without regard to the hierarchy.
The constraints are organized such that a comprehensive initial solution set of applicable materials is refined and reduced in size as the system proceeds through the different levels.
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:
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:
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.
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,
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.
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.